EOCENE DEFORMATION AT TUBBS HILL OF COEUR D’ALENE, IDAHO, SOUTHEAST PRIEST RIVER COMPLEX

Sensitive high-resolution ion microprobe analyses of igneous rims on zircons from deformed (mylonitic) granite dike rocks of Tubbs Hill, in downtown Coeur d’Alene, yielded U-Pb dates of 50.1±3.5 Ma and 49.5±1.1 Ma. The dike strikes northwest and is comprised of porphyritic biotite granodiorite with feldspar phenocrysts 0.5-0.75 cm in length. These robust dates provide a constraint on mylonitic deformation within the southeast part of the Priest River metamorphic complex (PRC). An Eocene igneous age from this Tubbs Hill dike indicates coeval emplacement with Eocene plutonic suites elsewhere in the PRC, such as the locally deformed Silver Point pluton quartz monzonite dated at 50.13±0.02 Ma (Stevens, 2015). The northern most mylonitic deformation within the Priest River Complex, 66 km north of Tubbs Hill, is constrained by a recent suite of monazite and zircon U-Pb dates of a leucocratic dike indicating crystallization was circa 49.9 Ma (Stevens, 2015). Mylonitic deformation in the Priest River complex appears to be similar age in both the north and south.