Tectonic switching of Southeast China in the Late Paleozoic

The identification of Late Carboniferous (317 ± 3 Ma) Daixi-Wufenglou S-type granites and Early Triassic (247 ± 2 Ma) Houzhang complex (comprising monzodiorite and monzogranite) in southeast China provides new insights into the bewildering tectonics of south China, before its final welding to the Eurasian continent. The Daixi-Wufenglou granites have high K₂O (4.01–6.54 wt%), A/CNK (1.02–1.34), high bulk initial ⁸⁷Sr/⁸⁶Sr (0.7094–0.7160) and low ε_Nd(t) (−8.87 to −6.31), and low zircon ε_Hf(t) (−8.1 to −1.9) and high δ¹⁸O (12.7–14.0‰), indicating derivation by partial melting of a Neoproterozoic basement as a result of the input of mantle-derived heat in a back-arc extensional setting, triggered by rollback of the Paleo-Pacific plate subduction beneath the outboard proto-Japan arc. The monzodiorites are shoshonitic, characterized by high K, Ni, Cr, and Mg^#, and the monzogranites are adakitic, defined by high Sr/Y, (La/Yb)_n and low Y, Yb. They have similar ⁸⁷Sr/⁸⁶Sr (0.7085–0.7087), ε_Nd(t) (−5.92 to −5.34), and ε_Hf(t) (−7.5 to −2.5), indicating their petrogenetic linkage. The monzodiorites and monzogranites represent early crystallization phases (cumulates) and residual melts of a mantle-derived magma, generated from a metasomatically enriched mantle. Integrating these data with other geological information from south China and neighboring countries, we propose a new model for the tectonic evolution of the Late Paleozoic south China, which was controlled by the subduction of the Paleo-Pacific Ocean plate. A distal continental back-arc setting was produced by slab rollback in the early stages, and an active continental margin environment was produced by the accretion of the proto-Japan arc in the later stages.