Ordovician Arc-Related Mafic Intrusions in South China: Implications for Plate Subduction along the Southeastern Margin of South China in the Early Paleozoic

Two models have been proposed to address the early Paleozoic tectonic evolution of Cathaysia in South China: the intracontinental model and the collisional model. Most geologists favor the intracontinental model because of the apparent absence of early Paleozoic ophiolite or arc igneous activity in Cathaysia. Geologists in favor of the collisional model suggest that the early Paleozoic amphibolite-granulite facies metamorphism and intensive igneous activities in Cathaysia share most features with collisional-type orogens. Here we report a new identification of 443–450 Ma arc-related mafic intrusions in Songxi, South China. The Ordovician Songxi mafic intrusions, outcropping near the Neoproterozoic Jiangshao suture zone in South China, are composed of pyroxenite, (hornblende) gabbro, and minor diorite. Major-element geochemistry of these rocks exhibits a tholeiitic trend with a wide range of SiO2 contents (41.39%–51.96%). The chemical compositions of the clinopyroxenes suggest an arc-related environment for these intrusions. As for the whole-rock trace elements, the mafic intrusions contain low total rare earth element (REE) contents (30.7–133.4 ppm) and high field strength elements (HFSEs), such as Nb, Ta, Zr, Hf, and Ti, and are systematically enriched in large ion lithophile elements and light rare earth elements (LREEs), leading to low HFSE/LREE ratios (Nb/La = 0.2–0.7). Trace element compositions share most features of Alaskan-type mafic intrusions. Isotopically, the Songxi mafic intrusions are characterized by negative whole-rock εNd(t) values (−4.6 to −6.0) and variably high (87Sr/86Sr)i values (0.7069–0.7115). Zircon εHf(t) values mostly range from −6.0 to 2.3 and form a Gaussian distribution that peaks at ca. −3.0, indicating their broadly coupled Nd and Hf isotope compositions. Chemical compositions of the rock-forming minerals and the presence of primary phlogopite and hornblende suggest that the mafic intrusions were likely formed via crystal fractionation/accumulation of a hydrous tholeiitic mafic magma derived from a metasomatized mantle source in a continental arc environment. The identification of the Ordovician arc-signature mafic intrusions in South China, in combination with early Paleozoic metamorphic features and the widely distributed middle Silurian to early Devonian postorogenic affinity igneous activities, reveals that Cathaysia preserves a tectonic evolution process of plate subduction transitioning to continent-continent collision before the early Silurian, followed by postorogenic collapse during the middle Silurian to early Devonian. This collision event possibly took place at 440–435 Ma, and the final postorogenic collapse at 435–400 Ma produced voluminous granitic intrusions and minor mafic rocks.

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