GEOCHEMISTRY OF THE EOCENE CRESCENT FORMATION BASALT, WASHINGTON STATE: IMPLICATIONS FOR MANTLE PLUME ACTIVITY

The Coast Range Basalt provinces of Oregon, Washington and Vancouver Island consist of an estimated 250,000 km³ of extrusive lavas generated between 62 and 50 Ma. Although previous studies have linked this Large Igneous Province (LIP) to subduction of a mid-ocean ridge (Haeussler et al., 2003) or accreted seamounts potentially associated with a mantle plume (Duncan, 1982), unambiguous evidence concerning the origin of Coast Range volcanism remains elusive. Ongoing investigation of the trace element and isotope composition of the Crescent Formation Basalts of Washington State demonstrates a range of basalt compositions consistent with an enriched mantle source. Temporal changes in composition between early-stage submarine volcanism (Lower Crescent Formation; Zr/Hf = ~30, Nb/Ta = ~18, eNd = ~6) and later-stage subaerial flows (Upper Crescent Formation; Zr/Hf = 35-39, Nb/Ta = 14-16, eNd = ~5) suggest changes in source reservoirs as volcanism progressed. This study presents an expanded geographical survey of Crescent Formation compositions in an effort to further constrain the origins of the Coast Range LIP.