Petrography of the Gyűrűfű Rhyolite Formation, Western Mecsek Mts, Hungary

Abstract

The Permian Gyűrűfű Rhyolite Formation is the only volcanic association within the Palaeozoic sequence of Southern Transdanubia which outcrops in the western foreland of the Mecsek Mts. Furthermore, this formation has long been well-known as the source of the gravel material of the younger sedimentary siliciclastic formations in the area. Due to the uranium ore exploration deep drillings of the second half of the 20th century, details about the subsurface regions of the formation are also well-documented; these regions include the Western Mecsek Mts, the northern foreland of the Villány Mts and the Máriakéménd–Bár–Báta Range.
According to the previous reports of uranium ore exploration and other studies based on them (SZEDERKÉNYI 1962, BARABÁSNÉ STUHL 1988, FÜLÖP 1994, BARABÁS & BARABÁSNÉ STUHL 1998, JAKAB 2005), the material of the outcrop is a lava rock. In the early stage of the research only PANTÓ (in BOCZÁN et al. 1966) mentioned the possibility of a pyroclastic (ignimbrite) origin. The later petrographic descriptions of the rocks recognised the common appearance of flattened, oriented pumices and this made it necessary to implement a process of reambulation. As the first step in this process, HIDASI et al. (2015) documented detailed petrographic descriptions of the deep-drilling and gravel material of the formation and, according his work, many textural features indicate a pyroclastic flow origin.
In this study, detailed macroscopic and microscopic documentation is given of surficial samples of Gyűrűfű Rhyolite (from area between the villages of Gyűrűfű and Dinnyeberki, Western Mecsek Mts). Many textural features (e.g. devitrified pumices, former glass shards, poor sorting, fragmented phenocrysts) of the pyroclastic flow deposits were also observed. In this paper the surficial rock material of the formation is presented as an altered, locally strongly-welded, crystal-rich, pumice-bearing lapilli tuff. According to the intensity of the welding, 2 lithofacies were distinguished. The eutaxitic, strongly-welded rock samples could have their origin in the medium or greater depth of the proximal part of the pyroclastic flows; the latter filled a former valley in the area of Gyűrűfű. The non-welded material is probably derived from the lowermost, upper, sideward or distal part of the pyroclastic flows.
Alongside the petrographic description, this study was supplemented with preliminary zircon examinations as part of the preparations for U-Pb radiometric dating. From the examined samples more than 200 pieces of zircon grains were documented. Based on this database, the respective zircon populations of the two lithofacies were analysed and compared
(using statistical parameters). Zircon crystals were found both in the matrix and within the phenocrysts of the rock samples. The high amount of idiomorphic and hipidomorphic zircon grains that appear in the groundmass shows the possibility of zircon crystallization within the rock-forming magma. Most of the zircon grains are fragmented and this fact serves as further evidence of a pyroclastic origin. The examined zircon crystals often show zonation and contain xenocrystic cores; these details need to be taken into account when carrying out U-Pb radiometric dating.