OIB-and P-type ophiolites along the Yarlung-Zangbo Suture Zone (YZSZ), Southern Tibet: Poly-Phase melt history and mantle sources of the Neotethyan oceanic lithosphere

Abstract

We present an overview of the internal structure of the ophiolite massifs along the Yarlung Zangbo suture zone (YZSZ) in southern Tibet with a focus on the geochemical character and tectonic evolution of the Ocean Island Basalt (OIB) and mafic alkaline rock assemblages associated with these ophiolites. The Jurassic-early Cretaceous lavas, massive diabase and gabbroic rocks are either tectonically intercalated with the early Cretaceous, subduction-influenced ophiolitic units, or occur as thrust sheets or blocks with an early Cretaceous mélange and in a Jurassic-Cretaceous flysch unit structurally beneath these ophiolites. They display uniform chondrite-normalized REE patterns with light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion, no obvious Eu anomalies or negative Nb, Ta and Ti anomalies, and primitive mantle normalized trace element patterns with significant large-ion lithophile element (LILE) enrichment, similar to those of modern OIB and the Hawaiian alkaline basalts. These mafic alkaline rock assemblages represent OIB-and Plume-type (P-type) oceanic crustal rocks (with no subduction influence) that formed from magmas produced by partial melting of plume-metasomatized asthenospheric mantle source during the early stages of the opening of a Neotethyan seaway between Proto-India and Eurasia. Subsequent consumption of this OIB-P-type mid-ocean ridge (MOR) oceanic lithosphere at an intraoceanic subduction zone produced the ∼130-120 Ma forearc to backarc, SSZ oceanic crust within the same Neotethys. The evolutionary history of the YZSZ ophiolites thus reflects a poly-phase melt history and different mantle melt sources. The final tectonic juxtaposition of the older OIBand P-type oceanic crustal rocks with SSZ-type oceanic lithosphere fragments took place as the northern passive continental margin of Proto-India collided with and underplated the intraoceanic subduction-accretion system in the late Cretaceous. The YZSZ displays a complete Wilson cycle record of the rift-drift, seafloor spreading and subduction zone tectonic evolution of the Mesozoic Neotethys.