Geodynamic evolution of the Pan-African lower crust in Sri Lanka : structural and petrological investigations into a high-grade gneiss terrain

Abstract

Some main objectives of present-day geological research are to assess the role of the lower crust in collision and extensional tectonics and to unravel the mechanism and timing of crustal growth. Both objectives require input from the study of high-grade gneiss terrains, notably data concerning their composition and structure and pressure-temperature-time evolution. The high-grade gneiss terrain of Sri Lanka is a suitable area to study deep-<:rustal processes because of good access, high outcrop density and large variation in rock composition. It comprises three provinces: the Highland Complex (HC) flanked by the Wanni and Kadugannawa Complexes (WKC) to the W and the Vijayan Complex (YC) to the E. The WKC lies on top of the HC, which tectonically overlies the Vc. The VC is dominated by amphibolite facies orthogneisses, while the other units contain granulite facies gneisses of meta-igneous and metasedimentary parentage. Peak metamorphism occurred between -610 and -550 Ma (HOIzI et aI., 1991) and KrOner (1991) therefore proposed a link with the Pan-African Mozambique Belt. This thesis discusses the geodynamic evolution of the Sri Lankan basement on the basis of new structural and petrological data and a review of earlier work. The petrological part of the thesis starts with a description of mineral reactions in sapphirine-bearing assemblages from the HC, which can be modelled in the FMAS system. Thermobarometry on critical assemblages indicates peak metamorphism at about 9 kbar and 830 oC, followed by isothermal decompression to -7.5 kbar.