Geochronology and geochemistry of early Palaeozoic ultramafic rocks along Jiangshan-Shaoxing Fault and their tectonic significance

Abstract

The origins of early Paleozoic orogen in South China have two different models: subduction model and intra-continental model. Here we report two new identification of ~440 Ma arc-related ultramafic intrusions in Tingzifan (TZF) and Fomuting (FMT) along Jiangshan-Shaoxing fault (JSF) in South China, respectively. The Silurian ultramafic intrusions are composed of olivine pyroxenite, the SiO₂, MgO and TiO₂ contents of olivine pyroxenites are 39.67–41.25 wt%, 28.98–31.38 wt% and 0.23–0.51 wt%, respectively. The geochemical compositions of the olivines, clinopyroxenes and hornblendes suggest an arc-related environment for these intrusions. As for the whole-rock trace elements, the ultramafic intrusions contain low total rare earth element (REE) contents (27.59–34.26 μg/g) and high field strength elements (HFSEs), such as Nb, Zr, Hf, Ti, and are systematically enriched in large ion lithophile elements and light rare earth elements (LREEs). Trace element compositions share most features of Alaskan-type ultramafic-mafic intrusions. Isotopically, the TZF and FMT ultramafic intrusions are characterized by negative Zircons ε_Hf(t) values (0.38–7.54). Combined with their whole-rock and mineral chemistry as well as zircon Hf isotope, we suggest that the Alaskan-type TZF and FMT pyroxenite were formed at the root of the continental arc by underplating and fractional crystallization of mafic magma which derived from subduction metasomatized mantle source. Thus, we proposed that the early Paleozoic ultramafic–mafic along Jiangshan-Shaoxing fault were most likely related to early Paleozoic subduction of the Paleo-South China Ocean between Cathaysia and Yangtze blocks, arguing that the origins of early Paleozoic orogen in the South China Block is a typical subduction-accretionary collisional-type orogenic belt rather than an intraplate belt.