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Geochemistry and U/PB geochronology of the eastern-most Peninsular Ranges Batholith of Southern California and northern Baja California
Moniz, Robert Edward
Kimbrough, David L.
Girty, Gary H.
Chatfield, Dale A.
There are numerous models explaining the widely debated Laramide orogenic event that occurred in western North America from ~80-40 Ma. The shallowing of the subduction slab, or more likely a section of the slab, may have created the compressional regime that was expressed well inboard of the trench. Geologic evidence of this contraction has been documented from Canada down into Sonora, Mexico and includes the under-thrusting of the amphibolite facies Rand-Orocopia-Pelona schist, basement-cored uplifts in the continental interior and the “migration” of magmatism reaching ~1000 km inboard of the continental margin. Despite the abundant geologic data for the Laramide and partially overlapping Sevier orogeny, the patterns of magmatic migration remain poorly understood. The characterization of these units may bring a better understanding of the transition to shallow slab subduction. Using zircon U/Pb geochronology, a new suite of latest Cretaceous early Tertiary (i.e. 86-70 Ma) granitic intrusives is recognized in the eastern-most Peninsular Ranges batholith. This suite is sparsely exposed for at least 300 km along strike and likely comprises >200 km2 in surface outcrop exposure. The units lie within three geographically distinct areas. The northernmost exposures encompass a small group of granitoids in the eastern Santa Rosa Mountains in southern California which were the only significant post-90 Ma plutonic rocks documented in the Jurassic-Cretaceous Peninsular Ranges. The youngest truly Laramide in age units (i.e. 80-70 Ma) were found in the Sierra Cucapas just south of the California-Baja California border whereas “transitional” aged units (i.e. 86-81 Ma) were identified in the Sierra Cucapa and further south in the Sierra San Felipe. Both the Laramide and transitional units in Mexico are found to be in cross-cutting relationship with the older La Posta-type plutonic rocks that comprise the main phase (100-90 Ma) of the eastern Peninsular Ranges batholith. The transitional units in the Santa Rosa Mountains occur to the east of La Posta-type units from which they are separated by the eastern Peninsular Ranges mylonite zone. Major and minor elemental geochemistry from 53 samples indicate that although the majority of granitoid rocks in the desert ranges to the east of the Peninsular Ranges escarpment are part of the La Posta suite, many do not follow the typical documented deep crustal root signature defined by high Sr/Y ratios. The Laramide and transitional aged samples of northern Baja California however were found to overlap significantly with the La Posa aged suite making characterization difficult. The eastern batholith has been described as “migrating” arc representing the initial stage of inboard migration of Laramide Cordilleran magmatism. The new U-Pb zircon ages however, suggests that following the voluminous La Posta magmatic flare-up at 100-90 Ma, magmatism stalled but continued intermittently until ~70 Ma in the eastern Peninsular Ranges batholith. With numerous examples of Late Cretaceous magmatism as far inland as Colorado and New Mexico, it now appears that magmatism did not simply “migrate” inboard and that current models of shallow subduction inadequately account for the chemically and spatially distinct suite of intrusions of this study. This suite may define a new zone in Peninsular Ranges batholith linking it with the Laramide orogeny and therefore provide a unique area that records the tectonics and magmatism associated with this major transition in Cordilleran geology.
San Diego State University
Master of Science (M.S.) San Diego State University, 2008
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