YOUNG LAVAS FROM MELTING OF A FOSSIL SLAB REMNANT, BAJA CALIFORNIA, MEXICO

More than a century of geologic study in Baja California has identified geochemically unusual, low-volume Miocene-Recent lavas whose origin is poorly understood. These lavas include adakites and bajaites with remarkably high Sr contents and Sr/Y ratios. Adakites in particular are often interpreted as partial melts of subducting young oceanic crust. However, the Baja high Sr/Y lavas erupted after Farallon Plate subduction ended locally at ~12 Ma. Other models invoked ridge subduction, following early plate reconstructions of the western North American margin. Bathymetric mapping of abandoned Farallon spreading centers west of Baja makes such models untenable. Thus, remaining possible explanations for the post-subduction lavas include melting thickened lower crust, or shear heating and melting of a fossil slab inferred from fast seismic velocities at depth beneath Baja. Here, we use thermodynamic phase equilibria modeling to test these hypotheses. We present an equilibrium pseudosection (7–47 kbar, 600–1400 °C) for average seafloor basalt recovered by DSDP cores on the Guadalupe and Magdalena microplates. We additionally calculated melt compositions along open-system isobaric heating paths (600–1200 °C at 10, 15, and 35 kbar). Modeled trace elements reproduce Baja adakite compositions, including strong Sr/Y ratio fractionation, but only with garnet-bearing eclogite residues. On this basis, we discard the mafic lower crust model as P_Moho < P_gt-in, and advocate for shear heating of a deep fossil slab. Although adakites and bajaites are not common lava types, we also propose that shear heating in subduction zones may produce a low-F, “background” slab-derived melt globally. In active arcs, this low-F melt is swamped by dominant flux melting and/or masked by melt-rock reaction between the source and the surface. Our models corroborate recent studies calling on shear heating to reconcile subduction zone thermal structure versus the exhumed rock record.