Chemical and mineralogic variations within four dikes of the Columbia River Basalt Group, southeastern Columbia Plateau

Abstract

Four dikes within the western portion of the Chief Joseph dike swarm of the Mio-Pliocene Columbia River Basalt Group show systematic major-element, mineralogic, and textural trends across their widths. Two of the dikes are of Wanapum Basalt (Dodge chemical type) and served as source vents for two flows of Dodge chemistry and petrography that are exposed in the study area. A third dike of Saddle Mountains Basalt served as a vent for the Wenaha flow (Elephant Mountain chemical type), also present in the area. Two additional dikes of probable Saddle Mountains Basalt cannot be correlated unambiguously with any flows exposed in the area, but one of them closely resembles the Buford flow chemically and petrographically. The chemical variations across all four dikes indicate that the chilled dike margins are slightly enriched in felsic components and incompatible elements (SiO 2 , P 2 O 5 , TiO 2 , and K 2 O) and depleted in MgO and FeO compared to dike interiors. Most of these chemical trends can be related to the marked enrichment in glass at chilled dike margins compared to their more crystalline interiors. Microprobe analyses of glasses from chilled dike margins, when compared to whole-rock analyses from dike interiors, agree with the above-described trends for SiO 2 , TiO 2 , MgO, and probably K 2 O. FeO is also concentrated in the three glasses analyzed, producing an opposing and stronger trend (increase) than shown by the whole-rock data. This probably reflects the greater concentration of opaque oxides within the marginal tachylite compared to the more crystalline dike interiors, which lack tachylite. This variation in glass content is, of course, controlled by the rapid quenching of magma against the relatively cold country rock and the progressively decreasing cooling rate toward the interior of a dike as intrusion continues. In addition to variations in crystallinity, fractionation and/or crustal contamination may also have contributed to or modified these major-element trends. Mineralogic trends inward from dike margins are from mafic-to-felsic, with plagioclase anorthite content, and perhaps the Ca content of augite, decreasing. Orthopyroxene (hypersthene-bronzite) is present only in the chilled margin of the Wenaha dike and exhibits well-developed reaction rims of augite. These mafic-to-felsic mineralogic trends were superposed on the felsic-to-mafic chemical trends.