西藏定结高压基性麻粒岩(退变榴辉岩)的变质P-T轨迹及构造意义  

Metamorphic P-TPath of High-Pressure Mafic Granulite(Retrograded Eclogite)from Dinggye of Tibet and Its Tectonic Implication

作　　者：丁自耕 仝来喜[1] 刘小汉[3] 刘兆[1,2] 周学君 

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]中国科学院大学,北京100049 [3]中国科学院青藏高原研究所,北京100085

出　　处：《地球科学》2018年第1期220-235,共16页Earth Science

基　　金：国家自然基金项目(No.41272261)

摘　　要：定结(Dinggye)位于藏南高喜马拉雅结晶岩系的中部,研究该区域麻粒岩的变质P-T轨迹对于理解青藏高原的碰撞和抬升过程至关重要.通过对该地区的高压基性麻粒岩(退变榴辉岩)的岩相学观察,确定了4期矿物组合:(1)峰期榴辉岩相矿物组合(M_1)由石榴子石(核部)+绿辉石(假象)+石英+金红石组成;(2)高压麻粒岩相矿物组合(M_2)主要由石榴子石(幔部)+单斜辉石+斜长石+钛铁矿+角闪石+黑云母组成;(3)中压麻粒岩相矿物组合(M_3)由石榴子石(边缘)+斜方辉石+斜长石+钛铁矿+黑云母组成;(4)角闪岩相矿物组合(M_4)主要由角闪石+斜长石组成.在NCFMASHTO体系下,用THERMOCALC软件对该高压基性麻粒岩进行了热力学模拟.结合传统温压计和平均温压计计算结果,求得M_2、M_3、M_4阶段的温压条件分别为786~826℃、0.78~0.96GPa;798~850℃、0.71~0.75GPa;610~666℃、0.51~0.60GPa,这指示了一条以峰期后近等温降压(ITD)为特征的顺时针P-T轨迹.结合已有地质资料,表明定结高压基性麻粒岩(退变榴辉岩)是喜马拉雅碰撞造山的产物,峰期后经历了近等温降压的构造抬升过程.Dinggye is located in the central part of the Greater Himalayan crystalline complex（GHC）in southern Tibet.It is essential to investigate the metamorphic P-T path of granulite in this area to better understand the collision and uplifting process of the Tibetan plateau.The petrological study of the high-pressure mafic granulite（retrograded eclogite）from the region indicates four stages：（1）peak eclogite facies mineral assemblage（M1）consists of garnet（core）＋omphacite（psedomorph）＋ quartz＋rutile;（2）high-pressure granulite facies mineral assemblage（M2）comprises garnet（mantle）＋clinopyroxene＋plagioclase＋ilmenite＋amphibole＋biotite;（3）medium-pressure granulite facies assemblage（M3）is composed of garnet（rim）＋orthopyroxene＋plagioclase＋biotite;（4）amphibolite facies mineral assemblage（M4）consists ofamphibole＋plagioclase.Using the THERMOCALC program,the thermodynamic modeling in the NCFMASHTO system has been undertaken for the high-pressure mafic granulite.Combined with the conventional thermobarometers and the average P-T estimates,the P-Tconditions of the different metamorphic stages are estimated to be 786-826℃,0.78-0.96 GPa（M2）;798-850℃,0.71-0.75 GPa（M3）;and 610-666℃,0.51-0.60 GPa（M4）,respectively,indicating apost-peak clockwise P-T path characterized by nearly isothermal decompression.Combined with geological data available,we propose that the highpressure mafic granulite（retrograded eclogite）of the Dinggye formed during the Himalayan collisional orogeny,and underwent apost-peak tectonic uplift process of nearly isothermal decompression.

关 键 词：高压基性麻粒岩 P-T轨迹 碰撞造山 高喜马拉雅 定结 岩石学 

分 类 号：P581[天文地球—岩石学]