THE VOLCANIC ROCKS OF MOUNT PERSIS: AN EOCENE CONTINENTAL ARC THAT CONTAINS ADAKITIC MAGMAS
The geochemistry and U-Pb ages suggest the Mt. Persis originated from a continental magmatic arc source that spanned ~36-48 Ma. The high Si peraluminous samples and xenocrystic zircons suggest assimilation occurred as this magmatic system evolved. Mapping suggests that several intrusive centers occur for the Mt. Persis (including the 39-42 Ma Youngs Creek igneous complex and 47 Ma Fuller Mt. Plug). The adakitic and E-MORB affinities found in the Mt. Persis are rare in other ancestral Cascades magmatism of similar age. However, Eocene forearc magmatism that is interpreted to have formed by slab windows resulting from subducting ridges occurs through the Cordillera (e.g., 37-42 Ma Grays River volcanics). A combination of adakitic and enriched MORB magmas that exist with transitional tholeiitic-calc-alkaline magmatism occurs in arcs that sit above subducting oceanic ridges. We suggest that the volcanic rocks of the Mt. Persis formed in a continental arc setting; and, typical continental arc magmas of the Mt. Persis mixed with magma generated by a subducted oceanic ridge that may have been generated at the Kula-Farallon boundary.