Pressure–temperature-fluid evolutionary history of orthopyroxene-bearing quartzofeldspathic and mafic granulites from northern parts of the Eastern Ghats Belt, India: implications for Indo-Antarctic correlation

Journal of Asian Earth Sciences 22 (2003) 81–100 www.elsevier.com/locate/jseaes

Pressure – temperature-fluid evolutionary history of orthopyroxenebearing quartzofeldspathic and mafic granulites from northern parts of the Eastern Ghats Belt, India: implications for Indo-Antarctic correlation Sankar Bosea,*, Supratim Pala, Masato Fukuokab b

a Department of Geology, Durgapur Govt. College, Durgapur 713 214, India Department of Earth and Planetary Sciences, Hiroshima University, Hiroshima, Japan

Received 28 February 2002; revised 15 July 2002; accepted 8 August 2002

Abstract Mafic and felsic orthogneisses form an integral component of the rock association of the Eastern Ghats Belt (EGB). Such rocks show similar P – T-fluid evolutionary history at two different localities of the EGB from where UHT metamorphism has already been recorded. Both mafic and felsic orthogneisses show contrasting petrological characters and are classified according to structural and mineralogical characteristics. All the varieties of mafic granulite and the gneissic enderbite preserve the strong regional foliation (S2/S3). The pegmatoidal enderbite is post deformational and hybrid in nature containing xenocrysts derived from associated rocks. Textural and thermobarometric data reveal different stages of metamorphism. Peak metamorphism (M1) occurs at 8 –9 kbar pressure and temperature in excess of 950 8C. Emplacement of mafic magma could be the principal cause of this metamorphism. This was followed by retrograde R1 stage when the rocks suffered a near isobaric cooling up to 700– 750 8C at 7 – 7.5 kbar. At the terminal retrograde stage (R2) fluid –rock interaction took place at 6 – 6.5 kbar, 600–650 8C. The fluid composition is calculated to be poor in H2O and CO2. Textural data predict that K-rich brine could be responsible for such composition. The observed P – T-fluid characteristics show similarity with other terranes belonging to erstwhile Gondwanaland. q 2003 Published by Elsevier Ltd. Keywords: Orthogneisses; Thermobarometry; Fluid activity; K-metasomatism; Eastern Ghats Belt

1. Introduction Orthopyroxene bearing quartzofeldspathic granulites and mafic granulites form common members of lithologic ensemble in regional granulite terranes (Harley, 1985, 1989). These rocks often show variable temporal relationships with the enclosing rocks. Traditionally such rocks draw less petrological attention in comparison to the high Mg – Al granulites and calc silicate granulites due to apparently monotonous textural features. In spite of this, such rocks can be profitably exploited to unravel P –T-fluiddeformational history of a granulite facies terrane. The Eastern Ghats Belt (EGB), a ca. 1000 km long regional granulite facies terrane comprises of rocks which are polymetamorphosed and polydeformed (Dasgupta and * Corresponding author. E-mail address: [email protected] (S. Bose). 1367-9120/03/$ - see front matter q 2003 Published by Elsevier Ltd. doi:10.1016/S1367-9120(02)00186-4

Sengupta, 2003, 2000; Dobmeier et al., 2001). The rocks of the EGB have attracted wide attention of the petrologists since last decade or so because of two prime reasons. Firstly, the rocks record ultrahigh temperature metamorphism achieved through an anticlockwise P – T path (Sengupta et al., 1990a; Dasgupta et al., 1995; reviewed in Harley, 1998), and secondly, according to the Gondwana reconstruction hypothesis, the EGB is inferred to be an erstwhile neighbour of the parts of east Antarctica (Yoshida, 1995; Unrug, 1996; Harley, 2003). Most of the petrological works on the rocks of the EGB, however, deal with the evolution of the high Mg –Al granulites (Lal et al., 1987; Kamineni and Rao, 1988; Sengupta et al., 1990a, 1991, 1999; Dasgupta et al., 1995; Mohan et al., 1997; Shaw and Arima, 1998; Bose et al., 2000) and calc silicate granulite (Dasgupta et al., 1993; Bhowmick et al., 1995; Sengupta et al., 1997). But for a few exceptions (Dasgupta et al., 1991, 1993; Sengupta et al., 1996), the mafic and the orthopyroxene-bearing

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(Fig. 1). The P –T-fluid evolutionary history of the EGB can be proved to be a powerful tool for much debated IndoAntarctic correlation considering its important position in the Gondwana assembly (Yoshida, 1995; Unrug, 1996; Harley, 2003).

2. Geological background

Fig. 1. Location of the study areas in the Eastern Ghats Belt. Inset map showing position of the Eastern Ghats Belt in India. 1 (A, B), 2, 3 and 4 are the four crustal domains of the EGB after Rickers et al. (2001).

quartzofeldspathic granulites have drawn much less attention till date. In this communication we evaluate the pressure – temperature-fluid evolutionary history for such rocks from two areas occurring within the northern EGB (Fig. 1). Recent geochronological data clearly demonstrated that the EGB represents amalgamation of four isotopically distinct crustal domains with characteristic thermotectonic history (Rickers et al., 2001 and references therein). Both these two study areas belong to the same crustal domain (Domain 2)

The areas chosen for the purpose of the present study occur to the north of the Godavari Rift cutting across the EGB (Fig. 1). The area Sunkarametta occurs about 100 km north of Visakhapatnam, whereas the area Narsapuram occurs just to the north of the Godavari Rift. In Sunkarametta nearly equal proportion of mafic and orthopyroxene bearing quartzofeldspathic rocks (represented as enderbite) occur as sub-parallel bands and lenses within leptynite and khondalite (Fig. 2). Moreover sapphirine –spinel bearing high Mg –Al granulite occurs within the host khondalite. Dasgupta et al. (1993) discussed the petrological characteristics of similar mafic granulites from Sunkarametta in a comprehensive way. They argued that the mafic granulites were emplaced as basic magma that suffered granulite facies metamorphism subsequent to emplacement. The regional gneissic foliation (S2) is strongly developed in the enderbite and weakly developed in the mafic granulites. A second variety of enderbite, which is pegmatoidal, often containing augen of plagioclase is distinctly massive without showing the development of S2.

Fig. 2. Geological map of the area in and around Sunkarametta.

S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

83

Fig. 3. Geological map of the area in and around Narsapuram.

Bose et al. (2000) documented heating-cooling P-T trajectory from the associated high Mg – A1 granulites where the peak metamorphism was achieved at ultrahigh temperature (UHT) condition. In Narsapuram, the mafic granulites and enderbites occur as bands, rafts and lenticular enclaves conformable to the regional gneissic foliation S3 within leptynite and khondalite (Fig. 3). These enderbites are both gneissic and pegmatoidal in nature, of which the latter is emplaced later than the development of the regional gneissic foliation S3. Field observations indicate that the enderbite gneiss and the mafic granulites share a common deformational history. Dasgupta et al. (1995) deduced from the associated spinel granulites that these rocks evolved through an anticlockwise P – T path comprising a shallow dP/dT prograde path culminating to UHT (. 950 8C) peak metamorphic condition at , 9 kbar and followed by a near isobaric cooling to , 750 8C at 8 kbar. These isobarically cooled crust was then reworked by a bear isothermal decompression down to , 6 kbar at 600– 650 8C.

3. Petrology of the mafic granulites In Sunkarametta, mafic granulites occur as thick to thin linear patches within khondalite and leptynite (Fig. 2). Apparently massive looking, such rocks locally show a crude gneissic foliation (S2). Based on the mineralogical assemblage we have divided these rocks into four types as discussed below. Compositions of coexisting minerals have been determined by JEOL-JXA 8600 Electron Probe

Microanalyzer at the University of Hiroshima and at Jadavpur University. In both the cases the instruments operated with 15 KV accelerating voltage, 10 nA specimen current and 2 mm beam diameter. Raw data were corrected by ZAF procedure. Elemental formula of the minerals are recalculated using the software MINFILE (version 9-89, Afifi and Essene, 1989) and are presented in Tables 1 – 4. Mineral abbreviations used are taken from Kretz (1983). 3.1. Type A mafic granulite Orthopyroxene, clinopyroxene, plagioclase and ilmenite form xenoblastic aggregate. Both ortho- and clinopyroxene show exsolved lamellae and patches of each other (Fig. 4). All the phases are variably deformed showing preferred grain alignment along S2. Plagioclase shows wedgeshaped deformation twins. Orthopyroxene is magnesian ðXMg ¼ 0:58 – 0:62Þand contains low alumina (0.8 – 2.5 wt% Al2O3) (Table 1). Clinopyroxene is also magnesian ðXMg ¼ 0:72Þ and shows slight drop in alumina content from core to rim. Plagioclase is Ca rich ðXAn ¼ 0:69Þ: Ilmenite contains low but variable amount of exsolved hematite ðXHem ¼ 0:03Þ and geikelite ðXGeik ¼ 0:08Þ components (Table 1). Some clinopyroxene grains show three distinct sets of exsolved lamellae of orthopyroxene. The most prominent set occurs along (100) plane of the host, while the other occurs at 358 to the former (Fig. 5). The third set is extremely fine and occurs at low angle (, 58) to the (100) plane.

84

Table 1 Representative mineral analyses from different mafic granulites of Sunkarametta Type A

Type B

Type C

Sample

16

14A

140

Phase

Opx

Mode

R

C

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O H2O

53.01 0.05 0.82 25.81 0.75 19.78 0.45 0.03 –

52.68 0.16 1.07 23.53 0.68 21.42 0.64 0.01 –

Total O2 basis Si Ti Al Feþ3 Feþ2 Mn Mg Ca Na K Cations XMg XAb XAn XOr XIlm XHm XGk XPrp XAlm XGrs XSps

100.70

100.19 6 1.970 0.005 0.047 0.004 0.732 0.022 1.194 0.026 0.001 – 4.000 0.62

1.995 0.001 0.037 – 0.813 0.024 1.110 0.018 0.002 – 4.000 0.58

Cpx

Pl

Ilm

Opx

C

C

C

X

C

C

X

C

50.68 – 30.54 0.13 – – 13.52 3.24 0.12

0.02 51.77 0.05 45.50 0.67 1.97 – 0.04 –

50.09 0.13 1.05 35.74 0.42 11.92 0.83 0.06 –

50.17 0.07 0.43 35.44 0.60 13.24 0.58 – 0.03

51.04 0.19 1.68 15.89 0.25 9.65 21.05 0.30 –

51.19 0.17 1.26 14.96 0.14 10.53 21.36 0.28 –

98.23 8 2.341 – 1.661 0.005 – – – 0.669 0.290 0.007 4.973

100.02 3 – 0.969 0.001 0.059 0.882 0.014 0.073 – 0.002 – 2.000

100.24

100.56 6 1.982 0.002 0.020 0.014 1.157 0.020 0.780 0.025 – 0.002 3.998 0.40

100.05

99.89

52.54 0.28 2.00 9.33 0.24 21.84 0.28 –

100.06 1.961 0.008 0.087 – 0.291 0.008 0.745 0.879 0.020 – 4.000 0.72

0.30 0.69 0.01

1.995 0.004 0.049 – 1.190 0.014 0.708 0.035 0.005 – 4.000 0.37

Cpx

1.960 0.005 0.076 0.016 0.495 0.008 0.552 0.866 0.022 – 4.000 0.53

Pl

1.958 0.005 0.056 0.037 0.441 0.005 0.601 0.876 0.021 – 4.000 0.58

Type D 17/2

Grt

Bt

Opx

R

R

Cr

C

R

S

50.89 0.02 30.47 0.24 – – 13.72 3.15 0.12

55.84 0.01 27.67 0.32 – – 10.19 5.15 0.20

38.27 0.03 21.17 29.29 1.06 3.29 7.23 – –

38.93 3.36 12.58 15.92 – 15.45 0.01 – 9.89 4.03

52.49 0.06 3.33 16.49 0.32 25.26 0.57 – – –

52.32 0.07 2.91 18.17 0.46 24.98 0.42 0.04 – –

52.48 0.03 3.34 17.31 0.36 25.51 0.54 – 0.02

98.61 8 2.343 0.001 1.652 0.008 – – – 0.677 0.281 0.007 4.969

99.38

100.34 12 3.024 0.002 1.970 – 1.934 0.071 0.388 0.612 – – 8.001

100.17 20 5.550 0.360 2.112 – 1.898 – 3.283 0.002 – 1.799 15.013 0.63

98.52

99.37

99.49 6

1.930 0.002 0.144 – 0.507 0.010 1.385 0.022 – – 4.000 0.73

1.917 0.002 0.126 0.038 0.518 0.014 1.365 0.016 0.003 – 4.000 0.72

1.914 0.001 0.143 0.028 0.500 0.011 1.381 0.021 – 0.001 3.999 0.73

0.29 0.70 0.01

2.523 – 1.472 0.011 – – – 0.493 0.451 0.012 4.962 0.47 0.52 0.01

0.892 0.031 0.076

Modal type: C ¼ Core of porphyroblast, R ¼ Rim of porphyroblast, Cr ¼ Corona, X ¼ Exsolved, S ¼ Symplectic.

0.129 0.644 0.204 0.003

Cpx C

140

Pl

Sl/l

Ol

Spl

Amp

Opx R

S

C

R

C

S

C

51.67 0.60 5.18 6.23 0.21 14.72 21.69 0.78 0.01

51.89 0.12 3.71 6.29 0.11 15.18 22.87 0.51 –

48.63 – 32.59 0.30 – – 15.75 2.33 0.01

50.42 – 30.97 0.02 – – 14.30 3.19 0.13

37.72 0.11 – 30.28 0.69 32.50 – 0.06 –

0.02 0.04 61.90 25.63 0.10 12.46 0.01 – –

41.69 1.18 15.01 8.60 0.10 14.79 11.90 2.30 1.55 2.05

50.66 0.22 3.85 25.07 0.48 20.51 0.28 0.14 0.01

101.09

100.68

99.61 8

99.03

101.36 4

100.16 4

99.17 22

101.22 6

2.231 – 1.761 0.010 – – – 0.774 0.207 0.001 4.984

2.316 – 1.676 0.001 – – – 0.704 0.284 0.008 4.989

0.21 0.79 –

0.28 0.71 0.01

1.882 0.017 0.222

1.890 0.003 0.159 0.090 0.101 0.003 0.824 0.892 0.036

– 0.190 0.006 0.799 0.846 0.055 0.001 4.017 0.81

– 4.000 0.89

1.005 0.002

0.001 0.001 1.937 0.060 0.502 0.002 0.493

– – 0.674 0.016 1.290 – 0.003 – 2.990 0.66

– – – 2.997 0.49

5.050 0.129 2.565 0.442 0.602 0.012 3.200 1.850 0.648 0.287 15.784 0.84

1.889 0.006 0.169 – 0.782 0.015 1.140 0.011 0.010 0.001 4.023 0.59

S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

Rock

S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

85

Table 2 Representation mineral composition of mafic granulites of Narsapuram Rock type

Metanorite

Sample no.

Rs 3b

Anal. no. Phase

11 Opx

SiO2 TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O Total Oxygen basis Si Ti Al Cr Fe3þ Fe2þ Mn Mg Ca Na K Total XMg XAn XAb XOr XAlm XPrp XGrs XSps XIlm XHem

27

26

12 Grt (R)

28 Grt (C)

30 Grt (R)

2 Ilm

17

51.46 0.06 0.99 0.01 0.82 27.73 0.49 18.12 0.55 0.05 0.04

51.21 0.10 0.93 0.00 0.99 27.16 0.35 18.43 0.63 0.02 0.02

46.12 0.00 33.82 0.00 0.28 0.00 0.00 0.00 17.16 1.66 0.10

46.16 0.00 34.27 0.00 0.48 0.00 0.00 0.00 17.83 1.21 0.06

38.32 0.04 21.45 0.05 0.26 28.47 1.15 4.46 6.12 – –

38.49 0.06 21.82 0.06 0.38 27.55 0.99 5.19 6.66 – –

38.35 0.04 21.64 0.08 0.29 26.88 1.07 5.12 6.98 – –

0.08 51.04 0.04 0.13 4.55 43.20 0.39 1.27 0.06 – –

0.07 50.16 0.04 0.07 5.92 43.20 0.24 1.15 0.06 – –

100.37

99.84

99.14 8 2.14 0.00 1.85 0.00 0.01 0.00 0.00 0.00 0.85 0.15 0.01

100.01

100.3

101.21 24 5.96 0.01 3.98 0.01 0.04 3.57 0.13 1.20 1.10 – –

100.45

100.76 3 0.00 0.95 0.00 0.00 0.08 0.90 0.01 0.05 0.00 – –

101.36

1.95 0.00 0.00 0.06 0.00 0.03 0.92 0.01 1.00 0.02 0.00

100.32 6 1.97 0.00 0.04 0.00 0.02 0.89 0.02 1.03 0.02 0.00 0.00

4.00 0.52

4.00 0.54

4.00 0.55

5.01

4.99

15.95

16.00

16.00

2.00

2.00

0.84 0.15 0.01

0.89 0.11 0.00 0.63 0.18 0.17 0.02

0.59 0.20 0.18 0.03

0.58 0.20 0.19 0.03 0.90 0.04

0.89 0.055

Two pyroxene granulite

Sample no. Phase

R-5-2 Opx

Total Oxygen basis Si Ti Al

10 Pl

50.97 0.13 1.33 0.00 0.91 28.55 0.40 17.53 0.50 0.04 0.01

Rock type

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O

29

R-5-3

1.96 0.00 0.05 0.00 0.03 0.87 0.01 1.05 0.03 0.00 0.00

R-6-1

R-6-2

2.13 0.00 1.86 0.00 0.02 0.00 0.00 0.00 0.88 0.11 0.00

R-5-2

6.00 0.00 0.00 3.96 0.01 0.03 3.73 0.15 1.04 1.03 –

R-5-3 Cpx

R-6-1

5.98 0.00 3.97 0.01 0.03 3.50 0.14 1.19 1.17 – –

0.00 0.94 0.00 0.00 0.11 0.89 0.01 0.04 0.00 – –

R-6-2

50.63 0.04 0.92 1.70 30.23 1.08 15.82 0.68 0.02 0.03

50.55 0.06 1.06 1.73 30.89 0.85 15.54 0.82 0.00 0.00

51.63 0.03 1.10 0.80 27.25 0.91 18.36 0.65 0.00 0.01

51.29 0.04 1.23 1.12 27.34 0.85 18.20 0.54 0.00 0.01

51.71 0.12 1.65 2.70 10.46 0.44 11.73 22.09 0.38 0.02

51.64 0.20 1.48 1.70 11.27 0.37 11.42 22.13 0.34 0.00

51.39 0.18 1.97 3.02 8.11 0.35 12.38 22.93 0.36 0.01

50.97 0.19 2.19 2.48 8.38 0.33 12.23 22.40 0.41 0.00

101.15

101.50

100.74

101.30

100.55

100.70

99.58

1.96 0.00 0.04

1.95 0.00 0.05

1.96 0.00 0.05

100.62 6 1.96 0.00 0.06

1.94 0.00 0.07

1.95 0.01 0.07

1.92 0.01 0.09

1.93 0.01 0.10 (continued on next page)

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S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

Table 2 (continued) Rock type

Two pyroxene granulite

Sample no. Phase

R-5-2 Opx

R-5-3

R-6-1

R-6-2

R-5-2

R-5-3 Cpx

R-6-1

R-6-2

Fe3þ Fe2þ Mn Mg Ca Na K

0.05 0.98 0.04 0.91 0.03 0.00 0.00

0.05 1.00 0.03 0.89 0.03 0.00 0.00

0.02 0.87 0.03 1.04 0.03 0.00 0.00

0.03 0.87 0.03 1.03 0.02 0.00 0.00

0.08 0.33 0.01 0.66 0.89 0.03 0.00

0.05 0.36 0.01 0.64 0.90 0.03 0.00

0.09 0.25 0.01 0.69 0.92 0.03 0.00

0.07 0.27 0.01 0.69 0.91 0.03 0.00

Total XMg XWo XEn XFs

4.01 0.48

4.00 0.47

4.00 0.55

4.00 0.54

4.01

4.02

4.01

4.02

0.43 0.38 0.19

0.44 0.36 0.20

0.44 0.41 0.15

0.44 0.41 0.15

Rock type

Two pyroxene granulite

Sample no. Phase

R-5-2 Ilm

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2 O Total Oxygen basis Si Ti Al Fe3þ Fe2þ Mn Mg Ca Na K Total XIlm XHem XAn XAb XOr XMg

0.01 50.72 0.07 0.00 48.72# 0.75 0.05 0.00 0.00 0.00

R-6-1

0.02 49.57 0.03 0.00 48.72# 0.72 1.00 0.03 0.00 0.00

R-5-3

R-6-2 Pl

R-5-3 Kfs

R-6-1 Amph

57.81 0.00 26.87 0.18 0.00 0.00 0.00 9.32 5.75 0.26

47.90 0.01 32.78 0.27 0.00 0.00 0.00 16.91 2.00 0.10

54.58 0.03 28.25 0.16 0.00 0.00 0.00 11.49 4.61 0.39

665.66 0.02 18.54 0.12 0.00 0.00 0.00 0.05 0.57 15.91

41.34 1.93 11.54 0.00 16.40 0.22 9.78 11.70 1.42 2.37

39.32 1.95 12.63 0.00 17.92 0.19 9.05 11.84 1.21 2.79

99.51

100.87

96.90

2.48 0.00 1.51 0.01 0.00 0.00 0.00 0.56 0.41 0.02

3.00 0.00 1.00 0.00 0.00 0.00 0.00 0.00 0.05 0.93

96.70 23 6.33 0.22 2.09 0.00 2.10 0.03 2.23 1.92 0.42 0.46 15.80

15.88

0.52

0.48

100.32 3 0.00 0.96 0.00 0.08 0.94 0.02 0.00 0.00 0.00 0.00

100.09

100.19

0.00 0.93 0.00 0.14 0.88 0.02 0.04 0.00 0.00 0.00

2.58 0.00 1.42 0.01 0.00 0.00 0.00 0.45 0.50 0.02

99.97 8 2.20 0.00 1.77 0.01 0.00 0.00 0.00 0.83 0.18 0.01

2.00 0.94 0.04

2.01 0.88 0.07

4.98

5.00

4.99

4.98

0.47 0.52 0.01

0.81 0.18 0.01

0.57 0.41 0.02

0.00 0.05 0.95

6.06 0.23 2.30 0.00 2.31 0.03 2.08 1.96 0.36 0.55

C ¼ Core of thick corona, R ¼ Rim of thick corona, – ¼ below detection limit, FeO#—FeO total.

3.2. Type B mafic granulite This variety contains garnet in addition to the phases present in type A mafic granulite. Garnet appears as continuous to discontinuous corona with/without intergrown quartz separating ortho- and clinopyroxenes from plagioclase (Fig. 6). It also forms symplectic intergrowth

with ilmenite surrounding coarser lobe shaped ilmenite grains (Fig. 7). Quartzofeldspathic segregates containing quartz and perthite occurs along S2. Plagioclase is commonly antiperthitic. All the ferromagnesian phases are distinctly Fe-rich than those in the type A mafic granulite (Table 1). Orthopyroxene ðXMg ¼ 0:37 – 0:40Þ is Al poor while clinopyroxene ðXDi ¼ 0:53 – 0:58Þ contains

S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

87

Table 3 Representative mineral analyses from enderbites of Sunkarametta Rock

Gneissic

Pegmatoidal

Sample

S2

97/2

97/5

97/2

Phase

Opx

Grt

Pl

Bt

Opx

Grt

Pl

Mode

M

Cr

C

Cr

C

XC

R

R

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O

50.25 0.21 1.13 30.00 0.78 15.89 0.66 0.15 0.01

37.91 0.04 21.82 27.84 0.87 8.39 2.60 – –

54.11 0.11 29.13 0.12 0.07 0.02 11.14 5.00 0.23

56.11 0.14 28.28 0.06 0.07 0.06 9.65 6.30 0.16

39.27 4.37 14.10 8.53 0.09 20.05 0.07 0.12 9.51

50.82 0.18 5.46 22.88 0.34 21.60 0.24 0.11 0.01

47.95 0.13 6.94 23.33 0.44 20.52 0.11 0.14 0.01

37.91 0.04 21.08 27.29 1.04 10.38 2.62 – –

56.03 0.13 27.51 0.07 0.07 0.04 9.71 5.51 0.42

Total O2 basis Si Ti Al Fe3þ Fe2þ Mn Mg Ca Na K Total XMg XAb XAn XOr XIlm XHm XGk XPrp XAlm XGrs XSps

99.07 6 1.969 0.006 0.052 – 0.983 0.026 0.928 0.028 0.011 0.001 4.003 0.49

99.47 12 2.958 0.002 2.007 – 1.817 0.057 0.976 0.217 – – 8.035

99.94 8 2.444 0.004 1.550 0.004 – 0.002 0.002 0.540 0.438 0.014 4.998

100.83

96.12 20 5.630 0.470 2.380 – 1.020 0.010 4.280 0.010 0.030 1.740 15.58 0.81

101.65 6 1.863 0.005 0.236 – 0.701 0.010 1.180 0.010 0.008 – 4.014 0.63

99.59

100.37 12 2.931 0.003 1.921 – 1.765 0.068 1.196 0.218 – – 8.052

99.48 8 2.530 0.004 1.464 0.002 – 0.002 0.002 0.470 0.482 0.024 4.984

0.44 0.54 0.02

R

2.504 0.004 1.488 0.002 – 0.002 0.004 0.462 0.544 0.010 5.022

1.808 0.004 0.308 – 0.735 0.014 1.154 0.004 0.010 0.001 4.039 0.61

0.54 0.45 0.01

0.49 0.48 0.03

0.32 0.59 0.07 0.02

0.37 0.54 0.07 0.02

Modal types: C ¼ Core of porphyroblast, R ¼ Rim of porphyroblast, M ¼ Matrix, Cr ¼ Corona, XC ¼ Xenocrystic.

low CaTs and negligible jadeite components in all textural modes. Plagioclase shows significant drop in anorthite component from core ðXAn ¼ 0:70Þ to rim ðXAn ¼ 0:52Þ at the contact of coronal garnet (Alm64Prp13Grs20Sps2). Late boitite ðXMg ¼ 0:63Þ replaces orthopyroxene along fractures. 3.3. Type C mafic granulite It contains alternate pyroxene-rich and plagioclase rich gneissic layering (S2). Olivine occurs as intergrain to orthopyroxene and is always separated from plagioclase by granular intergrowth of orthopyroxene – clinopyroxene – spinel (Fig. 8). Pargasitic amphibole replaces orthopyroxene and itself is replaced by biotite along S2 (Fig. 9). All the ferromagnesian phases are more magnesian in comparison

to the varieties of mafic granulites described above. Orthopyroxene ðXMg ¼ 0:71 – 0:73Þ is aluminous showing decrease in alumina from core (Al 2O 3 ¼ 3.3 Wt%) to rim (Al2O3 ¼ 2.91 Wt%) (Table 1). Clinopyroxene ðXMg ¼ 0:81 – 0:89Þ is also aluminous (Al 2O 3 ¼ 3.0 – 6.0 Wt%) Olivine is magnesian ðXMg ¼ 0:66Þ and spinel contains small amount of magnetite (, 5 mol%) component. Late pargasite and biotite replacing pyroxenes are also highly magnesian ðXMg ¼ 0:84Þ (Table 1). Plagioclase is calcic ðXAn ¼ 0:76 – 0:79Þ: 3.4. Type D mafic granulite This variety of mafic granulite occurs as centimeter thick layers within leptynite conformable to S2 developed in it. Mineralogically this rock can be considered as metanorite.

88

S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

Table 4 Representative mineral composition of enderbites of Narsapuram Rock type

Gneissic

Sample no.

R-4-1

R-4-3

R-4-1

Phase

Opx

Grt

Ilm

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2 O

50.55 0.07 1.06 0.71 31.73 0.47 15.40 0.63 – –

51.38 0.05 1.08 0.00 31.98 0.54 15.63 0.56 – –

38.97 0.04 21.30 1.12 28.01 1.32 3.91 6.92 – –

38.74 0.05 21.45 0.68 29.70 1.26 3.83 6.24 – –

0.00 49.89 0.06 – 48.92# 0.36 0.70 0.00 0.01 0.01

0.06 51.27 0.03 – 48.40# 0.40 0.74 0.00 0.01 0.00

39.85 1.76 12.39 – 20.28 0.15 7.86 11.51 1.41 2.58

Total Oxygen basis Si Ti Al Fe3þ Fe2þ Mn Mg Ca Na K

100.62 6 1.96 0.00 0.05 0.02 1.03 0.02 0.89 0.03 – –

101.22

101.59 24 6.00 0.01 3.87 0.13 3.61 0.17 0.90 1.14 – –

101.95

100.91

6.00 0.01 3.92 0.08 3.85 0.17 0.88 1.04

99.95 3 0.00 0.94 0.00 0.12 0.91 0.01 0.03 0.00 0.00 0.00

97.99 23 6.12 0.20 2.25 0.00 2.61 0.02 1.80 1.90 0.42 0.51

4.00 0.46

4.00 0.47

15.83

15.95

2.01

2.00

0.62 0.15 0.20 0.03

0.65 0.15 0.17 0.03 0.91 0.06

0.92 0.04

Total XMg XAlm XPrp XGrs XSps XIlm XHem

1.98 0.00 0.05 0.00 1.03 0.02 0.90 0.02 – –

– –

R-4-2

R-4-1 Amph

0.00 0.96 0.00 0.08 0.92 0.01 0.03 0.00 0.00 0.00

15.83 0.41

Rock type

Gneissic

Sample No.

R-4-1

Phase

Pl

SiO2 TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2 O

57.70 0.03 26.60 – 0.10 – – – 9.03 5.69 0.16

57.99 0.00 26.62 – 0.19 – – – 9.12 5.57 0.07

58.54 0.01 26.57 – 0.26 – – – 8.77 5.99 0.08

65.75 0.02 18.95 – 0.00 – – – 0.09 1.10 14.04

65.67 0.00 18.85 – 0.24 – – – 0.11 0.92 14.53

39.22 0.00 22.49 0.06 0.01 26.77 0.66 9.52 2.17 – –

39.46 0.00 22.43 0.02 0.13 25.25 0.70 10.84 1.62 0.00 0.00

99.31

99.56

99.95

100.32

2.60 0.00 1.41

3.00 0.00 1.02

3.00 0.00 1.01

100.90 24 5.96 0.00 4.03 0.01 0.00 3.40

100.45

2.60 0.00 1.41

100.22 8 2.61 0.00 1.40 – 0.01 –

Total Oxygen basis Si Ti Al Cr Fe3þ Fe2þ

Pegmatoidal R-4-2

R-4-1

R-4-2

Kfs

–

– 0.00

–

R-4-3

0.01 –

–

Grt

– 0.00

–

Rs 49c

0.01 –

5.98 0.00 4.00 0.00 0.01 3.20

S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

89

Table 4 (continued) Rock type

Two pyroxene granulite

Rock type

Gneissic

Sample No.

R-4-1

Phase

Pl

Mn Mg Ca Na K

– –

Total XAn XAb XOr XAlm XPrp XGrs XSps

Pegmatoidal R-4-2

R-4-3

R-4-1

R-4-2

Kfs – –

– –

– –

Grt – –

0.44 0.50 0.01

0.44 0.48 0.00

0.42 0.52 0.01

0.00 0.10 0.80

0.01 0.08 0.85

4.96 0.46 0.53 0.01

4.94 0.48 0.52 0.00

4.97 0.44 0.55 0.01

4.94 0.00 0.11 0.89

4.96 0.01 0.09 0.90

Rock type

Pegmatoidal enderbite

Sample No.

Rs 49c

Phase

Opx

Bt

Ilm

38.24 5.79 14.41 0.02 – 8.63 0.00 17.67 0.00 0.32 9.73 4.11

36.91 5.11 14.11 0.07 – 12.49 0.04 15.54 0.09 0.04 9.93 4.00

0.06 50.19 0.02 0.16 7.37 42.41 0.16 1.37 0.06 – – n.a.

56.07 0.00 27.52 0.00 0.11 0.00 0.00 0.00 9.93 5.85 0.15 n.a.

Total Oxygen basis Si Ti Al Cr Fe3þ Fe2þ Mn Mg Ca Na K

99.32 6 1.86 0.00 0.24 0.00 0.04 0.62 0.01 1.23 0.00 0.00 0.00

98.92 10 2.79 0.32 1.24 0.00 – 0.53 0.00 1.92 0.00 0.05 0.91

98.33

99.63

0.78 0.00 1.74 0.01 0.01 0.95

101.80 3 0.00 0.93 0.00 0.00 0.13 0.88 0.00 0.05 0.00 – –

4.00 0.66

7.76 0.78

7.80 0.69

2.00 –

Total XMg XIlm XHem XAn XAb XOr

FeO#—FeO total, –:below detection limit, n.a. not analysed.

0.09 2.45 0.26 0.00 0.00

16.00

16.00

0.57 0.36 0.06 0.01

0.53 0.41 0.04 0.01

– –

Kfs

50.8 0.06 5.44 0.05 1.30 19.87 0.19 22.17 0.11 0.04 0.01 n.a.

–

0.08 2.16 0.35

Pl

SiO2 TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O (H2O)

2.77 0.29 1.25 0.00

Rs 49c

56.23 0.00 27.44 0.01 0.08 0.00 0.00 0.00 9.78 5.83 0.26 n.a.

63.48 0.00 18.45 0.03 0.00 0.00 0.00 0.02 0.02 0.22 15.91 n.a.

99.63

98.13

2.53 0.00 1.46 0.00 0.00 0.00 0.00 0.00 0.48 0.51 0.01

2.54 0.00 1.46 0.00 0.00 0.00 0.00 0.00 0.47 0.51 0.01

2.99 0.00 1.02 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.96

5.00

5.00

4.99

0.48 0.51 0.01

0.47 0.52 0.01

0.00 0.02 0.98

8

0.88 0.065

90

S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

Fig. 4. Patches and lamellae of exsolved clinopyroxene (Cpx) in coarse orthoyroxene (Opx) in type A mafic granulite of Sunkarametta, Crossed Nicols. Bar ¼ 160 mm.

Fig. 6. Partial corona of garnet (Grt) intergrown with quartz (colourless) along the contacts of orthopyroxene (Opx) and plagioclase (Pl) in type B mafic granulite of Sunkarametta, Crossed Nicols. Bar ¼ 400 mm.

This rock contains orthopyroxene ðXMg ¼ 0:60Þ and plagioclase forming coarse xenoblastic aggregates. Orthopyroxene is aluminous (Al2O3 ¼ 3.85 Wt%) and replaced by pargasite and biotite. This rock is cut across by alkaline veins roughly along S2 containing megacrystic pethite and antiperthite.

Presence of intergrown orthopyroxene-clinopyroxene-Spinel surrounding relict olivine in type C mafic granulite suggest the mineral reaction

3.5. Mineral reactions

O1 þ P1 ! Opx þ Cpx þ Spl

ð3Þ

Reactions (1) – (3) have been documented by Dasgupta et al. (1993) who explained these reactions as manifestations of isobaric cooling from peak M1 assemblage. Occurrence of coronal garnet surrounding coarse lobe shaped ilmenite in type B mafic granulite suggest the deoxidation reaction

From textural study we have identified several important mineral reactions. The peak M 1 assemblages are orthopyroxene þ clinopyroxene þ plagioclase in Type A and Type B mafic granulite; Orthopyroxene þ clinopyroxene þ olivine þ plagioclase in Type C and Orthopyroxene þ plagioclase in Type D mafic granulite, respectively. In type B mafic granulite, formation of garnet corona (þ quartz) surrounding ortho- and clinopyroxene suggest the reactions

Products of reactions (1) – (4) constitute the early retrograde R1 assemblage. Pargasite replaces both ortho- and clinopyroxene against plagioclase. This implies the possible occurrence of the retrograde reaction in type C mafic granulite

Opx þ P1 ! Grt þ Qtz

ð1Þ

Opx þ Cpx þ Pl þ H2 O ! Parg þ Qtz

Cpx þ P1 ! Grt þ Qtz

ð2Þ

Occurrence of biotite along the S2 planar fabric is a conspicuous feature. Its presence as replacement of

Fig. 5. Three sets of exsolution lamellae of orthopyroxene (Opx) in coarse clinopyroxene (Cpx) in type A mafic granulite of Sunkarametta. Back scattered image.

Fig. 7. Symplectic intergrowth of garnet (Grt) and ilmenite (Ilm) on coarse ilmenite in type B mafic granulite of Sunkarametta. Plane polarized light. Bar ¼ 160 mm.

Fe2 O3 in Ilm þ Pl þ Qtz ! Grt þ O2

ð4Þ

ð5Þ

S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

Fig. 8. Olivine (Ol) is separated from plagioclase (Pl) by granular intergrowth of spinel– orthopyroxene – clinopyroxene – spinel (OCS) in type C mafic granulite of Sunkarametta. Plane polarized light. Bar ¼ 160 mm.

91

Fig. 10. Garnet (Grt) occurs as corona along the contacts of orthopyroxene (Opx) and plagioclase (Pl) in metanorite of Narsapuram. Crossed Nicols. Bar ¼ 160 mm.

In Narsapuram area, two broad compositional types of mafic/metabasic granulite occur (Fig. 3). These are metanorite and two pyroxene granulite. Metanorite is compositionally similar to type D mafic granulite of Sunkarametta. It occurs as lenses parallel to S3 gneissic foliation in leptynite and khondalite and itself is intensely deformed to yield a strong S4 shear foliation. Orthopyroxene, Plagioclase, ilmenite and minor magnetite constitute the primary mineral assemblage with local appearance of garnet corona over orthopyroxene, Plagioclase and ilmenite (Fig. 10). Garnet corona is overthickened at places producing skeletal inclusion ridden

coarser grains. Sheared grains of orthopyroxene and plagioclase occur as envelope over coarse corona of garnet that looks like moat over this thick garnet corona (Fig. 11). Quartz mostly occurs as clots along with plagioclase. Late amphibole and biotite replace garnet and orthopyroxene and are aligned along S4 shear foliation. Orthopyroxene ðXMg ¼ 0:52 – 0:55Þ is alumina poor ðAl2 O3 ¼ 0:93 – 1:33 wt%Þ and plagioclase is extremely calcic ðXAn ¼ 0:84 – 0:89Þ: Thick corona of garnet has almost homogenous composition from its core (Alm59Prp20Grs18Sps3) to rim (Alm58Prp20Grs19Sps3). It is locally slightly depleted in grossular and enriched in almandine (Alm63Prp18Grs17Sps02) near the rims at the contact with sheared grains of orthopyroxene and plagioclase (Table 2). Ilmenite contains 4 –5.5 mol% hematite. Formation of garnet corona can be explained by operation of reactions similar to (1) and (4) construed from the mafic granulites of Sunkarametta. Two pyroxene granulite occurs as bands and lenses in leptynite, Khondalite and enderbite gneiss. It contains clinopyroxene, orthopyroxene, plagioclase (often antiperthitic), ilmenite, magnetite, amphibole and minor quartz.

Fig. 9. Patchy amphibole (Amph) replaces orthopyroxene (Opx) and itself is replaced by biotite (Bt) in type C mafic granulite of Sunkarametta. Plane polarized light. Bar ¼ 160 mm.

Fig. 11. Sheared grains of orthopyroxene (Opx) and plagioclase (Pl) over thick corona of garnet (Grt) in metanorite of Narsapuram. Plane polarized light. Bar ¼ 160 mm.

orthopyroxene in almost all the mafic granulites suggest the reaction Opx þ Kfs þ H2 O ! Bt þ Qtz

ð6Þ

Products of reactions (5) and (6) constitute the late retrograde R2 assemblage. 3.6. Mafic granulites of Narsapuram

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Pyroxenes, plagioclase and ilmenite form the early granoblastic assemblage on which amphibole appeared during a later stage of hydration through reaction (5). Clinopyroxene contains (100) lamellae of orthopyroxene (Fig. 12) indicating subcalcic augite was initially stable. Compositional characteristics of the phases show that the banded granulite evolved from a more magnesian precursor (Table 2). Both the pyroxenes are more magnesian in the banded two pyroxene granulite than in the lenses (Table 2). Plagioclase is likewise more clacic in the banded variety, and is strongly reversibly zoned with D An ¼ 20 – 30 mol% from core to rim. This is apparently intriguing since garnet did not develop in the rock, and pristine igneous zoning is unlikely to be preserved considering the degree of metamorphism and deformation undergone by the rock. We would like to interpret this unusual zoning by metasomatic alteration, which is also vindicated by ubiquitous presence of patchy antiperthite in the rock. Ileminite contains maximum 7 mol% hermatite on stoichiometric consideration. Amphibole is ferroan pargasite to ferroan pargasitic hornblende (13 ex CNK method, Leake, 1978). K-feldspar blebs in antiperthite contain 95 mol% orthoclase component.

Fig. 13. Thin rim of garnet (Grt) occurring along the interfaces of coarse orthopyroxene (Opx) grains in pegmatoidal enderbite of Sunkarametta. Plane polarized light. Bar ¼ 160 mm.

In Sunkarametta enderbite occurs as thick layers and lenses in khondalite and leptynite. This rock is gneissic and fine grained, gneissosity being defined by alternate mm thick pyroxene rich layers and quartzfeldspathic bands. Orthopyroxene, plagioclase feldspar, ilmenite, magnetite, perthite and quartz form early stabilized assemblage and coronal garnet appeared next separating orthopyroxene and ilmenite from plagioclase possibly by reactions similar to (1) and (4). Late biotite develops on orthopyroxene and is possibly formed by the reactions similar to (1) and (4). Late

biotite develops on orthopyroxene and is possibly formed by the reaction (6). Orthopyroxene ðXMg ¼ 0:49 – 0:53Þ is low in alumina ðAl2 O3 ¼ max:1:1wt:%Þ (Table 3). Coronal garnet is rich in pyrope and almandine components and poor in grossular (up to 7 mol. %). Plagioclase shows minor drop in anorthite content ðDAn ¼ 5 mol:%Þ at contact of coronal garnet. Ilmenite contains small but variable amount of hematite component calculated on stoichiometric consideration (Table 3). Late biotite that replaces orthopyroxene is magnesian ðXMg ¼ 0:81Þ: Another variety of enderbite is massive and pegmatoidal in nature and occurs as irregular veins and patches in the fine grained variety. It contains xenoliths of high Mg – Al granulite. It shows almost similar mineralogical but different textural characters with respect to the gneissic variety. Coarse orthopyroxene forms subidioblastic mosaic with plagioclase, mesoperthite, quartz and garnet. Garnet shows a distinct mode of occurrence in this variety of enderbite. It occurs as thin rim as well as veins along the mutual grain boundaries of orthopyroxene (Fig. 13). It also forms decomposition lamellae in orthopyroxene (Fig. 14).

Fig. 12. Clinopyroxene (Cpx) containing orthopyroxene (Opx) lamellae along (100) crystallographic plane in two-pyroxene granulite of Narsapuram. Back scattered image.

Fig. 14. Decomposition lamellae of garnet (Grt) in coarse orthopyroxene (Opx) grains in pegmatoidal enderbite of Sunkarametta. Crossed Nicols. Bar ¼ 160 mm.

4. Petrology of orthopyroxene bearing quartzofeldspathic gneisses

S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

93

Harley (1985) observed similar textural features and argued that such textures result due to cooling. Coarse garnet grains include quartz, ilmenite and plagioclase. Plagioclase is often antiperthitic and its proportion is lower than in the gneissic variety. Orthopyroxene ðXMg ¼ 0:63Þ is aluminous (max. 5.5 wt% Al2O3) and show minor Al-depletion from core to rim. Textural study reveals that such coarse garnet and orthopyroxene grains are mostly xenocrystic in nature. Compositional data also corroborates this contention. Orthopyroxene xenocryst is extremely aluminous (6.9 wt% Al2O3). Garnet is compositionally pyropealmandine with low grossular (, 7 mol%) and spessartine component. Both the phases are comparable to similar phases in the associated high Mg – Al granulite in the study area (Bose et al., 2000). Furthermore, the garnet composition does not match with the composition of garnet porphyroblasts that crystallizes from a calc alkaline magma either from experimental constraints (Clemens and Wall, 1981; Sjkerlie and Johnston, 1992; Singh and Johannes, 1996) or from natural occurrence data (White and Clarke, 1994). Thus textural as well as field occurrence data clearly suggest that the rock is hybrid in nature. In Narsapuram, the gneissic enderbite contains plagioclase, quartz and mesoperthite in the leucoband and orthopyroxene, garnet, ilmenite and magnetite with minor quartz and K-feldspar in the mesobands. Coarse orthopyroxene, plagioclase (often antiperthitic) and quartz constitute the granoblastic fabric in the latter while garnet forms corona (with intergrown quartz) occurring at the interfaces of orthopyroxene and plagioclase (Fig. 15). Its formation can be explained by a mineral reaction similar to (1). Thin to thick corona of garnet with intergrown granular ilmenite separates coarser ilmenite from plagioclase and quartz. This textural feature can be explained by a mineral reaction similar to (4) construed for type B mafic granulite in Sunkarametta. Orthopyroxene, and locally garnet are often replaced by patchy late amphibole while veins of K-feldspar occur along the interfaces of garnet, plagioclase and ilmenite. Orthopyroxene ðXMg ¼ 0:47Þ is alumina poor

(Al2O3 ¼ 1.08 wt%) and plagioclase is andesine ðXAn ¼ 0:44 – 0:48Þ in composition (Table 4). Coronal garnet has the composition Alm62 – 65Prp15Grs17 – 20Sps3 (Table 4). Coarse ilmenite contains 4 –6 mol% hematite component calculated on stoichiometric considerations. Amphibole is ferroan pargasite. Another variety of enderbite in this area is pegmatoidal enderbite occurring as ramifying and cross-cutting veins within metanorite and gneissic enderbite. It consists of megacrystic plagioclase (locally antiperthitic), quartz, Kfeldspar with small idioblasts of orthopyroxene and very coarse porphyroblasts of garnet. Presence of megacrystic garnet is conspicuous as it contains inclusions of hercynite, quartz and stumpy biotite (Fig. 16). The composition of these megacrystic garnets is Alm53 – 57Prp36 – 40Grs4 – 6Sps1, similar to those in the spinel granulite (Dasgupta et al., 1995). Thus both texturally and compositionally this garnet resembles those in the spinel granulites. These garnets also contain inclusions of relatively coarse Fe – Ti oxides, particularly near the margins. The oxide phase is actually an intergrowth of titanohematite and ferrian ilmenite

Fig. 15. Corona of garnet (Grt) grown along the interfaces of orthopyroxene (Opx), ilmenite (Ilm) and plagioclase (Pl) in enderbite gneiss of Narsapuram. Plane polarized light. Bar ¼ 400 mm.

Fig. 17. Complex intergrowth pattern of ilmenite (Ilm) and hematite (Hem) in pegmatoidal enderbite of Narsapuram. Back scattered image.

Fig. 16. Coarse grain of garnet (Grt) contains inclusions of hercynite (Hc), biotite (Bt) and quartz (Qtz) in pegmatoidal enderbite of Narsapuram. Back scattered image.

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S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

Fig. 18. Overgrowth of garnet– quartz symplectite on coarse garnet (Grt) grain in pegmatoidal enderbite of Narsapuram. Plane polarized light. Bar ¼ 160 mm.

(Fig. 17), a feature common to the associated spinel granulite (Dasgupta et al., 1995). Therefore, we argue that the garnet is xenocrystic in nature. However, some of the garnet porphyroblasts contain an overgrowth of garnetquartz symplectite (Fig. 18), which is presumed to have formed due to re-equilibration/interaction of the porphyroblastic garnet with the invading enderbitic magma. Short prismatic orthopyroxene locally overgrows garnet, presumably crystallized from the enderbitic magma using garnet as a substrate. A second generation patchy biotite often replaces garnet along the boundaries. Orthopyroxene is strikingly Mg-rich ðXMg ¼ 0:66Þ and aluminous ðAl2 O3 ¼ 5:44 wt%Þ; as compared to that in the gneissic enderbrite and is compositionally similar to that in the spinel granulite. However, orthopyroxene in the latter is always a coronal variety, and such stumpy idioblasts are not commonly present. Plagioclase has, however, identical composition as that in the gneissic enderbite K-feldspar is nearly pure (98 mol% Or). Included biotite is Mg-rich and titaniferous, clearly comparable with that in the spinel granulite. The garnet-quartz overgrowth is possibly a product of interaction between the restite and the enderbitic magma. 5. Pressure-temperature-fluid evolutionary history 5.1. Thermobarometry The peak M1 stage of metamorphism is characterized by stabilization of coarse grained mineral assemblages in mafic and felsic gneisses in both the study areas. Garnet did not make an appearance at this stage. Temperature of M1 metamorphism could not be measured accurately using the conventional temperature sensors due to their thermal resetting at lower temperature (Frost and Chacko, 1989; Chakraborty and Ganguly, 1990). The temperature of M1 metamorphism has been constrained directly from associated spinel/sapphirine granulites that record UHT metamorphism in excess of 1000 8C at this stage (Dasgupta et al.,

1995 in Narsapuram; Bose et al., 2000 in Sunkarametta). Moreover, we have made some sincere effort to calculate metamorphic temperature at M1 stage using the technique of fossil thermometry (cf. Frost and Chacko, 1989). We have applied this technique to type A and B mafic granulites of Sunkarametta where both ortho- and clinopyroxene grains show frequent exsolution texture. The recomposed pyroxene (using BSE image analysis) would represent preexsolution pristine pyroxene composition, which yield maximum temperature of 980 8C (Table 5A) in the thermometric software QUILF (Anderson et al., 1993). The ortho- and clinopyroxene in type C mafic granulite of Sunkarametta gives a maximum temperature of 940 8C (Table 5A). The anorthite-CaTs – SiO 2 barometer (McCarthy and Patin˜o Douce, 1998) gives pressure estimate of 8.5 ^ 0.5 kbar from the same rock. These data corroborate the earlier estimate of Dasgupta et al. (1993) from similar rocks. Dasgupta et al. (1993) constrained the pressure of emplacement of mafic magma (precursor to the mafic granulites) at 8.5 kbar considering the experimental data on the stability of early assemblage olivine –orthopyroxene – clinopyroxene – plagioclase (Herzberg, 1978). Recent experimental data in CFMAS system further put an upper pressure limit of 10 kbar for the above assemblage (Gust and Prefit, 1987; Kuehner, 1992). P – T condition during R1 stage has been constrained mainly from type B mafic granulite in Sunkarametta and from metanorite and enderbite gneiss in Narsapuram. In all of these rocks appearance of coronal garnet marks the onset of this stage of metamorphism. In Sunkarametta metamorphic temperature is calculated using the composition of coronal garnet coexisting with ortho- and clinopyroxene. The garnet-clinopyroxene thermometer gives , 850 8C temperature using the model of Ganguly (1979), which is incidentally 100 8C higher than the estimate of Ellis and Green (1979). However, using the extended Ellis and Green (1979) model outlined by Sengupta et al. (1989), a maximum 930 8C temperature is estimated. Other models show lower estimates (Table 5A). The garnet –orthopyroxene pair gives the maximum temperature of 850 8C using the model of Lee and Ganguly (1988). Apart from Ganguly et al. (1996) other models (Harley, 1984; Bhattacharya et al., 1991) record significantly lower estimates (Table 5A). When the garnet corona is relatively thicker, its core and rim compositions are likely to reflect the physical realm of cooling. We have found that the rim compositions always give 80 –120 8C lower temperature estimates (Table 5A). Pressure estimates come mainly from garnet –pyroxene – plagioclase – quartz assemblages in which considerably contrasting results do appear for Fe and Mg end-member reactions. However, we have chosen PFe values in such cases, as the bulk composition of the type B mafic granulite is iron-rich. Maximum pressure, thus estimated is 9 kbar using the garnet – orthopyroxene – plagioclase – quartz (GOPQ) barometer (Perkins and Chipera, 1985). The estimated pressure is 10 kbar using the model of Bohlen

S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

95

Table 5 Summary of P– T estimates Rock type

Geothermobarometer used

Assumed P or T (kbar/(C)

Results P (kbar)

A. Sunkarametta Mafic granulite

Enderbitic gneiss

Two pyroxene QUILF Anderson et al., 1993 Two pyroxene Kertz, 1982 Ca–Mg exchange CaTs –An–Qtz McCarthy and PatinoDouce, 1998 Grt– Cpx Ganguly, 1979 Ellis and Green, 1979 Sengupta et al., 1989 (extd. EG) Ganguly et al., 1996 Grt– Opx Lee and Ganguly, 1988 Harley, 1984 Battacharya et al., 1991 Ganguly et al., 1996 GOPQ Perkins and Chipera, 1985 Bhattacharya et al., 1991 Bohlen et al., 1983 GCPQ Moecher et al., 1988 Two feldspar Fuhrman and Lindsley, 1988 Grt– Bt Dasgupta et al., 1991 Ganguly et al., 1996 Ferry and Spear, 1978 Opx– Bt Sengupta et al., 1990b Hbd– Pl Holland and Blundy, 1994 Grt– Opx Lee and Ganguly, 1988 Harley, 1984 Bhattacharya et al., 1991 Ganguly et al., 1996 GOPQ Newton and Perkins, 1982 Bhattacharya et al., 1991 Bohlen et al., 1983

T ((C)

8 kbar 900 8C

980 940 7.8 –9.0

8 kbar

850 (C) 810 (R) 750 (C) 710 (R) 930 (C) 860 (R) 810 (C) 760 (R)

8 kbar

880 (C) 780 (R) 730 (C) 640 (R) 780 (C) 700 (R) 850 (C) 760 (R)

880 8C

9.0 (Fe) 8.4 (Fe) 10.0 (Fe)

900 8C 8 kbar

7.8 (Fe) 650

6.5 kbar

580 480 410

6.5 kbar

640

6.5 kbar

830

8 kbar

850 750 780 750

850 8C

7.3 (Mg) 7.3 (Mg) 10.5 (Fe)

B. Narsapuram

Enderbite gnesiss

Metanorite

Grt– Opx Lee and Ganguly, 1988 GOPQ Bhattacharya et al., 1991 GOPQ Bohlen et al., 1983 Grt– Hbd Graham and Powell, 1984 Two feldspar Stormer and Whitney, 1977 Hbd– Pl Holland and Blundy, 1994 Grt– Opx Lee and Ganguly, 1988 GOPQ Bhattacharya et al., 1991 Opx– Ilm Bishop, 1980 Grt– Ilm Pownceby et al., 1987

780–790 7.5 8.7 670–725 560–620 5 kbar

800

6 kbar

790 (C) 740 (R)

750 8C

6.5 1015 (I) 855 (M) 855 (C) 790 (R) (continued on next page)

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Table 5 (continued) Rock type

Geothermobarometer used

Assumed P or T (kbar/(C)

Results P (kbar)

Pegmatoidal enderbite

Two pryoxene granulite

Grt–Bt Bhattacharya et al., 1992 Dasgupta et al., 1991 Ferry and Spear, 1978 Ganguly et al., 1996 Two pyroxene Kertz, 1982—Fe–Mg exchange Ca–Mg exchange Two pyroxene Lindsey, 1983 Opx–Ilm Bishop, 1980 Cpx–Ilm Bishop, 1980 Ca Ts–An–Qtz McCarthy and Patino Douce, 1998 Two feldspar Stormer and Whitney, 1977 Fuhrman and Lindsley, 1988 Hbd–Pl Holland and Blundy, 1994

T ((C)

580– 610 650– 700 630– 680 600– 660 550– 670 890 650– 700 740 580– 790 750 8C

4.2–4.5 440– 500 500 790– 830

C: Core, R: Rim, I; Inclusion, M: Matrix.

et al. (1983) (Table 5A). This estimate is clearly on the higher side considering the fact that this model uses simple mixing models for garnet and plagioclase. Slightly lower values are estimated from garnet –clinopyroxene – plagioclase –quartz (GCPQ) barometry (Table 5A). In Narsapuram the garnet-orthopyroxene thermometer gives maximum temperature of , 790 8C while GOPQ barometer estimates 7.0 ^ 0.5 kbar pressure for both metanorite and enderbite gneiss (Table 5B). The anorthite –CaTs– quartz barometer (McCarthy and Patin˜o Douce, 1998) iu two pyroxene granulite of Narsapuram gives very low estimate (4.5 – 5.0 kbar) either indicating condition of silica infiltration from outside of the system or represents an artifact more likely in view of the uncertainty in computing the CaTs component in clinopyroxene. Two pyroxene Fe – Mg exchange thermometer (Kretz, 1982) registers maximum temperature of 670 8C, which of course is cooling temperature. Similar temperature estimate comes from two feldspar thermometry (Fuhrman and Lindsley, 1988) in different rocks in the two areas. Enderbite gneiss of Sunkarametta gives maximum 850 8C temperature using the garnet –orthopyroxene pair (Lee and Ganguly, 1988). The GOPQ barometer gives maximum pressure around 7.3 kbar (PMg) (Newton and Perkins, 1982; Bhattacharya et al., 1991, Table 5A). R2 stage is characterized by extensive development of biotite and amphibole on R1 assemblages in different rocks. In Sunkarametta, orthopyroxene – biotite thermometer records 640 8C (Sengupta et al., 1990b) while garnet – biotite thermometer records , 580 8C (Table 5A) as maximum using the model of Dasgupta et al. (1991). In

Narsapuram, this temperature estimate is slightly higher i.e. 650 8C (Table 5B) using the same model. The model of Ganguly et al. (1996) gives comparable estimates in both the occasions, whereas the model of Ferry and Spear (1978) gives much lower estimate (Table 5A, B). The hornblende– plagioclase thermometer (Holland and Blundy, 1994) registers temperature in excess of 800 8C an all the rocks. This thermometer is known to be problematic and the temperature estimates are clearly at higher side since amphibole appeared late in the rocks. The garnet-hornblende thermometer (Graham and Powell, 1984) in gneissic enderbite of Narsapuram gives ca. 675 –725 8C. Pressure condition during this stage could not be measured directly from these rocks due to lack of suitable and reliable pressure sensors. However, we have taken the pressure to be , 6 kbar determined from associated spinel granulites (Dasgupta et al., 1995). In Sunkarametta the garnet – cordierite – sillimanite – quartz barometer in the high Mg – Al granulite gives maximum pressure of about 6.5 kbar determined using the model of Nichols et al. (1992). Hence P– T condition at this stage has been chosen as 650 8C, 6.5 kbar. Summarizing, all thermobarometric data it appears that the peak M1 metamorphism occurred at 8.5– 9 kbar pressure and temperature . 950 8C as recorded in two localities separated by nearly 200 km distance. Small baric difference from north to south (Narsapuram) is obvious. These P – T data corroborates well with those determined from the associated high Mg – Al granulites (Dasgupta et al., 1995; Bose et al., 2000). The R1 stage is marked by a pronounced retrogressive episode of cooling up to 700 – 750 8C at 7.0– 7.5 kbar in both areas. Stage R2 is clearly the stage of

S. Bose et al. / Journal of Asian Earth Sciences 22 (2003) 81–100

solid –fluid interaction, which possibly took place at 6 – 6.5 kbar, 600 – 650 8C. It is pertinent to mention here that the pegmatoidal enderbite records similar P –T estimates of R2, which renders its emplacement prior to R2 stage. 5.2. Characterization of metamorphic fluid Textural data demonstrate that M1 and R1 stages of metamorphism were characteristically fluid-deficient, if not absent, stabilizing dry mineral assemblages. We have calculated fluid activity during R2 stage using mineralfluid equilibria, a technique outlined by Ferry and Burt (1982). For this purpose reactions (5) and (6) are considered as model equilibria. Between the two, the former has been found to occur in almost all the rock types in both areas. The balanced reaction in the KFMASH system is 3 En þ 2 San þ 2 H2 O ¼ 2 Phl þ 6 Qtz We have taken the thermochemical parameters and RT ln fH2O values from internally consistent dataset of Holland and Powell (1998) and the activity of the solid solution phases are calculated using the software A-X. The aH2O values calculated at specified pressure and temperature of R2 stage, show a cluster around 0.1 irrespective of rock type. Similar estimate comes from the equilibrium reaction pertaining to reaction (5). The calculated aH2O value, therefore, show H2O deficient nature of the fluid during R2 stage. It is not possible to compute the activity of other gaseous species e.g. CO2, halogens etc. which might have been present in the fluid. However, fluid activity data from the associated calc silicate granulites reveal that during this stage aCO2 is around 0.41 in Narsapuram. Thus the activities of both H2O and CO2 are low in the fluid as calculated at this stage.

6. Discussion Mafic and orthopyroxene-bearing quartzofeldspathic gneisses constitute an important member in many regional granulite rock assemblages (see Harley, 1989). Field relation and textural and geochemical data on the mafic granulites of the EGB suggest that such rocks are originated from crystallization of basaltic magma (Dasgupta et al., 1993; Sengupta et al., 1996). The temperature of M1 metamorphism is comparable to the liquidus temperature of the basaltic magma. Furthermore, phase relation in the CFMAS system implies proximity between emplacement depth of the magma and peak metamorphic pressure (PMax). It may be recalled that the associated Mg – Al granulites record UHT metamorphism (Dasgupta et al., 1995; Bose et al., 2000) for which enormous amount of advective heat supply is the most possible explanation (Harley, 1989). In view of the close association of these rocks showing comparable P –T – D history we are tempted to argue that the crystallization of the mafic magma could have been

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responsible for the anomalous heat supply. Accretion of such basic magma at the base of the crust is consistent with P – T trajectory recorded from the high Mg – Al granulites of Sunkarametta (Bose et al., 2000) and other parts of the EGB (Dasgupta et al., 1997; Sengupta et al., 1999). The P– T path in Narsapuram is not a similar heating-cooling trajectory indicating that the effect of the mafic magma possibly was not as pronounced as in Sunkarametta as it is lower in volume in this area. However, the anticlockwise prograde path deduced from the area (Dasgupta et al., 1995) indicates probable magmatic under/intraplating in an extensional crust (Oxburgh, 1990). Pegmatoidal enderbite is characteristically a hybrid rock containing xenocrysts of garnet, orthopyroxene and oxide minerals from the associated leptynite and/or high Mg – Al granulites. This rock is massive without any development of the regional gneissic foliation. It certainly implies that this rock was emplaced in wide extent after the major deformational episodes. Available geochronological data suggest that the emplacement of the pegmatoidal enderbite could be manifestation of renewed anatexis during the Grenvillian orogeny (Rickers et al., 2001). This thermotectonic event in the EGB is accompanied by a isothermal decompression in between R1 and R2 (Sengupta et al., 1990a; Dasgupta et al., 1995; Dasgupta and Sengupta, 2000 and references therein). The decompression inferred by Dasgupta et al. (1995) from Narsapuram is not reflected in the thermobarometric data derived in this study. Decompression is principally invoked by Dasgupta et al. (1995) to explain formation of late cordierite in the spinal granulite. Since the stability of cordierite can be enhanced in the presence of a fluid, its formation in this rock can be explained by this alternative mechanism. Fluid composition during R2 stage is poor in both H2O and CO2. Recent experimental study shows that apart from CO2 reduction of aH2O can be brought about by K-rich brine (Aranovich and Newton, 1998). A combined effect of CO2rich fluid and K-rich brine is impossible to conceive due to immiscibility of the two at high grade condition (Duan et al., 1995). We would prefer the effect of K-rich brine as a possibility considering the following factors. 1. Presence of high concentration of F in late biotite in the associated high Mg –Al granulite is a key feature (Bose et al., 2000) suggesting halogens are the prime components of the brine. Although Cl is not measured for biotite its presence is a possibility. The brine solution, therefore, could be rich in KCl – KF. 2. Widespread development of K-feldspar-cordierite intergrowth in high Mg – Al granulite could be indicative of K-metasomatism. Although Bose et al. (2000) argued for the possibility of its origin directly from melt, an alternative explanation like K-metasomatism (B.J. Hensen pers. com.) is also possible. 3. Presence of patchy antiperthite and thin veins of Kfeldspar along the interfaces of other phases in enderbite

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and mafic gneisses may indicate activity of K-rich solution at this stage 4. Occurrence of pargasitic amphibole along the biotite in certain varieties of mafic granulite and enderbite gneiss could also be cited as evidence for the activity of concentrated chloride/fluoride solution (Aranovich and Newton, 1998) 5. Experimental data indicate that the alkali-enriched brine could be more effective for its ability to exchange alkali ions and greater infiltration in quartzofeldspathic rocks than relatively denser CO2 (Watson and Brenan, 1987). The source of this K-rich fluid is a matter of speculation. The most plausible source could be the crystallizing alkaline magma in the nearby areas (Karmakar, 1992). Moreover, extensive K-feldspathization is documented from the EGB (Bhattacharyya, 1972; Sengupta et al., 1990a). Bhowmik (2000) argued that K-metasomatism in the EGB is a manifestation of thermal perturbation during the PanAfrican orogeny (500 – 550 Ma). Recent isotopic data show wide spread of model ages from Late Archean (2.9 –2.5 Ga) to Middle Proterozoic (2.2 –1.4 Ga) for orthogneisses from this part (Domain 2) of the EGB (Rickers et al., 2001). The age of M1 metamorphism is not clear, but is distinctly pre-Grenvillian (1.4 Ga?? Jarick, 2000). It therefore renders a possibility that there is a causal relationship between mafic/felsic plutonism and UHT metamorphism. In the recent model of Indo-Antarctic reconstruction, the area belonging to the Domain 2 of the EGB is juxtaposed to the Proterozoic Rayner Complex of east Antarctica (Unrug, 1996; Sengupta et al., 1999; Harley, 2003). The Rayner Complex consists abundant mafic and felsic orthogneisses and is characterized by a strong isothermal decompressive event of Grenvillian age (Harley and Hensen, 1990; Harley, 2003 and references therein). Parts of the Domain 2 of the EGB also record this isothermal decompressive event following the isobaric cooling stage (Dasgupta et al., 1994; Paul et al., 1990; Mezger and Cosca, 1999). However, there are few isolated locales in the Rayner Complex, where the rocks dominantly preserve evidences of isobaric cooling history (like the present one) with little or minor decompression (Harley, 2003). Some of these rocks also bear evidences of pre-Grenvillian thermotectonic events and show P – T-fluid evolutionary history similar to one presented here. For the Antarctic granulites, this discrepancy of contrasting nature of the P –T trajectories has been explained by partitioning of strain during the Grenvillian orogeny in the Rayner Complex, in which, the low-strain zones apparently do not show evidences of the later decompressive events (Harley, 2003). A similar line of argument can be attributed to the rocks of the study area Sunkarametta, which can explain the complete absence of Grenvillian decompression. Unlike the case of East Antarctica, however, there are very limited peterological, structural and geochronological data from the EGB to arrive at a firm conclusion in this respect.

The R1 metamorphism can be correlated to the thermal relaxation postdating the M1 metamorphism. The R2 metamorphism is a separate episode and is a possible manifestation of deformation and fluid –rock interaction along the prominent shear zones of the erstwhile Rodinia during the Pan-African Orogeny.

7. Conclusions Textural and thermobarometric data derived from the mafic granulite and orthopyroxene-bearing quartzofeldspathic gneiss show a polymetamorphic evolutionary history. The peak M1 metamorphism exceeds 950 8C at 8 –9 kbar pressure. This is followed by R1 retrograde stage when the rocks suffer an episode of isobaric cooling up to 700– 750 8C at 7– 7.5 kbar. The late retrograde stage (R2) is an episode of fluid-rock interaction at 6 – 6.5 kbar, 600– 650 8C. The fluid composition is poor in H2O or CO2 and in all probability is enriched in K-rich brine.

Acknowledgements SB acknowledges University Grants Commission for awarding a Minor Research Fellowship (Grant No. PSW038/00-01(ERO)). SP and SB acknowledge CSIR and UGC, respectively, for their Doctoral Research Fellowship during which most of the samples of the present study were collected and analyzed. We are extremely indebted to Prof. S. Dasgupta and Dr P. Sengupta of Jadavpur University for many fruitful discussions and suggestions over the years. Prof. S.S. Sarkar, Head of the Department of Geology, Durgapur Govt. College is thankfully acknowledged for infrastructural help and encouragement. Special thanks are due to Profs B.J. Hensen and E.J. Essene for their comments on certain critical aspects of this work. Thought provoking and constructive reviews by Profs T.K. Biswal and A. Mohan helped immensely to improve the quality of this paper. This communication is a contribution to IGCP 368.

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