A 14−8 Ma coastal adakite belt in the western Peninsular Ranges produced by rift-related melting of the underplated subduction complex

A previously unrecognized, 160-km-long belt of small (0.5−7.2 km2) monogenetic adakite volcanic domes crops out in the southern California−northern Baja California coastal region. The belt appears related to Neogene transrotational rifting of the inner southern California borderland caused by propagation of the East Pacific Rise into the region. Volcanism from 18 Ma to 14 Ma attended peak rifting within the borderland. Adakites that erupted along the eastern rift flank are somewhat younger based on new 40Ar/39Ar data (14−8 Ma). The adakite domes are compositionally distinct from the mostly calc-alkaline borderland volcanics in having higher Na- and Al-contents, higher Na/K, depleted heavy rare earth elements, depleted Y, and elevated Sr and Sr/Y. We propose that they were generated by lateral heat transfer from upwelling asthenosphere beneath the inner borderland into the rift flank, where low volume adakite magmas were produced by partial melting of the Cretaceous subduction complex beneath the western Peninsular Ranges arc. This conductive heat transfer operated at slower time scales, lagging behind the more rapid advective processes active within the rifted borderland. Partial melts likely formed at 20−30 km depths for up to 6 m.y. after the main phase of borderland rifting. A similar process may have produced petrogenetically related rocks along the opposing rift flank in the northern Channel Islands and Santa Monica Mountains.