Plateau Uplift pp 352-363 | Cite as

Relation of Geophysical and Petrological Models of Upper Mantle Structure of the Rhenish Massif

  • K. Fuchs
  • K. H. Wedepohl
Conference paper

Abstract

The multi-disciplinary investigations of the plateau uplift of the Rhenish Massif provided an excellent opportunity to compare geophysical and petrological models of structure, composition and dynamics of the upper mantle. A low-velocity volume immediately below the crust-mantle boundary is observed east of the river Rhine whereas an absence of a sharp Moho is characteristic for the area west of this river. A low-velocity volume at 50 to 150 km depth in the Westeifel region is explained by the presence of 1% partial melt. This abnormal mantle could have caused 200–300 m uplift. The occurrence of anisotropy of P-velocities in the upper mantle can be correlated with depleted mantle compositions observed in peridotite xenoliths. The abundant depleted spinel-peridotite xenoliths sampled from Tertiary and Quaternary alkalic basalts in the Eifel, Westerwald, Hessian Depression areas contain on average about 70% olivine. These lherzolites and harzburgites cannot ex-plain the chemical composition of nepheline bearing basalts except by assuming very low degrees of partial melting of the former rocks (≤1%). A few percent (or less) partial melt in peridotites has a very low flow velocity in the porous space of peridotite and probably cannot form magma reservoirs of reasonable size in the upper mantle to feed volcanism. But a small proportion of immobile partial melt can explain low velocity volumes in the upper mantle without effecting the density in the area of the Rhenish Massif significantly. The cooling and solidification of this anomalous mantle needs time in excess of 10 m. y. Peridotite xenoliths are mainly equilibrated in the range from 900° to 1150°C related to a depth range from 60 to 80 km. Coarse grained xenoliths without recrystallization after heavy shearing are abundant in the Hessian Depression whereas porphyroclastic peridotites predominate in the volcanic areas of the Rhenish Massif. The latter probably indicate shearing in the border zones of uprising diapirs in the upper mantle. Metasomatically altered peridotites (with about 4% amphibole or phlogopite) occur as xenoliths in basalts from the sampled areas. They are probably the source rocks for the magmas of the nepheline containing basalts. Geothermal conditions as derived from petrological models for the upper mantle outside the Rhenish Massif have been presented in a tentative profile.

Keywords

Permeability Migration Quartz Dioxide Anisotropy 

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Copyright information

© Springer-Verlag Berlin Heidelberg 1983

Authors and Affiliations

  • K. Fuchs
    • 1
  • K. H. Wedepohl
    • 2
  1. 1.Geophysikalisches InstitutUniversität KarlsruheKarlsruhe 21Fed. Rep. of Germany
  2. 2.Geochemisches InstitutUniversität GöttingenGöttingenFed. Rep. of Germany

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