BASALT-RHYOLITE VOLCANISM BY MORB-CONTINENTAL CRUST INTERACTION - ND, SR-ISOTOPIC AND GEOCHEMICAL EVIDENCE FROM SOUTHERN SAN JOAQUIN BASIN, CALIFORNIA

The early Miocene Tecuya volcanic center in the southern San Joaquin basin of California consists of flows and tuffs of basalt and rhyolite that erupted, closely spaced in time, in both submarine and subaerial conditions. The rhyolites are overlain by the basalts and constitute approximately 45% of a total of at least 180 km3 of the Tecuya volcanic rocks. The basalts have epsilon-Nd(t) values of + 2 to + 6 and (Sr-87/Sr-86)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 subcontinental lithosphere, depleted upper mantle source (MORB), and local continental crust in the basalt petrogenesis. epsilon-Nd(t) values in rhyolites vary from + 1.5 to + 3.7 while (Sr-87/Sr-86)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 (Sr-87/Sr-86)i - epsilon-Nd(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 Tecuya volcanism at 22 Ma with interaction of a segment of the East Pacific Rise along the southern California margin. 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.