“Cobbling” together the age, composition, and origin of Jurassic mafic forearc basement in coastal and offshore southern California: Low-temperature metasomatism and its relationship to Cretaceous subduction

Fluids devolatilized from subducted sediments flux the mantle wedge, resulting in arc magmatism and discharge from accretionary prisms near the trench. Less is known about the interaction of these fluids with forearc basement. Here, we investigate the remnants of exhumed mafic forearc basement exposed across southern California, USA, including the Willows Plutonic Complex, Catalina Gabbro, and the underlying subduction complex (Catalina Schist). We also investigate cobbles that represent forearc detritus preserved within the San Onofre Breccia. We performed petrographic, zircon U-Pb age, and trace element analyses of mafic forearc basement to determine the cause and timing of low-temperature metamorphism and metasomatism. Magmatic ages (174−163 Ma) overlap those of the Coast Range and Cedros ophiolites. Cratonal zircon inheritance and trace element signatures require that southern California forearc basement formed along the continental margin adjacent to the southwestern North American craton. Most importantly, we have documented extensive Cretaceous zircon recrystallization that accompanied alteration. Three age maxima are resolved: 105 Ma, 89 Ma, and 77 Ma. These overlap with accretional timing of the Mid-Cretaceous Catalina Schist and later accreted Late Cretaceous Pelona-Orocopia-Rand schists that are presumed to underlie the Catalina Schist at depth. Results from a mafic basement clast preserved in the Catalina blueschist facies metaconglomerate reproduce an equivalent recrystallization history with corresponding age maxima, linking metasomatism to subduction processes. Collectively, this evidence implies significant infiltration and metasomatism of mafic forearc basement by subduction-related fluids.