Voluminous and crystal-rich igneous rocks of the Permian Wurzen volcanic system, northern Saxony, Germany: physical volcanology and geochemical characterization

Abstract

The North Saxon Volcanic Complex (NSVC) is a nested caldera edifice dominated by the c. 295 Ma Rochlitz Volcanic System and the c. 289 Ma Wurzen Volcanic System (WVS). The climactic activity of the WVS resembled a VEI ≥ 7 fissure ‘supereruption’ resulting in voluminous and crystal-rich caldera-fill ignimbrites (minimum volume c. 199 km3); caldera outflow facies is not known sofar. Precursory to the WVS ‘monotonous intermediates’, rhyolitic and rhyodacitic volcanic activity led to deposition of the low-volume Wermsdorf and Cannewitz ignimbrites. Modal analysis of the WVS pyroclastic units reveals an inhomogeneous crystal population (≤ 58 vol%) comprising k-feldspar, plagioclase, quartz, ortho- and clinopyroxene and minor amounts of biotite. The Wurzen caldera fill ignimbrites feature three types of fiamme: (1) felsic fiamme; (2) mafic fiamme; and (3) granite-porphyry fiamme. This, the modal variation, and the common presence of clinopyroxene and biotite indicate a strong magma mingling component in the WVS—characteristics which have not been observed in the precursory, Wermsdorf and Cannewitz ignimbrites. The caldera fill ignimbrites feature a large compositional variation from (basaltic) trachyandesite to rhyolite caused by basaltic injection and magma mingling. It is proposed that magmatic underplating led to reheating crystal mush and finally to convection processes within the WVS magma chamber. The predominance of either pyroxene or biotite as mafic mineral in the (trachy-) dacitic to rhyolitic ignimbrites indicates eruption of crystal mush from different magma batches. Prominent negative Nb and Ta anomalies of the Wurzen caldera fill ignimbrites, porphyries, and mafic dykes indicate enhanced melt–crust interaction or contamination of mantle melt. In the aftermath of the WVS caldera eruption, basaltic, trachyandesitic, andesitic and rhyolitic melts ascended puncturing the Wurzen-α and β ignimbrites leading to an array of NW–SE-trending dykes, subvolcanic bodies, and lava domes. Among these, voluminuous granite-to-syenite porphyries emplaced. The deeply eroded WVS caldera allows insight into one of the major magmatic processes that governed the post-collisional phase of the Variscan orogeny in Europe. The study of the deeply eroded supervolcano caldera will lead to the understanding of the connection between a monotonous intermediate ignimbrite and related post-eruptive intrusions.