U-Pb geochronology of zircons from river sediments in Sri Lanka: Implications on early Archean to late Cambrian magmatism and episodic crustal growth

摘要

Geochronology of zircons from river-sediments in Sri Lanka has never been attempted. Here we present Rare Earth Elements (REE) and U-Pb isotopes of -700 zircon grains by laser ablation ICP-MS technique, from sediments of the major rivers in the Sri Lankan basement (rivers of Mahaweli, Kelani, Kalu, Walawe, Maduru Oya and Maha Oya). Most of the studied zircons display oscillatory zoning and Th/U 0.1, confirming their igneous origin. U-Pb age distribution of these river-sand zircons exhibits two major populations depicting Mesoproterozoic to Archean ( similar to 30%; 1100-3100 Ma) and Neoproterozoic to late Cambrian ( similar to 70%; 430-1000 Ma) magmatism. The two populations could be further resolved into intensive and episodic magmatic peaks at 480-680 Ma, 680-1000 Ma, 1100-1300 Ma, 1300-1700 Ma, 1700-1900 Ma, 2100-2300 Ma, 2300-2600 Ma and 2600-3100 Ma. These river-sediment-zircon data are consistent with the principal episodes of global continental crustal growth and the recognized U-Pb ages of magmatism are mostly correlative with those obtained from basement rocks by previous studies of Sri Lanka. The older zircons found in this study may have mainly originated from reworked/recycled ancient (Palaoproterozoic to Archean) crust. A younger zircon group of 450-700 Ma with homogenous core-rim zoning texture and Th/U ratios 0.1 were inferred to be of metamorphic origin, indicating their possible crystallization at late Neoproterozoic metamorphic events, associated with the final assembly of the Gondwana Supercontinent. Thus, our data imply that the present-day continental crust of Sri Lanka may have predominantly formed and grown in the Neoproterozoic, while there is evidence that crustal growth began at least in the Paleoproterozoic. Further, the Mesoproterozoic peaks of the frequency distribution of U-Pb ages of river-sand zircons at similar to 1.6 Ga and similar to 1.1 Ga might reflect triggering of subduction in a paleo tectonic setting forming the Wanni and Vijayan dual arc systems, respectively.