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.
