The Deformation And Metamorphic Features Of Yadan Schist And Geological Significance In RongMa Area, Qiangtang, Tibet

Qiangtang is an important area of the evolution of the Paleo-Tethys tectonic evolution and Gondwana continent. In recent years, geologists continuously found Precambrian detrital zircon and magmatic zircons. These data reflect the possibility of Proterozoic crystalline basement. In western Yadan, during the Rong Ma 1: 50000 regional geology and mineral resources research. Amphibolite garnet mica schist and a series of metamorphic in low-greenschistfacies are found, which are greatly different from lower metamorphic facies in the whole survey area.The study from fieldwork and laboratory showed: This schist experienced the impact of two-stage metamorphism: high-amphibolite facies, which are followed by low-greenschist metamorphism, a widespread metamorphism in research area. The two-stage metamorphism resulted in a different metamorphic mineral assemblages. Stress minerals have been formed, including sillimanite + garnet + biotite + quartz + plagioclase, hornblende + almandine + plagioclase; Metamorphic mineral assemblages of lower-greenschist facies includes muscovite + quartz + plagioclase + garnet, actinolite + chlorite + albite + calcite.The deformations of episode I characterized by NW-SE-trending ductile shear which formed alignment of minerals and fold of syn-tectonic quartz veins. In episode II, E-W-trending compressive stress had effect onschistosity plane, and formed the flexural-slip fold. In episode Ⅲ, S-N-trending thrust in Cenozoic affected the strike-dip of schist terrane and the forming of axial surface of chevron fold. Further, in North America shale normalized graph, the REE and trace element characteristic of Yadan schist sample is different from the North America shale’s, which showed negative Ce anomaly and negative Eu anomaly, enrichment in LREE, stable in HREE, deficit in part of LPLE. It indicates that schist has both sedimentary and basic rock protolithic characteristics. As a result of Protolith Recovery, the sample could be divided into two parts, sub-alkaline or alkaline basaltic rocks and argillaceous siltstone and greywacke. The tectonic environment of basic rock and mud rock is continental or continental rift environment and continental island arc environment. In addition, the latter one may obtain tectonic environment characteristic ofprovenance. The age of magmatic zircons U-Pb points out that peak age is about 711.5 ~ 758.6Ma. The weighted average of peak ages is 746.3±3.7Ma, which records the time that Neoproterozoic Rodinia cracked. Moreover, it is revealed that the oldest piece of detrital zircons has an age of 3107.4±28.72 Ma that refers to tectonic thermal events of Mesoarchean, and 660±17.02 Ma for the youngest one, not recording Pan-Africa Movement. So as a result, not like the Pan-African nature of Lhasa, Qiangtang and Himalayaterrane, these age data does not prove that it has Pan-African experience. This result is a important condition for the existence of crystalline basement of Qiangtang terrane.