- UW Department of Geology and Geophysics
Field studies and U-Pb geochronology in the Sangre de Cristo Mountains, southern Colorado, provide new constraints on the Proterozoic tectonic evolution of southern Laurentia. Protoliths for basement gneisses and amphibolites were formed in an arc environment and underwent early penetrative deformation and metamorphism (D1 and M1) during formation of the Yavapai province. D1 deformation produced penetrative, subvertical, northwest-striking fabrics (S1) in rocks exposed throughout the range and is interpreted to have occurred during long-lived arc formation and accretion across northwest-striking tectonic boundaries. The ages of D1 and M1 are constrained by a suite of 1750–1730-Ma calc-alkaline intrusions in the southern part of the range and might have occurred as late as ca. 1710 Ma in the northern part of the range. The northeast-striking tectonic grain that was developed regionally during the culmination of the Yavapai orogeny is not recognized locally. Post-orogenic granitoid plutons were emplaced at 1695±2 Ma and 1682±3 Ma, broadly coeval with deposition of locally derived quartzite at the surface. Magmatism and sedimentation during this time are interpreted to represent contemporaneous responses to crustal extension during the ca. 60-m.y. inter-orogenic period between the Yavapai and Mazatzal orogenic events. D2 deformation is interpreted to represent the Mazatzal orogeny locally, and involved northwest-directed shortening and dextral shear localized along subvertical, northeast-trending high-strain zones. D2 was accompanied by amphibolite-facies metamorphism (M2) at 1637±6 Ma, and the quartzite is inferred to have been deformed during this time. Mesoproterozoic deformation (D3) produced a northeast-striking, subvertical tectonic foliation and localized shear zones between 1420 and 1412 Ma. D3 deformation was bracketed by the emplacement of two newly dated granitic intrusions at 1434±2 Ma and 1407±6 Ma. The map-scale geometry of these intrusions and coeval deformational fabrics suggest that ca. 1.4-Ga granites were emplaced into a broadly compressional stress field during subhorizontal northwest–southeast shortening. These new data and observations indicate that ca. 1.4-Ga granites are not anorogenic, consistent with tectonic models suggesting that widespread magmatism was broadly synchronous with intracontinental orogenesis at ca. 1.4 Ga.
Regional tectonic models for the Paleo- and Mesoproterozoic growth and evolution of southern Laurentia involve three distinct phases: early crust formation and orogenesis during ca. 1.8–1.6 Ga, a ca. 200-m.y. tectonic quiet period, and a ca. 1.4-Ga tectonothermal event that overprinted the Paleoproterozoic crust. These events are recognized throughout the southwestern United States and are generally well constrained (e.g., Reed et al., 1993; Karlstrom et al., 2004), but significant local and regional complexities remain to be resolved. Outstanding issues include: (1) the timing and tectonic setting of early crust formation and assembly; (2) the timing and tectonic significance of widespread clastic sedimentation along the Laurentian margin; and (3) the tectonic setting of widespread ca. 1.4-Ga granitic magmatism. These issues are directly related to more general questions concerning the timing and character of formation, accretion, stabilization, and reactivation of Proterozoic crust in southern Laurentia, and to what degree events were protracted or discrete and distributed or localized throughout the region.
This study presents new results from field mapping and U-Pb geochronology in the Sangre de Cristo Mountains, southern Colorado, that constrain the Proterozoic tectonic evolution of southern Laurentia and address some of the questions described above. Exposures of basement gneiss and amphibolite, quartzite, and granitic intrusive rocks throughout the Sangre de Cristo Mountains provide a relatively complete record of Proterozoic magmatism, deformation and metamorphism. Detailed field mapping of previously undated plutons combined with precise U-Pb geochronology in the central part of the range constrain the age of magmatism, metamorphism, and the development and reactivation of Proterozoic tectonic fabrics.
Combined with published geochronology from other parts of the mountain range, our new results illustrate the episodic nature of igneous and tectonic activity during the Proterozoic and provide new local constraints on regionally recognized tectonic events. These results also constrain an important change from early, northwest-striking, penetrative fabrics to localized, northeast-striking tectonic fabrics that characterize Proterozoic exposures throughout the southern Rocky Mountains. Furthermore, new structural observations suggest that penetrative fabrics developed during early, northeast-directed shortening fundamentally controlled the geometry and spatial distribution of subsequent deformation and magmatism throughout the Proterozoic and Phanerozoic. Detrital zircon geochronology constrains the maximum age of quartzite deposition and provides new information regarding sedimentary provenance. Cross-cutting relationships and new metamorphic U-Pb ages indirectly constrain the age of quartzite deformation.
Finally, we document previously unrecognized Mesoproterozoic (ca. 1.4 Ga) granitic intrusions that bracket an episode of coeval, northwest-directed, subhorizontal shortening. These new data and observations not only have important implications for constraining local and regional tectonic histories, but they also bear on the collective interpretation of the fundamental tectonic processes governing the growth, stabilization, and modification of southern Laurentia throughout the Meso-to Neoproterozoic.
Exposures of Precambrian crustal 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 as part of a protracted period of Laurentian crustal growth (Condie, 1982; Karlstrom and Bowring, 1988; Reed et al., 1993). These exposures have been divided into several orogenic provinces on the basis of rock ages and isotopic characteristics (Fig. 1). 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 between ca. 1.71 and 1.70 Ga (Karlstrom and Bowring, 1988) and was followed by a prolonged episode (ca. 40 m.y.) of voluminous post-orogenic granitoid magmatism (Anderson and Cullers, 1999). 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 1.66–1.62-Ga Mazatzal orogeny (Silver, 1965; Karlstrom and Bowring, 1988). Deformation related to the Mazatzal orogeny propagated northward into the southern part of the Yavapai province (Transition Zone, Fig. 1), and the Mazatzal front represents the approximate extent of these effects (Shaw and Karlstrom, 1999).
The Yavapai and Mazatzal orogenic events were separated by a ca. 50-m.y. interval during which thick (1–2 km) sequences of quartz arenite were deposited along the southern margin of the continent. These clastic sequences are preserved throughout the southwestern United States and include extensively exposed units (e.g., Uncompahgre Formation, Ortega Formation, Mazatzal quartzite) as well as numerous smaller, localized exposures throughout the region. Quartzites in the metasedimentary sequences are characterized by a high degree of compositional maturity and well-preserved primary sedimentary structures (Cox et al., 2002; Soegaard and Eriksson, 1985). They overlie poly-deformed basement assemblages consisting of compositionally diverse, but commonly mafic, rock types (e.g., Dubois and Cochetopa successions; Bickford and Boardman, 1984). Quartzite sequences and underlying basement assemblages were deformed and metamorphosed together, and quartzite exposures across southern Colorado commonly form synclinal “keels” interpreted to represent the roots of much larger folds that have been eroded (Reuss, 1974). Deformed quartzite sequences were intruded by coarse-grained granitoids at ca. 1.4 Ga (Harris et al., 1986; Gibson and Harris, 1992). Whereas published age constraints (U-Pb zircon) from Arizona and New Mexico require that quartzite deposition closely followed the ca. 1.7-Ga Yavapai orogeny (1703 Ma, Cox et al., 2002; Bauer and Williams, 1989), the age of quartzite deposition in southern Colorado is poorly constrained and regional correlations among isolated exposures are tenuous.
After a ca. 200-m.y. tectonic lull, 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 mafic diking, and regional high-temperature/low-pressure metamorphism occurred throughout the southwestern United States between 1.47 and 1.36 Ga (Reed et al., 1993; Williams, 1991; Williams et al., 1999) and has been interpreted as an intracontinental response to accretion of Mesoproterozoic crust to the south (Nyman et al., 1994). Ca. 1.4-Ga granites are characterized by distinct, A-type geochemical characteristics (Loiselle and Wones, 1979; Anderson, 1983) that are commonly associated with extensional tectonic environments (e.g., continental rifting; Emslie, 1978). However, regional evidence for deformation within the thermal aureoles of plutons and contemporaneous reactivation of northeast-striking crustal shear zones suggests that the magmatism was accompanied by regional northwest–southeast shortening at ca. 1.4 Ga (Graubard and Mattinson, 1990; Nyman et al., 1994; Shaw et al., 2001).
PROTEROZOIC GEOLOGY OF THE SANGRE DE CRISTO MOUNTAINS
The Sangre de Cristo Mountains, southern Colorado, are located in the southern part of the Yavapai province within the broad zone affected by Mazatzal-age orogenesis (Fig. 1). The range is located in a region of southern Colorado and New Mexico that preserves evidence of widespread deposition of quartzite during the Paleoproterozoic and extensive granitic magmatism at ca. 1.4 Ga. Although exposures of Proterozoic rocks throughout the Sangre de Cristo Mountains are locally segmented by Phanerozoic faults (Fig. 2), our reconnaissance mapping suggests that there is structural continuity among individual fault blocks along the length of the range. Exposures of Proterozoic rocks comprise a compositionally diverse assemblage of felsic to mafic metavolcanic rocks, interlayered metasedimentary rocks, and local cross-bedded quartzite that is intruded by voluminous mafic to granitic rocks of both Paleoproterozoic and Mesoproterozoic ages. New field mapping reveals that these rocks preserve evidence for at least three episodes of deformation and metamorphism (D1–D3 and M1–M3, respectively) and record a change in the orientation of regional deformational fabrics from northwest to northeast strikes between D1 and D2. Penetrative fabrics developed during D1 and M1 are preserved across the full length of the range (>100 km) and have influenced the geometry and localization of the effects of subsequent tectonism ranging from late Paleo- to Mesoproterozoic magmatism and deformation to multiple phases of Phanerozoic brittle deformation. In contrast, younger Proterozoic deformation (D2 and D3) produced fabrics that define a well-documented, regional northeast–southwest tectonic grain. These fabrics are localized into discrete domains of concentrated deformation throughout the range.
The following sections describe the Proterozoic rocks exposed in the Sangre de Cristo Mountains, organized from oldest to youngest. The dominant rock types are discussed in conjunction with the various structural elements they contain.
Mixed Gneiss and Amphibolite
The oldest rocks exposed in the Sangre de Cristo Mountains are a sequence of fine- to medium-grained amphibolite-facies gneisses and amphibolites that make up the host rock to all intrusive phases described below. These rocks comprise a varied assemblage described in detail by Johnson et al. (1987), and there is a general compositional progression from felsic gneiss and schist in the northern part of the range to mafic gneiss and amphibolite in the southern part of the range. The progression is gradational over tens of kilometers, and the different rock types are commonly interlayered on a scale of meters to centimeters in outcrop.
Structurally, gneisses and amphibolites across the range are characterized by a subvertical, northwest-striking foliation (S1) that is well developed in the northern and southern parts of the range (Fig. 2). In contrast, foliations in exposures of the same rock units in the central part of the range tend to strike east-northeast and dip steeply north. North-dipping foliations are more common on the western side of the range and are concentrated within localized, east-northeast-striking high-strain domains. However, many of these rocks are difficult to access, and their tectonic significance has yet to be determined. The dominant gneissic fabric is subparallel to compositional domains that likely reflect primary layering (S1 = S0). Individual compositional layers are locally folded by centimeter-scale isoclinal folds (Figs. 4A and 8A), and the main foliation (S1) is axial planar to these folds.
Blanca Peak Intrusive Suite
A suite of intermediate and mafic intrusive rocks is exposed at the southern end of the range south of Medano Pass in the Blanca Peak area (Fig. 2). These medium- to coarse-grained plutonic rocks, described in detail by Sabin (1994, after Johnson et al., 1987), include (in decreasing volumetric order) tonalite, diorite, granodiorite, quartz diorite, and gabbro. These rocks intrude the gneisses and amphibolite and cut across the northwest-striking gneissic foliation (S1). The plutonic rocks locally contain a weak solid-state foliation that does not display a preferred orientation. Sabin (1994) dated a suite of Blanca Peak intrusive rocks and identified two age populations: an older group with ages of ca. 1750 Ma, and a younger group with ages of ca. 1730 Ma (Fig. 2). Isotopic data (Sm-Nd and Rb-Sr) suggest that the intrusive suite was derived from a source with isotopic compositions similar to a depleted mantle reservoir and that no detectable amount of significantly older continental crust was involved in petrogenesis (Sabin, 1994). These rocks were not the focus of this study, but they provide useful minimum age constraints on the deposition and/or crystallization of basement gneisses and development of the early northwest-striking S1 fabric across the range. Furthermore, their isotopic characteristics help to constrain tectonic models for the formation of basement assemblages in the southern part of the range.
Marshall Gulch Syenite and Monzogranite (Xsm)
The Marshall Gulch pluton is exposed on the western flank of the Sangre de Cristo Mountains, ca. 10 km north of the town of Crestone (Fig. 2) between the Rito Alto and Wild Cherry Creek U.S. Forest Service trailheads. White to gray syenite and monzogranite are the main compositional phases of this granitic body, and modal mineral percentages are approximately 50–80 percent microcline, as much as 35 percent plagioclase, 1–5 percent quartz, as much as 10 percent biotite, minor muscovite, and accessory titanite, zircon, and apatite (Johnson et al., 1987). Rocks exposed are commonly coarse-grained to K-feldspar megacrystic with individual microcline phenocrysts up to 4 cm wide and 8 cm long, but fine-grained, leucocratic granite is locally abundant. The contact between the fine- and coarse-grained phases is gradational and commonly interfingering, such that they are interpreted to be part of the same general intrusive event. Although the Marshall Gulch pluton was previously undated, past workers correlated it with a regional suite of Paleoproterozoic granitic intrusions on the basis of its coarse-grained texture and the presence of deformational fabrics (Routt plutonic suite of Tweto, 1987; Tweto, 1979; Johnson et al., 1987). New U-Pb geochronology results described below support this correlation.
The Marshall Gulch pluton is exposed as an elongate, tabular body (2 km wide by 7 km long), oriented parallel to a well-developed foliation in host rock gneiss and amphibolite (S1; Fig. 3B) that strikes north–northwest and dips steeply west–southwest. Granitic rocks contain a strong foliation that is defined by aligned tabular feldspars and biotite grains that are oriented parallel to S1 (average strike/dip=330/70°S W, Fig. 3A). This foliation is interpreted to be magmatic in origin (Smagmatic), and it is best developed within 0.5 km of the pluton margin. Across the southernmost 1 km of the pluton exposure, fine- and coarse-grained granite also contain a solid-state foliation (S2) that cuts across and the magmatic foliation and is nearly perpendicular to S1 and Smagmatic (Fig. 3A). S2 also cuts across a large (0.5 km diameter) composite xenolith of gneiss and meta-tonalite in the southern part of the map area (Fig. 3B). In granitic rocks, S2 is primarily defined by biotite and is enhanced locally by grain-size reduction of the originally coarse-grained granitic matrix (Figs. 4B and 4D). S2 is parallel to the axial planes of tight, upright folds in thin (20–30 cm) dikes of granitic rock (Fig. 4C). A mineral lineation (L2) is defined by biotite and plunges shallowly west-southwest (average orientation = 24°/S69W; Fig. 3C) Where microcline megacrysts are locally preserved, they have asymmetric recrystallized tails indicating dextral shear sense (Fig. 4B). In outcrops where grain size reduction has not occurred, coarse-grained granite locally contains a composite (S-C) foliation that also records dextral shear sense (Fig. 4D). The spatial distribution of S2 fabrics and presence of both shortening- and shear-related deformation features suggest that the Marshall Gulch pluton experienced at least one episode of northwest–southeast directed shortening accompanied by localized dextral shear after crystallization.
Crestone Quartz Monzonite (Xqm)
The Crestone stock is exposed on the western flank of the Sangre de Cristo Mountains and forms a prominent, jagged ridge east-northeast of the town of Crestone (Fig. 2). This intrusion consists of light gray to light brown to tan, fine- to medium-grained quartz monzonite with modal percentages estimated at 35 percent plagioclase, 30 percent microcline, 30 percent quartz, 2–5 percent biotite, minor muscovite, and accessory magnetite, titanite, zircon, and apatite (Johnson et al., 1987). It is exposed as an elongate (2 km wide by 5 km long) body but is bounded by Phanerozoic thrust faults on two sides (Fig. 5). Host rocks include greenschist-facies amphibolite and fine-grained mafic gneiss and amphibolite that are only exposed as small (< 1 m) xenoliths in the heart of the intrusion and along its southern end. The stock sharply cuts across S1 in host-rock gneisses. Away from the southern contact, the granite is essentially undeformed with the exception of a locally developed foliation defined by muscovite and biotite that does not display a consistent orientation. The apparent lack of deformation across much of the Crestone stock and its fine-grained nature led previous workers to correlate this intrusion with a regional suite of granitic intrusions that are Mesoproterozoic in age (Berthoud plutonic suite of Tweto, 1987; Tweto, 1979; Johnson et al., 1987), but it was previously undated. New U-Pb geochronology reported below indicates that it was emplaced during the Paleoproterozoic.
The southernmost exposures of the Crestone stock contain a strong foliation defined by biotite and elongate quartz that strikes northeast–southwest and dips steeply southeast (Fig. 5A). This foliation is parallel to the dominant fabric in host-rock gneiss and amphibolite within 0.25 km of the margin of the stock (Fig. 5B), but the host-rock foliation changes to a more north–south orientation away from the granite to the south. The solid-state foliation is similar in orientation to both S2 of the Marshall Gulch pluton and S3 of the Music Pass pluton (described below), but timing relationships discussed below suggest that the foliation formed or was reactivated during Mesoproterozoic deformation (D3), and is, therefore, an S3 fabric.
In the central part of the range, there is a narrow exposure of a 100-m thick layer of cross-bedded quartzite (Fig. 6). Quartzite is heterogeneous with colors ranging from white to gray to red, is relatively pure (>90 percent quartz), and contains relict layering defined by seams rich in opaque minerals (Fig. 7). The nature of the contact between the quartzite and underlying basement rocks is difficult to determine because it is only locally exposed, 0.5 km southeast of Snowslide Mountain (Fig. 6), and is deeply weathered. However, based on subtle contrasts in the orientation of compositional layering in gneiss and quartzite and differing degrees of deformation between the two units, the quartzite must be in either fault or unconformable depositional contact with underlying basement. The timing of quartzite deposition is constrained locally by the cross-cutting Music Pass quartz monzonite (see below).
Music Pass Quartz Monzonite (Yqm)
The Music Pass quartz monzonite is exposed on the eastern flank of the Sangre de Cristo Mountains 20 km south of the town of Westcliffe (Fig. 2). Much of this pluton and its western margin are best exposed west of Sand Creek (Fig. 6); however, access to the mouth of the creek from the west side of the range is limited. Instead, exposures in this part of the range are best accessed from the east over Music Pass. The Music Pass pluton is characterized by grey to pink, coarse-grained to K-feldspar megacrystic quartz monzonite (Fig. 8C). Large pink to white microcline megacrysts up to 6 cm in length make up 25–45 percent of the rock, and estimated modal percentages of the groundmass are 60 percent plagioclase, 20 percent quartz, 10–20 percent biotite and amphibole, up to 1 percent titanite and magnetite, and accessory zircon and apatite (Johnson et al., 1987). Host rocks to the pluton include medium- to coarse-grained gneiss, amphibolite, and quartzite that contain a strong, subvertical gneissic foliation that strikes west-northwest (S1, Fig. 6A). Gneiss surrounding the Music Pass pluton locally contains abundant, small (10–15 cm) isoclinal folds that affect both compositional layering and the gneissic foliation (Fig. 8A). The intrusive rocks cut across the gneissic foliation of the host rocks, and a sharp vertical contact between quartz monzonite and host-rock gneiss is observed in the southeast face of Tijeras Peak (Fig. 8B). The pluton is xenolith-poor except along its eastern margin around Snowslide Mountain (Fig. 6) where it contains large (up to 10–15 m2 in cross-section) blocks of quartzite. Based on its coarse-grained texture and widespread evidence for solid-state deformation within the pluton (see below), the Music Pass quartz monzonite was correlated by previous workers with a regional suite of Paleoproterozoic granitic intrusions (Routt plutonic suite of Tweto, 1987; Tweto, 1979; Johnson et al., 1987). Thus, it was sampled for U-Pb geochronology to determine the minimum depositional age of the host-rock quartzite. New results described below indicate that the pluton was emplaced during the Mesoproterozoic (ca. 1.4 Ga).
The pluton contains a weakly- to moderately-developed foliation defined by large, tabular microcline megacrysts and biotite that is subvertical and strikes west-northwest (Fig. 6C). This fabric is strongest within 0.5–1.0 km of the pluton margin and is parallel to the contact and the dominant fabric in the host rocks (S1; Fig. 6A). The pluton fabric is interpreted to be magmatic in origin (Smagmatic). Detailed mapping within the pluton reveals that discrete zones of solid-state deformation up to a few meters thick are common. These deformation zones are characterized by grain-size reduction of the coarse-grained granitic matrix and development of a northeast-striking, biotite-dominated foliation that cuts across the magmatic fabric and dips steeply northwest. Although the orientation of this foliation is similar to S2 in the Marshall Gulch pluton, new U-Pb geochronology reported below indicates that the regional S2 predates the fabrics (S3) mapped within the Music Pass pluton. Strong flattening fabrics that are developed within many of these deformation zones suggests that northwest–southeast shortening dominated during deformation, but local asymmetric fabrics, offset pegmatite dikes, and local composite (S-C) fabrics in granitic rocks suggest that some component of both sinistral and dextral shear locally accompanied shortening (Fig. 8E). One zone of deformation along the ridge southeast of Music Pass displays mylonitic fabrics in which the granitic matrix has been reduced to sub-millimeter grain sizes, and microcline megacrysts are elongated into ribbons with aspect ratios of up to 20:1 (Fig. 8D).
Pegmatite Dikes (Yp)
The Marshall Gulch pluton is cut by a group of pegmatite dikes that do not appear to be directly related to the host. These dikes intrude both basement gneisses and granitic rocks and occur in a swarm of approximately 10 large intrusions with an average strike of 335° and near-vertical contacts (Figs. 3 and 9). These dikes are distinctively thick, averaging 10–15 m across. The dikes are leucocratic (white to light gray) and are composed of K-feldspar and quartz with minor muscovite, plagioclase, and black tourmaline. Contacts between the pegmatite and host rocks are commonly sharp and planar to angular, suggesting that the host rocks were cool relative to the intruding material. Locally, angular blocks of host gneiss up to a meter in size were incorporated into the margins of the dikes, and smaller swarms of veins and dikes extend up to a few meters beyond the contacts of the larger dikes. Although previously undated, these dikes were interpreted to be Paleoproterozoic in age because they were emplaced into a Paleoproterozoic pluton (Lindsey et al., 1985). However, new geochronology reported below indicates that they were emplaced during the Mesoproterozoic.
Mafic Dike (Ym?)
The Music Pass pluton is cut on its northern end by a thick (30 m), subvertical mafic dike. The north-northwest-striking dike is primarily composed of coarse-grained gabbro and has no detectable internal fabric. It is correlated with a swarm of similar mafic dikes with north-northwest trends exposed in the Sawatch Range, Park Range, and northern Front Range of Colorado (Tweto, 1987, and references therein). These mafic intrusions are interpreted to be broadly synchronous with ca. 1.4-Ga granitic magmatism based on hornblende K-Ar ages and spatial and cross-cutting relationships with dated plutons (Tweto, 1987). The dike was sampled for U-Pb geochronology but did not yield minerals suitable for dating.
U-Pb ZIRCON AND TITANITE GEOCHRONOLOGY
We sampled selected amphibolite host rocks, the three granitic plutons described above, and cross-cutting pegmatite dikes for U-Pb geochronology to constrain the age of metamorphism, magmatism, and deformation in the central Sangre de Cristo Mountains. We present isotopic data in Table 1 and associated concordia and isochron diagrams in Figures 10–⇓12. Zircon and titanite fractions were hand picked, examined using a petrographic microscope, characterized by cathodoluminesence, extensively abraded, and then subjected to a final optical re-evaluation before analysis. Complete analytical methods are presented in Appendix 1. Results described in the following section are grouped according to the main granitic intrusions that were targeted for new geochronology.
Marshall Gulch Syenite and Monzogranite (Xsm)
Marshall Gulch Monzogranite (J01-MG1)
A sample of foliated, K-feldspar megacrystic monzogranite from the southern part of the Marshall Gulch pluton (Fig. 3 for location) yielded a single population of large (0.5 mm on side), clear, blocky zircon interpreted to be fragments of larger zircon grains that were mechanically broken during mineral separation processes. Although no larger, whole zircon grains were recovered from coarser-grained sieve fractions, individual zircons up to 5 mm in length are visible in thin section. Two fractions (Z1 and Z2) overlap concordia (Fig. 10A) and have an average 207Pb/206Pb age of 1695±2 Ma (Table 1). This age is interpreted to represent the time of crystallization of this pluton.
This sample also yielded abundant pale yellow to clear fragments of titanite. In thin section, titanite occurs in clusters parallel to the solid-state biotite foliation. Three fractions (T1–T3) define a 238U/204Pb-206Pb/204Pb isochron with an age of 1637±6 Ma (MSWD=0.86, Fig. 10B). This age is significantly younger than the crystallization age of the monzogranite and thus could represent thermal resetting, growth of titanite during a post-crystallization metamorphic event, or recrystallization and resetting of existing titanite during metamorphism. Microtextures in the surrounding rock suggest that feldspar mega-crysts deformed by brittle cracking, and interstitial quartz deformed by dislocation creep accommodated by sub-grain rotation recrystallization. These mechanisms imply that metamorphic conditions did not exceed lower- to middle-amphibolite facies during deformation of the Marshall Gulch monzogranite. Temperatures associated with these metamorphic facies are generally thought to be lower than the U-Pb closure temperature for titanite (∼700°C; Pidgeon et al., 1996; Verts et al., 1996). Thus, the titanite age is interpreted to reflect resetting, either through metamorphic growth or recrystallization, during formation of the solid-state biotite foliation (S2) across the southern margin of the pluton.
Foliated Tonalite (J01-MG2)
We collected a sample of medium-grained, foliated tonalite from the large (0.5 km) composite xenolith in the southern part of the Marshall Gulch pluton (Fig. 3). It yielded a single population of equant, light pink to clear zircon grains. Some of the grains are euhedral with well-defined faces, but most are subhedral to anhedral fragments. Although we collected this sample to determine the timing of metamorphism accompanying deformation in the Marshall Gulch domain, cathodoluminescence (CL) imaging revealed that the zircons are concentrically zoned, reflecting igneous, rather than metamorphic, growth (Fig. 10C inset). Three zircon fractions (Z1, Z3, and Z4) define a line with intercepts of 1693±2 Ma and 51±351 Ma (Fig. 10C). A fourth fraction (Z2) plots beneath this line and is presumed to contain an inherited component. The upper intercept is interpreted as the age of crystallization of the tonalite in the xenolith and overlaps within analytical uncertainty with the age of the Marshall Gulch pluton. This relationship suggests that the Marshall Gulch monzogranite and tonalite were intruded at the same time and, thus, that magmatism during this time was more compositionally diverse than previously thought. The lower intercept age likely reflects Pb loss related to more recent thermal activity.
Pegmatite Dike (J01-MG3)
This sample is part of the suite of thick (10–15 m), northwest-striking, subvertical pegmatite dikes cutting across the southern part of the Marshall Gulch pluton (Figs. 3 and 9). It yielded a single population of pink to colorless, equant zircon, nearly half of which are euhedral and display prismatic faces that are typical of igneous growth. The other half of the population consists of subhedral grains with prismatic faces and anhedral fragments interpreted to represent remnants of larger grains. Four zircon fractions (Z1–Z4) define a line with intercepts of 1407±7 Ma and 22±54 Ma (Fig. 10D). The upper intercept is interpreted to represent the time of crystallization, and the lower intercept is attributed to recent Pb loss, possibly related to Laramide tectonism or Rio Grande rifting.
Crestone Quartz Monzonite (Xqm)
Crestone Quartz Monzonite (J01-RA60)
A sample of medium-grained, weakly foliated quartz monzonite, collected 0.5 km from the mouth of Burnt Gulch east of Crestone (Fig. 5), yielded a single population of colorless to light tan, equant, prismatic, euhedral to subhedral zircon typical of an igneous origin. Four zircon fractions (Z1–Z4) define a line with intercepts of 1682±3 Ma and 23±20 Ma (Fig. 11A). The upper intercept is interpreted to represent the age of igneous crystallization, and the lower intercept likely represents more recent Pb loss, possibly related to Laramide tectonism or Rio Grande rifting.
This sample also yielded abundant pale yellow, angular titanite fragments. Three titanite fractions (T1, T3, and T4) define a line with intercepts of 1420±4 Ma and 324±170 Ma (Fig. 11A). A fourth fraction (T2) does not fall on this line but, instead, overlaps concordia with a 207Pb/206Pb age of 1489 Ma. Both of these ages are significantly younger than the age of igneous crystallization. In thin section, microtextures suggest that feldspar grains deformed by brittle cracking, with some grains displaying evidence for fine-grained recrystallization along the fractures. Quartz is characterized by irregular, serrate grain boundaries that suggest deformation occurred by dislocation creep accommodated by grain boundary migration and minor sub-grain rotation recrystallization. These microtextures indicate that temperatures probably did not exceed upper greenschist facies conditions following crystallization of the pluton, and these temperatures are well beneath accepted U-Pb closure temperatures for titanite (∼700°C; Pidgeon et al., 1996; Verts et al., 1996). The upper intercept of the younger titanite population is interpreted to represent resetting that was likely synchronous with development of the solid-state biotite and quartz foliation in the monzogranite. The lower intercept represents more recent Pb loss, perhaps related to Ancestral Rockies tectonism. The older titanite fraction (T2) might represent a separate, poorly preserved recrystallization event at ca. 1489 Ma, but this age falls within a well-documented gap in magmatic and metamorphic ages between ca. 1.63 and 1.47 Ga across the southern Rocky Mountains (Reed et al., 1993).
We collected a sample of fine-grained amphibolite from the southern contact aureole of the Crestone stock along South Crestone Creek, 0.25 km from the Willow Creek U.S. Forest Service trailhead, to constrain the age of regional metamorphism. The sample yielded a population of colorless zircon with diverse morphologies that are generally equant but include euhedral grains with prismatic faces, subhedral grains with and without prismatic faces, blocky and angular anhedral grains, and rounded anhedral grains. Cathodoluminescence (CL) imaging of approximately 25 grains revealed an equally diverse array of internal structures ranging from well-developed, concentric zonation to irregular, patchy zonation (Fig. 11B insets). Analyses of seven zircon fractions representing various morphologies and internal structures suggest that there is no obvious correlation between age, morphology, and internal structure in this sample. Instead, the fractions plot within an envelope of ages defined by an older reference line with intercepts of 1760 Ma and 350 Ma and a younger reference line with intercepts of 1435 Ma and 350 Ma (Fig. 11B). Two fractions (Z3 and Z7) plot along the older line, and the upper intercept is consistent with published ages for basement rocks in neighboring areas (1770–1750 Ma, Gunnison-Salida volcanic-plutonic terrane of Bickford and Boardman, 1984). One fraction (Z4) plots along the younger reference line, and the upper intercept is consistent with titanite ages in the adjacent quartz monzonite that are interpreted to represent resetting during solid-state deformation. The lower intercept is broadly consistent with the timing of Ancestral Rockies orogenesis in southern Colorado (Kluth and Coney, 1981; Kluth, 1986). The remaining fractions plot between these reference lines and have 207Pb/206Pb ages that range from 1647 to 1509 Ma (Table 1). Although some of the ages suggested by zircon fractions from this sample provide promising correlations, further analyses coupled with careful CL characterization are required to precisely determine and interpret these ages. These new data are interpreted to represent complex U-Pb systematics likely resulting from multiple periods of zircon growth and varying degrees of recrystallization, regrowth, and/or Pb loss during Paleoproterozoic and Mesoproterozoic magmatism and metamorphism.
Music Pass Quartz Monzonite (Yqm)
Music Pass Quartz Monzonite (J01-MP1)
We collected a sample of foliated, K-feldspar megacrystic quartz monzonite along the ridgeline 0.6 km southeast of Music Pass on the eastern side of the Sangre de Cristo Mountains to determine the minimum age of quartzite deposition. The quartz monzonite locally intrudes the quartzite and was interpreted by previous workers to be Paleoproterozoic in age (Tweto, 1979; Johnson et al., 1987). This sample yielded a simple population of colorless to light pink, equant, euhedral to subhedral, prismatic zircon interpreted to be igneous in origin. Four fractions (Z1–Z4) define a line with intercepts of 1434±2 Ma and 0±92 Ma (Fig. 12A). The upper intercept is interpreted to represent the age of crystallization of the quartz monzonite, and the lower intercept is attributed to recent Pb loss, possibly related to Laramide tectonism or Rio Grande rifting.
This sample also yielded abundant brown to dark brown, angular titanite fragments. Three titanite fractions (T1–T3) define a line with intercepts of 1412±4 Ma and 136±480 Ma (Fig. 12A). Whereas quartz monzonite from the sampled locality contains only one foliation that is interpreted to be magmatic in origin (Smagmatic), thin sections from other foliated parts of the pluton contain clusters of titanite that occur parallel to the recrystallized biotite foliation (S3). Thus, the upper intercept is interpreted to reflect resetting of titanite, likely related to localized solid-state deformation of the pluton.
Pegmatite Dike (J01-MP2)
We sampled a thin (20–30 cm) pegmatite dike from the same outcrop as the Music Pass quartz monzonite (J01-MP1). The dike sharply cuts the biotite and K-feldspar fabric present in the quartz monzonite and it was sampled to provide a minimum age on the development of this foliation. The sample yielded a simple population of colorless to light pink, equant to slightly elongate (1.5:1 aspect ratio), euhedral, prismatic zircon that is consistent with an igneous origin. A regression of four zircon fractions analyzed (Z1–Z4) yields a line with a near-zero probability of fit. Two fractions (Z2 and Z4) have 207Pb/206Pb ages (1434 Ma, Table 1) that correspond with the age of the host-rock granite (1434±2 Ma, J01-MP1), and fraction Z4 overlaps concordia at 1434 Ma (Fig. 12B). Fractions Z1 and Z3 yielded 207Pb/206Pb ages of 1439 and 1440 Ma, respectively, and are interpreted to incorporate inherited zircon. Although these results did not permit precise determination of the age of the pegmatite dike, the two younger fractions suggest that it is not significantly younger than the Music Pass pluton.
Detrital Zircon Geochronology
We collected a sample of quartzite (J02-MP4) from the eastern margin of the Music Pass pluton (Fig. 6 for location) for detrital zircon geochronology to constrain the maximum age of sedimentation and to characterize the provenance of the metasedimentary sequence. Detrital zircon grains were separated from the quartzite using standard mineral separation procedures (Appendix 1) and were analyzed using laser-ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) techniques (Appendix 2). Complete analytical results are reported in Appendix 3. A total of 101 grains analyzed yielded a range of 207Pb/206Pb ages between 2694 and 1657 Ma with an uncertainty of 1–3 percent (ca. 50 m.y.) per analysis (Fig. 13 and Table 2). Only 3 percent of the grains analyzed yielded Archean ages, and most of the ages clustered between 1750 and 1650 Ma (Figs. 12A and 12B). Linear regression of all zircon grains younger than 1.8 Ga (n=90) yielded a mean detrital age of 1704±5 Ma (MSWD=0.68; Fig. 13C), with many of these grains being concordant (Figs. 13A and 13C). These results require that the quartzite is younger than ca. 1700 Ma and suggest that it might be also be younger than 1650(±50) Ma. The relatively restricted range of detrital zircon ages agrees well with published U-Pb zircon ages for rocks in surrounding areas (Reed et al., 1993), suggesting that the bulk of the detritus was derived from local sources.
PROTEROZOIC TECTONIC HISTORY OF THE SANGRE DE CRISTO MOUNTAINS
New and existing geochronology and structural data can be used to constrain the ages of Proterozoic magmatism, deformation, and metamorphism in the Sangre de Cristo Mountains. The Paleoproterozoic tectonic history of the range involved four major events: (1) formation of basement assemblages accompanied by early penetrative deformation (D1) and metamorphism (M1); (2) intermediate to mafic magmatism; (3) post-orogenic granitic magmatism and quartzite deposition; and (4) localized northwest–southeast shortening and dextral shear (D2) accompanied by metamorphism (M2). During the Mesoproterozoic, granitic magmatism, coeval mafic diking, and one episode of deformation (D3) occurred in localized exposures throughout the range. We summarize new and existing age constraints for these events in Table 3. The Proterozoic tectonic history of the range is summarized in the following section along with new geochronology constraining the relative and absolute ages of each event.
Early Deformation, Metamorphism, and Magmatism (D1/M1/I1)
Existing geochronology and cross-cutting relationships from the southern part of the range provide the best constraints on the age of basement assemblages that form the host rock to all subsequent intrusions. Basement gneisses and amphibolites are intruded to the south by the 1750–1730-Ma Blanca Peak suite of plutonic rocks (I1; Fig. 2), such that deposition and/or crystallization of the basement assemblages must predate the oldest intrusive unit of the Blanca Peak suite (1749±4 Ma; Sabin, 1994). Metarhyolite and other metavolcanic rocks comprising basement assemblages to the north yielded U-Pb zircon ages of ca. 1728–1713 Ma (Fig. 2; Bickford et al., 1989b). These ages are younger than the Blanca Peak intrusive suite, suggesting that basement assemblages exposed along the length of the Sangre de Cristo Mountains might represent at least two different age groups (i.e., pre-1750 Ma and post-1730 Ma). Northern basement assemblages are intruded by the ca. 1700-Ma Methodist Mountain granodiorite (Taylor et al., 1975; Johnson et al., 1987).
The Blanca Peak suite of intrusive rocks (I1) provides approximate age constraints on the earliest phase of deformation and metamorphism (D1 and M1) that affected the older (i.e., central and southern) basement assemblages in the Sangre de Cristo Mountains. Published geologic mapping (Johnson and Bruce, 1991; Bruce and Johnson, 1991; Johnson et al., 1987) and our reconnaissance mapping in the Blanca Peak area (Fig. 2) indicate that the suite of 1750–1730-Ma intrusive rocks cuts the subvertical, northwest-striking S1 fabric in the southern part of the range. Thus, the earliest phases of deformation (D1) and metamorphism (M1) locally occurred after deposition of the basement assemblage protoliths but prior to emplacement of the earliest phases of the Blanca Peak intrusive suite at 1750 Ma. Northwest-striking, subvertical S1 fabrics are also well developed in younger basement assemblages in the northern part of the range (Fig. 2) and likely formed between 1730 and 1710 Ma. Although there is an age progression from south to north, similarities in the nature and orientation of S1 along the entire length of the range suggest that D1 represents several pulses of coaxial deformation within a consistent tectonic context. The relatively rapid progression from protolith formation to metamorphism and penetrative deformation to intrusion combined with the abundance of intermediate to mafic lithologies exposed throughout the range (Johnson et al., 1987; Sabin, 1994) and the juvenile isotopic character of the Blanca Peak intrusive suite (Sabin, 1994) suggest that D1 occurred in a tectonically active environment, perhaps involving the formation and accretion of island arc terranes. The age progression of basement assemblages, deformation and metamorphism, and intrusive rocks from south to north is interpreted to represent continued arc formation and accretion across the same northwest-striking boundary for ca. 40 m.y. or more.
Granitic Magmatism (I2)
The Marshall Gulch monzogranite and Crestone quartz monzonite were emplaced at 1695±2 Ma and 1682±3 Ma, respectively (Figs. 10A and 11A), in the central Sangre de Cristo Mountains (Fig. 2). Foliated tonalite exposed within the southern part of the Marshall Gulch pluton was emplaced at 1693±2 Ma (Fig. 10C). These intrusive bodies are interpreted to represent a second generation of Proterozoic plutonism (I2) that postdates D1 contractional deformation. Although D1 and M1 might have occurred as late as ca. 1710 Ma in the northern part of the range, there is a magmatic gap to the south between ca. 1730 and 1695 Ma. Furthermore, the granitoids mark the beginning of a distinct compositional shift from dominantly mafic magmatism to felsic intrusions, and they were essentially undeformed during emplacement (i.e., post-tectonic with respect to D1). In contrast, granitoids emplaced prior to ca. 1700 Ma (e.g., Methodist Mountain granodiorite) commonly contain solid-state fabrics suggesting that they were contemporaneous with at least the youngest phases of D1 shortening. Thus, I2 is interpreted to represent post-orogenic magmatism in the newly accreted pre-1700 Ma crust.
New detrital zircon ages reported above require that quartzite was deposited after ca. 1700 Ma in the Sangre de Cristo Mountains, thus indicating that quartzite is younger than the basement gneisses and amphibolite with which it is in contact. This age relationship is consistent with an unconformable depositional contact or fault contact at the base of the quartzite and explains the marked contrast in penetrative deformation across the contact. Detrital zircon ages also indicate that sediment was primarily derived from local Paleoproterozoic basement with relatively minor input from more distal (i.e., Archean) sources. The only unit directly cutting the quartzite is the 1434±2 Ma Music Pass pluton. This relationship not only requires that deposition predated Mesoproterozoic magmatism, but also that the quartzite was buried to granite emplacement depths (10–15 km) prior to ca. 1434 Ma. D2 (see below) is the only deformation event recognized locally that postdates quartzite deposition but predates emplacement of the Music Pass pluton and is, therefore, interpreted to represent the best minimum age constraint on quartzite deposition. Thus, quartzite deposition occurred locally between 1700 and 1637 Ma.
Localized Northwest–southeast Shortening, Dextral Shear, and Metamorphism (D2/M2)
D2 deformation in the Sangre de Cristo Mountains involved the development of a 1-km-thick, subvertical zone of northeast-striking fabrics (S2) in southern part of the Marshall Gulch pluton and was accompanied by metamorphism at upper greenschist- to lower amphibolite-facies conditions (M2). Metamorphic conditions were estimated based on microtextures observed in deformed granitic rocks (described above) and mineral assemblages in host-rock mafic gneisses and amphibolites (hornblende + plagioclase). The style of D2 deformation and orientation of S2 fabrics suggest that deformation involved subhorizontal, northwest–southeast shortening with a component of oblique, dextral strike-slip displacement. Titanite that occurs parallel to S2 in the deformed granite yielded an age of 1637±6 Ma (Fig. 10B), a significantly younger date than the crystallization age of the pluton (1695±2 Ma). Titanite is thought to react readily during metamorphism (Frost et al., 2000), and evidence described above suggests that temperatures during M2 were likely too low to thermally reset titanite. Based on these lines of evidence and the observation that titanite occurs in clusters that are concentrated along and parallel to the solid-state biotite foliation (S2), we believe that the titanite age represents resetting during the formation of S2, and, thus, constrains the age of D2. Deformation and deep burial of quartzite is inferred to have occurred locally during D2 because D2 is the only deformation event recognized throughout the range that postdates the maximum depositional age (ca. 1700 Ma) but predates emplacement of the cross-cutting Music Pass pluton. The limited extent of quartzite exposures and locally intense Phanerozoic brecciation make it difficult to compare the orientation of fabrics and kinematics of deformation with those mapped and described in the Marshall Gulch area, so our interpretations are somewhat speculative. However, the observed structural relationships could have been produced during a single deformation event involving medium-grade metamorphism and the development of steeply-dipping shear zones at deeper structural levels contemporaneous with sediment burial, metamorphism, and fold-and-thrust style deformation at shallow structural levels.
Granitic Magmatism and Mafic Diking (I3)
The coarse-grained Music Pass quartz monzonite represents the third episode of Proterozoic plutonism (I3) in the Sangre de Cristo Mountains. New results indicate that the pluton was emplaced at 1434±2 Ma (Fig. 12A) as a tabular, elongate body parallel with gneissic foliation (S1) in the host rocks. A west-northwest-striking magmatic fabric defined by aligned K-feldspar megacrysts parallels S1 (Fig. 6), suggesting that magmatic flow occurred parallel to the host rock fabric. The pluton is cut by a thick (30 m), subvertical mafic dike that strikes northwest and contains no internal fabric. We prefer to interpret mafic diking as broadly contemporaneous with granitic magmatism, based on cross-cutting relationships between the dike and the Music Pass pluton and evidence for ca. 1.4-Ga mafic diking elsewhere in southern Colorado (Tweto, 1987). However, an attempt to date the dike was unsuccessful.
Localized Northwest–Southeast Shortening and Metamorphism (D3/M3)
After crystallization of the Music Pass pluton, numerous, discrete zones of solid-state, and locally mylonitic, fabrics formed during a third deformation event (D3). Subvertical zones of solid-state deformation are up to 1 m thick, strike northeast–southwest, and the boundary between deformed and undeformed granite is gradational over tens of centimeters. Local asymmetric fabrics include composite foliation (S-C fabric), and pegmatite dikes are offset by up to a few meters, but the presence of both dextral and sinistral kinematic indicators suggest that D3 involved local conjugate shearing during subhorizontal, northwest-directed shortening. The southern margin of the 1682±2-Ma Crestone quartz monzonite contains similar, coaxial, solid-state fabrics, and host-rock amphibolite and gneiss were broadly folded (100 m wavelength) against the pluton, producing moderately northeast-plunging axes. Age constraints from titanite that is interpreted to have dynamically recrystallized during the formation of S3 in the two granitic bodies constrain D3 and M3 to between 1420 and 1412 Ma.
Pegmatite Diking (I4)
The youngest recognized Proterozoic intrusive phase in the Sangre de Cristo Mountains is the swarm of thick (10–15 m) post-tectonic pegmatite dikes that cut the southern part of the Marshall Gulch pluton. These dikes are subvertical, strike northwest–southeast, and sharply cut all of the fabrics present in the granite and host gneisses. New results indicate that they were emplaced at 1407±6 Ma (Fig. 10D). A map-scale intrusion associated with these dikes has not been identified.
REGIONAL (SOUTHWESTERN UNITED STATES) CORRELATIONS
The tectonic history described above corresponds with published models for the regional tectonic evolution of the southwestern United States during the Proterozoic. In general, basement assemblages of the Sangre de Cristo Mountains can be texturally, compositionally, and geographically correlated with two successions of metavolcanic and metasedimentary rocks exposed along a belt extending across southwestern Colorado. Ca. 1750-Ma and older gneisses in the central and southern Sangre de Cristo Mountains are correlative with the Dubois succession, a bimodal suite of metavolcanic rocks that formed between 1770 and 1760 Ma (Condie and Nuter, 1981; Bickford and Boardman, 1984; Knoper and Condie, 1988). Younger metavolcanic rocks from the northern part of the range are correlated with the 1745–1730-Ma Cochetopa succession (Bickford and Boardman, 1984). This younger basement assemblage includes felsic metavolcanic rocks and volcaniclastic sediments with interlayered amphibolites (Bickford and Boardman, 1984; Knoper and Condie, 1988). The Dubois and Cochetopa successions have long been interpreted as having formed in arc settings based on their geochemical characteristics (e.g., Condie, 1986; Boardman and Condie, 1986). However, Hill and Bickford (2001) and Hill (2004) reported detrital zircon ages and Nd isotopic data that suggest the metavolcanic and metasedimentary successions might have been derived from and formed on top of pre-existing older crust. Although our results do not help to resolve these contrasting interpretations, we prefer the arc interpretation based on the juvenile isotopic characteristics of the Blanca Peak intrusive suite (Sabin, 1994), the abundance of intermediate to mafic rock types in the southern part of the range (Johnson et al., 1987), and widespread, penetrative crustal shortening (D1) that closely followed the formation of the basement assemblages.
The age and orientation of D1 and M1 in the Sangre de Cristo Mountains are consistent with early deformational and metamorphic fabrics that have been documented across southern Colorado and are interpreted to represent deformation associated with the amalgamation of the Yavapai province. The Iris syncline, south of Gunnison, Colorado, is a kilometer-scale F1 fold characterized by a northwest-trending axial surface and a steep, southeast-plunging fold axis (Afifi, 1981). The average orientation of S1 surrounding the fold is subvertical and striking northwest–southeast with an associated mineral lineation that plunges moderately to steeply northwest. Formation of the Iris syncline was synchronous with high-temperature metamorphism (M1; Afifi, 1981) and is constrained by cross-cutting igneous bodies to have formed between 1740 and 1725 Ma (Bickford and Boardman, 1984). Jessup et al. (2005) documented early subvertical, penetrative northwest-striking fabrics (S1) and northwest-trending folds (F1) in the Black Canyon of the Gunnison. Metamorphic zircon obtained from an amphibolite within the Black Canyon Succession of metasedimentary and metavolcanic gneisses yielded an age of ca. 1742 Ma and is interpreted to represent metamorphism (M1) accompanying development of these early fabrics (Jessup et al., 2006). These data suggest that an important regional episode of northeast–southwest shortening and metamorphism occurred prior to ca. 1710 Ma in rocks exposed throughout southern Colorado and affected rocks of the Dubois and Cochetopa successions. The prolonged deformation and metamorphism during this time are consistent with tectonic models for the progressive southward growth of Laurentia during the Yavapai orogeny by the formation and accretion of juvenile arc terranes, and the consistent orientation of D1 fabrics suggests that accretion likely occurred along long-lived northwest-striking tectonic boundaries (Jessup et al., 2005).
Post-D1 granitoids, ranging in age from 1695 to 1682 Ma, are correlative with a voluminous suite of granitoids emplaced between 1705 and 1663 Ma throughout southern Colorado (Arkansas River Gorge suite of Anderson and Cullers, 1999; Bickford et al., 1989a). The oldest of these plutons contain the strongest penetrative fabrics of the intrusive suite and were emplaced during the waning stages of deformation and metamorphism related to the Yavapai orogeny (i.e., D1). Younger granites of this suite are either weakly deformed or undeformed, and their geochemistry has been interpreted to reflect an increasing crustal component through time (Anderson and Cullers, 1999). The age of the youngest of these granites, the 1663±4-Ma Garell Peak pluton (Bickford et al, 1989a), overlaps with the onset of Mazatzal-age deformation in New Mexico (1664–1654 Ma; Bauer and Williams, 1994). Thus, we interpret this suite of granites to represent a nearly continuous inter-orogenic magmatic episode related to the late- to post-orogenic evolution of the newly accreted Yavapai-age crust. Magmatism persisted until the earliest stages of Mazatzal-age accretion to the south but effectively shut down in Colorado after ca. 1663 Ma as evidenced by a well-documented magmatic gap between ca. 1660 and 1470 Ma (Reed et al., 1993).
Ages of cross-cutting intrusive rocks described above only require that quartzite deposition occurred prior to 1434 Ma locally, but inferred local and regional constraints suggest that quartzite deposition likely coincided with 1.70–1.66-Ga inter-orogenic granitic magmatism. U-Pb zircon ages from coarse-grained granitoids underlying quartzite exposed 70 km north of the Sangre de Cristo Mountains at Blue Ridge, Colorado, require that quartzite was deposited unconformably on exhumed ca. 1700-Ma granitoids (Jones, 2005). Published ages (U-Pb zircon) from rhyolite underlying quartzites in New Mexico and Arizona indicate that the onset of widespread sedimentation closely followed culmination of the Yavapai orogeny at ca. 1700 Ma (Bauer and Williams, 1989; Cox et al., 2002). These results, combined with detrital zircon ages from this study, provide a maximum depositional age of ca. 1700 Ma. Quartzite exposed in the Sangre de Cristo Mountains is inferred to have been deformed during D2 locally, thus constraining the age of deposition to between 1700 and 1637 Ma. This interpretation defines a time window of ca. 60 m.y. for regional sedimentation during the period between Yavapai- and Mazatzal-age deformation. Williams (1991) described north-directed kilometer-scale folding and thrusting of the Ortega Quartzite and underlying assemblages in the Tusas Mountains, New Mexico, and the structural style and kinematics of deformation are consistent with documented Mazatzal-age deformation throughout the southwestern United States (e.g., Doe and Karlstrom, 1991).
Localized northwest–southeast shortening (D2) and metamorphism (M2) at 1637±6 Ma in the Sangre de Cristo Mountains are consistent with similar-age deformation and metamorphism documented throughout the region. Shaw et al. (2001) reported a monazite age of 1637±13 Ma from rocks with northeast-striking, subvertical fabrics within the Homestake shear zone of central Colorado. This monazite and another dated at 1658±5 Ma are interpreted to have grown broadly synchronously with a second phase of deformation (D2), locally involving the development of northeast-trending, subvertical foliation domains during deformation dominated by northwest–southeast contraction (Shaw et al., 2001). To the south in New Mexico, north-directed crustal shortening occurred across a series of northeast-striking structures after deposition of 1664±3 Ma supracrustal rocks and prior to 1654±1-Ma post-kinematic plutonism (Bauer and Williams, 1994). These observations and regional correlations suggest that an episode of northwest–southeast crustal shortening occurred throughout the region between 1660 and 1630 Ma, generally called the Mazatzal orogeny. D2 in the Sangre de Cristo Mountains is correlated with this orogenic event.
The 1434±2-Ma Music Pass pluton is correlative with a widespread suite of coarse-grained, A-type granites emplaced across the southwestern United States between 1440 and 1430 Ma (Reed et al., 1993). Granite plutons emplaced during this time are commonly deformed (e.g., Oak Creek pluton; Bickford et al., 1989a), and the Music Pass pluton contains a well developed northeast-striking, solid-state foliation (S3) and localized meter-thick high-strain zones. D3 deformation and metamorphism (M3) occurred locally between 1420 and 1412 Ma and involved subhorizontal, northwest–southeast shortening. D3 was followed by a second pulse of magmatism at 1407±6 Ma during which a suite of thick (∼15 m), northwest-striking pegmatite dikes was emplaced in the central Sangre de Cristo Mountains. A similar swarm of west- to west-northwest-striking pegmatite dikes was intruded into rocks exposed in the Black Canyon of the Gunnison at 1413±2 Ma (Jessup et al., 2006). Dike emplacement to the west occurred during the waning stages of dextral, transpressive deformation and was accompanied by amphibolite-facies metamorphism recorded by titanite growth and/or recrystallization in the 1434±2-Ma Vernal Mesa monzonite (Jessup et al., 2005).
REGIONAL (SOUTHWESTERN UNITED STATES) TECTONIC IMPLICATIONS
The new constraints on the Proterozoic tectonic history of the Sangre de Cristo Mountains have important implications for the Proterozoic tectonic evolution of southern Laurentia. Early (1750–1710 Ma), penetrative deformation (D1) affected rocks throughout the entire range (and surrounding region) and resulted in fabrics that exerted structural control during subsequent Proterozoic to Phanerozoic tectonism and magmatism, whereas younger deformation (D2–D3) was both temporally and spatially localized. These contrasting styles of structural behavior suggest that the dominant architecture of the juvenile crust was formed early in its history. Penetrative, subvertical, northwest-striking D1 fabrics preserved in exposures throughout the Sangre de Cristo Mountains and across large regions of southern Colorado suggest that Paleoproterozoic crustal growth was accompanied by widespread, northeast–southwest shortening. These early fabrics are perpendicular to the regionally pervasive northeast-striking tectonic grain that developed during the final phases of Yavapai-age accretion (Karlstrom and Humphreys, 1998). It is not clear how northwest-striking fabrics were formed, but there are at least two competing models. Jessup et al. (2005) suggested that early arc formation occurred along a complex, arcuate subduction system analogous to the modern-day Banda Sea. Early fabrics that were formed during northeast–southwest shortening within these arcs were rotated, tightened, and transposed into a northeast orientation or were overprinted by northwest-directed deformation during final assembly of the arc collage to southern Laurentia. Duebendorfer et al. (2001) proposed a model involving complex collisional histories, perhaps including collision and suturing of the Yavapai and Mojave provinces, that occurred outboard (south) of the Laurentian margin before final northward accretion.
Following early deformation, metamorphism, and magmatism related to formation and accretion of the Yavapai province, rocks exposed in the Sangre de Cristo Mountains behaved as a coherent tectonic block and record strongly localized, albeit temporally and kinematically compatible, responses to younger tectonic events. Northeast-striking fabrics developed regionally during the final collisional phases of the Yavapai orogeny are not recognized in exposures of the central and southern Sangre de Cristo Mountains. Wortman (1990) described evidence for the transition from northwest- to northeast-striking fabrics in exposures to the west near Gunnison, Colorado. Deformation of rocks of the Cochetopa succession involved refolding of northwest-trending fold axes about northeast-trending axes, and cross-cutting relationships indicate that northwest-directed shortening occurred between ca. 1741 and 1713 Ma (Wortman, 1990). The presence of northeast-striking, steeply-dipping foliations in ca. 1700-Ma intrusive rocks exposed in the northern Sangre de Cristo Mountains (Johnson et al., 1987) suggest that there might have been local northwest-directed shortening during the same time, but the age and extent of these fabrics are not well established. The earliest northeast-striking fabrics recognized during this study formed at 1637 Ma during deformation related to accretion of the Mazatzal province to the south. The lack of Yavapai-age northeast-striking fabrics in rocks exposed locally suggests that: (1) northeast-striking fabrics were formed much earlier but were reactivated and/or overprinted by subsequent coaxial deformation, (2) deformation recording the final phase(s) of northward accretion of the province to southern Laurentia was concentrated elsewhere to the north and/or south of current exposures, or (3) large parts of the southern Yavapai province were accreted across northwest-striking boundaries only to be modified by subsequent northwest-directed shortening during younger Mazatzal-age and/or Mesoproterozoic tectonism.
Direct and inferred local and regional constraints suggest that quartzite was deposited locally between 1700 and 1637 Ma during an inter-orogenic period between Yavapai- and Mazatzal-age deformation. The regional extent of quartzite exposures that are likely correlative requires that numerous depositional basins were developed during this time to accommodate the influx of locally derived sediment. The observation that quartzite is locally deposited on 1710–1700-Ma granitoids requires tens of kilometers of exhumation of mid-crustal rocks in the 5–10 m.y. preceding deposition (Willams et al., 2003; Jones, 2005). The inter-orogenic interval coincides with a prolonged episode of voluminous post-orogenic granitic magmatism in southern Colorado between 1695 and 1666 Ma (this study; Bickford et al., 1989a) that included two newly dated granitoids exposed in the Sangre de Cristo Mountains (described above). These relationships suggest that regional crustal extension might have occurred during this time, exhuming recently formed mid-crustal rocks and accommodating voluminous magmatism at deeper crustal levels and widespread sedimentation at the surface of the crust. The temporal overlap between quartzite deposition and granitic magmatism suggests that these events record different yet contemporaneous processes that were widespread throughout the southern part of the newly accreted Yavapai province and might represent collapse and stabilization of the orogen.
Newly recognized Mesoproterozoic (ca. 1.4 Ga) granitic and mafic magmatism and deformation in the Sangre de Cristo Mountains provide new insights regarding the timing and style of ca. 1.4-Ga deformation. D3 deformation occurred locally between 1420 and 1412 Ma and affected both the 1434±2-Ma Music Pass pluton and the 1682±2-Ma Crestone stock exposed 10 km to the north. Anorogenic models for widespread A-type Mesoproterozoic granitic magmatism suggest that deformation during this time simply represents structural responses to ambient intraplate stresses that are only expressed in the thermally softened aureoles of coeval intrusions (Ferguson et al., 2004). However, D3 deformation was not spatially restricted to the aureole of the Music Pass pluton. The orientation of S3 is consistent among exposures throughout the range and with other northeast-striking, subvertical ca. 1.4-Ga fabrics documented throughout the southwestern United States (e.g., Nyman et al., 1994; Shaw et al., 2001; McCoy et al., 2005). These fabrics are all compatible with widespread northwest-directed shortening at ca. 1.4 Ga and suggest that deformation during this time likely represents an organized and kinematically consistent intracontinental response to a regional stress field (Nyman et al., 1994). Locally, the orientation of pre-existing, host-rock fabrics (S1) at high angles to compressional stresses likely facilitated the emplacement and controlled the geometry of ca. 1.4-Ga intrusions (Nyman and Karlstrom, 1997).
During this same time period (ca. 1.48–1.37 Ga), granitic magmatism was widespread throughout much of southern Laurentia (Van Schmus et al., 1996). A voluminous suite of rhyolitic volcanic rocks and their epizonal plutonic equivalents dominate basement assemblages in the subsurface from Texas to Ohio and in localized surface exposures in Oklahoma and Missouri (Granite-Rhyolite province, Fig. 1). These rocks are largely undeformed, and yet they are compositionally and geochemically similar to contemporaneous granites exposed in the Rocky Mountains and southwestern United States that are locally strongly deformed. Siddoway et al. (2000) suggested that the depth of pluton emplacement at ca. 1.4 Ga influenced structural development and, thus, the current levels of exposure bear on the interpretation of dynamic versus anorogenic context for 1.4-Ga magmatism. We contend that this observation helps to explain the apparent contrast in deformation between mid-crustal exposures in the southern Rocky Mountains and shallower levels of exposure (or sampling in the subsurface) to the east. Additionally, we believe that continued field study combined with precise geochronology is necessary to further evaluate the regional temporal and kinematic organization of deformation and magmatism during an otherwise protracted ca. 1.4-Ga tectonothermal event. Increased temporal resolution will allow for more accurate regional correlations and, thus, will permit more rigorous evaluation of tectonic models for widespread Mesoproterozoic granitic magmatism throughout southern Laurentia.
Appendix 1. Isotope-dilution Thermal-ionization Mass Spectrometry (ID-TIMS) Analytical Methods
All sample processing was done at The University of Texas at Austin. 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 by using a binocular reflected-light microscope, a transmitted-light petrographic microscope (with condenser lens inserted to minimize edge refraction), and a scanning cathodoluminescence (CL) imaging system on a JEOL T330A scanning electron microscope (SEM).
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. All mineral fractions analyzed were strongly abraded (Krogh, 1982), were subsequently reevaluated optically, and then were washed successively in distilled 4N nitric acid, water, and acetone. They were loaded dry in to Teflon capsules with a mixed 205Pb-235U isotopic tracer solution and dissolved with appropriate acids (HF and HNO3 for silicates, 6.2N HCl for monazite). Chemical separation of U and Pb from zircon using minicolumns (0.044 mL resin volume; after Krogh, 1973) resulted in a total Pb procedural blank of 1–2 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 by 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 unpublished algorithms by L. Heaman (University of Alberta, Edmonton). Results are reported in Table 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 2s errors.
Appendix 2. Laser-ablation Inductively-coupled Plasma Mass Spectrometry (LA-ICP-MS) Analytical Methods
Zircon was extracted and concentrated from field samples using mineral separation procedures described in Appendix 1, and individual zircon grains were hand picked to ensure that all different sizes, colors, and morphologies were included in the detrital population to be analyzed. Picked zircons were placed on double-sided tape inside an aluminum ring (i.d. = 1 in) 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. He 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 ca. −450 to −500 V (soft extract voltage is ca. 0 to +100 V). 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 jumped to different values during the analysis, presumably due to internal age zonation. The data were culled to include only the longest coherent segment of the 20-second analysis. Mass fractionation and elemental bias between U and Pb were corrected by calibration to a well characterized (ID-TIMS) in-house standard, S97-19, that was analyzed both before and after the unknown sample.
Final generation of values for the measured ratios from the analysis was conducted offline using an Excel spreadsheet that allowed for the graphical and statistical evaluation of the data. The values reported (Appendix 3) constitute the “measured ratios” for the analysis after offline corrections for mass fractionation, elemental bias corrections, and blank and common Pb corrections. 207Pb/206Pb and 206Pb/204Pb ratios are averages of the accepted analyses, but 206Pb/238U values are typically observed to change with increasing depth/time during the course of an analysis. Therefore, 206Pb/238U ratios are determined by regression of the measured values back to the beginning of the analysis, and the intercept is taken as the experimentally determined value.
This research was funded by National Science Foundation grant EAR 0003528 awarded to J. Connelly and the Department of Geological Sciences and Geology Foundation at The University of Texas at Austin. Field assistance was provided by Adam Krawiec, and analytical assistance was provided by Kathy Manser, Todd Housch, and John Lansdown. Thorough and thoughtful reviews by Pat Bickford, Mike Williams, and Sarah Garlick greatly improved the organization, arguments, and clarity of this manuscript.
↵* Present Address: Geology Discipline, University of Minnesota Morris, Morris, MN 56267, U.S.A.
- Received June 6, 2006.
- Revision received October 5, 2006.
- Accepted November 1, 2006.