GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 116-7
Presentation Time: 9:00 AM-6:30 PM


CHIARO, Genna R., Department of Earth and Environmental Sciences, Vanderbilt University, Nashville, TN 37235, GUALDA, Guilherme A.R., Earth and Environmental Sciences, Vanderbilt University, Nashville, TN 37235, MORELLI, Corrado, Ufficio Geologia e Prove Materiali, Provincia Autonoma di Bolzano- Alto Adige, Cardano, 39053, Italy and GIORDANO, Guido, Dipartimento Scienze Geologiche Universita Roma TRE, LS Leonardo Murialdo 1, Roma, 00146, Italy

Supereruptions are paramount to volcanic studies because they can help us understand how vast amounts of eruptible melt are stored within the shallow crust. The Ora Ignimbrite (277 +/- 2 Ma) is a crystal-rich (25-55%) deposit formed during a supereruption that expelled >1300 km3 of material. Glacial incision of the Ora Caldera has resulted in exceptional cross-sectional exposures of the intracaldera fill, including a basal vitrophyre unit. Selective sampling of fiamme from this vitrophyre provides us with well-preserved juvenile material that records the distribution of different magma types within the Ora system.

We studied these samples with a petrographic microscope and backscatter electron images to document general textural features and to identify areas of interest for further study. We obtained glass and phenocryst major element compositions using quantitative EDS analysis on an SEM.

Our results show that there are at least three varieties of fiamme present: coarse-grained crystal rich (CGR), fine-grained crystal rich (FGR), and fine-grained crystal poor (FGP). The CGR samples from the Southern intracaldera deposit have high silica rhyolite (HSR) (76-77% SiO2) glass encompassing large crystals (1-3 mm) which are resorbed. The Northern extracaldera outflow CGR samples have fragmented clusters of feldspars (1-2 mm) and resorption textures as well. The FGR samples from that unit are heterogeneous with ribbons of lower silica (67-70% SiO2) throughout the sample. The FGP fiamma from the outflow has small (<0.5 mm) crystals and the fiamma is rimmed with HSR.

Detailed textural analyses of phenocrysts within the fiamme help illuminate the magmatic history of Ora. As a crystal rich high silica rhyolite, the individual crystal histories may help us to understand how the magmatic system transitioned from a state of potential rheological lock-up to a stage capable of eruption. For example, the resorption textures suggest that Ora underwent re-melting prior to eruption. Also, the presence of multiple glass populations within a single fiamma strongly suggests that the eruption occurred shortly after magma mingling. Further geochemical analyses and rhyolite MELTs modeling will be done to constrain P-T conditions of the magma bodies prior to eruption.