REASSESSING COMPOSITION AND DISTRIBUTION OF ~44 TO 17 MA VOLCANIC ACTIVITY ACROSS OREGON BEARING ON THE YELLOWSTONE- SILETZIA PLUME INTERACTION AHEAD OF COLUMBIA RIVER PROVINCE MAGMATISM

Despite extensive study, numerous questions remain regarding the Columbia River Basalt Group (CRBG) including the exact driving force of this magmatism. Most recently, a model is gaining popularity in which CRBG flood basalts do not result from the initial impingement of a deep-seated rising mantle plume but are a continuation of magmatism associated with the ~53 Ma Siletzia large igneous province. For this model, a critical time interval is the period when the North American plate started to override the plume to the onset of CRBG volcanism. Here, we focus on the composition and distribution of volcanic rocks in the intervening area between Siletzia and the CRBG dike swarms in this critical time window of ~44 to 17 Ma. The overall picture is that volcanism in this region of central and eastern Oregon was nearly continuous during the time period between 44 to ~22 Ma, yet evidence for that is sparser in areas covered by CRBG flood basalts along the Columbia River corridor as well as where covered by Miocene volcanic rocks along the High Lava Plains. Compositional data suggest that possibly except for the earliest part, volcanism was bimodal with abundant rhyolitic rocks (mostly A-type compositions) starting to erupt from 39 Ma onward belonging mostly to the John Day Formation and including now several recognized caldera systems. Most new studies, that include age dating of volcanic rocks of eastern Oregon, reveal ages that also fall in the mentioned bracket such as our new finding of widespread dacites and more mafic lavas with ages of 25 to 19 Ma in a broad area between Burns-John Day-Unity as well as 42-40 Ma basaltic andesite lava flows and andesitic ash-flow tuffs and 40 Ma rhyolite in the eastern part of the Clarno Formation that were on no previous geological maps. This highlights that our picture of ~44 to17 Ma volcanism in central to eastern Oregon (including the area of the early Cascades arc) is likely incomplete, however, currently available data suggest that the Yellowstone-Siletzia plume tracking across Oregon may have resulted in a more diffused volcanic surface expression than a relative narrow track.