Metamorphism Of Beishan Complex In Baishan Area Of Xinjiang

Beishan orogenic belt is located at Southern the Central Asian Orogenic Belt(CAOB),A large number of high-grade metamorphic rocks are widely distributed in several Beishan orogenic tectonic units,which were considered as Precambrian basements and referred to the ‘Beishan complex'.However,the age and setting of metamorphism are still unclear.Here,we present the type of rocks and metamorphism of the Beishan complex,which is located in the southern margin of the orogenic belt,and in the Baishan area of the Huaniushan-Shuangyingshan block.The research area of the Beishan complex is divided into three lithologic sections from south to north: amphibolite schist-biotite plagioclase gneiss,biotite quartz schist-thin siliceous marble,dimicaeous quartz schist-muscovite quartz schist,the metamorphic degree gradually weakened,the overall of metamorphic rocks are amphibolite facies.We present the metamorphism of garnet amphibolite.The mineral assemblage of inclusions of the garnet'core records the temperature and pressure conditions of the prograde metamorphic process.The metamorphic mineral assemblage are garnet+plagioclase+hornblende+ilmenite.The temperature conditions of the metamorphic process was measured by Amp-Pl thermometry and pressure conditions of the metamorphic process was measured by Grt-Amp-Pl barometry.The result are 723°C-736°C and 6798.47bar-7062.01 bar.Garnet,hornblende,plagioclase,and a small amount of ilmenite and biotite in the matrix represent the metamorphic mineral assemblage during the peak period.The temperature conditions of the metamorphic process was measured by Amp-Pl thermometry and pressure conditions of the metamorphic process was measured by Grt-Amp-Pl barometry.The result are 867°C-996°C and 3299.49bar-4526.16 bar.The assemblages of post peak metamorphic mineral are the amphibole and plagioclase which are distributed the marginal of the garnet.The temperature conditions of the metamorphic process was measured by Amp-Pl thermometry.The result are 838°C-885°C.The pressure condition of the structure is obviously lower than that of peak period The P-T-t path is clockwise.The type of P-T-t path usually occurre in the the orogenic process of subduction-collision-rapidly uplift.With the detailed observation we found the Symplectic of plagioclase and hypersthene in the edge of some garnet in the sample.According to the structure of the metamorphic reaction,it is considered a superimposed metamorphism.we also present the zircon morphology,REE and U-Pb isotope age characteristics of the garnet amphibolite and amphibole plagioclase schist.The zircon have the characteristics of the anatectic zircon and metamorphic zircon,Rare earth elements have the characteristics of heavy REE enrichment,Ce positive anomalies and negative Eu anomalies.At same time we found that there is a continuous age peak between 380-460 Ma,and there are two independent peak in ~300Ma and and ~230Ma.Based on the analysis of the characteristics and ages of zircon and the results of previous studies,the age of the first stage of metamorphism may be~900Ma,and the age of the second stage superimposed metamorphism is~230Ma.Therefore,the early Neoproterozoic orogeny formed the main part of Precambrian basement of the southern Beishan area.After that,the basement experienced multiple magmatic tectonic activities,including the process of subduction of the Paleoasian Ocean in late Ordovician to early Devonian and the mafic ultramafic magmatism related to subduction in late Carboniferous to early Permian.