Paleoproterozoic Orthogneisses from the Cerro Prieto Area: A New Addition to the Mexican Yavapai Province in NW Sonora

The Cerro Prieto area is located approximately 80 km NW of the town of Caborca and 40 km to the south of Quitovac in NW Sonora, Mexico. Three samples from an orthogneiss unit in this area have been dated by U-Pb zircon geochronology (SHRIMP-RG) yielding crystallization ages at 1687 ± 9, 1683 ± 9 and 1682 ± 9 Ma. This Paleoproterozoic orthogneiss unit at Cerro Prieto was overthrust by a middle-late Jurassic andesite-rhyolite unit during the Laramide Orogeny. Throughout the same compressional event, this volcanic unit was in turn overthrust by a unit of Mesoproterozoic (~1.1 Ga) metagranite.

Major and trace element geochemistry for two of the orthogneiss samples suggests that these rocks have affinities with granitic rocks formed in volcanic arc environments; the third sample plots in the within-plate field of granite formation (Rb vs Yb+Ta). Values for the sum of REE (129.76, 315.57 and 419.59), K2O (3.89, 5.03 and 3.73), TiO2 (0.29, 0.73 and 1.41), Fe2O3 total (2.55, 4.66 and 9.11) and the (Lu/Yb)N (13.89, 13.17 and 12.03) indicate an overall enrichment of these rocks with respect to the older (1.70-1.77 Ga) Paleoproterozoic Yavapai-type rocks from the Quitovac region. These geochemical differences could be explained by the fact that the Quitovac rocks, which are older, overall, could represent the source rocks for the Cerro Prieto orthogneisses.

Whole rock epsilon Nd(initial) values of +1.70, +1.40 and +0.52, and the corresponding Nd model ages (TDM) at 1.85, 1.86 and 1.91 Ga, suggest that these orthogneisses (magmas) were formed mostly from mantle sources with some degree of crustal contribution which is similar to the rocks from the Yavapai province in SW North America (including Quitovac). It is likely that if Mojave-type rocks exist in NW Sonora (our current working hypothesis), they would be located just west of the Cerro Prieto area.