The Major Element Variation of the Layered Series of the Skaergaard Intrusion and a Re-estimation of the Average Composition of the Hidden Layered Series and of the Successive Residual Magmas

Abstract

Using variation diagrams for the major elements in the layered rocks, estimates are made of the average amounts of the various elements in the total rock separating at successive stages. From the analyses of the chilled marginal gabbro, taken to represent the composition of the initial magma, and with the further likely assumption that the Skaergaard intrusion is a closed system, at any rate for most of the elements, various hypotheses on the relative volumes of the different parts of the intrusion are tested to find the one best fitting the known distribution of the elements in the observable rocks. Estimates are then made of (1) the overall composition of the hidden part of the intrusion by subtracting the amounts of an element in the observed rocks from the total in the initial magma, and (2) the composition of the successive residual magmas formed as a result of the crystal fractionation.