New 40 Ar/ 39 Ar ages and geochemistry of late Carboniferous-early Permian lamprophyres and related volcanic rocks in the Saxothuringian Zone of the Variscan Orogen (Germany)

Abstract

Abstract 40 Ar/ 39 Ar step-heating dating of mineral separates from a series of lamprophyre dykes in the Saxothuringian Zone of the Variscan Orogen yielded Viséan-Namurian (334–323 Ma) and Stephanian-early Permian (297–295 Ma) crystallization ages indicating magma generation over a period of 30 Ma. In many cases, dyke emplacement was controlled by faults. Many are composite or show evidence for mingling of primitive and evolved magmas, and, to a certain degree, contamination with crustal melts. The high MgO (6–7 wt%), Ni (75–270 ppm) and Cr (140–1250 ppm) contents and mafic phenocryst assemblage are evidence for derivation from a mantle source. Kersantites and minettes have similar incompatible trace-element and rare earth element (REE) patterns (light REE (LREE)- and medium REE (MREE)-enriched and heavy REE (HREE)-depleted) and high, but varying Th, Zr and Hf contents. Positive Ni v. Mg# (FeO=FeO tot ) correlations suggest early fractionation of olivine, and the general absence of negative Eu anomalies makes feldspar fractionation improbable. For the lamprophyres of the Spessart, the variations of Ba, Rb and TiO 2 indicate phlogopite fractionation. Negative Ta, Nb and Ti anomalies are common, and may be an artefact of the high large ion lithophile element (LILE) and REE contents, but are more likely to reflect derivation from a mantle source that was metasomatized during a previous (Devonian?) subduction event. The generation of the parent melts was possibly triggered by partial melting of metasomatized mantle due to lithosphere detachment, removal and replacement of metasomatized lithospheric mantle by upwelling hot asthenospheric mantle. Compared to the spessartites, the minettes and kersantites appear to have originated by partial melting of deeper-mantle sources. Lithospheric mantle detachment may have caused post-collisional Namurian uplift and cooling of the crust, and facilitated emplacement of lamprophyre dykes along fault zones at high crustal levels.