PRECAMBRIAN GEOLOGY OF YELLOWSTONE NATIONAL PARK (YNP) AND SURROUNDING AREAS: EXHUMATION OF PRECAMBRIAN GNEISSES AND APATITE (U-TH)/HE AGES

Surface uplift and erosional exhumation of the Precambrian rocks of the Beartooth Range and the South Snowy Range occurred in the Paleocene due to Laramide thrusting and more recently due to extension and advancement of the Yellowstone Hot Spot. Samples collected for low-temperature thermochronology included felsic and intermediate composition gneisses and granitoids from transects across the central Beartooth Range and the South Snowy block in Yellowstone National Park. Apatite (U-Th)/He (A-He) data place constraints on the exhumation history of the Archean rocks and the relative importance of Laramide and post-Laramide events in forming the topography. A-He apparent ages on the granitoids of Beartooth Plateau range from about 63 to 80 Ma and increase in age with elevation. These data are consistent with published apatite fission-track data and indicate that the Archean rocks were exhumed below ~70˚C during Paleogene time. In contrast the A-He analyses from Archean metasedimentary and plutonic rocks in the northern part of Yellowstone National Park give middle Miocene cooling ages, with single-grain ages ranging from about 6 to 25 Ma. These data indicate more recent exhumation of the Precambrian surface below ~70˚C either through erosion of Eocene volcanic and sedimentary rocks, or Paleozoic rocks. The scatter in single grain ages suggests the samples were within the He partial retention zone until after 10 Ma and that cooling commenced in late-Miocene-Pliocene times. The Miocene-Pliocene erosional exhumation was probably in response to development of the Yellowstone drainage system due to initiation of topographic relief associated with regional extension or the thermal bulge of the Yellowstone Hot Spot.