Volcanic geology and geochemistry of Waiotapu Ignimbrite ...

nie d eoch of Waiotapu Ignimbrite,

upo Vol nie Zon ,New land.

A thesis submitted in partial fulfilment

of the requirements for the Degree of

Master of Science at the

University of Canterbury by

Alistair B. H. Ritchie -;::;::.

University of Canterbury

1996

"/ have seen the truth and it makes no sense. J!

-Anon

Frontispiece

Waiotapu Ignimbrite at Wawa Quarry about to fall on me.

Waiotapu Ignimbrite (0.710 ± 0.06 Ma) is a predominantly densely welded, purple-grey coloured, pumice rich lenticulite, which is exposed on both eastern and western flanks of Taupo Volcanic Zone. The unit is uniform in terms of lithology and mineralogy over its entire extent and has been deposited as a single flow unit. The unit contains abundant pumice clasts which are often highly attenuated (aspect ratios of c.1 :30) and are evenly distributed throughout the deposit. Lithic fragments are rare, never exceeding 1 % of total rock volume at an outcrop and no proximal facies, such as lithic lag breccias, have been identified.

The deposit is densely welded to the base and only in more distal exposure does the ignimbrite become partially welded at the top of the deposit. Post-depositional devitrification is pervasive throughout the deposit, often destroying original vitroclastic texture in the matrix. Vapour phase alteration is extensive in welded and partially welded facies of the deposit.

Pumices within Waiotapu Ignimbrite appear to have been derived from two distinct magma batches, with differing Rb concentrations, that originated along different fractionation trends. Type-A pumices have significantly lower Rb than the subordinate type-B pumices. The presence of the pumices may represent the simultaneous evisceration of two spatially discrete magma chambers or the type-B chamber may have been intruded into type-A body, the magmas subsequently mingling prior to, or during, the eruption.

The source of Waiotapu Ignimbrite is poorly constrained, largely owing to the lack of meaningful maximum lithic data, and poor exposure of the unit. The distribution of the ignimbrite suggests that it was erupted from within Kapenga volcanic centre. If so the most proximal exposures of Waiotapu Ignimbrite are approximately 10km from the vent. Intensive and voluminous silicic volcanism, beginning with the eruption of the 0.33 Ma Whakamaru Group Ignimbrite eruptions, and extensive faulting within Kapenga volcanic centre will have obscured any intra-caldera Waiotapu Ignimbrite. The mechanism of eruption suggests that the source may not have been a caldera in the strictest sense, but instead a series of near linear fissures aligned with the trend of regional faulting.

Waiotapu Ignimbrite was generated in one sustained eruption and produced an energetic and high temperature pyroclastic flow. The lack of any recognised preceding plinian deposit, coupled with the energetic nature and paucity of lithics suggests eruption by an unusual mechanism. The eruption most likely resulted from the large scale collapse of a caldera block into the underlying chamber resulting in high discharge rates, which were no conducive to the development of a convecting column, and minimal vent erosion, resulting in negligible entrainment of lithics.

The density of welding and recrystallisation textures suggest that the flow retained heat to considerable distances which allowed the ignimbrite to weld densely to the base. The deposit was most likely progressively aggraded from the base, with material being supplied from an overriding particulate flow.

list of Figures list of Tables

Chapter One: INTRODUCTION

1.1 Field Area . . . . . . . . 1.2 Regional Geology

1.2.1 The Taupo Volcanic Zone 1.2.2 Volcanic History

1 .3 Previous Work 1.4 Objectives 1.5 Terminology. 1.6 Methods . .

Chapter Two: GEOLOGY OF WAIOTAPU IGNIMBRITE

Introduction . . . . . . 2.1 Distribution

2.1.1 Surface Exposure 2.1.2 Subsurface Distribution

2.2 Thickness and Volume 2.2.1 Thickness . 2.2.2 Aspect Ratio 2.2.3 Volume. .

2.3 Physical Character 2.3.1 Eastern TVZ 2.3.2 Western TVZ . 2.3.3 Lithic Fragments.

2.4 Welding and Density Variation 2.5 Wawa Quarry . . . . . . 2.6 Post Depositional Recrystallisation

2.6.1 Introduction 2.6.2 Western TVZ . 2.6.3 Eastern TVZ 2.6.4 Discussion. .

Page

iv vi

1

2 5 6 9 9 11

13

13 13

16 17 17

18 19 22 28 30

37 38 40 41

Chapter Three: WAIOTAPU IGNIMBRITE: PETROGRAPHIC AND GEOCHEMICAL VARIATION

Introduction . . . . . . . 3.1 Mineralogy

3.1.1 Mineral Descriptions 3.1.2 Ngapouri Ridge . . 3.1.3 Bison Road Quarry . 3.1.4 Wawa Quarry. . .

44

46 47 48 48

3.1.5 Lichfield Quarry 3.2 Geochemistry

3.2.1 Ngapouri Ridge 3.2.2 Bison Road Quarry . 3.2.3 Wawa Quarry. . . 3.2.4 Lichfield Quarry . .

3.3 Discussion: Vertical and Lateral Variation 3.3.1 Mineralogy. . 3.3.2 Geochemistry. . . . . . .

Chapter Four: GEOCHEMISTRY OF WAIOTAPU IGNIMBRITE

Introduction . . . . . . . . . . . . . . . . 4.1 Whole Rock VS. Pumice Chemistry - the Pros and Cons 4.2 General Characteristics . . . . . . . . . . . 4.3 Pumice Chemistry

4.3.1 Controls on Major and Trace Element Variation 4.3.2 Spider Diagrams.

4.4 Discussion . . . . . . . . . . . .

Chapter Five: GEOLOGY OF THE NGAPOURI RIDGE

Introduction . . . . 5.1 Stratigraphy. . . 5.2 Regional Structure 5.3 Ngapouri Rhyolite

5.3.1 Introduction 5.3.2 Petrography 5.3.3 Geochemistry 5.3.4 Discussion. .

5.4 The X-Files: Units Underlying Waiotapu Ignimbrite 5.4.1 Unit X . . . . . . . . . . 5.4.2 Akatawera AlRahopaka Ignimbrites

5.5 Paeroa Scarp Float Blocks 5.5.1 Introduction . . . . . 5.5.2 Characteristics . . . . 5.5.3 Discussion: Emplacement

Chapter Six: RAHOPAKA IGNIMBRITE

Introduction . . . . . 6.1 Stratigraphic Relations 6.2 Lithology . . 6.3 Geochemistry . 6.4 Petrology. . . 6.5 Lithic Fragments 6.6 Rahopaka Ignimbrite in Eastem TVZ? 6.7 Discussion . . . . . . . . .

Chapter Seven: DISCUSSION

Introduction . . . . . . . . . . 7.1 Ignimbrites: Eruption and Deposition.

Page

48

49 49 49 50

55 55

57 57 58

60 66 67

70 70 73

74 75 76 76

78 79

81 81 83

86 87 89 91 92 93 93 95

96 97

ii

7.2 Kapenga Volcanic Centre: the Source? 7.2.1 Introduction . . . . . . . 7.2.2 The Source ofWaiotapu Ignimbrite

7.3 Eruption and Deposition of Waiotapu Ignimbrite 7.3.1 Eruption. . . . . . . 7.3.2 Transport and Deposition. . . . . .

Page

98 101

103 104

Chapter Eight: CONCLUSIONS . . . . . . . . . . . . . . . . . 108

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . 110

REFERENCES . . . . . . .

~ppendix One: SAMPLE INDEX. . . . .

Appendix Two: GEOCHEMICAL ANALYSES

Appendix Three: MINERAL CHEMISTRY. .

Appendix Four: CLAST DENSITY MEASUREMENTS.

Appendix Five: POINT COUNT RESULTS

112

120

125

138

148

152

iii

Page

Chapter One: INTRODUCTION

Fig 1.1 Typical exposure of Waiotapu Ignimbrite. . . . . . . Fig 1 Map of Taupo Volcanic Zone. . . . . . . . . . .

1.3 Geophysical and geological profile of Taupo Volcanic Zone Fig 1.4 Recognised Taupo Volcanic Zone calderas. . . .

Chapter Two: WAIOTAPU IGNIMBRITE: FIELD GEOLOGY

2 3 4 6

Fig 2.1 Certain and probable distribution of Waiotapu Ignimbrite 14 Fig 2.2 Waiotapu Ignimbrite lithics at Butchers Boundary Road 16 Fig 2.3 Waiotapu Ignimbrite at Ngapouri Ridge . . . . . . 18 Fig 2.4 Graphic log of Waiotapu Ignimbrite at Ngapouri Ridge . 20 Fig Waiotapu Ignimbrite pumices. . . . . . . . . . 21 Fig 2.6 Graphic log of Waiotapu Ignimbrite at Bison Road Quarry 23 Fig 2.7 Graphic log of Waiotapu Ignimbrite at Rawhiti Road Quarry 24 Fig 2.8 Rawhiti Road Quarry. . . . . . . . . . . . . 25 Fig 2.9 Waiotapu Ignimbrite at Lichfield Quarry . . . . . . 26 Fig 2.10 Graphic log of Waiotapu Ignimbrite at Lichfield Quarry . 27 Fig 2.11 Welding textures in Waiotapu Ignimbrite. . . . . . 29 Fig 2.12 Wawa Quarry. . . . . . . . . . . . . . . 31 Fig 2.13 Isopach map ofWaiotapu Ignimbrite in Tokoroa-Kinleith region. 32 Fig 2.14 Graphic log of Waiotapu Ignimbrite at Wawa Quarry 33 Fig 2.15 Lower basal section at Wawa Quarry. . . . . 34 Fig 2.16 Upper basal section at Wawa Quarry. . . . . . 35 Fig 2.17 Generalised section through Wawa Quarry. . . . 36 Fig 2.18 Gas segregation structures in Waiotapu Ignimbrite . 37 Fig 2.19 Recrystallisation textures in Waiotapu Ignimbrite - western TVZ. 39 Fig 2.20 Recrystallisation textures at Ngapouri Ridge . . . . . . . 42

Chapter Three: WAIOTAPU IGNIMBRITE: PETROGRAPHIC AND GEOCHEMICAL VARIATION

Fig 3.1 Photomicrograph: Waiotapu Ignimbrite mineralogy Fig 3.2 Ab-An-Or plot of Waiotapu plagioclase . . . . Fig 3.3 Waiotapu Ignimbrite orthopyroxene compositions . Fig 3.4 Variation in Waiotapu Ignimbrite at Ngapouri Ridge Fig 3.5 Variation in Waiotapu Ignimbrite at Bison Road Quarry Fig 3.6 Variation in Waiotapu Ignimbrite at Wawa Quarry . Fig 3.7 Variation in Waiotapu Ignimbrite at Lichfield Quarry .

Chapter Four: GEOCHEMISTRY WAIOTAPU IGNIMBRITE

45 46 47 51 52 53 54

4.1 a) Total alkalis vs 8i02• b) K20 vs 8i02, Waiotapu Ignimbrite . . . . . 59

iv

Page

Fig 4.2 Selected major elements vs Si02 - Waiotapu Ignimbrite. 62 Fig Selected trace elements vs Si02 - Waiotapu Ignimbrite . 63 Fig Selected major elements vs Rb - Waiotapu Ignimbrite . 64 Fig 4.5 Selected trace elements vs Rb - Waiotapu Ignimbrite 65 Fig 4.6 Multi-element spider plot of selected Waiotapu Ignimbrite pumices 67 Fig 4.7 Sr vs Rb of TVZ ignimbrites, with Waiotapu analyses . . . . 68


Fig 5.1 Ngapouri Ridge . . . . . . . . . . . . . . . 71 Fig 5.2 Representative cross section of Waiotapu/Ngapouri Region 73 Fig 5.3 Ngapouri Rhyolite. . . . . . . . . . . . . . . 75 Fig 5.4 Oevitrification textures in Ngapouri Rhyolite. . . . . . 77 Fig 5.5 Variation diagrams comparing Waiotapu Ignimbrite with I\Igapouri Rhyolite 78 Fig 5.6 Outcrop of Unit X, Ngapouri Ridge . . . . . . . . . 79 Fig 5.7 Log of Unit X . . . . . . . . . . . . . . . 80 Fig 5.8 Blocks of Waiotapu Ignimbrite on the Paeroa Fault Scarp 82 Fig 5.9 Blocks at the base of the Paeroa Fault Scarp . . . . 84


Fig 6.1 Map of Matahana Basin. . . . . . . . . . . . . . Fig 6.2 Pukerimu Formation capped by Rahopaka Ignimbrite, Rusa Rd Fig 6.3 Rahopaka Ignimbrite near the end of Bob Rd Fig 6.4 Density profiles of Rahopaka Ignimbrite Fig 6.5 Lithics in Rahopaka Ignimbrite . . . . .

Chapter Seven: DISCUSSION AND CONCLUSIONS

86 87 90 91 94

Fig 7.1 The ignimbrite grade continuum. . . . . . . . . . . . 98 Fig 7.2 Gravity model of TVZ . . . . . . . . . . . . . . . 99 Fig 7.3 Temporal and spatial distribution of caldera forming activity in TVZ 101 Fig 7.4 Generalised isopach map of Waiotapu Ignimbrite 102 Fig 7.5 Eruption ofWaiotapu Ignimbrite. . . . . . . . . . . . 105

Appendix. Two: GEOCHEMICAL ANALYSES

Fig A2.1 Samples omitted from data set. . . . . . . . . . . . . . . 137

v

Page

Chapter Two: WAIOTAPU IGNIMBRITE: FIELD GEOLOGY

Table Thickness of Waiotapu Ignimbrite in Waiotapu Geothermal Field . 15 Table Welding facies in Waiotapu Ignimbrite. . . . . . . . .. 30

Chapter Four: GEOCHEMISTRY OF WAIOTAPU IGNIMBRITE

Table 4.1 Variation in major and trace elements in Waiotapu Ignimbrite Table 4.2 Analyses of representative Waiotapu Ignimbrite Pumices


60 61

Table 5.1 Stratigraphy of the Waiotapu Region ......... 72


Table 6.1 Generalised stratigraphy of Matahana Basin . . . . . .. 88 Table 6.2 Stratigraphy of Matahana Basin after Murphy (1977). . . .. 89 Table 6.3 Comparison of Rahopaka and Waiotapu Ignimbrite geochemistry 92 Table 6.4 Description of lithic fragments within Rahopaka Ignimbrite 93

Chapter Seven: DISCUSSION AND CONCLUSIONS

Table 7.1 Summary of volcanic activity at Kapenga Volcanic Centre . . . . . 100

vi

ONE

INTRODU ON

1.1 FIELD AREA

The Waiotapu Ignimbrite is exposed in two main areas of the Taupo Volcanic Zone,

one on the eastern side near Waiotapu, forming the prominent Ngapouri Ridge, and

the other on the western side where it comprises the bulk of the Tikorangi escarpment

in the Tokoroa forest, some 20 km east of Tokoroa (See Maps 1,2 and 3; in map

pocket). There are also scattered outliers in the Waikite Valley and within the

Matahana Basin. In addition it is exposed in quarries near Lichfield and to the south of

Kinleith Mill. Drillholes within the Waiotapu Geothermal Field have encountered large

thicknesses of Waiotapu Ignimbrite (Steiner, 1963). Large metre scale lithics occur

within the Kaingaroa Ignimbrite proximal lithic lag breccias which represent sub-surface

Waiotapu Ignimbrite on the eastern margin of the Reporoa Caldera.

The quality of outcrops is usually poor. At the time of field work much of the study

area within the Tokoroa forest was densely forested, consequently exposure was

restricted to three quarries, limited roadside outcrop and rare stream exposure (Figure

1.1). The areas of exposure of Waiotapu Ignimbrite south of Kinleith mapped by

Houghton et al (1987a) have now been reforested and no exposures of the ignimbrite

were found.

1.2 REGIONAL GEOLOGY

1.2.1 The Taupo Volcanic Zone

The Taupo Volcanic Zone (TVZ) (Figure 1.2) has been the main area of volcanic

activity in New Zealand over the last c. 2 Ma; the last c. 1.6 Ma being characterised by

intense rhyolitic volcanism. TVZ is 300 km long and 60 km wide and is the result of the

oblique convergence of the Pacific and Australian lithospheric plates, and the

subsequent subduction of the Pacific plate. It is the active eastern half of a wedge

shaped area of Quaternary volcanism termed the Central Volcanic Region (CVR),

originally defined by Thompson (1964) and subsequently redefined by Stern (1987) on

the basis of geophysical data. TVZ has a total extrusive magma flux of 0.3 m3 S-1 and

is the most productive and active silicic volcanic system on Earth (Wilson et ai, 1995).

The region is the site of numerous geothermal systems producing a total heat output of

1

Figure 1.1 Typical exposure of Waiotapu Ignimbrite. Throughout TVZ Waiotapu Ignimbrite is well exposed in three quarries and one cliff section. Elsewhere outcrop is restricted to sporadic outcrops such as this one on Beale Rd, Kinleith Forest.

4200 500 MW (Bibby et ai, 1995).

Estimates of the volume of material produced over the last 2 Ma vary from 10 000

km 3 (Cole, 1990) to 15 - 20 000 km3 (Wilson et ai, 1995). Eruptive products comprise

three major types; high alumina basalts (HABs), andesites, and the volumetrically

(>90%) dominant rhyolites; in addition there is a minor suite of dacites (Graham et ai,

1995). The HABs are evenly distributed throughout central TVZ and occur as a minor

part of a bimodal assemblage with the rhyolites from the caldera volcanoes. Modern

andesitic activity is mainly exposed in the northern and southern extremities of TVZ,

within the Tongariro region and Bay of Plenty respectively, but older andesitic

volcanism has been widespread throughout the region, as evidenced by various buried

andesite cones and pyroclastic flows encountered in drillholes in central TVZ (e.g

Steiner, 1963; Brown et ai, 1992). The minor dacites also occur in the central TVZ

(Graham et ai, 1995) and are associated with andesitic volcanoes such as Ruapehu,

White Island and Whale Island.

Cole (1990) described the TVZ in terms of an eastern volcanic front, or arc, that is

best developed around Tongariro in the south, while to the north it is represented by a

line of andesite/dacite volcanics on either side of the Whakatane graben. He

considered the 50 km wide tectonic Taupo-Rotorua basin, in the central TVZ to be an

2

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Figure 1.2 Map of the extent of Taupo Volcanic Zone. TVZ boundaries are after Davey et al (1995), Wright (1990) and Gamble et al (1993). Central Volcanic Region (CVR) after Stern (1985,1987). Inset: The setting of TVZ within North Island, New Zealand. VMFZ = Vening Meinesz Fracture Zone (after Gamble et ai, 1993). Adapted from Wilson et al (1995).

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Figure 1.3 Combined geophysical and geological profile of deep crustal and mantle structure beneath TVZ. From Stern and Davey (1987).

ensialic back arc basin dominated by rhyolitic volcanism, with minor basalt production.

Despite tentative support for this theory from geothermal system water chemistries

(Giggenbach, 1995) and basalt geochemistry (Gamble et ai, 1993), Wilson et al (1995)

do not concur. They cite the presence of andesites in drillholes in the central TVZ (e.g.

Steiner, 1963; Browne et ai, 1992) and consequently conclude that the andesites are

not restricted to the eastern TVZ. They conclude the TVZ is, therefore, unlikely to

represent andesite/dacite arc, and instead is most likely an arc disrupted by rifting.

The Taupo-Rotorua Basin is an extensional feature which has been spreading at

geodetically determined rates of 7-18 mm a-1 (Sissons, 1979), for approximately 0.9

Ma (Wilson et ai, 1995). Surficial faulting is normal with faults trending c. 0400 in the

vicinity of Taupo, while to the north at Okataina it is c. 0800• The change in orientation

of faulting is thought to be due to the influence of the dextral strike slip movement on

the North Island Shear Belt, which lies to the east (Cole, 1990).

The subsurface geology of the TVZ is subject to much debate and the true nature of

the subvolcanic basement is unknown. Drillholes on the eastern side have reached

greywacke while others in the central TVZ have not encountered basement despite

4

reaching depths of up to 2.5 km. Seismic reflection and refraction studies (Stern and

Davey, 1987) (Figure 1.3) suggest that the crust beneath the TVZ is 15 ± 2 km thick

and overlies upper mantle with anomalously low P wave velocities of c.7.5 km S-1,

North of Lake Taupo the upper c. 2.5 km of crust, thought to represent the Quaternary

volcanics, has a seismic velocity of 3.0 km S-1. Beneath this zone the seismic velocities

increase with depth from 5.1 km S-1 to 6.1 km S-1 before reaching the Moho. These

velocities are consistent with the presence of plutonic bodies of granitic or dioritic

composition, which is consistent with the high geothermal heat flux in the TVZ (Stern

and Davey, 1987).

1.2.2 Volcanic History

The TVZ encompasses an area of volcanic activity that began around 2 Ma in the form

of andesitic volcanism (Wilson et ai, 1995). From 1.6 Ma (Houghton et ai, 1995)

rhyolitic activity began and has dominated the zone to the present day. Eight rhyolitic

volcanic centres (Figure 1.4) have been active (Wilson et ai, 1995) with at least 34

caldera-forming events documented (Houghton et ai, 1995).

Wilson et al (1995) divide the history of the TVZ into three periods: old TVZ (c. 2 Ma

- 0.34 Ma) which encompasses activity from the inception of volcanism until the onset

of the Whakamaru Group eruptions, and young TVZ (0.34 Ma - present) which

includes all subsequent activity. The young TVZ also includes the modem TVZ which

refers to all activity since the eruption of the 65 ka Rotoiti Breccia

Activity began in the Mangakino Volcanic from c. 1.68 - 1.58 Ma and from c. 1.21 -

0.89 Ma (Houghton et ai, 1995) producing Ongatiti, Ahuroa, Rocky Hill and Marshall

Ignimbrites. Subsequently the locus of activity shifted to the Kapenga Volcanic Centre

which underwent an initial phase of activity from c. 0.89 - 0.71 Ma producing Tikorangi,

Matahana, Rahopaka and Waiotapu Ignimbrites (Wilson et ai, 1984; Houghton et ai,

1995). The end of this first phase of activity at Kapenga marked the beginning of an

apparent period of relative quiescence where no major caldera-forming eruptions are

present in the geological record until the eruption of the vast Whakamaru Group

Ignimbrites at 0.32 Ma (Houghton et ai, 1995). Subsequent active volcanic centres are;

Kapenga, Okataina, Reporoa, Rotorua, Maroa and Taupo (Wilson et ai, 1984;

Houghton et ai, 1995). Activity in the young TVZ has largely obscured deposits

produced prior to 0.32 Ma.

5

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Figure 1.4 Recognised rhyolitic volcanic centres and calderas active in the last 1.68 Ma. After Wilson et al (1995).

1.3 PREVIOUS WORK

There has been little detailed work on Waiotapu Ignimbrite prior to this study,

presumably because of the limited exposure of the unit. Much of this earlier work

related to establishing the unit's position within the stratigraphy of the TVZ, rather than

attempting to determine the nature of the ignimbrite.

The ignimbrite was initially identified in drillholes in the Waiotapu Geothermal Field,

although Marshall (1935) recorded the occurrence of "a pink wilsonite" near Putaruru,

now known to be Waiotapu Ignimbrite. The results of these drillhole investigations were

compiled in an unpublished report in 1957; a DSIR Research Bulletin detailing this

information, and incorporating subsequent revisions was published in 1963. The work

on the geological structure and history of the region was completed by Dr G.W.

Grindley, and his field mapping for the project was expanded and formed the basis of

the New Zealand Geological Survey 1 :63,360 series N85 (Waiotapu) sheet (Grindley,

1959). Grindley mapped large expanses of Waiotapu Ignimbrite in the Tokoroa Forest

6

and on the Ngapouri Ridge and proposed a source near Horohoro. The structural and

hydrothermal details of the region were published in Grindley (1963).

Seven cores were drilled for the geothermal project and logged, Waiotapu

Ignimbrite, termed Sheet II, was described as a lenticular ignimbrite and was notable

for "the absence of phenocrysts of quartz and biotite, and the presence of numerous

lenticles" (Steiner, 1963; p27) and was present in all 7 holes (Steiner, 1963). The

maximum thickness measured from the top to base of the ignimbrite was 350m,

although only 3 drill-holes actually reached the base of the ignimbrite. Martin (1961)

defined the type locality of the Waiotapu Ignimbrite as the eastern branch of the

Ngapouri Ridge (at N85/794767). He described a compound ignimbrite which was 305

m thick at Waiotapu and thinned westward to 9 m near Putaruru. Consequently the

source was inferred to be within the vicinity of Waiotapu. He described three layers

within the Waiotapu Ignimbrite on Ngapouri Ridge which lead to the interpretation that

the 110ws producing the ignimbrite were erupted in at least three pulses. Only one of

these flows was thought to have reached as far west as the Putaruru region as only

one flow unit was apparent in the area. Martin (1961) mapped Waiotapu Ignimbrite in

the Tokoroa Forest in a narrow strip along the Tikorangi Scarp. To the west of this

scarp the unit is unconformably overlain by the Mamaku Ignimbrite.

Grindley (1965) accepted Martin's (1961) source for the Waiotapu Ignimbrite and

further constrained the source to a vent on the Ngapouri Ridge. Following the eruption

he postulated that the vent was intruded by a rhyolite plug, then called the Waiotapu

Rhyolite, but which has since been renamed the Ngapouri Rhyolite (Grindley et ai,

1994).

Cox (1969) conducted a paleomagnetic survey of various North Island basalts,

andesites and ignimbrites. The collection of a normally orientated sample of Waiotapu

Ignimbrite from the Ngapouri Ridge lead to the conclusion that the ignimbrite post

dated the end of the Matuyama reversed polarity isochron, at the time thought to have

occurred at 0.68 Ma. He later noted a well defined change in the direction of magnetic

orientation between Marshall and Waiotapu Ignimbrites (both normally polarised) and

concluded that the orientations were consistent with the Marshall Ignimbrite being

older, with an interval of at least 103 years separating them (Cox, 1971).

Detailed field mapping of the Matahana Basin was carried out during 1975 by RP.

Murphy (Murphy, 1977). He accepted earlier conclusions concerning the source of the

ignimbrite and made no attempt to date it. Within the Matahana Basin Murphy

described the Waiotapu Ignimbrite as overlying the Marshall Ignimbrite (then dated at

0.56 Ma) and underlying the Mamaku Ignimbrite (then 0.17 Ma). However Murphy's

7

identification of the Marshall Ignimbrite within the Matahana Basin is now considered

incorrect (Houghton et ai, 1987a). At Bison Road Quarry (U16/771211) Murphy

conducted modal analyses on seven samples collected at intervals through the sheet,

finding no significant vertical variation in phenocryst proportions.

In addition Murphy recognised three new units beneath Waiotapu Ignimbrite,

including the Rahopaka Ignimbrite, the type section of which was defined at Pukerimu

Road (U16/789205). Paleomagnetic work found this unit to be reversely magnetised

and Murphy (1977) dated the ignimbrite between 0.85 and 0.69 Ma. A brief outline of

the stratigraphy of the Matahana Basin was published four years later (Murphy and

Seward, 1981).

During 1986 New Zealand Forest Products (NZFP) funded drilling and field mapping

in the Tokoroa/Kinleith area relating to groundwater and effluent disposal (Houghton et

ai, 1987 a). 13 drillholes in the region encountered Waiotapu Ignimbrite, varying from 1

to 50 m in thickness, in addition to surface exposures. Field relations and the dense

welding from top to near base, led to the conclusion that Waiotapu Ignimbrite was

"both a high temperature (i.e. highly welded) and energetic (i.e. very widespread)

deposit" (Houghton et ai, 1987a; p58). This conclusion is at odds with Murphy (1977)

who suggested that the Waiotapu Ignimbrite was largely topographically controlled.

The NZFP study, combined with the logging of a drillhole ENE of Tokoroa (Houghton et

ai, 1987b) in which Waiotapu Ignimbrite is present, helped to further constrain

ignimbrite stratigraphy on the western margin of TVZ.

In conjunction with the work of the New Zealand Geological Survey (Houghton et ai,

1987a) NZFP also funded the mapping of the south-west Tokoroa Plateau by M.G.

Gifford. Although the study emphasised the Marshall ignimbrites, Gifford did encounter

Waiotapu Ignimbrite (Gifford, 1988). He concluded, based on the presence of

paleosols and tuff units between the ignimbrites, and the degree of erosion of the

surface of the Marshall Ignimbrite, that the time break between the deposition of the

Marshall and Waiotapu ignimbrites was 104 - 105 years. Noting that no Waiotapu

Ignimbrite outcropped in his field area, Gifford concluded that either the unit had been

subsequently eroded or that it had failed to overtop nearby topographic highs,

therefore suggesting that the pyroclastic flows were not as energetic as suggested by

Houghton et al (1987a).

Grindley and Mumme (1991) carried out paleomagnetic work on 7 drillholes in the

western TVZ. Waiotapu Ignimbrite was identified in five of the drillholes and measured

samples were normally polarised. A unit that they correlated with the Rahopaka

8

Ignimbrite of Murphy (1977) was identified in two of the drillholes. They indicated the

Rahopaka Ignimbrite was stratigraphically below the Marshall Ignimbrites.

As part of a study of post depositional alteration in five ignimbrites exposed in

western TVZ, Dyah Hastuti (1992) studied welding and recrystallisation in Waiotapu

Ignimbrite at Wawa Quarry. Additional whole rock geochemistry of samples collected

from the unit suggested that the ignimbrite was erupted from a compositionally

homogenous magma chamber.

Recently Grindley et al (1994) have mapped the area around Waiotapu in some

detail, conducted further paleomagnetic investigations (though they did not measure

any samples of Waiotapu Ignimbrite) and undertook fission track dating of selected

units. Two samples of Waiotapu Ignimbrite gave fission track ages of 0.57 ± 0.05 Ma

and 0.58 ± 0.06 Ma (identical within error limits). The study also redefined the

previously accepted stratigraphy initiated by Steiner (1963).Finally new 40Ar_39Ar dates

of 0.71 ± 0.06 Ma for the Waiotapu Ignimbrite and 0.77 ± 0.03 Ma for the Rahopaka

Ignimbrite have been published by Houghton et al. (1995).

1.4 OBJECTIVES

The aim of this thesis is to take existing information and augment it with new data from

the field and laboratory analysis with a view to gaining a better understandi'ng of the

eruptive history and nature of the Waiotapu Ignimbrite. The main objectives are:

1. To determine the eruptive history of the Waiotapu event and clarify the nature of the

resulting pyroclastic flow from which the Waiotapu Ignimbrite was deposited.

2. Use maximum lithic data and information about the distribution of the unit to

determine the location of the source vent for the unit.

3. Determine vertical and lateral variations (if any) in the density, petrographic textures,

mineralogy and geochemistry of the ignimbrite.

4. Analyse the geochemistry of selected pumices within the Waiotapu Ignimbrite to

determine the nature of the magmatic system involved in the eruption.

5. Consider the relationship of the Rahopaka Ignimbrite (in the Matahana Basin), and

the I\Igapouri Rhyolite (on Ngapouri Ridge) to the Waiotapu Ignimbrite.

1.5 TERMINOLOGY

Ignimbrite: The term ignimbrite was first used by Marshall (1935) and is derived from

the Greek words ignis, meaning fire, and imber, meaning shower. Sparks et al

(1973) define an ignimbrite as: "the rock or deposit formed from pumiceous

pyroclastic flows irrespective of welding and volume." A degree of confusion exists

9

in the literature as the term can be used in a lithological (Le. describing a pyroclastic

deposit) or genetic (a deposit from a pyroclastic flow) sense (Cas and Wright,

1988). In the USA such deposits may be referred to as ash-flow tuffs. In New

Zealand all deposits from pyroclastic flows are termed ignimbrites.

Fiamme: extremely glassy attenuated pumices within a densely welded ignimbrite.

Vitroc/astic texture: Groundmass texture in ignimbrites. It is made up of broken

fragments (or shards) of pumices. Fragments commonly have cuspate

morphologies reflecting vesicle walls in pumices.

Lithic fragments: Exotic material incorporated into the pyroclastic flow and subsequent ~~~"- -<- -~ -<

deP9sit.. These may be either vent-derived, or accid~ntal lithics eroded and

entrained locally pya passing pyroclastic flow. Go-ignimbrite lag breCCias/

represent part of the proximal facies of anign)fl1~rite (Walker, 1985). Lithic size

generally decreases distally from the vent hence maximum lithic sizel

measurements (ML) are a useful means of constraining vent location.

Aspect Ratio: Ignimbrites may be categorised in two broad groups on the basis of the

ratio of their vertical dimension (usually average thickness) to an horizontal

dimension (commonly the radius of a circle enclosing the areal extent of the

deposit). High Aspect Ratio Ignimbrites (HARt) are generally low volume and

relatively passively emplaced and have aspect ratios upto 1 :400. Low Aspect Ratio

Ignimbrites (LARt) are characterised by large volumes and cataclysmic

emplacement with aspect ratios up to 1:100000 (Walker, 1983).

Welding: Commonly defined as the post-depositional sintering of hot pumice fragments

and glass shards under a compactional load (Ross & Smith, 1961). Welding may

also be a syn-depositional phenomena occurring during the progressive aggradation

of a passing pyroclastic flow (Branney & Kokelaar, 1992).

Ignimbrite grade: Depending on the degree of welding, ignimbrites may be considered

to be either high-grade or low-grade. Low-grade ignimbrites are non-welded

throughout, even when over SOm thick, while high-grade ignimbrites are densely

welded, even where they are less than Sm thick (Walker, 1983). Branney and

Kokelaar (1992) suggest an ignimbrite grade continuum ranging from extremely low

grade to extremely high grade, rather than defining strict end members.

Hackly fracture: Very rapid cooling can lead to the rapid development of innumerable,

closely spaced cooling joints which impart a rough hackly surface to the deposit

(Wilson, 1986).

Lithophysae: Cavities surrounded by a border of fine crystals which form as a result of

high temperature devitrification of coherent silicic glass during the early stages of

10

cooling, when the glass is still hot enough to deform plastically. Cavities form as

spherulites nucleate on small vesicles which expand due to exsolving volatiles with

continued spherulite development (McPhie et ai, 1993).

Oevitrification: S~~~~~~~~~~_is!~~!ITl?_~x~~mically unstable and consequently will readily

undergo post-eruption devitrification with the growth of crystallites of quartz and ,~~-"~~."",'~""-'-----~-- - - ----- - -

feldspar. Devitrification occurs in both lavas and pyroclastic deposits and can

develop to such an extent that original textures are completel~~e_~t:?¥~d (Cas &

Wright, 1988). Devitrification textures include; spherulites, bow-tie aggregates,

axiolites, orb texture and devitrification fronts (Ross & Smith, 1961). Extreme

devitrification results in the development of silica-feld§par mosaics which have been

referred to as granophyric interw()wths (e.g. Lofgren, 1971; Swanson et ai, 1989).

Other workers (e.g. Shelley, 1993) prefer to avoid this term due to the confusion

with granophyric intergrowth found in granites, and instead use the term <t~~;}ti~") /1 texture. The latter terminology is employed in this thesis.

Vapour phase alteration: occurs with or following devitrification and involves the

crystallisation of tridymite, cristobalite and alkali feldspar in open pore spaces.

These phases precipitate from volatiles derived from the precipitation of secondary

minerals during devitrification, but also volatiles, juvenile glass fragments and

heated ground water percolating through the deposit shortly after emplacement.

(Cas & Wright, 1988).

1.6 METHODS

Mineralogical and textural descriptioll~Lwere made by analysing thin secticJns of

representative samples. The sections were prepared by Rob Spiers at the University of

Canterbury using Logitech equipment.

Density of samples was determined using wax-coating methods described by

Houghton et al (1988). The results were tabulated and calculated using Microsoft Excel

5.0.

Major and trace element chemistry of both whole rock and pumice samples was

determined at the University of Canterbury by X-Ray Fluorescence spectrometry.

Samples were first crushed then milled to a powder in a tungsten carbide ring mill.

Following the general methods of Norrish and Hutton (1969) the powders were made

into glass fusion beads (to analyse major elements) and pressed powder pellets (to

analyse trace elements). The samples were run through a Philips PW1400 automatic

X-Ray Spectrometer by Stephen Brown. Detection limits and analytical uncertainty for

the results are outlined in Weaver et al (1990). Data were processed and plotted using

11

NEWPET geochemical software and the final graphs were produced using

CorelDRAW 4.0.

Several samples were analysed by X-Ray Diffraction to determine the nature of any

clay alteration in samples with high loss on ignition (from XRF work) or AI20 3 contents.

Samples were analysed using a Philips PW1729 x-ray generator and a Philips

PW1710 diffractometer control at the University of Canterbury by Stephen Brown.

Mineral and glass were analysed using a JEOL 8600 "Superprobe scanning

electron microprobe at the University of Otago (Dunedin) Geology Department with the

assistance of Dr Yosuke Kawachi. Polished sections of 11 samples (including 3

mineral separates) were made by Rob Spiers at the University of Canterbury.

12

CHAPTER Two

WAIOTAPU I NIM ITE

INTRODUCTION

This chapter will discuss the general field characteristics and internal stratigraphy of

Waiotapu Ignimbrite, with additional density and petrographic data.

Waiotapu Ignimbrite (0.71±0.06 Ma; Houghton et ai, 1995) is distinctive among TVZ

ignimbrite. It is welded throughout and has a characteristic blocky nature imparted by

numerous vertical and sub-horizontal cooling joints. In outcrop the unit ranges from

pale grey to dark purple and contains many highly attenuated pumices (10s of

centimetres in length), and, in more densely welded outcrop, black, glassy fiamme. In

hand specimen it is crystal poor (c.10-15% crystals) and has a distinct hackly fracture.

The ignimbrite is remarkably uniform in character, but has a well developed zone of

lithophysae near the base at Ngapouri Ridge.

2.1 DISTRIBUTION

2.1.1 Surface exposure

Waiotapu Ignimbrite is exposed on both the eastern and western margins of TVZ, in a

band 15km wide and 55 km long between Lichfield in the west and Waiotapu in the

east (see main map in back pocket). Since the onset of the eruption of the c.0.33Ma

Whakamaru Group ignimbrites (Brown, 1994), TVZ has been the site of intense and

voluminous rhyolitic activity that has obscured much of the older deposits on both

flanks of TVZ. Exposure of Waiotapu Ignimbrite is therefore poor, but does form most

of two major landforms: the Tikorangi escarpment in the west and the Ngapouri Ridge

in the east. Figure 2.1 is a generalised map of the certain and possible distributions of

Waiotapu Ignimbrite.

2.1.2 Subsurface distribution

Lithic component analyses (LeA) of younger ignimbrites reveals much about the

nature of units that lie within the calderas formed as a result of their eruption. With

additional drillhole data inferences can be made about the distribution of Waiotapu

Ignimbrite by documenting its occurrence in lithic lag breccias surrounding the study

area.

13

..>.

~

.. ~ .

", -, . '" ......... f)RoeAeL~ ...............

N

No Waiotapu Ignimbrite lithics have been recovered from Rotoiti and Earthquake Flat Breccias (Burt et ai, in press; this study) .

o km 10

Tokoroa

~ - ~- " ... • +

!.. ... : k' ...

:. + +

\. Kinleilh ..•.. '- ~

2.1 Certain (solid line) and probable (dotted line) distribution of Waiotapu Ignimbrite (black). The certain distribution encompasses known occurrences of Waiotapu constrained by outcrop, drillhole (+) and lithic data. The probable distribution is an estimate and accounts for the absence of the unit in Okataina volcanic centre and Whakamaru caldera eruptives .

'. ·""4 ••

................

' ..

.......

...............

".

.......

...........

................................. No Waiotapu Ignimbrite lithics reported in Whakamaru Group Ignimbrites (Brown, 1994).

Butchers Boundary Road: here Kaingaroa Ignimbrite lithic

lag breccias contain. 14% Waiotapu Ignimbrite lithics upto 3m in diameter.

No Waiotapu Ignimbrite was found as lithics in Whakamaru Ignimbrite (Brown,

1994) suggesting Waiotapu Ignimbrite does not occur beneath Whakamaru Caldera.

Nor have Waiotapu Ignimbrite lithics been recovered from Earthquake Flat or Rotoiti

Breccias which lie to the north (Burt et ai, in prep and reconnaissance field work

conducted by the author).

By contrast, in the east, LCA has revealed Waiotapu Ignimbrite to be a significant

component in Kaingaroa Ignimbrite lithic lag breccias (S.W. Beresford pers. comm.

1995). At Butchers Boundary Road (U17/062999), on the eastern margin of Reporoa

Caldera, clasts (some up to 1.5m across) of Waiotapu Ignimbrite are the second most

dominant lithic type, making up 14% of the proximal lithic lag breccia in Kaingaroa

Ignimbrite (Figure 2.2). Waiotapu Ignimbrite appears to be ubiquitous throughout

Kaingaroa Ignimbrite. The sheer abundance of Waiotapu Ignimbrite fragments

suggests that, at least along the eastern margin of Reporoa Caldera, the ignimbrite

was present in considerable thicknesses.

Keall (1988) has described Waiotapu-like lithics in Te Weta and Te Kopia Ignimbrite

lag breccias in Paeroa Scarp. Due to the confusion concerning the stratigraphy of the

region at the time (see Chapter 5.1) Waiotapu Ignimbrite was considered younger than

the Paeroa Range Group Ignimbrites and therefore Keall (1988) would not have

identified them as such. The sources for Te Kopia Ignimbrite (north of Paeroa Fault)

and Te Weta Ignimbrite (south of Ngapouri Ridge) defined by Keall (1988) both lie

within the likely distribution of Waiotapu Ignimbrite.

It is not known whether Waiotapu Ignimbrite travelled beyond Reporoa Caldera. On

northern Kaingaroa Plateau a series of 3 stratigraphic drillholes were drilled on

Northern Boundary Road (NBR1: V16/191115; NBR2: V16/207113; NBR3:

V16/196112; Nairn, 1984). Of these cores only one (NBR2) penetrated beneath

Rangitaiki Ignimbrite to a depth of 227m, into a grey-purple ignimbrite which graded

into a purple-grey lenticulite was logged at a depth of 143m, approximately the right

stratigraphic location for Waiotapu Ignimbrite. The mineralogy (plagioclase, quartz\

amphibole) is however inconsistent with Waiotapu Ignimbrite presenting two

possibilities: either Waiotapu Ignimbrite is present at greater depth and the drillholes

Table 2.1 Thicknesses of Waiotapu Ignimbrite encountered in drillcores within the Waiotapu Geothermal field (after Steiner, 1963; Hedenquist, 1983).

Drillho/e Wt1 Wt2 Wt3 Wt4 Wt5 Wt6 Wt7 Depth to top of unit (m) 240 170 250 230 230 175 130 Depth to base of unit (m) 485+ 455+ 454+ 580 455+ 415 360 Total thickness of unit (m) 245+ 285+ 204+ 350 225+ 240 230

15

Figure Large lithic fragments of Waiotapu Ignimbrite within Kaingaroa Ignimbrite lithic lag breccias at Butchers Boundary Rd (U17/062999), Kaingaroa Forest. Here Waiotapu Ignimbrite lithics make up c.14% of the breccia and are up to 3m in size. Sledgehammer is 55cm long.

penetrated younger, as yet unrecognised ignimbrites; or Waiotapu Ignimbrite is absent

and the lenticulite may correlate with the pre-Waiotapu Ignimbrite quartz/amphibole

bearing tuffs described in the Waiotapu Geothermal Field (Grindley et ai, 1995;

C.P.Wood pers. comm. 1996).

In western TVZ, drill hole data around Tokoroa and Kinleith suggests that the deposit

is at the limits of flow, thicknesses of the unit being <1 m in places. The subsurface

distribution of Waiotapu Ignimbrite around Tokoroa/Kinleith will be discussed in detail

in section 2.4.

2.2 THICKNESS AND VOLUME

2.2.1 Thickness

Within its outcrop area Waiotapu Ignimbrite ranges in thickness from 10 to 140 m,

however within the drill holes in the Waiotapu Geothermal Field the unit is reported to

be up to 350m thick (Steiner, 1963).

In the western TVZ Waiotapu Ignimbrite is present in thickness up to 140m along

the Tikorangi Escarpment. Drillhole TIK1 (U16/768272), north of the Tikorangi

Escarpment encountered 114m of the unit whereas a drillhole at Sutton Road

16

(U16/656303) encountered 30m of Waiotapu Ignimbrite beneath the Mamaku Plateau

(Grindley and Mumme, 1991). Drillholes near Tokoroa and Kinleith contain Waiotapu

Ignimbrite ranging from 1 to 50 m in thickness (see section 2.4).

In eastern TVl the ignimbrite is at least 100m thick at the Ngapouri Ridge and then

increases in thickness toward the Waiotapu Geothermal Field (Table 2.1). Below the

Paeroa Scarp on Te Kopia Road (U17/913066) drillhole TK2 has penetrated Waiotapu

Ignimbrite 150m thick (Grindley et ai, 1995). Beneath the eastern rim of Reporoa

Caldera the deposit is present in great thicknesses (see section 2.1).

2.2.2 Aspect ratio

The thickness of the ignimbrite is variable, and is controlled by factors such as

topography and proximity to source. The overall lateral extent of the deposit is

unknown and estimates of the aspect ratio vary. The vertical component, however, has

been defined as the average thickness of the deposit, which is 54m (calculated from all

drillhole and selected outcrop data).

Four aspect ratios, with varying lateral extent have been calculated:

a) Waiotapu Ignimbrite extending from Lichfield Quarry (T16/588400) to the

eastern margin of Reporoa Caldera, c. 62.5: 1 :1157

b) Waiotapu Ignimbrite confined to the extent of outcrop. c. 55km: 1:1019

c) Lateral extent defined by a circle with a radius of 38km, the distance from

Lichfield Quarry to Ngakuru, the arbitrarily defined point source of Waiotapu

Ignimbrite within Kapenga Volcanic Centre, c.76 km: 1 :1407.

These aspect ratios are remarkably low given the extent of the deposit, this is most

likely a function of the extreme thickening of the deposit as it ponds around Waiotapu

geothermal field and Reporoa caldera.

2.2.3 Volume

The limited outcrop of Waiotapu Ignimbrite precludes an accurate estimate of the

volume of the deposit. An estimate can be made by determining the volume of a series

of concentric cylinders whose radii were defined by the distance from the source

location of Ngakuru. The resulting estimate of 174 km 3 is a minimum figure as it fails to

account for any caldera-fill ignimbrite that may lie within the source region. In addition

the thickness of the unit east and south of Waiotapu is unknown, but is likely to be

considerable.

17

Figure Cliff forming Waiotapu Ignimbrite at the south end of Ngapouri Ridge (U171013099). Note the lithophysae zone near the base of the photo.

2.3 PHYSICAL CHARACTER

Waiotapu Ignimbrite shows minimal variation in character over the extent of its surficial

outcrop. Most variation, e.g lithophysae development, results from post depositional

phenomena. This section outlines the nature of key components of Waiotapu

Ignimbrite and their relationship to each other with a view to outlining possible eruptive

and depositional mechanisms for the unit.

2.3.1 Eastern TVZ

The bulk of the exposure of Waiotapu Ignimbrite in eastern TVZ is at Ngapourl Ridge

where it is at least 100m thick. Here the unit comprises the bulk of a ridge which

18

bifurcates to the south. At the southern end the ignimbrite is a major cliff forming unit,

forming exposures some 40m high (Figure 2.3). To the north the topography is steep

but exposure of the unit is restricted to isolated and small outcrops distributed across

the hill side. Within the core of the ridge are the eroded remains of a pre 0.75 Ma

rhyolite dome, the Ngapouri Rhyolite.

Here the ignimbrite shows the most variation in outcrop character than at any other

exposure (Figure 2.4). It is welded throughout (1.79-2.16 g cm -3) and has a uniform

dark purple colour. The unit is vertically and horizontally jointed, the intensity of jointing

being more marked at the southern end of the ridge. Horizontal jointing ranges from

<1 m to c.5m while vertical joining is commonly 1 m or greater. The apparent decrease

in jointing to the north of the ridge is most likely a function of small outcrop size.

Pumices are common and show considerable variation in size and aspect ratio, but

all are attenuated to some degree (Figure 2.5). The degree of attenuation of pumices

can be extreme, with aspect ratios varying from 1:S to 1:37, the mean being

approximately 1: 10. The majority of pumices are cream coloured, sparsely crystalline

and poorly vesicular (which probably reflects flattening), with most having grey rinds up

to Smm thick. Rare dark grey to black fiamme occur.

Throughout the entire ridge isolated lithophysae occur but at the south end they are

concentrated in two zones several metres thick. The zones are of variable thickness

and are typically lensoidal, occurring at the base of the cliff section and approximately

S m below the top of the ridge. Lithophysae are often elongate parallel to the

orientation offoliation within the ignimbrite, and are sometimes in excess of 10cm long,

although they are usually <Scm.

2.3.2 Western TVZ

Waiotapu Ignimbrite is exposed in three key localities in western TVZ; along Tikorangi

Escarpment, in Wawa Quarry and at Lichfield Quarry. Waiotapu Ignimbrite at Wawa

Quarry will be discussed in detail in section 2.4.

Along Tikorangi escarpment, Waiotapu Ignimbrite is up to 140m thick. The greatest

stratigraphic thickness of the ignimbrite is exposed in Bison Road and Rawhiti Road

Quarries; elsewhere outcrop is restricted to isolated exposures on the Waiotapu

Ignimbrite surface, in isolated cliffs in the upper section of the escarpment, to the west

of the scarp, or in streams to the south of the escarpment The unit is characterised by

numerous closely spaced (commonly <O.Sm) vertical and sub-horizontal joints, which

imparts a blocky nature to most outcrops. The sub-horizontal joints are more numerous

19

N o

I !!!. (0

;:r OJ cr o < CD

50

g- 25 I/J CD

~ o S. C'l a '0

]:

=--

-=

- I .

-=--'=" D

~-=

=:= ~

~

-o ,....

i 1

1:20 1:40 1.6 2.0 2.4 Pumice Aspect Ratio Density (g cmOs)

Uthophysae zone; numerous, highly elongate cavities, 15 cm in length.

Densely welded, dark purple, crystal-lithic poor, pumiceous ignimbrite.Occasionallithophysae.

Uthophysae zone; concentration of elongate cavities 1-20 cm long.

Pumice enriched zone (see Figure 2.5a).

Figure 2.4 Graphic log ofWaiotapu Ignimbrite Ngapouri Ridge (U17/015098).

o

Figure 2.5 Pumices within Waiotapu Ignimbrite from Ngapouri Ridge. a) From near the base of the unit (U 16/023127). Note the varying degree of development of the black rinds. Hammer head is 17.5 cm long. b) Large pumice from the north end of Ngapouri Ridge (U17/018098). Pencil is 14cm long.

21

than the vertical joints. Grey rinds on pumices in western TVZ are less developed than

at Ngapouri Ridge, and are generally <2mm thick.

Figures 2.6 and are graphic logs of Bison Road and Rawhiti Road Quarries.

Bison Road Quarry is uniform in nature throughout, showing little variation in density

and pumice aspect ratio. At the intersection of Bison Road and Tikorangi Road

(U161771207) 3m+ thick succession of non-welded, highly altered, buff coloured

material underlies Waiotapu Ignimbrite, this deposit is also exposed in the Tikorangi

Escarpment. This was originally interpreted by Murphy (1977) as one of the Marshall

Ignimbrites but Houghton et al (1987a) disagreed saying it was the unwelded base of

Waiotapu Ignimbrite. Mineralogically the deposit resembles Waiotapu Ignimbrite

(plag>px>mag>ilm), yet mineral chemistry reveals two pyroxene populations, both of

which differ greatly from Waiotapu Ignimbrite pyroxenes. The unit is consequently

considered to be unrelated to Waiotapu Ignimbrite.

At Rawhiti Road (Figure 2.8) the unit shows marked variation in colour, ranging from

a pale grey to a very dark purple. The colour shows no systematic variation with height

and degree of welding although in places colour change is clearly associated with soft

veins within the deposit. This may reflect varying degrees of vapour phase activity.

At Lichfield Quarry (Figure 2.9) the unit is uniform in character, again showing little

variation in density and pumice aspect ratio (Figure 10). Pumices are still common

although recorded aspect ratios are not as extreme. Rare black fiamme are present

towards the base of the deposit. Sub-horizontal jointing is poorly developed and the

ignimbrite has a more coherent appearance. Although the actual contact is not

observed, the ignimbrite overlies Ahuroa and Ongatiti Ignimbrites (1.18±0.02 Ma and

1.21±O.04 Ma respectively; Houghton et ai, 1995), and has apparently flowed over, or

around a paleohigh with a slope of c. 30°. Two ignimbrite facies normally result from

the interaction of a pyroclastic flow with changes in topography: Valley Ponded

Ignimbrite and Ignimbrite Veneer Deposit (Wilson and Walker, 1982), but there is no

variation in Waiotapu Ignimbrite between the top of the paleohigh and the valley

bottom.

2.3.3 lithic fragments

There are very few lithics in Waiotapu Ignimbrite. The largest lithic clast measured was

6cm at the north end of Ngapouri Ridge (U16/023127). Elsewhere rare lithic fragments

have been recorded in outcrop, not usually exceeding 1 cm. Most fragments are

rhyolite or greywacke, with rare dacite and possible trachyte. The problem is

exacerbated by the nature of outcrop of Waiotapu Ignimbrite, which makes it difficult to

N W

I (1)

tCi" ;:! 0> 0-0 < (1)

0-0> If) Cl)

9., 0 c:.

§ "0

]:

40

~

-='='"

20

~

-=

0"'------1 :20 1:40 1.6 2.0 2.4

Pumice Aspect Ratio Density (g em·:I)

Densely welded, purple coloured, crystal-lithic poor, pumiceous ignimbrite. No variation in character throughout exposure. Sub-horizontal jointing is intense and blocky character.

distinctive

Figure 2.6 Graphic log of Waiotapu Ignimbrite at Bison Rd Quarry, Kinleith Forest (U16/771211).

N .$:>.

Ol c-o <: (j)

c-Ol

'" (j)

a 0 c 0 0 "0

:[

8

4

o

0 ---

..c:::J -='" i

= ~

=l -=-----=~

~

I

1:20 1:40 PumiceAspect Ratio

1.6 2.0

(g

2.4

Figure 2.7 Graphic log of Waiotapu Ignimbrite at

Light grey to purple grey coloured, crystal-lithic poor, pumiceous ignimbrite. Shows considerable, but not systematic variation in colour throughout the exposure.

is intense and unit has distinctive

Rd Quarry, an71215).

---

P0

... _1\'-

-h. ,'" .".:' .,.', '-' -.-

. ,., ..... ~."""O>~

Figure 2.8 Rawhiti Road Quarry, Kinleith Forest (U16/77121S). Note the closely spaced,

~ --------------------------------------------------------------------------------------------------

N OJ

2.9 Waiotapu Ignimbrite at Uchfiled Quarry (T16/588400). Here a slope of 30°.

a paleohigh (Ongatiti and Ahuroa Ignimbrite)

::c (l)

to" ;:r III co <: (l)

IJ

10

fll 5 (l)

s, o c: no -0

~

o

T

grey/pink, welded, crystal-lithic poor ignimbrite.

1:20 1:40 1.5 1.7 1.9

Pumice Aspect Ratio Denslty (g cm·3)

Figure 2.10 Graphic Lag of Waiotapu Ignimbrite at Uchfield Quarry (T16/588400).

identify lithics. During point counting of samples of Waiotapu Ignimbrite at Wawa

Quarry, Dyah Hastuti (1992) noted that the percentage of lithics at that locality did not

exceed 0.4%.

The occurrence of coarse breccias in close proximity to ignimbrite source vents was

first reported by Wright and Walker (1977) in the Acaltan Ignimbrite, Mexico. Since

then such breccias have been recognised as the usual proximal facies of many

ignimbrites (e.g. Walker, 1985; Nairn et ai, 1994). It is likely the most proximal

Waiotapu Ignimbrite exposed is at least 10km from the source and lag breccias are

unlikely to be deposited at this distance from the vent. The lack of exposure of

proximal facies does not, however, explain the marked paucity of lit~lics elsewhere in

the deposit. It is possible that the flow did not incorporate large quantities of lithic

fragments. at the source. This may be due to limited fragmentation around the vent, the

lack of a caldera associated with the Waiotapu eruption (unlikely given the volume of

material erupted), or that caldera collapse was not syn-eruptive but followed the

generation of the pyroclastic flow.

2.4 WELDING AND DENSITY VARIATION

Whole rock density has been suggested as a reliable method of quantifying the

welding state of ignimbrites (Houghton et ai, 1988). Syn- and post-depositional

processes operating in ignimbrites produce characteristic textures in the ground mass

and, on hand specimen scale, in pumices. Consequently distinct welding facies can

usually be recognised in ignimbrites (e.g. Streck and Grunder, 1995). This section

aims to present bulk density data determined for Waiotapu Ignimbrite and relate it to

variations in groundmass texture, observed under the microscope, and the aspect ratio

and character of pumices in the deposit.

Syn- and post-depositional compaction and deformation are the most likely causes

of density variation in the deposit. However other post-depositional processes, such as

the preCipitation of vapour phase minerals into open pore space, will also increase the

density of the deposit. It has been suggested (Moon, 1994) that induration is a more

applicable term for the description of ignimbrite density and hardness, as welding

(sensu Smith, 1960a: "promotes the union or cohesion of glassy fragments in a

viscous state") is not readily observed. For the purposes of this thesis shard texture

and pumice deformation will be considered to be a result of depositional processes.

Density variation will reflect these process but will also have been affected by

recrystallisation.

Three welding facies are distinguishable in Waiotapu Ignimbrite and are

28

N to

Figure 2.11 Welding textures in Waiotapu Ignimbrite. All ppl, field of view 3mm. a) Partially welded. Shards are slightly deformed but

original morphology is still Welded. Shards are attenuated and are beginning

to deform around crystals. Densely welded. Shards are highly attenuated and are deforminq stronqlv around

Table 2.2 Welding facies recognised in Waiotapu Ignimbrite. Density values are the estimated range for each facies and are only a guide as the transition between facies is highly gradational.

Welding facies Densit! Clast deformation Colour (g cm-) Pumices Shards

Partially welded <1.60-1.75 Moderate Slight Light pink to purple

Welded 1.75-1.95 Moderate to extreme Moderate Purple to dark purple

Densely welded 1.95-2.30 Moderate to extreme Strong Dark purple or medium grey

summarised in Table 2.2. Classification is based on clast density, pumice deformation

(reflected by aspect ratio) and deformation of glass shards. In partially welded facies

(density:: 1.60-1.75 g cm-3) (Figure 2.11a) pumice aspect ratios are low, rarely

exceeding 1:10. Deformation of glass shards is slight and their original cuspate

morphologies are still evident. Shards do not deform around crystals. In welded facies

(1.75-1.95 g cm-3) (Figure 2.11b) the degree of pumice deformation is greater and

aspect ratios in excess of 1 :30 have been measured in outcrop. Deformation of shards

in the groundmass increases, although their original cuspate morphology is still

evident. Shards are beginning to deform around crystals. In densely welded ignimbrite

(1.95-2.30 g cm-3) (Figure 2;11 c) shards are increasingly attenuated and are strongly

deformed around crystals, often to the point where original shard texture is completely

obscured. There is no discernible increase in the degree of deformation of pumices

although black, glassy fiamme are more common and eventually dominate the pumice

population in samples with densities in excess of c.2.1 0 g cm-3. Hackly fracture is best

developed in samples with densities between 1.80 and 2.05 g cm-3.

2.5 WAWA QUARRY

The only exposure of Waiotapu Ignimbrite which offers a complete section from the

base of the unit to its eroded top is at Wawa Quarry (T16/627174) (Figure 2.12). Here

the ignimbrite has ponded in a valley or depression to a thickness of c.30m. This is

consistent with the topography prior to the eruption of Waiotapu Ignimbrite, evidenced

mainly by drillhole data, in which there is a marked erosional surface on Unit X, Ahuroa

Ignimbrite and Marshall Ignimbrite, in the Wawa Quarry area, and Ongatiti and Ahuroa

Ignimbrite to the north near Lichfield Quarry (Figure 2.13). Figure 2.14 is a graphic log

of Waiotapu Ignimbrite at Wawa Quarry.

The basal contact of the ignimbrite is sharp (Figure 2.15a) and is observed at two

locations in the quarry (Figure 2.15b and 2.16). The ignimbrite is densely welded to the

30

2 Waiotapu Ignimbrite at Wawa Quarry 6/771215). are exposed, at the bottom right and on the next bench up, to the left of the front end loader.

W ->.

Thickness in metres

30

60

Tokoroa

10

10 Scale 1 :50 000

o

o 1 2 3

km

Figure 2.13 Isopach map of Waiotapu Ignimbrite in the Tokoroa-Kinleith region. The thickness of the deposit varies considerably over short distances, reflecting the nature of the pre-Waiotapu Ignimbrite erosion surface in the underlying Marshall, Ahuroa and Ongatiti Ignimbrites. Drillhole data compiled from Houghton et al (1987a) and unpublished data used with permission of Carter Holt Harvey Ltd.

32

20 Grey, densely welded, crystal-lithic poor, lenticulite.

I'~ :t (!)

tCi' ;:r III c:r 0

Light pink, welded, pumiceous, crystal-lithic ignimbrite. <

l (!)

c:r III

'" (!)

a 10 1 l c: :::I :::::

i Grey, densely welded, crystal-lithic poor, lenticulite.

1 +

o Dark grey, densely welded, crystal-lithic poor, lenticulite. 1 :20 1 :40 1.4 1.8 2.2

Pumice Aspect Ratio Density (9 cm·3)

Figure 2.14 Graphic log of Waiotapu Ignimbrite at Wawa Quarry, Kinleith Forest 6/627174}.

w ..p".

I ro IB' ;r 0.25 ru 0-o ii 020 or ~ @ 0.15 g. c ~ 010

~ - 0.05

000

1.j 1 5 1 3 Z 0 2.2 2.4

DonsHy (9Icm')

Figure 2.15 The lower section of Waiotaou lanimbrite Wawa Quarry (T16/627174).

Close up of sharp contact with the underlvina non-welded ignimbrite. Marker pen is 12.5 cm b) basal section with density profiles illustrating welded nature of Waiotapu Ignimbrite at its base. intense in the first metre above the base (spacings of c.20cm) become more widely spaced with increasing height.

0.30

" g-O.15 " :;: s. ~ 010

g 05

000 " '::">-:'

1 B 2.0 22

Densily (Stem3)

w (J1

2.16 The upper exposure

Waiotapu Ignimbrite Basal Density Profile (3}

Wawa Quarrv (T16/627174)

0.20 Eli

I CD <3' 0.15 .. ;::: OJ c-o <: CD

g 0.10 (f)

CD 0 -. c 2. ~ 0.05 E-

0.00 . .11>

2.0 2.2 2.4

Density (g/cm3)

at Wawa Quarry.

26., .", J

:: 1 8~ :::r: 20 ./8 (1)

<0' 18 i ;:?;

Il> 16 ~ \ o::r

0 < 14j " CD \ o::r Il>

~~ 1 (II

.~ (1)

g, c

./" ~ :[ 6 J

: j I .------

o i ~ , ' j I e-

Upper Basal Section

Lower Basal Section 1.4 1.6 1.8 2.0 2.2

Density (g/cm3)

Figure 2.17 Generalised cross-section through Wawa Quarry showing the relative location of the base of the ignimbrite and bench surfaces.

base at both outcrops and becomes less welded with increasing height. Density

profiles at Wawa Quarry have previously been interpreted to infer the presence of 3

flow units within the ignimbrite (Dyah Hastuti, 1992), because of increases in welding

at c. 9 and 21 metres. However recent expansion of the quarry has revealed that the

welding increase at 9m was due to the proximity of the base (Figure 2.17). It is

therefore likely that welding variation may simply reflect the proximity of the base

rather than the presence of multiple flow units.

The density of welding at the base of the unit is anomalous. Density measurements

at the basal contact areiolflusually high, ranging from 1.80 to 2.20 g cm-3 (see Figures

2.1Sb and 2.16). Densities then increase in the next metre (1.87-2.27 g cm-3) before

gradually decreasing with increasing height. This increase in welding toward the base

is unusual as in classically zoned ignimbrites (e.g. Smith, 1960b) welding is usually

densest in the middle of the deposit and decreases towards the base. Groundmass

textures (see Figure 2.11) confirm that this is a primary phenomena, rather than being

produced by intense vapour phase alteration or diagenetic processes.

Elongate fines depleted pods have been identified in float blocks in the quarry

(Figure 2.18), but despite a search were not observed in the quarry wall. These have

been interpreted to represent small scale, localised gas segregation structures.

Gasses escaping through these structures are most probably from the combustion of

vegetation, or evaporation of water.

36

Figure 2.18 Gas segregation structures in Waiotapu Ignimbrite at Wawa Quarry. These structures are fines depleted and crystal enriched, lithics are exceedingly rare.

2.6 POST-DEPOSITIONAL RECRYSTALUSATION

2.6.1 Introduction

Glassy material in Waiotapu Ignimbrite (both shards and pumice) has undergone

pervasive devitrification and has almost certainly experienced vapour phase alteration.

The density of welding, and corresponding low porosity of the ignimbrite, has led to

lack of open space within the unit that means vapour phase alteration may have been

restricted in the more densely welded portions of the unit. Where the degree of welding

is below c.2.0 g cm-3 the intensity of recrystallisation in pumices (which are most likely

to contain vapour phase assemblages) increases but it is frequently not possible by

optical methods to distinguish between recrystallisation due to devitrification and that

37

caused by vapour phase alteration. The development of spherulitic and axiolitic

texture, however, can be considered to have resulted from devitrification alone.

Recrystallisation zones are defined on the degree of recrystallisation of pumice and

fiamme within the deposit and, to a lesser extent, recrystaliisation within the shard

matrix. The boundary between zones is gradational, for example spherulites may

develop in pumices to the extent that they are difficult to distinguish from felsitic texture

without the aid of a sensitive tint plate.

Recrystallisation within Waiotapu Ignimbrite differs between sections in western

(Bison Road Quarry, Wawa Quarry, and Lichfield Quarry) and eastern lYZ (Ngapouri

Ridge) and are described in the sections below.

2.6.2 Western TVZ

Two broad recrystallisation zones have been identified in western lYZ and are as

follows:

Spheru/itic Fiamme Zone: Oevitrification within fiamme is dominated by the

growth of spherulites (Figure 2.19a). Spherulites commonly nucleate on the

fiamme wall leading to the development of mosaics of incomplete spherulites.

Occasionally recrystallisation appears to have nucleated on crystals within the

fiamme and circular spherulites develop within the mosaic. Spherulite

development often fails to extend into the centre of the fiamme and relics of

the original fiamme texture may be retained. Within the shard matrix

vitroclastic texture is perfectly preserved and devitrification is restricted to the

development of weak axiolitic texture within shards, which do not extend

beyond shard walls.

Fe/sinc Pumice Zone: Within the felsitic pumice zone (Figure 2.19b) there is a

higher degree of de vitrification within both pumices and matrix. Pumices have

a felsitic texture consisting of intergrown silica and feldspar crystals which

have obliterated all original structures. Spherulites are rare, and when present

comprise relatively coarse radiating crystals. Within the shard matrix

vitroclastic texture is still present however there is greater devitrification and it

is variable. Variation ranges from almost complete preservation of shard

texture, to the destruction of the fine ash population leaving only larger shards

intact. Axiolitic texture in surviving shards remains within the confines of

fragments. In some examples patchy felsitic texture has begun to develop

within the matrix and in these areas «5mm across) vitroclastic texture has

been completely destroyed.

38

Figure 2.19 Recrystallisation textures in western TVZ. a) Spherulitic pumice zone. Most spherulites have nucleated on the pumice wall, although some do lie within the pumice and are completely spherical (ppl). b) Felsitic pumice zone. Pumices have completely recrystaliised, destroying all primary textures. Note the increased development ofaxiolitic texture in glass shards (cpl). Both samples are from Wawa Quarry. Field of view 3mm.

39

Bison Road Quarry

Waiotapu Ignimbrite exposed in Bison Road Quarry has undergone felsitic pumice

devitrification and shows little variation in recrystallisation texture with height. Some of

the pumices within the ignimbrite have O.5mm thick rims which appear to have resulted

from the development of coarse axiolitic texture, nucleating on the rim of the pumice.

The centre of these pumices is characterised by felsitic devitrification.

Within the matrix vitroclastic texture is well preserved, although in some areas the

texture within the fine ash fraction of the matrix has been recrystallised.

Wawa Quarry

Wawa Quarry is the only section to have both spherulitic fiamme and felsic pumice

zones. Up to 1 m above the base of the deposit the unit has undergone spherulitic

fiamme recrystallisation. Original fiamme textures are preserved in the cores of the

fiamme, but spherulite growth has completely overprinted such textures towards the

edge of the fiamme. Vitroclastic texture is almost perfectly preserved with only minor

development ofaxiolitic texture within shards.

Above the spherulitic fiamme zone the unit is characterised by felsitic pumice

recrystallisation~ All pumice textures have been destroyed and only the overall original

morphology of the pumice is retained. Within the matrix axiolitic texture (again confined

to within shard walls) is better developed and in places the fine ash fraction has been

destroyed by recrystallisation. In places devitrification has led to the development of

patchy felsitic texture in the matrix which obscures all original vitroclastic texture.

Variation within the felsitic pumice zone shows no systematic variation with height or

degree of welding.

Lichfield Quarry

As at Bison Road Quarry devitrification in Waiotapu Ignimbrite at Lichfield Quarry is

felsitic and shows no systematic variation with height. Here the matrix has experienced

a greater degree of devitrification and patchy felsic texture is more prevalent.

2.6.3 Eastern TVZ

At Ngapouri Ridge recrystallisation textures are significantly different to those on the

western margin of TVZ, reflecting increased recrystallisation following deposition of the

ignimbrite. Two recrystallisation zones have been recognised:

Fe/sitic pumice zone: texturally this zone is very similar to the felsitic pumice

zone of the western TVZ recrystallisation of pumices is more variable.

40

Pumices may show felsitic texture or a more complex arrangement of felsitic

crystallisation and well developed spherulites within the crystal mosaic (Figure

2.20a). In addition pumices may have a coarse axiolitic margin, up to 1 mm

thick, surrounding a felsitic core, which may contain further layers of differing

crystal size (Figure 2.20b).

Lithophysae zone: this zone is characterised by the development of large (c

7mm) lithophysae. Spherulites (up to 5mm in size) are also present. The

remainder of the matrix in this zone has either undergone total felsitic

recrystallisation, in which the vitroclastic texture is completely overprinted, or

very little, and the larger shards have remained intact.

The Ngapouri Ridge is characterised by felsitic texture at the base and in the top

half of the section. The lithophysae zone occurs near the base of the unit, within the

lower third, and is approximately 10m thick, although the thickness varies along the

cliff at the south end of the ridge. Isolated lithophysae occur elsewhere in the unit.

2.6.4 Discussion

The density of welding in Waiotapu Ignimbrite has limited the development of vapour

phase alteration in the deposit due to the limited availability of open pore space.

Pumices (as opposed to fiamme, which have extremely limited pore space) will contain

the most open space and are therefore the most likely to contain vapour phase

assemblages. Waiotapu Ignimbrite has almost certainly undergone vapour phase

alteration, as evidenced by the colour change in hand specimen and the considerably

degraded nature of several pumices recovered from Wawa Quarry. SEM work by Dyah

Hastuti (1992) also identified crystalline aggregates of minerals within pore space in

pumices that were interpreted to be the result of vapour phase preCipitation.

In western TVZ devitrification can be broadly classified on the basis of the nature of

recrystallisation in pumices and fiamme. In densely welded basal ignimbrite (above

c2.1 g cm -3) devitrification was hindered relative to the overlying material and

subsequently devitrification is in the spherulitic stage of Lofgren (1971). Throughout

the remainder of the unit the pumices are completely recrystallised and devitrification

in the matrix becomes more pronounced so that in places all original textures have

been destroyed. In both zones spherulites have circular morphologies suggesting that

the recrystallisation was occurring at temperatures below 400°C.

Waiotapu Ignimbrite at Ngapouri Ridge, in the eastern TVZ, has a longer

devitrification history than its western counterparts. Lithophysae have developed

41

Figure 2.20 Recrystallisation textures at Ngapouri Ridge. a) Spherulite within felsitic pumice (cpl). b) Layered pumice: spherulitic core (left), felsitic inner rim and thin coarsely recrystallised outer rim. Sensitive tint plate used to accentuate textures. Field of view 3mm.

42

across all other textures and must therefore post-date devitrification. The unit has

undergone considerable lithophysae formation in a c. 10m thick zone near the base.

Within the Kaingaroa ignimbrite lithic lag breccia, large numbers of lithophysae-rich

Waiotapu Ignimbrite lithics suggest that they comprise a significant portion of Waiotapu

Ignimbrite beneath Reporoa Caldera. Steiner (1963) reported lithophysae rich zones in

Waiotapu Geothermal Field drillcores. This abundance of lithophysae occurs only in

the eastern TVZ. This may reflect a greater supply of volatiles necessary for

lithophysae formation and growth, which involves the nucleation of spherulites on

vesicles (McPhie et ai, 1993). The abundance of lithophysae at the base of the section

suggests a process was operating, possibly geothermal activity, that concentrated the

volatiles necessary for lithophysae development. It also suggests that the unit retained

heat for longer as devitrification textures are advanced, but the unit was still hot

enough for glass to deform plastically during lithophysae formation.

43

INTRODUCTION

WAI HI

CHAPTERTH

I NIM AND EOCHEMICAl VARIATION

This chapter presents petrographic, whole rock and mineral chemical data to identify

variation in the mineralogical and chemical character of Waiotapu Ignimbrite, both in

terms of stratigraphic height and distance from the inferred source within the Kapenga

volcanic centre.

Lateral and vertical variation in the chemistry and mineralogy of Waiotapu Ignimbrite

have not previously been documented. Martin (1961) produced mineral histograms of

the phenocryst phases of Waiotapu Ignimbrite from localities spanning 55 km which

showed no significant variation in the relative proportions of minerals between

localities. Dyah Hastuti (1992) concluded from petrographic and whole rock

geochemical analysis of 11 samples of Waiotapu Ignimbrite from Wawa Quarry

(T16/627174) that there was little variation with height, and that the unit was erupted

from an homogenous magma chamber.

Petrographic and geochemical data presented here are from the four best

exposures of Waiotapu Ignimbrite, with greatest available stratigraphic thicknesses.

Samples were collected from the south end of Ngapouri Ridge (U16/015098), Bison

Road Quarry (U16/771211), Wawa Quarry (T16/627174) and Lichfield Quarry

(T16/588400). Thin sections were made of samples at regular intervals within the unit

while geochemicaJ samples were taken from at least the top middle and base of each

exposure.

It is recognised that whole rock analyses of tuffs are susceptible to a number of

influences which limit their use for petrogenetic modelling, however the difficulties

involved in extracting suitable pumices from the deposit have meant that whole rock

geochemistry was the only available means of defining vertical and lateral variation in

geochemistry. The controls on whole rock geochemistry and mitigating features of

Waiotapu Ignimbrite are outlined in Chapter 4.1.

Major and trace element geochemical data are presented in Appendix 2 and mineral

geochemical data are compiled in Appendix 3. Modal proportions of the phenocrysts

were determined by point counting (600 points per sample) thin sections.

44

Figure 3.1 Glomeroporphyritic aggregates of plagioclase feldspar, orthopyroxene and Fe-Ti oxides in Waiotapu Ignimbrite.

45

3.1 MINERALOGY

3.1.1 Mineral Descriptions

Waiotapu Ignimbrite is crystal poor (8-18%), although at the base of the north end of

the Ngapouri Ridge the crystal content increases to 30-40%; pumices within the unit

generally containing fewer crystals (c.5-10%). The remainder of the deposit is

,composed of welded glass shards or pumice and fiamme with varying degrees of

attenuation and distortion.

Plagioclase Feldspar

Dominant phenocrysts are plagioclase feldspar which usually occur singly, but may

form glomeroporphyritic aggregates of plagioclase, orthopyroxene and Fe-Ti oxides

(Figure 3.1). Plagioclase crystals, which make up 70-90% of the assemblage, are

usually isolated subrounded fragments within the matrix, or intergrowths of two or more

crystals. They are dominantly 1-2 mm in size, but fragments c. 0.1 mm occur in the

matrix. Most are andesine (An33-40) (Figure 3.2), display albite twinning and are

commonly unzoned or weakly normally zoned, a small percentage being strongly

zoned with andesine rims and labradorite cores (An35-55).

Ab

An

Legend

Wawa Quarry Core Rim

It

o Ngapouri Ridge Core Rim • o

Or

Figure Representative plagioclase compositions (Ab-An-Or) from Waiotapu Ignimbrite, analysed by scanning electron probe microanalysis (EDMA).

46

Orthopyroxene

Orthopyroxene makes up 5-18% of the crystal assemblage and occurs as <1 mm size

crystals, usually single, although occasionally as aggregates (see Figure 2.11 c). They

occur as equant subhedral crystals, although euhedral six sided crystals are present.

Throughout the unit the mineral has undergone alteration, but where the unit is most

densely welded degradation of crystals is less pronounced. ConsequentJy

orthopyroxenes are frequently present only as cores within cavities, which may be

surrounded by hematite alteration. Orthopyroxenes analysed by electron microprobe

were En45-48 (Figure 3.3).

Fe-Ti oxides

Fe-Ti oxides are present in similar quantities to orthopyroxene, but are considerably

smaller (c.O.1 mm). The dominant oxide phase is titanomagnetite which occurs as

subhedral crystals which are either isolated in the matrix, present as inclusions in

orthopyroxenes or, to a lesser extent, as inclusions in plagioclase. Ilmenite, which is

considerably less abundant than magnetite, occurs as tabular crystals. The oxides are

frequently altered, often occurring with haloes of hematite.

Iron-titanium oxides, which are known to have formed in equilibrium with one

another, may be used to provided estimates of the temperature of the host magma at

the time of crystallisation (Anderson and Lindsley, 1988). Magnetite-ilmenite pairs that

occur within larger crystals or pumices may be in equilibrium and are therefore suitable

for analysis. While such pairs were found during microprobe analysis no analyses

returned acceptable totals and most were therefore discarded. Estimates were limited

to one magnetite-ilmenite pair from AW051 (a pumice mineral separate from Bison

Road Quarry) which gave acceptable totals and satisfied the equilibrium test devised

by Bacon and Hirschmann (1988). This is based on the partitioning of Mg and Mn

between coexisting phases of titanomagnetite and ilmenite. The pair yielded

temperatures of 750.4 and 750.8°C. Magmatic temperatures were calculated using

Wo

I / v v

Enstatite v v

Ferrosilite \I \I

En

Figure 3.3 Waiotapu Ignimbrite orthopyroxene compositions as determined by electron microprobe. Pumice analysis is from a sample collected from Bison Rd Quarry. Classification after Morimoto (1988).

\ \( \

Fs

Legend Pumice /:;, Wawa Quarry 0 Ngapouri Ridge 0

47

PETMIN 2.0 geochemical software which uses the geothermometer devised by

Anderson and Lindsley (1988).

Hornblende

Hornblende is exceptionally rare within Waiotapu Ignimbrite; isolated crystals are found

in only a few samples. Hornblendes are typically <0.1mm in size and occur as

subhedral, green pleochroic crystals. Dyah Hastuti (1992) noted the presence of

hornblende in Waiotapu Ignimbrite at Wawa Quarry where it appeared in XRD

analyses, although she did not observe any in thin section.

3.1.2 Ngapouri Ridge

Eight samples from the south end of the Ngapouri Ridge were thin sectioned (Figure

3.4a) and include samples taken from near the base (at U16/015098) and the top

(U16/013099) of the ridge.

Crystal percentages range from 10 to 18% but show no systematic variation.

Phenocryst percentages in all samples were subject to error as many crystals were no

longer present, either due to alteration or mechanical plucking from the sample during

cutting of samples for thin sectioning. Plagioclase (75-92%) shows little significant

change with height, and orthopyroxene (7-18%) and Fe-Ti oxides (3-11 %) appear to

be controlled by plagioclase concentration, hence their plots tend to mirror those of

plagioclase percentages.

3.1.3 Bison Road Quarry

Six samples, collected at c.8m intervals from Bison Road Quarry were sectioned

(Figure 3.5a). Total crystals counted (12-18%) and plagioclase (80-90%) percentages

vary little with height. Orthopyroxene (7-15%) and Fe-Ti oxides (5-9%) increase

slightly, but again are controlled plagioclase concentration.

3.1.4 Wawa Quarry

Total crystal percentages and mineral percentage in the 11 samples collected from

Wawa Quarry (Figure 3.6a) vary little (8-16%). Orthopyroxene ranges from 4-19%, Fe

Ti oxide 8-14%, and plagioclase 70-83%.

3.1.5 Lichfield Quarry

The five samples from Lichfield Quarry (Figure 3.7a) show the lowest variation in

crystal percentage (8-11%) of the four sections. Plagioclase (73-85%) percentages

48

decrease with height and conversely orthopyroxene (8-16%) and oxides (6-11%) show

slight increases.

3.2 GEOCHEMISTRY

3.2.1 Ngapouri Ridge

At the south end of the Ngapouri Ridge, the type section of Waiotapu Ignimbrite

(Martin, 1961), seven whole rock samples and two pumices, from near the base, were

analysed by XRF. Two of the whole rock samples and both of the pumices were

discarded due to anomalous low iron contents (see Appendix 2 for a full explanation).

Throughout the whole stratigraphic section degradation of the mafics was considerable

and this, coupled with microprobe analysis (using EDS) revealed that the dominant

oxide was ilmenite. This may be due to the alteration of magnetite, suggesting that all

of the results must be treated with a degree of caution.

There is little significant variation in composition with height (Figure 3.4b) although

CaO (1.14-1.32%), Rb (111-115 ppm) and Sr (101-116 ppm) show slight increases

toward the top of the succession. Si02 (72.87-74.10%), Ti02 (0.28-0.35%), K20 (3.27-

3.42%) and Zr (244-283 ppm) show little significant change. Ti and Zr concentrations

parallel one another, possibly due to Zr behaving compatibly with the formation of

leucoxene (see Appendix 2).

3.2.2 Bison Road Quarry

Four whole rock samples from Bison Road Quarry (Figure 3.5b) were collected at

c.15m intervals and analysed by XRF. Si02 (70.51-72.55%), Ti02 (0.31-0.36%), K20

(3.11-3.21%) and Zr (257-279 ppm) show minimal variation with height. Overall CaO

(1.09-1.45%) and Sr (100-129 ppm) show a very minor decrease towards the top of

the succession, while Rb (105-114 ppm) shows a very minor increase. At 8m the unit

is depleted in Rb and enriched in CaO and Sr, possibly due to an increase in the

quantity of plagioclase in the sample. This is not backed up by the plagioclase crystal

percentage for that level, although there is a very slight increase in crystal percentage

at that height which may account for the increase in Rb.

3.2.3 Wawa Quarry

At Wawa Quarry (Figure 3.6b) nine whole rock samples were collected from the base

of the unit to the top, some 25 to 30m above. Three samples were collected in the first

meter above the base and the remained were collected at c. 5 m intervals.

49

At Wawa Quarry, Waiotapu Ignimbrite has ponded in a depression (see Chapter

2.5) and consequently samples collected on each bench are at unknown elevations

above an uneven base. Activity at the quarry after the collection of samples for

geochemistry revealed a new basal section and showed that AW109, at least, was

within c.1 m of the base.

SiOz (72.56-74.79%) increases from the base and then decreases slightly with

height. The marked drop over the last c.6m is probably a weathering phenomena as

Alz0 3 undergoes a corresponding increase. TiOz (0.23-0.29%), CaO (1.05-1.29%),

KzO (3.27-4.35%) and Sr (96-109 ppm) show little significant variation with height. The

increase in CaO and Sr at 9m is due to an increase in the percentage of plagioclase

crystals in the sample (see Figure 3.6b). The sample has undergone considerable

vapour phase alteration and devitrification, therefore the increase in KzO is most likely

an artefact of the secondary precipitation of alkali feldspar.

Rb (94-121 ppm) increases dramatically over the first meter and then shows little

variation with height. The low quantities of Rb at the base are possibly due to depletion

of Rb at the top of the magma chamber due to fluid removal of alkalis. Alternatively Rb

may have been removed by fluids from the underlying porous rocks percolating

through the base of the ignimbrite. As the base is not observed elsewhere it is

uncertain whether this phenomena occurs throughout the ignimbrite, hence is a

magmatic feature, or is only restricted to Wawa Quarry, and therefore post

depositional.

The increase in Zr (229-258 ppm) towards the top of the quarry is difficult to explain.

If the increase is related to magmatic processes then the same trend should be

observed at other sites, but this is not the case. The behaviour of the element is

inconsistent with other high field strength elements, such as Nb or V, which would be

expected to show similar behaviour if the increase was due to the presence of crystals

of zircon in these samples.

3.2.4 Lichfield Quarry

At Lichfield Quarry (Figure 3.7b) 3 samples for whole rock were taken from the top,

middle and base of the main exposure, the lowest sample is estimated to have come

from within 2m of the base.

SiOz (71.32-73.27%), CaO (0.87-1.05%), KzO (3.37-3.46%) and Sr (83-96 ppm)

show very slight decreases toward the top of the deposit, while TiOz (0.29-0.36%) and

Zr (245-286 ppm) increase with height. Rb (115-118 ppm) shows no appreciable

change.

50

40

35

30

I 25 :E .2' Q) 20

I

15

10.

5

0

70 75 80

Si02 (wt%)

'" b) 45 .

40

35

30

I 25 :E 01 'iii 20 I

15

10

5

o ., ... -, 0 10 20 60

Crys\ %

iii \ <II

\ <II

/lil "1 .-·'--T~T----·l

1.1 1.2 1.3 1.4 1.5 0.2

CaO(wt%)

80 100 o Plag %

0.3

Ti02 (wt%)

10 20

Opx%

0.4 3 3.1 3.2 3.3 3.4 3.5 200

K20 (wt%)

o 10 20

Oxides %

250

Zr (ppm)

300 100 110

Rb (ppm)

120 90 100 110 120 130 0.90 1.00 1.10 1.20

Sr (ppm) Rb/Sr

Figure 3.4 Variation in Waiotapu Ignimbrite at Ngapouri Ridge.

a) Variation in whole rock chemistry with increasing height. b) Variation in crystal percentages

I-!o.;,..hf refers to above

Cryst: crystals. Plag: plagioclase. Opx: orthopyroxene magnetite and ilmenite

(Jl N

a) 40

35

30

25 g :c 20 C'> 'iii J:

15

10

5

0

b) 40

35

30

25 g :c 20 C'> 'iii J:

15

10

5

o

65 75 85

Si02 (wt%)

o 10 20 60

Cryst %

III

/" 1.5 0.2 0.3 0.4 3

CaO Ti02

80 100 0 10 20 0

Plag % Opx%

3.2 3.4 200

K20

10 20

Oxide %

250

Zr(ppm)

300 100 110

Rb (ppm)

120 95

•

115

Sr (ppm)

135 0.75 0.95 1,15

Rb/Sr

Figure 3.5 Variation in Waiotapu Ignimbrite at Bison Quarry.

a) Variation in whole rock chemistry with increasing height. b) Variation in crystal percentages with height.

Height refers to height above base of

Cryst: crystals. Plag: plagioclase. Opx: orthopyroxene Oxides: magnetite and ilmenite

U1 W

25

20

I 15

:c .2' Q.)

I 10

5

o

25

20

I 15

:c 0) ·iii

I 10

5

72 74

Si02 (wt%)

o+--o 10

Cryst %

76

20 60

1.2

CaO (wt%)

80

Plag %

1.4 0 0.2

Ti02

100 0 10

Opx%

0.4 0

20 o

K20 (wt%)

10

Oxide (%)

5 220

20

240

Zr (ppm)

260 90 100 110 120 90 100 110 120 130 0.9 1 1.1 1.2 1.3

Rb (ppm) Sr (ppm) Rb/Sr

Figure 3.6 Variation in Waiotapu Ignimbrite at Wawa Quarry.

a) Variation in whole rock chemistry b) Variation in crystal percentages

increasing height.

Height refers to height above base of outcrop.


01 .j:>..

a) 8 , '" e e

7

6

5

I :c 4 ill $ 0& Ol 'iii I

3

2

o +--------". -----, ------"'r'-------,

70 75 80 0.5 1.5 0.2 0.3 0.4 3

Si02 (wt%) CaO (wt%) Ti02

b) 8.00

7.00

6.00

5.00

I :c 4.00 Ol . iii

I 3.00

2.00

1.00

o.oo+----~

0.00 10.00 20.00 60 80 100 0 10 20 0

Cryst % Plag % Opx %

ill

K20 (wt%)

10 20

Oxide %

.. e ~ '" ~ '"

+-------e------, ..1-----,--41----, -t----,--O!>-,--------, 41-,----------,

3.5 200 250

Zr (ppm)

300 100 110

Rb (ppm)

120 60 80 100 1201.1

Sr (ppm)

1.3

Rb/Sr

Figure 3.7 Variation in Waiotapu Ignimbrite at lichfield Quarry.

a) Variation in whole rock chemistry with increasing height. b) Variation in crystal percentages with height.

Height refers to height above base of outcrop.


1.5

3.3 DISCUSSION: VERTICAL AND LATERAL VARIATION

3.3.1 Mineralogy

Mean crystal percentages appear to increase towards the proposed source. At

I\Igapouri Ridge and Bison Road, which are both c.13km from source, mean crystal

percentages are higher (15% and 16% respectively). than at Wawa Quarry (25 km)

and Lichfield (40km) (12% and 11% respectively).

The inverse relationship between plagioclase and the orthopyroxenes and Fe-Ti

oxides is largely due to a constant sum effect whereby a decrease in the percentage of

plagioclase leads to an increase in the amount of the other mineral phases. However a

relative decrease in plagioclase content is usually coincident with a decrease in the

crystal percentage of the deposit.

At the four localities the percentage of phenocrysts and relative proportions of

minerals shows little significant variation with stratigraphic height. Ranges in crystal

percentages vary from 3% (at Lichfield Quarry) to 8% (at Bison Road Quarry). When

the influence of elutriation with changing flow dynamics on the degree of crystal

enrichment and the effects of point counting error on relative concentrations of crystals

are considered, this variation becomes insignificant.

3.3.2 Geochemistry

The geochemistry of Waiotapu Ignimbrite shows little significant variation with

stratigraphic height. Variation present can be explained in terms of crystal enrichment

or depletion in whole rock samples, or by post depositional alteration, such as

devitrification and vapour phase alteration.

Variation in concentrations of elements occurs between sections, but there appears

to be no correlation between element concentrations and distance from the inferred

source. Bison Road and Lichfield Quarry have lower Si02 (c. 72%) than the Ngapouri

Ridge and Wawa Quarry (c. 74%). Wawa Quarry has lower Ti02 than elsewhere,

which is not consistent with an increase in Fe-Ti oxides in the phenocryst phase. At

Wawa the Fe-Ti oxides and orthopyroxenes are not as degraded as at other localities

and therefore Ti concentrations may not have been elevated as a consequence of the

removal of elements such as Fe. CaO and Sr variation is consistent with the amount of

plagioclase in analysed samples and both behave in the same manner. At Wawa and

Lichfield Quarries, CaO and Sr are generally lower (CaO c. 1-1.1 and Sr c.90-99 ppm)

presumably reflecting less plagioclase at these localities. In addition the mobility of the

low field strength elements (Rb, Sr and K) mean that their concentrations are affected

by alteration, therefore variation is not necessarily due to magmatic processes. K20

55

concentrations may also affected by the degree of devitrification and vapour-phase

alteration. Zr does not vary much anywhere but Wawa Quarry, where concentrations

are anomalously low. Zr variation at Wawa is difficult to explain, especially when its

behaviour is inconsistent with that of other high field strength elements.

Neither the mineralogy or whole rock geochemistry of Waiotapu Ignimbrite shows

significant variation that can be inferred to be a result of primary magmatic processes.

This suggests that the unit was deposited from a pyroclastic flow originating from an

homogenous magma chamber. The nature of the Waiotapu Ignimbrite magma system

will be discussed in Chapter 4.

56

CHAPTER FOUR

G HEMI WAI NIM

INTRODUCTION

Few studies of the geochemistry of Waiotapu Ignimbrite have been conducted

previously. This chapter will outline the general geochemical nature of the ignimbrite

and attempt to determine what magmatic processes were operating prior to eruption

and the degree of compositional variation, if any, within the parental magma.

33 whole rock and 14 pumice samples of Waiotapu Ignimbrite were collected and

analysed by X-Ray Fluorescence to determine major and trace element compositions.

The results of these analyses are presented in Appendix 2. Whole rock samples were

collected at regular intervals within the ignimbrite, usually from the top, middle and

base of sections, although certain key sections were sampled in more detail.

4.1 WHOLE ROCK vs. PUMICE CHEMISTRY - THE PROS AND CONS.

When analysing pyroclastic deposits it is best to analyse individual pumices as

opposed to whole rock samples of the deposit as the composition of whole rock

samples is subject to a number of significant influences:

Elutriation: during transport a considerable amount of fine material is lost into the

atmosphere by elutriation which leads to fines depletion, and enrichment of

crystals and lithics within the final deposit As most of the lost material is glass

shards the geochemical character of the deposit becomes more mafic

reflecting the increased influence of crystal composition on the final chemistry.

Lithics: with the exception of cognate lithics, rock fragments within a deposit will

have different chemistries, unrelated to that of the parent magma of the

pyroclastic flow. As a result the chemistry of a deposit will vary depending on

the composition and percentage of the lithics within the ignimbrite (Walker,

1972).

Sampling of multiple magma compositions: unlike pumices, whole rock analyses

do not represent the chemistry of a magma at a given location within a

chamber at the time of eruption. A sample may be composed of material

(either pumices or glass shards) derived from different locations within the

parent magma body. Should this magma be compositionally zoned, the

chemistry of the erupted components will vary and the whole rock analysis will

57

reflect the mixing of these compositions. Consequently geochemical plots of

analyses will not reflect the evolution of the magma, but instead will plot on a

"mixing line" between end member compositions. As a result whole rock data

is of little use for petrogenetic modelling.

Waiotapu Ignimbrite is densely welded and the smooth nature of many outcrops

makes the recovery of pumices from outcrop very difficult. Few samples could

therefore be collected in-situ. Most were recovered from relatively fresh float material

in quarries of Waiotapu Ignimbrite in Kinleith Forest. To augment the limited pumice

data, whole rock samples were analysed in order to coarsely define the chemistry of

Waiotapu Ignimbrite. Several characteristics of Waiotapu Ignimbrite mitigate against

whole rock problems:

a) Waiotapu Ignimbrite is crystal poor (~10-15% phenocrysts) thus reducing the

influence of elutriation and crystal enrichment.

b) The paucity of lithic fragments within the ignimbrite lowers the risk of

contamination of the sample by xenolithic fragments. Extreme care was taken

during preparation of samples to remove any lithics from crushed samples before

they were milled to a powder for analysis.

Unfortunately the problem of sampling multiple magma compositions cannot be

totally avoided, and as a result the whole rock analyses will only be used to assist in

defining the bulk chemical character of the deposit and petrogenetic inferences will be

made using only pumice data.

4.2 GENERAL CHARACTERISTICS

The following are brief descriptions of the general character of Waiotapu Ignimbrite.

Unless specified otherwise, ranges quoted for elements are derived from combined

pumice and whole rock data. Table 4.1 presents elemental variations for pumice and

whole rock analyses separately. Table 4.2 compiles analyses of representative

pumices.

Waiotapu Ignimbrite is a rhyolite with a Si02 content of 70.51-75.31 wt% and plots

within the rhyolite field of the total alkalis v. silica diagram of Le Maitre et al (1989)

(Figure 4.1 a). The unit is dominantly medium-K (K20=3.11-3. 76 wt%) although several

pumices plot in the high-K field of Le Maitre et al (1989) (Figure 4.1 b). Fe203 content is

1.82-2.9 wt% however samples collected from the base of the Ngapouri Ridge at

U17/015098 have anomalously low Fe203 contents (1.08-1.38 wt%) which do not

parallel the behaviour of Ti02. Possible reasons for this anomalous behaviour will be

discussed at the end of Appendix 2, and the affected samples have been omitted from

the data set employed in this chapter.

58

a)

b)

~ 0

!. ~

15 After Le Maitre et al (1989)

[Whole Rock • Pumice

----'

10

5

Basalt

o 35 45

5 After Le Maitre et al (1989)

4.5 \0 Whole Rock I

4 ,. Pumice ~

3.5

3

2.5

2

1.5 basaltic

1 andesite

basalt

.5

0 45 55

Rhyolite , o

Basaltic-Andesite Andesite Dacite

55 65 75 Si02 (wt%)

high-K

medium-K

dacite and rhyolite

andesite

low-K

65 75

Si02 (wt %)

Figure 4.1 a} Total alkalis vs silica diagram for combined Waiotapu Ignimbrite pumice and whole rock geochemical analyses. b} K20 vs Si02 for Waiotapu Ignimbrite. Classification after Le Maitre et al (1989).

59

Table 4.1 Variations in the concentration of major and trace elements in Waiotapu Ignimbrite. (AS.I. = Alumina Saturation Index).

Major elements (a/l '.;'.::;!:;== wt%) elements (all "..;!~ .... _ ppm)

Element Pumices Whole Rock Element Pumices Whole Rock

Si02 70.98 - 75.31 70.51 - 75.49 V 11 15 10 - 21 Ti02 0.23 - 0.32 0.23 - 0.36 Cr <3 - 4 <3 - 6 AI20 3 12.63 -14.07 13.29 - 15.74 Ni <3 - 4 <3 Fe203 1.82 - 2.35 2.16 - 2.90 Pb 13 - 34 11 - 24 MnO 0.03 - 0.07 0.03 - 0.08 Zn 31 -43 33-64 MgO <0.05 -0.08 <0.05 - 0.23 Rb 116 161 94 - 121 CaO 0.86 - 1.26 0.87 -1.52 Sa 726 - 824 736 - 836 Na20 3.61 - 4.81 2.32 - 4.37 Sr 79 - 111 83 -129 K20 3.38 - 3.76 3.11-3.46 Ga 14 - 17 13 - 17 P20 5 0.03 - 0.09 0.01 - 0.06 Nb 7 -10 7 10

Zr 196 - 264 229 - 286 Y 20 - 37 8 - 48

Rb/Sr 0.71 - 1.73 0.81 - 1.39 Th 11 - 14 9 -14 KlRb 192 - 246 232 - 289 La 21 - 35 13 -45 A.S.1. 1.44 1.69 1.49 - 1.95 Ce 32 -70 22 -75

Nd 20 - 43 <10-41

The Alumina Saturation Index (AS.I.) of the ignimbrite ranges from 1.44 - 1.95 and

the unit is corundum normative indicating a peraluminous composition, like most other

ignimbrites in TVZ.

The unit contains 12.63-15.74 wt % A120 3. Changes in AI20 3 content can reflect the

development of clay minerals, but if so the AI20 3 content should show a linear increase

with loss on ignition (LOI), which is not observed. In addition XRD analysis of samples

with high LOI revealed no peaks related to the development of clays (see Appendix 5).

The high AI20 3 content of Waiotapu Ignimbrite samples is therefore considered a

primary feature.

Most trace elements show no systematic variation; Rb (94-161 ppm), Sr (79-129

ppm), Ba (726-836 ppm) and Zr (196-286 ppm) all show a wide range of values. Rb/Sr

varies from 0.71-1.73 and is controlled mainly by variation in Rb.

4.3 PUMICE CHEMISTRY

4.3.1 Controls on major and trace element variation

In attempting to determine magmatic processes controlling major and trace element

proportions in a melt, element concentrations are best plotted against a discriminant

that has behaved incompatibly during crystallisation (Wilson, 1989). Traditionally

Harker diagrams, with Si02 as a discriminant, are used (Rollinson, 1993), although it

has been suggested that in high silica rhyolites other elements may be more

60

Table 4.2 Geochemical analyses of selected Waiotapu Ignimbrite pumices. Samples AW85a, AW166 and AW189 are type-A pumices, whereas AW85b is a type-B pumice (see section 4.3).

Sample AW85a AW85b AW166 AW189 Lab No. 27386 27387 27391 27394 Si02 73.57 74.38 74.76 75.31

Ti02 0.32 0.27 0.23 0.32

AI20 3 13.99 13.82 13.69 12.90

Fe20 3 1.99 2.35 2.18 2.34

MnO 0.03 0.05 0.06 0.04 MgO 0.05 0.05 0.05 0.08 CaO 0.92 1.05 1.15 1.03 Na20 3.88 3.86 4.32 3.61

K20 3.53 3.70 3.41 3.52

P20 S 0.03 0.03 0.03 0.09

LOI 1.15 0.92 0.35 1.17 Total 99.39 100.45 100.21 100.40

V 12 13 12 15 Cr <3 <3 <3 <3 Ni 3 <3 <3 <3 Pb 26 16 16 13 Zn 40 35 41 42 Rb 126 153 116 122 Sa 748 794 804 734 Sr 83 95 104 91 Ga 17 16 15 15 Nb 9 8 9 10 Zr 240 250 236 264 Y 31 29 24 20 Th 11 12 13 12 La 31 28 28 21 Ce 60 63 60 38 Nd 41 33 25 20

A.S.I. 1.68 1.61 1.54 1.58 Rb/Sr 1.52 1.61 1.12 1.34 K1Rb 233 201 244 240

Q 35.30 34.61 33.15 38.06 C 2.15 1.56 0.80 1.44 Z 0.05 0.05 0.05 0.05 Or 21.30 22.06 20.26 21.05 Ab 33.44 32.86 36.66 30.83 An 4.69 5.30 5.78 4.80 Di 0.00 0.00 0.00 0.00 Hy 1.84 2.32 2.22 2.29 Mt 0.68 0.79 0.73 0.79 II 0.62 0.52 0.44 0.61 Ap 0.07 0.07 0.07 0.22 Total 100.14 100.15 100.15 100.15

61

.45 3.4 3.2 .4

3 ° 0

~ .35 0 fl

~ 2.8 ~ ~ 0 .. " ~ 2.6 0 0 0

~ 0" ' .. , fl

.3 0" 'It> 0

" 0 ·0

~ 2.4 o 0

0 0' 0 0 Bel' 00

F ... 0 .. .... .25 u.. 2.2 .. Ii _.0r/:.Otb .. 0

,p • 0 0

2 .. . 2

1.8 .15 1.6

73 74 75 76 73 74 75 76 SiOz(wt %) Si02(wt %)

1.8 4

1.6 3.8 .. • .. "

.. 11.4 ~ 3.6 ..

~ • 0 00 ....

<al 1.2 0 0 OIl" 0 .Ii' 0 0

" 0 ~3.4 0,. 0"0

0 . .; o o.ouo 0 0 00

0

0 0 o ,0 0 00

0 0 0 0"0 00 " .. .. 0

co 3.2 0

" .8 3

73 74 75 76 73 74 75 76 Si02(wt%) Si02(wt%)

17 5

16 .... oF • II 0 0

0 0 0 o gq, .. 0,\&_

~ 15 0 *' 4

0 00 ; D.o 0 0

~ ..

1 00 0 ..

° ~14

0 " , ... ~ " o.{/o .. ;,

e o.'tf Z 3 00 00 o 0

13

12 2 73 74 75 76 73 74 75 76

Si02(wt %) Si02(wt%)

Figure 4.2 Plots of selected major elements versus Si02. Note the weakly defined negative trends in whole rock (open circles) and pumice (closed circles) data for Ti02, Fe203 and A120 3.

62

140

130

120

E 110 a. .,e, en 100

90

80

" o

..

70~~ ______ ~ ____ ~ ________ ~

73 74 75 76 Si02 (wt %)

300 o

280 " ..

260 E a. .s 240 .... o N

220

200 - .. 180~~ ______ ~ ____ ~ ________ ~

73 74 75 Si02 (wt %)

1.5

.5

76

o

eo

" o "

170~~--~--~----~------~-'

160

150

E 140 a. .s 130 ..a n:: 120

110

100

.,

..

" o 0 oi &ct:.!0 .0\,

o DO 0 lib 0 0 0 0 ()

"

90~~ ______ ~ ______ ~ ____ ~ __ ~

900

850

800 E a. .s 750 ro

CD 700

650

73

"

74 75 76 S102 (wt %)

.. fit 0 0 ..,

" ., .0.' \I

.. <P .. o

o tPooo,poo·

600 '--~ ___ -'-___ "'--__ ~_-' 73 74 75 76

8i0 2 (wt %)

.. ..

o t9 IfII 0 .. 000

o~ .0:.00 0 0 0

73 74 75 76 S102 (wt %)

figure 4.3 Plots of selected trace elements versus Waiotapu Ignimbrite analyses. Note the disparity between the low Rb type A pumices, and whole rock data, and the high Rb type B pumices.

63

.5 4

3.5 .4

~ " ~ 3 0

~ 0) 0

& ?: " <d' " 1.3 .. .. ..

~2.5 0"" IIIlrb () .. 'II 8

" "ai .. '" " ., 0(JQ;)g :r 2 00 '1111" .. II 0

i= "

... .. . . 2

1.5

.1 1 90 100 110 120 130 140 150 160 170 90 100 110 120 130 140 150 160 170

Rb (ppm) Rb (ppm)

2 4.5

1.5 ~ 4 ~

""'''' ~

~ 0 ~ 0 .. 1 00 ,\.0. 0 e .. .. 1 3.5 " I>j-:.: o· 0

~ 0 III " jP.,

0 0

.5 3

0 2.5 90 100 110 120 130 140 150 160 170 90 100 110 120 130 140 150 160 170

Rb {ppm} Rb (ppm)

20 5 .. 18 o 00 t. II .. 0 .. 0 ..

~ 16 ~ 4

.,.... .. ~ 00 0 .. • 1 00 1 o cP 0 Ii)

'1>0

'" .,

~ ~14 o 0 0 ., 0:1).0" '1' • « " 00 z 3 ..

12

10 2 90 100 110 120 130 140 150 160 170 90 100 110 120 130 140 150 160 170

Rb (ppm) Rb (ppm)

Figure 4.4 Plots of selected major elements versus Rb for Waiotapu Ignimbrite.

64

140

130 " " 120

.. .. " 0 .. ..

"

E 110 Q. ,e, W 100 I>

90 ..

" 80 .. 70

90 100 110 120 130 140 150 160 170 Rb (ppm)

300 ,,"

280 " " 0

260 " If>. ° .. .. "" I>

E Q. 0 ,e, 240

" " .. " " .. It ..

" "": '10 ... " N

220

200

180~~ __ ~~ __ ~~ __ ~~~ 90 100 110 120 130 140 150 160 170

Rb (ppm)

2

" " 1.5 "

"" Jf " .... " " -ff! .. .. .0 1 " <>0 0:: '1,

.5

0 90 100 110 120 130 140 150 160 170

Rb (ppm)

900

850 " .. 1I ..

" 800 ~ .. ..

" " " E '\8 1/

Q. " ,e, 750 ';..B'l. " " III co

700

650

600~~ __ ~~ __ ~~ __ ~~~ 90 100 110 120 130 140 150 160 170

Rb (ppm)

Figure 4.5 Plots of selected trace elements versus Rb for Waiotapu Ignimbrite data.

appropriate (e.g. Bradshaw, 1992). Plots of 8i02 versus selected major elements

(Figure 4.2) and trace elements (Figure 4.3) are scattered to the extent that trends are

not readily discernible. With incorporation of whole rock data weak negative trends are

observed for Fe203, Ti02 and A120 3. In a high silica rock such as Waiotapu Ignimbrite

negative trends with increasing 8i02 may be spurious due to the constant sum effect

(Rollinson, 1993). By necessity major element compositions must total 100%. Data is

therefore not able to vary independently, so as the proportion of 8i02 increases the

sum of the remaining components must decrease. Often imparting a negative bias on

data as the percentage of minor constituents is depressed.

The range of KlRb ratios in Waiotapu Ignimbrite is narrow (192-246) and a plot of

K20 versus Rb defines a close trend (Figure 4.4) suggesting that K and Rb are

behaving perfectly incompatibly in the parent magma. The absence of biotite and alkali

feldspar phases into which K and Rb would fractionate supports this assumption.

Consequently Rb has been chosen as an index of differentiation (Figure 4.4 and 4.5).

Waiotapu Ignimbrite pumices can be divided into two types on the basis of Rb

content. Type A pumices are low Rb (116-128 ppm), and have a correspondingly low

65

Rb/Sr ratio (0.71-1.52), whereas Type B pumices have high Rb (141-161 ppm) and

Rb/Sr ratio (1.57-1.73). KlRb ratios differ between each type; type A (223-246) being

higher than type B (192-201).

Type A and B pumices do not lie on any apparent fractionation trend. Between the

two types Sr overlaps, however type A shows a far greater range in Sr (79-111 ppm)

than type B (93-95 ppm). If type B was derived from type A through plagioclase

fractionation then type B should have lower Sr. In addition CaO, AI20 3, Na20, Ti02 ,

Fe203, Zr and Ba all fail to show a change with increasing Rb and plot on a flat trend.

K20 has a shallow, but well defined, positive trend. On Sr v Rb plots, however, there is

a discernible negative trend consistent with the fractionation of plagioclase, whereas

type B pumices plot differently. CaO and Zr also show similar negative trends with

increasing Rb. A plot of Rb vs Si02 reveals the disparity in Rb concentrations between

groups and highlights the lack of a path between the two groups.

Whole rock samples do not reflect the type B composition. Pumice data suggests

the type B pumices are subordinate and therefore type A chemistry will be the major

control on whole rock composition. Also crystal enrichment will lead to a reduction in

the concentration of Rb, which will not have partitioned into any of the phenocryst

phases in the ignimbrite.

Whakamaru Ignimbrite type D pumices have anomalously high Rb that is of unclear

origin -(Brown, 1994). However, unlike Waiotapu Ignimbrite, these pumices have

distinct petrographic character and anomalously low 87Sr/86Sr. Unlike Waiotapu

Ignimbrite, where both pumice types plot within the field of TVZ ignimbrites (Figure

4.n), the type D Whakamaru pumices are c.60 ppm higher in Rb than all other TVZ

ignimbrites, suggesting a unique origin. It is therefore unlikely that Brown's (1994)

suggestion that the type D pumices are derived from a cumulate is applicable to

Waiotapu Ignimbrite.

The similarity of type B to type A in all other geochemical and petrographic respects

suggests derivation from a different magma batch with many chemical similarities,

perhaps sharing a common source.

4.3.2 Spider diagrams.

A primitive mantle normalised, multi element spider diagram for selected Waiotapu

Ignimbrite pumices is presented in Figure 4.6. The samples are from Rawhiti

Quarry(AW85a, AW85b), Wawa Quarry (AW166) and Ngapouri Ridge (AW189), and

represent both Type A (AW85a, AW166 and AW189) and Type B (AW85b) pumices.

The plots show a general negative trend as a result of large ion lithophile element

66

1000 Norm: Primitive Mantle

Sun & McDonough (1989) Legend:

A AW85a} D AW166 Type A pumice

<> AW189

o AW85b - Type B pumice 100

10

1 Rb 8a Th K Nb La . Ce Sr Nd Zr TI y

Figure 4.6 Primitive mantle normalised mUlti-element spider plot of selected Waiotapu Ignimbrite pumices. Normalised after Sun & McDonough (1989).

enrichment relative to high field strength elements. The negative trend and Nb

depletion is consistent with derivation from a subduction-related source (Wilson 1989).

The depletion in Sr reflects the fractionation of plagioclase, while Ti depletion indicates

the fractionation of ilmenite and titanomagnetite.

4.4 DISCUSSION

Throughout the world many examples of compositionally zoned ignimbrites have been

recorded and the variation has been interpreted as resulting from derivation from a

compositionally zoned magma chamber (Smith & Bailey, 1966; Lipman, 1967; Flood et

ai, 1989; Tait et ai, 1989; Smith, 1994). Smith (1979) suggested that systematic

compositional zonation should be observed in all ignimbrites that exceed 1 km3 in

volume, and subsequently compositional zonation has been considered a

characteristic feature in large volume ignimbrites.

Early geochemical studies in TVZ failed to show any significant degree of

systematic compositional zonation in large scale ignimbrites (Ewart, 1963; Froggatt,

1982; Smith, 1989; Dunbar et ai, 1989). TVZ magma bodies were, therefore,

considered not have developed compositional gradients, or pre- or syn-eruptive mixing

destroyed any compositional zonation. More recent work has revealed that some TVZ

67

400

350 l1korangi

300

250 ,,-..

E a.. ~ 200 L.. (/)

150

100

50

0 0

Ongatiti A---

50

Atiamuri

100 Rb (ppm)

Proposed source:

Taupo

Maroa

Rotorua Reporoa Whakamaru ....... .

Kapenga Mangakino

PokaiA Kaingaroa Whakamaru C

Ohakuri

150 200

Figure 4.7 Rb vs Sr plots of Waiotapu Ignimbrite pumice analyses (shaded dark grey; A: type A pumice, B: type B pumice) against fields enclosing other TVZ ignimbrites. Data from S.D. Weaver and B.F. Houghton (unpub. data), S.W. Beresford (unpub. data), Sutton et al (1995), Brown (1994), Karhunen (1993). and Briggs et al (1993). Proposed sources from Houghton et al (1995).

ignimbrites do display compositional zonation and that some TVZ magma systems are

chemically and thermally zoned (Briggs et ai, 1993; Karhunen, 1993; Brown, 1994;

S.W. Beresfordpers. comm. 1996).

Petrographic and bulk geochemical data outlined in Chapter 3 suggest that

Waiotapu Ignimbrite was derived from a compositionally homogenous magma

chamber. Pumice data suggests that two magma types were incorporated in the

pyroclastic flow during eruption, and that the type-A Waiotapu chamber was weakly

zoned, due to plagioclase fractionation. Possible origins for the batches are:

a) two separate magma chambers eruption simultaneously leading to the

incorporation of two magma types into the resulting pyroclastic flow.

b) integration of a smaller, type B, magma batch into a larger type A magma

chamber leading to magma mingling. It is possible that the influx of type B

magma may have triggered the Waiotapu eruption.

68

Two models have been proposed for the production of TVZ rhyolites: (1) melting of

crust resulting from heat transfer from mantle derived magmas or injection of mantle

material into crust already heated by plastic deformation (Ewart, 1966; Ewart et al.

1975; Reid, 1983; Graham et ai, 1992; Hochstein et al. 1993); (2) assimilation

fractional crystallisation of basaltic magmas (Blattner & Reid. 1982; Graham et ai,

1992; McCulloch et ai, 1994). Graham et al {1995} questioned these models as the

rhyolites do not show the uniform composition expected if the resulted from melting of

lower crustal rocks and there is little evidence for the quantities of basalt required to

produce 100 000 km3+ of rhyolite. Graham et al (1994) failed to come to an

unequivocal conclusion concerning the origin of TVZ rhyolites but felt that isotopic data

suggested that the rhyolites are derived from a combination of crystal fractionation and

crustal assimilation similar to (2) above.

Figure 4.7 compares analyses of Waiotapu Ignimbrite pumices with data for other

TVZ ignimbrites. Data is sourced from S.D. Weaver and B.F. Houghton {unpub. data},

S.W. Beresford (unpub. data), Sutton et al (1995). Brown (1994), Karhunen (1993).

and Briggs et al (1993). With the exception of Whakamaru Ignimbrite, Waiotapu

Ignimbrite pumice types show a wider disparity in Rb content than other TVZ

ignimbrites. Unlike many other ignimbrite pumice groups, Waiotapu Ignimbrite shows

no variation in Sr. These data have been interpreted to suggest that many TVZ

rhyolites have been generated in batches that have variable trace element

characteristics and have subsequently undergone unique fractionation trends (Brown,

1994).

Type-A pumice data for Waiotapu Ignimbrite suggests that the magma underwent

fractional crystallisation of plagioclase and Fe-Ti oxides. The erupted Waiotapu

Ignimbrite chemistry may simply represent the most evolved uppermost part of a

chamber that was not completely eviscerated during eruption. Alternatively mixing prior

to, or during eruption may have obscured the early crystallisation history.

There is insufficient data to determine the history of type-B magma, and its

relationship to type-A magma is equivocal. Isotopic data would be invaluable in

determining whether or not the type-A and B pumices are genetically related.

69

CHAPTER FIVE

TH N RI

INTRODUCTION

The Ngapouri Ridge (Figure. 5.1), situated 2 km southwest of Waiotapu, is a

topographic feature composed dominantly of Waiotapu Ignimbrite and Ngapouri

Rhyolite. A fault bounded scarp forming the Paeroa Range lies approximately 4km to

the west, while to the north is Maungaongaonga, an 825m high dacitic ediface. Along

the eastern margin of the ridge is the Trig 8566 rhyolite, the age of which is uncertain

however it is bracketed by the age of Waiotapu Ignimbrite (710 ka), which it intrudes,

and the eruption of Kaingaroa Ignimbrite (230 ka). The latter was erupted from

Reporoa Caldera, the bounding faults of which truncate the eastern side of the dome

(Nairn et ai, 1994).

This chapter will examine the units that are associated with Waiotapu Ignimbrite on

the ridge and surrounding area, and to try and determine what relationship, if any, they

have with Waiotapu Ignimbrite.

5.1 STRATIGRAPHY

The initial stratigraphy of the Waiotapu region (Table 5.1) was based on the logs of the

7 drillcores within the Waiotapu Geothermal Field (Steiner, 1963) and their correlation

with the sequence outcropping on the nearby Paeroa Scarp. Here Waiotapu Ignimbrite

appears to overlie the Paeroa Ignimbrite and the Paeroa Scarp sequence appears to

dip under the Ngapouri Ridge. Waiotapu Ignimbrite was also mapped on top of the

Paeroa Range, above Paeroa Ignimbrite, so it was concluded that the Waiotapu

Ignimbrite was younger than the Paeroa Ignimbrite. This stratigraphy was, until

recently, widely accepted (Grindley et ai, 1994).

More recent fission track and 40Ar_39Ar dating has forced a reappraisal of the

stratigraphy. Initial fission track dates (Kohn, 1986; Kohn et al 1992) showed that

Waiotapu Ignimbrite (0.58 ± 0.03 Ma) was older than Paeroa Ignimbrite (0.36 ± 0.03 to

0.38 ± 0.03 Ma) and Te Kopia Ignimbrite (mean age of 0.38 ± 0.09 Ma) which outcrop

on the Paeroa Scarp. Grindley et al (1994) then provided additional zircon fission track

ages of 0.57 ± 0.05 Ma and 0.58 ± 0.06 Ma for the Waiotapu Ignimbrite, and a mean

age of 0.36 ± 0.01 Ma for the Paeroa Ignimbrite and 0.36 ± .07 Ma for the Te Kopia

Ignimbrites, further supporting the revision of the stratigraphy. New 40Ar_39Ar dates of

70

·c

:::J o 0... m

Q

)

Z if)

"0

I-

m

$: o

...... 0... I-

co U

(f)

m

o I-<l.l co

0... <l.l .c: ...... E

o

..::: .c: t o c.: Q

) c.:

:sz o o ...J

71

Table 5.1 Stratigraphy of the Waiotapu Geothermal Field and surrounding region as originally defined by Steiner (1963) and subsequently redefined by Grindley et al (1994).

Description (After Steiner, 1963)

Interbedded siltstones & sandstones

Dacite

Rhyolitic pumice breccia with quartz feldspar and biotite

Biotite-bearing quartzose ignimbrite

Vitric tuff with interbedded pumiceous breccia

Lenticular ignimbrite

interbedded pumiceous breccias and tuffaceous sandstones

Quartzose ignimbrite with hornblende and pyroxene

Interbedded pumiceous breccias, tuffaceous breccias and tuffaceous sandstones

Rhyolite pumice shreds with crystals Df oligoclase andesine

Quartzose ignimbrite with hornblende and pyroxene

Interbedded pumiceous breccias and tuffaceous breccias

Augite-hypersphene andesite

Biotite-bearing quartzose ignimbrite

Unit Name Original Stratigraphy (Steiner, 1963)

Huka Group

Maungaongaonga Dacite

Pumiceous Breccia

Rangitaiki Ignimbrite

Huka Group

Waiotapu Ignimbrite

Lower Aquifer A

Paeroa Ignimbrite A

Lower Aquifer B

Pumiceous Tuffs

Paeroa Ignimbrite B

Lower Aquifer C

Ngakoro Andesite

Paeroa Ignimbrite C

Revised Stratigraphy (Grindley et ai, 1994)

Huka Group

Maungaongaonga Dacite

Onuku Pumice Tuff

Rangitaiki Ignimbrite

UnitW

Waiotapu Ignimbrite

UnitX

Ignimbrite A (I)

UnitY2

Unit Y1

Ignimbrite B(I)

UnitZ

Ngakoro Andesite

Ignimbrite C(I)

(I) Correlated with Akatawera Ignimbrites A & B identified in Braodlands-Ohaaki, Te Kopia and Orakeikorako geothermal fields (Grindley et ai, 1994).

tJ) No known correlative (Grindley et ai, 1994).

provide a much older age of 0.71 ± 0.06 Ma for the Waiotapu Ignimbrite and 0.33 ±

0.01 Ma for the Paeroa Ignimbrite (Houghton et ai, 1995).

It is now accepted that the original interpretation of the stratigraphy of the region

was incorrect and that Waiotapu Ignimbrite must pre-date the deposition of the

ignimbrites exposed in the Paeroa Range.

72

w

KEY: 0 Hydrothermal Breccia

Trig 8566 Rhyolite

Paeroa Ignimbrite

Te Weta Ignimbrite

Te Kopia Ignimbrite

Waiotapu Ignimbrite

Unit X

Ngapouri Rhyolite

) \ /

E

Rahopaka-Akatawera A Ignimbrite?

Figure 5.2 Representative cross-section of Waiotapu/Ngapouri Region (not to scale).

5.2 REGIONAL STRUCTURE

Faulting in the Ngapouri Region is complex and the exact nature and distribution of

faulting is difficult to resolve, given the poor exposure and subdued geomorphology

(Figure 5.2). Two major faults have been recognised and are agreed upon by all

previous workers (e.g. Grindley et ai, 1994; I.A. Nairn unpublished data; Keall, 1988).

In addition faulting related to the collapse of Reporoa Caldera and subsidence outside

the structural boundary of the caldera is likely to be imposed on the fault pattern in the

region, thus further complicating the fault distribution.

The major controlling fault in the region is the Paeroa Fault, which defines the

western boundary of the Paeroa Range. The trend of the fault is 040°. which is parallel

to the regional trend within TVZ and has a downthrow of 600m to the west, although

the fault scarp is only 500m high (Keall, 1988). Approximately 3 km to the south east is

the Ngapouri Fault (trending 055°) which has an appreciable down throw to the west

although the total amount of throw is unknown.

East of the Paeroa Fault the Paeroa Range Group Ignimbrites dip 7° to the south

east, before being covered by hydrothermal debris. Foliation within Waiotapu

Ignimbrite appears to have a concentric distribution, dipping in towards the Trig 8566

rhyolite. J. Healy (pers. comm. to Hedenquist. 1983) described this as a cup and

saucer relationship where the dome has been extruded through Waiotapu Ignimbrite

which has subsequently subsided due to the increased load.

73

5.3 NGAPOURI RHYOLITE

5.3.1 Introduction

The Ngapouri Rhyolite (Figure 5.3), originally called the Waiotapu Rhyolite by Grindley

(1965), is the eroded remains of a small rhyolite dome that comprises the central part

of the Ngapouri Ridge. No type locality has been defined, but a good reference section

can be found at U16/005114. Grindley (1965) originally considered the Waiotapu

Rhyolite to be a vent fill within the source of Waiotapu Ignimbrite, however later work

proved this was not the case (see Chapter 1.3) and the unit has since been renamed

the Ngapouri Rhyolite (Grindley et ai, 1994).

The exposures on the Ngapouri Ridge are the only known occurrence of the unit

within the TVZ. Inferences about the subsurface distribution of the rhyolite can be

made using lithic component analysis of lithic lag breccias within ignimbrites with

source calderas in the surrounding area. No lithics of Ngapouri Rhyolite (or

descriptions the match the rhyolite) have been reported in Whakamaru Ignimbrite

(Brown, 1994), Paeroa Range Group Ignimbrites (Keall, 1988) or Kaingaroa Ignimbrite

(S.W. Beresford pers. comm. 1996), suggesting that the distribution is restricted to the

area surrounding the Ngapouri Ridge (see section 5.5.3). Although it is possible that

there have been additional related domes within the vicinity they are no longer

exposed or extant.

In outcrop the unit is characterised by near vertical jointing, measured spacings

between joint sets ranging from 4 cm to 105 cm. Near the crest of the ridge the joints

trend almost east - west (096° to 102°, measured in outcrop) dipping between 76° to

84° to the north. Further to the north the orientation of the joints swings northeast (066°

to 054°) and the dip of the sets decreases to between 57° and 61° northwest.

Ngapouri Rhyolite can be divided into two main facies. On the crest of the ridge

(e.g. at U16/005114) the unit is a coherent purple or grey coloured flow banded rhyolite

containing an estimated 10% phenocrysts. Outcrops to the north are far more brittle

and samples have a pervasive hackly fracture; this makes the unit easily confused with

Waiotapu Ignimbrite, but with close examination flow banding is discernible within the

rhyolite.

The exact age of the unit is unknown but recent paleomagnetic polarity studies have

shown that the unit is reversely magnetised, therefore falling into the Matuyama

reversed polarity episode and giving a minimum age of 0.75 Ma (Grindley et ai, 1994).

Waiotapu Ignimbrite (0.71 Ma) appears to have flowed around the eroded remains of

the rhyolite.

74

Figure 5.3 Ngapouri Rhyolite, taken at the reference section at U16/005114 on Ngapouri Ridge. Here the unit is a purple coloured, coherent, flow banded rhyolite, further north the degre of devtrification intensifies imparting a pervasive hackly fracture. Hammer is 33cm long.

5.3.2 Petrography

Ngapouri Rhyolite is a purple, crystal poor (~10%) flow banded rhyolite. The

phenocrysts are 1-2 mm in size and the assemblage is dominated by plagioclase

(An35), many crystals of which are weakly zoned. In order of decreasing abundance

magnetite, orthopyroxene (of which most are almost completely altered) and quartz are

present in trace amounts.

The degree of devitri"flcation within the rhyolite intensi"fles away from the inferred

centre of the dome on the peak of the Ngapouri Ridge. Near the centre of the dome

devitrifiation is incipient and defines flow banding; in places there are spherulites up to

2 mm across. Further from the centre devitrification intensifies and spherulitic texture

becomes increasingly prominent (Figure 5.4a) until a mosaic of spherulites (0.5-2mm

across) completely obscures all primary textures (Figure. 5.4b). The extreme

devitrification, which is best observed near the contact with Waiotapu Ignimbrite

(U16/012114), has led to the development of a pseudo hackly fracture within the rock

which is caused by disaggregation of the rock along the boundaries between

spherulites within the mosaic.

5.3.3 Geochemistry

Two samples of I\Igapouri Rhyolite were analysed by XRF for major and trace

elements, and the results are presented in Appendix The unit is a high silica rhyolite

(74.83-75.22 wt% 8i02) with Rb/8r ratios of 1.41-1.54 and high potassium content

(4.02-4.12 wt% K20).

Figure 5.5 shows analyses of I\Igapouri Rhyolite plotted against fields enclosing

Waiotapu Ignimbrite pumice and whole rock analyses. Ngapouri Rhyolite conSistently

plots outside the fields of both whole rock and pumices. The location of the rhyolite on

the plots is difficult to explain by crystal fractionation, especially conSidering the age

difference between the units. When plotted against K20 the rhYOlite lies well away from

the Waiotapu Ignimbrite trend.

5.3.4 Discussion

While originally identified as a rhyolite by Grindley (1965) the similarities of the unit to

Waiotapu Ignimbrite in the field has meant that the rhyolite has been misidentified.

Martin (1961) described three layers within Waiotapu Ignimbrite at Ngapouri Ridge,

based on the presence of three levels of outcrop on the ridge. At least one of these

ridges, however, is entirely within Ngapouri Rhyolite. However the units can be

distinguished within the field, and near the crest of the ridge the unit is unequivocally a

rhyolite lava, which displays good flow banding. Even where pervasive spherulite

development imparts a pervasive "Waiotapu Ignimbrite-like" pseudo hackly fracture,

flow banding is still discernable.

Field relations, where Waiotapu Ignimbrite appears to have flowed around the

eroded remains of the Ngapouri Rhyolite dome, and the age discrepancy of at least 40

ka, suggests that the rhyolite and the ignimbrite are unrelated. Waiotapu Ignimbrite

and Ngapouri Rhyolite are petrologically similar, although Waiotapu Ignimbrite contains

a greater amount of orthopyroxene. Finally the chemistry of the rhyolite is inconsistent

with the unit being an older, and less evolved, product of the Waiotapu Ignimbrite

parental magma.

76

Figure 5.4 Devtrification textures in Ngapouri Rhyolite. a) "Tarantula" texture within coherent rhyolite which developed as a result of the growth of large spherulites. b) With increasing devtrification spherulites form a mosaic and the rock becomes brittle, failing along the margins of spherulites. Field of view 3mm.

77

2.0 ,-,---,-,----,-.......,-----,,-,--~ ... r ... _

1.S

1.6

1.4

... 1.2 ~ ~ 1.0

O.S

0.6

0.4

0.2

o

4

...... ~ ...... .1 ----''----'-----1------'_-'--_

80 90 100 110 120 130 140 150 160 170 Rb (ppm)

2L ... ~~ ..... L--~~~_~--'------'~ 80 90 100 110 120 130 140 150 160 170

Rb (ppm)

2.6 c---,--.......,-_ .... ,--,---.,_.,--,--.,_

2.4

2.2

2.0 '$. ! 1.S

~ 1.6 o

1.4

1.2

1.0

0.8 L~....L...~ .. __ L~.....L...--L .... _..L.---'------'.---' 80 90 100 110 120 130 140 150 160 110

Rb (ppm)

160

140

e-o. 8120

100

80

60L-·L .. ~ ....... ~~J ....... -L-----'-~--L ....... ~ 80 90 100 110 120 130 140 150 160 170

Rb (ppm)

Figure 5.5 Variation diagrams showing the relationship between Ngapouri Rhyolite (black squares) and fields containing Waiotapu Ignimbrite whole rock (dark grey) and pumice (light grey) analyses. Note the disparity between the trend of Rb vs K20 for Waiotapu Ignimbrite and Rahopaka Ignimbrite analyses.

5.4 THE X-FilES: UNITS UNDERLYING WAIOTAPU IGNIMBRITE

5.4.1 UnitX

Unit X (as defined by Grindley et ai, 1994) is a collection of interbedded pumiceous

sediments and tuffs that outcrop along the base of the northwestern side of the

Ngapouri Ridge (Figure. 5.6). Exposure of the unit is poor and much of it was mapped

on the basis of geomorphology, which suggests it extends south from Lake Opouri to

exposures with Kawaunui Stream. Unit X was logged in detail at the base of the

Ngapouri Ridge at U 16/004116 where it is exposed as a 6 m high outcrop consisting of

four distinct facies separated by gradational contacts. Figure 5.7 is a graphic log of the

outcrop.

Steiner (1963) and Hedenquist (1983) reported pumiceous breccias and crystal

lithic tuffs beneath Waiotapu Ignimbrite in the Waiotapu Geothermal Field drillcores

which Grindley et al (1994) believe correlate with Unit X. Successions of unwelded

ignimbrite, tephras and volcaniclastic sediments beneath Waiotapu Ignimbrite in the

western TVZ have also been reported (Houghton et ai, 1987a; Houghton et ai, 1987b;

78

Figure 5.6 Outcrop of Unit X on Ngapouri Ridge (U16/004116). See Figure 5.7 for graphic log of the outcrop. Length of hammer is 33cm.

Gifford, 1988).

Paleomagnetic work conducted by Grindley et al (1994) on samples of Unit X from

Kawaunui Stream (at U17/993098 and U17/993096) gave reversed magnetic

orientations, suggesting an upper age limit of 0.75 Ma, which supports a Waiotapu

over Unit X stratigraphy.

Unit X appears to represent the reworked pumiceous ignimbrite and tuffs formed

significantly earlier than the deposition of the Waiotapu Ignimbrite and the tuffs are

unlikely to represent any pre-eruptive activity related to Waiotapu Ignimbrite. The unit

has been defined elsewhere in TVZ (e.g. Houghton et ai, 1987a & b; Gifford, 1988)

and appears to include many pre-Waiotapu Ignimbrite tephras and sediment that are,

as yet, undifferentiated. In some areas where the unit occurs, Waiotapu Ignimbrite is

absent.

5.4.2 Akatawera A I Rahopaka Ignimbrites

Between the southern spurs of the Ngapouri Ridge at U16/005106 and U16/000104

are outcrops of a moderately welded ignimbrite beneath Waiotapu Ignimbrite. Grindley

et al (1994) named this unit Akatawera A and correlated it with units found within the

79

5 .() Q

Q

0 Q

Q 0

4 C>

0 . 6 CO,'

3

.. ", .. 2 .•..•

.D

1

O...l...... _______ --J

Pale brown to white, poorly indurated massive sandstone. Massive unit with sparse granule to pebble size clasts supported by a very-coarse sand matrix. Compositionally as below.

White to brown, indurated pumiceous sediment. Massive unit with pebble to cobble size clasts supported in a very coarse sand size matrix. Clasts are dominantly poorly vesicular, crystal poor pumices, matrix as below.

White-brown to red pumiceous sediment. Planar bedded, granule to pebble sized, subangular clasts supported in a coarse sand matrix. Composition as below. Red colouration due to hydrothermal alteration .

White brown coloured, moderate to poorly indurated pumice rich sediment. Poorly bedded unit containing granule to pebble sized, sub-angular clasts within a coarse sand matrix. Several boulder sized clasts present. Dominantly composed of ignimbrite and pumice clasts.

Figure 5.7 Log of Unit X at the base of Ngapouri Ridge (U16/004116).

80

Te Kopia, Orakeikorako and Ohaaki-Broadlands Geothermal Fields. C.P. Wood (pers.

comm. 1996) believes the ignimbrite may represent eastern exposures of Rahopaka

Ignimbrite, previously only mapped in the Kinleith Forest. These exposures will be

discussed in detail in Chapter 6.

5.5 PAEROA SCARP FLOAT BLOCKS

5.5.1 Introduction

For many years the stratigraphy of the region near the Ngapouri Ridge was confused

by the presence of what were thought to be in-situ outcrops of Waiotapu Ignimbrite

overlying the Paeroa Range ignimbrites (see Chapter 1.4). These blocks (Figure 5.8)

are exposed on the upthrown side of the Paeroa Range within an area of

approximately 4 km2, enclosed by a triangle extending from Trig 8533 (U16/987132), in

the north, to Trig 8551 (U16/975116), in the south, and to Kawaunui Stream

(U16/985103). More recent dating (e.g. Kohn et ai, 1992; Houghton et ai, 1995) forced

a revision of the local stratigraphy as it proved Waiotapu Ignimbrite was older than the

Paeroa Range Ignimbrites. Consequently the origin of the blocks of Waiotapu

Ignimbrite is now open to speculation, This section will outline the nature of the

exposed blocks and discuss possible origins.

5.5.2 Characteristics

The scattered blocks are of extremely limited composition; most are Waiotapu

Ignimbrite, but there are isolated blocks of Ngapouri Rhyolite. In both hand specimen

and thin section all samples are similar to Waiotapu Ignimbrite and Ngapouri Rhyolite

from the nearby Ngapouri Ridge. The blocks are commonly sub-angular although they

become sub-rounded with decreasing size. Some blocks may represent the

disaggregation of larger blocks but there is little evidence of fresh fracturing of the

material suggesting that rounding may, in part. be due to weathering following the final

emplacement of the blocks. Evidence of heat spalling has been reported in some of

the clasts (I.A. Nairn pers. comm. 1995; Grindley et ai, 1994) but was not observed

during this study.

The orientation of foliation (defined by fiamme and/or lithophysae) varies

considerably between blocks and their size is highly irregular. Clast sizes range from

centimetre scale blocks up to a large block forming a small hill in the vicinity of

U16/980107, reported by Grindley et al (1994). Here there is one outcrop

(U16/980106) which yielded strike and dip of foliation which is markedly discordant

with dips on the Ngapouri Ridge, suggesting the block has undergone subsidence.

81

00 N

Figure 5.8 Blocks of Waiotapu Fault scarp, eastern Left: U16/977122. Sledge is 55cm long. Below below: Blocks at U16/982125. Here bloCKS are numerous and range size from several cm to 2.3m.

\

5.5.3 Discussion: Emplacement

Since re-evaluation of the stratigraphy, alternative explanations of the presence of the

blocks have been proposed. Dr I.A Nairn (pers. comm. 1995) postulated that the

blocks represented the scattered remains of a lithic lag breccia within an ignimbrite

pre-dating the eruption of Kaingaroa Ignimbrite. He cited the presence of a number of

blocks that showed evidence for hot emplacement in the form of heat spalling.

Nairn's explanation, while plausible has limitations. The limited composition of the

blocks implies that the source was nearby and localised, otherwise lithic fragments of

other compositions would be expected. The Waiotapu Ignimbrite appears to have

flowed around the eroded remains of the Ngapouri Rhyolite, consequently it is unlikely

that the rhyolite extended significantly further than its current distribution. Also the

blocks are confined to the small saddle between Trigs 8533 and 8551. It would

therefore seem reasonable to assume that the blocks were derived from the Ngapouri

Ridge. In addition Grindley et al (1994) point out that the heat spalling of the blocks

cannot represent an extreme emplacement temperature, as Zircon fission track dates

from blocks have yielded dates matching those for in-situ Waiotapu Ignimbrite.

An alternative theory has been postulated by Grindley et al (1994). As a result of field

mapping in the region they felt it was necessary to infer a fault with a throw of

approximately 200m, the so-called Caldera Boundary Fault (Grindley et ai, 1994). The

structure is thought to represent the eastern margin of the proposed Paeroa Caldera

(presumably the source of the Paeroa Range Group Ignimbrites) and the float

represents exotic blocks spalled off the fault during successive episodes of caldera

collapse (Grindley et ai, 1995).

Grindley et al (1994) claim that where the base of any blocks are seen they lie on

pumiceous silts above the lower sheet of Paeroa Ignimbrite, implying that the blocks

were transported to their current location between the eruption of the two sheets and

were deposited under lacustrine conditions. Field work conducted for this study failed

to find any evidence for such conditions, and most blocks appear to be lying within

recent soils (see Figure 5.8). It is possible that the positions of the mapped blocks do

not represent their original locations after emplacement.

The evidence for the Caldera Boundary Fault is minimal and it does not have an

obvious surface expression in the field, however Grindley reports several hydrothermal

areas lie along the proposed trace of the fault. Detailed transects of the Paeroa Scarp

and subsequent interpretation of maximum lithic data (Keall, 1988) suggests that the

Te Weta and Paeroa Ignimbrites were sourced to the south of the Paeroa Range,

83

Figure 5.9 Blocks of Paeroa Range Group Ignimbrites lying at the base of the Paeroa fault scarp.

while Te Kopia Ignimbrite is derived from a northern source. Consequently serious

doubts must be thrown on Grindley's theories concerning the structure of the region.

The most plausible explanation for the presence of the blocks is that they are the result

of rock falls (probably related to episodes of fault movement) from a nearby

topographic high composed of Waiotapu Ignimbrite and Ngapouri Rhyolite. The limited

distribution of the Ngapouri Rhyolite suggests that the topographic high included the

modern Ngapouri Ridge, but was somewhat more extensive. The Ngapouri Fault runs

northeast along the base of the western side of the Ngapouri and activity on this fault is

most likely to have led to the destruction of portions of the ridge. A similar situation

occurs along the base of the Paeroa Scarp where blocks of Paeroa, Te Weta and Te

Kopia Ignimbrites have clearly fallen from the scarp above (Figure 5.9). If this is the

case then it is remarkable that the blocks have travelled some 2 - 3 km from their

original source (the blocks beneath the Paeroa Scarp are all found within half a

kilometre of the scarp). It is possible that the blocks are the remains of a much larger

amount of debris produced during a catastrophic failure of pali of the ridge which

generated large scale gravity slides with the subsequent deposit eroded away leaving

the current distribution. Some remobilisation due to activity on the Paeroa Fault is

84

possible, but this is unlikely to have transported the blocks very far. The large hill at

U16/980106 may represent subsidence of in-situ Waiotapu Ignimbrite with little, or no,

further transport.

The timing of emplacement is difficult to determine. Grindley et al (1994) believe the

blocks were emplaced between two sheets of Paeroa Ignimbrite, in which case the

blocks were deposited around 0.32 Ma (the age of emplacement of the Whakamaru

Group Ignimbrites - of which the. Paeroa Ignimbrite is considered a member (Brown,

1994)). Field observations show the blocks to be lying within soils, rather than directly

above Paeroa Ignimbrite, therefore the age of emplacement may be considerably later.

85

HAPTER SIX

IGNIM

INTRODUCTION

Rahopaka Ignimbrite (0.77 ± 0.03 Ma; Houghton et ai, 1995), a moderately welded,

hornblende-phyric tuff, was first described and named by Murphy (1977). who defined

the type locality at U 161789205, 150m east of the intersection of Pukerimu and

Tikorangi Roads. Reconnaissance mapping of Rahopaka Ignimbrite was conducted in

the Matahana Basin in order to establish the general nature of the unit and establish

any possible relationship with the overlying Waiotapu Ignimbrite.

At present the only known exposures of Rahopaka Ignimbrite are in the Matahana

Basin (Figure 6.1) from the end of Bob Rd in the west to Rusa Rd in the east. Much of

the area of interest is covered by younger deposits mainly the Pokai and Mamaku

Ignimbrites with additional undifferentiated pyroclastic deposits. There is speculation

that the unit has extended as far east as the base of the Ngapouri Ridge (C.P. Wood,

Waiotapu Ignimbrite

Rahopaka Ignimbrite

o

Figure 6.1 Map of the distribution of Waiotapu Ignimbrite, Rahopaka Ignimbrite and Pukerimu Formation within Matahana Basin, Kinleith Forest.

86

Figure 6.2 Inlier of Pukerimu Formation (the cliff forming unit exposed at the base of the hill) capped by Rahopaka Ignimbrite, seen from Rusa Rd.

pers. comm. 1995) where it has been mapped as Akatawera A by Grindley et al

(1994). The possible correlation of these two units will be examined in section 6.6.

Rahopaka Ignimbrite is poorly exposed, however it outcrops at three significant

localities within the Matahana Basin; at the end of Bob Rd (base of the Tikorangi

Escarpment), as an inlier around Pukerimu and Rhino Roads, and between Rusa and

Harry Johnson Roads, where it caps the older Pukerimu Formation and forms a

faulted, pre-Pokai Ignimbrite, paleohigh (Figure 6.2).

The thickness of the ignimbrite was difficult to determine as at most exposures the

top and base of the ignimbrite were not observed. Murphy (1977) gave a thickness of

61 m+ for Rahopaka Ignimbrite at Pukerimu Road.

Given the extremely limited distribution of outcrop of the ignimbrite and the absence

of recognised proximal facies the location of the source is poorly constrained. Current

speculation is that the unit originated from within the Kapenga volcanic centre

(Houghton et al. 1995).

6.1 STRATIGRAPHIC RELATIONS

Rahopaka Ignimbrite is one of the oldest exposed units in the Matahana Basin, a

region with a complex stratigraphy. Table 6.1 provides a generalised stratigraphy of

the region, while Table 6.2 details the stratigraphy of the basin as defined by Murphy

87

Table 6.1 Generalised stratigraphy of units within the Matahana Basin, Kinleith Forest. Data from this study with additional data from C.P. Wood, pers. comm. (1995), D. Dysart, pers. comm., (1996) and Murphy & Seward (1981). Ages are 40Ar}9Ar ages from Houghton et al (1995) except· which is a Fission Track age from Murphy and Seward (1981).

Unit

Undifferentiated material and lacustrine sediments

Johnson Road Basalts

Mamaku Ignimbrite

Ohakuri Ignimbrite

Pokai Ignimbrite

Whakamaru Ignimbrite

MatahanaA

Waiotapu Ignimbrite

Rahopaka Ignimbrite

Pukerimu Formation MatahanaB Tikorangi Ignimbrite Gradational contact Pukerimu Ignimbrite

Age (Ma)

0.22 ± 0.01

0.27 ± 0.03*

0.32 ± 0.02

0.68 ±0.04

0.71 ± 0.06

0.77 ± 0.03

0.89 ± 0.04

and Seward (1981). This section will examine the relationship between Rahopaka

Ignimbrite and units immediately above and below.

At the end of Bob Rd (U16/756185) Rahopaka Ignimbrite outcrops in cliffs in the

base of the Tikorangi Escarpment and is overlain by Waiotapu Ignimbrite. On

Pukerimu Road the Rahopaka Ignimbrite is topped by a small cap of Waiotapu

Ignimbrite.

Several undifferentiated units between Rahopaka and Waiotapu Ignimbrites are

exposed on Tikorangi Road and Rusa Road. On Tikorangi Road a well indurated,

accretionary lapilli bearing tuff and massive breccia are in unconformable contact with

Rahopaka Ignimbrite. To the east, on Rusa Road, Rahopaka Ignimbrite is overlain by a

poorly welded deposit with complexly interbedded zones of accretionary lapilli and

pumiceous ignimbrite. Murphy (1977) described three units younger than Rahopaka

Ignimbrite (Table 6.2).

Beneath Rahopaka Ignimbrite lies the informally named Pukerimu Formation which

consists of the Pukerimu and Tikorangi Ignimbrites (C.P. Wood pers. comm. 1995).

Murphy (1977) described a gradational contact between the Pukerimu Ignimbrite (a

grey, moderately welded, lenticular ignimbrite with abundant orange and grey pumices)

88

Table 6.2 Stratigraphy of the Matahana Basin as mapped by Murphy (1977). (After Murphy and Seward, 1981).

Formation

Undifferentiated volcanics and alluvium

Mamaku Ignimbrite

Unit D

Waiotapu Ignimbrite

Marshall Ignimbrite

Unit C

ms4 -welded ms3 -welded ms2 - unwelded ms1 - pumice fall

Undifferentiated pyroclastics B

Rahopaka Ignimbrite

Undifferentiated pyroclastics A

Tikorangi Ignimbrite

Pukerimu Ignimbrite

Lithology

Bedded fluviatile pumice, rhyolite, and quartz sands and boulders, slope wash

Grey-brown quartzose ignimbrite

Quartzose pumiceous breccia

Densely welded quartz-poor lenticulite

Poorly welded, crystal-poor, pumice breccia

Interbedded breccia and hard stratified lake silts

Densely welded crystal-rich ignimbrites and pisolitic tuff.

Grey-brown, moderately welded, hornblende rich ignimbrite.

Ignimbrite tuff, breccia and lacustrine silts

Grey-black, densely welded, quartz lenticulite

Pumiceous, crystal-poor, quartz-free ignimbrite

and Tikorangi Ignimbrite (a grey to black densely welded lenticulite with black fiamme).

Rahopaka Ignimbrite overlies the Pukerimu formation on Rusa Road although the

contact is not observed.

6.2 LITHOLOGY

Rahopaka Ignimbrite was logged and sampled at two localities in the Matahana Basin;

Bob Rd (U16/789205) and the type section on Pukerimu Road (U161756185). Outcrop

was limited at each locality with about 6 m of section and the unit showed little vertical

variation at each. These outcrops plus other smaller exposures in the vicinity of Rusa

Road, however, suggest that the degree of welding in the deposit decreases upwards.

At the end of Bob Road (Figure 6.3) the ignimbrite constitutes the lower part of the

Tikorangi Escarpment, and is inferred to be at least 40m thick. The ignimbrite is poorly

welded, with clast densities ranging from 0.98 - 1.43 g cm-3, welding appears to

89

Figure 6.3 Exposure of Rahopaka Ignimbrite near the end of Bob Rd (U16/756185).

increase with decreasing stratigraphic height (Figure 6.4a). Lithic fragments are sparse

and have an average size of ~3cm, and ML of 11 cm.

At Pukerimu Road the exposure is in the advanced stages of degradation however

samples collected at ~1 m intervals throughout the section suggest that the unit shows

little variation with increasing height. The ignimbrite is moderately welded (clast

densities ranging from 1.23 - 1.57 g cm-3, see Figure 6.4b) and again the degree of

welding appears to increase down through the unit. No large lithics were recovered

from this locality and here the unit also appears to contain few pumices.

Murphy (1977) noted a 70 rnm thick pumice ash zone beneath poorly welded

Rahopaka Ignimbrite at the end of Rusa Road. A poorly welded, pumiceous ignimbrite

imilar to that described by Murphy (1977) was mapped at the end of Rusa Road

90

Rahopaka Ignimbrite Density Profile

a) Bob Rd (U16f789205)

6~-----.------~

I I 5 Ell

~. \ ~ g. 4 •

L \ ; / 8- 2 •

i i \ :l ....... ,',., .. ,\ 0.5 0.7 0.9 1.1 1.3 1.5

Density (g/cm3)

Rahopaka Ignimbrite Density Profile

b) Pukerimu Rd (U16f756185)

1.00 1.20 1.40 1.60 1.80

Density (g/cm3)

Figure 6.4 Clast density profiles of Rahopaka Ignimbrite at (a) Bob Road and (b) Pukerimu Road, Kinleith Forest.

however it is equivocal as to whether this unit is Rahopaka Ignimbrite. Further to the

west, and approximately 20 m lower, Rahopaka Ignimbrite is seen to grade from a

poorly welded and pumiceous deposit into a moderately welded, poorly jointed unit

which is unequivocally Rahopaka Ignimbrite. The base of the unit is not observed on

Rusa Road.

6.3 GEOCHEMISTRY

In order to establish a possible relationship between Rahopaka and Waiotapu

Ignimbrites the general geochemical character of Rahopaka Ignimbrite was

established. Although pumices are present within the ignimbrite, they were either too

small to yield enough sample for analysis or were too altered to be of any use. So

whole rock samples of the unit were analysed. Three samples were cotlected from top,

middle and base of the 6 m high section at Pukerimu Road (U161756185). Major and

trace element analyses for these samples are presented in Appendix 2.

It is recognised that whole rock analyses of a tuff as poorly welded as Rahopaka

Ignimbrite are of dubious value and this must be considered before interpreting the

data (problems with whole rock geochemistry of tuffs are outlined in Chapter 4.2). In an

attempt to limit the influence of lithic fragments on the analyses, before samples were

91

Table 6.3 Comparison and ranges in concentration of selected elements between Rahopaka Ignimbrite and Waiotapu Ignimbrite. Rahopaka Ignimbrite analyses are from whole rock samples whereas Waiotapu Ignimbrite analyses are from whole rock and pumices.

Element

Si02 (%) Ti02 (%) K20 (%)

(ppm) Rb (ppm) Sr (ppm) RbISI'

Rahopaka (Whole rock)

70.25 - 70.78 0.40 - 0.59 2.84 - 2.91 182 - 194 98 -102 131 - 174 0.56 - 0.77

Waiotapu Waiotapu (Whole rock) (Pumice)

70.51 - 74.79 70.98 - 75.31 0.23 - 0.37 0.23 - 0.37 3.11 - 3.46 3.24 - 3.76 229 - 286 196 - 339 94 - 121 101 - 161 93 - 129 79 - 142 0.81 - 1.39 0.71 - 1.73

ground to a powder, lithics were removed by hand from crushed samples with the aid

of a binocular microscope.

The major and trace element geochemistry of Rahopaka Ignimbrite and Waiotapu

Ignimbrite appear to be distinct and ranges for elements seldom overlap (Table 6.3).

Rahopaka Ignimbrite has lower Si02, K20, Zr, Rb and Rb/Sr, and higher Ti02 and Sr

than Waiotapu Ignimbrite.

6.4 PETROLOGY

17 samples were collected at 1 to 2m intervals from the sections at the end of Bob

Road and on Pukerimu Road and were then thin sectioned in order to establish the

general petrographic character of Rahopaka Ignimbrite.

Rahopaka Ignimbrite (plag>qz>hbl>opx»mag>ilmenite) is a medium to fine grained

«1-3mm) tuff with crystal content ranging from c.10% (at Bob Road) to 25-30% (on

Pukerimu Road). The mineral assemblage is dominated (::::::70%) by plagioclase

feldspar (oligoclase, An27) which is commonly weakly zoned. Quartz occurs as sub

rounded to rounded crystals which are generally strongly em bayed. Hornblende, which

is diagnostic of Rahopaka Ignimbrite, occurs as euhedral to subhedral laths or

occasional six-sided prisms.

At Bob Road the unit is pumiceous, with the ground mass containing numerous

millimetre scale pumice fragments. Vitroclastic texture is common, although at times

the matrix is too fine to enable textures to be readily determined. By contrast

Rahopaka Ignimbrite on Pukerimu Road is pumice-poor and the groundmass is often

too fine to easily determine textures. Patchy felsitic devitrification occurs in some parts

of the deposit.

Hematitic alteration occurs throughout the unit, and many hornblende crystals have

92

Table Description of lithic fragments within Rahopaka Ignimbrite.

Type Description

Flow Banded Rhyolite Plagioclase, quartz, magnetite, and traces of orthopyroxene and ilmenite bearing rhyolite. Samples have undergone extensive devitrification, which clearly defines flow-banding. (Figure 6.5a)

Scoria Highly vesicular, crystal poor basaltic scoria. (Figure 6.5b)

Ignimbrite (?) I Intensively devitrified plagioclase, magnetite, orthopyroxene, quartz and biotite bearing unit. Devitrification is dominantly felsitic with occasional spherulites and patches of granophyric recrystalisation. All primary textures have been destroyed therefore the original nature of the fragment is equivocal. (Figure 6.5c)

Ignimbrite (?) II Very similar to Ignimbrite I, however the mineralogy is simpler (plagioclase, magnetite and quartz) and the patches of granophyric recrystallisation are larger and more widespread.

thin «O.5mm) rinds of hematite. Orthopyroxene crystals are generally (although not

always) degraded and occur as remnant cores within cavities.

6.5 LITHIC FRAGMENTS

Lithic fragments within Rahopaka Ignimbrite are rare and do not exceed c.12 cm size.

Lithic component analysis of fragments recovered from the section at the end of Bob

Road revealed 4 dominant lithic types that are readily distinguishable in hand

specimen. All types are present in roughly equal proportions and are summarised in

Table 6.4.

6.6 RAHOPAKA IGNIMBRITE IN THE EASTERN TVZ?

Rahopaka Ignimbrite (sensu stricto) is currently only exposed in and around the

Matahana Basin on the western margin of TVZ. A non-welded tuff at the base of the

Ngapouri Ridge (U16/005116) has been proposed as an eastern correlative of

Rahopaka ignimbrite by Dr C.P. Wood (pers. comm. 1995) on the basis of the

presence of cavities with morphologies resembling hornblende crystal habit. The unit

has been mapped as Akatawera A by Grindley et al (1994) (see section 5.4.1).

Petrographic analysis of samples collected for this study failed to yield any evidence of

hornblende crystals and the correlation with Rahopaka Ignimbrite is thought to be

erroneous.

93

Figure 6.5 Photomicrographs of lithic fragments Rahopaka Ignimbrite. view 3mm.

Flow banded rhvolite. Banding is defined devitrification. Basaltic scoria.

c) Recrystallised ignimbrite (?).

In addition a number of amphibole bearing ignimbrites of uncertain affinity have

been recovered as lithics from Kaingaroa Ignimbrite (S.W. Beresford, pers. comm.

1996). Most of these have pyroxene-amphibole ratios (px>amp) which differ from that

of type Rahopaka Ignimbrite (amp>px). Only one sample (KA 100) had similar px-amp

ratios. Amphibole mineral chemistries in KA 100 and a sample of Rahopaka Ignimbrite

(AW177) were analysed by scanning electron microprobe in order to establish any

possible genetic relationship between the units. The results of the probe work are

presented in Appendix 3. Hornblendes in KA 100 were cummingtonites, as opposed to

magnesio-hornblendes in Rahopaka Ignimbrite, suggesting that the two units are

unrelated (amphiboles were classified using criteria established by Leake, 1978).

6.7 DISCUSSION

On the basis of whole rock geochemistry, Rahopaka Ignimbrite appears to be distinct

from Waiotapu Ignimbrite. In addition Rahopaka Ignimbrite has a distinctive

hornblende-bearing mineralogy, unlike Waiotapu Ignimbrite where hornblendes are

rare.

Correlation of Rahopaka Ignimbrite with units in the eastern TVZ, in the base of the

Ngapouri Ridge and beneath Reporoa Caldera, cannot be sustained on mineralogical

grounds. At present the only known and unequivocal exposures of Rahopaka

Ignimbrite are in the Matahana Basin.

The complexity of the Matahana Basin and uncertain affinity of several of the units

means that a detailed study of Rahopaka Ignimbrite was beyond the scope of this

study. A detailed study of Rahopaka Ignimbrite would benefit considerably from the

examination of the underlying units (most notably the Pukerimu Formation) and the

undifferentiated deposits between Rahopaka Ignimbrite and Waiotapu Ignimbrite. It is

possible that not all hornblende-bearing deposits in the Matahana Basin are Rahopaka

Ignimbrite, and differentiation of these units will be important in understanding the

stratigraphy of the area.

95

INTRODUCTION

CHAPTER

DISCU N

"The facts, although interesting, are irrelevant." Anon

Waiotapu Ignimbrite is a voluminous (at least 175 km3) large scale (aspect ratio of c.

1: 1200) pyroclastic flow deposit which is remarkably uniform in terms of both physical

character and composition. Surface outcrop is limited but it is the only unit that has

been demonstrated to outcrop on both sides of TVZ. It was erupted around 0.71 Ma

and was followed by a period of relative quiescence in TVZ where no caldera forming

eruptions occurred until the eruption of the Whakamaru Group Ignimbrites around 0.33

Ma (Houghton et ai, 1995).

In outcrop the ignimbrite shows no evidence for multiple flow units with a

considerable degree of internal homogeneity, which is also reflected in density profiles,

suggesting that the unit was deposited during the passage of one sustained pyroclastic

flow. Lithic fragments are exceedingly rare, being «1 % throughout the entire exposed

extent. Nothing resembling a proximal facies is observed anywhere, and no lithic

concentration zones have been recorded, although any proximal facies could have

been obscured by younger deposits.

Waiotapu Ignimbrite is welded throughout, being most densely welded just above

the base. This feature is unusual in TVZ ignimbrites suggesting that processes akin to

those in high grade (Le. high temperature) ignimbrites were operating. The unit was

energetically emplaced, even at its most distal extent, around Lichfield and Tokoroa

the pyroclastic flow crossed the undulating paleotopography with ease, climbing

paleohighs with slopes of 30° at Lichfield Quarry, about 40 km from source.

No preceding plinian deposits are observed. Non-welded material beneath the

deposit at Ngapouri Ridge, Tikorangi Escarpment and Wawa Quarry have significantly

different mineral assemblages, or mineral chemistries. The lack of observed Waiotapu

plinian deposits may simply reflect a lack of suitable exposures, but it seems most

likely that a plinian phase did not precede eruption of Waiotapu Ignimbrite.

96

1.1 IGNIMBRITES: ERUPTION AND DEPOSITION

Most ignimbrite forming eruptions documented have a characteristic sequence

involving a preceding plinian eruption and then the deposition of an ignimbrite (Cas

and Wright, 1988). These eruptions are thought to represent the accumulation of a

volatile rich cap at the top of the magma chamber which is tapped first, leading to the

generation of a convecting plinian eruption column (Wilson, 1986). Once the column

loses its impetus, if it is denser than the surrounding atmosphere it will begin to

collapse, generating pyroclastic flows that will flow radially away from the vent. In other

situations the erupted material may be too dense to allow the formation of an eruption

column, or will undergo rapid lateral expansion on leaving the vent, and develop a low

pyroclastic fountain (e.g. Wilson et ai, 1980).

There are various depositional models for ignimbrites. Sparks (1976) considered

pyroclastic flow deposits were similar to sediment gravity flow deposits and thought

they represented the frozen remnants of high-concentration, non-turbulent, plug flows

deposited when the flow froze en masse once gravitational forces were no longer able

to overcome the strength of the flow. While this explained many HARls it was

unsatisfactory in explaining the nature of some LARI deposits. Wilson and Walker

(1982) proposed a mechanism whereby a pyroclastic flow could be considered in

terms of a head, body and tail, within which different depositional regimes operated,

leading to the deposition of differing depositional facies. Branney and Kokelaar (1992)

pointed out various problems with the model of Spark (1976) and its subsequent

refinements, most notably that en masse freezing of a flow was difficult to envisage

over its entire length. Instead they proposed that ignimbrites were deposited during the

passage of a sustained pyroclastic flow which had two distinct layers. Deposition would

occur in a basal agglutinate layer, which chills and freezes against the ground, the

layer thickening and aggrading with the supply of material from an overriding, non

particulate flow.

Branney and Kokelaar (1992) also defined an ignimbrite grade continuum (Figure

7.1), expanding on the concept of ignimbrite grade defined by Walker (1983).

Ignimbrites can range from being extremely high grade, where they are intensely

welded to the point of being lava-like, to low-grade ignimbrites, which exhibit little or no

evidence of welding. By definition of Branney and Kokelaar (1992), Waiotapu

Ignimbrite is between moderate grade (both welded and non-welded zones) and high

grade (predominantly welded with rheomorphic zones).

97

)-II: 0( II: I-10 II: 0(

(j) Z o E o z o o o z ~

(j) W (f) (f) W o o a: 0...

(j)

o i= ~ a: w Io ~ a: ~ :r: o Io :J o o a: 0..

EXTREMELY HIGH-GRADE

fountBln·lod lava-Uk., RheomarpMe Hon~the.otOO,phlc NOfl-W&fdGd Il1n.1:mbritu

lava· flows IgnlmbflloD IgntmbrU .. Waldod lonlmb,Uu

"" "'" (0,0. OraMGY a' oJ t9'Jli (00 !;cf'un,:j'\(ko .& $W".3r\10l'1 1067; to IJ flon t. SrfHlh tM'; n~f;'110

1 •. o.IluI~.1d 1m, 4 ~

e"," 01" 19",' Cha~ & to\¥Q41 HltQ: RoQ .. , S,",,,,,,,, 1012; (.QoWsJiM:dl~l

Or1i A Sh.ooid.an 19M) n ..... 101:l1

loctoasa In tompOtOltUlO. docroaso 11'\ palUdo 'WSOOSity <lnd '(+old suongfh

(IHllnSCaN» ----------.~ --~ -_.

lalofal pa,lkul.:alo now importanl during omplaromont

and

.---------------~~----~----~---

pumca imbllcaoon and grain J;)btics

non·pafticul.110 iinoilllons:, (ol<1tod lithiC'S nnd trow fold!>

Iobato -----"---- ---------. ------- - -------- -- ----------

kx4lisod ros.al and/or intarnal ;]ulohlo<xiJ

cmgulilr Of CUSpalo shatds p<os.orvod in ignimonlQ

Figure 7.1 The ignimbrite grade continuum, and associated eruption and emplacement processes, conditions and products. Solid lines indicate some characteristic features and dotted lines indicate possible features. Waiotapu Ignimbrite is best described as a non-rheomorphic welded ignimbrite. From Branney and Kokelaar (1992).

7.2 KAPENGA VOLCANIC CENTRE: THE SOURCE?

7.2.1 Introduction

Kapenga volcanic centre is located in the middle of the Taupo fault belt, and covers an

area of 250 km2, including the Ngakuru Graben in the southern portion of the structure

(Wilson et ai, 1984). The centre was first identified by Rogan (1982) from gravity and

98

HAURAKI GRABEN

30

----lSI)

~, .. ~' •• O

GRAVITY MODEL of the TAUPO VOLCANIC ZONE

O"noity contrast -600 kg m"3

Contour vtltue. In metres betow sea f(tve'

hit] Drttlhofe. tmlnlmumt depth to bsoemoot

~Lako

+ 5&'_",,10 refractIon line . ,

10" Yards East I

Figure 7.2 Contour map of the depth, below sea level, of the basement of TVZ based on a best fit model derived from gravity anomalies. Kapenga volcanic centre is located above the centre of the map and is clearly made up of two 3000m deep depressions. From Rogan (1982).

magnetic studies (Figure 7.2). These studies identified a 2.5km thick body of low

density, magnetised rocks which were interpreted to be rhyolitic volcanics infilling a

volcanic collapse structure. The centre had the largest extent of the five volcanic

centres recognised at the time.

Wilson et al (1984) later suggested separation of the centre into two collapse

structures. The northern end of the area corresponding with a 2.5 km deep basin, while

a larger, 3km deep basin was located in the centre and south of Kapenga. The

boundary between the Okataina and Kapenga volcanic centres is not apparent on the

IHI: UtlHAH1

UNfVERSITY Of CANTL:;Bum r.H~IRTCHlJRCH. NL

99

Table 1.1 Deposits thought to originate from Kapenga Volcanic Centre after Houghton et al (1995). Period refers to the location of activity at Kapenga to the three periods of volcanic activity defined by Houghton et al (1995). All ages are 40Ar_39Ar, unless otherwise specified.

Period liB

Unit

Ohakuri Ignimbrite Pokai Ignimbrite Chimp Ignimbrite·

Period IIIB

Age (Ma) Unit

0.27 ± 0.03+ Matahana A Ignimbrite Waiotapu Ignimbrite Rahopaka Ignimbrite Matahana B Ignimbrite Tikorangi Ignimbrite

Age (Ma)

0.68 ± 0.05 0.71 ± 0.06 0.77 ± 0.03

0.89 ± 0.04

+: unpublished fission track age (B.P. Kohn unpub data, held on GNS files) .: Highly equivocal as to whether Kapenga is the source area

surface and volcanism appears to overlap. The eruption of Earthquake Flat Breccia,

which occurred immediately after the eruption of Rotoiti Breccia (65ka, Houghton et ai,

1995), suggests its eruption was related to activity in Okataina but geophysical data

suggests a ridge separates the two centres (Wilson et ai, 1984).

Kapenga volcanic centre is extensively faulted, with faulting being more intense in

the north. To the south much of the centre is covered by Ohakuri Ignimbrite (0.27 ±

0.03 Ma; Houghton et ai, 1995) which post dates much of the faulting (Wilson et ai,

1984).

Originally, only Waiotapu Ignimbrite was considered to come from Kapenga,

however Wilson et al (1984) postulate that some of the units mapped within the

lVIatahana Basin by Murphy (1977) may also have had a Kapenga source.

Subsequently Karhunen (1993) proposed that Pokai Ignimbrite was sourced from

Kapenga, although Wood (1992) has suggested that this ignimbrite may be derived

from Rotorua Caldera. Houghton et al (1995) consider that 7 units (Table 2.2) have

originated from the centre during two periods of activity (Figure 7.3).

Wilson et al (1995) consider that Kapenga is a composite structure consisting of 4

temporally discrete, but spatially coincident vents. Three periods of activity are

recognised, two of which involve caldera-forming activity. The first period began

around 0.89 Ma and culminated in the eruption of Waiotapu Ignimbrite at 0.71 Ma. The

second period occurred between 0.32 0.22 Ma although it is poorly defined as no

ages have been obtained for Pokai and Chimp Ignimbrites. The last phase of activity in

the centre did not result in any further caldera formation but involved rhyolite dome

building activity and the 65 ka eruption of Earthquake Flat Breccia (Wilson et ai, 1995).

Wood (1995) has proposed that an episode of collapse within Kapenga may have

100

A

,...

D ilIA

/ /

b

/ /

/

Q

\17' I B

/ / ITA

/ /

/

/// Q /

/ I> /

/

/

C lIB

F me

~ / ~ /

~ /

/ /

/

/// Q

/ V /

~ /

Figure 7.3 Maps showing the spatial distribution of caldera forming activity in TVZ through time. Roman numerals refer to periods of volcanic activity as defined by Houghton et al (1995). Kapenga volcanic centre was active during period liB (0.77-0.68 Ma) and period IIIB (0.28-c.0.15 Ma). From Houghton et al (1995).

coincided with collapse of Rotorua Caldera during the eruption of the 0.22 Ma

(Houghton et ai, 1995) Mamaku Ignimbrite. The considerable downfaulting (c.300m)

and overthickening of Mamaku Ignimbrite as it crosses the north-west margin of

Kapenga volcanic centre suggests synchronous collapse as chambers beneath both

Rotorua and Kapenga centres were eviscerated and the material was erupted from the

south-west of Rotorua Caldera (Wood, 1995).

7.2.2 The source ofWaiotapu Ignimbrite?

Various means of constraining the source areas of pyroclastic flow deposits have been

proposed, these include: distribution of lithic fragments through a deposit (Walker,

1985); lateral changes in pumice size with distance from source (e.g. Streck and

Grunder, 1995); increased thickness and number of flow units with increase proximity

to source (e.g. Karhunen, 1993). The degree of internal uniformity within Waiotapu

Ignimbrite and paucity of lithic fragments means that none of the methods mentioned

are available to help define the source.

101

N Waialapu Geolhermal field AIea

l

10 15

Kilomelres

SCALE 1: 500 000

176'OO'fb'lg

Figure 7.4 Generalised isopach map of Waiotapu Ignimbrite based on outcrop and drillhole data. Data is too sparse to allow decisive conclusions however the western isopachs suggest the deposit is thickening towards the southern end of Kapenga volcanic centre. .

Previous workers (e.g. Wilson et ai, 1984) have cited the general distribution of

Waiotapu Ignimbrite as suggesting it originated from Kapenga volcanic centre.

Certainly the unit does appear to thicken around the Kapenga structure and an isopach

map (Figure 7.4) of the limited thickness data available for Waiotapu Ignimbrite

suggests that the thickness of the ignimbrite decreases away from the vicinity of

Ngakuru. This locality also coincides with a large, negative residual gravity anomaly

(Bibby et ai, 1995).

Thickness data suggests that Waiotapu Ignimbrite was derived from somewhere in

the vicinity of the Waiotapu region. Drillcores from the geothermal field however do not

yield any evidence of proximal facies of Waiotapu Ignimbrite, and indeed Waiotapu

Ignimbrite thickness in the region appears to be topographically controlled; it appears

to be ponding in a large depression beneath the modern Reporoa Caldera.

Without exposure of any recognised proximal facies the location of the source of

Waiotapu Ignimbrite must be open to debate. No features of the deposit (e.g. density

of welding) show any systematic variation that could possibly constrain a source. One

lithic, in the base of the north of Ngapouri Ridge, measure 6cm, suggests the locality

102

was relatively proximal, but by itself this measurement is meaningless.

Kapenga volcanic centre lies within the mapped distribution of Waiotapu Ignimbrite

and is considered to have been active around the time of the eruption of Waiotapu

Ignimbrite (Houghton et ai, 1995) and it is most likely that Waiotapu Ignimbrite came

from within the centre. Subsequent faulting and post 330ka volcanic activity may well

have resulted in the burial of most of the proximal facies of Waiotapu Ignimbrite. It is

almost certain that no trace of the caldera (if one was formed) associated with the

eruption of Waiotapu Ignimbrite is preserved. Kapenga volcanic centre comprises

several overlapping structures, and it is conceivable that subsequent caldera forming

activity has disrupted, and probably destroyed, the Waiotapu source. If this is the case

then presumably large quantities of intra-caldera Waiotapu Ignimbrite should be

present in later ignimbrites such as Pokai and Okakuri.

7.3 ERUPTION AND DEPOSITION OF WAIOTAPU IGNIMBRITE

7.3.1 Eruption

Few ignimbrites have features similar to Waiotapu Ignimbrite, suggesting that the unit

had an unusual mechanism of formation. The Cerro Galan Ignimbrite (Francis et ai,

1983), north-west Argentina, however has many key features that are similar. The unit

is a voluminous (c. 1000 km3) crystal rich, pumice poor ignimbrite, erupted violently

over a wide area reaching distances of up to 100km from the Cerro Galan caldera rim

(Sparks et ai, 1985).

Like Waiotapu Ignimbrite, Cerro Galan Ignimbrite is homogenous, having been

erupted energetically, depositing a single massive flow unit, from a magma chamber in

which zonation in weak or absent. The ignimbrite has no underlying plinian deposit and

contains very few lithics «<0.5%). Sparks et al (1985) have interpreted these features

to indicate that caldera collapse was initiated during the eruption. The shape and width

of the caldera was strongly influenced by regional fault patterns.

Sparks et al (1985) proposed that the ignimbrite was erupted as a consequence of

catastrophic foundering of a cauldron block into the underlying magma chamber along

outward dipping ring fractures. Subsidence resulted in magma being forced up the

bounding fractures at very high discharge rates so a convecting column never

develops. Lithic fragments are rare as the outward dipping ring fractures automatically

widen with subsidence and large scale erosion of the conduit wall (e.g. Wilson et ai,

1980) is unnecessary. Subsequent explosions are confined to the footwall of the

subsiding block. With considerable subsidence only finer or more buoyant material will

103

escape over the vent rim, consequently the lithics that are entrained are most likely

confined to intra~caldera material.

Fruendt and Schminke (1995) have reported a similar situation involving the Pi

basaltic welded ignimbrite from Gran Canaria. Here a low basaltic ash fountain was

erupted at high rates for a basaltic magma generating a turbulent, density-stratified,

hot ash flow. They considered the discharge rate was due, in part, to the subsidence of

the chamber roof into the Pi reservoir.

It is proposed that Waiotapu Ignimbrite was erupted under similar circumstances.

The likely source of Waiotapu Ignimbrite, Kapenga volcanic centre, lies within the

Taupo Fault Belt which is though to have begun spreading c.0.9 Ma (Wilson et ai,

1995). The Waiotapu magma chamber was consequently lying below an actively

extending region which would have been unstable due to the development of a series

of horsts and grabens (Figure 7.5a). Once the lithostatic pressure of this faulted

overlying material exceeded the magmatic pressure within the chamber subsidence of

one or more overlying blocks occurred, triggering the eruption (Figure 7.5b). The

subsiding block may well have been part of a horst structure and the outward dipping

ring fractures will have continuously widened as subsidence progressed facilitating the

rapid evisceration. of the chamber. The resulting deposit will have been emplaced

energetically from the passage of one sustained pyroclastic flow.

The trend of regional faulting is likely to been a major control on caldera formation.

Accordingly the eruption is likely to have taken place along a series of near linear

fissures aligned parallel to the 040° regional trend (Keall, 1988). This would explain the

elongate distribution of Waiotapu Ignimbrite as the resulting pyroclastic flow would

have flowed roughly north-west and south-east of the caldera with little material flowing

north-east, toward Okataina, or south-west, toward Taupo.

7.3.2 Transport and deposition

The exact nature of the pyroclastic flow from which Waiotapu Ignimbrite was deposited

is equivocal, however welding variation and post-depositional recrystallisation does

offer some clues. The density of welding at or near the base is unusual in pyroclastic

deposits that are not high grade. Classic ignimbrite welding zonation (e.g. Smith and

Bailey, 1966) suggests that the base of the deposit should be less dense as particulate

material chills against the substrate. Recrystallisation textures in Waiotapu Ignimbrite

suggest that the unit retained heat for a considerable period after deposition. The

circular habit of spherulites in both pumices and groundmass suggests that

recrystallisation was occurring at below 400°C. Experimental work by Lofgren (1971)

104

a) Immediately prior to onset of eruption'

\ ~/ 7

\ /

b) Simultaneous subsidence of caldera block and eruption

~~ ~ . . r- - . \.--~

./:.'" . /'"\.~

\ /

/'-r-. (": . . . . ________ " r-y .. .( .[::;r . ~C' ~ 0:£' "\~ ~', ',~ c- ~ .

. ' . .'.

'~-""----....j,~,

• I "

Figure 7.5 Generalised schematic interpretation of the eruption of Waiotapu Ignimbrite. a) The Waiotapu magma reservoir is intruded beneath the extending Taupo fault belt (arrows indicate direction of extension). The region is unstable due to faulting and minimal difference between the lithostatic pressure (Lp) of the overlying material and the magmatic pressure (Mp) within the chamber. b) The onset of the eruption and synchronous caldera collapse is generated by the catastrophic subsidence of the caldera block as Lp exceeds Mp. The automatic widening' of outwardly dipping fractures facilitates the rapid discharge of material, consequently a convecting column never develops. Erosion of conduit walls is minimal therefore few lithic fragments are produced, those that are entrained being most likely to be confined to intra-caldera deposits.

105

showed that for granophyric texture (referred to as felsitic texture in this study) to

develop in a rhyolite glass required prolonged periods at elevated temperatures. The

base of the ignimbrite did cool at a greater rate than the overlying deposit as

devitrification is less advanced than in overlying material and is limited to the

development of spherulites in fiamme and weak axiolitic texture in glass shards.

The Waiotapu pyroclastic flow is remarkable as it has managed to maintain high

temperatures at considerable distances from source (e.g. it is still densely welded at

Wawa Quarry), while travelling energetically and interacting with various topographic

obstacles. Walker (1983) stated that the grade of an ignimbrite, and consequently the

degree of welding, reflects the initial magmatic temperature prior to eruption. In the

case of Waiotapu Ignimbrite, Fe-Ti oxide thermometry suggests a magmatic

temperature. of 750°C, although it is possible that the temperatures were higher

(rhyolitic eruption temperatures range from 700-900°C; Cas and Wright, 1988). If an

ignimbrite is not accompanied by a preceding air-fall deposit then it is less likely to be

cooled (Sparks et ai, 1976), as the intake of air can cool an eruption column by as

much as 300°C (Sparks et ai, 1978). Flows that result from "boil over" eruptions are

typically less expanded, which would enable them to retain heat more efficiently during

transport. Heat loss can be minimal in a flow that is several times denser than the

surrounding atmosphere, thus reducing the intake of ambient air (Freundt and

Schminke, 1995). If a hot flow is too dense it may collapse due to coalescence of hot

particles, however it has also been proven that very hot particulate flows can travel as

expanded flows (Freundt, 1995).

With increasing stratigraphic height (e.g. at Wawa Quarry) shards become less

deformed and the intensity of welding decreases. This is probably a result of cooling of

the pyroclastic flow as it becomes less dense due to the removal and deposition of

material from the flow. Gas segregation structures found in float blocks at Wawa

Quarry suggest that the flow was more expanded at this locality.

Density profiles in Waiotapu Ignimbrite suggest that welding may have been largely

a syn-depositional process resulting from deposition by progressive aggradation (see

section 7.2). Material is welding as it is being deposited from a hot overriding

pyroclastic flow. What is unusual about Waiotapu Ignimbrite is that while it was

emplaced from a hot pyroclastic flow and it retained heat for a considerable period of

time after deposition there is little evidence for secondary flowage. In high grade

ignimbrites flowage of hot fragments after deposition can impart a strong layering, or

foliation on the final deposit (e.g. Schminke and Swanson, 1967; Chapin and Lowell,

1979). This layering has also been attributed to syn-depositional laminar shear of

106

agglutinating particles (Branney and Kokelaar, 1992). The lack of such features in

Waiotapu Ignimbrite suggests that particles were too cool and viscous to deform, and

that the shear strength of the deposit was too great, therefore inhibiting flowage.

107

CHAPTER

N SION

1) Waiotapu Ignimbrite is a lithologically homogeneous, low aspect ratio ignimbrite

(aspect ratio c. 1:1200) with a estimated minimum volume of 175 km3. The deposit

appears to be composed of a single flow unit, is moderately pumice rich and

contains very few lithics «<1%),

2) Waiotapu Ignimbrite appears to be unrelated to underlying deposits. In eastern TVZ

the unit has flowed around the eroded remains of Ngapouri Rhyolite. The rhyolite

was originally thought to be filling the Waiotapu Ignimbrite vent, but field relations,

and geochemistry have shown it to be considerably older and unrelated. In western

TVZ Rahopaka Ignimbrite lies directly beneath Waiotapu Ignimbrite, this ignimbrite

is also geochemically and mineralogically distinct and is c. 60ka older.

3) The source of Waiotapu Ignimbrite is poorly constrained, although the distribution of

the deposit suggests that the flow originated from within Kapenga volcanic centre, in

central TVZ.

4) No underlying plinian deposits are recognised, possibly due to a lack of exposure.

Non-welded deposits under the ignimbrite at Ngapouri Ridge and Tikorangi

Escarpment have distinct mineral chemistries.

5) Waiotapu Ignimbrite shows no significant vertical or lateral variation in mineral

content or whole rock geochemistry.

6) Pumice geochemistry has revealed that Waiotapu Ignimbrite involved the eruption

of two chemically distinct magma batches, with the volumetrically dominant type-A

magma having significantly lower rubidium concentration than the rare type-B

magma. The difference cannot be explained by fractionation and the magmas may

have occupied separate chambers which were eviscerated simultaneously during

eruption. Alternatively the type-A chamber may have been intruded by the type-B

body, the magmas subsequently mingling either prior to or during the eruption. The

type-A magma chamber may have been very weakly zoned due to fractionation of

plagioclase and Fe-Ti oxides.

108

7) Waiotapu Ignimbrite was erupted in one sustained eruption and the subsequent

pyroclastic flow was both high temperature and energetic. Even in its most distal

deposits the ignimbrite shows evidence of climbing topographic obstacles with

slopes on the order of 30°.

8) The apparent lack of a preceding plinian eruption, paucity of lithic fragments,

energetic nature, and evidence that it was erupted in one event suggests an

unusual eruption style. The eruption probably resulted from the catastrophic

collapse of the caldera roof into the underlying chamber, the caldera block being

bounded by outwardly dipping ring faults. Rapid widening of the vent and

subsidence of the caldera block facilitated high discharge rates and allowed for

minimal erosion of lithic fragments from conduit walls. Conditions suitable for the

development of a convecting plinian column were never attained.

9) It is likely Waiotapu Ignimbrite was not erupted from a caldera sensu stricto but

instead was erupted from a series of near linear fissure vents which developed

parallel to the regional fault pattern (c.0400) as the cauldron block subsided. As a

result Waiotapu Ignimbrite has an elongate distribution as most material flowed

WNW or ESE.

10) Waiotapu Ignimbrite is unusual as it is densely welded to the base and, in more

proximal exposures, welded throughout. The pyroclastic flow from which the

ignimbrite was deposited was able to retain heat to considerable distances from the

vent.

109

"There is absolutely no substitute for a genuine lack of preparation. " -Anon

Many people have contributed to the production of this wee tome, most of whom I have hopefully listed below. A large number have lent a considerable amount of support (often unwittingly) during the trials and tribulations (both in and out of University) encountered over the last couple of years. To everyone who kept me happy and sane (O.K. so that is a moot point) during the recent adventure: CHEERS.

First and foremost, I thank my supervisor Professor Jim Cole who suggested and organised this project and ensured that, despite my best efforts, this thesis was actually finished on time.

Dr Peter Wood at IGNS, Wairakei is to be thanked for his assistance both while I was lurching around the TVZ and back home in Christchurch (how did anyone survive without e-mail?).

To Steve Beresford, my unofficial supervisor, go big thanks. His enthusiasm, advice, help in the field and willingness to discuss the TVZ and volcanology plus cricket, the battle of the sexes and whose round it was were invaluable and not at all irrelevant.

Other members of staff contributed with useful discussions: Associate Professor Steve Weaver for geochemical problems, Associate Professor David Shelley on petrographic problems and Dr Rod Burt about geochemistry, depositional mechanisms ("Well it was sort of just put there.") and whether or not Judge Dredd was a good film ("Unparalleled special effects"?). Dave Bell and Dr John Bradshaw are thanked for hassling me endlessly without provocation, no you were really funny, honest.

The funding of the Mason Trust Fund is gratefully acknowledged, as is the financial assistance of Jan and Fred Swallow (who also provided good company while I was oop north). Without either this project would have never happened.

Various people helped while I was in the field. The Maxwells, Schiltzs, Renshaws, Daleys, Campbells, and Armours all let me wonder around their land so are consequently really cool people. Carter Holt Harvey Ltd provided invaluable access to Kinleith Forest. Steve, Rod, Dave Dysart, Angela Helbling and Sarah Gauden-Eng helped out at various times in the field, often during some unreasonably hot days (Dave and Jim especially responding above and beyond the call of duty during the scorcher on the Ngapouri Ridge). Steve Beresford has to be the worlds sexiest scale, although a half naked Rod Burt comes a close second (although I'm not really sure to what). Woo and Yvonne Koo, and Mr and Mrs Rout are thanked profusely for their hospitality while I was working around Tokoroa.

The efforts of the department technicians were greatly appreciated, Steven Brown for letting me play in the Geochem Lab, Cathy Knight for letting me into the Engineering Lab (and then out again), and for making sure that I was not only well equipped in the field, but that I looked good too, Rob Spiers for making my thin sections and pointing out that I had really ugly rocks (I hadn't noticed). Also Michelle Wright, Kerry Swanson,

110

Arthur Nicholas, Mike Finnemore and Craig Jones all contributed in ways to numerous to mention, so I wont.

To my classmates (in order of decreasing height) Jonny (yet another whose contribution cannot be underestimated), Rachael (on a box), Nick, Phil, Matt, Chris, Anna, Carl and Sarah, thanks for keeping quiet and being a sane studious bunch of people over the last few months ("Howls of derisive laughter, Bruce."). It has been nothing if not entertaining.

My family (who just happen to be the greatest - and that doesn't do them justice) for making the past years easy and putting up with my absence and bad moods are also thanked (incidentally my thesaurus is not thanked for failing to provided another suitable work for thanked). Without you all I wouldn't have made it this far, especially when you consider that I wouldn't have actually been born.

Richard Bentley helped out heaps with "my bloody computer" I solving various technical crises and cured the odd virus.

Also briefly thanks to: The New Zealand cricket team for playing badly at just the right time. The programmers of Doom II, Karts, T etris and Solitaire. The staff at the James Height and Engineering Cafes. Some guy I met in the supermarket the other day. Jen and Jane for pointing out spelling mistakes in these acnolegments. Scully and Mulder for providing an excuse to go home each Wednesday night.

111

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119

ApPENDIX E

SAMP DEX

Field No. Thin lab Grid location Description Secn No. Reference

AW011 T16/588400 Lichfield Quarry Ongatiti Ignimbrite AW012 Yes T16/588400 Lichfield Quarry Ahuroa Ignimbrite AW013 Yes T16/588400 Lichfield Quarry Ahuroa Ignimbrite AW014 Yes T16/588400 Lichfield Quarry Ahuroa Ignimbrite AW015 Yes 27270 T16/588400 Lichfield Quarry Ahuroa Ignimbrite AW016 Yes T16/588400 Lichfield Quarry Ahuroa Ignimbrite AW017 Yes T16/588400 Lichfield Quarry Ahuroa Ignimbrite AW018 27366 U16/004116 Ngapouri Ridge Unit X + ignimbrite clast AW019 U16/004116 Ngapouri Ridge Unit X + clasts AW020 U16/004116 Ngapouri Ridge Unit X + clasts AW021 U16/009116 Ngapouri Ridge Unkown ignimbrite AW022 Yes 27271 U16/005114 Ngapouri· Ridge Ngapouri Rhyolite AW023 Yes 27272 U16/005114 Ngapouri Ridge Ngapouri Rhyolite AW024 Yes U16/012114 Ngapouri Ridge Ngapouri Rhyolite AW025 T16/578375 Ngutuwera Stream Unknown Ignimbrite AW026 27367 U16/771211 Bison Rd Quarry Waiotapu Ignimbrite AW027 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW028 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW028 U16/771215 Rawhiti Rd Quarry Unkown pumice AW030 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW031 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW032 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW033 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW034 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW035 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW036 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW037 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW038 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW039 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW040 Yes U161792207 Pukerimu Rd Waiotapu Ignimbrite AW041 T16/627174 Wawa Quarry Waiotapu Ignimbrite AW042 T16/627174 Wawa Quarry Waiotapu Ignimbrite AW043 Yes T16/627174 Wawa Quarry Waiotapu Ignimbrite AW044 T16/627174 Wawa Quarry Unknown Ignimbrite AW045 T16/627174 Wawa Quarry Waiotapu Ignimbrite AW046.1 Yes U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite lithics AW046.2 Yes U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite lithics AW047 27368 U161771215 Rawhiti Rd Quarry Waiotapu Ignimbrite pumice AW048 Yes 27273 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW049 Yes U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW050 Yes 27274 U161771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW051 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW051 a 27369 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite pumice

120


AW051b 27370 U16n71215 Rawhiti Rd Quarry Waiotapu Ignimbrite pumice AW052 27371 U16n71215 Rawhiti Rd Quarry Waiotapu Ignimbrite pumice AW053 U16/771215 Rawhiti Rd Quarry Waiotapu Ignimbrite AW054 Yes 27275 U16/771211 Bison Rd Quarry Waiotapu Ignimbrite AW054 27372 U16/771211 Bison Rd Quarry Waiotapu Ignimbrite pumice AW055 Yes 27276 U16/771211 Bison Rd Quarry Waiotapu Ignimbrite AW056 Yes U16/771211 Bison Rd Quarry Waiotapu Ignimbrite AW057 Yes 27277 U16n71211 Bison Rd Quarry Waiotapu Ignimbrite AW057 27373 U16/771211 Bison Rd Quarry Waiotapu Ignimbrite pumice AW058 Yes U16/771211 Bison Rd Quarry Waiotapu Ignimbrite AW059 Yes 27278 U16/771211 Bison Rd Quarry Waiotapu Ignimbrite AW060 U16n71211 Bison Rd Quarry Lithic fragment in Waiotapu

Ignimbrite AW061 a Yes 27279 U17/062999 Butchers Boundary Waiotapu Ignimbrite lithic in

Rd Kaingaroa Ignimbrite AW061b T16/588400 Lichfield Quarry Ahuroa Ignimbrite AW062 T16/588400 Lichfield Quarry Ahuroa Ignimbrite AW063 T16/588400 Lichfield Quarry Waiotapu Ignimbrite AW064 T16/588400 Lichfield Quarry Waiotapu Ignimbrite AW065 T16/588400 Lichfield Quarry Waiotapu Ignimbrite AW066 Yes 27280 T16/588400 Lichfield Quarry Waiotapu Ignimbrite AW067 Yes T16/588400 Lichfield Quarry Waiotapu Ignimbrite AW068 Yes 27281 T16/588400 Lichfield Quarry Waiotapu Ignimbrite AW070 Yes ·27282 T16/588400 Lichfield Quarry Waiotapu Ignimbrite AW071 Yes T16/671188 SH1 Waiotapu Ignimbrite AW072 27283 T16/671188 SH1 Waiotapu Ignimbrite AW073 Yes T16/671188 SH1 Waiotapu Ignimbrite AW074 U16/745196 Rauna Rd Waiotapu Ignimbrite AW075 U16/745196 Rauna Rd Waiotapu Ignimbrite AW076 U16/743195 Rauna Rd Waiotapu Ignimbrite AW077 U16/743195 Rauna Rd Waiotapu Ignimbrite AW078 U16/743195 Rauna Rd Waiotapu Ignimbrite AW079 U16/745193 Rauna Rd Waiotapu Ignimbrite AW080 U16n49195 Rauna Rd Waiotapu Ignimbrite AW081 U16n49198 Rauna Rd Waiotapu Ignimbrite AW082 U16/749199 Rauna Rd Waiotapu Ignimbrite AW083 U16n49199 Rauna Rd Waiotapu Ignimbrite AW084 U16n49199 Rauna Rd Waiotapu Ignimbrite AW085a 27374 U16/771215 Rawhiti Rd Quarry Waiotapu Ig - samples for

pumices AW085b 27375 U16n71215 Rawhiti Rd Quarry Waiotapu Ig - samples for

pumices AW085c 27376 U16/771215 Rawhiti Rd Quarry Waiotapu Ig samples for

pumices AW086 U16/735192 Kudu Road Pokai Ignimbrite AW087 U16n50204 Beale Rd Waiotapu Ignimbrite AW088 27284 U16/750204 Beale Rd Waiotapu Ignimbrite AW089 U16/750204 Beale Rd Waiotapu Ignimbrite AW090 U16n51204 Beale Rd Waiotapu Ignimbrite AW091 U16/751204 Beale Rd Waiotapu Ignimbrite

121


AW092 U16n51204 Beale Rd Waiotapu Ignimbrite AW093 U16/751204 Beale Rd Waiotapu Ignimbrite AW094 U16/749202 Beale Rd Waiotapu Ignimbrite AW095 U16/751201 Mandril Rd Waiotapu Ignimbrite AW096 U16n55195 Jackal Rd Waiotapu Ignimbrite AW097 U16/755195 Jackal Rd Waiotapu Ignimbrite AW098 U16/755196 JackalRd Waiotapu Ignimbrite AW099 U16n57198 Jackal Rd Waiotapu Ignimbrite AW100 T16/627174 Wawa Quarry Ahuroa Ignimbrite (?) AW101 T16/627174 Wawa Quarry Ahuroa Ignimbrite (?) AW102 T16/627174 Wawa Quarry Ahuroa Ignimbrite (?) AW103 T16/627174 Wawa Quarry Ahuroa Ignimbrite (?) AW104 Yes 27285 T16/627174 Wawa Quarry Waiotapu Ignimbrite AW105 Yes T16/627174 Wawa Quarry Waiotapu Ignimbrite AW106 Yes 27386 T16/627174 Wawa Quarry Waiotapu Ignimbrite AW107 Yes 27387 T16/627174 Wawa Quarry Waiotapu Ignimbrite AW108 Yes 27388 T16/627174 Wawa Quarry Waiotapu Ignimbrite AW109 Yes 27286 T16/627174 Wawa Quarry Waiotapu Ignimbrite AW110 Yes 27389 T16/627174 Wawa Quarry Waiotapu Ignimbrite AW111 Yes 27390 T16/627174 Wawa Quarry Waiotapu Ignimbrite AW112 Yes 27287 T16/627174 Wawa Quarry Waiotapu Ignimbrite AW113 Yes T16/627174 Wawa Quarry Waiotapu Ignimbrite AW114 Yes T16/627174 Wawa Quarry Waiotapu Ignimbrite AW115 Yes 27288 T16/627174 Wawa Quarry Waiotapu Ignimbrite AW116 Yes 27377 T16/627174 Wawa Quarry Waiotapu Ignimbrite pumice AW117 27378 T16/627174 Wawa Quarry Waiotapu Ignimbrite pumice AW118 Yes U16/756185 Tikorangi Esc. Rahopaka Ignimbrite AW118.1 Yes U16/756185 Tikorangi Esc. Rahopaka Ignimbrite - lithic AW118.2 Yes U16/756185 Tikorangi Esc. Rahopaka Ignimbrite lithic AW118.3 Yes U16n56185 Tikorangi Esc. Rahopaka Ignimbrite - lithic AW118.4 Yes U16n56185 Tikorangi Esc. Rahopaka Ignimbrite - lithic AW119 Yes U16n56185 Tikorangi Esc. Rahopaka Ignimbrite AW120 Yes U16n56185 Tikorangi Esc. Rahopaka Ignimbrite AW121 Yes U16n56185 Tikorangi Esc. Rahopaka Ignimbrite AW122 Yes U16/756185 Tikorangi Esc. Rahopaka Ignimbrite AW123 Yes U16n56185 Tikorangi Esc. Rahopaka Ignimbrite AW124 Yes U16n56185 Tikorangi Esc. Rahopaka Ignimbrite AW125 Yes U16/756185 Tikorangi Esc. Rahopaka Ignimbrite AW126 Yes U16/791206 Pukerimu Rd Rahopaka Ignimbrite AW127 U16/984119 Paeroa Scarp Paeroa Ignimbrite AW128 Yes U16/977118 Paeroa Scarp Waiotapu Ignimbrite AW129 Yes U16/975119 Paeroa Scarp Paeroa Ignimbrite AW130 U16/975119 Paeroa Scarp Paeroa Ignimbrite AW131 Yes U16/977122 Paeroa Scarp Waiotapu Ignimbrite AW132 U16/977122 Paeroa Scarp Waiotapu Ignimbrite AW133 U16/977122 Paeroa Scarp Waiotapu Ignimbrite AW134 Yes U16/978117 Paeroa Scarp Waiotapu Ignimbrite and

Ngapouri Rhyolite AW135 U17/062999 Butchers Boundary Waiotapu Ignimbrite lithic in

Rd Kaingoroa ignimbrite

122

field No. Thin Grid location Description Secn No. Reference

AW136 U17/062999 Butchers Boundary Waiotapu Ignimbrite lithic in Rd Kaingoroa Ignimbrite

AW137 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW138 Yes 27391 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW139 Yes 27392 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW140 27393 U 17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW141 Yes 27394 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW142 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW143 Yes U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW144 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW145 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW146 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW147 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW148 Yes 27395 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW149 Yes U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW150 Yes U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW151 27396 U171015098 Ngapouri Ridge Waiotapu Ignimbrite AW152 Yes U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW153 Yes 27397 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite AW154 T16/627174 Wawa Quarry Sub-Waiotapu Ignimbrite

deposit AW155 T16/627174 Wawa Quarry Sub-Waiotapu Ignimbrite




deposit AW159.1 T16/627174 Wawa Quarry Waiotapu Ignimbrite basal

section AW159.2 T16/627174 Wawa Quarry Waiotapu Ignimbrite basal

section AW159.3 T16/627174 Wawa Quarry Waiotapu Ignimbrite basal

section AW160 T16/588400 Lichfield Quarry Sub-Waiotapu Ignimbrite

deposit AW161 T16/588400 Lichfield Quarry Sub-Waiotapu Ignimbrite

deposit AW162 T16/588400 Lichfield Quarry Sub-Waiotapu Ignimbrite

deposit AW163 Yes T16/588400 Lichfield Quarry Sub-Waiotapu Ignimbrite

deposit AW164 U16/083131 Kaingaroa Forest Waiotapu Ignimbrite Block AW165 Yes T16/588400 Lichfield Quarry Waiotapu Ignimbrite (?) AW166 27379 T16/627174 Wawa Quarry Waiotapu Ignimbrite Pumice AW167 27380 T16/627174 Wawa Quarry Waiotapu Ignimbrite Pumice AW168.1 Yes T16/627174 Wawa Quarry Basal Waiotapu Ig AW168.2 Yes T16/627174 Wawa Quarry Basal Waiotapu Ig AW168.3 Yes T16/627174 Wawa Quarry Basal Waiotapu Ig AW169.1 Yes T16/627174 Wawa Quarry Basal Waiotapu Ig

123

Field No. Thin Lab Grid Location Description Secn No. Reference

AW169.2 Yes T16/627174 Wawa Quarry Basal Waiotapu Ig AW169.3 Yes T16/627174 Wawa Quarry Basal Waiotapu Ig AW170 27381 T16/627174 Wawa Quarry Waiotapu Ignimbrite Pumice AW171 Yes T16/627174 Wawa Quarry Waiotapu Ignimbrite gas

segregation structure in float

AW172 U16/771207 Bison Road Sub-Waiotapu unwelded deposit

AW173 U16/771207 Bison Road Sub-Waiotapu unwelded deposit

AW174 U16/811186 Rusa Road Pukerimu Formation + lithics

AW175 Yes 27398 U16/789205 Pukerimu Road Rahopaka Ignimbrite AW176 Yes U16/789205 Pukerimu Road Rahopaka Ignimbrite AW177 Yes 27399 U16/789205 Pukerimu Road Rahopaka Ignimbrite AW178 Yes U16/789205 Pukerimu Road Rahopaka Ignimbrite AW179 Yes U16/789205 Pukerimu Road Rahopaka Ignimbrite AW180 Yes 27400 U16n89205 Pukerimu Road Rahopaka Ignimbrite AW181 Yes U16/980106 Ngapouri Road Waiotapu Ignimbrite AW182 Yes U16/987102 Ngapouri Road Waiotapu Ignimbrite AW183 Yes U16/986103 Ngapouri Road Waiotapu Ignimbrite AW184 Yes U16/983112 Ngapouri Road Waiotapu Ignimbrite AW185 U16/013117 Ngapouri Ridge Waiotapu Ignimbrite AW186 U16/011115 Ngapouri Ridge Ngapouri Rhyolite AW187 U16/013106 Ngapouri Ridge Ngapouri Rhyolite AW188 U16/014106 Ngapouri Ridge Ngapouri Rhyolite AW189 27382 U16/014117 Ngapouri Ridge Waiotapu Pumice AW190 U16/012119 Ngapouri Ridge Waiotapu Ignimbrite AW191 U16/015122 Ngapouri Ridge HT eruption breccia (?) AW192 U16/014122 Ngapouri Ridge Waiotapu Pumice AW193 U16/014122 Ngapouri Ridge Waiotapu Ignimbrite AW194 U16/019124 Ngapouri Ridge Waiotapu Pumice AW195 U16/019124 Ngapouri Ridge Waiotapu Ignimbrite AW196 Yes U16/023127 Ngapouri Ridge Waiotapu Ignimbrite AW197 Yes U16/023127 Ngapouri Ridge Waiotapu Ignimbrite AW198 U16/992123 Paeroa Scarp Paeroa Ignimbrite AW199 Yes U16/982125 Paeroa Scarp Waiotapu blocks AW200 Yes U16/005116 Ngapouri Ridge Rahopaka Ignimbrite (?) AW201 Yes U16/005116 Ngapouri Ridge Rahopaka Ignimbrite (?) AW202 Yes U16/005116 Ngapouri Ridge Rahopaka Ignimbrite (?) AW203 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite pumice AW204 Yes 27383 U 17/015098 Ngapouri Ridge Waiotapu Ignimbrite pumice AW205 27384 U17/015098 Ngapouri Ridge Waiotapu Ignimbrite pumice AW206 27385 U17J015098 Ngapouri Ridge Waiotapu Ignimbrite pumice

124

AW15 27270 Whole Rock

Si02 74.86 Ti02 0.29 AI20 s 13.80

Fe20S 2.28 MnO 0.03 MgO 0.09 CaO 1.22 Na20 3.82 K20 3.48 P20 S 0.02 LOI 0.59 Total 100.49

V 15 Cr <3 Ni <3 Pb 18 Zn 49 Rb 111 Sa 774 Sr 101 Ga 15 Nb 8 Zr 243 Y 22 Th 13 La 28 Ce 48 Nd 23

A.S.I. 1.62 Rb/Sr 1.10 KlRb 260

Q 35.68 C 1.51 Z 0.05 Or 20.67 Ab 32.41 An 6.19 Oi 0.00 Hy 2.27 Mt 0.77

" 0.55 Ap 0.05 Total 100.14

ApPENDIX Two

HEMICAL ANAL VS

AW22 AW23 AW48 27271 27272 27273 Whole Rock Whole Rock Whole Rock

75.22 74.83 72.24 0.15 0.19 0.29 13.52 13.71 14.11 1.72 1.55 2.46 0.01 0.01 0.05 0.13 0.11 0.12 1.15 1.24 1.24 3.15 3.32 4.02 4.12 4.02 3.20 0.04 0.02 0.04 1.23 0.65 2.37 100.44 99.67 100.13

12 13 15 <3 <3 <3 <3 <3 <3 15 14 15 29 37 38 139 142 113 834 867 768 90 101 110 15 14 15 7 7 8 149 149 256 23 22 40 12 13 13 25 27 39 50 53 75 22 22 41

1.61 1.60 1.67 1.54 1.41 1.03 246 235 235

38.07 37.02 33.54 1.82 1.63 1.84 0.03 0.03 0.05 24.63 24.08 19.43 26.90 28.41 34.86 5.76 6.37 6.29 0.00 0.00 0.00 1.97 1.67 2.63 0.58 0.52 0.84 0.29 0.36 0.56 0.10 0.05 0.10 100.14 100.15 100.15

AW50 27274 Whole Rock

72.71 0.29 13.93 2.32 0.03 0.06 1.15 3.96 3.26 0.01 1.29 99.00

14 <3 <3 15 39 116 744 103 15 8 252 35 12 45 50 36

1.66 1.13 233

34.45 1.79 0.05 19.80 34.35 6.03 0.00 2.29 0.80 0.56 0.02 100.15

125

Sample AW54 AW55 AW57 AW59 AW61a No. 27275 27276 27277 27278 27279

Type Whole Rock Whole Rock Whole Rock Whole Rock Whole Rock

Si02 70.51 71.62 72.55 72.11 74.18 Ti02 0.34 0.36 0.31 0.33 0.28 AI20 3 15.06 14.48 14.15 14.89 13.85 Fe203 2.90 2.72 2.50 2.63 2.39 MnO 0.05 0.08 0.05 0.08 0.06 MgO 0.13 0.10 0.07 0.05 0.09 CaO 1.22 1.45 1.31 1.09 1.52 Na20 3.41 4.31 4.32 3.82 4.14 K20 3.16 3.11 3.19 3.21 3.26 P20 5 0.02 0.03 0.02 0.03 0.04 LOI 1.80 0.62 0.48 1.57 0.66 Total 98.61 98.88 98.95 99.78 100.46

V 17 15 16 15 16 Cr <3 <3 <3 <3 <3 Ni <3 <3 <3 <3 <3 Pb 16 13 12 24 11 Zn 44 49 37 42 39 Rb 112 105 110 114 106 Ba 759 812 746 743 761 Sr 115 129 112 100 123 Ga 16 15 15 16 16 Nb 10 9 7 8 10 Zr 279 276 257 268 254 Y 26 33 17 18 32 Th 13 12 14 13 12 La 28 32 26 22 30 Ce 51 66 28 32 63 Nd 32 31 18 18 27

AS.I. 1.93 1.63 1.60 1.83 1.55 Rb/Sr 0.97 0.81 0.98 1.14 0.86 KlRb 234 246 241 234 255

Q 35.71 30.81 31.74 34.73 33.33 C 3.92 1.41 1.20 3.21 0.77 Z 0.06 0.06 0.05 0.05 0.05 Or 19.38 18.79 19.23 19.40 19.38 Ab 29.87 37.19 37.19 32.97 35.16 An 6.39 7.41 6.73 5.56 7.56 Oi 0.00 0.00 0.00 0.00 0.00 Hy 3.09 2.80 2.50 2.62 2.46 Mt 1.00 0.93 0.85 0.90 0.80 II 0.67 0.70 0.60 0.64 0.53 Ap 0.05 0.07 0.05 0.07 0.10 Total 100.15 100.16 100.15 100.15 100.15

126

Sample AW68 AW70 AW72 AW88 Lab No. 27280 27281 27282 27283 27284 Rock Whole Rock Whole Rock Whole Rock Whole Rock Whole Rock

Si02 72.84 73.27 71.32 73.87 72.66 Ti02 0.29 0.30 0.36 0.31 0.34 AI20 3 13.92 14.31 15.74 13.86 14.84 Fe203 2.29 2.49 2.76 2.42 2.69 MnO 0.03 0.04 0.05 0.04 0.05 MgO 0.05 0.06 0.20 0.23 0.11 CaO 1.05 1.05 0.87 1.26 1.06 Na20 2.93 2.99 2.32 4.04 3.53 K20 3.46 3.41 3.37 3.33 3.24 P20 S 0.02 0.02 0.03 0.03 0.03 LOI 2.39 1.97 3.29 0.56 1.91 Total 99.27 99.90 100.31 99.95 100.45

V 15 14 17 13 16 Cr 6 <3 <3 <3 <3 Ni <3 <3 <3 <3 <3 Pb 11 12 20 13 14 Zn 59 49 49 43 43 Rb 115 118 115 118 115 Sa 754 765 637 754 752 Sr 93 96 83 104 98 Ga 14 13 17 15 15 Nb 9 8 9 8 9 Zr 250 245 286 256 279 Y 23 29 ' 23 33 30 Th 13 12 13 13 14 La 22 32 18 32 29 Ce 39 44 23 61 40 Nd 18 34 16 39 32

A.S.I. 1.87 1.92 2.40 1.61 1.90 Rb/Sr 1.24 1.23 1.39 1.13 1.17 KfRb 250 240 243 234 234

Q 40.57 40.30 42.98 33.82 36.75 C 3.54 3.85 6.92 1.33 3.66 Z 0.05 0.05 0.06 0.05 0.06 Or 21.19 20.67 20.62 19.89 19.52 Ab 25.64 25.88 20.28 34.46 30.37 An 5.50 5.44 4.47 6.35 5.39 Di 0.00 0.00 0.00 0.00 0.00 Hy 2.25 2.47 3.08 2.77 2.76 Mt 0.79 0.85 0.95 0.82 0.92 II 0.57 0.58 0.71 0.59 0.66 Ap 0.05 0.05 0.08 0.07 0.07 Total 100.15 100.15 100.14 100.15 100.15

127

Sample AW104 AW106 AW107 AW108 AW109 lab No. 27285 27286 27287 27288 27367

Type Whole Rock Whole Rock Whole Rock Whole Rock Whole Rock

Si02 73.78 74.79 74.46 74.70 74.48 Ti02 0.23 0.27 0.26 0.29 0.27 AI20 3 13.62 13.51 13.29 13.54 13.36 Fe203 2.18 2.16 2.16 2.24 2.19 MnO 0.05 0.05 0.08 0.07 0.06 MgO 0.11 0.05 0.05 0.05 0.06 CaO 1.19 1.17 1.15 1.11 1.29 i'Ja20 4.02 4.37 4.21 4.05 4.35 K20 3.27 3.39 3.41 3.44 3.35 P20 5 0.04 0.03 0.02 0.03 0.04 LOI 1.00 -0.10 0.07 0.70 0.76 Total 99.50 99.68 99.17 100.20 100.21

V 10 13 12 12 13 Cr <3 <3 <3 <3 <3 i'Ji <3 <3 <3 <3 <3 Pb 15 14 12 11 12 Zn 46 44 40 43 44 Rb 94 110 118 119 115 Sa 826 799 826 836 809 Sr 97 96 100 98 109 Ga 14 16 15 16 15 Nb 10 9 8 9 8 Zr 239 238 229 240 241 Y 25 27 32 31 48 Th 12 12 13 13 10 La 42 29 31 30 33 Ce 61 58 63 60 64 Nd 40 32 36 34 34

A.S.1. 1.61 1.51 1.52 1.57 1.49 Rb/Sr 0.97 1.15 1.18 1.21 1.06 KlRb 289 256 240 240 242


128

Sample AW110 AW111 AW112 AW115 AW138 No. 27368 27369 27370 27371 27372

Rock Type Whole Rock Whole Rock Whole Rock Whole Rock Whole Rock


V 13 14 16 17 18 Cr <3 <3 <3 <3 <3 Ni <3 <3 <3 <3 <3 Pb 14 16 11 14 10 Zn 37 38 33 49 30 Rb 116 117 121 117 111 8a 799 774 756 736 716 Sr 102 97 98 96 116 Ga 15 14 15 16 14 Nb 9 8 9 9 9 Zr 239 256 257 258 253 Y 28 27 14 8 14 Th 13 13 13 9 10 La 28 30 31 16 17 Ce 53 49 51 22 34 Nd 36 32 37 <10 13

A.S.I. 1.61 1.67 1.67 1.84 1.56 Rb/Sr 1.14 1.21 1.23 1.22 0.96 KlRb 242 240 232 234 242

Q 34.48 34.62 35.26 36.45 34.07 C 1.44 1.97 1.93 3.29 0.86 Z 0.05 0.05 0.05 0.05 0.05 Or 20.23 20.19 20.24 19.98 19.46 Ab 34.70 34.21 33.02 30.88 35.95 An 5.66 5.30 5.46 5.64 6.78 Oi 0.00 0.00 0.00 0.00 0.00 Hy 2.24 2.37 2.64 2.44 1.65 Mt 0.75 0.81 0.91 0.83 0.63 II 0.52 0.56 0.56 0.52 0.62 Ap 0.10 0.07 0.07 0.05 0.07 Total 100.15 100.15 100.15 100.15 100.14

129

Sample AW139 AW140 AW141 AW148 AW151 Lab No. 27373 27374 27375 27376 27377 Rock Type Whole Rock Whole Rock Whole Rock Whole Rock Whole Rock

Si02 73.52 74.10 73.45 72.87 73.44 Ti02 0.35 0.28 0.30 0.31 0.29 AI20 3 13.87 13.40 13.87 15.33 13.50 Fe203 2.55 2.38 2.67 2.21 2.37 MnO 0.03 0.04 0.03 0.03 0.03 MgO 0.10 0.07 0.05 0.05 0.05 CaO 1.32 1.30 1.28 1.14 1.23 Na20 3.86 4.16 4.07 3.48 3.91 K20 3.27 3.30 3.23 3.24 3.42 P20S 0.02 0.04 0.03 0.04 0.06 LOI 1.12 0.43 0.91 1.76 1.60 Total 100.00 99.50 99.87 100.42 99.91

V 21 18 20 19 16 Cr <3 <3 3 <3 <3 Ni <3 <3 <3 <3 <3 Pb 14 13 14 11 13 Zn 64 33 46 56 42 Rb 112 113 114 111 115 Ba 747 763 747 742 744 Sr 116 109 114 101 107 Ga 17 15 16 16 15 Nb 9 8 8 8 9 Zr 283 244 257 261 258 Y 12 26 13 20 26 Th 9 12 9 12 13 La 16 26 13 22 30 Ce 25 49 26 46 52 Nd 13 32 12 24 35

AS.1. 1.64 1.53 1.62 1.95 1.58 Rb/Sr 0.97 1.04 1.00 1.10 1.07 KlRb 242 242 235 242 247


130

Sample AW153 AW26 AW47 AW51 a AW51b Lab No. 27378 27379 27380 27381 27382 Rock Whole Rock Pumice Pumice Pumice Pumice

Si02 74.57 74.06 70.98 75.18 74.18 Ti02 0.33 0.28 0.23 0.27 0.24 AI20 3 14.27 13.97 12.63 13.82 13.46 Fe203 1.38 2.26 1.82 2.25 2.22 MnO 0.02 0.04 0.04 0.05 0.06 MgO 0.05 0.05 0.05 0.05 0.05 CaO 1.43 0.97 0.91 1.10 1.04 Na20 4.35 3.80 3.67 4.05 4.01 K20 3.25 3.65 3.41 3.49 3.45 P20 5 0.02 0.03 0.03 0.03 0.03 LOI 0.62 1.37 6.48 -0.22 0.78 Total 100.26 100.42 100.22 100.03 99.46

V 20 11 13 11 14 Cr 4 <3 3 <3 <3 Ni <3 <3 <3 <3 <3 Pb 14 14 34 19 22 Zn 39 32 43 36 38 Rb 114 128 141 118 119 Sa 758 779 726 788 795 Sr . 122 79 90 92 88 Ga 15 14 14 14 15 Nb 8 8 8 8 9 Zr 263 196 237 244 246 Y 9 37 22 29 31 Th 11 13 13 12 14 La 15 35 22 32 32 Ce 31 70 32 60 63 Nd 16 43 26 32 34

A.S.1. 1.58 1.66 1.58 1.60 1.58 Rb/Sr 0.93 1.62 1.57 1.28 1.35 KlRb 237 237 201 246 241

Q 33.32 35.24 36.06 34.81 34.84 C 0.97 2.02 1.33 1.38 1.25 Z 0.05 0.04 0.05 0.05 0.05 Or 19.34 21.85 21.58 20.65 20.73 Ab 36.97 32.50 33.16 34.23 34.42 An 7.25 4.91 4.86 5.50 5.29 Oi 0.00 0.00 0.00 0.00 0.00 Hy 1.11 2.20 1.91 2.20 2.27 IVIt 0.46 0.76 0.65 0.75 0.75 II 0.63 0.54 0.47 0.51 0.46 Ap 0.05 0.07 0.08 0.07 0.07 Total 100.15 100.14 100.15 100.15 100.15

131

AW52 AW54 AW57 AW85a AW85b 27383 27384 27385 27386 27387 Pumice Pumice Pumice Pumice Pumice


V 14 12 13 Cr <3 <3 <3 Ni <3 3 <3 Pb 14 26 16 Zn 31 40 35 Rb 126 126 153 8a 767 748 794 Sr 100 83 95 Ga 16 17 16 Nb 9 9 8 Zr 239 240 250 Y 24 31 29 Th 14 11 12 La 26 31 28 Ce 46 60 63 Nd 33 41 33

A.S.I. 1.57 1.69 1.53 1.68 1.61 Rb/Sr 1.26 1.52 1.61 KlRb 223 233 201


132

Sample AW85c AW116 AW117 AW166 AW1 lab No. 27388 27389 27390 27391 27392 Rock Type Pumice Pumice Pumice Pumice Pumice

Si02 73.89 73.77 73.22 74.76 74.00 Ti02 0.29 0.30 0.29 0.23 0.24 AI20 3 13.52 13.82 13.99 13.69 14.04 Fe203 2.22 2.21 2.30 2.18 2.16 MnO 0.04 0.07 0.07 0.06 0.05 MgO 0.06 0.05 0.05 0.05 0.05 CaO 1.06 1.12 1.16 1.15 1.20 Na20 3.77 4.09 4.81 4.32 4.21 K20 3.73 3.41 3.52 3.41 3.38 P20 S 0.03 0.03 0.04 0.03 0.02 LOI 0.53 0.37 0.66 0.35 0.91 Total 99.14 99.17 100.05 100.21 100.23

V 14 14 14 12 13 Cr <3 <3 <3 <3 <3 Ni <3 <3 <3 <3 4 Pb 14 16 15 16 14 Zn 34 39 41 41 45 Rb 161 116 125 116 116 Sa 793 824 828 804 775 Sr 93 103 106 104 106 Ga 14 15 15 15 16 Nb 8 9 7 9 10 Zr 243 245 248 236 233 Y 35 25 22 24 26 Th 12 13 12 13 12 La 31 27 27 28 26 Ce 51 50 47 60 59 Nd 34 22 26 25 29

A.S.I. 1.58 1.60 1.47 1.54 1.60 Rb/Sr 1.73 1.13 1.18 1.12 1.09 KlRb 192 244 234 244 242

Q 34.91 33.93 28.43 33.15 33.21 C 1.37 1.38 0.18 0.80 1.26 Z 0.05 0.05 0.05 0.05 0.05 Or 22.46 20.47 21.01 20.26 20.19 Ab 32.40 35.06 40.99 36.66 35.91 An 5,40 5.70 5.80 5.78 6.12 Oi 0.00 0.00 0.00 0.00 0.00 Hy 2.18 2.17 2.28 2.22 2.17 Mt 0.75 0.75 0.78 0.73 0.73 II 0.56 0.58 0.55 0,44 0.46 Ap 0.07 0.07 0.10 0.07 0.05 Total 100.15 100.15 100.16 100.15 100.15

133

Sample AW170 AW189 AW204 AW205 AW206 Lab No. 27393 27394 27395 27396 27397

Type Pumice Pumice Pumice Pumice Pumice

Si02 74.14 75.31 74.71 73.87 73.10 Ti02 0.31 0.32 0.28 0.32 0.37 AI20 3 13.74 12.90 14.01 14.40 14.69 Fe203 2.28 2.34 1.08 1.37 1.24 MnO 0.07 0.04 0.04 0.01 0.01 MgO 0.05 0.08 0.05 0.05 0.05 CaO 1.26 1.03 1.17 1.48 1.57 l\Ia20 4.34 3.61 4.15 4.55 5.44 K20 3.39 3.52 3.54 3.24 3.16 P20 5 0.02 0.09 0.02 0.02 0.03 LOI 0.40 1.17 1.15 0.65 0.90 Total 99.97 100.40 100.15 99.90 100.50

V 13 15 18 13 14 Cr 4 <3 <3 <3 <3 l\Ii <3 <3 <3 <3 <3 Pb 16 13 14 13 13 Zn 38 42 25 45 37 Rb 119 122 124 106 101 Sa 792 734 778 754 752 Sr 111 91 101 136 142 Ga 15 15 14 17 16 Nb 9 10 9 9 9 Zr 244 264 250 339 337 Y 26 20 9 13 13 Th 14 12 10 11 10 La 29 21 18 20 19 Ce 56 38 25 31 30 Nd 26 20 15 18 13

A.S.1. 1.53 1.58 1.58 1.55 1.44 Rb/Sr 1.07 1.34 1.23 0.78 0.71 KlRb 236 240 237 254 260

Q 32.31 38.06 34.37 31.49 25.97 C 0.61 1.44 1.21 0.69 0.00 Z 0.05 0.05 0.05 0.07 0.07 Or 20.20 21.05 21.19 19.34 18.80 Ab 36.93 30.83 35.48 38.81 46.23 An 6.41 4.80 5.98 7.53 6.40 Di 0.00 0.00 0.00 0.00 1.30 Hy 2.22 2.29 0.91 1.10 0.21 Mt 0.77 0.79 0.37 0.46 0.42 /I 0.59 0.61 0.54 0.61 0.71 Ap 0.05 0.22 0.05 0.05 0.07 Total 100.15 100.15 100.15 100.16 100.16

134

Sample AW175 AW177 AW180 lab No. 27398 27399 27400 Rock Type Whole Rock Whole Rock Whole Rock

Si02 70.78 70.59 70.25 Ti02 0.45 0.59 0.40 AI20 a 15.16 15.01 16.25 Fe20a 3.17 3.09 3.18 MnO 0.07 0.05 0.10 MgO 0.32 0.30 0.27 CaO 2.31 2.17 1.63 Na20 3.99 3.63 2.69 K20 2.86 2.91 2.84 P20 5 0.03 0.03 0.01 LOI 0.34 1.19 2.74 Total 99.47 99.57 100.35

V 32 33 36 Cr <3 3 <3 Ni <3 <3 <3 Pb 11 11 12 Zn 38 37 45 Rb 98 102 101 Ba 738 742 903 Sr 174 165 131 Ga 15 16 16 Nb 7 7 7 Zr 182 182 194 Y 21 18 15 Th 12 10 13 La 26 26 22 Ce 43 40 42 Nd 24 22 16

A.S.I. 1.66 1.72 2.27 Rb/Sr 0.56 0.62 0.77 KlRb 242 237 233

Q 30.21 32.70 39.42 C 1.31 1.97 5.88 Z 0.04 0.04 0.04 Or 17.13 17.57 17.28 Ab 34.14 31.30 23.37 An 11.66 11.04 8.54 Di 0.00 0.00 0.00 Hy 3.65 3.26 3.73 Mt 1.07 1.05 1.09 II 0.86 1.14 0.78 Ap 0.07 0.07 0.02 Total 100.14 100.14 100.16

135

SAMPLES FROM DATASET

Five samples collected from the base of the Ngapouri Ridge were not considered for

geochemical interpretation due to anomalously low Fe203 concentrations. The

constant sum effect means that if Fe203 is depleted by alteration it will cause an

artificial increase in the concentrations of the other major elements in order to allow

concentrations to sum ~100%, thus rendering the analyses suspect. Figure A2.1

shows plots of SiOz vs. Fe203. TiOz and Alz0 3 for pumice samples AW204, AW205

and AW206. and whole rock samples AW138 and AW153 (all collected from

U17/015098) compared with fields enclosing all other whole rock and pumice analyses

of Waiotapu Ignimbrite.

Were this Fe203 depletion an effect of clay alteration in the samples then

elevated levels of AI20 3 would be expected. Plots for the samples show no great

enrichment in AI20 3 relative to the fields of Waiotapu Ignimbrite samples (Fig A2.1 b).

In addition loss on ignition values for the samples were reasonable (0.62 - 1.23 for the

whole rock analyses and 0.65 - 1.15 for the pumices) which is consistent with

insignificant clay mineral alteration. X-ray diffraction analysis of AW206 did not reveal

the presence of clay minerals.

The possibility that the Fe203 content is a magmatic artefact is unlikely. Fe203

and Ti02 behave in the same manner, fractionating into the same mineral phases (e.g.

ilmenite or titanomagnetite). consequently Fe203 and Ti02 trends should parallel one

another. Figure A2.1 a and A2.1 c show that these two elements show no such

relationship. The pumices may be exotic clasts within Waiotapu Ignimbrite, but whole

rock samples (which almost certainly represent a mix of all Waiotapu magma

compositions) show the same Fe203 depletion.

It is likely that the Fe203 content represents post depositional, probably

hydrothermal, alteration of Fe-bearing mineral phases which resulted in the removal of

Fe203 from the system. This is consistent with the considerable degree of degradation

of the mafic phases observed in thin sections of the same samples. TiOz was retained,

possibly into new mineral phases such as leucoxene (Ti02),an alteration product of

ilmenite. A polished section of mineral separates from AW205 The presence of

leucoxene may also explain the elevated Zr concentrations in AW205 and AW206 (Fig.

A2.1 d) as Zr will substitute for Ti in that mineral.

136

3 a) 17 b) 2.8 2.6 16

--. 2.4 ~ 15

~ ~ 2.2 ~ 2 1 0'"

.., (lJN1.8 g.14 u. 1.6 «

1.4 Ell 0 13 1.2 ,. 1 ,. 12

72 73 74 75 76 72 73 74 75 76 SiOz(wt %) Si02(wt %)

c) 340 d) Ell • 320 300 280

~ 0 E 260 .3; 240 = L.. 220 -N 200 -

-180 -

.2 160 140

.15 120 72 73 74 75 76 70 71 72 73 74 75 76 77

Si02(wt %) Si02(wt %)

Waiotapu Ignimbrite - Pumice Field • Pumice

Waiotapu Ignimbrite - Whole Rock Field a Whole Rock

Figure A2.1 Plots of selected major and trace elements vs Si02 showing the anomalous behaviour of samples AW138, AW153, AW204, AW205 and AW206.

137

DIX

IN H

The following are scanning electron microprobe analyses of ignimbrite mineral phases.

Samples are from Waiotapu Ignimbrite, expect for 172 (unknown ignimbrite), 177

(Rahopaka Ignimbrite), 201 (Akatawera A -Rahopaka Ignimbrite) and KA100 (unknown

ignimbrite from Kaingaroa Ignimbrite lithic lag breccias). All samples are whole rock,

with the exception of 51, which is a mineral separate derived from a pumice.

PLAGIOCLASE

Sample 1044 104-5 104-7 104-8 104-10 104-11 106-2 Core/Rim Core Rim Rim Core Rim Core Core Si02 49.733 61.173 63.732 61.4 61.348 61.996 54.729 AI203 0.279 26.929 25.998 27.136 25.291 25.07 29.216 Ti02 0.134 0 0 0.042 0.019 0.027 0.021 FeO 33.576 0.247 0.311 0.324 0.279 0.146 0.219 MnO 0.964 0 0.043 0 0 0.035 0.039 MgO 14.483 0 0 0 0 0 0 CaO 1.14 7.074 6.239 7.473 6.318 5.942 11.743 Na20 0.018 6.102 6.638 6.211 6.564 6.502 5.021 K20 0 0.373 0.403 0.373 0.436 0.434 0.156

Total 100.331 101.901 103.367 102.962 100.257 100.156 101.15

Oxygen 6 8 8 8 32 32 32 Si 1.9645 2.6612 2.725 2.6497 10.8411 10.9331 9.7889 AI 0.013 1.3807 1.3101 1.3802 5.2674 5.2106 6.1588 Ti 0.0039 0 0 0.0013 0.0025 0.0037 0.0029 Fe 1.1091 0.009 0.0111 0.0117 0.0412 0.0216 0.0327 Mn 0.0322 0 0.0015 0 0 0.0053 0.006 Mg 0.8529 0 0 0 0 0 0 Ca 0.0482 0.3297 0.2858 0.3455 1.1962 1.1227 2.2504 Na 0.0013 0.5146 0.5503 0.5197 2.2489 2.2232 1.7415 K 0 0.0207 0.022 0.0205 0.0984 0.0978 0.0358

Total 4.0256 4.9161 4.9061 4.9289 19.6962 19.6184 20.0174

138

Sample 106-3 106-4 1 106M 6 108-1 108-2 108-3 Core/Rim Core Rim Rim Core Core Core Rim Si02 57.56 61.369 61.662 61.761 61.264 62.78 60.352 AI203 26.688 25.462 25.227 24.274 25.282 24.963 25.337 Ti02 0.026 0 0 0.023 0.031 0 0 FeO 0.166 0.206 0.204 0.242 0.17 0.208 0.238 MnO 0 0 0 0 0 0 0 MgO 0 0 0 0 0 0.019 0 CaO 8.315 6.679 6.326 6.225 6.515 6.008 6.357 Na20 6.241 6.574 6.815 7.122 6.673 6.941 6.767 K20 0.321 0.368 0.431 0.434 0.439 0.44 0.398

Total 99.3205 100.661 100.669 100.085 100.378 101.363 99.4518

Oxygen 32 32 32 32 32 32 32 Si 10.3686 10.8086 10.8563 10.9477 10.8224 10.9573 10.7697 AI 5.6659 5.2853 5.2346 5.0711 5.2638 5.135 5.3288 Ti 0.0035 0 0 0.0031 0.0041 0 0 Fe 0.0251 0.0304 0.0301 0.036 0.0252 0.0304 0.0356 Mn 0 0 0 0 0 0 0 Mg 0 0 0 0 0 0.0049 0 Ca 1.6048 1.2604 1.1934 1.1822 1.2332 1.1236 1.2155 Na 2.1798 2.2449 2.3266 2.4479 2.2857 2.3489 2.3414 K 0.0738 0.0828 0.0969 0.0983 0.0989 0.098 0.0906

Total 19.9217 19.7126 19.7381 19.7867 19.7337 19.6986 19.7819

Sample 108-4 108-8 108-9 108-10 108-11 141 141-6 Core/Rim Rim Core Core Rim Rim Rim Rim Si02 60.406 56.112 55.055 60.218 60.352 60.422 61.263 AI203 25.501 27.572 27.078 25.339 24.466 26.25 26.156 Ti02 0 0 0 0 0.041 0.028 0 FeO 0.228 0.199 0.204 0.226 0.228 0.26 0.12 MnO 0 0 0 0 0 0 0 MgO 0 0 0 0 0 0 0 CaO 7.173 9.974 9.916 6.887 6.185 7.55 7.386 Na20 6.716 4.467 5.615 6.871 6.92 6.879 6.995 K20 0.398 0.202 0.221 0.385 0.427 0.374 0.43

Total 100.425 98.5296 98.0913 99.9291 98.6222 101.767 102.353

Oxygen 32 32 32 32 32 32 32 Si 10.7069 10.1859 10.1041 10.7227 10.8608 10.5939 10.6638 AI 5.3272 5.8988 5.857 5.3178 5.1891 5.4244 5.366 Ti 0 0 0 0 0.0056 0.0037 0 Fe 0.0338 0.0303 0.0313 0.0337 0.0343 0.0381 0.0174 Mn 0 0 0 0 0 0 0 Mg 0 0 0 0 0 0 0 Ca 1.3621 1.9399 1.9498 1.314 1.1925 1.4183 1.3774 Na 2.3081 1.5724 1.998 2.3722 2.4144 2.3387 2.3608 K 0.0901 0.0469 0.0517 0.0875 0.0982 0.0838 0.0956

Total 19.8285 19.6743 19.9923 19.8482 19.7953 19.9013 19.8814

139

Sample 141-1 112-8 1 1 1114 201-5 201-6 Core/Rim Core Core Core Core Core Core Rim Si02 59.701 59.435 60.438 60.149 60.412 60.979 61.458 AI203 24.369 26.227 26.481 26.896 26.534 26.71 26.863 Ti02 0.025 0.034 0.022 0 0 0.023 0 FeO 0.263 0.196 0.183 0.123 0.165 0.201 0.104 MnO 0.031 0 0.024 0 0 0 0.016 MgO 0 0 0 0 0 0 0 CaO 7.131 6.3 6.24 6.916 6.671 6.323 6.109 Na20 6.694 7.042 7.505 7.068 7.144 6.964 6.815 K20 0.495 0.384 0.407 0.427 0.463 0.444 0.485

Total 98.7145 99.622 101.303 101.582 101.391 101.647 101.854

Oxygen 32 32 32 32 32 32 32 Si 10.7801 10.6089 10.6194 10.5474 10.6078 10.6518 10.6899 AI 5.1862 5.5174 5.4838 5.5586 5.4912 5.4988 5.5069 Ti 0.0034 0.0046 0.0029 0 0 0.0031 0

0.0398 0.0292 0.0269 0.0181 0.0242 0.0294 0.0152 Mn 0.0048 0 0.0037 0 0 0 0.0024 Mg 0 0 0 0 0 0 0 Ca 1.3797 1.2049 1.1747 1.2994 1.255 1.1835 1.1386 Na 2.3438 2.4373 2.5569 2.4032 2.4323 2.3587 2.2986 K 0.1142 0.0874 0.0913 0.0956 0.1037 0.0989 0.1076

Total 19.8523 19.8901 19.9599 19.9226 19.9146 19.8244 19.7597

Sample KA 100-1 KA 100-2 KA 100-3 KA 1004 Core/Rim Core Core Rim Rim Si02 60.241 55.416 57.799 58.044 AI203 27.451 27.514 26.833 27.03 Ti02 0.022 0 0 0.025 FeO 0.132 0.175 0.153 0.172 MnO 0 0 0 0 MgO 0 0 0 0 CaO 7.43 8.795 6.895 7.146 Na20 6.827 6.014 7.21 6.823 K20 0.381 0.268 0.395 0.374

Total 102.487 98.1843 99.2877 99.6185

Oxygen 32 32 32 32 Si 10.4778 10.1306 10.4028 10.4016 AI 5.6272 5.928 5.6919 5.7089 Ti 0.0029 0 0 0.0034 Fe 0.0193 0.0268 0.023 0.0258 Mn 0 0 0 0 Mg 0 0 0 0 Ca 1.3846 1.7226 1.3296 1.3721 Na 2.3024 2.1316 2.5162 2.3709 K 0.0846 0.0626 0.0908 0.0856

Total 19.8991 20.0025 20.0547 19.9686

140

ORTHOPYROXENE

Sample -3 51-4 51-5 104-1 104-3 104-16 104-17 Si02 50.003 49.912 50.182 49.969 50.015 49.259 50.222 AI20 3 0.339 0.379 0.339 0.595 0.445 0.358 0.412 Ti02 0.173 0.16 0.15 0.177 0.139 0.17 0.112 FeO 31.053 30.178 30.546 32.057 32.164 33.052 32.306 MnO 1.49 1.343 1.401 1.486 1.358 1.254 1.419 MgO 15.212 15.43 14.896 15.774 15.562 14.872 15.994 CaO 1.145 1.298 1.171 1.372 1.143 1.283 1.218 Na20 0 0.019 0 0 0 0 0 K20 0 0 0 0 0 0 0

Total 99.419 98.7228 98.6899 101.432 100.83 100.253 101.686

Oxygen 6 6 6 6 6 6 6 Si 1.9737 1.9763 1.9889 1.9429 1.9549 1.9488 1.9477 AI 0.0157 0.0176 0.0158 0.0272 0.0205 0.0167 0.0188 Ti 0.0051 0.0047 0.0044 0.0051 0.0041 0.005 0.0032 Fe 1.025 0.9993 1.0124 1.0424 1.0513 1.0935 1.0477 Mn 0.0498 0.045 0.047 0.0489 0.0449 0.042 0.0466 Mg 0.8951 0.9108 0.8801 0.9143 0.9068 0.8771 0.9247 Ca 0.0484 0.055 0.0497 0.0571 0.0478 0.0544 0.0506 Na 0 0.0015 0 0 0 0 0 K 0 0 0 0 0 0 0

Total 4.0132 . 4.0107 3.9987 4.0383 4.0307 4.0378 4.0396

Sample 104-18 104-19 106-11 106-12 106-13 108-18 108-19 Si02 50.248 49.726 49.98 49.64 49.776 48.528 48.597 AI20 3 0.321 0.478 0.271 0.202 0.263 0.669 0.552 Ti02 0.15 0.182 0.128 0.119 0.156 0.284 0.215 FeO 32.217 31.286 32.449 32.397 32.578 29.974 31.605 MnO 1.41 0.911 1.516 1.336 1.439 1.176 1.255 MgO 15.666 15.438 15.464 15.521 15.584 16.133 15.171 CaO 1.33 1.357 1.229 1.097 1.147 1.156 1.82 Na20 0 0 0 0.038 0 0.026 0.026 K20 0 0 0 0 0 0 0

Total 101.346 99.382 101.039 100.354 100.946 97.9502 99.246

Oxygen 6 6 6 6 6 6 6 Si 1.955 1.9631 1.9545 1.9544 1.9497 1.9405 1.9363 AI 0.0147 0.0222 0.0124 0.0093 0.0121 0.0315 0.0259 Ti 0.0044 0.0054 0.0037 0.0035 0.0045 0.0085 0.0064

1.0482 1.0329 1.0612 1.0667 1.0671 1.0023 1.0531 Mn 0.0464 0.0304 0.0502 0.0445 0.0477 0.0398 0.0423 Mg 0.9086 0.9085 0.9015 0.9109 0.9099 0.9617 0.9011 Ca 0.0554 0.0574 0.0515 0.0462 0.0481 0.0495 0.0777 Na 0 0 0 0.0029 0 0.002 0.002 K 0 0 0 0 0 0 0

Total 4.0332 4.0203 4.0354 4.0388 4.0396 4.0362 4.0453

141

Sample 108-20 141 141 141 172-6 1 1 1 Si02 48.512 49.855 49.363 49.77 50.594 49.815 47.584 AI20 3 0.927 0.386 0.429 0.272 0.602 0.557 0.253 Ti02 0.337 0.148 0.195 0.132 0.173 0.161 0.178 FeO 31.816 30.392 30.759 32.27 23.968 23.537 34.454 MnO 0.914 1.279 1.343 1.43 0.91 0.98 1.505 MgO 15.037 16.26 16.293 15.584 19.666 19.721 12.426 CaO 1.856 1.125 1.105 1.198 1.225 1.311 1.379 Na20 0 0 0 0 0 0 0.02 K20 0 0 0 0 0 0 0

Total 99.4019 99.4475 99.4903 100.66 97.1406 96.0856 97.8036

Oxygen 6 6 6 6 6 6 6 Si 1.9281 1.9607 1.947 1.9525 1.9717 1.964 1.9555 AI 0.0434 0.0178 0.0199 0.0126 0.0276 0.0258 0.0122 Ti 0.01 0.0043 0.0057 0.0039 0.005 0.0047 0.0055 Fe 1.0575 0.9995 1.0146 1.0587 0.7811 0.7761 1.1841 Mn 0.0307 0.0426 0.0448 0.0475 0.03 0.0327 0.0524 Mg 0.8909 0.9533 0.9581 0.9114 1.1425 1.1591 0.7613 Ca 0.079 0.0474 0.0467 0.0503 0.0511 0.0554 0.0607 Na 0 0 0 0 0 0 0.0016 K 0 0 0 0 0 0 0

Total 4.04 4.0259 4.0372 4.0372 4.0094 4.0182 4.0336

Sample 172-12 201-1 201-2 201-3 201 Si02 47.907 50.095 50.227 50.338 49.631 AI20 3 0.385 0.533 0.293 0.365 0.357 Ti02 0.196 0.149 0.103 0.08 0.122 FeO 34.294 25.906 27.359 26.733 27.23 MnO 1.974 1.315 1.572 1.135 1.415 MgO 12.33 18.406 17.853 18.035 17.6 CaO 1.441 0.776 0.739 0.801 0.821 Na20 0.024 0 0 0 0 K20 0 0 0 0 0

Total 98.5555 97.1821 98.1492 97.4908 97.1792

Oxygen 6 6 6 6 6 Si 1.9541 1.9721 1.9729 1.9804 1.9699 AI 0.0185 0.0247 0.0135 0.0169 0.0167 Ti 0.006 0.0044 0.003 0.0023 0.0036 Fe 1.1698 0.8529 0.8987 0.8795 0.9038 Mn 0.0682 0.0438 0.0523 0.0378 0.0475 Mg 0.7498 1.0802 1.0454 1.0577 1.0414 Ca 0.0629 0.0327 0.0311 0.0337 0.0349 Na 0.0019 0 0 0 0 K 0 0 0 0 0

Total 4.0315 4.0111 4.0172 4.0087 4.0181

142

MAGNETITE

Sample 51-7 106-7 106-8 108-15 108-22 Si02 0.052 0.103 0.034 0.05 0.047 0.309 0.047 AI20 3 1.378 1.074 1.054 0.839 1.063 0.658 0.794 Ti02 9.948 7.324 11.029 13.45 13.391 14.455 14.199 FeO 83.534 84.113 80.915 82.344 83.048 78.573 79.494 MnO 0.783 0.736 0.661 0.379 0.59 1.592 0.868 MgO 0.259 0.186 0.312 0.353 0.367 0.261 0.24 CaO 0 0 0.016 0 0 0.039 0 Na20 0 0 0 0 0 0 0 K20 0 0 0 0 0 0 0

Total 95.9561 93.5374 94.0235 97.4182 98.5092 95.8907 95.645

Oxygen 32 32 32 32 32 32 32 Si 0.0189 0.0391 0.0126 0.0178 0.0164 0.1088 0.0168 AI 0.5881 0.4801 0.4556 0.346 0.4329 0.2728 0.3313 Ti 2.7082 2.0896 3.0431 3.537 3.48 3.8228 3.7792 Fe 25.2834 26.6804 24.822 24.0746 23.9952 23.1014 23.5236 Mn 0.24 0.2365 0.2055 0.1124 0.1728 0.4741 0.2601 Mg 0.1399 0.1054 0.1706 0.184 0.1894 0.137 0.1268 Ca 0 0 0.0065 0 0 0.0148 0 Na 0 0 0 0 0 0 0 K 0 0 0 0 0 0 0

Total 28.9788 29.6312 28.7163 28.2721 28.287 27.9319 28.0382

Sample 141-8 141 172-4 112-5 111-1 111-8 KA100-11 Si02 0 0.036 0.324 0.268 0.024 0.032 0 AI20 3 0.789 0.77 1.378 1.363 1.342 1.184 1.373 Ti02 6.702 7.921 17.778 17.249 8.548 8.443 11.044 FeO 85.699 86.971 69.386 71.257 82.436 82.388 80.679 MnO 0.169 0.914 1.209 0.775 0.623 0.431 0.455 MgO 0.082 0.332 0.501 0.549 0.39 0.319 0.432 CaO 0.016 0 0.028 0.019 0 0 0 Na20 0 0 0 0 0 0 0 K20 0 0 0 0 0 0 0

Total 93.4599 96.9469 90.6074 91.4834 93.3652 92.7986 93.9861

Oxygen 32 32 32 32 32 32 23 Si 0 0.0134 0.1159 0.0953 0.0091 0.0121 0 AI 0.3568 0.3324 0.5797 0.5715 0.5931 0.5278 0.4253 Ti 1.9337 2.1826 4.7726 4.6141 2.4114 2.4022 2.1826 Fe 27.4882 26.6436 20.7098 21.1913 25.8529 26.061 17.7259 Mn 0.0549 0.2837 0.3655 0.2335 0.1979 0.1381 0.1014 Mg 0.0471 0.1816 0.2667 0.2912 0.2181 0.1801 0.1693 Ca 0.0068 0 0.0109 0.0075 0 0 0 Na 0 0 0 0 0 0 0 K 0 0 0 0 0 0 0

Total 29.8879 29.6376 26.8215 27.0048 29.2828 29.3217 20.6047

143

Sample KA100-12 Si02 0.071 AI20 3 1.382 Ti02 10.849 FeO 80.213 MnO 0.399 MgO 0.421 CaO 0 Na20 0 K20 0

Total 93.3394

Oxygen 32 Si 0.0264 AI 0.5997 Ti 3.0032 Fe 24.6852 Mn 0.1245 Mg 0.2311 Ca 0 Na 0 K 0

Total 28.6704

144

ILMENITE

Sample 51-1 51-2 149-2 149-3 149-4 149-5 149-6 Si02 0 0 0 0 0 0 0 AI20 3 0.036 0 0.046 0.043 0.056 0.042 0.029 Ti02 49.509 49.697 50.89 50.512 49.898 49.896 50.503 FeO 47.194 48.087 47 46.975 46.736 48.012 46.625 MnO 2.157 1.717 1.559 1.536 1.374 1.079 1.232 MgO 0.759 0.674 0.461 0.534 0.531 0.336 0.363 CaO 0 0 0 0.029 0 0.014 0 Na20 0 0 0 0 0 0 0 K20 0 0 0.016 0 0 0.013 0.01

Total 99.6582 100.177699.9756 99.6319 98.5983 99.3954 98.7648

Oxygen 6 6 6 6 6 6 6 Si 0 0 0 0 0 0 0 AI 0.0021 0 0.0027 0.0026 0.0034 0.0025 0.0018 Ti 1.9102 1.9098 1.9469 1.9404 1.9378 1.9291 1.9545 Fe 2.0244 2.0545 1.9991 2.0063 2.018 2.0638 2.0062 Mn 0.0937 0.0743 0.0671 0.0664 0.0601 0.047 0.0536 Mg 0.058 0.0514 0.035 0.0406 0.0409 0.0258 0.0278 Ca 0 0 0 0.0016 0 0.0008 0 Na 0 0 0 0 0 0 0 K 0 0 0.001 0 0 0.0008 0.0006

Total 4.0887 4.0902 4.0522 4.0582 4.0604 4.07 4.0449

Sample 149-1 112-1 112-2 112-3 Si02 0 0 0 0 AI20 3 0 0.052 0.03 0.035 Ti02 49.523 48.862 49.231 46.845 FeO 47.579 46.829 47.302 47.175 MnO 0.816 1.235 0.927 1.244 MgO 0.384 1.027 0.986 1.031 CaO 0.016 0.018 0 0.015 Na20 0 0 0 0 K20 0 0 0 0

Total 98.3206 98.0256 98.4793 96.3488

Oxygen 6 6 6 6 Si 0 0 0 0 AI 0 0.0032 0.0018 0.0022 Ti 1.9338 1.9116 1.9165 1.8776 Fe 2.0656 2.0368 2.0472 2.1022 Mn 0.0359 0.0544 0.0406 0.0561 Mg 0.0297 0.0797 0.0761 0.0819 Ca 0.0009 0.001 0 0.0009 Na 0 0 0 0 K 0 0 0 0

Total 4.0661 4.0868 4.0825 4.1212

145

AMPHIBOLE

Sample 177-1 177-2 1 1 KA100-6 KA100-7 SiOz 44.743 45.666 42.149 43.014 47.892 47.983 Ti02 1.577 1.33 2.608 2.702 0.04 0.088 Alz0 3 7.214 6.388 9.623 9.5 0.195 0.341 FeO 16.92 17.268 14.203 13.665 29.43 29.51198 MnO 0.483 0.424 0.391 0.259 1.136 0.924 MgO 11.906 12.211 12.388 12.924 15.205 15.14 CaO 10.256 10.339 10.03 10.915 0.838 0.789 Na20 1.504 1.311 2.046 2.044 0 0 K20 0.363 0.372 0.328 0.344 0 0

Total 94.966 95.309 93.766 95.367 94.736 94.77701

Si1v 6.879064 6.992107 6.512863 6.52175 7.57952 7.580781

AI1V 1.120936 1.007893 1.487137 1.47825 3.64E-02 6.35E-02 Fe3+IV 0.4485 0.3894 0.6129 0.601 0 2.86E-06 Ti1v 0.0713 0.0726 0.0647 0.0665 4.76E-03 1.05E-02 T site 8 8 8 8 7.620653 7.654734 Alv1 0.186246 0.144858 0.265334 0.219344 0 0 Fe3+ 0 0 0 0 0 0

Ti 0.182329 0.15314 0.303049 0.308078 0 0 Mg(1) 2.728843 2.787243 2.853609 2.92119 3.58735 3.565832 Fe2+(1) 1.902581 1.914759 1.578008 1.551389 1.41265 1.434168 M1,2,3 5 5 5 5 5 5 Fez+(Z) 0.272927 0.296365 0.25735 0.181298 2.482507 2.46508 Mn 6.29E-02 5.50E-02 5.12E-02 3.33E-02 0.15228 0.123647 Ca(2) 1.664175 1.648647 1.660517 1.77311 0 0 Na(1) 0 0 3.10E-02 1.23E-02 0 0 M4 site 2 2 2 2 2.634787 2.588727 Ca(3) 0.025253 4.75E-02 0 0 0.142096 0.133555 Na(2) 0.448331 0.389193 0.582007 0.588541 0 0 K 7.12E-02 7.27E-02 6.47E-02 6.65E-02 0 0 A site 0.544783 0.509308 0.646664 0.655079 0.142096 0.133555 0 23 23 23 23 23 23 OH 0 0 0 0 0 0 Charge 3.81 E-06 3.81 E-06 3.81E-06 3.81E-06 3.81E-06 0

146

Sample KA 100-8 KA100-9 KA100-10 Si02 48.586 48.336 48.507 Ti02 0.091 0.086 0.09 AI20 3 0.256 0.315 0.271 FeO 29.85396 29.13997 29.62299 MnO 1.064 1.211 1.275 MgO 15.535 15.607 15.426 CaO 0.773 0.803 0.79 Na20 0 0 0 K20 0 0 0

Total 96.159 95.498 95.982

Si lV 7.570148 7.568951 7.572804

AIIV 4.70E-02 5.81E-02 4.99E-02 Fe3+IV 5.72E-06 3.81E-06 9.54E-07

Tilv 1.07E-02 1.01E-02 1.06E-02

Tsite 7.627826 7.637216 7.633235 Aivi 0 0 0 Fe3+ 0 0 0 Ti 0 0 0 Mg(1) 3.608386 3.643283 3.590163 Fe2+(1) 1.391614 1.356717 1.409837 M1,2,3 5 5 5 Fe2+(2) 2.4984 2.459298 2.457725 Mn 0.140417 0.160618 O~ 168596 Ca(2) 0 0 0 Na(1) 0 0 0 M4 site 2.638817 2.619917 2.626321 Ca(3) 0.129042 0.134722 0.132141 Na(2) 0 0 0 K 0 0 0 A site 0.129042 0.134722 0.132141 0 23 23 23 OH 0 0 0 Charge 3.81 E-06 3.81E-06 0

147

ApPENDIX R

N M REM

WAIOTAPU IGNIMBRITE

Sample WtAir WtWater Wax Density Height Grid Reference

Uchfield Quarry - Cliff Section

AW066 20.48 8.59 0.12 1.71 0.00 T16/588400

AW067 23.16 8.88 0.12 1.61 2.00 T16/588400

AW068 10.75 4.23 0.08 1.63 4.00 T16/588400

AW069 14.30 5.69 0.08 1.65 6.00 T16/588400

AW070 17.04 6.49 0.12 1.60 8.00 T16/588400

Bison Road Quarry

AW054 29.45 14.51 0.12 1.96 0.00 U161771211

AW055 16.94 8.81 0.08 2.06 8.00 U16/771211

AW056 15.37 8.00 0.08 2.06 16.00 U161771211

AW057 51.50 26.01 0.19 2.01 24.00 U161771211

AW058 24.19 12.27 0.12 2.01 32.00 U161771211

AW059 35.13 15.00 0.19 1.73 40.00 U161771211

State Highway One - T161671188

AW071 19.19 9.01 0.12 1.86 0.00 T16/671188

AW072 34.60 16.19 0.12 1.87 1.50 T16/671188

AW073 14.51 6.03 0.12 1.69 3.00 T16/671188

Rawhiti Rd Quarry - Section 1 a AW031 44.59 20.10 0.19 1.81 0.00 U16/771215

AW032 41.32 19.46 0.19 1.87 1.00 U161771215

AW033 44.33 21.08 0.19 1.89 2.00 U16/771215

AW034 28.44 12.59 0.12 1.78 3.00 U16/771215

AW035 56.00 24.48 0.19 1.77 4.50 U16/771215

AW036 48.34 21.94 0.19 1.82 5.50 U16/771215

AW037 37.29 14.83 0.19 1.65 6.00 U16/771215

AW038 45.59 19.48 0.23 1.73 7.50 U16/771215

AW039 44.96 19.68 0.19 1.77 9.00 U161771215

Rawhiti Rd Quarry - Section 1 b

AW048 29.16 13.76 0.12 1.88 U16/771215

AW049 28.45 13.12 0.12 1.84 U16/771215

AW050 48.01 22.99 0.19 1.90 U16/771215

148


Rawhiti Rd Quarry - Misc

AW051 35.25 18.11 0.12 2.04 U16/771215 AW053 37.33 15.96 0.19 1.73 U16n71215

Jackal Road

AW096 32.65 14.02 0.12 1.74 0.00 U16/755195 AW097 35.78 14.57 0.12 1.68 1.00 U16n55195 AW098 37.89 15.33 0.12 1.67 U16n55196 AW099 40.16 15.62 0.19 1.62 U16/757198

Rauna Road

AW074 25.24 10.26 0.12 1.67 0.00 U16/745196 AW075 49.13 20.28 0.19 1.69 3.00 U16/745196 AW076 45.40 22.25 0.19 1.95 0.00 U16/743195 AW077 52.46 25.05 0.19 1.90 1.50 U16/743195 AW078 25.84 11.83 0.12 1.83 3.00 U16n43195 AW079 31.65 14.39 0.19 1.81 U16/745193 AW080 25.06 9.79 0.12 1.63 U16/749195 AW081 53.17 23.00 0.19 1.75 U16n49198 AW082 23.32 10.46 0.12 1.80 0.00 U16/749199 AW083 51.56 24.18 0.19 1.87 1.50 U16/749199 AW084 45.18 20.20 0.19 1.79 3.00 U161749199

Beale Rd

AW087 62.43 29.09 0.23 1.86 0.00 U16/750204 AW088 45.78 20.33 0.19 1.79 1.50 U16/750204 AW089 36.93 16.30 0.19 1.77 2.50 U161750204 AW090 45.97 22.59 0.19 1.95 0.00 U16/751204 AW091 24.19 12.04 0.12 1.97 1.50 U16/751204 AW092 38.40 19.55 0.12 2.02 3.00 U16/751204 AW093 43.34 17.58 0.19 1.67 4.50 U16/751204 AW094 31.77 14.59 0.19 1.83 U16n49202

Ngapouri Ridge

AW148 9.69 4.81 0.08 1.95 0.00 U17/015098 AW149 21.47 11.10 0.08 2.05 2.00 U17/015098 AW150 16.56 8.22 0.08 1.97 3.00 U17/015098 AW152 18.18 9.84 0.08 2.16 5.00 U17/015098 AW153 17.57 7.85 0.08 1.79 10.00 U17/015098 AW138 20.01 10.12 0.12 2.00 30.00 U17/015098 AW139 16.61 8.47 0.08 2.02 42.00 U17/015098 AW141 14.59 6.62 0.08 1.81 46.00 U17/015098 AW143 10.64 5.20 0.08 1.93 50.00 U17/015098

Pukerimu Road

AW040 21.66 10.17 0.12 1.87 U16/792207

149


Butchers Boundry Road

AW061 a 22.35 11.44 0.12 2.03 U17/062999

Mandril Road

AW095 43.0B 20.6B 0.19 1.91 U16n51201

Wawa Quarry

AW104 24.44 13.43 O.OB 2.20 0.00 T16/627174 AW105 32.23 1B.13 0.12 2.27 0.50 T16/627174 AW106 32.01 17.90 0.12 2.25 1.00 T16/627174 AW107 20.3B 9.17 0.12 1.BO 3.00 T16/627174 AW10B 23.12 10.46 0.12 1.B1 6.00 T16/627174 AW109 1B.60 9.61 O.OB 2.05 9.00 T16/627174 AW110 19.24 9.00 O.OB 1.B6 12.00 T16/627174 AW111 26.21 11.B9 0.12 1.B2 15.00 T16/627174 AW112 12.BO 5.75 O.OB 1.BO 1B.00 T16/627174 AW113 21.49 10.96 0.12 2.02 21.00 T16/627174 AW114 20.45 B.71 0.12 1.72 24.00 T16/627174 AW115 . 16.B5 6.27 O.OB 1.5B 26.00 T16/627174

Wawa Quarry - Basal Sections

AW159: Upper Section 801.1 15.70 5.75 O.OB 1.57 0.00 T16/627174 801.2 29.45 13.B3 0.12 1.B7 0.05 T16/627174 801.3 15.30 7.34 O.OB 1.90 0.10 T16/627174 801.4 22.55 12.11 0.12 2.14 0.15 T16/627174 801.5 9.19 4.93 O.OB 2.12 0.20 T16/627174 801.6 24.B4 12.B3 O.OB 2.05 0.25 T16/627174 801.7 25.74 14.13 O.OB 2.20 0.35 T16/627174 AW16B: Lower Section I

802.1 B.73 4.35 O.OB 1.96 0.00 T16/627174 802.2 15.7B B.26 O.OB 2.0B 0.05 T16/627174 802.3 19.7B 10.37 O.OB 2.0B 0.10 T16/627174 802.4 16.19 B.51 O.OB 2.09 0.15 T16/627174 802.5 14.45 7.B2 O.OB 2.15 0.20 T16/627174 802.6 16.23 B.B1 O.OB 2.16 0.25 T16/627174 802.7 2.21 1.1B O.OB 1.99 0.30 T16/627174 AW169: Lower Section II

803.1 2B.01 14.99 0.12 2.13 0.00 T16/627174 803.2 16.51 9.07 O.OB 2.20 0.05 T16/627174 803.3 13.00 7.04 O.OB 2.15 0.10 T16/627174 803.4 24.0B 13.31 0.12 2.21 0.15 T16/627174 803.5 15.09 B.20 O.OB 2.16 0.20 T16/627174

150

RAHOPAKA IGNIMBRITE

Sample Wt WtWater Wax Density Height Grid

BobRd

AW119 10.00 3.08 0.08 1.43 0.00 U16/789205

AW120 13.53 3.60 0.08 1.35 1.00 U161789205

AW121 22.05 4.25 0.12 1.23 2.00 U16/789205

AW124 8.66 2.50 0.08 1.39 3.00 U16/789205

AW122 9.24 1.28 0.08 1.15 4.00 U16/789205

AW123 6.04 3.57 0.08 3.60 0.98 5.00

AW125 11.42 6.11 0.12 6.04 1.00 6.00 U16/789205

Pukerimu Rd

AW175 14.62 5.00 0.08 1.51 0.00 U16/756185

AW176 10.69 3.96 0.08 1.57 1.00 U16/756185

AW177 12.93 4.00 0.08 1.44 2.00 U16/756185

AW178 14.72 4.57 0.08 1.44 3.00 U16/756185

AW179 11.38 3.62 0.08 1.45 4.00 U16/756185

AW180 12.01 2.32 0.08 1.23 6.00 U161756185

Wt air: dry weight of sample (g)

Wt water: weight of wax coated sample immersed in water (g)

Wax: weight of wax coating in water (g). Wax is bouyant, therefore aU weights given are negative values.

Ballast: in ceratin cases low density samples will float therefore a ballast must be used to allow measurement of wt water.

Density: density of sample (g cm-3)

density:::: wt air - (wt water - ballast) - wax (Houghton et ai, 1988)

Height: height above base of outcrop (m), except at Wawa Quarry where all heights refer to height above base of the unit.

151

ApPENDIX FIVE

INT UNT

NGAPOURI RIDGE

Percentage (whole rock) Sample Plag Opx Fe-Ti Hbl Qz Matrix Crystal AW138 5.60 1.00 0.80 0.00 0.10 85.30 15.00 AW139 16.30 0.80 0.60 0.00 0.00 82.10 17.90 AW141 9.00 2.10 0.80 0.00 0.00 88.00 12.00 AW148 13.50 1.80 1.30 0.00 0.00 83.30 16.70 AW149 13.00 0.80 1.00 0.00 0.00 85.10 14.90 AW150 14.60 1.10 1.30 0.00 0.00 82.80 17.20 AW152 8.10 1.00 0.80 0.00 0.00 90.00 10.00 AW153 12.50 1.00 1.10 0.00 0.00 85.30 15.00

Percentage (mineral assemblage) Sample Plag Opx Fe-Ti Hbl Qz AW138 75.00 13.00 11.00 0.00 1.00 AW139 92.09 4.52 3.39 0.00 0.00 AW141 75.63 17.65 6.72 0.00 0.00 AW148 81.33 10.84 7.83 0.00 0.00 AW149 87.84 5.41 6.76 0.00 0.00 AW150 85.88 6.47 7.65 0.00 0.00 AW152 81.82 10.10 8.08 0.00 0.00 AW153 86.00 7.00 8.00 0.00 0.00

UCHFIELD QUARRY

Percentage (whole rock) Sample Plag Opx Fe-Ti Hbl Qz Matrix Crystal

AW66 8.60 0.80 1.00 0.00 0.00 89.30 10.70 AW67 9.10 1.00 0.60 0.00 0.00 89.10 10.90 AW68 8.50 1.50 1.00 0.00 0.00 89.00 11.00 AW69 9.80 2.10 1.50 0.00 0.00 86.50 13.50 AW70 6.00 1.30 0.80 0.00 . 0.00 91.80 8.20

Percentage (mineral assemblage) Sample Plag Opx Fe-Ti Hbl Qz

AW66 82.69 7.69 9.62 0.00 0.00 AW67 85.05 9.35 5.61 0.00 0.00 AW68 77.27 13.64 9.09 0.00 0.00 AW69 73.13 15.67 11.19 0.00 0.00 AW70 74.07 16.05 9.88 0.00 0.00

Plag: plagioclase. Opx: orthopyroxene. Fe-Ti: magnetite and ilmenite. Hbl: hornblende. Qz: quartz.

152

BISON ROAD QUARRY

Percentage (whole rock)

Sample Plag Opx Fe-Ti Hbl Q:z: Matrix Crystal AW54 14.00 1.60 0.80 0.00 0.00 83.30 16.70 AW55 15.60 1.50 1.00 0.00 0.00 81.60 18.40 AW56 11.60 1.10 1.10 0.00 0.30 81.60 18.40 AW57 14.00 1.10 0.50 0.00 0.00 84.30 15.70 AW58 9.60 1.80 0.60 0.00 0.00 87.80 12.20 AW59 9.10 1.50 1.00 0.00 0.00 82.80 17.20

Percentage (mineral assemblage)

Sample Plag Opx Fe-Ti Hbl Q:z: AW54 85.37 9.76 4.88 0.00 0.00 AW55 86.19 8.29 5.52 0.00 0.00 AW56 82.27 7.80 7.80 0.00 2.13 AW57 89.74 7.05 3.21 0.00 0.00 AW58 80.00 15.00 5.00 0.00 0.00 AW59 88.30 12.93 8.62 0.00 0.00

WAWAQUARRY

Percentage (whole rock) Sample Plag Opx Fe-Ti Hbl Q:z: Matrix Crystal AW104 8.30 2.30 1.10 0.10 0.00 88.00 12.00 AW105 9.00 2.00 1.00 0.00 0.00 88.00 12.00 AW106 7.80 1.30 1.00 0.00 0.00 89.80 10.20 AW107 8.60 1.10 0.80 0.00 0.00 89.30 10.70 AW108 5.80 1.30 1.10 0.00 0.00 91.60 8.40 AW109 11.60 0.60 1.30 0.00 0.00 86.30 13.70 AW110 10.30 1.80 1.60 0.00 0.00 86.10 13.90 AW112 11.30 2.30 2.30 0.00 0.00 84.00 16.00 AW113 9.60 1.30 0.60 0.00 0.00 88.30 11.70 AW114 10.80 1.80 1.60 0.00 0.00 85.60 14.40 AW115 10.80 1.50 2.00 0.00 0.00 85.60 14.40

Percentage (mineral assembalge) Sample Plag Opx Fe-Ti Hbl Qz AW104 70.34 19.49 9.32 0.85 0.00 AW105 75.00 16.67 8.33 0.00 0.00 AW106 77.23 12.87 9.90 0.00 0.00 AW107 81.90 10.48 7.62 0.00 0.00 AW108 70.73 15.85 13.41 0.00 0.00 AW109 85.93 4.44 9.63 0.00 0.00 AW110 75.18 13.14 11.68 0.00 0.00 AW112 71.07 14.47 14.47 0.00 0.00 AW113 83.48 11.30 5.22 0.00 0.00 AW114 76.06 12.68 11.27 0.00 0.00 AW115 75.52 10.49 13.99 0.00 0.00

153

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SAMPLE LOCALITY MAPS

Scale 1: 50 000

2 4 6 8 10 (km)

Lithologies Symbols

Trig 8566 Rhyolite -- Road

Waiotapu Ignimbrite 0 Drillhole

Ngapouri Rhyolite + Sample locality

Unit X x Waiotapu Ig. float block

Rahopaka Ignimbrite Spot height (m)

Unknown Ignimbrite

L Orotui

1

.. Wawa Quarry

T16. U16

MAP ONE: DISTRIBUTION OF WAIOTAPU IGNIMBRITE WITHIN TAUPO VOLCANIC ZONE.

o 2

Scale 1: 154 000

4 6 (km)

8

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# o ~

10

Map Two

l. Rotorua

c

/

Map Three

L.Okarr-~

5

l. Okataina

l. Tarawera

U16

U17

Volcanic geology and geochemistry of Waiotapu Ignimbrite ...

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