Abstract

A detailed geochemical characterization of 19 representative Proterozoic basement rocks in the Quitovac region in northwestern Sonora, Mexico, has identified two distinct Paleoproterozoic basement blocks that coincide spatially with the previously proposed Caborca and “North America” blocks. New U-Pb zircon geochronology revises their age ranges, the Caborca (1.78–1.69 Ga) and “North America” (1.71–1.66 Ga) blocks at Quitovac, and precludes a simple age differentiation between them. In addition, Grenvillian-age granitoids (ca. 1.1 Ga), spatially associated with the Caborca block have been identified at Quitovac.

Nd isotopes and major- and trace- element geochemistry support the distinction of these Paleoproterozoic blocks. Granitoids of the “North America” block are characterized by depleted εNd values (3.4–3.9) and younger Nd model ages (1800–1740 Ma) and have lower K2O, Y, Rb, Ba, Th, REE, and Fe/Mg values than coeval rocks of the Caborca block. The Caborca block granitoids are likewise characterized by slightly less depleted εNd (0.6–2.6) and older Nd model ages (2070–1880 Ma). Despite the subtle differences, granitoids from both the Caborca and “North America” blocks exhibit island arc-like affinities.

We propose that the Proterozoic basement rocks from the Quitovac region are an extension of the Proterozoic crustal provinces in the southwestern United States. Specifically, rocks of the Caborca block exhibit an affinity to rocks of either the Yavapai province or the Mojave– Yavapai transition zone, whereas rocks of the “North America” block have signatures similar to those of the Mazatzal province or possibly the Yavapai province of Arizona.

The new isotopic ages and geochemical data do not support the existence of the Late Jurassic Mojave–Sonora megashear at Quitovac, as originally proposed. However, the Quitovac region accounts only for a small fraction of the Proterozoic basement in Sonora, so these findings do not eliminate the possibility of a megashear elsewhere in northern Sonora.

Our new data create the possibility of alternative hypotheses for the distribution of Paleoproterozoic crustal provinces in southwestern North America that affect reconstructions of the original southwestern margin of Laurentia, and reduce uncertainties in the configuration, timing, and existence of the Proterozoic supercontinent, Rodinia.

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