Petrogenesis and Tectonic Setting of the Muhuguan High Ba-Sr Granitic Pluton in Xiao Qinling Area, Shaanxi Province

The Muhuguan granitic pluton is consisting of a various rock types, and located in the westernmost part of Xiaoqinling area. The study on its origin and formation environment can provide important evidence for the discussion of the formation background of the Late Mesozoic magmatism in this area and its genetic relationship with the coeval magmatism in the eastern mainland of China. Zircon U-Pb ages of 150 ~160 Ma have been achieve from the variety of rocks in the Muhuguan pluton, and zircon U-Pb ages of 150 Ma and 153 Ma have been obtained also from mafic enclaves and lamprophyre vein within the host granitoids, which suggest the earliest magmatic activity in the Xiaoqinling during Late Mesozoic. The granitoids are belong to high Ba-Sr granites with the characteristics of high Al₂O₃, high K₂O, enrichment in LREE and LILE, depletion of HREE and Y, weak Eu anomaly, and significantly high Ba and Sr. Their moderate MgO and low Cr, Ni as well as Nd isotopic composion between the basic-rocks and clastic rocks of the Qinling complex reveal that they were produced from the partial melting of the mixed materials of the Qinling complex. The occurrences of coeval mafic enclaves and mantle-derived dikes in host granitoids as well as wide range of Nb/Ta ratio of granitoids suggested that they were influenced by mantle-derived magmatic activity during the formation process. In addition, their K₂O, Rb, Rb/Sr and Rb/Ba ratios increase with the increase of SiO₂, while CaO, Sr, Ba and K/Rb are negatively correlated with SiO₂, indicating that there was fractional crystallization of some hornblande, clinopyroxene and minor plagioclase during magmatic evolution. All U-Pb chronology, petrogeochemistry and Nd isotope characteristics demenstrated above fully prove that the Muhuguan high Ba-Sr granitoids was most likely formed by the partial melting of the crustal material due to the heating from the upwelling of mantle material under the adjustment of the thickened crust resulted from the collision between the southern and northern continental blocks in the early Mesozoic as well as the extension and thinning of the lithosphere during late Jurassic, which has no directly connection with the magmatic activity caused by westward subduction of the Paleo-Pacific plate to the eastern mainland of China since late Jurassic of the late Mesozoic.