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.

The Contact Relationship of Charnockite and Biotite Gneiss at Bauchi, Northern Nigeria

“…The gradation from one to the other is therefore reflected by changes in both the colour and texture of the rocks. In both respects the gradation is extremely rapid, as has been observed in other cases (Cooray, 1962), taking place within a transitional zone varying from 4 inches to 1 foot for the colour and generally less than 1 inch for the texture. Nevertheless there is no doubt of the gradational nature of the junction.…”

“…It is for this reason that the charnockitic mineral assemblage was considered as typical of that facies. It is now well demonstrated that the rock may be found associated with gneisses of almandine-amphibolite facies (Pichamuthu, 1953 ;Cooray, 1962). The gneisses surrounding the charnockite which is the subject of this account appear to be of almandine-amphibolite facies (Oyawoye, 1959).…”

“…From the nature of the field relationships, three mutually exclusive petrogenetic conclusions may be made :-(i) that the biotite gneiss was derived from a once widespread charnockitic series which was formed during some previous period of granulitite facies of metamorphism, as suggested for the Ceylon charnockites (Cooray, 1962) ; (ii) that the charnockites represent some conformable body of igneous or other different rocks which together with biotite gneiss have been recrystallized together under conditions of high grade regional metamorphism, as advocated by Howie (1955) ; (iii) that the charnockite was derived from the biotite gneiss.…”

Proterozoic crustal evolution of southernmost India and Sri Lanka

“…Coates 1935;Wadia 1942;Cooray 1962Cooray , 1984Cooray , 1994 and names of basement traits have varied considerably. The knowledge of the basement geology of Sri Lanka increased rapidly in the period from 1987 to 1995, largely due to the efforts of Japanese-Sri Lankan and German-Sri Lankan cooperative projects (Hiroi & Motoyoshi 1990;Krrner 1991b;Raith & Hoernes 1994 Fig.…”

“…This pattern seems to be complicated, however, by the existence of a several hundred to a thousand metre thick zone, where rocks with apparently young Nd model ages seem to be interleaved with granulitic, partly metasedimentary, gneisses (Cooray 1962;BiJchel 1994;Kleinschrodt 1994).…”