新疆西天山超高压变质榴辉岩

Ultra-high pressure metamorphic eclogite in western Tianshan Mountains, Xinjiang

在新疆西天山 3类榴辉岩中均发现了超高压变质矿物:在第Ⅰ类与蓝片岩互层的榴辉岩中发现了柯石英假像;在第Ⅱ类枕状榴辉岩的绿辉石中发现了石英出溶叶片;在第Ⅲ类产于大理岩中呈透镜状的方解石/白云石榴辉岩中发现了变质成因的菱镁矿。详细的岩石矿物学研究表明,第Ⅰ类榴辉岩 3个阶段的变质演化温压条件为:UHP前变质阶段(35 6～ 4 33℃,0.8～ 1.0GPa)、峰期UHP榴辉岩相阶段(4 96～ 5 98℃,2.5 72～ 2.6 6 6GPa)和退变绿帘蓝片岩相阶段(5 0 0～ 5 30℃,1.0～ 1.2GPa),并建立了在NCMASCH (Na2 O -CaO -MgO -Al2 O3-SiO2 -CO2 -H2 O)体系中出现蓝闪石、石榴石、绿辉石、菱镁矿、白云石、黝帘石、柯石英、CO2 和H2 O的相平衡关系。经变质反应分析和相平衡理论计算西天山含菱镁矿的蓝闪石榴辉岩的变质峰期温压条件为:2.7～ 2.8GPa和 5 2 5～ 6 0 7℃,XCO2 低于 0.0 0 6。超高压变质矿物的发现表明西天山榴辉岩经历了超高压变质作用,所谓的南天山造山带可能是世界上规模最大的由洋壳俯冲形成的超高压变质带。

Ultra_high pressure metamorphic minerals were found in three types of eclogite in western Tianshan Mountains, inclusions of coesite pseudomorphs in garnets were detected in type I eclogites interlayered with mafic blueschists and carbonate eclogites, quartz exsolution lamellae in omphacites were recognized in type Ⅱ eclogites with pillow structure,and magnesite of metamorphic origin was discovered in type Ⅲ lenticular calcite/dolomite eclogites within marbles. Based on the study of mineralogy and petrology, the metamorphic evolution of typeⅠ eclogites can be divided into three stages, i.e. pre_peak ultra_high pressure (UHP) metamorphic stage (356～433℃, 0.8～1.0 GPa), peak UHP eclogite_facies stage (496～598℃, 2.572 ～2.666 GPa) and retrograde epidote blueschist_facies stage (500～530℃, 1.0～1.2 GPa). The authors have set up phase equilibrium in NCMASCH system (Na 2O-CaO-MgO-Al 2O 3-SiO 2-CO 2-H 2O), and the calculation shows that the peak metamorphism of magnesite_bearing glaucophane eclogites in western Tianshan Mountains occurred at 2.7～2.8 GPa and 525～607℃, X CO 2 <0.006. Consequently, the eclogites from western Tianshan Mountains have obviously undergone UHP metamorphism, and southern Tianshan orogenic belt might be the largest UHP metamorphic belt formed by subduction of oceanic crust in the world.