Ascertainment and environment of the OIB-type basalts from the Dongbo ophiolite in the western part of Yarlung Zangbo Suture Zone

The western part of the Yarlung Zangbo Suture Zone (YZSZ) is divided into two sub-belts: the genesis of the south plays a crucial role in the interpretation of the geological setting, magmatic and tectonic processes of the Neo-Tethyan Ocean. Previous research about the Dongbo ophiolite was poor, due to harsh natural environment and traffic inconvenience. This paper reports the data on petrography and geochemistry of OIB-type basalts and associated sedimentary rocks in the Dongbo ophiolite and discusses their source characteristics and geological significance. Dongbo ophiolite consists of peridotite (including harzburgite, minor Cpx enriched harzburgite and dunite), mafic dikes and submarine volcanic-sendimentary rock sequences, no typical cumulates and pillow lavas. The stable marine volcanic-sedimentary sequences mainly composed of siliceous limestone, red chert, shale interbedded sandstone, basalt and basaltic volcaniclastic rocks. OIB-type basalts are characterized by low SiO2 and MgO, high TiO2, P2O5 and (K2O + Na2O) content, enrichment of Nb, Ta and REE, depletion of Th, K, Pb, Sr. Trace elements and Sr, Nd, Pb isotope data show that these basaltic lavas were derived from 2% to 5% partial melting of garnet spinel lherzolite. Siliceous rocks are of biogenic origin, and show a close relation to terrigenous component and seamount or island arc basalts. Above evidences indicate that the Dongbo volcanosedimentary sequences have the typical seamount features, and the Dongbo ophiolite probably is related to a plume hotspot. Therefore, it can be considered, the plume hotspot underplating and interacting with the subcontinental lithospheric mantle of the Gondwana northern margin not only promoted opening the Daba-Xiugugabu Tethyan oceanic basin, but also prompted diamonds and other abnormal mantle minerals to be migrated upper mantle and wrapped in peridotite and chromite.