OF GRANOPHYRE AND GRAINS: MICROSTRUCTURE VARIATION IN THE WICHITA GRANITE GROUP, OKLAHOMA

The Wichita Granite Group (WGG) is composed of more than a dozen sheet-like plutons and associated intrusions emplaced in the shallow crust of the Southern Oklahoma Aulacogen during the early Cambrian. These alkali feldspar granites are silica-rich, ferroan, and alkali-calcic. The granite intrusions exhibit a fast emplacement tempo; recent dating places three distinct plutonic bodies within 500 thousand years of each other.

Despite gross compositional similarity and rapid emplacement rates, the granites exhibit a range of microstructures. The rocks are dominated by granophyric textures. These are largely porphyritic with minor volumes of sparsely phyric rocks. Other porphphyritic textures include spherulitic and allotriomorphic seriate matrices. Phenocrysts, dominantly perthitic feldspar and quartz, may exhibit significant resorption. Coarser grained rocks are seriate granular to poikilitic. These have equant glomerocrystic quartz with log-normal grain size distributions.

Models utilizing bulk-rock and mineral constraints indicate depth of emplacement to be a first-order control of WGG microstructure. Phenocrysts are consistent with ascent arrest at 7 km depth; decompression resorption increases with magmatic water content. The predominant granophyre texture reflects a hypabyssal setting (1 to 2 km depth), with spherulitic structure arising at near-surface or extrusive conditions. Granular textures result from intrusion further from the surface (up to 4 km). These slightly deeper conditions promote earlier and elevated vapor saturation. The enhanced vapor release, coupled with the reduced undercooling due to increased thermal insulation, permits grain coarsening that approaches textural equilibrium. The depth-dependence of microstructure suggests that texture analysis combined with precision dating may reveal the evolution of rift dynamics (e.g. unroofing vs. burial events).