THE VOLCANIC ROCKS OF MOUNT PERSIS: AN EOCENE CONTINENTAL ARC THAT CONTAINS ADAKITIC MAGMAS
MACDONALD Jr., James H., Marine & Ecological Sciences, Florida Gulf Coast University, 10501 FGCU Blvd South, Ft. Myers, FL 33965, DRAGOVICH, Joe D., Washington State Department of Natural Resources, Geology and Earth Resources Division, 1111 Washington St SE, PO Box 47007, Olympia, WA 98504-7007, LITTKE, Heather A., Washington Department of Natural Resources, Geology and Earth Resources Division, 1111 Washington St SE, PO Box 47007, Olympia, WA 98504-7007, ANDERSON, Megan, Geology Dept, Colorado College, 14 E. Cache La Poudre St, Colorado Springs, CO 80903 and DUFRANE, S. Andrew, Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building, Edmonton, AB T6G 2E3, Canada, jmacdona@fgcu.edu
The volcanic rocks of Mount Persis (Tabor et al., 1993), western Cascades, Washington, consist of flows, with lesser tuff, tuff breccia, lahar deposits, and interbedded sedimentary rocks. Published ages from the Mt. Persis are Eocene (~38 to 47 Ma). Several USGS STATEMAP studies have provided new age and chemical data for this formation. New U-Pb LA-ICP-MS zircon ages from two Mt. Persis tuffs are 36 ± 2.3 Ma & 43.7 ± 1.0 Ma. Both tuffs contain Mesozoic and Precambrian age zircons. Compositions for 64 samples analyzed for whole rock major and trace element geochemistry range from basalt to rhyolite with most samples being andesite or dacite. Lower Si samples are magnesian while samples with high Si are ferroan. About two thirds of the Mt. Persis samples are metaluminous, while about one third are peraluminous. The peraluminous samples tend to have higher Si. Samples are medium-K calc-alkaline with lower Si values transitional to tholeiitic. 7 samples, however, are transitional to E-MORB and 12 other samples have Si, Sr, and Y that indicate they are adakitic. These adakitic rocks also have high Al and Na, and low Yb.
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.
