Lithospheric mantle beneath the Mid-German Crystalline High Variscan unit: Breitenborn (Vogelsberg, Central Germany) case study

The Cenozoic volcanic field of Vogelsberg (part of CEVP in Central Germany) is located at the northern extension of the Upper Rhine Graben. Three Variscan basement units underlie Vogelsberg from NW to SE: the Rheno-Hercynian Zone, the Northern Phyllite Zone and the Mid-German Crystalline High. Xenoliths from the Breitenborn basanite sample lithospheric mantle (LM) beneath the Mid-German Crystalline High.

The Breitenborn suite comprises xenoliths of 3-7.5 cm in diameter: clinopyroxene-poor spinel lherzolites, spinel harzburgites and clinopyroxenites. Peridotites exhibit different degrees of deformation: porphyroclastic textures, foliation development and grain size reduction. Mineral components are chemically homogenous at the grain and xenolith scale. Forsterite content (Fo) in olivine ranges between 89.8 and 91.5% with exception of Fo ~89.0% in one xenolith. Orthopyroxene (opx) is characterized by Mg# of 0.900-0.923 and 0.06-0.18 atoms of Al pfu, whereas clinopyroxene (cpx) by Mg# of 0.894-0.931 and 0.11-0.23 atoms of Al pfu. Spinel Cr# ranges from 0.18 to 0.45. Clinopyroxenites exhibit protogranular textures with no deformation. They are significantly less magnesian (cpx Mg# 0.834-0.863) and more aluminous (0.25-0.31 atoms of Al pfu) than peridotites.

Peridotite cpx REE patterns show different degree of enrichment in LREE, except two xenoliths being strongly depleted in LREE. Opx from those two xenoliths exhibits patterns steeply depleted from HREE to LREE. The remaining opx shows mild depletion in LREE relative to HREE or slight LREE enrichment.

Temperatures calculated using REE content (T_REE) [1] range between 1030 and 1130°C for most of the xenoliths and show that pyroxenes are in REE equilibrium. Exceptions are LREE-depleted xenoliths which have 940-975°C and exhibit no LREE equilibrium. Temperatures calculated on the basis of pyroxene major element contents (T_BKN) [2] are ~40-140°C lower than T_REE.

During Cenozoic rifting which formed the Upper Rhine Graben, a diversity of melts interacted with the LM beneath Vogelsberg. LREE-enriched cpx and opx patterns suggest metasomatic alteration of LM by alkaline melts, which is typical of other studied sites in the area. A calculated hypothetical melt in equilibrium with clinopyroxenite cpx patterns resembles those of basanites and alkaline basalts occurring in Vogelsberg, which were possibly involved in the alkaline metasomatism of the LM. Varying discrepancy between T_REE and T_BKN indicate that the xenoliths experienced cooling after melt metasomatism of the LM, which was not followed by recrystallisation. Different degrees of LREE enrichment and gradual changes in major element compositions of peridotite minerals indicate the chromatographic character of the alkaline metasomatism. Strongly LREE-depleted cpx and opx patterns probably are effects of metasomatism by melts derived from depleted MORB mantle, which are typical products of advanced melting in continental rifting environments.

 

The study was funded by Polish National Science Centre to MZ (project UMO-2018/29/N/ST10/00259). EPMA analyses were done within the frame of the Polish-Austrian project WTZ PL 08/2018. MZ acknowledges the DAAD fellowship at the Goethe University in Frankfurt.

 

References

[1] Liang Y. et al. (2013). GeochimCosmochimAc 102, 246–260.

[2] Brey G. & Köhler T. (1990). JPetrol 31, 1353–1378.