Orogenic Andesites in an Anorogenic Floodbasalt Province: The Role of the Crust in Generating the Calc-alkaline Trend

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The geological past of Oregon is characterized by many volcanic eruptions that covered Oregon with layers and layers of volcanic rocks. One particularly volcanically active period was around 15 million years ago when fissures in the vicinity of Steens Mountain in SE Oregon, in the Wallow Mountains, NE Oregon, and other places in eastern Oregon opened to erupt massive amounts of basalt lavas collectively known as Columbia River Basalt Group (CRBG). All lavas of the CRBG are of one chemical type known as tholeiitic. However, in the geographic center defined by the eruptive sites of the CRBG, eruptions gave rise to the Strawberry Volcanics that are compositionally of a different kind known as calc-alkaline. Lavas with calc-alkaline compositions are characteristic for volcanic arcs like the Cascades Mountains that stretch along the West Coast from N. California into Canada. All volcanic arcs are situated at convergent plate boundaries where tectonic plates converge. The generation of volcanic rocks with calc-alkaline composition at convergent plate boundaries has been principally tied to processes going on deep in the mantle ( ~40 km depth). The calc-alkaline Strawberry Volcanics situated within a non-convergent plate setting and within a period dominated by tholeiitic lavas representing the magma supplied from the mantle at that time suggest that other ways must exist to produce such compositions. This study will investigate whether and how shallower processes going on solely in the earth crust ( ~40km depth) can impose calc-alkaline composition as first order process. Chemical, mineralogical and isotopic data will be used as means to research this question.In other words, this study will investigate the role of the crust in generating the calc-alkaline trend and associated orogenic-like andesites in a non-subduction, tholeiitic flood basalt province. This setting will minimize the uncertainty of what type of basalt is delivered from the mantle to the crust thus allowing to exclude that calc-alkaline signature of basaltic andesites and andesites/dacites was translated from a primitive basalt which acquired its calc-alkaline signature directly from the mantle source. The critical time window is ~17 to 15 Ma; that's when the dominant volume (~90%) of the CRBG erupted from fissures around the periphery to the Strawberry Volcanic Field that itself erupted during that time tholeitic basalt, calc-alkaline basaltic andesite, andesites and dacites, occasional tholeiitic intermediates, and rhyolites. Exposures of crustal rocks in and around this volcanic field provide the opportunity to obtain compositional data of the crust through which magmas of the Strawberry Volcanics passed and with which they may have interacted. The goal of this research is to comprehensively study the effect of crustal interactions (AFC, assimilation, mixing) in controlling the development of the calc-alkaline trend by divergence from the ambient tholeiitic trend. This is particularly important for determining the effect of crustal interactions during the early part of the liquid line of descent of the calc-alkaline trend that is not well understood. The calk-alkaline signature observed in the calc-alkaline suite of the Strawberry Volcanics is as strong as one observes in a very water-rich volcanic arc environment like Augustine volcano, Alaska and thus highlights why this study holds important implications not only for the understanding of orogenic andesites of non-subduction settings but also of those of volcanic arcs.