Emplacement and eruptive style of high-grade ignimbrites from fissure vents: The Las Mellizas Ignimbrite, Caviahue-Copahue volcanic complex, southern Andes

Abstract

The Pliocene to Recent Caviahue – Copahue volcanic complex is located in the Southern Volcanic Zone of the Andes. The Las Mellizas Ignimbrite is widely distributed in the intra-caldera region of the previously formed Caviahue caldera. Detailed field work led us to identify four facies associations for Las Mellizas Ignimbrite and to characterize the pyroclastic facies as dominantly vitrophyric, eutaxitic and rheomorphic, with limited occurrences of welded agglomerates and lithic breccias. Pyroclastic dykes with subvertical eutaxitic fabric were found and interpreted as fissure vents of this ignimbrite. Their orientations follow either the southern Caviahue caldera rim, or approximately the current extension direction as determined by previous neotectonic studies. Thermobarometry and hygrometry estimations were carried out based on compositional analyses of minerals (plagioclase, ortho- and clinopyroxenes, FeTi oxides) and glass. We interpret that Las Mellizas Ignimbrite was deposited from high-temperature and concentrated pyroclastic density currents, formed by almost fully collapsed low eruptive columns (pyroclastic fountaining). Low pre-eruptive pressures (0.04–0.06 GPa) and moderate water content (2.0–3.4 wt%) plus the low crystallinity of the magma (<10 vol% of phenocrysts) points to a shallow magma chamber and shallow fragmentation level, which is also supported by lithic clast composition. The eruption trigger is interpreted as mainly due to magmatic processes, and aided by tectonic activity. The presence of topographic scarps, outcrops distribution and stratigraphic discontinuities north of the Copahue volcano, in addition to co-ignimbritic lithic breccias, suggest that a small collapse may have occurred, nested on the western Caviahue caldera rim. In this context, the fissure vents represent peripheral extra-caldera pyroclastic conduits.