Exhumation kinetics of northern Sulu ultrahigh-pressure metamorphic belt, Rongcheng Area.
北苏鲁荣成地区超高压变质带的形成与折返动力学

Rongcheng of Shandong Province is located at northeast of Sulu UHP (ultrahigh-pressure) metamorphic terrane. It is composed of gneiss, paragneiss, eclogites, garnet pyroxenite, ultramafic rocks, granulites, quartzite, marble and amphibolites. Coesites as a reliable marker of ultra-high pressure environment are found in eclogites, quartzites, gneisses, marbles and amphibolites. The occurrence of mylonites extends NNE-SSW at the north part and NW-SE at the south part. Different parallel slices compositing the Rongcheng UHP metamorphic terranes are divided by some ductile shear zones (about 2km). Based on field work microstructural analyses and EBSD studies of quartz, a conclusion is arrived: These ductile shear zones are approximately SE trending normal faults. Lattice-preferred orientation (LPO) of omphacite and garnet in several eclogite samples of Rongcheng were determined from electron backscatter diffraction (EBSD) technique shows that: LPO of omphacite and rutile are characterized by high temperature type, where as garnet appears to be randomly oriented. From the original seismic reflection profile of Sulu UHP terrane an erosion antiform structure is shown. Rongcheng and Liushan normal faults are situated in the lower part of this antiform, and southern Sulu thrusts are situated in the above part. Ar-Ar dating study results indicates these normal faults were formed at 117~130Ma. Based on the petrologic, geochemical, structural studies, the extrusion model of Sulu UHP terrane is verified further.