Early Paleozoic oceanic slab subduction in South China: Evidence from adakite-like granodiorite and high-Mg diorite from Puyang pluton in the Wuyi orogenic belt

Abstract

The mechanism of Caledonian orogeny in South China is still controversial. The main argument focuses on the issue that whether there existed oceanic subduction. To answer this question, the complex Puyang pluton in the central part of Wuyi orogenic belt was selected for zircon U–Pb dating, in-situ Lu–Hf isotopic analysis and geochemical testing. The Puyang pluton is mainly composed of granodiorite and diorite. The results of geochronology indicate that the granodiorite and diorite emplaced at 450 ± 3.9 Ma and 443 ± 4.0 Ma. Their emplacement time were well corresponding to the subduction stage of the Yunkai orogeny in the southwestern part of Cathaysia block during the Early Paleozoic (460–440 Ma). The Puyang adakite-like granodiorite shows enrichments in Sr, but depletions in Y and Yb contents, high in Na₂O, K₂O and Al₂O₃, with ε_Hf(t) values range from −10.5 to −7.4 (mean of −9.0) and two-stage Hf model ages range from 1.91 to 2.10 Ga. These characteristics indicate that the magmas were generated by partial melting of subducted oceanic crust mixed with melts from the above wedgy mantle peridotite. The Puyang high-Mg diorite shows enrichments in Sr and Ba, depletions in Rb, Y and Yb contents, and strongly high in MgO and Al₂O₃ contents, with ε_Hf(t) values of −2.9 to 0 (mean of −1.4) and two-stage Hf model ages of 1.43 to 1.61 Ga. These indicate that the magmas were mainly generated by partial melting of the wedgy mantle peridotite, mixed by adakitic melts from subducted oceanic slab. Comprehensive analysis shows that Puyang adakite-like granodiorite and high-Mg diorite were formed in fore-arc setting, where the mantle and crustal magmas mixed during the oceanic subduction initiation period. By extension, this study offered important evidences to support the point that the Caledonian orogeny in South China was related to oceanic subduction which initiated prior to ~450 Ma.