特提斯喜马拉雅错那洞穹隆成因机制研究:来自构造变形与独居石U-Pb年代学的制约  

Formation of the Cuonadong Dome,Tethyan Himalaya:Constraints from Petro-structural Observations and Monazite U-Pb Geochronology

作　　者：王连峰 徐扛 李智勇 王强[1,2] 蒋映德[1,2] WANG Lianfeng;XU Kang;LI Zhiyong;WANG Qiang;JIANG Yingde(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,Guangdong,China;CAS Center for Excellence in Deep Earth Science,Guangzhou 510640,Guangdong,China;University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]中国科学院广州地球化学研究所,同位素地球化学国家重点实验室,广东广州510640 [2]中国科学院深地科学卓越创新中心,广东广州510640 [3]中国科学院大学,北京100049

出　　处：《大地构造与成矿学》2022年第2期253-271,共19页Geotectonica et Metallogenia

基　　金：第二次青藏高原科学考察研究计划(STEP)(2019QZKK0702)资助。

摘　　要：为厘清错那洞穹隆的成因机制,本文开展了详细的变质变形演化及独居石U-Pb年代学研究,结果表明错那洞地区发育了两期主要构造变形。早期变形以发育近水平变质面理为特征,而晚期变形以普遍发育轴向近东西的褶皱和高角度轴面劈理为特征。变质岩相学表明,早期面理伴随云母、石榴石、十字石等巴罗型特征变质矿物组合的发育,暗示地壳埋深加厚的进变质演化过程;而晚期变质面理则表现为长石、石英及云母的重结晶和重定向排列,无更高级变质矿物的发育,具剥露的退变质演化特征。伴随晚期变形,穹隆南侧核部与边部转换带发育一向南伸展的韧性剪切带,而在穹隆东西两侧分别发育具有右行及左行的水平走滑断层,表明穹隆边部相对于核部向南伸展拆离的剪切变形。独居石U-Pb年代学结果表明穹隆中淡色花岗岩的形成年龄约为19.9±1.0 Ma,而穹隆核部石榴石‒十字石云母片岩为19.3±2.2 Ma,后者为晚期面理的形成时间。综合区域已有研究成果,指出错那洞地区早期变形发生在中‒晚始新世,可能与该时期印度‒亚洲板块碰撞而产生的地壳垂向显著增厚相关;而第二期变形可能与中新世藏南拆离系活动的背景下,中上地壳发生南北向巨量缩短并伴随深部地壳物质经构造挤出在浅部地壳形成穹隆有关。The underlying mechanism accounting for the formation of the Cuonadong Dome remains obscure due to the lack of detailed investigations on the tectonic-metamorphic history of the dome and its associated metamorphic envelops.In order to fill this research gap,detailed petro-structural analysis and monazite U-Pb geochronological studies were carried out for the Cuonadong Dome.On the basis of structural investigations,two prominent episodes of deformation were identified,designated as D_(1) and D_(2).The first episode of deformation(D_(1))is characterized by the formation an ubiquitous sub-horizontal metamorphic foliation(S_(1))whereas the second one(D_(2))is marked by the widespread nearly west-east-trending close to open F_(2) folds in association with spaced high-angle axial plane cleavages S_(2).Metamorphic petrographic analysis revealed that the S_(1) foliations developed in the metamorphic envelope and in the core of the dome are up to biotite zone and staurolite zone conditions,respectively.Petro-structural analysis shows that biotite,garnet and staurolite index minerals grew synchronously with the S_(1) metamorphic foliations,suggesting D_(1) was probably associated with prograde Barrovian-type metamorphism and thus points to a progressive burial history.In contrast,during the D_(2) event,the S_(1) assemblage was affected by the F_(2) folding but remained stable,except for some quartz,feldspar and mica grains aligned parallel to the S_(2) foliations,which could represent re-orientated S_(1) assemblage or newly grew S_(2) assemblage.These features suggest that the S_(2) fabric probably corresponded with a retrograde evolution,implying an exhumation process.Besides,a prominent ductile shear zone developed syntectonically with D_(2) along the southern flank of the dome,as a transition zone separating the metamorphic envelop and the core of the dome.Meanwhile,the dextral and sinistral strike-slip movements developed along the eastern and western flanks of the dome,respectively.These features collectively suggest

关 键 词：特提斯喜马拉雅 错那洞穹隆 构造变形 独居石U-Pb年龄 穹隆成因 

分 类 号：P542[天文地球—构造地质学] P597[天文地球—地质学]