Geochronology and Geochemistry of the Shahewan and Caoping Granites in SouthQinling and Their Significance

This study presents the granites from the Shahewan and Caoping plutons in South Qinling area, including their geochemical, geochronological, and zircon Lu-Hf isotopic compositions. Based on the geochemical analyses, the two plutons exhibit high alkali and MgO contents and belong to metaluminous I-type granite of high-K calc-alkaline series. The samples show evident fractionation of light rare earth elements (LREE) and heavy rare earth elements (HREE), as the samples are enriched in LREE and large ion lithophile elements (LILE, e.g. Rb, Ba and Sr) and relatively depleted in HREE and high-field-strength elements (HFSE, e.g., Nb, Ta, Ti, Y, and Yb). Most of the samples exhibit slightly or no positive Euanomalies except for one sample showing moderate negative anomaly. LA-ICP-MS U-Pb zircon age analyses indicate that the magmatic crystallization age of the Shahewan pluton is 214-207 Ma while the Caoping pluton is 215-212 Ma. Zircon Hf isotopic analyses show that the εHf(t) values are between -1.38 and +1.81 and the two-stage model ages (tDM2) are 1.30-1.11 Ga for the Shahewan pluton. Correspondingly, the εHf(t) values are -1.0 to +3.1 and tDM2 values of 1.29-1.03 Ga for the Caoping pluton. Nearly all the samples in the Shahewan and Caoping plutons display similar characteristics of petrology, geochronology, geochemistry and zircon Lu-Hf isotopes, indicating homologous evolution. It is concluded that the parental magma of the two plutons is a mixture of Mesoproterozoic crust and mantle and the plutons formed during the extensional delamination stage of the post-collisional stage. The Mianlue Ocean closed completely before the Late Triassic, at which time the collisional orogeny between the Yangtze and North China Blocks was in the late stage.