ABSTRACT The Miocene Columbia River Basalt Group (CRBG) is the youngest and smallest continental flood basalt province on Earth. This flood basalt province is a succession of compositionally diverse volcanic rocks that record the passage of the Yellowstone plume beneath eastern Oregon. The compositionally and texturally varied suite of volcanic rocks are considered part of the La Grande–Owyhee eruptive axis (LOEA), an ~300-km-long, north-northwest–trending, Middle Miocene to Pliocene volcanic belt that extends along the eastern margin of the Columbia River flood basalt province. Volcanic rocks erupted from and preserved within the LOEA form an important regional stratigraphic link between the flood basalt–dominated Columbia Plateau to the north, the north and bimodal basalt-rhyolite volcanic fields of the Snake River Plain to the east, the Owyhee Plateau to the south, and the High Lava Plains to the south and east; the latter two have time transgressive rhyolite centers that young to the east and west, respectively. This field-trip guide details a four-day geologic excursion that will explore the stratigraphic and geochemical relationships among mafic rocks of the CRBG and coeval and compositionally diverse silicic rocks associated with the early trace of the Yellowstone plume and High Lava Plains in eastern Oregon. The trip on Day 1 begins in Portland then traverses across the western axis of the Blue Mountains, highlighting exposures of the widespread, Middle Miocene Dinner Creek Welded Tuff and aspects of the Picture Gorge Basalt lava flows and northwest-striking feeder dikes situated in the central part of the CRBG province. Travel on Day 2 progresses eastward toward the eastern margin of the LOEA, examining a transition linking the Columbia River Basalt province with a northwestward-younging magmatic trend of silicic volcanism of the High Lava Plains in eastern Oregon. Initial field stops on Day 2 focus on the volcanic stratigraphy northeast of the town of Burns, which includes regionally extensive Middle to Late Miocene ash-flow tuffs and lava flows assigned to the Strawberry Volcanics. Subsequent stops on Day 2 examine key outcrops demonstrating the intercalated nature of Middle Miocene tholeiitic CRBG flood basalts, temporally coeval prominent ash-flow tuffs, and “Snake River–type” large-volume rhyolite lava flows cropping out along the Malheur River. The Day 3 field route navigates to southern parts of the LOEA, where CRBG rocks are associated in space and time with lesser known and more complex silicic volcanic stratigraphy forming Middle Miocene, large-volume, bimodal basalt-rhyolite vent complexes. Key stops will provide a broad overview of the structure and stratigraphy of the Middle Miocene Mahogany Mountain caldera and of the significance of intercalated sedimentary beds and Middle to Late Miocene calc-alkaline lava flows of the Owyhee basalt. Initial stops on Day 4 will highlight exposures of Middle to Late Miocene silicic ash-flow tuffs, rhyolite domes, and calc-alkaline lava flows overlying the CRBG across the northern and central parts of the LOEA. The later stops on Day 4 examine more silicic lava flows and breccias that are overlain by early CRBG-related rhyolite eruptions. The return route to Portland on Day 4 traverses the Columbia River gorge westward from Baker City. The return route between Baker and Portland on Day 4 follows the Columbia River gorge and passes prominent basalt outcrops of large volume tholeiitic flood lavas of the Grande Ronde, Wanapum, and Saddle Mountains Formations of the CRBG. These sequences of basaltic and basaltic andesite lavas are typical of the well-studied flood basalt dominated Columbia Plateau, and interbedded silicic and calc-alkaline lavas are conspicuously absent. Correlation between the far-traveled CRBG lavas and calcalkaline and silicic lavas considered during the excursion relies on geochemical fingerprinting and dating of the mafic flows and dating of sparse intercalated ashes.

ABSTRACT The mid-Miocene Strawberry volcanic field of northeastern Oregon is an example of intracontinental flood volcanism that produced lavas of both tholeiitic and calcalkaline compositions derived by open-system processes. Until now, these dominantly calc-alkaline lavas have not been considered to have a petrogenetic origin similar to that of the flood basalts of the Pacific Northwest because of their calc-alkaline composition. These lavas are situated in between and co-erupted with the dominant volcanic field of the Columbia River Basalt Group (CRBG). Due to the timing, location, and diversity of these erupted units, the Strawberry Volcanics may hold valuable information about the role of crustal modification during large magmatic events such as hotspot volcanism. The earliest eruptions of the Strawberry Volcanics began at 16.2 Ma and appear continuous to 15.3 Ma, characterized by low-silica rhyolite. High-silica, A-type rhyolite eruptions followed at 15.3 Ma. The silicic eruptions continued until 14.6 Ma, with an estimated total volume up to ~100 km 3 . The first eruptions of the intermediate lava flows occurred at 15.6 Ma and continued with both tholeiitic and calc-alkaline, and transitional, lavas until 12.5 Ma. Volume estimates of the intermediate lavas are ~1100 km 3 . The mafic lavas are sparse (~2% of total volume) and are distributed throughout the upper sequences, and they appear to be near last to arrive at the surface. Herein, we show that the Strawberry Volcanics are not only related in time and space to the Columbia River Basalt, but they also share some chemical traits, specifically to the Steens Basalt. Evidence of this similarity includes: overlapping normalized incompatible trace-element patterns, selected trace-element ratios, and radiogenic isotopes. Furthermore, we compared the Strawberry rhyolites to the other mid-Miocene rhyolites of eastern Oregon associated with the inception of the Yellowstone–Snake River Plain hotspot and found overlapping eruption ages, trace and rare earth element compositions, and “A-type” rhyolite characteristics. This research concludes that the Strawberry Volcanics were part of the regional basalt to rhyolite magmatism of the Yellowstone–Snake River Plain hotspot.

Abstract The widely distributed, mid-Miocene lavas of the Strawberry Volcanics of NE Oregon are compositionally diverse, ranging from basalt to rhyolite. They are composed mainly of calc-alkaline and mildly tholeiitic basaltic andesite and andesite. Ar–Ar dating and stratigraphic relationships indicate that the volcanic field was active from >16 Ma to c. 12 Ma ago and thus is coeval for the first 1–2 Ma with strongly tholeiitic flood basalts of the Columbia River Province that encircle the Strawberry Volcanics. Tholeiitic and calc-alkaline compositions develop subtle but noticeable differences towards higher silica contents. At silica contents of <55 wt% SiO 2 , calc-alkaline and tholeiitic lavas are essentially indistinguishable. Trace element constraints among Strawberry Volcanics and crustal rocks indicate that open-system processes, such as assimilation or magma mixing, are responsible for evolution along a calc-alkaline trend leading to ‘orogenic’ andesites from tholeiite. Exclusively tholeiitic basalts carrying evidence for a metasomatized mantle source erupted during the mid-Miocene of eastern Oregon. Consequently, tholeiite imparted the ‘subduction signals’ and crustal processing generated the calc-alkaline character to end up with compositions of typical ‘orogenic’ andesites at the Strawberry Volcanics. No primitive calc-alkaline basalt from the mantle is needed as parental magma here and possibly at other similar intra-continental calc-alkaline suites.

ABSTRACT The John Day Formation of central and eastern Oregon, contains a widespread assemblage of both ash-flow and airfall tuffs, yet only a few corresponding caldera sources have been identified in the region. Investigators have long speculated on the sources of tuffs in the John Day Formation and have suggested that these pyroclastic rocks were vented from now buried eruptive centers in or marginal to a nascent Cascade Range. Recent detailed geologic mapping in the John Day and Clarno Formations, however, indicates the presence of at least three large-scale rhyolite caldera complexes centered along the northeast-trending axis of the Blue Mountains. This field guide describes a three-day geologic transect, from the scenic high desert of central Oregon eastward across the axis of the Blue Mountains, that will examine the physical volcanology and geologic setting of the 41.50-39.35 Ma Wildcat Mountain caldera exposed along the crest of the Ochoco Mountains, the 29.56 Ma Crooked River caldera at Prineville, and the 29.8 to 28.1 Ma Tower Mountain caldera near Ukiah.

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.