东昆仑地区昆仑河韧性剪切带的识别、时代厘定及构造意义

Recognition, age determination and tectonic significance of the Kunlun River ductile shear zone in the East Kunlun Mountains

位于东昆仑造山带的青海昆仑河地区发育一条走向近东西向的大型韧性剪切带,卷入韧性剪切变形的岩石主要为中—新元古界万宝沟群变质中基性火山岩、碎屑岩和大理岩,奥陶系纳赤台群变质碎屑岩、玄武岩和大理岩,以及古生代(志留纪)中酸性侵入岩,剪切带内的岩石多已发生糜棱岩化。剪切带中发育的σ型旋转碎斑、S-C组构、糜棱面理、石英脉的不对称褶皱等构造标志,总体指示以右行走滑剪切为主,晚期有左行走滑剪切作用叠加。LA-ICP-MS锆石U-Pb定年结果显示,剪切带中变形的花岗闪长岩及未变形的白云母花岗岩的结晶年龄分别为432.3±1.2Ma和423.3±1.5Ma,从而限定昆仑河韧性剪切带的形成时代介于中-晚志留世432~423Ma之间。结合区域构造演化特点,认为昆仑河韧性剪切带反映了东昆仑原特提斯洋关闭后,昆北与昆南地块于中-晚志留世碰撞造山运动的构造响应,晚期的左行走滑则可能受控于昆南断裂带燕山早期的左行走滑作用。

Along the Kunlun River of Qinghai Province in the East Kunlun Mountains,a large-scale ductile shear zone is developed.The rocks that involved in the ductile shear deformation and experienced extensive mylonitization include the metamorphosed basic to intermediate volcanic rocks,clastic rocks and marble of the Middle-Upper Proterozoic Wanbaogou Group,the metamorphosed clastic rocks,basalt and marble of the Ordovician Nachitai Group,and the Silurian intermediate to acid intrusive rocks.The structural marks observed in the shear zone,such as theσ-type rotated porphyroclasts,S-C fabrics and the asymmetrical folds of mylonite foliation,indicate in general a dextral strike-slip shear superimposed by a later sinistral shear.Zircon LA-ICP-MS U-Pb ages of the deformed granodiorite and the undeformed muscovite granite in the shear zone are 432.3±1.2Ma and 423.3±1.5Ma,respectively,suggesting that the shear zone formed between ca.432Ma and 423Ma.Combined with the regional tectonic evolution,it is suggested that the Kunlun River shear zone represents the tectonic response to the collisional orogeny in the Middle-Late Silurian between the North and South Kunlun blocks after the closing of the Proto-Tethys Ocean.The later sinistral shearing was controlled by the strike-slipping of the South Kunlun fault zone in the early Yanshanian period.