MINERALOGIC ASSESSMENT OF THE SOUTH RIM FORMATION: ADDITIONAL IMPLICATIONS FOR PETROGENETIC DIVERSITY

The South Rim Formation is comprised of two petrogenetically diverse suites extruded in the uppermost Chisos Mountains of Big Bend National Park, Texas, over a relatively short (≈ 200 ka) geologic time period. Major and trace element mass balance modeling, in addition to petrographic properties, suggests Pine Canyon Suite quartz-trachytes-rhyolites originated via assimilation of a shale wall rock and fractional crystallization of alkali basalt whereas Emory Peak Suite high silica rhyolites were likely produced via crustal anatexis of mafic granulite combined with volatile flux. Here, new mineralogic data are presented and compared with other peralkaline localities to evaluate proposed petrogenesis. Electron probe microanalysis reveals numerous mineralogic differences between each of these suites including: near anorthoclase Pine Canyon Suite sanidine versus more stable composition Emory Peak Suite sanidine, perthite exclusivity in Pine Canyon Suite, increased Ca and ^ivAl filling in Pine Canyon Suite sodic amphibole sites, aenigmatite formation in the Pine Canyon Suite versus biotite formation in the Emory Peak Suite, restriction of fayalite to Emory Peak Suite, and the presence of accessory pyrrhotite and bastnasite in the Pine Canyon Suite versus chevkinite in the Emory Peak Suite. Additionally, oxygen fugacities between -0.14 and -0.25 ΔFMQ were determined (using QUILF95) over a temperature range of 680-740 °C for newly discovered Emory Peak Suite fayalite microphenocrysts. These findings are consistent with and support the proposed petrogenesis.

Mineralogic properties of the two suites were also compared with five other peralkaline localities derived from similar petrogenetic mechanisms: volcanics from Pantelleria, (Italy), the Greater Olkaria Volcanic Complex and Eburru Volcanic Complex (Kenya), the Gedemsa caldera (Ethiopia), and intrusive material from Mounts Abu-Kharif and El-Dob (Egypt). For each location, mineral assemblages and chemistry resemble the petrogenetically respective suite in the South Rim Formation.