DETRITAL ZIRCON GEOCHRONOLOGY OF BLUESCHIST FACIES SEQUENCES IN THE ACCRETIONARY COMPLEX OF WEST-CENTRAL BAJA CALIFORNIA, MEXICO

Blueschist facies subduction accretion assemblages exist as fault-bounded blocks on Cedros Island and the San Benito Islands in west-central Baja California, Mexico. These sequences of blueschist facies metasediments and metabasites constitute the Western Baja terrane (WBt) and represent the exhumed products of tectonic underplating along the western North American plate margin. The WBt lies in normal fault contact beneath the mid-Jurassic ophiolite of the Choyal tectonostratigraphic terrane on Cedros Island. A serpentinite matrix mélange containing exotic blocks of blueschist, amphibolite, eclogite, and serpentinite crops out along the WBt-Choyal terrane contact zone and also separates the subterranes of the WBt. The WBt subterranes vary by protolith and degree of metamorphism, suggesting that they represent discrete packages of rock sequentially underplated beneath the accretionary prism. Past investigations yielded Early to mid-Cretaceous ages for blueschist facies metamorphism. We present new detrital zircon geochronologic data from metasediments representing each subterrane in the WBt. Ten samples of metasandstones and one granitic clast from a metaconglomerate were collected at different structural levels from the three subterranes that crop out on Cedros Island and the San Benito Islands. This work places new constraints on the maximum depositional ages and detrital sources of WBt metaclastic rocks, thereby clarifying regional sediment dispersal pathways; constraining the time at which the WBt subducted; revealing patterns between degree of metamorphism, current spatial position, and time; and permitting comparisons with the Catalina schist of southern California and the Franciscan complex of more northerly California. Results are discussed in the context of forearc and intra-arc thrusting in the northern Peninsular Ranges.