The Nagercoil Granulite Block, southern India: petrology, fluid inclusions and exhumation history

Journal of Asian Earth Sciences 22 (2003) 131–155 www.elsevier.com/locate/jseaes

The Nagercoil Granulite Block, southern India: petrology, fluid inclusions and exhumation history M. Santosha,*, M. Tagawaa, S. Taguchib, S. Yoshikuraa b

a Department of Natural Environmental Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japan Department of Earth System Science, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka-shi 814-0180, Japan

Received 7 February 2002; revised 7 July 2002; accepted 8 July 2002

Abstract The Nagercoil Granulite Block (NGB) constitutes the southernmost domain in the high-grade metamorphic terrain of southern India and dominantly comprises orthopyroxene –K-feldspar – plagioclase – quartz– biotite – ilmenite bearing massive charnockites. Where they are mixed with aluminous supracrustals, the charnockites contain garnet, often-displaying typical decompression textures with coronal orthopyroxene and plagioclase surrounding relict garnet grains. Orthopyroxene is preserved in near-pristine state in the charnockite suggesting low water activity. Garnet – orthopyroxene thermometry of NGB charnockites yields temperatures of 691– 934 8C. Garnet – biotite thermometry yield values that broadly overlap (538– 864 8C) with the above estimate, although some lower values indicative of retrograde Fe – Mg exchange are also obtained. The presence of mesoperthite in the rock and high fluorine content in biotites also suggest high metamorphic temperatures. Charnockite barometry using garnet –orthopyroxene – plagioclase – quartz assemblage yields 4.0– 6.3 kbar. An overall drop in pressure by over 2 kbar correlate with the isothermal decompression history inferred from textural criteria. Detailed fluid inclusion petrographic, microthermometric and laser Raman spectroscopic data presented here allow an evaluation of the fluid evolution history of NGB charnockites. We distinguish four groups of inclusions, with the earliest generation characterised by CO2 with variable but minor proportion of H2O. Subsequent generations are dominated by CO2. Locally, CH4 and N2 occur as trace components. CO2-rich inclusions dominate in all minerals: garnet, K-feldspar, plagioclase and apatite. Homogenisation temperatures assign densities of 0.92 – 0.93 g/cm3 for the CO2 involved in charnockite formation. A close correspondence of the isochores for CO2 þ H2O and CO2 inclusions with P– T data derived from mineral phase equilibria suggests fluid entrapment at peak metamorphic conditions. The metamorphic evolution of NGB and its exhumation path characterised by isothermal decompression are comparable with those of charnockites from Pan-African terrains elsewhere in East Gondwana. q 2003 Elsevier Ltd. All rights reserved. Keywords: Massive charnockite; Petrology; Fluid inclusions; Southern India; East Gondwana

1. Introduction East Gondwana incorporates a collage of polymetamorphic terrains with long-lived tectonic history from Early Archean to Neoproterozoic. The oldest cratonic areas are identified in South India (north of the Palghat – Cauvery Shear Zone) and East Antarctica (the Napier Complex). These terrains are remnants of an East Gondwana craton that underwent initial crustal growth during the Early Archean and granulite facies metamorphism at , 2.5 Ga. Both were virtually unaffected by the Grenvillian (1.1 Ga) * Corresponding author. Fax: þ 81-88-844-8278. E-mail address: [email protected] (M. Santosh). 1367-9120/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1367-9120(02)00176-1

and Pan-African (0.5 Ga.) orogenies. In contrast, the Proterozoic terrains were subjected to high-grade metamorphism during the Pan-African tectonothermal event at ca. 550 Ma (Harris et al., 1994, 1996). Like most other shield area of the globe, the southern Indian Peninsular shield is also composed of Archean cratonic nuclei surrounded by mobile belts of varying ages. Drury et al. (1984) divided the southern Indian shield into the Northern and Southern blocks separated by the E – W trending Palghat – Cauvery Shear Zone (PCSZ). The Northern Block includes the Dharwar craton, essentially a granite – greenstone belt, with a small component of granulites towards its southern margin. Charnockites and abundant swatches of granulite facies metasedimentary

132

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

lithologies dominate the Southern Block or the Southern Granulite Terrain. This terrain in punctured at several places by late Proterozoic magmatic intrusives. Harris et al. (1994) recognised that the network of Neoproterozoic or Early Palaeozoic shear zones which transect the Precambrian basement lithologies in southern India are also often terrain boundaries. Thus, they showed from Nd-isotope signatures that the PCSZ marks a major terrain boundary dividing an Archean craton to the north with accreted Proterozoic terrains to the south. The Proterozoic terrain south of PCSZ comprises a number of granulite blocks (Santosh, 1996). (1) The Madurai Granulite Block (MGB), bounded by PCSZ in the north and Achankovil Shear Zone (ACSZ) in the south, comprising the granulites of Madurai and Cardamom Hills and the metapelites of Kodaikkanal; (2) the Trivandrum Granulite Block (TGB) bounded by ACSZ in the north and the granulite massif of Nagercoil in the south, comprising the Kerala Khondalite Belt (KKB: Chacko et al., 1987) and (3) the Nagercoil Granulite Block (NGB) bounded by TGB in the north, and extending into the tip of the Peninsula in the south (Fig. 1). While all the granulite massifs are bounded by crustal-scale shear belts, the boundary between TGB and NGB is unclear. Therefore, the NGB and TGB were earlier considered as a single block (Harris et al., 1994), although subsequently, it has been assigned the status of an independent granulite block based on lithological characteristics, which contrast with the adjacent TGB (Santosh, 1996). Among the various granulite blocks above, very little information exists on NGB. Preliminary age data from mineral-whole rock Sm – Nd and Rb – Sr dating of the massive charnockite from Kottaram within NGB show metamorphic ages of 517 ^ 26 and 484 ^ 15 Ma, respectively (Warrier et al., 1995), confirming a late Pan-African tectonothermal event. The massive charnockite –pyroxene granulites association in this block, together with igneous intrusives of norite and syenite were considered to represent an ‘igneous charnockite’ suite (Santosh, 1996). The NGB has also been ascribed as the source for heat and fluids, which caused incipient charnockitico alterations in the adjacent TGB (Chacko et al., 1996). A syenite pluton at Puttetti, in the northwestern margin of NGB yielded whole rock Pb – Pb ages of 508 Ma and K – Ar (phlogopite) ages of 445– 454 Ma (Santosh et al., unpublished data), clearly indicating an intrusive magmatic phase that closely followed regional metamorphism in this block. Apart from preliminary reports (Srikantappa et al., 1985; Santosh, 1996) no detailed studies have so far been carried out on NGB. In this study, we report results from a detailed investigation of the petrology and fluid inclusions in the charnockites from NGB and evaluate the metamorphic and fluid evolution history of this crustal segment.

2. NGB charnockites The NGB provides excellent exposures of fresh rocks in several large working quarries as well as extensive rocky hills stretching over tens of kilometres from north of Nagercoil up to Kanya Kumari (Fig. 1). Two national highways, NH-47 and NH-7 envelope rocky hills of NGB which form part of the southern extremity of the Western Ghats hill range. Several active quarries in this region provide exposures at different levels to examine the geology in three dimensions. Although we studied about 15 localities within NGB in detail, here we summarise the lithological characteristics from four localities only, the samples from which were used for analytical work presented in this report. From south to north, these locations are Kottaram, Arakkakulam, Kozhikkotpothai and Kottorpotti (Fig. 1), the last one falling outside the main NGB massif, but considered to be part of the main charnockite unit. The lithological association and field relations in other localities examined within NGB are also generally comparable with the features summarised below. The Kottaram quarry is located 5 km north of Kanya Kumari in a hillock on the eastern side of NH-47. The quarry comprises mostly of massive charnockites. In the northern part of the quarry, a thick (8 – 10 m) band of metapelite is seen within the charnockite body. Some of the metapelite fragments enclosed within the charnockite are rotated, appearing as xenoliths. Also, some metapelite enclaves are crosscut by orthopyroxene bearing charnockitic pegmatites. The contact zone of the charnockite and metapelite is diffused at places due to partial assimilation where garnet appears in abundance in the charnockite. Away from the metapelite contact, garnet content gradually decreases within the charnockite. The charnockites further away are totally devoid of garnet. Towards the northern part of the quarry, thin remnant patches of the metapelite (less than 50 cm thick, 4– 20 m length lenses) occur within the charnockite. These lenses show clear indication of high degree of assimilation by the host charnockite, with very diffuse margins and laterally grading into the host charnockite. The charnockite adjacent to the layers and up to several metres are markedly rich in garnet. Where a series of such lenses occur in the quarry, a garnet-enriched charnockite zone develops, imparting a reddish colour to the rock, as against the greasy green type in other parts of the quarry and elsewhere in other localities of NGB. The metapelite contains an assemblage of garnet, sillimanite, and hercynite with plagioclase, K-feldspar, biotite and quartz. Fine-grained pyroxene granulite bands occur as thin concordant dykes and seams within the charnockite, and range in width from 20 to 60 cm. The Arakkakulam quarry (Fig. 1) is located 10 km north of Kanya Kumari along NH-47, a hillock on the western side of the road where an active quarry of 200 m width and nearly 50 m height exposes massive

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

133

Fig. 1. The NGB showing distribution of charnockites and locations of present study mentioned in the text. Inset shows the tectonic framework of southern India and major granulite blocks.

charnockites. The charnockite lacks any visible foliation, and is mostly garnet-free. A 12 m wide calc-silicate enclave occurs within the charnockite at the northern part of the quarry. The calc-silicate rock is coarse grained and contains wollastonite, scapolite, and diopside with minor calcite, quartz and plagioclase. The enclave also contains 2 –5 cm thick veins of wollastonite. Adjacent

to the calc-silicate enclave, the charnockite shows weak foliation and presence of sporadic garnet. A chain of quarries in the Kozhikotpothai village (Fig. 1) exposes charnockites and associated rocks. In the southern part of the area, the dominant rock is garnetfree massive charnockite in the absence of any visible foliation. Pyroxene granulite bands varying in width from

134

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

20 cm to 5 m occur within the charnockite, with a biotite-rich zone developing at the contact with charnockite. Two kilometre north of this location, along the same chain of hills, NW –SE trending quarries expose fresh rocks for over 300 m. Charnockite here is mediumto coarse-grained and massive, and contains garnet in some portions. Thin bands of garnet bearing felsic pegmatite occur within the charnockite. These leucosomic portions are rich in garnet, sometimes with grains ranging up to 3 cm in size. Charnockite adjacent to these leucosomes contains garnet. Pyroxene granulite bands up to 3 m width occur as near-horizontal sheets within the charnockite. An unmetamorphosed mafic dyke is exposed in this quarry which cuts across the charnockite and was traced for more than 200 m along ENE – WSW direction. The dyke has a near vertical dip and ranges in width from 20 to 40 cm, with a conspicuous dark and finegrained chilled margin (ca. 2 mm thick) on both sides of its contact with the charnockite. About 27 km north of Kanya Kumari along NH-7, a 4 km track to the west leads to a chain of quarries in the foothills of Kottorpotti, near the Anna Nagar settlement. Here, the inter-relationship among massive charnockite, garnet –biotite gneiss, pyroxene granulites, pegmatites and incipient charnockites is best exposed. The charnockite in all the quarries is medium- to coarse-grained, garnet bearing, and shows weak to moderate foliation. Pyroxene granulite bands of average 30 cm thickness occur subconcordantly within the charnockite. A number of pegmatites traverse the charnockite, broadly divisible into two types. Type I pegmatite contains very coarse Kfeldspar and biotite with large garnet grains at places, and carry spectacular incipient charnockite structures. The incipient charnockite formation is generally seen at the core portion of the pegmatites, with the development of coarse orthopyroxene-bearing greenish patches ranging in size from 1 to 30 cm. Incipient charnockite patches within the pegmatite are clearly restricted to zones rich in garnet. Where the charnockite patch forms, the garnet grain is partly to totally resorbed. Type II pegmatite cuts across the host (foliated) charnockite generating a ‘decharnockitised’ (bleached) zone along its margins within the charnockite, which in some places extend over a width of 10 m. These pegmatites contain an assemblage of quartz, feldspar and biotite. The bleached zone is light coloured, orthopyroxene absent, and rich in biotite. A network of inter-connected bleached zones follows the branching pegmatite veins. These pegmatites carry coarse flakes of graphite at places. Field studies in NGB reveal that the dominant lithological unit is massive charnockite with common presence of medium to coarse crystals of orthopyroxene. Apart from a weak foliation developed at places, the charnockite generally shows a massive texture. Garnet is present only in those areas where the charnockite encloses supracrustal lithologies, and shows pronounced foliation. This is in

contrast to the garnet- and graphite-bearing charnockites (paracharnockites) in the adjacent Trivandrum Granulite Block.

3. Sample preparation and analytical techniques Thin sections for optical and electron microprobe studies and wafers for fluid inclusion analysis were prepared from fresh rock samples collected from working quarries. Fluid inclusion wafers for optical, microthermometric and laser Raman microscopic studies were prepared from the same rock chips used for petrologic studies. Chemical analyses of minerals were performed on a JEOL SUPERPROBE JXA8600MX instrument housed at Kochi University. The analyses were performed at an accelerating voltage of 15 kV and 15 nA beam current with peak-counting times of 20 s for both major and minor elements. Instrument calibration was performed on natural and synthetic standards. The matrix corrections were done using an oxide ZAF-correction programme supplied by JEOL. Fluid inclusions were studied in doubly polished thin wafers ranging in thickness from 120 to 150 mm. Inclusion petrography involving the careful documentation of the nature of occurrence of inclusions, their distribution pattern, size, shape and phase categories was done under a petrologic microscope using varying magnifications and following procedures outlined in Roedder (1984), Touret (2001), and Van den Kerkhof and Hein (2001). The plates were initially observed under low magnification (generally 20 £ , 40 £ ) to precisely locate groups of fluid inclusions, arrays and fluid inclusion-rich zones and to characterise their distribution pattern. Subsequently, individual inclusions within groups or arrays were observed under high magnification (100 £ – 400 £ ) to record their phase composition at room temperature. The distribution pattern and phase composition of inclusions was carefully documented through a series of hand sketches and photographs. Fluid inclusion microthermometric analyses were done on a LINKAM THMS-600 Heating – Freezing stage attached to a Nikon petrologic microscope. The stage was calibrated using natural and synthetic standards of pure CO2 fluid inclusions. The precision and accuracy of measurements are ^ 0.1 8C for freezing runs. Laser Raman Microscopy (LRM) of fluid inclusions was performed using Jobin Yvon Inc. Model 64000 housed at Fukuoka University. Argon laser (514.5 nm stroke) was used, with 50 mW intensity and beam diameter of 1 mm. A Mitsutoyo long focus lens (100 £ ) objective was used for all measurements. Analyses were done using single scan spectra. Thirty seconds measurement time was given twice for each analysis and then the noises were reduced. Since organic materials give strong fluorescence and raise the background extremely, the samples were cleaned thoroughly before performing LRM.

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

4. Petrology and mineral chemistry Charnockites from NGB are generally massive in nature, although in some localities they show development of weak to moderate foliation. The index mineral orthopyroxene commonly occurs in all the charnockite localities examined, where it is found in association with plagioclase, K-feldspar and quartz (Fig. 2(a)). In garnet-bearing units orthopyroxene (up to 3 mm in size) occurs in equilibrium assemblage with garnet (0.1 – 5 mm) and plagioclase (0.4 –0.6 mm). Orthopyroxene also forms coronal rims (1 –0.3 mm) around partly digested, small (0.15 mm) relict gains of garnet (Fig. 2(b)), often in association with plagioclase (0.3 mm) and quartz. In massive charnockites, orthopyroxene occurs in close association with biotite laths (Fig. 2(c)). Relatively small

135

euhedral grains of orthopyroxene are often seen in association with ilmenite (Fig. 2(d)). The textures in the garnet-bearing charnockites are suggestive of orthopyroxene formation by the breakdown of garnet according to the decompression reaction: Garnet þ Biotite þ Quartz ¼ Orthopyroxene þ Plagioclase þ K-feldspar þ Ilmenite þ H2 O

ð1Þ

Similar reaction documented for the formation of orthopyroxene in charnockites of TGB (Santosh et al., 1990) proposes a reduction in H2O activity during the breakdown of garnet and biotite. In most cases, the orthopyroxene

Fig. 2. Thin section photomicrographs of NGB charnockites. (A) Orthopyroxene–K-feldspar–plagioclase–quartz assemblage in massive charnockite from NGB. (B) Corona of orthopyroxene and plagioclase surrounding relict garnet grain. (C) Orthopyroxene– biotite–quartz association. Orthopyroxene –ilmenite association biotite flake showing sutured margins and melting texture, associated with orthopyroxene, K-feldspar, plagioclase and quartz. Backscattered electron image of ilmenite grain showing Ti-rich margin. (A), (D) and (E) in crossed nicols; (B) and (C) in open nicols.

136

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

Table 1 Mineral chemistry data on orthopyroxene from NGB charnockite Arakkakulam (010906B2)

Kozhikotpothai (010905Cd1) Within corona

Discrete

Kottorpotti (010907Dc3)

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2 O

49.51 0.11 3.01 0.02 32.83 0.15 15.01 0.18 0.02 –

48.83 0.17 3.79 0.05 32.87 0.12 14.71 0.17 0.01 –

49.25 0.11 2.82 0.03 32.66 0.11 14.95 0.17 – –

49.08 0.18 3.94 0.05 31.76 0.05 14.70 0.23 – –

49.49 0.13 2.23 0.03 34.14 0.18 13.44 0.24 – –

49.26 0.09 2.20 – 35.19 0.22 13.77 0.17 – –

49.28 0.03 2.53 0.04 34.61 0.17 13.58 0.21 – –

48.95 0.09 2.83 0.05 34.92 0.15 13.18 0.22 0.02 –

48.58 0.17 1.21 0.07 39.52 0.42 8.92 0.31 – –

47.75 0.19 1.62 0.03 41.66 0.35 8.26 0.38 – –

48.37 0.20 1.15 – 41.72 0.38 8.90 0.31 0.01 –

Total

100.84

100.71

100.10

100.00

99.87

100.90

100.45

100.41

99.20

100.24

101.04

Cations on the basis of six oxygens Si 1.920 1.898 Ti 0.003 0.005 Al 0.138 0.173 Cr 0.001 0.001 Fe 1.065 1.068 Mn 0.005 0.004 Mg 0.868 0.852 Ca 0.008 0.007 Na 0.002 0.001 K – – Total

4.009

4.010

1.924 0.003 0.130 0.001 1.067 0.004 0.870 0.007 – –

1.909 0.005 0.181 0.002 1.033 0.002 0.853 0.009 – –

4.007

3.994

1.952 0.004 0.103 0.001 1.126 0.006 0.790 0.010 – – 3.992

grains are largely pristine and unaltered to hydrous minerals, suggesting the prevalence of anhydrous conditions. However, in some cases, orthopyroxene shows late alteration along cracks, with the appearance of tiny biotite flakes (0.8 mm) along cleavage traces. Orthopyroxenes in NGB charnockites have low Al2O3 contents (Table 1) in the range of 1.15 – 3.94 wt%. There is no major difference in Al2O3 content between discrete grains (2.52 –2.83) and those associated in corona texture (2.20 – 2.65). XMg (Mg/ Mg þ Fe) in the orthopyroxenes from different localities show notable variation (0.27 – 0.45), and plots in XMg versus Al and XMg versus Si (Fig. 3) demarcate two groups. Relatively higher Al content and XMg values are shown by orthopyroxene in the massive charnockites as compared to that from foliated varieties. An inverse relationship is also noted between Si and Al in the orthopyroxenes when plotted against XMg (Fig. 3). Garnets are mostly clear and occur as subhedral grains in association with plagioclase, quartz and orthopyroxene. In general, garnet grains in well-foliated charnockites are comparatively larger (3 – 5 mm) than those occurring in weakly foliated varieties. Small garnet grains form the core of coronal texture, surrounded by plagioclase and orthopyroxene. Relatively large garnet grains (2 – 5.5 mm) in weakly foliated charnockite enclose tiny (0.05 – 0.1 mm) biotite laths. Garnets adjacent to large biotite laths show sutured contact. Garnets in NGB charnockite are almandine-rich (Fig. 4), and show considerable chemical variation among

1.933 0.003 0.102

1.937 0.001 0.117 0.001 1.137 0.006 0.795 0.009

– 1.155 0.007 0.805 0.007 – –

– – 4.012

1.928 0.003 0.131 0.002 1.151 0.005 0.774 0.009 0.001 –

4.003

4.004

1.987 0.005 0.058 0.002 1.352 0.015 0.544 0.014 – – 3.977

1.955 0.006 0.078 0.001 1.426 0.012 0.504 0.017 – –

1.962 0.006 0.055 – 1.415 0.013 0.538 0.013 0.001 –

3.999

4.005

different localities (Table 2) ranging from Alm70Pyr26Gr4 to Alm81Pyr10Gr9. Individual grains do not show any marked core – rim compositional variation. Also there is no significant compositional difference between discrete garnet grains (Alm75Pyr20Gr5 – Alm77Pyr17Gr6) and those occurring within corona (Alm77Pyr17Gr6 – Alm79Pyr17Gr4). Garnet XMg values range from 0.12 to 0.27, with coronal garnets having relatively low values (0.18 – 0.19) as compared to discrete grains. Garnets in charnockites from the southern part of the area show higher XMg values. Although biotite is a common mineral in NGB charnockites, its nature and abundance varies among individual locations. In general, the foliated charnockites commonly contain biotite whereas biotite is rare in nonfoliated varieties. The rare biotites in the latter are euhedral, but those in the foliated varieties have irregular and sutured grain boundaries. In garnet-bearing charnockites, biotite shows breakdown textures with comb-like or serrated margins displayed by digested grains, within a mosaic of newly formed orthopyroxene, small grained garnet, quartz, K-feldspar and ilmenite (Fig. 2(e)). Biotites from NGB show elevated fluorine content (up to 3.68 wt%) (Table 3) which are markedly high when compared with their low XMg composition (0.56 –0.61). Plagioclase (0.3 –8 mm) in the charnockites occurs as subhedral laths showing lamellar twinning. In the massive and less foliated varieties it shows antiperthite texture (0.01 –0.5 mm). Plagioclase occurs both as discrete grains

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

Fig. 3. Compositional plots of orthopyroxene from NGB charnockites. (A) XMg versus Al. (B) XMg versus Si.

and in close association with orthopyroxene surrounding garnet grains within corona texture. An content in plagioclase (Table 4) ranges from 0.26 to 0.34 mol%. K-feldspar (0.3 – 4 mm) occurs as subhedral grains showing development of various perthitic textures ranging from small stringers (0.05) to coarser blebs (0.15 mm). In many cases, the volume ratio between perthitic component and host K-feldspar is almost 50:50, indicating mesoperthitic texture. The perthite lamellae vary in size from 1 to 30 mm. In non-foliated charnockite, the perthite stringers are mostly concentrated to the grain core of K-feldspar and the grain margins are perthite-free. The dominant opaque mineral in the NGB charnockite is ilmenite. Sometimes, ilmenite shows marginal to complete alteration to TiO2-enriched phase (Fig. 2(f)). Ilmenite is often associated with orthopyroxene and partly consumed biotite. The major accessory mineral is zircon, and large grains (4 mm) of zircon are found in

Fig. 4. Compositional plots of garnet from NGB charnockites.

137

138

Table 2 Mineral chemistry data on garnets of NGB charnockite Kottaram (KTR-C)

Arakkakulam (010906B2)

Kozhikotpothai (010905Cd1)

Kottorpotti (010907Dc3) Discrete

Core

Core

Rim

Rim

Core

Core

Rim

Rim

Core

Core

Rim

Rim

Core

Core

Rim

Rim

Core

Rim

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O

37.85 0.08 21.62 0.03 33.40 0.44 5.38 1.39 0.02 –

38.29 0.04 21.36 0.04 34.01 0.35 5.30 1.26 – –

38.67 0.03 21.97 0.03 32.66 0.43 5.14 1.47 0.01 –

37.92 0.49 21.25 0.02 33.48 0.42 5.23 1.07 0.02 –

37.85 0.08 21.62 0.03 33.40 0.44 5.38 1.39 0.02 –

38.29 0.04 21.36 0.04 34.01 0.35 5.30 1.26 – –

38.67 0.03 21.97 0.03 32.66 0.43 5.14 1.47 0.01 –

37.92 0.49 21.25 0.02 33.48 0.42 5.23 1.07 0.02 –

38.50 0.04 21.05 0.05 34.41 0.80 4.17 1.59 – –

37.72 0.04 21.29 0.08 35.75 0.73 4.28 1.68 – –

38.48 0.02 21.18 0.06 35.19 0.76 4.14 1.85 – –

38.47 0.02 21.67 0.01 34.59 0.69 4.34 1.73 0.01 –

38.41 0.01 21.43 – 33.74 0.64 5.21 1.55 0.01 –

38.73 – 21.46 0.08 34.20 0.59 4.15 1.93 0.01 –

39.13 0.07 21.60 0.01 34.02 0.53 5.11 1.97 – –

38.43 0.02 21.10 0.02 32.62 0.69 4.61 1.98 0.01 –

37.45 0.07 21.20 0.01 35.16 0.95 2.85 3.04 – –

37.86 0.04 21.30 – 34.19 0.81 2.78 3.22 0.04 –

Total

100.22

100.64

100.42

99.93

100.22

100.64

100.42

99.93

100.61

101.56

101.68

101.53

100.99

101.15

102.43

99.50

100.74

100.23

Cations on the basis of 12 oxygens Si 2.990 3.014 3.029 Ti 0.005 0.002 0.001 Al 2.013 1.982 2.028 Cr 0.002 0.003 0.002 Fe 2.206 2.239 2.139 Mn 0.029 0.023 0.029 Mg 0.634 0.622 0.600 Ca 0.118 0.106 0.123 Na 0.002 – 0.001 K – – – Total

7.999

7.991

7.954

3.003 0.029 1.983 0.001 2.217 0.028 0.617 0.091 0.003 – 7.974

2.990 0.005 2.013 0.002 2.206 0.029 0.634 0.118 0.002 – 7.999

3.014 0.002 1.982 0.003 2.239 0.023 0.622 0.106 – –

3.029 0.001 2.028 0.002 2.139 0.029 0.600 0.123 0.001 –

7.991

7.954

3.003 0.029 1.983 0.001 2.217 0.028 0.617 0.091 0.003 – 7.974

3.045 0.002 1.963 0.003 2.276 0.053 0.492 0.134 – –

2.979 0.003 1.982 0.005 2.361 0.049 0.504 0.143 0.003 –

7.969

8.027

3.024 0.001 1.961 0.004 2.313 0.050 0.484 0.156 – –

3.014 0.001 2.002 0.001 2.267 0.045 0.507 0.145 0.002 –

7.993

3.015 0.001 1.982

1.986 0.005 2.246 0.039 0.486 0.162 0.001

– 2.214 0.043 0.609 0.130 0.002 –

7.984

3.040

– 7.995

3.026 0.004 1.969 0.001 2.200 0.035 0.589 0.163

–

7.965

– – 7.986

3.052 0.001 1.975 0.002 2.166 0.047 0.546 0.169 0.002 – 7.960

2.990 0.004 1.995 0.001 2.348 0.064 0.339 0.260 – –

3.022 0.003 2.004 – 2.282 0.055 0.330 0.275 0.005 –

8.001

7.976

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

Within corona

Table 3 Mineral chemistry data on biotites from NGB charnockite Kottaram (KTR-C)

Arakkakulam (010906B2)

Kozhikotpothai (010905Cd1)

Kottorpotti

35.85 6.04 13.08 0.01 19.35 0.04 10.84 0.04 0.05 8.31 – –

35.81 6.01 13.03 0.04 18.78 – 11.01 – 0.04 8.65 – –

36.10 4.81 13.51 0.01 18.50 0.01 11.72 – 0.03 8.62 – –

36.37 5.68 13.44 – 15.28 0.03 13.67 1.00 0.14 8.62 – –

36.57 5.18 13.08 – 19.37 – 11.59 – 0.05 8.81 – –

36.89 5.07 13.18 0.06 19.52 0.07 11.73 0.01 0.07 8.70 – –

36.68 5.13 13.29 0.03 19.49 0.04 11.09 – 0.06 8.72 – –

37.45 4.97 13.14 – 20.08 – 11.54 – 0.06 8.32 – –

36.83 5.50 13.23 0.01 19.81 0.03 10.36 0.02 0.02 8.84 – –

37.25 3.58 13.52 0.07 17.90 0.01 14.04 – – 10.66 0.09 3.60

36.32 3.48 13.51 0.05 18.40 – 13.21 0.02 0.08 10.57 0.10 3.56

37.66 3.12 13.35 0.06 17.27 0.01 14.15 – 0.03 10.67 0.11 3.51

36.98 3.59 13.27 0.03 17.12 0.01 13.98 – 0.01 10.45 – 3.68

37.24 3.17 13.35 0.03 17.58 0.04 13.92 – 0.02 10.96 0.05 3.31

36.34 5.08 13.22 – 22.82 0.06 9.13 – 0.09 8.74 – –

36.98 3.83 13.19 – 22.28 0.03 9.67 0.02 0.09 8.48 – –

35.94 5.28 13.28 0.06 22.60 0.09 9.19 0.01 0.05 8.83 – –

36.28 5.06 13.10 – 22.51 0.02 9.08 – 0.08 8.90 – –

Total

93.62

93.36

93.31

94.22

94.65

95.30

94.54

95.58

94.64

99.21

97.81

98.46

97.57

98.27

95.47

94.56

95.31

95.03

Cations on the basis of 22 oxygens Si 5.567 5.571 5.600 Ti 0.705 0.703 0.561 Al 2.393 2.390 2.470 Cr 0.002 0.005 0.002 Fe 2.513 2.443 2.400 Mn 0.006 – 0.001 Mg 2.509 2.553 2.710 Ca 0.007 – – Na 0.014 0.011 0.010 K 1.646 1.716 1.705 Zn – – – F – – –

5.521 0.648 2.405 – 1.940 0.003 3.093 0.163 0.041 1.669 – –

5.618 0.598 2.369 – 2.489 – 2.653 – 0.016 1.728 – –

5.624 0.581 2.369 0.007 2.489 0.009 2.665 0.002 0.019 1.692 – –

5.638 0.592 3.638 0.592 2.408 0.004 2.505 0.005 0.541 – – –

5.682 0.567 2.349 – 2.548 – 2.610 – 0.019 1.609 – –

5.665 0.636 2.398 0.001 2.548 0.004 2.374 0.003 0.004 1.735 – –

5.587 0.403 2.391 0.008 2.245 0.001 3.139 – 0.001 2.040 0.010 1.710

5.555 0.400 2.436 0.006 2.354 – 3.011 0.004 0.023 2.063 0.011 1.722

5.668 0.354 2.369 0.007 2.174 0.001 3.175 – 0.008 2.048 0.012 1.672

5.622 0.410 2.378 0.004 2.176 0.002 3.169 – 0.002 2.026 – 1.768

5.633 0.360 2.381 0.003 2.224 0.006 3.139 – 0.005 2.116 0.006 1.582

5.626 0.591 2.412 – 2.954 0.007 2.108 – 0.026 1.727 – –

5.745 0.447 2.414 – 2.894 0.004 2.240 0.002 0.026 1.679 – –

5.579 0.616 2.430 0.007 2.934 0.012 2.126 0.002 0.014 1.748 – –

5.641 0.592 2.402 – 2.927 0.003 2.106 – 0.024 1.766 – –

Total

15.483

15.471

15.457

15.924

15.384

15.368

17.535

17.585

17.489

17.557

17.455

15.453

15.453

15.468

15.461

15.360

15.393

15.459

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Zn F

139

140

Table 4 Mineral chemistry data on feldspars and ilmenite from NGB charnockite Kottaram (KRT-C)

Arakkakulam (010906B2)

Kozhikotpothai (090105Cd1) Within corona

Discrete

Kottorpotti (010907Dc3)

Pl

Pl

Kf

Pl

Pl

Pl

Ilm

Ilm

Pl

Pl

Pl

Pl

Kf

Kf

Kf

Pl

Pl

Ilm

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O

61.07 0.04 23.73 – 0.23 – – 6.01 7.85 0.23

61.80 0.07 23.89 – – – – 6.18 7.06 0.60

60.42 0.01 24.06 0.03 0.04 0.01 – 6.36 7.51 0.35

65.12 0.07 18.23 0.03 – – – 0.06 0.33 14.51

62.04 0.05 23.37 0.02 0.01 0.05 – 5.31 8.14 0.35

61.78 0.03 23.30 – 0.07 0.02 – 5.38 8.31 0.32

62.03 – 23.58 0.01 0.04 – – 5.18 7.80 0.53

0.02 50.87 0.04 – 47.68 – 1.21 – – –

– 50.94 0.04 – 48.54 0.09 1.24 0.01 0.05 –

61.62 0.03 24.72 – 0.17 0.02 0.01 6.52 7.30 0.45

60.89 0.02 24.84 – 0.09 – – 7.01 7.24 0.43

62.07 0.03 24.68 – 0.12 0.04 – 6.33 7.56 0.30

61.65 – 24.57 – 0.06 0.02 – 6.50 7.11 0.45

67.19 – 19.08 0.01 0.04 0.04 – 0.21 2.79 11.26

66.42 0.03 18.91 0.03 0.05 0.01 – 0.18 1.81 12.18

65.58 0.04 18.65 – – – – 0.14 1.64 13.22

62.04 0.05 23.37 0.02 0.01 0.05 – 5.31 8.14 0.35

61.78 0.03 23.30 – 0.07 0.02 – 5.38 8.31 0.32

– 51.82 – 0.03 48.58 0.27 0.14 0.02 0.01 –

Total

99.15

99.60

98.79

98.37

99.36

99.28

99.25

99.82

100.91

100.84

100.51

101.11

100.36

100.60

99.63

99.26

99.33

99.21

100.86

and three 2.719 – 1.276 0.001 0.001 – – 0.307 0.655 0.020

oxygens for feldspars and ilmenite, respectively 3.024 2.766 2.757 2.766 – 0.003 0.002 0.001 – 0.970 0.998 1.228 1.227 1.239 0.001 0.001 0.001 – – – – – 0.003 0.002 1.011 – 0.002 0.001 – – – – – – 0.046 0.003 0.254 0.257 0.248 – 0.030 0.704 0.720 0.674 – 0.860 0.020 0.018 0.030 –

3.009 0.001 1.009 – – – – 0.007 0.146 0.774

2.766 0.002 1.228 0.001 – 0.002 – 0.254 0.704 0.020

2.757 0.001 1.227 – 0.003 0.001 – 0.257 0.720 0.018

–

4.945

4.976

4.984

Cations Si Ti Al Cr Fe Mn Mg Ca Na K Total

on the basis of eight 2.736 2.750 0.001 0.002 1.253 1.253 – – 0.008 – – – – – 0.289 0.295 0.682 0.609 0.013 0.034 4.983

4.943

4.979

4.919

4.977

4.987

4.962

2.028

–

2.716 0.001 1.284

0.002

0.308 0.624 0.025

0.333 0.622 0.025

0.298 0.643 0.017

0.308 0.610 0.025

0.010 0.243 0.644

3.017 0.001 1.013 0.001 0.002 – – 0.009 0.159 0.706

2.037

4.965

4.977

4.966

4.952

4.924

4.907

0.964 0.001 –

– 1.021 0.002 0.047

– –

2.697 0.001 1.297 –

0.006 0.001 –

2.725 0.001 1.277 –

0.003 – –

2.726 – 1.280 –

0.004 0.002 –

3.015 – 1.009 –

0.002 0.001 –

0.001 0.001 –

0.982 – 0.001 1.024 0.006 0.005 – – – 2.018

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

Pl

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

gneissic charnockite from Kottorpotti, in the northernmost part of the study area. Apatite (0.5 mm) also occurs commonly either as discrete grains or as inclusions within other minerals.

5. P –T estimates The charnockites of NGB offer equilibrium assemblages and mineral pairs that are suitable for the application of a number of mineral phase equilibria thermobarometers. In this study, we use the garnet – orthopyroxene and garnet – biotite pairs for thermometry and garnet – orthopyroxene – plagioclase – quartz assemblage for barometry. The compositions of garnets in equilibrium texture and reaction texture were used in separate computations to detect possible variations in P –T condition during exhumation. The co-existing garnet –orthopyroxene pairs provide one of the robust thermometers to estimate the equilibration of charnockite assemblage in NGB. We apply here the garnet – orthopyroxene thermometer of Lee and Ganguly (1988) which is based on experimental calibration of Fe – Mg exchange between garnet and orthopyroxene in the FMAS system at 20– 45 kbar and 975 – 1400 8C, and Sen and Battacharya (1984) method which is an empirical calibration based on thermodynamic data. At 5 kbar (average pressure estimate from NGB charnockites, see below), we obtain temperatures of 712 – 789 8C for Arakkakulam and 747 – 934 8C from Kottorpotti. From Kozhikotpothai, the discrete orthopyroxene grains yield values of 691 – 900 8C, while those associated with corona texture show 678 – 781 8C. The temperature ranges from garnet – orthopyroxene thermometry are fairly consistent and indicate charnockite formation at relatively higher temperatures as compared to the ca. 700 – 750 8C for charnockites in the adjacent TGB (Chacko et al., 1987; Santosh et al., 1990). Fe – Mg exchange between coexisting garnet and biotite has been used for the formulation of a number of geothermometers. Computations based on Bhattacharya et al. (1992), Perchuck and Lavrent’eva (1983)) and Thompson (1976) methods yield comparable results in the range of 648 –771 8C for Arakkakulam, 590 – 864 8C at Kottaram, 538 – 590 8C at Kozhikotpothai and 567– 646 8C at Kottorpotti (all temperatures computed at 5 kbar). The effect of retrograde Fe – Mg exchange (Frost and Chacko, 1989) is more pronounced in Kozhikotpothai and Kottorpotti in the northern part of the study area. Localities Arakkakulam and Kottaram in the southern part preserve higher temperatures. Charnockite barometry was performed using the equilibrium assemblage garnet – orthopyroxene – plagioclase – quartz, commonly present in the NGB charnockites. Perkins and Newton (1981) and Newton and Perkins (1982)

141

proposed two geobarometers based on thermodynamic modelling of the following reactions: Enstatite þ Anorthite ! Grossular þ Pyrope þ Quartz

ð2Þ

Diopside þ Anorthite ! Grossular þ Pyrope þ Quartz

ð3Þ

Geobarometric computations (at 800 8C) based on reaction (2) yield pressures in the range of 4.6 – 6.1 kbar at Arakkakulam. At Kozhikotpothai, computations using analyses from discrete orthopyroxene grains yield 4.3– 6.3 kbar, while those using orthopyroxene in corona texture yields 4.0 –5.7 kbar (Fig. 5). An average pressure estimate for NGB charnockites is inferred as 5 kbar. A drop in pressure by around 2 kbar is indicated by the pressure variation from ca. 6 to 4 kbar. This result is conformable with the decompression textures documented from petrologic studies.

6. Fluid inclusion studies 6.1. Inclusion petrography Fluid inclusions were observed in most of the minerals in the NGB rocks. Their size, distribution pattern and relative abundance varies among different minerals and different localities. In this study, fluid inclusions were recorded from apatite, garnet, feldspars and quartz. Other minerals such as orthopyroxene and tiny zircon grains also carry inclusions, but were not considered for detailed studies due to their small size and less abundance, which inhibit precise observations. Representative photographs and sketches of the distribution pattern of inclusions and their phase categories are given in Figs. 6 –8. Garnet in the charnockite samples is mostly clear and free of mineral inclusions. In some garnet grains we observed scattered and elongate ‘melt inclusions’ with a number of tiny solid phases, brownish glassy melt and dark fluid-like phase. These inclusions could suggest entrapment of a melt phase from which the garnets crystallised, or partial melts that infiltrated the rock at some stage. This category of inclusion is rare, and not considered further in this study. Garnet grains that are in close association with orthopyroxene and quartz commonly contain fluid inclusions distributed mostly towards the margins of the crystals. This could reflect a relationship between fluid inclusion entrapment and grain margin dissolution or recrystallisation of garnet. The fluid inclusions in garnet occur in two patterns. The first category is seen as azonal groups or clusters having elongate, ovoid, rounded or irregular vermicular cavities with size ranging from less than 10 mm to more than 50 mm. Some of the irregular inclusions show inter-connected pattern and define a large network of inclusion cavities. The second category is arraybound that cuts across the earlier generation of azonal inclusions. These arrays terminate within individual grains

142

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

Fig. 5. Pressure–temperature diagrams for Arakkakulam (A) and Kozhikotpothai (B). The limits of garnet– orthopyroxene (Grt–Opx) garnet–biotite (Grt–Bt) thermometry, and garnet– orthopyroxene–plagioclase–quartz (G– O–P –Q) barometry were derived from experimental and empirical calibrations referred in the text. In (B), pressure–temperature computations from discrete orthopyroxene grains and those associated with coronal textures are separately shown.

and do not transect grain boundaries. They are mostly rounded or ovoid, with an average size of 10 mm. Feldspars show abundant fluid inclusions distributed mostly as scattered groups or rarely along arrays. The inclusions in feldspars are characterised by rectangular cavities. In perthitic K-feldspar, the inclusions are oriented parallel to the long axis of the crystal, and distributed within the host K-feldspar in between perthite lamellae. These inclusions are relatively large, ranging up to 30 mm. Quartz from all the localities contain abundant fluid inclusions distributed in various patterns. Quartz in textural association with garnet, orthopyroxene and K-feldspar contains groups of azonal inclusions that show diamondshaped (negative-crystal), rounded, elongate or irregular cavities ranging in size from less than 5 mm up to 40 mm. Arrays of inclusions that pinch out within individual grains and late arrays that transect grain boundaries are also seen. Some apatite grains contain fluid inclusions that are azonally distributed within the crystal. These inclusions show ovoid, rounded or elongated cavities with an average

size of 10 mm. Some tiny arrays of inclusions also occur within this mineral, but are too small to observe. We adopt here a classification scheme of inclusions into various groups which combine information from distribution pattern and phase composition (discussed later), in order to discriminate the different generations of fluid inclusions in the NGB rocks (Table 5). This classification follows careful textural observations of individual, group and arrays of inclusions in the different minerals and their relative timing of entrapment as inferred from textural criteria. Group I: Group I inclusions are restricted to quartz and K-feldspar. These are bi-phase at room temperature (ca. 24 8C) and comprise a non-aqueous phase (CO2, see below) and an immiscible aqueous phase (H2O). These mixed CO2 – H2O inclusions are mostly present in feldspars and rarely in quartz, and have relatively large cavities (up to 30 mm). The H2O phase in these inclusions occupies an average of 20 volume percent of the inclusion cavity. Group I inclusions in feldspar have typical rectangular cavities and are mostly aligned

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

143

Fig. 6. Photomicrographs of fluid inclusions in NGB charnockites. Fluid inclusions showing rectangular cavities in feldspar from Arakkakulam. Oval, rounded and diamond-shaped inclusions in quartz from Kottaram. Oval and rounded inclusions in garnet from Kottaram. A pure CO2 fluid inclusion in feldspar from Kottorpotti. CO2 inclusions (shown by arrows) in apatite crystal from Kottorpotti. CO2 –CH4 –N2 inclusions in quartz from Kozhikotpothai.

parallel to the long axis of the crystal. In quartz, they occur as isolated groups or short inter-grain trails having round, oval, tabular or irregular cavities. Some of these inclusions, such as those from Arakkakulam have 30 volume percent of H2O. Group II: These are essentially monophase CO2-rich (see below) inclusions occurring as azonal groups or isolated clusters. They are found in almost all minerals from the different localities and constitute the most dominant category of inclusions in NGB. From textural criteria, they constitute the earliest generation of monophase inclusions. The inclusions range in size from 5 to 30 mm and show ovoid, rounded and diamond shapes. Group III: This group of monophase inclusions is found only in garnet and quartz. They form inter-grain trails that pinch out within individual grains, and sometimes cut across

clusters of Group II inclusions. They are relatively small (5 – 15 mm) and texturally, their entrapment post-dates Group II. Group IV: This group of monophase inclusions are restricted to garnet and quartz from only two localities (Arakkakulam and Kozhikotpothai) and are texturally the youngest generation of inclusions. They form arrays, which transgress grain boundaries and comprise inclusions of various sizes (less than 5 – 30 mm) and shapes (tabular, irregular, elongate, rounded, ovoid). At room temperature (ca. 24 8C), some of them contain a bubble (gaseous CO2), which homogenises on slight warming (see Section 6.4). 6.2. Melting experiments The melting and homogenisation data of the various groups of fluid inclusions in different minerals from the four

144

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

Fig. 7. Detailed sketch showing Group I (shaded) and Group IV (open) inclusions in quartz from Arakkakulam. The numerical values shown against inclusions are their homogenisation temperatures. Inset shows the area where the inclusions were observed.

localities are compiled in Table 5. Computed bulk densities and mole fraction of H2O in Group I inclusions are also given in the table. On freezing down to temperatures of 2 80 to 2 100 8C, the inclusions froze to a solid aggregate. Melting was observed during slow warming and was characterised by the sudden dissolution of the solid aggregate and instantaneous appearance of bubble, where solid, liquid and vapour coexist at the triple point. The rate of heating was kept low, at 0.1 8C/min to sharply constrain the phase transition and to record the temperature with utmost precision. Melting temperature data, compiled in histograms, are presented in Fig. 9. Group I mixed CO2 – H2O inclusions in K-feldspar show peak CO2 melting temperatures of 2 57.2 to 2 58.0 8C. The slight depression in the melting temperatures from that of pure CO2 (2 56.6 8C) may suggest traces of other nonaqueous volatiles such as CH4 and or N2. A similar slight melting point depression is also observed for Group I inclusions in quartz from Kottorpotti with a peak melting at 2 57.6 8C. However, CO2 in group I inclusions from Arakkakulam quartz show near-pure CO2 with peak melting at 2 56.8 8C. In mixed CO2 –H2O inclusions, CO2 being a non-polar liquid, non-aqueous phases such as CH4 and/or N2 if present, would preferentially enter this non-aqueous phase whilst NaCl and other soluble salts would strongly partition into the aqueous phase. Clathrate melting experiments were

performed in a few representative Group I CO2 – H2O inclusions. The temperature of final clathrate melting was inferred from the sudden disappearance of the angularity of the vapour bubble and its free movement within liquid CO2. In most cases, these temperatures were between 6 and 8 8C, suggesting low salt concentration in the aqueous phase. Group II monophase carbonic inclusions are present in all minerals, and constitutes the most abundant category in NGB charnockites. In apatite, these inclusions show peak melting at 2 59.4 8C, where as in garnet the peak melting temperatures lies between 2 56.8 and 2 58.1 8C. Group II inclusions in K-feldspar are restricted to a single locality at Kottorpotti where they show peak melting at 2 59.9 8C. Quartz from all localities show Group II inclusions with peak melting in the range of 2 56.7 to 2 58.1 8C, although in majority of the cases, the melting temperatures are below 2 57.0 8C and close to the triple point for pure CO2. Group III monophase inclusions occur in garnet and quartz. The peak melting temperatures of garnet-bound inclusions lie between 2 57.8 and 2 58.6 8C. In quartz, peak melting of the CO2-rich phase occurs between 2 56.6 and 2 59.9 8C. Group III inclusions thus show a spectrum from pure CO2 to dominantly CO2 with traces of other volatiles that depress the melting point. The last category, Group IV inclusions are restricted to quartz and garnet from Arakkakulam and quartz from

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

145

Fig. 8. Detailed sketch of fluid inclusions in feldspar from Arakkakulam. Homogenisation temperatures are also shown. Inset shows the area where the inclusions were observed.

Kozhikotpothai. At Arakkakulam, these inclusions are almost pure CO2 with peak melting temperatures of 2 56.6 8C for those in quartz and 2 56.7 8C for those in garnet. Quartz-bound Group IV inclusions in Kozhikotpothai show slight melting point depression, with a peak at 2 57.4 8C. The melting temperatures of carbonic phase in inclusions belonging to different categories within various minerals indicate a CO2-rich composition for the dominant fluid phase in NGB charnockites, with some amount of H2O in the early fluid as indicated by Group I inclusions. No systematic variation is observed in fluid composition either among different groups of inclusions or among inclusions in different minerals. This might suggest that the slight compositional variations as displayed in the depression of CO2 melting temperatures could be due to local controls which include post-entrapment modifications or local capture of heterogeneous fluids. The melting point depression observed in most cases is up to 2 8C below the triple point for pure CO2. From the homogenisation temperatures of these inclusions (discussed below), it can be estimated based on the thermodynamic data and experimental curves of Thiery et al. (1994) that CH4 or N2, if present, would be less than 10 mol%. The exceptions are melting point depression up to 2 61.5 8C in the case of

Group II inclusions within K-feldspar from Kottorpotti and 2 59.9 8C in Group III inclusions from quartz from Kozhikotpothai. Thiery’s et al. (1994) curves do not intersect for the observed homogenisation temperatures of these inclusions, and we have to assume that the depression in melting temperatures is caused by variable proportions of impurities such as CH4 and/or N2. The presence of both these volatiles was confirmed in some inclusions by Raman spectroscopy as discussed in Section 6.3. 6.3. Laser Raman microscopy LRM is used to define micron-sized unknown objects and molecular structures. The laser is guided to the object by an objective lens of a microscope. Raman scattering is focused into the objective and led to the CCD detector. A remarkable merit for the LRM is that it is capable for all commonbonded materials, and the technique offers a powerful tool for identifying and estimating the composition of fluids trapped within inclusions (Burke, 2001). Results from LRM studies on fluid inclusions in the NGB charnockites are shown in Figs. 10– 12. The figures compile information on the location of the mineral within the rock assemblage, the location of fluid inclusions within the different minerals, and the specific inclusions that were

146

Table 5 Fluid inclusion data on charnockite from the NGB

Arakkakulam

Sample number

Mineral

Kottorpotti

H2O mole fraction

Min

Max

Peak

Min

Max

Peak

Min

Max

Peak

Min

Max

Peak

258.60 256.70 256.80 256.70 256.60 256.60 257.20

20.70 27.50 8.30 5.70 16.20 28.50 11.70

23.30 29.70 15.00 15.50 23.90 30.10 19.00

22.20 28.80 13.20 8.30 21.80 29.20 14.20

0.74 0.61 0.88 0.82 0.73 0.59 0.83

0.77 0.67 0.92 0.89 0.81 0.64 0.88

0.75 0.64 0.89 0.87 0.75 0.62 0.87

– – 0.523 – – – 0.400

– – 0.538 – – – 0.419

– – 0.534 – – – 0.406

Group II Group III Group II Group III

– – – –

257.00 259.60 256.80 256.80

256.60 256.70 256.60 256.70

256.80 257.80 256.80 256.80

6.50 12.90 11.00 15.20

12.00 18.80 12.90 24.10

10.40 16.30 12.10 19.60

0.85 0.79 0.84 0.72

0.89 0.84 0.85 0.82

0.86 0.81 0.85 0.78

– – – –

– – – –

– – – –

Feldspar

Group II Group III Group IV Group I

– – – 20

259.90 259.90 257.60 258.00

257.00 259.90 257.40 257.50

257.00 259.90 257.40 258.00

11.60 20.00 24.70 9.00

15.30 20.00 29.10 13.40

13.60 20.00 25.10 12.00

0.82 0.77 0.63 0.87

0.85 0.77 0.72 0.90

0.83 0.77 0.71 0.88

– – – 0.395

– – – 0.404

– – – 0.401

Garnet Quartz Feldspar Apatite

Group II Group I Group II Group II

– 20 – –

258.80 259.90 261.50 259.40

258.10 256.70 258.70 259.60

258.10 257.60 259.90 259.40

8.40 4.00 7.90 10.40

14.90 28.60 14.60 13.40

11.60 12.40 10.50 11.00

0.82 0.71 0.83 0.84

0.88 0.93 0.88 0.86

0.85 0.88 0.86 0.85

– 0.386 – –

– 0.411 – –

– 0.402 – –

Quartz

010907DC3

Bulk density (g/cm3)

258.60 256.70 256.80 256.60 256.60 256.60 257.20

AK-C

010905CD1

Th (8C)

258.60 256.70 256.80 258.00 257.20 256.80 257.40

Garnet Quartz

Kozhikotpothai

Tm (8C)

– – 30 – – – 20

Garnet

KTR-C

H2O content, %vol. (approx.)

Group III Group IV Group I Group II Group III Group IV Group I

010906B2

Feldspar Kottaram

Inclusion type

Quartz

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

Locality

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

147

Fig. 9. Histograms assembling melting temperatures of carbonic inclusions in garnet (A), K-feldspar (B), quartz (C) and apatite (D) from NGB charnockites. Abbreviations for localities are: KTR, Kottaram; AK, Arakkakulam; KOT, Kottorpotti; KP, Kozhikkotpothai.

Fig. 10. Laser Raman data on fluid inclusions in garnet and feldspar from Arakkakulam charnockite in NGB. Location of mineral, analysed inclusion (marked by cross) and corresponding Raman spectra for quartz (A, B, C) and feldspar (D, E, F) are shown.

148

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

Fig. 11. Laser Raman data on fluid inclusions in quartz from Kottaram (A, B, C) and apatite from Kottorpotti (D, E, F).

analysed, along with the respective spectroscopic data. Although several LRM analyses were performed, only the representative Raman spectra are presented in the figures. The compiled information on the Raman shift and its implication on fluid species are given in Table 6. LRM analyses show distinct Raman shift at 1382– 1386 cm21 in most of the analysed inclusions, irrespective of the host mineral such as quartz, garnet, feldspar and apatite. This prominent and common peak corresponds to CO2. Peaks for CO2 were also obtained at range of 1279 – 1282 cm21 for inclusions in the different minerals. LRM studies confirm that the dominant fluid trapped within inclusions in NGB charnockite is pure CO2. In order to evaluate the compositional control on melting point depressions observed in some inclusions, we selected

Group III inclusions in quartz from Kozhikotpothai which show marked depression in melting point. LRM studies of these inclusions (Fig. 12) how clear Raman shifts for N2 and CH4 at 2327.9 and 2916 cm21, respectively (Fig. 12). Similar results were also obtained from Group II inclusions in K-feldspar from Kottorpotti. The same inclusions also yielded sharp peaks for CO2. Thus, we confirm the inference made from microthermometry that depression in melting temperatures in some groups of inclusions are caused by the presence of both CH4 and N2. 6.4. Homogenisation studies Homogenisation studies were made by continued warming of the inclusions after observing their melting behaviour

Fig. 12. Laser Raman data on fluid inclusions in quartz from Kozhikkotpothai. (A) shows sketch of the analysed grain. (B) and (C) show the analysed inclusions (marked by cross), and (D) and (E) represent the corresponding Raman spectra.

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155 Table 6 Data from Laser Raman Microscopic study on fluid inclusions in various minerals in NGB charnockites Locality

Sample number Host mineral Peaks (cm21)

Arakkakulam

010906B2

Fluid species

Garnet Garnet Feldspar Feldspar Feldspar

a 1281 1281 1282 1282 1280

b 1385 1386 1386 1386 1382

A CO2 CO2 CO2 CO2 CO2

B – – – – –

Garnet Garnet Garnet Quartz Quartz Quartz Feldspar Feldspar Feldspar

1281 1280 1288 1279 1279 1279 1279 1280 1279

1386 1385 1385 1384 1384 1384 1383 1384 1384

CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2

– – – – – – – – –

Kozhikotpothai 010905Cd1

Quartz Quartz Quartz Quartz Quartz Feldspar Feldspar

2328 2328 2327 1293 1283 1280 1281

2916 2916 2916 1386 1387 1385 1385

N2 N2 N2 CO2 CO2 CO2 CO2

CH4 CH4 CH4 – – – –

Kottorpotti

Apatite Feldspar Feldspar Quartz Quartz

1080 1280 1280 1281 1281

1385 1385 1385 1385 1383

CO2 CO2 CO2 CO2 CO2

– – – – –

AK–C

Kottaram

KTR-C

010907Dc3

at low temperatures. The homogenisation of carbonic fluid in all the groups of inclusions occurred into liquid phase, with gaseous CO2 bubble shrinking progressively and dissolving into the liquid CO2 phase. No vapour homogenisation or critical homogenisation was observed, even in those inclusions that showed homogenisation rather close to the critical temperature of CO2 (see below). The homogenisation data are compiled in histograms in Fig. 13. The CO2 in Group I mixed CO2 –H2O inclusions within feldspar shows peak homogenisation between 13.4 and 14.2 8C. Homogenisation temperatures in some of the inclusions are lower, down to 9 8C. Group I inclusions in quartz homogenise between 12.4 and 13.2 8C, with lowest homogenisation at 4 8C. Among Group II inclusions, the peak homogenisation occurs in the range of 10.4– 13.4 8C in apatite, 10.4– 11.6 8C in garnet, 10.5 8C in K-feldspar and 8.3 –12.1 8C in quartz. The lowest homogenisation at 5.7 8C was recorded from inclusions in quartz at Arakkakulam. Group III inclusions in garnet show peak homogenisation at 16.3– 22.2 8C and those in quartz homogenise at 19.6 –21.8 8C. The lowest homogenisation in Group III is recorded by inclusions in garnet from Kottaram, at 12.9 8C. Group IV inclusions show peak homogenisation at 28.8 8C in garnet and 25.1– 29.2 8C in quartz, with lowest value at 24.7 8C in quartz from Kozhikotpothai.

149

Table 5 also shows the bulk densities of the inclusions as computed from homogenisation data. Clearly, Group I inclusions show the highest densities among all varieties with quartz from Arakkakulam and Kottorpotti yielding 0.92 – 0.93 g/cm 3. Among the monophase carbonic inclusions, Group II shows the highest density (up to 0.92 g/cm3), comparable to Group I inclusions. Group III inclusions have moderate densities (maximum 0.84 g/cm3) and Group IV inclusions have low density fluids (0.59 – 0.72 g/cm3). Thus, a systematic variation in densities from higher to lower values is observed from Group I to Group IV. However, no systematic density variation exists in fluids trapped in the different minerals. 6.5. Fluid entrapment Knowledge on the composition and density of a fluid constrain it to lie along an isochore in P – T space. The isochores for the various groups of inclusions in individual minerals from the four localities are compiled in Fig. 14. For computations, we have used the highest density values from each group of inclusions. Isochores for Group I inclusions were computed by considering the mole fraction of H2O also, and the curves represent CO2 –H2O isochores which notably possess the steepest slope in P – T space among the various categories. Thus, at a temperature of 800 8C (average estimate for NGB rocks from mineral phase equilibria), Group I inclusions indicate entrapment pressures up to 6.8 kbar. The range of entrapment pressures for Group I inclusions lie between 4.7 and 6.3 kbar (with an exception of 3.4 kbar for the group in quartz from Kottorpotti). The entrapment pressures of Group I inclusions are thus consistent with the pressure estimates from mineral phase equilibria. Isochores for Group II inclusions show entrapment pressures in the range of 3.8 –4.2 kbar at 800 8C. The pressure estimates for Group III inclusions lie between 3.0 and 3.5 kbar, while those for Group IV inclusions show 2.1 – 2.6 kbar. If we consider that the very low-density inclusions belonging to Group IV were trapped at relatively lower temperature conditions (600 8C, from garnet – biotite thermometry), then the entrapment pressure comes down to 2 kbar or even less, indicating that Group IV inclusions represent fluids in the late stage of exhumation. The isochore diagrams for different groups of inclusions in various minerals indicate that quartz from Arakkakulam has the highest density fluids, with steep slopes. The same quartz sample also contains inclusions of later generation carrying the lowest density CO2 in the rocks examined in this study. Garnet and feldspar have moderate to highdensity fluids, which are also represented in quartz and apatite. Low-density fluids are represented in all minerals. Apatite, garnet and feldspar show a relatively narrow spectrum in fluid densities among the various inclusion types, whereas quartz shows a large spectrum of densities. In a locality-wise comparison, Arakkakulam shows

150

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

Fig. 13. Histograms showing homogenisation temperatures of carbonic inclusions in garnet (A), K-feldspar (B), quartz (C) and apatite (D) from NGB charnockites. Abbreviations for localities are: KTR, Kottaram; AK, Arakkakulam; KOT, Kottorpotti; KP, Kozhikkotpothai.

the highest spread in isochores and Kottaram shows the least spread. The other two localities, Kozhikotpothai and Kottorpotti have similar isochore slopes. However, the density values in majority of cases are comparable, yielding comparable isochores and pressure estimates in all the four localities. We interpret this feature to indicate that charnockites of NGB experienced a common fluid regime.

7. Discussion 7.1. P –T evolution The ubiquitous association of orthopyroxene in nearpristine state within charnockites of NGB suggests that these rocks equilibrated in a predominantly anhydrous environment, at relatively high temperatures (upper limit . 900 8C). In garnet-bearing localities, the textural evidence supports garnet breakdown to produce orthopyroxene, a feature which is commonly recorded from a number of localities in the adjacent Trivandrum Granulite Block (Santosh, 1987; Santosh et al., 1990). The coronal rims of orthopyroxene around garnet indicate that orthopyroxene formation occurred under a decompressional regime.

Pressure estimates indicate a drop from . 6 to , 4 kbar within a short interval of temperature. The lowest range of pressures is obtained from the garnet – orthopyroxene coronal assemblages. This drop in pressure indicates a rapid and virtually isothermal exhumation history. We shall address this aspect further in a Section 7.2 in combination with the fluid characteristics. The scarcity of biotite in non-foliated or weakly foliated massive type charnockites in NGB that carry pristine orthopyroxene grains (in which garnets are also rare or absent) further confirm the dry nature of the rocks. It has been proposed that charnockite massifs of southern India, such as NGB, originated as dry magmas that transported CO2 to effect incipient charnockite alterations in adjacent supracrustal domains (Chacko et al., 1996). Although data presented in our study are inadequate to test this hypothesis, the massive charnockite and pyroxene granulite suite of rocks in NGB might represent an original calc-alkaline magmatic suite that was subjected to granulite facies overprinting during the late Pan-African. Whether the abundant CO2-rich fluids in the charnockites recorded in present study preserve fluids inherited from original magmatic sources, or introduced during metamorphism remains to be evaluated in future studies.

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

151

Fig. 14. Isochores for groups of highest density fluid inclusions in different minerals from NGB localities. The P –T boxes for each locality are also shown.

7.2. Fluid evolution The common presence of CO2-rich fluid inclusions in most of the minerals in NGB rocks and their entrapment close to peak P – T conditions suggest that the charnockites equilibrated in a CO2-rich regime. Importantly, water-rich or brine-rich inclusions were not observed in any of the samples, although the possibility of tiny aqueous inclusions in some late arrays is not ruled out. Such arrays were not examined due to their small size and their relative scarcity. Visible H2O was documented only as an immiscible component in Group I inclusions. The dominant category is represented by monophase carbonic inclusions with moderate to high densities represented by Group II category. Melting temperatures of these inclusions lie mostly around 2 56.6 8C, denoting a near-pure CO2 composition. Laser Raman studies confirm the pure CO2 composition. The isochores for these inclusions show entrapment pressures, which are closely comparable with those from mineral phase equilibria at peak temperatures. This is a strong evidence to infer that Group II inclusions preserve the trace of charnockiteforming fluids. Presence of monophase carbonic inclusions with lower densities represented by Groups III and IV inclusions

indicate that the CO2-rich fluids were active even to lower temperatures and pressures. A generally dry environment was thus maintained during most part of the thermal event, which is also evident from the lack of any significant hydrous alterations in these rocks. Group I CO2 – H2O inclusions yield isochores with the steepest slopes and hence the highest entrapment pressures. These inclusions are not present in garnet and apatite, but occur as early groups in K-feldspar and quartz. If these inclusions represent the earliest fluid in the NGB rocks, then the presence of an aqueous phase, in addition to CO2, during the early high temperature stage is indicated. This observation is also supported by the occurrence of the hydrous mineral biotite as an early mineral in the massive charnockite. The high fluorine content in these biotites and the characteristic mesoperthitic nature of K-feldspar in the charnockites are in accordance with the high temperature history of the rock. Group I bi-phase inclusions are also found in K-feldspar. Based on these observations, we infer that Group I inclusions represent the earliest fluid phase, characterised by CO2 and minor amounts of H2O. Whether they represent original magmatic fluids associated with the ‘emplacement’ of charnockite remains uncertain, although by analogy with the common presence of CO2 – H2O rich fluids in the Pan-African magmatic suite in southern India

152

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

(Santosh et al., 2001), it is possible to envisage charnockitic magmas rich in CO2 þ H2O. The presence of a melt phase at high temperatures is also indicated by the occurrence of melt inclusions in some garnet grains, representing melt infiltration at high temperature stage. Both Group I and Group II inclusions contain pure CO2. The aqueous component in Group I was either originally present in the fluid system, or was introduced into the system. The H2O in the fluid system was subsequently consumed during the crystallisation of hydrous phases, leading to the prevalence of anhydrous fluids in later generations of inclusions belongs to Group II – IV. The absence of Group I inclusions in early minerals such as apatite might suggest that the aqueous phase, which was preferentially entrapped in feldspar and quartz, was of limited extent. The lack of Group I inclusions in garnet, and the presence of melt inclusions in some garnet grains is not clearly understood. It is possible that the garnet in the charnockite is an early product of high temperature interaction of the charnockite magma with the aluminous supracrustals, and the melt inclusions hosted by garnet correspond to the infiltration of high temperature melts. Garnet was subsequently involved in decompression reaction, as indicated by coronal orthopyroxene. The grain margins in contact with orthopyroxene, as well as adjacent quartz grains, commonly contain CO2rich inclusions of Groups II –IV, attesting to the role of CO2 in stabilising the dry mineral assemblage. Locally, the fluid composition was altered by additional volatile species as indicated by melting point depression in some inclusions. These anhydrous dilutants are mostly N2 and CH4 as revealed from Raman laser analyses of representative inclusions. Previous fluid inclusion data for charnockites in the adjacent TGB (Klatt et al., 1988; Santosh et al., 1991a) have shown that nitrogen is the dominant impurity in these inclusions (up to 14 mol%) and methane is rarely present, and that too only in negligible quantity (about 1 mol%). The high oxidation state of charnockites also preclude CH4 as a stable component (Santosh et al., 1990). A more detailed LRM study on the precise proportions of the dilutants is under way, which is expected to provide compositional information for quantitative modelling of fluid evolution in these rocks. Additional volatiles can result through either heterogeneous entrapment or fluid immiscibility from a complex CO2 – N2 – H2O (^ CH4) fluid system. This would result in permeation of different proportions of fluid mixtures in different zones. Post-trapping changes in the composition of the primary fluid as a result of the selective loss of its components can also lead to different fluid proportions within inclusions. A combination of such processes often operates in high-grade metamorphic terrains. Importantly, laser Raman data show that such ‘impure’ fluids are of local extent, and does not therefore alter the general inference of CO2 being the dominant fluid in NGB charnockites. We assume that these ‘impurities’ were introduced into the carbonic fluid through

interaction with supracrustal lithologies, for which there is common field evidence in NGB as described in Section 2. The timing of entrapment of fluids and fluid evolution in the NGB charnockites can be evaluated through a combination of data from mineral thermobarometry and fluid inclusions (Fig. 14). Peak P –T conditions derived from garnet – orthopyroxene thermometry and garnet –orthopyroxene –plagioclase barometry are plotted together with garnet – biotite thermometry limits and representative isochores for various groups of fluid inclusions in the different minerals from the four localities. In Arakkakulam, the peak P – T box is bound by Group I inclusions in quartz and feldspar and Group II in quartz, indicating that these inclusions entrapped traces of the peak metamorphic fluid. In fact, the steep isochore for Group I inclusions in quartz pass slightly above the P –T box for this locality. This is conceivable given the observation that orthopyroxene formed during decompression, in a CO2-rich environment, and the P – T window is defined from thermobarometery involving orthopyroxene. The temperature limits set by garnet – biotite thermometry in this locality is closely comparable with that of garnet – orthopyroxene, although the former shows a broader lower temperature limit, indicating continued Mg –Fe diffusion to lower temperatures, probably enhanced by the presence of fluids. The intersection of lower density isochores with this temperature limit without substantial change in temperature limit indicates near-isothermal decompression. In Kottaram, the isochores define a narrow region which passes through the peak P – T conditions of the rock estimated from mineral thermobarometry, yielding a more clear picture of CO2 entrapment during charnockite formation. At Kottorpotti also, isochores of Group I and Group II inclusions pass through the peak temperature window estimated from mineral phase equilibria. At Kozhikotpothai, it is possible to differentiate two P – T fields using discrete garnet and orthopyroxene grains (higher pressure) and the other employing partially consumed garnet grains mantled by orthopyroxene (lower pressure). The two P – T fields indicate a clear decompression trend. Isochores of Group I and Group II inclusions penetrate the upper and lower P – T boxes, further substantiating the ‘peak’ nature of CO2 and their role in the formation of orthopyroxene-bearing charnockites in NGB. The timing of entrapment of fluid inclusions in highgrade metamorphic rocks is often debated (Lamb et al., 1987; Santosh et al., 1991a). Although the concordance between fluid inclusion isochores and P – T condition estimated from mineral phase equilibria is an essential condition to infer ‘synmetamorphic’ nature of fluids and their capture at peak P – T conditions, this is in itself is not a conclusive evidence, since inclusions can be entrapped anywhere along the slope of the isochore in P– T space. Therefore, although the densities of carbonic inclusions in NGB charnockites are conformable with the P – T conditions

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

defined from mineral thermobarometry, additional evidence is required to confirm this inference. We address this problem by evaluating an example of extremely highdensity carbonic inclusions in a garnet-bearing massive charnockite below. A Late Archean (2.4 – 2.7 Ga) garnet-bearing charnockite from the Shevaroy Hills massif at the southern margin of the south Indian craton preserves fluid inclusions with the highest density CO2 yet reported from lower crustal rocks (Santosh and Tsunogae, 2003). The rock comprises garnet, clinopyroxene (salite), plagioclase, amphibole (pargasite), orthopyroxene (hypersthene), biotite, scapolite and quartz with accessory K-feldspar, ilmenite, apatite and ankerite. Pressure –temperature estimates of 9 –11 kbar and 740 – 800 8C are computed from electron microprobe data on equilibrium assemblages. The dominant category of inclusions trapped in garnet, feldspar and quartz from the granulite comprise monophase carbonic inclusions with melting temperatures close to 2 56.6 8C indicating a CO2rich composition for the fluid, further confirmed by laser Raman microscopic studies. The inclusions homogenise into the liquid phase at temperatures in the range of 2 55.3 to þ 9.4 8C. Primary CO2 inclusions in garnet show a sharply defined homogenisation temperature within the narrow range of 2 52.3 ^ 2.4 8C, translating into CO2 densities of 1.172 – 1.155 g/cm 3. While trail bound inclusions in garnet show densities of 1.137 –0.952 g/cm3, carbonic inclusions in plagioclase and quartz yield densities of 1.095 – 1.070 and 1.112 –1.086 g/cm3, respectively. The fluid densities when computed into isochores indicate entrapment of CO2 at peak pressure –temperature conditions estimated from mineral phase equilibria.

153

The above example of ‘ultrahigh density’ CO2 inclusions reveal that there is a close correspondence between density of the fluids and P –T estimates derived from mineral assemblages in rocks formed under high pressure conditions. In this case, the fluid inclusions show extremely high densities, consistent with entrapment at peak metamorphic conditions. In the case of NGB rocks, as well as those in the adjacent TGB, the relatively lower densities of CO2 fluids are in close correspondence with the lower pressure estimates obtained from mineral phase equilibria. A common post-peak fluid infiltration in the southern Indian terrain would not yield such systematic variations and patterns in fluid densities within rocks exhumed from various depths as observed above. The source of CO2 in NGB charnockites is speculative. Jackson (1990) analysed fluid inclusions in a series of mineral grains from the Kottaram quarry in NGB following a combination of stepped thermal extraction and visual decrepitation techniques described in Santosh et al. (1988). The results showed high content of CO2 (450 ppm) in the charnockite as compared to the adjacent gneiss (55 ppm) in this locality. Carbon isotope analyses of CO2 extracted from fluid inclusions in the charnockite yielded d 13C value of 2 7.8‰, correlating with ‘juvenile’ CO2 from subcontinental mantle source. Fluid inclusions in the metapelitic gneiss yielded lighter carbon-enriched values of 2 16.2‰, clearly indicating fluid mixing with crustal sources. These results suggest that the CO2-rich fluids in NGB charnockites may have been derived from sublithospheric sources, probably from mantle-derived magmas emplaced at depth, although further confirmation has to come from detailed stable

Fig. 15. Combined P –T diagram showing pressure–temperature ranges from mineral phase equilibria and isochores from fluid inclusions. The data indicate a T-convex exhumation path. See text for discussion.

154

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

isotopic investigations of fluid inclusions from the different localities. Additional evidence supporting a possible magmatic origin of the NGB charnockites is offered by oxygen isotope data from the Kottaram quarry (Santosh, 1996). An oxygen isotope traverse along the metapelite enclave – massive charnockite contact here shows d 18O values of þ 12‰ for the khondalite band, where as the massive charnockite away from the contact has a value of þ 8.9‰. The d 18O values decrease progressively while moving away from the metapelite into the charnockite, with the lowest value recorded from the sample farthest from the contact. At the contact, the charnockite shows d 18O value of þ 10.3‰. Srikantappa et al. (1985) reported preliminary whole rock geochemical data from NGB charnockites and noted linear correlation of Al2O3, FeOt, MgO and CaO with SiO2. High Na2O relative to K2O were also detected, and were interpreted to reflect a clear affinity to differentiated calcalkaline suite. Santosh (1996) from bulk geochemical characteristics of NGB charnockites proposed a possible affinity to C-type magmas.

The P –T fields from mineral phase equilibria correlate well with the peak densities of fluids and in individual localities and define a near-constant temperature window. However, a distinct trend towards lower pressures is indicated by these fields. Combined with the lower density isochores towards lower pressure –temperature regimes, it is possible to envisage a clock-wise exhumation path for NGB. This clock-wise path is identical to the isothermal decompression path proposed from combined mineral phase equilibria and fluid inclusion studies in the adjacent TGB. The path also compares closely with those defined from a combination of mineralogic thermobarometry and fluid inclusion studies on Pan-African charnockites of Lutzow – Holm Bay region in East Antarctica and Highland Complex in Sri Lanka (Fig. 16). The clock-wise P –T path may relate to the common tectonic framework of these terrains during late Pan-African when the collision of East and West Gondwana megacontinents was followed by extensional collapse (Santosh et al., 2001).

8. Conclusions 7.3. Exhumation path of NGB granulites A combined P – T diagram showing pressure – temperature estimates from different localities and representative fluid inclusion isochores is shown in Fig. 15. The P – T fields from mineralogic thermobarometry are shown by polygons with upper and lower ranges of pressure and temperature as obtained from garnet –orthopyroxene thermometry. Thin near-vertical lines in the P –T diagrams depict the limits of temperatures derived from garnet – biotite thermometry. The figure also shows the isochores derived from fluid inclusions in the various minerals from different localities.

Fig. 16. Comparison of exhumation paths for Pan-African charnockites from some East Gondwana crustal fragments including Trivandrum Granulite Block in southern India, Sri Lanka and Lutzow–Holm Bay in East Antarctica. Although several P–T paths are available on some of these terrains from more recent studies (Nanda-Kumar and Harley, 2000), most of them converge on late isothermal decompression. Only those studies in which P–T paths are derived from a combination of mineral phase equilibria and fluid inclusion are shown in the figure. (Santosh et al., 1991b; Santosh and Yoshida, 1992)

Charnockites in the NGB carry orthopyroxene in nearpristine state suggesting equilibration in a high temperature, anhydrous environment. The massive charnockite –pyroxene granulite suite of rocks in this block might represent an original calc-alkaline magmatic suite that was subjected to granulite facies metamorphism during the Pan-African. Mineral phase equilibria thermobarometry indicates that the metamorphic temperature was as high as 9348C and pressure up to 6.3 kbar. The NGB charnockites commonly contain CO2-rich fluid inclusions in various minerals with minor proportion of H2O. The carbonic fluids were entrapped at peak metamorphic conditions as inferred from the close correspondence of the fluid inclusion isochores with peak metamorphic P –T conditions. Sublithospheric magmas are suggested as the possible source for these carbonic fluids. Laser Raman spectroscopy confirm the pure CO2 composition of majority of inclusions, while minor CH4 and N2 were also identified locally within inclusions which were trapped during post-peak conditions. The combined fluid inclusion and mineral phase equilibria data define a clock-wise metamorphic exhumation path characterised by isothermal decompression.

Acknowledgements We thank Kochi University for facilities. This study is a contribution to the MONBUKAGAKUSHO international research project (No. 13377005) under the leadership of Prof. M. Arima (Yokohama National University). Reviews from Dr Anand Mohan and Dr M. Satish-Kumar are appreciated.

M. Santosh et al. / Journal of Asian Earth Sciences 22 (2003) 131–155

References Bhattacharya, A., Mohanty, L., Maji, A., Sen, S.K., Raith, M., 1992. Non-ideal mixing in the phlogopite–annite binary: constrains from experimental data on Fe–Mg partitioning and a reformulation of the biotite – garnet geothermometer. Contrib. Mineral. Petrol. 111, 87–93. Burke, E.A.J., 2001. Raman microspectrometry of fluid inclusions. Lithos 55, 139 –158. Chacko, T., Ravindra-Kumar, G.R., Newton, R.C., 1987. Metamorphic P– T conditions of the Kerala (S. India) Khondalite belt, a granulite facies supracrustal terrain. J. Geol. 95, 343– 358. Chacko, T., Lamb, M., Farquhar, J., 1996. Ultra-high temperature metamorphism in the Kerala Khondalite Belt. In: Santosh, M., Yoshida, M. (Eds.), The Archean and Proterozoic Terrains in Southern India within East Gondwana, Gondwana Research Group Memoir, vol. 3. Field Science Publications, Osaka, pp. 157–165. Drury, S.A., Harris, N.B.W., Holt, R.W., Reeves-Smith, G.F., Wightman, R.T., 1984. Precambrian tectonics and the evolution of South India. J. Geol. 92, 3–20. Frost, B.R., Chacko, T., 1989. The granulite uncertainty principle: limitations on thermobarometry in granulites. J. Geol. 97, 435 –450. Harris, N.B.W., Santosh, M., Taylor, P.N., 1994. Crustal evolution in South India: constraints from Nd isotopes. J. Geol. 102, 139– 150. Harris, N.B.W., Bartlett, J.M., Santosh, M., 1996. Neodymium isotope constraints on the tectonic evolution of East Gondwana. J. Southeast Asian Earth Sci. 14, 119–125. Jackson, D.H., 1990 Charnockite formation in South India. Unpublished PhD Thesis, Open University, Milton Keynes, UK. Klatt, E., Hoernes, S., Raith, M., 1988. Characterisation of fluids in the gneiss– charnockite transformation in southern Kerala (India). J. Geol. Soc. India 31, 57–59. Lamb, W., Brown, P.E., Valley, J.W., 1987. Post-metamorphic CO2-rich fluid inclusions in granulites. Contrib. Mineral. Petrol. 96, 485– 495. Lee, H.Y., Ganguly, J., 1988. Equilibrium compositions of coexisting garnet and orthopyroxene: experimental determinations in the system FeO–MgO–Al2O3 – SiO2, and applications. J. Petrol. 29, 93–113. Nanda-Kumar, V., Harley, S.L., 2000. A reappraisal of the pressure– temperature path of granulites from the Kerala Khondalite Belt, southern India. J. Geol. 108, 687–703. Newton, R.C., Perkins, D., 1982. Thermodynamic calibration of geobarometers based on the assemblage garnet– plagioclase–orthopyroxene(clinopyroxene)–quartz. Am. Mineral. 67, 203–222. Perchuck, L.L., Lavrent’eva, I.V., 1983. Experimental investigation of exchange equilibria in the system cordierite – garnet – biotite. In: Saxena, S.K., (Ed.), Kinetics and Equilibrium in Mineral Reaction, Springer, Berlin, pp. 199–239. Perkins, D., Newton, R.C., 1981. Charnockite geobarometers based on coexisting garnet – pyroxene – plagioclase – quartz. Nature 292, 144–146.

155

Roedder, E., 1984. Fluid inclusions. Rev. Mineral., Mineral. Soc. Am. 12, 644. Santosh, M., 1987. Cordierite gneisses of southern Kerala, India: petrology, fluid inclusions and implications for crustal uplift history. Contrib. Mineral. Petrol. 96, 343 –356. Santosh, M., 1996. The Trivandrum and Nagercoil Granulite Blocks. In: Santosh, M., Yoshida, M. (Eds.), The Archean and Proterozoic Terrains of Southern India within East Gondwana, Gondwana Research Group Memoir, vol. 3. Field Science Publications, Osaka, pp. 243–277, and references therein. Santosh, M., Tsunogae, T., 2003. Extremely high-density pure CO2 fluid inclusions in a garnet granulite from southern India, J. Geol 111, 1– 16. Santosh, M., Yoshida, M., 1992. A petrologic and fluid inclusion study of charnockites from the Lutzow–Holm Bay region, East Antarctica: evidence for fluid-rich metamorphism in the lower crust. Lithos 29, 107– 126. Santosh, M., Jackson, D.H., Mattey, D.P., Harris, N.B.W., 1988. Carbon stable isotopes of fluid inclusions in the granulites of southern India: implications for the source of CO2. J. Geol. Soc. India 32, 477 –493. Santosh, M., Harris, N.B.W., Jackson, D.H., Mattey, D.P., 1990. Dehydration and incipient charnockite formation: a phase equilibria and fluid inclusion study from South India. J. Geol. 98, 915–926. Santosh, M., Jackson, D.H., Harris, N.B.W., Mattey, D.P., 1991a. Carbonic fluid inclusions in South India granulites: evidence for entrapment during charnockite formation. Contrib. Mineral. Petrol. 108, 318–330. Santosh, M., Yoshida, M., Nada-Kumar, V., 1991b. Fluid characteristics across a gneiss–charnockite reaction front in Sri Lanka: implications for granulite formation in Gondwana deep crust. J. Min. Pet. Econ. Geol. 86, 27–44. Santosh, M., Yoshida, M., Yoshikura, S., 2001. Fluids in Gondwana crust. Gondwana Res. 4, 766–768. Sen, S.K., Battacharya, A., 1984. An orthopyroxene–garnet thermometer and its application to the Madras charnockites. Contrib. Mineral. Petrol. 88, 64–71. Srikantappa, C., Raith, M., Speiring, B., 1985. Progressive charnockitisation of a leptynite –khondalite suite in southern Kerala, India— evidence for formation of charnockites through decrease of fluid pressure? J. Geol. Soc. India 26, 849– 872. Thiery, R., Van den Kerkhof, A.M., Dubessey, J., 1994. vX properties of CH4 –CO2 and CO2 –N2. Modelling for fluid inclusions (T , 31 8C, P , 400 bar). Eur. J. Min. 6, 753–771. Thompson, A.B., 1976. Mineral reactions in Pelitic rocks: II. Calculation of some P–T –X (Fe–Mg) phase relations. Am. J. Sci. 276, 425–454. Touret, J.L.R., 2001. Fluids in metamorphic rocks. Lithos 55, 1–26. Van den Kerkhof, A.M., Hein, U.F., 2001. Fluid inclusion petrography. Lithos 55, 27–47. Warrier, U.C., Santosh, M., Yoshida, M., 1995. First report of Pan-African Sm–Nd and Rb –Sr mineral isochron ages from regional charnockites of southern India. Geol. Mag. 132, 253–260.