扬子西北缘碧口微地块南华系碎屑锆石物源示踪及其地质意义

Provenance Tracing of Detrital Zircons from Nanhua System in Bikou Microblock,Northwestern Margin of Yangtze and Its Geological Significance

扬子板块西北缘碧口微地块华严寺地区碧口群之上发育一套含砾沉积地层,其确切沉积时限对研究该区域的沉积—构造演化具有重要意义。本文对该套地层中的2件砂岩样品进行了碎屑锆石LA-ICP-MS U-Pb测年分析,结果显示,锆石晶型较好,无色透明,磨圆度较差,具典型岩浆生长振荡环带和韵律结构,Th/U比值为0.28~3.6,锆石U-Pb年龄介于706~2489Ma之间。存在3个主要的年龄组:新元古代年龄组(706~951 Ma),占92.9%,显著峰值为850 Ma和843 Ma;中元古代年龄组(1017~1080 Ma),占2.6%;古元古代年龄组(1628~2489 Ma),占4.5%。最小年龄组为706~715 Ma(峰值为711 Ma),结合区域地质和研究资料,华严寺地区沉积地层时代应属于南华纪,沉积时限约为720~635 Ma,物源区主要包括扬子板块西北缘碧口微地块以及南东侧的后龙门山构造带和汉南—米仓山微地块。新元古代晚期碧口微地块及扬子板块西北缘后碰撞—裂解阶段,华严寺地区南华纪沉积为碧口微地块及邻区新元古代岩浆岩在边缘裂谷环境中快速堆积形成。

A set of gravel-bearing sedimentary strata developed above the Bikou Group in the Huayansi area of the Bikou microblock on the northwestern margin of the Yangtze Plate.The exact time limit of its deposition is of great significance for studying the depositional characteristics and tectonic evolution of this area.In this paper,the detrital zircon LA-ICP-MS U-Pb dating analysis of the two sandstone samples in this set of formations shows that the two samples of detrital zircon have good average crystal form,colorless and transparent,and relatively roundness.Poor,with typical magma growth oscillation zone and rhythm structure inside,Th/U ratio is 0.28~3.6,zircon U-Pb age is between 706~2 489 Ma.There are three main age groups:Neoproterozoic age group(706~951 Ma)(92.9 percent),with significant peaks of 850 Ma and 843 Ma;Mesoproterozoic age group(1 017~1 080 Ma)(2.6 percent) and Paleoproterozoic age group(1 628~2 489 Ma)(4.5 percent),the smallest age group 706~715 Ma(peak value is711 Ma),combined with the regional geology and research dates,the sedimentary stratigraphic age of the Huayansi area should belong to the Nanhua Period,and the sedimentary time limit is about 720~635 Ma.The source area includes the Bikou micro-block on the northwestern margin of the Yangtze Plate,the Houlongmenshan tectonic belt and the Hannan-Micangshan microblock on the southeast side.In the late Neoproterozoic Bikou microblock and the post collision cracking stage of the northwest margin of the Yangtze plate,the Nanhua sedimentation in Huayansi area was formed by the rapid accumulation of Bikou microblock and Neoproterozoic magmatic rocks in the adjacent area in the marginal rift environment.