Mantle sources of Palaeozoic mafic rocks from the eastern Odenwald basement, Mid-German Crystalline Rise, Germany

Lithogeochemical and Sr-Nd-Pb isotope data obtained on gabbro, metagabbro, amphibolite, garnet amphibolite and diorite from the Palaeozoic Odenwald basement, Mid-German Crystalline Rise, Germany, show that their protolithic melts formed from different mantle source regions and were emplaced in different tectonic environments. Four geochemically different rock groups can be distinguished. The calc-alkaline Group I (mostly gabbro and amphibolite) and Group IV rocks (diorite) have low TiO₂ and high to intermediate Mg#, whereas the tholeiitic Group II and III rocks (predominantly garnet amphibolite) have intermediate to very high TiO₂ and low to intermediate Mg#. The Group I and II rocks have N- to E-MORB affinities, with the N-MORB type rocks having depleted Nd isotope compositions of e_{Nd, initial} =4.5-7.7. The precursor melts of all Group I and II rocks formed by partial melting in the shallow depleted mid-ocean ridge mantle and were emplaced in a divergent setting, possibly in a back-arc environment. Group III garnet amphibolite is strongly enriched in TiO₂, FeO^total and V (TiO₂ of up to 4 wt. % and FeO^total ranging from 14.4-17.6 wt. %). The parental melts of these high Ti-Fe rocks formed most likely by low-degree melting from a deep-seated, fractionated magma source. We propose that the melts were generated in an extensional setting, possibly in a continental rift environment during incipient rifting. The protolithic melts of the Group IV diorite formed by partial melting in the subcontinental lithospheric mantle in a supra-subduction setting (mature volcanic arc). The chemical features of the diorite are virtually identical to those of 340 Ma old western Odenwald and Spessart diorite. Thus, we propose that all diorite from the Spessart-Odenwald basement are part of one coherent intrusion that underlies the whole area. We think it likely that diorite formation was related to the presence of a mantle plume, which was also responsible for the widespread late Carboniferous magmatism and the associated high-temperature metamorphism in the Mid-German Crystalline Rise and other areas of the Variscan orogen. Most likely, this marks the beginning of lithospheric extension in the central European Variscides and may correlate with the incipient break-up of Pangaea.