Abstract

Variation in chemical composition within flows of the Columbia River Basalt Group (CRBG) is typically small, but one flow, the Huntzinger flow, exhibits a large intraflow compositional range. The composition varies both vertically and laterally, but mass-balance calculations for both major/minor oxides and trace elements show that the variation can be explained by simple binary mixing between two end members. These end-member compositions are identical to two Saddle Mountains Basalt flows, the basalt of Lapwai (Wilbur Creek Member) and the Asotin Member.

Primary intraflow features of the Huntzinger flow are highly variable, which is atypical for most CRBG flows. In addition to common features such as a scoriaceous flow top, an entablature, and a colonnade, the flow exhibits evidence of invasion into sediments, pillow-hyaloclastic zones, ophimottling, and veins. Ophimottling and veins can be directly related to compositional variations; veins occur in zones of mixing between the two end members, and ophimottling is confined to zones dominated by the Asotin composition.

The Huntzinger flow is dominated by clinopyroxene and plagioclase, with lesser amounts of olivine and variable amounts of glass. The mineralogies of the two end-member compositions are similar to those of the Asotin and Lapwai flows from the Clear-water embayment. The intermediate compositions of the Huntzinger flow contain characteristics of both flows, suggesting mixing of the two.

The compositional variation indicates that mixing increases from the eastern part of the Pasco Basin, where the Asotin composition occurs in the center of the flow and is surrounded by Lapwai, to the western side, where there is almost complete mixing of the Lapwai and Asotin. This pattern is analogous to that produced when two flows mix by free turbulent flow from a free jet. Based upon this analogy, a flow emplacement model is proposed in which the basalt of Lapwai is initially emplaced in the Pasco Basin and, prior to solidification, is invaded by and mixed with the Asotin lava.

This model places several petrogenetic constraints on this interval of the Saddle Mountains Basalt. The close spatial and temporal association of the Asotin Member and basalt of Lapwai with yet a third flow, the Wahluke flow (Wilbur Creek Member), allows the rate of eruption to be estimated and constrains the rate of magma recharge to a storage chamber. The role of crustal contamination can also be evaluated from this model. The conditions required for surface mixing suggest that this may not have been a unique occurrence and that other CRBG flows may be composite flows resulting from mixing.

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