Ridge subduction identified within the ancient Tianshan Ocean: Evidence from temporal variations of magmatism in the Tuwu-Yandong porphyry Cu belt, NW China

Abstract

Our knowledge of the many magmatic episodes and porphyry Cu belts north of the Kangguer Shear Zone in the Eastern Tianshan region is complicated by the ongoing controversy over the mechanism of the Tianshan Oceanic plate subducting during the Carboniferous epoch. Within the well-known Tuwu–Yandong porphyry Cu belt, we offer new zircon U–Pb dates, Hf isotopes, whole-rock geochemistry, and whole-rock Sr–Nd isotopes for granite porphyry, quartz diorite porphyry, and diorite. These findings shed light on I- and A-type granitoids as well as slab-derived adakites, offering fresh perspectives on the Tianshan Oceanic plate’s subduction process. The majority of the adakites are diorite and quartz diorite porphyry (334–331 Ma), which have low Y and Yb contents, high Sr concentrations, and higher Sr/Y ratios. These rocks yielded high positive zircon ε_Hf(t) values (+10.3 to +14.1) and whole-rock ε_Nd(t) (+6.8 to +7.5) values. The adakitic diorite is thought to originate from the melting of the oceanic slab, while the subsequent quartz diorite porphyry formed through fractional crystallization of the dioritic magma. The A-type granite porphyry (331 Ma) displays high contents of SiO₂ and Fe₂O₃^T, along with increased Zr concentrations and zircon saturation temperatures. This rock type also possesses zircon ε_Hf(t) (+10.5 to +11.8) and whole-rock ε_Nd(t) (+6.4 to +6.6) values, derived from the partial melting of the juvenile crust. The late I–type granite porphyry (322 Ma) is characterized by high SiO₂ and low P₂O₅ contents, as well as positive ε_Hf(t) values (+9.5 to +13.2) and whole-rock ε_Nd(t) values (+5.9 to +7.0). These rocks originated from the juvenile crust and underwent significant fractional crystallization. Our new data suggest that the ancient Tianshan Oceanic plate experienced a slab window during its northward subduction in the Carboniferous, creating a regional extensional setting. Through a detailed analysis of temporal variations in the regional magmatic rocks, we identified a distinguishable rock association of Nb-enriched, high-Mg, slab-derived adakites, and A-type rocks in the northern area of the Kangguer Shear Zone between 340 and 330 Ma. These rocks, along with the coeval regional porphyry Cu deposits, indicate the occurrence of a ridge subduction during that time.