Arrested charnockite formation in southern India and Sri Lanka

Contrib Mineral Petrol (1987) 96:225-244 Contributions to Mineralogy and Petrology �9 Springer-Verlag 1987

Arrested eharnoekite formation in southern India and Sri Lanka

E.C. Hansen 1, A.S. Janardhan 2, R.C. Newton 3, W.K.B.N. Prame 4, and G.R. Ravindra Kumar 5 l Department of Geology, Hope College, Holland, MI 49423, USA 2 Department of Geology, University of Mysore, Karnataka, 570006, India 3 Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA

Sri Lanka Geological Survey Department, Colombo, 2, Sri Lanka 5 Centre for Earth Science Studies, Trivandrum, Kerala, 695010, India

Abstract. Arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high-grade terrains of southern India and Sri Lanka. Two major kinds of orthopyroxene-producing reactions are recognized. Breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic/granitic "gray gneiss" produced the regional orthopyroxene isograd, manifest in charnockitic mottling and veining of "mixed-facies" exposures, as at Kabbal, Karnataka, and in the Kurunegala District of the Sri Lanka Central Highlands. Chemical and modal analyses of carefully chosen immediately-adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of C a t , MgO and FeO and gains of Sit2 and Na20. Rb and Y are depleted in the charnockite. Another kind of charnockitization is found in paragneisses throughout the southern high-grade area, and involves the reaction of biotite and quartz • garnet to produce orthopyroxene and K- feldspar. Although charnockite formation along shears and other deformation zones at such localities as Ponmudi, Ker- ala is highly reminiscent of Kabbal, close pair analyses are not as suggestive of open-system behavior. This type of charnockite formation is found in granulite facies areas where no prograde amphibole-bearing gneisses exist and connotes a higher-grade reaction than that of the orthopyroxene isograd. Metamorphic conditions of both Kabbal- type and Ponmudi-type localities were 700~176 and 5-6 kbar. Lower P(H20) in the Ponmudi-type metamorphism was probably the definitive factor.

CO2-rich fluid inclusions in quartz from the Kabbal- type localities support the concept that this type of charnockite formation was driven by influx of CO2 from some deep-seated source. The open-system behavior and high oxidation states of the metamorphism are in accord with the CO2-streaming hypothesis. CO2-rich inclusions in graphite- bearing charnockites of the Ponmudi type, however, commonly have Iow densities and compositions not predictable by vapor-mineral equilibrium calculations. These inclusions may have suffered post-metamorphic Hz leakage or some systematic contamination.

Neither the close-pair analyses nor the fluid inclusions strongly suggest an influx of CO2 drove charnockite formation of the Ponmudi type. The possibility remains that orthopyroxene and COz-rich fluids were produced by reaction of biotite with graphite without intervention of fluids of

Offprint requests to: E.C. Hansen

external origin. Further evidence, such as oxygen isotopes, is necessary to test the COz-streaming hypothesis for the Ponmudi-type localities.

Introduction

Granulite faeies metamorphism and fluid regimes

The role of fluid-rock interaction in granulite facies metamorphism is a topic of intense interest. The South India/Sri Lanka late Archaean charnockitic terrain has been cited numerous times and on varied criteria as the archetypal example of massive granulite facies development through streaming of pervasive, low-P(H20) vapors, possibly mantle-derived. Weaver (1980) showed that trace element patterns of Madras charnockites (orthopyroxene-bearing acid gneisses) indicate metasomatism during metamorphism. Condie et al. (1982) came to a similar conclusion for charnockites from the granulite facies transition zone south of Bangalore, Karnataka, particularly with respect to Rb and K patterns. Harris et al. (1982), in a survey ofpaleotemper- atures and paleopressures in the South Indian granulite terrain, concluded that the upiquitous paleotemperature range of 7000-800 ~ C for rocks of variable metamorphic pressure, 5-9 kbar, indicates that an "advective geotherm" operated during the late Archaean metamorphism, where massive CO2 streaming from a subcrustal source transported heat to middle and high levels of the crust. Elsewhere, Okeke et al. (1983) in the Scourian terrain, Sheraton and Collerson (1984) in the Vestfold Block of Antarctica, and Smalley et al. (1983) in the Bamble terrain of southern Norway, concluded that patterns of large-ion lithophile (LIL) element depletion, particularly of Rb, required partition into, and removal by, a pervasive vapor during granulite facies metamorphism. Hansen et al. (1984) calculated that such a fluid, to be in equilibrium with orthopyroxene in quartzofeldspathic rocks, must have had H20 less than 35 mole percent. Carbon dioxide is, by default, the likeliest vapor species that diluted H20 to this extent. High-density CO2 inclusions in minerals have been identified in the Kapuskas- ing high-grade belt of Ontario (Rudnick et al. 1984), the charnockite terrain of southern Karnataka, India (Hansen et al. 1984), and the Bamble terrain (Touret 1971). From these considerations, the possible role of CO2 streaming

226

14 ~

13 ~

12 ~

I1"

I0 ~

90

80

7 ~

6' N 74~ 75 ~ 76 ~ 77 ~ 78 ~ 79 ~ 80 ~ 81 ~

Fig. 1. Geological map of southern India and Sri Lanka, after various Geological Survey of India maps, Drury and Holt (1980), Drury et al. (1984) and the Sri Lanka Geological Survey Depart- ment map of Sri Lanka (1982). Charnockitic hill ranges: BH Bili- girirangan Hills; NH Nilgiri Hills; AH Annamalai Hills; KH Kol- laimalai Hills; SH Shevaroy Hills; CH Central Highlands. Pro- grade charnockitization localities: 1 Kabbal, Karnataka; 2 Pon- mudi, Kerala; 3 Kodamakod, Kerala; 4 Ervadi, Tamil Nadu; 5 Arni, Tamil Nadu; 6 Kurunegala, Sri Lanka

as a primary cause of deep-crustal metamorphism may be suspected to be of importance.

The Adirondack Highlands high-grade terrain of New York state offers an interesting contrast. Numerous features of the granulite facies rocks refute involvement of a pervasive high-pressure CO2 phase during metamorphism. Striking local pre-granulite facies gradients of oxygen fugacity and COz fugacity are preserved (Valley et al. 1983). Very large local variations in oxygen isotope composition are the result of pre-orogenic shallow contact metamorphism about anorthosite intrusions (Valley and O'Neil 1984). Low oxygen fugacities indicated by oxide minerals militate against the oxidizing influence of abundant high- pressure CO2 fluid (Lamb and Valley 1984). Preserved ig- neous textures and chilled margins of intrusive bodies imply that metasomatic redistribution was extremely limited. The Adirondack Highlands is therefore an example of an initial- ly dry terrain that underwent granulite facies metamorphism without widespread introduction of pervasive metamorphic fluidS. It will be of importance to characterize further the mineralogical and geochemical signatures of this kind of terrain and of the type of terrain where pervasive fluids seem to have been instrumental, such as southern India, the Bamble region, and the Buksefjorden area of SW Greenland (Wells 1979).

The South India/Sri Lanka high-grade terrain

The geology of southern India is summarized in Drury et al. (1984) and Newton and Hansen (1986). Figure 1 shows the regional relations. The older Precambrian Dharwar Craton,

a "granite-greenstone" continental nucleus, is composed of tonalitic-trondhjemitic gneisses ("Peninsular Gneiss"), some of which are as old as 3.4 Ga (Beckinsale et al. 1980), with infolded supracrustals, the Dharwar and Sargur schist belts. The Craton is traversed by N-S-trending elongate K-rich granites, including the Closepet Granite. The general N-S grain of the Dharwar Craton may be the result of late Archaean shearing or wrenching (Drury and Holt 1980).

The granite-greenstone craton increases southward in metamorphic grade (Pichamuthu 1965) through greenschist and amphibolite facies, to an orthopyroxene isograd in tonalitic-granitic gneisses (Subramaniam 1967). The onset of granulite facies seems to be continuous in several places where the transition zone has been studied. In the area south of Krishnagiri, Tamil Nadu, charnockites steadily increase southward in abundance relative to amphibole- bearing gneisses. The conversion of Peninsular Gneiss to charnockite is approximately isochemical in major and mi- nor elements (Condie et al. 1982). Further west, in the Sat- nur-Halaguru area of Karnataka (near area 1, Fig. 1), the same process is apparent. Charnockite patches appear mottling the gneisses and coalesce southward into the Biligirir- angan (BR) Hills charnockite massif, a uniformly granutite- facies terrain which contains some gneisses severely depleted in Rb (Newton and Hansen 1986).

The portion of the high-grade terrain adjacent to the Dharwar Craton is, in several places, structurally contigu- ous with it, and records a granulite-facies metamorphism of 2.5-2.6 Ga age (Crawford 1969). The relations of the vast high-grade area farther south are more complex. Indi- vidual charnockite highland massifs are bounded by roughly E-W shear zones which are certainly post-Archaean and may be mid-Proterozoic (Drury and Holt 1980). Large areas of amphibole-biotite gneiss with scattered granite bodies southwest of Madras are called Peninsular Gneiss on the Tamil Nadu State Geological Map (Karunakaran 1974) and are classified as Archaean, but may, in fact, be retrogressive after Archaean charnockites or accreted since 2.5 Ga ago (Hansen et al. 1985). The Sri Lanka corollary of the Tamil Nadu Peninsular Gneiss is the Vijayan Com- plex, which occupies two coastal belts athwart the charnockitic Central Highland Series. The Vijayan Complex is late Proterozoic, in contrast to the Highlands, which has an approximate age of 2100 Ma (Crawford and Oliver 1969). An extensive terrain of metasedimentary granulites, the Khondalite Belt, exists in southernmost Kerala and Ta- mil Nadu, at the tip of the peninsula. A few Sr-Rb whole- rock model ages are late Archaean to early Proterozoic (Crawford 1969).

Arrested charnockite forrnation

Numerous localities in southern India have been described which show clear evidence of arrested prograde charnockite formation in quartzofeldspathic gneisses. At the classic quarry exposure near Kabbal village in southern Karnataka (locality 1 in Fig. 1), Pichamuthu (1960) first described migmatitic Peninsular Gneiss veined and spotted by dark brown charnockite, characteristically along short shears, sometimes with drag-folding, as if deformation provided access to some sort of charnockitizing fluids. The reaction arrested in progress is that of amphibole and/or biotite in the gneiss to orthopyroxene. Although some observers of

Table 1. Close-pair rock analyses of gray gneiss (GN) and charnockite (CH), Kabbal-type localities

227

Kabbal, Karnataka Waraddana S.L. Udadigana S.L.

1 2 3 4 5 6 7

3-1A 3-1B 3-1A 3-1B 3-1A 3-1B 3-9A 3-9B 484-1A 484-1B D4-03 D4-03 D4-K1 D4-K1 CH GN CH GN CH GN CH GN CH GN CH GN CH GN

Wt% SiOz 71.4 68.4 71.8 71.4 71.4 68.7 70.7 69.6 73.9 73.6 69.3 68.4 69.2 68.0 TiO2 0.49 0.62 0.59 0.60 0.54 0.6l 0.53 0.54 0.29 0.44 0.50 0.62 0.41 0.51 AlzOa 13 .7 13.6 14.5 14,0 13.6 13.6 13.4 13.2 12.9 13.1 14.4 14.5 14.9 14.9 Fe2Oa 3.13 3.98 3.94 3.92 3.54 4.41 4.04 3.93 2.87 3.25 3.17 4.02 2.80 3.48 MgO 0.79 0.77 0.85 0.89 0.56 0.82 0.67 0.71 0.43 0.68 0.69 0.99 0.63 0.97 CaO 1.95 2.44 2.34 2.52 2.14 2.53 2.24 2.34 1.73 2.16 2.31 2.59 2.13 2.42 MnO 0.06 0.06 0.08 0.06 0.07 0.07 0.08 0.07 0.05 0.04 0.07 0.06 0.05 0.05 Na20 3.76 3.65 3.94 3.50 4.14 3.86 4.15 3.94 3.46 3.64 3.74 3.50 3.67 3.67 KzO 3.67 3.31 2.88 3.52 3.35 3.71 3.44 3.50 3.89 3.22 4.32 4.42 5.09 4.90 P205 0.12 0.17 0.15 0.17 0.14 0.17 0.03 0.02 0.09 0.15 0.17 0.20 0.14 0,18 C02 0.1 0.1 0.0 0.0 0.0 0.0 0.2 0.2 0,l 0.1 S 0.0 0.0 0.01 0.01 0,01 0.01 L.O.I. 0.47 0.31 0A7 0.14 0.23 0.00 0.54 0.08 0,44 0.08 0.77 0.47 0,39 0.31

ppm Rb 60 70 71 93 100 110 110 110 98 93 130 150 180 180 Sr 110 130 147 156 190 170 140 150 132 128 330 310 410 410 Zr 230 310 268 274 290 310 320 330 150 116 260 250 210 250 Nb 25 20 30 30 30 30 8 10 20 30 t0 20 Y 28 70 10 70 30 60 18 40 20 20 < 10 20 F 470 130 350 820 500 800 C1 95 120 < 50 50 < 50 50 U 1.0 1.4 0,9 1.1 1.1 1.8 1.7 1.5 0.7 0.4 1.2 2.1 1.9 1.2 Th 15 15 15 12 8 12 27 18 17 1.1 26 18 30 20

Analyses 1, 3, 4, 6, 7 by X-ray Assay Labs. Analysis 2 by present authors at Franklin and Marshall College. Analyses 5 from Allen et al. (1985)

this quarry have maintained that the amphibole-biotite gneiss is retrogressive after charnockite (Devaraju and Sadashi- vaiah 1969; Ray 1972), subsequent workers (Ramiengar e ta l . 1978; Janardhan eta l . 1982) have upheld Picha- muthu ' s original interpretation. Similar arrested charnockitization was described by Ramiengar et al. (1978) and Jan- ardhan et al. (1982) in several other quarries in southern Karnataka, and by Gopalakr ishna et al. (1986) in the east- ern margin of the Koorg granulite massif in southwestern Karnataka. All of these occurrences straddle the regional orthopyroxene isograd: they undoubtedly mark a threshold horizon of late-Archaean granulite-facies metamorphism.

Apparent ly similar exposure of partially charnockitized gneisses have been reported from extreme southern India, far south of the orthopyroxene isograd. Holt and Wight- man (1983) briefly described a quarry near Panaikkudi, Ta- mil Nadu (area 4, Fig. 1), in which charnockitic veins tra- verse acid biotite gneisses in a manner similar to Kabbal . Petrographic descriptions were not given. Ravindra K uma r et al. (1985) and Srikantappa et al. (1985) described very similar observations at several localities in the paragneiss Khondal i te Belt of southern Kerala (areas 2, 3, 4, Fig. 1). The prograde criteria of foliation - cutting charnockite veins with drag-folding about them are quite clear from these descriptions. The chemical analyses of Ravindra Ku- mar et al. (1985) indicate nearly isochemical conversion of biotite gneiss to charnockite.

It is important to establish the relationship of these ap-

parently widespread occurrences of arrested charnockitiza- t ion in southernmost India to those along the orthopyroxene isograd further north. A detailed analysis of the reactions producing orthopyroxene in the rocks is necessary to relate the various occurrences and to test the CO2- streaming mechanism as a possible agency of charnockite formation at the diverse localities.

M e t h o d s

Rock analyses. The present study makes use of close comparison of adjacent orthopyroxene-bearing and orthopyroxene-free rock samples (" close pairs ") in quarries, where vein-like or patchy alteration to charnockite is exposed. This kind of analysis provides a unique opportunity to study the charnockite-forming reactions, since there is complete control of the protolith and the alteration product. Previous attempts to demonstrate the nature of South Indian charnockitization have been unsatisfactory in that they have not, for the most part, made adequate use of close-pair analyses. In the present study, pair samples of about 2 kg each were selected from immediately-adjacent charnockitic and non-charnockitic por- tions of homogeneous protolith, a few to several cm apart, in fresh, active quarries. At Kabbal and similar localities, charnockite often occurs at the contacts of gneiss with metabasic lenses and with granite dikes; sampling of these contacts was avoided because metasomatic exchange could bias the results. Close-pair samples for analysis were cut perpendicular to gneissic foliation in order to minimize the effect of original variations in composition.

Major and trace element analyses were made at several different laboratories. The majority of the major element analyses were

228

Table 2. Mineral modes and plagioclase compositions

Locality Rock No. (Type)

Minerals present Percent abundance (molar An % of Plag; c core, r rim)

QTZ PLAG KSP BIO HBL OPX G A R CRD SIL MAG ILM GRA

Kabbal

3-1A (CH) 41.4 22.4 (21) 27.4 3.0 2.1 2.1 1~6 3-1B (GN) 36.4 26.8 (21) 24.3 4.3 6.1 2.1

3-9A (CH) 31.5 24.6 (20) 38.3 7.9 0.4 2.0 1.2 3-9B (GN) 29.5 25.1 (20) 33.4 6.8 3.1 2.0

Ponmudi

K 18-6A (CH) 28.1 25.3 (33) 27.3 4.8 3.1 10.0 1.3 K 18-6B (GN) 31.5 28.0 (32) 13.5 9.9 15.7 1.2

K 18-7 (CH) + (31) + + + + [ F

357-1A (CH) 22.3 31.6 32.3 2.1 5.4 5.5 0.8 357-1B (GN) 23.5 28.8 23.4 10.0 13.0 1.3

35-68A (KH) + (19) + + + (66) + +

+

+

Kodamakod

D I I -KC-7A (CH) 20.0 30.7 (22) 40.1 3.1 4.2 1.0 D l l -KC-7A (GN) 27.1 30.2 (22) 31.6 4.9 2.1 1.3

D 11-KC-6A (CH) + (20) + + + + + D I1-KC-6B (LP) + (20) + + + +

Ervadi

TN3-1A (CH) + (21c,24r) + + + + + + TN3-1B (GN) + (23) + + + + +

TN21-4 (CH) + (38) + + + + + + TN 2-1 (KH) + (26) + + + + + +

Arni

T6-30A (CH) + (23c,25r) + + + + + T6-30B (GN) + (23c,25r) + + + +

T13-83 (CH) + (27) + + + + + + T 4-16 (KH) + (25) + + + (74) + +

Waraddana

D4-03A (CH) 23.9 26.8 (26) 39.7 2.3 0.8 3.7 2.8 D4-03B (GN) 26.9 27.5 (26) 32.6 4.6 5.9 2.4

D4-N2 (BG) (39c,53r) + + + +

Udadigana

D4-K1A (CH) 18.7 30.4 (25) 38.8 5.8 2.9 1.8 1.6 D4-KIB (GN) 21.9 31.0 (25) 38.0 4.4 3.8 0.9

I

CH charnockite; K H khondalite; BG basic granulite. LP felsic gneiss. Precision of modes: + 10% relative. Mg number of two cordierites given

made at X-ray Assay Laboratories, Don Mills, Ontario, by X-ray fluorescence. Detection limits and accuracies of all the analyses are indicated in Table 1. XRF analyses were also done at the De- partment of Geology, Franklin and Marshall College, the Depart- ment of Geology, Miami University of Ohio, and the Department of Geosciences, the New Mexico Institute of Mining and Technolo- gy. Analyses for Rb, Sr, Zr, Y, F, C1, C, CO2 and S were done by conventional XRF methods, and of U and Th by neutron activation by X-ray Assay Laboratories. A few analyses of U and Th by ultra-high precision instrumental neutron activation were done at the Los Alamos National Laboratory.

Modal analyses. Modal mineral abundances were determined by point-counting standard thinsections of key close pairs stained with

sodium cobaltinitrite. Two to three thin-sections were counted for each sample. Counting was done with a Leitz mechanical traversing stage with 0.5 mm increment. About 1000 counts were collected for each sample.

Mineral analyses. Minerals were analyzed with an ARL electron microprobe at the University of Chicago. Both energy dispersive and wavelength dispersive methods were used. The methods were identical to those described in Janardhan et al. (1982). Wavelength dispersive analysis of F and C1 in biotites and hornblendes were made with synthetic fluorphlogopite and a natural scapolite as standards.

Fluid inclusion analyses. Fluid inclusions were studied with a Fluid Inc. adapted U.S.G.S. gas-flow heating/freezing microscope stage

229

Table 3. Analyses of biotites from gray gneiss (GN), and charnockite (CH) of Kabbal-type localities, both primary and retrogressive (RT)

Kabbal Waraddana Udadigana

3-1 3-9 D4-03 D4-K1

Wt% GN CH CH-RT G N CH G N CH CH-RT G N CH CH-RT

SiO2 37.7 36.9 37.4 36.9 37.0 37.1 37.2 37.5 36.8 36.8 38.6 TiO2 -4.7 4.7 1.6 4.8 4.3 5.0 4.7 2.0 5.1 4.7 2.9 A120 3 13.1 13.2 15.4 13.5 13.7 13.0 13.2 18.7 12.9 13.0 13.8 FeO 21.9 21.9 19.6 22.2 21.5 21.3 21.1 15.1 21.0 20.8 18.9 MgO 9.4 9.8 10.9 9.3 9.9 10 .0 10.7 14.0 10.1 10.4 13.6 KzO 9.3 9.2 9.0 9.2 9.2 9.1 9.3 9.0 9.1 9.1 9.8 F 1.4 1.3 < 0.2 2.0 2.0 3.0 3.0 2.0 2.8 2.9 2.0 C1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 0.1 0.1 < 0.1 0.1 0.1 < 0.1

Total 97.5 97.0 94.3 97.9 97.6 98.6 99.3 98.3 97.9 97.8 99.6

Atoms/11(O)

Si 2.88 2.83 2.88 2.82 2.83 2.85 2.84 2.72 2.84 2.85 2.86 Ti 0.27 0.27 0.09 0.28 0.25 0.29 0.27 0.11 0.30 0.27 0.16 A1 1.19 1.20 1.40 1.23 1.24 1.18 1.18 1.60 1.18 1.18 1.20 Fe 1.41 1.41 1.26 1.43 1.38 1.37 1.34 0.91 1.36 1.34 1.17 Mg 1.08 1.13 1.25 1.06 1.14 1.15 1.21 1.59 1.16 1.20 1.51 K 0.91 0.90 0.88 0.91 0.90 0.89 0.90 0.86 0.89 0.89 0.92 F 0.34 0.31 0.0 0.49 0.49 0.71 0.71 0.49 0.67 0.70 0.49 C1 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 0.01 0.01 < 0.01 0.01 0.01 < 0.01

M g / M g + Fe 0.43 0.45 0.50 0.43 0.45 0.46 0.47 0.63 0.46 0.47 0.56

Table 4. Analyses of biotites from gray gneiss (GN), charnockite (CH) and khondali te (KH) of Ponmudi-type localities, both primary and retrogressive (RT)

Ponmudi, Kerala Kodamakod Ervadi, Tamil Nadu

K18-6 K18-7 147-214 35-68A D l l - K C - 7 TN3-1 TN2-1

Wt% G N CH CH CH K H G N CH G N CH G N

Arni, Tamil Nadu

T6-30 T4-16

G N CH CH-RT KH

SiOz 35.7 35.9 38.3 37.3 37.1 37.6 38.0 37.9 36.3 TiOz 5.3 5,0 4.8 5.5 4.7 5.8 4.1 5.3 4.7 AlzO3 13.6 13.9 14.3 14.2 15.6 14.7 14.4 15.1 15.3 FeO 20.1 20,9 17.4 19.4 14.1 19.5 19.7 17.5 16.7 MgO 10.3 10.3 11.2 10.4 13.5 10.3 11.1 11.4 11.7 K20 9.0 9.0 9.6 9.3 10.1 9.4 9.5 9.6 9.4 F 3.3 3.7 3.0 2.1 2.5 2.4 2.3 3.0 2.9 C1 0.4 0.4 0.4 0.6 0.0 0.4 0.4

38.1 36.3 36.3 36.4 37.1 5.3 4.8 4.8 3.5 2.5

15.8 15.0 14.9 15.0 17.5 13.4 20.3 20.2 20.2 17.3 14.2 9.9 10.0 11.8 12.4 9.6 9.2 9.3 9.2 0.1 2.7 0.8 0.8 0.7 1.1 0.2 < 0.1 < 0.I < 0.1 < 0.1

Total 97.7 99.1 99.0 98.8 97.6 100.1 99.5 99.8 97.0 99.3 96.3 96.3 96.8 97.0

Atoms/11(O)

Si 2.78 2.77 2.88 2.82 2.78 2.80 2.85 2.8l 2.77 2.78 2.77 2.78 2.76 2.76 Ti 0.31 0.29 0.27 0.31 0.26 0.33 0.23 0.30 0.27 0.29 0.28 0.28 0.20 0.14 A1 1.25 1.27 1.26 1.27 1.36 1.29 1.27 1.32 1.38 1.36 1.35 1.34 1.34 1.54 Fe 1.31 1.35 1.09 1.23 0.89 1.22 1.24 1.09 1.06 0.82 1.30 1.29 1.28 1.08 Mg 1.19 1.18 1.25 1.17 1.50 1.15 1.24 1.26 1.33 1.55 1.13 1.14 1.34 1.38 K 0.89 0.88 0.91 0.90 0.96 0.89 0.91 0.91 0.91 0.89 0.89 0.90 0.89 0.86 F 0.79 0.88 0.71 0.51 0.60 0.57 0.54 0.71 0.68 0.64 0.19 0.19 0.17 0.27 CI 0.05 0.05 0.05 0.07 0.00 0.05 0.05 0.03 < 0.01< 0.01< 0.01 < 0.01

M g / M g + F e 0.48 0.47 0.53 0.49 0.63 0.49 0.50 0.54 0.56 0.65 0.44 0.47 0.49 0.56

(Werre et al. 1979) in the Department of Geology, Hope College. Doubly-polished rock sections of about 0.1 mm thickness were examined. Synthetic mixed H20-CO2 fluid inclusions (Sterner and Bodnar 1984) in a similar doubly polished section were used as a melting point calibrant for CO2.

S a m p l e loca l i t i es

Kabbal, Karnataka. T h e K a b b a l q u a r r y ha s b e e n desc r ibed in a n u m b e r o f paper s , i n c l u d i n g R a m i e n g a r et al. (1978), J a n a r d h a n et al. (1982) a n d F r i e n d (1983). G r a y a m p h i b o -

230

Table 5. Analyses of hornblendes from gray gneiss (GN) and charnockite (CH), from Kabbal-type localities, both primary and retrogressive (R73

Kabbal, Karna taka Waraddana

3-1 3-9 D4-03

Wt% G N G N G N - R T CH-RT CH-RT G N CH-RT G N CH

Udadigana

D4-K1

G N CH CH-RT

SiO2 42.4 43.5 43.6 43.4 44.3 43.0 43.9 43.7 43.6 TiOz 1.9 2.1 1.1 0.6 0.4 1.7 1.3 2.0 2.0 A1203 10.0 10.3 10,3 10.1 10.2 10.3 9.8 9.7 9.6 FeO 21.1 21.3 20,7 20.6 19.0 21.0 20.6 20.8 20.6 MnO 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 MgO 8.6 9.0 8.6 9.3 10.4 8.2 8.7 9.0 9.0 CaO 10.7 11.0 10.6 10.4 10.6 10.9 10.8 10.9 11.0 NazO 1.4 0.0 0.8 0.8 0.0 1.1 1.2 0.4 0.3 K20 1.3 1.6 1.5 1.2 1.3 1.5 1.4 1.4 1.4 F 0.8 0.8 0.0 0.0 0.0 1 .t 0.0 1.4 1.2 C1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

43.6 43.8 53.0 2.0 1.6 0.0 9.9 9.7 2.9

20.8 20.2 16.6 0.0 0.0 0.6 9.1 9.4 14.1

11.0 11.0 10.8 0.4 0.3 0.00 1.4 1.4 0.00 1.1 0.6 0.23 0.0 0.0 0.0

Total 98.6 99.6 97.2 96.4 96.2 98.8 99.5 99.3 98.7 99.3 98.0 98.2

Atoms/23(O)

Si 6.47 6.57 6.68 6.68 6.76 6.57 6.70 6.64 6.65 Ti 0.22 0.24 0.13 0.07 0.05 0.20 0.15 0.23 0.23 A1 1.80 1.83 1.86 1.83 1.83 1.88 1.76 1.74 1.73 Fe 2.69 2.69 2.65 2.65 2.43 2.68 2.62 2.64 2.63 Mn 0.05 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mg 1.96 2.03 1.96 2.14 2.35 1.87 1.98 2.04 2.05 Ca 1.75 1,77 1.73 1.72 1.73 1,79 1.76 1.78 1.80 Na 0.41 0.00 0.23 0.24 0.00 0.34 0.34 0.12 0.09 K 0.25 0.31 0.28 0.24 0.26 0.30 0.24 0.27 0.27 F 0.39 0.39 0.00 0.00 0.00 0.54 0.00 0.69 0.59 C1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

M g / M g + F e 0.42 0.43 0.42 0.45 0.49 0.41 0.43 0.44 0.43

6.62 6.67 7.70 0.23 0.18 0.00 1.77 1.74 0.49 2.64 2,57 2.02 0.00 0,00 0.07 2.06 2.13 3.07 1.79 1.80 1.68 0.12 0.09 0.00 0.27 0.27 0.04 0.54 0.30 0.00 0.00 0.00 0.00

0.43 0.45 0.43

Table 6. Analyses of garnets

Ponmudi, Kerala Arni, Tamil Nadu

K18-6 K18-7 147-214 35-68A T13-83 T4-16

Wt% Gneiss Charnockite Charnockite Charnockite Khondali te Charnockite Khondali te

Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim

SiOz 36.7 36.9 36.8 37.1 38.1 38.2 37.9 38.5 38.5 37.2 38.4 38.4 37.8 37.8 A120 3 21.0 21.4 21.2 21.6 21.6 21.7 21.3 21.2 22.2 21.9 21.7 21.7 22.2 22.0 FeO 34.8 34.5 34.5 34.3 33.1 33.0 32.0 32.0 32.1 31.7 31.7 32.2 32.7 32.5 MnO 0.4 0.5 0.5 0.8 0.6 0.7 1.3 1.5 0.5 0.7 0.7 0.8 1.2 1.1 MgO 4.2 4.3 4.4 4.4 4.2 4.3 5.0 5.1 7.3 7.0 5.7 5.5 6.3 6.2 CaO 2.4 2.4 2.4 2.4 2.4 2.4 2.0 1.8 0.7 0.6 2.0 2.0 0.8 0.7

Total 99.6 99.9 99.8 100.5 100.0 100.3 99.5 100.1 101.3 99.1 100.2 100.6 101.0 100.3

Atoms/12(0)

Si 2.96 2.96 2.95 2.96 3.03 3.03 3.04 3.04 2.97 2.94 A1 2.00 2.02 2.01 2.02 2.03 2.03 1.98 1.97 2.02 2.04 Fe 2.35 2.31 2.32 2.28 2.20 2.19 2.12 2.11 2.07 2.10 M n 0.03 0.03 0.04 0.05 0.04 0.05 0.09 0.10 0.03 0.05 Mg 0.51 0.51 0.52 0.52 0.50 0.51 0.59 0.60 0.84 0.83 Ca 0.20 0.20 0.21 0.20 0.21 0.20 0.17 0.15 0.06 0.05

Z'2 + 3.09 3.05 3.09 3.05 2.95 2.95 2.97 2.96 3.01 3,02

M g / M g + F e 0.178 0,181 0.183 0.183 0.185 0.189 0.218 0.221 0.289 0.283

3.02 3.01 2.96 2.98 2.01 2.01 2.04 2.04 2.08 2.11 2.14 2.14 0.05 0.05 0.08 0.07 0.66 0.64 0.74 0.72 0.17 0.17 0.07 0.06

2.96 2.97 3.03 2.99

0.241 0.233 0.257 0.252

lite-biotite gneiss is heavily migmatized with granitic and pegmatitic veins, ranging from diffuse feldspathic blurring to discrete dikes. Bulk compositions of the gneiss range continuously from tonalitic to granitic with degree of mig- matization. Small areas of diffuse darkening occur throughout the quarry. On close inspection, the dark patches prove to be very coarse charnockite, with orthopyroxene crystals up to one cm across. Charnockite grades in and out of normal pink granite or occludes meter-sized areas of migmatitic gneiss. Distribution of the charnockite is deformation-related: it occurs often in diffuse transverse veins parallel to subvertical N-S shear system which cuts an earlier foliation, in the hinges of drag folds along this shear system, and in tension gashes oblique to the axes of the new folds (see Fig. 3, Newton and Hansen 1986). The near-contem- poraneity of the charnockite and granite is obvious, although the charnockite is generally somewhat later. Friend (1983) and Janardhan et al. (1982) concluded that K-rich metasomatizing fluids traveled along deformation zones, and that those fluids were closely followed by H20-remov- ing fluids with less K20. Some anatexis was promoted by the earlier fluids. This conclusion is identical to that of Weaver (1980) for the Madras charnockites, based on geochemical patterns.

Table 1 shows five close-pair analyses for the Kabbal rocks, including three new sets of the present study. The previous conclusion from one set of close pairs from Kabbal (Janardhan et al. 1982) and from averages of charnockites and gray gneisses from the Krishnagiri area (Condie et al. 1982) was that the metamorphism was isochemical or nearly so. The present study shows, however, that there are indeed subtle chemical changes in charnockitization. The close-pair analyses show decreases of CaO (av. 0.3 wt%), iron oxide as Fe203 (av. 0.5 wt%), and MgO (av. 0.2 wt%), and increase of SiO 2 (av. 1.2 wt%) and possible increase of Na20 (av. 0.2 wt%). There is also a marked decrease of Y and slight decrease of Rb.

231

Table 2 gives the modal compositions of two of the Kab- bal close pairs. The point-counts show that the amount of hornblende decreases greatly as charnockite is formed and that biotite may decrease somewhat, perhaps more than the modal counts reveal, because some biotite in the charnockite is definitely retrogressive after orthopyroxene (Jan- ardhan et al. 1982). The amounts of plagioclase, alkali feldspar and quartz remain constant, within the precision of the measurements, which is about 10% of the amounts listed. Both gneiss and its charnockite counterpart have similar amounts of magnetite and ilmenite. These results will be used in the Discussion section to define the orthopyroxene-producing reaction.

Tables 2-7 give the mineral compositions. The compositions for Kabbal are the averages of 30 analyses of different grains of each mineral. The tables show that there is no significant compositional difference between plagioclase of gray gneiss and charnockite. Hornblendes in the gray gneisses have variable Na/K ratios, with some nearly Na-free analyses. There is a significant difference between gray gneiss hornblende and the small amount of hornblende in the charnockite. The latter hornblende is significantly lower in Ti and F and may therefore be retrogressive. This deduc- tion rests on the direct strong relationship between Ti content of hornblende southward with increasing grade in metabasic rocks of the Craton (Raase et al. 1986). Thus, the mineral analyses show that the reaction to form orthopyroxene involves mainly the breakdown of hornblende, and that a certain amount of biotite breakdown may be involved. The conversion of hornblende to orthopyroxene in orthogneiss compositions will subsequently be referred to as charnockitization of the Kabbal Type.

Fluid inclusions in the Kabbal rocks have been described in detail elsewhere (Hansen et al. 1984). In summary, charnockitic veins and nebulous migmatite patches contain large numbers of high density pseudosecondary or secondary CO2-rich inclusions, best displayed in healed fractures in

w t %

Kodamakod, Kerala Ervadi, Tamil Nadu Waraddana, S.L.

D11-KC-7 D11-KC-6 TN3-t TN21-4 TN2-1 D4-N2

Gneiss Felsite G n e i s s Charnockite Charnockite Khondalite Metabasite

Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim

SiO2 38.7 38.4 38.7 38.5 38.6 38.4 37.4 37.7 38.1 38.8 38.3 39.2 38.4 38.2 AlzO3 20.8 20.8 21.8 22.0 21.9 22.0 21.7 21.7 21.3 21.2 21.8 22.4 21.5 21.4 FeO 33.9 33.9 32.6 32.7 31.7 31.9 32.7 32.6 33.4 33.5 31.3 29.3 26.1 26.4 MnO 0.0 0.0 0.0 0.0 0.5 0.4 0.4 0.5 0.5 0.6 0.0 0.3 2.6 3.0 MgO 5.3 5.4 6.3 6.3 6.4 5.9 6.5 6.6 3.7 3.2 8.0 8.4 5.9 5.6 CaO 1.3 1.2 1.2 1.0 1.3 1.1 1.3 1.1 3.3 3.4 0.7 0.7 6.1 5.7

Total 99.5 99.7 100.6 100.5 100.4 99.7 100.t 100.2 100.3 100.7 100.1 100.4 100.6 100.3

Atoms/12(0)

Si 3.05 3.05 3.03 3.01 3.02 3.03 2.95 2.96 3.03 3.08 3.08 2.98 3.03 2.98 Al 1.95 1.95 2.01 2.03 2.02 2.05 2.02 2.02 2.00 1.98 2.00 2.04 1.98 1.97 Fe 2.26 2.26 2.13 2.14 2.07 2.11 2.16 2.15 2.22 2.23 2.04 1.89 1.70 /,73 Mn 0.00 0.00 0.00 0.00 0.03 0.03 0.03 0.03 0.03 0.04 0.00 0.02 0.17 0.20 Mg 0.63 0.64 0.73 0.74 0.75 0.70 0.77 0.78 0.44 0.38 0.93 0.97 0.69 0.66 Ca 0.11 0.10 0.10 0.08 0.11 0.09 0.11 0.09 0.28 0.29 0.06 0.06 0.51 0.48 Z2 + 3.00 3.00 2.97 2.96 2.96 2.93 3.07 3.05 2.97 2.94 3.03 2.94 3.07 3.07 Mg/Mg+Fe 0.218 0.221 0.255 0.257 0.266 0.249 0.260 0.270 0.165 0.146 0.313 0.339 0.291 0.284

232

Tab le 7. Analyses of orthopyroxenes of charnockites (CH) and a basic granulite (BG), and of one clinopyroxene (C)

Wt%

Kabbal, Ponmudi, Kodamakod, Ervadi, Arni, Waraddana, Udadigana, KA KE KE T.N. T.N. S.L S.L.

3-1A 3-9A K18-6 K18-7 147-214 DI1- D l l - TN3-1 TN21-4 T6-30 T13-83 D4-03 D4-N2 D4-K1 KC-6 KC-7

CH CH CH CH CH CH CH CH CH CH CH CH BG CH CH-C

SiO2 48.5 50.6 48.9 49.2 49.5 48.4 48.5 49.9 48.6 50.1 49.4 50.0 49.9 51.8 51.7 Al~O3 0.7 0.7 1.8 2.1 2.5 3.5 3.4 3.3 1.5 3.3 2.6 0.3 2.0 0.2 0.9 FeO 34.7 33 .1 38.6 36.3 33.7 34.6 34.3 32.3 37.8 32.3 31.9 33.6 28.2 32+1 14.7 MnO 1.5 1.8 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.6 0.9 2.0 0.8 MgO 12.6 12.7 11.3 12.0 14.3 13.7 14.2 I4.7 11.0 14.7 15.4 13.9 17.7 14.3 11.0 CaO 0.7 0.7 0.1 0.4 0.2 0.0 0.0 0.0 0.4 0.0 0.0 1.0 0.5 1.0 20.4

Total 98.7 99.9 100.8 100.0 100 .2 100.2 100.4 100.2 99.3 100.2 99.2 100.4 99.2 101.4 99.5

Atoms/6(0)

Si 1.97 102 1.95 1.96 1.94 1.90 1.90 1.94 1.97 1.95 1.94 1.97 1.93 2.01 1.99 AI 0.03 0.03 0.09 0.10 0.12 0.16 0.16 0.15 0.07 0.15 0.12 0.01 0.09 0.01 0.04 Fe 1.17 1.I0 1.29 1.21 1.10 1.14 1.12 1.05 1.28 1.05 1.05 1.11 0.91 1.04 0.47 Mn 0.05 0.06 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.03 0.07 0.03 Mg 0.76 0.76 0.67 0.71 0.83 0.80 0.83 0.85 0.66 0.85 0.90 0.82 1.02 0.83 0.63 Ca 0.03 0.03 0.00 0.02 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.04 0.02 0.04 0.84

Z2+ 2.01 1 .98 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.01 2.00 1.98 2.00

Mg/Mg+Fe 0.39 0.41 0.34 0.37 0.43 0.41 0.42 0.45 0.34 0.45 0.46 0.42 0.52 0.44 0.57

quartz, whereas gray gneiss little affected by such alter- at ions has no large fluid inclusions o f workable size ( > 3 pro). Rare fluid inclusions found in leucogranites show immiscible COz and HEO in roughly equal amounts . These may represent an early (pre-granuli te facies) metamorph ic N fluid. The densities o f CO2-rich inclusions are a round 1.0 gm/cm a, consistent with an ent rapment pressure of 9~ about 5.6 kbar at the mineralogical ly-inferred metamorph ic temperature o f 750 ~ C, a pressure the same as inferred from regional geobarometry. Freezing temperatures are near - 5 6 . 6 ~ C, indicat ing pure COa. However, immiscible H 2 0 in amounts less than about 20 tool%, if present, would exist 8045 ~ as a vir tual ly undetectable film on inclusion walls. In addi- t ion to large CO2 inclusions, charnocki t ic and granitic patches show clearly secondary arrays of t iny aqueous inclusions which cut across the arrays of CO~ inclusions. These arrays are sometimes speckled with chlorite and cal- cite and may be associated with the weak retrogression ~ ~ which converted some or thopyroxene to chlorite and thus darkened the charnocki te (Howie 1967).

Kerala Khondatite Belt. At the southernmost t ip of India, a terrain o f layered granulites covers an area of approximately 200 km by 100 km in Kera la and Tamil Nadu (Fig. 2). The section strikes consistently northwest with variable dip to nor theast and southwest, a l though southerly dips pevail (Narayanaswamy and Lakshmi 1967; Jacob 1962). Perhaps most abundan t among the diverse l i tholo- gies are light gray garnetiferous, graphi te-bear ing bioti te gneisses. Khondal i tes (garnet-sil l imanite metapelites) and graphi te-bear ing charnockites are nearly equal in abundance. Marbles, talc-sil icates and quartzites, though locally conspicuous, are subordinate. Basic layers are rare, and no ul t ramafic bands have been reported. The gray gneisses and khondal i tes are invar iably migmatized, with quar tzo- feldspathic bands and patches of variable thickness. An-

~ ; _ ; . ;.o',PC 2DI

. , ~ ' , % o 0 \ / , , . \ - o v ~ , o ~ . o

~ N ' ~ v' "~,~** -~- - - . ~ , ~ , ~ ~ ~ "~. ~ " ~ " I

I " "w l y . __ at , v

5o'I,,. {"-" g ~ ~"i. . ++x • ~ l l ~ Shear Zonel eErvod i .4" 74~'~ ~rlvondfum -li '- At'li%de of I . '%v"~N',. I Polio'i'ion I Y '>, . . . .

.I ILoyered garnet, motite, g r o p h i f i c [ gneiss,voriobly chornock i l i c ~ ~ ~

I~]Moss ve chornockite I-t-H=i=i~ . . . . . . . . . . . . ~ . . . . ' = l I I III IIi i i ', ', ', ', ', ' ' ' ~.~',," r~TC~Gr~ rn'c~ gne'ss ~ ~ [_T~]Composite b io t i te , amphibole ~ . ~

gneiss ~ \,~5) \~''- ~ M o f i c pyroxene gronulite 0 I0 ?-0 X~A'Co Ic -g ronu l i te km

~176176 Ou~ I I I I

77~ ' E 77~ ' 77030 , 77045 '

Fig. 2. Geological map of the Khondalite Belt of metasedimentary granulites of southernmost India and adjacent regions, after Nar- ayanaswamy and Lakshmi (1967), Chacko et al. (1987), Karunak- aran (1974), Varadan (1975) and present observations. Koda- makod, Ponmudi, Servalar Dam and Ervadi sample localities shown

233

�9 �9 o 0 5 1 _ ~ N �9 : . , . . . , , i , / I .%..'

�9 " �9 km . I-4~u . . . -...,. ~ - ~ ~ , , , �9 . �9 " , , I :." ".. �9 . �9 �9 �9 �9 " . - - N ' . l <,,,~,"~, . �9 . . �9 . . . . . . . "

�9 P 1 8 ~ " . �9

Ponmudi

C h a r n o c k i t e ~ K h o n d a l i t e F o l d a x i s ~ G a r n e t - b i o t i t e gneiss 7 \ `

I . ' l P a r t i a l l y c h a r n o c k i t i c / 4 I F o l i a t i o n

Fig. 3. Sketch map of metasedimentary geology of the Ponmudi, Kerala locality, showing sample localities. The partially charnockitized garnet-biotite gneiss appears to be a discrete stratum at the axis of a tight sycline

Fig. 4. Charnockitic veins (dark) through graphite-bearing garnet- biotite gneiss in the quarry near Ponmudi, Kerala. Hammer on quarry floor points northeast. Note warping of migmatitic layering about charnockite-filled shear zone and charnockitic discoloration proceeding laterally along foliation planes in gneiss, indicating access of charnockitizing fluids along deformation zones

other important alteration of gray paragneiss is patchy conversion to a dark green charnockite. This kind of arrested charnockite formation far south of the orthopyroxene isograd appears to be a wide-spread, characteristic feature of the layered granulites of southern India.

The Khondalite Belt is terminated on the northeast by a terrain of lower-grade acid gneisses and intrusive granites which probably represent, in large part, retrogressive alteration of older rocks. Gneisses and granites in a quarry near Madurai, just north of the map area of Fig. 3, were dated at 550+_15 Ma by a whole-rock Rb-Sr isochron (Hansen et al. 1985). Retrogression of charnockite near granite dikes is common in the northern Khondalite Belt and in the West- ern Ghats charnockites west of Madurai. However, Srikan- tappa et al. (1985) consider a lower concordia intercept of 550 Ma in zircons from the Khondalite Belt to date charnockitic metamorphism rather than retrogression.

On its northwestern margin, the Khondalite Belt gives way to a monotonous, massif charnockite terrain which covers a substantial portion of the state of Kerala. These gneisses lack graphite or obvious layering, and, in the few places examined are poor or lacking in biotite and amphibole: they may be an orthogneiss basement to the Khonda- lite Belt layered series. The boundary of the Khondalite Belt with the Kerala charnockite massif is the Achankovil Zone, a lineament similar in straightness and geometry to the Proterozoic shear belts farther north in Tamil Nadu and Kerala. A thin zone of granite bodies and migmatites intervenes between the Khondalite Belt, and the Kerala Massif along the Achankovil Zone (Fig. 2).

More massive charnockites occur at the extreme southern end of the Khondalite Belt, near Nagercoil and Trivan- drum. These are biotite- and garnet-rich, and, at Trivan- drum, contain graphite and felsic interlayers. The Trivan- drum charnockites are metasedimentary in origin, interlayered with the khondalite sequence.

Ponmudi. A quarry near the hill resort of Ponmudi, Kerala, exposes a graphitic biotite-garnet acid gneiss mottled and veined by dark green charnockite in large amounts which in places, can be seen to be a transgressive alteration which

disrupts the dominant NW-SE foliation, with warping of foliation about charnockite-filled shears in a manner extremely similar to Kabbal. The mixed gneiss-charnockite body is undoubtedly a stratigraphic layer, interbedded with, and structurally above, a khondalite in the core of a syncline (Fig. 3). The charnockite-filled deformation zones tend to have a maximum frequency of strike near N45E and subvertical dip, though charnockite veins commonly swerve, as if around fold noses (Fig. 4) and are highly variable in orientation. In many places in the quarry they coalesce to form homogeneous charnockite many meters across.

The charnockite in hand specimen and thin section is a drastic coarsening and darkening of the fne, even-grained host gneiss. Feldspar crystals 1 cm across are common. The decrease of biotite is striking: rare biotite in some of the charnockites is found mainly as inclusions in garnet and orthopyroxene. The reaction thus principally involves biotite and quartz to orthopyroxene, K-feldspar and vapor. Garnet and ilmenite may be involved to a lesser extent in the reaction. Graphite is abundant in both gneiss and charnockite. Magnetite is absent.

Table 8 gives analyses of close-pairs of biotite gneiss and charnockite. The nearly isochemical nature of the conversion to charnockite is again apparent. Close inspection of the Ponmudi analyses, and the others in Table 8 that per- tain to charnockitization of the Ponmudi type (caused by the biotite-quartz reaction) shows small increases of SiO2 and Na20 and possible slight losses of F%O3 and MgO in the charnockite-forming process. There is no consistent loss of CaO, as there is in the Kabbal-type charnockite formation. Among the minor elements, there are consistent decreases in Rb and Y in charnockite. The Rb loss is undoubtedly due to the instability of biotite, but a product phase, presumably K-feldspar, can accommodate substantial quantities of Rb. The Ponmudi charnockite is thus of the undepleted type, with relatively low K/Rb.

Mineral compositions are given in Tables 2-7. Red- brown, high-Ti biotite in the gneisses and preserved biotite in their close-pair charnockites have identical compositions. This finding contrasts with that of Srikantappa et al. (1985), who found evidence of significant increase of Xvo in the

234

Table 8. "Close-pair" rock analyses of gray gneiss (GN), charnockite (CH) and felsic gneiss (LP), Ponmudi-type localities

Ponmudi, Kerala Kodamakod, Kerala

K18-6 K18-8 P18-85 Dl l -KC-I DI1-KC-6

Wt% CH G N CH G N CH G N CH G N CH LP

Ervadi, T.N. Arni, T.N.

TN-3-1 T6-30

CH GN CH GN

SiOz 67.1 65.6 65.1 64.8 67.0 66.4 72.1 72.3 71.1 69.9 TiOz 0.97 0.97 0.85 0.78 0.77 0.93 0.33 0.27 0.34 0.39 A1203 15.2 15.2 14.9 15.9 14.3 14.6 14.7 14.4 14.9 15.5 FezO3 5.29 6.87 6.30 6.24 5.90 7.10 1.36 2.01 2.32 3.41 MgO 0.95 1.13 1.18 1.11 1.01 1.15 0.40 0.54 0.71 0.57 CaO 2.40 2.24 2.07 2.10 2.17 2.29 2.19 1.95 2.18 2.27 MnO 0.06 0.10 0.06 0.08 0.06 0.10 < 0.01 0.01 0.01 0.04 Na20 2.70 2.33 2.46 2.31 2.73 2.53 4.68 4.37 4.75 4.56 K20 4.60 4.03 5.42 5.06 5.30 4.09 3.07 2.97 2.99 2.85 P2Os 0.20 0.20 0.20 0.17 0.16 0.20 0.07 0.06 0.13 0.21 CO2 0.31 0.14 0.5 0.1 0.1 0.l S 0.01 0.02 NIL NIL NIL NIL L.O.I. 1.17 1.07 0.81 0.87 0.47 0.16 0.93 0.39 0.39 0.16

72.37 71.26 71.4 71.1 0.27 0.25 0.13 0.34

15.53 15.20 14.6 15.3 2.65 3.07 3.13 2.21 0.78 0.81 0.86 0.65 2.85 2.66 2.95 3.23 0.03 0.04 0.08 0.03 4.58 4.08 4.88 5.25 1.37 1.47 1.66 1.35 0.06 0.06 0.08 0.09

0.30 0.34 0.23 0.39

pplTi

Rb 186 207 228 247 210 240 110 120 100 90 Sr 165 142 184 166 190 140 310 290 330 310 Zr 350 400 150 200 150 1290 Nb 20 20 10 10 10 20 Y 47 60 54 80 10 40 < 10 < 10 < 10 10 F 1500 200 170 310 140 140 C1 < 50 200 100 150 50 100 U 2.9 2.2 1.2 1.1 1.4 0.8 Th 57 30 14 13 12 3

22 49 30 50 250 312 360 370

110 120 < 10 20

13 21 < 10 < 10

0.7 0.4 7 3

K18-6, K18-8, TN3-1 : Franklin and Marshall College (E. Hansen) P18-85, DII-KC-1, Dlt-KC-6, T6-30; X-ray Assay Lab

biotites of charnocki t ized gneisses at several Khonda l i t e Belt localities. The bioti tes in both gneiss and charnocki te show quite high F (about 3.3-3.7 wt%) and significant C1 (about 0.4 wt%). Similarly, the garnets of gneiss and charnockite have identical composi t ions. They show little or no zonation. Plagioclase in the charnocki te may be slightly more calcic than in the gneiss. Or thopyroxene has a somewhat lower Mg number than biotite. The mineralogic difference between veins and host rock is thus one of moda l p ropor t ions ra ther than of mineral chemistry. Minerals from a massive graphi te-bear ing charnocki te (147-214) below the khondal i te unit (Fig. 3) have different composi t ions from those of the mixed charnockite-gneiss unit.

Modal analyses (Table 2) show decrease of bioti te and increase of alkali feldspar in the charnockites relative to the host gneisses. The moda l abundances of plagioclase and quar tz are not markedly different in close pairs. There is, however, a marked decline of garnet in the charnockites, which indicates that garnet is consumed in the or thopyroxene-forming reaction.

Both gneiss and charnockites contain arrays of dense C02-richfluid inclusions. Most of the fluid inclusions occur in quar tz ; however, they are occasionally present in garnet as well. All the fluid inclusions occur in p lanar arrays, presumably occupying healed fractures. In the quartz grains many of the trails contain some part ial ly or completely decrepi ta ted inclusions - the undecrepi ta ted inclusions are in the range 5 to 20 lam with i rregular equant to slightly elongated shapes. The fluid inclusions in the garnet are larger, with elongated, wormy, occasionally branching

forms; no inclusions of negative garnet geometry of the sort described by Coolen (1982) were found. Some quar tz grains contain arrays of small ( < 5 gm), two-phase, apparently aqueous inclusions, a l though they are not as common as at Kabbal . Where these cross trails o f COz-rich inclusions the lat ter are d is rupted: " h a l o s " o f lower density (liquid-gas) CO2 fluid inclusions can be traced along either side of many o f these intersections. Large mixed CO2-H20 inclusions like those repor ted by Sr ikan tappa et al. (1985) from some o f the Kera la localities were not observed.

Figure 6 shows the freezing and homogenizat ion temperatures for the CO2-rich fluid inclusions in two Ponmudi samples, a charnocki te and adjacent gneiss. The results of the micro thermomet ry were very similar for both samples and hence they have been combined. The freezing temperatures are depressed from 0.6 to 2.3 ~ C below the CO2 triple point of - 5 6 . 5 ~ C which indicates a depar ture from pure CO2. This is confirmed in some inclusions by melting over a small (0.2-0.3 ~ C) temperature range. Methane is the most likely secondary component ; ni trogen is also a possibil i ty (Touret 1981). The maximum amount of CH4 is est imated from the diagrams of Burruss (1981) to be about I0 mol%. Homogeniza t ion temperatures range from + 6 to + 1 9 ~ with a max imum at + 9 ~ C. The implied density (about 0.90 gm/cm 3) is lower than would be expected if the fluid were en t rapped at metamorph ic pressures o f 5 kbar or greater.

Kodamakod. A number of other localities where incomplete conversion of garnet-biot i te gneiss to charnocki te is exposed

235

are found in the vicinity of Ponmudi, and some of these may belong to the same paragneiss stratum which crops out at Ponmudi. A similar occurrence is at Kodamakod, a small hill village 20 km to the northwest along the regional strike from Ponmudi (Fig. 2). Here, the charnockite quarry relations are very similar, but the felsic migmatite component is more abundant. Coarse feldspars and quartz and centimeter-sized red garnets are mostly in bands parallel to foliation, but also in nebulitic patches which blur foliation. Charnockite is nebulous in aspect, but is conspi- cuously associated with deformed zones about which foliation is warped, as at Ponmudi.

Table 8 gives the analyses of immediately adjacent charnockite and gray gneiss, including a felsic band. As at Pon- mudi, the charnockite and gneiss are nearly isochemical. The charnockite is not significantly depleted in the large-ion lithophiles. The charnockite is different in a number of re- spects from an adjacent felsic band, which has substantially higher A1 and Fe, and is greatly enriched in Zr and depleted in Th relative to the charnockite. Modes of a gneiss and charnockite close pair are given in Table 2. The results are very similar to those for Ponmudi. The charnockite lacks garnet, which was consumed in the orthopyroxene-forming reaction. Mineral compositions of closely associated gneiss, charnockite and felsic band are given in Tables ~ 7 . As at Ponmudi, biotite and plagioclase are nearly the same in gneiss and charnockite close pairs. Biotite is high in TiOz and F. Fluid inclusions in the charnockite and most migmatitic gneiss are very rare. None were found which could be characterized.

Ervadi. This locality is 60 km SE of Ponmudi in Tamil Nadu, nearly along the regional strike, and about 10 km north of the town of Panagudi (Panaikkudi). The quarry exposes biotite-garnet paragneiss in much the same state of arrested charnockitization as at Ponmudi, although the amount of charnockite is considerably less. Although much of the charnockite is discordant, in places occupying obvious shear zones, many charnockite patches are concordant with the foliation in the gneisses. Some of these concordant patches branch off from discordant charnockite veins. In general, the charnockite is much coarser-grained than the gneiss. Both rock types contain coarse graphite. Late granite veins are prominent in parts of the quarry; care was taken to avoid these areas in our study of prograde charnockitization. A photograph of the quarry was given by Holt and Wightman (1983), who correctly noted the prograde, transgressive nature of the coarse charnockite veins cutting the gneiss (see their Fig. 1). Small outcrops and quarries with mixed charnockite-gneiss occur sporadically up to about 20 km north of this locality.

Chemical analyses of charnockite from a one-meter- thick seam and from gneiss host rock 10 cm away are given in Table 8. The results follow the pattern of the other Pon- mudi-type localities. There is slight loss of Fe and gain of Si and Na, and significant decrease in Rb and Y in the charnockite compared to the close-pair host gneiss. Mineral analyses show the same pattern of nearly identical chemistries in charnockite and gneiss. Plagioclase in the charnockite is slightly zoned, with more calcic rims. The biotite is rich in F and Ti. Mineral analyses from a massive charnockite from this locality and from a khondalite found 5 km to the east were used to calculate metamorphic temperature and pressure.

z~ 'i t.

~ 20 I "~ I0

~ o ~ %

E

I I

,il,

rARADOANA 0403

IL, I ' I

PONMU01 K 18-6

ERVAOI TN3-1.~ 2O o 5, 7 0 IL. I I , I I a ~ l l L

t 1 1

I

20 SERVALAR DAM KHONDALITE ,o; o , d , I

+30 +~0 +lb 6 H0rnogenizeti0n

1 _d -tb -~0 -30-56 -Jr -se -59 -60 T Deg C Freezing T Deg C

Fig. 5. Histograms of freezing and homogenization temperatures of CO2-rich fluid inclusions from southernmost India and Sri Lanka granulite facies localities. Depression of the freezing temperatures by up to 3.5~ indicates probable CH4 admixtures of up to 15 mol%, which apparently correlate with, but do not account for, the trend of densities indicated by decreases in homogenization temperature

Thefluid inclusions in quartz in charnockite are remark- ably similar to those of Ponmudi and Kodamakod. They are CO2-rich, texturally late (in healed fractures) with many decrepitated inclusions. Figure 5 gives the freezing data, which indicate the presence of some component miscible with CO2, probably about 10% CH4. Homogenization temperatures cluster about + 6 ~ C, indicating somewhat higher densities than at Ponmudi.

Arni Area, South Arcot District, Tamil Nadu. This area is close to the regional orthopyroxene-in isograd (Fig. 1). The surrounding area is dominantly charnockite, but there are abundant light-colored gneisses, and less abundant metapelites, quartzites and metabasic layers. The maps of Suga- vanam et al. (1977) and Jesudossan (1985) show widespread biotite gneisses in irregular patches within charnockite; these may be preserved precursors in charnockite or may be retrogressive after charnockite in the vicinity of younger granites such as the Gingee Granite.

A biotite gneiss just north of the village of Somantangal, 12 km northwest of Arni, is mottled with dark ovoids, one cm to several cm across (Fig. 6). Although they generally appear to be fairly regularly spaced along foliation planes, in rare instances the ovoids coalesce to form irregular stringers, some of which distinctly cut across gneissic foliation. In some places the entire outcrop is light green, with the ovoids occurring as darker green patches. Sugavanam et al.'s (1977) map of the area, combined with our own reconnaissance, suggests that the mottled rock occurs as part of a thick gneissic layer structurally above a khondalite and below a metabasic unit. The layered sequence near Somantangal is tightly folded, with limbs dipping 400-80 ~ and striking northeast.

The dark ovoids of the spotted gneiss are cored by clus- ters of orthopyroxene crystals surrounded by coarse quartz and feldspar with very little biotite. The drawing of Fig. 7

236

Fig. 6. Outcrop appearance of patchy charnockitic alteration in biotite gneiss on vertical quarry face at Somantangal, near Arni, northern Tamil Nadu. Note vague tendency of dark orthopyroxene-bearing spots to be aligned, as in stringers (shear zones?) dipping 20 ~ to the right just above coin, and parallel to pre-existing foliations dipping 60 ~ to the right. The deformation control of charnockite formation is much less evident than at other partially charnockitized localities

Fig. 7. Dark ovoid in spotted gneiss near Arni, Tamil Nadu. The spot is cored by an elongate row of anhedral orthopyroxene crystals (cross-hatched) nearly in optical continuity. Surrounding quartz and feldspar are recrystallized relative to fine-grained host rock, and darkened by chloritic veinlets adjacent to pyroxene, enveloping grain boundaries and penetrating along cleavages (plagioclase) and irregular fractures (quartz). Biotite (bold with cleavage) indicates foliation in host rock, is absent from margins of ovoid, and assumes a bladed form, intergrown with quartz, in center of ovoid, indicating probable retrogression after pyroxene. Length of thin section view is 1 cm. Black grains are ilmenite

shows the essential elements of a dark ovoid in thin-section. Typically, a dozen or more anhedral, interstitial orthopyroxene crystals occur at the center of an ovoid, among sub- hedral plagioclase and anhedral quartz whose average grain sizes are much larger than typical of the host biotite gneiss. The orthopyroxene crystals commonly all have nearly the same optical orientation, with only 10~ ~ variation in extinction direction. Quartz tends to be elongated parallel to the long axis of an ovoid. Darkening is produced by myriads of tiny chlorite veinlets permeating straight cleavages and twin planes in feldspars and fractures in quartz

surrounding the orthopyroxene. Such chlorite veinlets are the principal cause of the typical dark color of charnockite (Howie 1967). The dark centers of the ovoids are commonly surrounded by blanched quartz and plagioclase margins which contain no biotite. Apparently, the mafic components migrated inward to pyroxene nucleii from immediately adjacent regions as biotite was destroyed. Outside the blanched zones the normal, even-grained biotite gneiss re- sumes. Potassium feldspar is rare in the spotted gneiss specimens, occurring as small interstitial blebs in both host gneiss and charnockitic ovoids. Ilmenite and rare magnetite, but not graphite, are present in both rock types.

A curious bladed form of biotite intergrown with quartz occurs in the interiors of the ovoids, sometimes apparently overprinting orthopyroxene. This is interpreted as retrogressive, formed by the reaction of K-bearing solutions with orthopyroxene.

Analyses of large samples from a spotted gneiss and from immediately-adjacent little-altered biotite gneiss are given in Table 8. As with other charnockitization of the Ponmudi type (biotite-quartz reaction), there is little change except for a significant decrease in Rb in the charnockite. Lack of significant differences in K20 makes it doubtful that the spotted gneiss is a partial melt segregate in low-P(H20) anatexis, for example by the reaction biotite+ quartz + K- feldspar + CO2-rich vapor = orthopyroxene + liquid (Wend- landt 1981). Although control of charnockite formation by deformation is less evident here than in other occurrences, the near-isochemical conversion is similar.

The mineral analyses of charnockitic spotted gneiss and host biotite gneiss are given in Tables 2 7. Orthopyroxene and biotite are typical of incipient charnockites. The bladed biotite intimately intergrown with quartz, and sometimes overgrowing orthopyroxene, has lower TiO2, which suggests retrogressive origin. Biotite is lower in F than in the other localities. The inhomogeneous distribution of minerals in the ovoids made accurate point-counting impossible.

Fluid inclusions in the spotted gneiss are strikingly few and small. Healed fractures in quartz contain rare very small bubbles, tubules or sheets of one-phase fluid which is probably CO2. These were too small for accurate determi- nation of homogenization or freezing temperatures.

Kurunegala District, Sri Lanka. We observed widespread arrested charnockite formation in the Central Highlands of Sri Lanka, in the districts of Colombo, Kurunegala and Galle. Most of these localities show Ponmudi-type charnockite formation. The most spectacular development is in the central part of the Kurunegala District, on the NW margin of the Central Highlands, near the contact of the Vijayan Complex (Fig. 1). In three quarries (Udadigana, Angangala and Waraddana, Fig. 8), a hornblende-biotite gray gneiss without graphite or garnet is partially converted to charnockite. In many ways the phenomena of charnockite distribution are quite like at Kabbal. Diffuse, very coarse charnockite occurs in short shear zones marked by warping of gneissic foliation (Fig. 9). Shear veins occasionally contain large black amphibole crystals instead of, or in addition to, orthopyroxene. Shears have a rough preferred N-S orientation direction with steep dips. However, like at Ervadi, charnockite patches also occur concordant with gneissic foliation, especially at Angangala. Pink granite veins are very evident at Waraddana but rare at the other localities; their formation appears to have preceded the development of

237

7~ ' N

80o25 ' E Fig. 8. Areal geology of Central Highlands charnockitic terrain around Kurunegala, Sri Lanka. Arrested charnockitization localities of Waraddana, Udadigana and Angangala are possibly all part of the same biotite-hornblende gray gneiss unit interbedded with metapelites, quartzites, and charnockitic gneisses

the charnockite. In places pyroxene crystals, up to 1 cm across, are clustered along the boundaries between diffuse granite patches and less altered gneiss. The local geological relationships suggest that all three partially charnockitic

localities are part of a thick stratum interlayered in a plung- ing syncline with quartzites and metasediments (Fig. 8).

Close-pair analyses of specimens from the Udadigana and Waraddana localities are given in Table 1. The comparison of host gneiss and charnockite alteration is much the same as for Kabbal: the metamorphism is very nearly isochemical, but there is a significant depletion of CaO, FeO and MgO in the charnockite compensated by an increase of SiOa. Rb is slightly decreased in one specimen and F greatly decreased in both. Th is very high in these rocks, and seems to increase in the charnockites.

The mineral analyses of Tables 2-7 show a familiar pattern: plagioclase, primary biotite and hornblende compositions are identical in both rock types. Primary biotite is high in F and Ti. Retrogressive biotite in the charnockites, sometimes of the bladed form, is lower in Ti and F. Modal data (Table 2) show that hornblende is depleted in the charnockite, and decline of primary biotite is probable, although there is an overall increase of biotite in the charnockite from Udadigana. There is little change in the abundances of the other minerals.

Fluid inclusions were relatively rare in most of the samples we examined from these localities. The one exception is a coarse-grained charnockite (D4-03) from Waraddana. All of the fluid inclusions occur in planar arrays within quartz. Mixed CO2-H20 and CO2 inclusions are the most common. Large, empty decrepitated inclusions are abundant in these trails. Water was observed wetting the walls in about half of the larger (> 20 gm) undecrepitated carbonic fluid inclusions. The amount of water is variable, ranging from about 50 to about 25 vol.%. A clathrate with a dissociation temperature of 7.8 ~ C (implying low salinity) was found in one of the largest of the mixed CO2-H20 inclusions. Thermometric data on CO2 in both the CO2 and CO2-H20 inclusions are given in Fig. 5. Melting points are close to the normal melting point of - 56.6 ~ C, indicating lack of CH~ and other miscible components. Homogeni-

Fig. 9. Outcrop appearance of partially charnockitized gray gneiss in quarry at Waraddana, Kurunegala District, Sri Lanka. Apparent deformational control of charnockite formation is strikingly similar to Kabbal and Ponmudi localities

238

zation temperatures are very high, indicating densities much lower than would be expected for trapped samples of the peak metamorphic fluid. Trails of aqueous inclusions also occur in the charnockite, although they are much rarer than the carbonic inclusions. Petrographic relationships are not completely unambiguous, but do appear to indicate that these aqueous inclusions cut and are therefore later than the carbonic inclusions. Final melting temperatures were measured on a few of the largest aqueous inclusions. They ranged from - 0 . 2 to - 0 . 4 ~ C, indicating relatively low salinities. Homogenization temperatures of the same inclusions ranged from 192 ~ to 282 ~ C.

Discussion and conclusions

Charnockite facies and charnockite-producing reactions

The present work establishes that arrested charnockite formation is widely distributed south of the orthopyroxene-in isograd of southern India and is found also in the Sri Lanka Central Highlands. Hornblende is absent from acid gneisses over most of this region, except in gneisses retrogressive after charnockite. The conditions for incipient instability of biotite and quartz were preserved over a very broad area of southern India with widespread occurrence of garnet-biotite gneiss partially converted to charnockite. It is not yet established that all of these localities are of latest Archaean-early Proterozoic age, although this remains the simplest hypothesis to relate the various localities.

The charnockite-gneiss associations of the present work cannot be adequately represented in conventional ACF or AFM diagrams. A projection of Froese (1978) is adequate for representation of the incipient charnockite assemblages and related rocks. In the Froese projection, the coordinates are A: A l a O 3 - ( N a 2 0 + C a O + K 2 0 ) ; F: FeO- (FezO3 + TiOa); and M: MgO. The projection is thus from quartz, magnetite, ilmenite, K-feldspar and a plagioclase of fixed composition. In the rocks studied here, most of the plagioclase is oligoclase of a limited range of compositions and is the same in both charnockite and host gneiss.

The Froese projection leads to the recognition of three facies types in acid gneisses in the S. India-Sri Lanka high- grade area (Fig. 10). The lowest-grade facies, called "Penin- sular Gneiss facies," does not include orthopyroxene. Acid gneisses of a wide compositional range contain primary hornblende. This facies is distributed north of the orthopyroxene isograd in southern Karnataka and northern Tamil Nadu. Garnet-sillimanite metapelites are found throughout this region (Harris and Jayaram 1982). Orthopyroxene appears by the isograd reaction of hornblende, botite and quartz, giving rise to the second facies type, called "Kabba l facies". With progressive metamorphism, the orthopyroxene-biotite-hornblende triangle enlarges. Biotite is stable with quartz over a broad range of compositions, but orthopyroxene is not found with garnet. Biotite reacts with garnet and quartz to form orthopyroxene and K-feldspar at still higher grade to give the third or "Ponmudi facies". Biotite and hornblende are not stable together except in the more magnesian, usually mafic, bulk compositions. Most acid rocks which could crystallize amphibole at lower grades are charnockites in the Ponmudi facies. The three facies types are related by increasing temperature, decreasing pressure, or decreasing H20 pressure. Geothermometry and geobarometry (see below) favor the latter hypothesis.

Peninsular Gneiss Facies Kabbal Facies Ponmudi Focies ASIL A A

' ISIL SIL

F ' , ' , , ' ' , ,&M FZ-~d~.~cd,,f/', ', ' , ~ , FA~x ,'-,-'Xr ',', ',_.X, . y . , , , , . " , . ' , " . ' , . . / . . , , , , ,. ,, > , , L ~ l l l , ~ l \',~,',', \

Fig. 10. Schematic Froese (1978) projection of mineral compositions in high-grade quartzfeldspathic gneisses. Peninsular Gneiss Facies does not have orthopyroxene. Kabbal Facies has orthopyroxene, but not with garnet. Ponmudi Facies has stable garnet- orthopyroxene. The prograde sequence going to the right may be the result of increasing temperature, decreasing pressure, or, more probably, decreasing H20 activity

I.O,t, SlL KABBAL FACIES / / \ \ o OPX+BIO

o.~/ \ \ ~, OPX+BIO+HBL / I \ ~ �9 GAR+BIO+SIL(+CRD)

0,6 o?1%~ \ \

X

/L HBL-~ I :~ "xX

�9 0 0.2 0.4 0.6 0.8 1.0 M

M+F Fig. 11. Mineral compositions in gneisses from Kabbal-type localities and related localities plotted on the Froese (/978) projection. A vertex: A12Oa - (KzO+Na20+CaO); F vertex: FeO - (F%O3 + TiO=) ; M vertex: MgO

,.o.8)@ ,.slL :0jRR;0%,, ~ PONMUDI FACIES ~'--k k �9 OPX+B]O

A 0.6,/- / \ ~ �9 GAR+BIO+SIL{+CRD)

/

-0 .4 / , / ~ 7 0 0.2 0.4 0.6 0.8 1.0

M

M+F Fig. 12. Mineral compositions in gneisses from Ponmudi-type localities, plotted on Froese (1978) projection

Mineral compositions of the Kabbal facies localities are consistently represented in the Froese plot of Fig. 1 I. The most magnesian orthopyroxenes are those which coexist with hornblende as well as biotite. The most magnesian biotite shown is from a sillimanite-bearing metapelite 7 km south of Kabbal, from Harris and Jayaram (1982). The assemblage orthopyroxene-garnet-biotite was not found in localities adjacent to the orthopyroxene isograd. Mineral compositions of the Ponmudi facies are illustrated in the Froese plot of Fig. 12. The most Fe-rich garnets and ortho-

239

pyroxenes are those from charnockites without biotite. The most magnesian garnets and biotites are from khondalites. Hornblende was not found in any charnockites of the Pon- mudi type but could conceivably occur in compositions of high Mg/Mg + Fe ratio. The width of the sillimanite-biotite two-phase field is so small that minor additional chemical degrees of freedom, such as Mn in garnet, allow the stable assemblage garnet-sillimanite-cordierite-botite.

The facies type represented by the spotted gneiss locality in North Arcot, Tamil Nadu, is ambiguous in this classifica- tion: a biotite-orthopyroxene two-phase field occurs in both charnockitic facies types. The North Arcot occurrence may be basically of the Ponmudi facies for the reasons that garnet charnockites are found close by (rock T13-83, Table 2), because most of the regional gneisses approaching orthogneiss compositions are charnockitic in this area, and because the reaction to produce orthopyroxene in the spotted gneiss is basically that of biotite and quartz.

Using the average mineral analyses for Kabbal charnockite and gneiss 3-1, one may balance a closed-system reaction among the observed phases amphibole, biotite, orthopyroxene, plagioclase, K-feldspar, quartz, ilmenite and magnetite. In order to achieve the chemical balance, it is necessary to determine the coefficients of albite and anorthite components of plagioclase separately; this is equiva- lent to recognition that the orthopyroxene-forming reaction is at least divariant. Hornblende of composition Ko.25- Nao.41Cal.7 sMg1.96Fe2.69All.80Tio.22Si6.4vOzz(OH)2, biotite of composition Ko.91All.~gTio.27Mgl.0sFel.41Si2.ss- O~o(OH)2, and othopyroxene of composition Mgo.76- Fe~,lvAlo.o3Sil.9706 give the following reaction:

hornblende + 10.28 biotite + 31.50 quartz + 2.01 magnetite = 0.41 albite + 1.75 anorthite + 9.60 K-feldspar + 17.28 orthopyroxene + 3.00 ilmenite + 11.27(H20 + F) + 1.01 02 (1).

The postulated closed-system reaction grossly violates the modal data of Table 2 : it predicts that five times as much biotite by mass or volume as hornblende is reacted to produce orthopyroxene; that is, that the Kabbal-type reaction is essentially that of biotite breakdown. The closed-system reaction further predicts that large amounts of quartz are consumed and that the resulting plagioclase will increase in anorthite content. None of these trends are supported by the data of Table 2, which show that quartz and feldspars are in nearly the same amounts in charnockite and close-pair host gneiss, that biotite decreases only slightly and that the plagioclase composition does not change.

An open-system approach provides a reaction more consistent with the modal data. The most consistent refer- ence frame for a metasomatic reaction is conservation of a relatively inert component, such as A1203 (Ferry 1982). Recalculating the close-pair oxide differences in Table I relative to fixed A1203, the average amphibole gneiss-to-charnockite differences of the three 3-1 pairs are SiO2: + 1.06% ; FeaO3: --0.63%; MgO: - 0 . 1 % ; CaO: - 0 . 3 4 % ; Na20: + 0.10 %, and the others negligible. A balanced orthopyroxene-producing reaction in a system open to all components except A1203, and with plagioclase composition fixed at An21 is thereby:

hornblende + 1.30 biotite + 1.24 quartz + 4.23 SiOz (added from fluid)+ 0.82 magnetite + 0.38 Na20 (added from fluid) = 1.49 plagioclase (An21)

+ 1.43 K-feldspar + 3.67 orthopyroxene + 0.57 ilmenite + [1.44 CaO +2.12 FeO +0.58 MgO] (removed in fluid) +2.30 ( H 2 0 + F ) + 0 . 4 1 02 (2).

For a given amount of orthopyroxene produced, the amount of biotite consumed is volumetrically only about half the hornblende consumed. The amounts of quartz and plagioclase involved are quite minor. There is a moderate increase of K-feldspar, as shown by all of the Kabbal-type charnockites. It is concluded that the appearance of orthopyroxene in the Kabbal-type localities results from an open- system reaction similar to reaction (2). The slight apparent decrease in O2 in reaction (2) may be an artifact of not considering the Fe 3 § content of orthopyroxene and ilmenite.

According to the modal data of Table 2, the charnockite-forming reaction in the Ponmudi-type localities consumes biotite, garnet and quartz and produces orthopyroxene, K-feldspar and ihnenite. The reaction may be modelled by a closed-system equation, using average mineral compositions for the K18-6 pair:

biotite + 0.99 garnet + 2.37 quartz = 0.21 anorthite + 0.89 K-feldspar + 2.53 orthopyroxene + 0.31 ilmenite + (H20 + F2) (3).

In contrast with the Kabbal-type localities, the closed system reaction for Ponmudi appears consistent with the modal data. Garnet is consumed in relatively large amounts and K-feldspar increases in the charnockite. Plagioclase may increase very slightly in anorthite content in K18-6 charnockite. The ability of an isochemical reaction to pre- dict the modal data is in keeping with the tendency of the Ponmudi-type close-pair analyses to show only small chemical differences. An open-system reaction in which a small amount of iron oxide is lost and SiO2 is gained gives reaction coefficients that are not significantly different from those of reaction (3).

Geothermometry and geobarometry

The mineral analyses may be used to estimate temperatures and pressures during charnockite formation. The most useful geothermometers are based on Fe-Mg exchanges between garnet and biotite (Ferry and Spear 1978) and between garnet and orthopyroxene (Harley 1984; Sen and Bhattacharya 1984). The most useful geobarometers are the orthopyroxene-plagioclase-garnet-quartz equilibrium (New- ton and Perkins 1982, Mg-reaction; Bohlen et al. 1983b, Fe-reaction). The somewhat more temperature-dependent equilibrium of plagioclase-garnet-sillimanite-quartz (New- ton and Haselton 1981) is useful for associated khondalites. Table 9 lists the paleotemperatures calculated at an assumed pressure of 5 kbar and the paleopressures calculated at an assumed temperature of 750 ~ C.

It is very likely that the P-T conditions for incipient charnockites apply to the period of charnockite formation, when the coarsely-crystalline orthopyroxene-bearing assemblages were formed. During this transient event the systems were fluxed by fluids which included some H20 released from hornblende and biotite breakdown. It is probable that this fluid was removed on a relatively short time scale and that mineral chemistries and isotopic systems were effective- ly frozen in when the vapor phase migrated out of the system. The temperatures and pressures inferred for charnockitization do not necessarily correspond to the maxi-

240

Table 9. Paleotemperatures (7) and paleopressures (P)

Locality Tn TsB TFs PPN PB PNn Prow I.D.No. (Type)

Ponmudi K18-6 (CH) 814 958 596 5.7 4.8 - - K18-7(CH) 755 867 618 5.6 5.2 - - 147-214 (CH) 736 860 778 5.5 4.9 - - 35-68A - - 691 - - 6.3 5.2

Kodamakod Dt t-KC-7 (GN) - - 780 . . . .

Ervadi TN3-t (CH) 815 946 795 6.9 5.8 - - TN21-4(CH) 759 876 725 5.8 5.2 - - TN2-1 TKH) - 690 - - 4.9 -

Arni T13-83 (CH) 721 810 - 6.0 6.0 - - T4-16 (KH) - - 739 - - 6.0 5.6

Waraddana D4-N2 (BG) 754 852 - <7.2" <6.6 a - -

a Using plagioclase rims (An53); upper limit in absence of quartz CH Charnockite; KH Khondalite; GN Gt.-Biot. Gneiss; BG Basic Granulite; T~,TsB Garnet-Opx K w-Mg (Harley 1984; Sen and Bhattacharya 1984). Tvs Biotite-Garnet K ve-Mg (Ferry and Spear 1979). PeN, PB Garnet-Opx-Plag-Qtz (Perkins and Newton 1981, eq. 9; Bohlen et al. 1983 b). Puw Garnet-Cordierite-Sillimanite-Qtz, av. for P(HzO)=PTotal and P(H20)=0 (Newton and Wood 1979); PNn Garnet-Plag-Sil-Qtz (Newton and Haselton 1981)

mum conditions realized in the rock, but only those associated with the appearance of orthopyroxene.

Useful geothermometers and geobarometers are not present at Kabbal. However, the geographic position in the apparently continuous P-T progression of southern Karnataka suggests conditions of 750~ and 5.5 kbar (Hansen et al. 1984). The temperature estimate is supported by the coexistence of orthopyroxene and an acid melt under 5 kbar CO2 pressure (Wendlandt 1981). The metamorphic conditions at North Arcot and Kurunegala are similarly undefined in the lack of garnet in the incipient charnockites, but garnet-bearing charnockites and khondalites occur in close association at North Arcot.

Garnet-orthopyroxene temperatures show the least scat- ter among the geothermometers. The empirical calibration of Sen and Bhattacharya yields very high temperatures, some above 900 ~ C. Temperatures in this range have been inferred for the sapphirine-osumilite terrain of Enderby Land, Antarctica (Grew 1982), but seem too high for the lower-grade terrains discussed here. Harley's thermometer is consistent with the experiments on rock melting and agrees with the biotite-garnet temperatures for a number of rocks. Lower biotite-garnet temperatures, as at Ponmudi, may be retrogressive. The probable temperature range for all of the localities is 690~ ~ C.

The two garnet-orthopyroxene-plagioclase barometers give similar readings, mostly in the range 5-6 kbar for all of the localities. Pressures of associated khondalites are generally consistent with those of charnockites. Maximum pressures of 6.6-7.2 kbar are recorded by garnet-orthopyroxene-plagioclase in a metabasite from Waraddana, using the more calcic rims of plagioclase. Zonation of plagioclase suggests that the rock experienced higher pressures before final equilibration.

Entrapment conditions of fluid inclusions

Most of the fluid inclusions in quartz in the S. India-Sri Lanka incipient charnockites have densities below 1.0 gm/ cm a, indicative of entrapment at relatively low pressures. It has been shown (Hansen et al. 1984) that the entrapment pressures of Kabbal CO2-(H20) inclusions at a reasonable temperature of 750 ~ C are 5-6 kbar (i.e., consistent with mineral barometry). For the Ponmudi locality, homogenization temperatures near 10~ indicate entrapment at 3-4 kbar at 750 ~ C, using the P-T isochores for pure CO2 given by Touret and Bottinga (1979). A pressure 500 bars higher is obtained for Ervadi. The presence of about 10 mol% CH, or N2 in these inclusions makes the pure COz estimates upper limits. These pressures are too low to be compatible with entrapment at peak metamorphic conditions as indicated by the mineralogic geobarometers. Data for COz-rich inclusions in a khondalite from Servalar Dam, near Ambasamudram (Fig. 2) are given in Fig. 5 for comparison. This is the only locality found in the Khonda- lite Belt where CO2 densities are high enough to have been trapped near peak metamorphic pressures. The correlation of freezing temperatures with homogenization temperatures is striking: the higher the homogenization temperatures of the inclusions, the purer the CO2 fluid. The correlation cannot be simply the result of miscible impurities: the maximum amount of 15 mol% CH4 is too small to lower the homogenization temperatures by 40 ~ C, based on the diagrams of Burruss (1981). The low densities of fluid inclusions from this area are almost certainly the result of subsequent mechanical disburbance, probably in association with retrogression which created the mixed CO2-H20 inclusions by hydrous contamination.

Nature of the metamorphic JTuid

The compositon of C-O-H fluid in a rock containing graphite, biotite, orthopyroxene, K-feldspar and quartz may be estimated from mineral-fluid equilibria, using procedures similar to those of Ohmoto and Kerrick (1977), Walther (1983) and Lamb and Valley (1984). The MRK procedure was used with the end-member fugacities of Ryzhenko and Volkov (1971) and the mixing properties of CO2, H20 and C H 4 of Jacobs and Kerrick (1981). Figure 13A shows H20 contents of C-O-H fluids in equilibrium with incipient biotite-bearing charnockite, based on the biotite-quartz-orthopyroxene-K-feldspar-fluid equilibrium. For the P-T conditions recorded by the minerals of the south Asian rocks, the mol fraction of H20 in a vapor phase in equilibrium with incipient charnockites was less than 0.35, and may have been near 0.20. The additional chemical factors not considered in this analysis, including FeO, Fe2Oa, TiOz and F, are likely to have the effect of stabilizing biotite to even lower H20 activities (Hansen et al. 1984). Fig- ure 13 B shows the mol fractions of gas species present in the C-O-H gas in equilibrium with graphite at 727 ~ C and 5000 bars. There are only two regions where the HzO activity is equal to or less than 0,35: a region of essentially binary CO2-HzO at high oxygen fugacity near the graphite-CO2 buffer, and a region dominated by CH4 with a small amount of Hz at much lower oxygen fugacity. The region near f(O2)--10 -16 bars is much more probable, because the fluid inclusions indicate COz-rich, rather than CH4- rich, fluids. The possibility of CO2-N2 fluids, with perhaps smaller amounts of HzO, cannot be ruled out.

241

8 en eto1198 g

#o.# 4 o.50 o[ ~ /o~5./~*~UT. I9631 ~ 0.25

600 700 800 -17.0 -18.0 -19.0 ~.-12 , t ' I I.O0 1 7pTor ~nnnR., C "~ ~ 5000 Bars

1 '< "~ 0.25 o -18

-~ K, I, , I 0 600 700 800 -17.0 -18.0 -19.0

Temperature Deg C Log Oxygen Fugacity

Fig. I3A-D. Calculated composition and activity relations in metamorphic fluids associated with South India/Sri Lanka charnockite formation. A low-P (H20) character of fluid, assumed CO2-dominated, in Kabbal-type localities without graphite, inferred from equilibrium of biotite-orthopyroxene-K-feldspar-quartz. B MRK calculation of fluid species in C-O-H gas with graphite, showing two allowable vapor compositions, pts. a and b, which could coexist with incipient orthopyroxene at inferred metamorphic P-T conditions (see text). C Similar diagram at lower pressure showing that observed fluid inclusions in quartz in charnockites could not be the result of re-entrapment of fluids from inclusions decrepitated during uplift (see text). D oxidation states of Kabbal-type charnockites (open circles) calculated from orthopyroxene-magnetite- quartz assemblage, and of graphite-bearing Ponmudi-type charnockites (dosed circles) inferred from isothermal isobaric vapor calculations as in B

The observed fluid inclusions, if they are survivors of the orthopyroxene-producing metamorphism, must have been considerably modified by subsequent processes to produce the low densities. Such processes might have included: 1) Reaction leakage, as postulated by Hollister and Burruss (1976), whereby H20 and CH4 may react in an inclusion after entrapment to produce CO2 and H2. The H2 diffuses out of the crystal, increasing CO2 and lowering density and H20 content. 2) Decrepitation during uplift. Hollister et al. (1979) argued that fluid inclusions in quartz will burst if pressure is dropped 1-2 kbar below the entrapment isochores. The fluid released in decrepitation could react with the minerals and graphite at some different P-T conditions, producing a modified fluid which could have been re-trapped. 3) Late entrapment of volatiles from some remote source, at temperatures and pressures removed from peak metamorphism. These fluids, if channellized, might have had only limited contact with graphite.

None of the above three possibilities seems adequate. In order for the mechanism of reaction leakage to have been effective in producing CO2-CH4 inclusions, the original composition of the vapor must have been about 60% CO2 and 20% each of H20 and CH~. This is more CH4 than the M R K calculations indicate is possible in equilibrium with graphite, in CO2-dominated fluids. Reaction of decrepitated fluids with graphite at lower pressures during uplift would not produce a fluid rich in both CO2 and CH4, as shown by Fig. 13C. A completely remote source of late-entering volatiles seems dubious, inasmuch as CO2- CH4 inclusions are found only in graphite-bearing rocks

(Hollister and Burruss 1976; Rudnick et al. 1984). The fluid inclusions observed in the south Asian granulite paragneisses are therefore enigmatic in their low densities and non-equilibrium compositions, if CH4 is the freezing point depressant.

Oxygen fugacities can be inferred for the granulites lacking graphite based on the assemblage orthopyroxene-magnetite-ilmenite-quartz (Hansen et al. 1984). Figure 13D shows the metamorphic oxidation states of Kabbal, Sri Lanka and North Arcot rocks. The observed high-density CO2 inclusions with probable subsidiary H20 could well have been in equilibrium with orthopyroxene at peak metamorphic conditions at elevated oxygen fugacity, and are thus compatible with influx of CO~. This is also true of the graphite-bearing parageneses. CO2 entering the rocks would have driven the oxidation state towards the graphite- CO2 buffer. As long as biotite was still present, the oxygen fugacity would have been buffered just below graphite-CO/, as shown by the points in Fig. 13 D.

Petrologic interpretations

The present study describes various kinds of arrested charnockite formation which are widespread in southern India and Sri Lanka. There were two types of reactions that generated orthopyroxene. One, at Kabbal, Karnataka and in the Kurunegala district of Sri Lanka, was the breakdown of calcic amphibole in graphite-free gray gneisses. It was an open system reaction in which basic components were lost, including the Ca0 formerly in the amphibole, and SiO2 and probably NagO were gained, This reaction marks the regional orthopyroxene-in isograd. The other type of reaction is characteristic of graphitic paragneisses in extreme southern India and in Sri Lanka, originally described at Ponmudi, Kerala. Here, the reaction consumes biotite and quartz, with the participation of garnet in several localities. A major conclusion is that this reaction occurred at higher grade than the Kabbal reaction. Biotite and quartz are stable together at Kabbal and similar localities. Because the paleotemperatures and paleopressures are nearly the same at both types of localities, the discriminating parame- ter is inferred to be lower H20 activity in the Ponmudi-type metamorphism.

A major interpretation of our study is that the whole of the earlier Precambrian terrain of southern India south of the orthopyroxene isograd is in the granulite facies. We have never found an amphibole-bearing gray gneiss except where there is clear evidence of retrogression. All rocks of approximately calc-alkaline composition are charnockites. This may not apply to the northern part of the Sri Lanka Central Highlands, where we found Kabbal-type charnockite formation. Sri Lanka may be an independent crustal block, duplicating, on a smaller scale, the South India terrain, or may be less directly related. The age of the Kurunegala charnockitization is not yet known, and may be younger than the similar occurrences in India.

The Kabbal-type occurrences are compatible in most of their features with the CO2-streaming hypothesis. The main evidences are:

1) The field evidence of deformation-aided access of fluids. 2) The necessity that these fluids, coexisting with orthopyroxene, must be low in H20. 3) CO2-rich inclusions in incipient charnockites in the Kab- bal area, with densities consistent with entrapment at high

242

pressures, and fluid chemistries (CO2 + 2 0 % H20) which are compatible with the high oxidation state inferred from the presence of magnetite and intermediate hypersthene, and 4) The evidence from the close-pair analyses of an open- system orthopyroxene-producing reaction. This evidence is supported by the need for chemical transport in order to explain the modal data with a balanced reaction.

The origin o f the COz is unknown. It must have been from a deep-seated source, either from the lowermost crust or the upper mantle. Possible crustal sources could be exso- lution from solidifying basaltic underplates (Touret 1971) or sedimentary carbonate deeply buried by thrusting (Glassley 1983). Possible subcrustal sources could have included decarbonation of a diapiric mantle (Wyllie and Huang 1976) or deeply subducted marine carbonates (Helmstaedt and Gurney 1984).

The Ponmudi-type charnockite is less obviously the result of pervasive CO2-streaming. Charnockite formed in shear zones at Ponmudi is indeed reminiscent of Kabbal. However, the need for an open-system charnockite-forming reaction is not as clear-cut as at Kabbal, either from the close-pair analyses or from the modal data. Furthermore, the fluid inclusions at Ponmudi and similar localities have lower densities than predicted from mineralogic geobarometers, and chemistries not predictable from C-O-H vapor calculations.

The idea o f Srikantappa et al. (1985) that the Ponmudi- type charnockitization was caused by decrease of fluid pressure during fracture of the host gneisses is appealing in explaining the association of incipient charnockite with shears and other deformation features. However, it seems unlikely that passageways o f considerably reduced Pnuie could exist during the high-temperature and high-pressure conditions indicated by the charnockite mineralogy. I f reaction o f biotite with graphite to produce a COe-rich vapor could have been driven by small pressure differentials, of the order of a few hundred bars, it might be possible that this mechanism was effective. Production of charnockite would thus be an internally-generated phenomenon in the Ponmudi rocks, brought about by the presence of graphite.

Study of oxygen isotopes may provide a means of decid- ing between CO2 influx and internal generation of a vapor phase that could have carried HzO out o f the Ponmudi rocks. Archaean paragneisses and other metasediments o f amphibolite grade in the Superior Province of western On- tario average about 11%o in 6180 (Longstaffe and Schwarcz 1977). Paragneisses o f granulite facies from the same region are much lower in 180, averaging about 9%o (Longstaffe ] 979). Such a contrast between amphibolite facies and granulite facies paragneisses was also described by Shieh and Schwarcz (1974) in the Grenville Province of southwestern Ontario. A drop in 6180 from amphibolite facies to granulite facies could not have been produced by simple dehydra- tion reactions, which would have increased 1sO slightly (Hoernes and Hoffer 1985). Shieh and Schwarcz (1974) concluded that the granulites in the western Grenville commun- icated with a pervasive fluid emanating from a large large mafic or ultramafic reservoir (i.e., the lowermost crust or upper mantle). A similar conclusion was reached by Four- cade and Javoy (1973) for ~80-depleted granulite paragneisses from Algeria. Characterization of the oxygen isotopes in the southern Indian and Sri Lanka paragneisses may provide important evidence on an internal or external

source of the CO2-rich fluids in the formation of charnock- ire.

Acknowledgements. Alphabetical authorship order was adopted by the authors of this paper in recognition that the work is a team effort in which it is impossible to assay the relative magnitudes of the individual contributions.

The material help of many people contributed substantially to the carrying out of this project. D.D. Jayawardena made the services of the Sri Lanka Geological Survey Department available, including the use of a field vehicle, without which the Sri Lanka part of our project could not have been carried out. H.K. Gupta and the Centre of Earth Science Studies similarly made available to us crucial transportation and lodgings assistance in Kerala. S. Saravanan, through Tamin Ltd., provided us with several days' field transportation and accommodations in Tamil Nadu: to him our grateful thanks. G. Raghavan provided key logistical help in Madras. V. Venkatachelapathy made available to us the resources of his Department of Geology at Mysore University.

Numerous geologists guided the authors to key localities, including P. Allen and U.S. Ramachandran to the North Arcot localities, L.R.K. Perera to the Kurunegala localities and R.T. Wight- man to the Panagudi localities. C.R.L. Friend and T. Chacko helped with field collections and discussions. S.C. Jacob, N. Leelan- anda Rao and P.W. Vitanage are thanked for valuable information and discussions. R.A. Wiebe made available his XRF analysis facil- ities. P. Allen provided an analysis of the 3-1 charnockite and its close-pair gneiss. Wendy Hunt helped with the fluid inclusion measurements.

U.S. National Science Foundation grant INT 82-19140 (RCN) provided field expenses in India and grant EAR 82-19248 provided international travel and U.S.-based study of the rocks.

The detailed and critical comments of J.M. Ferry, S.N. Olsen and W.E. Glassley created major changes in this paper.

We thank Cassandra Spooner for her skill and patience in the typing several versions of the manuscript.

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Received September 23, 1986 / Accepted February 18, 1987

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Arrested charnockite formation in southern India and Sri Lanka