The Kraemer Island macrodyke, East Greenland: solidification of a flood basalt conduit

Abstract

The Kraemer Island macrodyke that is exclusively exposed on Kraemer Island about 7 km west of the Skaergaard Intrusion belongs to a regional dyke swarm termed the ‘Skaergaard-like dykes’ (or FG-1 dykes). Weakly modally layered olivine gabbros dominate the exposed parts of the intrusion that has a width of 650 m to 1000 m. Plagioclase (core An68±2) and Ca-rich pyroxene (core Mg no. 79±1) grains are normally zoned, whereas olivine grains (Fo50–65) are homogeneous. Calculated mineral–magma equilibria, based on experimentally determined Mg–Fe magma–olivine and magma–clinopyroxene partition coefficients, suggest that the observed olivine and clinopyroxene compositions in the gabbros cannot have formed from a common parental magma. The unzoned nature of olivine grains and their iron-rich com-positions relative to clinopyroxene suggest post-cumulus Mg–Fe exchange between olivine and interstitial melt. A gabbroic pegmatite is developed in the centre of the intrusion along its entire exposed 5 km strike length. Here, mineral zonation is limited and compositions are similar to rims of cumulus minerals in the enveloping olivine gabbros. The pegmatite could therefore represent interstitial melt mobilized from gabbroic cumulates that later accumulated and crystallized at its present stratigraphic location. Cumulus olivines in the gabbros are close to equilibrium with Ca-rich pyroxene in the pegmatite. This is interpreted as reflecting interstitial melt mobility during the late stages of solidification of the macrodyke. Chilled margins are well preserved at the intrusion margins and are rich in Fe (14–15.3% FeOTOT), Ti (3–3.3% TiO2) and light rare-earth elements ([La/Sm]N = 1.2–1.3), similar to magmas well represented in the overlying sequence of contemporaneous High-Ti Series flood basalts. It is therefore likely that extensive wall-rock melting adjacent to the macrodyke reflects continuous feeding of the overlying flood basalts through the Kraemer Island macrodyke.