Primitive and contaminated basalts from the Southern Rocky Mountains, U.S.A
Basalts in the Southern Rocky Mountains province have been analyzed to determine if any of them are primitive. Alkali plagioclase xenocrysts armored with calcic plagioclase seem to be the best petrographic indicator of contamination. The next best indicator of contamination is quartz xenocrysts armored with clinopyroxene. On the rocks and the region studied, K 2 O apparently is the only major element with promise of separating primitive basalt from contaminated basalt inasmuch as it constitutes more than 1 % in all the obviously contaminated basalts. K 2 O: lead (> 4 ppm) and thorium (> 2 ppm) contents and Rb/Sr (> 0.035) are the most indicative of the trace elements studied. Using these criteria, three basalt samples are primitive (although one contains 1.7% K 2 O) and are similar in traceelement contents to Hawaiian and Eastern Honshu, Japan, primitive basalts. Contamination causes lead isotope ratios, 206 Pb/ 204 Pb and 208 Pb/ 204 Pb, to become less radiogenic, but it has little or no effect on 87 Sr/ 86 Sr. We interpret the effect on lead isotopes to be due to assimilation either of lower crustal granitic rocks, which contain 5–10 times as much lead as basalt and which have been low in U/Pb and Th/Pb since Precambrian times, or of upper crustal Precambrian or Paleozoic rocks, which have lost much of their radiogenic lead because of heating prior to assimilation. The lack of definite effects on strontium isotopes may be due to the lesser strontium contents of granitic crustal rocks relative to basaltic rocks coupled with lack of a large radiogenic enrichment in the crustal rocks. Lead isotope ratios were found to be less radiogenic in plagioclase separates from an obviously contaminated basalt than in the primitive basalts. The feldspar separate that is rich in sodic plagioclase xenocrysts was found to be similar to the whole-rock composition for 206 Pb/ 204 Pb and 208 Pb/ 204 Pb whereas a more dense fraction probably enriched in more calcic plagioclase phenocrysts is more similar to the primitive basalts in lead isotope ratios. The primitive basalts have: 206 Pb/ 204 Pb ∼ 18.09–18.34, 207 Pb/ 204 Pb ∼ 15.5, 208 Pb/ 204 Pb ∼ 37.6–37.9, 87 Sr/ 86 Sr ∼ 0.704–0.705. In the primitive basalts from the Southern Rocky Mountains the values of 206 Pb/ 204 Pb are similar to values reported by others for Hawaiian and eastern Honshu basalts and abyssal basalts, whereas 208 Pb/ 204 Pb tends to be equal to or a little less radiogenic than those from the oceanic localities. 87 Sr/ 86 Sr appears to be equal to or a little greater than those of the oceanic localities. These 206 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios are distinctly less radiogenic and 87 Sr/ 86 Sr values are about equal to those reported by others for volcanic islands on oceanic ridges and rises.