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

Using variation diagrams for the major elements in the layeredrocks, estimates are made of the average amounts of the variouselements in the total rock separating at successive stages.From the analyses of the chilled marginal gabbro, taken to representthe composition of the initial magma, and with the further likelyassumption that the Skaergaard intrusion is a closed system,at any rate for most of the elements, various hypotheses onthe relative volumes of the different parts of the intrusionare tested to find the one best fitting the known distributionof the elements in the observable rocks. Estimates are thenmade of (1) the overall composition of the hidden part of theintrusion by subtracting the amounts of an element in the observedrocks from the total in the initial magma, and (2) the compositionof the successive residual magmas formed as a result of thecrystal fractionation.