含流体相条件下白云岩的低温流动变形——破裂与溶解-结晶的耦合过程

LOW TEMPERATURE FLOW OF DOLOMITE ROCKS AT THE PRESENCE OF FLUID PHASE—— Coupled fracturing and dissolution-crystallization

应用光学显微分析、阴极发光显微分析和TEM亚微分析,对于取自阿尔卑斯逆冲断层系底部的变形白云岩开展了详细的显微构造与岩石流动机制分析,光学显微构造和阴极发光分析表明岩石具有典型的类碎裂结构,微角砾常常是白云石颗粒的集合体组成,常常呈浑圆的形态和不规则的边界。基质颗粒中以新生结晶颗粒集合体为主,部分具有溶蚀特点的残余微角砾,而且它们经常具有新生结晶边。对于碎裂岩中变形残斑的透射电镜分析揭示出岩石的脆性特点,主要表现为各种不规则和缠结位错亚微构造特点的出现,而变形基质颗粒却很少具有或仅有微弱发育的位错亚结构。阴极发光显微结构分析进一步证实基质颗粒与微角砾之间在微量元素成分及形成时代上的差异。提出破裂(碎裂)及随后发生的溶解-迁移-结晶过程是断层带岩石细粒化与岩石应变的重要机制,认为破裂与溶解-结晶的耦合以及由此所致的岩石弱化为阿尔卑斯逆冲断层带巨大位移的形成做出了重要的贡献。

Detailed optical, cathodoluminesce and Transmission Electron microscopic studies are conducted on micro-and submicro-structures and flow mechanisms of deformed dolomite rocks from the base of the Alpine thrust systems,so as to unravle the significance of fluid phases during the Alpine thrust faulting. It is shown from optical and cathodoluminescence microscopic analysis that the fault rocks are characterized by cataclastic textures. Clasts are often of dolomite grain aggregates, subrounded in shape with irregular boundaries. Extensive fractures and microfractures are observed in the rocks and in highly deformed clasts. Grains often show intragranular deformation features, including undulatory extinction, patchy extinction and occasionally subgrains. Clasts often have boundaries indicating processes of dissolution. Progressive grain size reduction starting from primary rocks (of about 0.05 mm) is compatible with increasing strain. Matrix is composed mainly of new crystallized fine grains,in which dissolved relic clasts are often observed. They often show internal zoning with a rim of crystallization. Larger crystals in the matrix contain inclusions of finer grains. It is obvious that some grains in the matrix are nucleated from deformed relics. Transmission Electron Microscope analysis on deformed clasts in the cataclasites indicates the brittle behavior of the deforming rocks, by the presence of irregular and tangled dislocations. Grains in the matrix are often dislocation free or contain very few dislocations. Crystallized new grains often have straight and regular boundaries and triple points at their contacts. Cathodoluminescence microscopic analysis shows differences of matrix grains and clasts grains in minor elements, either varieties or amounts, suggesting different stages of crystallization and involvement of fluid phases during the formation of matrix materials. It is proposed that fracturing and dissolution-migration-crystallization are the dominant processes responsible for the grain size reduction and strain of rocks along the fault zone. Coupled fracturing and dissolution and crystallization which lead to the weakening of the dolomitic rocks has contributed to the great displacement of the Alpine thrust faulting.