Petrographic analysis of the Gyűrűfű Rhyolite Formation using the thin section collection of Mecsekérc Company

Abstract

Facies interpretation of volcanic rocks is of critical importance for the reconstruction of eruptive processes, particularly in ancient and sometimes strongly altered successions with a limited outcrop. However, this is often a difficult task since some pyroclastic rocks such as high-grade ignimbrites and felsic lavas may develop similar textures during emplacement, cooling and post-depositional alteration. In this study the samples of the ”Vulkanitok, etalon kollekció” thin-section collection (’Volcanic rocks, reference collection”, Mecsekérc Ltd) were examined, representing the rocks of the Permian Gyűrűfű Rhyolite Formation and their resedimented clasts from the Cserdi Conglomerate Formation (in southern Transdanubia, Mecsek Mountains, Hungary). This area represents a felsic igneous province in the post-collisional Variscan foreland; here the Gyűrűfű Rhyolite has been interpreted traditionally as a rather monotonous complex of lava flows. The most conspicuous feature of the studied samples is the apparent porphyritic texture comprising abundant, but unevenly distributed, mostly broken quartz and feldspar  phenocrysts and opaque pseudomorphs after biotite. An important indicator of the volcaniclastic origin is, however, the presence of relict, coarsely porphyritic pumice lapilli which have been  flattened during compaction. Examination of a thin-section provides clear evidence of recrystallised shards in the matrix, showing strongly-altered vitriclastic textures. Some shards have recognisable rod and bubble-wall shapes, but those at the edges of quartz and feldspar crystals are strongly deformed and indicate welding compaction. The formerly glassy shards show  remnants of an axiolitic devitrification texture. In the relict pumice clasts, the internal vesicular microstructure has been destroyed. The brown rims of pumice clasts show axiolitic and spherulitic
devitrification. Their central parts consist of a mosaic of fine-grained quartz. Axiolitic devitrification develops during primary cooling and crystallisation of hot volcanic glass, and is a good indicator of the primary emplacement of volcaniclastic deposits. In the framework of the Cserdi Conglomerate samples, volcanic rock fragments with felsitic and altered vitriclastic textures are dominant. The data acquired for this study show that previously identified lavas are best interpreted as ignimbrites and that, as a result, the importance of explosive volcanism has been underestimated in the western part of the Mecsek Mts. This research has been supported by the Hungarian Scientific Research Found (OTKA; No. PD 83511) and by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences (AV).