We present new trace element and radiogenic isotope data for a 150 km across arc transect that from west to east includes: Boring Volcanic Field (BVF), Mount St. Helens (MSH), Indian Heaven (IH), Mt. Adams (MA) and the Simcoe Mountains Volcanic Field (SVF). The basaltic magmatism across this transect is the most diverse within the Cascades Arc as a whole. Calc-alkalic basalts (CAB) and their derivatives are most abundant at all locations except for SVF. The most primitive CAB show a range in trace element and isotopic compositions at each volcanic center, but there is no systematic variation across the transect. Next in abundance are low-K tholeiites (LKT) that occur throughout the transect except at SVF and vary little in composition along the transect. A third group of OIB-like basalts are present across the transect. From BVF to MA, they comprise ~10% of basaltic lavas with similar compositions, but the easternmost OIB of SVF are proportionately more abundant (~80%) and compositionally different. Overall, the OIB form two parallel arrays in Pb isotope space with SVF being more radiogenic in 206
Pb than the remainder of the transect, indicating a time integrated more enriched source that the other OIB-type magmas. This difference in Pb-isotopes between the OIB-types is not observed for Sr and Nd-isotopes. The enrichment is also observed in lower Zr/Nb, and Sr/Nd ratios and higher Ce/Pb and Dy/Yb ratios.
Magmas throughout the transect show enrichment in Ba over Th indicating their source has seen an enrichment in a fluid-like component. The enrichments are extreme in the high K-CAB which also shows large enrichments in LIL and Sr. The limited compositional variability of all CAB, OIB and LKT types across a large part of the transect is remarkable and requires the presence of three distinct source domains at depth beneath each volcanic center as opposed to lateral transport of diverse magmas from either the east or west. Seismic studies of the subsurface indicates that, if present, the slab would be at 50 km depth beneath the western most volcanic field, while the Moho is at 40 km depth. A potential scenario is a gap in the slab that allows magma to be generated over a larger depth range that just 10km below the Moho and thus across arc differences are minimized.