PICTURE GORGE BASALT (PGB) VERSUS PGB-LIKE BASALT: A STATISTICAL AND ELEMENTAL EVALUATION

The Picture Gorge Basalt subgroup (PGB) was previously interpreted as a member of the Grande Ronde Basalt (GRB) group of the Columbia River Basalt Group (CRBG). Initially inferred from field observations, lavas of the PGB and lavas of the GRB were thought to have erupted concurrently. However high-precision 40Ar/39Ar geochronology has refined the eruptive timing of the PGB, with ages ranging from 17.23 to 15.76 Ma. New age data illustrate that the PGB is the earliest and longest eruptive unit compared to other main-phase CRBG formations, and more widely distributed than previously alleged (up to 100km).

This study examines geochemical characteristics of PGB lavas and dikes outside the original distribution area in south-eastern Oregon near the towns of Brogan and Huntington, OR,. Samples collected in the greater Brogan area were geochemically correlated to be PGB when compared to all other main-phase units. PGB basalts show comparable SiO₂ wt.% ranges of 49–53 wt.% to the Steens and Imnaha Basalts, PGB contains lower concentrations of Th, high field strength elements (HFSEs), light rare earth elements (LREEs) and Zr/Y values.

Geochemical correlation of these new samples as PGB suggests (1) a new distal eruption site away from the type locality for PGB (2) expands the distribution of PGB eruptive products and (3) brings to light the question of PGB like vs. PGB basalts. We use statistical analysis to compare the geochemistry of basaltic samples collected from the extended distribution (this study) to PGB samples from the type locality. This provides clues as to elements that are indicators of PGB and elements that are indicators of PGB-like basalts, and how these vary with distribution across eastern Oregon. In context with their ages, these geochemical signatures demonstrate a PGB-like mantle component influenced eruptive products across a wider region (not spatially localized).