Different melting conditions and petrogenesis of peraluminous granites in western Qinling, China, and tectonic implications

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The Qinling orogen records the prolonged amalgamation history between the North China Block (NCB) and the South China Block (SCB) accompanied by major granitoid intrusions during the Triassic. However, the petrogenesis of these granitoids, especially that of the peraluminous granites, remains a matter of debate. Detailed study of these peraluminous granites, which are the products of crustal melting, will give insights into the melting process and petrogenesis of peraluminous magmas and will further constrain the regional geological evolution. In this study, Xiahe (XH) tourmaline-bearing two-mica granite and Baijiazhuang (BJZ) two-mica granite were selected for systematic analysis of their mineralogy, petrology, zircon U-Pb geochronology, and geochemistry. The zircon U-Pb dating showed that the XH and BJZ granites were emplaced at approximately 242 Ma and approximately 214 Ma, respectively. The presence of muscovite and tourmaline, high peraluminosity at >1.1, low oxygen fugacity (ƒO2) conditions below the fayalite–magnetite–quartz (FMQ) buffer, and low zircon saturation temperatures indicate sedimentary protolith (S-type) granitic affinity. The low CaO/Na2O ratios <0.4 suggest that the granites formed by partial melting of a pelitic source. The low heavy rare earth element (HREE) concentration and weak negative Eu anomaly in the XH granite implies garnet residue in the magma source, whereas an obvious Eu anomaly in the BJZ granite indicates no residual garnet. The Rb/Sr ratios remained constant in the XH granite with decreasing Sr and Ba concentrations; however, these ratios increased in the BJZ granite. This observation reflects that the XH (BJZ) granite was the product of fluid-present (fluid-absent) muscovite melting. This finding is in accord with the different geochemical features and the estimates of zircon and monazite saturation temperature between the XH and BJZ granites. In western Qinling, low-Sr/high-REE (group A) and high-Sr/low-REE (group B) peraluminous granites have been identified. We propose that the former were generated during fluid-absent muscovite melting, whereas the latter were derived from fluid-present muscovite melting. This hypothesis was further confirmed by higher zircon and monazite saturation temperature estimates in addition to higher REE, Th, Nb, and Ta concentrations in the group A granitoids. From regional geological considerations, it can be concluded that the Middle Triassic granitoids in western Qinling formed during subduction of the A'nimaqing–Mianlue oceanic crust, whereas the Late Triassic granitoids are related to syn- or post-collisional processes.