Ultra-high pressure metamorphic eclogite in western Tianshan Mountains, Xinjiang
Ultra-high pressure metamorphic eclogite in western Tianshan Mountains, Xinjiang
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发表时间：
2002
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通讯作者：
Zhangqun Li;A. Yong
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作者：
Zhangqun Li;A. Yong
在西天山3种类型榴辉岩中发现超高压变质矿物，在与镁铁质蓝片岩和碳酸盐榴辉岩互层的I型榴辉岩中发现石榴石中柯石英假像包裹体，在枕状构造的Ⅱ型榴辉岩中发现绿辉石中石英出溶片晶，在大理岩中的Ⅲ型透镜状方解石/白云岩榴辉岩中发现变质成因的菱镁矿。根据矿物学和岩石学研究，Ⅰ型榴辉岩的变质演化可划分为峰前超高压（UHP）变质阶段（356~433℃，0.8~ 1.0GPa），超高压榴辉岩相高峰期（496~598℃，2.572 ~ 2.666GPa）和退变质绿帘石蓝片岩相阶段（500~530℃，1.0~ 1.2GPa）。建立了NCMASCH体系（Na 2 O-CaO-MgO-Al 2 O3-SiO2-CO2-H2 O）的相平衡，计算表明西天山含菱镁矿蓝闪石榴辉岩的峰期变质作用发生在2.7 ~ 2.8GPa，5 2 5 ~ 6 0 7 ℃，XCO 2 0.0 0 6。因此，西天山榴辉岩经历了明显的超高压变质作用，南天山造山带可能是世界上最大的洋壳俯冲超高压变质带。
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.