Process-Response Models Based on Heavy-Mineral Content of the San Isabel Granite, Colorado

Three-dimensional variability of the San Isabel Granite is studied by trend-surface analysis based on 61 samples. Specific gravity of crushed rock samples and weight percentages of total heavy minerals, magnetic heavy minerals, and nonmagnetic heavy minerals appear to show that in the center of the granite there are significant vertical and small lateral changes, whereas toward the margins the opposite is true. Although the deviation patterns of these variables arc very similar, the geologic factors responsible for the deviations have not yet been identified.

These data have been used to present a case history and as a basis for testing the correctness of three possible conceptual process-response models. Such methods are useful in development of a framework for making significant quantitative objective decisions about petrogenesis.