THE HOLM,KAP,GUSTAV TERTIARY-PLUTONIC-CENTER, EAST GREENLAND

The plutonic centre is exposed on the narrow headland of Kap Gustav Holm in East Greenland. It was emplaced into the hinge zone of a major crustal flexure that developed during initial rifting between Greenland and northwest Europe. The earliest intrusion is a funnel-shaped tholeiitic gabbro that was emplaced into the developing flexure, probably about 55 Ma ago. The intrusion comprises a Basal Contact Zone of gabbro 100 m thick, showing syn-plutonic deformation and recrystallization; a Lower Zone of massive olivine gabbro 250-1500 m thick; and an Upper Zone of layered gabbro with a minimum thickness of 500-700 m. All are cut by sheets of gabbro pegmatite. Abrupt reversals in cryptic and phase layering suggest that the gabbro intrusion is the result of several injections of magma. The gabbro intrusion was tilted steeply seawards before emplacement of plutons and sheets of monzonite, syenite and granite about 50 Ma ago. The main monzonite body forms a vertical cylinder 2.5 km in diameter, with saucer-shaped mineral-graded layering and abundant volcanic inclusions. Regular stratigraphic variation in the composition of plagioclase and augite with structural height is interrupted by an abrupt reversal, suggesting emplacement of a new pulse of magma. Syenite forms ring dykes and partial ring dykes of two distinct compositions: an older subalkaline syenite related to the monzonite and gabbro, and a younger mildly peralkaline syenite. Granite forms a small composite pluton. Abundant syn-plutonic bodies of microdioite are associated with monzonite, syenite and granite intrusions.