Geochemistry of the Coastal Sonora batholith, Northwestern Mexico

Abstract

New major- and trace-element and radiogenic Sr and Nd data from granitic rocks help to characterize the geochemical composition of the Coastal Sonora batholith, which represents the westernmost portion of the Laramide Magmatic arc (~80–40 Ma) in Mexico. Compared with more inland-located regions of the arc in central and eastern Sonora (Inner Granites), the plutons within the coastal batholith have higher MgO, FeO, and CaO and lower K2O contents, and tend to be more depleted in Nb, U, and to a lesser extent Rb, but relatively enriched in Sr. These rocks display flatter slopes of rare-earth elements, with La abundances around 80 to 200 times chondrite values and smaller but well-developed negative Eu anomalies. Initial 87Sr/86Sr ratios and εNd values from a few studied samples of the Coastal Sonora batholith range from 0.70586 to 0.70679 and from –2.3 to –4.7, respectively. These isotopic signatures allow recognition of a more mantle-related magma source for this batholith as compared with the Inner Granites, which have higher Sr and lower Nd ratios. In a more regional context, the compositions of the Coastal Sonora batholith rocks resemble those of the eastern portion of the Peninsular Ranges batholith to the northwest. Based on our observations, we propose that partial melting of a crust thickened by overlapping periods of Jurassic through Cretaceous arc magmatism along the southwestern edge of the North American continent may account for the geochemical and isotopic characteristics displayed by the Coastal Sonora batholith granitic rocks.