大别造山带东部假玄武玻璃的显微构造特征及其意义

Microstructure Characteristics of Pseudotachylites from the Eastern Dabieshan Orogenic Belt and Their Tectonic Implications

最近,在大别造山带东部发现了广泛的地震成因假玄武玻璃,这些假玄武玻璃主要呈简单脉状沿NE—SW向走滑断裂带或剪切带发育,后者大多与郯庐断裂带平行并穿切了包括白垩纪花岗岩在内的地质体。某些假玄武玻璃内发育由暗色石英条纹构成的糜棱质条带。本文通过普通光学显微镜和扫描电镜观察分析,详细研究对比了不同断裂带内部发育的假玄武玻璃及其围岩在显微构造特征上的差异及联系。根据岩石的组构特征,证实所发现的这些假玄武玻璃主要是由母岩的超碎裂岩化形成的,但在点1发育的假玄武玻璃基质的扫描电镜影象特征上,沿某些残斑矿物(钾长石、斜长石、黄铁矿)的边缘可以看到些许代表摩擦熔融成因的熔蚀状港湾结构,说明假玄武玻璃形成过程中曾经发生了程度较低的局部熔融作用。肉眼所见到的糜棱质石英条纹在显微镜下证实为早期的糜棱面理。同时,岩石组构的叠加显示,含假玄武玻璃的断裂带及假玄武玻璃本身普遍具有多期性,且晚期构造产物(或假玄武玻璃)总是较早期产物(或假玄武玻璃)碎裂岩化作用更加强烈,说明先期存在的构造软化带在控制假玄武玻璃形成过程中起着非常重要的作用,即构造带抬升过程中伴随的多期构造及细粒化是形成假玄武玻璃的基础。岩石的变形序列总体上显示为韧性韧脆性脆性的演化过程。

Recent field survey in eastern Dabie Mountain has revealed extensive existence of pseudotachylite. The pseudotachylite tends to occur in the NE-SW trending fracture zones or shear zones which parallel the Tan-Lu fault zone and cut the early-Cretaceous granites. Mylonitic bands are developed in some pseudotachylites. This paper mainly discusses the characteristics of microstructures between the pseudotachylites and their host rocks through observed mainly by optical microscope and SEM imaging. The microstructure characteristics of pseudotachylites and their respective wall rocks indicate that the pseudotachylites were formed mainly by ultracataclasis of wall rocks. Nevertheless, a small amount of corrosion borders along the margins of some porphyroid crystals of feldspar, plagioclase and pyrite from the SEM imaging of the sample Loc.1 suggest that there might have somewhat existed melt du-ring the pseudotachylite generation. The macro-mylonitic quartz bands have been proved to be the mylonitic foliation developed in early mylonitic breccias. The superimposition of microstructures shows the multi-stage development of the pseudotachylite-bearing fault zone and the pseudotachylite itself. In addition, the veins show features of progressive fracturing of the wall rocks interspersed with many rock fragments, which strongly suggests that antecedent cataclasis or pre-existing tectonic soften belt was a reworking and forming the pseudotachylite, so, pseudotachylites were formed in which they occurred. The deformation succession between the pseudotachylite and its wall rock generally exhibits the evolutionary process from ductile through ductile-brittle to brittle, which shows that the pseudotachylites were generally formed during the uplift of the orogenic belt instead of during the early subduction.