Reconstruction of the Precambrian sedimentary basin in the granulite belt of Sri Lanka

Abstract

Of the two major Precambrian lithological divisions of Sri Lanka, viz. the Highland Group and the Vijayan Complex, the former shows evidence of a highly metamorphosed assemblage of marine-deposited volcanic, volcanoclastic and sedimentary rocks. Using the available major- and trace-element data for the rocks of the Highland Group, an attempt has been made to elucidate the primary features of the depositional environment of the Highland Group in the granulite belt of Sri Lanka. The garnet gneisses belonging to the Khondalite suite have an overall sedimentary parentage, the original clastic sediments being derived from a continental crust. The garnet gneisses of the Highland Group are comparable to early Proterozoic—Archaean shales, a significant depletion in total Fe and a slight Mn enrichment being evident. The charnockites and allied rocks which form an integral part of the Highland Group correspond to an igneous differentiation sequence of basaltic composition. The presence of graphite closely associated with sulphides in the western part of the basin indicates an anoxic—sulphidic type of environment whereas in the eastern part, the conditions were more oxidizing with abundant dolomitic marbles, quartzites and banded Fe formations. In its entirety, the Highland Group of Sri Lanka can be interpreted as a Precambrian sedimentary basin having shelf—slope characteristics and located at a plate junction predating a continent—continent type of collision.