ARC MIGRATION DURING THE LATE CRETACEOUS TO EOCENE: GEOCHRONOLOGY OF GRANITOIDS FROM NORTHWESTERN MEXICO

Restauration of Miocene to Quaternary displacements of the Baja California peninsula, along the San Andreas fault system (>300 km), allow the correct evaluation of Late Cretaceous to Eocene magmatic arc migration in northwestern Mexico.

Previous studies have suggested an eastward arc migration using extensive K-Ar, Rb-Sr, Ar-Ar and U-Pb geochronology of a variety of igneous lithologies from NW Mexico and SW USA (e.g., Coney and Reynolds, 1977). This migration has been explained as connected to an increase in rate of convergence during subduction, resulting in slab flattening of this portion of the North America Cordillera.

In order to evaluate this arc migration, we collected a suite of 28 samples, from granite to gabbro, along an E-W transect from Sierra San Pedro Mártir in Baja California, through Isla Tiburón and Hermosillo in Sonora, all the way to Chihuahua city. Granitoids located east of the Main Mártir Thrust in Sierra San Pedro Mártir yielded U-Pb ages between 98–91 Ma (La Posta type), while samples in the San Felipe region present zircon crystallization ages ranging between 90–85 Ma. Granites in Isla Tiburón have ages between 90–83 Ma, very similarly to their equivalent rocks in the San Felipe area. Coastal Sonora intrusive units are slightly younger ranging between 83–70 Ma, and as we progress towards central Sonora we obtain ages ranging between 70–50 Ma. Lastly, granitoids in Chihuahua have U-Pb ages between 58–33 Ma. This collection of new crystallization ages supports the previous idea of magmatic arc migration during the Late Cretaceous to the Eocene (~98–33 Ma) in the northwestern part of the Mexican Cordillera.

Another objective of this study is to assess the presence of old Proterozoic basement along the studied E-W crustal transect by using U-Pb geochronology of inherited zircons present in this suite of granitoids. Samples collected across the transect from Sierra San Pedro Mártir to Hermosillo show an absence of such old inherited zircons suggesting that Proterozoic basement may not be present in that crustal section as suggested by recent geophysical studies. In contrast, most of the granitic samples from Hermosillo through Chihuahua present abundant Proterozoic inherited zircons suggesting that old basement participated in the magma genesis of these granitoids.

Project PAPIIT-UNAM (IN111718)