南天山基性麻粒岩中变形石英和石榴子石的TEM研究

基性麻粒岩产于大陆地壳深部,它的研究可以为大陆岩石圈的性质、组成、演化和深部动力学过程提供重要信息。南天山库米什榆树沟地区发育总厚达1000多米的基性麻粒岩构造岩片,走向南东,朝北东推覆。剖面上和石炭纪蛇绿混杂岩共存,并发生了石榴子石的旋转变形和强烈的石英-长石矿物相韧性变形。然而它们的变形条件至今尚未被研究。本文运用透射电镜(TEM)超微组构技术对榆树沟基性麻粒岩中石英和石榴子石进行了详细研究,结果表明,石英中存在着丰富的超显微变形构造,自由位错、亚颗粒和位错网非常发育,这表明研究区基性麻粒岩在折返抬升过程中曾遭受过较强的石英-长石相韧性变形作用;石榴子石中的超微变形构造则极为少见,只在其中发现几根自由位错,并在其颗粒边缘发现少量亚颗粒构造,说明该矿物主体属于脆性变形。这两种矿物变形性质的差异,反映了石榴子石的晶体结构在该条件下比石英更稳定,而且说明石榴子石的变质温度没有超过900℃,也就是说,本区麻粒岩的变质作用并非发生在极高温度的条件下。

A TEM Study of Deformed Quartz and Garnet in the Mafic Granulite from the Southern Tianshan,China

The mafic granulite occurs in the depth of the continental crust. Research on the mafic granulite would provide important information to understand the characteristics, component, polyphase evolution and deep geody-namic process of continental lithosphere. Several SE-trending mafic granulite blocks with a total thickness more than 1000m occur in and coexist with Carboniferous ophiolitic melange slabs in Yushugou, Kumishi, Southern Tianshan, China. All minerals in the granulite were deformed, an obvious rotation deformion is found in the garnet grains, various ductile deformed structures are well developed in the granulite. Previous kinematic studies suggest a NE-verging thrust. But their deformation condition is unclear up to now due to an absence of sub-micro-fabric study. This paper reports a primary study by the TEM technique on the quartz and the garnet grains from the Yushugou granulite. Our results show that there are a lot of submicroscopic deformation structures in the quartz, including free dislocations, subgrains and dislocation nets, which suggest that the Yushugou granulite has undergone strong ductile deformation during a tectonic exhumation. The submicroscopic deformation structures in the garnet are not developed and few free dislocations are observed, implying that the major part of the garnet has undergone only brittle deformation. This reveals likely that the crystal structure of the garnet is more stable than that of the quartz under the middle crust environment. And the deformed temperature of the garnet was less than 900℃. Namely, the metamorphism of the mafic granulite in this area did not take place under extremely high temperature.