Compositional signatures of dolerite dykes from the Purang ultramafic massif, Tibet: Implications for garnet-bearing components in the Neo-Tethyan mantle

Abstract

We present geological, whole-rock geochemical, Sr–Nd isotopic and U–Pb geochronological data for the mafic dykes from the Purang ultramafic massif, South Tibet (China). The mafic dykes are composed of dolerite within a mylonitized harzburgite body. These mafic dykes are dated at 138.0–138.5 Ma using zircon and apatite U–Pb methods, and are characterized by enrichments in large-ion lithophile elements (LILEs) and high-field-strength elements (HFSEs), resembling those of typical ocean island basalts (OIB). They have initial 87Sr/86Sr ratios of 0.70612 to 0.70670, initial 143Nd/144Nd ratios of 0.51231 to 0.51233 and high εNd(t) values of −3.1 to −2.6. Geochemical modeling suggests that the parental melts of dolerites were derived by ~50% melting of garnet pyroxenite or eclogite veins in the lithospheric mantle. The formation of the dyke-filling melts can thus be linked to melting of a thin mantle layer containing old low buoyancy plume (LPB)-components in the form of garnet-bearing veins. Our results provide a scenario for the tectono-magmatic evolution of the oceanic lithosphere beneath the Neo-Tethys in the South Tibet. We conclude that the existence of a thermal plume might not be essential to the petrogenesis of the Yarlung-Zangbo Suture Zone (YZSZ) ophiolites.