Abstract
A graphical procedure is described by which analytical data for a suite of metavolcanic rocks may be tested to detect, and correct for, later metasomatic modification of individual sample compositions. The method involves plotting the analytical data in terms of oxide molecular proportion ratios in the form log XZ vs. log YZ, where X, Y, and Z are the oxide molecular proportions (e.g. SiO2, Al2O3…) calculated from the whole-rock weight per cent analyses. For metasomatism involving several components, a single X, Y, Z diagram does not provide a unique solution for possible changes in X, Y, and Z, but for a suite of metasomatised samples, a unique solution can be tested for by using a set of such diagrams. Two initial assumptions are required: firstly, that the altered rocks initially had compositions which, when plotted on the diagrams used, would conform with the well-defined trends shown to be present in relatively unaltered post-Mesozoic suites; secondly, that Al2O3 remained immobile during alteration. For the method to be applied to any particular set of analytical data a second relatively immobile component must also be identified, using the first assumption above. The method has been applied to major-element analyses of approximately 250 samples of mafic to felsic volcanic rocks from the Archean greenstone belt at Timagami, northeast Ontario. It has shown that alkalies, lime, magnesia, and iron have moved appreciably and differently in different parts of the belt. We therefore suggest caution in the characterization of Archean volcanics as low-K tholeiites, calc-alkaline, and so on, by simple comparison of their compositions with those of young volcanics.
Published Version
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