Cryptorhythmic and macrorhythmic layering in aegirine lujavrite, Ilimaussaq alkaline intrusion, South Greenland

Abstract

Cryptorhythmic layering (Wilson 1992) has been established in a 200 m drill core through a layered aegirine-eudialyte nepheline syenite, aegirine lujavrite I, of the Ilimaussaq alkaline intrusion. Within each cryptorhythmic unit, the content of Y in cumulus eudialyte is constant, but there is a sharp increase to a new fixed value in the next unit. Each cryptorhythmic unit contains from two to four macrorhythmic units which show moderate to poor density sorting and consist of three layers defined by successively high contents of cumulus aegirine, then cumulus eudialyte, microcline and nepheline, and finally intercumulus zeolites and arfvedsonite. The "dry" density of both cumulus and intercumulus material decreases in the higher cryptorhythmic units and is thought to reflect the decreasing density of the parental liquid layers. The appearance of a new cryptorhythmic unit does not coincide with the end of a macrorhythmic unit and is apparently unrelated to pulses of nucleation and settling out of cumulus phases. Crystallization of cryptorhythmic units from chemically distinct double-diffusive liquid layers in the aegirine lujavrite I magma and of macrorhythmic units by repeated in situ crystallization on the floor of the Ilimaussaq magma chamber is discussed.