CHARNOCKITES AND THEIR ASSOCIATED GNEISSES IN THE PRE-CAMBRIAN OF CEYLON

Abstract

The Pre-Cambrian rocks of Ceylon are divided into the granulite-facies Highland Series (metasediments and charnockites) and the almandine-amphibolite facies Vijayan Series (granites, granitic gneisses, and migmatites). The Vijayan Series represents, in part, Highland Series rocks which were further altered during a later metamorphic episode or episodes. At the junction of the two series is a transitional zone which possesses lithological, structural, and metamorphic features characteristic of each. Charnockites are abundant in the Highland Series and rare in the Vijayan, and charnockitic biotite-gneisses are widely developed in the transitional zones. The field-relations, structures, tectonics, lithology, and grades of metamorphism of the major Pre-Cambrian units are described and the relations of these units to each other are discussed. It is shown that in the course of retrogressive metamorphism (probably in the presence of water) that converted Highland Series rocks to Vijayan types the charnockites were first altered to charnockitic biotite-gneisses and ultimately to non-charnockitic biotite- and hornblende-bearing rocks.