DECIPHERING CRETACEOUS AND EOCENE GRANITOIDS IN THE SPOKANE AREA, EASTERN WASHINGTON

Multiple phases of felsic to intermediate magmatism occurred in the Spokane area. The felsic intrusive rocks serve as an ideal way to measure and decipher the tectonic events that formed eastern Washington, which is similar to many other parts of the Cordilleran. This project focuses on the comparison of Cretaceous and Eocene granitoids, which may correlate with two major Cordilleran orogenies. A total of fifteen samples from the area were collected and compared based on whole-rock geochemical composition, radiogenic age determination, and mineral chemistries measured using a field emission electron microprobe. As a result of the whole rock data, these rock types have been classified as peraluminous granite to granodiorite. Liquidus temperatures ranged from 700 to 950 degrees Celsius. Trace element comparison was similar between ages, though Cretaceous granitoids did not show pronounced Eu-anomalies. Whole rock data did not generally provide a strong distinction between the Cretaceous and Eocene granitoids. However, based upon two-feldspar geothermometry it appears that Eocene granitoids had higher temperatures during plagioclase feldspar crystallization. We hope that further analyses will provide a more distinct difference between Cretaceous and Eocene magmatism as they have regional tectonic significance.