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

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、S-C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Ar-Ar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,pegmatite,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:top to S thrust (D1),early approximately N-S 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