Origin and Evolution of the Sierra Nevada and Walker Lane themed issue

Origin of the Mineral Belt

Charles E. Chapin New Bureau of and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA

ABSTRACT Laramide plutons (ca. 75–43 Ma) are mainly may have aided the rise of magma bodies into alkaline monzonites and quartz monzonites in the upper crust from batholiths at depth, but had The (CMB) is a the northeastern CMB, but dominantly calc- no role in the generation of those batholiths. northeast-trending, ~500-km-long, 25–50-km- alkaline granodiorites in the central CMB. There is the crux of the enigma. wide belt of plutons and mining districts (Colo- Geochemical and isotopic studies indicate From several decades of fi eld work in the rado, United States) that developed within that CMB magmas were generated mainly states of Colorado, New Mexico, and , an ~1200-km-wide Late –Paleo- in metasomatized Proterozoic intermediate I became aware of signifi cant differences in geo- gene magma gap overlying subhorizontally to felsic lower crustal granulites and mafi c logic features on opposite sides of the CMB. My subducted segments of the . rocks (± mantle). Late roll- goal in this paper is to summarize these differ- Of the known volcanic gaps overlying fl at back superimposed on the CMB ences, integrate them with the regional tectonic slabs in zones around the Pacifi c during waning of Laramide compression and geochronologic framework, and thereby Basin, none contains zones of magmatism (ca. 43–37 Ma) resulted in world-class sul- gain insight into the origin of the CMB. The analogous to the CMB. I suggest that the fi de replacement ores in the Leadville area. differences are primarily contrasts in: (1) the primary control of the CMB was a north- Overprinting of the CMB by Rio Grande orientation of Laramide structures, (2) Late Cre- east-trending segment boundary within the Rift extension beginning ca. 33 Ma resulted taceous–Eocene and , underlying Farallon fl at slab. The boundary in intrusion of evolved alkali-feldspar granites and (3) the nature and distribution of middle was dilated during warping of slab segments and generation of major porphyry molybde- magmatism. The paper is essentially by the overriding thick (~200 km) litho- num deposits at Climax and Red Mountain. the story of what happened with the Farallon spheres of the Wyoming Archean craton slab after it shut off magmatism in the Sierra and the continental interior craton during INTRODUCTION Nevada ca. 85 Ma. acceleration of Farallon–North American convergence beginning in mid-Campanian The origin of the Colorado Mineral Belt (CMB) PLATE TECTONIC SETTING time (ca. 75 Ma). Because the primary con- has been a long-standing geologic enigma. The trol was not in the North American plate, CMB trends ~N43°E from the The basic elements of the plate tectonic the CMB cut indiscriminately across the area on the to near Boulder, framework can be visualized in Figure 1, sup- geologic grain of Colorado, seemingly inde- Colorado (United States; Fig. 1) and is marked plemented by the geochronologic chart of Fig- pendent of the tectonic elements it crossed. by numerous igneous intrusions and many of ure 2. Figure 1 shows the Laramide CMB (after A series of discontinuous shear zones of the metal mining districts of Colorado. Since Mutschler et al., 1987) as a narrow magmatic Proterozoic ancestry provided some local the classic paper by Tweto and Sims (1963), the lineament extending northeastward ~500 km control at the district level but were not the origin of the CMB has usually been ascribed to from the Four Corners area of the eastern Colo- primary control. localization of Laramide and younger intrusions rado Plateau to the Rocky Mountain front near Geologic contrasts north and south of the by a northeast-trending Colorado lineament con- Boulder, Colorado. The CMB occurs within CMB refl ect its relationship to a segment sisting of multiple shear zones of Proterozoic the eastern bulge of the Cordillera formed by boundary in the Farallon plate. The domi- ancestry. Detailed geologic mapping by many basement block uplifts and arches of Laramide nant trends of Laramide basement-cored geologists provided evidence of local structural age (Erslev, 1993). The uplifts formed as the uplifts are northwestward north of the CMB control of and younger intru- subhorizontally subducted Farallon slab trans- but northward south of the CMB. Laramide sions by zones of the Colorado lineament lated compressive stresses northeastward via sedimentary deposits of Late Cretaceous and (e.g., Lovering, 1933; Lovering and Goddard, viscous coupling with the overlying North age (exclusive of the Sevier fore- 1950; Tweto and Sims, 1963; Braddock, 1969; American plate (Coney, 1972, 1978; Coney and deep) are as much as 6 km thick north of the Tweto, 1975; Bookstrom, 1990; Wallace, 1995). Reynolds, 1977; Cross and Pilger, 1978a; Bird, CMB versus only ≤3 km south of the CMB. However, as observed by Tweto and Sims 1984; Cross, 1986). The uplifts of Laramide The Farallon segment south of the CMB (1963), the CMB cuts indiscriminately across age are mostly north-trending south of the rolled back to the southwest and sank into the geologic grain of Colorado with remarkable CMB but northwest-trending north of the CMB the mantle beginning ca. 37 Ma with resultant continuity, seemingly independent of the tec- (Fig. 1). The eastern bulge of the Cordillera major ignimbrite volcanism and genera- tonic elements it crosses. Tweto (1975) further and its component uplifts and basins bridge tion of the large San Juan and Mogollon- stated that the only unifying structural feature an ~1200-km-wide gap in the Laramide (75– Datil volcanic fi elds. Volcanism in the Rocky within the belt is a system of discontinuous and 43 Ma) subduction-related volcanic arc (Fig. 1). Mountains north of the CMB was sparse. overlapping shear zones, which The northwestern boundary of the fl at slab was

Geosphere; February 2012; v. 8; no. 1; p. 28–43; doi: 10.1130/GES00694.1; 10 fi gures.

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the gaps coincide with the collision or subduc- tion of an aseismic ridge or oceanic plateau. Several authors (Livacarri et al., 1981; Hender- son et al., 1984; Liu et al., 2010) have proposed that the Laramide fl at slab and magma gap of the southwestern U.S. was caused by subduc- tion of oceanic plateaus, possibly conjugates of the Shatsky Rise and Hess Plateau. The vol- canic gaps tabulated by the above-cited authors ranged from 200 to 800 km in width, except for the Peru and southern Chile gaps, which are 1500 and 1000 km wide, respectively. The Peru gap (Fig. 3) is a composite gap formed by subduction of the Inca Plateau and Nazca Ridge (Gutscher et al., 2000b). The exceptional width (~1200 km) of the southwestern U.S. magma gap raises the possibility of a composite origin, as explored by Liu et al. (2010). A fundamental question must be addressed before continuing. If the primary control of the CMB was a leaky segment boundary in the underlying subhorizontally subducting Faral- lon plate, how could the CMB have main- tained the same N43°E trend and geographic position on the North American plate through ~40 m.y. of convergent-margin tectonism? This seemingly insoluble problem arises because of the segmented interlocking nature of the Pacifi c-Farallon plate boundary and the appar- ent northward to northwestward movement of the Pacifi c plate through Late Cretaceous and Paleogene time (Engebretson et al., 1985; Stock and Molnar, 1988; Atwater, 1989). Apparent changes in Farallon–North American conver- gence direction with time (Page and Engebret- son, 1984; Engebret son et al., 1985; Stock and Figure 1. Laramide paleotectonic map of western United States and northern Mexico show- Molnar, 1988; Saleeby, 2003; Jones et al., 2011) ing relationships of the Colorado Mineral Belt (CMB) to the gap in the Laramide volcanic also pose a problem. However, the revisionist arc, major Laramide uplifts and basins of the Rocky Mountain broken foreland, and the geo dynamic concepts of Hamilton (2007) offer subsidence anomaly (red area) of the Wyoming province. Map is modifi ed from Seager a solution. Hamilton emphasized that subduc- (2004). Inset at lower left shows northeastward trajectory of a point on the Farallon plate tion provides the primary drive for both upper from 100 to 60 Ma (from Engebretson et al., 1985). Trace of CMB is from Mutschler et al. and lower plates, and that plates move toward (1987). Outline of subsidence anomaly is after McGookey et al. (1972, fi g. 22 therein), and subduction zones as subducting slabs sink Cross (1986). Dashed lines show inferred boundaries of the fl at slab. more steeply than they dip, causing subduction hinges to retreat oceanward: an overriding plate is drawn forward to maintain contact with the located along the northeast-trending Humboldt the lateral extent of the Laramide magma gap; retreating hinge and falling slab. structural zone (Mabey et al., 1978), which in boundaries of the fl at slab are also located at The earliest record of subhorizontal subduc- late Cenozoic time appears to have controlled major discontinuities in structural style. How- tion of the Farallon slab is the extinguishing the eastern Snake River Plain–Yellow stone ever, other slab segments to the north and south of magmatism in the Sierra Nevada batholith of trend (Christiansen et al., 2002). The south- were also relatively fl at and underwent similar California ca. 85 Ma (Santonian; Evernden and eastern boundary of the fl at slab trended north- slab rollback, as evidenced by caldera migra- Kistler, 1970; Chen and Moore, 1982; Saleeby, eastward through southern New Mexico along tion in Nevada and to the north and trans- 2003). On the opposite side of the overrid- a zone marked by the Pecos buckles (Fig. 1), a Pecos Texas to the south (see also Henry et al., ing North American plate, seafl oor spreading zone that separated Laramide block uplifts in 1991, 2010; Chapin et al., 2004b; Schmandt and began in the northwest-trending Labrador southwest New Mexico from the fold and thrust Humphreys , 2011). Trough prior to 84 Ma (anomaly 34, Santonian; belt of the Sierra Madre Oriental and the thin- McGeary et al. (1985) and Gutscher et al. Srivastava and Tapscott, 1986; Ziegler, 1988), as skinned of the Chihuahua trough (2000a) tabulated magmatic gaps (>200 km began to separate from Green- in northern Mexico (de Cserna, 1989; Seager, wide) in subduction-related, active volcanic land and Eurasia. Seafl oor spreading slowed in 2004). The fl at slab in Figure 1 is based on chains around the Pacifi c Basin. At least 10 of the Labrador Sea after ca. 50 Ma (anomaly 21,

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Figure 2. Chart showing chronology of tec- tonic, magmatic, sedimentation and/or sub- sidence, and major mineralization events affecting the origin and evolution of the Colo- rado Mineral Belt (CMB). Bold numbers refer to specifi c events with key references in the following list. 1—Weimer (1960), Jordan (1981), De Celles (1994); 2—Haun and Kent (1965), McGookey et al. (1972), Cross and Pilger (1978b), Bird (1984); 3—Coney (1978), Jurdy (1984), Engebretson et al. (1985); 4—Gries (1983), Mutschler et al. (1987), Dickinson et al. (1988); 5—Coney and Reynolds (1977), Cross and Pilger (1978a), Lipman (1980); 6—Coney and Reynolds (1977), Lipman (1980), Chapin et al. (2004a, 2004b); 7—Tweto (1979), Chapin and Cather (1994); 8—Ducea (2001), Saleeby et al. (2007, 2008); 9—Mutschler et al. (1987), Book- strom (1990), Wallace (1995); 10—Chapin et al. (2004a, 2004b), Lipman (2007), Lipman and McIntosh (2008); 11—Geissman et al. (1992), Wallace and Bookstrom (1993), Shan- non et al. (2004); 12—Beaty et al. (1990), Johansing and Thompson (1990), Thompson and Arehart (1990). Star at lower left marks opening of Labrador Trough and beginning LABRADOR TROUGH of separation of North America from Green- land and Eurasia ca. 84 Ma (anomaly 34; Srivastava and Tapscott, 1986; Ziegler, 1988). RGR—; FA–NA—Faral- lon–North American convergence; moly.— molybdenum; epoch and stage boundaries are from Gradstein et al. (2004).

Middle Eocene) and stopped completely ORIENTATION OF LARAMIDE gap, oriented north-south? I suggest that the prior to ca. 36 Ma (anomaly 13, Late Eocene) STRUCTURES frontal ranges of the southern (Srivastava and Tapscott, 1986; Ziegler, 1988). owe their northward orientation to resistance to Note the correlation with the time span of the Basement-cored arches and tilted-block uplifts deformation by the cooler, thicker, lithosphere of Laramide (Fig. 2). Apparently, as of Laramide age south of the CMB are mostly the North American interior craton. the Farallon plate subducted to the northeast, the north trending (Fig. 1), whereas those north of Using a tomographic inversion of tele- subduction hinge retreated to the southwest the CMB are mainly northwest trending; excep- seismic shear waves, Lee and Grand (1996) and drew the overriding North American plate tions include the north-trending southern Lara- found an abrupt increase in shear wave veloc- after it, thus opening the Labrador Trough. Vis- mie Range, the east-west–trending Uinta Range ity in the upper 200 km of the mantle near the cous coupling of the Farallon fl at slab with the (Fig. 1), and structures of variable orientation Colorado-Kansas border that they interpreted overriding North American plate over an area on the Colorado Plateau (Davis and Bump, as the boundary of the cratonic lithosphere. ~1200 km wide by 1000 km long, as evidenced 2009). Other exceptions are structures inherited West et al. (2004), using surface wave veloci- by the magma gap and severe deformation of from the late Ancestral Rocky Moun- ties, mapped a transition in lithospheric thick- the overriding plate, may also have helped keep tains. The northwest-trending uplifts are nearly ness from ~200 km under the Great Plains to the Farallon–North American convergence on perpendicular to the CMB, the margins of the 45–55 km beneath the Rio Grande Rift and 120– a steady N43°E trend, as marked by the CMB. magma gap, and presumably the trajectory of 150 km beneath the Colorado Plateau. Yuan As pointed out by Henry et al. (2010), other the fl at slab. Viscous coupling of a northeast- and Romanowicz (2010) used changes in the segments of the Farallon plate subducted at moving fl at slab with the overlying North Ameri- direction of seismically determined azimuthal relatively shallow angles and rolled back to the can plate should result in northwest-trending anisotropy to measure lithospheric thickness southwest, as indicated by southwest-migrating contractional structures. Why then are the uplifts (~180–240 km) of the stable cratonic interior caldera complexes. south of the CMB, but still within the magma of North America; they found that the north-

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maps show northward fold orientations along the Rocky Mountain front, whereas northwest trends dominate western Colorado and much of Wyoming. Bolay-Koenig and Erslev (2003) reported that the most striking and systematic change in Laramide structural fabric is the pro- gressive change in fold orientations from north- west to north-south as they approach the eastern boundary of the Laramide province. The chemical compositions of igneous rocks and mineral deposits also refl ect the presence of the cratonic boundary. Alkaline intrusives, lavas, breccia pipes, and diatremes were emplaced along the eastern frontal ranges of the Rocky Mountains during the Late Cretaceous and Cenozoic (Mutschler et al., 1987; McLemore, 1996; Kelley and Ludington, 2002), indicating a thicker, cooler lithosphere. Mineral deposits containing gold tellurides, native gold, tungsten, uranium, and other rare elements also refl ect the alkaline environment of the Rocky Moun- tain front (Lovering and Goddard, 1950; Davis and Streufert, 1990; McLemore, 1996; Kelley and Luding ton, 2002). The alkaline compositions are thought to be due to smaller degrees of par- tial melting at greater depth, probably within the lithospheric mantle and/or lower crust, and pos- sibly with enrichment of source rocks by meta- somatic activity (Mutschler et al., 1987; Kelley and Ludington, 2002; Pilet et al., 2008). This explanation of the northward trend of Laramide uplifts south of the CMB appears quite straightforward, but raises questions about the northwest-trending uplifts north of the CMB. Why was the Wyoming Archean craton severely disrupted by a series of alternating Laramide uplifts and interspersed basins? Liu et al. (2010) used conceptual models and inverse convection modeling, starting with plate recon- structions and seismic tomography, to track sub- duction of conjugates of the Shatsky Rise and Hess Plateau during Laramide fl at subduction of the Farallon plate. Their reconstruction has the Figure 3. Tectonic setting of the Andean convergent margin; fl at slab segments are indicated Shatsky Rise conjugate follow a N23E trajec- by thick brackets and subducting oceanic plateaus and ridges are shaded gray. The tory from an entry point off southern California are defi ned by the 2000 m contour; active volcanoes are shown as black triangles. Inferred ca. 90 Ma with the leading edge arriving beneath subducted Inca Plateau and uncertain continuations of Cocos, Carnegie, Nazca, and Juan southern Wyoming ca. 68 Ma (Liu et al., 2010, Fernandez Ridges (R) are dashed. Plate convergence vectors are based on global kinematic fi g. 3 therein). Their model has the Hess con- model (DeMets et al., 1990). Carib.—Caribbean; FZ—fracture zone; B—basin; Bl—Choco jugate arriving at the latitude of northern Baja block. After Gutscher et al. (2000b) with permission of American Geophysical Union (paper ca. 65 Ma and following a N46°E trajectory number 0278–7407/00/1999TC001152). across northern Mexico with its northern edge approximately along the New Mexico–Mexico border (Liu et al., 2010, fi g. 3 therein). The tra- trending Rocky Mountain front in New Mex- (Yuan and Romanowicz, 2010). It should not jectories of both the Shatsky and Hess conju- ico and Colorado marks the western boundary be surprising, then, to see the frontal ranges of gates are perpendicular to thrusting in Wyoming of the craton and that the chemically depleted the aligned along the and the Sierra Madre Oriental, respectively. The upper layer of the cratonic lithosphere is absent north-trending barrier of the cratonic boundary. tomographic images of present-day seismic west of the boundary. Cratonic lithospheres are The Laramide folds compiled by Bolay-Koenig anomalies in the mantle beneath the eastern characteristically thick, cold, relatively buoy- and Erslev (2003, fi g. 12 therein) from geo- United States were interpreted as remnants of ant, highly viscous, and resistant to deformation graphic information system–enhanced tectonic the Shatsky and Hess conjugates.

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The model of Liu et al. (2010) ignores the taceous deposits and concluded that additional Zuni III stratigraphic subsequence (latest Early CMB but provides an interesting perspective that subsidence was required beyond that related to Cretaceous to Early ; ca. 100–61 Ma) may explain why the Archean craton of Wyo- thrusting and accumulation; they sug- along and east of the Sevier thrust belt. In refer- ming underwent the greatest Laramide tectonic gested that the depocenter had been pulled down ring to the intensity of the downwarp, Sloss disruption. If the segment of the Farallon fl at from below between 80 and 70 Ma by subcrustal (1988, p. 45) stated that “the rates exceed any- slab north of the CMB carried the Shatsky Rise, loading and cooling induced by a shallowly sub- thing recorded inboard of the margins since the its greater thickness and relatively head-on col- ducted oceanic plate. Jordan (1995) concurred Absaroka II subsidence of the Permian Basin lision with the thick Archean craton could have with this interpretation, as have others who refer some 160 m.y. earlier.” resulted in exceptionally strong compression. to it as dynamic subsidence due to underplating Figure 5 (reproduced from Liu and Num- For example, the east-west–trending Uinta uplift and mantle fl ow induced by shallow subduction medal, 2004) details the ages and restored owes its anomalous trend to tectonic inversion of (e.g., Liu and Nummedal, 2004; Jones et al., thicknesses of Late Cretaceous strata (ca. 90.4– a half-graben fi lled with ~8 km of Paleoprotero- 2011). The subsidence feature is anomalous in 73.0 Ma) along a cross section extending zoic continental deposited along the its areal extent, thickness of fi ll, and its occur- ~500 km eastward from the Wyoming salient Archean-Proterozoic province boundary (Burch- rence in the spatial position of a backbulge depo- of the Sevier thrust belt. Liu and Nummedal fi el et al., 1992). To the north, the Wind River center, which normally is a shallow depression (2004) reported that total subsidence during the Range was uplifted by a Laramide thrust, traced with thin sedimentary fi ll on the craton side modeled interval exceeded that due to fl exure seismically to at least 24 km depth, with an esti- of the forebulge (Jordan, 1995; DeCelles and by ~1 km with a wavelength that exceeded the mated 21 km horizontal displacement and 13 km Giles, 1996). Figure 4 (modifi ed from Sloss, length of the studied profi le (500 km). From (Smithson et al., 1978). 1988, p. 46) shows the net subsidence rate of the measured thicknesses in Figure 5, it appears that The Liu et al. (2010) model also estimates that the trajectory of the Hess Plateau was northeast- ward beneath northern Mexico. If so, the Hess Plateau would have been riding on a segment of the Farallon slab whose northern boundary coincided with the southern boundary of the Southern Rocky Mountain segment (Fig. 1). The northwest-trending belt of intrusives and calderas that extends from the Big Bend area of Texas to southern New Mexico (Henry et al., 1994) may represent middle Cenozoic rollback volcanism of this southern segment. The south- west younging of calderas in the trans-Pecos volcanic fi eld is very similar in trend, timing belt (37.5–27.7 Ma), and episodicity to that in the San Juan and Mogollon-Datil volcanic fi elds (Chapin et al., 2004a, 2004b; Henry et al., thrust 2010). In summarizing the ignimbrite fl are-up in western North America, Henry et al. (2010) Sevier found that peak activity occurred between 37 and 22 Ma, with voluminous caldera activity migrating southwestward through time as roll- back of the Farallon slab fragmented into a few major panels. Apparent alignment of the west Texas panel with the Mogollon-Datil volcanic fi eld (Chapin et al., 2004b) and the conspicuous offset of the latter from the San Juan volcanic fi eld suggest differences in rollback behavior that have yet to be addressed.


Pioneering stratigraphic studies by Weimer Figure 4. Zuni III subsequence (Late Cretaceous to Early Paleocene) (1960) and Weimer and Haun (1960) showed net subsidence rate (Sloss, 1988). 1—Wind River uplift; 2—Front that a broad depocenter (~500 × 600 km across) Range uplift; 3—Kaibab uplift; 4—Red Desert–Hanna basin; 5— east of the Wyoming salient of the Sevier fold and Piceance-Washakie basin; 6— Basin; 7—Pedregosa Basin. thrust belt contains 2–3 km of Late Cretaceous Trace of Colorado Mineral Belt (solid black line) is from Mutschler sedimentary deposits (locally up to 5 km thick). et al. (1987). Boundaries of fl at slab (dashed black lines) are from Cross and Pilger (1978b) and Cross (1986) Figure 1. East-west white line in southern Wyoming shows location drew attention to the unusually thick Late Cre- of ~500 km cross section in Figure 5. Modifi ed from Sloss (1988).

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Figure 5. (A) Map of southern Wyoming from the Idaho-Wyoming salient of the Sevier thrust belt on the west to the Cheyenne basin on the east showing line of section for B and location of well control. (Cross section also shown by white line in Fig. 4.) (B) Section based on control of 19 well logs covering the stratigraphic interval from middle Cenomanian base of the Frontier Formation to the late Campanian Eric- son Formation. Five reference levels corresponding to the ages 97.2, 90.4, 83.9, 78.5, and 73.4 Ma (40Ar/39Ar-derived Late Cretaceous time scale of Obradovich, 1993) were used to calculate decompacted sediment thicknesses in modeling study by Liu and Nummedal (2004) to determine the dynamic component of subsidence, which ranged from ~800 m in eastern Wyoming to ~1.8 km near the thrust belt. Note that this only includes part of the Late Cretaceous subsidence and none of the Paleogene. After Liu and Nummedal (2004). For the distribution, thickness, and ages of Paleogene sediments, see McDonald (1972, 1975) and Lillegraven (1993).

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sediment accumulation was fastest during the Precambrian-Phanerozoic interface south of the lon fl at slab that extended from the CMB south- interval 83.9 Ma to 78.5 Ma (150–280 m/m.y.). CMB is ~2000 m below sea level (in the San ward to southern New Mexico (Fig. 8), and Liu and Nummedal (2004) concluded that Juan and Denver Basins) while north of the (2) extension of the Rio Grande Rift through the accommodation for the westward-thickening CMB the interface is commonly –4000 m to CMB in the Leadville area. The fl at slab broke wedge of sediments was provided by combined –5000 m beneath Laramide basins, but deep- beneath what later became the Rio Grande Rift fl exural loads and dynamic subsidence due to ens to more than 7500 m below sea level in the (Fig. 8) and rolled back to the southwest, more mantle fl ow generated by eastward subduc- northern Green River Basin and in the Hanna or less opposite to the initial emplacement direc- tion of the Farallon plate beneath North Amer- Basin. Not all was Laramide subsidence, of tion. The timing and direction of rollback are ica. Note that the cross section and modeling course, but these fi gures give a sense of the dif- evident from the migration direction of caldera by Liu and Nummedal (2004) only involve the ferences across the CMB. clusters as seen in Figure 8 (Chapin et al., 2004a, interval 97.2–73.4 Ma, whereas thrusting con- Besides its unusual thickness of sedimen- 2004b). The earliest calderas (ca. 37–36 Ma) tinued episodically to ca. 50 Ma (DeCelles, tary fi ll for a backbulge depocenter, the Late formed at opposite ends of the slab segment, 1994). The Zuni III subsequence of Sloss (1988) Cretaceous–Paleogene subsidence anomaly is where Laramide magmatism in the CMB and shown in Figure 4 extends from the latest Albian unusual in several other respects. It adjoins the southern New Mexico had increased heat fl ow (ca. 100 Ma) through the Danian Stage of the most cratonward salient of the Sevier fold and and perhaps created ascent routes for magmas. Early Paleocene (ca. 61 Ma). thrust belt and contains the thickest accumulation The remarkable correlation of pulses of Most authors have agreed with the original of Laramide sedimentary deposits in the West- ignim brite volcanism (Fig. 9) from the CMB interpretation by Cross and Pilger (1978b) that ern Interior foreland basin outside of the Sevier to the Mexican border indicates that rollback the anomalous subsidence was caused by sub- foredeep. It hosted several large, long-lived of the Farallon fl at slab occurred in an episodic, crustal loading induced by a shallowly subducted lacustrine systems and became the terminal regional manner between ca. 37 and 23 Ma Farallon plate that was slightly denser than the depocenter for major orogen-parallel Eocene (Chapin et al., 2004a, 2004b). The San Juan vol- it displaced. However, consider- rivers, such as the Idaho (Carroll et al., 2008; canic fi eld in Colorado (Lipman and McIntosh, ation of space-time relationships raises questions. Chetel et al., 2010) and California (Davis et al., 2008) and the Mogollon-Datil and Boot Heel Temporally, the anomalous sub sidence coincides 2010), rivers that drained large areas of the volcanic fi elds in New Mexico (McIntosh and with Late Cretaceous–Early Eocene major thrust- western Cordillera. These characteristics testify Bryan, 2000; Chapin et al., 2004a, 2004b) con- ing in the Sevier belt (Fig. 2) with subsidence in to persistent subsidence beginning in the Late tain clusters of calderas that are progressively the backbulge depocenter beginning ca. 90 Ma Cretaceous (ca. 90 Ma) and continuing into the younger to the southwest (Fig. 8), indicating the (Weimer and Haun, 1960; Jordan, 1981; Molenaar Middle Eocene. The subsidence anomaly was rollback direction of the fl at slab. The rollback and Rice, 1988; DeCelles, 1994; Roberts and essentially the sump for erosional products of a has also been described as falling away in pieces Kirschbaum, 1995; Liu and Nummedal, 2004). large portion of the western Cordillera. Since the (Atwater, 1989); however, Henry et al. (2010) The 90 Ma onset of sub sidence began near the Wyoming province was a relatively stable shelf described migration of ignimbrite volcanism in Turonian-Coniacian boundary, ~5 m.y. before blanketed by comparatively thin Triassic through western North America consistent with rollback shutoff of major intrusions in the Sierra Nevada Early Cretaceous strata (Mallory, 1972), the Late to the southwest of a slab fragmented into a few batholith ca. 85 Ma, generally considered to Cretaceous–Paleogene subsidence anomaly major panels. The important point is that as the be the earliest evidence for fl at slab subduc- stands out as a signifi cant feature of the Laramide asthenosphere encroached between the sink- tion of the Farallon plate (Coney, 1972; Coney orogeny. The subsided area accumulated clastic ing slab and the overlying lithosphere, a surge and Reynolds, 1977; Saleeby et al., 2008). The sediments ranging up to 6–11 km in maximum of fl uids and/or melts came in contact with the 90 Ma onset of subsidence also preceded by thickness, about equally divided between Late lithosphere and began the magma generation 10–15 m.y. the widespread tectonic partitioning Cretaceous and Paleogene deposits (McGookey processes. This is an example of the magmatic- of the foreland that began ca. 80–75 Ma (Gries, et al., 1972; McDonald, 1972, 1975). power input that Lipman (2007) called on to 1983; Mutschler et al., 1987; Cather, 2004) and generate major pulses of ignimbrite volcanism. is commonly ascribed to fl at subduction of the MIDDLE CENOZOIC MAGMATISM That ignimbrite volcanism is so episodic indi- Farallon plate. cates that rollback of the fl at slab was a stick- The intrusion of Laramide plutons along First impressions of the CMB tend to focus slip process, possibly augmented by fl uctuations the CMB began ca. 75 Ma (Mutschler et al., on the Laramide plutons (ca. 75–43 Ma) that in regional stresses. 1987; Bookstrom, 1990), approximately coeval distinguish the CMB (Figs. 1 and 6) as an enig- Middle Cenozoic magmatism was gener- with widespread tectonic partitioning of the matic northeast-trending magmatic lineament ally absent in the Rocky Mountains above the Laramide fl at-slab area into a complex array of within a broad magma gap. What is not so well fl at slab segment north of the CMB, except basement-cored uplifts and intervening basins. known is that middle Cenozoic magmatism was where the Absaroka volcanic fi eld (55–44 Ma; Both orogenic events coincided temporally with superimposed on the central part of the CMB Mutschler et al., 1987) spilled across the north- the sharp increase in Farallon–North American and resulted in large mineral deposits. Both the ern margin, and in , where a convergence (Fig. 2) ca. 75 Ma (Engebretson time-distance plot of CMB igneous rocks by few scattered volcanic rocks occur north of the et al., 1985). If the subsidence anomaly was Bookstrom (1990) and magmatic event histo- CMB (36–26 Ma; Mutschler et al., 1987; Cole caused by subcrustal loading by the fl at slab, grams (in Chapin et al., 2004a, 2004b; Eaton, et al., 2010). why did subsidence begin 10–15 m.y. before 2008) show that middle Cenozoic magmatism these other Laramide events, and why was there began sporadically ca. 43 Ma and culminated in PLUTONS AND ORE DEPOSITS not similar subsidence of the North American an impressive peak between ca. 37 and 18 Ma lithosphere above the fl at-slab segment south (Fig. 7). Middle Cenozoic magmatism was gen- The CMB is a composite of three main age of the CMB? The tectonic map of Muehlberger erated by two major overlapping events: (1) the groups of igneous rocks: (1) Late Cretaceous– (1992) shows that the maximum depth to the rollback and sinking of the segment of the Faral- Middle Eocene (Laramide, ca. 75–43 Ma),

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Figure 6. Shape and extent of the Colorado Mineral Belt (CMB) as drawn by different authors. (A) Outline of the CMB as defi ned by principal mining districts of Laramide age versus maximum boundary when mid- dle Cenozoic mining districts are included (modified from Tweto and Sims, 1963). (B) Inset showing CMB (COMB) as defi ned by only Laramide plu- tons (modifi ed from Mutschler et al., 1987). Note different scale, and that there is no age pro- gression of Laramide plutons along the CMB. Isotopic dat- ing has determined a ca. 75 Ma age for the Jamestown stock at the northeast end of the CMB (McDowell, 1971; Gable, 1984) and confi rmed the 70 Ma age for the Carrizo Mountains at the southwest end (Semken and McIntosh, 1997). A 75 has been added to B for the James- town stock. The 70 Ma Carrizo laccolith and 72–71 Ma Ute 43 lacco lith at the southwest end of the CMB are labeled C and U, respectively. Other in the Four Corners area are mid- Tertiary in age (Mutschler et al., 1987).

(2) middle Cenozoic (ca. 43–18 Ma), and (3) late the central CMB was caused by: (1) a north- in a fl atly subducting oceanic slab that was being Cenozoic (ca. 18–0 Ma). The tectonic environ- northwest–trending transverse alignment of over ridden by thick continental lithosphere, ment varies between age groups, as do the com- Laramide stocks (ca. 72–60 Ma; Cunningham as much as 1000 km from the trench, is there positions of igneous rocks and ore deposits. In et al., 1994) extending from Salida on the south- anything unusual about the magmas generated? map view, the outline of the CMB varies among east to Fulford on the northwest (Fig. 6); and Mutschler et al. (1987) summarized the geo- authors depending upon how inclusive they are (2) tectono magmatic overprint by middle Ceno- chronology, igneous , and geochem- (compare, for example, in Figure 6; Mutschler zoic intrusions related to rollback of the Faral- istry of intrusions along the CMB. The minor et al., 1987; and Tweto and Sims, 1963). The lon slab beginning ca. 37 Ma and Rio Grande element and strontium isotope geochemical Laramide record of the CMB consists of a nar- Rift extension beginning ca. 33 Ma. The bulge study of Simmons and Hedge (1978) and the rare row alignment (25–50 km wide) of deeply contains several of the richest mining districts earth element (REE) and samarium-neodymium eroded plutons trending ~N43°E and extending in Colorado and documents the dramatic effects isotopic studies of Stein and Crock (1990) fl esh from the Four Corners area on the Colorado Pla- changes in tectonic stress can have on magma out the geochemical framework. Bookstrom teau to the northern near Boulder, compositions and associated ore deposits. Since (1990) and Cunningham et al. (1994) provided Colorado (Fig. 6B). Volcanic detritus is pres- the CMB developed in a relatively unique tec- summaries of the ages and mineralization asso- ent in the bordering Laramide basins (Tweto, tonic environment, interpreted as a magmatic ciated with the intrusions. The following sum- 1975, fi g. 7 therein). A conspicuous bulge in lineament overlying a leaky segment boundary mary is largely based on these papers.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/1/28/3342088/28.pdf by guest on 25 September 2021 Chapin nit, so c plu- , 2008) is very similar except for the greater the greater except for , 2008) is very similar mbers of additional basalts not shown (due to es rock types; abbreviations identify specifi types; abbreviations es rock cant errors were not used. Color-coded legend identifi not used. Color-coded were cant errors Ar ages of igneous rocks in New Mexico, Late Cretaceous to present. Only one point is plotted for each event or stratigraphic u each event or Only one point is plotted for to present. in New Mexico, Late Cretaceous ages of igneous rocks Ar 39 Ar/ 40 Figure 7. Histogram of K-Ar and 7. Histogram of K-Ar Figure suspected to have signifi as could be ascertained. Dates known or far tons (see Chapin et al., 2004a, 2004b, for lists of plutons). Numbers in (PL.) and Quaternary (Q.) columns indicate nu tons (see Chapin et al., 2004a, 2004b, for size restrictions). Holocene dates are shown to right of zero. A histogram for Colorado and easternmost Utah and Arizona (Eaton Colorado and easternmost Utah histogram for A shown to right of zero. Holocene dates are size restrictions). abundance of Late Miocene to Quaternary basalts in New Mexico.

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quartz monzonite suite is the most abundant rock type and dominates the central CMB, but can occur anywhere along the CMB. The granites have lower REE concentrations with patterns that form distinct U shapes by swing- ing upward at the heavy REE end, and display pronounced negative europium anomalies. (For additional chemical and isotopic data, see the papers cited in the previous paragraph.) The monzonite suite is the most variable in composition and was emplaced during Laramide compression; it also refl ects its proximity to the thicker, colder lithosphere of the continental interior craton through its highly alkaline com- positions and associated mineralization (e.g., gold, tellurium, tungsten, uranium). The REE patterns that characterize the Laramide monzo- nite stocks are typical of alkalic, silica-saturated rocks worldwide (Stein and Crock, 1990). It is also true that the most highly alkaline rocks are along the eastern, cratonward side of con- tinental margin volcanic arcs in both the west- ern U.S. and South America (Kay and Gordillo, 1994; McLemore, 1996; Kelley and Ludington, 2002). Stein and Crock (1990) stated that direct involvement of upper mantle is not necessary, and that metasomatized amphibolitic to granu- litic mafi c lower crust enriched in potassium, sodium, and light REEs was the most likely source material; they also concluded that an eclogitic (garnet bearing) residual assemblage remained after generation of the alkaline mon- zonites, and may indicate that the monzonites were generated deeper than the granodiorites. Plutons of the granodiorite–quartz monzonite suite include both Laramide and middle Tertiary

rocks, generally contain >63% SiO2, have total alkalies ~7%, and almost all contain at least 20% quartz. Simmons and Hedge (1978) con- cluded that the granodiorite suite was derived by Figure 8. Map showing the Colorado Mineral Belt (CMB) and middle Cenozoic volcanic partial melting of a mixed source that yielded fi elds and calderas. Dark gray—middle Cenozoic volcanic fi elds; light gray—basin fi ll of residues of pyroxene granulite or pyroxenite. Rio Grande Rift and . Calderas color coded by age as shown in Stein and Crock (1990) stated that the REE pat- legend and taken from McIntosh et al. (1992), McIntosh and Bryan (2000), Lipman, (2000), terns for these intermediate composition intru- Chapin et al. (2004a, 2004b), McIntosh and Chapin (2004), and Lipman and McIntosh sives are typical of patterns displayed by other (2008). CMB is from Mutschler et al. (1987). Base is modifi ed from Muelhberger (1992). granodiorites and quartz monzonites developed Figure is updated from Chapin et al. (2004a, 2004b). Bold dashed line at bottom is inferred on continental crust. margin of fl at slab from Figure 1. The north-central and south-central portions of the CMB are underlain by two major com- posite batholiths marked by prominent negative The chemical composition of CMB intrusives The monzonites characteristically have ini- gravity anomalies that merge along the trans- varies with age (tectonic stress) and geographic tial 87Sr/86Sr ratios <0.706, strontium contents verse bulge of the CMB; the bulge contains at position along the belt. They are commonly >1000 ppm, and linear, steep REE patterns that least six Laramide plutons (ca. 72–60 Ma) and divided into three rock suites: (1) a silica-satu- lack europium anomalies. The granodiorites fi ve middle Tertiary calderas (ca. 37–33 Ma). rated, high-alkali, monzonite suite; (2) a silica- and quartz monzonites are characterized by ini- The south-central gravity anomaly is due largely oversaturated, granodiorite–quartz monzonite tial 87Sr/86Sr ratios >0.707, strontium contents to development of the huge middle Tertiary San suite; and (3) a highly evolved alkali feldspar <1000 ppm, and moderately steep REE pat- Juan volcanic fi eld that overlaps and partially granite suite. The monzonite suite dominates terns that fl atten and curve slightly upward at obscures the CMB. The last 50–100 km at the the northeast portion of the CMB and includes the heavy REE end, and also have very small to ends of the CMB are outside the major gravity alkali and mafi c monzonites and quartz syenites. absent europium anomalies. The granodiorite– anomalies and presumably lack large batholithic

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Ignimbrite volcanism by many to be Laramide in age, based on fi eld relationships; however, detailed studies utilizing Boot Mogollon Latir Central San fi ssion-track thermochronology and K-Ar dat- Heel Datil Colorado Juan ing of sericitic alteration have shown the main 18 mineralizing event to be Middle to Late Eocene 19 (39.6 ± 1.7 Ma; Thompson and Arehart, 1990). The dominant ore deposits at Leadville are tabu- 20 lar zinc, lead, silver, and gold sulfi de replace- ment bodies (mantos) in dolostones, mainly in 21 the Mississippian Leadville Limestone (Tweto, 22 1968; Bookstrom, 1990; Thompson and Arehart, 1990). Additional silver, lead, zinc, and barium 23 production (the controversial Sherman type) has come from karst zones developed on the Lead- 24 Pulse 3 ville Limestone (De Voto, 1990; Johansing and 25.1–23.0 Ma Thompson, 1990; Tschauder et al., 1990). Since 25 1860, the Leadville district has been in produc- 26 1.8 m.y. gap tion nearly continuously (except 1957–1971), producing 24.1 × 106 mt of ore valued in excess 27 of $5.4 billion at 1989 metal prices; the total included 8.84 × 106 kg of silver and 106,616 kg 28 Pulse 2 of gold (Thompson and Arehart, 1990). 29.4–26.9 Ma 29 Similar lead, zinc, and silver manto ores were discovered 32 km north of Leadville

Age (Ma) 30 at Gilman (Fig. 6) in 1879 and became the 1.9 m.y. gap well-known Eagle mine of the New Jersey 31 Zinc Company. The ores at Leadville and Gil- man were similar in many respects; both were 32 mainly tabular sulfi de replacement ores in the 33 Leadville Limestone, the mineralizing fl uids coming from unexposed stocks of Late Eocene 34 Pulse 1 age (39.6 ± 1.7 Ma, Leadville; Thompson and 36.9–31.3 Ma Arehart, 1990; 35.8 ± 2.0 Ma, Gilman; Beaty 35 et al., 1990). Delineation of thermal anomalies 36 by use of fi ssion-track thermochronology was instrumental in dating both systems. Both the 37 Leadville and Gilman deposits are located in the belt of Paleozoic formations on the northeast 38 fl ank of the Laramide Sawatch uplift. Later- ally extensive pre-ore porphyritic sills provided 39 seals to hydrothermal fl uids above the Leadville Limestone. The segment of the Farallon slab 40 south of the CMB broke along the trend of the Figure 9. Graph showing episodic ignimbrite volcanism in New Mexico and Colorado. Ages are incipient Rio Grande Rift in which Leadville is mostly 40Ar/39Ar ages from references listed in Figure 8 (updated from Chapin et al., 2004a). located, and a surge of magmatism developed as the asthenosphere fl owed in between the litho- sphere and the sinking slab. The earliest cal deras roots. The central CMB may have dilated more melting of lower crustal granulites that may formed along the west fl ank of, or within, what than the ends and had a greater fl ux of mantle have undergone rubidium and fl uorine meta- became basins of the Rio Grande Rift (Fig. 8). and/or slab fl uids and melts and greater heat somatism (Mutschler et al., 1987). As regional extension increased in the Early fl ow, giving rise to magma generation at higher, Middle Cenozoic magmatism began in the Oligocene, magmatism along the intersection of more intermediate crustal levels. CMB ca. 43 Ma during a 6 m.y. transition period the incipient Rio Grande Rift and CMB transi- The alkali feldspar granite suite originated with waning Laramide compression (Fig. 2). tioned to intrusion of alkali-feldspar granites, from highly evolved magmas emplaced in an The initial activity was concentrated in the Lead- some with radial swarms of rhyolite dikes and extensional tectonic environment. Their high ville-Breckenridge area (Fig. 6) with intrusion minor associated lamprophyres (Bookstrom, silica compositions show signifi cant lithophile of several stocks and a large number of dikes 1990). Two world-class molybdenum-rich com- element enrichment and pronounced nega- and laterally extensive sills (Bookstrom, 1990; posite intrusive systems consisting of multiple tive europium anomalies. On the basis of lead, Thompson and Arehart, 1990; Cunningham intrusions of leucocratic alkali-feldspar rhyolite strontium, and neodymium isotopic studies, the et al., 1994). Mineralization in the world-class and/or granite porphyry were emplaced at Cli- granite suite is thought to represent minimal Leadville mining district (Fig. 6) was thought max (ca. 33–24 Ma; Wallace and Bookstrom,

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1993) and Red Mountain (ca. 30–27 Ma; Shan- Other molybdenum prospects have been found 9. The segment of the Farallon fl at slab south non et al., 2004). Both deposits are similar in where Neogene volcanic fi elds overlie or are of the CMB was present until ca. 37 Ma, when it age, composition, and geometrical relationships in close proximity to the CMB. Why magmas began to rollback to the southwest, as indicated of overlapping, partly nested, shell-like stock- continued to intrude along the CMB long after by southwest-younging caldera clusters during work orebodies related to a series of porphyry the Laramide is an interesting question. Chang- the interval 37–23 Ma (Chapin et al., 2004a, intrusions. From 1918 to 1987, the Climax ing tectonic conditions, as with rollback mag- 2004b). mine produced 464.6 × 106 mt of ore averaging matism or regional extension, are an obvious 10. Volcanic gaps with underlying seismically

0.410% molybdenum sulfi de (MoS2) (Wallace factor , but the processes by which magmas pene- mapped subhorizontal segments of the sub- and Bookstrom, 1993). These authors estimated trate the crust seem to create pathways often ducting Nazca plate along the Andean conver- that prior to pre-mining erosion, the Climax used by subsequent intrusions. gent margin provide analogs for the Laramide deposit may have exceeded 1 × 109 mt of plus magma gap and underlying fl at segments in

0.40% MoS2 and was, as far as is known, the DISCUSSION the southwestern U.S. (James and Sacks, 1999; world’s greatest deposit of molybdenite. Gutscher et al., 2000b), but not for the CMB. The Red Mountain intrusive complex (Fig. 6) A framework for discussion of the origin of 11. Subduction of aseismic ridges or oceanic includes two separate orebodies. The larger, the CMB is provided by the following observable plateaus at magma gaps in modern volcanic arcs deeper Henderson deposit is an umbrella- characteristics and well-documented analogs. around the Pacifi c Basin (McGeary et al., 1985; shaped stockwork that began production in 1. The CMB is an ~500-km-long, 25–50-km- Gutscher et al., 2000b) indicates that fl at sub- 1976 with initial combined proven and probable wide magmatic lineament that formed within an duction is caused by subduction of anomalously 6 reserves of 303 × 10 mt at 0.49% MoS2 with anomalously wide (~1200 km) magma gap in buoyant oceanic lithosphere.

a 0.2% MoS2 cutoff (Wallace et al., 1978). The the Laramide convergent-margin volcanic arc. 12. Gaps in modern volcanic arcs around 40Ar/39Ar ages of the Urad-Henderson deposits 2. Ages of igneous rocks show no consistent the Pacifi c Basin are mostly 300–500 km wide span the interval 29.9–26.95 Ma (Geissman trend along the CMB (Mutschler et al., 1987; (McGeary et al., 1985). The 1500-km-wide et al., 1992; Shannon et al., 2004), similar to Barker and Stein, 1990). Peruvian magma gap is anomalous in width ages reported in Tweto (1979) and Chapin and 3. The calc-alkaline to alkaline composi- because of subduction of the Nazca Ridge and Cather (1994) for initiation of the Rio Grande tions of plutons along the CMB are typical of Inca Plateau. Rift in . Shannon et al. (2004) convergent-margin volcanic arcs (Gill, 1981; 13. Seismic imaging of the subducted Nazca reported that the Red Mountain intrusive suite Mutschler et al., 1987). plate beneath the Peruvian magma gap shows includes at least 23 intrusive events that were 4. The ~N43°E trend of the Laramide CMB two morphologic highs, corresponding to esti- emplaced over a 3 m.y. interval, creating a bull’s is roughly parallel to the transport direction of mated positions of the Nazca Ridge and Inca eye exploration target that was further enhanced the underlying Farallon fl at slab (Coney, 1978; Plateau, separated by a 20–40-km-deep sag on by radial and concentric dikes. Engebretson et al., 1985; Barker and Stein, trend with an offshore segment boundary, the Isotopic studies indicate that the Climax-type 1990). Mendaña fracture zone (Gutscher et al., 2000b). magmas were derived by low-percentage par- 5. The CMB cuts indiscriminately across the 14. The CMB occupies a reentrant in ~200- tial melting of lower crustal Proterozoic source geologic grain of Colorado with remarkable con- km-thick cratonic lithospheres of the Wyoming rocks (Simmons and Hedge, 1978; Bookstrom tinuity, seemingly independent of the tectonic Archean craton on the north (Yuan and Dueker, et al., 1988; Stein and Crock, 1990). As sug- elements it crosses (Tweto and Sims, 1963). 2005) and the continental interior craton on the gested by Wallace and Bookstrom (1993), the 6. The Proterozoic shear zones to which the east (West et al., 2004). repetition of nearly identical magmatic-hydro- CMB is usually correlated are discontinuous, Arc volcanism requires the ascent of fl uids thermal events in the same restricted area over ending within Proterozoic batholiths or failing and/or melts from the asthenosphere and/or sub- a time span of 9 m.y. at Climax and 3 m.y. at to project in comparable magnitude between ducting slab, accompanied by greatly increased Red Mountain requires a long-enduring intru- adjacent mountain ranges, and vary in trend heat fl ow, to induce melting of the lithosphere of sive cupola replenished periodically from a from N50° to N60°E (Tweto and Sims, 1963; the overlying plate (Gill, 1981; Gutscher et al., large reservoir at depth. Wallace et al. (1968) McCoy et al., 2005; Shaw et al., 2005). 2000a; Grove et al., 2009). Therefore, it seems estimated that ~125 km3 of magma would be 7. Compilation of structural and mineral likely that the CMB developed along a tear or required to produce the Climax ores; Wallace deposit data in the Front Range suggests that segment boundary in the fl at slab that provided and Bookstrom (1993) thought a more realis- Proterozoic inheritance was not the primary access for fl uids and/or melts to reach the over- tic fi gure might exceed 400 km3. Wallace et al. control of mineral deposit location, orientation, riding North American lithosphere (see also the (1978) suggested that the master reservoir at or permeability structure (Caine et al., 2010). convection model of Jones et al., 2011). That depth may have been a differentiated of 8. There are three contrasts in geologic fea- the CMB trends ~N43°E, roughly parallel to the a large batholith, indicated by gravity surveys tures that exist on opposite sides of the CMB. transport direction of the Farallon slab, and sepa- (Tweto and Case, 1972; Behrendt and Bajwa, (1) Laramide uplifts are dominantly northwest rates contrasting Laramide geologic features to 1974; Cordell et al., 1982; Mutschler et al., trending on the north side versus north trend- either side, strongly suggests that the CMB marks 1987) to underlie the central CMB. A third ing on the south side. (2) Late Cretaceous– a segment boundary in the fl at slab. The CMB is major, but undeveloped, porphyry molybde- Paleogene subsidence and sedimentation were a unique geologic feature. There are no known num deposit was generated beneath Mount anomalously great on the north side but com- analogs among the many fl at slab and/or magma Emmons near , Colorado, in the paratively modest on the south side. (3) Middle gaps in modern convergent-margin volcanic west-central CMB ca. 17 Ma as regional exten- Cenozoic volcanic rocks are sparse on the north arcs. The closest analogy is with the 20–40-km- sion became more widespread. A multiphase side compared to the voluminous Late Eocene– deep sag between the subducted Inca Plateau stock of rhyolite-granite porphyry is the host Oligocene volcanic fi elds and widely scattered and Nazca Ridge (Fig. 3) on the 1500-km-wide (Dowsett et al., 1981; Stein and Crock, 1990). igneous rocks on the south side. Peruvian fl at slab (Gutscher et al., 2000b). If

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further subduction pulls open the sag, the result could be a Peruvian mineral belt. Note that the sag occurs online with the Medaña transform fault in the Nazca plate (Fig. 3). What pulled open the segment boundary in the Farallon fl at slab and allowed astheno- spheric fl uids and/or melts access to the over- riding North American lithosphere, thereby generating the CMB? Temporally, the begin- ning of Laramide magmatism along the CMB (ca. 75 Ma; Fig. 2) coincided with widespread tectonic partitioning of the broad Wyoming basin into basement-cored uplifts and inter- vening basins (Gries, 1983; Dickinson et al., 1988). The onset of both CMB magmatism and tectonic partitioning coincided with a sharp increase in the velocity of Farallon–North American convergence ca. 75 Ma (Fig. 2), and both waned as convergence slowed in Middle Eocene time (Gries, 1983; Engebretson et al., 1985; Dickinson et al., 1988). The association with rapid convergence suggests a possible solution. Both segments of the Farallon fl at slab had reached the northeast end of the CMB at the Rocky Mountain front by 70–75 Ma, as evidenced by isotopic ages of stocks at several points along the CMB. At a convergence rate of 100 km/m.y. (Fig. 2) the Farallon fl at slab would have underthrust western North America Figure 10. Paleotectonic map from Figure 1 modifi ed to show loca- by ~1000 km since shutting off pluton emplace- tion of Colorado Mineral Belt (CMB) relative to cratonic litho- ment in the Sierra Nevada batholith ca. 85 Ma spheres of the Wyoming Archean province (Karlstrom et al., 2005) (Fig. 2). The Farallon segment north of the and the continental interior craton (West et al., 2004). Bdry— CMB would have fl exed downward beneath boundary. Large arrows show transport direction of the Farallon the thick (~200 km) Archean lithosphere of fl at slab. Narrow arrows show tectonic stresses imposed on the fl at the Wyoming province, and the segment south slab by warping of the slab required to obliquely underslide the of the CMB would have fl exed down as it was thick (≤200 km) cratonic lithospheres. Such stresses are inferred to overridden by the comparably thick (~200 km) have dilated a segment boundary in the fl at slab, allowing the rise of lithosphere of the continental interior craton fl uids and/or melts into the overriding North American lithosphere. (Snoke, 1993; Deep Probe Working Group, 1998; van der Lee, 2001; Goes and van der Lee, 2002; CD-ROM Working Group, 2002; Humphreys et al., 2003; West et al., 2004; Yuan tal interior craton. As long as the Farallon slab lowed by an overprint of bimodel Rio Grande and Dueker, 2005). However, because of the continued to be rapidly overridden by the North Rift magmatism. The largest ore deposits of northeastward trajectory of the Farallon slab, American plate as it continued to converge on the CMB were generated during these middle both segments would encounter their respective the same northeast trend, the downwarps in Cenozoic stress and magmatic transitions. Simi- cratonic lithosphere at oblique angles (Fig. 10). the plate segments along the cratonic margins lar effects have been noted in the Andes where With sharply increased convergence ca. 75 Ma, would tend to hold open the segment boundary many porphyry copper deposits formed under the downwarps imposed on the fl at slab seg- and the CMB would continue to host magmatic conditions of near-neutral stress (Tosdal and ments by the overriding thick cratonic litho- intrusions. However, Farallon–North American Richards, 2001; Kay and Mpodozis, 2001; see spheres would generate extensional stresses convergence decreased by half (Fig. 2) in two also Barton, 1996; Anthony, 2005; Muntean approximately perpendicular to the segment sharp steps ca. 43 and 37 Ma (Engebretson et al., et al., 2011). A period of near-neutral stress boundary that underlays the CMB (Fig. 10). An 1985). At 37 Ma the Farallon segment south of was also present in Colorado during the middle oceanic plateau or aseismic ridge riding on the the CMB broke along the trend of the incipient Cenozoic transition, as evidenced by the wide- Farallon slab would increase the warping nec- Rio Grande Rift and began to roll back to the spread Middle Eocene erosion surface (Epis essary to pass under the cratonic barriers. southwest accompanied by major volcanism and Chapin, 1975) of low relief capped by the As shown in Figure 10, the segment boundary and the ignimbrite fl are-up (Figs. 7, 8, and 9). 37 Ma Wall Mountain Tuff. of the Farallon slab overlain by the CMB occu- The effect on the CMB was a transition between Tweto and Sims (1963) and Tweto (1975) rec- pies a reentrant formed by the west-southwest– 43 and 37 Ma (Fig. 2) of waning Laramide com- ognized the association of the CMB with a belt trending margin of the Wyoming Archean craton pression replaced ca. 37 Ma by the beginning of overlapping and discontinuous shear zones of and the north-trending margin of the continen- of widespread middle Cenozoic volcanism fol- Precambrian ancestry, but were clearly puzzled as

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to the origin of the magmas. Tweto (1975, p. 37) Bookstrom, A.A., Carten, R.B., Shannon, J.R., and Smith, to ancient: Geological Society of America Field Guide stated, “The problem … is not so much why or R.P., 1988, Origin of bimodal leucogranite-lampro- 18, p. 55–76, doi: 10.1130/2010.0018(03). phyre suites, Climax and Red Mountain porphyry Coney, P.J., 1972, Cordilleran tectonics and North America how magmas were generated, but why magmatic molybdenum systems, Colorado, in Drexler, J.W., and plate motion: American Journal of Science, v. 272, activity took the pattern it did—that is, of a rather Larson, E.E., eds., Cenozoic volcanism in the south- p. 603–628, doi: 10.2475/ajs.272.7.603. ern Rocky Mountains revisited: A tribute to Rudy C. 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