An 40Ar/39Ar study of metamorphic and volcanic rocks from Magdalena and Santa Margarita Islands, Baja California Sur, Mexico, provides evidence for at least four events that can be related to the tectonic evolution of this portion of the southern Baja California continental borderland prior to, and during, the transition from a convergent to a transform plate boundary system. (1) Metamorphism of amphibolites and quartz-mica schist from upper plate units occurred in Late Jurassic and Early Cretaceous time (∼158–138 Ma). Analogous metamorphic rocks from upper plate rocks in the Cedros–Vizcaíno–San Benito terranes of west-central Baja California have not yet been identified. However, the Santa Margarita amphibolites are similar in age to amphibolite blocks from melange of the Puerto Nuevo terrane. (2) Metabasite and metapelitic float blocks associated with serpentinized ultramafic rocks were metamorphosed and subsequently cooled in Early Cretaceous time (129–115 Ma). Their corresponding 40Ar/39Ar age spectra do not show pronounced age gradients, and they are dissimilar in age, as compared to Late Jurassic (160–170 Ma) amphibolite and late Early Cretaceous (100–115 Ma) blueschist blocks from melange in the Cedros–San Benito terranes. (3) A 30.7 Ma lamprophyre dike that crosscuts normal faults within ophiolitic rocks indicates that synsubduction extension of the upper plate on Santa Margarita Island occurred prior to mid-Oligocene time. (4) Latest Miocene–early Pliocene age (5–6 Ma) adakites were erupted into serpentinite-matrix melange on Santa Margarita Island and indicate derivation by partial melting of a young subducted slab prior to collision of ancestral East Pacific Rise ridge segments. The discovery of adakites in the continental borderland of Baja California Sur provides additional constraints for the Neogene thermal evolution of this convergent margin.

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