The Heldburg Phonolite, Central Germany: Reactions between phonolite and xenocrysts from the upper mantle and lower crust

Abstract The Heldburg Phonolite, Central Germany contains abundant fragments of xenolithic material most commonly seen as single xenocrysts of olivine and orthopyroxene as well as larger poly-mineralic micro-xenoliths. The xenocryst and micro-xenolith compositions indicate two cumulate source rocks; a spinel-bearing lherzolite and a pyroxene rich gabbro-norite. Disequilibrium between the host melt and xenocrysts lead to the formation of phlogopite–diopside double rims on olivine and either amphibole–phlogopite or amphibole–diopside double rims on orthopyroxene. The rim assemblages and infiltration of melt into some micro-xenoliths suggest that the xenolithic material was sampled by the phonolite directly from their source rocks. The idea that the phonolite originated from the upper mantle is supported by thermobarometry of amphibole and clinopyroxene phenocrysts. The reaction rims therefore provide small-scale analogues of a metasomatic event involving an evolved alkali enriched melt and upper mantle and lower crust wall rocks. Chemical zoning within the rims and the inheritance of compositional features from their hosts indicates rapid rim growth rates with slow diffusion rates of components through the rims. Residence times for the xenocrysts in the melt are in the order of several months to a year.