CRYSTALLIZATION HISTORY OF A RHYOLITE DIKE: INSIGHTS FROM CRYSTAL SIZE DISTRIBUTION ANALYSIS

Crystal Size Distribution (CSD) analysis of a porphyritic rhyolite dike from the Wichita Mountains, Oklahoma is being used to constrain the magmatic history and textural development of felsic igneous rocks. This dike has the textural appearance of rhyolite, but field relationships indicate intrusive relationships with Mt. Scott Granite. Phenocrysts constitute 19% of the rock and consist of quartz (25%) and alkali feldspar (75%) crystals set in a fine grain matrix (80%) of quartz and feldspar. Quartz phenocrysts are typically 0-1mm in size subhedral to euhedral and can be embayed. K-feldspar phenocrysts are 0-7mm long, euhedral and tabular. Both types of phenocrysts occur as individual crystals and as glomerocrysts. Vesicles (1%) are 2-3mm in size and occur throughout the rock. Comparison with the Q-AB-Or-H₂O ternary indicates “equilibrium” crystallization conditions of 1.0 kb and 730°C, corresponding to a depth of 3.5 km. Outlines of quartz and feldspar phenocrysts were traced from multiple scanned images approximately 900 mm². These “maps” were exported as greyscale images and converted to binary images. The individual crystals were fitted to an ellipse and the minor and major axis lengths were determined. Length data was analyzed using CSD Corrections (Higgins 2002) to determine the frequency distribution of the area number density of the quartz and feldspar crystal populations and to fit a logarithmic-normal curve to the results. 1042 feldspar and 899 quartz crystals within a total area of 11016 mm² were analyzed. Both populations yield inverted “S” shaped curves. Regression of the total straight lines segments yielded a slope of -0.429 and an intercept of -3.57 (R²=0.53) for feldspar and a slope of -1.20 and an intercept of -0.72 (R²=0.80) for quartz. Linear correlations indicate uniform nucleation and growth. Convex tails indicated under representation of smaller crystals sizes and concave tails indicated over representation of larger crystals sizes and indicate a deviation from uniform nucleation and growth. Under representation of small crystals may reflect dissolution in response to decompression during magma ascent. Over representation of larger crystals is attributed to the presence of glomerocrysts formed as a result of either synneusis or resorption during crystallization.