Petrogenesis Of Low-Sr And Low-Yb Granites In The Biliya Mountains,Xing'an Block: Geochronological And Geochemical Evidence

The granites in Biliya mountain,which are exposed in the southeast of Tuohe Tree Farm,Elunchun Autonomous Banner,Inner Mongolia,are mainly composed of quartz and alkali feldspar,and they are alkali-feldspar granites.Trace elements are characterized by low Sr and low Yb.They are granites of"low-Sr and low-Yb type".Zircons from the Biliya Mountain granites have positive Ce/Ce*anomalies and negative Eu/Eu*anomalies,indicating an obvious magmatic origin.They yielded a weighted-mean U–Pb age of 137.6±2.2 Ma,indicating a formation in the Early Cretaceous.Major-element compositions are characterized by high Si and alkali contents,and low Fe,Ca,and Mg contents,suggesting these granites are metaluminous to weakly peraluminous and high-K calc-alkaline series.Trace-element compositions are enriched in Rb,Th,U,Ce,Zr,and Hf,and depleted in Nb,Ta,Sr,Ba,P,and Ti.Compatible Co,Ni,Cr,Sr and Yb contents are low.Sr<300×10^-6?Yb<2×10^-6,indicating they are low-Sr and low-Yb granites.Rare-earth element?REE?compositions exhibit significant light-REE fractionation,weak heavy-REE fractionation,and also show a depletion of middle REEs?Yb_N>Ho_N?,with weak–medium positive Ce anomalies and negative Eu anomalies,suggesting the rocks were formed in a strongly oxidizing environment.Zircon?Hf?t?values are positive?average+10.6?and their two-stage Hf model ages can be divided into two periods:Neoproterozoic?763–558 Ma?and Paleozoic?488–335Ma?,indicating that their magma was generated by partial melting of basalt that probably originated in a depleted mantle during the Neoproterozoic?632 Ma?.Residual mineral phases of partially melted source rocks are plagioclase+garnet+amphibole+pyroxene.Magma was mixed with upper crust materials such as clastic rocks in the early Paleozoic?410Ma?during its ascent process.According to the partial-melting experiments of rock with low-Sr/low-Yb and hydrous tholeiite,combined with zircon saturation temperature and crystallization temperature of biotite chemical composition,the diagenetic conditions of"low-Sr and low-Yb"granite are estimated as follows:magma source pressure is 0.8¹.5Gpa,crust thickness is 40⁵0km,consolidation temperature is 720⁸40?,oxygen fugacity f?O₂?value 10^-1110^-12kbar.The discovery of"low-Sr and low-Yb"granites in the Biliya Mountain indicates that there was a collisional orogeny and crustal thickening event in the Daxinganling region at the junction of J₃-K₁,which is a response to the"East Asia multi-direction convergence"event.In the Rb vs?Y+Nb?tectonic discrimination diagram,the granite samples plot in the post-collision granite field,implying that this crustal thickening event has begun to enter the stage of extensional orogenic collapse in the early Cretaceous?137.6±2.2Ma?.