Phonolitic melt production by carbonatite Mantle metasomatism: evidence from Eger Graben xenoliths

Abstract

Ultramafic xenoliths and green-core clinopyroxene xenocrysts from a basanite from Spitzberg, Cotta, Germany were investigated. The Spitzberg volcano on the northern shoulder of the Eger (Ohře) Graben is part of the Central European Cenozoic Volcanic suite. Inclusions of (tephri-)phonolitic, carbonatitic and sulfidic melt and associated reaction textures in the ultramafic xenoliths show evidence for metasomatic alteration of lherzolite to wehrlite by carbonatitic melt, which produced the (tephri-)phonolitic melt as a reaction byproduct. The investigated xenoliths and xenocrysts were sampled and brought to the surface by a basanitic eruption. Green-core clinopyroxene (gc-Cpx) xenocrysts found abundantly in the samples have similar inclusions as the wehrlite and are interpreted to have crystallized from the aforementioned (tephri-)phonolitic melt during a later interaction with additional carbonatite melt. Major and trace element chemistry as well as a complex suite of inclusion phases clearly link the melts in both cases. The growth textures of the gc-Cpx indicate that it grew in two stages: first quickly as a result of interaction between (tephri-)phonolitic silicate melt and carbonatite melt; then more slowly and free-floating in magma with no contact to Mantle or Crustal rocks. We conclude that Mantle metasomatism has in this case directly produced (tephri-)phonolitic magma of sufficient volume and proportion to allow extraction from the original host rock, not merely some interstitial phonolitic melt. However, surface outcrops of such phonolite have so far not been identified in continental settings.