Z. E. Peterman R. G. Coleman E. D. Ghent 1973 <p>Six samples of group C eclogites from California and&nbsp;southwestern Oregon have initial Sr⁸⁷/Sr⁸⁶ ratios in the range of&nbsp;0.7028 to 0.7051; Rb contents from less than 1 to 53.4 ppm; and Sr&nbsp;contents from 147 to 270 ppm. These data and major-element&nbsp;compositions suggest that the eclogites were derived from basalts older&nbsp;than but similar in composition to those intercalated with Mesozoic&nbsp;cugeosynclinal sedimentary rocks in California and western Oregon.&nbsp;The isotopic heterogeneity of the eclogites and associated graywackes&nbsp;contrasts with a uniformity of Sr⁸⁷/Sr⁸⁶ in many younger calc-alkaline&nbsp;volcanic rocks of the circum-Pacific province. Derivation of the younger&nbsp;volcanic rocks from an eclogite-blueschist melange isotopically similar&nbsp;to those investigated here would require isotopic homogenization or&nbsp;integrated partial melting over sufficient volumes so as to reduce these&nbsp;variations in the derived melts.</p> application/pdf en U.S. Geological Survey Sr87/Sr86, K, Na, Rb, and Sr in some eclogites and associated basalts from California and southwestern Oregon article