REVISITING COMPOSITION AND TIMING OF EARLIEST CRBG VOLCANISM AND CONTEXT TO LATER CRBG UNITS AS WELL AS TO ASSOCIATED SILICIC MAGMATISM

For the last 40 years, research contributions on the lava flows of the Columbia River Basalt Group by the honoree Vic Camp have made lasting impacts and have stimulated new research. Building on this work, one focus in our research has been to better understand the Picture Gorge Basalt (PGB) formation in relation to compositionally similar lavas within the Imnaha and the Steens Basalt. Another focus has been on the distribution, composition, and timing of abundant rhyolitic units of eastern Oregon that traditionally received less attention. Recently, our focus has turned to the timing and composition of dikes from the associated dike swarms.

Our new radiometric data along with literature data suggest the initial activity that formed basal lava flows of PGB, Imnaha, and Steens Basalt occurred around 17.1 (± 0.2) Ma. Onset of this province-wide activity is supported by waxing rhyolite activity from 17.5 to 16.4 Ma across the province within and around the three observed dike swarms. All of the earliest CRBG lavas, including the PGB, record a somewhat depleted and metasomatized mantle, yet compositional provinciality is expressed in trace element signatures (e.g., Zr/Y, La/Yb, Ba/Nb, Tb/Yb) and radiogenic isotopes, which in turn suggest that lavas were tapped relatively locally, and emplacement was not widespread enough to disrupt the observed regional variations at this eruption stage. While the PGB stratigraphy indicates a relatively narrow compositional array at a given higher MgO, the compositional stratigraphy of the Imnaha and Steens Basalt suggest reoccurrence of more depleted lavas at higher stratigraphic levels. The existence of crustal CRBG magma reservoirs beneath rhyolites seems inevitable, and hence, age and distribution of rhyolites suggest the following. After the waxing phase of rhyolite activity due to the thermal pulses of PGB, of Imnaha Basalt in the north, and of Steens Basalt magmas in the south, the largest number of rhyolite centers were active between 16.3–15.4 Ma, overlapping with and following the voluminous Grande Ronde Basalt and which may record the stage of greater coalescence of regional crustal reservoirs. The waning rhyolite phase from 15.3 to 14.5 Ma was accompanied with eruption of highly evolved icelandites starting at ~16 Ma, marking a temporary waning of more primitive mafic magmatism.