IAVCEI – 5IMC, Querétaro, México
IAVCEI – 5IMC Conference
PS3-2.2
Querétaro,Abstracts México,Volume 2014
Episodic volcanism in the West Eifel Volcanic Field: indicating an asthenospheric trigger Förster, Michael W.
Michael W. Förster 1
1
Johannes Gutenberg University, Institute for Geosciences, J.-J.-Becher-Weg 21, D-55099 Mainz, Germany.
[email protected]
Keywords: Tephrochronology, Thermobarometry, Eifel.
The West Eifel Volcanic Field, situated in southwest Germany was dominated by two phases of volcanism. The older phase formed the center of the volcanic field and occurred 750,000 to 400,000 BP. Following a hiatus of 300,000 years, a younger phase of volcanism started around 130,000 BP. This phase was dominated by two contrasting magma sources. The first was active during the older phase and is strongly foiditic, comprising magmas of melilitite, nephelinite and potassium-rich leucitite. The second source is less foiditic and comprises very mafic olivine nephelinites and sodic basanites. These two sources were described by Mertes and Schmincke (1985) as the F-suite and ONB-suite, respectively. The last 130,000 years of the eruption history in the West Eifel Volcanic Field were determined from the positions of the ash-layers in the drill-cores of the Eifel Laminated Sediment Archive (ELSA) project (Sirocko et al. 2013). The activity is concentrated in two brief periods of time. The positions of the ash-layers from the drill-cores show a characteristic pattern (see Fig. 2). Phreatomagmatic ashes from maar volcanoes were distinguished from strombolian scoria-cone ashes by their high abundance of accidentally introduced host-rock of up to 90 vol.-%. It is stated here, that each phase began with maar volcanoes and ended with the formation of scoria cones. Specifically, the first period began at 130,000 BP with a cluster of maar volcanoes forming until 100,000 BP and was followed by the eruptions of scoria-cones around 90,000 BP. After a lull in activity a second cluster of maar formation occurred from 60,000 BP to 40,000 BP followed by the formation of several large scoria-cones at 30,000 BP. Only one volcano erupted after the second period, the small Ulmener Maar at 11,000 BP. The scoria cones belong to the younger ONBsuite, whereas the maar volcanoes with exception of Boos and Mosbruch belong to the F-suite. The thermobarometric equations of Lee et al. (2009) are used to calculate the depth of origin for both the F-suite and ONB-suite magmas. The calculated source region of the F-suite magma results in 2.0 to
2.5 GPa. This corresponds to 60 - 90 km depth for mean density of continental lithosphere. The calculated temperatures amount to 1250°C to 1320°C. The calculated depth of the ONB-suite magmas range from 3.2 to 3.8 GPa. That equals to 100 - 125 km depth with a temperature of 1430°C to 1480°C, grouping around the 1400°C adiabat (see Fig. 1). These two domains show clear differences in pressure and temperature: The ONB-suite magma comprises partial-melting at greater pressure and temperature than the F-suite. Both sources lie well below the solidus for dry melting (line starting at 1100°C), thus explaining the volumetric minor volcanism.
Fig. 1 – The thermobarometric calculations result in two pressure/temperature distributions for the magmas of the Fsuite (open circles) and ONB-suite (filled circles) (modified after Lee et al. 2009, data from Mertes and Schmincke (1985)).
It is assumed here, that the formation of the Fsuite magma ceased at the end of the older phase around 400,000 BP, in the West Eifel Volcanic Field. The sodium and magnesium-rich magma of the ONB-suite appeared here first about 80,000 BP. This very mafic melt rose from the asthenosphere and infiltrated the base of the lithosphere, generating high-Mg veins with type-I clinopyroxene megacrysts PS3-2.2
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as described by Shaw and Eyzaguirre (2000). The source of the F-suite is situated at the base of the lithosphere. ONB-suite magmas rose from the underlying asthenosphere and displaced the remaining batches of F-suite magma, triggering their eruption. Due to the high ascent rates of the volatile rich F-suite magmas, maar volcanoes were preferential formed by the rapid encounter of groundwater. The ONB-suite magmas are on the other hand characterized by less groundwater interaction- because their lower rising velocity caused evaporation prior to contact. As a result voluminous scoria cones are formed preferentially.
IAVCEI – 5IMC, Querétaro, México
References Lee, C. T. A., Luffi, P., Plank, T., Dalton, H., & Leeman, W. P. (2009). Constraints on the depths and temperatures of basaltic magma generation on Earth and other terrestrial planets using new thermobarometers for mafic magmas. Earth and Planetary Science Letters, 279(1), 20-33. Mertes, H., & Schmincke, H. U. (1985). Mafic potassic lavas of the Quaternary West Eifel volcanic field. Contributions to Mineralogy and Petrology, 89(4), 330-345. Shaw, C. S., & Eyzaguirre, J. (2000). Origin of megacrysts in the mafic alkaline lavas of the West Eifel volcanic field, Germany. Lithos, 50(1), 75-95. Sirocko, F., Dietrich, S., Veres, D., Grootes, P. M., Schaber-Mohr, K., Seelos, K., ... & Grim, S. (2013). Multi-proxy dating of Holocene maar lakes and Pleistocene dry maar sediments in the Eifel, Germany. Quaternary Science Reviews, 62, 56-76.
Fig. 2 – The activity pattern of the West Eifel Volcanic Field derived from the position of ash-layers in drill-cores: black scoria-cone eruptions, grey - maar eruptions, light grey plinian eruptions from the distant East Eifel Volcanic Field
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