大别山超高压变质岩的变形历史及折返过程

大别山南部的超高压变质岩在其形成及折返过程中经过5期变形。D₁变形为榴辉岩相前变形,形成于扬子板块北缘陆壳基底的俯冲过程中;D₂变形形成于折返初期(220-210Ma)即超高压变质岩在浮力驱动下折返至下地壳底部的过程中,变形以块状榴辉岩的糜棱岩化及层状榴辉岩和基质的紧密-同斜褶皱为特征;D₃变形发生在折返中期(200-180Ma)即超高压变质岩在南北陆块持续碰撞作用下被挤出并向北逆冲折返至中地壳的过程中,变形以榴辉岩的布丁化和基质的强烈韧性剪切变形为特征;D₄变形是折返晚期(130-110Ma)超高压变质岩在地壳浅部伸展体制下向南滑脱所致;在折返至近地表时,超高压变质岩受到NE向断层(D₅)的切割。

THE DEFORMATION HISTORY AND EXHUMATION PROCESS OF THE UHPM ROCKS IN THE DABIESHAN

The ultra high pressure metamorphic (UHPM) rocks in the Dabie Shan suffered five generations of deformation during their formation and exhumation, and provide an important structural constraints to their exhumation. The first generation of deformation (D₁) developed before the metamorphism of eclogite facies and are represented by the relic foliation composed of epidote, barroisite, quartz and phengite inclusions in garnets of eclogites. The D₂ is preserved as the mylonitic zones on the margin of eclogite blocks and tight isoclinal folds of stratoid, tabular eclogites and their matrix, which developed after the peak metamorphism of eclogite facies. The D₃ represented by the boudins of eclogites and the ductile shearing deformation of their matrix is approximately synchronized with the amphibolite facies retrogression. The shear sense in the D₃ indicate an up to north thrusting. D₄ developed under the metamorphic condition of green schist facies, and is shown by the ductile detachment shear zone and asymmetric tight to open folds in the UHPM rocks. D₅ is brittle deformation, in which the UHPM rocks were cut by the NE trending strike slip faults. The exhumation process of the UHPM rocks can be understood by their deformation history with petrology and isotope chronology information. The continental crust on the north margin of the Yongtze plate underthrust to the mantle depth of more than 100km and suffered the UHP metamorphism of eclogite facies during the late Permian to the early Triassic, and then quickly exhumated to the shallow crust: at the first stage (220-210Ma), the UHPM rocks rapidly exhumatd to the bottom of the lower crust by the buoyancy of the subducted continental crust and developed the ductile deformation of eclogite facies; and then at second stage (200-180Ma), the UHPM rocks were extruded and overthrust northward to the middle crust; at last stage (130-110Ma), the uppermost crust over the UHPM rocks detached southward by the post collisional extension, the UHPM rocks further exhumated to the shallow crust. When approaching the earth surface, the UHPM rocks were cut by the NE-trending strike slip faults.