EXTRUSIVE CARBONATITE FROM THE QUATERNARY ROCKESKYLL COMPLEX, WEST EIFEL, GERMANY
TEAL R. RILEY*, D. KEN BAILEY and FELICITY E. LLOYD
Department of Geology, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, U.K.
* Present address: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, U.K.
Abstract
Spheroidal carbonate lapilli are present in unconsolidated ash from the upper unit of a diatreme in the Quaternary West Eifel volcanic province
of Germany. The cm-sized lapilli are carbonatite autoliths with a groundmass mineralogy dominated (<50%) by polycrystalline quenched
carbonate. They are typically nucleated on fragmental material and are concentrically layered around the central nuclei. The carbonatitic groundmass
includes clinopyroxene (diopside), melilite, apatite, vishnevite, magnetite, schorlomite and trace sanidine, representing the preserved composition of
the erupted melt. The composition of the calcite (SrO 2%, BaO 0.5% and La/Yb 125-215) implies a magmatic origin, and the equilibrium
assemblage of minerals indicates a high temperature of eruption. The presence of melilite suggests a genetic link with alkali-mafic, melilite-bearing
nephelinitic magma erupted as lava and pyroclastic rocks near the diatreme. Sanidine with melilite are widely considered to be incompatible;
however, their coexistence in the carbonatitic autoliths and the presence of Or70-85 feldspar in syenite xenoliths and their phonolite hosts, as
well as megacrysts erupted from the diatreme, indicate that this composition has a broad petrological stability and may form a major reservoir of
potassium at low aH2O conditions in the upper mantle.