Concentration trends and frequency distribution patterns for elements in igneous rock types

In samples of any defined igneous rock type some compositions will approach those of associated and earlier-formed rock types and others will approach those of associated and later-formed rock types. Thus in an area showing genetically related granites, adamellite and granodiorite, a large selection of samples properly described as adamellite would be expected to show all compositional variations towards granodiorite or granite. The trends of concentration changes of elements plotted against a common measure of evolution within any defined rock type are discussed. Frequency distribution patterns for data for a reasonable number of samples of the rock type are shown to vary from element to element and the importance of analytical errors is illustrated. The degree and sense of skewness in the frequency distribution diagrams is entirely relative and depends on the method of sample selection as well as on analytical error.