Subduction of solidifying melt lens beneath ultraslow-spreading ridges revealed by (Grt-)amphibolites and dolerites in the Xiugugabu ophiolitic mélange, southwestern Tibet

Abstract

Ridge subduction is an inevitable process during the evolution history of a vanished ocean. Due to its special characteristics, such as buoyant density, high heat fluxes and active magmatism, subduction of a ridge segment would leave specific fingerprints of high geothermal gradients and unique metamorphism. Therefore, investigations on ridge subduction could provide invaluable details of the evolution of oceanic lithosphere, and also help to understand the emplacement and preservation mechanism of worldwide ophiolites. This study mainly focuses on the mafic blocks of garnet-(Grt-)free and Grt-bearing amphibolites, foliated dolerites and dolerite dykes intruding into other blocks outcropped in the Xiugugabu ophiolitic mélanges of the west segment of the Yarlung-Tsangpo Suture Zone (YTSZ). On the basis of the detailed field, petrographical and geochemical investigations, combined with previous research on the other YTSZ ophiolites, this study highlights that the (grt-)amphibolites in the ophiolitic mélange are metamorphosed products under conditions of high geothermal gradients with protoliths to be subducted cumulate layered gabbros, which were likely to be newly crystallized in solidifying melt lenses beneath ridges. Hence, the generation of the XGGB ophiolite was contemporary with Neo-Tethyan ridge subduction. Intruding dolerite dykes show identical UPb ages to their intruded metamorphic soles, indicating that these amphibolites had gone through extremely rapid exhumation after peak conditions. Finally, scenarios of ridge destruction and subduction were proposed to complete the evolution history of the Neo-Tethyan Ocean and to provide an explanation for ophiolite emplacement and extremely rapid exhumation of metamorphic soles.