Petrogenesis and tectonic setting of Early Permian–Late Triassic granitoids in the Qinling Orogenic Belt: Constraints from petrology, geochemistry and zircon U–Pb–Hf isotopes

Abstract

The widely distributed Late Hercynian–Indosinian granites in the West Qinling Orogenic Belt are keys to better understand the tectonic–magmatic evolution of the West Qinling Orogenic Belt. In this paper, granitoids from the Zhacanggou area of Guide Basin in the western section of the West Qinling Orogenic Belt were studied on petrography, geochemistry, zircon U–Pb dating, and Lu–Hf isotopes. Zircon U–Pb dating yield granitoids in the Zhacanggou area were emplaced at 228.3 ± 4.4 Ma, 265.2 ± 2.2 Ma and 277.8 ± 2.5 Ma, respectively. The whole-rock geochemical compositions of these granitoids belong to the weakly peraluminous calc-alkaline to metaluminous high-K calc-alkaline series, and exhibit different degrees of negative Eu anomalies, loss of high-field strength elements (Nb, P and Ti), and enrichment of large ion lithophile elements (Rb, Th, U and Sr). The ε_Hf (t) values of Early–Middle Permian and Late Triassic granitoids range from −12.0 to 5.3 and –10.6 to −5.0, and corresponding two-stage model ages (T_DM2) of 956 to 2051 Ma and 1581 to 1936 Ma, respectively. A summary of geochronology for granitoids formed in the West Qinling Orogenic Belt during the Late Hercynian–Indosinian indicating magmatic activities concentrated in the Late Triassic (210–230 Ma) and Permian–Middle Triassic (235–277 Ma). These granitoids were both formed by partial melting of ancient crust, which then mixed with enriched lithospheric mantle, and the former has a higher mantle contribution than the latter. The early granitoids were associated to northward subduction of the Mianlue oceanic slab, while the late granitoids were formed in the transition from collision to extension.