Sixteen mafic and intermediate lava flows of Eocene to Pleistocene age from the northwestern Great Basin have initial Sr⁸⁷/Sr⁸⁶ ratios of from 0.7029 to 0.7047. Seven upper Miocene mafic and intermediate lava flows have initial ratios of from 0.7037 to 0.7041, suggesting a common source for the Steens Basalt and contemporaneous rocks of the northwestern Great Basin. Values of 0.7047 and 0.7033 obtained on olivine basalts of early Miocene and Quaternary ages, respectively, suggest that these lavas were derived from different source materials than were the late Miocene rocks. Unusually low values of 0.7029 obtained on two specimens of andesite from the Eocene Cedarville Series of Russell (1928) suggest that these lava flows were derived from still another source material.

Ten silicic volcanic rocks, most from widespread and voluminous ash-flow sheets and lava complexes, have initial ratios of approximately 0.7023 to 0.7057. Some of the more radiogenic values, obtained on very highly differentiated and strontium-poor rock units, provide only an upper limit for the initial Sr⁸⁷/Sr⁸⁶ ratios of the parent magmas. The close similiarity of the strontium isotopic composition of the silicic rocks to those of spatially and temporally associated intermediate lavas supports the concept that the widespread Miocene silicic volcanic rocks of the northwestern Great Basin were produced by the fractional crystallization of enormous volumes of mantle-derived mafic magma. The data also provide indirect support for the interpretation that the relatively radiogenic character of many salic volcanic rocks from other parts of the Great Basin largely reflects the presence of an unusually radiogenic mantle rather than the involvement of crustal material.