Mineralogy,Petrochemistry,and Petrogenesis Of Nb-Ta-rich Alkaline Rhyolite Of The Tuohe Forest Farm In The Northern Da Hinggan Mountains

The high Nb-Ta alkaline rhyolite is exposed in the western Tuohe Forest farm in Keyihe Town,Oroqen Autonomous Banner and the northern Xing'an Massif,with an area of approximately 3km².The research area is located in the suture zone of Pacific tectonism,eastern Central Asian Orogenic Belt and Mongolia-Okhotsk ocean tectonic domain.This paper researched the chronology,petrology,petrochemistry and mineralogical characteristics of high Nb-Ta alkaline rhyolite and discussed it's tectonic setting,petrogenesis mechanism and magmatic source.The whole rock data of high Nb-Ta alkaline rhyolite shows that SiO₂?72.35%⁷3.22%?and Al₂O₃?13.18%¹3.60%?are relatively high.The alkaline?Na₂O+K₂O?varied from 9.39%to 11.43%,showing alkali-rich characteristics.MgO,TFeO,CaO and TiO₂ are relatively low.The rare earth elements?REE?in the rocks is in the range of86.87×10^-6161.72×10^-6,which is relatively high,the LREE/HREE is in the range of3.40⁶.87,showing a medium-light fractionation,Gd_N/Yb_N is between 0.54⁰.80,and Sr/Y is in the range of 0.04⁰.35,indicating low diagenesis pressure and shallow depth;Ce/Ce*is a positive anomaly between 1.05¹.31;Eu/Eu*value is in the range of0.39⁰.44,and the loss is obvious.The trace elements show that the rocks are rich in high field strength elements Nb,Ta,Zr,Hf,Th,U and lager ion lithophie elements Rb.Rock Sr content 1.86×10^-68.8×10^-6,Yb content 3.35×10^-64.65×10^-6,?very low Sr high Yb type?rhyolite.The U-Pb age of ziron is 111¹16Ma,indicating that the main body of the rock formed in the late Early Cretaceous.By calculating the saturation temperature of ziron,the magma formation temperature of high Nb-Ta alkaline rhyolite is 870.1?⁹24.2?,with an average of 881?.In the discriminatory digram,it is shown that the rock is A-type granite,indicating the alkaline rhyolite formed in the Tuohe Forestry Center is under condition of low pressure,high temperature,and high oxygen fugacity,indicating the magma source area is the upper crust.It is the high magma formed by the melting of th upper crust.In the Ta-Yb,Y-Nb tectonic environment diagrams,R1-R2 diagrams,and Hf-Rb/10-3Ta,Hf-Rb/30-3Ta tectonic environmental discriminant diagrams,rocks belong to intraplate granites.The high Nb-Ta alkaline rhyolite is formed in the plate extension structure environment.The Nd isotopic model age T_2DMDM of the two-age depleted mantle is between 799 to 813Ma,indicating that there have been crustal hyperplasia events in the Neoproterozoic period,indicating that the diagenetic material was derived from the partial melting if the depleted methane source or the young crust material of the newly added source.It is speculated that the source rocks are Neoproterozoic metamorphic surface rocks of the Xinghuadukou Group.