GEOCHEMICAL EVOLUTION OF PERIDOTITES AND PYROXENITES FROM THE SUBCONTINENTAL LITHOSPHERICMANTLE OF THEAGUA POCA VOLCANO, CUYANIA TERRANE, ARGENTINA

Abstract

Anhydrous spinel-bearing peridotite and pyroxenite xenoliths in Pleistocene alkali basalts from the Agua Poca volcano, Argentina, were sampled for this study and represent the Cuyania Subcontinental Lithospheric Mantle (Cuyania SCLM). Agua Poca mantle xenoliths are depleted, but is possible to identify metasomatic event through the incompatible elements enrichment, due the interaction of the mantle lithosphere with aqueous fluid derived from slab subduction in a continental extra back-arc tectonic setting. Geochemical signatures of the studied samples show negative correlation for main oxides against Mg#, and depletion of incompatible elements compared to the Primitive Mantle (PM). Considering non-modal batch melting model, it is possible to attribute to SCLM below Cuyania Terrane 1 to 10% of partial melting DMM or 8 to 17% of partial melting Primitive Mantle. According to geochemical models considering mixing between end members (melting residue from PM/DMM and fluid or sediment compositions related to subduction tectonic setting) it is possible to suggest up to 3% of metasomatic enrichment in the Agua  Poca SCLM. ⁸⁷Sr/ ⁸⁶Sr ratios (0.702874 - 0.704999, with average of 0.704035) are similar to those 87 86 defined to OIB sources peridotites ( Sr/ Sr = 0.70244 to 0.70502) and are close to Depleted Mantle  ratios (DM; ⁸⁷Sr/ ⁸⁶Sr = 0.7023 to 0.7032). Agua Poca mantle xenoliths show ⁸⁷Sr/ ⁸⁶Sr lower than those defined to metasomatized peridotites (usually > 0.705). Nb/Ta ratios suggest a refractory eclogitic reservoir, which by partial melting generated alkaline/carbonatitic magmas with superchondritic Nb/Ta ratios.