CONTRASTING DETRITAL ZIRCON ASSEMBLAGES AND PROVENANCE OF UPPER JURASSIC AND LOWER CRETACEOUS STRATA, SOUTHEAST ARIZONA AND SOUTHWEST NEW MEXICO

The stratigraphic sections that include the Jurassic Canelo Hills Volcanics and Glance Conglomerate and the Lower Cretaceous Bisbee Group (coeval Morita and Hell-to-Finish Formations) record a dramatic shift in Mesozoic sediment sourcing and basin connectivity. LA-ICPMS U-Pb ages from zircons in tuffs of the Canelo Hills Volcanics and Glance Conglomerate of the Huachuca Mountains and Canelo Hills of southeastern Arizona yield Early to Middle Jurassic ages. The thick upper caldera-fill tuff of the Canelo Hills Volcanics yielded an age of 178.4 ± 1.1 Ma (n = 36; MSWD = 1.3). Two outflow tuffs interbedded with rhyolite- and limestone-clast conglomerate, assigned to the Canelo Pass member of the Glance Conglomerate, have ages of 174.8 ± 1.0 Ma (n = 29; MSWD = 1.06) and 172.7 ± 0.9 Ma (n = 36; MSWD = 1.11). Formerly, the outflow tuffs were considered Late Jurassic on the basis of whole-rock Rb-Sr isochron ages. A detrital zircon sample from a volcanic litharenite high in the Glance Conglomerate in SW Huachuca Mountains contains dominantly Early to Middle Jurassic grains (81 of 96 grains), indicating it was derived almost exclusively from underlying volcanic strata. In contrast, a detrital zircon sample from a volcanic litharenite in the overlying Morita Formation has few Early to Middle Jurassic grains (11 of 97 grains) but abundant Early Cretaceous grains (ca. 145 – 120 Ma) and pre-Jurassic grains, suggesting a distant source to the west. In the Little Hatchet Mountains, New Mexico, the Glance Conglomerate is absent, and the Hell-to-Finish Formation conformably overlies Late Jurassic marine strata. The Hell-to-Finish sandstones are litharenites with abundant chert and carbonate clasts and no volcanic fragments. A Hell-to-Finish detrital age sample was almost exclusively pre-Jurassic (78 of 79 grains) and was sourced primarily from the Burro uplift to the north. U-Pb geochronology indicates that southeast Arizona is a locally important igneous source area for Early to Middle Jurassic zircons. Contrasting age data suggest the Hell-to-Finish Formation of Little Hatchet Mountains was deposited in a basin isolated from the Morita Formation of Huachuca Mountains.