The Quartz Diorite Boundary Line in the Western United States

Compilation of geologic mapping and study of chemical analyses of Mesozoic and Cenozoic granitic rocks in the western United States show that there is a relation between the composition and the geographic distribution of the rocks. Rocks of the western zone are dominantly quartz diorite; rocks of the eastern zone are dominantly quartz monzonite and granodiorite. The line separating these two zones is called the "quartz diorite boundary line" and is traced on a map. The line crosses the Washington-Canadian border near the 120th meridian, strikes southeast through the western margin of the Idaho batholith, swings around to a southwest trend across eastern Oregon, strikes south through California, remaining in the western margin of the Sierra Nevada batholith. The contrasting compositions of granitic rocks in the two zones is believed to be inherited from contrasting compositional differences in the crust existing before emplacement of the granitic rocks.