Palaeozoic high-Ti basalt, an indicator for incipient rifting, at the Saxo-Thuringian passive margin, NE-Bavaria (Germany)

Abstract We investigated mafic igneous rocks found in the vicinity of the allochthonous Münchberg Massif in the Saxo-Thuringian Zone of the European Variscides, NE Bavaria. The basaltic magmas were injected during the Ordovician, Silurian and Devonian into contemporaneous marine sedimentary rocks of the Saxo-Thuringian Basin. Ordovician sills east and Devonian volcanic to subvolcanic rocks west of the Münchberg Massif show high Fe and Ti contents that place them in the rarely occurring group of high Fe–Ti basalts with FeO content of up to 17.6 wt% and up to 5.7 wt% TiO 2 . Covariation of Nb, Y and Zr with major elements indicates compositional control by fractionation. Trace element patterns are similar to those of ocean island basalt and continental intraplate magmatic rocks. Nd isotope signatures of the mafic rocks from the Early Palaeozoic until the Devonian resemble those of the regional continental lithospheric mantle of the Saxo-Thuringian and Bohemian zones. Their Sr isotope signatures show the variable influence of post-magmatic mobilisation. The major source of these mafic rocks is likely the subcontinental lithospheric mantle. The Pb isotope compositions of the mafic rocks do not allow distinguishing between mantle and crustal sources but resemble the regional crustal signatures. Crustal contamination in the mafic rocks is minor. The geochemical signatures of the mafic igneous rocks and continuous deposition of their host sediments indicate a long-standing passive margin setting between the northern part of Gondwana and the Saxo-Thuringian. In contrast, the trace element patterns and isotope characteristics of post-Devonian mafic dykes in the allochthonous Münchberg Massif are significantly different: They are related to a late stage of subduction during the Variscan orogeny. Graphical abstract