The form and internal structure of the alkaline Kangerdlugssuaq intrusion, East Greenland

Summary

The Kangerdlugssuaq intrusion (68′ 20′ N. 32′30″ W., roughly circular and 33 km across) is of Tertiary age, like the nearby Skaergaard intrusion. At the present level of erosion the outer part is of nordmarkite and inwards there are successive rings of transitional pulaskite (without quartz or nepheline), main pulaskite and, in the centre, foyaite. The different rock types are due to gradual changes in the amounts of certain of the minerals and there are no sharp junctions such as would be expected to result from successive injections.

Zones of inclusions of basalt, fallen from above, dip inwards at 30 to 60° and, in places, there is a platy parallelism of the tabular alkali feldspars dipping similarly inwards. The three-dimensional form of the intrusion is pictured as resembling a pile of saucers of decreasing size, the sequence of formation being from the outer saucer of nordmarkite to the inner, and smaller saucers of increasingly nepheline-rich syenites. Bottom accumulation of crystals is postulated to explain the disposition of the rock types, but the cause of the succession from quartz-bearing to felspathoidal syenites remains uncertain.