The Geology and Petrology of the Fifes Peak Formation in the Cliffdell Area, Central Cascades, Washington

The Fifes Peak Formation in the Cliffdell area is comprised of two structurally, lithologically and geochemically distinct members separated by a pronounced unconformity. The older member (Edgar Rock member) is comprised of highly brecciated lava flows, coarse laharic breccia and lesser volcaniclastic sediments and tuffs. A thickness of at least 1,800 m is exposed near Edgar Rock. K-Ar ages indicate a latest Oligocene age of about 24 to 27 Ma. Lava compositions range from basalt to dacite, but are mostly basaltic andesite. These lavas are typically highly porphyritic and contain an anhydrous phenocryst assemblage of plagioclase, olivine, minor clinopyroxene and magnetite. Gabbroic inclusions, various pyroxene clots and green spinel are present in some basaltic andesite flows. The most common type of laharic breccia contains clasts and matrix constituents petrographically similar to the abundant basaltic andesite lavas. A less cemmon type of lahar-like breccia contains abundant silicic lapilli and variable amounts of basaltic andesite. Clasts of non-Fifes Peak Fm. lithologies, including gabbroic plutonic rocks, sandstone and altered volcanic rocks, are most commonly associated with these units. Beds in the Edgar Rock member define the circular Edgar Rock dome. An extensive system of radial dikes, truncated at the top of the member, has a focal area near the structural center of the done. Dike compositions range from basaltic andesite to dacite and are petrographically diverse, indicating a history of multiple intrusions. Mafic dikes broadly resemble the mafic flows in the Edgar Rock member, but andesite and dacite dikes are petrographically distinct from flows of similar composition. The Edgar Rock dome is probably the site of a major volcanic center in tile Fifes Peak Fm. Present steep dips are a combination of primary depositional attitudes and syn- and post-depositional uplift, perhaps partly related to the intrusion of the radial dike system. Volcaniclastic interbeds and laharic breccias constrain primary attitudes in the dome from nearly horizontal up to about 15 degrees. The Edgar Rock member on the flanks of the dome represents the proximal apron facies of a cone, mostly removed by erosion, located near the focus of the dike system. The younger member of the Fifes Peak Fm. (Nile Creek Member) unconformably overlies the Edgar Rock dome. The Nile Creek member is comprised mostly of andesite flows and coarse laharic breccias. Lavas are petrographically uniform, highly porphyritic and contain phenocrysts of plagioclase, clinopyroxene, orthopyroxene, magnetite and olivine. K-Ar ages indicate an early Miocene age of about 24 to 19.5 Ma. An ash-flow tuff has a fission-track age of 23.3 Ma. The source of the Nile Creek lavas was not identified in this study. The principal structure of the study area, the Edgar Rock dome, formed during Fifes Peak time (pre-24 Ma). A NW-trending monocline offsets the Edgar Rock member and is coeval with the adjacent Little Naches Syncline. The Cliffdell area lavas and dikes have geochemical features common to continental margin, subduction-related lavas. Enrichments in K and related LIL and light-REE are characteristic of calc-alkaline lavas, but high FeO/MgO ratios, especially of the Edgar Rock suite, are typical of tholeiitic lavas. High AI2O3 and low MgO, Ni and Cr of mafic Edgar Rock lavas are characteristic of orogenic high—alumina basalts. Edgar Rock lavas and dikes are enriched in LIL, REE and HFS elements relative to the Nile Creek lavas. The Cliffdell suites, especially the Edgar Rock, are Fe-enriched relative to the coeval Cascade batholiths. Separate source regions or magmatic differentiation processes, or both, are indicated