Miocene phreatomagmatic volcanism at Tihany (Pannonian Basin, Hungary)

Abstract

A late Miocene (7.56Ma) maar volcanic complex (Tihany Maar Volcanic Complex — TMVC) is preserved in the Pannonian Basin and is part of the Bakony–Balaton Highland Volcanic Field. Base surge and fallout deposits were formed around maars by phreatomagmatic explosions, caused by interactions between water-saturated sediments and alkali basalt magma carrying peridotite lherzolite xenoliths as well as pyroxene and olivine megacrysts. Subsequently, nested maars functioned as a sediment trap where deposition built up Gilbert-type delta sequences. At the onset of eruption, magma began to interact with a moderate amount of groundwater in the water-saturated sand. As eruption continued phreatomagmatic blasts excavated downward into limestones, providing access to abundant karst water and deeper to sandstones and schist both providing large amount of fracture-filling water. At the surface, this ‘wet’ eruption led to the emplacement of massive tuff breccias by fall, surge, mudflow and gravity flow deposition. The nature of the TMVC maar eruptions and their deposits appears to depend on the hydrological condition of the karst and/or fracture-filling aquifer, which varies seasonally with rainfall and spring runoff. The West and East Maar volcanoes of TMVC are interpreted to represent low water input from the karst and/or fracture-filling aquifer (‘summer vent’), whereas the East Maar is interpreted to have formed when abundant karst and/or fracture-filling water was available (‘spring vent’).