Tertiary peralkaline rhyolite dikes from the Skærgaard area, Kangerdlugssuaq, East Greenland.

Resumé

Tertiary peralkaline acid rocks (comendites) are described from East Greenland. These are dike rocks, which may be aphyric, carry sparse phenocrysts of alkali feldspar, or more abundant quartz, alkali feldspar and katophorite phenocrysts. Regarded as being closely related, are a group of peralkaline granitic rocks associated with the nordmarkitic syenites of the area, which contain quartz, alkali feldspars, arfvedsonite, pyroxenes ranging from hedenbergitic to acmitic, and assessories including zircon, Mn-rich ilmenite, astrophyllite and chevkinite. Analyses are presented for most of these phases and the chemistry of the rocks and that of the crystallization process in the dike centres is discussed. These rocks have an origin clearly distinct from that of the nearby Skaergaard granophyres, which show no peralkaline tendencies, and it is postulated that the comendites have arisen from magmas of nordmarkitic composition which, in turn, arose at depth from a transitional basalt. This suite of rocks seems to reflect closely the tectonic setting, occurring in many areas of crustal attenuation, a situation in accordance with that believed to be present here.