大别—苏鲁超高压变质带内的块状榴辉岩及其构造意义

大别—苏鲁超高压(> 27× 108Pa) 变质带内的榴辉岩, 在大陆深俯冲、碰撞和折返剥露过程中, 大都遭受了强烈的变形和变质作用的重置与再造.但是, 大型榴辉岩体核部以及包裹于大理岩和石榴橄榄岩体内部的块状榴辉岩, 往往保留其初始简单的矿物组合、中-细粒状变晶结构和块状构造.详细地分析了块状榴辉岩的几何学、岩相学及变质作用特征, 指出它们是超高压榴辉岩递进及多期变质变形分解作用的残留体, 位于尺度不同的弱应变域内, 是大陆深俯冲及碰撞作用的真正记录. 

Massive Eclogites and Their Tectonic Significance in Dabie-Sulu UHP Metamorphic Belt, East-Central China

Detailed study of the structures in the Dabie-Sulu UHP metamorphic belt, east-central China indicates that most of Triassic UHP rocks were reworked and reset under eclogite- and amphibolite-facies conditions, following the peak UHP metamorphism. However, some of fresh coesite- and microdiamond-bearing massive eclogite bodies which are enclosed by marbles, foliated eclogites and garnet-peridotites respectively, remained almost undeformed on the centimeter to hundred of meter scale. The massive eclogites consist of garnet, omphacite, rutile and coesite/quartz with medium to fine-grained massive features in appearance. Most of diamond-bearing eclogites come from the eclogite lenses enclosed by the marbles which are surrounded, in turn, by the amphibolite facies gneisses and orthogneisses. Based on the analyses of petrography, structure and metamorphism of the massive eclogites, it can be inferred that these eclogites may record p-T conditions for the deep continental subduction, collision and peak UHP metamorphism. The massive eclogites have been equilibrated in the range of > 800 ℃ and 32×108-40×108 Pa. Furthermore, 250-220 Ma is considered the best estimate for the age of the peak UHP metamorphism and deep continental subduction. A comparative study of deformation features in the massive eclogite lenses and the country rocks shows that metamorphism and deformation partitioning played an important role in the preservation of the fresh massive eclogites as record-keepers of the deep subduction within the Dabie-Sulu UHP metamorphic belt. The episodic or progressive metamorphism and deformation partitioning has resulted in lens-shaped domains of low strain separated by anastomosing domains of high strain. The massive eclogites which occur within the domains, at different scales, of low strain are likely to survive a retrogressive metamorphism and deformation resetting, and consequently to retain the near peak metamorphic signature. Therefore, only these eclogites preserve the record of early UHP metamorphic and deformational events. A possible tectonic model is also given for the Dabie-Sulu UHP metamorphic belt, showing the general geometry of the Dabie-Sulu lithosphere at the time of peak metamorphism during the Triassic continental subduction and collision.