Melt evolution of upper mantle peridotites and mafic dikes in the northern ophiolite belt of the western Yarlung Zangbo suture zone (southern Tibet)

The Yarlung Zangbo suture zone (YZSZ) in southern Tibet is divided by the Zhongba terrane into two subparallel belts in its western end. The northern belt (NB) is tectonically juxtaposed against an accretionary prism complex and the Gangdese magmatic arc of Eurasia along dextral oblique-slip faults. Peridotite massifs in this belt are intruded by mafic dikes, providing critical geochemical, geochronological, and isotopic information about the melting-melt extraction history of the Tethyan mantle. Peridotites consist of harzburgite and clinopyroxene harzburgite with minor Iherzolite and dunite-chromitite. Dolerite and microgabbro dikes crosscutting these peridotites display U-Pb zircon ages of 128-122 Ma, and show normal mid-oceanic ridge basalt (N-MORB) like rare earth element patterns with negative Nb, Ta, and Ti anomalies, high epsilon(Nd)(t) values (+8.39 to +9.28), and (Nd-143/Nd-144)(t = 128 Ma) ratios of 0.51290-0.51295. They display high (Sr-87/Sr-86) t ratios of 0.70433-0.70489, and Pb-206/Pb-204 of 17.546-17.670, Pb-207/Pb-204 of 15.432-15.581, and Pb-208/Pb-204 of 37.724-37.845, suggesting that their N-MORB-like mantle source was modified by island arc melts. Slab rollback-induced extension in an arc-trench system along the Eurasian continental margin led to similar to 7%-12% partial melting of subduction-influenced, spinel lherzolite peridotites, producing dike magmas. The NB peridotite massifs and ophiolites thus represent a suprasubduction zone oceanic lithosphere formed in close proximity to the late Mesozoic active continental margin of Eurasia.