GEOLOGY OF YELLOWSTONE NATIONAL PARK (YNP) AND SURROUNDING AREAS: PLUTONIC ROCKS OF THE COOKE CITY AREA

The Cooke City area of Southern Montana hosts expansive exposures of Precambrian basement rocks of the Wyoming Craton, typified by intermediate to felsic plutonic rocks. Modal mineralogy indicates that most rocks are of the tonalite-trondhjemite-granodiorite (TTG) suite, typical of Archean cratons worldwide. Additional rock types in the area include: granite, diorite, quartz diorite, quartz syenite, and mafic amphibolites. Emplacement styles of the igneous bodies differ systematically with location. In the Lamar Valley of Yellowstone National Park to the west, Archean metasedimentary rocks are intruded by plutonic bodies that range in scale from 100’s of meters to kilometers, while in the Beartooth Mountains to the east, rocks of similar composition described by Mueller et al. (2010) as the Long Lake Magmatic Complex (LLMC); form a series of coeval sheet-like intrusions that occur on meter to tens of meters scale. The plutonic bodies contain biotite-quartz schist xenoliths, with partially assimilated boundaries, which may be equivalent to the low-grade Jardine Metasedimentary Sequence to the west. Field relations in the Cooke City area suggest that TTG suite bodies were emplaced in metasedimentary country rock, which was later intruded by felsic to mafic bodies. The plutonic rocks are characterized by hypidiomorphic texture, however, undulatory extinction patterns of quartz grains and a weak mineral preferred orientation of biotite indicate some deformation. The presence of magmatic epidote indicates a crystallization pressure of at least 8 kbar. In addition, many of the plutonic rocks are cut by shear zones with a strong mylonitic fabric, particularly in the Colter campground area. Ductile deformation, as evidenced by discrete meter-scale shear zones, grain size reduction, and development of a strong lineation, is pervasive in this area. Investigation using whole-rock geochemistry (XRF), hornblende thermobarometry (EMPA), and zircon geochronology should allow determination of the bodies’ magmatic origins. Preliminary evidence indicates that the time of emplacement of the plutons near Cooke City is similar to the LLMC in the Beartooth massif, although the larger scale of the plutonic bodies may indicate a different depth of emplacement compared to the LLMC.