Petrological and seismic studies of the lithosphere in the earthquake swarm region Vogtland/NW Bohemia, central Europe

New petrological and geochemical data of upper mantle and lower crustal xenoliths from a Quaternary tephra deposit in Mýtina, Czech Republic, are discussed in the frame of previous geophysical results (receiver functions, reflection seismology) of the western Eger/Ohře Rift area. The Vogtland/NW Bohemia region is well known for intraplate earthquake swarms, which are usually associated with volcanic activity. As previously reported, 3He/4He data of CO2 emissions in mofettes and mineral-water springs point at ongoing magmatic processes in this area. Using teleseismic P receiver functions, an approximately 40-km-wide Moho updoming (from 31 to 27 km) and indications for a seismic discontinuity at 50 to 60 km depth were observed beneath the active CO2-degassing field. The studied xenolith suite probes a lithospheric profile within the structural and gas geochemical anomaly field of the western Eger Rift.With regard to texture, composition, p–T estimates and origin, five xenolith groups can be discriminated. Upper crustal xenoliths (quartzites, phyllites, mica schists) resemble crystalline country rocks at surface. One noritic xenolith (6 kbar, 800 °C) could represent a sample of the lower crust. Clinopyroxenites and hornblendites probably represent cumulates of the nephelinitic magma or fragments of magmatic veins. Porous wehrlites and one hornblende peridotite xenolith reflect a metasomatied upper mantle. Megacrysts of Ti-rich amphibole, olivine, clinopyroxene, and phlogopite could be fragments of pegmatitic veins or high-pressure phenocrysts. Most of the ultramafic nodules (xenoliths and megacrysts) formed at pressures between 6 and 11 kbar (22 to 38 km depth), at temperatures well above regional geotherms of the Bohemian Massif calculated from surface heat flow studies. Orthopyroxene-bearing spinel-lherzolite xenoliths were not observed. Our petrographical, geochemical, and thermobarometric results indicate a lithospheric mantle strongly altered by magmatic processes. This metasomatism can cause slower than typical uppermost-mantle seismic velocities in a greater area and might help to explain observed seismic anomalies.