Age and origin of the Amdo ophiolite, central Tibet: New constraints on the evolution of the Bangong-Nujiang archipelagic ocean

Abstract

The central segment of the Bangong-Nujiang suture zone features multiple branching sub-sutures, yet the tectonic characteristics and evolutionary history of the associated ocean basin remain poorly understood. This study presents a detailed analysis of the field geology, geochronology, and petrogeochemical features of the Amdo ophiolite, located in the northernmost part of the suture zone. The Amdo ophiolite preserves a relatively complete sequence of oceanic lithosphere, including mantle peridotites, cumulate gabbros, sheeted dikes, and pillow lavas, arranged from bottom to top. The oceanic crust sequence is unusually thin, measuring less than 3 km, suggesting formation in a restricted oceanic basin. Zircon U-Pb dating of mafic rocks and plagiogranites reveals that the Amdo ophiolite formed during 184181 Ma, consistent with the age of the Dongqiao ophiolite located to its west. Geochemically, the mantle peridotites display refractory compositions akin to forearc peridotites, while the mafic rocks exhibit high MgO, low TiO₂, and rare earth element (REE) patterns similar to N-MORB, with notable depletion in Nb, Ta, and Ti and enrichment in large-ion lithophile elements (LILE). These geochemical features are indicative of supra-subduction zone (SSZ) ophiolites. Combined with evidence from the regional forearc mélange, oceanic crust-type retrograde eclogite, and associated island arc magmatic rocks, the data suggest that the Amdo ophiolite originated from a trench-arc system. It represents the remnants of Early Jurassic forearc extension within the northern branch of the Bangong-Nujiang Ocean. Thus, our study further elucidates the early evolutionary processes of the northern branch within the Bangong-Nujiang archipelagic ocean. It provides crucial insights into the evolution of an archipelagic ocean and the complex orogenic processes involving the multiphase assembly of microcontinents and accretionary terranes.