Geochemical characteristics and petrogenesis of phonolites and trachytic rocks from the České Středohoří Volcanic Complex, the Ohře Rift, Bohemian Massif

Trachyandesites, trachytes and phonolites represent the most evolved rock types within the České Středohoří Volcanic Complex (CSVC) in the Ohře/Eger Rift. The K–Ar ages of the suite range from ~33.8 to ~25.8Ma. Major and trace element variation in the basanite – trachybasalt – trachyandesite series can be explained by several stages of modification of parental magmas by assimilation-fractional crystallization (AFC) involving fractionation of olivine, clinopyroxene, apatite, amphibole and Ti-oxide and bulk continental crust (BCC) as an assimilate. Relative to plausible basanitic starting compositions, the trachytes are moderately depleted in Sr, exhibit more pronounced depletions in P and Ti and some of them also show mild MREE depletion. Such composition requires variable amphibole, clinopyroxene, plagioclase±apatite, titanite and/or Ti-magnetite fractionation and BCC assimilation. Two types of phonolites (type A and B phonolite) can be distinguished on the basis of overall REE patterns, Gd/Gd* ratios and Ba and Sr contents. Type B phonolites are depleted in Ba, Sr and MREE as a result of extensive alkali feldspar, plagioclase and amphibole fractionation. Modelling of trace element distributions implies basanitic magmas as the most likely parental composition of the basanite – trachybasalt – trachyandesite – trachyte – phonolite suite formed through magmatic differentiation. The Sr–Nd isotopic compositions in the samples can be explained with the assimilation of continental crust by such parental magmas. The highly radiogenic 87Sr/86Sr found in some phonolites are contrasted by uniform Nd isotopic signature; this feature may be explained by contamination and/or overprint of source magmas by Na–Rb-rich material with radiogenic Sr signature formed due to high-Rb (>200ppm) character of these melts/fluids. The nature of such contaminant is further evidenced by elevated Li (and Cs in some cases) abundances in type B phonolites although at least two distinct fluids are implicated from the Li–Cs correlations. The derivation of these melts/liquids from sedimentary and/or meta-sedimentary crustal sources is underscored by variable but overall light Li isotopic compositions. Some phonolites exhibit enrichments in high-field-strength elements coupled with increased Zr/Nb ratios. In contrast to previous studies, we show that this feature, apparent in many volcanic rocks from the Bohemian Massif, can be explained with progressive melt fractionation of parental magmas involving amphibole and plagioclase.