Provenance Analysis of the Middle-Lower Jurassic Clastic Rocks in the Southern Margin of the Yili Basin and Their Insight into the Evolution of Southern Tianshan Orogenic Belt
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摘要: 伊犁盆地南缘中-下侏罗统碎屑岩的物源特征，可为南天山造山带的演化提供重要证据。对其碎屑岩锆石U-Pb定年研究结果表明，伊犁盆地南缘坎乡下侏罗统八道湾组砂岩的碎屑锆石年龄集中在290~260 Ma，而下侏罗统三工河组的碎屑锆石年龄集中在350~290 Ma和460~390 Ma，中侏罗统西山窑组的碎屑锆石年龄集中在370~320 Ma和450~390 Ma。所有测试样品中前寒武纪的年龄记录非常少。这些特征表明，伊犁盆地南缘中生代碎屑沉积物主要来自于伊犁-中天山地块南部。测试样品中几乎不存在晚二叠世-中三叠世的碎屑锆石，与南天山造山带的岩浆岩记录一致，暗示在晚二叠世-中三叠世南天山地区并没有发生强烈的与碰撞或后碰撞相关的岩浆活动。该结果不支持塔里木克拉通与伊犁-中天山地块在晚二叠世-中三叠世碰撞的观点。结合高压-超高压变质岩的数据和地层记录，认为塔里木克拉通与伊犁-中天山地块的碰撞发生在晚石炭世。同时，样品中最年轻锆石的年龄数据从早侏罗世到中侏罗世逐渐增大，显示了揭顶沉积的特点。对伊犁盆地南部中生代的锆石年龄数据与同时代南天山地区的锆石年龄数据进行综合对比表明在早-中侏罗世发生构造沉积夷平的特征。Abstract: The provenance features of the middle and lower Jurassic clastic rocks in southern margin of the Yili Basin can provide important information for insights into the evolution of southern Tianshan Orogenic Belt. Detrital zircon age of the Badaowan Formation sandstone of lower Jurassic in southern margin of the Yili Basin demonstrates a prominent age population at 290-260 Ma, whereas the Sangonghe Formation of lower Jurassic shows two prominent age populations at 350-290 Ma and 460-390 Ma, respectively. The Xishanyao Formation of middle Jurassic has two prominent detrital zircon age populations at 370-320 Ma and 450-390 Ma. All study samples have little zircon age record of Precambrian period. These features indicate that Mesozoic detrital sediments in southern margin of the Yili Basin predominantly might be derived from the south part of the Yili-Central Tianshan Block. The late Permian to middle Triassic zircons are nearly absent, consistenting with magmatic record in the South Tianshan Orogenic Belt and probably indicating no strong magmatic activities associated with collision or post-collision occurred in South Tianshan Block during late Permian-middle Triassic periods, which does not support the idea that the collision between the Tarim Craton and Yili-CTS Block occurred in the late Permian-middle Triassic periods. Combined with previously published data of tectonic exhumation of high to ultrahigh pressure metamorphic rocks and stratigraphic records, these data support the idea that the collision between the Tarim Craton and Yili-CTS Block occurred in the late Carboniferous period. The youngest detrital zircon ages of the studied samples are increasing from the middle-late Triassic to early Jurassic periods to middle Jurassic period, indicating a feature of unroofing and sedimentation. Integration of the provenance data of Mesozoic strata in the southern Yili Basin with the time equivalent of the South Tianshan Belt, provides an integrated record of the tectonic subsidence and planation in the early-middle Jurassic period.
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