COMPARISON OF TWO CRETACEOUS AGE GRANITES ALONG THE SPOKANE RIVER USING ESTIMATED AGE, MINERAL COMPOSITION, AND APPLICABLE GEOTHERMOBAROMETRY

The goal of this study is to compare two granite samples obtained from hydroelectric dams along the Spokane River in Eastern Washington. Samples obtained are from Little Falls Dam and Long Lake Dam which is located 7.7 km upstream from Little Falls. Previous studies conducted by Engles and Miller (1975) indicate different ages of intrusion, differing geothermobarometry, and provides new insight to Spokane’s regional geology. The granitoids at Long Lake and Little Falls are mapped as Eocene and Cretaceous on a 1:100,000 scale. To determine ages a LA-ICP-MS is used to radiometric date by means of U-Pb decay (McCollum and Pritchard 2012). Results from this study indicate Eocene and Cretaceous age granites are present at both locations. Cretaceous granites have been associated to the Cordilleran magmatism and Eocene granite and granodiorite have been associated with uplift of the Spokane Dome, or Priest River Complex. Observations using a petrographic light microscope conclude the Long Lake sample underwent extensive chlorite alteration and the Little Falls sample has preserved amphibole and biotite grains in abundance. Ternary diagrams were used to further classify the samples based on the abundance of common igneous forming minerals including quartz, orthoclase, anorthite, and albite. Temperatures of the granitoids have been estimated using a two-feldspar thermometer and a hornblende-plagioclase (770°C and 3kBar for Little Falls dam) thermometer. Multiple types of analysis have been used to compare the different Cretaceous granitoids and these rocks preserve a great deal of the pre-neogene history of the area. With recent events in Oroville California understanding structures of dams and the environment surrounding can be paramount in predicting hazards and implicating safety procedures.