Abstract

The primary objective of this study is to assess the origin of granophyre in the A-type Long Mountain Granite of southwestern Oklahoma. The magma was emplaced as a thin sheet of crystal-poor, water-undersaturated liquid at or near the base of ∼1.4-km-thick, comagmatic rhyolite cover. Phenocrysts of anorthoclase and quartz were the first products of crystallization, and skeletal to subhedral morphologies indicate their growth involved ∼50 °C of thermal and constitutional undercooling. This initial crystallization, ∼50 vol% of the magma in a wt% ratio of anorthoclase:quartz ≈3:1, resulted from decompression and concomitant migration of the alkali feldspar–quartz cotectic boundary. This residual magma represents the liquid present when crystallization culminated with granophyric intergrowth of quartz and alkali feldspar. Sluggish diffusion of silica away from the growth surfaces of anorthoclase phenocrysts resulted in boundary-layer liquids that were supersaturated in quartz, producing initial granophyric intergrowths in which the modal proportions of quartz were greater than that of the liquid composition. From initial quartz-rich compositions, intergrowths do not follow a consistent trend of fractionation. Instead, following variable or oscillating chemical pathways, they approach and hover around a composition appropriate to the pressure and low water content of magma at the emplacement level. Compositional trends of granophyre demonstrate that the intergrowth records a process in which its growth rate exceeded the rate at which aluminosilicate components could diffuse away through magma. Granophyre in both nature and experiment is formed under a variety of pressures and activities of water but in all cases involves the simultaneous crystallization of quartz and alkali feldspar from a viscous granitic liquid that is cooled well below its liquidus temperature. The estimated undercooling for granophyre growth at Long Mountain, and observed in experiments with granitic compositions, is in the range of 70–150 °C. This magnitude of undercooling is similar to that which produces graphic granite, and it is in the cooling regime between spherulitic obsidian (or felsite) and hypidiomorphic-granular granite.

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