GEOLOGICAL JOURNAL, VOL. 30, 307-318 (1995)

Ultrametamorphism in Precambrian granulite terranes: evidence from Mg-Al granulites and calc-silicate granulites of the

Eastern Ghats, India

SOMNATH DASGUPTA and PULAK SENGUPTA Department of Geological Sciences, Jadavpur University, Calcutta- 700 032, India

High Mg-A1 granulites and calc-silicate granulites provide evidence for ultra-high temperatures of metamorphism (ca. 1000°C) at moderate pressures (9-10 kbar) in the Eastern Ghats Belt, India. Lack of proper geochronological data prevents the dating of this extreme metamorphism. High Mg-A1 granulites contain different subsets of mineral assemblages involving spinel, quartz, sapphirine, cordierite, orthopyroxene, garnet and sillimanite coexisting with either rutile-ilmenite or titanohaematite-ferrianilmenite. These high Mg-A1 rocks are poor in Zn and Cr, as reflected primarily in the composition of spinel. Evidence of ultra-high temperature metamorphism comes from (i) textural interpretation of the former coexistence of spinekordierite-quartz and sapphirine-quartz and stabilization of the assemblages orthopyroxene-sillimaniteecordierite and spinel4uartz-sapphirine-garnet and (ii) the high A1203 content of orthopyroxene coexisting with garnet and/or cordierite. Consideration of the sequence of deduced mineral reactions in petrogenetic grids in the system FMAS attests to an anticlockwise P-T path of evolution for the granulites. In calc- silicate granulites stabilization of nearly pure meionite and of the wollastonite-plagioclase-andradite-rich garnet, wollastonite-scapolite-grandite garnet-calcite association corroborate high temperatures of metamorphism. Conven- tional mineralogical geothermobarometry in all the rocks record lower temperatures (maximum 950°C) at 9-1 0 kbar pressures, attributed to resetting of the mineral compositions during cooling. Following peak metamorphism, the rocks firstly experienced near-isobaric cooling followed by near-isothermal decompression. On the basis of the available evidence it appears that non-extensional lithospheric thinning and/or heat input from basic/enderbitic magma are the causes of such ultra-high temperature metamorphism on an anticlockwise path in the Eastern Ghats Belt.

KEY WORDS ultrametamorphism; Eastern Ghats; India; Mg-A1 granulites; calc-silicate granulites

1. INTRODUCTION

Ultra-high temperature ( > 9OOOC) regional metamorphism signifies anomalously high thermal input and, hence, is of fundamental importance in understanding the thermal structure and related petrological processes that shaped the continental crust through geological time. Experiments on biotite-bearing quartzo-feldspathic assemblages predict extensive melting even under vapour-absent conditions, leading to the formation and emplacement of voluminous granitic magma at shallow crustal depths, which leads to large-scale crustal differentiation. Along with some other regional granulite facies terrains of the world (Napier Complex, East Antarctica-Ellis 1980, 1983; Harley 1985; Labwor Hill, Uganda-Sandiford et al. 1987; the central zone of the Arunta Complex, Australia-Warren 1983; Goscombe 1992; Wilson Lake, Canada-Currie and Gittins 1988; Sipiwesk Lake, Canada-Arima and Barnett 1984; Ouzzal, Algeria- Bertrand et al. 1992) among more than 90 well-studied terranes (Harley 1989), the Eastern Ghats Belt preserves unequivocal evidence of ultra-high temperature metamorphism (Sengupta et al. 1990, 199 1; Dasgupta et al. 1991). In this paper, we discuss the criteria of ultra-high temperature metamorphism from the Eastern Ghats Belt, citing evidence from the Mg-A1 granulites. Additional supportive evidence by calc- silicate granulites occurring in the belt.

CCC 0072-l050/95/040307-12 0 1995 by John Wiley & Sons, Ltd.

Received 14 October 1994 Revised 14 March 1995

308 S. DASGUPTA AND P. SENGUPTA

76"E 96"E

24"N -

8"N -

M.R. Mahanadi River G.R. Godavari River

1. Anantagiri 2. Rajahmundri 3. Paderu 4. Garavidi 5. Borra 6. Anakapalle

- 200 km.

Figure I . Location of the Eastern Ghats granulite belt and the study areas

2. GEOLOGICAL BACKGROUND

The Eastern Ghats granulite belt (Figure 1) is comprised of metapelitic gneisses (garnet-sillimanite- perthite-quartz gneiss or khondalite), leptynite (garnet-plagioclasequartz-perthite gneiss), ortho- pyroxene-bearing quartzo-feldspathic gneisses (mainly enderbitic), two-pyroxene mafic granulite, high Mg-AI granulites and calc-silicate granulites. In this study, we will focus on the last two rock types. The areas chosen for the present study are located in the southern part of the Eastern Ghats belt in the state of Andhra Pradesh (Figure 1). The rocks of the belt show a dominant NNE-SSW to NE-SW gneissic foliation and have suffered at least three phases of deformation (Sarkar et al. 1981; Halden et al. 1982; Bhattacharya et al. 1994). Minerals that developed during retrogression and occur as coronas and symplectites bear the imprint of the first phase of deformation. It follows that the first recognizable deformation post-dates peak granulite facies metamorphism.

Geochronological data on the Eastern Ghats show wise scatter (see review in La1 et al. 1987 and Sengupta et al. 1990) and many of the dates require careful re-evaluation with multiple isotopic systematics. However, a granulite facies metamorphism at ca. 1000-900 Ma has been convincingly demonstrated (Grew and Manton 1986; Aftalion et al. 1988; Paul et al. 1990) from both the northern and southern parts of the Eastern Ghats Belt. An earlier (c. 2200 Ma?) granulite metamorphism has been postulated (Grew and Manton 1986), which requires confirmation. Pan-African reworking of the granulites is indicated by K-Ar dating of biotite (Aswathanarayan 1964), but the extent and the exact nature of reworking is yet to be evaluated.

The Mg-A1 granulites studied in this work occur as impersistent lenses in leptynite (near Rajamundry), at the contact of khondalite and mafic granulite (at Paderu) and in enderbitic gneisses (at Anantagiri) (Figure 1). The calc-silicate granulites studied here have diverse modes of occurrence. At Garividi and Borra, calc-silicate granulites form major rock units associated with leptynite and khondalite (see figure 1 in Bhowmik et al. 1995 for the Borra occurrence). Near Rajamundry, the calc-silicate granulites occur as impersistent lenses enclosed in leptynite and enderbitic gneisses.

ULTRAMETAMORPHISM IN GRANULITE TERRANES 309

3. PETROLOGY OF Mg-A1 GRANULITES

Spinel-sapphirine-bearing high Mg-A1 granulites from the Eastern Ghats Belt have been the focus of petrological investigation in recent years (La1 et al. 1987; Kamineni and Rao 1988a, 1988b; Sengupta et al. 1990, 1991; Dasgupta et al. 1994, 1995). These rocks have provided a wealth of information on the P-T evolutionary history of the terrane (references as cited above). It is now evident that phase relations in the simplified system FMAS are complicated by the influence of Zn, Cr, bulk Fe/Mg ratio and fo , (Hensen 1986; Harley 1986; Sengupta et al. 1991; Nichols et al. 1992). These components enlarge the stability limits of sapphirine and/or spinel to significantly lower temperatures (5CrlOO0C) and higher pressures than those predicted from the experimental studies in the simple FMAS system (Harley 1986). Hence to document anomalously high peak metamorphic temperatures in Eastern Ghats Belt we will restrict attention here to Mg-A1 granulites low in Zn and Cr as revealed primarily from the chemical composition of spinel. This will minimize the uncertainties associated with extrapolation of the FMAS invariant points due to the stabilizing effects of Zn and Cr in spinel. The important reactions in the system FMAS are otherwise fairly well constrained from experimental and theoretical studies (Hensen 1971; Hensen and Green 1973; Annersten and Seifert et al. Bertrand et al. 1991; or from natural occurence (Grant 1985; Dasgupta et al. 1995).

Low Zn-Cr spinel-sapphirine granulites studied in this work can be divided into two groups: those containing ilmenite (with little or no FezO3)-rutile (Anantagiri) and those with ferrianilmenite- titanohaematite (Paderu and Rajamundry). In low Zn-Cr bulk compositions the former group has presumably equilibrated at low fo, and the latter at high f O , (cf. Hensen 1986). Consequently, mineral reactions in the two groups of rocks need to be considered in the low and highfo, grids, respectively, in the system FMAS.

All the studied rocks are migmatitic, containing impersistent quartzo-feldspathic segregations, which possibly represent a melt fraction (Dasgupta et al. 1995). Mineral associations and textural features described in the following are from the dark coloured bands well away from the contact with quartzo- feldspathic segregations. We have paid particular attention to determine whether there is any systematic pattern in the abundance of quartz with respect to sample distance from the quartzo-feldspathic segrega- tions by carrying out modal calculations. No such systematic pattern emerged from this attempt.

3a. Assemblage with rut ile-ilmenite (Anan t agiri)

The following mineral association is noted in the Mg-A1 granulites of Anantagiri: spinel-sapphirine- cordierite-quartz-orthopyroxene-sillimanite-garnet~ilmenite-rutile. Table 1 lists the subsets of mineral assemblages and the reaction textures studied in these rocks. There is no grain contact between spinel and quartz and sapphirine and quartz. Sapphirine ( X M g = 0.74) contains inclusion of spinel ( X M g = 0.44). Sapphirine is peraluminous and is closer to a 7 : 9 : 3 composition. These characteristics indicate the FMAS divariant reaction

Spinel + quartz + sillimanite = sapphirine (1)

Sapphirine also contains inclusions of an early generation of cordierite (XMg = 0.87). Compositional colinearity between cordierite-sapphirine-quartz for one set of particular compositions in (FM)-A-S compositional space (Sengupta et al. 1990) and the above textural feature indicate the degenerate reaction

Cordierite = sapphirine + quartz (2)

Sapphirine and quartz are separated by orthopyroxene (XMg = 0.66-0.69) and sillimanite, suggesting the FMAS divariant reaction

Sapphirine + quartz = orthopyroxene + sillimanite (3)

Coronal garnets (PyS1Alm4SGr3Sp1) with intergrown quartz develop at the contact of orthopyroxene and

310 S. DASGUPTA AND P. SENGUPTA

Table I . List of mineral assemblages, localities and characteristic reaction textures in Mg-AI granulites and calc-silicate granulites referred to in the text.

Assemblage -

Locality Reaction textures

1. Mg-A1 granulites (1) Low fb, assemblages coexisting with ilmenite-rutile

(i) Sap+Opx+Cd(2)+Sil+Sp+Qz (ii) Sap+Cd( I)+Opx+Sil+Gt+Qz

(iii) Gt(P)+Sp+Cd(l)+Sil+Qz+Cd(2)+Opx (iv) Sp tCd(l)+Opx+Sil (v) Sp+Qz+Sap+Opx+Sil (vi) Sp+Qz+Sap+Gt (vii) Sp+Qz+Sap+Gt+Sil (viii) Sp+Qz+Opx+Sil

( 2 ) High /& assemblages coexisting with ferrianilmenite and titanohaematite

11. Calc-silicate granulites (ix) Cc+PI+Sc (x) Wo+Pl+Gt+Qz (xi) Wo+Sc+Gt+Cc+Qz

Anantagiri Anantagiri

Rajamundry Rajamundry

Paderu Rajamundry Rajamundry

Paderu

Borra Rajamundry

Garividi

Mineral abbreviations: Sap, sapphirine; Opx, orthopyroxene; Cd, cordierite; (1) and (2) denote early and late, respectively; Sil, sillimanite; Qz. quartz; Sp, spinel; Gt, garnet; Gt(P), porphyroblastic garnet; Cc, calcite; Sc, scapolite; Wo, wollastonite; and PI, plagioclase Reaction textures: (a) Sap contains inclusion of Cd; (b) symplectitic Opx and Sil separate Sap from Qz; (c) Cd(2) rims Si!, Opx and Qz; (d) Cd(l) occurs as inclusions in Sap; (e) coronal Gt-Qz symplectite separate Opx from Sil; (0 symplectitic Opx and Sil separate Sp from Cd(1); (g) prophyroblastic garnet contains inclusions of Sp, Cd and Qz; (h) Sap rims Sp; (i) Sap rimmed by symplectitic Opx and Sil; (j) coronal Gt rims Sp against Qz and Sap contains Sp inclusions; (k ) Sp and Sap are rimmed by Gt and Sil; (I) coronal Opx and Cd(2) around Ct(p) against Qz; (m) coronal Cd(2) rims Gt(P) against Sil and Qz; (n) corona of Opx-Sil over Sp and Qz; (0) PI Cc intergrowth over Sc; (p) coronal Gt-Qz symplectite over Wo against PI; (4) coronal Gt-QzCc symplectite over Wo and Sc

sillimanite, which implies the FMAS divariant reaction

Orthopyroxene + sillimanite = garnet + quartz (4)

A second generation cordierite (XMg = 0.82) rims orthopyroxene, sillimanite and quartz, suggesting another divariant reaction

Orthopyroxene + sillimanite + quartz = cordierite ( 5 )

The sequence of mineral reactions described above requires P-T variation from an initial spinel-quartz- sillimanite-cordierite assemblages to sapphirine-quartz to orthopyroxene-sillimanite-quartz-garnet and finally to a cordierite-orthopyroxene-sillimanite-quartz assemblage. Figure 2 shows the FMAS petro- genetic grid at lowfo, (after Hensen 1971; Hensen and Green 1973; Hensen 1986; Bertrand et al. 1991). The stability fields of these assemblages are shown as shaded regions in the diagram. It is evident that the sequence of mineral reactions define an anticlockwise P-T trajectory extending from a low P-high T prograde path reaching P - T M ~ ~ of ca. 9 kbar and 1000°C, followed by near-isobaric cooling and then subsequent near-isothermal decompression. The deduced trajectory is essentially similar to that described in Sengupta et al. (1990) from the same area. The important point to be noted here is that during peak metamorphism the mineral reactions occurred at temperatures above the invariant point [Sp].

3h. Assemblages with ferrianihenite-titanohaematite (Rajamundry, Paderu)

The mineral association in the Mg-A1 granulites are as follows (each coexisting with ferrianilmentite- titanohaematite): spinel-sapphirine-sillimanite~ordierite-quartz-garnet-orthopyroxene. The coexisting subsets of mineral assemblages and the reaction textures are listed in Table 1. At Rajamundry, garnet occurs both as porphyroblasts (with inclusions of cordierite-spinel-quartz-biotite) and coronas over spinel.

ULTRAMETAMORPHISM IN GRANULITE TERRANES

12

11

10

9 .

a 8 .

7 .

6 -

5 .

31 1

-0

900 1200

The system FMAS at low f 0 2

Figure 2. Petrogenetic grid in the system FMAS at lowfo, showing the stability fields of the different assemblages (after Hensen 1986; Bertrand et al. 1991). Cd, Cordierite; Gt, garnet; Opx, orthopyroxene; Sa, sapphirine; Sil, sillimanite; Sp, spinel; and Qz, quartz

Cordiertie also occurs as porphyroblasts in the matrix and as coronas over porphyroblastic garnet. From a detailed textural and mineralxhemical study Dasgupta et al. (1995) showed that dehydration melting of an initial biotite-sillimanite-quartz assemblage produced a cordierite (porphyroblastic) and spinel association in these rocks. Subsequently, porphyroblastic garnet appeared through the breakdown of spinel-biotite- quartz. Orthopyroxene-sillimanite symplectite separates spinel from porphyroblastic cordierite and also from quartz. These features can be explained in terms of divariant equilibria associated with the FMAS univariant reaction

Spinel + cordierite + quartz = orthopyroxene + sillimanite (6)

La1 et al. (1987) presented textural and compositional evidence in support of the coexistence of sapphirine- spinelquartz during peak metamorphism from the Paderu area.

Textural features thus show an early stability of spinel-quartzxordierite in the rock, which was followed by the development of prophyroblastic garnet, presumably during prograde metamorphism. Spinel is clouded with exsolved magnetite and reintegration mostly gives an initial spinal composition of (Hc, Sp)sOMtsO. Sapphirine is particularly developed in domains where spinel is Zn-poor. Sapphirine (XMg = 0.73-0.81) rims spinel ( X M ~ =, 0.41) and is in turn rimmed by an intergrowth of coronal orthopyroxene ( X M ~ = 0.65-0.74) and sillimanite. These textural features indicate operations of divariant equilibria consistent with the FMAS univariant reaction

Spinel + sapphirine -t quartz = orthopyroxene + sillimanite (7)

Locally, in sillimanite-free domains in the rock, coronal garnet ( P ~ ~ ~ A l m ~ 4 G r ~ S p ~ ) rims spinel (XMg = 0.46) against quartz and sapphirine ( X M ~ = 0.77) and contains inclusions of spinel. These textures suggest the FMAS divariant reaction

Spinel + quartz = sapphirine + garnet (8)

312

9 -

h L (d

2 a v

m m B 0

S. DASGUPTA AND P. SENGUPTA

I

- (Sil)

Sp+Cd

Sa+Qz

Opx+Sil

Gt+Cd

I I I 900 950

T "C The system FMAS at high f 0 2

Figure 3. Petrogenetic grid in the system FMAS at highfo, showing the stability fields of the different assemblages (after Hensen 1986; Dasgupta et al. 1995). Cd, Cordierite; Gt, garnet; Opx, orthopyroxene; Sa, sapphirine; Sil, sillimanite; Sp, spinel; and Qz,

quartz

Coronal garnet and sillimanite develop around sapphirine and spinel in other domains, which can be explained in terms of divariant equilibria related to the FMAS univariant reaction

Spinel + sapphirine + quartz = garnet + sillimanite (9)

Similar textural features are present in the Mg-A1 granulites of Paderu and some of these were earlier described by La1 et al. (1987).

Subsequently, porphyroblastic garnet and quartz (with or without sillimanite) broke down via two FM AS divariant reactions

Garnet + quartz = orthopyroxene + cordierite (10)

Garnet + sillimanite + quartz = cordierite (1 1 )

Thus the Mg-A1 granulites record a sequence of reaction textures, which imply a transition from an early spinel-quartz-cordierite assemblage to sapphirine-spinel-quartz to orthopyroxene-garnet-sillimanite to orthopyroxene-cordierite-garnet assemblage. Figure 3 shows the high fo, grid for the system FMAS (after Hensen 1986; Dasgupta et al. 1995). The stability fields of the above-mentioned assemblages are shaded in the diagram. Figure 3 clearly shows that the rocks evolved through an anticlockwise trajectory reaching T,,, beyond [Cd], followed sequentially by near-isobaric cooling and near-isothermal decompression. Thus both the oxidized and reduced Mg-A1 granulites from the Eastern Ghats record similar P-T evolutionary histories. The occurrence of fingerprint-like symplectites comprising K-feldspar-cordierite-orthopyroxene in Mg-A1 granulites from Paderu (La1 et al. 1987) and elsewhere indicates the former existence of osumilite in the rocks, which broke down during post-peak isobaric cooling.

ULTRAMETAMORPHISM IN GRANULITE TERRANES 313

Table 2. Representative chemical analyses of phases from calc-silicate granulites

Analysis No: I 2 3 4 5 6 Mineral: Garnet Scapolite Plagioclase

Area: G R G B R B

Si02 TiOz A1203

MnO MgO CaO K20 NazO Total Oxygen basis

Si Ti A1 Fe3 + Fe2+ Mn

Ca K Na Total

Fez03

Mg

XAnd

XAlm XGrs E q A n

X A n aGr

aMe aAr

X A t

37.73 0.07

13.79 10.69 0.29 0.06

34.20 -

- 97.97

24

6.04 0.01 2.60 1.29 0.15 0.04 0.01 5.86 - -

16.00

0.33 0.03 0.64

0.57

38.64 0.20

18.18 3.53 0.37 0.20

35.34 -

- 99.69

24

5.99 0.02 3.32 0.41 0.42 0.05 0.04 5.87 - -

16.12

0.11 0.08 0.81

0.70

45.52

28.35 0.14

0.05 18.87 0.12 2.65

95.76 25

6.65

4.88 0.02

-

-

-

-

-

0.01 2.95 0.02 0.75

15.28

62.70

0.48

38.02

31.01 0.06

0.03 23.06 0.13 0.30

92.60 25

6.09

5.86 0.01

-

-

-

-

-

0.01 3.96 0.03 0.09

16.05

95.20

0.95

46.25

34.48

0.01

17.9 1 0.06 1.45

100.23 8

2.12

1.87

-

-

-

-

-

- -

0.88

0.13 5.00

-

0.13 0.87

0.82

41.42

35.25 -

-

- -

20.14 -

-

96.99 8

1.98

1.99 -

-

-

- 1.03 - -

5.00

0 1 .oo

1 .oo X A " ~ = Fe3 '/Fe3 + +AIVi. EqAn = 9 x 100 when Si+AI= 12. XA, = Ca/(Ca+Na).

4. PETROLOGY OF CALC-SILICATE GRANULITES

Calc-silicate granulites studied here are from Borra, Garividi and Rajamundry areas (Figure 1). Details accounts of petrological evolution of calc-silicate granulites from different parts of the Eastern Ghats Belt have been presented elsewhere (Dasgupta er al. 1992; Dasgupta 1993; Bhowmik et al. 1995; Sengupta et al. unpublished data). In the context of this paper, only those petrochemical attributes of the calc-silicate granulites which attest to very high temperatures of metamorphism are emphasized. Representative chemical compositions of the phases are given in Table 2 and the mineral assemblages and reaction textures are listed in Table 1.

At Borra, nearly pure meionite (Eq An = 95) broke down to a symplectite of anorthite and calcite according to the reaction

Meionite = anorthite + calcite (12)

This reaction with near-zero volume change occurs at 877°C (Baker and Newton 1991).

314 S. DASGUPTA AND P. SENGUPTA

10

9

8 h

5 g7 n

6

5

600 700 800 900 1000

T ("C)

p-T FROM INDFPFNDENT SOURCES:

1. GARIVIDI: T (charnockite) = 860 "C

(Gt-Opx, Lee & Ganguiy 1988) P (metapelite) = 8.6 kbar (GRAIL)

2. RAJAMUNDRY:

T = 950 "c P = 9-10 kbar

Spinel - Sapphirine Granulite

Dasgupta et al. 1995 1 Petrogenetic Grid [ 3. BORRA:

Orthopyroxene Granulite P = 9.1 kbar (Harley & Green 1982) T = 860 "C (Gt-Opx, Lee & Ganguly 1988

Figure 4. Fluid-absent reactions in the studied calc granulites plotted in P-T space. End-member reactions are shown along with the positions of the reactions using the compositions of the studied phases (see Table 2 for chemical data). The shaded region represents calculated peak pressure values in the areas with uncertainties. An, Anorthite; Cc, calcite; Gr, grossular; Me, meionite; Qz, quartz; Sc,

scapolite; and Wol, wollastonite

At Rajamundry, thin rims of garnet (Gr8,AndrI IAlmo8) develop at the contact of wollastonite and plagioclase (XAn = 0.87). This can be explained by the solid-solid reaction

Wollastonite + plagioclase = garnet + quartz (13)

In this area peak metamorphic pressure has been calculated to be 9-10 kbar using GRAIL (Ghent and Stout 1984) and GRIPS (Bohlen and Liotta 1986) barometers in associated rocks (Dasgupta et al. 1995). The peak pressure estimate is corroborated by petrogenetic grid considerations in Mg-A1 granulites (Figure 3). In this area, coronal garnet (+ quartz) developed around orthopyroxene and plagioclase in closely associated orthopyroxene granulite. Geothermobarometry in this GOPS assemblage has indicated the formation of garnet during post-peak near-isobaric cooling (ca. 9 kbar, 8OOOC) (Dasgupta el al. 1995). Hence it is concluded that in the calc-silicate granulites also, the formation of garnet according to reaction (1 3 ) occurred at pressures close to the peak condition (ca. 9 kbar). The end-member reaction (13) computed from the internally consistent dataset of Holland and Powell (1990) is shown in Figure 4. Using the activity-composition relationships of garnet according to the modified version of Engi and Wersin (1 987, cited in Harley et al. 1994 and Fitzsimons and Harley 1994) and of plagioclase according to Newton (1983), the occurrence of reaction (1 3) at Rajamundry implies peak metamorphic temperatures exceeding 900°C at 9 kbar (Figure 4). Further, early stabilization of wollastonite-plagioclase in preference to garnet indicates high temperatures and/or low pressures and thus corroborates the anticlockwise P-T path deduced from the Mg-A1 granulites.

At Garividi, garnet ( G ~ - @ A n d r ~ ~ A l m ~ ~ ) corona, intergrown with calcite and quartz, rims scapolite (Eq An = 63) and wollastonite. This texture can be attributed to the reaction

Wollastonite + scapolite = garnet + calcite + quartz (14)

In this area, the peak pressure has been estimated as ca. 8.5 kbar using well-calibrated geobarometers in associated metapelites and orthopyroxene granulites (Dasgupta er al. 1992). Following the same logic as in Rajamundry, garnet formation in calc-silicate granulites is considered to have taken place close to the pressure maxima. Using the thermochemical parameters of Holland and Powell (1990) and the a-x relationships of garnet according to Fitzsimons and Harley (1994) and of scapolite according to Oterdoom

ULTRAMETAMORPHISM IN GRANULITE TERRANES 315

and Gunter (1983), reaction (14) is located at ca. 920°C at 8.5 kbar. Hence the three vapour-absent reactions (12) to (14) in calc-silicate granulites indicate peak metamorphic temperatures in excess of 900°C at 8.5-10 kbar pressures. This supports peak granulite metamorphism at ultra-high temperatures in the Eastern Ghats as deduced from the high Mg-A1 granulites.

5. THERMOBAROMETRY

Mineral assemblages in enclosing metapelitic granulites and pyroxene granulites are particularly suitable for geothermobarometry. However, conventional mineralogical thermometers register lower temperatures than that predicted by the petrogenetic grids in the FMAS system. This is obviously related to down- temperature resetting as the closure temperature of Fe-Mg diffusion between the phases is much lower than 900°C (Chakraborty and Ganguly 1990). In the GOPS assemblage in associated orthopyroxene-bearing quartzo-feldspathic gneisses cores of ca. 4 mm diameter garnets and orthopyroxenes register temperatures ranging from 900-950°C (following Lee and Ganguly 1988), 800-840°C (following Harley 1984a) and 840- 900°C (following Carswell and Harley 1990).

Notwithstanding the problems associated with the various formulations of the garnet-orthopyroxene thermometer and the compositional dependence of the diffusion coefficients, it can be safely argued from the above that the peak metamorphic temperature was well above 900°C. Textural features indicate the former presence of a spinel of composition approaching (Sp, Hc)SOMtSO during peak metamorphism in spinel granulites. Following the solvus of Turnock and Eugster (1962), this would also imply temperatures in excess of 860°C. Frost and Chacko (1989) argued that 'fossil thermometers' may be used to extract information regarding peak metamorphism. Mg-A1 granulites invariably contain mesoperthites coexisting with plagioclase. Reintegration of the mesoperthite composition and the application of two-feldspar thermometry yield temperatures in excess of 950°C following the methods of Fuhrman and Lindsley (1988) and Kroll et al. (1993). Further evidence for extreme thermal conditions comes from the A1203 content in orthopyroxene coexisting with garnet in Mg-A1 granulites. The orthopyroxene in this rock is a coronal variety, which had formed during post-peak near-isobaric cooling (Figures 3 and 4). The composition of orthopyroxene and garnet, when considered in the best-fit P-T net of Hensen and Harley (1990), gives temperatures in the range 900-950°C at estimated pressures of 9-10 kbar. This would obviously imply a higher temperature during peak metamorphism.

6. DISCUSSION

The mineral reactions during peak metamorphism in the low fo, Mg-A1 granulites from Anantagiri occurred beyond the invariant point [Sp] in the FMAS system (Figure 2), the location of which has been experimentally constrained to be > 1000°C (Bertrand et al. 1991). Similarly, in the high fo, Mg-A1 granulites from Rajamundry and Paderu, mineral reactions during peak metamorphism occurred at temperatures beyond the invariant point [Cd]. The location of [Cd] has been fixed at T> 950°C (Dasgupta et al. 1995). Thus consideration of (1) mineral equilibria in oxidized and reduced Mg-A1 granulites in relevant petrogenetic grids, (2) vapour-absent equilibria in calc-silicate granulites in P-T space and (3) thermometric evidence from a large part of the Eastern Ghats Belt document regional granulite facies metamorphism under extreme thermal conditions (ca. lO0O'C) at moderate pressures (9-10 kbar). There is no available evidence of regional thermal gradient along the length of the Belt. Early stabilization of assemblages such as spinel-quartzxordierite in Mg-A1 granulites and of wollastonite-plagioclase in calc- silicate granulites attest to relatively high-temperature and low-pressure conditions during prograde metamorphism. Successive stabilization of mineral assemblages in Mg-A1 granulites (Figures 2 and 3) indicates that the Eastern Ghats granulites evolved through an anticlockwise P-T trajectory leading to peak metamorphism at ca. 1000°C and 9-10 kbar. This was followed by near-isobaric cooling. The IBC granulites were then reworked by near-isothermal decompression.

3 16 S. DASGUPTA AND P. SENGUPTA

At Anakapalle, however, a slight decompression at peak metamorphism condition has been recorded preceding isobaric cooling (Dasgupta et al. 1994). In other areas such an early decompression could not be documented. In the absence of geochronological data, the age of granulite facies metamorphism under extreme thermal conditions remains indeterminate. At Anakapalle, the early decompression has been dated to be ca. 1000 Ma (Grew and Manton 1986). On the other hand, the Anakapalle granulites do not record evidence of prograde metamorphism nor the extreme thermal condition at peak metamorphism. This could imply an earlier granulite facies metamorphism (before ca. 1000 Ma) under extreme thermal conditions in the Eastern Ghats. Petrographic evidence for an earlier event at Anakapalle is preserved in the form of sapphirine inclusions in porphyroblastic garnet (Grew and Manton 1986; Dasgupta et al. 1994). Detailed geochronological work is necessary to proceed any further with an interpretation of the thermal history.

Notwithstanding the problem of the absolute age, the ultra-high temperature metamorphism in the Eastern Ghats signifies an anomalously high thermal budget at 30-35 km (corresponding to 9-10 kbar pressure) depth. Granulite metamorphism on an anticlockwise P-T trajectory and at extreme temperatures could be caused by magmatic under-/intraplating (Bohlen 199 1). Magmatic rocks (two-pyroxene granulites and enderbites) are present in the Eastern Ghats, although their age relationship with the peak granulite metamorphism is not substantiated by geochronological data. Nor has the cooling rate of the granulites been calculated. In view of these constraints, we can only speculate about the cause of an anomalously high thermal budget during granulite facies metamorphism in the Eastern Ghats. Theoretical calculations by Oxburgh (1990) show that in the case of advective heating by magmas, the volume of the required magma is of the same magnitude as the terrane itself. For such a large terrane as the Eastern Ghats (Figure l), the requirement is too high to be compatible with the field data. Subsurface geophysical data are not available so that a firm conclusion cannot be drawn. Nevertheless, the available geological evidence does not support the contention that magmatic accretion is the cause of the ultra-high temperature metamorphism. Alternatively, anomalously high temperatures can be obtained from lithospheric thinning (extensional or non-extensional), which may also involve some magmatic heating (Sandiford and Powell 1986; Oxburgh 1990; Waters 1990). At the present state of knowledge, we are inclined to invoke this model as the cause of ultra-high temperature metamorphism in the Eastern Ghats.

ACKNOWLEDGEMENTS

This paper was presented orally at the symposium on Ultra-High Temperature Metamorphism held during the Conference on Controls of Metamorphism at Liverpool, 12-14 September 1994. Partial financial support for the first author to attend the conference was provided by the IGCP Project 304 (Lower Crustal Processes), for which SD is thankful. Financial assistance for this work was obtained from the Council of Scientific and Industrial Research. We thank S. Pal, S. Mitra, U. Bhui, S. Bose, D. K. Mukhopadhyay, S. Karmakar and K. Das for their help. Constructive criticisms by Drs S. L. Harley and D. J. Waters on an earlier draft and efficient editorial handling by Dr A. P. Boyle are very much appreciated. This paper is a contribution to IGCP Project 304 (Lower Crustal Processes).

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