超高压变质岩的塑性流变：显微构造和变形机制

超高压变质岩是大陆深俯冲作用的产物。超高压变质岩在深俯冲和快速折返过程中,经历了长距离地构造搬运和构造力的作用。其构造变形主要集中在韧性剪切带中,并发生强烈地塑性流变。研究超高压变质构造岩的显微构造及其变形机制对于深入了解大陆壳岩石在深俯冲过程中的流变学行为有十分重要的意义,山东仰口的超高压韧性剪切带中榴辉岩质和花岗质糜棱岩记录了超高压变形的历史。在超高压条件下的稳定矿物绿辉石、多硅白云母、兰晶石和钾长石具有不规则波状消光、亚晶界、核幔构造和动态重结晶等显微构造特征,TEM 研究揭示了大量的位错构造,表明位错蠕变是其主要的变形机制。在花岗质糜棱岩中,金红石在刚性矿物的压力影中沉积,细粒的石榴石条带平行片理延伸,都说明超高压变形过程中有流体存在,流体助力的物质扩散迁移是又一个重要的变形机制。依据现有的流变学定律估算的流变应力应该在几十兆帕以上。

Plastic flow of ultrahigh pressure metamorphic rocks: microstructure and deformation mechanisms

Ultrahigh pressure (UHP) metamorphic rocks are products of deep continental subduction and have experienced long distance transportation and subjected to tectonic stresses during deep continental subduction and rapid exhumation.High strain and strong plastic deformation are mainly concentrated into ductile shear zones.Study of structures,microstructm'es and deformation mechanisms of the UHP rocks is important in understanding the rheological behavior of continental crust in the deep subduction processes.The eclogite and granitic mylonites in the ultrahigh pressure ductile shear zone at Yangkou,Shangdong Province,have recorded UHP deformation history of the rocks.Minerals stable at UHP conditions,omphacite,phengite,kyanite and K-feldspar have irregular undulatory extinction,subgrain boundary,core-mantle structure and dynamic recrystallization microstructures,indicating dislocation creep is a major deformation mechanism.TEM also reveals abundant dislocation structures,edge and screw dislocations,tilt and twist walls.In granitic mylonite rutile precipitates preferentially in the pressure shadows of porphyroclasts and garnet occnrs as fine grains in narrow zones parallel to foliation,which all suggest that fluid is present in deformation of the UHP rocks and fluid-enhance diffusion mass transfer is another important deformation mechanism during UHP deformation.Stress level is estimated at tens of MPa based on existing flow laws.