UC San Diego

The dynamic tectonic history of peninsular Baja California commenced at approximately 13 Ma, when subduction along the northwestern coast of Mexico ceased. Subsequently, the Gulf of California opened, and transverse faults formed parallel to the ancestral trench. Post-subduction arc-related magmatism continued while the Baja peninsula was forming until approximately one million years ago. The post-subduction lavas that erupted in the peninsula have variable compositions ranging from calc-alkalic to tholeiitic arc basalts. Bajaite is a collective term that describes andesites and basaltic andesites found in Baja California that, like adakites, are characterized by elevated magnesium, silica, and incompatible trace element concentrations. However, in contrast, bajaites exhibit relative enrichment in Sr—their origin is the subject of discussion. It has been proposed as the product of partial melting of subducted oceanic crust - similar to the proposed origin of adakite - or partial melting of the metasomatized mantle wedge due to its arc lava-like features, such as depletion in Nb and Ta, and relative enrichment in Sr. Bajaite has also been proposed as a mixture of differentiated and mafic arc lavas. The composition of bajaite is similar to the bulk continental crust and, thus, its true origin can explain a potential mechanism of continental growth. In this study, I use the petrographic, major and trace element, and Sr-Nd-Pb isotopic data for bajaites from the Jaraguay and San Borja volcanic fields in Baja California Norte, Mexico to show that their source is plausibly the depleted mantle wedge metasomatized by a subduction component.