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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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Abhishek Saha ¹, Sohini Ganguly ¹, Jyotisankar Ray ¹, Avik Dhang ²

Affiliations
1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata 700 019, IN
2 Aurum S.P.R.L., 59, Avenue Colnol Muzemba, Lubumbashi, CD

Abstract

A new occurrence of (syenite-hosted) Vanadium bearing titaniferous magnetite ore body has been reported from Ganjang (26°09'35" N: 93°20' E), Karbi-Anglong, Northeastern India. The magnetite ore bodies have lumpy and sporadic occurrences within the host syenite pluton intrusive into gneissic country rocks. Ore microscopic studies reveal that magnetite is often associated with haematite and ilmenite depicting different textural patterns. Critical consideration of several elemental patterns suggests magmatic differentiation to be main ore-forming process. The ore body is suggested to have been formed as late stage segregation from a differentiating alkaline magma in a fluid enriched milieu.

Keywords

Vanadium Bearing Titaniferous Magnetite, Karbi-Anglong, Widmanstatten Texture, Magmatic Differentiation, Northeastern India.

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