Jesús Roberto Vidal-Solano

Geochemical and geochronological constraints on the geologic evolution of the western Sonobari Complex, northwestern Mexico

In the southern Sierra Sonobari, NW Mexico, U-Pb and 40Ar/39Ar geochronology studies allowed to define the provenance and maximum depositional age of the Francisco Gneiss basement of the Sonobari terrane, and to establish the age of some magmatic events in that area. The youngest zircon cluster in paragneisses of the Francisco Gneiss indicates a maximum depositional age of 509 ± 29Ma. The main peaks of the relative probability plot yield ages of 1690 and 1404Ma with minor peaks at 1156, 921, and 517Ma. Major peaks suggest that the main source of sediments was the Paleo- and Mesoproterozoic crust of Laurentia. Orthogneiss from the Francisco Gneiss yields a U-Pb zircon upper intercept age of 248 ± 28Ma, which is interpreted as the crystallization age. Crosscutting dykes of metabasite yield an 40Ar/39Ar age of 67 ± 5Ma, which is interpreted as indicating cooling after either a latest Early Cretaceous orogenic event or Late Cretaceous contact metamorphism. Granodiorite intruding the Francisco Gneiss yields a U-Pb age of 64 ± 1Ma, which is interpreted as a magmatic age. The hornblende-plagioclase Macochin Gabbro yields 40Ar/39Ar isochron ages of 54 ± 10Ma and 47 ± 5Ma, which are interpreted as cooling ages after the gabbro intrusion. Geochemical data indicate that the mafic rocks of the Francisco Gneiss correspond to subalkaline basalts of tholeiitic affinity with concentrations of high field strength elements similar to oceanic basalts, suggesting an asthenospheric upper mantle source. However, according to the variation in Th/Yb and U/Yb, the amphibolites display a significant influence of the upper continental crust. The Macochin Gabbro also has a geochemical signature characteristic of subalkaline basalt with tholeiitic affinity, and high field strength elements similar to oceanic basalts. Tectonic discrimination diagrams and elemental distribution suggest that the mafic rocks of both the Francisco Gneiss and Macochin Gabbro were emplaced during rifting in a back-arc setting.