DETAILED FIELD MAPPING OF THE LOST MINE TRAIL AND CASA GRANDE AREA INCLUDING PETROLOGICAL ANALYSIS OF THE CASA GRANDE LAVA DOME AND LOCALIZED DIKES, CHISOS MOUNTAINS, BIG BEND NATIONAL PARK, TEXAS

The suspected lava domes in the Chisos Mountains are of particular interest after ideas of a semi-circular series of post Pine Canyon Caldera (PCC) ring vents associated with the South Rim Formation eruptions were proposed by Urbanczyk and White (2000). The presence of these domes may be the evidence for the locations of other vents associated with the magma chamber that fed the lavas and tuffs. These vents erupted gas-rich pyroclastic deposits, then emplaced lava domes. The association of the lava domes with the rest of the stratigraphy of the South Rim Formation is not well understood, and further detailed mapping is needed. The area between Lost Mine Peak and Casa Grande is thought to be significant since it contains many dikes that are thought to be associated with the Casa Grande lava dome, and lies just beyond the western margin of the PCC. A large intrusion is also located in the area, which caused uplift and folding of older formations and may or may not be associated with the local dikes and lava domes. Mapping in the past has not been detailed enough to address these concerns and a 1:10,000 map of the area can lead to better conclusions concerning the volcanic activity in and around the Chisos Mountains after the collapse of the PCC. Preliminary thin-section work has suggested similar rock types for Casa Grande and a large dike that cross-cuts Casa Grande on the south-east side. Other dikes south of Casa Grande in Juniper Canyon show evidence of devitrification, and more work is to be done analyzing these characteristics. The petrology of Casa Grande and dikes will be analyzed in thin-section as well as XRF to form better conclusions of the geochemical relationships these lava domes and dikes have with the rest of the rock members in the area in and around the Chisos Mountains.