The evolution of the lithospheric mantle beneath Ia Bang, Pleiku plateau, Central Vietnam

Abstract

Mantle xenoliths are scattered in alkali basalt in central and southern Vietnam. The xenoliths from Ia Bang, Pleiku plateau, consist of fertile spinel lherzolites and refractory spinel harzburgites. The spinel peridotites display varying textures from protogranular through porphyroclastic to equigranular. Primitive mantle-normalized rare earth elements (REE) of clinopyroxene in the peridotites reveal depleted, spoon-shaped, and enriched patterns, suggesting a depletion event followed by various enrichment processes. The correlation between (Yb)n and (Y)n of clinopyroxene in spinel peridotites indicates that the subcontinental lithospheric mantle (SCLM) beneath Ia Bang has been affected by 1–20% fractional melting. The presence of glass and secondary clinopyroxene, coupled with the enrichment of light REEs (LREEs), high field strength elements (HFSEs), and large ion lithophile elements (LILEs) without introductions of new minerals suggests that the xenoliths underwent cryptic metasomatism. Metasomatic agents were identified as Na-alkali silicate, carbonate-rich melts, and H2O-CO2 fluids. The peridotites were at equilibrium temperatures of 841–1131 °C before being brought up by alkali basalt from depths of 36 to 50 km. The SCLM beneath Ia Bang experienced a thinning process in which the refractory lithosphere was thinned and replaced by asthenosphere during the Phanerozoic, following the asthenosphere-lithospheric interaction.