Investigation of Basal Imnaha Basalt Flows and their Relationship to the Picture Gorge Basalt of the Columbia River Basalt Group

Abstract

The lower American Bar flows (AB 1 and 2) of the Imnaha Basalt have previously been observed as being chemically similar to those of the Picture Gorge Basalt (PGB). Previous age data separate eruption of the Imnaha and Picture Gorge Basalts by approximately 1.5 Ma, precluding the possibility of a genetic link between the two basalt units. New age data for the Picture Gorge Basalt has expanded the eruptive timeframe and the earliest flows coincide with the eruption of the Imnaha Basalt, making it possible that the lower American Bar flows of the Imnaha Basalt and the Picture Gorge Basalt may be more closely related than previously thought. This research focuses on expanding Imnaha Basalt chemical dataset by sampling basalt exposures in Hells Canyon of northeastern OR and northwestern ID. A total of forty-eight samples of the Imnaha Basalt were analyzed and classified. Basal American Bar samples were compared to the overlying American Bar flows and the Picture Gorge Basalt. Magma evolution models suggest that fractionation alone cannot derive the main American Bar flows from the basal flows, suggesting that two different sources must have been tapped to produce the two chemical types. The expanded chemical dataset corroborates the findings of previous researchers in that the lowest flows, American Bar flows 1 and 2, are very similar to the PGB, especially in their incompatible element concentrations. In line with observations from other researchers, AB flows 1 and 2 from south of the Wallowa Mountains possess some chemical characteristics different from those in the northern part of Hells Canyon in that they are more enriched in Ba and depleted in Nb along with being more primitive in their major element compositions.