Geological Society of America Bulletin, published online on 26 January 2011 as doi:10.1130/B30206.1 Age and tectonic setting of the Mesozoic McCoy Mountains Formation in western Arizona, USA Jon E. Spencer1*, Stephen M. Richard1†, George E. Gehrels2§, James D. Gleason3#, and William R. Dickinson2** 1Arizona Geological Survey, 416 W. Congress Street, #100, Tucson, Arizona 85701, USA 2Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA 3Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA ABSTRACT tion was deposited during rifting within the rifting in Upper Jurassic mafi c lava fl ows and western extension of the Sabinas-Chihuahua- sills within the McCoy Mountains Formation The McCoy Mountains Formation consists Bisbee rift belt. Abundant 190–240 Ma zir- in western Arizona and within the lower Bisbee of Upper Jurassic to Upper Cretaceous silt- con sand grains were derived from nearby, Group in southeastern Arizona (Gleason et al., stone, sandstone, and conglomerate exposed unidentifi ed Triassic magmatic-arc rocks in 1999; Lawton and McMillan, 1999). Recogni- in an east-west–trending belt in southwest- areas that were unaffected by younger Juras- tion of a major angular unconformity within the ern Arizona and southeastern California. sic magmatism. A sandstone from the upper McCoy Mountains Formation in the Dome Rock At least three different tectonic settings have McCoy Mountains Formation in the Dome Mountains and Livingstone Hills of western been proposed for McCoy deposition, and Rock Mountains (Arizona) yielded numer- Arizona and a 79 ± 2 Ma tuff in strata above the multiple tectonic settings are likely over the ous 80–108 Ma zircon grains and almost no unconformity led to hybrid models for McCoy ~80 m.y. age range of deposition. U-Pb iso- 190–240 Ma grains, revealing a major reor- genesis, with the Upper Cretaceous succession topic analysis of 396 zircon sand grains from ganization in sediment-dispersal pathways derived from uplifted metamorphic and granitic at or near the top of McCoy sections in the and/or modifi cation of source rocks that had rocks in the south-vergent Maria fold and thrust southern Little Harquahala, Granite Wash, occurred by ca. 80 Ma. belt, which forms the northern limit of McCoy New Water, and southern Plomosa Moun- outcrops (Stone et al., 1987; Tosdal and Stone, tains, all in western Arizona, identifi ed only INTRODUCTION 1994; Dickinson and Lawton, 2001b). Jurassic or older zircons. A basaltic lava U-Pb geochronologic analysis of detrital- fl ow near the top of the section in the New The McCoy Mountains Formation is a zircon grains from the McCoy Mountains For- Water Mountains yielded a U-Pb zircon date several-kilometer-thick succession of Upper mation in its type area in the McCoy Mountains of 154.4 ± 2.1 Ma. Geochemically similar Jurassic and Cretaceous sandstone, siltstone, and of California allows division into a three-part lava fl ows and sills in the Granite Wash and conglomerate exposed in an east-west–trending succession (Barth et al., 2004), as follows: southern Plomosa Mountains are inferred to belt in western Arizona and southeastern Cali- (1) The lowest McCoy member, basal sand- be approximately the same age. We interpret fornia (Fig. 1; Harding and Coney, 1985; Lau- stone member 1 of Harding and Coney (1985; these new analyses to indicate that Meso- bach et al., 1987; Stone and Pelka, 1989; Tosdal member A of Stone and Pelka, 1989), consists zoic clastic strata in these areas are Upper and Stone, 1994). It rests on ~160 Ma, felsic of ~500 m of metasiltstone, quartzite, and chert- Jurassic and are broadly correlative with the volcanic rocks (Reynolds et al., 1987; Fackler- and quartzite-clast conglomerate. In the nearby lowermost McCoy Mountains Formation in Adams et al., 1997) that overlie lower Mesozoic Palen Mountains, this unit is interpreted to be the Dome Rock, McCoy, and Palen Moun- and Paleozoic strata of the North American cra- locally interbedded with underlying ca. 160 Ma tains farther west. Six samples of numerous ton (Miller, 1970; Stone et al., 1983). Subsi dence volcanic rocks or deposited within the inter- Upper Jurassic basaltic sills and lava fl ows that led to initial Late Jurassic clastic infl ux and stices of lava fl ow-top breccias before fi lling by in the McCoy Mountains Formation in the deposition of the McCoy Mountains Formation volcanic-derived weathering products (Busby- Granite Wash, New Water, and southern has been attributed to (1) transtensional rift- Spera et al., 1990; Fackler-Adams et al., 1997). ε Plomosa Mountains yielded initial Nd values ing along the postulated, left-lateral, Mojave- Of 65 detrital-zircon grains from this member (at t = 150 Ma) of between +4 and +6. The Sonora megashear (Harding and Coney, 1985; that were analyzed for 238U/206Pb, none were geochemistry and geochronology of this igne- Anderson and Nourse, 2005), (2) foreland-basin younger than 179 Ma, which is consistent with ous suite, and detrital-zircon geochronology genesis adjacent to the Cordilleran fold-thrust a Late Jurassic age suggested by association with of the sandstones, support the interpretation belt (Drewes, 1991), and (3) tectonic extension underlying volcanic rocks (Barth et al., 2004). that the lower McCoy Mountains Forma- along the western projection of the Sabinas- (2) Analysis of 82 zircon grains from two samples Chihuahua-Bisbee rift belt (Dickinson et al., of overlying basal sandstone member 2 (member 1989; Busby-Spera et al., 1990; Dickinson and C of Stone and Pelka, 1989) yielded nine grains *E-mail: [email protected] Lawton, 2001b). This last interpretation was younger than 125 Ma, eight of which are 116– †E-mail: [email protected] §E-mail: [email protected] strengthened by identifi cation of high positive 124 Ma. (3) Thirty-three of 120 zircon grains # ε E-mail: [email protected] Nd values and other geochemical indicators of analyzed from four samples of the upper McCoy **E-mail: [email protected] GSA Bulletin; Month/Month 2010; v. 1xx; no. X/X; p. 000–000; doi: 10.1130/B30206.1; 14 fi gures; 2 tables; Data Repository item 2011100. For permission to copy, contact [email protected] © 2011 Geological Society of America Geological Society of America Bulletin, published online on 26 January 2011 as doi:10.1130/B30206.1 Spencer et al. R ° ° i 115 E v 114 E e r s ° C 34 N id . o e ts x M a r i a M F o l d a n d T h r u s t B e l t M c ts. aarr o uv m c . Fig. 3 arrcuv s b t . H M s M a t l Lit ha t M s tl a Palen e . M Mts. a a rqu r s a ia Granite H M B o t ig s. M m Wash o a l M r Mts. c i a P C M . o t L N i y s tt M . le . ts H s ar 10 . D t qu N ahal o a Blythe M Mts e . m w a e s W o R a 10 Mu te m E le M o r ag . ts c o M le . th l ta rust k ts il ts P . M M ts. M . t s S e a l i . n u for N M i al a C on riz Kofa Mts. A 005500 km Tertiary volcanic rocks California Conglomerate, sandstone, and siltstone members (79 Ma tuff; <80 Ma dz) McCOY Arizona New Basal sandstone member 2 and mudstone member (<110 Ma dz) MOUNTAINS Mexico Basal sandstone member 1 (<160 Ma; <180 Ma dz) FORMATION Fig. 1 Jurassic volcanic rocks (160–175 Ma) Paleozoic and Mesozoic metasedimentary rocks Sonora Tertiary to Proterozoic crystalline rocks Figure 1. Geologic map of the McCoy Mountains Formation and the adjacent Maria fold and thrust belt and Mule Mountains thrust. Sources of mapping include Miller (1970), Stone and Pelka (1989), Sherrod et al. (1990), Tosdal (1990), Richard et al. (1994b), and Tosdal and Stone (1994). Abbreviations: dz—maximum age from detrital-zircon geochronologic analysis; Mts.—Mountains. Mountains Formation yielded 238U/206Pb dates of fl ows within the McCoy Mountains Formation arc axis (Busby-Spera et al., 1990; Riggs et al., 84–107 Ma and likely were derived from volu- to evaluate the tectonic signifi cance of syndepo- 1993), and emplacement of the Late Jurassic minous igneous rocks of the late Cretaceous sitional mafi c magmatism. Results of this study Independence dike swarm in California (James, Cordilleran magmatic arc (Barth et al., 2004). indicate much of the McCoy Mountains Forma- 1989; Glazner et al., 1999). The McCoy Moun- These geochronologic constraints on the age of tion in Arizona was deposited in a Late Juras- tains Formation was not deposited in an intra- the McCoy Mountains Formation led Barth et sic rift that formed a western extension of the arc setting, however, as indicated by the fact that al. (2004) to conclude that 90% of the McCoy Sabinas-Chihuahua-Bisbee rift belt. it contains essentially no arc volcanic rocks in Mountains Formation was deposited after ca. any of its numerous exposed sections. Exten- 116 Ma and is unrelated to regional rifting and THE JURASSIC MAGMATIC ARC sion also has been attributed to propagation of a extension that also produced the Bisbee basin. rift (aulacogen) during separation of the Yucatan Although strata in the Dome Rock Mountains The McCoy Mountains Formation was block from Texas and opening of the Gulf of and Livingston Hills in western Arizona can be deposited immediately following widespread Mexico, an alternative that does not require confi dently correlated with specifi c members and voluminous, ca.