Thermal evolution of the mantle underneath the Mid-German Crystalline Rise: evidence from mantle xenoliths from the Rhone area (Central Germany)

A suite of ultramafic xenoliths (spinel peridotites, one olivine-clinopyroxene hornblendite, and one spinel pyroxenite) from Tertiary basalt vents and lava flows of the Rhon area (Central Germany) were investigated petrologically and geochemically. With regard to P-T estimates two distinct groups of peridotite xenoliths can be discriminated: (I) A low-to intermediate-temperature group of spinel lherzolites and wehrlites mainly displaying coarse equant textures yielded temperature estimates in the range of 840-1050 degrees C at rather variable pressures of 11-24 kbar. The strong variability of the P-T estimates is attributed to mineral chemical disequilibria and different diffusion rates of the elements used for geothermometry and geobarometry. Spinel-pyroxene simplicities within part of these xenoliths point to a former position in the stability field of garnet Iherzolite. These xenoliths are variably depleted in the basaltic component by partial melt extraction. They often show an enrichment in LREE and MREE which is due to a later overprinting by cryptic metasomatism. (II) A high-temperature group of xenoliths, which mainly consists of porphyroclastic and subordinate coarse equant spinel lherzolites and harzburgites, experienced temperatures of 1190-1270 degrees C at 19-26 kbar. The P-T values for these xenoliths fall close to a geothermal gradient of about 90mW/m(2) and illustrate intense heating processes in the mantle which were often coupled with ductile deformation caused by lithospheric stretching. The thermal disturbance which led to the reequilibration of these peridotites must have occurred during the Tertiary magmatic event as indicated by the absence of retrograde mineral zoning, missing textural reequilibration, and the presence of partial melting phenomena in clinopyroxene. Unlike sheared xenoliths from other locations, the porphyroclastic high-temperature peridotites from the Rhon are depleted in basaltic component, in HREE, Y, and Sc. An olivine-clinopyroxene hornblendite is classified as some kind of basaltic cumulate which - according to its P-T estimate of about 1150 degrees C at 9 kbar - originates from the transition zone between the lower crust and the upper mantle. Ar-40-Ar-39 dating of kaersutite from this sample indicates an age of about 25 Ma which is in accordance with the beginning of Tertiary volcanism in the Rhon area. These investigations show that part of the lithospheric mantle underneath the Rhon area experienced a thermal reequilibration during the Tertiary magmatic event while other parts give evidence of an older history, i.e. a cryptic metasomatism and a transition from the garnet-to the spinel-lherzolite field. A possible geotectonic scenario for the transition could be the post-Variscan crustal reequilibration.