Geochemistry of Proterozoic granulites from northern Prince Charles Mountains, East Antarctica

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Anfarcfic Science 4 (I): 59-69 (1992)

Geochemistry of Proterozoic granulites from northern Prince Charles Mountains, East Antarctica N.C. MUNKSGAARD', D.E. THOST and B.J. HENSEN*

'School of Earth Sciences, Macquarie University, New South Wales 2109, Australia 2Department of Applied Geology, Universiry of New South Wales, Kensington, New South Wales 2033, Australia

Abstract: The late Proterozoic basement of the Porthos Range northern Prince Charles Mountains, east Antarctica, is dominated by a suite of felsic to mafic granulites derived from igneous and, less importantly, sedimentary protoliths. Compositionally, they are broadly similar to granulites occurring along the Mac. Robertson Land coast and southern Prince Charles Mountains. Ultramafic to mafic orthopyroxene + clinopyroxene granulites with relict igneous layering occur as lenses within the felsic to mafic granulites, and show compositional evidence of a cumulate origin. The felsic to mafic granulites are intruded by several large charnockite bodies that have similarities to the Mawson Charnockite, and may have formed via a two-stage partial melting process. The charnockite and host granulites are chemically very similar, and both may have been derived from a common middle to lower crustal source region. Undepleted K/Rb ratios suggest retention of original chemistry, with variations being due to fractionation processes. Normalized trace element patterns resembling modern-day arc settings suggest that the Porthos Range granulites were possibly generated in a subduction zone environment.

Received 10 December 1990, accepted 5 August 1991

Key words: Antarctica, charnockites, felsic to mafic granulites, geochemistry, Proterozoic

Introduction

This paper presents a geochemical reconnaissance study of predominantly granulite facies rocks from the Porthos Range, northern Prince Charles Mountains (NPCM), located at approx. 70" 30's between 64" 30E and 66" 30'E (Fig. 1). The Prince Charles Mountains constitute a well-exposed cross section through the East Antarctic metamorphic shield which is composed of a number of Archaean cores preserved among larger areas of younger, mainly Proterozoic, metamorphic rocks. The East Antarctic Shield has been the subject of several recent studies on the origin and thermal and baric evolution of granulite terranes (e.g. Harley 1988, Harley & Hensen 1990, Thost et al. 1991). This study forms part of a project which aims to elucidate the pressure-temperature evolution of the granulite facies rocks of the NPCM, and to establish an absolute time framework for the thermal and tectonic events.

Previous geological investigations

During 1954-56 the Australian National Antarctic Research Expeditions made the first overland journeys to the Prince Charles Mountains, investigating the geology and glaciology

(Lower Crustal Processes). This paper is a contribution to IGCP Project 304

of the region (Crohn 1959). McLeod (1964) later refined this work. Regional mapping of the northern and southern Prince Charles Mountains was undertaken by the Australian Bureau of Mineral Resources during the 1969-74 summer field seasons, the results ofwhich have beensummarized by Tingey (1972, 1982). Associated regional geochronological studies have been published by Arriens (1975). McKelvey & Stephenson(l990) conducted detailed fieldworkintheRadok Lake area, south-east of the Porthos Range, in 1988, sampling two 1 km traverses in the Proterozoicgneisses. They broadly identified two lithological associations, subjected to at least three major deformational episodes. The following outline is from Tingey (1982).

In the SPCM, Archaean granitic basement rocks (approximately 2800 Ma; Rb-Sr whole-rock isochrons) are unconformably overlain by metasedimentary rocks, both displaying amphibolite to lower amphibolite facies assemblages. Locally these metasediments are cross-cut by late Archaean to early Proterozoic granites and pegmatites. One pegmatite has yielded a Rb-Srmuscovite age of 2580Ma. All Archaean rock units are intruded by amphibolitized basic dykes of probable middle Proterozoic age.

In the NPCM a major metamorphic episode, between 800 and 1100 Ma ago (Rb-Sr whole-rock isochrons), produced widespread amphibolite to granulite facies metamorphicrocks of predominantly felsic composition, with minor metasedimentary and metabasic rocks. Syntectonic bodies of intrusive charnockite (orthopyroxene2garnet-bearing granite) occur throughout the NPCM. Greenschist to amphibolite

59

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60 N.C. MUNKSGAARD ef a/.

PEGMATITES

DEFORMATION (D6, regional; D7, small-scale flexures and shear zones)

LEUCOGNEISS I1

DEFORMATION (DS)

MAFIC DYKES (2 generations)

CHARNOCKITE (approx. 960 Ma?)

LEUCOGNEISS I

DEFORMATION (DI-D4)

Undifferentiated gneiss 'tR? ;s. ChsrnOckite (chk) 18, "* 25

Lsucogneiss (Igl

/ Dykes: b - basaltic a s - alkali meia-syenite I a

E

2 I.

- : 2 0 V

P

zl m

.- d ... .- - 8

MT GAVAGHAN MT I

MARTIN MS.

Fig. 1. Geological sketch map of the Porthos and part of the Athos ranges, northern Prince Charles Mountains (NPCM). Inset shows area of the main map. Numbers indicate sample locations and refer to geochemical analyses in Table 11.

facies metamorphism in the southern Prince Charles Mountains is probably the marginal manifestation of the late Proterozoic metamorphic episode in the NPCM.

Granitic stocks and pegmatites were intruded into both the southern and northern Prince Charles Mountains about 500 Ma ago (Rb-Sr mica ages). A few alkaline basic intrusive rocks with a wide range of Phanerozoic ages intersect the basement. Sheraton (1983) studied the chemistry of mafic dykes in the NPCM.

Geology of Porthos Range

Accounts of the interpretation of the geology of the Porthos and Aramis ranges based on field workin the 1988-89Austral summer are given by Fitzsimons & Thost (1992). ThePorthosRangeconsistsofisolatednunataks and massifs

(locally with extensive moraine fields) covering up to 25 km2, separated by areas of blue ice. Access is restricted by the development of extensive wind scours, generally on the western (windward) facing side of outcrops. The main lithological unit in the Porthos Range is composed of upper amphibolite to granulite facies, felsic to mafic granulites displaying a complex structural and thermal history with several discrete phases of deformation. Lenses of ultramafic to maficgranulites preserve early fabrics. The felsic to mafic granulites have been intruded by at least two generations of relatively undeformed metabasic dykes. Field observations show that the granulites have been subjected to a number of partial-melting events resulting in the formation of intrusive leucogneiss bodies.

Intrusive bodies of charnockite are common in the Porthos Range. These have all been affected to varying degrees by deformation events resulting in foliation development. Post- peak metamorphic activity includes further deformation (mylonites, pseudotachylites) and the intrusion of pegmatites

Table I. Schematic outline of geological events in the Porthos Range, after Fitzsimons & Thost (1992). Youngest event at the top.

QTZ VEINS ALKALlNE MAFIC DYKES

and alkaline basaltic dykes. Table I outlines the sequence of geological events in the Porthos Range.

The lithologies sampled for geochemical analyses are briefly described below:

Granulites

Felsic granulites composed predominantly of quartz + K-feldspar t plagioclase, and containing various amounts of orthopyroxene and/or hornblende, biotite and rarely, garnet are the most common rocks in the Porthos Range. Mafic granulites (essentially two-pyroxene t plagioclase 2 quartz 2 biotite) and amphibolites occur within the felsic granulites as discrete concordant layers and boudinaged pods, and as layers with much morediffuse boundaries. The relationship between felsic, intermediate and mafic rock typesvary fromintimately layered to massive outcrops with little lithological variation.



PROTEROZOIC GRANULITES FROM PRINCE CHARLES MOUNTAINS 61

Ultramafic to mafic orthopyroxene t clinopyroxene granulites

Ultramafic tomafic orthopyroxene + clinopyroxene granulites occur as large (up to 10 m) boudinaged layers within the granulites at Crohn Massif and Mount Kirkby, and as smaller pods at Cony Massif, and possibly represent original dykes or sills. They are well layered on a centimetre scale with orthopyroxene crystals up to 15 cm in length. Their layering, although displaying a metamorphic overprint, may be of igneous origin. At Mount Creighton and Corry Massif, an unusual association occurs. Blocks of ultramafic granulites, preserving early fabrics, are suspended in a matrix of coarse- grained pegmatites, which are foliated only at their margins, and occur as lensoid bodieswithinwell-foliated felsic granulite.

Pelitic gneisses

Although pelitic and semi-pelitic migmatitic gneisses are common at Carter Peaks in the Athos Range, these lithologies are very rare in the Porthos Range. One occurrence is a highly deformed inclusion within a coarse-grained garnet leucogneiss from Corry Massif, and Tingey (1981) reported calc-silicate and pelitic gneiss from the western end of Mount McCarthy. The pelitic gneiss from the Carter Peaks area is medium- to coarse-grained with large (up to 3 cm) garnet porphyroblasts and fresh cordierite in a quartz t K-feldspar rich matrix. Siilimanite constitutes less than 5% of the rock mass, and occurs as 5-10mmlong crystals in the matrix, and as rare (and much smaller) inclusions along with cordierite and quartz in the garnet. Textures vary from well-developed gneissic layering to migmatitic.

Charnockite

The charnockite bodies of the Porthos Range contain large simply twinned K-feldsparphenocrysts, as well as plagioclase and blue quartz lenticules, with minor orthopyroxene +- hornblende 2 biotite and, less commonly, garnet. Texturally the charnockite ranges from almost undeformed to highly foliated. The least deformed examples (e.g. at Crohn Massif) arevery similar to charnockite in theFramnesMountains near Mawson Base, approximately 300 km to the north (Mawson Charnockite, Tingey 1972). At CrohnMassif the charnockite- granulite contact isclearly intrusive, whereas at other localities the contact with the host rock is concordant and gradational. In these cases the original contact appears to have been modified by intense deformation.

Leucogneiss

Leucocratic gneissic bodies with parallel to discordant boundaries intrude the granulite, pelitic gneiss and charnockite. There are at least three temporally distinct generations of leucogneiss. At Mount Kirkby, a leucogneiss layer is isoclinally

folded within felsic gneiss, and two generations of leucogneiss intrude charnockite at Mount McCarthy (Fitzsimons & Thost 1992). All are quartz + K-feldspar-rich and have variable amounts of garnet and minor biotite. They have a medium- to coarse-grained texture, and large bodies (e.g. those at Cony Massif) contain schlieren and locally, blocks of felsic gneiss and pelitic gneiss.

Mafic dykes

At least two generations of metamorphosed mafic dykes occur in the Porthos Range. They are fine- to medium-grained amphibolites, generally less than one metre in width. They posess avariable fabric: in low strainzones, they clearly cross- cut the foliation of their host gneiss and are themselves unfoliated to weakly foliated and may exhibit minor folds, whereas, in zones of high strain, they are drawn out parallel to the host gneiss foliation. In both cases, the dykes are generally dismemberedboudinaged.

Unmetamorphosed dykes were encountered at two localities. Clinopyroxene t olivine-phyric alkali olivine basalt dykes occur at Mount Kirkby and a K-rich alkaline dyke was discovered at northern Webster Peaks (Fig. 1).

Geochemistry

Twenty-nine whole-rock samples were analysed for their major- and trace-element compositions. Whole-rock and trace element analyses were determinedby X-ray fluorescence techniquesusing a Siemens SRS-1 spectrometer at Macquarie University following the method of Norrish & Hutton (1969). Sample locations are shown on Fig. 1. Chemical analyses are presented in Table I1 and Harker diagrams in Fig. 2.

The areally dominant granulites (including mafic to intermediate compositions) form well defined variation trends in some Harker diagrams (e.g. A1,0,, MgO, CaO) whereas in others, mainly those for trace elements, no clear trends are discernable (e.g. Zr, Y). One sample (NM05) of a relatively undeformed but metamorphosed mafic dyke, is compositionally similar to the mafic members of the granulites in both major and trace elements.

The pelitic gneiss samples form a compositional grouping distinct from the granulites in most variation diagrams (e.g. FeO*, Na,O, K,O, Cr) and show relatively little scatter.

Without exception the charnockite samples have elemental compositions very similar to those of the silica-rich members of the granulites (SO,> 52 wt%). However, the charnockite samples show considerably less compositional scatter.

The two analysed samples of leucogneiss have high silica contents (73-76 wt.%), and for most elements are of similar composition to the high-silica end of the granulites. An exception is sample NM52 which has substantially higher Na,O and lower q0 content than the silica-rich granulites. In thin section the primary K-feldspar in NM52 is to a large extent replaced by coarse and irregular patches of albite,



m ru Tabk II. Whole-rock compositions of major and trace elements for samples from the Porthos Range. A Felsic to mafic granulites B Charnockite C Dykes (a: metamorphosed dyke, Mount Tam. b alkali mela- syenite dyke, Webster Peaks. c basalt dyke, Mount Kirkby) D Mafic granulites E Pelitic gneisses F: Leucogneiss.

A B C

Sampleno. NM06

Analysisno. 1

SiO, 5630 TiO, 0.57

17.08 1.66

FCO 7.14 MnO 0.15 MgO 4.05 CaO 5.62

3.68 2.87 0.36 0.37 0.06 0.26

Total 100.17

40, %O,

Na20 qo P A 40+ 40- COZ

Trace elements, ppm

Ba 2406 cr 61 cu 27 Ga 13 Nb 3 Ni 9 Pb 23 Rb 66 SI 374 Th 1 U 0 V 188 Y 17 zcl 65 n 19

WRb 361

NM07

2

57.97 1.59

15.57 3.95 6.98 0.18 2.58 5.48 3.38 1.51 0.52 0.45 0.07 0.13

100.36

1510 53 32 21 24 6

20 33

291 5 2

152 38

124 288

380

NM14

3

52.99 1.05

16.63 2.47 7.23 0.15 6.02 8.52 2.94 1.13 0.28 0.52 0.10 0.22

100.25

1256 172 23 18 9 46 13 30

304 2 1

167 28 96

101

313

NM3 1

4

69.57 0.57

14.85 1.27 2.60 0.07 1.23 1.84 2.66 4.31 0.07 0.45 0.04 0.05

99.58

869 62 8

19 13 25 40

192 171 18 3

69 18 77

149

186

NM03A

5

67.71 0.38

15.21 2.28 2.05 0.06 0.95 3.22 2.52 4.91 0.19 0.29 0.07 0.04

99.88

2497 7

19 13 2 5

24 203 344

3 2

40 7

37 49

201

DT040D NMO2A

6 7

64.13 68.68 0.75 0.41

16.91 14.90 0.87 1.18 2.07 3.12 0.01 0.05 2.37 0.33 3.77 2.23 3.51 2.55 3.48 5.78 0.33 0.18 0.83 0.40 0.12 0.04 0.28 0.08

99.43 99.93

1874 1945 55 12 46 10 20 23 10 15 15 3 30 51

152 189 533 179

4 49 1 0

107 15 16 45 31 69

252 302

210 254

NMOl

8

44.33 0.28

15.41 4.81 6.68 0.18

12.88 11.93 1.33 0.72 0.07 1.57 0.07 0.15

100.41

158 209 93 9 1

108 4

33 224

3 0

143 6

68 8

181

NM03B

9

46.69 0.50

15.29 4.23 6.50 0.17

10.54 1295 1.21 0.74 0.10 0.97 0.12 0.12

100.13

97 527 78 12 1

202 4

22 162

2 0

299 10 65 13

279

NM17 NM27 NM28 DT013 DTU56

13 14 10 11 12

61.75 66.25 67.09 0.98 0.74 0.77

16.50 14.60 14.15 1.90 1.95 2.07 5.09 3.07 2.88 0.11 0.07 0.09 1.63 1.34 1.25 4.85 3.89 3.66 3.46 2.75 2.62 2.81 3.85 3.92 0.31 0.23 0.24 0.33 0.45 0.50 0.10 0.08 0.08 0.12 0.47 0.23

68.68 0.58

15.16 1.10 3.02 0.07 0.95 3.10 2.65 3.31 0.12 0.39 0.1 1 0.13

63.81 1.03

14.28 2.26 4.62 0.12 1.87 4.15 3.05 3.45 0.30 0.35 0.10 0.01

99.94 99.74 99.55 99.31 99.46

2726 38 22 21 16 8

27 59

321 1 0

90 23 78

250

2324 17 15 16 11 3

26 98

359 7 0

66 28 57

242

2204 1913 1454 20 34 34

20 20 13 17 19 15 11 18 12

3 13 5 24 31 24

107 103 85 349 210 265

8 22 0 1 0 1

75 60 106 27 43 40

42 72 53 285 241 238

395 326 304 267 337

NM05 NMll DTU70

a b C

47.65 51.87 49.05 1.42 2.51 1.72

15.12 12.16 14.67 4.03 4.20 1.65 9.61 3.20 6.87 0.22 0.12 0.14 7.07 7.14 6.74 9.47 5.01 7.45 2.87 0.92 3.25 0.50 9.99 3.60 0.19 0.68 0.48 2 1.08 1.41 2.10 0 0.12 0.16 0.30 c 0.37 0.44 1.86

E : 99.72 99.81 99.88

n 0

310 4706 1181 % 104 234 180 c 78 30 22 18 22 19 3 101 62

57 153 94 10 9 9 13 303 128

159 571 602 4 17 10 0 1 0

248 119 138 38 16 22 63 81 85 63 515 252

319 274 233



Table II continued

D F E

NM49 21 62.83 0.93

16.31 5.51 4.70 0.14 3.57 1.08 0.95 3.15 0.05 0.60 0.13 0.41

Sampleno. NM24 Analysisno. 15 SiO, 47.46 TiO, 0.20

Fe,O, 5.25 A1203 8.77

FeO 6.22 MnO 0.20 MgO 19.35 CaO 8.87

1.04 0.29 0.04 1.32 0.05 0.34

N%O 50 PZO, YO+ Yo- co2

DTlO8 16 51.67 0.31

11.07 2.91 6.78 0.21

10.06 11.73 2.49 1.27 0.27 0.69 0.07 0.32

DTWB 17 51.94 0.10 6.11 2.80 6.62 0.18

19.87 10.42 0.80 0.31 0.01 0.57 0.05 0.08

DTo4oC 18 55.04 0.20 6.99 2.24 8.74 0.20

18.57 4.17 1.33 0.89 0.00 0.59 0.1 1 0.41

NM44 20 74.19 0.88

11.24 5.09 1.47 0.12 1.82 2.33 1.58 0.52 0.05 0.30 0.07 0.09

DT055B 22 66.25 1.02

15.03 2.14 6.55 0.12 3.00 1.13 1.44 2.19 0.03 0.68 0.08 0.20

NMlO 19 68.18 1.01

13.10 5.48 3.50 0.11 2.47 2.07 1.50 2.22 0.02 0.46 0.05 0.12

NM41 NM18 DTO52B 23 24 25 64.68 80.20 73.15 0.96 0.20 0.08

15.43 10.12 14.41 7.91 0.62 0.25 1.66 1.11 0.36 0.13 0.17 0.02 3.16 0.68 0.11 1.48 124 1 .00 1.28 2.96 2.44 2.53 1.73 7.18 0.07 0.01 0.04 0.53 0.28 0.34 0.10 0.06 0.10 1.01 0.21 0.11

NM52 26 75.93 T 0.03 R

0.06 0.25 0.02

D 2.29 4.67 R

0.00 0.27 si rn cn 0.06 0.08 n n

0 99.64 x

14.02 2 E

1.91 P F

rn I]

0.05 :

100.29 99.75 100.36 Total 99.40 99.85 99.86 99.48 99.86 100.93 99.59 99.59

Trace elements, ppm

865 1718

29 10 5

365 16 76

231 11 2

208 16 78 33

262 2638

12 11 6

290 9

54 127

3 2

137 20

106 28

622 140 17 15 11 80 19 78

172 7 1

304 44 91

21 1

122 797 114 209

9 11 13 18 10 11 17 10 11 34 21 101

141 129 15 16 2 0

135 178 36 50 65 116

193 189

586 248

19 18 15 58 19

107 133 13 1

158 44

100 208

96 1 991 195 6 50 1 15 9 9 2

83 3 19 11 79 25

185 146 12 3 0 1

186 5 35 3

112 28 154 94

98 1 1305 3 8 0 4

11 15 1 1 6 5

89 19 192 61 114 532 15 7 0 1 4 6 7 2 7 4

15 92

Ba 45 Cr 2578 c u 16 Ga 8 Nb 3 Ni 429 Pb 4 Rb 10 Sr 35 Th 3 U 2 V 149 Y 5 Zn 79 n 38

8 3017

14 6 2

421 4 6

28 1 1

129 4

54 14

WRb 241 139 429 137 236 206 259 1 70 231 574 310 260

8



64

L A

N.C. MUNKSGAARD eta/.

t 2

a 18

b - . C -

+. nd a . 4

. + + DDf7,. np- . A

I 1 I " 1

14

* I203

10

- 0.6

- p205

- 0.2

6

0 -

TiO,

. I I I I

FeO'

MgO

. B g

. .. a C

0

* b + 0

0

+ 1 . I

I I I

0

a ' + .

o f 7 -. .

C

ti O

0

0 Fig

I I I I

+ I + 20

40 60 80

SiO,

~~ . . +

a + + .

I I I I ' 0

B

Fig. 2. Harker diagram plots for samples from the Porthos Range. Major elements in wt % and trace elements in ppm. The letters a, b and c refer to dykes NM05, N M l l and DT070 respectively.



PROTEROZOIC GRANULITES FROM PRINCE CHARLES MOUNTAINS

0 1

65

+ . I , I t 1 I

0 I

b 1000

+ $ +

. 0 . 8. A

A . ' a M. 0

i 10

. 0

M. 0

I I I I

10000 I

1000

Cr 100

10

1

4000

Ba 2000

+ + +

a

I I I I

b '

400

Rb 200

b .

. C .

- "A

0 40 60

SiO,

80

. a * t 300 0

t 2oo

100

'' . b . a

a .

El

0%

. A 0 0

. a . 'b

E l 0 * + + I I I

b '

. A. C ' A B A A

0 0

40 60 80

SiO,

600

Sr

200

0

40 Y

20

0

400

Zr

200

0

Key to symbols: A Charnockite; . Dyke; 0 Granulite; ULeucogneiss; t Mafic granulite; 0 Pelitic gneiss



66 N.C. MUNKSGAARD et a/.

K (%)

1 10 100 1000

Rb (PPm)

Fig. 3. Plot of K (wt %) against Rb (ppm). Lines of constant K/ Rb ratio are indicated, MT: Main Trend of Shaw (1968) for continental granitoids. Symbols as for Fig. 2.

suggesting possible loss of K and gain of Na during post- magmatic alteration. The geochemistry of the leucogneiss is consistent with the field evidence reported by Fitzsimons & Thost (1992) which indicates that several generations of intrusive leucogneiss bodies were derived from the high-&O silica-rich members of the granulites by partial melting.

The four samples of utramafic-mafic orthopyroxene t clinopyroxenegranulite form a compositionally distinct group from the basic members of the granulite group for certain elements (i.e. lower TiO, and Al,O,, higher MgO, Ni, Cr).

The compositionof sample DT070, a lateunmetamorphosed mafic alkaline dyke from Mount Kirkby, corresponds closely to the analysis published by Sheraton (1983) for the same dyke. Sample NMll from northern Webster Peaks is an unmetamorphosed K-rich peralkaline dyke (lamproite) of similar composition to dykes from Mount Bayliss in the southern Prince Charles Mountains and Priestly Peak in Enderby Land (Sheraton & England 1980). Relative to the Mount Bayliss dyke however, NMll has much higher A1,0,, but lower TiO,, P,O,, Sr and Zr.

KIRb ratios

WRb ratiosforthePorthos Range(Fig. 3)rangefrom 137-574 with an averagevalue of 277. Thesevalues are similar to other rocks from Mac. Robertson Land (Sheraton & Black 1983), which also have low values (< 500). This contrasts with the depleted Archaean Napier Complex gneisses which exhibit very highWbratios(occasional1y > 2000) and have apparently undergone loss of Rb relative to K during high grade metamorphisddeformation (Sheraton & Black 1983). The Porthos Range granulites have “undepleted” geochemical characteristics, with the possible exception of Th, similar to the “Main Trend” of Shaw (1968), and show no evidence for loss of K or Rb. Thus the simplest interpretation of the data is that the granulites appear to have retained their original chemistry, with variations being mainly due to igneous fractionation processes.

L

100 =

10 :

P b R b Ba Th K N b L a Ce Sr P Zr TI Y _ _ ~ _ _ _ _ _ _ _ _ _ _ _ _ - ~

b I 100

P b Rb Ba Th K N b La Ce Sr P Zr Ti Y ___- _ _

t P b Rb Ba Th K N b La Ce Sr P Zr T i Y

Fig. 4. RocMprimordial mantle normalized plots (values from Sheraton & Black 1988, as modified from Sun 1980). a. Porthos Range felsic granulites (stippled area) compared with average of 20 orthopyroxene + quartz t feldspar gneisses from the Rayner Complex (+): data from Sheraton & Black (1983). La and Ce data for Porthos Range from four unpublished analyses (J.W. Sheraton, personal communication

b. Porthos Range mafic granulites (stippled area) compared with average mafic granulites from the southern PCM (hornblende + clinopyroxene t orthopyroxene + plagioclase; O), average of five mafic granulites from the NPCM and Mac. Robertson Land (hornblende t clinopyroxene t orthopyroxene + plagioclase; o), and average of eight amphibolites from the southern Prince Charles Mountains (m). Data from Sheraton (1984). Porthos Range data includes two mafic members of the granulites (NMO1 and NM03B) and excludes extreme high and low values. c. Average of five Porthos Range charnockites (A) compared with average Mawson charnockite (+, Sheraton & Black, 1988) and average of six Archaean Proclamation Island Granite gneisses, Napier Complex (0, Sheraton & Black 1988).

1990).




Discussion

The analytical results introduce certain constraints on the origin of the various major lithological units of probable igneousorigin. Herewe focus attentionon thefelsicgranulites, the ultramafic-mafic orthopyroxene-clinopyroxene granulite, and the charnockite.

Granulites

Normative corundum contents average 0.94 wt.% and molar Al,O,/CaO + Na,O + &O ratios average 1.02 for the silica- richmembers of thegranulites. These geochemical parameters suggest that the granulites are predominantly orthogneiss (I-type: Chappell &White 1974).

The range of basic to acidic compositions within the granulites shows continous covariation in many Harker diagrams (Fig. 2). Some of the covariance may be attributable to igneous fractionation processes during partial melting of pre-existing crustal rocks and/or fractional crystallization. For example, the “boomerang”shaped trends of increasing Sr and Ba to approximately 60-65 wt% SiO, where calcic plagioclase is dominant, followed by decreasing contents in the more silica-rich samples where Na- and/or K- feldspar becomes more abundant. Positively correlated Rb and SiO,, even to high SiO, values of 75 wt%, are also characteristic of igneous fractionation processes. In contrast, metasediments often show decreasing Rb with increasing silica contents (e.g. Wyborn & Chappelll983, Munksgaard 1988).

Primordial-mantle normalized trace element patterns (Sun etal. 1979, Wood etal. 1979) are shown in Fig. 4 for the felsic andmaficgranulitesfrom the PorthosRange, and are compared with other similar rock types from East Antarctica.

The PorthosRangefelsicgranulites(Fig. 4a)exhibit patterns that are broadly similar to mineralogically equivalent rocks from the Rayner Complex, but show relative enrichment in Ba, and to a lesser extent, P and Ti, and relative depletion in Th and Zr.

The Porthos Range felsic granulites have a signature that broadly resembles those of modern day arc calc-alkaline magmas, if such characteristics can realistically be applied to high grade metamorphic Proterozoic rocks. Both show a strong negative Nb anomaly (also typical of continental crustal material, i.e. granitoids and sediments: Sun 1980, Tarney et al, 1982, Weaver & Tarney 1981,1983, Thompson et al. 1983, Pearce 1983, as quoted in Jahn 1990) and a less pronounced negative anomaly for Th, possibly a result of metamorphic Th depletion, with positive Ba and K.

Ultramafic to mafic orthopyroxene + clinopyroxene granulites

The ultramafic to mafic orthopyroxene + clinopyroxene granulite boudins show some compositional characteristics commonly found in igneous cumulates. Whole rock Mg#

(atomic Mg/(Mg+ZFe)) are high, averaging0.74 compared to 0.58 for the mafic members of the granulites. Orthopyroxene and clinopyroxene microprobe analyses on samples from the boudins showMg# up to 0.77 and 0.85, respectively (N.C.M. unpublished data). Also, Cr whole-rock contents are very high (1700-3000ppm)asare Ni contents(290-430ppm), compared with 100-500 ppm Cr and 50-200 ppm Ni for the mafic members of the granulites. Early removal of Cr and Ni in magnesian pyroxenes from a magma body undergoing fractional crystallization would account for these geochemical differences.

The mafic granulites (Fig. 4b) are variably enriched in Rb, Ba and Th, relative to similar rocks from elsewhere in Mac. Robertson Land. As with the felsicgranulites, a marked negative Nb anomaly is apparent, a distinctive feature of subduction-related magmas. Notable too are low values of P, Zr, Y and particularly Ti, relative to other Mac. Robertson Land mafic rocks. Low TiO, values, relative Nb depletion, and K-enrichment in the mafic granulites are also comparable to arc basalts (e.g. Jahn 1990). The possibility that these patterns are the result of crustal contamination of mantle derived magmas cannot be excluded (Wilson 1989, p. 21).

Charnockites.

A comparison with the geochemical data for the charnockites of the Mac. Robertson Land Coast (Sheraton & Black 1988, Sheraton, unpublished data) shows that in terms of major and trace element compositions the Porthos Range charnockites are broadly similar, although Na,O, Sr, and Ba are higher, and TiO, and Zr are slightly lower in the Porthos Range charnockites (Fig 4c).

Compositionally the Porthos Range charnockites are similar to the silica-rich members of the granulites with respect to all elements analysed. Considering the trace element characteristics as potential fingerprints of source-rock compositions, we suggest that the broad compositional similarity of the silica-rich members of the granulites and the charnockite may indicate that they have both been derived from the same middle to lower crustal source region in which water availability was variable. Both the charnockite and granulites show only moderate negative Sr anomalies when compared with more evolved granitoids (Figs. 4a,c), suggesting that the amount of residual plagioclase was not as great as for more typical upper crustal granitoids.

The source of the charnockitic magmas was presumably less siliceous (i.e. intermediate) particularly if it had already undergone an earlier melting phase (J.W. Sheraton, personal communication 1990). Melting of the presently exposed granulites appears to have been only minor, and no field evidence has been found for in-situ formation of the charnockites.

Sheraton (1982) argued that the Mac. Robertson Land charnockites were derived by two-stage partial melting of granulite facies quartzo-feIdspathic gneiss similar to those


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68 N.C. MUNKSGAARD eta/.

found in the NPCM and along the Mac. Robertson Land coast. Initial melting under "wet" amphibolite facies conditions would produce graniticliquids, whereas further melting of the quartz-syenite residues under "dry" granulite facies conditions would produce the potassic charnockite melts. The intrusive relationship of the charnockites with respect to the felsic granulites of the Porthos Range is consistent with such a model.

Sr isotope data for the charnockites and the felsic granulites (authors' unpublished data) are consistent with a common source region for both these rock types. 87SrP6Sr initial ratios range from 0.7069-0.7095 and 0.7048-0.7095 for samples of the charnockites and the silica-rich members of the granulites, respectively.

Preliminary E Sr/E Nd ratios that form well defined mixing trends between mantle and continental crust values, as well as whole-rock Sm/Nd model ages ranging from 1600 to 1300 Ma (N.C.M., unpublished data) are consistent with the conclusion that older crustal material may have been involved in the genesis of the igneous protoliths.

Conclusion

The basement rocks of the Porthos Range are dominated by a suite of felsic to maficgranulites, derived predominantly from igneousprecursors, with sedimentary input being subordinate and obvious only in the Athos Range to the immediate north. These felsic to mafic granulites host a number of other rock types, and are intruded by charnockite plutons.

Our interptretation of the geochemical results, suggests that the igneous rocks which constitute the protolith to the Porthos Range granulites may have been produced in a subduction zone environment (e.g. Jahn 1990). K/Rb ratios suggest possible retention of original chemistry, and normalized trace element patterns are similar to those found in modern-day arc settings.

Acknowledgements

We wish to thank the officers and expeditioners involved in the 1988-89 Australian National Antarctic Research Expedition for excellent support in the field as well as financial assistance through an ASAC grant. This research project is also supported by the Australian Research Council and a Macquarie University Research Grant. D.E.T. acknowledges the support of an Australian Postgraduate Research Award. We are very grateful to Drs. J.W. Sheraton, S.L. Harley, I.C.W. Fitzsimons and Professor D.R. Hunter for thorough and constructive reviews.

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Geochemistry of Proterozoic granulites from northern Prince Charles Mountains, East Antarctica

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