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

Indrani Mukhopadhyay ¹, ]yotisankar Ray ¹, Sudipto Nath ¹

Affiliations
1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Calcutta-700019, IN

Abstract

This paper describes the results of field investigation and structural geology of the rocks of the area between Shantinagar - Uppalchelka in Khammam district of Andhra Pradesh (latitudes 17°14'15" to 17°18'43" N and longitudes 80°41'40".to 80°44'44" E). A zone of silicification (quartz - mylonite with well preserved colour banding) with patches of tectonic conglomerate (quartz-pebble) is found to occur within the generally dominant amphibolites. Structural analysis indicates the dominant influence offolding deformation (plungeoffold axis 42° -> 74°) in the area. All microstructural elements (e.g. polygonization, subgrain formation, "core-mantle" structure, ribbon like appearance of quartz grains) are deduced to be related to ductile deformation, recovery phenomena and recrystallisation process. Progressive development of tectonic conglomerate fabric and decrease in grain size are typically indicative of incremental strain magnitude across shear zone in the studied area. A "thin skinned" tectonic model invoking a network of thrusts has been proposed.

Keywords

Structural Geology, Mylonites, Eastern Ghat Belt, Andhra Pradesh.

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Sudipto Nath ¹, Chittaranjan Bhattacharyya ¹

Affiliations
1 Department of Geology, University of Calcutta, Kolkata - 700 019, IN

Abstract

Abundant small lenses and bands of different types of amphibolites belonging to amphibolite facies occur within Proterozoic (1638±38 Ma) Kuilapal granitoid gneiss body (12 8 Km × 64 Km) and outside it in the metapelitic country. The different petrographic types are (A) PI hb-Mc amphibolite, (B) Ap-mt-hb-M amphibolite, (C) Monomineralic hb-UM amphibolite,(D) Tr-act-UM amphibolite and (E) MI-hb MC amphibolite Petrographic and geochemical studies suggest that all these types of amphibolites except (E) are orthoamphibolites which are similar in chemical composition to tholentic ferrobasalt and exhibit trace element characters corresponding to ocean floor basalt. The parent basaltic magma for these amphibolites appears to show signature of a magnetic history connected with Dalma basalts derived from Iron-Enriched mantle source. However, the parent basaltic magma of the Kuilapal area, in all probability represents an earlier phase of basic magmatism than Dalma basalt (1547±20 Ma). The age of emplacement of ferrotholentic magma in the present area should be earlier than 1638±38 Ma, which is the age of Kuilapal granatoid rocks. The chemical composition of tr actinolite amphibolite, ap mt hb amphibolite and pl ap mt-hb amphibolite have similarity with such igneous rocks as pyroxenite, ap-mt pyroxenite and gabbroic rocks respectively. The type (E) amphibolite represents metamorphosed impure marly sediment EPMA data of hornblendes show systematic increase of Si and Mg and decrease of Fe from almandine zone to sillimanite zone metamorphsm Hornblende-plagioclase geothermometry suggests the variation of temperature from about 730°C to 795°C from garnet/staurolite to sillimanite zone.

Keywords

Petrology, Amphibolite, Kuilapal Granitoid Gneiss, Hornblende, Geothermometry-Geobarometry, Dalma Basalts, Puruliya, West Bengal.

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