Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

U. P. Radhika ¹, M. Santosh ¹, H. Wada ²

Affiliations
1 Centre for Earth Science Studies, Thuruvikkal Post, Trivandrum 695 031, IN
2 Institute of Geosciences, Shizuoka University, Shizuoka 422, JP

Abstract

Graphite is commonly associated with the metapelites of upper amphibolite to granulite grade in the Kerala Khondalite Belt (KKB). A number of distinct associations of graphite are identified in this study such as: (I) strataboundand stratiform disseminations in metapelites, (2) veins, coarse flakes, pods and lenses associated with pegmatites, (3) enrichment along shear zones, (4) flakes and books in calc-silicate-charnockite associations and (5) residual concentrations in weathered rocks and laterites. While graphite disseminations in the metapelites (khondalites) occur as small flakes oriented parallel to the metamorphic fabric, the vein and pegmatite hosted graphites are found as coarse Rakes and books. Graphite which fills mesoscopic shears and faults also form coarse aggregates. X-ray diffraction studies indicate a high degree of crystallographic ordering for all the graphite types. Scanning Electron Microscopy reveals perfect crystal forms for graphites from the vein and pegmatite assemblages. Carbon stable isotope analysis of twenty three graphite samples are presented. The disseminated graphite yielded δ¹³C values in the range of -17.5 to -32.1 per mil, characteristic of biogenic origin. Graphite associated with pegmatites shows δ¹³C in the range of -10 to -15.1%, while those from shear planes have values ranging from -8.2 to -12.4%. Heavier carbon enrichment in these cases is suggestive of precipitation from CO₂-rich fluids. The mechanisms which lead to the precipitation of graphite from CO₂-rich fluids are evaluated. Thermodynamic considerations indicate that introduction of CO₂-rich fluids into country rocks under low fO₂, will result in the precipitationof graphite on a quantitative basis. The vein-, pegmatite-, and shear-hosted graphite in Kerala may therefore represent large scale infiltration of CO₂,-rich fluids along structurally controlled pathways.

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M. Santosh ¹, U. P. Radhika ¹

Affiliations
1 Centre for Earth Science Studies, P.B. 7250, Akkulam, Thuruvikkal Post, Trivandrum 695 031, IN

Abstract

Field relations in an orthogneiss quarry within the Palghat Gap region illustrate the development of charnockite alteration halos surrounding alkaline pegmatite dykes. Altered orthopyroxenes in the charnockitic zones are artifacts from concordant melt pockets within the gneisses, where orthopyroxene has developed prior to the charnockitie alteration during regional metamorphic, in a process equivalent to dehydration melting. The charnockitic halos represent metasomatic alteration zones developed through the infiltration of CO₂-H₂O fluids expelled from the dykes at shallow crustal levels. Such examples of greasy green charnockitie alterations could be a widespread phenomenon in different crustal blocks of varying metamorphic grades, and are unrelated to regional prograde granulite processes.

Keywords

Charnockitcs, Fluid Inclusions, Petrology, Palghat Gap, Kerala.

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