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古南天山洋闭合过程中,由于洋壳俯冲产生的岛弧岩浆作用加热大陆地壳,在新疆西南天山木扎尔特一带形成了一套低压高温泥质麻粒岩相变质岩石。本文用Theriak-Domino热力学软件对该套岩石中的堇青石榴夕线石黑云母片麻岩和含夕线石堇青石榴黑云母片麻岩进行了岩石学相平衡计算研究,得到它们峰期变质的温压条件分别是:T=630～674℃,P=5.2～5.5kbar和T=645～684℃,P=5.4～5.7kbar。并采用独居石Th-U-Pb电子探针定年方法,对样品WQ006中的3颗独居石进行了原位年龄测定(38个分析点),得到2组等时线年龄,分别是376±8Ma和280±8Ma(2σ)。结合独居石的岩相学特征,提出了新疆西南天山低压高温麻粒岩相峰期变质作用的时代为280±8Ma,而376±8Ma(2σ)可能为原沉积岩的原岩/成岩年龄。表明西南天山洋壳开始俯冲发生在晚古生代,进一步证明了西南天山造山带俯冲碰撞发生在晚二叠纪之后的观点。
During the closure of the Shannan ocean in the southern South China Sea, a set of low-pressure and high-temperature argillaceous granulite granulite facies metamorphic rocks were formed in Muzartt, Tianshan Mountains, southwestern Xinjiang, due to the island arc magma produced by oceanic crust subduction. In this paper, we use the Theriak-Domino thermodynamic software to study the lithophile dumbbell biotite mica gneisses and the ettringite-garnet biotite gneisses in this set of rocks, and obtain their peak metamorphic Temperature and pressure conditions are: T = 630 ~ 674 ℃, P = 5.2 ~ 5.5kbar and T = 645 ~ 684 ℃, P = 5.4 ~ 5.7kbar. In-situ dating (38 analysis points) was performed on three monazite samples in sample WQ006 using the monazite Th-U-Pb electron probe dating method. Two sets of isochron age were obtained, which were 376 ± 8Ma And 280 ± 8Ma (2σ). According to the lithofacies characteristics of monazite, the age of peak metamorphism of low pressure and high temperature granulite facies in the southwestern Tianshan Mountains in Xinjiang is 280 ± 8Ma, while that of 376 ± 8Ma (2σ) may be the protolith / diagenetic age of the original sedimentary rocks. It shows that the subduction of the oceanic crust in the southwest Tianshan occurred in the Late Paleozoic and further proved that the subduction collision of the southwestern Tianshan orogenic belt occurred after the Late Permian.