A DEFORMED GRANITIC PLUTON IN THE SOUTHERN PRIEST RIVER COMPLEX, SPOKANE VALLEY, WASHINGTON

The pluton being studied is located in the northeastern corner of the Dishman Hills Natural Area in Spokane Valley, WA. This small pluton is composed of a moderately foliated equigranular granite, and has surface exposures of approximately 0.5 mi by 0.1 mi. The rock’s dominant mineral assemblage includes; plagioclase, orthoclase, quartz, biotite, with accessory sphene, apatite, zircon, monazite, and opaque minerals. Rare muscovite was observed in thin section.

The foliation averages 172˚ at 40˚ SW. Characteristic features of the pluton are oblong schlieren, which are thin (less than 3 cm), biotite-rich, tabular accumulations (10 cm to 2 m in length) that occur within the foliation. Outcrops exhibit two distinct sets of lineations. The first set is defined by the elongated schlieren and have an average plunge and azimuth of 31˚ at 252˚. The second set of lineations is defined by rod-like quartz bands that have an average plunge and azimuth of 65˚ at 256˚. The steeper quartz lineations crosscut the biotite lineations. Thin sections reveal fine grain polygranular quartz, with lobate grain boundaries, suggesting variable degrees of recrystallization.

Geochemical plots of the major element oxides, indicate that the pluton is a calc-alkaline granite. Based on the geochemistry, this pluton compares favorably to the Rathdrum Mountain Granite (dated at 47 Ma), located approximately 10 miles NE, and to granitic dikes found at Tubbs Hill (dated at 50 Ma), located in Coeur d’Alene, ID (24 miles E).

From this study, the pluton appears to correlate with the Eocene-aged Rathdrum Mountain Granite, which intruded during development of the core complex. The pluton exhibits two distinct sets of lineations; early deformation is defined by lineated biotite schlieren, and late deformation is defined by cross-cutting, quartz-rich lineations.