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

Abstract

Purely effusive activity built up a sequence of nine lavas on Úhosď Hill. The oldest erupted lavas are tephrites/basanites (28.66 ą1.06 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 ą1.04 Ma). The entire succession is crowned by alkali basalt lavas (22.09 ą0.73 Ma). All lavas appear to be emitted from a common conduit. With respect to geochemical and isotopic data, however, 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 basalts form another group differing from the tephrites/basanites and picrobasalts in more evolved isotopic compositions of Sr and Nd as well as in incompatible-element ratios. The isotopic composition of the lavas seems to reflect mixed mantle 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.