Nd-Sr isotopic geochemistry and tectonics of ridge subduction and middle Cenozoic volcanism in western California Ronald B. Cole* } Department of Geological Sciences, University of Rochester, Rochester, New York 14627 Asish R. Basu ABSTRACT Episodes of middle Cenozoic near-trench volcanism in California occurred during the transition from convergent to transform plate boundaries as segments of the East Pacific Rise intersected a subduction zone along western North America. Geochemical features of volcanic rocks from the Coast Range Province and Santa Maria Province, which represent two near-trench volcanic episodes, indicate that magmas from each province were derived from depleted mantle and evolved by assimilation-fractional crys- tallization processes to form predominantly bimodal suites. Basalt and basaltic andesite from both provinces yielded  Nd (t) values between 9.3 and 2.4 and 87 Sr/ 86 Sr(t) ra- tios of 0.702 58 – 0.706 72. The observed  Nd (t) values that cluster around 9 and the 87 Sr/ 86 Sr(t) ratios <0.7029 imply a source of depleted mantle, analogous to mid-ocean-ridge basalt (MORB) sources, for these rocks. Th/Ta and Ba/Ta ratios as low as 0.49 and 35.78, respectively, for the basalt are similar to those of MORB and also suggest a magma source from depleted mantle. Acidic rocks, including rhyolite, da- cite, and trachyte samples have  Nd (t) val- ues between 6.3 and 3.2 and 87 Sr/ 86 Sr(t) ratios of 0.703 93 to 0.711 31. The variation among the Coast Range and Santa Maria Provinces volcanic rocks in Nd-Sr isotope ratio space suggests that mixing occurred between the depleted mantle-derived basal- tic end-member and an incompatible-ele- ment– enriched crustal reservoir through which these rocks erupted. The observed negative correlation of  Nd (t) and positive correlation of 87 Sr/ 86 Sr(t) ratios with SiO 2 , respectively, also suggest assimilation of an isotopically distinct crustal component by depleted mantle-derived melts. The ages and paleogeographic distributions of these volcanic rocks indicate that they were erupted during episodes when segments of the East Pacific Rise intersected southern California. Depleted mantle that was em- placed beneath the continental margin dur- ing ridge subduction became a source of magma for the episodes of near-trench vol- canism as a new strike-slip regime evolved along the continental margin. INTRODUCTION The nature of volcanism in California changed markedly during the middle Ceno- zoic Era when segments of the East Pacific Rise approached and intersected a subduc- tion zone along western North America (Fig. 1) (Atwater, 1970, 1989; Lipman et al., 1972; Christiansen and Lipman, 1972). Prior to early Tertiary time, a belt of calc-alkalic Andean-type magmatism characterized eastern California along the Sierra-Nevada arc during subduction of the Farallon plate beneath North America (Lipman et al., 1972; Christiansen and Lipman, 1972; Sny- der et al., 1976). As parts of the Farallon plate were consumed at the subduction zone and the East Pacific Rise approached west- ern North America, increasingly younger oceanic lithosphere was subducted (At- water, 1970; Dickinson and Snyder, 1979a; Severinghaus and Atwater, 1990). Subduc- tion of young, buoyant oceanic crust and the ridge-trench interactions that ensued re- sulted in the termination of subduction, de- velopment of a no-slab region beneath the continental margin, cessation of Sierran arc magmatism, and the transition from a con- vergent to a transform plate boundary along western North America (Atwater, 1970, 1989; Dickinson and Snyder, 1979a, 1979b; Severinghaus and Atwater, 1990). Magma- tism in California during this tectonic tran- *Present Address: Department of Geology, Al- legheny College, Meadville, Pennsylvania 16335. Figure 1. Model for middle Tertiary North American (NA)–Pacific (PA)–Farallon (FA) plate interactions. From Atwater (1970, 1989) and Severinghaus and At- water (1990). Arrows show relative motion be- tween Pacific and North American plates along the San Andreas trans- form boundary. EPR  East Pacific Rise; MTJ  Mendocino triple junc- tion; RTJ  Rivera triple junction. GSA Bulletin; February 1995; v. 107; no. 2; p. 167–179; 9 figures; 4 tables. 167