MANTLE PROCESSES AND SOURCES OF PLIOCENE BASALTS FROM THE MESETA DEL VIENTO, ARGENTINA

Pliocene Patagonian lavas (49.5°S, 72°W) from the Meseta del Viento can be associated with the opening of an asthenospheric slab window. Previous K-Ar data from the southwestern area of the meseta gives an age of ~3.5 Ma (Mercer, 1976). Currently another sample is being analyzed by Ar-Ar dating to confirm the 3.5 Ma age. The meseta lies 360 km east of the Peru-Chile trench, southeast of the Chile Triple Junction (46.5°S), and is located in the back-arc region, east of the Andes. The Meseta del Viento has an area of approximately 1200 km² and the plateau is overlain by 3-12 layered basalt flows, each being 5-10 m thick. This study deals with 12 samples that were taken from 8 locations, all from the eastern half of the meseta. Major element classification based on a silica-alkali diagram yields a strong alkalic chemical signature with SiO₂ content ranging from 48.4-49.8% and Na₂O + K₂O ranging from 4.5-6%. One outlier sample plotted as tholeiitic with a SiO₂ content of 52.7%. The tholeiitic sample has a REE signature identical to that of the alkalic samples, which can be explained by a shallower depth of melting and/or increased amounts of partial melting of a relatively homogenous mantle. Among the alkalic suite, there were three samples from the same location with increased but parallel REE concentrations had the least amount of MgO, Cr, and Ni, which is most consistent with shallow-level crystal fractionation of olivine. Based on trace element data and REE patterns the samples can be classified as ocean island-like basalts. The variations in the chemistries of the flows from the Meseta del Viento can be attributed to different depths of melting, varying percentages of partial melting of a homogenous mantle, and subsurface crystal fractionation. The ocean-island basalt like geochemical signature provides evidence for OIB-like mantle source left behind from the passing asthenospheric slab window.