Petrology of the Earliest Cascade Volcanic Units in Washington: the Northcraft, Tukwila, and Rattlesnake Mountain Formations and Implications for the Evolution of the Cascade Arc

Cascade arc magmatism began ~45 million years ago, following the reconfiguration of the Farallon-North American subduction zone. This project seeks to classify the path of magmatism throughout the history of the Cascade arc, as well as categorize the earliest expressions of the Cascade arc. Three volcanic units, the Northcraft Formation (NF), Tukwila Formation (TF), and Rattlesnake Mountain Formation (RM) will serve as examples of the earliest Cascades-related lavas. Their categorization by field setting, geochemical characteristics, isotopic data, and age will create a better understanding of how Cascades magmatism began and developed over time. Analysis has shown these units to contain 51.6-70.5% SiO2 (NF), 56.5-63.0% SiO2 (TF), and 53.2-65.1% SiO2 (RM). All three units are majority calc-alkaline, evolved in their compositions, and demonstrate arc-like tendencies. Categorically, these units contain lower levels of K2O than the modern arc and few early Cascades samples contain greater than 6% MgO, which is notable as the modern arc sees many samples up to 9% MgO. In trace data, early Cascades samples show much lower Ba/Nb ratios, which is attributed to their lower Ba and higher Nb values. Furthermore, analysis of another HFSE, La, yields similar results when related to Yb values. These analyses support the hypothesis that over time, slab interaction in the arc has increased, positing greater amounts of fluids and incompatible elements over time. This input of fluids allowed for flux melting, which in turn fed the Cascades, resulting in lavas with more mineralogical maturity and compositional diversity