Model for three-dimensional mineralogical variation in granitic plutons based on the Glen Alpine stock, Sierra Nevada, California

Polynomial surfaces fitted by least-squares methods to modal data indicate that, near the center of the Glen Alpine stock the vertical rate of mineralogical change is large compared to the horizontal rate of change; near the boundaries the opposite is true. Variation is most pronounced in modal potassium feldspar and color index, and less pronounced in modal quartz and plagioclase. Results suggest a three-dimensional model of mineralogic variation may be appropriate for other plutons. Vertical gradients are an important feature of the model, implying that mineralogical composition of each exposed level of a pluton is unique. Thus, observations of a two-dimensional exposed surface must be used cautiously in determining the genesis of a three-dimensional pluton