Petrogenesis and Tectonic Implications of the Yuhuashan A-Type Volcanic-Intrusive Complex and Mafic Microgranular Enclaves in the Gan-Hang Volcanic Belt, Southeast China

Early Cretaceous felsic volcanic-intrusive complexes are widespread in the Gan-Hang Volcanic Belt (GHVB) and accompany abundant uranium ore resources. However, the petrogenesis and tectonic significance of these rocks are not well understood. We present zircon U-Pb geochronology, major- and trace-element geochemistry, and Sr-Nd-Hf isotopic compositions of the Yuhuashan Complex and hosted mafic microgranular enclaves (MMEs) from the southwestern part of the GHVB. The Yuhuashan Complex consists of rhyolitic porphyritic lava and subvolcanic porphyritic granite that are metaluminous to weakly peraluminous and have pronounced A-type geochemical characteristics. They were formed at a high temperature and can be further classified as of A(2) subtype affinity. The Yuhuashan A-type granitic rocks were generated largely by partial melting of Proterozoic metamorphic rocks. Our new data, together with previously published data, suggest that the Early Cretaceous A(2)-type granitic rocks formed along the GHVB during an extensional event. The MMEs hosted in the Yuhuashan porphyritic granite exhibit igneous textures, suggesting quenching of mafic magmas that comingled with the host granitic magma. Zircons from the MMEs have similar U-Pb ages but higher epsilon(Hf)(t) values, compared to the host granite. Hence, the MMEs and host granite likely crystallized from different magma sources, providing direct evidence for mafic-felsic magma-mingling processes. The MMEs have high MgO and K2O contents and show shoshonitic affinities. Geochemical and isotopic data imply that the MMEs were derived from a phlogopite-bearing lithospheric mantle source that had been metasomatized by previous subduction-related fluids or melt. Underplating of such high-temperature mafic magmas could have induced lower-crustal rocks to partially melt and generate the Yuhuashan A-type complex. A backarc extensional setting, related to the rollback of a subducted paleo-Pacific-derived plate, is favored to explain the petrogenesis of the Yuhuashan Complex and MMEs.