EXTRUSIVE CARBONATITE FROM THE QUATERNARY ROCKESKYLL COMPLEX, WEST EIFEL, GERMANY

Spheroidal carbonate lapilli are present in unconsolidated ash from th e upper unit of a diatreme in the Quaternary West Eifel volcanic provi nce of Germany. The cm-sized lapilli are carbonatite autoliths with a groundmass mineralogy dominated (>50%) by polycrystalline quenched car bonate. They are typically nucleated on fragmental material and are co ncentrically layered around the central nuclei. The carbonatitic groun dmass includes clinopyroxene (diopside), melilite, apatite, vishnevite , magnetite, schorlomite and trace sanidine, representing the preserve d composition of the erupted melt. The composition of the calcite (SrO similar to 2%, BaO similar to 0.5% and La/Yb 125-215) implies a magma tic origin, and the equilibrium assemblage of minerals indicates a hig h temperature of eruption. The presence of melilite suggests a genetic link with alkali-mafic, melilite-bearing nephelinitic magma erupted a s lava and pyroclastic rocks near the diatreme. Sanidine and melilite are widely considered to be incompatible; however, their coexistence i n the carbonatitic autoliths and the presence of Or(70-85) feldspar in syenite xenoliths and their phonolite hosts, as well as megacrysts er upted from the diatreme, indicate that this composition has a broad pe trological stability and may form a major reservoir of potassium al lo w aH(2)O conditions in the upper mantle.