Origin of Holocene trachyte lavas of the Quetrupillán volcanic complex, Chile: examples of residual melts in a rejuvenated crystalline mush reservoir

The Quetrupillán Volcanic Complex (QVC) is a stratovolcano placed in the center of a NW-SE volcanic chain, between Villarrica volcano and Lanín volcano, in the Central Southern Volcanic Zone of the Andes. Its youngest effusive products are dominated by crystal-poor (most samples with <9 vol% phenocrysts), crystal clot-bearing trachytes (from 64.6 up to 66.2 wt% SiO₂), whereas the oldest units are mainly basaltic andesites. Two-stage generation of QVC trachytes by differentiation at shallow depth (<1 kbar) and NNO-QFM oxidation conditions were obtained from initial melt compositions equivalent to the Huililco basalts, a small eruptive centre located ca. 12 km NE of the QVC main vent. Pyroxene-bearing crystal clots, locally abundant in the trachytes, were formed at 900–960 °C (±55 °C) and represent a dismembered crystal mush from which interstitial trachytic melts were extracted and transported upward before eruption. Heating of the crystal mush by a hotter magma recharge is inferred from complex zoned plagioclases formed at higher crystallization temperatures (50–90 °C) than those obtained from pyroxene. Ca-rich plagioclase overgrowths around more albitic cores, followed by an external rim of similar composition to the core are interpreted as restoration to the initial conditions of plagioclase crystallization after the mentioned heating event. Additionally, a late heating of up to 150 °C just prior to eruption is recorded by Fe-Ti oxide thermometry.