Multiple Magma Bodies: Understanding the Pre-Eruptive Architecture and Magmatic Processes of Supereruptions Based on Textural, Mineralogical, and Geochemical Features of Fiamme from the Ora Ignimbrite (Permian, Italy)

Abstract

The investigation of the Ora Ignimbrite (~275 Ma) helps further our understanding of how vast amounts (>1,000 km3) of melt are generated, stored, and erupted from the shallow crust. As the last eruptive product of a slab rollback ignimbrite flareup that lasted for 10 Ma, Ora’s glacially incised outcrops tower over 1,300 m above Bolzano, Italy.

Two key outcrops, early-erupted intracaldera vitrophyre and late-erupted outflow vitrophyre, provide well-preserved, glass-bearing juvenile material. Petrographic optical and electronic (back-scattered electron) analysis was used to document the textural features of minerals and glass. Glass and mineral major-element compositions were obtained using Energy-Dispersive X-ray (EDX) analysis on a Scanning Electron Microscope (SEM). Glass with low Na/high K concentrations and A/CNK ratios > 1.1 was deemed altered.

Intracaldera vitrophyre contains two distinct fiamma types: very coarse-grained, crystal-rich (VCCR) and fine-grained (FG) fiamme. Glass in VCCR fiamme is homogeneous high-silica rhyolite (76.5-77.5 wt. % SiO2; normalized anhydrous) with low K2O values (3-3.5 wt. %). The FG fiamme have a broader SiO2 range (75-78 wt. % and 72-78 wt. %) and higher K2O values (3-4.5 wt. %).

Outflow vitrophyre has medium-grained (MG) and fine-grained, crystal-poor (FGCP) fiamme. The MG fiamme have homogeneous high-silica rhyolite glass (76-78 wt. % SiO2) with lower K2O (2-3 wt. % K2O). Glass in three FGCP fiamma form compositional continua from 68-78 wt. %, 67-79 wt. %, and 72-78 wt. % SiO2, and K2O varies substantially (0.5-3.5 wt. %). These results demonstrate mingling and mixing and suggest that multiple melt-rich zones contributed to the erupting magma.

Mineral textures in the VCCR and MG fiamme, such as interlocking growth textures observed between plagioclase and alkali feldspar, suggest that the Ora Ignimbrite may have remobilized material that was once mostly crystallized. Differences in biotite content, biotite and plagioclase major element compositions, and fiamme glass major element compositions suggest that the VCCR fiamme originated from a different source than the MG fiamme.

We propose that at least four separate magma bodies contributed to the Ora Eruption. Each one evolved independently, leading to variable amounts of magma mingling and mixing. We also suggest that the Ora Ignimbrite tapped either two or more discrete mush zones or two or more heterogeneous regions within a continuous mush zone, remobilizing previously crystallized material. These results illuminate the subsurface architecture of a large silicic system during the final episodes of an ignimbrite flareup.