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

Sampa Hazra ¹, Jyotisankar Ray ¹, Abhishek Saha ¹

Affiliations
1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata - 700 019, IN

Abstract

The Proterozoic mafic suite of rocks within the Khasi greenstone belt of the Meghalaya plateau, north eastern India, occurs as intrusives into weakly metamorphosed Shillong Group of rocks. This paper highlights field, petrographic and mineral-chemical studies of mafic suite of rocks of the Khasi greenstone occurring near the type area Laitlyngkot in east Khasi Hills district of Meghalaya. Field relations and petrographic studies bring out presence of two broad lithotypes viz. 'massive' and 'foliated' which are integral parts of the Khasi greenstone. Chemical characters of constituent pyroxene grains indicate that they belong to augite field. Amphiboles are 'calcic' in nature and may be further discriminated as magnesio-hornblende, ferro-hornblende, tschermakitic hornblende, ferro-tschermakitic hornblende, tschermakite and ferro-tschermakite. Composition of plagioclase suggests initial magmatic crystallization which was partly re-equilibrated by later metamorphic adjustment. Chemistry of accessory mineral phases like opaque oxides, sphene and biotite is also presented. Use of relevant thermobarometric methods involving pyroxene, amphibole and plagioclase yields a temperature of ∼1200°C to ∼510°C at a pressure of ∼10 Kb to ∼1 Kb. It signifies initiation of magmatic intrusion corresponding to the peak of ideal igneous temperature followed by substantial drop of temperature corresponding to lower range of magmatic crystallization at hydrous condition or later metamorphic reconstitution. Chondrite normalized REE patterns of the mafic suite of rocks indicate a common parent magma with variable degree of differentiation. Importance of PGE-studies to constrain petrogenesis of these rocks has also been discussed.

Keywords

Mineral Chemistry, Thermobarometry, Cooling History, Re-Equilibration, Khasi Greenstone.

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Joytisankar Ray ¹, Abhishek Saha ¹, Sampa Hazra ¹

Affiliations
1 Dept. of Geology, University of Calcutta, Kolkata, IN

Abstract

No Abstract.

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Sampa Hazra ¹, Pritam Saha ¹, Jyotisankar Ray ¹, Avijit Podder ¹

Affiliations
1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata - 700 019, IN

Abstract

The porphyritic granite body (∼600 Ma) around Mylliem (25°32'N:91°52'E), east Khasi hills district, Meghalaya occurs as a distinct intrusive body into the host rock of low-grade meta-sediments belonging to Proterozoic Shillong Group. The porphyritic granite body manifests prominent lath-shaped feldspar phenocrysts giving rise to dominant porphyritic texture. Further, this porphyritic granite body is characterized by primary foliation (defined by parallely arranged feldspar laths), tongues and appophyses and xenoliths of older metamorphics. Petrographically, the Mylliem porphyritic granite is grey leucocratic, coarse grained, phanerocrystalline with modal variants ranging from granite to granodiorite tending to tonalite. Simple statistical studies based on correlation coefficient values involving modal variables of the Mylliem porphyritic granite bodies appear significant in majority of the cases indicating magmatic crystallization. Use of relevant thermometric method indicates temperature of equilibration of the body in the range of 369°C to 507°C. The crystallization of the Mylliem porphyritic granite pluton initiated at an average lithostatic pressure in the tune of ∼11 Kb followed by dominant P_H2O controlled milieu. Spatially projected mineralogical parameters suggest a general trend of the magmatic cooling of the body from margin inward with sudden influx of volatile matters that occasionally offsets mineralogical trends.

Keywords

Mylliem Porphyritic Granite, Mineral Chemistry, Magmatic Crystallization, Statistical Studies, Correlation Coefficients, Meghalaya.

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