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StructureswithinlargevolumerhyolitelavaflowsoftheDevonianComerongVolcanics,southeasternAustralia,andthe...

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DOI:10.1016/0377-0273(92)90113-R

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Journal 21" Volcanology and Geothermal Research, 54 ( 1992 ) 33-51 33 Elsevier Science Publishers B.V., Amsterdam

Structures within large volume rhyolite lava flows of the Devonian Comerong Volcanics, southeastern Australia, and the

Pleistocene Ngongotaha lava dome, New Zealand

Kelsie A. Dadd Department of Applied Geology, University of Technology, Broadway, P.O. Box 123. Sydney, N.S. 14 ~ 2007, Australia

(Received March 3, 1992; revised version accepted May 18, 1992 )

ABSTRACT

Dadd, K.A., 1992. Structures within large volume rhyolite lava flows of the Devonian Comerong Volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome, New Zealand. J. Volcanol. Geotherm. Res., 54:33-51.

Many rhyolitic units within the Late Devonian Comerong Volcanics erupted as lava flows and domes, some up to 18 km long and 350 m thick. The textural and structural characteristics that distinguish the flows and domes as lava rather than rheomorphic ignimbrite include unbroken phenocrysts, zones of autobrecciation, and finely developed flow layering with individual layers continuous for several metres. The flow layering is typically contorted into isoclinal folds with forms suggesting fluidal deformation and is interlayered with and gradational into restricted zones of pumice-rich lapilli tuff and zones of lenticulite breccia. The lenticulite breccia comprises discontinuous, lenticular rhyolite fragments, the long axes of which define a foliation parallel to flow layering. Lenticles in the breccia vary from elongate layers up to 1 m long and several millimetres thick to short fragments less than 10 cm long and several cenlimetres wide. Similar zones of lenticulite breccia consisting of glassy lenticular clasts in a devitrified, spherulitic "'matrix" of cristobalite and albite, exist within the Late Pleistocene Ngongotaha dome near Rotorua, New Zealand. The lenticulite breccia is considered to form by aqueous diffusion and selective devitrification of the rhyolite along anastomosing fluid paths and to be modified by mechanical fracturing of the lava in a zone of high shear stress.

Geochemically the rhyolites are high-Si and A-type, with high Zr and Y contents indicating that they formed from high- temperature, relatively anhydrous, F-rich melts. A-type granitoids crop out intermittently along the length of. and adjacent to the volcanic complex and are comagmatic with the rhyolite. The high temperature, low bubble and phenocryst content, and a high eruptive rate of the rhyolite, likely resulted in a low effective viscosity and extensive flow units.

Introduction

The Late Devonian Comerong Volcanics (Fig. 1 ) provide an excellent opportunity to study the evolution o fa bimodal volcanic field (Dadd, 1992). The volcanic and sedimentary rocks that constitute the Comerong Volcanics crop out as two thin belts approximately 1 km wide on the eastern and western limbs of the Budawang Synclinorium, a structural subdivi-

Correspondence to: K.A. Dadd, Depar tment of Applied Geology, Universi ty of Technology, Broadway, P.O. Box 123, Sydney, N.S.W. 2007, Australia.

sion of the Lachlan Fold Belt in southeastern Australia. Within this structure the sequence is exposed from its base, an unconformity with Ordovician metasedimentary rocks (Powell, 1983), to its top, where it is interbedded with fluvial siliciclastic sedimentary rocks of the overlying Merrimbula Group.

Two thick units of rhyolite of enigmatic ori- gin crop out on the eastern limb of the syncli- norium (Fig. 2). Measured sections through the rhyolite units are as thick as 350 m, and outcrop can be traced for over 50 km along strike (Dadd, 1989). The lateral extent of sin- gle flows is unknown due to the discontinuous

0377-0273/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

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Fig. 1. Location of the Comerong Volcanics, southeastern Australia, and the area of detailed study. Points labelled 1-3 indicate the position of columns in Fig. 2.

nature of the outcrop, but thickness variations along strike and variations in structure within the flows susgest that the units consist of sev- eral overlapping flows, some as long as t 8 kin. Estimating volumes for individual flows or for the package as a whole is difficult, because the east-west dimensions of flow units cannot be measured.

The Comerong rhyolites have the distribu- tion and aspect ratio of an ash flow tuff but show features typical of siticic lava flows (Dadd, 1986). The origin of lava-like silicic units of great areal extent is controversial (e.g., Ekren et al., 1984; Twist and Harmer, 1986; Bonnichsen and Kauffman, 1987; Henry et al., 1988, 1989); such units are interpreted as

RHYOLITE LAVA FLOWS OF THE COMERONG VOLCAN1CS AND THE NGONGOTAHA LAVA DOME 35

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either ash flow tufts that have undergone ex- treme rheomorphism or hot and unusually low- viscosity lava flows.

Rheomorphic ignimbrites are deposited as pyroclastic flows and undergo mass flowage either directly after deposition (Schmincke and Swanson, 1967; Noble, 1968; Walker and Swanson, 1968; Chapin and Lowell, 1979) or in the final stages of deposition (Hoover, 1964; Ragan and Sheridan, 1972; Wolff and Wright, 1981 ) due to their high temperature, low vis- cosity and deposition on steep slopes. As a re- sult of this secondary flowage, the ignimbrite develops characteristics similar to those of a silicic lava flow and may be virtually indistin- guishable. The original pyroclastic textures of a rheomorphic tuff may be preserved within the basal poorly welded to non-welded zone (Ek-

ren et al., 1984). In cases of extreme rheo- morphism however, when the temperature of ejecta is high enough to allow it to coalesce and flow, even the basal zone may be modified to a lava-like structure and be indistinguishable from a cohesive flow.

Similarly, extensive silicic lava flows may result from extremely high eruption tempera- tures leading to flows of low viscosity and causing the structure and texture of the lava flow to diverge from that of a high aspect ratio flow. An unequivocal set of parameters to dis- tinguish tufts modified by extreme rheo- morphism from extensive lava flows has not yet been developed from modern well-exposed ex- amples; the origin of such rhyolite units in an- cient sequences will even more often be ambiguous.

310 L .A I) a, D D

Structural and textural features of the exten- sive Comerong rhyolites that suggest a lava- flow origin include contorted and planar flow layering, unbroken phenocrysts and few (typ- ically < 1%) lithic fragments. Sections of the rhyolite units, however, have a clastic struc- ture defined by elongate lenticular fragments that resemble the flattened pumice clasts of a rheomorphic ignimbrite and cause some con- fusion in assigning an origin to the rhyolite units. Similar breccia, comprising lenticular glassy rhyolite fragments in a devitrified ma- trix, is found in the silicic Ngongotaha lava dome near Rotorua, New Zealand. The brec- cia is interlayered with and grades into zones of flow-layered rhyolite. The lenticular frag- ments resemble flattened pumice but occur within the lava dome. This lenticulite breccia structure, previously undocumented in silicic lava flows, provides an analogue for the brec- cia within the Comerong rhyolite.

In this paper I describe the large rhyolite units of the Comerong Volcanics, including their meso- and microscale textural and struc- tural characteristics, and suggest two possible origins for the formation of the lenticulite breccia. The rhyolite units are interpreted as several extensive lava flows and smaller flows and domes based on the lack of pyroclastic tex- tures. The bases of flows however, are not well exposed and so their origin is equivocal.

Rhyolite lavas of the Comerong Volcanics: meso- and microstructure

The rhyolite lava flows of the Comerong Volcanics exhibit a range of structural types in which variation in field and microstructural characteristics are inferred to be the result of several physical parameters, including the magma viscosity, volatile content and degree of devitrification and crystallization.

Massive structural type

The massive structural type consists of sparsely porphyritic rhyolite with approxi-

mately 5% phenocrysts of quartz, albite, minor alkali feldspar and altered mica in a ground- mass with poikilocrystic or "snowflake" tex- ture (Anderson, 1969; Lofgren, 1971). Quartz and feldspar phenocrysts are typically unbro- ken, and many are well rounded through re- sorption. The poikilocrystic texture comprises spherical, very fine-grained, optically continu- ous quartz patches with inclusions of alkali feldspar, surrounded by darker-coloured areas of albite and orthoclase. Poikilocrysts are up to 1 mm in diameter and have a mottled texture, as the included mineral grains are irregular in shape.

Flow-layered structural type

This is the most widespread structural type, and many flows consist of a thick sequence of flow-layered rhyolite with scattered brecciated zones. Flow layering is defined by colour (Fig. 3), with lighter layers having larger poikilo- crysts than darker layers, and by a parallel pre- ferred parting plane in the outcrop. Also par- allel to the flow layering are fine, l mm wide, quartz veinlets and irregular to lenticular areas of fine intergrown needles of feldspar and quartz in a poikilocrystic groundmass. Indi- vidual layers range from 1 mm to several cen- timetres in width, averaging 1-2 mm.

Flow layering varies from planar and con- stant in orientation to open folds or complexly deformed structures, commonly with refolded isoclinal folds suggesting fluidal deformation. The fold axes in some areas show little consis- tency, and zones of complex folding are sand- wiched between zones of planar layering. In- dividual layers, only millimetres thick, are in places continuous for several metres.

Lenticulite breccia

Lenticulite breccia, in which discontinuous, lenticular fragments define a foliation, is al- ways spatially associated with the flow-layered structure. Lenticles vary from elongate layers

RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME 3 7

Fig. 3. Rhyolite outcrop in the Comerong Volcanics showing flow banding defined by a colour variation in outcrop. View taken through water.

Fig. 4. Rhyolite outcrop in the Comerong Volcanics showing long thin lenticles intermediate in structure between flow banding and stubby lenticles.

Fig. 5. In situ lenticulite breccia in the Comerong Volcanics with stubby, black lenticles which stand out as prominent features on the outcrop surface.

Fig. 6. Photomicrograph of elongate lenticles. The lenticles have an axiolitic rim surrounding an intergrowth of coarse- grained quartz and feldspar. The surrounding "matrix" consists of micropoikilitic quartz grains with a flow-banded struc- ture parallel to the elongate lenticles. Under crossed polarizers. Bar scale: 1 ram.

RHYOLITE LAVA FLOWS OF THE COMERONG VOLCAN1CS AND THE NGONGOTAHA LAVA DOME 39

up to 1 m long and several millimetres thick (Fig. 4 ) to short fragments less than 10 cm long and several centimetres thick (Fig. 5 ).

There are two broad groups of lenticles, elongate and stubby. Both groups contain up to 5% euhedral to rounded quartz and feldspar phenocrysts, as in the groundmass. The stubby lenticles are darker and more resistant than the surrounding rhyolite, and are defined by areas of large micropoikilitic quartz grains. Elongate lenticles are commonly recessive features and consist of a rim of axiolitic spherulites with a central region of coarse-grained quartz and feldspar and large spherulites. Many of the len- ticles are surrounded by a dark halo (Fig. 6), consisting presumably of material excluded during spherulite crystallization as shown by the experiments of Lofgren ( 1971 ). These len- ticles typically are more altered, as indicated by the presence of white mica and chlorite, than the surrounding rhyolite. The coarse-grained intergrowth of quartz and feldspar within the lenticles suggests a decrease in volume during devitrification of the glassy rhyolite, creating a

low-pressure area and promoting the growth of vapour-phase minerals at these sites.

Brecciated structural type

Brecciation, a common feature of all other structural types, ranges from closely spaced fractures where there has been little movement of the clasts to zones of angular rhyolite clasts in a jumbled block arrangement. A common structure is a "jigsaw puzzle" arrangement of clasts with rhyolite of a similar colour and tex- ture as the cementing material. This structure may result from the autobrecciation of a par- tially solidified or devitrified rhyolite lava.

The zones of brecciation are both concor- dant and discordant with layering in the out- crop and vary from microscale zones to areas of several square metres. Contacts between the brecciated and unbrecciated rhyolite vary from sharp to gradational. The breccia cement is typically similar in composition to the clasts but in some cases consists of silica (Fig. 7). Brecciated zones occur throughout the flow

Fig. 7. Rhyolite breccia in the Comerong Volcanics. Angular clasts of rhyolite in a "jigsaw puzzle" arrangement with a siliceous cement. View taken through water.

4f~ ~ • DAI)I>

units and are not preferentially concentrated at any one level in the flow.

Pumice-rich lapilli tuff

The pumice-rich lapilli tuff is associated with the flow-layered structure, and contacts be- tween the two are often irregular. The lapilli tuff has a restricted distribution, occurring in only two adjacent sections. Layers of the tuff have a maximum thickness of approximately 50 m. Clasts within the tuffrange from several millimetres to 15 cm in diameter, averaging 2- 3 cm, are irregular in shape, randomly distrib- uted and constitute up to 50% of the outcrop. The clasts are amygdaloidal and are possibly pumice. A few clasts contain spherulites and were once glassy. Pumice is often associated with rhyolite lava flows (Bristow and Duncan, 1983; Fink, 1983; Fink and Manley, 1987; Heiken and Wohletz, 1987 ), either as precur- sor pumice eruptions or as frothy vesicular lay- ers within the flow. The individual pumice clasts within the tuff are more typical of a near vent air-fall pumice deposit.

Basal section of lava flows

The basal part of each lava flow is covered or poorly exposed with a thick cover of lichen and moss. At one place (Fig. 8), the contact is covered for 2.5 m between the top of the un- derlying basalt unit and the rhyolite. In the basal metre of exposed rhyolite the unit is brecciated and contains vesicular basalt frag- ments and patches and angular clasts of jasper. These accidental clasts may have been incor- porated into the flow from the palaeo-ground surface. One metre above the exposed base, the rhyolite has a poorly developed foliation de- fined by an alignment of sparse feldspar, laths. A faint, wavy colour layering is visible on some surfaces. The unit grades vertically into flow- layered rhyolite.

In the second section, the rhyolite is exposed for 5 m above the contact with a bedded silt-

stone sequence. The basal section of the rhyo- lite comprises a 5-m-thick, polymictic breccia with a non-porphyritic rhyolite matrix. Clasts in the breccia include rhyolite, similar to the matrix, as well as siltstone, basalt, and banded porphyritic rhyolite. Clasts are mostly angular except those close in composition to the ma- trix, which have diffuse edges. They are in a close-packed arrangement with clasts up to 30 cm in width. The next 3 m of outcrop consists of a matrix-supported breccia in which the clasts are almost entirely porphyritic rhyolite and have diffuse contacts with the matrix. The unit grades upward into an intermingling of porphyritic rhyolite and the non-porphyritic rhyolite that formed the matrix to the breccia.

Extent of individual lava flows

Vertical continuity in mapped sections is good, but the lateral extent of individual flows is difficult to establish due to the uniform tex- ture and phenocryst content of the rhyolite and to breaks in outcrop along strike. No flows can be physically traced between measured sec- tions, which are typically separated by several kilometres of poor outcrop. Abrupt structural and lithological changes occur between some measured sections. Rhyolite flows tend to be steep sided, so many of these changes are likely depositional features. However in some cases, the boundaries between different lithological units correlate with Landsat and air photo li- neaments and are probably fault controlled.

The chemical composition of the rhyolite, proved unreliable as a tool for distinguishing separate flows. The scatter of elemental abun- dances, due largely to alteration, now masks any primary differences in flow units.

Much of the rhyolite outcrop occupies the same stratigraphic position in adjacent sec- tions (Fig. 2 ) and can be divided into two main eruptive episodes, the last being most volumi- nous. The upper unit can be correlated for ap- proximately 50 km along strike, 18 km of which is so similar in texture and flow layering ori-

RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME 41

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Auto-brecciated bands with no systematic orientation; BST/ open to isoclinal folds.

BX / Light-coloured lenticles and lithic clasts up to 2 cm; BST [ clasts decrease in abundance upward.

LB ~ , Large black lenticles up to 5 x 1 cm.

~ Spherulitic light-coloured lenticles up to 10 x 3 cm; rounded 'pumiceous' fragments. Covered section.

BST Alternating buff and purple bands; very open folds; some lighter coloured bands are spherulitic and lenticular in shape.

J B l a c k up to x cm; no grading. lenticles 20 2

LB J O c c a s i o n a l 'pumiceous-looking' clasts up to 3 cm. BX LB Black lenticles; av. 3 x 0.5 cm.

Light coloured lenticles up to 20 x 1.5 cm; normally graded.

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BST

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A streaky, indented lineation and alignment of phenocrysts; planar.

Colurrmar jointing perpendicular to flow banding.

A streaky, indented lineation and alignment of phenocrysts; planar.

Faint colour banding; folds with wavelengths 5 to 10 cm.

Indented parting planes; grades from underlying wispy structure.

Dark-coloured wisps and streaks.

Bands <1 to 3 cm wide.

BX Polymictic breccia with rhyolite and vesicular basalt fragments.

Fig. 8. S t ra t igraphic c o l u m n showing the u p p e r rhyol i te uni t on the eas te rn l imb o f the B udawang Sync l inor ium. The c o l u m n i l lustrates the r e l a t ionsh ip b e t w e e n c o n t i n u o u s f l o w - b a n d e d rhyol i te and in situ lent icul i te breccia . BST= b a n d e d s t ructura l type; BX= brecc i a t ed s t ructura l type; LB = lent icul i te breccia; M S T = mass ive s t ructural type.

42 K.L I)ADD

entation that it may represent a single, large rhyolite flow. Rhyoli te lava flows of this scale occur elsewhere, for example, the Plateau Rhy- olite o f the Yellowstone caldera (Wyoming, U.S.A.; Christiansen and Blank, 1972), where some flows are as much as 300 m thick and ex- tend for 20 km from their source.

The origin of some larger silicic volcanic units is a matter of debate due to the similari- ties of rhyolite lava flows with rheomorphic ig- nimbrites. In ancient volcanic provinces the distinction between rheomorphic ignimbrites

and large volume lavas could be especially un- clear due to the effects of devitrification, re- crystallization and tectonic disruption of the units (cf. Allen, 1988). The large extent of the Comerong rhyolites and the enigmatic lenti- culite breccia they contain are features similar to rheomorphic ignimbrites. Rheomorphic ig- nimbrites do occur elsewhere in the Comerong Volcanics and show significant differences to the lava flows in the study area, including rec- ognizable fiamme, abundant lithic clasts (Fig. 9a), lithic-rich horizons and a discontinuous

~ - ~ ~ - _ . _ . ~ _ _

| _ l _ _ _ _ - - :-I

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Fig. 9. (a) Foliation developed in rheomorphic ignimbrite in the Comerong Votcanics, showing flattened pumice and lithic ctasts (solid black). Bar scale is 5 cm. (b) Lineation developed on foliation surface of outcrop in (a), defined by elongate, flattened pumice. Note the discon- tinuous nature of the lineation. Bar scale is 5 cm. (c) Lineation developed on flow-banding surface in a rhyolite lava flow. Note the continuous nature of the lineation and absence of lithic clasts.

RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME 43

lineation developed on the foliation plane (Fig. 9b). In contrast, the rhyolite lavas have con- t inuous flow layering, an interlayered and gra- dational relationship between the lenticulite breccia and flow-layered rhyolite, few lithics and a more continuous lineation developed on the flow layering surface (Fig. 9c).

Many rheomorphic ignimbrites in the Trans- Pecos, Texas, U.S.A. (Henry et al., 1988 ) have a poorly welded to non-welded base in which pumice or ash textures are still visible. Above this zone the unit grades into more lava-like features. No ash-rich basal zones have been re- cognised in the Comerong rhyolites; howe'~er, such bases may be covered or obscured due to devitrification.

Controls on the viscosity of the Comerong rhyolites

The Comerong rhyolite lavas are predomi- nantly alkalic and high SiO2. Major elements show a broad range of abundances likely due to alteration. Y, Zr and Nb abundances are high with values intermediate to those of peralkalic (Mahood, 1981 ) and calc-alkalic (Ewart et al., 1968; Hildreth, 1979) rhyolite. They are most likely comagmatic with A-type, alkalic granites suggesting that the melt was high temperature (possibly greater than 830 ° C ), anhydrous and had a high F content (Collins et al., 1982; Cle- mens et al., 1986; Whalen et al., 1987). A high F content leads to a low viscosity (Dingwell et al., 1985 ), particularly in high-silica melts and thus may have been an important control on viscosity in the high-Si Comerong rhyolites.

Both an increase in bubble content and an increase in the abundance and size of crystals in a melt lead to an decrease in fluidity (Mc- Birney and Murase, 1984). The Comerong rhyolites have a low crystal content and small crystals. The lack of lithophysal cavities and vesicular horizons in the flows, except for the rare pumice-rich lapilli tuff, suggests the magma may also have had a low bubble content.

Comparison with modern lava flows: Ngongotaha lava dome and flow, New Zealand

The structures developed in the rhyolite units of the Comerong Volcanics are similar to those in the Pleistocene Ngongotaha lava dome, near Rotorua, New Zealand (Fig. 10). The lava dome, situated on the edge of Rotorua Cald- era, is approximately 4 km in diameter. It is older than the 50,000 yr B.P. rise of the lake level and younger than 140,000 yr B.P. (B. Houghton, pers. commun. , 1988). The flow- layered character and local brecciated areas are excellently exposed in a quarry on the north- eastern face of the dome.

Flow-layered structures

Flow layering in the Ngongotaha lava dome is defined by colour variation and by a change from obsidian to light-coloured devitrified rhyolite consisting of cristobalite and albite. Layers are continuous for several metres, are commonly folded and have wispy, discontin- uous and interfingering contacts. Elongate lithophysae and layers rich in lithophysae oc- cur parallel to the colour layering. Some litho- physae, however, are wrapped by the foliation, so that they developed late in the formation of the layering while the rhyolite could still be- have plastically.

Brecciated structures

The flow layering in the lava dome is brec- ciated in several areas. These include a brec- ciated carapace; internal areas discordant with the flow layering in the dome, in which large blocks of the rhyolite are in a jumbled arrange- ment, and areas of brecciation, concordant with the trend of layering in the dome, in which fragments are lenticular and lie within the plane parallel to flow layering. In this paper this breccia type is called lenticulite breccia.

The first two breccia types are well known in lava flows and domes. In the lenticulite brec-

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~k Huka Group Haparaagi Rhyolite Mamaku Ignimbrite Haparangi Rhyolite

Geological f boundary

Fault "J /

Volcanic centre and / caldera margin ~ /

km 0 5 r "' IIII " I IIJ ]

Fig. 10. Location of the Ngongotaha lava dome and flow near Rotorua, New Zealand. Simplified geology taken from Thompson (1974).

cia, lenticular to wispy segments of obsidian are contained within a light-coloured, devitrified rhyolite that constitutes a "matrix". The obsi- dian clasts have both feathered and planar ends (Figs. 11 and 12). They range from massive with sparse spherulites and perlitic cracking to clasts "veined" with devitrified rhyolite and having a wispy texture. The lenticular clasts range from 1 mm to approximately 2 cm thick and are up to 10 cm long in the samples studied.

The flow-layered and lenticulite breccia structures occur together within the Ngongo- taha dome, grading vertically one from the other in a similar style to that developed in the Comerong lava flows.

Microscale features

The lenticular, obsidian clasts are typically flow layered, and many exhibit microfolding with a fluidal appearance (Fig. 13a). Pheno- crysts make up less than 5% of all structural types in the Ngongotaha lava dome and are typically unbroken. Fragmental phenocrysts are more abundant in areas with a lenticulite breccia structure, occurring in trains concen- trated at the boundary between breccia and continuous flow layering (Fig. 13b and c).

Devitrification of the rhyolite commenced parallel to layering and in apparently random spherical structures. Although large devitrified

RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME 45

2-1 Fig. 11. A zone of in situ lenticulite breccia in the Ngon- gotaha Dome, bounded by flow-banded rhyolite and with gradational contacts. The contact between breccia and flow-banded rhyolite is indicated by a dashed line. Indi- vidual glassy clasts in the breccia are highlighted in solid black. Bar scale is 5 cm.

patches in some samples have developed in- dependent of, and obscuring, the flow layering, the devitrification more typically enhances the layered and lenticulite breccia structure.

Discussion

Many clastic structures in rhyolite lava flows and domes result from a continual change in the rheology of the moving mass. As part of the rhyolite devitrifies sufficiently to become brit- tle it behaves differently from the remaining ductile rhyolite. With continued movement, the brittle component reaches its yield point and fractures to form breccia. At any stage an increase in temperature, possibly caused by thermal feedback within the flow (Nelson, 1981 ), can change the behaviour of the lava mass and the breccia can be re-cemented. At the top and sides of the flow, where cooling is more rapid than the interior, a brecciated car- apace develops. Many other breccia structures are produced by: ( 1 ) contact with groundwa- ter and subsequent water movement through the lava; and (2) water expansion on heating, especially that trapped in pores (Heiken and Wohletz, 1987).

Within the interior of the moving lava mass during laminar flow, shear produces a folia-

Fig. 12. Black lenticular clasts in the Ngongotaha lenticulite breccia are glassy and contain large light-coloured spherulites. The surrounding white-coloured rhyolite is totally devitrified. The black glassy clasts resemble flattened pumice.

46 ~,.a, 1)4I ~i)

Fig. 13. Photomicrographs of lenticulite breccia from the Ngongotaha dome. Bar scales: 1 mm.(a) Clasts have both planar, banded structures and folded flow banding (plane-polarized light). (b) and (c). A train of broken phenocrysts occurs along the contact of the lenticulite breccia domain (lower part of photo) and a cohesive flow-banded domain. The phenocrysts in the flow-banded domain are unbroken. (b) Plane-polar- ized light. (c) Under crossed polarizers.

RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME 47

Fig. 13. (Continued).

tion. This foliation is typically de fned by col- our variation and enhanced by devitrification structures. The formation oflenticulite breccia involves sections of flow layering within this portion of the flow.

Textural changes accompanying devitrification

The flow-layered rhyolite and fragments in the lenticulite breccia both show microscale colour layering, folding of the flow layering and a similar percentage of phenocrysts, suggesting they form initially by the same processes. The few spherulites found in the obsidian layers and fragments of the Ngongotaha samples are larger than those in the devitrified "matrix"; thus a completely devitrified lava of similar compo- sition should contain areas or layers of more coarse-grained devitrification textures.

Similarly, in the Comerong lavas, the micro- poikilitic quartz grains and spherulites in the lenticulite breccia fragments are larger than

those in the surrounding devitrified rhyolite, suggesting by analogy that the fragments re- mained glassy while the surrounding rhyolite devitrified. After some cooling of the rhyolite mass, the glassy lenticles devitrified at a lower temperature (Lofgren, 1971). The textural differences between the stubby and elongate lenticles in the Comerong rhyolite therefore probably reflect variation in the rate and tem- perature of devitrification, volume changes ac- companying devitrification, and post-devitri- fication alteration.

Features similar to these lenticles are de- scribed as early formed, planar lithophysal cavities by Hausback (1987) in a Miocene rhyodacite lava flow in Baja California, Mex- ico. The cavities were shear planes active in the last stages of laminar flow and were later filled with vapour-phase minerals. The lenticular zones in the Comerong flows, however, con- tain isolated phenocrysts, a feature inconsist- ent with a lithophysal origin.

The role of water

Lenticulite breccia in the Ngongotaha dome is interlayered with flow-layered rhyolite. These layers may develop different textures due to water and other volatile movement within the lava mass, which would reduce the viscos- ity and shear strength of the lava. Water may diffuse from a lava flow or dome on cooling and decompression (Heiken and Wohletz, 1987 ). The water would be at higher pressure than the atmosphere and may cause fracturing and explosion of the lava mass (Heiken and Wohletz, 1987).

Two cases of diffusion that Heiken and Wohletz ( 1987 ) consider are molecular trans- port of one material through another, and mix- ing of lava with groundwater in the vent dur- ing extrusion. Apart from the obvious role of water pressure in producing a brecciated struc- ture, the first of these processes may be impor- tant in the development of flow-layered and

1. Development of cracks in the rhyolite due to shear during flow. Volatiles begin to move through the rhyolite.

2. With continued [ ' ~ "~ 1 volatile movement ~ -~ ~" ~ ~ along inter- -~ "~ "~'~" ~ connecting fractures,

I "~ -~ ~, the rhyolite [--~ "-~ ~, ~ selectively

devitrifies.

3. The pattern of devitrified zones

~ (volatile movement paths) and

~ undevitrified glassy I~ ,~""~- '1 ~ zones, produces a

"pseudobreccia" texture.

Fig. 14. Anastomosing volatile movement paths produc- ing a "pseudobreccia" texture.

lenticulite breccia structure, by migration of magmatic volatiles from the lava to fracture planes, vesicles and lithophysae (Heiken and Wohletz, 1987) accompanied by chemical al- teration of the medium. Lofgren ( 1971 ) sug- gested that a rhyolitic glass devitrifies most rapidly in the presence of alkali-rich solutions and that these solutions might occur locally within a cooling rhyolitic magma. Movement of such fluids in a glassy rhyolite mass, along fractures parallel to the flow layering, may lead to accelerated alteration and devitrification of selected layers and the development of a "pseudobreccia structure" (cf. Allen, 1988) by isolating lenticles of fresh obsidian where fluid paths anastomose (Fig. 14 ).

Models for the formation of lenticulite breccia

The lenticulite breccia is interlayered with continuously flow-layered rhyolite within the non-pumiceous section of both large and small rhyolite lava flows and domes. Two possible models for the formation of the lenticulite breccia are outlined. The breccia may have formed solely by one mechanism or by a com- bination of the two.

( 1 ) Brecciation of flow layering may have formed by drawing out and breaking of plastic glassy layers with low shear strength, due to an increase in the shear stresses at particular lev-

A B C

Fig. 15. The stretching and brecciation of flow-banded rhyolite in a plastic state to produce the lenticular seg- ments of the lenticulite breccia structure. (A) Banding is developed due to shear in the moving rhyolite flow. Bands are both glassy and partially devitrified leading to differ- ences in rheological behaviour. (B) Stretching of bands during flow produces a boudinage structure while the rhy- olite behaves in a plastic manner.(C) The more glassy bands with a lower yield strength than the devitrified bands break to form lenticular segments.

RHYOLITE LAVA FLOWS OF THE COMERONG VOLCANICS AND THE NGONGOTAHA LAVA DOME 49

els in the rhyolite mass (Fig. 15 ). This process is similar to the development of shear zones, as they are both planar zones of concentrated deformation that help to accommodate a local strain rate the country rock cannot accommo- date by bulk deformation (White et al., 1980). The shear zone develops parallel to the major heterogeneity in the rock, that is, the flow lay- ering, which itself is produced by shear during flow of the lava mass. Obsidian zones or layers deform plastically and with a low shear strength during flow until they reach their yield point and fracture. Clasts in the lenticulite breccia have both high- and low-angle terminations relative to flow layering and suggest a range in the ductility & t h e clasts prior to failure (White et al., 1980).

The development of isolated zones of lenti- culite breccia or shear zones within the rhyo- lite mass may be due to an increase in water content or water movement in these zones. The presence of a pore fluid can both reduce the strength of a rock (Fyfe et al., 1978) and in- crease the rate of devitrification, thus creating a heterogeneity and promoting the develop- ment of a shear layer. Alternatively, the brec- cia horizon may develop because of an in- creased strain rate due to pulsing flow rate or some heterogeneity in the flow of the lava.

Features of the lenticulite breccia that sup- port a brecciation model include sharp termi- nations of clasts (Fig. 13a), rotation of clasts within the breccia zones, the termination of clast fabric at the edges of breccia zones (Fig. 13a), and the concentration of broken pheno- crysts within the breccia zones indicating movement . The "trains" of phenocryst frag- ments at the contact of the breccia zones and cohesive rhyolite (Fig. 13b) suggest that the contact is the site of a high strain rate.

(2) The second model for the development of lenticulite breccia is the formation of a "pseudobreccia" by a process of aqueous dif- fusion and selective devitrification. Clasts in this model are not formed by mechanical frac- turing of the lava. Features that support this

process, as outlined above and in Figure 14, in- clude the juxtaposition of clasts with similar internal fabrics, and the gradation of devitrifi- cation within clasts in the Ngongotaha lenti- culite breccia from glassy, through partially devitrified to totally devitrified "matrix". The gradation may reflect the extent of aqueous diffusion and acceleration of devitrification processes in the glass. Following the develop- ment of a lenticulite breccia horizon by this second process, the zone could be modified by shear, as it would act as a more heterogeneous layer than the flow foliation and hence concen- trate strain.

Conclusions

The Comerong rhyolites erupted as lava with several large flow units as well as smaller dome- like bodies and flows. A typical section through a flow unit consists of continuously flow-lay- ered rhyolite with planar flow layering inter- layered with lenticulite breccia and rare pum- ice-rich lapilli tuff. The layering in a few localities is contorted into fluidal flow folds and cut by auto-brecciated zones.

The lenticulite breccia consists of clasts of rhyolitic composit ion similar to the host rhyo- lite but differing in devitrification texture and degree of alteration. They may have formed by aqueous diffusion and selective devitrification of the rhyolite with anastomosing fluid paths and have been modified by mechanical frac- turing in a zone of high shear stress.

Lenticulite breccia and flattened pumice lenticles could be easily confused in devitrified rhyolite, but the Comerong rhyolite lavas lack other features diagnostic of ignimbrites. The similarity of the lenticulite breccia in the Com- erong rhyolites with those in the Ngongotaha lava dome indicates a lava-flow origin.

The Comerong rhyolites typically have a low crystal content, very few lithic clasts and few lithophysal cavities. Their distinctive A-type chemistry and high temperatures, possibly ac- companied by the rapid extrusion of the rhyo-

lite, led to a low effective viscosity of the magma and to large eruptive units.

Acknowledgements

This research was supported by an Austra- lian Commonwealth Postgraduate Research Award. I would like to thank R.H. Flood, C. Henry, D.A. Swanson and an anonymous re- viewer for critical appraisal of the manuscript. The time and effort given by C. Henry and J. Wolff in assistance with many aspects of this paper is greatly appreciated.

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