Strongly undersaturated Tertiary volcanic rocks from the Kangerdlugssuaq area, east Greenland

Erratic blocks from the interior of Kangerdlugssuaq fjord are described with chemical analyses and microprobe data for their constituent minerals. A strongly peralkaline phonolite, unlike any previously known from the province, shows that some magmas which precipitated alkali feldspar, nepheline and amphibole (magnesio-katophorite) differentiated along a trend towards increasing peralkalinity, although this trend is unknown in the nearby plutons. Feldspar-free nephelinite and less undersaturated limburgite are also encountered, which apparently provided a petrographic link between the strongly undersatrated ultramafic rocks recently discovered in the area and the alkali basalts of Prinsen af Wales Bjerge.

Recognition of these rock types provides further important analogies with the plume-generated triple junctions of the African Rift and affords samples approximating closely to the compositions of the more unusual magmas of the area.