The petrologic study of a microsyenite intrusion in Brewster County, Texas

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The Mountain U32O microsyenite intrusion consists of a large, elliptical-shaped, hypabyssal stock or laccolith (the main mass), two elongate sills (the north and south prongs) and numerous small dikes. These masses are intrusive into sediments of Upper Cretaceous age. The intrusive rocks are a holocrystalline, porphyritic mixture of medium-grained, subhedral alkali feldspar phenocrysts and rare pyroxene phenocrysts in a fine-grained allotriomorphic granular groundmass. The groundmass is predominantly subhedral, alkali feldspar laths with felted to fluidal textures. The alkali feldspar phenocryst and felsic groundmass minerals account for more than 83 per cent of the bulk of these rocks. 2V's of the alkali feldspars range from 18° to 57°. Phenocryst samples from the center of the main mass appear to have larger 2V values than samples at its periphery. The 2V data indicate a low sanidine structural state for the samples analyzed. Composition analyses (estimation of mole % Or by x-ray diffraction) show the phenocrysts of these rocks to vary from 28 to 41 mole per cent Or. Based on these analyses, the rocks of the main mass are divisible into chilled zone or periphery samples (phenocrysts >36 mole % Or) and interior or core samples (phenocrysts < 36 mole % Or). The south prong, north prong and dikes have average phenocryst compositions of 39, 41, and 40 mole per cent Or respectively. Groundmass compositions range from 40 to 46 mole per cent Or. Most of the alkali feldspars are cryptoperthitic mixtures of K and Na phases which have anomalous cell dimensions. The K phase of these mixtures has low sanidine structural states. Samples from the main mass periphery, the prongs, and the dikes, however, contain both the cryptoperthitic mixtures and a homogeneous phase or a homogeneous phase alone. These samples have low to high sanidine structural state equivalents and normal cell dimen- sions. Further study is required to determine if these latter samples are sanidines or anorthoclases. The crystallization history of these rocks (based primarily on the alkali feldspar relations) can be accurately traced in the system Ab-Or-An-Q. The crystallization sequence derived for these rocks agrees well with previous laboratory and natural rock studies in this system.