Paralavas in the Cretaceous Paraná volcanic province, Brazil - A genetic interpretation of the volcanic rocks containing phenocrysts and glass.

Sérgio B Baggio,Rosa M.S Bello,Léo A Hartmann

doi:10.1590/0001-3765201620150088

Abstract

The occurrences of glassy rocks containing long and curved phenocrysts in the Paraná volcanic province, South America, are here interpreted as paralavas. The large number of thin (0.1-0.5 m) dikes and sills of glassy volcanic rocks with hopper, hollow or curved, large crystals of clinopyroxene (up to 10 cm), plagioclase (up to 1 cm), magnetite and apatite are contained in the core of thick (>70 m) pahoehoe flows. They are strongly concentrated in the state of Paraná, coincident with the presence of the large number of dikes in the Ponta Grossa arch. These rocks were previously defined as pegmatites, although other names have also been used. A paralava is here interpreted as the product of melting of basaltic rocks following varied, successive processes of sill emplacement in high-kerogen bituminous shale and ascent of the resultant methane. As the gas reached the lower portion of the most recent lava flow of the volcanic pile, the methane reacted with the silicate and oxide minerals of the host volcanic rock (1,000 ºC) and thus elevated the local temperature to 1,600 ºC. The affected area of host rock remelted (possibly 75 wt.%) and injected buoyantly the central and upper portion of the core. This methane-related mechanism explains the evidence found in the paralavas from this volcanic province, one of the largest in the continents.

Highlights

In the Paraná volcanic province (Figure 1), there are occurrences of paralavas that are genetically related to remelting of the holocrystalline host rock at 1,600 oC
We found the evidence in outcrops of the Paraná volcanic province where reactive relationships are present between the host basalt and the surrounding paralava
The study of paralavas in the Paraná volcanic province derived from several years of research of hydrothermal mineralization hosted in volcanic rocks (e.g., Duarte et al 2009, 2011, Hartmann et al 2012a, b, Rosenstengel and Hartmann 2012) and sand injectite structures (Hartmann et al 2013)

Summary

Introduction

In the Paraná volcanic province (Figure 1), there are occurrences of paralavas that are genetically related to remelting of the holocrystalline host rock at 1,600 oC. These rocks were previously denominated pegmatites or segregation sheets and occur in all continental volcanic provinces and plutonic rocks of basaltic composition from many provinces. The formation of these rocks is usually. The presence of the voluminous, glassy matrix in coarse-grained basalt is a recurring feature in large volcanic provinces but is still a paradox that remains to be solved.

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Discussion

Conclusion