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1 Geological Quarterly

1 2008

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Geochemical variations within the Upper Oligocene-Lower Miocene lava succession of Úhošt' Hill (NE margin of Doupovské hory Mts., Czech Republic)

Rapprich V., Holub V. F.

Geological Quarterly

2008

Vol. 52, No 3

253-268

Purely effusive activity built up a sequence of nine lavas on Úhoš' Hill. The oldest erupted lavas are tephrites/basanites (28.66 ± 1.06 Ma) and are of slightly evolved character. These are overlain by more differentiated alkali basalt. The succession continues with picrobasaltic lavas rich in phenocrysts (ankaramites 26.38 ± 1.04 Ma). The entire succession is crowned by alkali ba alt lavas (22.09 ± 0.73 Ma). All lavas appear to be emitted from a common conduit. With respect to geochemical and isotopic data, how ever, the studied sequence could not be interpreted in terms of the exhausting of a simple magma chamber. We propose the role of a more complex reservoir or, of two independent reservoirs attached to a single conduit system. The tephrites/basanites and picrobasalts (ankaramites) are closely related, the latter being derived by accumulation of mafic phenocrysts, namely clinopyroxene. The bas alts form an other group differing from the tephrites/basanites and picrobasalts in more evolved isotopic compositions of Sr and Nd as well as in in compatible-elementratios. The isotopic composition of the lavas seems to reflect mixed man tle sources derived from the European Asthenospheric Reservoir through the addition of components similar to Enriched Mantle (EM 1 and EM 2). The basalts originated by fractionation of different magma batches that contained a higher proportion of the enriched mantle component(s) or that were more contaminated with crustal material.