RECOGNITION OF LARAMIDE-AGE INTRUSIVES IN THE EASTERN PENINSULAR RANGES BATHOLITH OF NORTHERN BAJA CALIFORNIA; IMPLICATIONS FOR MAGMATISM AND TECTONICS AT THE ONSET OF THE LARAMIDE OROGENY

There are numerous models explaining the widely debated Laramide low-angle subduction event that occurred on the western edge of North America from ~80-40 Ma. Evidence for shallow subduction has been documented from eastern California through Wyoming and down into Sonora, Mexico and includes the under-thrusting of the amphibolite facies Rand-Orocopia-Pelona schists to the uplifting of the continental interior. Zircon U/Pb ages measured by LA-ICP-MS document the presence of latest Cretaceous (i.e., 86-70 Ma) granitic intrusives in the eastern-most Peninsular Ranges batholith. This suite appears to be dominated by leucocratic biotite granodiorite that typically contains minor garnet and/or muscovite. These rocks are exposed for at least 200 km along strike and comprises >300 km2 in surface outcrop exposure mainly in the Sierra Cucapas through the Sierra San Felipe in northern Baja California, Mexico. They intrude La Posta-type plutonic rocks that comprise the main phase of the eastern batholith. This suite records the tectonics and magmatism associated with a major transition in Cordilleran geology. The eastern Peninsular Ranges batholith has been described as a “migrating” arc representing the initial stage of inboard migration of Laramide Cordilleran magmatism. The temporal gap coupled with the absence of a spatial gap however suggests that following the voluminous La Posta magmatic flare-up at 100-90 Ma, magmatism stalled but continued intermittently until ~70 mybp in the eastern Peninsular Ranges batholith before inboard migration was initiated.