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Anna Ladenberger, Uppsala University (Sweden)
David W. Peate, University of Iowa (United States)
Cestmir Tomek, University of Salzburg (Austria)
Marek Michalik, Jagiellonian University (Poland)
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We present preliminary results of isotopic composition of Cenozoic volcanic rocks from western part of Bohemian Massif. The Bohemian Massif is the largest surface exposure of basement rocks in central Europe. In Cenozoic times the widespread rifting and alkaline volcanism occurred within the Bohemian Massif in relation to the Atlantic Ocean opening and Alpine orogeny. In the Bohemian Massif, which formed the northern foreland of the Alpine orogen, the Variscan shear zones were reactivated during Cenozoic extension and provided channels for alkaline magmas. Volcanic activity occurred in a wide time span, from 0.26 to 79 Ma, with the main phase of syn-rift volcanism in the èeské støedohoøí from Late Eocene to Middle Miocene (42-18 Ma) which produced the basanite-phonolite series (Ulrych & Pivec 1997, Ulrych et al., 2008).
Studied volcanic rocks are c. 12 Ma old and according to TAS classification belong to the alkaline volcanic series. They represent basanites, basaltic trachyandesites, trachybasalts and trachytes with wide range of SiO2 (41 - 67 wt %) and MgO (0.1 - 14.6 wt %) contents. Higher SiO2 and lower MgO abundances are attributed to advanced fractional crystallization and crustal contamination processes. The double-spike Pb data reveal values from 18.86 to 20.04 for 206Pb/204Pb, 15.63 to 15.66 for 207Pb/204Pb and 38.93 to 39.68 for 208Pb/204Pb. Basanite from Krasny Les has the highest values of Pb isotopic ratios and the lowest Sr isotopic ratios (87Sr/86Sr=0.7033).
Pb isotopic ratios are typical of OIB volcanism and show values similar to German part of CEVP but lower than easternmost part of CEVP (SW Poland). High 207Pb/204Pb ratios diverge into the direction of EMII mantle source. Higher Sr isotopic ratios and known from literature lower 143Nd/144Nd (Ulrych et al. 2002) also indicate the influence of EM component in the mantle source of Bohemian volcanics and modification of magma composition by crustal contamination and assimilation processes.
References:
Ulrych J., Pivec E., 1997: Age-related contrasting alkaline volcanic series in North Bohemia. Chem. Erde 57, 311-336.
Ulrych J., Svobodova J., Balogh K., 2002: The source of Cenozoic volcanism in the èeské støedohoøí Mts., Bohemian Massif. N.Jb.Miner.Abh. 177,2, 133-162.
Ulrych J., Dostal J., Hegner E., Balogh K., Ackerman L., 2008: Late Cretaceous to Paleocene melilitic rocks of the Ohøe/Eger Rift in northern Bohemia, Czech Republic: Insights into the initial stages of continental rifting. Lithos, 101, 2008, 141-161.
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