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

P. Perraju ¹, V. Natarajan ¹

Affiliations
1 Geological Survey of India, Hyderabad, IN

Abstract

The' Peninsular Gneiss' in the northern parts of Andhra Pradesh can be described briefly as a group of composite gneisses, mostly banded or layered, formed as a result of migmatisation of the older meta-pelites (predominantly biotite-schists), metapsammites, and meta-basics (predominantly amphibolites and hornblende schists), by a grey quartzo-feldspathic injection. The resultant migmatitic gneisses enclose inclusions of all shapes and sizes of the older rocks and exhibit different degrees of interaction and assimilation of them. This complex was later intruded by a 'Pink Granite' the effect of which is also seen as a ' second period of migmatisation '. The two major injections appear to be fairly well separated in time.

These two granitic injections and their corresponding migmatisation effects on the older group of diverse rocks has produced a heterogeneous complex of rock types which show wide variations in composition as well as in texture over the region. Both granites have their own groups of aplite, pegmatite and quartz veins. The white quartz and pegmatite veins traversing the grey porphyroblastic biotite gneisses in Karimnagar district carry molybdenite and sulphide mineralisation. All the rocks were later injected by quartz, epidote and at places by fluorite veins. Fluorite is also found as a component mineral of the' pink granites' in Karimnagar district. Dolerite dykes which cut across all the above, mark the last phase of igneous activity.

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P. Perraju ¹, Adam Kovach ², E. Svingor ²

Affiliations
1 Geological Survey of India, Southern Region, Hyderabad-500001, IN
2 Institute of Nuclear Research of the Hungarian Academy of Sciences, H 4001, Debrecen 1, Pf. 51, HU

Abstract

Garnet-sillimanite-graphite gneiss (khondalite) from Puri district, Orissa has given an age of 3090 m.y. while another sample of similar composition from Yerada hill near Visakhapatnam has yielded an age of 2482 m.y. A sample of garnetiferous hypersthene gneiss from Kasipatnam in Visakhapatnam district is found to be 2695 m.y. in age and a feldspathised variety of the same rock has yielded an age of 2129 m.y. indicating the feldspathisation process to be a later event. The present paper highlights a hitherto unreported granitic activity around 816 ± 40 m.y. in the Eastern Ghats area.

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