www.jgeosci.org Journal of Geosciences, 58 (2012), 199–219 DOI: 10.3190/jgeosci.125 Original paper Geochemistry and petrology of pyroxenite xenoliths from Cenozoic alkaline basalts, Bohemian Massif Lukáš ACKERMAN 1, 2* , Petr ŠPAČEK 3 , Gordon MEDARIS, Jr. 4 , Ernst HEGNER 5 , Martin SVOJTKA 1 , Jaromír ULRYCH 1 1 Institute of Geology v.v.i., Academy of Sciences of the Czech Republic, Rozvojová 269, 165 00 Prague 6, Czech Republic; ackerman@gli.cas.cz 2 Czech Geological Survey, Geologická 6, 150 00 Prague 5, Czech Republic 3 Institute of Geophysics v.v.i., Academy of Sciences of the Czech Republic, Boční II, 141 34 Prague 4, Czech Republic 4 Department of Geoscience, University of Wisconsin-Madison, WI 53706, USA 5 Department für Geowissenschaften, Universität München, Theresienstraße 41, D-8033 München, Germany * Corresponding author Pyroxenites occur as rare, but important, mantle xenoliths within Cenozoic volcanic rocks of the Central European Vol- canic 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), Dobkovičky and Kuzov (Ohře/Eger Rift), and Lutynia (SW Poland). Three Kozákov xenoliths record a complex nature and evolution. As suggested by contrasting estimated temperatures (596–663 ºC and 1008–1067 ºC) 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 ºC) correspond to derivation from depths of 55–69 km. The composition of websterite from Dobkovičky 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 posi- tion 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 CO 2 -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. Keywords: pyroxenite, xenolith, Cenozoic, basalt, Sr–Nd isotopes, geothermobarometry Received: 7 August 2012; accepted: 13 November 2012; handling editor: J. Kotková 1. Introduction Pyroxenite is a volumetrically minor, but petrologically significant, mantle rock type found in mafic–ultramafic series. Pyroxenites usually form layers, veins, or dikes in orogenic peridotites and occur as xenoliths of variable sizes and shapes in volcanic rocks. Their compositions range from orthopyroxenite through websterite to clino- pyroxenite, with variable modal contents of olivine and the common presence of garnet and/or spinel. The origin of pyroxenites is still largely debated (Downes 2007; Gonzaga et al. 2010a, b), and diverse processes of forma- tion have been proposed, such as crystallization from sili- cate melts passing through the lithospheric mantle (e.g., Frey 1980; Loubet and Allègre 1982; Witt-Eickschen and Kramm 1998; Puziewicz et al. 2011), in-situ melting/ dissolution of pyroxenes and their subsequent precipita- tion in pyroxenite layers (Dick and Sinton 1979; Chen et al. 2001), melt–rock reaction between peridotite and transient basaltic melts (Porreca et al. 2006; Ackerman et al. 2009), and high-pressure crystal accumulation from subduction-related melts (Davies et al. 1993; Pearson et al. 1993; Medaris et al. 1995). In this paper, we present the petrography, major- and trace-element compositions of minerals, and Sr and Nd isotopic data for clinopyroxene in six mantle pyroxenite xenoliths from the Bohemian Massif. The xenoliths are hosted by Tertiary–Quaternary alkaline basalts of the Central European Volcanic Province (CEVP; Wilson and Downes 1991). The data reveal a spectrum of xenolith compositions and characteristics that are interpreted in the context of various magmatic processes and mantle sources.