北京周口店三好砾岩的时代、物源背景及其古地理意义:来自沉积学和碎屑锆石年代学的证据

Zircon U-Pb Chronology and Provenance of the Sanhao Conglomerate in Zhoukoudian,Beijing:Implications for Coeval Paleogeography

三好砾岩出露于北京周口店地区太平山北坡,不整合覆盖于中奥陶统马家沟组之上,以分选好、磨圆好、胶结好为特征。研究表明:三好砾岩是一套二次改造的滨岸砾岩,代表了晚石炭世海侵过程中的底砾岩。来自于砾岩和其夹层中砂岩层的碎屑岩浆锆石年龄均集中在330~300 Ma之间,峰值年龄为约310 Ma,结合区域地层展布情况与华北北缘的构造热事件记录认为,它是晚石炭世至早二叠世沉积,具备准同生沉积特征。借助上述U-Pb年代学数据,结合碎屑组分特征、古流向分析及同期华北北缘岩相古地理分布特征表明,三好砾岩的物源区为内蒙古隆起。以三好砾岩为代表的上石炭统底砾岩反映了内蒙古隆起迅速隆升、剥蚀、搬运和沉积的过程,这套砾岩亦可以作为当时盆山边界的标识物。三好砾岩中高含量的晚石炭世岩浆锆石暗示了内蒙古隆起初始隆升的时限应在330~300 Ma之间。

This paper reports in situ U-Pb dating on detrital zircons from the basal conglomerate of Upper Carboniferous in Beijing which constrains the evolution and paleogeography of the northern part of the North China Craton（NCC）,the Inner Mongolia Paleo-Uplift.Isotopic dating results from the conglomerate and interbedded sandstone reveal a significant concentration around 310 Ma.Furthermore,the minimum single grain represents（300±3）Ma.The stratigraphic distribution suggest that the basal conglomerate above the unconformity surface of the Middle Ordovician should have been deposited between the Late Carboniferous and the Early Permian.Beyond that,there is a good coherence between the ages of Neoarchean and Paleoproterozoic zircons and coeval tectonothermal events distribution.Some Phanerozoic zircons have similar geochemical characteristics to those of the igneous zircons from intrusive rocks acquired from the NCC,indicating the source of the conglomerate was locally derived.A significant concentration of the Late Carboniferous zircon implies a connection between magmatism and a sharp uplifting event of Inner Mongolia.In this case,the uplifting process should have occurred during 330-300 Ma.In a broader sense,the basal conglomerate of Upper Carboniferous provides implications for calibrating the basin-mountain boundary near Inner Mongolia Paleo-Uplift and certain reference to the palaeogeography reconstruction.