U-Pb and 40Ar/39Ar geochronology of the coastal Sonora batholith:: New insights on Laramide continental arc magmatism

The coastal Sonora batholith comprises a series of Cretaceous granitoids taut intruded a metasedimentary basement of possible Mesozoic age. They are partially covered by Tertiary volcanic flows and pyroclastic rocks. In order to elucidate the crystallization and cooling history of the granitoids, nine rock samples were collected from Bahia Kino to Punta Tepopa. Eight samples dated by U-Pb zircon geochronology show that the Coastal Sonora batholith was emplaced during the Late Cretaceous, between 90.1 +/- 1.1 and 69.4 +/- 1.2 Ma. The interval of similar to 20 Ma between the different stages of crystallization indicate that mogmatism was relatively static within coastal Sonora, although the magmatic are recorded an eastward migration as a whole during Cretaceous and Paleogene. In addition, three of these samples were also dated by 40Ar/39Ar in biotite and K-feldspar separates. Ages vary from similar to 74 to 67 Ma in biotite and from similar to 68 to 42 Ma in K-feldspar We interpret these ages as the cooling progression of the batholith, associated with exhumation of the region before the Basin and Range extension. Furthermore, these results show a local trend towards younger ages to the north of the batholith, and they are in good agreement with the model of a general eastward migration of the Cretaceous-Tertiary magmatic arc in northwestern Mexico. In general, the available ages suggest that the arc moved slowly across Baja California between 140 and 105 Ma, and continued its eastward migration across the eastern portion of Baja California and Sonora between 105 and similar to 60 Ma. According to the isotopic ages, the Coastal Sonora botholith would be the westernmost part of the Laramide magmatic event (similar to 90-40 Ma). Thus, on the basis of new and available geochronologic, petrographic, and geochemical data, we propose that the Coastal Sonora batholith and the eastern portion of the Peninsular Ranges batholith belong to a single magmatic arc, which was separated during the continental breakup and rifting of the Gulf of California in the Tertiary.