Temporal constraints on the eastward migration of the Late Cretaceous–early Tertiary magmatic arc of NW Mexico based on new 40Ar/39Ar hornblende geochronology of granitic rocks

Hornblende step-heating 40Ar/39Ar dating for granitic plutons along an E–W transect of central Sonora was carried out to constrain the Late Cretaceous–early Tertiary migration of the cordilleran magmatic arc across northwestern Mexico. Geochronological data from previous studies offer a good estimate of the overall process, but because they come from different dating schemes performed on a variety of rocks and/or minerals with a wide range of closure temperatures, the ages largely overlap when plotted on a map. Previous data suggest that the Cordilleran magmatic arc was nearly static in the western portion of the Peninsular Ranges batholith in Baja California (140–105 Ma), then the axis of magmatism migrated east at approximately 10 km/Ma and reached coastal Sonora approximately 90 Ma ago. The locus of the plutonic emplacement continued to migrate inland during the Laramide magmatic pulse (80–40 Ma), penetrating up to central Chihuahua. New argon data indicate that granitic plutons intruded the region northeast of Bahía Kino, in coastal Sonora, approximately 77 Ma ago. Magmatism subsequently moved east to the area surrounding the city of Hermosillo approximately 69 Ma ago and continued its easterly migration, reaching the Sonora–Chihuahua state boundary 59 Ma ago. However, the granitic rocks of east-central Sonora yield ages in a relatively wide range of 62–56 Ma. Synchronic plutons reported farther east in central Chihuahua suggest an unusually broad magmatic arc, which appears difficult to explain on the basis of the traditional subduction model assumed for southwestern North America during this time and may reflect particular – and little understood – tectonic conditions derived from the relatively flat subduction regime prior to the extinction of the Laramide magmatic arc. Moreover, volcanic rocks exposed in east-central Sonora yield fairly old U–Pb zircon dates of 90–70 Ma, which have no known contemporaneous plutons, and complicate the scenario for the Laramide event in Sonora, perhaps requiring the existence of a second volcanic arc. Considering solely the evidence from granitic plutons, the data provide a systematic way to evaluate the shift of magmatic activity across Sonora. It needs a proper restitution for the conspicuous Cenozoic extension affecting the region. After restoring the cumulative extension of 90% estimated for east-central Sonora, a rate of approximately 8.5 km/Ma of eastward migration can be roughly estimated for the Laramide arc across Sonora.