中国阿尔泰造山带西部二叠纪构造演化:来自构造地质及独居石U-Pb年代学的制约

Permian Deformational History of Western Chinese Altai Orogenic Belt: Insights from Structural and Monazite U-Pb Data

晚古生代是中国阿尔泰造山带构造演化的关键时期,对于其大地构造环境一直有着碰撞后伸展或持续俯冲增生的争论。而究其根源主要是缺乏对该时期演化的相关构造变形制约。当前,对于中国阿尔泰造山带晚古生代岩石和构造的研究多集中于其南缘额尔齐斯断裂带,而对于造山带内部构造变形的研究则略显薄弱。本文对中国阿尔泰造山带西部布尔津县-冲乎尔乡一带开展了详细的构造地质学和独居石U-Pb年代学研究。研究结果表明,中国阿尔泰西部存在着三期主要变形,第一期(D1)以发育近水平面理为特征,第二(D2)、第三期(D3)均受到NE-SW向构造挤压形成NW-SE向近直立轴面劈理。本文对同D3期变形的伟晶岩脉和发生褶皱变形的片岩、片麻岩样品进行独居石U-Pb定年分别获得~275 Ma和~267 Ma的年龄,代表了第三期构造挤压作用时代为二叠纪中期。结合区域资料,认为中国阿尔泰造山带在二叠纪中期仍处于构造挤压而非伸展的大地构造环境。

The late Paleozoic tectonic regime of the Chinese Altai Orogenic Belt has been discussed controversially in different models,involving subduction-accretion related compression or post-collisional extension.Up to date,comparative structural and geochronological studies from the hinterland of the Chinese Altai are rare and data available are mainly restricted on the south Chinese Altai orogen along the Erqis Fault Zone.Consequently,our knowledge of the late Paleozoic tectono-thermal evolution of the region remains fragmentary.Here we present new structural and geochronological data from the western Chinese Altai in order to decipher the geodynamic evolution of the region.Three major episodes of tectonic deformation were identified.D1 is featured by a penetrative sub-horizontal fabric S1.D2 is represent by upright folds(F2)with NW-SE-striking axial planes and spaced foliations(S2)in relatively shallow crustal levels and by subvertical transposition foliation(S2)in the relatively deep crustal levels.D3 is marked by upright folds(F3)with NW-SE-striking axial planes,steep dipping hinges and spaced sub-vertical foliation(S3)parallel to S2,implying NE-SW directed compression.Monazite minerals from syn-D3 pegmatite veins and metapelitic schist-gneiss yielded U-Pb age at^275 Ma and^267 Ma,respectively,interpreted as the timing of D3.Combined with regional geological data available,our data suggest that the Chinese Altai was under compressional rather than extensional tectonic regime in the late Paleozoic.