Petrogenesis and Geochronology of Tianshui Granites from Western Qinling Orogen, Central China: Implications for Caledonian and Indosinian Orogenies on the Asian Plate

The precise timing, petrogenesis, and geodynamic significance of three granitoid bodies (Beidao granite, Caochuanpu granite, Yuanlongzhen granite, and the Roche type rock) of the Tianshui area in the Western Qinling Orogen, central China, are poorly constrained. We performed an integrated study of petrology, geochemistry, and zircon U-Pb dating to constrain their genesis and tectonic implication. Petrographic investigation of the granites shows that the rocks are mainly monzogranites. The Al saturation index (A/CNK versus SiO2) of the granitoid samples indicates meta-aluminous to peraluminous I-type granites. Their magmas were likely generated by the partial melting of igneous protoliths during the syn-collisional tectonic regime. Rare-earth-elements data further support their origin from a magma that was formed by the partial melting of lower continental crust. The Beidao, Caochuanpu, and Yuanlongzhen granites yielded U-Pb zircon weighted mean ages of 417 ± 5 Ma, 216 ± 3 Ma, and 219 ± 3 Ma, respectively. This study shows that the Beidao granite possibly formed in syn- to post-collision tectonic settings due to the subduction of the Proto-Tethys under the North China Block, and can be linked to the generally reported Caledonian orogeny (440–400 Ma) in the western segment of the North Qinling belt, whereas Yuanlongzhen and Caochuanpu granites can be linked to the widely known Indosinian orogeny (255–210 Ma). These granitoids formed due to the subduction of the oceanic lithospheres of the Proto-Tethyan Qinling and Paleo-Tethyan Qinling. The Roche type rock, tourmaline-rich, was possibly formed from the hydrothermal fluids as indicated by the higher concentrations of boron leftover during the late-stages of magmatic crystallization of the granites.