Detrital zircon geochronology of quartzites from the southern Madurai Block,India:Implications for Gondwana reconstruction

摘要

Detrital zircons are important proxies for crustal provenance and have been widely used in tracing source characteristics and continental reconstructions.Southern Peninsular India constituted the central segment of the late Neoproterozoic supercontinent Gondwana and is composed of crustal blocks ranging in age from Mesoarchean to late Neoproterozoic-Cambrian.Here we investigate detrital zircon grains from a suite of quartzites accreted along the southern part of the Madura) Block.Our LA-ICPMS U-Pb dating reveals multiple populations of magmatic zircons,among which the oldest group ranges in age from Mesoarchean to Paleoproterozoic(ca.2980-1670 Ma,with peaks at 2900-2800 Ma,2700-2600 Ma,2500-2300 Ma,2100-2000 Ma).Zircons in two samples show magmatic zircons with dominantly Neoproterozoic(950-550 Ma) ages.The metamorphic zircons from the quartzites define ages in the range of 580-500 Ma,correlating with the timing of metamorphism reported from the adjacent Trivandrum Block as well as from other adjacent crustal fragments within the Gondwana assembly.The zircon trace element data are mostly characterized by LREE depletion and HREE enrichment,positive Ce,Sm anomalies and negative Eu,Pr,Nd anomalies.The Mesoarchean to Neoproterozoic age range and the contrasting petrogenetic features as indicated from zircon chemistry suggest that the detritus were sourced from multiple provenances involving a range of lithologies of varying ages.Since the exposed basement of the southern Madurai Block is largely composed of Neoproterozoic orthogneisses,the data presented in our study indicate derivation of the detritus from distal source regions implying an open ocean environment.Samples carrying exclusive Neoproterozoic detrital zircon population in the absence of older zircons suggest proximal sources in the southern Madurai Block.Our results suggest that a branch of the Mozambique ocean might have separated the southern Madurai Block to the north and the Nagercoil Block to the south,with the metasediments of the khondalite belt in Trivandrum Block marking the zone of ocean closure,part of which were accreted onto the southern Madurai Block during the collisional amalgamation of the Gondwana supercontinent in latest Neoproterozoic-Cambrian.