| Popis: |
Extrusion tectonics has been invoked to explain recent magmatism across Indochina (e.g., Hoang & Flower, 1998). Lithospheric mantle xenoliths sampled from two alkali basalts in south-central Vietnam, Pleiku and Xuan Loc, comprise fertile spinel peridotites. To better determine the origins of the Indochinese subcontinental lithospheric mantle (SCLM), including impacts of tectonic extrusion, we present major and trace elements, and 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb 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 depleted isotopic signatures (e.g., 87Sr/86Sr = 0.70238–0.70337 and 143Nd/144Nd = 0.512921–0.514190). A small group of refractory peridotites have “enriched” REE patterns suggesting more extensive metasomatism, and enriched isotope ratios (87Sr/86Sr = 0.70405 and 143Nd/144Nd = 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 °C and pressures of 11.7-15.8 kbar, while refractory xenoliths have comparable temperatures of 923-1,006 ºC, but lower pressures of 7.1-10.0 kbar, suggesting refractory rocks are dominantly present at shallower depths. We suggest that the regional lithospheric mantle has experienced variable melt extraction, producing heterogeneous 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. |
