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

  • Ernesto Ramos-Velázquez Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Apartado Postal 1039, Hermosillo, Sonora, 83000, México. Universidad Autónoma de Baja California Sur, Departamento de Geología, Apartado Postal 19-B, La Paz, B.C.S., 23080, México.
  • Thierry Calmus Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Apartado Postal 1039, Hermosillo, Sonora, 83000, México.
  • Victor Valencia University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA.
  • Alexander Iriondo Centro de Geociencias, Universidad Nacional Autónoma de México (UNAM), Campus Juriquilla 76230, Juriquilla, Querétaro, México. Department of Geological Sciences, University of Colorado at Boulder, Boulder, Colorado 80309, USA.
  • Martín Valencia-Moreno Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Apartado Postal 1039, Hermosillo, Sonora, 83000, México.
  • Hervé Bellon UMR 6538, Domaines Océaniques, IUEM, Université de Bretagne Occidentale, 6, Av. Le Gorgeu, BP 809, F-29285, Brest Cedex, France.
Keywords: geochronology, U-Pb, 40Ar/39Ar, Cretaceous-Tertiary magmatic arc, Laramide, Coastal Sonora batholith, Mexico.

Abstract

The coastal Sonora batholith comprises a series of Cretaceous granitoids that 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 Bahía 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 ~20 Ma between the different stages of crystallization indicate that magmatism was relatively static within coastal Sonora, although the magmatic arc 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 ~74 to 67 Ma in biotite and from ~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 ~60 Ma. According to the isotopic ages, the Coastal Sonora batholith would be the westernmost part of the Laramide magmatic event (~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.

Published
2018-01-16
Section
Articles