EOCENE DEFORMATION AND ASSIMILATION IN THE WILLOW LAKE NATURAL AREA, SPOKANE COUNTY, WASHINGTON

The rocks along the southern and western banks of Willow Lake Natural Area preserve a snapshot of time from about 49 million years ago when magma was forcing its way through the crust. The area displays multiple forms of deformation and preserves various degrees of basement rocks being assimilated (or melted) into the granite. This project included mapping, geochemical identification, and U-Pb age determination to address the formation of the Medical Lake granite and the Willow Lake aureole. The Willow Lake aureole is found in Proterozoic (~1.45 billion years old) calcareous-silicates of the Wallace Formation, Piegan Group, the upper most portion of the Belt Super group found in the Spokane area. The aureole was formed by Cretaceous to Eocene granites with amphibolite coronas hosting metallic and silicic mineralization. Bedding and foliation dip to the east at 30 to 90 degrees, likely related to folding and the emplacement of the Medical Lake granite leading to a minimum uplift of 536 meters. The Wallace Formation in this area is generally green and thinly layered (laminated) mudstone and quartzite. Soft sediment deformation prior to lithification can be difficult to distinguish from post-depositional deformation and metamorphism. Based upon plagioclase – amphibole geobarometery the minimum uplift of the area is on the order of 7 km in the last 49 Ma, or an estimated uplift rate of 5.8 mm/ year. Zircon cores in the granite preserve Proterozoic ages suggesting that the granite is a partial melt of the crust. The granite has been cross-cut by mafic dikes that contain large feldspar xenocrysts. Age determination of the intrusions corresponds to the Priest River Complex.