- UW Department of Geology and Geophysics
We report new U-Pb zircon ages for four previously undated Proterozoic granitoid intrusions exposed in the southern Sawatch Range, central Colorado. Coarse-grained to K-feldspar megacrystic granite (Henry Mountain granite) exposed along Taylor Canyon 20 km north of Gunnison, Colorado, crystallized at 1697 ± 7 Ma. It cuts across high-temperature deformational fabrics in metavolcanic and metasedimentary country rocks, thus bracketing at least one Paleoproterozoic tectonic event locally. This granite also contains a well-developed northeast-striking, subvertical foliation that postdates emplacement. The three other intrusions all yielded Mesoproterozoic ages. The Monarch Pass pluton, comprising coarse- to medium-grained granodiorite exposed 50 km east of Gunnison, crystallized at 1447 ± 9 Ma. It cuts across well-developed fabrics in metavolcanic host rocks and contains a widespread biotite foliation. Coarse-grained to K-feldspar megacrystic granite (Horsethief granite) exposed 5–10 km northwest of Taylor Park Reservoir was emplaced at 1437 ± 5 Ma, and it is locally deformed. Fine-grained, muscovite-biotite granite (Taylor River granite) that cuts across the southwestern part of the Henry Mountain pluton crystallized at 1428 ± 23 Ma. A subvertical, northeast-striking biotite foliation cuts across the contact between these two intrusions, suggesting that northwest-directed subhorizontal shortening occurred locally during the Mesoproterozoic. These ages provide new opportunities to constrain the age of tectonism in central Colorado and to further understand the Proterozoic tectonic evolution of southern Laurentia.
Tectonic models for the Paleo- and Mesoproterozoic growth and evolution of southern Laurentia involve three distinct phases: early crust formation and orogenesis between ca. 1.8 and 1.60 Ga, a ca. 200-m.y. tectonic quiet period, and a ca. 1.4-Ga tectonothermal event that overprinted the Paleoproterozoic crust. Some significant local and regional complexities, however, remain to be resolved. Outstanding issues include the timing and tectonic setting of early crust formation and assembly, and the tectonic setting of widespread granitic magmatism at ca. 1.4 Ga. These issues are directly related to questions concerning the formation, accretion, stabilization, and reactivation of Proterozoic-aged crust in southern Laurentia, and to what degree events were prolonged versus episodic and distributed versus localized throughout the region.
Widespread exposures of Proterozoic rocks occur in the southern Sawatch Range near Gunnison, Colorado (Fig. 1; e.g., Bickford and Boardman, 1984; Jessup et al., 2006) and provide an opportunity to address some of the outstanding issues in the Proterozoic tectonic evolution of southern Laurentia. Three separate Paleoproterozoic basement assemblages are recognized in this region, and they are dominated by metavolcanic and metasedimentary rocks that preserve evidence for multiple deformational and metamorphic events. Numerous Paleoproterozoic igneous intrusions cut across the basement assemblages and help determine the age of tectonic events throughout the region. Decades of research in this area have generated a substantial database of rock ages (Fig. 1; Bickford et al., 1989b) and have led to the development of tectonic models for early crust formation (e.g., Condie, 1982; Condie, 1992; Jessup et al., 2005; Whitmeyer and Karlstrom, 2007). However, there still exist a number of igneous intrusions in this area whose ages and tectonic significance are not well known (Tweto, 1979). The extent of Mesoproterozoic (ca. 1.4 Ga) magmatism and deformation is also not well established, and numerous studies indicate that both magmatism and deformation occurred ca. 1.4 Ga in surrounding areas (Fig. 1) like the Black Canyon of the Gunnison (Jessup et al., 2006), northern Sawatch Range (Shaw et al., 2001), and Sangre de Cristo Mountains (Jones and Connelly, 2006).
In this study, we focused on four previously undated granitic plutons exposed in the southern Sawatch Range east and northeast of Gunnison, Colorado (Fig. 1). All four granites have textural characteristics that are similar to both Paleoproterozoic and Mesoproterozoic intrusions documented throughout the region (e.g., Tweto, 1987; Reed et al., 1993; Anderson and Cullers, 1999). Furthermore, the granites cut across deformational fabrics and metamorphic mineral assemblages in the country rock and, in some cases, also contain evidence for solid-state deformation. Our results from new field mapping and U-Pb zircon geochronology lead to a better understanding of the age and extent of Proterozoic magmatism and to the tectonic evolution of Paleoproterozoic basement assemblages. We also document three previously unrecognized Mesoproterozoic (ca. 1.4 Ga) granitic intrusions, at least one of which was coeval with northwest-directed, subhorizontal shortening. Combined with published geochronology from exposures in the surrounding region, our findings illustrate the episodic nature of magmatism and deformation during the Proterozoic and provide new constraints on the tectonic processes that governed the growth, stabilization, and modification of southern Laurentia throughout the Middle to Late Proterozoic.
Exposures of Precambrian rocks across the southwestern United States consist of a diverse assemblage of metavolcanic rocks, metasedimentary rocks, and mafic and granitoid plutons that were formed and accreted to the southern margin of the Archean Wyoming province between 1.8 and 1.6 Ga (Condie, 1982; Karlstrom and Bowring, 1988; Reed et al., 1993) as part of a protracted period of Laurentian crustal growth (Whitmeyer and Karlstrom, 2007). These exposures have been divided into several orogenic provinces on the basis of rock ages and isotopic characteristics (Fig. 1 inset). The Yavapai province is interpreted to represent a complex collage of predominantly juvenile arc terranes characterized by rocks with Nd model ages between 2.0 and 1.8 Ga (Bennett and DePaolo, 1987). Rocks of the Yavapai province were accreted to the Laurentian margin between 1.78 and 1.70 Ga along a belt stretching from Colorado to Arizona and New Mexico. The final collisional phase of this long-lived, progressive orogenic event occurred ca. 1.71–1.70 Ga (Karlstrom and Bowring, 1988), and was followed by a 40-m.y. episode of voluminous post-orogenic granitoid magmatism (Anderson and Cullers, 1999) and by a relatively brief interval of widespread sedimentation (Jones et al., 2009). The Mazatzal province lies to the south of the Yavapai province and extends across central and southern New Mexico and Arizona. Mazatzal province rocks are characterized by Nd model ages between 1.8 and 1.7 Ga (Bennett and DePaolo, 1987) and were accreted to southern Laurentia during the 1.66–1.60-Ga Mazatzal orogeny (Silver, 1965; Karlstrom and Bowring, 1988; Amato et al., 2008). Mazatzal-aged deformation affected a large foreland region of the southern Yavapai province (transition zone, Fig. 1; Karlstrom and Humphreys, 1998), and the Mazatzal deformation front represents the approximate northern extent of these effects (Shaw and Karlstrom, 1999). Various workers have challenged the juvenile arc accretion model for the Yavapai and Mazatzal orogenies on the basis of zircon ages, lithological associations, and limited Hf isotopic data (Bickford and Hill, 2007a; Bickford et al., 2008), but alternative models are still being evaluated and debated (Duebendorfer, 2007; Karlstrom et al., 2007; Bickford and Hill, 2007b).
After a ca. 150-m.y. tectonic lull from 1.60 to 1.45 Ga, renewed southward growth of Laurentia is inferred to have occurred during the Mesoproterozoic. This interpretation is based on a large crustal province with Nd model ages of 1.5–1.3 Ga extending from northern Mexico to Labrador, Canada (Bennett and DePaolo, 1987; Patchett and Ruiz, 1989; Karlstrom et al., 2001). An episode of widespread granitic magmatism, local emplacement of mafic dikes, and regional high-temperature, low-pressure metamorphism occurred throughout the southwestern United States between 1.47 and 1.36 Ga (Williams, 1991; Reed et al., 1993; Williams et al., 1999), and rocks of this age currently account for nearly 20 percent of all Precambrian exposures across the region (Fig. 1 inset). Circa 1.4-Ga granites, previously described as being A-type because of their alkalinity, anhydrous character, and presumed anorogenic tectonic setting (Loiselle and Wones, 1979; Anderson, 1983), are ferroan in nature (Frost et al., 2001), a geochemical characteristic that is indicative of mantle influence (Frost and Frost, 1997) and is generally associated with extensional tectonic environments such as continental rifting (Emslie, 1978; Whalen et al., 1987; Eby, 1990). However, regional evidence exists for contractional to strike-slip deformation within the thermal aureoles of plutons and contemporaneous reactivation of northeast-striking crustal shear zones in the Rocky Mountains and southwestern United States (Graubard and Mattinson, 1990; Shaw et al., 2001; McCoy et al. 2005; Jessup et al., 2006; Jones et al., in press). Nyman et al. (1994) suggested that ca. 1.4-Ga magmatism coincided with regional contraction arising from a convergent plate boundary on a distal southern margin of Laurentia. These events were all part of a prolonged, ca. 800-m.y. episode of crustal growth along southern Laurentia along a long-lived “southern” plate margin that culminated in the Grenville orogeny and assembly of the supercontinent Rodinia at ca. 1.1 Ga (Karlstrom et al., 2001).
PROTEROZOIC GEOLOGY OF THE SAWATCH RANGE
The Sawatch Range is located in the central part of the Yavapai province and at the northern edge of the broad zone that was affected by Mazatzal-aged orogenesis (Fig. 1). Lithologic associations in the southern Sawatch Range are dominated by interlayered felsic and hornblendic gneisses that include metarhyolites, metabasalts, and interbedded metagraywackes. Biotite gneiss is common, and it is locally migmatitic and contains minor interlayered hornblende gneiss and calc-silicate rocks (Tweto et al., 1976). Locally abundant metasedimentary rocks include quartzites, quartz-mica schists, and quartzofeldspathic gneiss (Tweto et al., 1976; Dewitt et al., 1985; Hetherington, 1991; Fridrich et al., 1998).
These basement rocks are broadly correlated with three different Paleoproterozoic successions that are classified on the basis of geochemical and lithologic characteristics and radiometric ages (e.g., Knoper and Condie, 1988). The ca. 1770–1760-Ma Dubois and ca. 1740–1730-Ma Cochetopa successions are made up of metavolcanic and metasedimentary rocks that are intruded by gabbroic sheets and plutons and represent the oldest exposed rocks in the Gunnison, Colorado area (Bickford and Boardman, 1984; Bickford et al., 1989b). The metavolcanic rocks have bimodal chemistry with end-member compositions that range from tholeiitic basalts to rhyolites. Primary structures in the metavolcanic rocks include pyroclastic sheets, basalt flows with amygdaloidal tops, pillow lavas, and breccias (Fig. 2A; Olson and Hedlund, 1973; Bickford and Boardman, 1984; Bickford et al., 1989b).
The Black Canyon succession consists of quartz-rich metasedimentary rocks, amphibolite, and schists that are distinct from the metavolcanic Dubois and Cochetopa successions (Hansen and Peterman, 1968; Hansen, 1971, 1981; Bickford and Boardman, 1984; Jessup et al., 2005, 2006). In the Black Canyon of the Gunnison, quartzofeldspathic paragneisses locally preserve primary structures such as bedding, ripple marks, and cross-bedding. Paragneisses commonly grade into subordinate amounts of pelitic schist with varying mineral assemblages. Age data suggest that the Black Canyon succession is coeval with the Cochetopa succession (Bickford et al., 1989b; Jessup et al., 2006), and Jessup et al. (2005) proposed that the contact between rocks of the Dubois and Black Canyon successions was originally depositional. In general, however, structural and stratigraphic relationships between the three basement successions are not well understood at the regional scale (Bickford et al., 1989b). Where possible contacts are exposed, they tend to be highly tectonized (Jessup et al., 2005).
The Dubois succession is interpreted to have formed in an island-arc system that was accreted to the southern Wyoming province, likely between ca. 1760 and 1740 Ma, during the early stages of the Yavapai orogeny (Condie and Nuter, 1981; Condie, 1982; Chamberlain, 1998; Tyson et al., 2002; Jessup et al., 2005). Inherited zircon in Dubois succession rocks together with isotopic characteristics of rocks in the surrounding region indicate that the island arc may have built on pre-existing Proterozoic crust of Trans-Hudson or Penokean age (ca. 1850 Ma; Hill and Bickford, 2001; Bickford and Hill, 2007a). However, crust of this age is not presently exposed, and its extent is not well established. Alternatively, the inherited zircon could have been derived from Yavapai-age metasedimentary rocks that contain older Paleoproterozoic (1880–1800 Ma; Premo et al., 2007) and late Archean detrital zircon (Premo et al., 2007; Jones et al., 2009). The Cochetopa succession was formed in an intra-arc basin developed within a continental-margin arc along southern Laurentia (Boardman and Condie, 1986), and the Black Canyon succession likely formed at approximately the same time in an associated back-arc basin (Knoper and Condie, 1988; Hetherington, 1991; Jessup et al., 2006).
In the southern Sawatch Range, primary layering (S0) in basement rocks is indicated by pillow structures in metavolcanic rocks (Fig. 2A) and sedimentary structures such as cross-bedding in quartzose metasedimentary rocks (Fig. 2B). Primary layering is locally folded by meter-scale isoclinal F1 folds (Fig. 2C). An axial-planar S0/S1 composite foliation defined by compositional layering and aligned mica is the dominant fabric observed in exposures throughout the study area (Fig. 2C,D). The orientation of S0/S1 varies widely with an average east–west strike and steep northward dip (average orientation = 186, 71° N; Fig. 3A). We attribute the variation to reorientation of the early fabric during subsequent deformation. S0/S1 is folded in outcrop by meter-scale, tight open F2 folds (Fig. 2D) with shallowly plunging hinge lines. The F2 folds contain a well-developed axial planar fabric (S2) defined by aligned mica that cuts across S0/S1 (Fig. 2D). S2 strikes east-northeast and is generally subvertical (average orientation = 078, 87° SE; Fig. 3B).
Numerous granitoid plutons of both Paleoproterozoic and Mesoproterozoic age are exposed in the Sawatch Range and in the Cochetopa Hills south and southwest of Gunnison, Colorado (Fig. 1). In general, Paleoproterozoic intrusions have ages that range from ca. 1757 to 1669 Ma (Fig. 1; Bickford et al., 1989b; Reed et al., 1993). The oldest of these plutons occur in exposures southwest of Gunnison and have ages of 1757 ± 10 Ma and 1751 ± 6 Ma (Fig. 1; Bickford et al., 1989b). These plutons are granitic in composition and are moderately to strongly foliated. Their ages and cross-cutting relationships indicate that they were broadly coeval with metavolcanic rocks of the Dubois succession (Bickford et al., 1989b). Granitic plutons with ages ranging from ca. 1730 to 1713 Ma are exposed near Gunnison, Colorado (Fig. 1), and are interpreted to be associated with 1745–1728-Ma metavolcanic rocks of the Cochetopa succession (Bickford et al., 1989b). The youngest Paleoproterozoic plutons are exposed in the southern Sawatch Range east of Gunnison and in the Collegiate Peaks east of Taylor Park (Fig. 1). These plutons, with ages ranging from 1701 to 1669 Ma (Fig. 1; Bickford et al., 1989b; Reed et al., 1993; Fridrich et al., 1998), postdate the formation of the basement associations and are broadly interpreted to be post-tectonic (Bickford et al., 1989b).
Circa 1.4-Ga granites are widespread in exposures throughout the region (e.g., Reed et al., 1993; Anderson and Cullers, 1999) and a number of granitic intrusions in the Sawatch Range are known or presumed to be Mesoproterozoic in age (Fig. 1). These granites tend to be either coarse-grained to K-feldspar megacrystic or fine-grained, and their ages range from ca. 1434 to 1396 Ma (Fig. 1; Bickford et al., 1989b; Reed et al., 1993; Leader and Jones, 2008). Many of the ca. 1.4-Ga granites in the Sawatch Range occur within the Collegiate Peaks Wilderness Area to the east (Fig. 1; Fridrich et al., 1998, Leader and Jones, 2008).
In this study, we focused on four granitic intrusions exposed in the southern Sawatch Range whose ages were previously not well constrained. The Henry Mountain granite is a coarse-grained to K-feldspar megacrystic biotite (± muscovite) granite exposed over a broad area of approximately 100 km2 between Gunnison and Taylor Park, Colorado (Fig. 1; Dewitt et al., 1985; Hetherington, 1991). It contains tabular K-feldspar grains up to 5 cm in length (Fig. 4A) that are commonly aligned in a northwest–southeast-striking, subvertical foliation (average orientation = 218, 79° SW; Fig. 3C) that is interpreted to be magmatic in origin. The magmatic fabric is cut by a solid-state foliation defined by coarse-grained biotite, ribboned quartz, and, locally, recrystallized feldspar that strikes northeast–southwest and is subvertical (average orientation = 115, 87°NW; Fig. 3C).
A large body of granite is exposed northwest of Taylor Park (Fig. 1) that, like the Henry Mountain granite, is made up of coarse-grained to K-feldspar megacrystic biotite granite that is locally fluorite-bearing. Tweto et al. (1976) and Tweto (1979) correlated this granite with the Henry Mountain granite on the basis of similarities in texture and outcrop appearance. However, there is a distinct break in the map pattern and the two intrusions are not continuous at the surface (Fig. 1). We informally refer to this intrusion as the Horsethief granite after Horsethief Lake, a small alpine lake approximately 1 mile northwest of the sampled locality (see below). The Horsethief granite contains a variably developed fabric defined by aligned K-feldspar megacrysts that is interpreted to be magmatic in origin (Fig. 4D). This fabric is locally cut by a solid-state foliation defined by medium- to coarse-grained biotite, quartz, and, locally, recrystallized tails of K-feldspar megacrysts, but the overall extent and orientation of this fabric are not well known. In general, though, the Horsethief granite does not appear to be as strongly deformed as the Henry Mountain granite, thus adding additional uncertainty to the correlation of the two igneous units.
The Monarch Pass pluton is exposed in the southern Sawatch Range between Salida and Gunnison, Colorado (Fig. 1). It is composed of medium- to coarse-grained biotite-hornblende granodiorite (Fig. 4B) and locally contains a northeast-striking, moderately northwest-dipping foliation defined by biotite and aligned feldspar megacrysts. We interpret the dominant foliation to be magmatic in origin because it is primarily defined by aligned feldspar megacrysts up to 4 cm long. However, some of the megacrysts appear to be locally kinked and folded. Additionally, zones of concentrated, strongly aligned biotite up to a few centimeters thick suggest that deformation occurred or continued to occur after crystallization of the pluton. The extent and orientation of fabrics in the Monarch Pass pluton are not known.
The Taylor River granite, exposed on the north side of Taylor River canyon northeast of Gunnison, Colorado (Fig. 1), is made up of equigranular, medium- to fine-grained biotite-muscovite granite (Fig. 4C). It cuts across granite of the Henry Mountain pluton, thus establishing at least relative age relationships for these two intrusions. The Taylor River granite contains a widely developed, but relatively weak foliation defined by aligned mica and, locally, recrystallized quartz and feldspar. We interpret this fabric to be solid-state in origin, and it is very well developed within a few hundred meters of the contact with the Henry Mountain granite. The average foliation strikes northwest–southeast and dips steeply southwest (average orientation = 217, 75° SW; Fig. 3D) and locally parallels the geometry of the contact between the Henry Mountain and Taylor River granites. However, we also observed the foliation locally cutting across the contact between the two units (Fig. 5A). The Taylor River granite contains numerous xenoliths of quartzose gneiss, amphibolite, and schist, and it cuts across compositional layering (S0/S1) in the basement assemblages. However, the granite also appears to be folded locally by F2-type folds and, in these localities, the axial-planar fabric in the xenolith (S2) is subparallel to the solid-state foliation (S3) in the fine-grained granite (Fig. 5B).
U-Pb ZIRCON AND TITANITE GEOCHRONOLOGY
We sampled the four previously undated granitic plutons described above for new U-Pb geochronology to determine the age of Proterozoic plutonism in the southern Sawatch Range, Colorado. Three of the plutons—the Henry Mountain granite, Horsethief granite, and Monarch Pass pluton—are represented with the map symbol “YXg” on the Geological Map of Colorado (Tweto, 1979), indicating that their ages are not known and that they have field characteristics compatible with regional suites of both ca. 1.7-Ga and 1.4-Ga plutons (Tweto, 1979). The fourth pluton—the Taylor River granite—was initially correlated with a regional suite of ca. 1.7-Ga plutons (the Routt plutonic suite of Tweto, 1987; Tweto et al., 1976), but later mapped as a ca. 1.4-Ga intrusion by Tweto (1979). This uncertainty in age assignments highlights the difficulty of regional correlation based solely on textural criteria and, in part, provided the initial motivation for this study. It also demonstrates the need for U-Pb geochronology to completely understand the regional extent and age range of ca. 1.7-Ga versus ca. 1.4-Ga plutons and, thus, their tectonic setting.
All sample processing was done at The University of Texas at Austin. Bulk rock samples were crushed to mineral size under clean conditions by using a jaw crusher and disc pulverizer. Initial mineral separation was done using a Wilfley table and heavy-mineral components were processed further with sieves, heavy liquids, and a Frantz magnetic separator. Mineral fractions were characterized using a binocular, reflected-light microscope, a transmitted-light petrographic microscope with condenser lens inserted to minimize edge refraction, and, in some cases, a scanning cathodoluminesence (CL) imaging system on a JEOL T330A scanning electron microscope. Multiple or single grains of each population were selected for analysis on the basis of optical properties to ensure that only the highest-quality grains were analyzed. We present analytical data and location coordinates for each sample in Appendices 1 and 2. Associated U-Pb concordia diagrams are presented in Figures 6 and 7.
Isotope-Dilution Thermal-Ionization Mass Spectrometry (ID-TIMS) Techniques
For samples analyzed by isotope-dilution thermal-ionization mass spectrometry (ID-TIMS) techniques, the mineral fractions analyzed were strongly air-abraded (Krogh, 1982), were subsequently re-evaluated optically, and then were washed successively in distilled 4N nitric acid, water, and acetone. They were loaded dry into Teflon capsules with a mixed 205Pb-235U isotopic tracer solution and dissolved with HF and HNO3. Chemical separation of U and Pb from zircon using minicolumns with a 0.044 mL resin volume (after Krogh, 1973) resulted in a total Pb procedural blank of 1 pg over the period of analyses. Chemical separation from titanite on 0.250 mL columns resulted in a total procedural Pb blank of 1–2 pg. The U procedural blank is estimated to have been 0.5 pg for both column types. Pb and U were loaded together with silica gel and phosphoric acid onto an outgassed filament of zone-refined rhenium ribbon and analyzed on a multicollector MAT 261 thermal-ionization mass spectrometer, either operating in static mode with 204Pb in the axial secondary-electron-multiplier (SEM) ion-counting system or dynamic mode with all masses measured sequentially by the SEM-ion-counting system. Ages were calculated using decay constants of Jaffey et al. (1971). Errors on isotopic ratios were calculated by propagating uncertainties in measurement of isotopic ratios, fractionation, and amount of blank with a program modified after algorithms by L. Heaman (University of Alberta, Edmonton). Results are reported in Appendix 1 with 2s errors. Linear regressions were performed with the procedure of Davis (1982). The goodness of fit of a regressed line is represented as a probability of fit, where 10 percent or better is considered acceptable and corresponds to a mean square of weighted deviates (MSWD) of 2 or less. Ages listed in the text, tables, and figures are given with 2σ errors.
Laser-Ablation Inductively-Coupled Plasma Mass Spectrometery (LA-ICP-MS) Techniques
For samples analyzed by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) techniques, hand-picked zircon grains were placed on double-sided tape inside an aluminum ring with an inner diameter of 1 in. (2.54 cm) that was coated with a Teflon releasing agent. Grains were mounted with Struers Epofix resin, and the cured mount was polished with diamond paste until the zircon surfaces were exposed. Individual grains were ablated with a Merchantek New-Wave LUV213 laser, a frequency-quadrupled 213-nm Nd:YAG laser. Ablation was done using the following parameters: ca. 30–45 J/cm2 energy density, 10-Hz laser cycle, and 50-mm beam diameter. Helium was used as the laser carrier gas, and Ar was added as a make-up gas before the sample entered the plasma. Zircon grains were pre-ablated prior to analysis with a wide (ca. 280 mm), low energy (ca. 2–3 J/cm2) beam to remove any surface residue or contamination.
U-Pb isotopic data were collected during 20 one-second integrations in static multicollection mode on the IsoProbe, a multicollector, magnetic-sector, inductively-coupled plasma mass spectrometer produced by GV Instruments. Analysis was made in hard extract mode, meaning that voltage on the collimator cone was −450 to −500 V as compared to 0 to +100 V in soft extract mode. Hard extract has the advantage of greater sensitivity, but backgrounds are somewhat higher. Signal intensities of 238U, 207Pb, and 206Pb were measured on Faraday collectors; 204Pb-Hg and 202Hg were measured using channeltrons in ion-counting mode. 204Pb was corrected for an isobaric 204Hg interference using 202Hg; the Hg contribution to the total 204 signal was generally >95 percent. In some grains, isotope ratios shifted to different values during the analysis, presumably due to internal age zonation of the zircon. The data were culled to include only the longest segment of similar isotope ratios of the 20-second analysis. Mass fractionation and elemental bias between U and Pb were corrected by calibration to a well-characterized in-house standard, S97-19, with an age of 1087 Ma (ID-TIMS). We analyzed the standard both before and after each unknown sample.
Data were reduced offline using an Excel spreadsheet, and the values reported in Appendix 2 constitute the corrected ratios for each analysis after offline corrections for mass fractionation, elemental bias corrections, and common Pb. 207Pb/206Pb and 206Pb/204Pb ratios are averages of the accepted analyses, but 206Pb/238U values typically change with increasing depth/time during the course of an analysis. Therefore, 206Pb/238U ratios are determined by regression of the accepted measured values back to the beginning of the analysis, and the intercept is taken as the experimentally determined value.
Henry Mountain granite (J02-SC2)
We collected a sample of K-feldspar megacrystic granite on the south side of Gunnison National Forest Road 742 approximately 1.4 miles (2.25 km) northeast of the intersection with Spring Creek Road (Forest Road 744). The sample contains a well developed north—northeast-striking, subvertical foliation defined by aligned feldspar megacrysts and biotite. Fine-grained granitic dikes correlated with the Taylor River granite (J02-SC1, see below) sharply cut across the foliation. Granite from this locality is continuous in outcrop with the granite of Henry Mountain as described and mapped by Dewitt et al. (1985).
This sample yielded a relatively simple population of colorless to light pink, euhedral to subhedral, prismatic zircon interpreted to be igneous. Many of the grains are needle-like, doubly-terminated prisms with aspect ratios of up to 10:1, and we did not observe any evidence for overgrowths or xenocrystic cores. Nine zircon grains analyzed by LA-ICP-MS techniques were <3 percent discordant had 207Pb/206Pb ages that overlapped within 2σ uncertainty (Fig. 6A). The weighted mean of the overlapping 207Pb/206Pb ages was 1697 ± 7 Ma (MSWD = 0.88).
Horsethief granite (J02-SC3)
We collected a sample of coarse-grained to K-feldspar megacrystic granite from outcrops along the north side of Gunnison National Forest Road 748 approximately 3.25 miles (5.23 km) west of the intersection with Forest Road 742. The sampled outcrop contains a northeast-striking, subvertical foliation defined by aligned K-feldspar megacrysts and biotite that is locally cut by small pegmatite dikes up to 5 cm in thickness. This granite is continuous in outcrop from the north side of Forest Road 748 to beyond the north end of Taylor Park (Fig. 1), and it was originally correlated with the Henry Mountain granite on the basis of their close proximity and similar composition and texture (see above; Tweto et al., 1976; Tweto, 1979).
This sample yielded a relatively simple population of colorless to light pink, euhedral to subhedral zircon interpreted to be igneous. We did not observe any evidence for overgrowths or xenocrystic cores. We initially analyzed zircon from this sample by LA-ICP-MS techniques to test the correlation with the Henry Mountain granite, and seven zircon grains were concordant or nearly concordant (Appendix 2) with a weighted mean 207Pb/206Pb age of 1433 ± 15 Ma (Fig. 6C). We subsequently analyzed three additional single zircon grains by more precise ID-TIMS techniques to verify this age determination and to minimize the uncertainty. The three fractions (Z1–Z3; Appendix 1) plot along a line that has an upper-intercept age of 1437 ± 5 Ma and a lower-intercept age of 193 ± 225 Ma (Fig. 6D).
Monarch Pass granodiorite (J01-M1)
We collected a sample of coarse-grained to K-feldspar megacrystic granodiorite on the west side of U.S. Highway 50 approximately 0.35 miles (0.56 km) south of the Monarch Ski Area. The sampled outcrop contains a northeast-striking, moderately northwest-dipping foliation defined by biotite and aligned feldspar megacrysts. We interpret the dominant foliation to be magmatic in origin because it is primarily defined by aligned feldspar megacrysts up to 4 cm long. However, some of the megacrysts appear to be locally kinked and folded. Additionally, zones of concentrated, strongly aligned biotite up to a few centimeters thick suggest that deformation occurred or continued after crystallization of the pluton.
This sample yielded a relatively simple population of colorless to light pink, euhedral to subhedral zircon interpreted to be igneous. We did not see any evidence for overgrowth or xenocrystic cores. Zircon from this sample were analyzed by ID-TIMS techniques, and three multi-grain fractions (Z1–Z3) define a line with intercepts of 1447 ± 9 Ma and 796 ± 475 Ma (Fig. 6B). This sample also yielded abundant medium to dark brown, angular titanite fragments. Two concordant fractions (T1 and T3) have 207Pb/206Pb ages of 1430 Ma and 1424 Ma, respectively (Appendix 1). A third fraction, T2, is nearly concordant with a 207Pb/206Pb age of 1437 Ma and plots in a linear array with the three zircon fractions (Fig. 6B; Appendix 1).
Taylor River Granite (J02-SC1)
We collected a sample of fine-grained biotite-muscovite granite on the north side of Gunnison National Forest Road 742 approximately 0.65 miles (1.05 km) northeast of the intersection with Spring Creek Road (Forest Road 744). The granite is variably foliated and is texturally fairly homogenous with the exception of coarse-grained biotite clusters that occur as knots up to 4 cm in diameter or elongate lenses up to 4 cm wide by 40 cm long.
This sample yielded a relatively simple population of zircon that ranged in size and color but were generally euhedral and prismatic and, therefore, interpreted to be igneous. In some grains, we observed semi-opaque xenocrystic cores that were surrounded by colorless prismatic overgrowths. However, these grains were small enough that we were unable to mechanically separate the overgrowths from the core materials or fit a laser-ablation spot within a single growth domain. Thirty-six zircon grains analyzed by LA-ICP-MS techniques yielded 207Pb/206Pb ages that ranged from 1875 to 1373 Ma (Fig. 7A,B; Appendix 2). The seven youngest grains analyzed plot in a linear array, with two of the analyses overlapping concordia. Regression of these data with the lower intercept forced through the origin (0 ± 50 Ma) produces a line with an upper-intercept age of 1428 ± 23 (Fig. 7C). The remaining zircon plot to the right of this line and yield ages to 1875 Ma with discordance to 46 percent (Appendix 2).
Interpretation of U-Pb geochronology results
We interpret the average 207Pb/206Pb zircon age of 1697 ± 7 Ma from the Henry Mountain granite to represent crystallization of the pluton. This age agrees with early U-Th-Pb and Rb-Sr data for the Henry Mountain granite (Aldrich et al., 1956; Wetherill and Bickford, 1965; Fridrich et al., 1998) and indicates that it is similar in age to numerous granitoids exposed south of Gunnison, Colorado (Fig. 1; Bickford et al., 1989b). Our new results also indicate that the Henry Mountain granite is age-correlative with the Arkansas River granites, a voluminous suite of 1700–1660-Ma granitoids exposed to the east and southeast (Anderson and Cullers, 1999) of the Sawatch Range. Both the LA-ICP-MS and ID-TIMS analytical approaches produced the same age within analytical uncertainty for the Horsethief granite, and we prefer the more precise upper intercept age of 1437 ± 5 Ma from the ID-TIMS analyses as the crystallization age for the granite. This result indicates that the Horsethief granite is not correlative with the Henry Mountain granite, and the close agreement in ages produced by the two different analytical approaches gives us added confidence in the LA-ICP-MS approach for igneous age determinations.
We interpret the upper-intercept age of 1447 ± 9 Ma for zircon from the Monarch Pass pluton as its crystallization age. This age is in agreement with an unpublished SHRIMP zircon age of 1441 ± 9 Ma for Monarch Pass granodiorite reported by W. Premo (written communication, 2009). We attribute the slight discordance to Phanerozoic Pb loss. We did not observe any color or textural variation in the titanite recovered from this sample, and all of the grains were similar in size. We did not observe any core/rim relationships that might represent multiple generations of titanite, and the U content for the three analyzed fractions was fairly consistent (135–118 ppm; Appendix 1). On the basis of these observations, we do not believe that the younger titanite ages represent growth of new titanite. Instead, we attribute the spread of titanite ages to variable resetting during the thermal pulse that accompanied widespread ca. 1.4-Ga magmatism throughout the region (Shaw et al., 2005).
The Taylor River granite is the youngest intrusive unit recognized in this study, and we interpret the upper-intercept age of 1428 ± 23 Ma for the seven youngest grains to represent the crystallization age. This age is consistent with observed cross-cutting relationships with the Henry Mountain granite and agrees with early Rb-Sr geochronology (Wetherill and Bickford, 1965) and a preliminary single-fraction zircon 207Pb/206Pb age of ca. 1406 Ma reported in Fridrich et al. (1998). We attribute the abundant older zircon ages in this sample to inheritance (Fig. 7A, B), and the ages of these grains are consistent with the age of plutons and basement supracrustal and metavolcanic successions exposed locally and throughout the surrounding region (Bickford et al., 1989b; Hill and Bickford, 2001). Two discordant grains with 207Pb/206Pb ages >1800 Ma (Appendix 2) are consistent with the findings of Hill and Bickford (2001) in rocks exposed south of Gunnison area, but we note the absence of any Archean grains among all of the samples in this study.
The observation that the Henry Mountain granite cuts across S0/S1 in basement assemblages requires that at least one deformation event (D1) occurred prior to emplacement of the granite at 1697 ± 7 Ma. D1 involved isoclinal folding and development of a pervasive layer-parallel foliation throughout the area. The orientation of D1 deformation, however, is difficult to determine because of reorientation and overprinting of S0/S1 during subsequent deformation. Following crystallization of the Henry Mountain granite, a second deformation event (D2) occurred that affected both the pluton and metasedimentary and metavolcanic country rocks. D2 produced a well-developed biotite foliation in the granite along with localized high-strain zones defined by concentrations of aligned biotite and highly elongated quartz and feldspar. The northeast strike of S2 in the granite suggests that D2 deformation involved northwest—southeast-directed shortening. The high-strain zones contain asymmetric mineral fabrics with an associated steeply plunging mineral lineation, and kinematic indicators show that D2 shortening was accompanied locally by dip-slip, west-side-up movement (Hetherington, 1991). D2 produced tight, upright folds in basement assemblages and an associated axial-planar fabric (S2) that varies somewhat in orientation, but has an average east-northeast strike. Hetherington (1991) noted that the orientation of S2 in the basement assemblages is broadly consistent with north–south directed shortening, but also varies in a systematic pattern around the Henry Mountain granite and generally reflects the mapped geometry of the pluton. Hetherington (1991) also noted that the distribution of strain varied with distance from the pluton, and he recognized that deformation was concentrated along the contact between metavolcanic and metasedimentary rocks in the western portion of the southern Sawatch Range. Thus, he proposed a structural model (Fig. 8) to explain the distribution and geometry of D2 deformation involving a thrust ramp between the two lithologies that was likely influenced by the Henry Mountain granite pluton.
F2 folds and the S2 fabric in basement assemblages are cut by the Taylor River granite (Fig. 5B), requiring that D2 occurred prior to 1428 ± 23 Ma. The Taylor River granite contains a northeast-striking biotite foliation (S3) that is best developed within a few hundred meters of the contact with the Henry Mountain granite and locally cuts across the contact between the two intrusive units. This fabric is also parallel with the northeast-striking foliation in the Henry Mountain granite, indicating that additional fabric development or reactivation or the pre-existing fabric (S2) occurred in the ca. 1697-Ma granite during a third deformation event (D3) at ca. 1.4 Ga. Although we observed that the 1437 ± 5-Ma Horsethief granite and 1447 ± 9-Ma Monarch Pass pluton contain local evidence for solid-state deformation, our current mapping does not indicate that ca. 1.4-Ga deformation was penetrative throughout these intrusions or throughout the southern Sawatch Range in general. Instead, we believe that deformation at ca. 1.4 Ga was primarily concentrated along the margins of the 1428 ± 23-Ma Taylor River granite. The overall northeast strike of S3 in the Taylor River granite suggests that D3 involved northwest–southeast shortening, but the orientation of the fabric might also have been influenced by the pre-existing northeast-striking foliation in the Henry Mountain granite (Fig. 3C) or by the geometry of the contact between the two intrusions (Fig. 1).
The Henry Mountain granite appears to have influenced the distribution and orientation of deformation during both D2 and D3, and the interpreted shortening directions for both events indicate that they were broadly coaxial. However, D2 deformation involved widespread folding, fabric development, local shear fabric development, and possibly thrust faulting within the basement lithologic units. D3 deformation involved development of foliation that was concentrated along the contact between two plutons and otherwise localized within other larger ca. 1.4-Ga plutons. The localized nature of D3 effects throughout the study area contrasts sharply with widespread evidence for D2 deformation in both basement assemblages and the Henry Mountain granite, suggesting that the two events likely do not represent a single, progressive deformation event at ca. 1.4 Ga. Although our results only require that D2 occurred between ca. 1697 and 1428 Ma, we prefer the interpretation that D2 occurred during the Paleoproterozoic following emplacement and crystallization of the Henry Mountain granite.
D1 is the oldest deformation event recognized in the southern Sawatch Range and affected rocks of the Cochetopa and Black Canyon successions. Thus, we interpret D1 to represent deformation that occurred as these rocks were accreted to southern Laurentia during the late stages of the Yavapai orogeny, ca. 1730–1700 Ma (Karlstrom and Bowring, 1988; Jessup et al., 2006). Various workers have recognized broadly coeval deformation with similar characteristics and fabric orientations in surrounding areas (e.g., Shaw et al., 2001; Jessup et al., 2006; Jones and Connelly, 2006; Gonzales and Van Schmus, 2007), supporting the interpretation that D1 represented a major regional event. A voluminous suite of late- to post-tectonic granitic intrusions was emplaced throughout central and southern Colorado during the late stages of the Yavapai orogeny (Bickford et al., 1989a, b; Anderson and Cullers, 1999; Jessup et al., 2006; Jones and Connelly, 2006). These intrusions help to establish the minimum age for Yavapai-related deformation and are represented locally by the 1697 ± 7-Ma Henry Mountain granite.
Following the Yavapai orogeny, accretion of the Mazatzal province to the south caused renewed deformation and metamorphism throughout the southern Yavapai province (Fig. 1; transition zone of Shaw and Karlstrom, 1999). Deformation occurred between ca. 1680 and 1630 Ma, but varied widely in space (e.g., Shaw and Karlstrom, 1999; Shaw et al., 2001; Jones and Connelly, 2006). On the basis of arguments outlined above, we contend that D2 in the southern Sawatch Range occurred during the Paleoproterozoic following crystallization of the Henry Mountain granite, and we interpret D2 to represent local effects of Mazatzal-aged tectonism. Development of a major thrust fault along the contact between the Black Canyon and Cochetopa successions during this time (Fig. 8; Hetherington, 1991) is consistent with fold-and-thrust style deformation that resulted in the burial of thick successions of post-Yavapai sedimentary rocks in the surrounding region (Williams, 1991; Jessup et al., 2006; Jones et al., 2009;). D2 folding in basement assemblages and foliation development and shearing along the margins of the Henry Mountain granite are similar to Mazatzal-related deformation in the Homestake shear zone to the northeast (Fig. 1; Shaw et al., 2001) and Sangre de Cristo Mountains to the southeast (Fig. 1; Jones and Connelly, 2006), providing additional evidence for deformation in the foreland of the Yavapai–Mazatzal transition zone (Fig. 1; Karlstrom and Humphreys, 1998).
The 1447 ± 9-Ma Monarch Pass pluton, 1437 ± 5-Ma Horsethief granite, and 1428 ± 23-Ma Taylor River granite are all correlated with a widespread suite of A-type granites emplaced across the southwestern United States during the Mesoproterozoic (Reed et al., 1993; Anderson and Cullers, 1999). These granites are part of a prolonged regional tectonothermal event that began as early as ca. 1470 Ma (Bickford et al., 1989a; Cullers et al., 1993) and lasted until ca. 1360 Ma in some areas (Bickford et al., 1989a; Cullers et al., 1992). Limited geochemical data are only available for the Taylor River granite (Fridrich et al., 1998), but the presence of accessory fluorite in the Horsethief granite is consistent with A-type geochemistry (Collins et al., 1982; Whalen et al., 1987). Granites emplaced at ca. 1.4 Ga are commonly deformed in exposures throughout the Rocky Mountains (e.g., Mount Evans batholith; Aleinikoff et al., 1993), and, in some cases, magmatism accompanied movement along major crustal shear zones (Shaw et al., 2001; Jessup et al., 2006). In the southern Sawatch Range, D3 deformation followed magmatism at ca. 1428 Ma, producing a weak, but penetrative, northeast-striking foliation within the Taylor River granite and localized shear fabrics along the contact with the Henry Mountain granite. These fabrics are compatible with northwest-directed shortening at ca. 1.4 Ga, and they are similar in orientation with broadly contemporaneous fabrics and shear zones documented throughout the southwestern United States (e.g., Nyman et al., 1994; Duebendorfer and Christensen, 1995; Shaw et al., 2001; Ferguson et al., 2004; McCoy et al., 2005; Jessup et al., 2006; Jones et al., 2010). Our initial mapping suggests that the Monarch Pass pluton and Horsethief granite might have also been deformed during the same time, but we have not yet established the extent, nature, and orientation of ca. 1.4-Ga fabrics in these granites or their immediate country rocks.
Four newly dated granitic plutons exposed in the southern Sawatch Range, central Colorado, provide new insights into the age and extent of Paleoproterozoic and Mesoproterozoic magmatism and lead to a better understanding of the age and extent of events that occurred during the Proterozoic growth and modification of southern Laurentia. Early deformation of arc-related metavolcanic and metasedimentary rocks involved isoclinal folding and penetrative fabric development. We interpret the early deformation to represent the late stages of the Yavapai orogeny, and the post-orogenic Henry Mountain granite brackets the minimum age of orogenesis locally at 1697 ± 7 Ma. Initial deformation of the Henry Mountain granite involved development of penetrative foliation and local shearing, and was accompanied by widespread folding, thrust faulting, and fabric development in surrounding basement rocks. We believe that this event represents ca. 1.65-Ga deformation of the southern Yavapai province during accretion of the Mazatzal province to the south. Three separate plutons—the Monarch Pass granodiorite, Horsethief granite, and Taylor River granite—were emplaced between ca. 1447 and 1428 Ma as part of a regional tectonothermal event involving widespread A-type granitic magmatism that affected the newly accreted Proterozoic lithosphere south of the Archean Wyoming province. Deformation of the 1428 ± 23-Ma Taylor River granite involved northwest-directed shortening that produced a subvertical, northeast-striking foliation and localized shear fabrics along the contact with the Henry Mountain granite. This finding is consistent with models for intracontinental orogenesis involving localized yet kinematically consistent deformation in exposures throughout the southwestern United States during the same time.
This research was funded by National Science Foundation grant EAR-0003528 awarded to J. Connelly and by the Department of Geological Sciences and Geology Foundation at The University of Texas at Austin. Field assistance was provided by Adam Krawiec and Chris Rhea. Analytical assistance was provided by Kathy Manser and Todd Housh. Reviews by Paul Mueller and Wayne Premo helped to clarify and improve the paper.
- Received November 6, 2009.
- Revision received March 15, 2010.
- Accepted April 2, 2010.