连云山岩体韧性走滑剪切带变形特征及其动力学

Deformation Characteristics and Dynamics of the Ductile Strike-slip Shear Zones in the Lianyunshan Pluton

晚侏罗世末‒早白垩世早期中国东部岩石圈经历了从挤压到伸展的构造转变,这一过程反映了古太平洋板片活动方式的变化。连云山岩体位于江南造山带中段,发育E-W向和NE-SW向走滑剪切带。E-W向走滑剪切带(F1)弥散地发育于二云母花岗岩中,而NE-SW向走滑剪切带(F2)以应变集中带形式切过黑云母花岗岩和斑状花岗岩岩株,叠加在F1之上。锆石U-Pb定年结果表明,连云山岩体中剪切变形的黑云母二长花岗岩和斑状花岗岩的侵位年龄分别150±2 Ma和138±5 Ma。从F2剪切带斑状花岗岩中心向边缘,石英C轴组构揭示糜棱岩中石英滑移系从柱面<c>到底面,表明从岩体中心向边缘变形温度从500℃降至约300℃。据此可以确定F1变形时间为150 Ma之后,F2变形时间为斑状花岗岩冷却至中心温度约400~500℃时,接近138 Ma。应力分析揭示两条剪切带发育的最大主应力(σ1)为NEE-SWW向,指示早白垩世早期古太平洋板块向扬子板块低角度俯冲方向为NEE-SWW向,深俯冲板片“回绕”时间略晚于138 Ma。

The lithosphere of the eastern China underwent a tectonic shift from compression to extension at the end of the Late Jurassic or the beginning of the Early Cretaceous,which is of great significance for revealing the changes of the subduction type of the Palaeopacific plate.However,the deformation time and kinematics and dynamics of the compression of the lithosphere of the eastern China remain controversial.Two strike slip shear zones,striking along E-W(F1)and NE-SW(F2),respectively,developed in the Lianyunshan pluton that is located in the middle section of the Jiangnan orogenic belt.Foliations of F1 dispersedly penetrate into the biotite granite and overprinted by the foliations of F2,whereas the foliations of F2 occur as a strain concentration belt that cut through the biotite granite and porphyritic granite stocks.U-Pb dating of zircon grains from the deformed two-mica granite and porphyritic granite yielded ages of 150±2 Ma and 138±5 Ma,respectively.Patterns of the crystallographic preferred orientation of quartz show a variation in slip system from prismatic<c>to basalfrom center to boundary of F2,corresponding to the variation of the deformation temperature of ca.500℃to ca.300℃.Combing the ages of the intrusions with the deformation temperature,the deformation time of F2 can be limited at about 138 Ma.Stress analysis shows that the maximum principal stress(σ1)for F1 and F2 was in the NEE-SWW direction.We believe that the compression resulted from the low angle subduction of the Palaeopacific plate toward the Yangtze plate,and the subducted Palaeopacific plate rolled back at somewhat later than 138 Ma.