Characterizing Peridotite Xenoliths From Southern Vietnam: Insight Into the Underlying Lithospheric Mantle

We present data for lithospheric mantle xenoliths sampled from two alkali basalts in south-central Vietnam, Pleiku and Xuan Loc, including fertile spinel peridotites. To better determine the origins of the Indochinese subcontinental lithospheric mantle (SCLM), including impacts of posited tectonic extrusion, we present major and trace elements, and Sr-87/Sr-86, Nd-143/Nd-144, Hf-176/Hf-177, Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 in xenolith mineral separates. Most peridotites from Pleiku and Xuan Loc have fertile major element compositions, "depleted" and "spoon-shaped" rare earth element (REE) patterns, interpreted to record prior melt depletion followed by melt metasomatism, and variable but generally depleted isotopic signatures (e.g., Sr-87/Sr-86 = 0.70238-0.70337 and Nd-143/Nd-144 = 0.512921-0.514190). A small group of refractory peridotites have "enriched" REE patterns suggesting more extensive metasomatism and enriched isotope ratios (Sr-87/Sr-86 = 0.70405 and Nd-143/Nd-144 = 0.512755-0.512800). The presence of both fertile and refractory xenoliths records a heterogeneous SCLM beneath Vietnam. Based on geothermobarometry calculations, fertile xenoliths have equilibrium temperatures of 923-1,034 degrees C and pressures of 11.7-15.8 kbar, while refractory xenoliths have comparable temperatures of 923-1,006 degrees C, but lower pressures of 7.1-10.0 kbar, suggesting refractory rocks are dominantly present at shallower depths. We suggest that the lithospheric mantle has experienced variable melt extraction around 1.0-1.3 Ga, producing heterogeneous major element compositions. While we cannot rule out partial removal and replacement of the lithosphere, large-scale delamination is not necessary to explain observed characteristics. The entire SCLM was more recently metasomatized by melts resembling Cenozoic basalts, suggesting recent asthenospheric melting has modified the SCLM by melt infiltration.