PETROGENESIS OF “DEVONITE”: A GEOLOGIC TREASURE OF MISSOURI

“Devonite”, described and named by Albert Johannsen (1937), is a megacrystic porphyritic diabase dike from the Proterozoic St. Francois Mountains of Missouri. The dike, ~2.1 m in width, crops out on Mt. Devon with an orientation of ~000 65-70NE. Contacts with the surrounding rhyolite are sharp. Along the contact zone there is a 30 cm wide zone of fine grained equigranular diabase. Beyond this contact zone, the inner 152 cm of the dike is porphyritic with abundant and large (~3 by 4 cm) phenocrysts of plagioclase. The phenocrysts constitute approximately 75-80 % of the rock. The plagioclase phenocrysts have been replaced by chlorite, kaolin, calcite, sericite, biotite, and small amounts of hematite, leaving a slight green-tinge to these crystals in hand sample. Adjacent to the contact zone the plagioclase phenocrysts are larger (~3 by 4 cm) and aligned. Towards the center of the dike, plagioclase phenocrysts become smaller (<2cm) and more randomly oriented. Outlines of the phenocrysts were traced from scanned images of sawed slabs using Corel Draw to determine the major and minor axis lengths. Length data was analyzed using CSD Corrections (Higgins, 2002) to determine the frequency distribution of the area number density of the feldspar crystal populations and to fit a logarithmic-normal curve. There were 175 phenocrysts over an area of 31,496.64 mm². The major-axis of this preliminary test yielded a regressive slope of -1.18 and intercept of -4.40 (R²=0.27). CSD patterns for this dike are concave rather than linear (e.g., low R² values), indicating processes other than constant nucleation and growth are affecting the sizes of crystals present in the phenocryst population. These more complex patterns may reflect under-representation of smaller phenocrysts, overlap of scanned images, or the presence of glomerophyric crystals (see O'Donnell et al., this session). Once these issues have been addressed, the effects of size sorting during dike flow and/or changes in nucleation and growth rate during dike emplacement on the CSD patterns can be evaluated.