东昆仑古特提斯后碰撞阶段伸展作用:来自晚三叠世岩浆岩的证据

通过对东昆仑造山带晚三叠世岩浆岩的岩石类型、形成时代、岩石地球化学和同位素地球化学资料综合分析，对岩浆岩的岩石组合、分布特征和岩石成因进行研究，探讨东昆仑造山带晚三叠世构造演化的地球动力学背景。东昆仑造山带晚三叠世是古特提斯演化过程中重要的构造转换期，岩浆岩岩石类型多样，主要包括辉长岩、花岗闪长岩、二长花岗岩和正长花岗岩，并且广泛出露具埃达克质特征的岩浆岩和A型花岗岩。晚三叠世岩浆岩的出露规模与俯冲阶段相比，规模较小，一般以小岩体、岩株和岩脉侵入于早期岩体和地层中。东昆仑晚三叠世岩浆岩主体为准铝弱过铝质高钾钙碱性钾玄岩系列，轻重稀土元素具有一定分异，富集大离子亲石元素，亏损高场强元素，岩石类型不同时分异程度、富集和亏损程度有一定差异。大部分晚三叠世花岗质岩浆岩的同位素特征与晚二叠世—三叠纪镁铁质岩浆岩近似，部分具有更高的εNd(t)和εHf(t)值。镁铁质岩浆岩、普通花岗岩、埃达克质岩浆岩在东昆仑各个构造带皆有分布，A型花岗岩主要分布在祁漫塔格构造带(东昆北)的阿牙克库木湖—香日德断裂附近。东昆仑晚三叠世镁铁质岩浆岩具有弧岩浆岩特征，为俯冲流体交代的地幔楔部分熔融产物。普通花岗岩和埃达克质岩浆岩多为新生下地壳部分熔融产物，少量埃达克质岩浆岩由于与地幔的交代作用，具有幔源特征。A型花岗岩为残留下地壳部分熔融的产物。部分普通花岗岩、埃达克质岩浆岩和A型花岗岩由于岩浆混合作用，具幔源特征。构造环境研究表明，东昆仑在晚三叠世进入古特提斯演化的后碰撞阶段。巴颜喀拉地块同东昆仑地块的持续碰撞导致地壳加厚，密度增大，使岩石圈重力不稳定发生拆沉作用，引发岩石圈地幔减压熔融，产生大量的镁铁质岩浆岩；镁铁质岩浆底侵不同类型地壳熔融及拆沉地壳部分熔融而形成的岩浆交代地幔，以及岩浆混合和岩浆后期演化，形成了东昆仑造山带晚三叠世丰富多样的岩浆岩。

Lithospheric extension of the post-collision stage of the Paleo-Tethys oceanic system in the East Kunlun Orogenic Belt: insights from Late Triassic plutons

We studied in detail the rock assembly, distribution and petrogenesis of Late Triassic plutons in the East Kunlun Orogenic Belt (EKOB) by performing a comprehensive analysis of rock types, intrusion ages, petrogeochemistry and isotopic geochemistry characteristics of the plutons. The Late Triassic is a tectonically pivotal transitional period during the evolution of the Paleo-Tethys oceanic system in EKOB when various magmatic rocks, including gabbro, granodiorite, monzonite and syenite, crystallized extensively while adakitic magmatic rock and A-type granite were extensively exposed. Compared to plutons intruded during subduction stage, the Late Triassic plutons possess smaller scale outcrop, such as small intrusions, stocks and dykes intruding into earlier magmatite and strata. Geochemically, the plutons mainly belong to metaluminous to weakly peraluminous high-K to shoshonite series. They are enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE), Rb, Th, Ba and Cs, and depleted in heavy rare earth elements (HREE) and high field strength elements (HFSE), Nb, Ta and Ti, with varying degrees of differentiation, enrichment and depletion from rock to rock. Most of the plutons have similar isotopic characteristics to that of Late PermianTriassic mafic magmatic rocks; furthermore, some rocks have higher εNd(t) and εHf(t) values. Mafic magmatite, ordinary granite and adakitic magmatite are ubiquitous in EKOB. However, A-type granites are mainly developed in the Qimantagh tectonic zone, near the Ayakekumulake-Xiangride fault. The mafic plutons are most likely derived from partial melting of metasomatic mantle wedge with subduction fluid based on their arc magmatite features. Most of the ordinary granites and adakitic magmatites are partial-melting products of juvenile lower crust, except for some mantle-derived adakitic magmatites with mantle magma mixing, as well as some A-type granites from relict of partial-melting of lower crust. All studies indicate that EKOB stepped into the post-collision stage of the Paleo-Tethys oceanic system in the Late Triassic period. Crust thickening and density increasing, triggered by continuous collision between the Bayanhar block and EKOB, led to lithospheric delamination due to gravitational instability, which resulted in lithospheric mantle decompressional melting to yield much of mafic magma. Mafic magma later on intruded into different crust melts and partial-melts of delaminated crust to form metasomatic mantle magma, which, through magmatic mixing and later stage evolution, produced the rich and diverse plutons of the Late Triassic in EKOB.