Lavas or ignimbrites? Permian felsic volcanic rocks of the Tisza Mega-unit (SE Hungary) revisited: A petrographic study.

Máté Szemerédi,Elemér Pál-Molnár,János Szepesi,Réka Lukács,Andrea Varga

doi:10.1556/24.2020.00003

Abstract

Permian felsic volcanic rocks were encountered in petroleum exploration boreholes in SE Hungary (eastern Pannonian Basin, Tisza Mega-unit, Békés-Codru Unit) during the second half of the 20th century. They were considered to be predominantly lavas (the so-called "Battonya quartz-porphyry") and were genetically connected to the underlying "Battonya granite." New petrographic observations, however, showed that the presumed lavas are crystal-poor (8-20 vol%) rhyolitic ignimbrites near Battonya and resedimented pyroclastic or volcanogenic sedimentary rocks in the Tótkomlós and the Biharugra areas, respectively. The studied ignimbrites are usually massive, matrix-supported, fiamme-bearing lapilli tuffs with eutaxitic texture as a result of welding processes. Some samples lack vitroclastic matrix and show low crystal breakage, but consist of oriented, devitrified fiammes as well. Textural features suggest that the latter are high-grade rheomorphic ignimbrites.Felsic volcanic rocks in SE Hungary belong to the Permian volcanic system of the Tisza Mega-unit; however, they show remarkable petrographic differences as compared to the other Permian felsic volcanic rocks of the mega-unit. In contrast to the crystal-poor rhyolitic ignimbrites of SE Hungary with rare biotite, the predominantly rhyodacitic-dacitic pyroclastic rocks of the Tisza Mega-unit are crystal-rich (40-45 vol%) and often contain biotite, pyroxene, and garnet. Additionally, some geochemical and geochronological differences between them were also observed by previous studies. Therefore, the Permian felsic volcanic rocks in SE Hungary might represent the most evolved, crystal-poor rhyolitic melt of a large-volume felsic (rhyodacitic-dacitic) volcanic system.The Permian volcanic rocks of the studied area do not show any evident correlations with either the Permian felsic ignimbrites in the Finiş Nappe (Apuseni Mts, Romania), as was supposed so far, or the similar rocks in any nappe of the Codru Nappe System. Moreover, no relevant plutonic-volcanic connection was found between the studied samples and the underlying "Battonya granite."

Highlights

Permian felsic volcanic rocks were encountered in petroleum exploration boreholes in SE Hungary during the second half of the 20th century
Felsic volcanic rocks in SE Hungary belong to the Permian volcanic system of the Tisza Mega-unit; they show remarkable petrographic differences as compared to the other Permian felsic volcanic rocks of the mega-unit
Permian felsic volcanic rocks in SE Hungary were previously described and interpreted in the archive reports of petroleum exploration predominantly as lavas (“Battonya quartz-porphyry”). We showed that they are predominantly welded or rheomorphic (Battonya area), rarely lava-like ignimbrites (Totkomlos-I borehole), and reworked pyroclastic/volcanogenic sedimentary rocks (Totkomlos and Biharugra areas)

Summary

Introduction

Permo-Carboniferous large-volume silicic magmatism is a common feature of the European Variscides that was genetically controlled by a post-collisional to extensional tectonic setting (Cortesogno et al 1998; Awdankiewicz 1999; Wilson et al 2004; Paulick and Breitkreuz 2005; Vozarova et al 2009, 2015, 2016; Seghedi 2010; Wilcock et al 2013; Letsch et al 2014; Unauthenticated | Downloaded 11/02/21 03:18 PM UTCCentral European Geology 63 (2020) 1, 1–18Nicolae et al 2014; Repstock et al 2017; Ondrejka et al 2018). Ancient volcanic rocks might have undergone various processes of syn- and post-volcanic alteration; it could be a real challenge to determine their original volcanic facies. Primary textural features could have been overprinted or modified, making the genetic interpretation (e.g., pyroclastic rock or lava) of such rocks a difficult task for volcanologists (e.g., Allen 1988; Branney and Kokelaar 1992; Branney et al 1992; Henry and Wolff 1992; Gifkins et al 2005a, b). False pyroclastic textures (false shards, false eutaxitic texture) as well as false massive textures can have been formed, usually causing significant difficulties in the reliable interpretation of ancient volcanic rocks (Allen 1988; Gifkins et al 2005a, b)

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