Pages: 53 - 64
Abstract: The Middle to Late Miocene intraplate alkaline volcanism of W Bohemia is associated with the uplift of the NE flank of the Cheb-Domazlice Graben. Two coexisting cogenetic volcanic series have been recognised there: (i) weakly alkaline series (WAS) basanite – trachybasalt – (basaltic) trachyandesite – trachyte – rhyolite (15.9-11.4 Ma) and (ii) strongly alkaline series (SAS) olivine nephelinite – tephrite (16.5-8.3 Ma). Similar patterns in olivine nephelinite and basanite PM-normalised incompatible trace elements point to a single mantle source that was geochemically similar to EM. Negative Rb and K anomalies probably reflect residual amphibole in the source. Magma modelling showed that olivine nephelinite and basanite could form by different degrees of partial melting of the mantle source, with amphibole, garnet, olivine and clinopyroxene in the residuum. The mantle source was probably represented by lithospheric mantle metasomatised by plume-like material. Evolution of the SAS is limited in extent and rather unclear. The WAS is well developed and its evolution can be modelled by fractionation of olivine, clinopyroxene, Fe-Ti oxide, and a small proportion of plagioclase, and in late stages by alkali feldspar, Fe-Ti oxide, clinopyroxene and apatite. Crustal contamination cannot be ruled out but there is no substantial evidence of this in felsic derivatives.
Keywords: W Bohemia, Miocene volcanism, alkaline rocks, geochemistryDownload PDF document