Geochemical diversity of Late Miocene volcanism in southern Baja California, Mexico:: Implication of mantle and crustal sources during the opening of an asthenospheric window

Five main petrologic and geochemical groups can be identified among the Middle to Late Miocene lavas from the western part of southern Baja California: (1) calc-alkaline and K-rich andesites emplaced between 15.5 and 11.7 Ma; (2) adakites and (3) associated niobium-rich basalts erupted between 11.7 and 8.5 Ma in the Santa Clara volcanic field, Vizcaino Peninsula; (4) 10.6-9.2 Ma tholeiitic basalts and basaltic andesites that form large tabular plateaus near San Ignacio; and (5) magnesian and basaltic andesites of adakitic affinity whose emplacement started at 11.7 Ma south of San Ignacio and between 9.7 and 8.8 Ma near La Purisima. These lavas, although spatially and temporally related, display very different geochemical signatures. Their trace elements and isotopic characteristics suggest that three different magma sources were involved in their genesis. Partial melts of subducting altered oceanic crust produced the adakites when erupted directly at the surface. These magmas were eventually trapped in the mantle wedge where they reacted with ultramafic lithologies. Such slab-melt-metasomatized mantle could then melt to produce niobium-rich basalts or magnesian andesites, depending on the pressure that controlled the stability of garnet into the mantle wedge. The melting of fluid-metasomatized mantle wedge led to the emplacement of andesites. In southern Baja California, the opening of a slab window following active ridge subduction resulted in the additional contribution of partial melts from the suboceanic mantle uprising through the tear in the slab. This process might be responsible for the occurrence of tholeiitic basalts and basaltic andesites near San Ignacio. The studied association can be considered as a modern analog of high-thermal-regime Archean subductions.