RAPAKIVI TEXTURE IN THE MOUNT-SCOTT GRANITE, WICHITA MOUNTAINS, OKLAHOMA

Mount Scott Granite is the largest recognized A-type sheet granite of the Cambrian Wichita Igneous Province, southwestern Oklahoma. Despite surprising compositional uniformity over a substantial areal extent, M ount Scott Granite can be subdivided into medium-grained granophyre-po or and fine-grained granophyre-rich facies both of which are character ized by five texturally and compositionally distinct feldspar populati ons. Phenocrysts of ovoid anorthoclase-N (antiperthitic) were the firs t feldspar to precipitate, followed by morphologically similar, but mo re potassic, anorthoclase-K. Both ovoid phases are rimmed by sodic pla gioclase resulting in rapakivi texture. The granophyre-rich facies con tains fewer plagioclase-mantled grains than the granophyre-poor facies ; the majority of ovoids have incomplete or absent rims. Rapakivi-text ure phenocrysts are in turn surrounded by a mantle of perthitic alkali feldspar and quartz. This mantle of alkali feldspar merges with alkal i feldspar of the rock matrix. Additionally, scarce plagioclase microp henocrysts occur in the matrix. Their significance in the petrogenesis of Mount Scott granite remains problematic. Although primary feldspar compositions have not been directly preserved, igneous relationships have been interpreted through a screen of later equilibration by compa rison with experimentally determined feldspar phase relationships and by textural relationships. Mount Scott granite has been previously sho wn to have undergone an earlier period of crystallization in a deeper- level storage chamber at similar to 7-8 km. The presence of rapakivi t exture reflects depressurization during ascent to the emplacement leve l of similar to 1 km. The inconsistency in the abundance of plagioclas e mantled ovoids between granophyre-rich and -poor samples indicates p lagioclase instability resulting from variable fF(2) within the Mount Scott magma.