Early stages in the differentiation of the Skaergård magma, as revealed by a closely-related suite of dike-rocks

Certain dikes in the Kangerdlugssuaq area of east Greenland correspond rather closely with the presumed bulk composition of the nearby Skaergaard intrusion and therefore allow a better characterization of the chemistry of the Skaergaard initial magma than has so far been possible. They indicate that Wager's chilled margin composition needs only minor revision, except in the case of Al2O3. Other dikes belonging to the same suite show a range of compositions extending up to the differentiation stage represented by the lower part of the Skaergaard middle zone and these allow an investigation, using computer least squares and Rayleigh fractionation modelling, of the early part of the Skaergaard differentiation trend which is otherwise unexposed. We conclude that the only important cumulus phases at this stage were olivine and plagioclase but clmopyroxene grew by heteradcumulus growth and effectively eliminated all intercumulus liquid. The liquids described here are similar to many ocean-floor basalts.