Subduction‐related mafic to felsic magmatism in the Xiangpishan concentric calc‐alkaline complex, Northeast Tibetan Plateau

Abstract

The Xiangpishan complex in the Zongwulong‐Qinghainanshan Tectonic Belt, Northeast (NE) Tibetan Plateau, is a composite concentric pluton consisting of a felsic core (granodiorite) surrounded by quartz diorite in the middle to diorite and minor gabbro at the margin with locally less volume of monzogranite. Many dioritic enclaves are unevenly distributed within host granitoids. We evaluate the petrogenesis of the complex by geochemical, geochronological, and Hf isotopic data in tandem with regional data in the belt. The results show that felsic rocks were emplaced during the Late Permian (ca. 262–256 Ma); whilst gabbros yielded a younger age of ca. 249 Ma. As the hybrid phase from mixing between felsic and mafic magmas, enclaves and diorites have analogous ages (ca. 257–254 Ma) to both. Zircons from gabbro and enclaves are marked by higher εHf(t) value up to +1.82; whilst granodiorites have lower εHf(t) value of −5.48, consistent with hybrid diorites possessing intermediate εHf(t) values of −3.14 to 0.34. Furthermore, the quantitative calculation from Mass Balances Modelling suggests that the mass of mafic magma (ca. 67%–79%) is involved to achieve the hybridization. Geochemically, these rocks crystallized from calc‐alkaline magma with different sources, but demonstrated consistent arc‐like signatures, as they are enriched in large‐ion lithophile elements and light rare earth element (LREE), and depleted in high‐ field‐strength elements and heavy rare earth element (HREE). Besides, they present negative Nb–Ta anomalies together with significant P and Ti troughs. Finally, an evolutional model has been proposed that the asthenosphere‐lithosphere interaction played an important role during the emplacement of the complex, where the limited volumes of mantle‐derived melt act as the suppliers of heat and mass (mainly volatile components) to induce partial melting of the juvenile mafic lower crust and mixed (or mingled) with the produced crust‐derived magma during the oceanic subduction, which led to the generation of diorites as well as mafic microgranular enclaves.