THE WEST SIDE STORY OF THE SPOKANE DOME OF THE PRIEST RIVER METAMORPHIC CORE COMPLEX (PRC), WASHINGTON AND IDAHO

The Spokane “dome” is a NNE trending, Challis-age antiform in the southern 60% of the PRC. The Spokane Dome mylonite zone (SDMZ) in the core of the antiform pervasively affects all lithologic units > 46 Ma. The bounding West Side Transition Zone (WSTZ) contains amphibolite facies rocks that are not pervasively mylonitic, and which have cm-scale ultramylonites, relict folds in metamorphic rocks, and some relict igneous textures. We bring 1:24,000 mapping to SDMZ and WSTZ in the Mount Spokane quadrangle (MSqd) and two new U/Pb dates from zircons.

Much of the antiform is underlain by the semi-pelitic, sillimanite-bearing, 1.45 Ga Hauser Lake Gneiss (HLG), which is meta-Prichard Formation. Peak metamorphism was 72 or 68-64 Ma. Overlying HLG is the biotitic Newman Lake Gneiss (NLG). From this orthogneiss, we obtained a date of 65.4 ± 0.9 Ma. Both HLG and NLG are blastomylonitic.

An orthogneiss within MSqd, previously regarded as Precambrian, typically has 10 to 20-cm thick biotite-rich and biotite-poor bands. It has screens of HLG. Our U/Pb date from a sample with a relict igneous texture in WSTZ is 98.9 ± 1.2 Ma, so metamorphism in WSTZ is younger. A few zircons in this gneiss and NLG range from 1584 to 1899 Ma.

The three gneisses are overlain by the 76.5 Ma, sill-like Mount Spokane two-mica granite (MS2MG), which is nonmylonitic. The contact dips westerly about 20⁰, indicating that MS2MG probably is < 4 km thick. The portion of WSTZ below MS2MG is about a kilometer thick.

Others have mapped similar relationships between MS2MG and metamorphic rocks at Beacon Hill and Chester Creek, 33 and 45 km to the southwest, respectively. No mappers have reported chloritic breccia below or above MS2MG; so, no detachment fault bounds the WSTZ. Near Chester Creek, MS2MG occupies the unconformity at the base of middle Cambrian quartzite. The WSTZ is a carapace of essentially autochthonous rocks above the SDMZ.