Age and tectonic setting of the Mesozoic McCoy Mountains Formation in western Arizona, USA

Abstract

The McCoy Mountains Formation consists of Upper Jurassic to Upper Cretaceous silt- stone, sandstone, and conglomerate exposed in an east-west-trending belt in southwest- ern Arizona and southeastern California. At least three different tectonic settings have been proposed for McCoy deposition, and multiple tectonic settings are likely over the ~80 m.y. age range of deposition. U-Pb iso- topic analysis of 396 zircon sand grains from at or near the top of McCoy sections in the southern Little Harquahala, Granite Wash, New Water, and southern Plomosa Moun- tains, all in western Arizona, identifi ed only Jurassic or older zircons. A basaltic lava fl ow near the top of the section in the New Water Mountains yielded a U-Pb zircon date of 154.4 ± 2.1 Ma. Geochemically similar lava fl ows and sills in the Granite Wash and southern Plomosa Mountains are inferred to be approximately the same age. We interpret these new analyses to indicate that Meso- zoic clastic strata in these areas are Upper Jurassic and are broadly correlative with the lowermost McCoy Mountains Formation in the Dome Rock, McCoy, and Palen Moun- tains farther west. Six samples of numerous Upper Jurassic basaltic sills and lava fl ows in the McCoy Mountains Formation in the Granite Wash, New Water, and southern Plomosa Mountains yielded initial e Nd values (at t = 150 Ma) of between +4 and +6. The geochemistry and geochronology of this igne- ous suite, and detrital-zircon geochronology of the sandstones, support the interpretation that the lower McCoy Mountains Forma- tion was deposited during rifting within the western extension of the Sabinas-Chihuahua- Bisbee rift belt. Abundant 190-240 Ma zir- con sand grains were derived from nearby, unidentifi ed Triassic magmatic-arc rocks in areas that were unaffected by younger Juras- sic magmatism. A sandstone from the upper McCoy Mountains Formation in the Dome Rock Mountains (Arizona) yielded numer- ous 80-108 Ma zircon grains and almost no 190-240 Ma grains, revealing a major reor- ganization in sediment-dispersal pathways and/or modifi cation of source rocks that had occurred by ca. 80 Ma.