Significance of Upper-Mantle Geochemical Signals of the Picture Gorge Basalt (PGM) Member in Light of an Extended Distribution and New Age Data Placing PGB at Onset of Columbia River Basalt Group Eruptions

Picture Gorge Basalt (PGB) lavas erupted from NNW trending feeder dikes of the Monument Dike swarm, eastern Oregon. Lavas with PGB composition, recently geochemically correlated to the original outcrop area around the type locality at Picture Gorge, nearly doubles the spatial distribution of the PGB. We report new, high-precision ⁴⁰Ar/³⁹Ar age dates on samples from well-known and newly-correlated PGB locations. Ages range between 17.23 ± 0.04 and 16.06 ± 0.14 Ma for this previously poorly dated CRBG unit. These dates indicate the PGB erupted earlier and longer than other CRBG main phase units, and place the onset of CRBG eruptions shortly before an increase in global temperatures that peaked in the Miocene Climatic Optimum (MCO).

Long known geochemical signatures of the PGB are typified by overall large-ion-lithophile element (LILE) enrichment and ⁸⁷Sr/⁸⁶Sr values on the low end of the range displayed by other CRBG lavas, but exhibit relative depletion of high-field strength elements (HFSE) typical of other CRBG main-phase units. The relative strong enrichment of LILE and depletion of HFSE has suggested a metasomatized upper mantle as the primary magmatic source for the PGB. Early geochemical modeling of PGB utilized the composition of two high-MgO primitive dikes exposed in the northern portion of the Monument Dike Swarm as parental melt. However, these dikes are distinct in geochemical space from any known PGB flow, and we question their petrogenetic connection to PGB.

Our data reveals that PGB lavas erupted first and throughout eruptions of main phase CRBG units (Steens, Imnaha, Grande Ronde Basalt). PGB volcanism occurred in two pulses, demonstrated by a ~0.4 Ma temporal gap in our reported ages, 16.62 to 16.23 Ma. Earlier ages (older than 16.62 Ma) have almost exclusively lower Mg#'s and elevated Ba/Nb ratios. Coupling our ages with strong relative elemental abundances of Ba, Sr, and K within PGB among other CRBG units, indicates shallowest metasomatized mantle was erupted first with later CRBG members (Imnaha, Steens) showing increased influence of a deeper, potentially plume-like source.