Early Cretaceous (～138-134 Ma) Forearc Ophiolite and Tectonomagmatic Patterns in Central Tibet: Subduction Termination and Re-initiation of Meso-Tethys Ocean Caused by Collision of an Oceanic Plateau at the Continental Margin?

摘要

The Bangong-Nujiang Suture Zone (BNSZ) in central Tibet represents relics of the Meso-Tethys Ocean that once separated the Lhasa and Qiangtang terranes. When and how this ocean closed has been a matter of debate, and the cause of the uncertainty stems primarily from a lack of consensus as to whether Early Cretaceous ophiolite exists within the BNSZ. New geochronological and geochemical data presented here identify similar to 138-134 Ma MORB-like and OIB-type mafic rocks in the Kangqiong area of the western BNSZ. These data, together with the spatially associated peridotite and marine sediment, provide the first firm evidence for the presence of Early Cretaceous ophiolite within the BNSZ. Our integrated study of geological, mineralogical, and geochemical data suggests that the Kangqiong Ophiolite formed in a forearc setting related to the subduction re-initiation of the Meso-Tethys Ocean. By combining the present results with a synthesis of tectonomagmatism in central Tibet, including the existence of an Early Jurassic oceanic plateau within the Meso-Tethys Ocean, the magmatic lull (similar to 145-130 Ma) in the Qiangtang Terrane, an angular unconformity between Lower Jurassic oceanic rocks and Upper Jurassic shallow-marine strata, and the compositional differences and migration of magmatism before and after the magmatic lull, we propose a new model of the evolution of the Meso-Tethys Ocean. This model proposes that the collision of an oceanic plateau with the Qiangtang continental margin caused the termination of Meso-Tethyan subduction during the Late Jurassic and subsequent re-initiation behind the oceanic plateau during the Early Cretaceous.