Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 39-8
Presentation Time: 8:30 AM-6:00 PM


REID, Allison1, TAPANES, Steven1, WEBBER, Jeffrey1, KEMBLE, Richardjames1 and ELLIOTT, Colleen2, (1)Geology Program, Stockton University, 101 Vera King Farris Dr, Galloway, NJ 08205, (2)Montana Bureau of Mines and Geology, Montana Tech, Butte, MT 59701

The Mount Powell Batholith (MPB) in western Montana is exposed across a 13 km transect through the Dempsey Creek watershed providing an ideal opportunity to investigate structural and intrusive relationships of Late Cretaceous to Paleocene plutonic rocks within the Eocene-Oligocene Anaconda Metamorphic Core Complex (AMCC). We present preliminary results from geologic mapping at a scale of 1:10000 and petrologic analyses motivated to better refine the context of Cretaceous to Eocene magmatism and crustal extension. Our results indicate an overall tabular geometry of the MPB with a regional dip of 9° to the northeast. The MPB intrudes host rock of marble, quartzite, and phyllite that is commonly intercalated with altered quartz diorite. The base of the intrusive complex is characterized by a tabular zone of quartz diorite intruded by more felsic granitoids. The younger granitoids display a variety of textures and compositions ranging from garnet-bearing aplitic dikes to granite pegmatite intrusions. The orientations of felsic dikes are consistent with the overall dip of the intrusive complex. The minimum thickness of the intercalated zone is ~400 m and the minimum thickness of the entire intrusive complex is ~1300 m. Samples of younger granitoid display evidence for predominantly static late-to-post-magmatic hydration that is not pervasively developed in older quartz diorite suggesting regionally extensive, but locally restricted fluid infiltration. Field-based magnetic susceptibility measurements from granitoids of the MPB are on the order of 7.5x10-4 SI, whereas measurements collected from unaltered quartz diorite are consistently larger, on the order of 2.5x10-2 SI. This suggests that the abundance of magnetite is minimal within the granitoids and that positive aeromagnetic anomalies observed at the regional scale likely correspond to exposures of the intercalated quartz diorite. The eastern flank of the complex contains exposures of Phanerozoic sedimentary rocks that are locally brecciated and faulted defining the eastern margin of the MPB and AMCC. Ongoing work is aimed at constraining the temperature and style of deformation recorded in quartzite of the host rock through quartz CPO analysis and petrologic characterization of the intrusive phases including a rare occurrence of orbicular granite.