Late Jurassic magmatism in the Neo-Tethys Ocean: Evidence from zircon U-Pb ages and geochemistry of dolerites in the Bainang Terrane, southern Tibet

The Yarlung Zangbo suture zone(YZSZ) in southern Tibet marks the collision between the Eurasia plate and the Indian subcontinent, and the tectonic affinity of the ocean island-type(OIB-type) basaltic rocks within this suture remains controversial.The Bainang terrane in the middle segment of the YZSZ is a NE-SW-oriented tectonic slice, which is composed of radiolarian chert, siliceous mudstone, siliceous mudstone, shale, tuff, micritic limestone, basalt, dolerite and gabbro.The dolerite and gabbro are dikes or sills intruding into strata.The dolerites formed in the Late Jurassic, with a concordant zircon ²⁰⁶Pb/²³⁸U age of 150.3±0.8 Ma(n=39, MSWD=1.8).They are chemically characterized by LREE enrichment, variable enrichment of large ion lithophile elements(LILEs, e.g.Rb, Sr and Pb) and high field strength elements(HFSEs, Th, Nb, Ta, Zr, Hf, and Ti), indicating OIB affinity with little or no continental crust contamination.These features are similar to Late Jurassic-Early Cretaceous OIB in the YZSZ.It is suggested that these OIB rocks were derived from the intraplate environment within the Neo-Tethys and represented the remnants of seamounts.In combination with regional geological material, it is proposed that the Jurassic mantle plume in Neo-Tethys might drive the oceanic lithosphere northward subduction under the Lhasa terrane, and formed the Andean continental margin.During the Early Cretaceous, the Neo-Tethys oceanic lithosphere was decoupled from the overlying plate, and retreated southward to induce the Gangdese forearc extension to form the present ophiolites preserved in the YZSZ.