Petrographic analyses of a new mafic maar-diatreme volcano in the Seidewitztal near Pirna (Saxony)

Investigations of core material from a borehole that was drilled in the Elbe Zone south of Dresden suggest the existence of a unique volcanic structure. It is situated in a complex geological area with units of different ages, e.g. Neoproterozoic greywackes of the Weesenstein Group, metamorphosed Palaeozoic sediments and igneous rocks of the Elbtalschiefergebirge and sediments of the North Bohemian Cretaceous Basin.

The borehole gives insight into a 50 m thick succession of a volcanic breccia underneath a few metres of Quaternary fluvial gravel. The colour varies between greenish-grey at the uppermost part to greenish-black down to the final depth of 55.3 m. Clast analyses allow to classify the rock as bomb-bearing lapillistone. Besides pyroclasts of different modal composition and texture, xenoliths occur representing ultramafic mantle xenoliths and diverse crustal fragments. Notable are amphibole megacrysts up to 10 cm in size.

Among the juvenile components, it can be distinguished between dominating scoria lapilli and subordinate crystallized melt fragments, which contain olivine, clinopyroxene, sometimes plagioclase in a glassy, partly altered groundmass (mesostasis). The mafic clasts are accompanied by xenoliths of greywacke, gneiss, granite and claystone with foraminifera of supposed Upper Cretaceous age. Such mixture suggests a Strombolian eruption of an alkaline basalt magma, accompanied by phreatomagmatic pulses. The mineralogical variation of the juvenile components hints to different magma batches in a complex feeding systems for this monogenetic volcano. A Tertiary age can be assumed which is common for the Lusatian Volcanic Field further east.