GEOCHRONOLOGIC AND GEOCHEMICAL EVIDENCE FOR EXTENSION OF THE BISBEE TROUGH TO THE LOWER PART OF THE MCCOY MOUNTAINS FORMATION IN SOUTHWESTERN ARIZONA

The McCoy Mountains Formation (MMF) consists of Jurassic and Cretaceous siltstone, sandstone, and conglomerate exposed in an east-west trending belt in southwestern Arizona and southeastern California. In the Dome Rock, McCoy, and Palen Mountains, the MMF consists of two fining-upward sequences, each broken into three members by Harding and Coney (1985, GSAB). The youngest zircons identified in a recent detrital zircon U-Pb study (Barth et al., 2004, GSAB) are middle Jurassic (179 Ma) for sandstone of basal member 1, middle Cretaceous (109 Ma) for overlying basal member 2, and upper Cretaceous for the top two members. Detrital zircons from four fault-bounded MMF sequences in western Arizona were analyzed to determine if these sequences could be correlated with specific parts of the MMF farther west. Detrital zircons from at or near the top of sections in the southern Little Harquahala, Granite Wash, New Water, and southern Plomosa Mountains all had only Jurassic or older zircons (396 grains analyzed by LA-ICPMS). An andesitic lava flow near the top of the section in the New Water Mountains yielded a U-Pb zircon date of 154.4 +/- 2.1 Ma (18 zircon grains). We interpret these new analyses to indicate that Mesozoic clastic strata in these areas are upper Jurassic and are broadly correlative with basal member 1 of the McCoy Mountains Formation. Six of twelve samples of the numerous basaltic sills and lava flows in the McCoy Mountains Formation in the Granite Wash, New Water, and southern Plomosa Mountains analyzed for Nd isotopes yielded initial epsilon Nd values (at t = 150 Ma) of between +4 and +6. These isotopic analyses, as well as zircon geochronology, support the interpretation that the lower MMF was deposited during rifting within the western extension of the Bisbee trough. Another sandstone sample, from the upper MMF in the Dome Rock Mountains, contained 80-102 Ma zircon grains (14 of 99 grains). Correlative strata in the Livingstone Hills are more feldspathic and have higher K-feldspar/plagioclase than lower McCoy sandstones. We infer that arkosic sands carrying a significant fraction of 80-110 Ma zircons and probably derived from the Cretaceous magmatic arc to the southwest reached at least as far east as the Livingstone Hills, but these sands are not represented in any of the MMF sections farther east in Arizona.