MAGMATIC EVOLUTION OF THE GULF OF CALIFORNIA EXTENSIONAL PROVINCE

Volcanism in the Gulf of California Extensional Province (GEP) records the transition from subduction (>12 Ma) to continental rifting (11-5 Ma) and to oceanic rifting in the last 4 Ma. After the end of subduction, extension was localized along the former volcanic arc. This produced a change in the eruptive processes and magma composition from dominantly calc-alkaline dacitic-andesitic composite volcanoes along the eastern margin of Baja California (23-12 Ma); to bimodal alkaline and sub-alkaline mafic and rhyolite ignimbrite deposits (<12 Ma). Calc-alkaline rhyolite to dacite dome complexes, caldera-type ignimbrite deposits and composite andesitic volcanoes continued to erupt along the eastern margin of the Baja Peninsula and in some islands within the gulf. Additionally from ~11 Ma up to present alkali basalt and andesite lavas also flowed west of the gulf depression in Baja California, and to a lesser extent in the opposite side in central Sonora. The alkalic lavas in Baja California and coastal Sonora have distinctive geochemical patterns with low Nb-Ta, and high LILE concentration, principally Ba, Sr, and light REE likely inherited from subduction. In Quaternary time more typical intra-plate alkaline basalts erupted on both sides of the gulf. Their trace element and isotopic composition indicates a source in the asthenospheric mantle. Low-K sub-alkaline basalt (transitional tholeiite), and differentiates where produced from 10 to 5 Ma along both margins, which is especially notable over a broad region in southern Baja California. Later (<5 Ma), these sub-alkaline rocks occurred in a more restricted area within the Gulf, and evolved into N-MORB lavas in the deep basins from the southern Gulf. Arc volcanism likely provided thermal conditions that favored extension and rupturing of the continental lithosphere along the Gulf. Syn-rift volcanism in the GEP although comparatively less voluminous suggest compositional heterogeneities in the source related to 1) subduction metazomatism, 2) melts from a subducted slab, 3) partial melting of a depleted lithospheric mantle, and 4) partial melting of the asthenospheric mantle.