Magmatic Architecture and Processes in a Supereruption-scale System: The Crystal-Rich Ora Ignimbrite (Permian, Northern Italy)

Abstract

The Ora Ignimbrite (~275 Ma) is a crystal-rich (~40%), heterogeneous, supereruption-sized (~1,300 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. We investigate magma storage conditions and the dynamic processes that occur in large silicic systems by studying geochemical and petrological characteristics of juvenile material from the Ora Ignimbrite. Two key vitrophyre outcrops contain well-preserved, glass-bearing, crystal-rich (20-50%) and crystal-poor (<20%) fiamme. Textural analysis of these fiamme reveals four types: coarse-grained crystal-rich (VCCR) and fine-grained crystal-poor (I-FCP) intracaldera fiamme and coarse-grained crystal-rich (CCR) and fine-grained crystal-poor (O-FCP) outflow fiamme. Differences in fiamma textures, mineral rim compositions, and glass trace-element compositions indicate that at least two separate magma reservoirs, each containing multiple crystal-rich magma bodies, erupted to form the Ora Ignimbrite. High-precision40Ar/39Ar sanidine ages suggest that the eruption of each reservoir may have been separated in time. Discrete and tightly-clustered crystal-rich (VCCR & CCR) fiamma glass trace-element populations support the existence of multiple magma bodies in the Ora system. Plagioclase and sanidine rim compositions plot in distinct domains, reproducing clusters observed in glass trace-element compositions, and euhedral rims suggest these crystals recorded the final melt compositions of five separate magma bodies. Three CCR populations can also be differentiated by plagioclase traverse compositions, bulk fiamma compositions, and textural features unique to each population; rhyolite-MELTS quartz and two feldspar storage pressures suggest the CCR magmas are stored at slightly different depths, from 115-160 MPa. VCCR magmas appear to be stored at similar depths (135-150 MPa). 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 in crystal-rich (VCCR & CCR) fiamme indicate that these magmas contained parcels of disaggregated mush. Complex feldspar textures with Ba-enriched zones surrounding resorbed cores suggest that silicic recharge facilitated mush entrainment into the crystal-rich magmas. Mingled crystal-poor fiamme with high- and low-silica rhyolite glasses provide further evidence for silicic recharge in the Ora System.