Recent mapping and bulk chemical analyses have established the existence of a long-lived field of andesitic and dacitic volcanism in the Mt. Jefferson area of the High Cascade Range in Oregon. The project aims to further constrain the temporal evolution of this volcanic field with precise K-Ar age dating, and to better understand the petrogenesis of the filed with rare earth element (REE) and isotopic (O, Sr, and Nd) analyses. The volcanic field around Mt. Jefferson is particularly suitable for this study as it has been mapped in detail over a period of five years, and an extensive and detailed stratigraphic framework involving some 150 map units has been established. Few parts of arcs anywhere are known in such detail. An extensive bulk composition and electron microprobe data base has also been acquired on the Mt. Jefferson area rocks. These data demonstrate the importance of open system processes, presumably magma mixing, in these rocks. this conclusion applies to all rock types examined, including basaltic andesites, andesites, and dactites.
