Synchronous eruption of compositionally distinct near-solidus rhyolitic and trachydacitic magmas from a zoned upper crustal reservoir in the Permian Wurzen caldera (Germany): evidence from reconnaissance drillcore B1/19S

Abstract Supereruptions and caldera formations are rare but catastrophic events in Earth’s history, and their underlying processes and internal structure often remain inaccessible. This study presents results from a 648 m deep, continuously cored profile (drilling B1/19S) through the caldera fill deposits of the Lower Permian Wurzen Volcanic System (WVS) at Großsteinberg quarry (Grimma, Saxony, Germany), offering valuable insights into deep caldera sequences of a large-volume silicic volcanic complex. Monotonous intermediates are conventionally regarded as fully homogenised magma bodies. The combination of modal, geochemical, and thermobarometric analyses reveals systematic, depth-dependent variations within the ignimbrite sequence, demonstrating that the Wurzen monotonous intermediate was incompletely homogenised prior to eruption. This is evidenced by the coexistence of compositionally distinct rhyolitic and trachydacitic magma batches within the Wurzen ignimbrite, which were tapped and erupted synchronously, as well as by the occurrence of rhyolitic fiamme throughout the succession. Zircon and apatite saturation temperatures (T Zrn and T Ap ), obtained from bulk rock and juvenile fiamme analyses indicate thermal and compositional zoning of the magma reservoir. Significant temperature variations of up to 140 °C in the hydrated, rhyolitic magma domains point to derivation from a differentiated, zoned magma chamber, whereas the minor variation (−6.9 °C) in the anhydrous trachydacitic magma also reflects intense but incomplete mixing within the Wurzen reservoir. Moreover, trace-element analyses demonstrate that the crystal-poor lower unit of the drilled sequence corresponds to the Cannewitz ignimbrite, likely erupted from a silicic cap above the voluminous Wurzen magma reservoir. This unit marks the initial eruption of the WVS, which transitions into the climactic supereruption of the large-volume Wurzen magma. To emphasise the coherence of this eruptive event and to highlight the previously unrecognised genetic relationship between the units, the terms ‘Cannewitz member’ and ‘Wurzen member’ are introduced. Graphical Abstract