Grain-transition probabilities among four mineral phases are used to analyze variation in grain sequence among 60 samples distributed throughout the zoned Cornelia pluton of southwestern Arizona. Six basic patterns in grain sequence are recognized, and similarity among them defines three major groups. These three predominantly record (1) plagioclase replacement by K-feldspar; (2) primary, magmatic intergrowth of K-feldspar and quartz; and (3) an extreme poikilitic habit n in which quartz encloses sodic plagioclase.
Variation in these patterns among units of the pluton is responsible for initiating a completely new petrologic understanding of the Cornelia stock. Detailed correspondence is found with modern models of pegmatite derivation, which involves inward crystallization of early, anhydrous phases; concentration of volatiles beneath the encroaching roof; evolution of potassium-rich aqueous fluids that impregnated chamber walls, rose to concentration in the stock’s apex, and left quenched residue behind; and consequent enrichment in sodium and silicon in the rest magma leading to evolution of hydrothermal fluids.
The three textural features above, which gave rise to the grain-sequence groups, are genetically related to (1) impregnation of rocks along the roof of the conduit that was followed by evolving pegmatitic fluid, (2) orthomagmatic crystallization at depth in the rest magma, and (3) mixing of crystallization products from the potassium-depleted residual magma and a newly developed aqueous fluid.