RADIOGENIC ISOTOPE EVIDENCE ON THE ORIGIN OF MESOPROTEROZOIC ANORTHOSITES IN NORTHERN SONORA, MEXICO

Whole rock Nd, Sr, and zircon U-Pb and Hf isotopic data were obtained from a suite of Mesoproterozoic anorthosites and granitic rocks in northern Sonora to determine the sources of their parental magmas, refine Precambrian terrane boundaries, and to assess if these rocks were likely sources of ~1.1Ga detrital zircons in Neoproterozoic and Cambrian siliciclastic sedimentary rocks in southwestern North America. Six localities were sampled ranging from Sonoyta to the north and Cerro Prieto near Caborca to the south. Measured whole rock ε_Nd(0) values for the anorthosites range from -5.2 to -11.8, and both anorthosites and granitic rocks plot along ~1.1 Ga isochron with an initial ε_Nd ~-4. The Nd isotopic compositions overlap those of Yavapai/Mazatzal Paleoproterozoic basement and support the conclusion that the anorthosites were derived from parental melts that were derived from, or that interacted extensively with, local Precambrian basement rocks. High initial whole rock ⁸⁷Sr/⁸⁶Sr ratios of ~0.7095 are also consistent with a crustal origin for the anorthosites. Zircons in the anorthosites are low in abundance but were obtained from rocks in Quitovac and Cerro Prieto. Laser ablation ICPMS U-Pb ages from these zircons range are 1095 +/- 29 Ma and 1089 +/- 13 Ma, respectively, and are consistent with narrow range of zircon U-Pb ages observed for other Mesoproterozoic igneous rocks in northern Mexico. Hf isotopic compositions of these same zircon grains range from ε_Hf(0) =-24 to -20, overlapping the values observed for zircons in Mesoproterozoic igneous rocks intruding Yavapai/Mazatzal basement in northcentral Sonora (Escuadra granite) and the western U.S. (Pikes Peak). Given the low zircon fertility of the anorthosites, these rocks cannot be sources for the abundant Grenvillian detrital zircons deposited in western North America. In contrast, the Mesoproterozoic granitic rocks in Sonora are a potential source of such zircon grains, but only for low measured ε_Hf(0)(<-20) zircons found in Neoproterozoic basal sandstones in Sonora and not for the more abundant and widespread, higher ε_Hf(0) (-20 to -15), detrital zircons present in Cambrian sandstones throughout SW North America. The high eHf zircons are more likely derived from the erosion of Grenville age igneous rocks in Texas and Oklahoma.