Contrib Mineral Petrol (1991) 109:159 172 Contributions to Mineralogy and Petrology 9 Springer-Verlag1991 Basalt-Rhyolite Volcanism by MORB-Continental Crust Interaction: Nd, Sr-Isotopic and Geochemical Evidence from Southern San Joaquin Basin, California Mukul Sharma, Asish R. Basu, Ronald B. Cole, and Peter G. DeCelles Department of Geological Sciences, Universityof Rochester, Rochester, NY 14627, USA Received December 13, 1990/AcceptedJune 13, 1991 Abstract. The early Miocene Tecuya volcanic center in the southern San Joaquin basin of California consists of flows and tufts of basalt and rhyolite that erupted, closely spaced in time, in both submarine and subaeriat condi- tions. The rhyolites are overlain by the basalts and consti- tute approximately 45% of a total of at least 180 km 3 of the Tecuya volcanic rocks. The basalts have eNd(t) values of + 2 to + 6 and (878r/86Sr)i values between 0.7035 and 0.7052. These rocks show LREE enrichment [(La/Yb)N = 2.4-5.5; La = 28 150 times chondrite] and higher Th/U, Th/Ta, Rb/Ta, Ba/Ta, Cs/Rb but lower K/Rb ratios than MORB. Combined major- and trace-element, and Sr-Nd isotopic data suggest the involvement of sub- continental lithosphere, depleted upper mantle source (MORB), and local continental crust in the basalt petroge- nesis, eNa(t) values in rhyolites vary from + 1.5 to + 3.7 while (87Sr/86Sr)i ratios range from 0.7051 to 0.7064. The rhyolites display LREE enrichment [(La/Yb)N = 10; La = 100 times chondrite] along with a distinct negative Eu anomaly (Eu/Eu* = 0.75) and depletion of Ti and P. Mixing relations in (878r/86Sr)i- ~Na(t) space among basalts, rhyolites, and local continental crust indicate that the Tecuya rhyolites were produced by assimilation of variable amounts of continental crust by MORB-related magmas and subcontinental lithosphere-derived melts. This conclusion is supported by the synchroneity of Te- cuya volcanism at 22 Ma with interaction of a segment of the East Pacific Rise along the southern California mar- gin. The Tecuya volcanic rocks thus provide an example for the generation of rhyolitic melts owing to crustal assimilation by basaltic melts during mid-oceanic ridge- induced magmatism along a continental margin. Introduction Several recent studies have evaluated the comparative roles of mantle-derived and continental crustal sources for the formation of relatively large volumes of acidic magmas in the continental environment (e.g., Barker 1981; Hil- Offprint requests to: A. R. Basu dreth 1981; Watson 1982; Musselwhite et al. 1989). The role of fractional crystallization alone in the petrogenesis and evolution of silicic magma bodies (Bowen 1928) from mantle-derived basalts is now considered inadequate, mostly on grounds that many acidic magma bodies are found in association with relatively minor volumes of basalt. Instead, two competing models can be found in the current literature, both attempting to address the central question of the evolution of silicic magmatic systems along with the problem of the coexistence of marie and silicic volcanic rocks, generally basalts and high-silica rhyolites. In one of these models, mantle-derived basaltic magmas are considered to induce melting of the local continental crust, generating rhyolites; the other model, supported by isotopic and trace elemental data, requires fractional crystallization of the basaltic magma accom- panied by assimilation of the continental crust. We report the results of an isotopic and geochemical study of the 22.5 Ma (Turner 1970; Weigand and Thomas 1990) Tecuya volcanic member (informal designation) in southern California (Fig. 1). This study has a direct bear- ing on the questions just discussed and, particularly, on crustal versus mantle-derived source contributions in a bimodal suite of basalts and rhyolites. In addition, the evolution of the Tecuya bimodal suite can be correlated with tectonic events as the causal mechanism for the observed volcanism. We will show from the combined Nd- and St-isotopic and trace-elemental data that the more silicic of the Tecuya magmas, including dacites and rhyol- ites, were derived from MORB and subcontinental litho- sphere-derived melts which assimilated variable amounts of local continental crust. In light of the field evidence and the proposed models for the plate-tectonic evolution of western North America, our new geochemical data imply that the bimodal Tecuya volcanism was a result of the collision of a segment of the East Pacific Rise with the North American plate. Plate-tectonic and geologic setting During the late Cenozoic, segmentsof the East Pacific Rise (EPR) collided with the North Americanplate (Fig. 1) along a trench that