Early Cretaceous Dongbo ophiolite in the western part of the Yarlung Zangbo Suture Zone, Tibet: Constraints on multi -stages from ultra -slow spreading influenced by a mantle plume to intraoceanic subduction

The genesis of several large ultramafic massifs outcropped in the western part of the Yarlung Zangbo Suture Zone, such as the Dongbo and Purang massifs, are still controversial. Geological mapping and an 1, 002m-length scientific drilling (DSD-1) are carried out on the Dongbo ophiolite in recent years to solve this problem. Our study indicates that the Dongbo ophiolite is characterized by a " thick mantle and very thin crust" structure which recorded multi-stage magmatic events. The evolution of the major events of this massif can be diveided into three stages as follows: (1) The similar to 137Ma OIB-like basalt, 138 similar to 144 Ma OIB-like dolerite and gabbro dikes, and their wall-rock depleted peridotite with a melting degree of more than 30% reflect the influence of mantle plume activity on the ancient ocean basin in the Early Cretaceous; (2) The similar to 129Ma isotropic gabbro (type I), 128Ma gabbro dike (type II) and 18m thick dolerite (type III) in the DSD-1 drill core have similar geochemical compositions to the Southwest Indian ocean ridge basalts, which were produced during the emplacement of the ocean core complex near the slow-ultraslow spreading ridge; (3) The dolerite dikes (type N) dated by SIMS and SHRIMP zircon U-Pb with ages of 121 123 Ma have abnormally low contents of rare earth elements and high field-strength elements, and displayed obvious negative Th, Nb, Zr and Hf anomalies in their whole rock geochemical compositions. These features are similar to those of medium-titanium basalts without the addition of subduction components in the Albanide-Hellenide ophiolites, indicating that these dolerite dikes were formed in an initial intra-ocean subduction environment. In a word, the Dongbo ophiolite records an ultra-slow spreading influenced by a mantle plume and initial intra-ocean subduction process during the Early Cretaceous.