共和盆地东北部新生代沉积物源及其地质意义

Provenance of Cenozoic sediments in the northeastern Gonghe Basin and its geological significance.

共和盆地处于秦岭、祁连、昆仑造山系相互叠置的关键区域,盆地及周缘造山带记录的构造、岩浆、变质、沉积作用事件复杂多样,是研究青藏高原北部构造-沉积演化的关键位置。本文以多口钻井的岩心为基础,结合区域地质调查资料对共和盆地东北部进行了岩石组合、沉积类型和沉积相特征分析、碎屑锆石U-Pb年代学、砂岩组分和重矿物分析,明确了共和凹陷新生代的沉积特征、地层发育格架、沉积相类型、沉积物源及构造-沉积演化过程。碎屑锆石年代学分析结果揭示了共和凹陷新生代砂岩具有200~300Ma、400~500Ma、750~1000Ma、1800~2000Ma以及2400~2550Ma的年龄峰值,且主要集中在200~300Ma和400~500Ma,表明三叠纪的侵入岩及浅变质岩是其主要物源。砂岩组分和重矿物分析结果记录到共和盆地东北部在咸水河组和临夏组沉积之间发生过一次构造环境转变,其时共和凹陷的沉积物源发生变化,来自青海南山的物质供给增加。共和盆地东北部共和凹陷新生代沉积物在西宁组-咸水河组沉积时物源方向为NEE方向,而在临夏组沉积时物源方向为NNE方向。青海南山的隆起主要发生在晚中新世-上新世,分隔了青海湖盆地与共和盆地。

Gonghe Basin records a series of complex and diverse tectonic,magmatic,metamorphic and sedimentary events of the Qinling,Qilian and Kunlun orogenic systems around it,and it is a key position to study the tectonic-sedimentary evolution of the northern Tibetan Plateau.Combined with regional geological survey data in this area,and based on systematic analyses of the lithologies and rock assemblages,detrital-zircon geochronology,sandstone components,heavy mineral compositions together with sedimentary facies of the Cenozoic strata from drilling cores,we reconstructed the stratigraphic correlation framework of the strata and discussed the tectonic-sedimentary evolution and sedimentary provenance in the Gonghe sag of the Gonghe Basin during Cenozoic.The chronological analysis of detrital zircons of the Cenozoic sandstones(sediments)in Gonghe sag shows that they have age peaks of 200~300Ma,400~500Ma,750~1000Ma,1800~2000Ma and 2400~2550Ma,respectively,and they are mainly concentrated in between 250~300Ma and 400~500Ma,which means that the Triassic intrusive rocks and low-grade metamorphic rocks are the main provenance.The analysis results of sandstone components and heavy minerals recorded a transition of tectonic environment in between the Xianshuihe Formation and Linxia Formation in the northeastern part of the Gonghe Basin.The sources of the Cenozoic sediments in the Gonghe sag were from its NEE direction during the sedimentation of the Xining Formation-Xianshuihe Formation,while those were from the NNE direction during the sedimentation of the Linxia Formation.Based on previous research and provenance analysis results of this study,it is believed that the uplift of the Qinghai Nanshan occurred during the Late Miocene-Pliocene,separating the Qinghai Lake Basin and the Gonghe Basin.