The Mesozoic prebatholitic terranes of central Baja California include distinct volcanic sequences associated with sediments, ranging in age from Late Triassic to Early Cretaceous. These volcanic-sedimentary sequences are parts of a large “nappe” lying on top of blueschist-bearing melanges. The magmatic parentages of basic lavas from the allochthonous units have been determined from major- and trace-element analyses of total rocks and microprobe data from their calcic clinopyroxene phenocrysts. The results show two major types of basic volcanism, linked to extensional processes (nonorogenic tholeiites) and to compressional processes (orogenic tholeiites), respectively. This study documents a complex Mesozoic geodynamic evolution for this area before nappe emplacement during the Albian. On the basis of these data, the Late Triassic to Albian geologic history of this area is interpreted in terms of two successive episodes of marginal oceanic basin development, followed, respectively, by subduction and collision processes.

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