摘要
超高压变质带的研究是国际地质学界研究的前沿课题,对超高压变质带的研究,不仅可以深入理解板块边界俯冲过程、壳幔作用和造山带动力学过程,而且可以推动对大陆以及全球动力学过程的研究,并可能从更深层次完善地球动力学理论。在超高压变质带的研究中最受关注的问题是大陆壳如何被俯冲到地幔的深度义返回地表的。这一问题的解答依赖变质岩石学、地球化学、同位素年代学和构造地质学等多学科综合研究。研究超高压变质岩超微构造特征相关的变形机制与变形历史以及与特殊出溶结构相关的俯冲深度信息可以为大陆深俯冲和折返的构造模式提供重要依据。本文以透射电子显微分析方法为土,结合电子探针分析方法,详细研究了大别—苏鲁超高压变质带榴辉岩、石榴橄榄岩及硬玉石英岩中主要矿物的超微构造特征,并探讨了其地质意义。取得了如下主要结论: 1、对中国大陆科学钻探主孔岩心中面理化榴辉岩和块状榴辉岩的石榴石变形特征进行了比较研究。结果表明:(1)面理化榴辉岩中石榴石的位错密度比块状榴辉岩中石榴石的位错密度高大约3个数量级,位错密度达1.2×10~(10)/cm~2。并且有水分子团与高密度位错共存,表明主导石榴石发生强烈塑性变形的机制为水弱化的位错滑移。(2)块状榴辉岩中石榴石的位错密度低,没有观察到水分子团,虽然在局部发现有位错网,但总体上仍呈刚性。(3)两类榴辉岩的石榴石中占主导地位的位错滑移系都是1/2{110},其次为{010}。对变形石榴石晶体中水的赋存状态、水弱化机制及变形石榴石形态优选方位与晶格优选方位不协调问题进行了讨论:(1)水在石榴石晶体中有三种可能赋存状态:第一种是吸附于位错管道中的极化水分子;第二种是通过晶体表面以及位错管道扩散进入晶格的间隙水分子;第三种是以4OH~-替代SiO_4~(4-)形式的结构水。(2)石榴石水弱化机制有两种可能:一种是水弱化促进的位错滑移机制,在面理化榴辉岩石榴石的塑性变形中起主导作用;另一种是水弱化促进的扩散和颗粒边界滑移机制,在面理化榴辉岩石榴石的塑性变形中只起次要作用。(3)变形石榴石不产生明显的LPO是由其晶体结构本身特征所决定的,并不能排除其主导变形机制仍然是位错滑移的可能性。 2、对双河榴辉岩绿辉石的透射电镜研究表明,在P2/n绿辉石中存在由位错壁围成的C2/c绿辉石条带。沿C2/c绿辉石条带分布有冻监闪石及钠长石出溶体。(1)根据冻监闪石出溶体、钠长石出溶体、C2/c绿辉石及P2/n绿辉石之间的关系,结合大别双河榴辉岩和硬玉石英岩的退变质P-T轨迹资料,探讨了其成因机制。冻监闪石出溶体和钠长石出溶体是在绿辉石C2/c-P2/n相变之前,即大约在700℃以上的温度条件下形成的,而且冻监闪石出溶体在前,钠长石出溶体在后,钠长石的初始出溶相应该为高钠长石。该演化过程表明Liou等(1997)给出的大别双河榴辉岩和硬玉石英岩的退变质P-T轨迹应该向高温方向作适当的平移。(2)冻蓝闪石出溶体在绿辉石C2/c-P2/n相变之前的高压条件下形成,表
Research of ultrahigh-pressure (UHP) metamorphic belt was significant for improve comprehension of plate boundary subduction process, crust-mantle interaction and orogenic belt dynamic course, continent and global dynamics course. An important Problem was how continent crust subduct and exhumate. To answer the problem needed integrated research of metamorphic petrology, geochemistry, isotope chronology, structural geology and so on. Research of deformational mechanism and deformational history correlate microstructural characteristics of UHP metamorphic rock and research of subduction depth information correlate special exsolution structure could provided important evidence for tectonic model of continent crust subduction and exhumation. Used transmission electron microscopy (TEM) and electron probe microanalysis (EMPA), we particular researched ultramicrostructural characteristics of main minerals of eclogite, garnet-peridotite and Jadeite-quartzite in Dabie-Sulu UHP metamorphic belt and discussed its geological significance. Conclusions as follows:1, According to the deformational degree, UHP eclogites in continental collision orogens can be divided into foliated eclogites and massive eclogites. The deformation characteristics of garnets in both types of eclogites, sampled from the borehole of the Chinese Continental Scientific Drilling (CCSD), have been studied by TEM. The following results have been obtained: (1) Dislocation density of garnet in foliated eclogites is about 1.2×l0~(10)/cm~2, three order of magnitudes higher than that of massive eclogite. Clusters of water molecules coexisting with dense dislocations indicate that the dominant deformation mechanism of the elongated garnet in foliated eclogite is hydrolysis weakening dislocation glide. (2) In the massive eclogites, the garnets with low dislocation density and with no visible clusters of water molecules are mainly rigid although a few dislocation nets have been observed. (3) In both types of eclogite the dominant slip system of garnet is 1/2{110}, and the minor is {010}.Three possible forms of water in elongated garnets are discussed in this paper. The first form is polarization water molecules, which is absorbed in dislocation tubes. The second form is interstitial water molecules diffusing into crystal lattice through surface and dislocation tubes. The third form is 4OH~- water molecules replacing SiO_4~(4-). In addition, possible hydrolysis weakening mechanisms of garnets in foliated eclogites have been discussed: the major mechanism is water induced dislocation glide meanwhile water induced diffusion and grain boundary glide play a minor role. Moreover, the problem of shape preferred orientation (SPO) of deformed garnets not corresponding with lattice preferred orientation (LPO) is also discussed. Elongated garnets with weak LPO have been determined by the characteristic of their crystal structure. Therefore, the possibility that dislocation creep is the dominant deformation mechanism of garnets can not be excluded.2、 Research of omphacite from eclogite at shuanghe in Dabie mountains by TEM indicated