Extreme 230Th excesses in magnesian andesites from Baja California

Four Late Pleistocene magnesian andesites (locally called “bajaites”) from Baja California have been dated by the unspiked K–Ar method and analyzed for major, trace elements and Sr, Nd, Pb, Th and U isotopes. They display very large 230Th excesses (50% to 120%) relative to 238U. They also have very high Ba and Sr contents and Sr/Y and La/Yb ratios, together with suprachondritic Nb/Ta ratios (22 to 26). From correlations between 230Th excess and major elements, trace element and isotopic ratios, we show that the unusual geochemical signature of these lavas can be ascribed to very low partial melting degrees of a mantle source containing residual rutile. This mantle source derived from interactions between the Baja California lithospheric mantle and older magnesian andesite melts carrying a sedimentary component, which were formed during the Late Miocene opening of the Baja California asthenospheric window. During the Quaternary, this metasomatized source experienced dehydration melting triggered by the hot thermal regime due to the uprise of the subslab Pacific asthenosphere beneath Baja California and the Gulf of California.