摘要
大陆碰撞造山带中超高压榴辉岩从变形程度上通常可以分为面理化榴辉岩和块状榴辉岩两类。本文利用透射电子显微镜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1.2×1010/cm2。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}。本文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SiO44-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。
According to the deformational degree, ultrahigh-pressure 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 transmission electron microscopy (TEM). The following results have been obtained :(1) dislocation density of garnet in foliated eclogites is about 1.2×1010/cm2 , 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 40H water molecules replacing SiO44- . 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.