THE SILVER HILL ISOLATED TERRANE: A KLIPPE OF PALEOZOICS ON THE WESTERN FLANK OF THE SPOKANE DOME

A sequence of contact-metamorphosed sedimentary rocks occur on Silver Hill south of Spokane, WA. They are surrounded by Miocene basalt and juxtaposed against amphibolite facies rocks of the Priest River Complex to the east. They are thin to medium bedded, fine-grained micaceous orange-weathering quartzite and locally medium-grained, white to lavender, vitreous, non-micaceous quartzite. The quartzites are interbedded with thin phyllites and locally graphitic schist that reach 5 meters in thickness. A weak foliation along with randomly oriented metamorphic minerals (biotite + andalusite + orthoclase + sillimanite +/- muscovite +/- cordierite), some reaching 3 cm in size, record contact metamorphism at pressures below 300 MPa.

Similar epizonal metamorphism characterizes rocks outside of the Priest River complex, which were at shallow depths during the intrusion of arc-related magmas during the Jurassic and Early Cretaceous.

Silver Hill occurs on the west side of the Spokane Dome above the top of the west-dipping, but east-verging, Spokane Dome mylonite zone. It is an extensional klippe resting on the back-rotated western flank of the core complex.

Previously, the weakly metamorphosed sediments at Silver Hill were considered to be Archean or Proterozoic, but meter thick intervals of graphitic or carbon-rich strata are unknown in most Precambrian rocks that crop out in this part of North America. However, graphitic or carbon-rich strata have been reported from Late Precambrian and Paleozoic strata elsewhere in northeast Washington. The lack of distinctive late Precambrian rock types (diamictites and greenstones) makes a Paleozoic correlation the only logical choice. We infer that the sequence at Silver Hill correlates with Cambrian strata, either the Addy Quartzite or the quartzite of Chewelah, found in the Kootenay Arc of northeastern Washington.