四川盆地西部中二叠统栖霞组中鞍形白云石的去白云化作用

Dedolomitization in saddle dolomites of Middle Permian Qixia Formation,western Sichuan Basin,China

对四川盆地西部的中二叠统栖霞组白云岩进行岩石学观察,发现遭受了去白云化作用的白云石尤其是鞍形白云石中的去白云化作用。栖霞组白云岩中发生去白云化作用的岩石学证据包括:(1)破损的白云石晶体边界;(2)方解石胶结物中呈"漂浮状"的白云石晶体的碎片;(3)白云石晶体中的不规则方解石斑块;(4)大部分或完全被方解石所交代的白云石晶体假象。去白云化作用由方解石对白云石的等摩尔交代过程和白云石先溶解后由方解石就地沉淀的过程综合作用,可以跨组构或不跨组构发生。促进去白云化作用发生的流体通道可以分为孔、洞等先存的流体通道以及后生的裂缝。裂缝可以由不同机制产生,还会对先存流体通道进行改造。去白云化作用发生在埋藏环境。

Based on petrographic observations, the dedolomitization was recognized in the dolomite intervals of Middle Permian Qixia Formation in western Sichuan Basin, especially in the saddle dolomites. The petrographic evidences for dedolomitization include： （1） the corroded dolomite boundaries; （2） the floating dolomite fragments in calcite cements; （3） the irregular calcite patches in dolomite crystals; （4） the completely or mostly calcite-replaced dolomites and the preservation of optical properties of saddle dolomite in calcite pseudomorph. The dedolomitization is a combination process of mole to mole calcite-dolomite replacement and dolomite-dissolution-calcite-precipitation. And the dedolomitization can be fabric-selective or non-fabric-selective, The fluid pathway for improving dedolomitization includes the pre-existing pathways （e.g. pores and voids） and post-dolomitization fractures. The fractures are of various origins, and will modify the pre-existing pathways. Comprehensive research on dedolomitized samples confines the dedolomitization to the burial environment.