内蒙古西乌旗罕乌拉地区下二叠统寿山沟组碎屑锆石LA-ICP-MS U-Pb年龄及其地质意义

摘要

中亚造山带南缘二叠纪的构造背景一直存在争议.以内蒙古西乌旗罕乌拉地区发育的寿山沟组为研究对象,开展了野外地质、岩石学、碎屑锆石LA-ICP-MS U-Pb年龄研究.2个样品锆石阴极发光图像和Th/U值指示,锆石为岩浆成因锆石.103颗碎屑锆石年龄测试结果显示,年龄信息可划分为5组:285～328Ma,峰期年龄302Ma;338～361Ma;455～490Ma;757Ma;1278Ma、2380Ma.最年轻的年龄为285Ma,结合侵入其中的花岗岩同位素年龄,指示西乌旗罕乌拉地区寿山沟组沉积时限介于285～280Ma之间,主体沉积时代应为早二叠世Sakmarian期—Artinskian期.寿山沟组碎屑锆石反映出近源、快速沉积的特点,沉积物源中含有较多的火山碎屑物,可能代表弧后盆地沉积,为早二叠世古亚洲洋闭合前洋壳俯冲消减作用的沉积响应.结合区域资料,寿山沟组碎屑锆石的年龄对应于东北地区的变质基底及其后的构造岩浆事件,物源区物质主要来自于苏尼特左旗—锡林浩特—西乌旗一带早石炭世末—晚石炭世岩浆弧及贺根山—东乌旗一带,并进一步限制了华北与西伯利亚两大板块的缝合线应位于寿山沟组发育地区的南部,即索伦缝合带,拼合时代最可能为晚二叠世—早三叠世.%There exist different opinions concerning the structural setting of southern Central Asia Orogenic Belt (CAOB) in the Permian period.The authors studied the Shoushangou Formation in Hanwula area of Xi Ujimqin Banner,Inner Mongoha,in such aspects as field occurrence,petrology,and detrital zircon U-Pb isotopic geochronology.The result shows that the cathodoluminescence (CL) images of analyzed zircons of two samples from Shoushangou Formation and their Th/U ratios imply the igneous origin.And the ages of total 103 detrital zircons can be mainly divided into 5 groups:285～328Ma (peak age being 302Ma),338～361Ma,455～490Ma (Early Paleozoic),757Ma (Neoproterozoic) and the fifth group including 1278Ma and 2380Ma (Mesoproterozoic-Paleoproterozoic).The youngest age of dating zircons is 285Ma.Together with the age of the granite that intruded into it,the authors have come to the conclusion that the sedimentary age of the Shoushangou Formation ranges from 285Ma to 280Ma,with the main deposition epoch being the Sakmarian-Artinskian stage of Early Permian.The detrital zircons of Shoushangou Formation indicate that the investigated rocks are immature,probably due to the results of rapid erosion,transport,and diagenetic process and nearby supply derived from the nearby source.Additionally,lots of volcanic clastics were found in the provenance of the Shoushangou Formation.All these characteristics suggest that the Shoushangou Formation was most probably formed in a back-arc spreading environment triggered by oceanic crust subduction before closure of the Paleoasian Ocean in Early Penman.Combined with the regional geology and timing of the magmatism,it is concluded that the detrital zircons of the Shoushangou Formation reflect the information of the metamorphic basement and following tectonomagmatic event in Northeast China,and the provenance of Shoushangou Formation mainly came from the Paleozoic granitoids and volcanic rocks in the Sonid Zuoqi-Xilin Hot-Xi Ujimqin Banner area,with the probable addition of materials from Hegenshan-Dong Ujimqin Banner area.Therefore,the suture zone between the North China and Siberia plate is situated in the south of Shoushangou Formation,which is located along the the Solonker suture zone,and the timing for the final collision might be Late Permian-Early Triassic.