THE JURASSIC EASTERN PENINSULAR RANGES BATHOLITH IN BAJA CALIFORNIA, MEXICO: NEW AGES FROM AN ANCIENT CONTINENTAL ARC

New U-Pb zircon SHRIMP ages of 164.3 ± 2.3 and 164.4 ± 1.2 Ma from extensive exposures of orthogneiss in the eastern zone of the Peninsular Ranges batholith (PRB) at latitude 30.5° N in Baja California, Mexico, indicate that robust Middle Jurassic arc magmatism continued well south of San Diego County at 33° N latitude where orthogneiss of similar lithology and age has been previously described. These biotite, muscovite orthogneisses show relict igneous plagioclase and intrude Paleozoic miogeoclinal rocks along the eastern margin of the batholith. Similar, presently undated, lithologies are found as far south as latitude 29° N on the Baja Peninsula.

Even after restoring Neogene extension, this belt of Jurassic magmatism in Baja California lies well to the SW of the Late Triassic to Middle Jurassic arc magmatic belt identified in southern California, southern Arizona, and NE Sonora, Mexico. We speculate that slab roll-back that initiated intra-arc rifting identified for the Triassic-Jurassic magmatic belt farther north caused the arc to shift trenchward to a position in Baja California. The Jurassic strain field for the PRB is unclear, but was potentially extensional as indicated by the development of a poorly understood basin along the axis of the present PRB in which volcanic, volcaniclastic and turbidite assemblages were deposited. Notwithstanding, contractional deformation was prevalent by 132 Ma. Magmatism continued after 164 Ma in the eastern PRB of this region as indicated by pluton ages in the range 134 to 100 Ma. Thus, contrary to some reconstructions of western Mexico for Middle Jurassic-Early Cretaceous time, west-facing subduction was active in Baja California up to and following the time of collision of Guerrero terrane island arc fragments with North America to form parts of the western PRB at 115-108 Ma.