Abstract

Isluga is an active, fumarole-bearing volcano with erupted products, showing a homogeneous plagioclase + pyroxene + amphibole +/- biotite +/- Fe-Ti oxides mineralogical assemblage. These products are mainly high-K calc-alkaline trachy-andesite with subordinated trachy-dacite and andesite (56-65 wt% SiO2), which exhibit decreasing FeO, MgO, CaO, Co, and Sr concentrations, and increasing K2O and Rb concentrations with differentiation. Primitive mantle normalized-rare earth elements (REE) present light (L)-REE enriched - heavy (H)-REE depleted patterns, with Eu/Eu & lowast; ratios 0.7 to 1.0. Overall, Sr-87/Sr-86 ratios (0.7056-0.7059) vary slightly without correlation with differentiation but positive correlation with increasing Sr content (523-1007 ppm). We proposed that the parental magmas of Isluga were generated from a sub-arc mantle-derived primary magma (Sr-87/Sr-86 similar to 0.704), which assimilated lower crustal material with fractional crystallization in a garnet-dominated environment (>40 km depth), generating an Andean-type parental magma (Sr-87/Sr-86 = 0.7055). Combination of Sr/Y, Gd/Dy, and Eu/Eu & lowast; ratios suggest that garnet became unstable during the ascent of the magmas towards a plagioclase-dominated evolution (<35 km depth). Finally, a late stage of crystallization of amphibole + biotite + plagioclase in magmatic chambers located at shallow depths (<18 km depth) is proposed. When compared with other volcanoes of the Central Andes (e.g., Taapaca, Parinacota, and Tata Sabaya), Isluga shows a mantle-array trend, and lower radiogenic Sr-87/Sr-86, and Sr/Y and Gd/Dy values. This implies a lesser extent of crustal contributions, both at lower and upper crustal levels, related to a regime of high rates of sub-arc mantle-derived mantle inputs for Isluga.