西秦岭造山带徽县-成县盆地早白垩世沉积物源分析——锆石LA-ICP-MS U-Pb年代学的约束

    rovenance analysis of Early Cretaceous Huixian-Chengxian basin, western Qinling orogenic belt, China: constraints from zircon U-Pb geochronology

    • 摘要: 运用LA-ICP-MS U-Pb方法,对徽县-成县盆地下白垩统田家坝组花岗岩砾石和周家湾组、鸡山组砂岩中的锆石进行U-Pb年龄测定,分别获得了25组、89组和123组U-Pb有效年龄。结果表明:田家坝组花岗岩砾石年龄有2期,分别是210.8Ma±0.6Ma和218.8Ma±1.8Ma,与前人测得的迷坝岩体的年龄一致。周家湾组砂岩中的碎屑锆石U-Pb年龄主要分布在186~240Ma和629~1182Ma 2个区间,而鸡山组砂岩中的碎屑锆石U-Pb年龄集中在179~233Ma、305~459Ma和632~1281Ma 3个区间,表明物源区存在差异。综合分析锆石的U-Pb年龄数据,认为田家坝组物源主要来自于盆地南缘的迷坝岩体。周家湾组物源除迷坝岩体外,大量元古宙地层为其提供了沉积物。少量石炭系和泥盆系—奥陶系则成为鸡山组的次要物源区。碎屑锆石年龄结果还表明,徽成盆地南缘没有沉积下三叠统—二叠系,徽成盆地南部地区在207 Ma(206 Ma)、448Ma(428Ma)、791 Ma(874Ma)等发育多期岩浆活动,915Ma(910Ma)发生区域变质作用。周家湾组1个太古宙碎屑锆石年龄和已有的研究成果,有力地证明了西秦岭造山带存在新太古代基底。

       

      Abstract: Detrital zircon U-Pb ages of three samples from the granite and sandstones in Huixian-Chengxian basin were measured by the LA-ICP-MS method. The sample of 09TB4, 09DPL25 and 09DPL41 respectively records 25, 89 and 123 age data. The granite boulder ages are shown in 210.8±0.6Ma and 218.8±1.8Ma, which are consistent with Miba granite age dated by Sun et al. Statistical analyses show that the provenances of Zhoujiawan Formation are 186~240Ma and 629~1182Ma rocks while main provenance ages of Jishan Formation are 179~233Ma, 305~459Ma and 632~1281Ma. Detrital zircon data show that different distribution and provenance are variable. All of zircons U-Pb ages indicate that sediments of Tianjiaba Formation are mainly from Miba granite, and Proterozoic rocks provide sediments for Zhoujiawan Formation besides Miba granite. The Carboniferous, Devonian to Ordovician rocks are subordinate provenance for Jishan Formation. These also showed that the lower Triassic-Permian rocks didn′t occur in southern margin of Huicheng basin. There occured a few magmatic activities during 207 Ma (206 Ma), 448Ma (428Ma), 791 Ma (874Ma) periods and developed regional metamorphism in 915Ma (910Ma) in the southern areas of Huicheng basin.Together with the exsiting data, one detrital zircon age in Zhoujiawan Formatin strongly suggests that the basement rocks in the western Qinling orogenic belt were formed in the Neo-Archean period.

       

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