I . metamorphic Ceol., 1991, 9, 451-459

Petrology of spinel granulites from Araku, Eastern Ghats, India, and a petrogenetic grid for sapphirine-free rocks in the system FMAS P. SENGUPTA, S . KARMAKAR, S . DASGUPTA A N D M . FUKUOKA* Department of Geological Sciences, Jadavpur University, Calcutta-7ooO32, India *Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan

ABSTRACT Spinel-quartz-cordierite and spinel-quartz are found as relic prograde assemblages in Fe-rich granulites from the Araku area, Eastern Ghats belt, India. Subsequent reactions produced orthopyroxene + sillimanite in the former association and garnet + sillimanite in the latter. The first reaction is univariant in the FMAS system, but is trivariant in the present case because of the presence of Zn and Fe3+ in spinel. The second reaction also has high variance because of Zn and Fe3+, but also because of the presence of Ca in garnet. Thermobarometry shows that the metamorphic conditions were approximately 950" C and 8.5 kbar and the fo, was near the NNO buffer. In Fe-rich bulk compositions and low-P-high-T conditions of metamorphism, two of the univariant reactions around the invariant point [Sa], namely (Sa, Hy) and (Sa, Cd), change topology due to reverse partitioning of Fe-Mg between coexisting garnet and spinel. An alternative partial petrogenetic grid in the system FMAS is constructed for such conditions and is applied satisfactorily to several sapphirine-free spinel granulites. It is shown that bulk composition (X, and Zn) exerts greater control on the stability of spinel +quartz than f,,. The effect of the presence of Zn and Fe3+ in spinel on the proposed grid is evaluated. Reaction textures in the Araku spinel granulites can be explained from the petrogenetic grid as due to near-isobaric cooling.

Key words: Araku; corona texture; FMAS system; petrogenetic grid; spinel granulite; thermobarometry.

INTRODUCTION

Mineral assemblages containing spinel and/or sapphirine with quartz represent more or less extreme conditions of metamorphism (Hensen & Green, 1973; Sandiford et al., 1987; Powell & Sandiford, 1988; Harley, 1989). These often preserve spectacular corona structures reflecting retrograde equilibration and can, therefore, provide insights into the tectonic setting of the metamorphism (summarized in Harley, 1989). It has been suggested recently that mineral reactions in the system FMAS are dependent on f,, and that the invariant points [Sa], [Gal and [Cd] are stabilized at high fo,, and [Sp], [Qz], [Hy] and [Sill at low f,, (Hensen, 1986). Powell & Sandiford (1988) and Clarke et al. (1989) have further considered the effects of TiOz and Fe203 on the stability relations of the phases in the system FMASTO. Based on theoretical considerations and natural assemblages (Sandiford et al., 1987), Powell & Sandiford (1988) concluded that the high-f,, assemblages are buffered by magnetite- ilmenite, and the low-f,, assemblages by ilmenite-rutile. The P-T-f,, conditions of magnetite-ilmenite- buffered assemblages of spinel-sapphirine-gamet-quartz- sillimanite-hypersthene, sapphirine-spinel-cordierite-

hypersthene-sillimanite-quartz and sillimanite-spinel- cordierite-garnet-hypersthene-quartz are not well con- strained from experimental and natural occurrences (cf. Powell & Sandiford, 1988).

Two notable natural occurrences include sapphirine- spinel-quartz-hypersthene-sillimanite-hematite-magnetite (Caporuscio & Morse, 1978) and spinel-sillimanite- hypersthene-quartz-magnetite-ilmenite (Sandiford er al., 1987). It has been argued that these occurrences support both the experimental data of Annersten & Seifert (1981) and the theoretical prediction of Hensen (1986) (Powell & Sandiford, 1988). The topological inversion in the system FMASO is basically related to enlargement of the spinel + quartz stability at high fo, due to incorporation of Fe3+ in spinel (Hensen, 1986). A similar effect also occurs because of the presence of Zn in the bulk composition, which is preferentially partitioned into spinel (Harley, 1986). The stabilization of [Sa] in the FMASO system supposedly at high f,, (Hensen, 1986) is of particular interest because the stability of sapphirine is also known to be a function of XMg in the bulk composition and is favoured by higher XMg (Berg, 1977; La1 ef al., 1978; Ellis, 1980; Ellis et al., 1980, Vielzeuf, 1983). In this communication, we evaluate these findings in relation to

651

452 P . SENCUPTA ET AL.

Gondwana to Cenozoic cwer [T3sedirnentory rocks a Deccan basalt

[=I crystal l ine rocks Precgrn b r i m

unmetamorphosed basin

HT. RANGES

1 _---- RiwarS O H Y CITIES

--- Chornockite l ine fFcrmor.1936)

Fig. 1. Map of India showing the Eastern Ghats belt and the study area. MA = Madras, V = Visakhapatnam, HY = Hyderabad, AR = Araku, AN = Anantagiri, BA = Bangalore.

the petrological evolution of two unusual sapphirine- East Antarctica, and the second one has been described by absent assemblages occumng in Fe-rich spinel granulites Stiiwe & Powell (1989) from the Larsemann Hills, East from the Araku area, Eastern Ghats, India (Fig. 1). We Antarctica. In association (i), spinel is commonly rimmed propose a partial petrogenetic grid in the system FMAS by sillimanite against cordierite porphyroblasts (Fig. 2a). which is applicable to Fe-rich bulk compositions and is Orthopyroxene is conspicuously intergrown with silliman- complementary to the grid proposed by Hensen (1986). ite in the corona (Fig. 2b). Cordierite porphyroblasts are The Eastern Ghats belt, particularly the Araku and again rimmed by sillimanite and orthopyroxene (Fig. 2c). adjoining areas, bears signatures of extremely high-T There is no grain contact between spinel and quartz. (c. 950" C) metamorphism (Dasgupta et af., 1990; Seng- Biotite replaces cordierite and orthopyroxene. In associa- upta et al . , 1990). tion (ii), spinel is rimmed by sillimanite (Fig. 2d) and

garnet (Fig. 2e). There is again no grain contact between

GEOLOGICAL BACKGROUND

spinel and quartz. Exsolved magnetite blebs occur in spinel (Fig. 2d) and discrete magnetite occurs also in contact with spinel and rimming it. Garnet and sillimanite also rim

The Eastern Ghats granulite belt along the east coast of coarse Fe-Ti oxides (Fig. 2d,e). The Fe-Ti oxides show India (Fig. 1) exposes several types of ortho- and complex intergrowth patterns (Fig. 2f). These grains are paragneisses intruded by anorthosite and alkaline rocks composed of about equal proportions of ilmenite and (Naqvi & Rogers, 1987). In the Araku area, the principal magnetite. Texturally, two types of ilmenite lamellae are rock types are khondalite (garnet-quartz-perthite- present (Fig. 2f). Type I lamellae are coarse (c. 15 pm), sillimanite gneiss), orthopyroxene granulite (ortho- whereas type I1 lamellae are finer (c. 10pm) and both pyroxene-plagioclase-perthite-quartz-garnet gneiss), types occur in a single grain (Fig. 2f). Coarse ilmenite matic granulite (orthopyroxene-clinopyroxene-garnet- lamellae also contain finer (c. 10 pm) magnetite lamellae plagioclase-quartz gneiss), calc-granulite (scapolite- (Fig. 2f) similar to type 11. Similar complex intergrowth of wollastonite-garnet-clinopyroxene-calcite gneiss) and Fe-Ti oxides is also reported from very high-grade leptynite (garnet-quartz-perthite-plagioclase gneiss) (>8W C) metamorphic terranes (Rollinson, 1980; Per- (Dasgupta et al., 1990). Spinel-bearing rocks occur as rault & Martignole, 1988). From the textural relationships, isolated lenses in leptynite and orthopyroxene granulite. a two-stage evolution of the intergrowth can be All the rocks show a pervasive NE-SW planar structure in deciphered. In the first stage, coarse ilmenite lamellae the form of gneissose banding. Geochronological data on developed in the magnetite host obviously from the the Eastern .Ghats granulites are meagre and are breakdown of an ulvospinel solid solution, and in the summarized in La1 er af. (1987) and Sengupta er al. (1990). second stage ilmenite exsolved from magnetite and vice

PETROGRAPHY

versa. Perthite, plagioclase and quartz usually occur as megacrysts in the assemblages (Fig 2a-e). Garnet locally rims these phases in association (ii).

The textural relationships suggest that in association (i), Two distinct mineral associations have developed in the spinel-cordierite-quartz were stabilized early and prob- spinel-bearing granulites in closely spaced domains: (i) ably represent a prograde assemblage. Subsequently, spinel-quartz-cordierite-orthopyroxene-sillimanite-perth- sillimanite and orthopyroxene appeared in the assemblage. ite-biotite; (ii) spinel-garnet-quartz-sillimanite-perthite- In association (ii), on the other hand, early stabilization of biotite-magnetite-ilmenite. spinel-quartz-Fe-Ti oxides is indicated by the textural

The first association is similar to one of the assemblages relationships. Garnet and sillimanite appeared later in the described by.Ellis er al. (1980) and Harley (1986) from assemblage. The presence of a melt phase is indicated by

SPINEL GRANULITES, EASTERN GHATS, I N D I A 453

Fig. 2. (a) Spinel (white) enclosed in sillimanite (dark) against a cordierite (C) porphyroblast (enlarged from b). (b) Orthopyroxene (0) and sillimanite (dark) corona around spinel (white). (c) Cordierite (C) porphyroblast partially rimmed by sillimanite (dark) and orthopyroxene (0). (d) Spinel (S) rimmed by sillimanite (SI). Exsolved magnetite (white) in spinel. (e) Spinel (S) rimmed by garnet (G). Magnetite (white) exsolved in spinel. Garnet and sillimanite (black) rimmed coarse Fe-Ti oxides (white). Note coarse quartz (Q) and K-feldspar (K). (f) Intergrowth of titaniferous magnetite (white) in hemoilmenite (black).

454 P. SENCUPTA E T A L .

the megacrysts of quartz and feldspar and melting took place quite early in the evolution of the rock.

CHEMICAL CHARACTERISTICS

Bhk chemical compositions, determined by XRF, of two representative samples containing the two mineral associations are given in Table 1. Association (i) is characterized by a relatively high magnesian bulk composition [MgO/(MgO + Fe203 (tot)) of 0.31, com- pared to 0.19 in association (ii)]. However, both associations are conspicuously more Fe-rich, compared to

Table 1. Bulk chemical analysis of representative rock types.

Assemblage (i) Assemblage (ii)

SiO, TiO,

Fe,O, (tot) MnO MgO CaO Na,O K2O PZO,

A1203

Total

56.78 2.30

21.38 9.05 0.02 4.38 5.38 0.10 0.24 0.01

99.66

53.43 1.72

20.73 13.21 0.14 3.42 3.03 1.71 2.18 0.01

99.58

other reported sapphirine granulites (with or without spinel) (see Sengupta et al., 1990, for details).

Chemical compositions of the coexisting phases were determined using a JEOL-JXA 733 and 8600 EPMA system, with natural minerals as standards. The results are given in Tables 2 & 3. Spinel in association (i) contains a significant gahnite component (maximum 6.6 mol.%), but contains less Zn in association (ii) (Table 2). It is primarily a spinel-hercynite solid solution with an X,.,, [Mg/(Mg + Fe")] of 0.47 in association (i), and 0.35-0.42 in association (ii). Recalculation of spinel composition according to stoichiometry (Bohlen & Essene, 1977) gives 2.3-4 wt% Fe,O, in association (ii) but <1 wt% Fe,O, in association (i) (Table 2). Type I ilmenite lamellae contain appreciable amounts of Fez03 (6.4-7.7 wt%) and X,,,, is close to 0.94. The magnetite host is enriched in ulvospinel (31-40mol.%, Table 3). The compositions of type I1 ilmenite lamellae and the similar magnetite lamellae in ilmenite could not be determined. Similarly, the composition of magnetite blebs exsolved in spinel could not be determined. Discrete magnetite grains occurring at the contact of spinel approach end-member composition.

Sillimanite contains 1.2-1.8 wt% Fe203 in association (ii) but is very poor in Fe,O, in association (i) (Table 2). Cordierite (association i) is highly magnesian with Xu, = 0.84 (Table 2). Orthopyroxene (association i) is aluminous (AI,03 = 6.5 wt%) and is essentially enstatitic

Table 2. EPMA analysis of representative spinel, sillimanite, cordierite and hypersthene from spinel granulite.

Sample no. E2-2 E2-2 E6-1 E7-2 E2-2 E6-1 E6-3 E2-1 E2-2' E2-2 E2-2'

Assemblage (i) ( 9 (ii) (ii) (i) (ii) (ii)

Spinel Sillimanite Cordierite Hypersthene

SiO, TiO, A W 3 Fe203 FeO MnO

CaO ZnO Na,O

Total

MgO

Si Ti A1 Fe3+ Fe2+ Mn2+ Mg ca Zn Na

0.04

63.08 0.81

21.98 0.05

11.11

3.33 0.06

*-

-

0.02 0.00

62.06 60.33 0.91 4.17

21.68 25.38 0.04 0.04

10.99 10.39

3.02 0.43 0.09 0.03

- -

- -

100.46 98.81 100.77

Cations to 4 oxygens

- - - 1.98 1.98 1.91 0.01 0.02 0.08 0.49 0.49 0.57

0.44 0.44 0.42

0.06 0.06 0.01

2.98 2.99 2.99

- - -

- - -

- - -

0.03 0.05

59.45 2.21

26.63 0.00 8.15

1.88 0.08

98.48

-

37.05 36.62 36.77 0.05 - 0.03

62.08 61.42 61.25 0.27 1.80 1.24

- - - 0.01 - - 0.01

- 0.07 0.05 - 0.01 0.01

99.45 99.93 99.36

- -

~

50.41 49.98 0.04 -

33.33 33.74

3.81 3.85

11.21 11.29 0.04 -

0.06 0.04

98.90 98.90

- -

- -

- -

- 1.95 0.04 0.62

0.34

0.04

2.99

- - -

2.98 2.98 2.99

Cations to 18 oxygens

5.04 5.00

3.93 3.98

0.32 0.32

1.67 1 . a

- -

- - - -

- - - -

0.01 0.01

10.97 10.99

50.64 50.49 0.21 0.19 6.56 6.66

20.93 21.80 0.01 0.04

21.63 21.81 - 0.01 - 0.09 0.02 0.02

100.00 101.11

Cations to 6 oxygens

- -

1.86 I .85

0.28 0.28

0.64 0.66

1.18 1.19

- -

- -

- -

- -

3.% 3.98

* Below detection limit.

SPINEL G R A N U L I T E S , EASTERN G H A T S , I N D I A 455

Table 3. EPMA analysis of representative garnet, magnetite and ilmenite from spinel granulite.

Sample no. E6-1 E2-3 E6-3 E7-3 E6-3 E7-3

Assemblage (ii) (ii) (ii) (ii) (ii) (ii)

Garnet Magnetite * Ilmenite Core Rim

SiO, 38.95 38.81 38.25 0.03 0.04 - - TiO, 0.02 -t - 10.61 15.36 49.62 50.05

21.99 22.41 21.92 0.20 0.16 - 0.06 Fed33 1.16 - 0.22 47.35 38.69 7.73 6.41 FeO 26.16 24.79 26.96 40.45 44.73 43.00 43.00 MnO 0.54 0.40 0.42 0.01 0.04 0.04 0.14

10.51 11.38 9.92 0.08 0.25 0.88 1.04 - 0.01

MgO CaO

Total 100.69 98.75 98.45 98.73 99.27 101.27 100.71

Cations to 12 oxygens

A1203

1.36 0.96 0.76 - -

Cations to 12 oxygens Cations to 4 oxygens

Si Ti Al Fe3+ Fez+ Mn2+ Mg Ca

2.96

1.97 0.06 1.66 0.03 1.19 0.11

7.98

- 2.98

2.03

1.59 0.02 1.30 0.08

7.98

-

-

2.98

2.01 0.01 1.75 0.03 1.15 0.06 7.99

- -

0.30 0.01 1.37 1.30 - - -

2.98

- 0.44 0.01 1.11 1.42

0.01

2.99

-

-

- 3.71

0.58 3.57

0.13

7.99

-

-

-

- 3.75 0.01 0.48 3.58 0.01 0.15 - 7.98

Mixture of magnetite and ilmenite (exsolved). t Below detection limit.

(XMg = 0.65) (Table 2). Cores of garnet in association (ii) are enriched slightly in pyrope (XMg = 0.43) relative to the rims (XMg=0.38) (Table 3). The grossular content of garnets is <3% in all cases.

The greater proportion of Fe203 in sillimanite and spinel in association (ii), as compared to association (i), could be related to the presence of Fe,03-saturating Fe-Ti oxides and/or higher fo,. In the absence of suitable fo , sensors in association (i), it is not possible to assess the exact role of fo, for such compositional variations. However, the differences in the initial mineralogy of the two associations can be attributed to variations in bulk rock composition. Early stabilization of cordierite in assemblage (i) is related to higher XMg in the bulk composition.

M I N E R A L R E A C T I O N S

Textural relationships and compositional characteristics of the phases in association (i) suggest early stabilization of the assemblage spinel-cordierite-quartz (Sa, Ga, Hy, Sil). The assemblage is at least bivariant in the FMAS system. However, the presence of appreciable Zn and Fe” in spinel raises the variance to four. Textural relationships, such as the rimming of cordierite and spinel by orthopyroxene-sillimanite symplectite and the composi- tional plots in the AFM plane (Fig. 3a) and in the (FM)AS system (Fig. 3b), indicate the reaction:

spinel + cordierite + quartz = orthopyroxene + sillimanite (Sa, Ga). (1)

The reaction is univariant in the FMAS system, but is tnvanant in the system ZnFMASO. Finally, the (Sa, Ga) assemblage is replaced by one of lower variance (Sa, Ga, Sp) since spinel in the association is almost invariably rimmed by either sillimanite or a sillimanite- orthopyroxene symplectite.

In association (ii), spinel-quartz-Fe-Ti oxides constit- uted the early assemblage. Spinel at this stage must have incorporated appreciable Fe3+ which was later exsolved as magnetite. In places, the magnetite moved outside the grain boundaries of spinel and this explains the occurrence of Ti-free magnetite at the margin of spinel. Reaction textures and compositional plots in the (FM)AS system (Fig. 4a) suggest that the garnet-sillimanite corona formed due to the equilibrium:

(2)

The reaction is bivariant in the system FMAS. However, due to the presence of Zn and Fe3+ in spinel and of Ca in garnet, the variance is increased. Compositional plots of these phases on the AFM plane (Fig. 4b) reveal co-linearity among spinel, garnet and sillimanite. This is a result of equal partitioning of Fe-Mg between garnet and spinel. Because of the co-linearity, reaction (2) becomes degenerate in the FMAS system. Such equipartitioning of Fe-Mg between coexisting garnet and spinel is reported from other spinel granulites (cf. Vielzeuf, 1983) and has a marked influence on the topology of the FMAS system, which is discussed in a later section.

The presence of a sillimanite rim around magnetite

spinel +quartz = garnet + sillimanite (Sa, Cd, Hy).

456 P. SENGUPTA ET AL.

Fig. 3. (a) AFM projection of coexisting phases in assemblage (i). (b) (FM)AS projection of coexisting phases in assemblage (i).

Fig. 4. (a) (FM)AS projection of coexisting phases in assemblage (ii). (b) AFh4 projection of coexisting phases in assemblage (ii).

grains in contact with spinel, which presumably exsolved from a spinel-magnetite solid solution, indicates that exsolution took place prior to the onset of reaction (2). Complex intergrowth of magnetite (with Usp = 30- 41 mol.%) and ilmenite may be a product of unmixing of ulvospinel through the reaction:

6Fe,Ti04 + 0, = 6FeTi0, + 2Fe304. (3) This reaction is driven towards the right with a fall in temperature. The presence of two types of ilmenite lamellae suggests that such unmixing proceeded in at least two stages during the cooling of the complex (cf. Perrault & Martignole, 1988). Sillimanite rimming the Fe-Ti oxides is enriched in Fe20, compared to that in association (i). It has already been mentioned that the magnetite exsolved from spinel is Ti-free. It therefore follows that the Fe-Ti oxides acted only as a source for Fe", and the assemblage

sillimanite-garnet-spinel-quartz can be adequately de- scribed in the system Fh4AS with Fe3+ and Zn as additional variables.

P -T-fo2 CONDITIONS OF METAMORPHISM

The mineral assemblages in the spinel granulites do not permit estimation of the P-T conditions of metamorphism through well-constrained thermobarometers. However, the magnetite-spinel solvus of Turnock & Eugster (1%2) suggests a temperature in excess of 860°C for the peak metamorphism. The spinel-cordierite thermometer of Vielzeuf (1983) for association (i) yields a temperature of 960" C which is in surprising agreement with the maximum temperature recorded in the Araku area (c. 950"C, Dasgupta el al., 1990). The rutile-absent association (ii)

SPINEL GRANULITES, EASTERN GHATS, INDIA 457

yields a maximum pressure of 9 kbar at this temperature using the GRAIL barometer (Ghent & Stout, 1984). Similar values (7.5-8.5 kbar) are obtained from GAPQ barometry for the same assemblage. In assemblage (i), the cordierite-orthopyroxene-sillimanite-quartz barometer of Harris & Holland (1984) gives pressures of 8.5 kbar at 960°C. Thus, from the above results, the maximum P-T conditions of the studied spinel granulites can be set at 8.5 kbar and 960" C. These values are consistent with those derived from adjacent areas (Dasgupta et al., 1990; Sengupta et al., 1990). Compositions of coexisting Mt-Usp, in association (ii) give log fo, = -15 at 850" c , and log fo, = - 17 at 750" C following Spencer & Lindsley (1981). The log fo,-T values plot just above the NNO buffer (Lindsley, 1976) and confirm the 'oxidized' nature of the spinel granulites. The magnetite-ilmenite tempera- tures obviously indicate post-peak equilibration. The spinel granulites of Araku, therefore, are part of a unique granulite suite that record a very high metamorphic temperature (c. 950" C).

A PETROCENETIC G R I D FOR SPINEL C R A N U LlTES

Hensen (1971) constructed a P-T grid in the system FMAS in which the invariant points [Sa], [Gal and [Cd] were metastable. Subsequently, this grid, with minor modifications, was successfully applied to several high- grade metamorphic terranes (cf. Ellis et al., 1980). Vielzeuf (1983) pointed out that the grid cannot explain the experimental data of Annersten & Seifert (1981) in which (Sa, Cd) was stable and the natural rock data of Capourscio & Morse (1978) in which (Sa, Cd, Ga) was stable. Both indicate the stability of [Sa] which was deemed metastable in Hensen (1971). Accordingly, Vielzeuf (1983) suggested an alternative grid in the FMAS system where the stabilities of spinel + quartz and sapphirine+quartz did not overlap. This point was criticized by Hensen (1986) who showed that there is an overlap in the stabilities of spinel + quartz and sapphirine + quartz (references cited in Hensen, 1986). He, however, agreed with Vielzeuf (1983) that the invariant point [Sa] could be stable. He argued that an increase in fo, leads to the incorporation of Fe'+ in spinel and the univariant reactions in the system FMAS move parallel to themselves in such a way that the stability field of spinel + quartz is enlarged. Even though no new phase is produced, the enhanced stability of spinel + quartz at high fo, causes a topological inversion so that [Sa], [Gal and [Cd] become stable. Harley (1986) concluded that the presence of Zn in spinel has an analogous effect of increasing the stability of spinel + quartz.

On the other hand, the theoretical analyses of Berg (1977) and La1 et al. (1978) and natural rock data of Ellis ef al. (1980), Harris (1981) and Vielzeuf (1983) suggest that the stability field of spinel +quartz is enlarged in Fe-rich bulk composition [ X , = FeO/(FeO + MgO) 2 0.401. Ellis et af. (1980) showed that in granulites metamorphosed under identical low-fo, conditions (indicated by oxide

mineralogy), sapphirine appeared only in Mg-rich bulk compositions. Several univariant and bivariant as- semblages related to [Gal, [Sa] and [Cd] invariant points have been reported from granulites metamorphosed under lower fo, conditions (Berg, 1977; La1 et al., 1978; Vielzeuf, 1983; Sengupta et al., 1990). The non-appearance of sapphirine and stabilization of the spinel + quartz as- semblage in Fe-rich bulk compositions (X, = 0.8) is also confirmed from the experimental work of Annersten & Seifert (1981). It is apparent from the foregoing that the enlargement of the stability field of spinel + quartz and the consequent topological inversion leading to stabilization of [Sa], [Gal and [Cd] is dependent not only on fo, but also on the presence of Zn and a higher FeIMg ratio in the bulk composition. It is evident from the studies of Ellis et al. (1980) and Harley (1986) that the last two factors can override the effect of fo,.

The 'high-foi grid of Hensen (1986), where [Sa], [Gal and [Cd] are stable, was constructed on the assumption that XMg in the coexisting phases increases in the order Cd-Sa-Hy-Sp-Ga. Natural compositions of garnet and spinel in many spinel granulites show that the Fe-Mg K, value approaches unity and inversion of the distribution relationship occurs as a function of PIT and bulk composition. For high-P conditions and magnesian bulk compositions, the XMg in spinel is greater than XH8 in garnet, as suggested by Hensen (1986). However, for high-T, low-P conditions and Fe-rich bulk compositions, which are typical for most of the reported sapphirine-free spinel granulites (Berg, 1977; La1 et al., 1978; Vielzeuf, 1983; this communication), XZg is either less than X g or equal, so that the increase in XMg is in the order Cd-Hy-Ga 2 Sp.

The reverse partitioning of Fe and Mg between garnet and spinel in Fe-rich bulk compositions changes two of the univariant reactions around the invariant point [Sa]:

Ga + Sil = Sp + Cd + Qz (Sa, Hy) Ga + Sil = Sp + Hy + Qz (Sa, Cd).

Furthermore, a K , inversion on a univariant boundary creates a singular point (where X"M", = X g g ) at which the amount of one of the reactants reduces to zero and the reaction becomes degenerate, i.e. (Sa, Cd, Hy) (Hensen & Harley, 1990). This leads to one less reaction in M A S , i.e. (Sa, Cd, Hy), instead of (Cd, Sa) and (Sa, Hy).

It is thus evident horn the foregoing discussion that for low?, high-T and Fe-rich bulk compositions, which are appropriate for the presently studied spinel granulites as well as other sapphirine-free parageneses, the order of increase in XMg of the coexisting phases and the chemographic relations are not similar to those predicted by Hensen (1986). This necessitates construction of an alternative grid. Figure 5(a) shows a partial petrogenetic grid for the system FMAS where [Sa] is stable in Fe-rich bulk compositions and XMg in the phases increases in the order Cd-Hy-Ga-Sp. The bivariant reactions related to the univariant reactions are shown in Fig. 5(a). The reaction vectors deduced from textural relationships of

458 P . SENGUPTA € T AL.

T r Fii. 5. A partial petrogenetic grid for the system FMAS. (a) The thick line denotes univariant equilibria around the invariant point [Sa] in the FMAS system. Thin lines denote bivariant equilibria. The grid is drawn where X,,, in the associated phases is in the order Cd > Hy > Ga > Sp. Arrows indicate P-T paths in different terranes: (1) Nain Complex, Labrador (Berg, 1977); (2) Sonapahar, India (La1 et al., 1978); (3) Talanta, Italy (Vielzuef, 1983); (4) Cortland, New York (Caporuscio & Morse, 1978); (5) Uganda (Sandiford et al., 1987). (b) As in (a) but with the condition X& = Xg. The arrow indicates the P-Tpath of evolution of the Araku spinel granulites.

spinel granulites from several terranes are shown by arrows. The proposed petrogenetic grid (Fig. 5a) explains satisfactorily the P-T path of these spinel granulites and is consistent with those predicted for the respective areas.

P-T EVOLUTION OF THE ARAKU SPINEL GRANULITES IN THE CONTEXT OF THE GRID

For the compositional relation Xg = X& applicable for the Araku granulite, the reaction (Sa, Cd) becomes degenerate (Sa, Cd, Hy). The topology around the invariant point [Sa] in such a situation is shown in Fig. 5(b). Incorporation of Zn in spinel would enlarge the stability field of spinel + quartz and the [Sa] invariant point becomes univariant. It translates along the line (Sa, Sp) towards the FMAS invariant point [Sp], i.e. towards lower temperature. Presence of Fe3+ in spinel would further accentuate this translation. However, as w o n as spinel becomes rimmed by the product phases, such as orthopyroxene + sillimanite (association i) or by garnet + sillirnanite (association ii) which do not incorporate Zn, reactions (1) and (2) will jump back to the original FMAS

positions (cf. Harley, 1986). The reaction vector for the studied spinel granulites, as deduced from textural relationships, is shown in Fig. 5(b). The path is one of near-isobaric cooling subsequent to early stabilization of spinel + quartz f cordierite. Sapphirine granulites from an adjoining area showed an anticlockwise P-T path of evolution comprising a prograde trajectory of high dTldP, followed sequentially by near-isobaric cooling and near-isothermal decompression (Sengupta el af., 1990). Thus, the studied spinel granulites preserve a partial retrograde P-T trajectory. Early stabilization of spinel + quartz f cordierite in the spinel granulites attests to a prograde path of high dTldP (cf. Vielzeuf, 1983). Thermobarometric data show that isobaric cooling commenced from a temperature of c. 950°C at 8-8.5 kbar and continued up to c. 750" C.

ACKNOWLEDGEMENTS

P.S. acknowledges the Council of Scientific and Industrial Research and S.K. and S.D. acknowledge the University Grants Commission for financial support. We thank Drs S. L. Harley, S. K. Sen and D. Robinson for their

SPINEL GRANULITES, EASTERN GHATS, INDIA 459

constructive criticism and helpful suggestions o n an earlier draft of the manuscript.

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Received 11 July 1990; revision accepted 6 December 1990.