Late Cenozoic Alkali-Rich Basalt from the Western Grand Canyon Area, Utah and Arizona: Isotopic Composition of Strontium

Twenty samples of Pliocene-Holocene basanite, hawaiite, and quartz-bearing basaltic andesite from the western Grand Canyon district have Sr⁸⁷/Sr⁸⁶ ratios that range from 0.7028 to 0.7069. Sixteen of these are lava flows that have Sr⁸⁷/Sr⁸⁶ (0.7028 to 0.7041), K/Rb (500 to 790), and Rb/Sr (0.016 to 0.047) ratios, which suggest a probable mantle origin. These data are typical of many late Cenozoic basaltic flows from the southern Great Basin and southern Colorado Plateau. In the remaining four Grand Canyon samples (3 hawaiites, 1 basaltic andesite) Sr⁸⁷/Sr⁸⁶, K/Rb, and Sr concentrations are suggestive of possible crustal contamination. Correlation coefficients for chemical and isotopic compositions of the Grand Canyon basalt are significant at the 95 percent level for covariances between Sr⁸⁷/Sr⁸⁶ and only TiO₂, Rb/Sr, and Sr. The latter two correlations are mainly influenced by data for the four possibly contaminated basalt samples and are insignificant when those samples are omitted.

The isotopic and chemical data are compatible with the model proposed by Best and Brimhall (1974) in which the Grand Canyon basalt flows are derived by variable degrees of partial melting of mantle material over a range in depth and modified by polybaric crystal fractionation during ascent to the surface. Small variations in isotopic composition in the bulk of these samples probably reflect heterogeneities in their mantle source regions rather than the effects of crustal contamination. It is inferred that the mantle underlying the western Grand Canyon district is similar in its isotopic characteristics to typical oceanic-type mantle regions.