Early activity of the Kerguelen Mantle plume: geochronology, geochemistry and Sr-Nd-Pb isotopes of mafic dykes and sills from the Tethyan Himalaya

The remnant Comei igneous province in SE Tibet has been recognized as a product of early Kerguelen mantle plume activity. In this paper, we investigate mafic dykes and sills intruded into Lakang Formation in the southeastern Tethyan Himalaya. Based on new whole-rock geochemical, Sr-Nd-Pb isotopic and SHRIMP zircon U-Pb geochronological data, these rocks can be divided into three groups. Group 1 is composed of OIB-like mafic dykes and sills with lower ε_Nd(<i>t</i>) (+1.78-+2.55) and dated at 124.8 ± 1.7 and 133.3 ± 1.5 Ma. Group 2 consists of N-MORB-like mafic dykes and sills with lower TiO₂ and P₂O₅ contents. Group 3 has geochemical compositions transitional between Group 1 and Group 2, with ε_Nd(<i>t</i>) ranging from +1.98 to +2.24 and an age of 125.1 ± 2.1 Ma. We interpret Group 1 as being derived from an OIB-like source related to a mantle plume. Groups 2 and 3 are considered to have been generated from melts of the asthenosphere and the lower crust, with some involvement of melts from the upper crust. Considering the oldest (ca. 147 Ma) and the youngest (ca. 125 Ma) ages of the OIB-like mafic dykes and sills related to the Kerguelen mantle plume and their palaeogeographic location, we suggest that the Kerguelen mantle plume head had reached the lithosphere during the late Jurassic and was later directly underneath the Cona area. We propose a new model to describe the early activity of Kerguelen mantle plume, which put the mantle plume below the mid-ocean ridge and can well explain the long-lived magmatism related to Kerguelen mantle plume.