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

摘要

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.