K/Na ratio of Cenozoic igneous rocks of the western United States

The potassium and sodium content of chemically analysed Cenozoic igneous rocks from about 150 areas of the western United States has been examined. For each area a plot of the molecular proportion K2O(K2O + Na2O)">K₂O(K₂O + Na₂O) [Niggli's k-value] is shown, and the projected k-value determined at 50 and 60 weight per cent SiO₂. The k-values are plotted and contoured on maps of the western United States.

These maps show that potassium is least abundant relative to total alkali (when rocks of the same SiO₂ content are compared) in a zone along the Pacific Coast, becomes more abundant eastward, and is highest in the Colorado Plateau and Northern Rocky Mountains.

These k-value variations can be related to regional variations in the abundance of certain trace elements and of different types of older granitic rocks, and to Bouguer gravity maps. This correspondence indicates that the alkali ratio of Cenozoic igneous rocks is closely related to the character of the crust where the rocks are formed.