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A. T. Rao ¹, V. Divakara Rao ², M. Yoshida ³, M. Arima ⁴

Affiliations
1 Department of Geology, Andhra University, Visakhapatnam- 530 003, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN
3 Department of Geosciences, Osaka City University, Osaka - 558, JP
4 Department of Geological Science, Yokhoma National University, Yokahoma, JP

Abstract

Oxygen isotope values in metapelites including garnet-sillimanite gneisses (khondalites), quartzo-feldspathic gneisses (leptynites), cordierite-sapphirine gneisses and the associated marbles, calc-silicates, quartzites, basic granulites and charnockites from the Eastern Ghats Granulite Belt are presented and their significance discussed. Metapefites show a wide variation in δ¹⁸O values (4.50 to 13.90‰) that is controlled largely by the mineralogy of the rocks. Coarse grained porphyritic charnockites have higher δ¹⁸O values (8.60-11.20‰) than that of medium grained ones (5.90-6.60‰) Basic granulites have the minimum δ¹⁸O values (∼5.0‰). Oxygen isotope values of different rock units in the granulite belt are constrained mostly by the composition of the source rocks of the sediments and of the protoliths in the case of charnockites. Post-depositional metamorphic and metasomatic fluids had little contribution to the δ¹⁸O variations in these rocks.

Keywords

O-Isotopes, Eastern Ghats Granulite Belt, Khondalites, Charnockites, Andhra Pradesh.

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B. Hari Prasad ¹, T. Okudaira ¹, Y. Hayasaka ², M. Yoshida ¹, R. S. Divi ³

Affiliations
1 Department of Geosciences, Osaka City University, Osaka 558-8585, JP
2 Deparlrnent of Earth and Planetary Systems and Science, Hiroshima University, Higashi-Hiroshima 739-8526, JP
3 Department of Geology, Kuwait University, KW

Abstract

Amphibolites from the Nellore-Khammam schist belt, Khammam district, Andhra Pradesh, were metamorphosed at 700 ± 50 °C, 1.1 ± 0.1 GPa. Bulk-rock chemical characteristics for relatively immobile element ratios (e.g. Zr-Y-Ti) of the amphibolites suggest two different tectonic settings: oceanic island arc and continental margin island arc affinities. Coexistence of rocks for two different tectonic settings in the schist belt can be explained by accretion at trench as protoliths of the highpressure metamorphic amphibolites. Because the Nellore-Khammam schist belt is located between two terrains, such as Dhanvar Craton and Eastern Ghats Granulite Terrain, their mode of occurrence implicitly suggests that formation of the Nellore-Khammam schist belt was related to the continentcontinent collision tectonics. Consequently, the Nellore-Khammam schist belt may have formed at Ieast in two steps: (1) accretion of the rocks originating from different tectonic settings at trench and (2) subsequent high-pressure metamorphism and exhumation of the rocks caused by collision tectonics.

Keywords

Petrology, Geochemistry, P-T Conditions, Amphibolites, Nellore-Khammam Schist Belt, Andhra Pradesh.

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M. Yoshida ¹, M. Funaki ², P. W. Vitanage ³

Affiliations
1 Department of Geosciences, Faculty of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558, JP
2 National Institute of Polar Research, Kaga, ltabashi-ku, Tokyo 173, JP
3 Department of Geology, University of Peradeniya, Peradeniya, LK

Abstract

A dolerite dike from southwestern Sri Lanka gave whole-rock K-Ar ages of 152.6 ± 7.6 Ma and 143.3 ± 7.2 Ma. Many of the other dolerite dikes of Sri Lanka are considered to be of Mesozoic ages judging from the present age data and tectonometamorphic history of Sri Lanka. Petrographic similarities should not be used for age correlations, because dolerites of different age may have the same petrography.

Preliminary natural remanent magnetization (NRM) after AF and thermal demagnetization gave a mean inclination of 24.6° and declination of 67.5° with α95=21.7°. A virtual geomagnetic pole position calculated from the mean NRM was rotated relative to Antarctica so as to fit with that obtained from the Jurassic Ferrar dolerite of Antarctica. This rotation results in the location and attitude of Sri Lanka to attach with Antarctica at Lutzow-Holm Bay as suggested by Barron et al. (1978).

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T. Radhakrishna ¹, John Mathai ¹, M. Yoshida ²

Affiliations
1 Centre for Earth Science Studies, P. B. No. 7250, Trivandrum 695031, IN
2 Department of Geosciences, Osaka City University, Osaka 58, JP

Abstract

The Punalur-Achankovil sector is a southwestern segment of the high-grade granulite terrain of South India. It is characterised by charnockite and garnet-biotite. Quartz-feldspar (GBQF) gneiss with minor occurrences of sillimanite and graphitebearing gneiss and basic granulite. Structurally. The terrain has undergone three phases of intense deformation and metamorphism and a late less intense deformation. The first deformation (D₁) has disrupted original depositional layering with development of quartzo-feldspathic bands; the accompanying metamorphism (M₁) is represented by granulite facies sillimanite-bearing gneiss and basic granulite. The second deformation (D₂) resulted in isoclinal folding and the corresponding metamorphism (M₂) produced large neosome now represented by the GBQF gneiss. Lithologies representing the D₃-M₃ event are the charnockite and minor granite bodies developed under high-grade metamorphic conditions; open to tight" asymmetric folds are characteristic of this deformation. The final deformation (D₄) is less penetrative and is characterised by open folds. Structural data from the area do not indicate any major shear zone features nor does the region constitute an intercratonic suture belt as suggested earlier. The topographic expression along Achankovil-Punalur represents only a neotectonic lineament.

Keywords

Structural Geology, Granulites, Punalur-Achankovil, Kerala.

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Radhi D. Menon ¹, M. Santosh ¹, M. Yoshida ²

Affiliations
1 Centre for Earth Science Studies, P.B. 7250, Akkulam, Trivandrum 695031, IN
2 Department of Geosciences, Osaka City University, Osaka 558, JP

Abstract

Within the gem field in southern Kerala and adjacent Tamil Nadu region, extending over an area of 70 X 35 sq km, a variety of precious and semi-precious stones occur such as chrysoberyl (cymophane or "eat's eye", and alexandrite varieties), ruby, sapphire, emerald, topaz, zircon, and amethyst. The primary mineralization occurs within zoned, complex pegmatites of Pan-African age, emplaced within granulite facies metapelites (khonda1ites), and variably weathered and lateritised. Gemstone deposits of secondary type are associated with stream gravels and placers.

Preliminary fluid inclusion results provide evidence for the involvement of CO₂-enriched felsic melts which interacted with the aluminous supracrustals in the formation of the pegmatite-hosted gem mineralization. Thennal decrepitation offluid inclusions under high vacuum, extraction and quantification of the fluids from inclusions indicate the presence of variable amounts of CO₂ in quartz (72.7 ppm), garnet (141.2 ppm) and chrysoberyl (51.6 ppm). The carbon isotopic composition of CO₂ in quartz indicates a 'Juvenile" magmatic nature, with δ¹³C va1ue of -8 per mil. The gem-bearing pegmatites may therefore represent cryptic pathways through which large volumes of felsic melts migrated, a process which can be correlated to deep-seated extension in the Gondwana crustal fragments during the PanAfrican.

Gem recovery in Kerala, despite vast untapped potential, is largely done through illegal means. We emphasize the need for a gem industry in the State, and the requirement to bring the local prospectors under the fold of artisans so as to make use of their experience and expertise, in gem prospecting.

Keywords

Gemstones, Pegmatites, Fluid Inclusions, Economic Geology, Kerala.

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M. Satish Kumar ¹, M. Santosh ¹, S. Yoshikura ², M. Yoshida ¹

Affiliations
1 Department of Geosciences, Osaka City University, Osaka 558, JP
2 Department of Geology, Kochi University, Kochi 780, JP

Abstract

Colour-coded compositional mapping under an electron microprobe provides useful information on chemical gradients within the domain of individual minerals. Here we apply this technique to granulite facies reaction textures from a calc-silicate-charnockite interface at Nuliyam, southern Kerala, to understand the element mobility patterns and mass balance related to fluid-induced metasomatism during deep crustal metamorphism. The contact zone assemblage at Nuliyam is characterized by plagioclase + quartz symplectites after K-feldspar. From element mapping of this reaction interface on micron - scale, and from quantitative analyses of the individal mineral phases under an electron microprobe, we trace the diffusion of K, and addition of Na and Ca, through an open system migration dUling the influx of externally-derived CO₂-rich fluids. Our study provides the first direct evidence for microscale chemical diffusion associated with CO₂-induced charnockite formation in southern Kerala, and illustrates the potential of colour-coded compositional mapping in tracing element mobility patterns.

Keywords

Metamorphic Petrology, EPMA - Compositional Mapping, Calc-Silicate, Charnockite, Nuliyam, Southern Kerala.

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A. T. Rao ¹, V. Divakara Rao ², M. Yoshida ³, M. Arima ⁴

Affiliations
1 Department of Geology, Andhra University, Visakhapatnam-530 003, IN
2 National Geophysical Research Institute, Hyderabad- 500 007, IN
3 Department of Geosciences, Osaka City University, Osaka 558, JP
4 Department of Geological Sciences, Yokohama National University, Yokohama, JP

Abstract

The whole rock analyses confirm the basic to intermediate nature of the protolith (SiO₂, > 53% - < 67%) with high alumina (highest 22.18%) and high K₂O (∼ 4%). Metamorphic segregation as well as source rock compositions appear to have contributed -to the formation of high alumina granulites. The temperatures of metamorphism estimated using garnet - hypersthene/cordierite pairs range from 680-980°C, The gamet-cordierite reference temperatures with narrow range 700-750°C, indicate a pressure range of 5-7 kbar for the garnet-orthopyroxene-plagioclase-quartz assemblage in the aluminous granulites. Textural relationships of cordierite breakdown and reappearance of cordierite supplement the two generations of cordierite from aluminous granulites in the Eastern Ghats Granulites Belt.

Keywords

Geochemistry, Aluminous Granulites, P-T Estimates, Eastern Ghats Granulite Belt (EGGB), Andhra Pradesh.

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