Earliest Carboniferous magmatic event in the eastern section of the northern margin of the North China plate and its tectonic implications—Evidence from rock association and zircon U-Pb geochronology

Abstract

LA-ICP-MS zircon U-Pb dating results and rock association are reported for the metavolcanic rocks from the Panling Formation in the eastern section of the northern margin of the North China plate,and their tectonic setting has been discussed,with the aim of constraining the tectonic evolution of the eastern section of the northern margin of the North China plate in the Late Paleozoic period.The Panling Formation in the Xia'ertai Group consists of albite-amphibole schist whose protolith is basaltic andesite,metarhyolite,micaquartz schist,and minor metamorphic siltstone.Zircons from the albite-amphibole schist and metarhyolite in the Panling Formation are euhedral-subhedral in shape and display fine-scale oscillatory growth zoning as well as high Th/U ratios(0.77-1.79),implying their magmatic origin.The 206Pb/238U ages of 15 spots of zircons from the albite-amphibole schist are between 337Ma±11Ma and 346Ma±13Ma,with a weighted mean 206Pb/238U age of 342Ma±6Ma(MSWD=0.053,n=15),whereas the 206Pb/238U ages of 16 spots of zircons from the metarhyolite in the Panling Formation range from 341Ma±12Ma to 351Ma±14Ma,with a weighted mean 206Pb/238U age of 347Ma±5Ma(MSWD=0.116,n=16).The above dating results indicate that these metavolcanic rocks were formed in the earliest Carboniferous,i.e.,the Xia'ertai Group were formed in the Late Paleozoic rather than in the Early Paleozoic as previously thought.The earliest Carboniferous volcanic rocks are composed mainly of basaltic rocks and rhyolites,suggesting a typical bimodal volcanic rock association and implying an extensional environment.Combined with regional geological data,the authors hold that the eastern section of the northern margin of the North China plate might have been in a passive continental-margin setting in the earliest Carboniferous.