PROVENANCE CONTRASTS REVEALED BY U-PB AGE POPULATIONS OF DETRITAL ZIRCONS IN CHINLE SANDSTONES OF THE FOUR CORNERS REGION, SOUTHWEST US

Detrital zircons (DZ) were sampled from five fluvial sandstone units of the Upper Triassic Chinle Group in the Four Corners region of the Colorado Plateau: Karnian lower Chinle (Shinarump) near Cameron (NW AZ) and Sanders (NE AZ), and Norian upper Chinle on the Defiance Plateau (Sonsela of NE AZ), in the Chama basin (Poleo of NW NM), and near Elk Ridge (Moss Back of SE UT). Shinarump and Sonsela in Arizona were deposited by streams flowing north, whereas Poleo and Moss Back farther north were deposited by northwesterly flow. U-Pb ages were determined for 500 individual DZ grains (~100 per sample) by laser ablation ICPMS using a beam diameter of 35 microns. Analytical data were filtered to exclude DZ grains with >20% age discordance or poor precision, leaving 448 reliably dated grains. The DZ grains range dominantly from 210 Ma to ~1800 Ma, with a few older Paleoproterozoic and Archean grains present in all except Cameron Shinarump. A prominent Permian-Triassic age spike of 225-295 Ma in southern samples may reflect derivation of detritus in part from the coeval Cordilleran arc assemblage, although the older (>245 Ma) fraction of that age population was probably derived instead from the Gondwanan East Mexico arc (232-284 Ma). Penecontemporaneous grains of arc derivation are present only in Cameron Shinarump (~220 Ma) and Sonsela (~210 Ma) samples. Abundant Paleozoic-Panafrican (300-950 Ma) grains in northern samples were probably reworked from the Ouachita orogen, but are less abundant in southern samples. Grains of Grenville age (950-1350 Ma) in most samples were probably recycled from older sedimentary successions, but are nearly absent from the Cameron Shinarump sample. Southern samples contain numerous 1400-1800 Ma grains derived from basement rocks of southwest Laurentia, but that age population is less abundant in northern samples. The DZ age pattern implies that southern samples were derived mainly from exposed basement and arc assemblages lying south of the Colorado Plateau, whereas detritus in northern samples were fed mainly from eastern headwaters flanking the Ouachita orogen. The DZ age spectra for Moss Back and Poleo are closely comparable to those for coeval Trujillo Formation (Chinle-Dockum Group) exposed farther east and closer to the Ouachita orogen on the High Plains of northeast New Mexico and northwest Texas.