Nd isotope constraints on sources of Neoproterozoic to Early Cambrian siliciclastic sedimentary rocks in northern Sonora

The chemical and Nd isotopic compositions of 10 Neoproterozoic to Early Cambrian siltstones and sandstones from the Caborca area in northwestern Sonora, Mexico, were determined to assess the provenance of the original sediments and to compare the sources of correlative miogeoclinal sedimentary rocks in northern Mexico and in the southwest United States. The sandstones and siltstones generally have trace element characteristics typical of material eroded from upper continental crust, with the exception of volcaniclastic sandstones from Unit 1 of the early Cambrian Puerto Blanco Formation that lack negative Eu, Nb, Ta, and Ti anomalies. Measured epsilon (Nd) values vary from -22.1 to -4.2, with Nd model ages (T-DM) varying from 0.9-2.6 Ga. The least negative epsilon (Nd) values correspond to the chemically distinct sandstone samples from Unit I of the Puerto Blanco Formation. These data indicate that much of the siliciclastic detritus represented by the Neoproterozoic to Early Cambrian miogeocline in northern Sonora was derived from subjacent Paleoproterozoic crust of western North America. Despite the greater proximity of the Sonora portions of the Cordilleran miogeocline to 1.0-1.3 Ga Grenville crust relative to its northern portions, there is little Nd isotope evidence that the Grenville crust provided significant amounts of detritus to this portion of the miogeocline. Only Unit I of the Puerto Blanco has sufficiently high epsilon (Nd) values to represent detritus derived from Grenville crust, but the high epsilon (Nd) values can also be attributed to the presence of Cambrian-age volcaniclastic material within the original sediments that comprise this unit. The Nd isotope stratigraphy of the Carborca miogeoclinal rocks is similar to that of correlative sedimentary rocks in the southern Great Basin of the western United States, but more detailed isotopic and petrographic data will be necessary before it can be concluded that the Caborca block is allochthonous with respect to eastern portions of northern Mexico and has been tectonically transported to its current position along the Mojave-Sonora megashear. (C) 2001 Elsevier Science Ltd. All rights reserved.