Rapid ascent and contamination of magmas through the thick crust of the CVZ (Andes, Ollagüe region): evidence from a nearly aphyric high-K andesite with skeletal olivines

Described in this study are the mineralogy and petrology of a nearly aphyric high-K andesite (herein referred to as the SC2 sample) located in the Ollagüe volcano region, characterized by microcrystalline to glassy groundmass and very few olivine and pyroxene microphenocrysts (≪3 vol.%). Olivine (Fo72–82) exhibits a wide range of skeletal crystal morphologies due to the high cooling rate and geometrically-different entrapped inclusions of glass, ranging in composition from intermediate to acid (SiO2 53–72 wt.%). Plagioclase (An56–64), meanwhile, often shows a quench texture with holed and swallow-tailed forms. Ferroan enstatite locally occurs as subhedral microphenocrysts whereas ferroan diopsides to magnesium-rich augites are common in the groundmass. Opaque minerals (Ti–Al-magnetites and Cr-spinels) are rare and generally found as tiny euhedral inclusions in the olivines. Although SC2 magma should have contained a relatively high amount of H2O (ca. 6 wt.%), hydrous minerals such as amphibole were never on the liquidus because a high temperature (≥1000 °C) was maintained by the rising magma up to the shallow crustal levels. The SC2 incompatible trace element pattern has a typical arc-like signature with Sr–Nd isotopic ratios falling within the range of the Quaternary volcanic rocks of the CVZ, straddling between the less contaminated basaltic andesites and the more contaminated intermediate to acid extrusives of the Ollagüe volcano. Pb isotopic values of the studied lava also fit very well with the overall volcanic rocks of the Southern CVZ and suggest that the SC2 magma underwent a contamination from the Paleozoic basement inferred to characterize this region of the Andean Cordillera. During decompression, the H2O-rich mantle-derived magma reached a superheated status (as the aphyric character of the SC2 sample demand) and was contaminated to some extent by high-T, “fertile” crustal wall-rocks. The contamination process of the superheated magma probably occurred at the meso-microscale as a complex function of diffusion rates of components in the magma and gave rise to a wide compositional range of liquids, frozen within the skeletal olivines before complete homogenization. However, this does not necessarily imply the existence of geologically significant batches of magmas with compositions similar to those of the glass inclusions. Neutral to extensional stresses during a Quaternary tectonic phase of crustal relaxation of the Altiplano–Puna Volcanic Complex should have permitted the fast uprising of the SC2 magma through the thick CVZ crust without any storage and differentiation in magma chambers. The observed data indicate that in the Andean CVZ, some mantle-derived magmas, although modified by crustal contamination “en route” to the surface, were able to rise with negligible fractional crystallization, by-passing the flanks of Quaternary stratovolcanoes.