Origin and chronology for hornblende gabbroic intrusions in Urad Zhongqi of western Inner Mongolia and their tectonic implications

The Urad Zhongqi mafic intrusions crop out in western Inner Mongolia and are dominated by hornblende gabbro.Zircon LA-ICP-MS U-Pb dating indicates that the mafic pluton was emplaced in the Early Permian(272 ± 3Ma).The hornblende gabbro rocks have low concentrations of Si O2,high total Fe2O3,Mg O,Ni,Co,Sc and Cr.The rocks are characterized by selective enrichment of large ion lithophile elements(LILE) and light rare earth elements(LREE),and depletion in high field strength elements(HFSE)(e.g.Nb,Ta,Zr,Hf,P and Ti).It suggests that they formed from a primary magma generated by the partial melting of lithospheric mantle material had been previously modified by subduction-related fluids.According to the geochemical data and the geology of the region,the authors found that the Early Permian hornblende gabbroic in western Inner Mongolia formed in a post-collisional extensional setting.