Whole-rock Nd isotopic data and U-Pb zircon geochronology from Precambrian crystalline rocks in the Caborca area, northern Sonora, reveal that these rocks are most likely a segment of the Paleoproterozoic Mojave province. Supporting this conclusion are the observations that paragneiss from the ≥1.75 Ga Bamori Complex has a 2.4 Ga Nd model age and contains detrital zircons ranging in age from Paleoproterozoic (1.75 Ga) to Archean (3.2 Ga). Paragneisses with similar age and isotopic characteristics occur in the Mojave province in southern California. In addition, “A-type” granite exposed at the southern end of Cerro Rajon has ca 2.0 Ga Nd model age and a U-Pb zircon age of 1.71 Ga, which are similar to those of Paleoproterozoic granites in the Mojave province. Unlike the U.S. Mojave province, the Caborcan crust contains ca. 1.1 Ga granite (Aibo Granite), which our new Nd isotopic data suggest is largely the product of anatexis of the local Precambrian basement. Detrital zircons from Neoproterozoic to early Cambrian miogeoclinal arenites at Caborca show dominant populations ca. 1.7 Ga, ca. 1.4 Ga, and ca. 1.1 Ga, with subordinate Early Cambrian and Archean zircons. These zircons were likely derived predominately from North American crust to the east and northeast, and not from the underlying Caborcan basement. The general age and isotopic similarities between Mojave province basement and overlying miogeoclinal sedimentary rocks in Sonora and southern California is necessary, but not sufficient, proof of the hypothesis that Sonoran crust is allochthonous and was transported to its current position during the Mesozoic along the proposed Mojave-Sonora megashear. One viable alternative model is that the Caborcan Precambrian crust is an isolated, autochthonous segment of Mojave province crust that shares a similar, but not identical, Proterozoic geological history with Mojave province crust found in the southwest United States

Structural relief resulting from middle Tertiary extensional deformation in the Chemehuevi Mountains exposes a unique cross section through a temporally and compositionally zoned (both vertically and horizontally), laccolith-shaped intrusion of Late Cretaceous age. The calc-alkalic, metaluminous to peraluminous Chemehuevi Mountains Plutonic Suite exhibits crude normal, vertical, and temporal zonation. The zones are progressively younger and more felsic away from the roof and walls; the most differentiated material is concentrated toward the center and floor of the intrusion. Hornblende-biotite- and biotite granodiorite are metaluminous and form the outer margin of the intrusion along the northern and southern walls, and sill-like bodies in an older suite of granitoids and Proterozoic basement rocks. Locally these rocks bear a sub-horizontal, southwest-trending, mylonitic lineation, considered to be synchronous with regional mylonitic deformation. Later and more evolved units are subequigranular to porphyritic, metaluminous to weakly peraluminous biotite granodiorite to granite, and make up the greatest proportion of the intrusion. The youngest, most leucocratic members of the suite are undeformed, locally garnetiferous muscovite granite and granodiorite that form the central part of the intrusion. Major, trace, and rare earth element data indicate that the magmas of the Cheme-huevi Mountains Plutonic Suite became progressively enriched in Si, K, Rb, Mn, Y, U, and heavy rare earth elements (REE). Fractional crystallization of some REE–rich accessory minerals was important in producing some of these trends. Although modest compositional breaks occur across internal contacts, the general continuity of trends from field, modal, and chemical data suggests that these rocks constitute a comagmatic intrusive suite. Estimates for the pressure of emplacement of the suite vary from 4 to 6 kbar, or a minimum depth of 12 km. Preliminary Pb-, Sr-, and oxygen-isotopic data, together with the REE chemistry, suggest that the Chemehuevi Mountains Plutonic Suite was derived from a heterogeneous crustal source. Compositional variations within the plutonic suite are consistent with open-system fractionation, involving fractional crystallization of discrete batches of magma derived from the melting of a heterogeneous crustal source under H 2 O-saturated conditions.