On the basis of the distribution of major elements, as represented by bulk chemical analyses of 180 samples and with the aid of a new spatial derivative technique, the Criffell pluton is shown to be composite, with a complex boundary zone of intermediate composition. Sr isotope data on 15 samples indicate that the boundary zone may be explained by interaction between chemically distinct melts. The granitic melt was derived from a source richer in radiogenic Sr than the granodiorite melt, and the granite in the interior of the pluton underwent crystal fractionation. The apparently simple zoned Criffell pluton is actually a composite of unzoned and zoned components, and the boundary between components is of a transitional character of variable chemical gradient.

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