Original paper

Crustal xenoliths from the Quaternary Wehr volcano (East Eifel)

Wörner, G.; Schmincke, H.-U.; Schreyer, W.

Neues Jahrbuch für Mineralogie - Abhandlungen Band 144 Heft 1 (1982), p. 29 - 55

57 references

published: Jun 1, 1982

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ArtNo. ESP154014401002, Price: 29.00 €

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Abstract

A large spectrum of crustal xenoliths (chiefly Devonian slates and higher grade metamorphic rocks such as amphibolite facies mica schists and metaquartzites) occur in the youngest pyroclastic deposit of the Quaternary phonolitic to trachytic Wehr volcano (East Eifel, West Germany). Most of the higher-grade metamorphic rocks have been changed by contact metamorphism to spotted mica schists and partially molten rocks. A hypothetical crustal model for the East Eifel area is based on the study of East Eifel xenoliths and available seismic data: Devonian and low-grade metamorphic rocks are separated by a horizontal thrust plane from the underlying higher-grade metamorphic basement. Granulites are postulated to form the lower crust in the East Eifel area. Contact metamorphism in most spotted mica schists was isochemical as is shown by chemical compositions of spotted mica schists and equivalent rocks not affected by thermal metamorphism. Silica deficient, restitic hornfelses, enriched in Al2O3, TO02 and FeOtot, were formed by partial melting, extraction of the „granitic“ components (degranitization) and recrystallization. Pumiceous buchites (highly vesicular, glass-rich rocks) represent partially molten rocks with partial melts retained in the fragments. In contrast to the high-grade metamorphic xenoliths, here referred to as basement rocks, none of the Devonian fragments or greenschist facies rocks show evidence of prolonged contact metamorphism. Thus, a postulated phonolitic to trachytic magma chamber which caused the low pressure contact metamorphism probably resided at a shallow crustal level within the metamorphic rocks but close to the boundary to the overlying Devonian rocks generally believed to lie at a depth of 4 to 5 km. Temperatures during contact metamorphism, estimated from relevant mineral assemblages range from 500-650 °C in spotted mica schists up to a maximum temperature of 700-800 °C reached in some partially molten rocks. Partial melts possibly migrated from the surrounding wall rocks into the magma causing contamination of the primarily phonolitic magma towards a trachytic composition. A significant transfer of heat and material is indicated by these results between the Wehr magma chamber and the country rock at depth.

Keywords

Crustal structurexenolithmagma chamber (high level -)volcanismQuaternaryphonolitetrachytecontact metamorphismgranulitechemical composition; Eifel (Wehr volcano)