Paleoproterozoic Granulite Facies Metamorphism For Gneisses From Lvliang Complex In Xiyupi Area And Its Tectonic Significance

Lvliang complex is located in the western margin of central segment of the Trans-North China Orogen (TNCO), North China Craton (NCC), and predominantly composed of the paleoproterozoic gnessic granitoids, metamorphic supracrustal rocks and minor meta-mafic dikes. Although some geochronology for these gneisses have being done, the metamorphic evolution process for them was quiet few till now. As an important part of the Trans-North China Orogen, the study on the metamorphic ages and metamorphic evolution of Lvliang complex can provide important clue for undersdanding the early tectonic evolution of North China Craton. This paper concentrated on studies for the field features, petrography, mineral composition and metamorphic ages of gneisses and clinopyroxene-orthopyroxene granulites in Xiyupi area to southeastern of Lvliang area. Furthemore, their metamorphic stages, peak temperature and P-T path have been determined, and then their tectonic evolution processes was discussed.The gneisses are mainly composed of sillimanite biotite two-feldspar gneisses associated with some anatectic felsic veins and mafic granulites. The mafic granulites outcropped as a dyke within the gneisses. Petrographic studies reveal that the sillimanite biotite two-feldspar gneisses have a mineral assemblages of garnet+sillimanite+biotite+K-feldspar +plagioclase+quartz, and the main mineral assemblage of mafic granulites is garnet+orthopyroxene+clinopyroxene+amphibole+plagioclase, both showing mineral assemblages similar to those of granulite facies.The garnets from gneisses are high in Fe, Mg and low in Mn, Ca, with a texture of homogeneous core and an inhomogeneous rim, indicating the retrograde metamorphic zone. The biotites in matrix are high in Mg. In contrast, biotites surrounded in garnets have various Fe, Mg contents, implying the Mg-Fe exchange had been occurred along the baoundary between garnet and biotite in matrix during the retrograde metamorphism. Garnets in mafic granulites are high in Fe, Ca and low in Mg, Mn, which, like garnet in gneisses, also have retrograde metamorphism zones. Most of the orthopyroxenes are hypersthene with high Mg and low Al contents. The clinopyroxenes are diopsides, which are characterized by the high Ca, low Fe and Mg.The peak temperature and pressure for the gneisses and mafic granulites have been evaluated using methods such as the geothermometer, geobarometer and average P-T calculation. In which, the peak temperature of ～700 ℃ and pressure of 6 kbar have been obtained from gneisses, and the retrograde metamorphic temperature of ～530℃ and pressure of 2 kbar were confirmed. Meanwhile, we got the peak temperature of 497 ℃ and pressure of 8.7 kbar for the mafic granulites. According to the calculated results above, a clockwise memamorphic P-T path have been obtained.Zircon U-Pb dating of the anatectic granitic vein developed in the gneisses yielded the age of 1981±17Ma, defining the maximum metamorphic age of the gneisses. Zircons from the mafic granulites gave two age populations of 1945±20Ma and 1928±25Ma. The age of 1945Ma is interpreted as metamorphic peak age of granulitic faces and 1928 Ma as retrograded metamorphic age of amphiblitic faces during later crustal uplift. Combined with the previous EPMA Th-U-Pb monazite dating, we suggested that the peak metamorphic age of the gneisses is around 1947～1942Ma and the retrograde metamorphic age could be occurred from 1928 Ma to 1857Ma. Combined with the previous researches, we proposed that highly metamorphic gneisses and mafic granulites from Lvliang complex strongly metamorphosed to granulite faces at 1.95 Ga due to crustal thickness resulted from continent-continent collision. After that, the metamorphism of amphibolite faces had been occurred around 1.93 Ga because the crustal uplifting, which superposed granulite. Like the gneisses and granulites developed in other areas of the TNCO, the highly metamorphic rocks including gneisses and mafic granulites well recorded two metamorphic events from granulite to amphibltic faces, indicating that the Lvliang complexe experienced a tectonic evolutionary process similar to those occurred in other areas of the TNCO.