CRYSTAL SIZE DISTRIBUTION ANALYSIS OF A RHYOLITE DIKE, WICHITA MOUNTAINS OKLAHOMA

Crystal Size Distribution (CSD) analysis is being applied to a “rhyolite dike” from the Wichita Mountains, Oklahoma to reveal its magmatic history. This dike is unique in that, texturally, it has the appearance of rhyolite, but field relationships clearly show that it is intrusive into the Mt. Scott Granite. The porphyritic rhyolite dike consists of quartz (25%) and alkali feldspar (75%) phenocrysts, which constitute 19% of the rock. Embayed quartz phenocrysts (subhedral with 2-3 euhedral sides) are 0-1mm in size. Euhedral tabular K-feldspar phenocrysts are 0-7mm long. Both minerals occur as individual phenocrysts and as glomerocrysts. Vesicles (1%), which indicate volatile saturation at the time of emplacement, are present and are 2-3mm in diameter. The other 80% is a fine grain matrix of quartz and feldspar. Comparison with the Q-AB-Or-H₂O ternary indicates the melt composition coincides with 1.0 kb and 730°C, corresponding to a depth of 3.5 km. The rhyolitic texture indicates a near surface emplacement for the dike. The magma underwent a period of crystallization at depth, ascended, and quenched. CSD analysis (in progress) is being applied to quantify the early textural development of this magma. Application of CSD consists of mapping individual crystals in a computer drawing program. This image is exported as a binary image and the area and length of individual crystals within a population is determined with image analysis software. CSD Corrections (Higgins 2002) is then used to analyze this data and determine the frequency distribution of the area number density of these populations and fit a logarithmic-normal curve to the results. Growth histories of crystals can be inferred from the shape and slopes of these curves. Linear correlations indicate uniform nucleation and growth. For the rhyolite dike we expect clustering of quartz and feldspar will produce spikes in the data and skew the frequency distribution. We also expect a kink in the quartz data corresponding to resorption of quartz crystals in response to decompression during magma ascent. The nature of the CSD curve for the rhyolite dike should help to define its magmatic history and potentially provide insight into the nascent stages of development of “granitic” textures.