U-Pb Isotopic Systematics of Zircons from Prograde and Retrograde Transition Zones in High-Grade Orthogneisses, Sri Lanka

We present U-Pb zircon isotopic data from locally restricted prograde (arrested in situ charnockitization) and retrograde metamorphic transition zones, which are well exposed in Proterozoic orthogneisses tectonically interbanded with granulite facies supracrustal rocks of the Highland Group in Sri Lanka. These granitoid rocks yield apparent ages of 1942 ± 22 Ma, ~770 Ma, ~660 Ma, and ~560 Ma. All samples show severe Pb-loss some 550-560 Ma ago. The main phase of granulite-formation could not be dated unambiguously but is bracketed between ~660 Ma and ~550 Ma. The pervasive Pb-loss event around 550-560 Ma reflects the end of this period of high-grade metamorphism and was associated with widespread igneous activity and retrogression. This is constrained by the 550 ± 3 Ma intrusion age for a post-tectonic granite. We relate this late phase of thermal activity to crustal uplift of the Sri Lankan granulites. Our data unambiguously prove the high-grade history of the Sri Lanka gneisses to be a late Precambrian event that may be related to the Pan-African evolution along the eastern part of Africa.