Arc-like volcanic rocks from the southern Lancangjiang zone,SW China:Geochronological and geochemical constraints on their petrogenesis and tectonic implications

摘要

Triassic volcanic rocks are mainly distributed in the central part of the San-Jiang orogenic belt in southwestern China,and are exposed dominantly in the eastern segment of the Lancangjiang zone.Interpreted as the products of subduction during closure of the paleo-Tethyan Ocean,these volcanic rocks are important for reconstructions of the evolution of the paleo-Tethyan Ocean in southwestern China.Petrologically,the Triassic volcanic rocks in the zone are dominated by andesites with minor basaltic andesites.An andesite sample yielded a SHRIMP U-Pb zircon age of 248.5±6.3 Ma,suggesting that these volcanic rocks erupted in the Early Triassic and not the Middle Triassic as previously assumed.Geochemically,these volcanic rocks have enrichments in LILE and LREE and depletions in HFSEs,significantly similar to typical subduction-related arc volcanic rocks.They are also characterized by relatively high Al2O3 contents and radiogenic Sr-Pb isotopic compositions,indicating that their mantle source could have experienced the modification of subducted sediments and slightly fluid/melt metasomatism from subducted pelagic sediments.The results of mixing calculations further confirm that the mantle source of the Early Triassic volcanic rocks in the area could have been predominantly modified by involvement of 5-7% subducted pelagic sediments into an Indian MORB-like mantle source.Associated with large volumes of Middle-Triassic(~230 Ma)syn-collision granites in the region,these volcanic rocks in the southern Lancangjiang zone,southwestern China,are considered to have developed in a continental margin volcanic arc setting during Early Triassic time,which suggests that the paleo-Tethyan Ocean had not been closed until Middle Triassic when the continent-continent or continent-arc collision occurred.