A Permian andesitic tuff ring at Rožmitál (the Intra-Sudetic Basin, Czech Republic) – evolution from explosive to effusive and high-level intrusive activity

Abstract

Mafic, monogenetic volcanism is increasingly recognized as a common manifestation of post-collisional volcanism in late Variscan, Permo-Carboniferous intramontane basins of Central Europe. Although identification of individual eruptive centres is not easy in these ancient successions, the Permian Rožmitál andesites in the Intra-Sudetic Basin (NE Bohemian Massif) provide an exceptionally detailed record of explosive, effusive and high-level intrusive activity. Based on field study and petrographic and geochemical data on pyroclastic and coherent rocks, the Rožmitál succession is interpreted as the proximal part of a tuff ring several hundred metres in diameter. Initial accumulation of pyroclastic fall and surge deposits occurred during phreatomagmatic eruptions, with transitions towards Strombolian eruptions. Gullies filled with reworked tephra document periods of erosion and redeposition. Andesitic blocky lavas capped the volcaniclastic succession. Invasion of lavas into unconsolidated sediments and emplacement of shallow-level intrusions in near-vent sections resulted in the formation of jigsaw- and randomly-textured peperites. Most geochemical differences between coherent andesites and pyroclastic rocks can be linked to incorporation of quartz-rich sediments during the explosive eruptive processes and to later cementation of the volcaniclastic deposits by dolomite. The Rožmitál tuff ring could have been one of several phreatomagmatic centres in a monogenetic volcanic field located on an alluvial plain.