Subduction Initiation of the Southern Branch of the Paleo‐Asian Ocean in the Middle Ordovician in the Southern Beishan Orogen

Abstract

Abstract The subduction initiation and the early‐stage tectonics of the southern Paleo‐Asian Ocean have rarely been discussed. To address these problems, we report new geochemical and geochronological data of arc‐related volcanic rocks in the Huaniushan arc of the southern Beishan. Basaltic andesites and high‐Mg rocks yield zircon U‐Pb ages of 449–422 Ma and 460–437 Ma, respectively. The basaltic andesite is a continental arc‐related tholeiite with moderate amounts of TiO2; and is enriched in light rare earth elements (LREE), such as Rb, Ba, Th and Sr; it is also depleted in Nb, Ta and Eu; and has moderate εHf (t) (−0.3 to +5.5) and εNd (t) (−3.48 to +0.03) values; and high (87Sr/86Sr)i (0.7066–0.7079) values. The high‐Mg andesite and dacite are sanukitic‐type high‐Mg rocks, which have high MgO (Mg# = 49–62) contents, A/CNK (0.82–2.36) and (La/Sm)N values, low Sr/Y (3–15) and Ba/Th (11–182) ratios, moderate to low εHf (t) (−16.7 to +7.6) and εNd (t) (−3.74 to +0.05), and high (87Sr/86Sr)i values. All geochemical data indicate that they were sourced from mixing between mantle‐peridotite‐derived melts and silicic melts generated by the subducting oceanic slab and/or sediments in a hot subduction setting. After integrating the Sanukitic high‐Mg andesites (HMAs) with previously reported Nb‐enriched basalt, volumes of arc‐related tholeiitic to calc‐alkaline volcanic rocks, I‐ to S‐type granites, and HT eclogites, we conclude that the subduction initiation was one of the most likely geodynamic process for the metamorphism and magmatism in the Middle Ordovician to early Silurian.

Keywords

• high‐Mg andesite
• middle Ordovician
• subduction initiation
• old and cold slab
• southern Altaids