CRYSTAL-MUSH COMPACTION AND DIFFERENTIATION IN THE COHASSETT FLOOD-BASALT FLOW, HANFORD, WASHINGTON

The Columbia River Cohassett flood-basalt flow has complex chemical profiles in the central Columbia Basin that suggest it was formed by inflation, with the earlier top and bottom of the flow having compositions similar to the underlying TiO2-rich McCoy Canyon flow and the later central part having a composition similar to that of the overlying TiO2-poor Rocky Coulee-Museum flow. Considerable mixing of these magmas took place. In the flow’s central part, however, the profiles were also modified when residual liquid was expelled upward by compaction of the crystal mush. The central part of the flow consequently has S-shaped profiles of compatible elements and Z-shaped profiles of incompatible elements.

Textures in the flow’s central part preserve a clear record of compaction. Most crystallization occurred rapidly in the roof zone from which plumes of dense crystal mush sank to the floor. As mush accumulated on the floor, the rapidly grown pyroxene crystals from above recrystallized into fine-grained granular aggregates of augite and pigeonite. With compaction, these granular patches flattened, and plagioclase phenocrysts were rotated toward the horizontal, many becoming bent or broken. Groundmass plagioclase laths surrounding pyroxene crystals in the roof zone are randomly oriented and are separated by large volumes of glass. In the compaction zone, however, these same laths surrounding granular patches of pyroxene rotated into clusters of tightly packed parallel crystals, many in an imbricate pattern, and by so doing reduced the percentage of interstitial liquid. A striking compaction texture, for which we propose the name "lintel" texture, is formed where groundmass plagioclase laths have packed down on horizontal plagioclase phenocrysts (the lintel). Immediately beneath such phenocrysts, the groundmass laths tend to be more randomly and openly stacked, with considerable quantities of interstitial residual liquid that was prevented from rising past the phenocryst cap. This asymmetric texture is clearly visible in vertically oriented thin sections. Quantitative measures of the textural anisotropy indicate that compaction reached a 35% maximum where the incompatible element concentrations in the flow reached a minimum.