Tertiary volcanism in the Kangerdlugssuaq region, E Greenland: trace-element geochemistry of the Lower Basalts and tholeiitic dyke swarms

Abstract

Summary Tholeiitic dyke swarms in the Kangerdlugssuaq area can be divided, like the Lower Basalts, into two broad categories: (1) an early tholeiitic ‘Picrite-ankaramite Series’ (PAS) with relatively high levels of incompatible elements; and (2) a less incompatible-element enriched ‘Tholeiitic Series’ (TS) to which the majority of the dykes belong. The latest of the tholeiitic dykes tend to have the lowest incompatible element contents. The PAS dyke samples have steep light-enriched rare earth patterns and strongly fractionated spidergrams ([Nb/Yb] N >20), with no conclusive elemental evidence of significant crustal contamination. TS dykes have less steep rare earth element (REE) patterns and spidergrams, the latter having negative Sr anomalies. Corresponding PAS and TS lavas have similar patterns, although the Nb/La, Ta/La and P/Nd ratios are significantly lower. The PAS dyke samples are more enriched in highly incompatible elements than typical ocean island basalt (OIB) tholeiites. Derivation of these magmas from a sub-lithospheric OIB source, such as that presently contributing to Icelandic volcanism, is tenable only if the picritic magmas underwent subsequent enrichment by incorporation of incompatible elements from sub-continental lithosphere or from continental crust. It is difficult to derive the observed enrichment entirely from the continental crust. The alternative is to envisage an OIB-source jet (the precursor to that currently situated under Iceland) that penetrates upward into the sub-continental lithosphere, and supplies parental picritic magmas which, in passing to the surface, scavenge incompatible elements from enriched lithospheric mantle. Later tholeiitic lavas and dykes tend to be characterized by less steep incompatible-element profiles, resulting in two late dykes with spidergrams identical to sub-aerial Icelandic tholeiites. This trend may be interpreted in terms of a decreasing contribution of highly incompatible elements from the progressively thinner lithosphere.