GEOLOGIC MAPPING OF INTERMEDIATE PORPHYRITIC INTRUSIONS ON THE LINE CREEK PLATEAU, SOUTHEASTERN BEARTOOTH MOUNTAINS, MONTANA AND WYOMING

The Line Creek Plateau in the southeastern Beartooth Mountains (Montana/Wyoming) contains porphyritic intrusions. These were previously mapped as "Tertiary or Late Cretaceous" and have been recently dated 2-6 miles west and north of our study area to 98–96 Ma. To better understand these porphyry intrusions, we conducted 1:24,000 scale geologic mapping, mostly in the Mt. Maurice 7.5-minute quadrangle, to document their distribution and extent. This area has previously been mapped at 1:62,500 in Wyoming (1960's-70's), 1:126,720 in Montana, approximately 1:27,000 (1950's), and 1:100,000 (2001). These maps are all either small scale, out of date, or do not match up over the Montana-Wyoming border. Field mapping was conducted over three weeks in July-August 2024 using GPS units, Brunton compasses, and field maps with satellite imagery and overlain topographic contours. Units mapped included the porphyry (unit of interest), Archean undifferentiated metamorphics, Archean quartzite, Quaternary glacial till, and Quaternary alluvium. Due to limited exposure, many bedrock contacts were mapped as approximate via float identification and walking contacts. Four samples of porphyry were collected for petrographic analysis, geochemistry, and U-Pb dating. Plagioclase feldspar phenocrysts made up 30-50% of the composition, ranging in size from 1-6 mm. Three samples contained large quartz crystals; they were 0.2-0.5 cm in one and reached 1 cm in the other two. One sample contained orthoclase megacrysts in addition to plagioclase. Amphibole and biotite were also present. The groundmass was light to medium grey; weathering colors included yellow, brown, and pink. Thin sections are complete and petrographic analysis is currently underway. A spatial analysis was conducted using ArcMap comparing the surface area of our mapped porphyry exposures to previous 1:100,000 scale mapping. There was a significant increase in area, with numerous distinct exposures not previously mapped. U-Pb dating and geochemistry are currently in progress. We hypothesize the ages will be consistent with previous nearby dates that show a mid-Cretaceous age, and we will compare the geochemistry with other nearby porphyry. These results will have implications for the magmatic history of south-central Montana.