Petrogenesis and its implications for the lithospheric thinning of the Wushijiazi pluton in Chifeng, Inner Mongolia

This paper is the result of geological survey engineering.

Objective The Wushijiazi pluton in Chifeng area, Inner Mongolia, is located in the southwestern part of the metallogenic belt of the southern Great Xing'an Range. A systematic geochronological and geochemical study can provide insights into understanding of regional tectonic-magmatic evolution and metallogenic regularity.

Methods Based on the petrological and geochemistry study, this paper uses LA-ICP-MS zircon U−Pb dating, major and trace element analysis and Zircon Hf isotopic analysis methods to analyze the genesis of the pluton.

Results LA-ICP-MS zircon U−Pb dating results show that the porphyritic biotite-bearing monzogranite, porphyritic biotite-bearing syenogranite and porphyritic biotite syenogranite in the Wushijiazi pluton were formed at (150.3±1.3) Ma, (145.9±1.8)Ma and (137.1±2.2)Ma, respectively, which are the magmatic products during the Late Jurassic to Early Cretaceous. The pluton is geochemically characterized by high contents of silica and alkalis, but low contents of aluminium and calcium, belonging to alkaline and quasi aluminous-weakly peraluminous A-type granites. Zircon Hf isotopic analysis displays that the porphyritic biotite-bearing syenogranite has positive ε_Hf(t) values (+7.5-+14.3) and juvenile two-stage (t_DM2) Hf model ages (t_DM2=285-718 Ma), which was consistent with those of Late Mesozoic granites in the southern of Great Xingan Range. Therefore, it is suggested that the contribution of the juvenile lower crust is dominant in the source region. The occurrence of mafic microgranular enclaves in porphyritic biotite-bearing monzogranite indicates magma critical role of mantle-crust interaction.

Conclusions Combined with regional geological background, our study suggests that the asthenosphere upwelling resulted in partial melting of the juvenile lower crust and formed the primitive magma mush under the late Mesozoic lithospheric extension and thinning environment, and then the magma mush mixed with the mantle-derived magma. The highly fractionated mixed magma was emplaced in the shallow level, and finally formed the Wushijiazi granitic pluton. Wushijiazi pluton is characterized by the high degree of differentiation, as well as deep-source but shallow-emplacement, which is closely related to the tin-polymetallic mineralization in this area.