Enigmatic postsubduction lavas from a fossil slab remnant beneath Baja California, Mexico

Geochemically unusual, low-volume lavas have erupted along the Baja California Peninsula (BCP) in Mexico since the Miocene. These rocks have remarkably high Sr contents and Sr/Y ratios. High-Sr/Y magmas are commonly ascribed to partial melting of subducting young oceanic crust (adakites) or melting of mantle previously metasomatized by slab melts (bajaites). However, BCP adakites and bajaites erupted after Farallon plate subduction ended locally. We leverage a novel approach to studying the Baja Californian adakites by combining phase equilibria and trace element modeling. Our calculated melts of a relict slab fragment (imaged geophysically beneath the BCP) are consistent with the trace element compositions of local adakites but only at high pressures when residues comprise garnet-bearing eclogite. We infer the high thermal regime needed to melt the fossil slab and overlying metasomatized mantle may originate via shear heating, sub-slab mantle flow, or a combination thereof. Recent studies have invoked slab melting to reconcile geochemical mass balance at active arcs. The high-Sr/Y BCP lavas may offer a unique, isolated view into this slab-derived component because dominant flux melting overwhelms such a signal in active subduction zones.