New age constraints on magmatism and metamorphism of the Western Sonobari Complex and their implications for an earliest Late Cretaceous orogeny on northwestern Mexico.

Abstract

The Western Sonobari Complex in northwestern Mexico is composed of orogenic metamorphic rocks intruded by a variety of unmetamorphosed plutons and dikes. Petrologic studies and U-Pb geochronology allow dividing the protolith of orthogneisses in the next groups: a) Lower Triassic granodiorite and quartz monzodiorite (249.6¿241.3 Ma); b) Upper Triassic granodiorite (213.7¿203.5 Ma); c) Upper Jurassic tonalite and granodiorite (162.9¿159.1 Ma); and d) Lower Cretaceous diorite (99.9¿98.8 Ma). Most of these rocks display amphibolite facies metamorphism, pervasive foliation and several stages of folding. Recrystallized zircon rims yield U-Pb ages of 92.3±4.1 and 90.1±1.3 Ma, which are interpreted to date the orogenic metamorphism. Metamorphic rocks are intruded by numerous post-orogenic granitic dikes dated at 83.9±0.5 to 80.6±1.7 Ma. Geochronology of igneous rocks indicates that the Cordilleran magmatic belts including Triassic and Jurassic plutons continue through northwestern-central Mexico apparently without displacement by the Mojave-Sonora megashear.