Petrogenesis of Silurian intrusive rocks in the Zhenghe area, S. China: constraints on the geodynamics of the early Paleozoic Wuyi-Yunkai orogeny

The geodynamic mechanism of the early-middle Paleozoic Wuyi-Yunkai orogeny in South China remains controversial. The main debate focuses on whether oceanic slab subduction took place. Our recent investigations have identified subduction-related intrusive rocks in the Zhenghe area, Cathaysia. The Lintun intrusive suite is composed of diorites, monzodiorites, and quartz diorites. LA-ICP-MS zircon U-Pb dating shows that the diorites, monzodiorites, and quartz diorites were emplaced at 437 ± 4 Ma, 437 ± 4 Ma and 435 ± 6 Ma, respectively. The diorites are characterized by low SiO2, elevated MgO, high abundances of Ni and Cr, and high Mg# values, resembling sanukitic high-Mg andesites. They exhibit a moderate radiogenic initial 87Sr/86Sr ratio of 0.705863 and εNd(t) value of −0.8, along with low Nb/Th ratios, which are indicative of fluid-component contributions. Combined with the positive zircon εHf(t) values (+5.7 to +9.0), the diorites were most likely originated from depleted mantle that was metasomatized by slab-derived fluids. In contrast, the geochemical characteristics of monzodiorites and quartz diorites are distinct from those of diorites. They are characterized by moderate silica and alkali, a metaluminous to weakly peraluminous nature. They are enriched in Rb, Th, U and K and depleted in Nb, Ta and Ti, indicating an affinity with subduction-related arc rocks. Additionally, they exhibit heterogeneous positive zircon εHf(t) values (+2.4 to +11.0) and positive εNd(t) values (+1.4 to +1.6), along with relatively high Mg# values. These features suggest that they were likely generated through partial melting of the juvenile crust with input of mantle components. Integrating these findings with previous research, we propose that the Lintun intrusive suite was formed in a continental arc setting during the subduction of the Proto-Tethys ocean towards the West Cathaysia Block. This further supports the view that the Wuyi-Yunkai orogeny in South China was related to oceanic slab subduction.