Geochemistry of Fe-rich peridotites and associated pyroxenites from Horní Bory, Bohemian Massif: Insights into subduction-related melt–rock reactions

Abstract

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, 87Sr/ 86Sr = 0.7046–0.7068, ε Nd = + 4.1 to + 5.3), and another of Fe–dunite/wehrlite (Mg-number = 83–88, 87Sr/ 86Sr = 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.