Geochemical comparison between minettes and kersantites from the Western European Hercynian orogen: trace element and Pb [sbnd]Sr [sbnd]Nd isotope constraints on their origin

Abstract

Trace element and isotopic characteristics of late Carboniferous to early Permian minettes and kersantites have been determined. These lamprophyres have been sampled throughout the Western European Hercynian orogen, from Brittany to the west to Schwarzwald to the east. In spite of sharp petrological differences reflected by mineralogy and major element geochemistry, minettes and kersantites exhibit close identity with respect to trace element and isotopic features. These features comprise enrichment in incompatible elements, highCs/Rb and lowCe/Pb ratios, Ta and Ti relative depletion, high abundance in transition elements and highNi/Mg ratios. Pb isotope ratios are undistinguishable from those measured on Hercynian continental crust. Initial 143Nd/ 144Nd ratios are between0.5120 (ε i ≅ −5) and0.5122 (ε i ≅ −1) for minettes and kersantites whereas initial 87Sr/ 86Sr ratios vary between 0.7055–0.710 for minettes and 0.707–0.708 for kersantites. No simple mixing relations are visible on Rb Sr and Sm Nd isochron diagrams. The exceptional homogeneity of these geochemical characteristics along a 1000 km traverse does not allow for an hypothesis of enrichment through upper level assimilation and thus leads to propose that these rocks originated through melting of a mantle enriched by recycling of crustal material.