Geology of the High Grade Proterozoic Terrains of Sri Lanka, and the Assembly of Gondwana: an Update on Recent Developments

Abstract

Nd model ages determined for the high-grade rocks of Sri Lanka delineate three crustal units, viz., the Highland Complex (HC), the Wanni Complex WC), and the Vijayan Complex (VC). The distribution of these three units differs considerably from the three geological divisions demarcated previously on the basis of geological mapping. The centrally located HC comprises mainly granulite grade charnockitic rocks, and metasediments characterized by older Nd model ages (2.0–3.4 Ga). The Highland sedimentary pile was thickened by intermittent granitoid intrusions, most of which are now charnockitic gneiss, and granulites, and basaltic sills, and dikes. All these metaigneous rocks now occur as conformable bands or layers due to intense polyphase deformation. The HC is bounded on the east by the amphibolite grade VC, composed mainly of granitic gneisses, basic gneisses, and migmatites, and they have ‘younger’ Nd model ages (1.1–1.8 Ga). The isotopic, and geochemical characteristics identify the precursors to the Vijayan rocks as I-type calc-alkaline granitoids originated at an ‘arc’-related tectonic environment. Thus, the earlier interpretation that the Vijayan rocks represent reworked HC was rejected. The granulite inliers within the VC, earlier considered as “resisters” to re-working, are now shown as overthrust klippen or rotated rafts of the HC. The WC, demarcated on the basis of Nd model ages (1.1–1.8 Ga) similar to those of the VC, lies west of the HC. It consists mainly of granitic gneisses, charnockitic gneisses, and migmatites, and the metamorphic grade ranges from amphibolite to granulite. Comprehensive geothermobarometric surveys constrain the P-T evolution of the three crustal units, and indicate that both the HC, and WC underwent near isobaric cooling, followed by a decompression with decreasing temperature. Extensive isotopic studies (U-Pb, Pb-Pb, Sm-Nd, Rb-Sr) have established a new geochronological framework for these high-grade rocks of Sri Lanka. The new framework has bracketed the age of high grade metamorphism in the three crustal units at 550–600 Ma. The recent advances in knowledge of the geology of Sri Lanka favour a strong geological correlation of the HC, and the VC of Sri Lanka, respectively, with the Lutzöw-Holm Complex, and the Yatmato-Belgica Complex in the East Antarctica. The geology of the WC suggests a possible correlation with Madagascar, and East Africa. The amalgamation of the three crustal units of Sri Lanka, is apparently related to the two distinct orogenic events that resulted in the assembly of the Gondwana supercontinent.