Volcanism in a late Variscan intramontane trough: Carboniferous and Permian volcanic centres of the Intra-Sudetic Basin, SW Poland

Abstract

The Intra-Sudetic Basin represents a late Variscan intramontane trough situated near the NE margin of the Bohemian Massif. The Carboniferous-Permian molasse succession in the northern part of the basin provides evidence of three stages of volcanic activity during: 1) the latest Visean/earliest Tournaisian, 2) the late Westphalian-Stephanian, and 3) the early Permian, the latter corresponding to the climax of volcanism. Rhyodacites, andesites and basaltic andesites were characteristic of the earlier stages (1 and 2), while basaltic trachyandesites, trachyandesites and rhyolites erupted during the later stages (2 and 3). The earliest volcanism occurred near the northern margin of the Intra-Sudetic Basin and the successive Carboniferous and Permian volcanoes shifted SE-wards with time, consistently with the intrabasinal depositional centres. The location of the volcanoes was controlled by NNW-SSE to NW-SE aligned fault zones. The magmas intruded thicker accumulations of sedimentary rocks within intrabasinal troughs, and erupted through thinner sequences outside the troughs. Effusive to extrusive activity created lava-dominated, composite volcanic centres to the north and west. In the eastern part of the basin the most evolved acidic magmas erupted explosively, with the formation of: 1) a maar belt (late Carboniferous) and 2) a major caldera (early Permian), with subsequent emplacement of subvolcanic intrusions in both cases. The volcanic edifices represented intrabasinal elevations subjected to substantial erosion, with the largest supply of volcanogenic debris into the basin following the most voluminous rhyolitc eruptions in Permian times. The caldera was a centre of lacustrine sedimentation.