Volcanic glass in Cretaceous dacites and rhyolites of the Parana Magmatic Province, southern Brazil: Characterization and quantification by XRD-Rietveld

Author(s)

Dias de Andrade, Fabio Ramos ; Polo, Liza Angelica ; Janasi, Valdecir de Assis ; de Souza Carvalho, Flavio Machado

• Document type: Journal article
• Source: JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH; v. 355, p. 13-pg., 2018-04-15.

Abstract

Acidic rocks are a significant component of the Cretaceous Parana Magmatic Province, occurring in different stratigraphic positions, and often forming deposits of complex and as yet poorly defined architecture. Vitrophyric varieties are surprisingly abundant for a volcanic sequence of this age, and are composed of predominant glass plus plagioclase (labradorite-andesine), pyroxenes (augite +/- pigeonite and orthopyroxene), Ti-rich magnetite, and traces of apatite. Hypocrystalline rocks, largely derived from devitrification, additionally contain sanidine, cristobalite, and quartz. The negative correlation between the abundance of these minerals and the amount of glass suggests that these latter phases formed by devitrification. Modal analysis using a combined XRD Rietveld-RIR method detected glass contents between 0 and 85 wt% % in a set of representative samples of Palmas-type acidic rocks from southern Brazil with dacite to rhyolite composition. Modal compositions determined by XRD and by scanning electron microscope are in good agreement with each other, and were checked against whole-rock XRF chemical data. Water contents up to 4 wt% show a positive linear correlation with the amount of glass, and are inferred to be mostly secondary, as original (pre-eruptive) H2O dissolved in melts is inferred to have been <1.5 wt% in all rocks. Glass is the only water bearing phase in the studied samples, which lack low temperature hydrated phases. Water loss during devitrification appears to have occurred along fractures, and was accompanied by Na loss and, in some samples, also Ca, Rb and Sr loss. The rapid and inexpensive method of modal analyses of glassy rocks developed here may be a useful tool for mapping acidic volcanic rocks in southem Parana Magmatic Province, and also to identify the architecture of these deposits. (C) 2017 Published by Elsevier B.V. (AU)

• FAPESP's process: 12/06082-6 - The Paraná-Etendeka magmatic province in Brazil: temporal and petrologic relationships between the tholeiitic and alkaline magmatism and geodynamic implications
• Grantee: Excelso Ruberti
• Support Opportunities: Research Projects - Thematic Grants