Age and Tectonic Setting of the Mesozoic Mccoy Mountains Formation in Western Arizona, USA

Home , Dome Rock Mountains, Granite Wash Mountains, New Water Mountains, Plomosa Mountains

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.

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

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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. 160–180 Ma arc magma- associated arc magmatism (Bilodeau, 1982). of the McCoy Mountains Formation in Cali- tism along the southwestern continental mar- fornia (Harding and Coney, 1985; Tosdal and gin of North America (e.g., Tosdal et al., 1989; THE McCOY MOUNTAINS Stone, 1994), similar strata elsewhere in west- Mayo et al., 1998; Barth et al., 2008; Haxel et FORMATION IN WESTERN ARIZONA ern Arizona are of uncertain detailed correlation al., 2008b). Termination of magmatism has because of deformation, metamorphism, and been attributed to slab rollback that was associ- The most extensive exposures of the McCoy complex, regionally variable stratigraphy. We ated with intra-arc to backarc extension in the Mountains Formation, located in the Dome undertook two complementary studies, reported McCoy and Bisbee basins (Bilodeau, 1982; Rock Mountains in Arizona and the McCoy and here, to clarify the age and tectonic signifi cance Lawton and McMillan, 1999; Dickinson and Palen Mountains in California (Fig. 1), form of these Arizona strata: U-Pb isotopic analysis Lawton, 2001a, 2001b). Intra-arc extension is two broadly fi ning-upward successions with a of detrital zircons to constrain age and sediment suggested by the geochemistry of some late arc total thickness of ~5–8 km (right two columns sources, and geochronological, geochemical, magmatism (Young et al., 1992; Haxel et al., in Fig. 2; Harding and Coney, 1985; Stone and and Nd isotopic analysis of mafi c sills and lava 2008a), accumulation of sediment within the Pelka, 1989; Tosdal and Stone, 1994). Each of

2 Geological Society of America Bulletin, Month/Month 2009 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 these two successions was divided by Harding ing stratigraphic order, the (4) conglomerate, scour overlain by medium to coarse sandstone and Coney (1985) into three members, which (5) sandstone, and (6) siltstone members. These and pebble to cobble conglomerate, in turn are, from base to top, as follows: (1) Basal sand- members are distinctly arkosic and contain a overlain by generally fi ning upward sandstone stone member 1 consists of quartz arenite with signifi cant fraction of K-feldspar, in contrast and siltstone (Robison, 1980; Harding and associated maroon siltstone and mudstone and to quartzose and lithic-rich sands of underlying Coney, 1985). Tabular and festoon cross beds minor quartzite-clast conglomerate. (2) Basal members. In the Dome Rock Mountains and in are common in the sandstones. Siltstones at the sandstone member 2 consists of sandstone and the Livingston Hills in western Arizona (Figs. 2 top of each cycle may include calcareous silt- siltstone, with less quartzose sandstone that and 3), the conglomerate member rests with stone, gypsum, and desiccation cracks. These contains substantial plagioclase and volcanic angular unconformity on underlying members strata have been interpreted as cyclothems rep- and sedimentary lithic grains. Sparse conglom- (Tosdal and Stone, 1994). resenting meandering stream deposits (Robi- erate commonly contains a signifi cant frac- Depositional environments of the McCoy son, 1980). The overlying mudstone member tion of volcanic-rock clasts. (3) The mudstone Mountains Formation are generally interpreted includes similar strata but with a much greater member consists of siltstone and mudstone with as fl uvial, deltaic, and lacustrine. The lower two proportion of siltstone and mudstone. These less common sandstone and minor conglomer- McCoy members are characterized by fi ning- strata have been interpreted as partially lacus- ate. The three highest members are, in ascend- upward cycles ~5–20 m thick, each with a basal trine. The conglomerate and sandstone members in the upper McCoy Mountains Formation contain lenticular, poorly sorted conglomerate and McCoy Mts. sandstone beds with basal scours interpreted as 8 (Stone and Pelka, 1989) alluvial fan and braided stream deposits (Hard- ing and Coney, 1985). The uppermost siltstone L K member contains freshwater brachiopods and J fossil logs that are consistent with inferred distal 7 Siltstone I fl uvial, deltaic, and lacustrine depositional envi- H member ronments (Harding and Coney, 1985). Paleocur- Dome Rock Mts. G rent data indicate dominantly southward and 6 New Water (Tosdal and Stone, Mts. (Richard 1994) F southeastward sediment transport throughout and Spencer, deposition of the McCoy Mountains Formation 1994) Upper McCoy79 Ma Sandstone (Robison, 1980; Harding and Coney, 1985). F member 5 Lower McCoy tuff Other strata correlated with the McCoy Mountains Formation are exposed in about a Conglomerate dozen areas in western Arizona east of the Dome 4 F member Rock Mountains (Fig. 3). The strata are gener- km ally lithologically similar to McCoy exposures 154 Ma farther west, with similar inferred depositional 3 Mudstone environments, but lithologic and stratigraphic E member dissimilarities generally preclude confi dent cor- SW Plomosa relation to specifi c McCoy members (except 2 for strata in the Livingstone Hills and quartz- Mts. (Richard D et al., 1993) rich sandstone in the southwestern Plomosa Basal Mountains and northwestern Little Harquahala sandstone 1 C member 2 Mountains). Correlation with specifi c McCoy members is also hindered by moderate to severe B Basal deformation within the late Cretaceous, east- A sandstone 0 member 1 west–trending Maria fold and thrust belt (Reyn- olds et al., 1986; Spencer and Reynolds, 1990; Crystal Hill formation Detrital zircons ≥116 Ma Tosdal and Stone, 1994) and greenschist-grade (Barth et al., 2004) metamorphism related to thrust burial and late Lower McCoy Mountains Middle and Upper Cretaceous granitoid intrusion (Hoisch, 1987; Formation McCoy Mountains Reynolds et al., 1988). Furthermore, the outcrop Formation areas are separated from each other by faults, Mafic lava flows and sills Siltstone intrusions, and Cenozoic volcanic cover and Fine grained, volcanic-lithic basins. East-northeast–west-southwest extension sandstone Sandstone within the Basin and Range province, including Volcanic-lithic sandstone Conglomerate extensional detachment faulting, generally tilted Quartz-rich, basal and separated fault blocks and, in the Harqua- sandstone member 1 Mudstone hala Mountains, uncovered McCoy strata previ- Unit designations from Unit designations from (1985) and Coney Harding Unit designations from Unit designations from (1989) Stone and Pelka ously buried by thrusting (Richard et al., 1990a, Figure 2. Stratigraphic columns for four successions of the McCoy Mountains Formation. 1990b; Spencer and Reynolds, 1991). All four successions rest on Jurassic volcanic rocks. See Figure 1 for locations of ranges. Previous workers established the general Mts.—Mountains. petrologic character of the McCoy Mountains

Geological Society of America Bulletin, Month/Month 2009 3 Geological Society of America Bulletin, published online on 26 January 2011 as doi:10.1130/B30206.1

Spencer et al.

114° 00′ E Arrastre Gulch Northern . ts M Plomosa a al Mts. Granite h qua “S” Mountain ar Wash H Plomosa detachment fault Mts.

Weldon Ranegras Hill area Southeast . Red s Little Little t Hills M Harquahala Harquahala New Harquar Mts. sa Water Mts. o Figure 5 m

o l

P N e . w Wate Black S Rock 10 Hills Apache r Wash Mts . Ea 33° 30′ N glet ail Crystal Hill M ts. Kofa Mts. Livingston Hills

Tertiary volcanic rocks Detrital zircon sample Conglomerate, sandstone, Nd isotope and geochemistry sample N and siltstone members McCOY MOUNTAINS Undivided FORMATION Sandstone petrology sample area Basal sandstone member 1 Harquar Jurassic volcanic rocks (160–175 Ma) Paleozoic and Mesozoic metasedimentary rocks Faults, dotted where concealed Tertiary to Proterozoic crystalline rocks Thrust Low-angle normal km 050 High-angle normal

Figure 3. Geologic map of eastern exposures of the McCoy Mountains Formation and other rock units, showing sample locations (except for the detrital-zircon sample from the southern Dome Rock Mountains). See Figure 1 for location. Sources of mapping include Miller (1970), Spencer et al. (1985), Sherrod et al. (1990), Reynolds et al. (1991), Richard et al. (1993, 1994b), and Tosdal and Stone (1994). Mts.—Mountains.

Formation by determining detrital mineral com- 1985; Richard et al., 1987, 1993). Sandstone in Koch, 1987). The less metamorphosed and positions of 180 sandstone and metasandstone the Livingstone Hills is moderately feldspathic deformed New Water succession includes strata samples (Robison, 1979; Harding, 1980, 1982; and contains signifi cant K-feldspar, consistent with graded beds with fl ute and groove casts that Harding and Coney, 1985; Laubach et al., 1987; with previous correlation to the upper McCoy have been interpreted as lacustrine turbidites Richard et al., 1987; Fackler-Adams et al., 1997; Mountains Formation (Fig. 4; Harding and (Sherrod and Koch, 1987). The two successions Barth et al., 2004). Many of the samples studied, Coney, 1985; Tosdal and Stone, 1994). are separated by a northeast-dipping, Cenozoic however, have been subjected to greenschist- extensional detachment fault with an estimated grade metamorphism and penetrative deforma- New Water and Granite Wash Mountains 25 km of displacement (Spencer and Reynolds, tion, which obscured original detrital textures 1991). Restoration of this displacement places and compositions to varying degrees. Analysis Two western Arizona successions, located in the two successions much closer together and of 30 thin sections from six well-mapped strati- the New Water Mountains (sedimentary rocks approximately on strike. Sandstones in the graphic successions in western Arizona that are of Ramsey Mine of Sherrod and Koch, 1987) Granite Wash succession contain plagioclase not strongly affected by metamorphism and and Granite Wash Mountains, are subvertical, but not K-feldspar, consistent with correlation deformation indicate great diversity in McCoy northeast-striking, top to the southeast, contain with the lower McCoy Mountains Formation detrital modes (Fig. 4; Dickinson, 2000). Sand- numerous mafi c sills and lava fl ows, and rest (Laubach et al., 1987). However, the New Water stones from the Crystal Hill Formation (Figs. 2 directly on Jurassic volcanic rocks (Figs. 2, 3, McCoy sandstones are dominated by volcanic and 3) and the Ranegras unit (Fig. 3) are strongly and 5; Sherrod and Koch, 1987; Reynolds et lithic grains and cannot be readily correlated dominated by quartz (Fig. 4), consistent with al., 1989b, 1991; Richard and Spencer, 1994). with any specifi c member of the McCoy Moun- previous correlation to basal sandstone member These two successions were correlated on the tains Formation on the basis of modal mineral- 1 (Fig. 2; Robison, 1980; Harding and Coney, basis of lithologic similarity (e.g., Sherrod and ogy (Fig. 4). Both successions were correlated

4 Geological Society of America Bulletin, Month/Month 2009 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

Qt Crystal Hill Qm and no plagioclase (Richard et al., 1987), simi- Ranegras lar to the K-feldspar–bearing upper McCoy Apache Wash New Water Mountains Formation and unlike basal sand- Harquar stone members 1 and 2. ABLivingston Hills DETRITAL-ZIRCON GEOCHRONOLOGY

Methods

Nine sandstone samples from fi ve stratal successions correlated with the McCoy Moun- tains Formation were collected for U-Pb isoto- F L F Lt pic geochronologic analysis of detrital-zircon Qp Qm grains. For four of the fi ve successions, a pair of samples from up to 340 m apart were combined for analysis in order to increase the probability of sampling a more geochronologically diverse CDrange of sand sources (GSA Data Repository Table DR11). This was considered benefi cial because the primary goal of detrital-zircon analysis was to constrain the age of each succession by identifying the youngest grains (Dickinson and Gehrels, 2009c). Five sandstone samples (four representing combined fi eld-sample pairs) were crushed, and zircon grains were separated by density and Lvm Lsm P K magnetic susceptibility. Zircon grains were then embedded in epoxy along with a geochronologic Qt = Quartz, total F = Feldspar, total Lvm = Lithics, volcanic Qm = Quartz, monocrystalline P = Plagioclase Lsm = Lithics, sedimentary standard zircon from a Sri Lanka granite and Qp = Quartz, polycrystalline K = K-feldspar L = Lvm + Lsm National Institute of Standards and Technology Lt = L + Qp (NIST) Standard Reference Materials (SRM) trace-element standard glass (see Gehrels et al., Figure 4. Detrital modes of selected sandstone samples from the McCoy Mountains Forma- 2008, and Dickinson and Gehrels, 2008, for addition in Arizona (data from Dickinson, 2000). Volcanic and sedimentary lithic fragments, tional details on laser ablation– multicollector– visible in thin section, were not differentiated from metamorphosed equivalents because inductively coupled plasma mass spectrometry of uncertainty in determining whether metamorphism was predepositional or postdeposi- [LA-MC-ICPMS] analytical procedures and tional. See Figure 3 for locations for each of the six sample areas. data reduction and presentation). The epoxy mounts were then abraded to a depth of ~20 µm, polished, imaged, and cleaned before placement with basal sandstone member 2 of the McCoy ard, 1992; Richard et al., 1993). The presence in the laser-ablation chamber of the mass spec- Mountains Formation on the basis of litho- of basal breccia and conglomerate suggest cor- trometer. An ~35 µm spot on each zircon grain logic similarity, with basal sandstone member relation with upper McCoy strata. However, the was vaporized with an excimer laser, mobilized 1 absent (Sherrod and Koch, 1987; Laubach et Apache Wash sandstone is high in monocrys- in a helium carrier gas, and accelerated down the al., 1987). Stratigraphically higher mudstone in talline quartz relative to feldspar (Fig. 4D) and mass-spectrometer fl ight tube in static magnetic the southern Granite Wash Mountains was cor- contains abundant sedimentary lithic grains like mode with multiple Faraday collectors, one for related with the overlying McCoy mudstone Crystal Hill strata (Fig. 4C), suggesting cor- each analyzed isotope (238U, 232Th, 208Pb, 207Pb, member (Laubach et al., 1987). relation with basal members 1 or 2. A broadly 206Pb, and 204Pb) (see Table DR2 [footnote 1] for fi ning-upward succession in the southeastern analytical results). Upper Apache Wash and Southeastern Little Harquahala Mountains, consisting almost The ~1%–2% 2σ uncertainty in calculated Little Harquahala Mountains entirely of sandstone and siltstone, rests on 206Pb/238U age for each laser-ablation event Jurassic volcanic rocks and contains no basal results from uncertainty in measured 206Pb/238U Two other western Arizona McCoy succes- conglomerate. Fine-grained strata include non- and 206Pb/204Pb. Because of low concentra- sions of uncertain specifi c correlation are the marine brachiopods indicating a lacustrine envi- tions, 235U is not measured but is calculated upper Apache Wash sandstone in the south- ronment (Richard et al., 1987). Its dominantly ern Plomosa Mountains and similar strata in sandy composition and position directly above the southeastern Little Harquahala Mountains Jurassic volcanic rocks suggest correlation with 1GSA Data Repository item 2011100, tables of (Fig. 3). The Apache Wash succession includes lower McCoy strata. However, three of fi ve sample locations, U-Pb geochronologic data, and trace-element geochemical data, is available at http:// a basal breccia and conglomerate unit overlain petrographic thin sections analyzed for modal www.geosociety.org/pubs/ft2011.htm or by request by sandstone in turn overlain by siltstone (Rich- mineralogy contained 17%–33% K- feldspar to [email protected].

Geological Society of America Bulletin, Month/Month 2009 5 Geological Society of America Bulletin, published online on 26 January 2011 as doi:10.1130/B30206.1

Spencer et al.

A A′ reference to determine how many grains are rep- 5 km resented by a probability peak or set of peaks. m asl 1000 154.4 ± 2.1 Ma U-Pb zircon Jvs ε (150) = -3.3 ε (150) = +4.4 dz sample ε (150) = +5.5 Jv Nd Nd Nd Results

500 Detrital-Zircon Age Constraints Of the fi ve sandstone samples analyzed for 0 Jvq detrital-zircon U-Pb isotopic analysis, four are from stratal successions that were previously of -500 uncertain correlation with any specifi c member of the McCoy Mountains Formation (Figs. 1 o o Jv 113 59’59' 70 113 57’57' and 3). Samples were collected near the top of Qs 80 Js these successions in order to maximize the pos- Ps Jv Tsv sibility of capturing zircon grains derived from the youngest igneous rocks in sediment source Jvq 60 A 55 areas. Two samples, from the southern Little 10 Tsv ε =-3.3 38 o Harquahala Mountains and upper Apache Wash 84 70 Nd 33 37’37' 50 in the southern Plomosa Mountains, are from 72 50 ε =+5.5 very near the stratigraphically highest levels of Nd exposure. The other two samples were collected ε =+4.4 U-Pb=154 Ma Nd Jm 57 74 22 from high stratigraphic levels but below the Jv 63 stratigraphically highest mafi c sill in the Granite Qs 44 Qs Wash Mountains and below the highest mafi c 55 Jvs dz sample Jm 76 35 33 lava fl ow in the New Water Mountains (Fig. 5). 68 42 All of the 396 zircon grains analyzed from these 68 70 Jsf 56 Qs four samples are older than 145 Ma, and all but 46 60 Js 85 50 two are older than 157 Ma (Fig. 6). This is con- 45 20 Js 85 48 sistent with a Late Jurassic age for these McCoy Jc 64 56 50 successions, and allows correlation with the 67 44 o 61 33 36’36' McCoy Mountains Formation basal sandstone 67 member 1, which rests on ca. 160 Ma volcanic 79 Qs 52 rocks (Fackler-Adams et al., 1997) and con- 48 N AA'′ tained no zircons younger than 179 Ma in two 66 60 Jm 52 Jsf Tsv samples analyzed by Barth et al. (2004). Barth et 48 Js Jc al. (2004) identifi ed nine grains (out of 83 ana- Tsv 62 lyzed) dated at 109–124 Ma (eight of nine grains are 116–124 Ma) in two samples of basal sand- Tvs Oligo-Miocene volcanic and sedimentary rocks Mi. 0 1 stone member 2 in the McCoy Mountains, but Jm Jurassic mafic igneous rocks km 0 1 we did not identify any grains of this age range in our four samples. We conclude that our four Jc Js Jsf McCoy Mountains Formation: Jc—conglomerate; Js—sandstone; Jsf—fine grained sandstone McCoy samples are younger than ca. 157 Ma based on the presence of only two zircon grains Jvq Jv Jvs Jurassic volcanic rocks: Jvq—massive quartz porphyry; Jv—volcanic rocks, undivided; Jvs—volcanic-lithic sandstone and conglomerate younger than this age and stratigraphic position Pzs Paleozoic metasedimentary rocks above ca. 160 Ma volcanic rocks. The lack of 116–124 Ma grains suggests that the Arizona Figure 5. Geologic map and cross section of the Ramsey Mine area in the New Water McCoy successions are older than basal mem- Mountains (simplifi ed from Richard and Spencer, 1994). Abbreviations: asl—above ber 2 (which is younger than ca. 116 Ma). sea level; dz—detrital zircon. See Figure 3 for location. A fi fth sample, from sandstone beds within the siltstone member of the upper McCoy Mountains Formation in the southern Dome from measured 238U (235U = 238U/137.88). The and 206Pb/238U for younger ages. Each age- Rock Mountains, is stratigraphically above a 207Pb/235U age is calculated from measured probability plot (age-distribution curve) is the tuff dated by U-Pb at 79 ± 2 Ma (Tosdal and 206Pb/238U and measured 207Pb/206Pb [207Pb/235U = sum of all ages represented on the corresponding Stone, 1994). This siltstone unit is correlative (206Pb/(238U/137.88))/(206Pb/207Pb)]. Two-sigma histogram with each age determination repre- with strata in the McCoy Mountains that yielded uncertainty in measured 206Pb/207Pb age is sented by a normal probability distribution with 24 grains dated at 84–108 Ma (26% of 92 grains ~1%–2% for zircon crystals older than ~1 Ga, equal area (vertical scale is arbitrary). The age- from three samples; Barth et al., 2004) and was but it is higher for younger grains because of distribution curves more accurately represent analyzed to determine if similar-age zircons are low concentrations of 207Pb (Gehrels et al., raw data because, unlike the histograms, they also common in upper McCoy sands in Arizona. 2008). Histograms of age determinations are include representation of analytical uncertain- The Dome Rock Mountains sample yielded 15 based on 206Pb/207Pb for ages older than ~1 Ga ties. The histograms, in contrast, provide a ready grains dated at 80–102 Ma (15% of 99 grains),

6 Geological Society of America Bulletin, Month/Month 2009 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 0.9 Ga) – (>1.8 Ga) (1.4–1.5 Ga) (150–180 Ma) (190–240 Ma) (1.6–1.8 Ga) Appalachian and Early Cordilleran terranes (0.3 terranes Cordilleran (<0.3 Ga) Cordilleran Yavapai - Mazatzal Yavapai Northern Laurentia Jurassic magmatic arc Jurassic Grenville (1.0–1.3 Ga) Grenville Laramide (80–110 Ma) Laramide Anorogenic magmatism Late Cretaceous - early 18 40 accreted peri-Gondwanan 16 Dome Rock 35 Dome Rock 14 12 30 10 25 8 20 6 15 4 2 10 0 5 38 0 36 Apache Wash 55 34 50 Apache Wash 32 30 45 28 40 26 35 24 30 22 25 20 18 20 16 15 14 10 12 5 10 8 0 6 35 4 30 Little Harquahala 2 25 0 20 14 15 12 Little Harquahala 10 10 8 5 6 0 4 55 2 50 Granite Wash 0 26 45 24 Granite Wash 40 22 35 20 30 18 25 16 14 20 12 15 10 10 8 5 6 4 0 2 25 0 20 New Water 10 15 8 New Water 10 6 4 5 2 0 1100 1200 1300 1400 1500 1600 1700 1800 >1900 200 300 400 500 600 700 800 900 1000 100 0

0 – 100 80–90 90–100 100–110 110–120 120–130 130–140 140–150 150–160 160–170 170–180 180–190 190–200 200–210 210–220 220–230 230–240 240–250 250–260 260–270 – – – – – – – – – 200 300 400 500 600 700 800 900 1000 – – – – – – – – – 1100 1200 1300 1400 1500 1600 1700 1800 1900

Age range (Ma) Age range (Ma)

Figure 6. Histograms and age-probability plots of U-Pb dates of detrital-zircon grains from sandstone samples of the McCoy Mountains Formation collected in ﬁ ve areas in western Arizona (see Table DR1 [footnote 1] and Figure 3 for sample locations, and Table DR2 [footnote 1] for analytical data). Histograms are shown so that the number of zircons represented by the peaks can be determined and the corresponding statistical signiﬁ cance of each peak evaluated.

Geological Society of America Bulletin, Month/Month 2009 7 Geological Society of America Bulletin, published online on 26 January 2011 as doi:10.1130/B30206.1

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TABLE 1. MAJOR-ELEMENT ANALYSES, MESOZOIC MAFIC SILLS AND LAVA FLOWS, WESTERN ARIZONA*

Sample no. SiO2 (%) Al2O3 (%) Fe2O3 (%) MgO (%) CaO (%) Na2O (%) K2O (%) TiO2 (%) P2O5 (%) MnO (%) LOI (%) Sum (%) 11-3-83-1 52.70 16.70 10.55 2.96 5.01 5.84 3.01 2.17 0.88 0.14 0.89 100.85 11-17-83-1 47.30 18.50 9.82 8.74 8.55 3.55 0.28 1.49 0.26 0.15 2.26 100.90 11-17-83-3 58.53 15.9 7.60 2.16 3.17 5.12 4.51 1.66 0.60 0.14 1.35 100.74 10-14-94-2 48.35 16.99 9.58 6.45 9.50 3.71 1.76 1.47 0.48 0.15 1.44 99.88 10-17-94-1 45.09 18.95 10.14 8.46 9.82 2.77 0.24 1.24 0.18 0.15 2.92 99.98 12-11-92-1 52.81 18.66 7.55 4.33 3.91 6.43 1.46 1.08 0.54 0.06 3.97 100.80 12-11-92-4b 45.40 16.00 10.23 10.2 9.87 2.40 0.71 1.09 0.37 0.14 3.97 100.38 10-14-94-4 58.09 18.45 6.20 1.34 1.48 6.50 4.15 0.93 0.32 0.09 1.91 99.46 10-17-94-6 47.06 18.59 9.41 7.73 9.70 3.80 0.58 1.38 0.22 0.13 1.46 100.06 10-14-94-1 47.89 17.41 9.84 6.45 9.38 3.65 1.31 1.42 0.42 0.14 1.96 99.87 3-25-04-2 40.93 17.74 8.83 6.13 9.24 1.60 2.49 1.35 0.40 0.15 11.5 100.36 3-25-04-3 47.98 16.71 7.50 5.44 7.39 2.63 1.74 1.38 0.43 0.13 9.0 100.33 3-25-04-5 52.87 17.95 7.20 3.90 6.21 4.88 2.15 1.32 0.51 0.10 3.2 100.29 3-25-04-5B 53.25 17.97 7.15 3.65 6.97 4.6 2.03 1.19 0.47 0.10 2.9 100.28 3-25-04-6 50.50 15.58 8.85 7.04 5.26 3.43 0.45 1.11 0.16 0.14 7.8 100.32 3-25-04-9 43.84 15.02 10.41 8.60 8.33 2.86 0.04 1.36 0.32 0.16 9.5 100.44 3-25-04-10 45.22 15.53 9.99 7.32 8.28 3.83 0.05 1.50 0.48 0.14 8.0 100.34 *Analyses by X-ray ﬂ uorescence by Actlabs (sample numbers beginning with “3”) or by Chemex Labs (all others). LOI—loss on ignition.

which supports previous correlations (Harding were sampled for major-element, trace-element, from 0.04 wt% to 4.5 wt%. On an alkali-silica and Coney, 1985; Tosdal and Stone, 1994). and neodymium-isotope geochemistry (see diagram (Le Bas et al., 1986), most samples plot Table DR1 [footnote 1] for sample locations). in the alkaline fi eld (Fig. 7). The alkali elements MAFIC SILLS AND LAVA FLOWS Seventeen samples were analyzed for major ele- tend to be among the most mobile during defor- ments by X-ray fl uorescence (Table 1). Twelve mation and metamorphism, processes which Mafi c sills and lava fl ows are common in the of these samples were also analyzed for neo- have clearly affected these rocks. However,

McCoy Mountains Formation in the Granite dymium and samarium isotopes (Table 2) and the strong correlation between SiO2 and K2O +

Wash and New Water Mountains, and are pres- trace elements (Tables DR3 and DR4 [footnote Na2O (Fig. 7) suggests that alkali elements may ent but less abundant in the nearby southern Plo- 1]). SiO2 ranges from 40.9 wt% to 58.5 wt%, but nonetheless be somewhat reliable geochemical mosa Mountains. In the southern Dome Rock some samples are signiﬁ cantly hydrated, pre- indicators of original igneous compositions for Mountains, diorite and andesite within basal sumably by postcrystallization alteration. Loss- these rocks. sandstone member 1 of the McCoy Mountains on-ignition totals range from ~1 wt% to almost Neodymium-isotope compositions fall into ε Formation (Tosdal and Stone, 1994) are possi- 12 wt%, consistent with the abundance of two groups, one with Nd between +4 and +6 ε − bly part of the same igneous suite, as are maﬁ c hydrous secondary assemblages visible in thin (six samples) and one with Nd between 6 and ε igneous rocks at the south end of the McCoy sections. Normalized to anhydrous, SiO2 ranges +2 (six samples; Table 2). High Nd values cor- and Palen Mountains (Stone and Pelka, 1989). from 45.6 wt% to 59.3 wt%, and Al2O3 varies respond to low SiO2, low total alkalis, and low

Most of the maﬁ c igneous rocks in the Granite from 16.1 wt% to 19.8 wt %. Measured Na2O Nd (Fig. 8). Chondrite-normalized, rare-earth

Wash and southern Plomosa Mountains are dio- varies from 1.6 wt% to 6.5 wt%, and K2O varies element patterns show light rare-earth element ritic to gabbroic sills up to 30 m thick, with variable grain size ranging from fi ne-grained chilled margins to coarse-grained interiors (Laubach et TABLE 2. Sm-Nd ISOTOPE DATA FOR MESOZOIC al., 1987; Reynolds et al., 1989b, 1991; Richard MAFIC SILLS AND LAVA FLOWS, WESTERN ARIZONA* et al., 1993). In the Granite Wash Mountains, 147 144 †143144 § ε # Sample no. Location Sm (ppm) Nd (ppm) Sm/ Nd Nd/ Nd Nd(150) the sills and fl ows are metamorphosed so that Granite Wash 11-3-83-1 9.70 46.34 0.1265 0.512598 ± 7 0.56 pyroxene, hornblende, and feldspar have been Mountains Granite Wash 11-17-83-1 4.01 16.59 0.1462 0.512836 ± 6 4.82 largely altered to actinolite, chlorite, biotite, ser- Mountains Granite Wash icite, epidote, and other secondary minerals, but 11-17-83-3 9.53 46.74 0.1233 0.512429 ± 9 −2.68 Mountains relict igneous textures are apparent (Laubach et Granite Wash 10-14-94-2 5.65 26.97 0.1266 0.512546 ± 5 −0.45 al., 1987). Contorted beds and load casts within Mountains Granite Wash sediments directly adjacent to the intrusive mar- 10-17-94-1 2.96 11.24 0.1592 0.512864 ± 7 5.12 Mountains gins of sills suggest some sills were intruded Southern Plomosa 12-11-92-1 7.03 35.24 0.1206 0.512251 ± 7 −6.10 into semiconsolidated sediments (Laubach et Mountains Southern Plomosa 12-11-92-4b 3.55 15.93 0.1348 0.512675 ± 8 1.91 al., 1987). Lava fl ows in the New Water Moun- Mountains tains are characterized by autobrecciated, locally 3-25-04-3 New Water Mountains 3.864 18.643 0.125297 0.512848 ± 8 5.49 3-25-04-5 New Water Mountains 6.394 35.631 0.108488 0.512381 ± 7 −3.32 vesicular rinds and massive cores, and most are 3-25-04-6 New Water Mountains 2.801 11.174 0.151584 0.512819 ± 7 4.43 Southern Plomosa present in the middle part of the steeply dipping, 3-25-04-9 4.453 18.646 0.144387 0.512796 ± 7 4.10 Mountains southeast-facing succession (Fig. 5; Sherrod and Southern Plomosa 3-25-04-10 5.302 24.669 0.129954 0.512840 ± 9 5.23 Koch, 1987; Richard and Spencer, 1994). Mountains *Nd isotope analysis followed methods of Patchett and Ruiz (1987). †Uncertainties in 147Sm/144Nd ratios are <0.5%. Igneous Geochemistry §Measured Nd isotopic ratios are normalized to 146Nd/144Nd = 0.7219 (±2σ errors refl ect in-run precision only). # ε 4 143 144 143 144 − Initial Nd = 10 [( Nd/ Nd(t)sample)/( Nd/ Nd(t)CHUR) 1] using present-day chondritic uniform reservoir 143 144 147 144 Sills and lava fl ows from the Granite Wash, (CHUR) values of Nd/ Nd = 0.512638 and Sm/ Nd = 0.1966; t = 150 Ma (initial ratios reproducible to 0.5 ε units). New Water, and southern Plomosa Mountains Nd

8 Geological Society of America Bulletin, Month/Month 2009 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

12 sample 3-25-04-2). Previous attempts to extract ablation events yielded U-Pb dates that over- Trachyandesite zircon phenocrysts from geochemically simi- lap within 1σ analytical uncertainty (Fig. 10) lar mafi c sills in the Granite Wash Mountains regardless of laser-target location within each 10 Basaltic had been unsuccessful, but ~10 zircon crystals grain. Assimilation of underlying McCoy sands trachyandesite were separated from this lava fl ow. These zir- and incorporation of detrital zircon into the 8 cons yielded a well-defi ned 238U/206Pb weighted basaltic lava fl ow can be ruled out because the O (%) 2 Trachybasalt kaline mean age of 154.4 ± 2.1 Ma (Fig. 10). This age detrital-zircon ages from the directly underly- alAlkaline is compatible with detrital-zircon data from the ing sandstone include none of Late Jurassic age 6 sub-alkalineSub-alkaline

O + K host sandstone; all 100 analyzed zircon grains (all 100 analyzed grains are older than 180 Ma;

2 Tephrite are ≥180 Ma. Table DR2 [footnote 1]). In addition, none of

Na 238 206 4 The 154.4 ± 2.1 Ma U/ Pb age for the the faces on any of the separated zircons are basalt lava ﬂ ow is interpreted as its primary rounded, as might be expected if the crystals are Basaltic Picro- Basalt igneous age. Two samples of the lava ﬂ ow xenocrysts derived from sand grains. In sum-

andesite Andesite 2 basalt (Table 1, samples 3-25-04-2 and 3-25-04-3) mary, all geochemical, geochronologic, and 40 45 50 55 60 contain 40.9% and 48.0% SiO , and are strongly petrographic data support a primary magmatic SiO (%) 2 2 hydrated (11.5% and 9.0% volatiles, respec- origin for the dated zircon crystals, and we fi nd tively). Normalized to volatile-free composi- no evidence that they are xenocrysts. Figure 7. Measured SiO versus K O + Na O 2 2 2 tions, SiO contents are 45.6% and 52.3%, for mafi c sills and lava fl ows from the Gran- 2 which more likely refl ect original igneous com- DISCUSSION ite Wash, New Water, and southern Plomosa positions. The fact that a lava of such mafi c com- Mountains plotted on the classifi cation position yielded zircon phenocrysts, which are McCoy Mountains Formation Sandstones diagram for igneous rocks of Le Bas et al. rare in basaltic rocks (e.g., Baines et al., 2009), (1986). Alkaline-subalkaline boundary from prompts concern that the dated zircons are xeno- Mesozoic Detrital-Zircon Geochronology Le Bas and Streckeisen (1991). (See Table 1 crysts inherited from underlying Jurassic felsic Dominant Mesozoic sand sources in our for analytical data.) volcanic rocks or deeper, concealed Jurassic McCoy sandstone samples refl ect three peaks granitoids. If this were the case, assimilation of in magmatism separated by periods of relative crustal rocks during basalt-magma ascent would magmatic inactivity (Fig. 6). Only the sample ε (LREE) abundances slightly to moderately likely have reduced magma Nd. Nd concentra- from the Dome Rock Mountains contained Cre- ε enriched over heavy rare-earth element (HREE) tions in high- Nd mafi c rocks are low, generally taceous grains, consistent with previous corre- abundances (LaN/YbN = 2.5–13.3), with moder- 10–20 ppm, which means that assimilation of lation to the upper McCoy Mountains Forma- ately to steeply sloping LREE and gently slop- even small amounts of crustal rock with signifi - tion. The other four samples, from successions ing HREE abundances (Fig. 9). The six samples cantly higher Nd content would likely reduce that were previously of uncertain correlation ε ε with high positive Nd correspond to a group Nd, a trend shown clearly on a plot of Nd con- to specifi c McCoy members, contained only ε ε with low REE abundances (Fig. 9). centration versus Nd (Fig. 8C). However, Nd = Jurassic or older grains. The absence of Cre- +5.5 for the dated fl ow (Table 2, sample 3-25- taceous grains suggests that these four succes- U-Pb Geochronology 04-3), which indicates little or no assimilation sions do not correlate to basal member 2, which of crustal or lithospheric material. Differences contains a signifi cant zircon population in the A vesicular mafi c lava fl ow in the New Water in rim and core ages would be expected if zircon 116–124 Ma range in the McCoy Mountains Mountains containing ~10%, 1–3 mm feldspar xenocrysts were older than the host magma and in California (Barth et al., 2004). The 154 ± phenocrysts was sampled for U-Pb zircon geo- zircon overgrowths had developed during resi- 2 Ma U-Pb date on a basalt fl ow in the New chronologic analysis (Table DR1 [footnote 1], dence in a younger magma. However, all laser- Water Mountains McCoy succession indicates

8 8 8 6 AB6 6 C 4 4 4 2 2 2 Nd Nd

Nd 0 0 0 ε ε ε -2 -2 -2 Granite Wash Mts. -4 New Water Mts. -4 -4 -6 Apache Wash -6 -6 Dripping Spring Wash -8 -8 -8 40 45 50 55 60 0 2468 10 0 10 20 30 40 50 K O + Na O (%) Nd (ppm) SiO2 (%) 2 2 ε ε Figure 8. Measured Nd(150) versus weight percent (A) SiO2 and (B) K2O + Na2O; and (C) measured Nd(150) versus ppm Nd for sills and lava ﬂ ows from the Granite Wash, New Water, and southern Plomosa Mountains (Mts.). (See Fig. 3 and Table DR1 [footnote 1] for sample locations; see Tables 1 and 2 and Tables DR3 and DR4 [footnote 1] for analytical data.)

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deposition of several kilometers of siliclastic 300 sediments immediately following termination ε Nd = +4 to +6 of widespread, ca. 160 Ma felsic magmatism, ε < +1 which would disallow correlation to any McCoy Nd member other than basal unit 1. The three detrital-zircon samples from the Little Harquahala, Granite Wash, and New 100 Water Mountains yielded only three grains in the 160–180 Ma range, in contrast to a total of 113 grains in the 190–240 Ma range. These Mesozoic zircon populations are similar to Mesozoic zircon populations in two samples of basal member 1 from the McCoy Moun- tains for which nine of 11 analyzed Mesozoic 30 grains are 189–229 Ma (Barth et al., 2004). The fourth sample in our group, from Apache Sample/chondrite Wash in the southern Plomosa Mountains, was dominated by 150–180 Ma zircon grains (n = 47), with a subordinate but substantial number of 190–240 Ma zircons (n = 20). If our four samples are all about the same age, abundant 10 150–180 Ma grains in one sample but not the La Ce Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu other three indicate that 150–180 Ma igneous Rare-earth element rocks were locally but not regionally important sources of lower McCoy sands. Alternatively, the Apache Wash succession is younger than Figure 9. Chondrite-normalized, rare-earth–element abundances for ten samples of lava the other successions, and its abundant 150– fl ows and sills from the McCoy Mountains Formation in western Arizona, divided into two 180 Ma grains refl ect a paleogeography fol- groups based on ε . Normalization based on chondrite abundances from McDonough Nd(150) lowing faulting and disruption of lower McCoy and Sun (1995). (See Table 1 and Tables DR3 and DR4 [footnote 1] for analytical data.) successions and tectonic exhumation of underlying Jurassic igneous rocks. This is consistent with limestone fragments identifi ed in thin section (Dickinson, 2000) and interbedded rock- 166 Sample #3-25-04-2, Ramsey Mine area, New Water Mountains avalanche breccias containing Paleozoic rocks (Richard, 1992), both features without parallel Single-grain zircon isotopic analyses by LA-ICPMS in the other three successions. 162 The most likely source for the abundant 190–240 Ma zircon grains in basal McCoy 158 sandstones is the Triassic magmatic arc that was constructed across southern California and, with poorly understood extent and signifi cance, 154 across southern Arizona and northwestern Mexico (Barth et al., 1997; Barth and Wooden, 2006). Most of these rocks have since been U age (Ma) 150 assimilated by Jurassic and Cretaceous intru- 238 sions, buried, or eroded away, and it is diffi cult to assess their extent in lower McCoy time. An Pb/ 146 alternative source of 190–240 Ma zircons from 206 recycling of Lower and Middle Jurassic Colo- Age = 154.4 ± 2.1 Ma rado Plateau eolianites and related strata, which 142 Mean = 154.4 ± 1.5 Ma are the inferred source of 190–240 Ma zircons MSWD = 0.39 (2σ) in the quartzose, Lower Cretaceous Cintura 1σ data-point uncertainty 138 sandstone in the Bisbee Group (Dickinson et al., 2009), can be eliminated for western Ari- zona McCoy strata. Modal mineral analysis of Figure 10. 206Pb/238U dates of zircon grains from a basalt lava fl ow in the Ramsey Mine McCoy sandstones from the New Water Moun- area of the New Water Mountains that is interbedded with the McCoy Mountains For- tains identifi ed abundant volcanic-lithic grains mation (see Table DR2 [footnote 1] for analytical data and Fig. 3 and Table DR1 [foot- (26%–72% of grains) and generally minor note 1] for sample location). Abbreviations: LA–ICPMS—laser ablation–inductively monocrystalline quartz (8%–19%; Fig. 4; Dick- coupled plasma mass spectrometry; MSWD—mean square of weighted deviates. inson, 2000), which is completely unlike very

10 Geological Society of America Bulletin, Month/Month 2009 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 quartz-rich Colorado Plateau eolianites. The igneous rocks contain Zr at ~3.5 times the izing for elevated zircon content of these two detrital-zircon U-Pb age spectra for the Gran- concentration characteristic of all other major age groups allows representation of sand frac- ite Wash and Little Harquahala samples are North American age groups except the 1.4– tions contributed from different sources rather very similar to that for the New Water sample 1.5 Ga granite suite, which contains Zr at ~2.5 than zircon fractions. The relative detrital-zircon (Fig. 6). For these three samples, 28%–52% of times the more typical level (Dickinson, 2008). concentrations of McCoy sands and other Juras- analyzed grains are 190–240 Ma, in contrast Assuming that zircon content in granitoids sic sandstone units in Arizona and on the Colo- to Colorado Plateau Jurassic eolianites, which accurately reﬂ ects Zr concentration, Grenvillian rado Plateau, normalized for differential zircon contain, on average, 5% zircons of this age zircon grains in sandstones are overrepresented fertility and divided into three groups, are plot- range. We conclude that abundant 190–240 Ma by ~3.5× relative to the amount of sand actually ted on Figure 11. The three major sand compo- zircon grains in our McCoy samples were contributed, and 1.4–1.5 Ga grains are overrep- nents shown in Figure 11 are as follows: (A) far- derived from nearby igneous rocks of the Trias- resented by 2.5× (Dickinson, 2008). Normal- traveled sand derived from the Mesoproterozoic sic magmatic arc. The near absence of <190 Ma grains in three of our McCoy samples indicates that early Mesozoic rocks in the source region for McCoy sands were unaffected by Middle Detrital-zircon ages of sandstones divided into three sand components, Jurassic magmatism. normalized for differential zircon fertility of source terranes

Pre-Mesozoic Detrital-Zircon Geochronology Zircon source A Pre-Mesozoic McCoy zircon grains fall into (northeast Laurentia: 0.3–1.3 Ga, >1.8 Ga) three primary age groups (Fig. 6), as follows: Upper McCoy (1) 1.6–1.8 Ga, derived from igneous and meta- This study Jurassic sandstones of the morphic rocks of the Yavapai-Mazatzal super- Barth et al., 2004 90% southern Colorado Plateau group of southwestern Laurentia or equivalent (Dickinson and Gehrels, 2009b) units farther east (Whitmeyer and Karlstrom, Lower McCoy This study San Rafael Group 2007). (2) 1.4–1.5 Ga, derived from mid- (Middle to Upper Jurassic) Proterozoic granitic plutons that were intruded Barth et al., 2004 across a large swath of Laurentia (anorogenic 70% Glen Canyon Group Mount Wrightson granite suite of Anderson, 1989; Anderson and (uppermost Triassic to Morrison, 2005) including western Arizona Cintura Formation Middle Jurassic) and southeastern California (e.g., Gleason et (Bisbee Group) al., 1994; Bryant et al., 2001). (3) 1.0–1.3 Ga, 50% derived from Grenville age basement with likely distant sources. Rocks of Grenville age are com- McCoy mon in eastern Arizona and are locally present member C elsewhere in the Mojave-Sonora desert region, McCoy member A Springdale but most consist of diabase (Wrucke, 1989) that 30% Little Harquahala contains little or no zircon. Zircons of this age New Water are abundant in Jurassic sandstones of the Colo- rado Plateau, and were ultimately derived from Granite Wash eastern North America (Dickinson and Gehrels, 10% 2008, 2009a, 2009b). These Plateau sandstone Dome Rock Apache units, most of which are eolianites of the San Wash Rafael and Entrada Groups, are older than the post–160 Ma McCoy Mountains Formation, Zircon source B Zircon source C indicating that any infl ux of Jurassic Plateau (SW Laurentia: 1.3–1.8 Ga) (Mesozoic magmatic sands to the McCoy Mountains Formation arc: <0.3 Ga) resulted from erosion and dispersal of previously deposited Jurassic sands (e.g., Dickinson Figure 11. Relative proportions of provenance sands in Jurassic and Cretaceous sand- et al., 2009). All three pre-Mesozoic population stones of the American Southwest plotted on a ternary diagram in which the apices groups are represented in the four older sand- represent provenance age groups. Samples are from the McCoy Mountains Formation, stone samples reported here (Fig. 6). The upper Jurassic Colorado Plateau eolianites and fl uvial sandstone, and related units in south- McCoy sample contained signifi cant popula- eastern Arizona. Points are derived from detrital-zircon geochronologic data normal- tions of 1.6–1.8 Ga and 1.4–1.5 Ga zircons, but ized to correct for the elevated zircon fertility of Grenville (1.0–1.3 Ga) igneous rocks almost no Grenville-age zircons. (3.5×) and 1.4–1.5 Ga granitoids (2.5×) (Dickinson, 2008). Zircon grains >1.8 Ga are included with 0.3–1.3 Ga grains because both are interpreted as derived from distant Sources of McCoy Sands sources (northern and eastern Laurentia, respectively) with mixing during transport to Detrital-zircon populations represent diverse southwestern North America. Data from Dickinson and Gehrels (2009b), Dickinson et igneous epochs that include two epochs with al. (2009), Barth et al. (2004), and Table DR2 [footnote 1]. McCoy member A (Stone generally elevated concentrations of high fi eld- and Pelka, 1989) is part of basal sandstone member 1 (Harding and Coney, 1985), while strength elements. Grenville age (1.0–1.3 Ga) McCoy member C is part of basal sandstone member 2.

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Grenville and Paleozoic Appalachian orogens, Harquahala Mountains, 154 ± 5 Ma in the ~158–153 Ma, which overlaps with the 154 ± Neoproterozoic peri-Gondwanan terranes now Buckskin Mountains (both dates from Reyn- 2 Ma U-Pb date reported here for basalt inter- forming much of the Appalachian Piedmont, olds et al., 1987), 162 ± 3 Ma and 155 ± 8 Ma bedded with the McCoy Mountains Formation. and >1.8 Ga sands derived most likely from from the Palen Mountains (Fackler-Adams et The synchronous or nearly synchronous nature central and northern Laurentia; (B) sand derived al., 1997), and 165 ± 2 Ma from the northern of geochemically similar basaltic volcanism from southwest Laurentia, which consists of McCoy Mountains (Barth et al., 2004). The associated with extension in the Bisbee trough 1.6–1.8 Ga igneous and metamorphic rocks that 154 ± 2 Ma U-Pb date reported here for the is consistent with a rifting environment during represent genesis of the continental crust in the basalt interbedded with the McCoy Mountains initial McCoy deposition within a western con- region, and intruding, largely undeformed, ca. Formation is within the analytical uncertainty tinuation, or right-stepping en echelon segment, 1.4 Ga granites; and (C) sand derived from the of three of fi ve dates reported for regionally of the Sabinas-Chihuahua-Bisbee rift belt. Mesozoic Cordilleran magmatic arc. underlying Jurassic volcanic rocks. The sedi- Additional evidence for association of mafi c The Jurassic quartz-rich sands that make up mentary succession containing the basalt forms McCoy magmatism with rifting is provided ε the San Rafael and Glen Canyon groups on a southeast-dipping homocline with ~4 km of by correlated La/Ta ratios and Nd values. Tan- the Colorado Plateau, with their abundant far- structural separation between the Jurassic fel- talum is one of the most diagnostic elements traveled zircons (average 71% after normal- sic volcanic rocks and the dated basalt (Fig. 5 for distinguishing subduction-modifi ed sources izing), were originally carried westward by cross section). The succession is deformed, from depleted mantle that has no arc compo- a transcontinental river system to west-coast however, and original stratigraphic separation nent because Ta is preferentially removed from regions where generally southward-directed is uncertain, but stratal separation is not obvi- ascending magmas during passage through winds (modern coordinates) carried the sands ously greatly different than structural separa- hydrated mantle above subduction zones to the Jurassic sand ergs of the Colorado Pla- tion. Regardless of how many kilometers of (Baier et al., 2008). Furthermore, assimilation teau region (Dickinson and Gehrels, 2009a, strata originally separated the two volcanic of lithosphere during magma ascent should be 2009b). The fraction of these sands derived units, initial McCoy deposition must have been identifi able in asthenosphere-derived magmas from Cordilleran magmatic-arc sources aver- rapid, perhaps ~1 km/m.y. Furthermore, abrupt in southwestern North America because the ages only 16%, while the fraction derived from termination of silicic volcanism is indicated by Paleoproterozoic lithosphere has developed sig- ε 1.3 to 1.8 Ga sources averages 14% (Fig. 11). In the complete lack of felsic volcanic rocks in nifi cantly negative Nd values. On a diagram of ε contrast to the eolianites and recycled eolianite the lower McCoy Mountains Formation in all La/Ta versus Nd, fi ve of ten mafi c McCoy sands in the Cintura Formation of the Bisbee exposures in Arizona and California (with the igneous-rock samples plot within a fi eld defi ned ε Group (Dickinson et al., 2009), our four lower exception of minor interbedding at the basal by La/Ta <20 and Nd = +2 to +6 (box labeled McCoy samples are dominated by zircon grains contact in the Palen Mountains). “Mesozoic rifts” in Fig. 12). All eight mafi c from the Mesozoic magmatic arc (average 59% Lower McCoy deposition was approximately igneous-rock samples from the basal Bisbee normalized) and contain a signifi cant sand frac- synchronous with extensional genesis of the Group plot in this fi eld (Lawton and McMillan, tion from 1.3 to 1.8 Ga sources (average 30%), Bisbee basin in southeastern Arizona, north- 1999), as do four samples from the Fogo Sea- with an average of only 11% of sand derived eastern Sonora, and southwestern New Mexico mounts east of Nova Scotia that were erupted from distant, eastern and northern Laurentian (Dickinson and Lawton, 2001b). The Upper during early separation of North America from sources. We conclude that the McCoy Moun- Jurassic to Upper Cretaceous Bisbee Group, Iberia (Pe-Piper et al., 2007). Four McCoy sam- tains Formation received sands from diverse deposited during rifting and subsequent ther- ples that plot below and slightly to the right of and varied sources that were mostly within motectonic subsidence, locally includes mafi c this fi eld are interpreted to have an assimilated ε southwestern North America, with large frac- lava fl ows deposited during initial rifting. Eight lithospheric component with negative Nd val- tions of sand from eastern and northern North samples of mafi c igneous rocks associated ues and elevated La/Ta (Gleason et al., 1999). America only apparent in two lower McCoy with the basal Bisbee Group in the Chiricahua For comparison, intraoceanic arc magmatism, samples from California analyzed by Barth et Mountains in southeastern Arizona, 450 km to as in the Izu-Bonin arc, is strongly affected by al. (2004). Sparse quartz-rich varieties of basal the east-southeast of McCoy exposures in west- Ta depletion, but neodymium isotopes are unaf- ε McCoy strata in Arizona (Fig. 4; Crystal Hill ern Arizona, are characterized by Nd values (at fected by assimilation of ancient lithosphere ε Formation and Ranegras unit) probably also t = 150 Ma) between +2.9 and +5.1 and La/Ta (Ishizuka et al., 2006). In fact, at +6 to +7, Nd contain a signifi cant fraction of sand derived ratios of 12.2–16.2, consistent with primitive is slightly higher in Izu-Bonin arc rocks, appar- from 0.3 to 1.3 Ga and >1.8 Ga sources. (oceanic and asthenospheric) mantle sources ently because of the complete absence of old (Lawton and McMillan, 1999; Fig. 12). Some lithospheric mantle beneath the arc. Upper Mafi c Igneous Rocks Associated with the of these rift-related basaltic rocks are interbed- Cenozoic subduction-related igneous rocks McCoy Mountains Formation ded with siliciclastic and carbonate strata that in the southern Andes have elevated La/Ta, as ε contain microfossils of Callovian (165–161 Ma) expected, and Nd >0, consistent with the upper Deposition of the McCoy Mountains Forma- to Oxfordian (161–156 Ma) age and ammonites Paleozoic to Triassic age of local crust (Kay et tion occurred in an area underlain by an ~1-km- of late Kimmeridgian (156–151 Ma) age (Law- al., 2005). thick cratonal Paleozoic succession overlain ton and Olmstead, 1995; Olmstead and Young, Also plotted on Figure 12 are analyses of locally by fi ne-grained Triassic strata, Lower 2000; numeric ages from Walker and Geissman, dikes from the ca. 148 Ma Independence dike Jurassic quartz-rich sandstones (Laubach et 2009). Olmstead and Young (2000) consid- swarm in eastern California that are possibly al., 1987; Reynolds et al., 1989a; Lerch, 1992), ered the ammonites to be early Kimmeridgian related to the same extensional tectonic event and Middle to lower Upper Jurassic volcanic but allowed that they could be late Oxfordian that produced the McCoy and Bisbee basins. rocks. These volcanic rocks, including felsic because of their association with late Oxford- However, Independence suite La/Ta is >30 and ε − tuffs, domes, and fl ows, yielded U-Pb zircon ian microfossils. Paleontology thus places the Nd is < 1 for all samples reported by Glazner et ε dates of 156 ± 10 Ma in the southern Little high Nd basalts of the Chiricahua Mountains at al. (2008), and none plot near the Mesozoic rift

12 Geological Society of America Bulletin, Month/Month 2009 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

fi eld, indicating a signifi cant lithospheric com- that also produced the Bisbee basin. However, Formation also occurred in response to ther- ponent to dike magmas. This does not mean that marine strata within the Bisbee Group, which motectonic subsidence following Late Jurassic the stress regime leading to dike emplacement form a westward-thinning tongue extending rifting, as with the Bisbee basin (Dickinson and was unaffected by the geotectonic environment from the Chihuahua trough almost to Tucson Lawton, 2001b). of rifting that produced the Bisbee and McCoy in southeastern Arizona, encompass the ca. basins, but rather that Independence dike-swarm 112 Ma Albian-Aptian stage boundary and so CONCLUSION geochemistry was infl uenced by hydrated man- are about the same age as basal sandstone mem- tle and old lithosphere. ber 2 of the McCoy Mountains Formation. The Subsidence in the McCoy basin trapped marine incursion into the Bisbee basin is attrib- clastic sediments from diverse sources. The Thermotectonic Subsidence and uted to thermotectonic subsidence following Mesozoic magmatic arc was the most impor- McCoy Deposition tectonic extension and basaltic magmatism ~40 tant source of McCoy sands. Sands from east- million years earlier, which is consistent with ern and northern Laurentia that make up most The detrital-zircon study of McCoy strata in the long timeframe of subsidence following of the Lower to Middle Jurassic sandstone units the McCoy Mountains of California by Barth intracontinental rifting (Xie and Heller, 2009). on the Colorado Plateau, identifi able by their et al. (2004) concluded that 90% of the McCoy We conclude that it is possible that deposition U-Pb zircon age spectra, make up a signifi cant Mountains Formation was deposited after ca. of basal sandstone member 2 and the overlying component of lowermost McCoy sediments in 116 Ma and is unrelated to regional extension mudstone member of the McCoy Mountains two samples analyzed by Barth et al. (2004) but are only a minor component in our samples (Fig. 11). Sand from 1.3 to 1.8 Ga sources in 8 southwestern North America represents a large fraction of some lower McCoy samples, appar- Mesozoic ently because tectonic activity in upstream areas rifts had uplifted Proterozoic basement, and Pha- 6 nerozoic cover had been removed. A nearby source for the abundant 190–240 Ma zircons identifi ed in four lower McCoy samples has 4 not been identifi ed but is thought to have been part of a Triassic magmatic arc emplaced across McCoy Mts. Fm southwestern North America that was largely unaffected by <190 Ma Jurassic magmatism. 2 Granite Wash Mts. A tectonic environment of rifting during ini- New Water Mts. tial deposition of the McCoy Mountains For- mation in western Arizona is supported by the Dripping Spring

Nd 0 geochemistry of associated mafi c magmatism. Wash ε Specifi cally, several widely spaced samples analyzed for Nd isotopes are characterized Bisbee Group by high positive ε values (at t = 150 Ma) of – 2 Nd Fogo Seamounts, between +4 and +6, and low La/Ta of 10–19 Grand Banks area (Fig. 12), indicating primitive (rift-like) mantle sources similar to rift-related Bisbee basin – 4 Izu-Bonin arc basalts. Some of the mafi c igneous rocks associated with the McCoy Mountains Formation are Southern Andes ε characterized by La/Ta >20 and Nd <+1, which – 6 Independence we interpret to result from interaction with dike swarm Proterozoic lithosphere. One McCoy basalt fl ow yielded a U-Pb date of 154 ± 2 Ma, which – 8 overlaps with the age range of geochemically 0 20 40 60 80 100 120 140 similar magmatism that occurred during early rifting in the Bisbee basin (Fig. 13A; Lawton La/Ta and McMillan, 1999). We interpret McCoy basaltic rocks as rift related on the basis of their ε Figure 12. Plot of Nd versus La/Ta for mafi c sills and lava fl ows in the McCoy geochemistry, and infer that abrupt initiation of Mountains Formation (Mts. Fm.) and, for comparison, igneous rocks from rapid subsidence and McCoy deposition was other areas of rifting (Bisbee and Fogo) and subduction (Izu-Bonin and a consequence of rifting that immediately fol- southern Andes). Data from the 148 Ma Independence dike swarm (all plot- lowed ca. 160 Ma arc magmatism.

ted samples <60% SiO2) do not fall in Mesozoic rift ﬁ eld, and the dike-swarm Thermotectonic subsidence ~35–50 m.y. magmas are interpreted here as partially lithospheric in origin. Sources of after rifting, similar to subsidence in the Bisbee data: Bisbee Group (Lawton and McMillan, 1999); Fogo Seamounts (Pe- basin that led to Aptian-Albian marine incur- Piper et al., 2007); Izu-Bonin arc (Ishizuka et al., 2006); southern Andes (Kay sion, is possibly reﬂ ected in accumulation of et al., 2005); and Independence dike swarm (Glazner et al., 2008). basal sandstone member 2 and the overlying

Geological Society of America Bulletin, Month/Month 2009 13 Geological Society of America Bulletin, published online on 26 January 2011 as doi:10.1130/B30206.1

Spencer et al.

mudstone member in the McCoy Mountains Latest Jurassic A C l a s t i c Formation (Fig. 13B). This was followed by (150 Ma) deposition of the upper McCoy Mountains For-

M f o r e d e e p

a N mation, identiﬁ ed by its basal conglomerate, its

S g 0 500 T h r u s t b e l t position above an unconformity with the lower u m km

b a McCoy Mountains Formation in Arizona, its

d t

u i elevated content of K-feldspar, and its abundant c 150–155 Ma basalt c M o ε t g = +4 to +6 80–108 Ma zircons. Uplift of the Maria fold and i o Nd(150) a l o l r o n n thrust belt to the north (Fig. 13C), and deforma- c H i g h l a n d s tion within the McCoy basin, are suspected as z o Bisbee Chihuahua the immediate cause of the unconformities and n trough e basin the renewed inﬂ ux of coarse clastic sediments (Reynolds et al., 1986; Tosdal and Stone, 1994; McCoy basal Barth et al., 2004). However, the Maria fold Sab inas basin and thrust belt does not contain abundant Cre- member 1 Coahuila platform Rio Grande taceous igneous rocks, and the ultimate source Embayment of most of the 80–108 Ma zircons was likely more distant. New geochemical, geochronologic, and min- B C l a s t i c Mid-Cretaceous (Albian - 110–100 Ma) eralogic data from the McCoy Mountains For-

M f o r e d e e p mation and integration with geotectonic setting

N a allow division of the McCoy Mountains For-

g 0 500 S T h r u s t b e l t m u km mation into a three-part sequence (Fig. 14), as

b a

t follows: (1) The four lower McCoy successions d

i u c evaluated for detrital-zircon geochronology,

t three of which contain rift-related maﬁ c igne- i a o r n c ous rocks, are interpreted as a rift-axis facies, Mural limestone, Cintura Fm

U-Bar Fm, Mojado Fm while thinner, quartz-rich sandstones of basal z o n Aptian, Albian, and sandstone member 1 and the Crystal Hill For-

e Cenomanian marine mation are interpreted as a rift-ﬂ ank facies. Both sediments facies apparently received recycled sands from McCoy basal Colorado Plateau eolianites, identiﬁ ed by their member 2 far-traveled Grenville and associated zircons, but such sands were greatly diluted in the rift- axis facies by abundant, more locally derived sands. (2) Overlying clastic sedimentary units C l a s t i c Late Cretaceous correlative with basal sandstone member 2 and C the mudstone member are possibly related to (75 Ma) M f o r e d e e p thermotectonic subsidence following rifting, N a

as inferred for similar-age units in the Bisbee g 0 500 S

m T h r u s t b e l t u km Group (Dickinson and Lawton, 2001b) and

b a

d t consistent with the long timeframe for such sub-

i u c

c sidence (Xie and Heller, 2009). (3) The upper

t Maria fold and thrust belt

i a McCoy Mountains Formation formed a fore- o r n Early Laramide faulting, folding, c deep to the developing Maria fold and thrust

magmatism, and sedimentation

z belt, as with previous interpretations (e.g., Tos- o n Chihuahua tectonic belt dal and Stone, 1994). e

Tornillo basin ACKNOWLEDGMENTS Upper McCoy (conglomerate, We thank Jon Patchett for making the Radiogenic sandstone, and siltstone) Isotope Laboratory at the University of Arizona available for Sm-Nd isotopic analyses. We also thank Jon Patchett and Theresa Kayzar for additional Sm-Nd isotopic analyses. Bill Boynton and Dolores Hill of Figure 13. Summary evolution and tectonic setting of the Sabinas- the Arizona Lunar and Planetary Laboratory are grate- Chihuahua-Bisbee-McCoy rift belt. The narrow connection between the fully acknowledged for providing access to instru- McCoy Mountains Formation (Fm) area of deposition and the Bisbee mental neutron activation analysis (INAA) facilities and for assistance with INAA data processing for trough is hypothetical because no Mesozoic volcanic or sedimentary ﬁ ve samples from the Granite Wash Mountains. For rocks are exposed in this region. It is also possible that the two areas of petrographic analysis of sandstones, most samples extension were arranged en echelon and were entirely separate. were collected, and thin sections made, with support

14 Geological Society of America Bulletin, Month/Month 2009 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

McCoy Busby-Spera, C.J., Mattinson, J.M., Riggs, N.R., and McCoy Mountains Formation Schermer, E.R., 1990, The Triassic-Jurassic magmatic 8 Tectonic interpretation Mts. arc in the Mojave-Sonoran Deserts and the Sierran- Klamath region: Similarities and differences in paleo- geographic evolution, in Harwood, D.S., and Miller, M.M., eds., Paleozoic and Early Mesozoic Paleogeo- 7 Siltstone graphic Relations: Sierra Nevada, Klamath Mountains, member and Related Terranes: Geological Society of America Special Paper 255, p. 325–337. Dickinson, W.R., 2000, Detrital modes of selected sand- 6 Dome Rock stone samples from the McCoy Mountains Formation Mts. and correlative units in southwestern Arizona: Arizona New Water Geological Survey Contributed Report CR-00-A, 18 p. Mts. Sandstone Dickinson, W.R., 2008, Impact of differential zircon fertil- 5 member ity of granitoid basement rocks in North America on Upper McCoy age populations of detrital zircons and implications for granite petrogenesis: Earth and Planetary Science Let- Lower McCoy Thrust-belt foredeep thrust-belt foredeep ters, v. 275, p. 80–92, doi: 10.1016/j.epsl.2008.08.003. Conglomerate Dickinson, W.R., and Gehrels, G.E., 2008, Sediment deliv- 4 member ery to the Cordilleran foreland basin: Insights from km U-Pb ages of detrital zircons in Upper Jurassic and Cretaceous strata of the Colorado Plateau: Ameri- can Journal of Science, v. 308, p. 1041–1082, doi: 3 Mudstone 10.2475/10.2008.01. member Dickinson, W.R., and Gehrels, G.E., 2009a, Insights into North American Paleogeography and Paleotectonics riftRift axisaxis SW from U-Pb ages of detrital zircons in Mesozoic strata 2 Plomosa of the Colorado Plateau, USA: International Journal of Mts. Earth Sciences, doi: 10.1007/s00531-009-0462-0. Basal Dickinson, W.R., and Gehrels, G.E., 2009b, U-Pb ages of sandstone detrital zircons in Jurassic eolian and associated sand- 1 thermotectonicThermotectonic subsidence subsidencesubsidence member 2 stones of the Colorado Plateau: Evidence for transcontinental dispersal and intraregional recycling of sediment: Geological Society of America Bulletin, v. 121, Riftrift flankflank Basal sandstone no. 3/4, p. 408–433, doi: 10.1130/B26406.1. 0 Dickinson, W.R., and Gehrels, G.E., 2009c, Use of U-Pb member 1 ages of detrital zircons to infer maximum depositional ages of strata: A test against a Colorado Plateau Meso- Figure 14. Interpretation of tectonic environments for deposition of the McCoy Moun- zoic database: Earth and Planetary Science Letters, tains (Mts.) Formation based on stratigraphic columns in Figure 2. The three sequen- v. 288, p. 115–125, doi: 10.1016/j.epsl.2009.09.013. Dickinson, W.R., and Lawton, T.F., 2001a, Carbon- tial tectonic settings correspond to the three regional tectonics settings represented in iferous to Cretaceous assembly and fragmenta- Figure 13. tion of Mexico: Geological Society of Amer- ica Bulletin, v. 113, no. 9, p. 1142–1160, doi: 10.1130/0016-7606(2001)113<1142:CTCAAF>2.0 .CO;2. Dickinson, W.R., and Lawton, T.F., 2001b, Tectonic set- from National Science Foundation (NSF) grant EAR- Pb/U zircon ages constrain gabbroic crustal accretion ting and sandstone petrofacies of the Bisbee basin 8417106 (to W.R.D.). Arizona LaserChron Center at Atlantis Bank on the ultraslow-spreading Southwest (USA–Mexico): Journal of South American Earth activities were supported by NSF Instrumentation Indian Ridge: Earth and Planetary Science Letters, Sciences, v. 14, p. 475–504, doi: 10.1016/S0895 and Facilities award EAR-0732436. Finally, we thank v. 287, p. 540–550, doi: 10.1016/j.epsl.2009.09.002. -9811(01)00046-3. Barth, A.P., and Wooden, J.L., 2006, Timing of magmatism Carl Jacobson, Steve Reynolds, and Associate Editor Dickinson, W.R., Fiorillo, A.R., Hall, D.L., Monreal, R., following initial convergence at a passive margin, Potochnik, A.R., and Swift, P.N., 1989, Cretaceous Rob Rainbird for careful reviews, and Editor Nancy southwestern U.S. Cordillera, and ages of lower crustal strata of southern Arizona, in Reynolds, S.J., and Wilt, Riggs for review and editorial oversight, all of which magma sources: The Journal of Geology, v. 114, J.C., eds., Geologic Evolution of Arizona: Arizona resulted in considerable improvement. p. 231–245, doi: 10.1086/499573. Geological Society Digest, v. 17, p. 463–483. 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16 Geological Society of America Bulletin, Month/Month 2009 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

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