ABSTRACT A large part of the northwestern United States has undergone extensive late Cenozoic magmatic activity yielding one of the great continental volcanic provinces on Earth. Within this broader area lies the High Lava Plains province, the focus of this field guide. For our purposes, the High Lava Plains is a middle and late Cenozoic volcanic upland, contiguous with and gradational into the Basin and Range province to the south. The High Lava Plains province of southeastern Oregon is characterized by thin, widespread Miocene-Pleistocene lava flows of primitive basalt and a belt of silicic eruptive centers. The rhyolitic rocks generally are successively younger to the northwest, describing a mirror image to the basalt plateau and rhyolite age progression of the Snake River Plain. The High Lava Plains is associated with a zone of numerous, small northwest-striking faults and lies at the northern limit of major Basin and Range normal faults. The abundant late Cenozoic bimodal volcanism occupies an enigmatic intracontinental tectonic setting affected by Cascadia subduction, Basin and Range extension, the Yellowstone plume, and lithospheric topography at the edge of the North American craton. The purpose of this field trip is to focus on the late Cenozoic lithospheric evolution of this region, through the lens of the High Lava Plains, by considering structural, geophysical, petrologic, and temporal perspectives. A grand tour southeast from Bend to Valley Falls, north to Burns, and then east to Venator, Oregon, takes participants from the eastern edge of the Cascade volcanic arc, across several basins and ranges in eastern Oregon, and onto the volcanic plateau of the High Lava Plains. Day 1 provides an overview of Newberry Volcano and the western edge of Basin and Range, including the Ana River and Summer Lake fault zones. On Day 2, the early magmatic and extensional history of the region is explored along the Abert Rim range-front fault. Participants are introduced to the bimodal volcanism within the High Lava Plains, with focus on the Harney Basin and Rattlesnake ignimbrite event. An evening session will highlight geophysical results from the High Lava Plains, including new data from one of the largest active-source seismic experiments to be conducted in North America. Day 3 activities examine early bimodal volcanic history of the eastern High Lava Plains and the late Miocene and Pliocene subsidence history on the east edge of the Harney Basin east of Burns, Oregon.

The White Mountain granitoid suite represents an isolated window into Cretaceous age magma intruded into Wrangellia terrane basement. Although the total area of exposed granitoid at White Mountain is relatively small (∼1 km 2 ), substantial textural, chemical, and isotopic complexities exist. The granitoid suite consists of six surficially isolated bodies, all of which are calc-alkaline and metaluminous, ranging in composition from hornblendebiotite quartz diorite to biotite granodiorite. Three 40 Ar/ 39 Ar analyses provide cooling ages between 113.3 ± 1.3 and 117.38 ± 0.54 Ma, suggesting at least two pulses of magmatism are represented in the granitoid suite. Two of the bodies, comprising ∼20% of the total exposed granitoid, are enclave-bearing, with the hosts representing the most chemically evolved material at White Mountain and the enclaves among the least evolved. The enclaves typically are <15 cm in size and circular to oval in shape, are dominated by plagioclase and amphibole, and are intermediate in composition (∼54 wt% SiO 2 ). Enclave rare-earth element patterns and isotopic characteristics, and the lack of petrographic evidence for quenched margins, suggest that they are cumulates from liquids chemically similar to but isotopically distinct from their host materials. One granitoid hand specimen exhibits textural and geochemical evidence for mixing at the low MgO end of the compositional spectrum. Although the granitoid suite exhibits a narrow range in whole rock isotopic compositions (ε Nd(115 Ma) 7.2–9.1 and 87 Sr/ 86 Sr (i) 0.7032–0.7043) further suggesting open system differentiation, these compositions do not require an appreciable role for ancient, evolved continental lithosphere in the White Mountain magmatic system(s). Rather, the dominant source reservoir was depleted mantle. This conclusion provides evidence that mid-Cretaceous magmatism in this region was generated in either an intraoceanic island arc or an “immature” (proto-) continental arc tectonic setting.

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Field, petrographic, geochemical, and limited chronologic information allow for construction of a composite stratigraphic section for the approximately 1-km-thick exposure of basaltic lavas in the Pueblo Mountains region of the Oregon Plateau. Comparison of the Pueblo basalts with those of Steens Mountain and the Columbia Plateau suggests that these flood-basalts were derived from a common mid-Miocene magmatic event, possibly marking the initiation of back-arc spreading along the western margin of the Wyoming craton. Geochemical and Sr-isotopic variations within the Pueblo basalts and, more specifically, vertical variations within the Pueblo composite section, define two distinct groups of basalt and three phases of evolution. Within-group variations are modeled by low-pressure fractional crystallization, whereas the between-group variations are attributed primarily to different degrees of interaction between crustal materials and ascending Pueblo primary magmas during the three distinct phases of volcanism.