TEXTURES IN A POIKILITIC OLIVINE GABBRO CUMULATE - EVIDENCE FOR SUPERCOOLING

Supercooling of repeated influxes of high-MgO (similar to 13.5 wt% MgO) tholeiitic magma emplaced along the floor of the Kap Edvard Helm magma chamber led to the formation of fine grained, intraplutonic quench zones that define the bases of 12 macrorhythmic units. The quench zones grade upwards into poikilitic cumulates containing olivine dendrites, suggesting supercooling by similar to 40 degrees C. Supercooling led to early, precumulus crystallization of metastable, non-equilibrium plagioclase, followed by co-precipitation of cumulus plagioclase and olivine. Later nucleation of clinopyroxene followed by rapid growth led to crystallization of zoned oikocrysts during the cumulus stage, followed by postcumulus overgrowth and crystallization of discrete intercumulus grains. At least four generations of plagioclase crystals appear to be preserved.