Petrology and Chemistry of the Rock Creek Flow, Columbia River Basalt, Idaho

Abstract

A detailed re-examination of the Rock Creek flow, one of the thickest known pre-Yakima basalt flows of the Columbia Plateau, includes 5 wet-chemical and 55 rapid x-ray fluorescent analyses for major elements. The samples were collected at regular intervals from five sections through the flow over a distance of ~ 20 mi. A revised description of the flow9s field characteristics and petrography is included. The Rock Creek flow is typical of the pre-Yakima basalt in most of its field, petrographic, and chemical properties and is quite distinct in all these properties from the Yakima basalt. It differs from the Picture Gorge basalt type in its higher olivine, TiO 2 ; and K 2 O content, and is regarded as a member of a discrete Rock Creek basalt type which will form a valuable marker horizon in the southeast part of the plateau. In the Whitebird Creek section, the lower part of the Rock Creek flow cooling unit is formed from a different chemical type with a different mineralogy, similar to the Roza and Frenchman Springs chemical types as defined by Wright and others (1973). The mineralogy of the main part of the flow is typical of alkali basalt. Plagioclase and olivine were stable before eruption and were joined by a high-calcium pyroxene soon after eruption. Olivine continued to crystallize with the plagioclase and pyroxene, which were later joined by ilmenite and then apatite. Late chabazite and secondary analcite occur in the scoria. Lack of crystal settling and the high potassium content of the unusually thick scoriaceous top, which is locally replaced by a pyroclastic capping, may be associated with exceptionally vigorous degassing of the magma. Chemically, the Rock Creek flow is an olivine tholeiite, but its alkali basalt mineralogy makes either of these terms misleading. Together with many other basalts possessing a similar combination of mineralogy and chemistry, it is classified as a transitional basalt. The mineralogy of the residual material suggests a peralkaline composition.