Abstract
Compositional data on a 1,250-m-thick sequence of sparsely porphyritic lavas that comprise the Geikie Plateau Formation, part of the approximately 55 Ma break-up-related flood basalts in E Greenland is presented. Major element compositions are relatively restricted (6.3-7.6 wt% MgO; 2.2-2.4 wt% TiO (sub 2) ), with two excursions to more evolved compositions (2.4-3.4 wt% TiO (sub 2) ) that are similar to the inferred parental magma of the nearby Skaergaard Intrusion. Major and trace element calculations show that fractional crystallisation is the principal control on magma compositions, and the cyclical sequential variations imply regular magma chamber replenishment events. Isotopic data indicate minor crustal assimilation, but with different contaminants for the main group (amphibolitic gneiss) and evolved cycles (granulitic gneiss). Rifting episodes may have allowed more primitive magmas to ascend to shallow crustal levels and subsequently fractionate to more evolved compositions in a separate chamber, which was perhaps similar to the source of the Skaergaard Intrusion.