Temporal geochemical evolution of Neogene volcanism in northern Baja California (27°-30° N): Insights on the origin of post-subduction magnesian andesites

New field, major and trace element and whole rock K-Ar data on the Neogene volcanic fields of northern Baja California (Jaraguay and San Borja) show that the calc-alkaline activity of the Comondu arc vanished around 11 Ma, following the end of the subduction of the young oceanic Farallon plate. It was immediately replaced by magnesian andesite volcanism, showing paroxysmal activities between 8.5 and 3.8 Ma in Jaraguay, and 3.7 to 1.8 Ma in San Borja. Our modelling results show that the characteristic geochemical signature of the magnesian andesite suite (high MgO, Cr, Co and Ni contents; highly fractionated rare earth element (REE) patterns with strong depletion in heavy REE and Y; exceptional enrichments in Sr and Ba and relative depletion in Rb and Th) is consistent with an origin through dehydration melting of pargasite-rich lithospheric mantle at depths of ca. 80 km. The temporal geochemical evolution of the Baja California magnesian andesite suite is studied using a set of 98 K-Ar dated whole rock samples ranging in age from Upper Miocene to Pleistocene. It is marked by an increase of incompatible elements concentrated in pargasitic amphibole (Sr, Ba, K, light REE), which were released in large amounts during dehydration melting, and conversely by the progressive exhaustion in Rb and Th hosted in small quantities by this mineral. The progressive temporal depletion in Y and heavy REE of the suite is consistent with the increase of the amount of residual garnet in the lithospheric mantle as a product of the dehydration melting reaction. This melting occurred at minimal temperatures of 1050-1075 degrees C, consistent with a high thermal flux in the mantle wedge. This flux was linked to the opening of an asthenospheric window following ridge-trench collision, and later to the no-slab regime which followed the sinking of the Farallon plate into the deep mantle. (C) 2008 Elsevier B.V. All rights reserved.