Geochemistry of Fe-rich peridotites and associated pyroxenites from Horní Bory, Bohemian Massif: Insights into subduction-related melt–rock reactions Lukáš Ackerman a,b, ⁎, Emil Jelínek b , Gordon Medaris Jr. c , Josef Ježek d , Wolfgang Siebel e , Ladislav Strnad f a Institute of Geology v.v.i., Academy of Sciences of the Czech Republic, Rozvojová 269,165 00, Praha 6, Czech Republic b Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6,128 43, Praha 2, Czech Republic c Department of Geology and Geophysics, University of Wisconsin-Madison, WI 53706, USA d Faculty of Science, Charles University, Albertov 6, 128 43, Praha 2, Czech Republic e Institute of Geosciences, Eberhard-Karls-University Tübingen, Wilhelmstraße 56, 72074 Tübingen, Germany f Laboratories of the Geological Institutes, Faculty of Science, Charles University, Albertov 6, 128 43, Praha 2, Czech Republic abstract article info Article history: Received 26 June 2008 Received in revised form 23 October 2008 Accepted 29 October 2008 Editor: R.L. Rudnick Keywords: Peridotite Dunite–wehrlite Melt–rock reaction Subduction zone Sm–Nd geochronology Bohemian Massif Variscan, mantle-derived peridotites and associated pyroxenites occur as boudins in Moldanubian granulite near the town of Horní Bory in western Moravia. The peridotites consist of two compositionally distinct suites, one of Mg–lherzolite (Mg-number = 89–91, 87 Sr/ 86 Sr = 0.7046–0.7068, ε Nd = +4.1 to +5.3), and another of Fe–dunite/wehrlite (Mg-number = 83–88, 87 Sr/ 86 Sr = 0.7079–0.7087, ε Nd = - 2.8 to - 1.3). Modelling of Mg–Fe exchange between peridotite and Fe-rich melts reveals that the modal and chemical composition of the Fe– dunite/wehrlite suite can be produced by melt–rock reactions between lherzolite and SiO 2 -undersaturated melts of basaltic composition at melt/rock ratios ranging from 0.3 to 2. In such a model, pyroxenites represent the crystalline product (±trapped liquid) of melts migrating along conduits in peridotite. The Fe– dunite/wehrlite suite and pyroxenites are enriched in the LILE and depleted in the HFSE. The trace element and Sr–Nd isotopic compositions of Horní Bory peridotites and pyroxenites point to a significant component of crustal material in the invasive melts. The melt–rock reactions recorded in the Horní Bory ultramafic boudins are attributed to melt percolation in a mantle wedge above a Variscan subduction zone. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Peridotites occurring in orogenic massifs, ophiolite complexes and as xenoliths in volcanic rocks provide direct evidence for mantle evolution throughout Earth history. The lithology of mantle peridotite ranges from fertile lherzolite to refractory harzburgite and dunite, and such variation has commonly been ascribed to depletion of fertile mantle by partial melting and extraction of fusible components (e.g., Arai, 1994; Pearson et al., 2003 and references therein). In contrast to the partial melting model, there is evidence that some harzburgite, wehrlite, and dunite form by the reaction of transient melts with fertile mantle wall rocks (e.g., Kelemen et al., 1990; Takazawa et al., 1992). Harzburgite is thought to develop by reaction of host peridotite with SiO 2 -saturated, subduction-related melts (e.g., Kelemen et al., 1992, 1998), and the formation of wehrlite and associated dunite has been attributed to reaction with carbonate-rich liquids (Yaxley et al., 1991) or alkaline SiO 2 -undersaturated basaltic melts (Batanova et al., 1998; Peslier et al., 2002; Ionov et al., 2005). Mantle-derived peridotites and pyroxenites are widespread in several tectonostratigraphic units of the Bohemian Massif, where they occur as bodies of different sizes in Late Paleozoic (Carboniferous) Variscan terranes and as Neogene-Quaternary mantle xenoliths. The uppermost tectonic unit in the Moldanubian Zone of the Bohemian Massif, the Gföhl Nappe, contains numerous disrupted bodies of spinel and garnet peridotites (Machart, 1984). Based on their mineralogies, pressure–temperature (P–T) conditions, and chemical compositions, the Gföhl peridotites have been divided into three groups, including a Mg–Cr type of suboceanic origin, another Mg–Cr type of subconti- nental derivation, and an Fe-rich type associated with abundant pyroxenite (Medaris et al., 1990, 2005). The Bory granulite in the Gföhl Nappe contains conspicuous peridotite, pyroxenite, and eclogite boudins, which are well exposed in a quarry near the town of Horní Bory (Mísař and Jelínek, 1981; Mísař et al., 1984). Although some peridotite boudins in the quarry are of the Mg–Cr type, most of the peridotite boudins have relatively low Mg- numbers [100 ⁎ Mg / (Mg+ Fe)], are interlayered with pyroxenite, and belong to the Fe-rich type of peridotite defined by Medaris et al. (2005). Here, we present petrographic, mineralogical, geochemical and isotopic data for the Horní Bory ultramafic suite, including Mg– lherzolite, Fe-rich dunite to wehrlite, and pyroxenite, in order to interpret their origin and evolution. We show that melt–rock reactions Chemical Geology 259 (2009) 152–167 ⁎ Corresponding author. Institute of Geology v.v.i., Academy of Sciences of the Czech Republic, Rozvojová 269,165 00, Praha 6, Czech Republic. Tel.: +420 2 233087240; fax: +420 2 20922670. E-mail address: ackerman@gli.cas.cz (L. Ackerman). 0009-2541/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2008.10.042 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo