Petrology of monogenetic volcanoes, Mount Bailey area, Cascade Range, Oregon

The Mt. Bailey area encompasses the contact between Western Cascade and High Cascade volcanic rocks in southern Oregon. Western Cascade rocks are represented by a thick stack of magnetically reversed lavas and intercalated laharic deposits (the "Devils Canyon sequence") that range in composition from calc-alkaline basalt to andesite. The oldest (magnetically reversed) rocks of the High Cascade sequence formed two small andesitic shield volcanoes and, in the northern part of the area, several intracanyon flows of high-alumina olivine tholeiite (HAOT). These eruptions were followed by formation of a basaltic shield volcano (Sherwood Butte) and then by the eruption of andesitic lavas to form Mt. Bailey, and basaltic andesite which formed the adjacent cone of Garwood Butte. The Devils Canyon lavas are enriched in total Fe, total alkalis, K₂O, P₂O₅, and TiO₂ relative to the High Cascade lavas. High Cascade lavas display calc-alkaline affinities (e.g., enrichment of large-ion lithophile element and light rare earth elements) except for the intracanyon HAOT, which has trace-element characteristics typical of MORB. Rb/Zr ratios suggest that calc-alkaline lavas from each of the High Cascade volcanoes represent distinct episodes of partial melting. The source of the calc-alkaline magmas was probably mantle enriched by fluids derived from subducted oceanic crust, whereas the source of the HOAT was depleted mantle similar to the source of MORB.