Petrology of the Leptynites and Garnetiferous Granite-Gneisses Around Garbham, Srikakulam District, Andhra Pradesh

Petrography, mineralogy and chemistry of leptynites and garnetiferous granitegneisses of Garbham area have been described. On the basis of texture, structure and mineral assemblage, the leptynites have been classified mainly into three types A, B and C, all of which have been formed by the granulite-facies metamorphism of sediments of appropriate compositions. A fourth type of leptynite has been shown to have formed by migmatisation of khondalite; with increasing migmatisation-granitisation, this is locally converted into one-feldspar (Kvfeklspar) garnetiferous granite-gneiss. One-feldspar garnetiferous granite-gneiss has been locally formed by the granitisation of khondalite and quartzite. Two-feldspar garnetiferous granite-gneisses are predominant and have evolved by the granulite-facies metamorphism of sediments, possibly shale and/or greywackes, followed by granitisation in the form of K-feldspathisation. Two-feldspar garnetiferous granite-gneisses are suggested to be essentially the variants of acid charnockites. Both the acid charnockites and garnetiferous granite-gneisses suffered K-feldspathisation, the latter having been also locally derived by K-feldspathisat ion of acid charnockite.