Geochemical characteristics of the volcanic rocks from Zhongtanggou area of Mianlue suture zone in South Qinling Mountains and their geological significance

A detailed geological cross- sectional survey was conducted for the volcanic rocks in Zhongtanggou area on the western side of Sanchazi along Mianlue suture zone. The volcanic rocks are mainly greenschist facies metamorphic rocks, which comprise mainly metamorphic andesite rocks and subordinately metamorphic basalt and metamorphic dacite rocks, belonging mainly to the calc- alkaline series. Their REE distribution patterns are characterized by LREE enrichment, right- oblique shape and depletion of Eu. Their trace elements generally have features of enrichment of Cs, Rb and Ba, obvious depletion of Nb and Ta, and slight depletion of P and Ti. Trace elements such as Zr and Hf are relatively stable. In the La- La/Nb, Nb- Nb/Th and La/Yb- Sc/Ni discrimination diagrams, the metamorphic basalt and metamorphic andesite samples mainly fall in the region of island arc volcanic rocks. These characteristics, in combination with features of volcanic rocks, indicate that the volcanic rocks in Zhongtanggou area were formed in a typical tectonic setting of island arc. The authors hold that the material source of metamorphic andesite and metamorphic basalt rocks might have been closely related to a locally depleted area in the mantle wedge with the addition of continental crustal material. The material source of metamorphic dacite was probably related to the partial melting of the crustal granitic magma.