The U-Pb ages and Hf isotopes of zircons in the metadiabase and gneissic granite,Beishan orogenic belt,Inner Mongolia,China and its significance
LIU Xue-min1,CHEN Yue-long1,LI Da-peng1,WANG Zhong2,LIU Jin-bao21. School of the Earth Sciences and Mineral Resources,China University of Geosciences,Beijing 100083,China; 2. Geological Survey of Inner Mongolia Autonomous Region,Hohhot 010010,Inner Mongolia,China
By means of integrated study,including petrology,trace element geochemistry,zircon cathodoluminescence images,zircon SHRIMP U-Pb dating and LA-MC-ICP MS zircon Hf isotopic composition of the metadiabase and gneissic granite in Beishan orogenic belt,Inner Mongolia,together with the previous results,following conclusions are drawn. The metadiabase is characterized by slightly right-inclined REE distribution pattern,without obvious Eu anomaly,but with depletion of HSFE,such as Ta,Nb in a primitive mantle-normalized spider diagrams. All the detected zircon domains are characterized by igneous origin with 0.1 values of Th/ U ratio. The gneissic granite displays a right-inclined REE distribution pattern with negative Eu anomaly and depletion of Nb,Ta,Zr,Hf and HREE in a primitive mantle-normalized spider diagrams. The main zircon crystalline ages of the metadiabase are about 115Ma,which indicates its tectonic episode of Yanshanian. The main zircon crystalline ages of the gneissic granite are 423Ma,which suggests that the rock was intruded in Late Caledonian. Therefore,the two samples were not Paleo-proterozoic or Carboniferous rocks as suggested by previous researchers. The metadiabase assimilated crustal component during the upwelling/intrusion,indicated by the inheritance zircons with Mesoarchaean to Early Cretaceous ages. From Late Mesoarchaean to Late Palaeoproterozoic,there are at least five episodes of crustal accretion from the upper mantle in the region,which led to formation of the regional crustal basement.