Formation of the Xigaze Metamorphic Sole under the Tibetan continental margin reveals generic characteristics of the subduction initiation process

Abstract Metamorphic soles found under allochthonous oceanic lithosphere, or ophiolites, are derived from the earliest lower plate oceanic crust accreted to upper plate mantle and are considered diagnostic of intraoceanic subduction initiation. Their metamorphic evolution is inferred to reflect the thermal structure at the site of subduction nucleation, with granulite-bearing soles linked to initiation at hot spreading centers. Here we present garnet Lu-Hf geochronology for the granulite-bearing sole of the Xigaze ophiolite in South Tibet, whose oceanic crust formed around 130 Ma through continental forearc extension. Our study shows that sole metamorphism was ongoing by 144 Ma, 14 million years before oceanic forearc spreading. The granulite-bearing sole thus formed following subduction initiation under a continental margin and not a spreading center. Our results imply that the metamorphic characteristics of soles are independent of the specific tectonic setting at the subduction nucleation site and rather provide generic constraints on the subduction initiation process.