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The Miocene Kaiserstuhl volcanic complex in the Rhine graben rift is known for simultaneously exposing both intrusive and erupted (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 lapillistone beds covering a debrite fan on the western flank of the volcano. The debrites are interpreted as lahar (debris flow) and possibly also debris-avalanche deposits. Based on the 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. The edifice failure ultimately exposed the intrusive system, and 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 breakdown of Ca-carbonate may facilitate calciocarbonatite eruption. We suggest that the sudden exposure and decompression 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 the overall scarcity of erupted calciocarbonatites. [ABSTRACT FROM AUTHOR]

The sequence of alkaline basaltic lavas and associated pyroclastic deposits that fill Bažina maar in western Bohemia is transected by a vein of apatite-dominated rock revealed in the S4 borehole. Aside from solid vein fill, apatite also impregnates a significant part of the drilled volcanic sequence. In the main body (depth 60.00--66.60 m), apatite occurs in two texturally distinct types: common cauliflower-type apatite (type 1) is transected by veinlets of coarser (0.2 mm long) apatite crystals (type 2). Both types have grown into open spaces suggesting precipitation from fluids rather than crystallization from melt, which is consistent with generally low trace element contents with slight enrichment in Cs, Sr, U and Pb. Relative to other local Plio-Pleistocene volcanic rocks, apatite vein-fill is shifted in 87Sr/86Sr towards more radiogenic values (0.7053--0.7054). This suggests that fluids carrying mantle-derived ions mixed with groundwater interacting with granitic country-rock, as also evidenced by the purely crustal character of noble gases trapped in apatite. Unlike the mofettes occurring along the Mariánské Lázně Fault on the eastern margin of the Cheb Basin, the 4He/3He ratios from Bažina apatite do not indicate mantle noble gas contribution. According to our model, the originally Sr--P-rich fluids with low 87Sr/86Sr mixed with Cs--U-enriched groundwater circulating in country-rock granites with radiogenic 87Sr/86Sr. Therefore, present-day CO2 outgassing through mofettes related mainly to the Mariánské Lázně Fault (eastern margin of the Cheb Basin) is likely independent of, and unrelated to, volcanism of the Cheb--Domažlice Graben. [ABSTRACT FROM AUTHOR]

A group of pyroclastic cones is dispersed in the North Chamo Volcanic Field, i.e. in the northern surroundings of the Chamo Lake and over neighbouring part of the Nech Sar plains (southern termination of the Main Ethiopian Rift). The activity of scattered cinder cones was partly coeval with that of Tosa Sucha Volcano (Calabrian), but continued also after Tosa Sucha's extinction until Middle Pleistocene (c. 0.5 Ma). Whereas scoria cones on the Nech Sar plains displayed a rather simple Strombolian eruptive style, the cones located within the northern part of Chamo Lake were characterized by more complex evolution. Ganjulle scoria cone, with a uniform olivine basalt composition, started with a Surtseyan-style eruption, which turned into Strombolian as the volcano grew above the water level. An even more complex history was documented for the Ganta cone. Compositional zoning of pyroclastic rocks is explained by zoned-chamber exhaustion. The transition from magmatic to phreatomagmatic style of the eruption was then most likely linked to syn-eruptive subsidence of the area on the Chamo Lake banks. Subsequent transition back to Strombolian style reflected the growth of the cone above water level. The Sr--Nd--Pb isotopes, together with major-element-based thermodynamic modelling, demonstrate that magmas parental to the North Chamo alkaline volcanic rocks (alkali basalt, through trachybasalt and trachyandesite to trachyte) evolved initially by closed-system fractionation of olivine, later joined by clinopyroxene, spinel and calcic plagioclase. The subsequent stage was characterized by a substantial (c. 25% by mass) assimilation of country-rock felsic igneous material, perhaps corresponding to the Paleogene ignimbrites. [ABSTRACT FROM AUTHOR]

Timing of magmatic activity of the late-Variscan Altenberg–Teplice Caldera was rather vaguely constrained. In this paper, we present five new laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon ages, which along with published data indicate ~13 Myr lifetime. Formation of the Altenberg–Teplice Caldera commenced with the emplacement of the pre-caldera pluton at ~325–319 Ma, and terminated with the intrusion of the syn-collapse ring dykes and post-collapse granites at ~312 Ma. The main ignimbrite volcanism in the area of the ATC occurred at ~318–313 Ma and peaked at ~314–313 Ma. The latter corresponds to the age of the caldera-forming eruptions, which sourced the extra-caldera pyroclastic deposits. The same age yielded the rhyolite dyke swarm that fed the ignimbrite eruptions. Some pyroclastic fall deposits preserved in adjacent Carboniferous basins indicate similar ages of ~314–312 Ma including the widespread ~314 Ma Bělka tuff, which represents the main chronostratigraphic marker of this area. Its thickness and grain size progressively degrease from the ATC towards S and SW. This, together with the isopach map distribution and the available geochronological data suggest that the Bělka tuff was sourced from the Altenberg–Teplice Caldera. The Bělka tuff distribution and its association with the extra-caldera ignimbrites of the Altenberg–Teplice Caldera imply that this tuff is a co-ignimbrite fall deposit that accompanied pyroclastic density currents sourced from the caldera. The calculated minimum volume of fallout ash-tuffs and extra-caldera ignimbrite facies contribute 30 km3 to the 350 km3 (dense rock equivalent) estimates of the total Altenberg–Teplice Caldera products. Such volumes correspond well to similar modern analogues of collapse calderas of intermediate size. [ABSTRACT FROM AUTHOR]