Identification of ca. 470 Ma continental margin arc volcanism in the Cathaysia Block interior: Constrains on the geodynamic process of the early Paleozoic orogeny in South China

Abstract

Due to the lack of evidence related to oceanic subduction, it has been debated whether the early Paleozoic orogeny in South China Block belongs to an intracontinental orogeny or a subduction-collision orogeny. To better understand the nature and geodynamic processes of this orogeny, we conducted geochronological, geochemical, and isotopic studies on pyroclastic rocks, including crystal tuffs and sedimentary volcanic breccias, which outcrop in the Zhenghe area of the Cathaysia Block. The crystal tuffs have experienced strong chloritization and have high FeO^t (25–31.3 wt%) and high Cr (60.7–143 ppm) and Ni (33.8–93.6 ppm) contents. Immobile elements indicate that the crystal tuffs are products of basic-intermediate volcanism. SIMS and LA-ICP-MS UPb dating show that the crystal tuffs were erupted during 471–465 Ma. The crystal tuffs exhibit positive zircon ε_Hf(t) (+7.8 to +14.2) and low δ¹⁸O values (5.05 to 5.89 ‰), suggesting that they were derived from depleted mantle. They exhibit subduction-related geochemical signatures, with enrichment in Th but depletion in Nb and Ta. Additionally, high U/Yb and low Nb/Yb values of the zircons suggest that the crystal tuffs were generated in a continental arc environment. The interlayered sedimentary volcanic breccias are products of intermediate-acidic volcanism. They share similar geochemical characteristics with the crystal tuffs, including zircon UPb ages (ca. 467 Ma), whole-rock trace element patterns, zircon CL images, zircon ε_Hf(t) values (+ 9.9 to +13.3) and zircon trace element ratios, implying that the two types of volcanic rocks originated from similar sources or were genetically related. Notably, the presence of few continental clastic materials in these pyroclastic rocks provides evidence that the volcanic arc was situated offshore from the continent. The identification of the ca. 470 Ma continental margin arc volcanism in the Zhenghe-Dapu fault, along with the previously discovered ca. 520 Ma MOR-type ophiolite in the same fault, and the widespread presence of late-Ordovician to early-Devonian igneous activities and Silurian high-grade metamorphism in the Cathaysia Block, supports that the early Paleozoic orogeny in South China was a subduction-collision orogeny, with the Zhenghe-Dapu fault serving as a suture zone. Consequently, it is believed that this orogeny resulted from the collision between the West Cathaysia Block and either the East Cathaysia Block or the northern margin of Gondwana. The geodynamic process involved the initiation of oceanic plate subduction during the middle Ordovician, transitioning to continent-continent collision by the middle Silurian, and concluding with post-orogenic collapse from the late Silurian to early Devonian.