Small isolated late Cenozoic basaltic lava flows, dikes, and plugs are widely distributed on the west slope of the Sierra Nevada (Figs. 1 and 2). More voluminous basalt, commonly associated with rhyolite, is present along the east escarpment of the Sierra Nevada and farther east within the Basin and Range Province. All of these basaltic rocks are alkalic in composition. The most alkalic volcanic rocks, which predominate west of the Sierra Nevada crest, are highly potassic and include leucite-bearing varieties. The recent discovery of leucite-bearing rocks at nine new localities demonstrates that this region, principally the west slope of the Sierra Nevada, is an important province of mafic, potassic-alkalic magmatism.
The presence of potassium-rich volcanic rocks in the Sierra Nevada was documented by Nockolds and Allen (1954). Hamilton and Neuerburg (1956) described potassic lavas in the Huntington Lake area and attributed their origin to assimilation of the granitic rocks. Moore (1962) demonstrated that this potassic suite is restricted to a region of low Bouguer gravity and thick crust. Nash and Nelson (1964) and Tidy (1970) described leucite-bearing rocks from Deep Spring Valley, and Peck (1976) reported similar rocks in the San Joaquin River drainage of the western Sierra Nevada. Van Kooten and Peck (1977) and Van Kooten (1980) suggested that the potassic lavas could have been produced by deep-seated incongruent melting of a phlogopite-bearing ultramafic source. Moore (1978) mapped small occurrences of leucite phonolite in the Marion Peak quadrangle, and Moore and Dodge (1977) described potassic lavas in the Kings River drainage of the western Sierra Nevada and compared them to other occurrences within the Sierra Nevada.
In this study, we describe the geographic distribution and composition of volcanic rocks of the southern Sierra Nevada and the extreme western Basin and Range Province and explore the origin of the highly potassic basalt. We also examine the petrology and distribution of rock types and, in a companion paper (Dodge and Moore, in prep.), we consider the variations in their major and minor chemical constituents.