A sheared garnet peridotite mantle xenolith from the Delitzsch Carbonatite Complex, Germany - Evidence for thickened Cretaceous lithosphere beneath Central Europe

Abstract

Sheared garnet peridotites occur in the deep lithosphere and record processes of deformation, melt interaction, and metasomatism along the lithosphere-asthenosphere boundary (LAB). While sheared peridotites are typically found in kimberlites and related volcanic rocks within cratonic settings, this study reports on an exceptionally fresh occurrence found in an ultramafic lamprophyre (UML) from the Delitzsch Carbonatite Complex (DCC) in Saxony, Germany. The xenolith was emplaced during the main magmatic activity, previously dated to between 72 and 83 Ma during the late Cretaceous period. The xenolith contains porphyroclasts of garnet, clinopyroxene and orthopyroxene in a fine-grained matrix of olivine, and thus classifies as garnet-lherzolite. Evidence of deformation and metasomatism is recorded in its mosaic to fluidal mosaic texture, characterized by olivine neoblasts, elongated garnet grains, and reaction rims around garnet (kelyphite) and clinopyroxene (spongy rims). The xenolith represents a relatively fertile mantle composition with magnesium numbers (Mg# = Mg/[Mg + Fe²⁺] × 100) of 89.4 for olivines and clinopyroxenes, 90.8 for orthopyroxenes and 82.6 for garnets. Measured trace element compositions suggest high temperature interaction with primitive, fertile melts, as shown by an enrichment of middle rare earth elements (MREE) as well as high field strength elements (HFSE) such as Ti. Reconstructed P-T conditions indicate equilibration at 61.1 ± 5.1 kbar and 1330 ± 23 °C. The corresponding depth of ∼190 km is substantially deeper than the depth of the modern LAB at 120-140 km. This not only establishes the sheared garnet peridotite xenolith as the deepest sample ever recorded from Germany but also has significant implications for mantle dynamics beneath the Mesozoic Central European crust. The sheared garnet peridotite provides evidence of the destabilization and destruction (e.g., by delamination) of a formerly thick lithosphere at least beneath Eastern Germany and possibly beneath Central Europe. Such thick, craton-like lithosphere could have originally underlain the Bohemian Massif and extended to the NW at upper mantle depth.