Mineral Reactions and Geothermobarometry in a Suite of Granulite Facies Rocks from Paderu, Eastern Ghats Granulite Belt: a Reappraisal of the P-T Trajectory

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Mineral reactions and geothermobarometry in a suite of granulite facies rocks from Paderu, Eastern Ghats granulite belt: A reappraisal of the P-T trajectory

SUPRATIM PAL and SANKAR BOSE Department of Geological Sciences, Jadavpur University, Calcutta 700 032, India

High Mg-A1 spinel-sapphirine granulites, orthopyroxene-bearing quartzofeldspathic granulites, two pyroxene-bearing mafic granulites and metapelitic gneisses are exposed around Pa~leru, Eastern Ghats Belt. Geothermobarometry in orthopyroxene-bearing quartzofeldspathic granulites and mafic granulites indicate near isobaric cooling through 90~ from ca. 720~ to 630~ at 8.0 kbar. However, signatures of ultrahigh temperature metamorphism are recorded from the mineralogy and reaction textures in the high Mg-A1 granulites. Mineral reactions deduced in this work, when combined with others described by Lal et al (1987) from the same area and plotted in an appropriate petrogenetic grid in the system FMASO indicate an ACW path comprising a high d T/dP prograde arm reaching Pm~x- Tma~ = 9.5kbar, 1000~ followed by near-isobaric cooling down to 9kbar, 900~ and subsequent decompressive reworking.

1. Introduction pressure-temperature evolution history is best recorded in Mg-A1 granulites for which semi-quanti- It has become increasingly clear in recent years that tative to quantitative petrogenetic grids in the several granulite terranes of the world experienced systems FMAS and KFMASH have become recently ultrahigh temperatures (ca. 1000~ of metamor- available (Hensen 1986; Hensen and Harley 1990; phism at mid- to deep crustal levels correspond- Dasgupta et al 1995; Carrington and Harley 1995). ing to 7-10kbar pressures. Examples include the However, mineralogical geothermobarometry can Napier Complex (Ellis 1983), the Arunta Complex provide some constraints on the retrograde evolution (Goscombe 1992), the Labwor Hills (Sandiford et al of the terranes and is still useful. In this communica- 1987) and the Eastern Ghats Belt, India (Lal et al tion, we deduce the pressure-temperature history of 1987; Sengupta et al 1990; Dasgupta and Sengupta the granulite facies rocks occurring near Paxieru, 1995; Sen et al 1995). It is well known that minera- Eastern Ghats belt, through a combination of a logical geothermometry cannot act as sensor for the petrogenetic grid approach for high Mg-A1 granulites peak thermal conditions in case of such ultra high and geothermobarometric approach in associated temperature metamorphism due to down-temperature mafic and orthopyroxene-bearing quartzofeldspathic resetting of mineral compositions (Frost and Chacko gneisses. On the basis of the deduced P-T trajectory 1989). However, in some cases attempt has been made from the study area as well as from adjoining areas to evaluate almost near-peak condition of meta- in the Eastern Ghats belt (Sengupta et al 1990, morphism (Harley and Fitzsimons 1991). Recently 1991; Dasgupta et al 1994, 1995), we will attempt to Fitzsimons and Harley (1994) have used a conver- model the geodynamic evolution and tectonic milieu gence technique for the recovery of peak (or at least of this belt of ultra-high temperature granulite facies near-peak) metamorphic conditions. In such terranes, rocks.

Keywords. Mineral reactions; geothermobarometry; Paderu, Eastern Ghats, ACW P-T trajectory.

Proe. lndian Acad. Sci. (Earth Planet. Sci.), 106, No. 3, September 1997, pp. 77-89 Printed in India 77 78 Supratim Pal and Sankar Bose

quartz-perthite-sillimanite-garnet-plagioclase gneiss M.R h~rusl or khondalite, quartz-plagioclase-garnet-perthite 1 203 Km I gneiss or leptynite, orthopyroxene-bearing charnocki- IND~ tic and enderbitic quartzofeldspathic gneiss, two M,R- iv~nodi River 5.R-(3o(~vori River pyroxene-bearing mafic granulite and small lenses of high Mg-A1 granulites occurring within leptynite. All G.R the rock types are characterized by the presence of a pervasive gneissic foliation striking NNE-SSW, which is later folded into an antiformal structure. Thus, the rocks bear evidence of at least two episodes of deformation. Later ductile shear zones traverse the oo rocks dominantly in a NW-SE direction. The gneissic foliation is defined by deformed granulite facies ~0 0 75*0' 96'0' mineral assemblages and thus post-dates peak granulite facies metamorphism. The predominant gneissic Figure 1. The position of The Eastern Ghats Mobile Belt in foliation is defined by alternating layers of quartz- India and the position of the study area Paderu in it. feldspar and mafic minerals and is particularly well developed in khondalite and leptynite. 2. Geologic background 3. Brief petrology of the enclosing rock types The present study area is near the village Talarsingi just outside the Paderu town in Visakhapatnam 3.1 Metapelites district of Andhra Pradesh (18 ~ 041; 82 ~ 38 I) falling in the southern sector of the Eastern Ghats belt Khondalites and Leptynites are the major metapelitic (figure 1). The dominant rock types in this area are lithounits of this area. Porphyroblastic garnet, coarse

Table 1. Representative mineral compositions of mafic granulite. Sample no. G 66 Anal. no. 21(C) 20(R) 23(C) 22(R) 9(R) 2 Mineral Orthopyroxene Clinopyroxene Hornblende SiO2 48.49 48.24 47.65 48.24 48.81 40.58 40.55 TiO2 0.12 0.05 0.40 0.22 0.22 2.02 1.48 A1203 1.25 1.34 3.25 2.34 2.23 11.23 10.95 Cr2 0 3 0.03 0.02 0.03 0.09 0.05 0.18 0.11 Fe203 6.60 6.52 4.20 4.37 5.11 1.57 2.14 FeO 18.83 18.51 6.07 5.90 5.36 11.28 10.74 MnQ 0.68 0.69 0.32 0.25 0.25 0.15 0.09 MgO 21.16 21.22 13.12 13.75 13.81 13.72 14.26 CaO 0.63 0.52 19.99 20.09 20.86 11.46 11.60 Na20 0.01 -- 0.42 0.32 0.34 1.35 1.33 K20 -- 0.01 -- -- 0.01 3.04 2.70 H20 ..... 1.98 1.96 Total 97.80 97.11 95.45 95.57 97.05 98.56 97.91 Oxygen basis 6 23 Si 1.87 1.87 1.87 1.89 1.88 6.16 6.18 Ti -- -- 0.01 0.01 0.01 0.23 0.17 A1W 0.06 0.06 0.13 0.11 0.10 1.91 1.91 A1w -- -- 0.02 -- -- 0.07 0.03 Cr ..... 0.02 0.01 Fe z+ 0.19 0.19 0.12 0.13 0.15 0.18 0.25 Fe 2+ 0.61 0.60 0.20 0.19 0.17 1.43 1.37 Mn 0.02 0.02 0.01 0.01 0.01 0.02 0.01 Mg 1.22 1.23 0.77 0.80 0.79 3.10 3.24 Ca 0.03 0.02 0.84 0.84 0.86 1.86 1.89 Na -- -- 0.03 0.02 0.03 0.40 0.39 K ..... 0.59 0.53 Total 4.00 4.00 4.00 4.00 4.00 16.00 16.00 XMg 0.650 0.660 0.420 0.440 0.430 XFe 0.330 0.320 0.120 0.100 0.100 Xca 0.020 0.020 0.460 0.460 0.470 (Continued) Mineral reactions and geothermobarometry in granulite facies rocks 79

Table 1. (Continued) Sample no. C 66 Anal. no. 26(C) 27(R) 10 7(Cr) 11 Mineral Plagioclase Magnetite Ilmenite Biotite SiO2 46.97 45.89 0.04 0.03 37.77 TiO2 0.02 -- 0.09 45.26 3.13 A1203 32.12 32.98 0.28 0.03 12.36 Fe203 -- -- 65.48 13,48 -- Cr203 0.02 -- 1.42 0.09 0.01 FeO 0.19 0.34 30.40 39.06 7.95

MnO -- 0.06 0.02 0.46 0.04 MgO 0.01 -- 0.03 0.69 21.45 CaO 16.59 17.59 0.09 -- 0.03 Na20 2.05 1.53 -- -- 0.09 K20 0.13 0.05 -- -- 10.41 H20 4.02 Total 98.09 98.44 97.85 99.10 97.25 Oxygen basis 16 16 3 22 Si 4.40 4.30 0.01 -- 5.63 Ti -- -- 0.01 0.87 0.35 A1 3.54 3.64 0.05 -- 2.17 Fe 3+ -- -- 7.74 0.26 -- Cr -- -- 0.18 -- -- Fe 2+ 0.01 0.03 3.99 0.83 0.99

Mn ------0.01 --

Mg -- -- 0.01 0.03 4.77 Ca 1.66 1.76 0.02 -- -- Na 0.37 0.28 -- -- 0.03 K 0.01 0.01 -- -- 1.98 Total 9.99 10.02 12.00 2.00 15.92 XAb 0.180 0.140 XAn 0.810 0.860 Xor 0.010 -- Xnm 0.830 XHem 0.130 XF~+ 0.660 XMg 0.780 XFe 0.160 XTi 0.060 C--Core; R--Rim; Cr=Coronal; Xi=i/~i where i=Fe 2+, Mg, Ca Ti (for biotite); XAD=Na/Ca+ Na+K; XAn = Ca/Ca + Na + K; Xor = K/Ca + Na + K. Fe Xnm =- Fe 2+ + Mg + Mn + Fe3+/2 "

sillimanite, quartz, mesoperthite and minor plagio- filling mineral with or without biotite, replacing the clase and rutile are present in khondalite. In pyroxenes. Magnetite occurs as porphyroblasts. Ilme- leptynite, sillimanite is absent; otherwise, the miner- nite occurs as fine granules around hornblende. alogy remains the same as that of khondalite. Plagioclase is both porphyroblastic and fine grained. However, modally plagioclase dominates over meso- Bulk chemical analysis of this rock (by XRF at the perthites in leptynite. Khondalites in the study area University of Bonn) reveals that it is hypersthene are extremely weathered and are unsuitable for normative basalt with MgO/(MgO+FeOtotal) chemical analyses. -- 0.38 -0.53. Composition of the phases was determined by CAMEBAX-MICROBEAM electron probe microanalyzer at the University of Bonn. The 3.2 Mafic granulite structural formulae of the minerals have been Orthopyroxene, clinopyroxene, hornblende, plagio- determined using the computer program MINFILE, clase and magnetite constitute this rock. Besides, Version 1-89, by A M Afifi and E J Essene. Repre- ilmenite, quartz and biotite occur as minor constitu- sentative mineral compositions are given in table 1. ents. Both orthopyroxene and clinopyroxene occur as Amphiboles are magnesio-hastingsite in composition, porphyroblasts. Hornblende is mostly a late fracture- calculated on 13 ex-CNK basis. Orthopyroxenes 80 Supratim Pal and Sankar Bose are enstatitic (XMg ---- 0.65-0.66) with low A1203 Fe = 0.42-0.44) with low A1203 (2.23-3.25wt.%) contents (1.25-1.34 weight per cent). Clinopyroxenes contents. Plagioclase is anorthite rich (XAn ---- 0.81- are similarly diopsidic (XMg---- Mg/Mg+ Ca+ 0.86). The ilmenites contain substantial amount oI

Figure 2(a). Orthopyroxene (O) and ilmenite (I) separated Figure 2(b). Symplectite of garnet (G) and quartz (Q) from plagioclase (P) by coronal garnet (G) and intergrown separating magnetite (M) with exsolved hercynite (Hc) from quartz (Q) in orthopyroxene-bearing quartzofeldspathic gneiss plagioclase (P) in orthopyroxene-bearing quartzofeldspathic (G20/1). Plane polarized light, scale bar = 140 pin. gneiss (G40B). Plane polarized light, scale bar -- 56 ~m.

Table 2. Representative mineral compositions of enderbitic gneisses. Sample no. G40B Anal. no. 69(C) 68(R) 4 6 67 11 75 10 Mineral Orthopyroxene Garnet Biotite SiO2 49.62 50.64 37.87 37.87 37.49 37.46 38.41 39.33 TiO2 0.11 0.09 0.53 -- 0.02 0.03 0.02 3.58 A1203 2.85 1.79 21.18 21.12 20.19 20.67 20.87 12.77 Cr203 0.01 0.02 0.01 0.01 -- 0.01 -- 0.01 Fe203 1.95 1.21 1.11 1.26 1.57 0.98 1.32 -- FeO 26.90 27.16 30.87 28.80 28.76 29.61 27.66 8.39 MnO 0.36 0.30 0.87 0.97 1.27 0.89 0.66 -- MgO 17.70 18.38 5.28 6.40 4.41 5.45 6.16 21.38 CaO 0.35 0.28 3.75 3.53 3.57 3.65 4.18 0.04 Nar 0.03 0.01 ..... 0.07 K20 0.01 ...... 10.50 H20 4.15 Total 99.89 99.88 101.47 99.96 97.28 98.75 99.28 100.92 Oxygen basis 6 24 22 Si 1.91 1.94 5.91 5.94 6.00 5.99 6.00 5.68 Ti -- -- 0.06 .... 0.39 Alw 0.09 0.06 0.09 0.06 -- 0.01 -- -- AlvI 0.03 0.02 3.81 3.85 3.81 3.83 3.84 2.17 Cr ...... Fe3+ 0.06 0.03 0.13 0.15 0.19 0.12 0.16 -- Fe2+ 0.86 0.87 4.03 3.78 3.85 3.96 3.61 1.01 Mn 0.01 0.01 0.12 0.13 0.17 0.12 0.09 -- Mg 1.01 1.05 1.23 1.50 1.05 1.30 1.43 4.60 Ca 0.01 0.01 0.63 0.59 0.61 0.62 0.70 0.01 Na ...... 0.02 K ...... 1.93 Total 4.00 4.00 16.00 16.00 15.81 16.00 15.83 15.81 Xpy 0.540 0.540 0.205 0.250 0.184 0.220 0.245 XAlm 0.460 0.450 0.670 0.630 0.676 0.660 0.620 XGr 0.000 0.010 0.105 0.098 0.107 0.100 0.120 Xsp 0.000 0.000 0.020 0.022 0.033 0.020 0.015 XMg 0.770 XFe 0.170 XTi 0.060 (Continued) Mineral reactions and geothervnobarometry in granulitc facies rocks 81

Table 2. (Continued) Sample no. G40B Anal. no. 61 62 7 1 2 5 70 73 74 Mineral Plagioclase Ilmenite Spinel Magnetite SiO2 56.29 56.49 55.27 -- 0.04 0.69 0.03 0.01 0.01 TiO2 ------49.33 48.96 48.00 48.25 0.25 1.18 Al203 28.11 28.14 27.89 0.04 0.06 0.37 0.06 60.46 0.85 Cr203 0.02 0.02 -- 0.04 -- 0.01 0.03 0.73 0.51 Fe203 ------6.36 6.16 6.92 5.27 0.51 65.99 FeO 0.13 0.42 0.21 42.64 42.22 41.32 41.72 28.60 32.40 MnO 0.04 -- 0.01 0.13 0.17 0.28 0.23 0.11 0.02

MgO -- 0.03 -- 0.92 0.94 1.02 0.83 8.38 0.14 CaO 10.51 10.51 10.32 -- 0.02 0.01 0.01 -- -- Na20 5.55 5.62 5.50 -- 0.02 -- 0.01 -- -- K20 0.38 0.39 0.38 -- 0.01 0.01 ------Total 101.02 101.63 99.57 99.46 98.60 98.72 96.43 99.04 101.09 Oxygen basis 16 3 16 Si 5.02 5.01 5.00 -- -- 0.02 ------Ti ------0.94 0.94 0.91 0.94 0.02 0.13 A1TM 2.96 2.94 ------0.01 ------A1VI -- -- 2.97 .... 7.85 0.15 Cr ...... 0.06 0.06 Fe 3+ ------0.12 0.12 0.10 0.10 0.04 7.52 Fe 2+ 0.01 0.03 0.02 0.90 0.90 0.87 0.90 2.63 4.10 Mn ...... 0.01 Mg ------0.03 0.03 0.04 0.03 1.38 0.03 Ca 1.00 1.00 1.00 ...... Na 0.96 0.97 0.96 ...... K 0.04 0.04 0.04 ...... Total 10.00 10.00 10.00 1.99 1.99 1.98 1.97 12.00 12.00 XAb 0.480 0.480 0.480 Xgn 0.500 0.500 0.500 Xor 0.020 0.020 0.020 XI~ 0.914 0.910 0.890 0.920 XHem 0.055 0.061 0.066 0.050 XHc 0.650 XFez+ 0.650 C = Core; R = Rim; Anal no. 4 = Corona at contact of ilmenite; 6 = Corona at contact of plagioclase; 67 -- Corona at contact of orthopyroxene; 11 = Replaced by biotite; 75 = Corona at contact of magnetite, XAtm = FeZ+/Fe2+ + Mg + Ca + Mn; Xpy = Mg/Fe 2+ + Mg + Ca + Mn; XGr = Ca/Fe 2+ + Mg + Ca + Mn; Xsp = Mn/Fe 2+ + Mg + Ca + Mn; Xi = i/~ i where i = Fe 2+, Mg. XAb = Na/Ca + Na + K, XAn = Ca/Ca + Na + K, Xor = K/Ca + Na + K; Fe3+ XIhn = Fe2+/Fe2+ + Mg + Mn -~.F; 2

Fe3+ Fe3+ XHem -- 2 //Fe2+ + Mg + Mn + -- 2 XHc = Fe2+/Fe2+ + Mg;

X FeZ+Mt ~__ Fe3+/Fe3+ q- Fe 2+

hematite in solid solution (~ 13 mol.%). Magnetite is oblastic phases. Garnet coronas of variable thickness nearly pure. (with or without quartz intergrowth) have formed on plagioclase, orthopyroxene, ilmenite and locally on magnetite (containing exsolved green spinel) 3.30rthopyroxene-bearing quartzofeldspathic (figure 2b). K-feldspar is a minor constituent in this gneisses rock and biotite is distinctly a late phase replacing As pointed out earlier, both enderbitic and charno- garnet and orthopyroxene. ckitic gneisses are present in this area. The mutual Orthopyroxenes are enstatitic (En54) with 1.79 to relation between the two varieties could not be 2.85 wt% A1203 (table 2). There is a distinct depletion studied. In the enderbitic variety, orthopyroxene, in alumina content towards rims at the contact plagioclase, ilmenite and magnetite occur as porphyr- with coronal garnet. Plagioclase is Ab4sAn50. Coronal 82 Supratirn Pal and ~qankar Bose

garnet shows microdomaina] variation in composi- variable amount of ilmenite, magnetite and biotite. tion depending on the nature of the contact phases. Orthopyroxene, perthite, plagioclase, ilmenite and It varies from Alm6r-6sPyls-20Grlo-uSp03 at the magnetite occur as porphyroblasts. Garnet, when contact with orthopyroxene and ilmenite to present, occurs as coronas of variable thickness with Alm63Py25GrloSp02 at the contact with plagioclase or without intergrown quartz, over orthopyroxene, to Alm62Py24Gh2Sp02 at the contact with magnetite. plagioclase and ilmenite porphyroblasts (figure 2a). Garnet at the contact of biotite replacing it along the Composition of the coexisting phases is given in margin has a composition of Alm66PY22Gr10Sp02. table 3. Orthopyroxenes are comparatively enstatite- Ilmenite contains 5-6.6 mol % hematite. Exsolved spi- rich (XMg = 0.67-0.68) in garnet-rich domains com- nel in magnetite is hercynite-rich (XFe = 0.65).Cr203 pared to garnet-free domains (XFe = 0.50-0.52). It is is always very low (0.73 wt%). well known that the assemblage orthopyroxene + The charnockitic gneisses are characterized by a plagioclase has larger stability field in rocks strong predominance of K-feldspar over plagioclase. having higher bulk MgO/(MgO + FeO). Thus, it is Garnet has sporadic occurrence. Other phases are expected that in the rocks with lower MgO/ perthite, orthopyroxene, plagioclase, quartz with (MgO+FeO), coronal garnet would appear earlier

Table 3a. Representative mineral compositions of charnockitic gneisses. Garnet-bearing domain. Sample no. G20/1 Anal. no. 2(C) I(R) 4 3 23 I 17 5 22 Mineral Orthopyroxene Garnet ] Plag Biotite SiO2 49.52 50.10 38.27 38.33 37.21 63.72 40.41 40.12 TiO2 0.09 0.06 0.03 0.02 0.05 0.01 2.79 4.32 A12Oa 5.05 4.12 21.71 21.36 21.94 23.32 13.34 13.06 F%O3 3.89 4.12 0.47 0.46 1.08 ------FeO 18.09 19.03 27.47 28.37 27.90 0.16 5.54 5.61 MnO 0.14 0.16 0.46 0.40 0.51 -- 0.01 -- MgO 22.03 22.76 9.17 8.27 8.78 0.04 23.87 22.48 CaO 0.11 0.08 1.42 1.20 1.42 6.17 0.02 -- Na20 ..... 8.07 0.10 0.19 K~O ..... 0.09 11.15 10.95 H20 4.25 4.23 Total 100.58 100.43 99.00 98.42 98.89 101.61 101.48 100.96 Oxygen basis 6 24 16 22 Si 1.83 1.85 5.96 6.00 5.82 5.60 5.70 5.69 Ti .... 0.01 -- 0.29 0.46 AITM 0.22 0.15 0,04 -- 0.18 ------A1V1 -- 0.03 3.94 3.94 3.86 2.42 2.22 2.18 Fe3+ 0.11 0.11 0.06 0.05 0.13 ------Fe2-~ 0.61 0.59 3.57 3.71 3.65 0.01 0.65 0.66 Mn -- 0.01 0.06 0.05 0.07 ------Mg 1.22 1.25 2.13 1.93 2.05 -- 5.01 4.75 Ca -- -- 0.24 0.20 0.24 0.58 -- -- Na ..... 1.38 0.03 0.05 K 0.01 2.00 1.98 Total 4.00 4.00 16.00 16.00 16.00 10.00 15.90 15.77 XMg 0.670 0.680 0.355 0.327 0.342 0.840 0.810 XFe 0.330 0.320 0.595 0.629 0.608 0.110 0.110 Xca -- -- 0.040 0.034 0.040 XM~ -- -- 0.010 0,010 0.010 XAb 0.700 XAn 0.290 Xor 0.010 iT i 0.050 0.080 C =- Core; R = Rim; Anal no. 4 = Core of thick corona; Anal. no. 3 = Rim of thick corona; Anal no. 23 = replaced by biotite; Xi = i/~ i where i = Fe 2+, Mg, Ca, Mn; Fe a+ Xnm = Fe2+/Fe 2+ + Mg + Mn + -~--;

Fe3+/Fe2+ Fe3+ XHe m = -~ . + Mg + Mn + --~; Xnb := Na/Ca + Na + K; XAn = Ca/Ca + Na + K; Xor = K/Ca + Na + K. Mineral reactions and geothermobarometry in granulite facies rocks 83 Table 3b. Representative mineral compositions of charnockitic gneisses. Garnet-free domain. Sample no. G22/1

Anal. no. 65(C) 62(R) 64(C) 63(R) I 44(C) 36 53 Mineral Orthopyroxene Ilmenite I Plag Magnetite SiO2 46.95 46.08 -- 0.01 55.68 0.08 0.07 TiO2 0.09 0.38 51.19 59.70 0.01 1.73 2.41 A1203 1.64 1.46 0.06 0.05 26.07 1.09 0.73 Fe203 3.57 5.48 3.49 1.00 -- 63.49 61.04 FeO 29.42 27.69 44.26 51.53 0.10 32.47 32.25 MnO 0.94 0.99 0.74 0.96 0.01 0.06 0.16 MgO 14.01 14.67 0.61 0.68 0.02 0.07 0.03 CaO 0.61 0.47 0.01 -- 9.10 -- 0.03 Na20 0.02 ------6.04 -- -- K20 .... 0.40 -- -- Total 97.25 97.22 100.39 113.94 97.43 99.04 96.77 Oxygen basis 6 3 12 16 Si 1.90 1.87 -- -- 5.13 0.01 0.01 Ti -- 0.01 0.97 0.99 -- 0.20 0.30 Al 0.08 0.07 -- -- 2.83 0.20 0.14 Fe 3+ 0.11 0.17 0.07 0.02 -- 7.37 7.23 Fe 2+ 1.00 0.94 0.93 0.95 0.01 4.19 4.28 Mn 0.03 0.03 0.02 0.02 -- 0.01 0.03 Mg 0.85 0.89 0.02 0.02 -- 0.02 -- Ca 0.03 0.02 -- -- 0.90 -- -- Na .... 1.08 -- -- K .... 0.05 -- -- Total 4.00 4.00 2.00 2.00 I0.00 11.99 11.99 XMg 0.440 0.470 XFe 0.520 0.500 Xc~ 0.020 0.010 XMn 0.020 0.020 XIlm 0.920 0.950 XHem 0.030 0.010 XAb 0.530 XAn 0.440 Xor 0.030 XFea+ 0.64 0.63 C = Core; R = Rim; Xi = i/~ i where i = Fe 2+, Mg, Ca; Fe 3+ Xnm = Fe2+/Fe 2+ + Mg + Mn+ 2 ; Fe3+ XHem- Fe3+2 / /Fe 2++Mg+Mn+ 2 ; XAb = Na/Ca + Na + K; XAn = Ca/Ca + Na + K; Xor = K/Ca + Na + K.

which is not observed in this case. The only orthopyroxene + plagioclase + / - hemoilmenite explanation of the observed magnesian pyroxene in = garnet + quartz + / - ilmenite + / - O2 garnet-bearing domains is the down temperature (orthopyroxene granulite) .... (I). resetting of the former mineral during cooling in contact with garnet. This retrograde Fe-Mg (cf. McLelland and Whitney 1977). exchange in garnet-orthopyroxene pairs is further corroborated by the rimwards increase in almandine 3.4 High Mg-Al granulites in thick garnet corona (table 3a). Porphyroblastic ilmenite in garnet-free domains contains little Fe203 The high Mg-A1 granulites are both massive and mig- (1-3.5m01%) in solid solution. Magnetite is nearly matitic. In the latter, impersistent quartzofeldspathic pure. segregations impart migmatitic appearance. These The formation of coronal garnet-quartz in these segregations could presumably represent melt frac- quartzofeldspathic gneisses can be attributed to the tions (see Dasgupta et al 1995). The presently studied reaction, Mg-A1 granulites comprise different subsets of the 84 Supratim Pal and Sankar Bose

Figure 3. Inclusion of spinel (Sp) within stumpy grain of Figure 5. Spinel (Sp) separated from quartz (Q) by successive sapphirine (Sa) in high Mg-A1 granulite (G85A/2). Plane corona of sillimanite (Si) and orthopyroxene (O) in high Mg-A1 polarized light, scale bar = 140 pm. granulite (G85A/2). Plane polarized light, scale bar = 140 ~m.

Figure 4. Spinel (Sp) rimmed by sapphirine (Sa) against prismatic sillimanite (Si) in high Mg-A1 granulite (G85A). Figure 6. Sapphirine (Sa) with included spinel (Sp) is Crossed Nicols, scale bar = 140 ~m. separated from quartz (Q) by symplectitic intergrowth of sillimanite and orthopyroxene (Sy) in high Mg-A1 granulite (G85A/2). Plane polarized light, scale bar -- 140 ~m. following mineral association: spinel-sapphirine-cor- diexite-quartz-orthopyroxene-sillimanite-garnet-hem- spinel + quartz --- orthopyroxene § sillimanite .... oilmenite-biotite. Former grain boundary contacts between spinel and quartz and sapphirine and quartz (m). are now separated by complex corona/symplectite of 9Sapphirine (containing included spinel) is likewise different phases described later. Important textural separated from quartz by coronitic sillimanite- features and the deduced mineral reactions in the high orthopyroxene symplectite (figure 6). This suggests Mg-A1 granulites are given below: the reaction, 9Porphyroblastic spinel (with exsolved magnetite), sapphirine+quartz = orthopyroxene § sillimanite .... cordierite, quartz, and prismatic sillimanite con- (iv). stitute the earliest recognizable mineral association in the rock. Sapphirine occurs as stumpy grains 9Spinel and quartz are again separated from each with inclusions of spinel (figure 3). Further, sap- other by successive coronas of sillimanite and phirine separates spinel from prismatic sillimanite garnet (figure 7). The last two minerals also rim (figure 4). These two textural features signify sapphirine against quartz (figure 8). These features movement across the reaction, suggest the reactions, spinel + quartz ---- garnet + sillimanite .... (V). spinel § quartz § sillimanite = sapphirine .... (II). sapphirine + quartz = garnet + sillimanite .... (VI). 9Spinel is separated from quartz by corona of sillimanite and orthopyroxene, with the former 9Preferential rimming of sapphirine and spinel by growing preferentially over spinel (figure 5). This sillimanite produced through reactions (II)-(VI) indicates the reaction, indicates the relatively immobile nature of A1. Mineral reactions and geothermobarometry in granulite facies rocks 85

Figure 7. Spinel (Sp) successively rimmed by sillimanite (Si) Figure 9. Sapphirine (Sa) separated from porphyroblastic and garnet (G) against quartz (Q) in high Mg-A1 grannlite cordierite (C) by symplectitic intergrowth of sillimanite and (G85A/1). Plane polarized light, scale bar = 140 ~tm. orthopyroxene (Sy) in high Mg-A1 granulite (GS5A/4). Plane polarized light, scale bar = 140 ~m.

Figure 8. Sapphirine (Sa) rimming spinel (Sp) is separated from quartz (Q) by thin corona of sillimanite (S) followed by Figure 10. Spinel (Sp) separated from quartz (Q) by thin rim thick corona of garnet (G) in high Mg-A1 granulite (GS5A/1), of cordierite (C) in high Mg-A1 granulite (G67B/1). Plane sillimanite is preferentially formed on sapphirine. Partially polarized light, scale bar = 140 ~m. crossed Nicols, scale bar = 140 ~m.

ral compositions in high Mg-A1 granulites to calculate 9Porphyroblastic cordierite is separated from sap- pressure and temperature and obtained a range of phirine by a symplectite of orthopyroxene-sillima- values from 900 4-60~ 6.5 90.7kbar (core) to ca. nite (figure 9), which suggests the reaction, 760 + 50~ 5 • 0.6 kbar (rim) from garnet-orthopyr- cordierite § sapphirine -- orthopyroxene oxene-plagioclase-quartz geothermobarometry and GRAIL barometry. When considered in the best-fit +sillimanite .... (VH). petrogenetic grid of Hensen and Harley (1990), the 9Development of a second generation of cordierite is alumina content of orthopyroxene, coexisting with documented by its rimming of coarse spinel against garnet, (data from Lal et al 1987) gives P-T value of quartz in the leucocratic bands of the high Mg-A1 ca. 8 kbar, 950~ which is consistent with the 'peak' granulite (figure 10). This can be attributed to the conditions deduced from figure 11. However, it is to degenerate reaction be noted that the said grid involves a number of assumptions which may or may not be correct. The spinel + quartz = cordierite .... (VIII). obtained P-T value is thus semi-quantitative. The method of Newton and Perkins (1982) is clearly unsuitable for garnet and orthopyroxene compositions 4. Geothermobarometry in high Mg-A1 granulites owing to unknown effects of A1 in orthopyroxene and low Ca contents in garnet The mineral assemblages in orthopyroxene-bearing which introduce error in pressure estimates. We have granulites are particularly amenable to mineralogical applied GOPS thermobarometric method for ortho- geothermobarometry. Lal et al (1987) used the mine- pyroxene-bearing garnetiferous granulites in the study 86 Supratim Pal and Sankar Bose

Gt+ Sa 10-

"Qz

"c o

v Q.

'Q2 Cd Sa § " ~il [Sa]

-..:'. : .; - :.':..~.: .:...-:. ~6-

900 950 T('C) Figure 11. A partial petrogenetic grid in the system FMASO at high fO2 (cordierite composition considered is anhydrous). The location of the invariant points have been taken from Dasgupta and Sengupta (1995). Thick and thin lines represent uni- and bivariant reactions respectively. The reactions shown are taken from this study (roman numerals) and from Lal et a11987 (arabic numerals). The P-T evolutionary path of the studied high Mg-A1 granulite is shown by the stippled arrow. Inset represents bivariant reaction bundles emanating from (Cd, Opx) and (Cd, Gt) univariant equilibria. (Mineral abbreviations used: Cd - Cordierite, Gt - Garnet, Opx - Orthopyroxene, Sa - Sapphirine, Sil - Sillimanite, Sp - Spinel, Qz - Quartz).

Table 4. Bulk chemical analysis of the mafic granuIite. Sample no. G 73 G 66 G 73 G 66 wt% mole% wt% mole% CIPW Norm SiO2 47.02 50.47 44.35 47.15 Quartz 0.00 0.00 TiO2 2.57 2.07 1.65 1.32 Corundum 0.00 0.00 A1203 13.25 8.38 16.10 10.09 Orthoclase 2.96 3.84 FeO# 17.33 15.58 13.95 12.41 Albite 12.44 7.53 MnO 0.29 0.26 0.23 0.21 Anorthite 28.08 38.02 MgO 5.95 9.52 9.03 14.31 Diopside 17.41 12.09 CaO 10.18 11.71 11.33 12.91 Hypersthene 27.36 13.28 Na2 O 1.47 1.53 0.89 0.92 Olivine 0.00 19.55 K20 0.50 0.34 0.65 0.44 Ilmenite 4.88 3.13 P205 0.28 0.13 0.55 0.25 Apatite 0.66 1.30 Cr203 0.07 0.03 0.03 0.01 Mg Number 0.38 0.53 Mineral reactions and geothermobarometry in granulite facies rocks 87

Table 5. Pressure-temperature estimated from various geothermobarometries in enderbitic and charnockitic gneisses and mafic granulite. Rock type Enderbite Charnockite Mafic granulite Sample no. G 40B G20/1 G22/1 G66 Thermobarometry GOPSpe PMg (kbar) 8.0 8.0 PF~ (kbar) GOPSB ~t ~l PMg (kbar) 6.6 6.5 PFe (kbar) GOLG T~ 666 656 721 626 GOB ~t ol T~ 512 446 681 578 GOsB T~ 594 582 677 576 OIB T~ 660 652 620 581 OCK(Ca-Mg) T~ 750 OCK(Fe-Mg) T~ 700-650 OCDL T~ 650-600 TFsw T~ 646 622 TFFL T~ 537 629 GBpL T~ 436 440 GBFs T~ 345 353 GBD et aZ T~ 486 404 GOPSpc - GOPS barometry after Perkins and Chipera (1985); GOPSB et ~l - GOPS barometry after Bhattacharya et al (1991); GOLG - GO thermometry after Lee and Ganguly (1988); GOsB - GO thermometry after Sen and Bhattaeharya (1984); OIB - OI thermometry after Bishop (1980); OCK - Two pyroxene thermometry after Kretz (1982); OCDL -- Two pyroxene thermometry after Davidson and Lindsley (1989); TFsw - Two feldspar thermometry after Stormer and Whitney (1977); TFFL - Two feldspar thermometry after Fuhrman and Lindsley (1988); GBpL - GB thermometry after Perehuk and Lavranteva (1983); GBFs - GB thermometry after Ferry and Spear (1978); GBD ~t ~z - GB thermometry after Dasgupta et al (1991). area (following the formulations of Lee and Ganguly through their consideration in appropriate petroge- 1988; Perkins and Chipera 1985; Sen and Bhatta- netic grid. Lal et al (1987), working in the same area, charya 1984 and Bhattacharya et a11991). The results also inferred reactions II through VIII (their reaction are given in table 5. Garnet in these rocks is a coronal numbers 1, 3, 7, 8 & 17 corresponding to II, III, IV, V variety and is, therefore, unlikely to record 'peak' and VIII of this study respectively). Additionally, metamorphic compositions. For relatively thick gar- they deduced reactions from textural and composi- net coronas, the P-T estimate (through simultaneous tional criteria (table 2 in Lal et al 1987). Since their solution of the net-transfer and exchange equilibria) is study area overlaps with ours, we propose to consider ca. 8.0kbar, 720~ at the central part and 8.0kbar, all the reactions inferred by them in a suitable 630~ at the outer part. The thermometric formula- petrogenetic grid to deduce the P-T trajectory. We tion of Sen and Bhattacharya (1984) and thermo- begin our analysis with the petrogenetic grid in the barometric formula of Bhattacharya et al (1991) give system FMAS considered by Lal et al (1987). They lower P-T estimates (table 5). Even considering that considered a grid in which the invariant points [Sp], the temperature calculated from outer part of the [Opx], [Sil] and [Cd] are stable (figure 8 in Lal et al corona could just be a blocking temperature for the oi9. cit) and, by implication, [Sa], [Gt] and [Qz] are ion-exchange equilibria, for the coronal nature of the metastable. They obtained a gentle dT/dP near- garnet (clearly a product of static growth during isobaric cooling path for the Mg-A1 granulites, which cooling, Tracy and McLellan 1985), we interpret the traversed from [Cd] to [Sp]. There are serious data as indicative of nearly isobaric cooling through problems with the grid of Lal et al (1987) and because ca. 90~ for similar pressure. Coexisting pyroxenes of their erroneous assumptions in the construction of in mafic granulite give temperatures in the order of the grid, their conclusions are untenable. This is 650-750~ following the methods of Kretz (1982) and precisely the reason why some of the mineral reactions Davidson and Lindsley (1989) (table 5). (reaction 21 and the related bivariant reactions) having large positive AV were shown to be the product of near isobaric cooling in the grid of Lal et al 5. Discussion (1987). In the multisystem FMAS, there are seven invariant points [Sp], [Qz], [Cd], [Opx], [Sill, [Gt] The sequence of mineral reactions deduced from and [Sa] in silica-saturated bulk composition. In silica- successive development of coronitic textures can be undersaturated bulk composition, corundum takes utilized to predict the pressure-temperature (-time) the place of quartz. In the rocks studied by Lal evolutionary history of the studied granulite complex et al (1987) and this work, the former situation is 88 Supratim Pal and Sankar Bose

applicable. Topological constraints explicitly demand that an observed near-isobaric cooling could have that two alternate combinations of the invariant been an apparent one lying between two temporally points can be stable at any given situation - either unrelated decompressive events. This can not be [Sp], [Qz], [Opx], [Sill or [Sa], [Gt], [Cd] (Heusen 1971, evaluated in the present circumstances. This near- 1986; Hensen and Harley 1990 and references therein). IBC was followed by a near isothermal decompressive There could be several competiting reasons for the arm down to P = 6 kb when reactions (17), (21), (22), stabilization of the alternate bundles e.g., fO2 (24) were intersected. Reaction (25) deduced by Lal et (nensen 1986), bulk Fe/Mg ratio (Sengupta et al al (1987) on the basis of projective analysis alone, 1991), role of Zn in the stabilizing spinel (Harley 1987; would imply a period of cooling subsequent to Sengupta et al 1991). Whatever may be the reason, decompression (figure 11). This is not unexpected in no way is the combination [Opx], [Sill, [Sp] and because the decompressed crust would relax thermally [Cd] stable. In order to make [Cd] stable along with to achieve stable thermal configuration. Near isobaric the other three, the grid of Lal et al (1987) rendered cooling subsequent to peak metamorphism is by [Qz] metastable (see figure 8 of Lal et a11987) and the itself not indicative of either CW or ACW P-T reactions (Cd, Gt) and (Cd, Sa) were placed at higher path, for which the prograde arm has to be deduced. P than [Cd] - both are untenable from topological Several lines of evidence, however, suggest that constraints and natural occurrence data (Hensen the prograde metamorphic arm traversed through a 1986). It is obvious that conclusions derived from low P-high T regime prior to peak metamorphism their erroneous petrogenetic grid are untenable. viz., Having discarded the grid of Lal et al (1987) we will proceed to deduce the P-T trajectory of the (a) early stabilization of spinel + quartz and spinel + Mg-A1 granulites from an appropriate one. The cordierite as deduced by Lal et al (1987) indicate choice is obvious because two univariant reac- high d T/dP condition (Waters 1991; Goscombe tions (Sp + Sa + Qz = Opx+Sill and Sp + Qz = Gt+ 1992), Opx+Sil) and one related divariant reaction (b) early stabilization of osumilite + cordierite + (Sp + Qz -- Opx + Sil) are deduced in the studied orthopyroxene (Lal et al 1987) suggests low P rocks (reaction IH of this work and reactions 6, 9, 7 metamorphism (Carrington and Harley 1995), respectively in Lal et al 1987). These reactions are (c) complete absence of relicts of high pressure permitted only in a grid where [Sa], [Gt] and [Cd] assemblage (e.g., pyrope-rich garnet, kyanite, are stable. This was considered to be the high fO2 orthopyroxene + sillimanite + quartz) in the grid by Hensen (1986). That [Sp] was not stable in studied rocks. the studied rocks is also indicated by the participation Collectively, these would imply an ACW path of spinel in thirteen of the total eighteen early of evolution for the studied granulite complex stage reactions deduced by Lal et al (1987). (figure ii). The deduced P-T trajectory comprising The presence of hemoilmenite-titanohematite in an ACW path having high d T/dP prograde arm the assemblages (Lal et al, op cit) does indicate high reaching Pma~ -- Tmax = 9.5 kb, " 1000~ followed by fO2. In the absence of bulk compositional data it is a near IBC down to 9kb," 900~ and subsequent not possible to evaluate whether Mg/Fe ratio exerted decompressive reworking producing late cordierite is decisive control or not in the present situation. strikingly similar to that deduced from adjoining areas Notwithstanding the actual reason for stabilization of the EGB (Sengupta et al 1990; Dasgtlpta et al of the said invariant points, it is indisputable that 1995). the evolution of the studied assemblages must be considered in the high fO2 grid of Hensen (1986). Using available experimental and natural occurrence Acknowledgement data, Dasgupta and Sengupta (1995) approximately positioned the invariant points [Sa], [Gt] and [Cd] We are grateful to Prof. M Raith and Prof. S Hoernes in P-T space. This grid constructed in the system for providing EPMA and XRF data used in this FMASO (for the sake of simplicity, Cd is assumed study. We acknowledge Council of Scientific and anhydrous), along with the divariant reactions Industrial Research (SP) and University Grants deduced by us and by Lal et al (1987), is shown in Commission (SB) for financial support through figure 11. research schemes. We acknowledge constructive sug- It is evident from figure 11 that the sequence of gestions from Dr. S Dasgupta and Dr. P Sengupta deduced mineral reactions defines an initial near during preparation of the manuscript. SP acknowl- isobaric cooling arm down to P = 9kb, T = 900~ edges Dr. S Karmakar for stimulating discussions and from peak metamorphic condition of P--9.hkb, help in various forms during the field work. A T =~ 1000~ when reactions (1), (2), (3), (4), (5), thorough and thought provoking review by Dr. Ian (6), (7), (8), (9), (10), (23) were intersected. However, Fitzsimons and comments of an anonymous reviewer it is noteworthy here that Hand et al (1992) argued helped a lot to improve the quality of the paper. Mineral reactions and geothermobarometry in granulite facies rocks 89

This work is a contribution to IGCP projects 348 Hensen B J 1986 Theoretical phase relations involving cor- and 368. dierite and garnet revisited: The influence of oxygen fugacity on the stability of sapphirine and spinel in the system Mg- Fe-A1-Si-O; Contrib. Mineral. Petrol. 92 362-367 Hensen B J 1971 Theoretical phase relations involving References cordierite and garnet in the system MgO-FeO-A1203-SiO2; Contrib. Mineral. Petrol. 33 191-214 Bhattacharya A, Krishnakumar K R, Raith M and Sen S K Hensen B J and Harley S L 1990 Graphical analysis of P-T-X 1991 An improved set of a-x parameters for Fe-Mg-Ca relations in granulite facies metapelites; In High temperature garnets and refinements of the orthopyroxene-garnet ther- metamorphism and crustal anatexis (eds) J R Ashworth and mometers and the orthopyroxene-garnet-plagioclase-quartz M Brown (London: Unwin Hyman) 19-56 barometer; J. 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