Alleged Caboniferous (Viséan) volcanism at the eastern margin of the Moravo-Silesian Basin, Kraków region, Southern Poland

The volcanic rocks, represented by clasts in the breccias occurring within the Viséan Culm sequence at Głuchówki, previously were considered to provide evidence of the earliest record of Variscan volcanism on the eastern margin of Moravo-Silesian Basin in the Kraków area. The breccias have been described as the deposits of submarine slides, accompanying the Viséan volcanic activity. This paper provides evidence of a different mode of origin. The present study, based on new field observations, combined with petrological and geochemical data, showed that volcanic rocks are major framework constituents of a breccia and are accompanied by clasts of spotty hornfels and Culm mudstone. Some clasts originated in erosion of the nearby Permian Zalas rhyodacite laccolith and the spotty hornfelses of the contact aureole, while other clasts came from the Culm mudstones. The breccia framework components are not rounded and poorly rounded, which reflects their short distance of transport. Therefore, an alternative scenario for the genesis of the breccia is proposed. The breccias represent material deposited in erosional pockets formed along the bedding of the Culm mudstones. The red iron oxide shells around clasts may indicate weathering in a warm continental climate, thus constraining the breccia formation to either the Permian or the Early or Late Triassic. Thus, the only well documented Viséan igneous rock in the study area remains the diabase, known from the Klucze borehole. Conversely, no volcanic rocks of this age have been documented in the area under consideration.