Geochemical constraints on the evolution of the lithospheric mantle beneath central and southern Vietnam

Abstract

We present comprehensive geochemical and isotopic (Sr-Nd-Hf) datasets for two suites of ultramafic rock in Vietnam, namely xenoliths of spinel lherzolite entrained in late Cenozoic alkali basalts, and Paleozoic ultramafic massifs that occur along the Tam Ky-Phuoc Son suture zone in central and southern Vietnam. The ultramafic massifs are the products of high degrees of melt extraction (up to 40%), and have relatively low equilibrium temperatures of 603 to 778 °C. The lherzolites are residues of relatively low degrees of fractional melting (< 1% up to 20%). The compositions of minerals in the xenoliths reveal that the late Cenozoic lithospheric mantle beneath central and southern Vietnam was hotter (825–1058 °C) than during the Paleozoic. The calculated trace element patterns of metasomatic melts that equilibrated with clinopyroxenes in the LREE-enriched xenoliths show enrichments in Th, U, and LREEs, and depletions in Nb. These data, together with the elevated Ti/Eu ratios of the clinopyroxenes, reflect the role of hydrous silicate melts as the main agents of metasomatism. The ultramafic rocks of the Paleozoic massifs contain spinel with TiO2 contents higher than expected for residual spinel, which suggests the influence of boninitic melt(s). In the lherzolite xenoliths, the clinopyroxenes have MORB-like depleted Sr-Nd-Hf isotopic compositions (87Sr/86Sr = 0.70241–0.70416; eNd = +6.6 to 12.3; eHf = +13.1 to 25.1), suggesting metasomatic melts/fluids from upwelling asthenosphere. We suggest that subduction of the (Paleo-)Pacific Plate and continental collision during the Permian–Triassic played key roles in lithospheric replacement and thinning beneath central and southern Vietnam.