GNEISSES OF THE NW SONORA-SW ARIZONA BORDER REGION: TWO DISTINCT PALEOPROTEROZOIC COMPLEXES AT THE SOUTHWEST EDGE OF LAURENTIA

New geological mapping, SHRIMP U-Pb zircon analyses, and Sm-Nd isotopic data from the international border region southeast of Yuma distinguish two contrasting Paleoproterozoic gneiss complexes near the truncated margin of Laurentia. The western complex is composed of finely banded felsic, intermediate, and mafic orthogneisses intruded by hornblende-biotite granodiorite (1729±18 Ma), porphyritric biotite granite (1726±15 Ma), and leucocratic biotite monzogranite (1696±11 Ma). Exposed in a west-northwest trending fold belt, these rocks preserve penetrative southwest-vergent shear fabrics annealed by crystalloblastic textures. Gneisses of the eastern complex consist of recrystallized arkose, quartzite, and biotite-quartz-feldspar schist interlayered with strongly foliated biotite granite (augen gneiss). Detrital zircons from four meta-sandstone samples indicate a maximum depositional age of 1690 Ma. Three samples of augen gneiss yield concordia upper intercept ages between 1655 Ma and 1642 Ma. The augen gneisses and host strata display southeasterly lineation and crystalloblastic foliation that is folded about south trending hinges and sharply intruded by Mesoproterozoic granite (1405±10 Ma) and 1.1 Ga(?) diabase dikes. Lead-loss trajectories suggest that regional metamorphism persisted until ~1600 Ma. Analyses of rims indicate new zircon growth at ~1430 Ma. Both gneiss complexes preserve depleted eNd values (+1 to +4), distinct from the Mojave province, but overlapping with Yavapai and Mazatzal provinces of central Arizona. Granitoids of the western complex have “Yavapai-type” isotopic characteristics but display structures discordant to the southwesterly Yavapai trend in central Arizona. The eastern complex has the appropriate age/isotopic signature to be an extension of the Mazatzal province, but records major tectonism at ~1.6 Ga that postdates the Mazatzal orogeny. These characteristics raise intriguing questions about possible oroclinal bending or tectonic disruption of the Yavapai-Mazatzal belt along the Sonoran margin of Laurentia.