U-Pb Zircon Ages and Lu-Hf Isotope Systematics Across Northwestern Mexico: Implications for Cretaceous to Paleocene Tectonomagmatic Evolution During Farallon Subduction

This study presents U-Pb zircon ages and Lu-Hf zircon isotope data for Cretaceous-Paleocene plutonic rocks along a W-E transect in northwestern Mexico. These data are combined with tectonic reconstruction that restores Late Cenozoic extensional deformation and shows the position of magmatism at 36 Ma. Zircon U-Pb ages results span from 142 to 58 Ma and demonstrate that the continental arc migrated northeastward at 1-2.5 km/Myr. These rates are slower than previously interpreted, but consistent with landward arc migration rates observed in the Andes. Weighted mean initial epsilon hafnium (epsilon Hf(t)) values of plutonic rocks along the transect range from + 8.8 to -9.1. The heterogeneity in the zircon epsilon Hf(t) is spatially related to the pre-Cretaceous basement provinces that the intrusive rocks were emplaced into. Zircon epsilon Hf(t) values of western Baja California display positive values ranging from + 8.8 to + 2.6 suggesting they were formed from a moderately depleted mantle and were emplaced into the Guerrero-Alisitos-Vizcaino terrane. Zircon epsilon Hf(t) values in the eastern part of Baja California and most of Sonora are heterogeneous ranging between -0.7 and -9.1 and may be formed from a relatively slightly more evolved mantle source and end up more evolved after crustal assimilation of metasediments. Zircon epsilon Hf(t) values ranging from + 8.7 to + 2.9 in Chihuahua are consistent with a depleted-mantle derived melt and assimilation of Grenville lithospheric province. Our results highlight how Hf isotopic signatures help to constrain the pre-Cretaceous basement configuration in northwestern Mexico despite the few exposed outcrops along the transect.