Insight into the upper mantle beneath an active extensional zone: the spinel-peridotite xenoliths from San Quintin (Baja California, Mexico)

Abstract

Many of the peridotite xenoliths included in the San Quintin (Baja California Norte, Mexico) quaternary alkali-basalts have undergone a very intense shear deformation (deviatoric stresses up to 0.1 GPa), hence a first-order classification into coarse-grained lherzolites and deformed peridotites (porphyroclastic and mosaic textures) has been applied. All of these rocks show a very limited compositional variability in the Mg/(Mg+Fe²⁺) ratios (olivine: 0.894–0.905±0.005; orthopyroxene: 0.899–0.9105±0.005), and the observed trends in the Cr/(Cr+Al) spinel ratios (from 0.1 to 0.6) can be interpreted as resulting from gradual partial melting followed by homogenization of the bulk phases. A later and less accentuated melting event is also evidenced by internal core-rim variations in the spinels from a few samples and ascribed to the thermal effect of the host lava.

Simultaneous application of exchange geothermometers which give the latest equilibrium temperatures (i.e. at the time of eruption: Fe-Mg exchange between olivine and spinel) and of pyroxene transfer thermobarometers yields two distinct behaviours: the porphyroclastic and mosaic peridotites record an event of deformation and recrystallization and were equilibrated at 800°–950° C and P≲-1 GPa at the time of eruption, but have also retained evidence of higher temperatures (1000°–1050° C) and pressures; the coarsegrained lherzolites, which yield conditions of 1000°–1050° C and P<-2 GPa at the time of eruption, were originally equilibrated at higher temperature and pressure conditions and were subsequently re-equilibrated to 1000°–1050° C by solid-state bulk diffusion, without exsolution.

Clinopyroxenite veins provide evidence of magma injection into the host-peridotite, before deformation but after the major melting event.

To explain the simultaneous sampling of both groups of peridotites by the San Quintin alkali basalts, we suggest that the ascending magma reached the critical limit for hydraulic fracturing in the coarse-grained lherzolites. At shallower depth, the magma cross-cut an active shear zone, sampling prophyroclastic and mosaic samples of the strained peridotites.

Our model is consistent with the regional tectonic context: upwelling of the mantle by isostatic re-equilibration after the end of the subduction processes and subsequent opening of the California Gulf. The only questionable parameter of the model remains the geometry of the shearzone, high or low angle orientation.