THE GENERATION OF HOT-SPOT RELATED CALC-ALKALINE ANDESITES OF THE STRAWBERRY VOLCANICS, NORTHEAST OREGON

The mid-Miocene Strawberry Volcanics of northeastern Oregon erupted between 16.2 and 12.5 Ma, coeval with and geographically surrounded by the Columbia River Flood Basalt Group (CRBG). Within <2 m.y. period, the Strawberry Volcanics erupted tholeiitic basalt, tholeiitic and calc-alkaline basaltic andesite to andesite, and rhyolite lavas. Here we investigate how magmas differentiate from parental tholeiitic basaltic magmas to tholeiitic and calc-alkaline intermediate lavas in an intra-continental setting. Trace elements and isotopic ratios indicate that the magmas that generated both calc-alkaline and tholeiitic intermediate magma compositions were initially basaltic tholeiites. The least evolved magmas (>6 wt.% MgO) share similar trace element and isotopic concentrations including enrichments and depletions patterns in spider diagrams to the Imnaha and Steens basalts of the CRGB. The tholeiitic intermediate magmas were produced by fractional crystallization of the mafic magmas while the calc-alkaline magmas are a result of mixtures of tholeiitic basalt, rhyolite, and assimilated crust. Mineral chemistry shows that the calc-alkaline intermediate lavas contain the most primitive minerals, and therefore, must have had an interaction with a more primitive magma. Mafic silicate phenocrysts have upper magnesian compositions of Fo₈₄, Mg_83, and Mg₈₄ of olivine, clinopyroxene, and orthopyroxene, respectively. This research shows that there is no need for a primitive calc-alkaline magma or extensive fractional crystallization to generate the calc-alkaline andesites seen in the Strawberry Volcanics. The transition from tholeiitic to calc-alkaline magmas occurred as a product of magma mixing and crustal assimilation.