摘要
利用透射电子显微镜对中国大陆科学钻探(CCSD)主孔超高压石榴橄榄岩中石榴石、橄榄石及透辉石的超微构造特征进行了详细研究。石榴石、橄榄石及透辉石中的大部分区域都没有位错,在局部区域发育高密度位错及位错壁。石榴石局部区域的位错密度达到1.34×109/cm2以上,滑移系以1/2{110}为主。在石榴石中发现一部分熔融条带,条带由熔体和磁铁矿球粒构成,在附近的石榴石基体中有高密度的小位错圈。橄榄石局部区域的位错密度达到2.64×108/cm2以上,并发育杆状钛铁矿和含铬钛磁铁矿出溶体。透辉石局部区域发育有位错壁,在部分位错壁部位形成了微裂隙。透辉石内部存在类“多边化”区域,不同区域之间存在小角度差。分析认为该石榴橄榄岩加入俯冲板片之后,在超高压变质峰期及早期快速折返阶段发生了较强的塑性变形,在石榴石、橄榄石及透辉石中形成了高密度的位错、位错壁及亚颗粒。在折返过程中,该石榴橄榄岩曾经发生了减压部分熔融和静态重结晶,导致石榴石、橄榄石及透辉石中大部分区域的位错消失。石榴石中部分熔融条带得以保存,表明在发生减压部分熔融和静态重结晶之后所经历的折返过程非常迅速,具有很高的抬升速率及降温速率。
The microstructural characteristics of garnets, olivines and diopsides in ultrahigh-pressure garnet-peridotite, sampled from the borehole of Chinese Continental Scientific Drilling (CCSD ), have been studied by transmission electron microscopy (TEM ). Free dislocations with high density, dislocation walls and sub grains have been observed in some very local areas of garnet, olivine and diopside. Dislocation density of garnet is about 1. 34×109/cm2, and the dominant slip system of it is 1/2 {110}. A partial melting belt, consisting of melt and magnetite grains, has been observed in a garnet crystal containing small dislocation loops. Dislocation density of olivine is about 2. 64×108/cm2. Ilmentite and magnetite exsolutions are present in olivine. Polygonal areas with small offset of orientation between each other are found in diopside. This indicates that, after the garnet-peridotite entered the subduction channel, strong plastic deformation took place in the garnet-peridotite during the ultrahigh-pressure metamorphic peak stage and the first rapid exhumation stage; this explains the abundance of free dislocations, dislocation walls and sub grains in garnet, olivine and diopside. Then, partial melting and static state recrystallization occurred in these minerals, so the dislocations in the most part areas of garnet, olivine and diopside vanished. The partial melting belt existing in garnet indicates that the subsequent exhumation process must had a very high uplift rate and a very high cooling rate.