Origin and Evolution of the Sierra Nevada and Walker Lane themed issue Origin of the Colorado Mineral Belt Charles E. Chapin New Mexico Bureau of Geology 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 Colorado Mineral Belt (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 Wyoming, an ~1200-km-wide Late Cretaceous–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 Farallon plate. rocks (± mantle). Late Eocene–Oligo cene roll- goal in this paper is to summarize these differ- Of the known volcanic gaps overlying fl at back magmatism superimposed on the CMB ences, integrate them with the regional tectonic slabs in subduction 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 subsidence and sedimentation, 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- Cenozoic 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 Four Corners The basic elements of the plate tectonic the CMB cut indiscriminately across the area on the Colorado Plateau 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 Late Cretaceous and younger intru- subhorizontally subducted Farallon slab trans- but northward south of the CMB. Laramide sions by fault 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 Paleogene 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 Precambrian 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. 28 For permission to copy, contact [email protected] © 2012 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/1/28/3342088/28.pdf by guest on 25 September 2021 Colorado Mineral Belt 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).