Claudio Contreras

The scarcity of historical rhyolite eruptions means that volcanological and petrological studies of past eruptions are a key tool for assessing the potential for future hazardous activity and improving interpretations of unrest signals. For the last 18 ky, the Laguna del Maule volcanic complex (LdM) in Chile has erupted primarily rhyolites but with differing magma compositions, eruption styles and eruptive volumes. Rapid surface uplift and episodic seismic activity at LdM over the last two decades has emphasized the need to understand both the recent evolution of the magmatic system and the most likely future eruption scenarios. Using mineral composition, geothermobarometry and MELTS modelling, we assess the influence of the magma storage and ascent conditions on the magnitude and styles of three LdM rhyolites. Magmas of the first and largest Plinian-ignimbrite eruption (Laguna del Maule: rdm; >17 km³ DRE) form a distinct mineral assemblage with An_28-60 plagioclase, amphibole and quartz derived from a magma plumbing system over a large pressure range (90-350 MPa). We suggest that the rdm eruption was triggered by magma recharge and overpressure within a ~90 MPa magma chamber of high-silica (>76.5 wt.% SiO₂) rhyolitic melt. The rdm eruption appears to have reset the LdM storage conditions, such that subsequent rhyolite eruptions have been smaller (3 DRE), involved less evolved melt (2) and produced (sub)Plinian-effusive eruption sequences. Of these, we have studied the earliest (Los Espejos: rle) and the most recent (Las Nieblas: rln). Both have mineral assemblages with An_20-34 plagioclase and biotite formed in water-saturated crystal-poor rhyolite batches over limited pressure ranges (ΔP ~ 50 MPa) within a rhyodacitic mush reservoir. We suggest that rle and rln were triggered by injection of water-rich magma and volatile exsolution; the transition to effusive behavior occurred because of changes in magma temperature (rle) or magma decompression rate (rln). Similarities in the limited pressure range and high volatile content inferred for the current low-density magma body with the petrologically-determined source conditions of magmas that fed past subplinian-effusive eruptions suggest that the next LdM eruption will be similar.

Along the Southern Volcanic Zone (SVZ) of the Andes, the Liquiñe Ofqui Fault Zone (LOFZ) controls the occurrence and distribution of small eruptive centres (SECs) and stratovolcanoes. The Fui Group are clusters of Holocene SECs <5.5 km west of the LOFZ and 8 – 16 km ENE from the Mocho-Choshuenco Volcanic Complex (MCVC). Although separated ~2 km from each other, the Fui Norte (north) volcanic products are olivine-rich basalts (49 – 52 wt% SiO 2 ) and enriched in incompatible elements, whilst the Fui Sur (south) materials are basaltic andesites to andesites (55 – 58 wt% SiO 2 ) with less amount of olivine but more plagioclase and clinopyroxene and depleted in incompatible elements. Crystal clots of olivine, plagioclase and clinopyroxene are abundant in all the studied samples. We found two compositional zones of olivine and plagioclase: High-Fo, High-An (Zone 1, Fo 78-89 and An 76-93 ) and Low-Fo, Low-An (Zone 2, Fo 60-78 and An 57-73 ); only Zone 2 are in equilibrium with the groundmass (representing the melt) compositions. We determined shallow reservoir conditions (up to 7.6 and 18.9 km in the Fui Norte and Fui Sur SECs, respectively) for Zone 2 crystallisation using geothermobarometers and thermodynamic modelling by rhyolite-MELTS v.1.0.2 freeware. Additionally, we determined water content dissolved in melt of 1 – 2.5 wt% and 2 – 3.8 wt% for Zone 2 compositions of the Fui Norte and Fui Sur SECs, respectively, and oxidation conditions for Zone 2 of QFM, QFM-1 and NNO. The relative more primitive compositions and the common occurrence as part of crystal clots of Zone 1 crystals suggest an earlier stage of crystallisation in a mushy zone of the shallow reservoirs. Although their geographical proximity, we propose that the magmatic origins of Fui Norte and Fui Sur SECs are independent due to their differing petrographic and geochemical features and the likely genetic relation of Fui Sur SECs magmas with those erupted from MCVC, where the structures associated to the LOFZ aids the magmatic ascent and emplacement of the volcanic products.