Gondwana Research, V. 4, No. 3, pp. 337-357. 0 2001 International Association for Gondwana Research, Japan. ISSN: 1342-937X

Polymetamorphism in the Schirmacher Hills Granulites, East Antarctica: Implications for Tectonothermal Reworking of an Isobarically Cooled Deep Continental Crust

Somnath Dasguptal, Sudipta Senguptal, Santanu Bosel, Masato Fukuoka2 and Sreemati Dasguptal

Department of Geological Sciences, Jadavpur University, Calcutta - 700 032, India Department of Earth and Planetary Sciences, Hiroshima University, Higashi-Hiroshima, Japan

(Manuscript received March 18,2000; accepted January 30,2001)

Abstract

The Precambrian basement of the Schirmacher Hills, Queen Maud Land, East Antarctica has evolved through multiple episodes of deformation and metamorphism. The rocks have suffered at least five phases of deformation. The imprint of the early deformation, D,, is preserved in some mafic isolated enclaves. The second and the third deformations (D, and D,)arc the dominant deformations of this area and produced isoclinal folds with transposition of earlier cleavages. The later deformations, D, and D,, produced two sets of open, upright folds. Detailed mineralogical, textural, mineral chemical studies and geothermobarometry on khondalite, leptynite as well as different varieties of enderbite and mafic granulites have revealed that the rocks suffered two phases of metamorphism under granulite facies conditions followed by an amphibolite facies overprint. M, is broadly coeval with D, only in mafic granulite enclaves within enderbitic gneiss, and took placc at ca. 10 Kbar, 900" C. The mafic magma, parental to the enclaves, probably crystallized at 11.2 Kbar. Following post-peak near isobaric cooling, the mafic granulites were transported to shallower levels by the enderbitic magma. M,, recorded in all the lithologies, occurred at ca. 8 Kbar, 800 - 850" C and synchronous with D,. Post peak M, evolution of the rocks was characterized by a pressure - temperature drop of 2 Kbar and 200C respectively and textures indicative of both cooling and decompression are preserved in different rocks. The relative timing of the two, however, cannot be worked out. M,, synchronous with D,, took place at 6 Kbar, 600 - 650C and evolved hydrous fluid flux. Correlation with available structural and geochronological data shows that both M, and M, could be of Grenvillian event. M, could well be Pan-African age.

Key words: Antarctica, Schirmacher hills, granulite facies, polymetamorphism, cooling.

Introduction

Granulite facies rocks provide powerful constraints on thermotectonic evolution of ancient mid- to lower continental crust through interpretation of their pressure- temperature-time paths of evolution (Harley, 1989). Such rocks occurring in East Antarctica, particularly in the Enderby Land, Mac Robertson Land, Lutzow Holm Bay, Larsemann Hills and Prydz Bay became the focus of intensive petrological and isotopic studies over the last two decades. As a result of these studies, several tectonometamorphic age provinces could be delineated, which range in age from early Archaean to Cambrian. These form the basis of intercontinental correlation , delineation of the loci of major orogenic belts and development of models of supercontinent cycle involving

Antarctica (Moores, 1991; Dalziel, 1991; Hoffman, 1991; Unrug, 1992; Yoshida, 1995). The metamorphic history of the Queen Maud Land (Fig. l ) , which is supposed to be contiguous with the Natal and Mozambique Belts (Unrug, 1995), is relatively less well known, although polymetamorphism under both granulite and amphibolite facies conditions have been reported. However, the Schirmacher Hills (Fig. 1) is one of the well studied area in the Queen Maud Land where a number workers carried out studies since 1964 (Ravich and Kamenev, 1975; Grew 1983; Stackebrandt et al., 1988; Paech and Stackebrandt, 1995). In recent years the structural history of the Schirmacher Hills has been studied in details by Sengupta (1986,1988,1993,1994) and Sengupta and Bose (1997). In this work, we present petrological data on the rocks of the Schirmacher Hills in the Queen Maud Land (Fig. l ) ,

338 S. DASGUPTA ET AL.

/

Africa

: r (

c- - ,5 x x

x x x

r E a s t -- :/

Pan-African (WO m.y I

(some parts reworkedin 500Ma) Archaean and Polaeoproterozoic cratons and mobile belts

a trenvillian ~ 1100 Ma

L R L e P Fig. 1. Gondwanaland reconstruction showing the position of Queen

Maud Land (after Grantham et al., 1988).

and document granulite and amphibolite facies overprints on an isobarically cooled granulite facies metamorphosed lower continental crust. We correlate polyphase metamorphism with the multiple phases of deformation and igneous activities.

Geological Background of the Schirmacher Hills

The Precambrian basement of the Schirmacher Hills has evolved through a complex geological history of

multiple episodes of deformation, metamorphism and migmatization with emplacement of granitic and mafic bodies of different ages. Moreover, the fabrics of rocks have undergone several stages of modification due to multiple stages of folding, shearing and development of successive generations of foliation under different metamorphic conditions.

In the absence of stratigraphic markers the lithological units are differentiated on the basis of compositional differences and mesoscopic fabrics (Sengupta, 1986, 1988). In recent years detailed mapping has been carried out in the Schirmacher Hills in the scale of 1:25,000 and the earlier map ( Sengupta, 1988 )was modified (Bose, 1999). A lithological map of the Schirmacher Hills is given in figure 2. The major units are : (1) different types (streaky and augen) of granite gneiss, (2) leptynite , ( 3 ) garnet-biotite gneiss, (4) calc gneiss, khondalites and associated migmatites, (5) enderbitic gneiss with bands of mafic granulite and (6) small mafic-ultramafic granulite enclaves in (Fig. 2). Granitic gneisses are largely restricted to the western part, while the banded gneiss along with different types of granulites constitute the eastern part of the area (Fig. 2). The contact between these two units is a layer-parallel shear zone with a thrusting sense of movement (Sengupta and Bose, 1997). A thick unit of interbanded calc gneiss, khondalite, mafic granulite and enderbite occurs in the central part.

In the banded gneisses the individual mafic bands, varying in width from a few centimetres to a few metres, have both sharp well-defined and gradational contacts with the enderbites. The rocks are foliated and the foliation is traced out by the pyroxene and plagioclase grains.

There are several isolated ultramafic and mafic enclaves within the banded gneiss (Sengupta, 1993). The size of the enclaves varies from a few centimetres to a few tens of metres. These enclaves show a prominent compositional

LITHOLOGICAL MAP OF THE SCH I R MACHE R HILLS, EAST ANTARCTIC

p 5 p COpoMeters

I Mafic-uhmafic enclave mi MaM (Indian Station1 /' Limit of exposure

a Leptynite Mybnitic gneiss

Fig. 2. Lithological map of the Schirmacher Hills, East Antarctica.

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TECTONOTHERMAL HISTORY OF SCHIRMACHER GRANULITES, EAST ANTARCTICA 339

banding with a well-developed foliation parallel to it (Figs. 3 a , 3b, also Sengupta, 1993, Figs. 2 and 3). The different bands are composed of layers of lherzolite, websterite, pyroxenite, metagabbro and anorthosite. These bands represent remnants of a layered igneous complex. The large ultramafic enclaves are often fractured and invaded by later pegmatites. The pegmatite is very coarse-grained with a poorly developed foliation and contains mostly plagioclase feldspar with some quartz. The ultramafic rock

Fig. 3a. Banded ultramafic rocks occur as enclaves within the banded gneiss. The foliation within the enclave is at high angle with the surrounding granulite gneiss.

Fig. 3b. Emplacement of hypersthene bearing pegmatite within the pyroxene granulite.

in these places contains coarse unstrained needles of hornblende.

Enderbite constitutes nearly 30% of the banded gneiss. The majority of these rocks contain garnet. In general, two types of enderbites can be identified from the mesoscopic character of the rocks (however, see later). The banded enderbite is a well-foliated, medium grained rock that occurs interlayered with mafic granulites (Fig. 3a). The pegmatitic enderbite, on the other hand, is a

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340 S. DASGUPTA ET AL.

very coarse grained rock and occurs as diffused patches cutting across mafic granulites. A crude foliation can be traced within these pegmatitic enderbites, which runs parallel to the foliation of the associated granulites. Since the rock cuts across the D, foliation but itself shows the same foliation it must have been emplaced at some later stage of D, deformation.

Leptynite (referred to as alaskite in our earlier papers) occurs mostly within the eastern part of the Schirmacher Hills as long narrow bands within the banded gneiss (Fig. 2). The rocks often show a foliation marked by thin translucent flattened grains of quartz.

Garnet-biotite gneiss occupying the central part of the Schirmacher Hills (Fig. 2) is characterized by the presence of large elongate dark clots of mafic minerals separated by quartzofeldspathic materials. It is a coarse grained rock that contains mainly quartz-plagioclase-K-feldspar-garnet- biotite. In thin section, the rock shows banding of alternate quartz and feldspar. Partial replacement of garnet by biotite is common.

Although calc gneiss, khondalites and mafic granulites occur in isolated bands within the banded gneiss and granite gneiss, they mainly occupy the central part of Schirmacher Hills interlayered with thick veins of granite gneiss (Fig. 2). Calc gneiss occupies nearly 25% of this central zone of interlayered granulite and granites. In most places the bands of calc gneiss are extensively sheared and are invaded by quartzofeldspathic materials. Khondalite is generally coarse grained with large platy sillimanite grains defining a foliation.

The western part of the Schirmacher Hills, though occurring at a lower tectonic level, is dominantly constituted of different types of granite gneiss (both augen gneiss and streaky gneiss). The augen gneiss is well foliated and contains quartz-K-feldspar-plagioclase-biotite with some garnet and hornblende. The augen structure is mostly represented by very coarse elongate grains of K-feldspar while the matrix contains equant grains of quartz, plagioclase, hornblende and biotite. The streaky gneiss is a fine grained rock with well developed foliation defined by fine grained flakes of biotite and within a quartzofeldspathic matrix. In some cases, within a shear zone, the augen gneiss became extremely stretched and finally the rock became indistinguishable from streaky gneiss. The general assemblage of the streaky gneiss is quartz-plagioclase-biotite with rare remnant garnet and pyroxene.

In the central part of the Schirmacher Hills a thick zone of mylonitic gneiss occurs. Along this wide zone, pyroxene granulite, garnet biotite gneiss and augen gneiss are affected by a northeasterly oriented shear zone. As a result the rocks have become mylonitized. This sheared unit as

a whole has been referred to as mylonitic gneiss. Within this band there is a wide variation of composition and microstructure. However, all these rocks contain hornblende, biotite, plagioclase and quartz in different proportions. A few isolated remnant lenses of mafic granulite occur within the mylonitic gneiss.

Successive Phases of Deformation in the Schirmacher Hills

The deformation history of the Schirmacher Hills can be divided into five groups of events. Each group consists of broadly contemporaneous processes of folding, shearing, metamorphism and emplacement of quartzofeldspathic and basic bodies.

Earliest deformation D,

The signature of the earliest deformation is preserved in some isolated mafic and ultramafic enclaves (Figs. 4a and b; also Figs. 2,3,16 of Sengupta, 1993). These enclaves show a prominent compositional banding with a well- developed foliation parallel to it. I t is possible that the foliation is a composite one and represents more than one deformation. However, we shall consider all early deformations as a single one and refer to it as D, No associated fold is found in these enclaves. This early foliation D, is traced by granulite facies minerals, indicating that the early deformation is contemporaneous with a phase of granulite facies metamorphism (MJ. In some of the enclaves high strain zones parallel to D, foliation is present indicating that ductile shearing took place at great crustal depth.

The second deformation D,

The D, foliation of the enclaves is truncated by the D, foliation of the enveloping granulites (Figs. 4a and b). The angle between the two foliations varies considerably. Usually, the enclaves with D, foliation has been mostly rotated and deformed by later deformations. Elongated enclaves with a layer-parallel D, foliation is folded by the D, deformation. Figure 5a shows such an elongated enclave with banding-parallel D, foliation isoclinally folded by L), fold. Within the enclave the D, foliation wraps around the hinge of the fold. The foliation of the host gneiss is axial planar to the fold and is marked out by granulite facies minerals, indicating thereby that the D, deformation took place under granulite facies conditions (MJ. The D, foliation is the dominant foliation within the mafic granulites, enderbites, khondalites and calc gneisses. It has an average easterly strike with moderate dip towards south. Axial planar nature of this foliation may also be identified in some isolated D, folds, which

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'TECTONOTHERMAL HISTORY OF SCHIRMACHER GRANULITES, EAST ANTARCTICA 341

occurs on bands of mafic granulites, khondalites or calc gneisses.

The third deformation D,

The third group of events includes isoclinal folding, development of a dominant axial planar foliation and layer parallel ductile shearing under amphibolite facies conditions (M,). The D, deformation transposed and re-transposed all the earlier foliations. The D, folds (Fig. 5b) are isoclinal, inclined to reclined with a m.oderate plunge of the fold axis towards southwest. The D, foliation has an easterly strike with a moderate dip

Fig. 4a. Elongated enclave with discordant internal foliation.

Fig. 4b. Part of a large enclave with concordant boundary but with discordant internal foliation.

towards south. D, deformation is also associated with extensive emplacement of granitic materials and synchronous ductile shear which developed subparallel to the D, foliation. Zones of intense noncoaxial deformation were often produced by shearing out of the middle limb of asymmetric folds. There was syntectonic emplacement of granitic material along these shear zones. The shear zone rocks show excellent development of mylonites (Sengupta, 1988,1994). Along a thrust zone in the central part, the granulites were stacked over the amphibolite facies rocks to produce a tectonic inversion.

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342 S. DASGUPTA ET AL.

Later group of events

The axial surfaces of D, folds have been deformed by two sets of folds with steep axial surfaces, D, and D,. The D, axial surfaces have a northerly trend with southwesterly trending fold hinges parallel to or at low angles to the D, hinges. The D, folds are subhorizontal with an easterly trending steep axial surfaces. The intensity of deformation during the development of D, is much weaker than that of the previous deformations. Two dominant sets of shear zones developed during this period. One set has an easterly trend and a steep dip of mylonitic foliation. The

Fig. 5a. Elongated enclave with D, foliation folded by D, fold. The regional D, foliation of the host gneiss is parallel to the axial plane of this fold.

Fig. 5b. D, fold on interbanded mafic granulite and augen gneiss. Axial planar D, foliation is marked by long axes of the feldspar augen.

mylonitic lineation has a moderate to gentle plunge towards southwest. The second set has a northeasterly trend with a steep dip. A wide shear zone of this type occurs in the central part of the Schirmacher Hills. The mylonitic lineation has a variable plunge from subhorizontal to gentle indicating a large component of strike slip movement. Assemblages of the newly crystallized minerals in the newly formed mylonitic foliation suggest that the ductile shearing of both the easterly and northeasterly striking shear zones took place under amphibolite facies conditions.

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TECTONOTHERMAL HISTORY OF SCHIRMACHER GRANULITES, EAST ANTARCTICA 343

Petrography and Mineral Chemistry

We have carried out detail petrological studies on some of the rock types from the Schirmacher Hills which provide the best constraints on thermobaric evolution of the terrane.

A. Khondalite (or garnet-sillimanite bearing metapelite)

Khondalite contains porphyroblastic garnet, mesoperthite, sillimanite, quartz and locally plagioclase. In some places the porphyroblast garnet contain an internal schistosity defined by relatively finer grained sillimanite, spinel, biotite and quartz (Fig. 6). Coarse sillimanite prisms, ribbons of quartz and feldspar are aligned in one direction defining an external foliation (D,), which is at an angle to the internal schistosity. A crude compositional banding is preserved in the khondalite in

Fig. 6. Porphyroblastic garnet (Grt) showing an internal schistosity defined by sillimanite (Sill), quartz (Qtz) and spinel (Sp) in khondalite. EPMA back scattered image.

Fig. 7. Second generation biotite (Bt) clearly replaces porphyroblastic garnet (Grt) in khondalite. Plagioclase (Pl) partially rims garnet. EPMA back scattered image.

the form of alternate quartzofeldspathic and garnet- sillimanite rich layers. This banding is parallel to the D, foliation. The porphyroblastic phases may also be wrapped around by a still later schistosity (D,) (Fig. 7) marked by biotite flakes. The flakes of biotite, locally intergrown with quartz, clearly replace garnet and mesoperthite (Fig. 7). Angular relationships are locally observed between D, and D,, which help to distinguish between the two. Further, D, is defined by relatively lower grade hydrous minerals, which replace pre-existing garnet and mesoperthite. Plagioclase further occurs as partial rims over garnet against sillimanite and quartz (Fig. 7). Relatively coarse flakes of graphite are abundant in some samples of khondalite. The graphite flakes are aligned parallel to the D, foliation. Ilmenite and rutile are accessory phases. Although Ravikant and Kundu (1998) described cordierite from this rock on petrographic criterion, we could not detect presence of cordierite in the khondalites despite assiduous search.

The chemical composition of the phases in these rocks was determined with JEOL - JXA 733 Superprobe at Hiroshima University, Japan and JEOL - JXA 8600 Superprobe at Jadavpur University. The instrumental conditions for both the cases were 15KV accelerating voltage, lOnA specimen current and 2-5 mm beam diameter. Natural mineral standards were used and raw data was corrected by ZAF. Representative chemical composition of the phases in khondalite are given in table 1. Porphyroblastic garnet has the composition Alm,, ,7 Prp28-31 Grs04-05 Sps,,,, in the cores, and is zoned to lower Prp ( 20-25 mol %) and higherAlm(71-72 mol%) contents at the rims against biotite (XMg = 0.74, TiO, G 2 wt. %) at the contact. Included biotite has slightly lower XMg (0.68) and higher TiO, (3-3.5 wt. %). Porphyroblastic plagioclase (An30-35)is distinctly less calcic than that which forms rims (An,,) around garnet. Spinel included in the garnet is zincian (ZnO = 7-9 wt. %) with G, = 0.35-0.4. Therefore, G, in spinel is higher than that in garnet for the studied rocks.

B. Leptynite

Leptynite is differentiated from khondalite by the absence of sillimanite and dominance of plagioclase over mesoperthite. Porphyroblastic garnet, plagioclase, quartz, mesoperthite and ilmenite are the major phases in leptynite. Representative mineral chemical data are given in table 2. Plagioclase has identical composition (An,,.,,) as in the khondalite. Garnet has core compositions of Alm,,Prp,,Grs,,Sps,, and is zoned to Alm,,Prp,,Grs,,Sps,, at the contact of secondary biotite (XMg = 0.45-0.50). Garnet has been extensively replaced by biotite. Ilmenite contains negligible Fe,O,, calculated on the basis of stoichiometry.

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344 S. DASGUPTA ET AL.

Table 1. Representative chemical analyses of minerals in khondalite.

Analyses No. 1 2 3 4 5 6 7 8 9 10 Mineral SPl SPl Grt Grt Grt Bt PI P1 Ilm Sill Sample 240B1 240B1 240B1 240B1 240B1 240B1 240B1 240B1 240B1 240B1

SiO, TiO, A1203 FeO MnO MgO CaO K2O Na,O H P C r P , ZnO Total Oxygen basis Si Ti A1 FeIL MnI2

Ca K Na Cr Zn Total

Mg

X M X

x, e

58.46 22.18

0.01 7.92 0.02

0.32

0.71 9.1

98.72 4

1.94 0.48

0.33

0.02 0.19 2.96

0.05 57.52 21.88 0.02 6.54 0.04

0.41

0.84 8.8

96.1 4

1.97 0.53

0.28

0.02 0.19 2.99

38.46 0.03

22.27 29.05

0.66 8.06 1.47

0.01

100.01 12

2.982 0.002 2.035 1.883 0.043 0.932 1.122

0.002

9.001 0.31 0.63 0.04 0.02

38.81

22.62 31.13

0.65 7.34 1.48 0.03 0.01

0.03

102.1 12

2.97

2.04 1.993 0.042 0.837 0.121 0.003 0.001 0.002

8.009 0.28 0.67 0.04 0.01

38.18

22.16 31.67

1.32 5.1

1.86 0.02

0.08

100.39 12

2.998

2.051 2.079 0.088 0.597 0.156 0.002

0.005

7.976 0.02 0.71 0.05 0.04

37.1 2.11

18.24 10.46

17.15 0.05 9.36 0.21 4.1

0.19

98.97 22

2.714 0.116 1.572 0.64

1.87 0.004 0.873 0.03

0.011

7.83 0.75 0.25

58.38

25.8

7.37 0.1 7.6

99.25 8

2.628

1.369

0.356 0.006 0.663

5.022

0.35

53.57

28.83

10.99 0.06 5.23

98.68 8

2.446

1.552

0.538 0.003 0.463

5.002

0.54

0.07 52.63

44.37 0.3

0.65 0.05 0.03

0.1

98.2 12

0.007 4.031

3.779 0.026 0.099 0.005 0.004

0.008

7.959 0.03 0.96

0.01

36.92

62.2

0.02

0.02

99.16

1.002

1.989

0.001

2.992

1,2 - Spinel included in garnet; 3,4 - Core of porphyroblastic garnet; 5 - Rim of porphyroblastic garnet against biotite; 6 - Biotite at the contact of garnet; 7 - Porphyroblastic plagioclase; 8 - Plagioclase rimming garnet; 9 - Ilmenite; 10 - Sillimanite; Dashes - below detection limit.

C. Mafic granulite

Mafic granulite contains the mineral association clinopyroxene-orthopyroxene-plagioclase-garnet-ilmenite- hornblende-biotite-minor quartz-K-feldspar. Ortho- and clinopyroxene, together with plagioclase and ilmenite form a granoblastic mosaic texture. At places where hydrous minerals are poorly developed, a relict interlocking igneous texture is discernible. Coarse clinopyroxene often contains (100) orthopyroxene lamellae (Fig. 8). Plagioclase porphyroblasts contain curved twin lamellae. Recrystallization of plagioclase and the pyroxene porphyroblasts along the margins into smaller strain-free grains is a common feature. Garnet occurs as a coronal phase in the mafic granulites. However, the thickness of the coronae varies widely even within a sing1e thin and garnet Fig. 8, Coarse clinopyroxene (Cpx) contains (1 00) lamellae of grains can be physically traced from fine thread -like orthopyroxene in mafic granulite. EPMA back scattered image. corona. Garnet, intergrown with very fine grained ilmenite, porphyroblastic ilmenite and plagioclase (Fig. 9) . On the other hand, garnet is intergrown with quartz when it separates porphyroblastic orthopyroxene

and clinopyroxene from plagioclase (Fig. 10). Locally, a finer variety of clinopyroxene is intergrown with garnet and quartz. Two types of amphibole are present in the

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TECTONOTHERMAL HISTORY OF SCHIRMACHER GRANULITES, EAST ANTARCTICA 345

Table 2. Representative chemical analyses of minerals in leptynite.

Analyses No. 1 2 3 4 5 6 Mineral Grt Grt Bt Bt Bt Ilm Sample 218A 218.4 218A 218A 218A 218.4 SiO, 37.238 36.443 34.196 35.97 37.255 0.039 TiO, 0.257 0.257 3.217 4.116 3.315 55.033 AL03 23.339 22.79 17.567 17.857 16.451 0.151

FeO 30.608 30.969 19.046 18.305 10.698 46.647

MnO 0.678 0.361 0.058 0.019 0.09 0.098 MgO 6.432 6.246 8.871 10.363 17.403 0.422 CaO 1.976 1.81 0.03 0.042 0.035 0.049 K,O 0.01 0.01 9.595 7.01 9.827 0.009 Na,O 0.118 0.119 0.132 0.022 0.099 0.17 H,O Cr,03 0.048 0.022 0.057 0.085 0.052 0.054 Total 100.704 99.027 92.769 93.789 95.225 102.672 Oxygen basis 24 24 22 22 22 24 Si 5.7964 5.7858 5.8689 5.9489 5.96 0.0075 Ti 0.03 0.0307 0.4152 0.0512 0.4 8.0666 A1 4.2822 4.2648 3.5537 3.481 3.1 0.0356 Fe+3 Fef2 3.9846 4.1119 2.7338 2.532 1.43 7.6034 Mn'" Mn" 0.0894 0.0486 0.0085 0.0027 0.01 0.0162 Mg 1.4924 1.478 1 2.2697 2.5548 4.15 0.1227 Ca 0.3257 0.3079 0.0055 0.0075 0.0103 K 0.002 0.002 2.101 1.479 2 0.0023 Na 0.0358 0.0367 0.0438 0.0069 0.03 0.0642 Cr 0.006 0.0027 0.0077 0.0111 0.0084 Total 16.0445 16.0692 17.0078 16.5359 17.08 15.9372

0.25 0.25 0.45 0.5 0.74 0.02 Xk 0.68 0.69 0.55 0.5 0.26 0.98

Fez93

M n P ,

X M g

XG3 0.06 0.05 %I,, 0.01 0.01

1,2 - Core of garnet; 3,4 - Biotite replacing garnet; 5 - Included biotite; 6 - Ilmenite.

Fig. 9. Coronal garnet (Crt) of variable thickness and intergrown with fine grained ilmenite (Ilm) rim of porphyroblastic ilmenite and plagioclase (PI) in mafic granulite. EPMA back scattered image.

~ i ~ . 10. coronal garnet ( G ~ ~ ) intergrown with quartz separates porphyroblastic plagioclase (Pl) and orthopyroxene (Opx) in mafic granulite. Orthopyroxene has been replaced by biotite

mafic granulites. Calcic amphibole, although replacing both types of pyroxene, forms relatively coarse polygonal

(Bt). Please see right side of the figure 9 where thin garnet - quartz corona separates orthopyroxene from plagioclase. EPMA back scattered image.

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346 S. DASGUPTA ET AL.

Table 3. Representative chemical analyses of mafic granulite.

Analyses No. 1 2 3 4 5 6 7 8 9 10 Miner a 1 OPX OPX OPX CPX CPX CPX P1 P1 P1 PI Sample 182 182 180 301 182 180 301 301 180 180

SiO, TiO, '41,03 FeP3 FeO Mn203 MnO MgO CaO YO Na,O H,O Cr,O, Total Oxygen basis Si Ti Al Fe+3 FefZ Mn+3 Mn+, Mg Ca K Na Cr Total

XM, Xk Xca x,

49.85 0.13 0.91

32.38

0.34 14.8 0.65

0.01

99.07 6

1.972 0.004 0.042

1.071

0.011 0.873 0.028

0.001

4.002 0.45 0.55

50.26 0.13 1.02

33.49 0.44

13.79 0.84

99.97 6

1.979 0.004 0.047

1.103

0.015 0.81

0.035

3.993 0.42 0.58

50.06 0.03 0.62

35.13

0.45 13.18 0.68

0.03

100.18 6

1.984 0.001 0.029

1.164

0.015 0.779 0.029

0.002

4.003 0.4 0.6

50.49 0.32 1.98

11.74

0.32 10.64 22.15 0.02 0.22

97.88 6

1.956 0.009

0.09

0.38

0.011 0.615 0.919 0.001 0.017

3.998 0.61 0.39

50.6 0.1

1.75

12.85

0.27 10.99 21.52

0.38

0.03 98.49

6 1.955 0.003

0.08

0.415

0.009 0.633 0.891

0.028 0.001 4.015

0.6 0.4

50.79 0.13 1.55

15.98

0.21 9.93

22.09

0.46

101.14 6

1.941 0.004 0.07

0.511

0.007 0.566 0.905

0.034

4.038 0.53 0.47

55.12 0.02

27.63

0.05

9.84 0.15 5.68

98.49 8

2.515 0.001 1.486

0.002

0.481 0.009 0.503

4.997

0.48

55.87 0.01

28.31 0.3

9.9 0.09 5.83

100.31 8

2.505

1.496 0.01

0.476 0.005 0.507

4.999

0.48

57.55

26.59 0.02

0.01 8.31 0.23 6.97

99.68 8

2.587

1.409 0.001

0.001 0.4

0.013 0.608

5.019

0.4

57.34

26.88 0.13

0.03

8.54 0.11 6.76

99.79 8

2.575

1.423 0.004

0.001

0.411 0.006 0.589

5.009

0.41

%I"

1,2,3 - Porphyroblastic Opx; 4,5,6 - Porphyroblastic Cpx; 7,8,9,10 - Core of plagioclase; 11 - Rim of plagioclase against garnet; 12,13 - Ilmenite; 14 - Core of thick corona of garnet; 15 - Rim of thick corona of garnet; 16 - Coronal garnet against plagioclase and ilmenite; 17 - Coronal garnet against Cpx and plagioclase; 18 - Coronal garnet against Opx and plagioclase; 19 - Calcic amphibole replacing garnet.

grains, which often show preferred orientation. Fe-Mg amphibole is typically patchy in appearance and fibrous and replaces calcic amphibole. K-feldspar forms coarse grains that embay pyroxene, plagioclase and hornblende. Locally, a second variety of plagioclase embays the ferromagnesian phases along with K-feldspar.

Chemical composition of the co-existing phases is given in table 3. Porphyroblastic orthopyroxene has rather constant composition of XMP = 0.40-0.45 (0.42-0.92 wt. % A1,OJ. Porphyroblastic clinopyroxene has slightly more variable composition (k, = 0.52-0.62, A1,0, = 1-2 wt. %). Plagioclase porphyroblasts range in composition from An,, to An,,, with slight lowering of X,, towards rims against coronal garnet. Coronal garnet shows domainal variations in the composition depending on the nature of the rimmed phases e.g., Alm6,Prpo9Grs,,SpsOs against plagioclase and ilmenite, Alm6,Prp,,Grs,,SpsOs against orthopyroxene and plagioclase and AlrnblPrplo

Grs,,Spso4 against clinopyroxene and plagioclase. Thick coronas have core compositions of Alm63Prpl,Grs,3Sps,4 and rim compositions of Alm,,Prpl,Grsl,Sps,,. Late plagioclase is more calcic (Ansa). Porphyroblastic ilmenite contains 2-3 mol Yo of hematite. Secondary calcic amphibole shows wide Tschermak and Fe-Mg exchange and vary in composition from magnesio hornblende to ferroan pargasitic hornblende to ferroan pargasite (nomenclature after Robinson et al., 1971; Leake, 1978). Non calcic amphiboles belonging to the cummingtonite - anthophyllite series replace the calcic amphiboles as fibrous grains.

D. Enderbite From the point of view of structural relationships,

textural characteristics and chemistry of the minerals, three types of enderbites are distinguished. The pegmatoidal enderbite (which was emplaced later than

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TECTONOTHERMAL HISTORY OF SCHIRMACHER GRANULITES, EAST ANTARCTICA 347

Table 3. Contd.

Analyses No. 11 12 13 14 15 16 17 18 19 Mineral PI Ilm Ilm Grt Grt Grt Grt Grt Amp Sample 301 180 180 301 301 301 301 301 182

SiO, TiO, A1203 F e P 3 FeO Mn,O, MnO MgO CaO K,O Na,O H,O %O3 Total Oxygen basis Si Ti Al Fef3 Fe+, Mnf3 Mn+,

Ca K Na Cr Total

Mg

% xFe

xca

50.29 0.04

30.95 0.04

13.87 0.07 3.65

98.91

8

2.314 0.001 1.679 0.001

0.684 0.004 0.326

5.009

0.68

0.04 49.28

5.99 43.26

0.39 0.3

0.08 0.09 0.01

0.05 99.49

12

0.004 3.765

0.458 3.675

0.034 0.045 0.009 0.012 0.002 0.004 8.008

0.01 0.98

0.01

0.04 52.15

1.85 45.77

0.41 0.4

0.01 0.03 0.02

0.08 100.76

12

0.004 3.923

0.139 3.828

0.035 0.06

0.001 0.004 0.004 0.006 8.004

0.01 0.98

0.01

38.53 0.25

21.67

28.44

1.33 2.65 8.06 0.03

100.96

12

3.017 0.015

2

1.862

0.088 0.309 0.676 0.003

7.97

0.1 0.63 0.23 0.04

37.03 1.12

21.89 0.09

31.96

1.65 2.47 5.61 0.03

101.85

12

2.945 0.007 2.052 0.006 2.126

0.111 0.293 0.478 0.003

8.021

0.09 0.7

0.15 0.06

37.48 0.1

21.3 0.74

29.17

2.06 2.36 7.76 0.03 0.01

0.01 101.02

12

2.967 0.006 1.987 0.044 1.931

0.138 0.278 0.658 0.003 0.002 0.001

,8.015

0.09 0.64 0.21 0.06

38.02 37.2 0.15 0.12

21.74 21.19

27.55 30.91

1.57 1.88 2.62 1.8 8.63 6.57 0.03 0.02 0.01

100.32 99.69

12 12

2.997 2.992 0.009 0.007 2.019 2.008

1.816 2.079

0.105 0.128 0.308 0.216 0.729 0.566 0.003 0.002 0.002

7.988 7.998

0.1 0.07 0.61 0.7 0.25 0.19 0.04 0.04

40.54 1.41

12.53

18.56

0.07 8.63

11.38 1.92 1.35

96.39

23

6.278 0.164 2.287

2.404

1.992 1.88

0.379 0.405

15.789

0.45 0.55

enderbitic gneiss) is characterized by megacrystic orthopyroxene, plagioclase, perthite, garnet and quartz. Garnet and pyroxene megacrysts may be more than a centimeter in diameter and are mostly euhedral containing inclusion of mesoperthite, quartz and plagioclase. Garnet has been extensively replaced by biotite, that forms folia warping around the former. Orthopyroxene megacrysts are likewise replaced by biotite. Replacement is at places very advanced and only relict grains float in mats of biotite.

Chemical composition of the phases is given in table 4. Plagioclase porphyroblasts have the composition An,,.,,, while that occurring as lamellae in mesoperthite is An,,,,. Orthopyroxene has X,, = 0.46-0.56 and is aluminous (A120, wt. % 4.4 max). Garnet is essentially almandine-pyrope binary (Alm63~65Prp29~32Grso4-05Spsol in the cores and Alm7,,,Prp19~22Grso4~05Spsol in the rims at the contact of biotite). Biotite is phlogopitic (k, = 0.72- 0.82) and titaniferous (TiO, = 2.8 wt. %).

The enderbitic gneiss is characterized by all the phases present in the pegmatoidal variety, but contains

additionally ilmenite and amphibole. The grain size of the minerals is distinctly smaller and the plagioclase : perthite modal ratio is higher. In some localities garnet and orthopyroxene porphyroblasts are extensively replaced by amphibole and to a lesser degree by biotite. The chemical composition of the phases is given in table 5. Garnet is distinctly richer in grossular and poorer in pyrope content than in the pegmatoidal variety (Alm73Grsl,PrplSSpso,) but shows a sharp increase in grossular towards the rim against plagioclase (Alm,,Grs,,Prp,,Sps,,). There is a slight depletion in An content of plagioclase from core to rim (An39-40 to An,,). Orthopyroxene varies in composition from ;yM, = 0.46 to 0.57). Late biotite has 4-5 wt. % TiO, with &, = 0.45. The late amphibole is compositionally cummingtonite- anthophyllite.

A third variety of enderbitic rock has been distinguished on the basis of mineral composition only. This rock is also characterized by the presence of porphyroblastic garnet, plagioclase, orthopyroxene, quartz and lesser amount of perthite. However, garnet shows sharp increase in

Gondwana Research, V. 4, No. 3,2001

348 S. DASGUPTA ET AL.

Table 4. Representative chemical analyses of minerals in pegmatoidal enderbite.

Analyses No. 1 2 3 4 5 6 7 8 9 10 11 Mineral P1 PI OPX OPX OPX Grt Grt Grt Grt Bt Bt Sample 179/1 179/1 179/1 179/1 179/1 179/1 179/1 179/1 179/1 17911 17912

SiO, 60.71 TiO, Al,O, 24.26 Fez03 0.03 FeO MnzO, 0.05 MnO MgO CaO 5.79 KZO 0.1 Na,O 8.19 HZO Cr,O, ZnO Total 99.13 Oxygen basis 8

Si 2.719 Ti A1 1.281 Fe+3 0.001 Fe+2 Mni3 0.002 Mni2 Mg Ca 0.278 K 0.006 Na 0.711 Cr Zn Total 4.998

X M P

x, xca 0.28

59.95

25.04

0.01

6.49 0.1

8.09

0.09

99.77 8

2.677

1.318

0.311 0.006

0.7 0.003

5.015

0.3

48.91 0.04

3.5

31.51

0.32 15.23 0.13

0.01

0.13

99.78 6

1.909 0.001 0.161

1.029

0.011 0.886 0.005

0.001 0.004

4.007 0.46 0.54

49.66 0.12 2.65

29.06

0.2 17.73

0.1

0.02

0.2

99.74 6

1.917 0.003 0.121

0.938

0.007 1.02

0.004

0.001 0.006

4.017 0.52 0.48

49.33 0.02 4.85

26.53

0.19 18.54 0.11

0.04

0.14

99.75 6

1.878 0.001 0.218

0.845

0.006 1.052 0.004

0.003 0.004

4.011 0.55 0.45

39.29 0.03

22.25

30.34

0.36 7.57 1.74 0.01 0.01

0.21

101.81 12

3.002 0.002 2.004

1.939

0.023 0.862 0.142 0.001 0.001 0.013

7.989 0.29 0.65 0.04 0.02

38.27 0.02

22.24

30.02

0.43 7.4 1.7

0.03

0.16

100.27 12

2.974 0.001 2.037

1.951

0.028 0.857 0.142

0.005 0.01

8.005 0.29 0.66 0.04 0.01

38.5 0.02

22.04

32.59

0.5 5.78

1.9 0.01 0.04

0.17

101.55 12

2.989 0.001 2.017

2.116

0.033 0.669 0.158 0.001 0.006

0.01

8 0.22 0.71 0.05 0.02

38.04 0.05

21.91

32.92

0.6 4.82 2.07 0.01 0.03

0.13

100.58 12

2.993 0.003 2.032

2.166

0.04 0.565 0.175 0.001 0.005 0.008

7.988 0.19 0.74 0.06 0.01

38.18 2.8

14.98

12.81

0.06 17.02 0.01 9.93 0.17 4.09 0.25

100.3 12

2.8 0.154 1.295

0.786

0.004 1.861 0.001 0.929 0.024 0.014

7.868 0.7 0.3

38.6 2.52

15.75

10.59

17.84

10.08 0.31 4.12 0.18

99.99

2.808 0.138

1.35

0.644

1.934

0.935 0.044

0.01

7.863 0.82 0.18

1,2 - Core of plagioclase; 3,4,5 - Core of Opx; 6,7 - Core of porphyroblastic garnet; 8,9 - Rim of porphyroblastic garnet; 10,11 - Biotite replacing garnet.

grossular content from core to rim (Alm,,Grs,,Prp,,Sps,, to Alm69Grs,sPrp,,Spsos) at the contact with plagioclase (AnSo) (Table 6). However, garnet shows increase in almandine and decrease in pyrope at nearly constant grossular against secondary biotite (XMg = 0.6) (Alm,,Prp,,Grs,,Sps,, to Alm,,Grs,,Prp,,Sps,,). These compositional variations attest to domainal equilibrium controlled by adjoining phases. There is a coronal variety of garnet in this rock (Fig. 11) rimming plagioclase, which has the composition Alm,,Prp,,Grs,,Sps,,. Orthopyroxene porphyroblasts have XMg = 0.43 and (A1,0, wt.% 0.9), but highly altered relict grains floating in biotite matrix have higher XMg (0.53) and A1,0, (4.14 wt.%). Late amDhiboles varv in comDosition from ferroan pargasitic hornblende to acti;lolitic hornblende to cummingtonite-anthophyllite (Robinson et al., 1971;

Fig. 11. Coronal garnet (Grt) rimming plagioclase (PI) against orthopyroxene (Opx) in the third variety of enderbite. Note that orthopyroxene has been variously replaced by biotite (Bt).

Leake, 1978). EPMA back scattered image.

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TECTONOTHERMAL HISTORY OF SCHIRMACHER GRANULITES, FAST ANTARCTICA 349

Table 5. Representative chemical analyses of minerals in enderbitic gneiss

Analyses No. 1 2 3 4 5 6 7 8 Mineral OPX Grt Grt Grt P1 PI Bt Bt Sample 162 new 162 162 162 162 162 162 162

sio,

A1203 Fe20,

Ti0,

FeO Mn,O, MnO

CaO

Na,O

MgO

K*O

HA0 Cr,O, ZnO Total Oxygen basis si Ti A1 Fe t 3 Fef2 Mn+? Mn+2

Ca K Na Cr Zn Total

Mg

xMP

x, P x, 1

53.045 0.26

0.999

29.012

0.199 13.706

1.893 0.068 0.118

0.038

99.338 6

2.047 0.01

0.045

0.937

0.006 0.789 0.078 0.003 0.008 0.001

3.924 0.46 0.54

37.39 0.02

21.52 0.06

32.78

0.96 3.58 3.91 0.01 0.01

0.1

100.34 12

2.975 0.001 2.018 0.004 2.181

0.065 0.425 0.333 0.001 0.002 0.006

8.011 0.14 0.73 0.11 0.02

38.2 0.06

21.35 0.16

33.15

0.96 3.95 3.46 0.02

101.31 12

3.005 0.004 1.979 0.01

2.181

0.064 0.463 0.292 0.002

8 0.15 0.73 0.11 0.02

38.26 0.05

21.54

30.82

0.77 3.45 6.18

0.03

0.13

101.23 12

3 0.003

1.99

2.021

0.051 0.403 0.519

0.005 0.008

8 0.13 0.68 0.17 0.02

58.92 0.03

25.27 0.05

0.07

7.35 0.22 6.48

98.39 8

2.663 0.001 1.346 0.002

0.002

0.356 0.013 0.568

4.951

0.39

59.81

25.64 0.09

6.86 0.08 7.38

99186 8

2.664

1.346 0.003

0.327 0.005 0.637

4.982

0.34

36.16 4.22

14.33

22.87

0.07 9.08 0.11 9.74 0.04 3.9

100.52 22

2.78 0.244 1.298

1.47

0.005 1.041 0.009 0.955 0.006

7.808 0.41 0.59

35.56 4.17

14.41

21.79

0.04 9.07 0.14 9.46 0.02 3.84 0.05

98.55 22

2.776 0.245 1.326

1.423

0.003 1.056 0.012 0.942 0.003 0.003

7.789 0.43 0.57

1 - Core of Opx; 2,3 - Core of garnet; 4 - Rim of garnet against plagioclase; 5 - Core of plagioclase; 6 - Rim of plagioclase; 7,8 - Secondary biotite.

E. Calc gneiss

Detailed petrological studies on calc gneiss will be presented elsewhere, particularly in view of their importance on characterization of metamorphic fluids. However, the salient features are noted below. This rock, intimately associated with khondalite, contains the mineral association garnet-calcite-scapolite- clinopyroxene-plagioclase-zoisite/epiodote-quartz- titanite. Wollastonite has not been detected so far. Garnet varies in composition from nearly pure grossular to almandine-pyrope-grossular solid solution. Andradite content is typically low. Peak metamorphic assemblage of clinopyroxene + plagioclase + scapolite + titanite + calcite + quartz was overprinted by two generations of garnet during cooling. The rocks were finally retrogressed into the stability fields of tremolitic amphibole and zoisite/ epidote in appropriate bulk compositions.

E The enclaves

The enderbitic gne,isses and mafic granulites contain enclaves of several mafic and ultramafic rocks and these preserve an early foliation at an angle to that in the host rocks (Sengupta, 1993). These enclaves represent fragments of a chromitite - bearing layered igneous complex, comprising of high Mg gabbro - metanorite - websterite - olivine-spinel bearing enstatite - spinel lherzolite. These are similar to the rocks described by Harley et al. (1998) from the Rauer Group. Petrogenesis of the layered complex is the subject of a separate communication. However, one component of these layered complex provides important constraints on the P - T history of the study area and, is, therefore discussed in details below.

Mafic granulites, occurring as enclaves in the enderbitic gneiss, preserve an early foliation (D,) defined by a crude

Gondwana Research, V. 4, No. 3,2001

350 S. DASGUPTA ET AL.

Table 6. Representative chemical analyses of minerals in the third variety of enderbite.

Analyses No. 1 2 3 4 5 6 7 8 9 10 Mineral Grt Grt Grt Grt PI PI Grt OPX OPX Bt Sample 251g/2 251g/2 251g/l 251g/l 251g/l 251g/l 251g/l 251g/l 251g/l 251g/2

SiO, TiO, AlP, Fe,O, FeO Mn,O, MnO MgO CaO K P Na,O H P Cr,O, ZnO Total Oxygen basis Si Ti A1 Fe+, Fe+, Mnf3 Mnf2 Mg Ca K Na Cr Zn Total

X M R

Xk XI2 Xh",,

38.14 0.15

21.54

33.42

1.18 3.64 3.38 0.01

0.08

101.54 12

2.998 0.009 1.995

2.197

0.079 0.426 0.285 0.001

0.005

0

0.14 0.74 0.09 0.03

37.91 0.29

21.67

31.62

2.09 2.79 5.33 0.02 0.03

0.21

101.96 12

2.974 0.017 2.004

2.075

0.139 0.326 0.448 0.002 0.005 0.013

0

0.11 0.69 0.14 0.06

37.88

21.31

28.31

1.01 3.21 7.51

0.03

0.27

99.53 12

3.007

1.994

1.88

0.068 0.38

0.639

0.005 0.017

0

0.13 0.63 0.22 0.02

37.62 0.12

20.65 0.38

28.42

2.93 2.56 6.88 0.01 0.02

0.02

99.61 12

3.013 0.007 1.949 0.023 1.904

0.199 0.306

0.59 0.001 0.003 0.001

1.1 0.1

0.67 0.19 0.03

55.12

27.48 0.19

9.81 0.07 5.61

98.28 8

2.519

1.48 0.007

0.48 0.004 0.497

0.981

0.49

54.65

27.72 0.17

0.01 10.2 0.05 5.72

98.52 8

2.498

1.493 0.006

0.001 0.5

0.003 0.507

2.081

0.5

38.41 0.01 21.3 0.31

28.57

1.17 3.53 7.65

0.18

101.13 12

3.005 0.001 1.964 0.019 1.869

0.078 0.412 0.641

0.01 1

0.011

0.14 0.62 0.2

0.04

49.68 0.06 1.23

33.13

0.66 14.27 0.71

0.05

0.17

99.96 6

1.959 0.002 0.057

1.092

0.022 0.839 0.03

0.004 0.005

0.009 0.43 0.57

50.04 0.12 0.88

31.58

0.6 14.82 0.79

0.02

0.12

98.97 6

1.977 0.004 0.041

1.044

0.02 0.873 0.033

0.002 0.004

0.02

0.46 0.54

38.28 2.62

14.39

15.94

0.05 15.08 0.01 9.69 0.07 4.05

0.3

100.48 22

2.837 0.146 1.257

0.988

0.003 1.666 0.001 0.916

0.01 0.018

2.614 0.63 0.37

1,3 - Core of porphyroblastic garnet; 2 - Rim of porphyroblastic garnet; 4 - Rim of garnet against biotite; 5,6 - Porphyroblastic plagioclase; 7 - Coronal garnet against plagioclase; 8,9 - Porphyroblastic Opx; 10 - Biotite replacing garnet.

parallel alignment of ortho- and clinopyroxene and minor ilmenite. Additionally, this rock contains garnet, plagioclase and amphibole. Both texturally and compositionally, the enclave mafic granulites are distinct from the bands of mafic granulite interlayered with the enderbitic gneiss (parallel to D,). These D, foliation- parallel mafic granulites have already been described. In the enclave mafic granulites, ortho-, clinopyroxene, plagioclase constitute the porphyroblastic phases along with a variety of garnet (Fig. 12). It may be recalled that garnet is only a coronitic phase in the other type of mafic granulite. Coarse plagioclase often embays ortho-, clinopyroxene and porphyroblastic garnet (Fig. 12). Relict orthopyroxene occurs in coarse plagioclase. Within coarse plagioclase grains occur rare inclusions of another plagioclase of lower R.1 (compositionally An,, discussed later). A coarse variety of amphibole (ferroan pargasitic hornblende) replaces orthopyroxene and plagioclase

Fig. 12. Porphyroblastic clinopyroxene (Cpx), orthopyroxene (Opx), plagioclase (Pl) and garnet (Grt) from a granoblastic texture in enclave mafic granulite. Plagioclase embays partially orthopyroxene, clinopyroxene and garnet. EPMA back scattered image.

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TECTONOTHERMAL HISTORY OF SCHIRMACHER GRANULITES, EAST ANTARCTICA 351

Table 7. Representative chemical analyses of minerals in mafic enclaves.

Analyses No. 1 2 3 4 5 6 7 8 9 Mineral Grt Grt Grt P1 P1 OPX OPX CPX CPX Sample 338 338 338 338 338 338 338 338 338 SiO, TiO, A1203 Fe,O, FeO MnP, MnO MgO CaO K,O Na,O HP c r p , ZnO Total Oxygen basis Si Ti A1 Fe+3 Fe c2 Mn+3 MnI2

Ca K Na Cr Zn Total

Mg

39.59

22.19

22.98

1.09 6.59 6.93

0.08

99.45 12

3.047

2.013

1.479

0.071 0.756 0.572

0.005

7.943 0.26 0.51 0.2

0.04

38.98

21.76

27.24

1.16 4.69 6.85

0.02

0.11

100.81 12

3.024

1.99

1.767

0.076 0.542 0.569

0.003 0.007

7.978 0.18 0.6

0.19 0.02

38.58

21.61

26.97

1.35 4.47

7.4

0.01

0.1

100.49 12

3.01

1.987

1.76

0.089 0.52

0.619

0.002 0.006

7.993 0.17 0.59 0.21 0.03

45.06

35.3

18.82

0.99

100.17 S

2.076

1.917

0.929

0.088

5.01

0.91

44.41

35.3

19.51

0.77

99.99 8

2.055

1.925

0.967

0.069

5.016

0.93

50.79

1.84

24.83

0.37 19.7 0.47

0.01

98.01 6

1.956

0.084

0.8

0.012 1.131 0.019

0.001

4.003 0.59 0.41

51.3

1.81

25.89

0.34 19.57 0.47

0.03

0.03

99.44 6

1.955

0.081

0.825

0.011 1.112 0.019

0.002 0.001

4.006 0.57 0.43

49.53 0.4

4.26

9.9

0.18 12.23 22.51

0.41

0.14

99.56 6

1.875 0.011 0.19

0.313

0.006 0.69

0.913

0.03 0.004

4.032 0.68 0.32

50.8 0.15 2.8

8.01

0.17 13.34 23.14

0.31

0.02

98.74 6

1.922 0.004 0.125

0.253

0.005 0.752 0.938

0.023 0.001

4.023 0.75 0.25

1 - Core of porphyroblastic plagioclase; 2,3 - Coronal garnet; 4,5 - Core of plagioclase; 6,7 - Porphyroblastic Opx; 8,9 - porphyroblastic Cpx.

Fig. 13. Plagioclase (Pl) embaying garnet (Grt) and orthopyroxene (Opx) in enclave mafic granulite. Coarse amphiboles (Amph) replace orthopyroxene and is locally intergrown with second generation garnet. EPMA back scattered image.

(Fig. 13) and is locally intergrown with a second variety of garnet (Fig. 13). Garnet also forms a coronitic phase over orthopyroxene and plagioclase (Fig. 14).

Fig. 14. Garnet (Grt) corona over orthopyroxene (Opx) and plagioclase (Pl) in enclave mafic granulite. EPMA back scattered image.

Interestingly, quartz is totally absent in this rock. There are two other varieties of amphibole in this rock; both replace pargasitic hornblende - actinolitic hornblende and cummingtonite - anthophyllite (Table 6) .

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352 S. DASGUPTA ET AL.

The last two varieties of amphibole are patchy in appearance.

Porphyroblastic garnet has the composition Alm,oPrp,,Grs,,Spso,. High - calcic plagioclase contains (rarely) less calcic cores (AnJ (Table 7). Both ortho- and clinopyroxene are more magnesian and aluminous in this rock, as compared to those in the other mafic granulites (Opx: XMg = 0.57- 0.62, A1,0, wt.%

Orthopyroxene shows negligible Al zoning. Coronal garnet has the composition Alm,,Grs,,Prp,,Sps,, at the contact with orthopyroxene and plagioclase (An,,). Garnet intergrown with pargasitic hornblende has composition similar to that of coronal garnet.

1.86); CPX: XMg = 0.68-0.75, A1,0, wt.% 4.26).

Metamorphic Evolution of the Rocks

Textural relationships and compositional characteristics of the rocks attest to some key mineral reactions which occurred in course of evolution. Garnet - sillimanite - mesoperthite - plagioclase - quartz constituted the peak assemblage in the khondalite. Inclusion - porphyroblast relationship suggests that biotite + spinel were stabilized prior to garnet and the reaction

Biotite + spinel + quartz + garnet + sillimanite f occurred at the initial stage leading to the formation

of garnet and coarse prisms of sillimanite. Oriented sillimanite inclusions in garnet would then have to be considered as inert phase in course of this reaction. The D,-parallel quartzofeldspathic layers could represent the melt fraction. Development of plagioclase rims over garnet against sillimanite signifies progress of the model end- member reaction

Grossular + sillimanite + quartz + anorthite (2) Subsequently, garnet broke down to biotite + quartz

Garnet + K- feldspar + V -+ biotite + quartz (3) In the mafic granulites the two types of pyroxene,

plagioclase and ilmenite were stabilized early, presumably at the magmatic stage as testified by occasional relict interlocking texture. There is no indication that these minerals had an amphibolite facies precursor, indicating emplacement and cooling of the magma under high grade condition (dry metamorphism). Exsolution of (100) orthopyroxene in clinopyroxene could be a result of either subsolidus cooling or cooling from metamorphic peak. coronal garnet appeared subsequently according to the reactions,

Orthopyroxene + plagioclase -+ garnet + quartz k clinopyroxene (4) Ilmenite,,, + plagioclase + quartz -+ garnet + ilmenite,,, + 0, (5)

Kfs + vapor/melt (1)

accompanying the development of D, by the reaction

Reaction (4) occurs in response to cooling (Harley, 1989), while reaction (5), albeit controlled by fO,, could also occur due to cooling (Ellis and Green, 1985). Amphibole and biotite developed last in the rock at the expense of the anhydrous phases synchronous with D, due to cooling and hydration.

In the enclave mafic granulite, ortho-, clinopyroxene, ilmenite and low calcic plagioclase were the early stabilized phases, along with the porphyroblastic garnet. The possibility that garnet crystallized directly from the magma cannot be ruled out. High calcic plagioclase replaced all these early stabilized phases. Pargasitic amphibole and coronal garnet appeared through the reaction,

Orthopyroxene + plagioclase + H,O + garnet + The other varieties of amphiboles are the products of

still later hydration. Orthopyroxene, plagioclase, quartz, perthite and garnet

constitute the early stabilized phases in all the varieties of enderbitic rocks. It is significant that garnet, orthopyroxene and plagioclase have compositional differences in the three varieties. Biotite and amphibole appeared later in all the rocks. Garnet shows Fe- enrichment at the contact of late biotite indicating Fe - Mg re-equilibration during cooling. Remarkable similarity in composition of garnets in khondalite and pegmatoidal enderbite is noteworthy. This raises a possibility that the garnet in the latter is entrained from pre-existing khondalite. In the other two varieties of enderbite, garnet is distinctly grossular - rich and pyrope - poor, and becomes more grossular - rich towards the rims against plagioclase (which shows Ca - depletion). Interestingly the coronal garnet rimming plagioclase in one of the rocks is even more grossular - rich. All these features are consistent with progressive breakdown of anorthite component in plagioclase in course of evolution of the rocks. The significance of such variations in this rock as well as in others discussed earlier in terms of P-T evolution will be discussed in the following section.

In calc - silicate granulites grossular rich garnet was stabilized at a later stage of evolution at the expense of scapolite, calcite, plagioclase and clinopyroxene. This could occur in response to cooling (Warren et al., 1987). Tremolitic amphibole and zoisite / epidote appeared later in response to hydration and/or further cooling.

amphibole (6)

Geothermobarometry

Resetting of mineral compositions during down P-T evolution is a major hindrance to estimate peak metamorphic conditions for granulite facies rocks. Further,

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T E C T O N O T H E M L HISTORY OF SCHIRMACHER GRANULITES, EAST ANTARCTICA 353

thermobarometric data are difficult to evaluate in polymetamorphic terrains. These data, when used in conjunction with the sequence of mineral reactions deduced from textural criteria, however, provide better constraints on thermobaric evolution of terrains.

Two pyroxene thermometry (Kretz, 1982) in the enclave mafic granulite gives 760-8OO0C, which is obviously cooling temperature. Porphyroblastic garnet - orthopyroxene core compositions give 850-900C at assumed P of 10 Kbar (Lee and Ganguly, 1988), while at the same pressure garnet - clinopyroxene thermometer (Ellis and Green, 1979) registers 780-840C. Since quartz is absent in the rock, "peak" pressure is estimated from A1 content of orthopyroxene coexisting with porphyroblastic garnet (Harley, 1984a, b) as 11.2-11.5 Kbar (at T = 900C). Temperature in the range of 650-750C were obtained using coronal garnet - orthopyroxene, - clinopyroxene and - hornblende compositions at an assumed P of 10 Kbar (methods after Lee and Ganguly, 1988; Ellis and Green, 1979 and Graham and Powell, 1984). GOAL barometer gives 10.2 Kbar (after Harley, 1984a) and 10.8 Kbar at 800C for the compositions of coronal garnet and rims of adjacent orthopyroxene. Although the GOAL barometer is subject to some uncertainties owing to the Fe-rich nature of garnet and low Al- content of orthopyroxene (Fitzsimons and Harley, 1994), we are constrained to use it since no other geobarometers can be applied to this rock to estimate physical conditions of metamorphism. Application of Fitzsimons and Harley (1994) retrieval method at 900C gives slightly higher pressures. Summarizing, the enclave mafic granulites record imprints of a rather high pressure (at ca. 10-1 1 Kbar) cooling when coronal garnet appeared.

P-T conditions of metamorphism of pelites ascertained from simultaneous solution of GASP (Koziol and Newton, 1988), GRAIL (Bohlen et al., 1986) and GRIPS (Bohlen and Liotta, 1986) barometric expressions and garnet (core) - biotite (matrix) thermometric expression (Dasgupta et al., 1991). The results converge at 8-8.5 Kbar, 770-820C for the core compositions and 6-6.2 Kbar, 600-650C for rim compositions. Garnet - biotite thermometer is particularly susceptible to down - temperature re - adjustment, hence we chose biotite grains away from any ferromagnesian mineral. Nevertheless, the computed temperature should be considered as minimum estimate. Progress of reaction (3) would imply decrease in pressure. GASP barometry using the composition of moat plagioclase and rim of garnet gives 5.2 Kbar at an assumed temperature of 650C (Koziol and Newton, 1988). Garnet - late biotite temperatures cluster around 500-560C at P = 6 Kbar.

Simultaneous solution of GOPS barometer (Bhattacharya et al., 1991) and garnet - orthopyroxene thermometer (Lee and Ganguly, 1988) using the compositions of coronal garnet and adjoining phases in mafic granulites gives P = 7-8 Kbar, T = 680-720C . Similar calculations for core compositions of the phases in the enderbitic gneiss gives 8-8.5 Kbar, 780-810C and 5-5.5 Kbar, 640-660C for rim compositions. The pegmatoidal enderbite and the third variety of enderbite both give 5-5.5 Kbar, 740-760C. Two pyroxene thermometer (Kretz, 1982) registers 700-740C in the mafic granulites. GRIPS barometry (Bohlen and Liotta, 1986) in the enderbitic gneiss gives 8 Kbar at 800C. Garnet - late biotite temperatures in mafic granulite and enderbite cluster around 700C (after Dasgupta et al., 1991). Orthopyroxene - late biotite thermometry (Sengupta et al., 1990) in these rocks show a scatter from 650-720C at an assumed pressure of 6 Kbar. Garnet - late hornblende thermometry (Graham and Powell, 1984) registers 570-640C at 6 Kbar. As discussed earlier, progressive enrichment in grossular component in garnet from enderbites can be explained by cooling (Harley, 1989). This is also true for the formation of grossular - rich garnet in calc - silicate granulites (Warren et al., 1987). Therefore, the rocks, other than the enclave mafic granulites, show "peak" metamorphic conditions of ca. 8 Kbar, 800-850C , and were subjected to both cooling and decompression during retrogression (AT z 200 "C, AP G 2 Kbar). P-T conditions of hydration (formation of biotite and amphiboles) can be constrained as Ca. 5-6 Kbar, 600rt 50C.

Tectonothermal Evolution

Meso-, microstructural characteristics (Sengupta, 1988), petrographic, mineral chemical and geothermobarometric data can now be synthesized to bring out the tectonothermal history of the study area. At the outset, it may be pointed out that geochronological data on these rocks is sparse (see for summary Rameshwar Rao et al., 2000). This problem must be addressed in future. On the basis of available information two major tectonothermal events, one at ca. 1100-1000 Ma and the other at ca. 550-500 Ma, have been recognized. The earlier one is equivalent to the Rayner orogeny (= Grenvillian) and the latter to the Pan- African orogeny. Whether the rocks have an Achaean history or not can only be speculated at present. However, the remarkable similarity between the early history recorded in the mafic - ultramafic enclaves of the study area and those from the Rauer Group (Harley et al., 1998) and from. Western Dronning Maud Land (Grantham et al., 1995; Groenewald et al., 1995) is noteworthy.

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354 S. DASGUPTA ET AL.

Polymetamorphism under granulite and amphibolite facies conditions for the rocks of the Schirmacher Hills has been suggested by earlier workers (Sengupta, 1993; Ravikant and Kundu, 1998; references therein). Kundu et al. (1998) deduced similar polymetamorphic history from Central Dronning Maud Land. Ravikant and Kundu (1998) suggested four metamorphic events: M, granulite facies metamorphism (P-T conditions unknown) recorded in mafic granulite and khondalite enclaves, M, (7 Kbar, 800 - 850C, data from Sengupta, 1990) characterized by near isothermal decompression, M, characterized by near - isobaric cooling, and M, upper amphibolite facies event (5.1 Kbar, 6OO0C), also characterized by hydration and cooling. They documented both decompression and cooling from textural features (resorption of garnet and formation of coronal garnet respectively), but did not attempt quantification. Further they did not provide any evidence (geochronologic or otherwise) in favor of three discrete metamorphic events (Ml, M, and M,) under granulite facies condition. Their M, could really be the retrogressive arm of M, (M,) and their M, isobaric cooling could again be related to Mlr, caused by thermal relaxation of the decompressed crust (Harley, 1989). Although coronal garnet formation is clearly indicative of cooling, it is not clear how a dehydration - reaction (reaction 10 in their paper) could be caused by cooling. The M, amphibolite facies metamorphism (Ravikant and Kundu, 1998) is, however, distinct. They have invoked decompression (accompanying cooling and hydration) during M,, as indicated by the development of cordierite in metapelites. Kundu et al. (1998) ascribed the same reaction to decompression under granulite facies condition from Central Dronning Maud Land. Kundu et al. (1998) deduced isothermal decompression subsequent to cooling, while Ravikant and Kundu (1998) thought otherwise. This contradiction underscores problems associated with constructing P-T paths from different rock types. There are other problems with Kundu et al. (1998). They obtained anomolously high temperatures from garnet- biotite thermometry, which exceeds those from garnet- orthopyroxene thermometry. It is not clear why biotite, which has lower blocking temperature for Fe-Mg exchange than orthopyroxene, preserved pristine compositions. Since the rocks studied by them showed isobaric cooling at the initial stage, mineral compositions are likely to have been affected. The mineral reactions deduced by Grew (1983) from high Mg-A1 granulites attest to ultrahigh temperatures. However, garnet-biotite thermometer is unlikely to preserve such UHT conditions. Secondly, the spinel described by them is excessively rich in Cr and Zn, and it will be extremely difficult to retrieve P and T from

such composition, unless proper activity corrections are employed. It is not clear what methods were adopted by these workers. We re-calculated the position of the equilibrium Spinel + Quartz = Garnet + Sillimanite using the formulation of,Nichols et al. (1992) in the system ZnFMAS and the compositions reported by Kundu et al. (1998) and obtained much lower P-T estimates. The actual estimate will be even lower after correction for Cr. Thirdly, cordierite stability is strongly dependent on presence/absence of fluid in the channels, and therefore, it could form simply due to increase in fluid activity.

Rameshwar Rao (2000), on the other hand, deduced only one phase of granulite facies metamorphism (at 827*29"C, 7.3k 0.3 Kbar, followed by near-isobaric cooling) and a later amphibolite facies (at 6:4+ 27"C, 5.4-t 0.4 Kbar) overprint and accompanying granitization.

Grew (1983) described two occurrences of high Mg-Al granulites from the Schirmacher Hills, which provide evidence of much higher metamorphic temperatures. Unfortunately, these rocks were not encountered in course of the present study.

Data presented in this work brings out a different thermal history for the Schirmacher Hills granulites. Although we deduce polymetamorphism under granulite facies conditions followed by an amphibolite facies overprint, the major difference lies in the thermobaric evolution of the rocks during the earlier granulite facies metamorphism (Fig. 15). This metamorphism (M,) is recorded only in the mafic enclaves in the enderbitic gneiss. Because of suitable bulk composition and high pressure, garnet appeared early in this rock, which is key to understanding of MI P-T condition. The ultramafic enclaves, on the other hand, show evidence of recrystallization, but the condition at which it took place remains undetermined. Temperature estimates in the mafic enclaves, obtained from Fe - Mg exchange thermometers, place only lower limits owing to down - temperature re - equilibration. The mafic magma probably crystallized at a depth of ca. 35km, corresponding to 11.2 Kbar, when porphyroblastic garnet appeared. I t may be pointed out here that this conclusion hinges on the argument whether garnet is a primary magmatic phase and, therefore, has some uncertainty. However, coronal garnet (reaction 6) formed during retrograde metamorphism from M, peak (MI) at ca.10 Kbar. Reaction (6) proceeds to the right with increasing a H,O and cooling (Harley, 1989; Thost et al., 1991). This constrains MI metamorphism also at lower crustal depths. Imprint of this high pressure metamorphism was hitherto unrecognized in the Schirmacher Hills. D, was synchronous with M,. Subsequent to cooling at depth,

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TECTONOTHERMAL HISTORY OF SCHIRMACHER GRANULITES, EAST ANTARCTICA 355

11

10

L H .- P 9

2 a v)

8

7

6

5

D(1) Defmation

Transportation of the enclaves by enderbitic magma to higher levels I I I

M(2) "Peak" condltlon rec0rd.d In dlmnnt lithologios D(2) Mommtion

/ I I

Emplacement of pegmatoidal enderbite

M(3) 0 D(3) Deformation

500 600 4 0 do '0 Temperature in o C

Fig. 15. Pressure - temperature - deformation history of the Schirmacher Hills granulites.

the mafic granulites were transported to shallower crustal levels by the enderbitic magma.

There is at present no evidence that the supracrustals were buried to depths corresponding to that of MI. The high Mg-A1 granulites described by Grew (1983) could provide such evidence. All the rocks, other than the enclaves, bear evidence of M, granulite facies metamorphism at ca. 8 Kbar, 800-850C (Fig. 15). D, was synchronous with M,. M, deduced in this work corresponds to the M, of Ravikant and Kundu (1998) and the granulite facies event discussed by Rameshwar Rao (2000). While both the enderbite and mafic granulite experienced essentially "dry metamorphism" evidence for prograde path of M, comes from reaction (1) in the pelites. This reaction has steep slope in P-T space and can be intersected during heating (Hensen and Harley, 1990; Dasgupta and Sengupta, 1995). However, estimation of pressure at which the reaction was crossed is difficult to obtain because of the high ZnO content in the spinel.

P-T condition of equilibration of the rims of the porphyroblastic phases in all the rocks is constrained to be 5.5-6 Kbar, 600-650C (Fig. 15). Interestingly, even the cores of the minerals in pegmatoidal enderbite register this P-T condition. It appears therefore that this rock was emplaced subsequent to peak M,. The retrograde path of M, (M,) is, therefore, characterized by AP =: 2 Kbar, AT E 200C. Development of coronal garnet in the mafic granulite and enderbite and the compositional variations

of garnet, orthopyroxene and plagioclase in enderbite attest to a dominantly cooling trajectory during MZr. On the other hand, reaction (2) in the khondalite is triggered by decompression. It is evident, therefore, that MZr involved both cooling and decompression. This raises the possibility that MZr may not be a continuous down P-T trajectory, but may have discontinuous P-T trajectory - one involving cooling and the other decompression (Fig. 15). Indeed it is being realized recently that high grade rocks are commonly characterized by such compound P-T paths (Harley and Fitzsimons, 1995). Even if such a two stage retrograde path is constructed for the present studied area, the relative timing of cooling and decompression cannot be worked out because these are not recorded in the same rock. Nor is there any overprinting reaction textures indicative of cooling and decompression in the same rock. However, from adjoining areas Ravikant and Kundu (1998) suggested cooling subsequent to decompression. Unfortunately, convincing reaction textures (Fig. 2c in their work) indicative of decompression are not studied by us.

M,, synchronous with D,, is an amphibolite facies event and involves influx of H,O resulting in the formation of biotite and amphiboles in different rocks. Geothermometry shows T = 600+ 50C for M, at P E 6 Kbar (Fig. 15).

Grantham et a1.(1995), Groenewald et a1.(1995) and Asami et al. (1992) discussed the tectonometamorphic history of the Western Dronning Maud Land and Sor

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356 S. DASGUPTA ET AL

Rondane mountains respectively. These areas are located to the west and east of Schirmacher Hills (Fig. 1). The presently deduced deformational and metamorphic history can be compared with those described by these authors. Asami et al. (1992) showed peak metamorphic conditions of 750-800C, 7-8 Kbar, which was followetl by retrograde P-T conditions of 5.5 Kbar, 530-580C. They conceived a subsequent period of contact metamorphism. They documented preservation of initial high pressure minerals (kyanite) as inclusions in MI garnet (also see Ravikant and Kundu, 1998). The petrological descriptions of Asami et al. (1992) shows that their MI is probably equivalent to the M, described in this work. M,deduced presently, corresponds to their M,.

There is a striking similarity between the metamorphic history recorded by Groenewald et a1.(1995) and Grantham et a1.(1995) from the Western Dronning Maud Land and the present work. These authors also recorded an initial high pressure metamorphism (MI, 12-15 Kbar) only in mafic boudins. Their M,(8-9 Kbar, 850C) and M3 (6 Kbar, 600C) affected all the lithologies and correspond to identically numbered events described here. Rameshwar Rao et al. (2000) found an average T,, age of 1200 Ma for the mafic granulites from the presently studied area, and reported a metamorphic age of ca. 960 Ma for the same. The model age can be interpreted as the age of emplacement of the mafic magma (assuming that there was not much difference between the same and separation of the magma from the mantle), and could well be correlated with the early M I granulite metamorphism deduced in the present work. M, could then correspond to the Grenvillian granulite facies event recorded by Rameshwar Rao et al. (2000). Groenewald et al. (1995) also noted a spread in age (1200-900 Ma) for their early high pressure event, but correlated their M, (amphibolite facies) with ca. 500 Ma Pan-African event. In the present area, the amphibolite facies M, could well be of Pan-African age.

Acknowledgments

We would like to thank Department of Ocean Development, Govt. of India, for providing facility of field works in Antarctica. We are grateful to CSIR, UGC and DST for assistance. We thank an anonymous reviewer for comments. This is a contribution to the IGCP Project 368.

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