ORIGIN OF GRANOPHYRIC INTERGROWTHS DEDUCED FROM NATURAL AND EXPERIMENTAL OCCURRENCES

The granophyric intergrowth of alkali feldspar and quartz (GI) occurs most commonly in shallowly-emplaced, usually A-type, granites, but also along the contacts of mafic intrusions in crustal rocks and in bolide impacts. We characterized the modal mineralogy of progressive growth increments of GI in the Long Mountain Granite, a classic, shallow A-type granite of the Wichitas Igneous Province, to see if compositions track a trend of bulk or fractional crystallization. Growth increments do not record any constant or consistent compositional path, and interior composition correlates with mineralogy of the growth substrate. Where nucleated on liquidus anorthoclase, initial growth increments are strongly quartz-enriched; intergrowth around resorbed quartz or plagioclase is not. These relations reflect a disequilibrium process in which magmatic undercooling (-ΔT) drives rapid, skeletal intergrowth. Sluggish diffusion of Al and Si through viscous melt explains compositional coupling of initial granophyre composition with substrate type. Growth of anorthoclase phenocrysts produced metastable liquid boundary layers enriched in SiO₂, recorded by the high mode of quartz in adjacent GI. Two-feldspar geothermometry utilizing rare plagioclase with both anorthoclase and granophyric Afs suggests GI formed at -ΔT = 80^o-165^oC. This is the within the range of -ΔT that we have produced GI in simple to complex granitic liquids in both vapor-saturated and -undersaturated experiments at 2 kb, placing GI formation in the cooling regime between quenched felsite and hypidiomorphic-granular granite. The production of GI from both water-undersaturated and -saturated conditions indicates that neither the action nor loss of a vapor phase is required to produce granophyre. Undercooling of high-viscosity silicate liquid is the only process that can be ascribed to all environments of granophyre occurrence. An increase in undercooling for GI, relative to phenocryst growth, at Long Mountain indicates that a hiatus in crystallization must have occurred prior to GI growth. Such metastable resistance to crystallization is a common attribute of near-eutectic granitic liquids, and can account for undercooling adjacent to large mafic bodies that cool more slowly than shallow granitic intrusions.