北喜马拉雅E-W向伸展变形时限:来自藏南错那洞穹隆Ar-Ar年代学证据  

Timing of E-W Extension Deformation in North Himalaya:Evidences from Ar-Ar Age in the Cuonadong Dome,South Tibet

作　　者：付建刚[1,2] 李光明[1] 王根厚[2] 张林奎[1] 梁维[1] 张志[1] 董随亮[1] 黄勇[1] Fu Jiangang;Li Guangming;Wang Genhou;Zhang Linkui;Liang Wei;Zhang Zhi;Dong Suiliang;Huang Yong(Chengdu Center of China Geological Survey,Chengdu 610081,China;School of Earth Sciences and Resources,China University of Geosciences,Beijing 100083,China)

机构地区：[1]中国地质调查局成都地质调查中心,四川成都610081 [2]中国地质大学地球科学与资源学院,北京100083

出　　处：《地球科学》2018年第8期2638-2650,共13页Earth Science

基　　金：中国地质调查局项目(No.DD20160015);国家自然科学基金项目(No.41602214);国家重点研发计划项目(No.2016YFC060308)

摘　　要：特提斯喜马拉雅带以广泛发育近E-W向和近S-N向断裂以及北喜马拉雅片麻岩穹隆带为典型特征.藏南错那洞穹隆位于特提斯喜马拉带的东部,是近两年新发现并厘定的穹隆构造.该穹隆从外向内主要由3部分组成:上部单元(盖层)、中部单元(滑脱系)和下部单元(核部),其中滑脱系主要由一套强烈变形的片岩、伟晶岩、花岗岩、大理岩和矽卡岩组成,片岩包括含石榴石云母片岩、含石榴石十字石云母片岩、含蓝晶石石榴石十字石片岩、含矽线石蓝晶石石榴石片岩和云母石英片岩.野外构造变形特征表明滑脱系为一条强烈变形的韧性剪切带,发育大量的鞘褶皱、"Z"形揉褶皱和眼球状构造、石榴石的旋转碎斑、SC组构和压力影构造.错那洞穹隆记录了4期构造变形:第1期由北向南的逆冲挤压构造、第2期由南向北的韧性伸展构造、第3期近E-W向的韧性伸展构造变形和第4期成穹后的脆性垮塌构造.通过对滑脱系中含石榴石云母片岩的白云母进行ArAr同位素测年,获得坪年龄为14.0±0.2 Ma,等时线年龄为13.7±0.5 Ma,二者基本一致,同时微观构造特征显示石英呈亚颗粒旋转重结晶(SGR),其韧性变形的温度为450~550℃,该变形温度高于白云母的封闭温度.因此,白云母Ar-Ar坪年龄(14.0±0.2 Ma)代表错那洞穹隆近E-W向伸展变形的时间,也即近S-N向桑日-错那裂谷的活动时间.结合构造变形和年代学特征,认为错那洞穹隆是STDS向北伸展拆离的主导机制叠加后期近E-W向韧性伸展活动的结果.The Tethys Himalaya is characterized by the S-N trending and E-W trending structures,and the North Himalaya gneiss domes（NHGD）.The Cuonadong dome,located at the eastern part of the North Himalaya,is a recently newly identified dome.The Cuonadong dome is divided into three units from outer to inner：the upper unit（the cover rocks）,middle unit（the detachment layer）and lower unit（the core）.The middle unit mainly consists of a series of strong deformation schist,pegma-tite,granite,marble,and skarn.The main schist types include garnet mica schist,garnet staurolite schist,kyanite garnet staurolite schist,sillimanite kyanite garnet schist,and mica quartz schist.This unit is characterized by a ductile shear zone including the sheath fold,augen structure,rotating porphyroclast,S-C structure,and pressure shadow structure.The Cuonadong dome preserves evidences for four major deformational events：N-S thrust（D1）,early approximately S-N extensional deformation（D2）,approximately E-W extensional deformation（D3）,and collapse structural deformation（D4）.Ar-Ar dating of muscovite from the mylonitic schist in the Cuonadong dome yielded Ar-Ar plateau age of 14.0±0.2 Ma and inverse isochron age of 13.7±0.5 Ma,meanwhile the presence of subgrain rotation recrystallization（SGR）in quartz shows that the schist was deformed under high deformation temperature（450~550 ℃）,which is clearly higher than the closure temperature of muscovite.Therefore,we suggest that the Ar-Ar plateau age of 14.0±0.2 Ma represents the age of the E-W extensional deformation in the Cuonadong dome,also the age of the S-N trending Sangri-Cuona rift.Combined with the structural deformation and thermochronology,we suggest that the formation of the Cuonadong dome resulted from both the earlier S-N and later E-W extensional deformations,especially the S-N extensional deformation,i.e.the STDS.

关 键 词：AR-AR定年 伸展变形 错那洞穹隆 北喜马拉雅 地球化学 地质年代学 

分 类 号：P597[天文地球—地球化学]