Regional Variations in the Mineralogy of Metasomatic Assemblages in Mantle Xenoliths from the West Eifel Volcanic Field, Germany

Abstract

Xenoliths record two distinct events in the mantle below the Quarternary West Eifel Volcanic Field, Germany. The first, during the Hercynian Orogeny, led to widespread formation of secondary, Ti-poor amphibole, clinopyroxene and phlogopite. The signature of the second event, related to Quaternary volcanism, varies across the field. At Dreiser Weiher and Meerfelder Maar, this event is characterized by amphibole–phlogopite–clinopyroxene veins, hosted in lherzolite and harzburgite xenoliths brought to the surface by sodic olivine nephelinite–basanite suite lavas. These veins formed from crystallization of sodic magma that flowed along fractures in the mantle. At Rockeskyller Kopf, Gees and Baarley, the Quaternary event is characterized by wehrlite xenoliths, many of which have phlogopite–clinopyroxene veins, that were transported by potassic foid suite lavas. Wehrlite formed by reaction of lherzolite– harzburgite, with a large volume of potassic magma that flowed along grain boundaries rather than in fractures. During reaction, orthopyroxene was consumed and secondary clinopyroxene, olivine and phlogopite precipitated. Veins formed in wehrlites only during periodic over-pressure events. The composition of the magmas parental to the veins is similar to the lavas that carried the xenoliths to surface, indicating that the source of foid and olivine nephelinite–basanite suite magma is domainal, as was the flow regime and magma flux.