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

B. Bhaskar Rao ¹, G. Venkataraman ¹

Affiliations
1 Department of Civil Engineering, I. I. T. Bombay, IN

Abstract

Zircon studies have been carried out on the granitic rocks occurring around Sagtala, Panchmahal district, Gujarat, The granitic rocks are coloured grey or pink and can be classified into three types: (1) Grey quartz-monzonite, (2) Grey granodiorite, (3) Pink granite. Elongation frequency diagrams drawn for the zircons of the different granitic rocks show maxima in the range 2 - 2.5. Positional as well as slope differences are observed in the reduced major axes of the zircons of quartz-monzonite, granodiorite and granite. On the basis of zircon studies, it is concluded that the Sagtala granite is a composite intrusive consisting of three distinct intrusive phases whose emplacements were separated in time.

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B. Bhaskar Rao ¹, R. M. Ramanathan ¹

Affiliations
1 Department of Civil Engineering, Indian Institute of Technology, Powai, Bombay 400076, IN

Abstract

The restricted occurrence of chloritoid in some schists around Adyal is attributed to their bulk rock chemistry. Optical and X-ray studies reveal that the chloritoid is triclinic. From the co-existing mineral assemblages it is inferred that the maximum temperature attained by these rocks is well below 540°C. The minimum pressure is inferred to be around 3.6 Kbar.

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K. V. SubbaRao ¹, M. J. Le Bas ², B. Bhaskar Rao ¹

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology, Powai, Bombay 400 076, IN
2 Department of Geology, University of Leicester, Leicester LEI 7RH, GB

Abstract

New whole-rock rare earth element (REE) data for the metacarbonate rocks interbedded with schists at Vinjamur in the Nellore Schist Belt of Andhra Pradesh, show low total REE contents (Σ9-128 ppm) that are inconsistent with an igneous carbonatitic origin but which correspond more closely with a sedimentary limestone origin. The REE data of these rocks however, do not give absolute discrimination between marbles of meta-limestone and metacarbonatite origin. Micro-probe analytical data give better discrimination, and the chemical compositions of the calcite, micas, amphibole, plagioclase, apatite, monazite and staurolite in the Vinjamur marbles give strong and consistent evidence of a metamorphosed sedimentary rather than an igneous origin.

Keywords

Carbonatites, Vinjamur, Nellore Schist Belt, Andhra Pradesh.

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