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

Sunil Kumar ¹, R. Dhana Raju ², H. M. Varma ², N. K. Dougall ¹

Affiliations
1 Atomic Minerals Division, Department of Atomic Energy, Skyline View, G S Road, Mawtai, Shiliong 793 008, IN
2 Atomic Minerals Division, Department of Atomic Energy, 110-153, Begumpet, Hyderabad 500 016, IN

Abstract

Rocks of alkaline affinity, namely cancrinite-tinguaite and K-rich trachyte occur as dykes in the Plecambrian gneissic and migmatitic country rocks in the Nongcharam. Darugiri area of the East Garo Hills district Meghalaya Cancrinite-tinguaite is characterized mineralogically by the presence of phenocrysts of high temperature K-feldspar cancrinite nepheline aegirine-augite aeggitine, diopside, biotite and occasional barkevikite in a groundmass consisting of K-feldspar albite anorthoclase, aegirinec aegirinec-augite, nepheline, cancrinite, sodalite and chlorite and chemically by an agpaitic index' of about 1 2 K-rich trachyte exhibits porphytitic texture, and is chemically characterized by low NaO(0 45 wt %), high K₂O(13 25 wt %) resulting to high K ₄O/Na₂O ratio of nearly 3.

This occurrence of alkaline and related rocks together With the earlier reported occurence of an alkaline-carbonatite complex from the nearby Wah Sung valley, Meghalaya points to an alkaline phase of magmatic activity in this part of the Indian subcontinent.

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A. N. Basu ¹, G. S. Sharma ¹, H. M. Varma ¹, R. Dhana Raju ¹, N. K. Dougall ¹, N. V. Raju ¹

Affiliations
1 Atomic Minerals Division, Department of Atomic Energy, Hyderabad 500016, IN

Abstract

At Maturigiri and Dhurkantagiri, mineralised coarse-grained quartz syenite bodies (up to 10 x 2 m) occur as intrusives along the NNW-SSE trending joints in the gneissic country rock. Mineralogically. the quartz syenite is characterised by over 50% K-feld-spar, lesser plagioclase, quartz, mica, garnet and chlorite, together with accessory uraninite, monazite, molybdenite and pyrite. Uraninite occurs as euhedral to subhedral inclusions and mineralisation is syngenetic. Two types of monazite, Th-poor and Th-rich, are present. Molybdenite occurs as veins along fractures and appears to be of late hydrothermal origin. The mineralogical and textural features suggest that quartz syenite was formed from a residual melt of granitic magma of crustal origin.

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