Magmatic Origins of Plutonic Rocks of the Cooke City Area, Montana

Archean bedrock outcrops exposed in the Cooke City area of southern Montana are part of the crystalline basement of the Wyoming Craton. These rocks are plutonic intermediate to felsic, and modal petrographic analyses characterize them as tonalite, trondhjemite, and granodiorite (TTG) as well as granite. Field relations indicate that magmatic bodies are small, and several generations of magma appear coeval. The presence of epidote, interpreted as a primary magmatic phase, indicates a mid- to lower crustal emplacement depth. Chemically, these rocks are similar to other Archean TTG, they are metaluminous with high AI2O3 (>15%), low Ti02 (1.5 GPa) melting environment and appears consistent with the geochemical signature imparted by melting a hydrous basalt at depth. Additionally, HFSE depletion and trace element tectonic discrimination diagrams indicate that these samples have a chemical signature similar to modern subduction zone rocks. Field and chemical evidence indicate that rocks in the Cooke City area are similar in nature to plutonic rocks of the Long Lake Magmatic Complex exposed to the east. The Archean plutonic rocks in the Cooke City area appear to have formed from coeval injections of magma bodies with differing sources and processes.