Geochemistry of laramide granitic rocks across the southern margin of the paleozoic north american continent, central sonora, Mexico

The geochemistry of Laramide granitic rocks from central Sonora was studied to relate possible compositional variations to the assimilation of different crustal sources. Most of the studied rocks are granodiorites collected near the southern boundary of the Paleozoic North American continent. North of this boundary, the Laramide plutons intruded a thick section of Upper Proterozoic and Paleozoic miogeoclinal strata, whereas south of it, the intrusives were emplaced in Lower Ordovician to Permian eugeoclinal rocks accreted to the continental margin between Late Permian and Middle Triassic times. Whole-rock Na₂O, TiO₂, and P₂O₅ contents are slightly higher in plutons intruded in cratonic and miogeoclinal rocks, whereas MgO and CaO seem to be higher in plutons emplaced in eugeoclinal rocks. The samples located in the north are characterized by steeper chondrite-normal-ized REE slopes and generally well developed negative europium anomalies, whereas the group of granitoids in the south have flatter REE slopes and generally smaller negative europium anomalies. Available isotope data show ⁸⁷Sr/⁸⁶Sr initial ratios above 0.7070, and ϵ_Nd initial values between −4.6 and −4.2 in plutons to the north. In a narrow E-W region just south of the Paleozoic continental margin, the samples yielded similar ⁸⁷Sr/⁸⁶Sr ratios but unexpectedly low ϵ_Nd values between −5.4 and −5.1. South of this region, the isotope signatures are slightly more primitive, with ⁸⁷Sr/⁸⁶Sr initial ratios between 0.7067 and 0.7057 and initial ϵ_Nd values between −3.9 and −3.7. The results of this study suggest that the nature of the assimilated crust may have influenced the final composition of the Laramide granitic rocks of central Sonora.