The petrology and age of alkaline mafic lavas from the nunatak zone of central East Greenland

Abstract

Alkaline basic lavas occurring as outcrops of limited extent in the nunatak zone of East Greenland beyond the head of Kejser Franz Josephs Fjord have been examined in detail on the basis of new collections. These rocks show considerable diversity and include highly magnesian, perovskite-bearing nephelinites, slightly less magnesian nephelinites, basanitoids and nepheline hawaiites. Phonolite, reported earlier, is absent from the new collection. Apart from the perovskite-bearing nephelinites, probably primary magmas derived by small degrees of partial melting of normal mantle, these rock types have evolved in a complicated way and do not appear to be related by low pressure processes. While it is possible that alkaline basaltic magmas have been a widespread feature in the late stages of volcanism in this part of East Greenland, the nephelinites appear to be restricted to intracontinental areas. They represent small-scale flank activity possibly related to the prolongation of the Jan Mayen fracture zone.