Metamorphism Of Mica Schist And Zircon Chronology In Linxi Area Of Inner Mongolia

The Xingmeng Orogenic Belt composes the central regions of Inner Mongolia and Northeast China,which is an important zone of the southeastern part of the Central Asian Orogenic Belt.It is an important hot topic for studying the location,age and the process of the closure of the Paleo-Asian Ocean,which is considered to be located between the North China Plate and the Siberian Plate.The formation of the Xingmeng Orogenic Belt is a subduction-collision-cracking-reclosing composite orogenic evolution process.The location and the age of the closure of the Paleo-Asian Ocean and the tectonic properties of the Late Paleozoic in the Xingmeng Orogenic Belt are considerably debated.In this paper,the Shuangjing schist in the Linxi area is mainly studied on the basis of petrology,mineral chemistry,phase equilibrium simulation and geochronology.The kyanite is found in the sample,which represents that the sample should be likely equivalent to the metamorphic conditions of the kyanite zone in the Barrow metamorphic belt.Compared with previous research results,the mineral assemblage analysis indicates that the sample should be in a relatively higher metamorphic condition.The X_almvalue in the garnet is about 0.64-0.69,the X_pyvalue is about 0.03-0.04,the X_spsvalue is about 0.07-0.10,and the X_grvalue is about 0.20-0.23.The X_spsvalue in the garnet porphyroblast shows a weak decrease from the core to the edge,and that of X_grvalue shows a slight upward trend from the core to the edge.Using the phase equilibrium simulation software,the isolines are made using the values of garnet X_alm,X_sps,and X_gr.The results show that the garnet records a short metamorphic evolution from the nucleus to the edge as a clockwise P-T trajectory.The phase equilibrium simulation results indicate a process of tectonic thickening and thinning and the medium-pressure phase system.The geothermal gradient is in the range of15-35oC/km,which is obviously different from the typical high-pressure subduction environment.According to the regional geological background,we can speculate that it may relate to the closure of the finite ocean basin or the thickening of the crust that has undergone tectonic thinning.Geochronological studies show that the youngest group of U-Pb zircon ages limit the uppermost deposition age of the protolith,which is in the Late Permian.Zircon has no obvious metamorphic zircon characteristics,so the metamorphic age cannot be obtained.