Geochemistry and petrology of pyroxenite xenoliths from Cenozoic alkaline basalts, Bohemian Massif

Pyroxenites occur as rare, but important, mantle xenoliths within Cenozoic volcanic rocks of the Central European Volcanic Province (CEVP). We report the petrography, geothermobarometry, mineral chemistry as well as clinopyroxene trace-element and Sr-Nd isotopic compositions of six pyroxenite xenoliths hosted by Tertiary-Quaternary volcanic rocks from Kozákov (NE Bohemia), Dobkovi ky and Kuzov (Oh e/Eger Rift), and Lutynia (SW Poland). Three Kozákov xenoliths record a complex nature and evolution. As suggested by contrasting estimated temperatures (596-663 °C and 1008-1067 °C) and textures, olivine clinopyroxenite and one websterite were likely derived from different depths and may represent crystallization products of transient melts. Another websterite contains symplectite pseudomorphs after garnet which originated by exsolution from highly aluminous clinopyroxene, and the garnet breakdown was associated with melt introduction. This websterite P-T conditions (17.3 to 21.4 kbar and 1080-1200 °C) correspond to derivation from depths of 55-69 km. The composition of websterite from Dobkovi ky suggests its origin as a cumulate from melt derived from a highly depleted mantle source with a composition similar to depleted MORB mantle (DMM). The position and petrological significance of websterite from Kuzov remains unclear because of a lack of data from associated peridotites. At Lutynia, cryptic metasomatism of peridotites by a CO2-bearing alkaline melt was described previously, and the composition of melt calculated to be in equilibrium with Lutynia websterite clinopyroxene is very similar to that in equilibrium with metasomatized amphibole-bearing peridotite. Therefore, the Lutynia websterite may represent a cumulate from percolating melt that metasomatized the lithospheric mantle in this area.