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

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（M₁）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（M₂）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（M₃）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（M₄）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M₂、M₃、M₄阶段的温压条件分别为786~826 ℃、0.78~0.96 GPa；798~850 ℃、0.71~0.75 GPa；610~666 ℃、0.51~0.60 GPa，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. 

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

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 of amphibole+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-T conditions 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 a post-peak clockwise P-T path characterized by nearly isothermal decompression. Combined with geological data available, we propose that the high-pressure mafic granulite (retrograded eclogite) of the Dinggye formed during the Himalayan collisional orogeny, and underwent a post-peak tectonic uplift process of nearly isothermal decompression.