OWYHEE BASALT - DEVELOPMENT OF CALC-ALKALINE VOLCANISM ON THE HEELS OF THOLEIITIC FLOOD BASALTS OF THE COLUMBIA RIVER PROVINCE

Flood basalt volcanism initiating at ~17 Ma has been attributed to the Yellowstone plume impinging beneath North American lithosphere. Basalt magmas are prone to mixing or assimilation with a crustal component as they propagate towards the surface, resulting in compositionally diverse magmas. Within the La Grande-Owyhee eruptive axis of eastern Oregon, mid Miocene calc-alkaline lavas have been shown to be spatially and chronologically related to flood basalts. A transition from tholeiitic mafic to calc-alkaline intermediate magmas during the waning stages of CRBG flood basalt volcanism has been proposed (Steiner & Streck, 2013, GSL, Spec Pub, 385).

Calc-alkaline units within the region prove to be petrologically important when making inferences about the magmatic evolution of the CRBG province (Brueseke & Hart, 2009, JVGR 188). One of the calc-alkaline units of this region is the Owyhee Basalt, located within the Oregon-Idaho graben and it serves as an important transitional unit immediately after ~coevally erupting CRBG tholeiitic and silicic lavas. Although some inferences can be made from pre-existing data of other calc-alkaline units in the region, the Owyhee Basalt remains relatively unstudied and thus a better understanding of it will add to the significance of calc-alkaline units within the region.

This project aims to detail petrologic characteristics of the little studied Owyhee Basalt along transects across 360m of superbly exposed volcanic stratigraphy. Geochemical data show that these lavas range from basalt to andesite in composition with MgO values ranging from 6.60-2.75. Despite the differences in the major element data, incompatible trace elements such as Ba, Sm, and Th are nearly constant throughout the suite with values of roughly 700, 5.5, and 2.5 ppm respectively. Petrographic analysis shows that most of the samples contain zoned plagioclase or two populations of plagioclase potentially indicative of magma mixing or assimilation. Owyhee Basalt units also have a similar trace element signatures and systematics to adjacent calc-alkaline units, such as the Strawberry Volcanics, providing additional evidence that a transition from tholeiitic to calc-alkaline lavas during the latter stages of CRBG volcanism is a widespread feature.