Some problems on granites and vertical growth of the continental crust in the eastern Tianshan Mountains,NW China.
Some problems on granites and vertical growth of the continental crust in the eastern Tianshan Mountains,NW China.
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作者：
Gu Lian
东天山大陆碰撞后花岗岩的形成可分为三个阶段：压缩-伸展过渡阶段（310~ 285 Ma）碰撞后伸展阶段（285~ 250 Ma）和板内阶段（250~ 208 Ma），各期花岗岩的岩石圈厚度、等温线形状和地球动力学特征决定了各期花岗岩在岩体形态、岩石组合、岩石学和地球化学方面的显著特征，除底侵作用、内侵作用能引起不同地壳层次的部分熔融外，是花岗岩岩浆生成和地壳生长的另一重要机制，东天山部分片麻状花岗岩是由变质作用形成的，变质后的峰期韧性剪切带是片麻状花岗岩形成的最有利场所，尽管大多数片麻状花岗岩侵位在地壳深部，但它们的形成和演化与岩浆的底侵作用和内侵作用有着密切的关系。康古尔韧性剪切带的形成除与构造力有关外，还与大陆垂直生长，特别是内板作用密切相关，并包含构造作用、岩浆作用、变质作用博格达造山带碰撞前和碰撞后岩浆岩均具有正Nd（t）值特征，这表明造山带的地幔在碰撞前已经亏损，碰撞后亏损可能继承了碰撞前的地幔东天山存在三叠纪地壳垂直生长事件，伴随该事件的岩浆活动和成矿作用与古特提斯洋俯冲和碰撞有关，代表了东天山由中亚区向特提斯区的构造过渡。
Granite formation after major continental collision in the eastern Tianshan Mountains can be divided into three stages:the compression-extension transition stage(310～285Ma),the post-collision extension stage(285～250Ma)and the intra-plate stage (250～208Ma).Distinguishable granite features of each stage in pluton morphology,rock association,petrography and geochemistry depend upon lithosphere thickness,isotherm shapes and geodynamics of these stages.Apart from underplating,intraplating,which is capable to induce partial melting at various crustal levels,is another essential mechanism for generation of granite magmas and crustal growth.Some of the gneissic granites in eastern Tianshan were formed by metamorphism-metasomatism with ultimate genetic affiliation to post-collisional underplating and intraplating of mantle magmas.Post-metamorphic peak ductile shear zones with highly reduced grain sizes are most favorable sites for the formation of gneissic granites.Despite that most of the gneissic granites are emplaced at deep crustal levels,they can also be formed at relatively shallower levels depending upon the assistance of heat from underplating.Apart from tectonic forces,formation of the Kanguer ductile shear zone is closely connected to continental vertical growth,particularly intraplating,and includes complicated feedback between tectonism,magmatism,metamorphism,and fluid migration.Both the pre- collisional and post-collisional magmatic rocks from the Bogda orogen are characterized by positivee_(Nd)(t)values,indicating that the mantle of this orogen were already depleted before collision and that post-collisional depletion could have been inherited from pre- collisional mantle.An Indosinian(Triassic)event of vertical crustal growth is discerned in eastern Tianshan.Magmatism and ore- formation in accompany with this event was related to the subduction and subsequent collision of the Palaeo-Tethys,and hence represent the tectonic transition of eastern Tianshan from the Central Asia regime to the Tethys regime.