MAGNESIAN ANDESITES FROM BAJA CALIFORNIA, MEXICO: THE ROLE OF SLAB MELTS

Along Baja California Peninsula, magnesian andesites and associated basalts occur in five main volcanic fields. From north to south they are respectively: (1) the Jaraguay volcanic field, a rather flat plateau overlying the Mesozoic basement. Magnesian basaltic andesites occur mainly as massive lava flows, erupted from almost one hundred small cones and domes; (2) the San Borja volcanic field, mainly represented by isolated basaltic flows resting over Mesozoic plutons; (3) the San Ignacio basaltic andesitic flows, emitted from strombolian cones and covering Tertiary sediments and tholeiitic lava flows; (4) the Santa Rosalia lava flows pile, immediately below the Upper Miocene Boleo Formation, where it is possible to distinguish two volcanic phases, the older one with clinopyroxene-bearing flows, and the younger one with a few hornblende-bearing flows; (5) the La Purisima – San José Comondú volcanic field, with two units: The 9 to 8 Ma old magnesian andesites represented by thick lava flows forming mesas, and the Plio-Quaternary aa and block lava flows.

The magnesian andesites range in age from 11 Ma to Pleistocene, and display very specific geochemical characteristics: SiO2=50 to 58%, MgO up to 9%, very low FeO*/MgO ratios usually less than 1.5, highly fractionated REE patterns with low Y and heavy REE, and very high Sr (up to 3000 ppm) and Ba (up to 2300 ppm) contents. These results suggest that most of their incompatible element ratios, which vary significantly in space and time, reflect source heterogeneities rather than partial melting, fractional crystallisation or crustal contamination effects. Their slab melt imprint, traduced e.g. by high Ba/Y, Sr/Y, Nb/Y, Sm/Yb ratios, increases both from northwest towards southeast and with time.

These features reflect a magmatic origin through melting of mantle peridotites from the upper plate, previously metasomatised by slab melts (adakites). Melting was initiated either by high thermal regime accompanying ridge subduction during Miocene at least in northern Baja California, or later during Plio-Pleistocene thermal pulses linked to the rupture of the slab, or to the opening of the Gulf of California.