Thermal evolution of the mantle underneath the Mid-German crystalline rise: Evidence from mantle xenoliths from the Rhön area (Central Germany)

摘要: A suite of ultramafic xenoliths (spinel peridotites, one olivine-clinopyroxene hornblendite, and spinel pyroxenite) from Tertiary basalt vents lava flows the Rhon area (Central Germany) were investigated petrologically geochemically. With regard to P-T estimates two distinct groups peridotite can be discriminated: (I) low- intermediate-temperature group lherzolites wehrlites mainly displaying coarse equant textures yielded temperature in range 840–1050 °C at rather variable pressures 11–24 kbar. The strong variability is attributed mineral chemical disequilibria different diffusion rates elements used for geothermometry geobarometry. Spinel-pyroxene symplectites within part these point a former position stability field garnet lherzolite. These are variably depleted basaltic component by partial melt extraction. They often show an enrichment LREE MREE which due later overprinting cryptic metasomatism. (II) high-temperature xenoliths, consists porphyroclastic subordinate harzburgites, experienced temperatures 1190–1270 19–26 values fall close geothermal gradient about 90 mW/m2 illustrate intense heating processes mantle coupled with ductile deformation caused lithospheric stretching. thermal disturbance led reequilibration peridotites must have occurred during magmatic event as indicated absence retrograde zoning, missing textural reequilibration, presence melting phenomena clinopyroxene. Unlike sheared other locations, component, HREE, Y, Sc. An hornblendite classified some kind cumulate - according its estimate 1150 9 kbar originates hte transition zone between lower crust upper mantle.40Ar-39Ar dating kaersutite this sample indicates age 25 Ma accordance beginning volcanism area. investigations that underneath while parts give evidence older history, i.e. metasomatism garnet- spinel-lherzolite field. possible geotectonic scenario could post-Variscan crustal reequilibration.