Isotopic and geochemical investigation of a carbonatite-syenite-phonolite diatreme, West Eifel (Germany)

The Rockeskyll complex in the north, central part of the Quaternary West Ei fel volcanic field encapsulates an association of carbonatite, nephelinite and phonolite. The volcanic complex is dominated by three eruptive centres, which are distinct in their magma chemistry and their mode of emplacement. The Auf Dickel diatreme forms one centre and has erupted the only known ca rbonatite in the West Eifel, along with a broad range of alkaline rock type s. Extrusive carbonatitic volcanism is represented by spheroidal autoliths, which preserve an equilibrium assemblage. The diatreme has also erupted xe noliths of calcite-bearing feldspathoidal syenite, phonolite and sanidine a nd clinopyroxene megacrysts, which are interpreted as fragments of a sub-vo lcanic complex. The carbonate phase of volcanism has several manifestations ; extrusive lapilli, recrystallized ashes and calcite-bearing syenites, fra gmented during diatreme emplacement. A petrogenetic link between carbonatites and alkali mafic magmas is confirm ed from Sr and Nd isotope systematics, and an upper mantle origin for the f elsic rocks is suggested. The chemistry and mineralogy of mantle xenoliths erupted throughout the West Eifel indicate enrichment in those elements inc ompatible in the mantle. In addition, the evidence from trace element signa tures and melts trapped as glasses support interaction between depleted man tle and small volume carbonate and felsic melts. This close association bet ween carbonate and felsic melts in the mantle is mirrored in the surface er uptives of Auf Dickel and at numerous alkaline-carbonatite provinces worldw ide.