| 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（YbN>HoN）,with weak–medium positive Ce anomalies and negative Eu anomalies,suggesting the rocks were formed in a strongly oxidizing environment.ZirconεHf（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）. |
