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Michael Abratis, Universitaet Jena (Germany)
Lothar Viereck-Goette, Universitaet Jena (Germany)
Jaromir Ulrych, Academy of Sciences of the Czech Republic (Czech Republic)
Dirk Munsel, Universitaet Karlsruhe (Germany)
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W Bohemia (Czech Republic) with the neighbouring Vogtland (Germany) is part of the Cenozoic Central European Volcanic Province and currently one of the most seismically active areas in Central Europe: The region is characterized by periodic occurrences of earthquake swarms, abundant CO2-rich mineral springs and massive gas emanations with mantle isotopic signatures of C, O and He, all of which indicate magma migration from depth. Miocene to Quaternary volcanism documents the persistent magmatic potential.
The area is marked by the intersection of the NNW-SSE trending Cheb-Domazlice Graben with the SW-NE trending Ohre (Eger) Rift. Quaternary volcanism is restricted to the Cheb Basin. However, Miocene volcanism contemporaneous with the formation of the graben extends further to the N along the trace of the Mariánské Lázne fault zone into the Vogtland (Germany).
Strongly silica-undersaturated alkaline volcanics including olivine melilitites, melilite-bearing and melilite-free olivine nephelinites are characteristic for the NW Bohemian/Vogtland areas and built up volcanic necks, dikes and scoria cones, respectively. The typical mineralogy of the rocks comprises abundant olivine, diopside, nepheline and melilite as well as minerals such as perovskite, nosean, hayne, phlogopite and amphibole that are rich in incompatible and volatile elements . Whole rock geochemistry shows that the rocks are highly enriched in incompatible elements such as HFSE (Zr: 280-450 ppm, Nb: 90-170 ppm, TiO2: 2.6-3.3 %, P2O5: 0.9-1.3 %), LREE (La: 65-110 ppm, Ce 150-220 ppm) and other LILE (Ba: 700-1400 ppm, Sr: 900-1600 ppm). All rock types are uniform in their Sr- and Nd- isotopic signature exhibiting the typical European subcontinental mantle composition (European Asthenospheric Reservoir - EAR / Low Velocity Component - LVC) with only minor effects by assimilation: 87Sr/86Sr = 0.7032 - 0.7036, 143Nd/144Nd = 0.51282 - 0.51287. Rocks enriched in radiogenic Sr are macroscopically characterized by crustal xenoliths (schists and granites). Trace element ratios (e.g., Gd/Yb: 4.6-5.9) suggest small degrees of melting at rather deep levels >80km in a mantle source considerably enriched in fluids and LILE.
Many of the rocks give evidence of hydrothermal alteration, though others still contain fresh glass. The degree of alteration is determined by the proximity to geodynamically active zones. Carbonate often occurs as a secondary mineral, but seems to be present as a primary phase (carbonate melt droplets) in some of the rocks as well.
The abundance of silica-undersaturated melilite-rich rocks in the W Bohemia / Vogtland area may give evidence for carbonate metasomatism of the subcontinental mantle in the area where the Mariánské Lázne fault zone deeply dissects the lithosphere.
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