Rapakivi texture in the Mount Scott Granite, Wichita Mountains, Oklahoma
Price, Jonathan D.; Hogan, John P.; Gilbert, Charles
European Journal of Mineralogy Volume 8 Number 2 (1996), p. 435 - 452
published: Apr 26, 1996
manuscript accepted: Dec 6, 1995
manuscript received: Feb 2, 1995
ArtNo. ESP147050802023, Price: 29.00 €
Abstract 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, Mount Scott Granite can be subdivided into medium-grained granophyre-poor and fine-grained granophyre-rich facies both of which are characterized by five texturally and compositionally distinct feldspar populations. Phenocrysts of ovoid anorthoclase-N (antiperthitic) were the first feldspar to precipitate, followed by morphologically similar, but more potassic, anorthoclase-K. Both ovoid phases are rimmed by sodic plagioclase resulting in rapakivi texture. The granophyre-rich facies contains fewer plagioclase-mantled grains than the granophyre-poor facies; the majority of ovoids have incomplete or absent rims. Rapakivi-texture phenocrysts are in turn surrounded by a mantle of perthitic alkali feldspar and quartz. This mantle of alkali feldspar merges with alkali feldspar of the rock matrix. Additionally, scarce plagioclase microphenocrysts 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 comparison with experimentally determined feldspar phase relationships and by textural relationships. Mount Scott granite has been previously shown to have undergone an earlier period of crystallization in a deeper-level storage chamber at ~ 7-8 km. The presence of rapakivi texture reflects depressurization during ascent to the emplacement level of ~ 1 km. The inconsistency in the abundance of plagioclase mantled ovoids between granophyre-rich and -poor samples indicates plagioclase instability resulting from variable fF2 within the Mount Scott magma.