The Proterozoic of NW Mexico revisited: U–Pb geochronology and Hf isotopes of Sonoran rocks and their tectonic implications

Several Proterozoic basement units crop out in the Sonora State of NW Mexico, and the same can be correlated with crustal provinces of southern Laurentia in the neighboring southwestern USA. Zircon U–Pb and Hf isotopic determinations in more than 300 grains separated from igneous and metaigneous rocks from these units indicate that the crystalline basement in Sonora is made up of different components, which are from west to east: (1) The Caborca–Mojave province to the west, characterized by the so-called Bámori Complex, have U–Pb ages between 1696 and 1772 Ma, with moderately juvenile to slightly evolved εHf values, yielding T_DM ages of ca. 2.1–2.4 Ga; (2) in the intermediate area, east of Hermosillo, the Palofierral and La Ramada orthogneiss units yield an age of 1640 and 1703 Ma, respectively, both having juvenile εHf with the Palofierral overlapping the depleted mantle curve at ca. 1.65 Ga; and (3) in the northeastern Sonora, samples from the southern extension of the Mazatzal province, represented by the Pinal Schist, yielded ages between 1674 and 1694 Ma, with moderately juvenile to juvenile εHf values and a T_DM age of ca. 1.9 Ga. In addition, a suite of post-tectonic granites was also studied in Caborca (San Luis granite) as well as in northeastern Sonora (Cananea granite), both yielding ages of ca. 1.44 Ga with moderately juvenile εHf values ranging from −1 to +8 and T_DM dates of ca. 1.8–1.9 Ga and 1.6–1.7 Ga, respectively. These two isotopically contrasting provinces may imply the existence of a Proterozoic paleo-suture. However, if the Palofierral gneiss, of which the Hf signature straddles the depleted mantle array, is taken as the source for the 1.44 Ga Cananea granite, then the location of such a suture zone should lay farther south than the proposed trace of the Mojave–Sonora megashear.