Petrogenesis of the quartz diorite porphyry in the Zaozigou, West Qinling and its geological implications

The West Qinling Orogenic Belt, located between the North China Block and the South China Block, is one of the most important gold regions in China. It records the evolution of the northeastern branch of the Paleo-Tethys in East Asia, which is of great significance for understanding the collision of the North China Block and the South China Block. Most of the Early-Middle Triassic igneous rocks are associated with the subduction process of the Mianlue Ocean, however, its closure time remains unclear. In present study, the zircon U-Pb geochronology, major- and trace-element geochemistry, and Lu-Hf isotopic analyses are employed to investigate the petrogenesis and geodynamic setting of the quartz diorite porphyry from the Zaozigou, and to constrain the closure time of the Mianlue Ocean. Zircon LA-ICP-MS U-Pb age suggests the quartz diorite porphyry intruded at ca. 232Ma, as the products of Middle Triassic magmatism. The quartz diorite porphyry is characterized by relatively high SiO2 values (56.18%similar to 68.26%), high K2O contents (1.82% similar to 3.70%) and intermediate A/CNK values (0.73 similar to 1.07). Their metaluminous to weakly peraluminous and potassium-rich characteristics reveal that the Triassic granitoids are generated by partial melting of a medium- to high-K mafic lower crust. Moreover, they are enriched in LREEs, depleted in HREEs, and characterized by negative Eu anomalies in the chondrite-normalized REE patterns, which is typical of continental crust rocks. The high Mg-# values (47.57 similar to 70.35) of the quartz diorite porphyry indicate the involvement of lithospheric mantle materials and/or magma mixing. The relatively wide ranges of e(Hf)(t) from -12.9 to -7.12 and two-stage Hf model ages (1.70 similar to 2.10Ga) imply that the magma originated from partial melting of the Paleo-Proterozoic lower crust. We conclude that the quartz diorite porphyry was formed in the active continental margin, with characteristics of pre-collision granitic rock, and formed under the background of northward subduction of the Mianlue ocean crust, supporting that the Mianlue oceanic crust closed after similar to 232Ma.