U-TH/PB GEOCHRONOLOGY AND PRESSURE-TEMPERATURE CONSTRAINTS ON METAMORPHISM IN THE FREEZEOUT RIDGE AREA, CLEARWATER METAMORPHIC CORE COMPLEX, NORTHERN IDAHO

Recent geochronologic and metamorphic data from the Freezeout Ridge area, northern Idaho, reevaluate stratigraphic relationships and further define conditions of metamorphism in the western portion of the Clearwater metamorphic core complex (CMCC). The Freezeout Ridge area was previously mapped by Hietanen [1968] as metasedimentary rocks of the Mesoproterozoic Belt Supergroup (~1.45 Ga); however, these metasediments are now recognized as exposures of basement rocks upon which the Belt Supergroup was deposited. U/Th-Pb monazite and xenotime geochronological data collected by laser ablation split stream (LASS) dating records ages as old as 1.5 Ga in the Freezeout Ridge schist (FRS). These ages predate the deposition of the Belt Supergroup, suggesting the protoliths of the FRS are supracrustal basement rocks. Doughty et al. [2007] concluded peak metamorphic conditions to be Cretaceous (82-80 Ma and 72-70 Ma) in the external zone of the CMCC. This study will further constrain peak metamorphic conditions in the Freezeout Ridge area through scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS) and thermobarometry. SEM/EDS will analyze the equilibria between different minerals in two samples of garnet-bearing amphibolite. Mineral chemistry will be used to calculate the P-T conditions of metamorphism. If the peak conditions of metamorphism in the Freezeout Ridge amphibolite samples match the conditions of other localities, the resulting metamorphic events that affected those rocks are assumed to be correlative. Interpreting age data collected from LASS petrochronology and analyzing conditions of metamorphism through SEM/EDS and thermobarometry in the Freezeout Ridge rocks will allow for a better understanding of the geologic processes within core complexes, which is a vital part of the geologic history of northern Idaho.