Abstract

Strontium isotope and major- and trace-element data from the voluminous middle Miocene tholeiitic basalts of the Columbia River Group differentiate the lavas stratigraphically and reflect the origin of the basalt. The Picture Gorge Basalt has relatively low Sr87/Sr86 ratios (0.7035 to 0.7039) and a major- and trace-element composition consistent with an origin from the upper mantle. The Picture Gorge Basalt is clearly distinguished from the Imnaha basalt in northeast Oregon and from the widespread, mainly younger Yakima Basalt. The Yakima Basalt, including lava flows in the Vantage, Grande Ronde, and Imnaha regions, forms a relatively coherent geochemical group with moderate to high initial Sr87/Sr86 ratios (0.7045 to 0.7080), which increase with decreasing age of the lava and show an inverse relation with Sr abundance. Possible explanations include progressive contamination of parental mantle—derived magmas by crustal material or derivation from an inhomogeneous upper mantle.

It is suggested that the Yakima Basalt formed during a major readjustment episode related to plate motions in an extensional tectonic regime. This caused diapiric upwelling of upper mantle peridotite with formation of great volumes of olivine tholeiite magma, which underwent extensive olivine fractionation. The Columbia Plateau may be a continental analogue of the marginal seas of the western Pacific Ocean.

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