LOCATION OF THE SOUTHWESTERN EDGE OF PROTEROZOIC NORTH AMERICA BASED ON SR AND ND ISOTOPIC DATA FROM LARAMIDE GRANITIC ROCKS OF SONORA, NW MEXICO

Numerous exposures of Paleo- and Mesoproterozoic crystalline rocks and Neoproterozoic platform strata occur in northern and central Sonora. However, how far the Proterozoic North American crust extends into Mexico is unknown because of the Phanerozoic cover rocks. Paleogeographic reconstructions suggest that the Paleozoic passive continental margin of North America extended approximately E-W by southern Sonora, but its original position was modified between Late Permian and Middle Triassic times, when the deep-water sequences were compressionaly-transported onto the shallow-water rocks. This facies boundary may continue in the same direction to coastal Sonora, where it presumably bent northwest, until dissected by the San Andreas Fault system. Other important paleogeographic feature takes place to the north of the Sonora-Sinaloa limit, where the northernmost extension of the mid- to late Mezosoic island arc-related Guerrero terrane is expected to occur. Radiogenic isotope data from granitic rocks along the broad Laramide belt that intruded these basement assemblages in a NW-SE direction reveal important clues to elucidate the nature of the underlying crust. Departing from relatively old -but still valid- concepts, assimilation of old North American rocks may have caused initial strontium ratios of granitic magmas to rise above 0.7060. Assuming this, a line marking the southwestern edge of Proterozoic North America can be traced NW-SE from north of Isla Tiburón down to Guaymas. Then, the line bends in a nearly E-W direction north of Alamos, approximately 200 km south of Hermosillo, continuing eastwards beneath the mid-Tertiary Sierra Madre Occidental volcanic province. Moreover, our data suggest a possible analogue line characterized by an epsilon Nd value of -3.4, which essentially describes a similar trajectory. North of this boundary, the Phanerozoic units are assumed to be underlain by Precambrian North American rocks, whereas south of it, the more primitive Sr and Nd isotopic signatures suggest a more juvenile source. Connecting earlier studies of granitic rocks in Nevada and California, our results accomplish a coherent model to configurate the southwestern edge of Proterozoic North America.