柴北缘北部中吾农山构造带及邻区中吾农山群物源和构造环境:来自地球化学与锆石年代学的证据

Provenance and Tectonic Setting of the Zhongwunongshan Group from the Zhongwunongshan Structural Belt and its Adjacent Areas in North Qaidam,China:Evidence from Geochemistry and Detrital Zircon Geochronology

中吾农山构造带位于南祁连地块和柴北缘欧龙布鲁克地块之间,其构造特征研究对邻区的构造演化有重要的启示意义。目前关于中吾农山构造带加里东期构造-地层-热事件的研究很少,且对于该构造带及邻区中吾农山群的物质来源和其形成的构造环境仍存在不同观点。本文对中吾农山构造带及邻区中吾农山群开展岩石学、地球化学、碎屑锆石U-Pb年代学及其微区原位Hf同位素分析,结果显示中吾农山群碎屑岩具有较低的化学蚀变指数(CIA<72)和斜长石蚀变指数(PIA=50.26~68.12),较高的成分变异指数(ICV=1.21~1.58),表明成熟度较低,经历了弱-中等的化学风化、不具"沉积再循环"特征。地球化学指标(La/Sc,Th/Sc,Zr/Sc,Cr/Th,Th/Co等)和判别图解(Zr/Sc-Th/Sc和La/Sc-Co/Th)显示源岩主要为长英质岩石。锆石U-Pb年龄谱存在新元古代(0.8~1.0 Ga)和早古生代(0.4~0.45 Ga)两个主要峰值,对应ε_(Hf)(t)分别为-9.6^-3.09和-15.87~9.5,而最年轻碎屑锆石年龄为410 Ma。结合前人研究,推测中吾农山群可能形成于晚奥陶世-早泥盆世早期,而柴北缘鱼卡-沙柳河高压-超高压变质带内新元古代花岗片麻岩和早古生代滩间山蛇绿杂岩带内大陆弧型花岗岩为其主要物源,其沉积形成于早古生代活动大陆边缘-大陆岛弧环境下的弧后盆地。

The Zhongwunongshan structural belt is on the northern margin of the Qaidam Basin which lies between the South Qilian block and Olongbruk block of North Qaidam.The Caledonian tectonic-stratigraphic-thermal events of the Zhongwunongshan structural belt are not well-documented,and the tectonic setting and provenance of the Zhongwunongshan Group in the structural belt and its adjacent areas are controversial.In this study,a comprehensive study of mineralogy,geochemistry,and geochronology was carried out on the Zhongwunongshan Group clastic rocks from the Zhongwunongshan structural belt and its adjacent areas in North Qaidam,to provide constraints on the tectonic setting and provenance.The low Chemical Index of Alteration(mean CIA<72)and Plagioclase Index of Alteration(mean PIA=50.2668.12),as well as relatively high Index of Compositional Variability?(mean ICV=1.21?1.58),indicated a relatively low maturity and weak to moderate chemical weathering,without significant sedimentary recycling.The geochemical analysis results suggested that the sediments were mainly derived from felsic rocks.The detrital zircon U-Pb dating results of the Zhongwunongshan Group clastic rocks showed two major populations with the minimum age of410Ma.The first was Neoproterozoic(0.8?1.0Ga)with aεHf(t)=?15.87to9.5.The other was determined to be Early Paleozoic(0.4?0.45Ga)withεHf(t)=?9.6to?3.09.Integrated with the regional geological research,it can be inferred that the dominated source rocks were the Neoproterozoic granitic gneiss of the Yuqia?Shaliuhe HP-UHP metamorphic belt and the Early Paleozoic continental-arc granite of the Tanjianshan ophiolitevolcanic arc belt in North Qaidam.By combining with previous studies,it was determined that the sediments of the Zhongwunongshan Group were deposited in a back-arc extensional basin which was related to an active continental margin?continental arc environment during the Late Ordovician?Early Devonian.