María Magdalena Island, Mexico: A geochemical, isotopic, and geochronological study of an ocean floor sequence within the shelf of the Gulf of California

María Magdalena Island is part of the Islas Marías Archipelago, located at the mouth of the Gulf of California. The island is composed of a gabbroic and basaltic sill complex that intrudes a sedimentary sequence. Its geology is drastically different to the rest of the archipelago that reflects a protracted geological history with continental affinity. It constitutes a rare example of an exposed ocean floor sequence. Neighboring María Madre, María Cleofas, and San Juanito islands contain silicic volcanic and granitoid rocks of mostly upper Cretaceous age. In contrast, 40Ar/39Ar ages of the gabbroic and basaltic sills from María Magdalena Island show younger magmatic pulses of late Miocene age (~ 8.7 and 6.9 Ma). María Magdalena samples are isotopically more evolved (87Sr/86Sr ~ 0.7027–0.7049, 143Nd/144Nd ~ 0.51305–0.51310) in comparison to the MORB-type Rivera plate basalts (87Sr/86Sr ~ 0.7024–0.7025, 143Nd/144Nd ~ 0.51323–0.51325). The presence of detrital zircons in the hosting sedimentary rocks and their siliciclastic nature suggest that María Magdalena Island was formed and uplifted in situ to its present position in the archipelago.Our data suggest that María Magdalena Island was formed in an evolved oceanic magmatic environment within a thinned continental crust setting since ~8.7 Ma, possibly related to the extensional tectonic regime associated to the separation of Baja California Peninsula from mainland Mexico. María Magdalena Island may be an extinct spreading center oriented in a ~ N-S direction that emplaced the Magdalena sills in an ~E-W direction. Magmatism ceased as extension progressed.