Gravitational collapse of a volcano edifice as a trigger for explosive carbonatite eruption? – a lesson from Kaiserstuhl

The Miocene Kaiserstuhl Volcanic Complex in the Upper Rhine Graben is known for simultaneously exposing both intrusive and pyroclastic calciocarbonatites. This makes Kaiserstuhl a promising candidate for studying the field and genetic relations between intrusive calciocarbonatite and its eruptive equivalent, and the processes enabling eruption of the calciocarbonatite at the surface in particular. Eruptive calciocarbonatites in Kaiserstuhl are represented by carbonatite tuff and lapilli-stone beds covering a agglomerate fan on the western flank of the volcano. The debrites represent lahar (debris flow) and possibly also debris avalanche deposits. Based on observed textures, the debris flows were most likely derived by water-dilution from debris avalanches resulting from edifice failure, which occurred in the central part of the Kaiserstuhl Volcanic Complex and ultimately exposed the intrusive system. The carbonatite pyroclasts (lapilli and ash) were ejected from narrow vents represented by open framework tuff-breccias aligned along the detachment scarp. Since the Ca-carbonates break down rapidly at high temperatures and low pressures, calciocarbonatites are unlikely to form surface lavas. On the other hand, the presence of the calciocarbonatite pyroclastic deposits suggests that some geological process faster than the high-temperature break-down of Ca-carbonate may facilitate calciocarbonatite eruption. Prompt exposure of a suprasolidus high-level carbonatite intrusion by edifice collapse may be a suitable scenario enabling calciocarbonatite eruption. The absence of edifice failures on alkaline volcanoes, where carbonatite intrusion is either supposed or exposed, may explain overall scarcity of erupted calciocarbonatites.