Lynne J. Elkins
Date of this Version
Hobbs, K., 2020, Characterizing peridotite xenoliths from southern Vietnam: insight into the underlying lithospheric mantle [M.S. thesis]: University of Nebraska-Lincoln
Extrusion tectonics has been invoked to explain the extensive basaltic magmatism that has erupted over Indochina within the last 17 Ma. The basalts display two-stage eruptive cycles consisting of tholeiites followed by alkaline basalts. Lithospheric mantle xenoliths recently sampled from the alkaline basalts of two volcanic centers, Pleiku and Xuan Loc, primarily consist of fertile spinel lherzolites, and Xuan Loc also contains refractory spinel harzburgites. We measured major elements in xenolith mineral separates, trace elements in clinopyroxenes and orthopyroxenes, and Pb-Sr-Nd isotopic compositions in clinopyroxenes to determine the origin and history of the subcontinental lithospheric mantle (SCLM) beneath Vietnam. Most peridotites from Pleiku and Xuan Loc exhibit fertile major element compositions, “depleted” and “spoon-shaped” rare earth element (REE) patterns, and isotopic signatures ranging from typical depleted MORB mantle to an even more depleted source (87Sr/86Sr = 0.702381 - 0.703365 and εNd = +8.84 - +30.28). A smaller group of peridotites from Xuan Loc show distinct refractory major element compositions, “enriched” REE patterns, and more incompatible element enriched isotopic signatures (87Sr/86Sr = 0.704050 and εNd = +3.16 in one sample) than the fertile peridotites. Based on their major and trace element compositions, Pleiku and Xuan Loc xenoliths have calculated equilibrium temperatures of 807-1052 ÅãC which indicate extraction depths of 30 to 45 km. We interpret the fertile peridotites from Pleiku and Xuan Loc to sample recently emplaced lithospheric mantle from the convecting asthenosphere, whereas the refractory peridotites from Xuan Loc may represent partial melting residues derived from older SCLM. We conclude that the extrusion of Indochina initiated regional asthenospheric upwelling, resulting in the partial removal and replacement of the lithospheric mantle.
Advisor: Lynne J. Elkins