Indexed on: 15 Jun '16Published on: 11 Jun '16Published in: Precambrian Research
Recent tectonic models of Sri Lankan basement suggest that the Highland Complex (HC), which is dominantly composed of metasedimentary rocks, forms part of an accretionary complex developed during Neoproterozoic subduction-collision tectonics between the Wanni and Vijayan Complexes. Here we present new U-Pb geochronological data on detrital zircon in metasediments from the HC and adjacent regions to investigate the source characteristics, deposition and regional correlations with similar rocks in adjacent Gondwana fragments. Cores of detrital zircon grains in twelve metasediment samples (quartzite, pelitic gneiss, leptynite, and metamorphosed banded iron formation) characterized by higher Th/U ratios indicating derivation from magmatic protoliths yield Neoarchean to Paleoproterozoic ages (ca. 2700-1700 Ma) with minor Paleoarchean (ca. 3500 Ma), Mesoproterozoic (ca. 1200 Ma), and Early to Middle Neoproterozoic (ca. 800-600 Ma) components. The rims of most grains as well as homogeneous grains show Late Neoproterozoic ages (582-533 Ma) with lower Th/U ratios that can be interpreted as metamorphic ages. Our results constrain the depositional age of the HC sediments to be Late Neoproterozoic (ca. 600-550 Ma) based on the ages of the youngest detrital zircon grains and metamorphism. Although exposed Archean magmatic units are absent in Sri Lanka, several crustal blocks including the Dharwar Craton, Coorg Block, and Salem Block in the adjacent Gondwana fragment of India carry Archean (ca. 3400-2500 Ma) basement, which we consider to be the major source for the Sri Lankan detrital zircons. The Paleoproterozoic zircons might have been derived from the Congo-Tanzania-Bangweulu Block in East Africa that includes Archean to Paleoproterozoic blocks (e.g., ca. ∼2700 Ma Tanzania Craton; ca. 2300-1800 Ma Usagaran-Ubendian belt). The Kibaran belt (ca. 1400-1000 Ma) in the Congo-Tanzania-Bangweulu Block might have served as the source of minor Mesoproterozoic zircons. Early to Mid Neoproterozoic grains might have come from the adjacent Wanni, Vijayan, and Kadugannawa Complexes in Sri Lanka (ca. 1100-750 Ma). The similarity in detrital age spectra of the metasediments in the HC and those of the northern Madurai Block in southern India suggests that they could have similar provenances (Archean cratons in India and/or East Africa). Similar age distributions of detrital zircon cores reported from the Itremo Group (central Madagascar) indicate that the metasedimentary terranes in central Madagascar, southern India, and Sri Lanka could have been juxtaposed, and with sediment source including those in the East African cratons. Similar magmatic features in central Madagascar, southern India, Sri Lanka, and eastern Antarctica also suggest a series of volcanic arcs developed through a complex subduction-accretion process in these regions before the final amalgamation of Gondwana.