Subduction Recycling and Closure of the Paleo-Asian Ocean in the Beishan Orogen: Constraints From the Niujuanzi Ophiolitic Mélange and Associated Intrusions

Abstract

Ophiolitic mélanges and associated intrusions provide key archives for reconstructing the evolution of ancient ocean basins and subduction-related mantle modification. The Hongliuhe–Niujuanzi–Xichangjing ophiolitic mélange in the central Beishan Orogen preserves important records of the Paleo-Asian Ocean, but the timing of oceanic subduction, terminal closure, and subsequent postcollisional reworking in the Niujuanzi remain debated. Here we integrate field observations, zircon U–Pb geochronology, whole-rock and mineral geochemistry, and Sr–Nd–Hf–Pb isotopic data from the newly dated and geochemically characterized ophiolitic rocks and associated dioritic to granodioritic intrusions. Zircon U–Pb ages define three Paleozoic magmatic episodes: Early Silurian diorites at 435–427 Ma, Late Silurian ophiolitic gabbros at 423–418 Ma, and Early Devonian granodiorites at ca. 403 Ma. The diorites display continental arc-like geochemical signatures and were derived from a lithospheric mantle source modified by slab-derived fluids. The gabbros show N-MORB-like affinities with weak subduction-related enrichment, consistent with formation in an SSZ setting. The granodiorites were generated mainly by partial melting of the ancient lower crust with minor mantle input, recording a transition to postcollisional extension. Silicate in situ Pb isotopes exhibit a temporal shift toward Indian MORB-like signatures from the Early Silurian to Early Devonian, indicating progressive mantle source modification by recycled subduction components. These results demonstrate that the Niujuanzi basin remained active until the Late Silurian, closed by the end-Silurian, closed during the latest Silurian to the earliest Devonian (ca. 418 Ma), and transitioned to postcollisional extension in the Early Devonian, providing a refined geodynamic model for the central Beishan Orogen.