EARLY CORDILLERAN ARC MAGMATISM RECORDED BY DETRITAL ZIRCON GRAINS IN LOWER TO MIDDLE TRIASSIC STRATA OF SOUTHWESTERN NORTH AMERICA

Lower to Middle Triassic strata in SW Laurentia were deposited in a pro-foreland basin that formed in flexural response to emplacement of the Golconda allochthon during the Sonoma orogeny. Simultaneously, NE-directed subduction beneath Laurentia in NW Sonora and the northern Mojave Desert generated magmatism from 275-240 Ma, forming the early Cordilleran magmatic arc. The effect of subduction and arc magmatism on Lower to Middle Triassic sedimentation has been challenging to resolve due to subsequent tectonism southwest of the Colorado Plateau. In order to test the contribution of the Permo-Triassic Cordilleran magmatic arc to Lower to Middle Triassic strata, samples were collected for detrital zircon analysis from three locations along a roughly N-S transect from southern Nevada to NW Sonora. The samples are from units interpreted to be equivalent to the basal Moenkopi Formation and metasedimentary Buckskin Formation. A sample of calcareous sandstone from the Spring Mountains contains Permian to Neoarchean grains with age peaks related to the Acadian orogeny, Gondwanan crust of the Ouachita-Marathon-Sonora belt, the Grenville province, and the Mazatzal province. A quartzite sample from the New York Mountains contains three grains younger than 280 Ma that are likely derived from the Cordilleran arc but otherwise has similar age peaks as the Spring Mountains sample. In NW Sonora, basal Buckskin Formation schist contains Triassic through Archean grains with peaks related to the Cordilleran magmatic arc, Paleozoic Appalachian orogenies, Gondwanan crust, and Grenville, Yavapai, and Mazatzal provinces. An overlying metaconglomerate bed is dominated by Cordilleran arc grains that suggest continuous magmatism from at least 268-248 Ma in Sonora. These results indicate that the Cordilleran magmatic arc was a significant source of sediment in Lower to Middle Triassic strata in the Sonora region and not significant in the northern Mojave region. Based on a compilation of stratigraphic thickness in the arc-proximal region the NE-trending isopachs of the Colorado Plateau continued to the SW, so the Sonora segment of the arc was in an area of significantly less flexural subsidence than the northern Mojave segment, and also upstream with respect to the regional paleocurrent direction, thus having a higher potential to become a significant sediment source.