TRANSITION FROM THOLEIITIC FLOOD BASALT MAGMATISM TO CALC-ALKALINE AND ALKALINE VOLCANISM ALONG AN ERUPTIVE AXIS COINCIDENT WITH THE EAST MARGIN OF THE GRANDE RONDE VALLEY, NORTHEASTERN OREGON

Detailed mapping supplemented by geochemical analysis and Ar/Ar age dates indicate that the east side of the modern Grande Ronde Valley is part of a large volcanic complex that first produced tholeiitic flood basalt (Columbia River Basalt Group lavas) and later evolved to produce calc-alkaline andesite and dacite flows and alkalic trachyandesite and basanite flows (Powder River Volcanic Field lavas). Columbia River Basalt eruptive centers are marked by spatter mounds, diatremes, dikes, and fringing hydrovolcanic breccias. Powder River Volcanic Field calc-alkaline eruptions formed small composite and shield volcanos that largely buried the older CRB vents. About a 3 million year hiatus separates the last flood basalt eruption (the evolved, high silica, iron-rich ferro andesite of Fiddlers Hell) from the first eruption of calc-alkaline andesite and dacite flows.