Mush Architecture and Processes in the Reservoirs of a Supereruption-Scale Magma System, Permian Ora Ignimbrite (Northern Italy)

Abstract

Abstract The Ora Ignimbrite (~275 Ma) is a crystal-rich (~40%), heterogeneous, supereruption-sized (~1300 km3) rhyolite in northern Italy and the final eruptive product of the Athesian Volcanic Group (289–274 Ma), the largest Permian volcanic district in Europe. Two key outcrops, an intracaldera vitrophyre and an outflow vitrophyre, contain well-preserved, glass-bearing juvenile material. We investigate at what scale heterogeneities existed in the Ora magmatic system by studying the geochemical and petrological characteristics of crystal-rich (20–50%) and crystal-poor (<20%) fiamme from the Ora vitrophyres, allowing us to reconstruct critical aspects of Ora's magmatic architecture and pre-eruptive history and granting us further insight into the storage and dynamic processes that occur in large, crystal-rich, silicic systems. Textural analysis of fiamme reveals four types: the intracaldera vitrophyre has very coarse-grained crystal-rich (VCCR) and fine-grained crystal-poor (I-FCP) fiamme, while the outflow vitrophyre has coarse-grained crystal-rich (CCR) and fine-grained crystal-poor (O-FCP) fiamme. The diversity of crystal-poor (I-FCP & O-FCP) fiamme glass trace-element compositions suggests that multiple small crystal-poor magma bodies were dispersed throughout the Ora magmatic system. Compositionally and texturally distinct glass types in single O-FCP fiamme preserve evidence of variable degrees of magma mixing and record different mingling to eruption timescales, revealing that the Ora magmatic system experienced multiple silicic recharge events. In the crystal-rich (VCCR & CCR) fiamme, high crystal contents (VCCR: 40–50%; CCR: 20–40%), large crystals (max grain size: VCCR: 4–10 mm; CCR: 3–4 mm), and interlocking growth textures suggest that these fiamme contain parcels of disaggregated magma mush. High-silica rhyolite glass compositions (77.2–78.3 wt.% SiO2) and extremely depleted Sr (VCCR: <20; CCR: <5 ppm) and Ba (VCCR: <70; CCR: <20 ppm) glass trace-element concentrations indicate that the crystal-rich fiamme glasses represent highly fractionated melts. Crystal-rich fiamme glass trace-element compositions also form discrete, tightly clustered populations, suggesting that multiple crystal-rich, melt-dominated magma bodies existed prior to eruption. We propose that the coexistence of large-scale heterogeneity (e.g. deposit-scale and outcrop-scale) with small-scale homogeneity observed in crystal-rich fiamme is resolved by a model in which at least two separate magma reservoirs, each containing multiple melt-dominant magma bodies, erupted to form the Ora Ignimbrite.