Linking ocean subduction with early Paleozoic intracontinental orogeny in South China: Insights from the Xiaying complex in eastern Guangxi Province

Abstract

The tectonic setting of the early Paleozoic orogeny in the South China Block (SCB) has been hotly controversial. Although most studies have ascribed it to an intracontinental orogeny, there are various views on the geodynamic drivers of the intracontinental orogeny. This study reports on the Ordovician Xiaying igneous complex in the eastern part of Guangxi province, southern South China. This complex is composed of granodiorite, monzogranite and micro-granular diorite enclosed by granodiorite. The zircon U–Pb dating suggests that these three types of rocks emplaced synchronously at around 464–442 Ma, significantly older than those S- and I-type granites (450–400 Ma) distributed in the interior of the SCB. One monzogranite collected in the border of the complex has an older age (480 Ma). The Xiaying hornblende–bearing granodiorites have low SiO2 (61.8–68.7 wt%), P2O5 (0.07–0.10 wt%), Rb/Sr (0.52–1.30) and Rb/Ba (0.13–0.34) and high Fe2O3t (3.84–5.57 wt%), MgO (1.48–2.35 wt%), CaO (2.28–4.54 wt%) and Na2O/K2O (0.90–1.91), with metaluminous to weakly peraluminous features (most A/CNK = 0.87–1.07), showing typical I-type granitoid characteristics. The Nd–Hf isotopic model ages (ca 1.6–1.8 Ga) indicate that the Xiaying granodiorites were probably formed by partial melting of a late Paleoproterozoic basement in the deep crust. The micro–granular dioritic enclaves (MMEs) have similar formation ages and Nd–Hf isotopic compositions to the host granodiorites, and show geochemical evolution relation with granodiorites, suggesting that they resulted from the mixing of mafic and granodioritic magmas, which derived from a depleted mantle and a Paleoproterozoic crust, respectively. Based on their similar formation ages and zircon Hf-isotope compositions and the assimilation and fractional crystallization (AFC) modelling, it is suggested that major monzogranites were generated from the granodioritic magma through the AFC process. The rock associations and arc-related geochemical characteristics of the granodiorites and dioritic MMEs suggest that they probably formed in a subduction-related tectonic setting. Combined with the spatial distribution of the rock types and formation ages of these early Ordovician to early Devonian magmatic rocks in the SCB from southeast to northwest, it is inferred that there was an Ordovician northwestward subduction of the proto–South China Ocean occurring to the south of the SCB, which may connect with the Tam Ky–Phuoc Son Suture Zone in the eastern Indochina Block. It was the subduction of the proto–South China Ocean and subsequent collision between the southern SCB and an unknown continent that triggered early Ordovician to early Devonian intracontinental orogeny in the interior of the SCB.