The evolution of the southern California Borderland during mid-Miocene time was dependent on the interactions between the North American continent and the converging East Pacific Rise. Evidence from geological and geochemical investigations of the Conejo volcanic suite, whose petrogenesis is linked to these interactions, suggests an intimate relation between the subducting ridge and the volcanic rocks. The whole-rock chemistry of this suite (K2O < 0.4%; 87Sr/86Sr = 0.70248–0.70372, average = 0.70306; TiO2 = 1.2%–1.8%) and restricted range of pyroxene and plagioclase compositions support a model in which active ridge volcanism, fractionation, and mixing of primitive liquids with later differentiates can account for the observed petrologic evolution. The interaction of the ridge with the subduction zone may result in the cessation of subduction, the preferential subduction of the trenchward ridge flank, or localized volcanism in the vicinity of the ridge–subduction zone intersection.

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