Petrogenesis of the Cenozoic alkaline volcanic rock series of the České Středohoří Complex (Bohemian Massif), Czech Republic: A case for two lineages

Abstract

Cenozoic České Středohoří volcanic complex (CSVC) of the Bohemian Massif (Czech Republic) forms the eastern part of the Central European Volcanic Province and is associated with the Ohře/Eger graben which belongs to the rift system stretching from Spain and France through Germany to the Czech Republic and Poland. CSVC is about 90 km long and up to 25 km wide. The main pulse of the magmatic activities took place from Late Eocene to Middle Miocene and peaked from ∼32 to ∼24 Ma. CSVC is composed of lava flows, volcanosedimentary deposits and subvolcanic intrusions. The volcanic rocks are sodic (Na₂O>K₂O) alkaline silica-undersaturated types, which form two main associations: (1) basanite-phonolite and (2) subordinate trachybasalt-trachyte series, which differ particularly by the major element variations and Nd-Sr isotopic compositions. The mafic rocks strongly predominate over the felsic types (∼6%). The intermediate rocks are rare. Petrological modeling using MELTS software shows that both suites were generated from two distinct parental magmas (basanitic and basalt/trachybasaltic) by fractional crystallization without noticeable crustal contamination. Some phonolites, particularly their abundances of several incompatible trace elements such as heavy REE and Zr, were modified by late- to post-magmatic fluids. The compositional differences among the mafic rocks are, in part, related to variably metasomatically enriched lithospheric mantle. The Nd-Sr isotopic and trace element composition of the mafic magma indicates that the mantle source was slightly heterogeneous amphibole-and/or phlogopite-bearing garnet peridotite. The presence of garnet, amphibole and/or phlogopite indicates lithospheric melting took place close to the asthenosphere-lithosphere boundary. The lithospheric mantle source was probably metasomatically enriched by fluids or melts from an upwelling asthenospheric mantle. The enrichment may have taken place during the later stages of the Variscan orogeny. The two mafic parent magmas may be derived from a similar source which showed subtle differences in the source mineralogy mode and Nd-Sr isotopic compositions.