Subduction Polarity And Processes Of The Bangong-Nujiang Tethys:Insights From Igneous Rocks And Detrial Zircons

Subduction of oceanic lithosphere is the major driver of plate motion and ultimately leads to ocean closure and collisional orogenesis.Subduction results in the generation of an arc-trench system in the overriding plate.The recognition of such systems in the rock archive is fundamental for reconstructing the evolution of ancient orogens and delineates ancient plate margins.The Bangong–Nujiang suture zone(BNS)in Tibet represents the remnants of the Bangong Tethys.The subduction and closure of the Bangong Tethys lead the Lhasa Terrane collide with Qiangtang Terrane and uplift of the Tibetan Plateau.However,the subduction polarity and processes of the Bangong Tethys remains unclear.This work studies the arc-trench systems,including detrital zircons and magmatism within the BNS and adjacent regions of the Lhasa and Qiangtang terranes,to reconstruct the evolution of the Bangong Tethys.The Late Jurassic(~165 Ma)Daruco high-Mg andesites-dacites-rhyolites within the BNS imply a subducted sediment origin with melt-mantle interaction during its ascent.The BNS can be divided into two belts in the Baila-Amdo area,i.e.,the Dongqiao belt and the Baila belts.Sandstone petrography coupled with detrital zircon U-Pb ages of these mélange indicates these mélange represent accretionary prism and show a Qiangtang Terrane provenance.The accretion of Dongqiao belt lasted to 169 Ma,which were angular unconformities overlain by the Shamuluo Formation during 169-153 Ma,and the accretion of Baila belt lasted to 148 Ma.This feature and the uplift of the accretionary prism revealed by the detrital zircon ages and angular unconformities,along with integration of magmatism and metamorphism,argue for a special subduction event.We develop a two stages subduction processes,i.e.,parallel ridge-trench interaction(170-160 Ma)and ridge subduction(160-150 Ma).Integration of the Yanhu volcanic rocks(135-116 Ma)indicates the development the Baingoin-Yanhu arc in the northern Lhasa Terrane.This observation,in combination with the distribution of Early Cretaceous arc magmatism across the Lhasa Terrane,which prior to subsequent deformation had a width of at least 600 km,requires the existence of two arc systems flanking the Lhasa Terrane and related to opposed oceanic plate subduction: north dipping subduction of the Neo-Tethys and south dipping subduction of the Bangong Tethys.Compositional change from medium-K to high-K calc-alkaline volcanism around Yanhu records changing mantle geodynamics,which we infer to reflect rollback and breakoff of the south dipping Bangong Tethyan slab.This observation,in combination with the Xiabie-Gerze-Rutog magmatism(125-110 Ma)with compositional diversity in the Qiangtang Terrane and OIB-type rocks(120-110 Ma)within the BNS,along with lithostratigraphic and metamorphic data,indicated double side subduction and breakoff of the Bangong Ocean lithosphere.