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| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
1 Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA
2 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
3 Department of Geosciences, University of Massachusetts, 611 North Pleasant Street, Amherst, Massachusetts 01003, USA
4 Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, Colorado 80401, USA
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FOOTNOTES |
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Present address: Department of Earth Sciences, University of Arkansas at Little Rock, Little Rock, Arkansas 72204, USA; e-mail: jvjones{at}ualr.edu 
GSA Data Repository Item 2008213, containing detailed rock descriptions and complete U-Pb isotopic data, is available at www.geosociety.org/pubs/ft2008.htm. Requests may also be sent to editing{at}geosociety.org.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGIC BACKGROUNDBLUE RIDGE, COLORADO, QUARTZITE...QUARTZITE DETRITAL ZIRCON...DISCUSSIONTECTONIC IMPLICATIONSTHE QUARTZITE COMPOSITIONAL...CONCLUSIONSREFERENCES CITED |
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Our model for syntectonic deposition involves extensional basin development followed by thrust closure, possibly due to opening and closing of slab rollback basins related to outboard subduction. The first-cycle origin of orthoquartzites near the end of the arc collisions of the Yavapai orogeny seems to contrast sharply with their extreme compositional maturity. This can be explained in terms of protracted, extreme diagenesis and/or special environmental influences that enhanced chemical weathering but were unique to the transitional atmosphere and ocean chemistry of the Proterozoic. Similarities among quartzites exposed throughout the southwestern United States and along the Laurentian margin suggest that they represent a widespread regional, and perhaps global, episode of sedimentation involving a distinctive syntectonic setting and unique climatic conditions, a combination that might make these units a signature lithology for Paleoproterozoic time.
Key Words: quartzite • Proterozoic • Colorado • New Mexico • U-Pb geochronology • detrital zircon • laser-ablation • ICP-MS
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGIC BACKGROUNDBLUE RIDGE, COLORADO, QUARTZITE...QUARTZITE DETRITAL ZIRCON...DISCUSSIONTECTONIC IMPLICATIONSTHE QUARTZITE COMPOSITIONAL...CONCLUSIONSREFERENCES CITED |
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We also discuss Paleoproterozoic orthoquartzites as potential secular markers that will be useful for understanding Proterozoic tectonics and climatic conditions. Orthoquartzite successions may provide an important tool for correlating Proterozoic rocks around the world and, thus, might be used to test proposed contiguous plate margins of ancient supercontinents (Hoffman, 1988; Karlstrom et al., 2001; Rogers, 1996; Sears and Price, 2003). The contrast between the compositional maturity of quartzites and their juvenile detrital character seemingly requires protracted diagenesis and/or extreme chemical weathering prior to and during deposition. The Proterozoic represents a critical time in the evolution of Earth's ocean-atmosphere system as it transitioned from a CO2-H2S–dominated system to one of excess oxygen (Canfield et al., 2000; Canfield and Teske, 1996), ultimately setting the stage for the explosion of life at the end of the Precambrian. This transition is commonly thought to have resulted in the disappearance of banded iron formation and the complementary appearance of red sandstones and shales through the late Paleoproterozoic to early Mesoproterozoic (Holland, 1984; Canfield, 1998). The appearance of thick Paleoproterozoic quartzites in other parts of southern Laurentia during this transition is attributed to distinctive Proterozoic weathering conditions (e.g., Medaris et al., 2003) involving some combination of (1) high CO2 and CH4 and low pH coupled with the absence of stabilizing plants, (2) presence of microbial mats (Dott, 2003), (3) depositional conditions involving extreme wind and water abrasion (Dott, 2003), and/or (4) diagenetic conditions involving the breakdown and removal of labile materials (Cox et al., 2002a). We briefly examine possible correlations between Paleoproterozoic quartzite successions throughout southern Laurentia in light of new age constraints presented herein and suggest that orthoquartzite deposition may be a widespread but temporally restricted phenomenon and, thus, a signature event in Earth history.
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GEOLOGIC BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGIC BACKGROUNDBLUE RIDGE, COLORADO, QUARTZITE...QUARTZITE DETRITAL ZIRCON...DISCUSSIONTECTONIC IMPLICATIONSTHE QUARTZITE COMPOSITIONAL...CONCLUSIONSREFERENCES CITED |
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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 they were accreted to the southern Yavapai Province during the 1.66–1.60 Ga Mazatzal orogeny (Silver, 1965; Karlstrom and Bowring, 1988; Luther, 2006; 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). After an ~150 m.y. tectonic lull, renewed southward growth of Laurentia is inferred to have occurred during the Mesoproterozoic. Reactivation of the Paleoproterozoic lithosphere accompanied a widespread regional pulse of A-type granitic magmatism between 1.45 and 1.36 Ga (Reed et al., 1993), possibly in response to renewed crustal accretion along a distal southern margin (Nyman et al., 1994; Whitmeyer and Karlstrom, 2007). These events were all part of a prolonged, ~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).
During the southward growth of Laurentia, thick (1–2 km) successions of quartz sandstone were deposited within the orogenic belts. Some of the best examples of these deposits are exposed in the Lake Superior region of the northern United States (Baraboo interval; Dott, 1983) and throughout the southwestern United States (Fig. 1). Quartzite successions occur in the Yavapai, Mazatzal, and Mojave Provinces (Fig. 1), and exposures include extensively exposed units like the Ortega Formation (New Mexico), Mazatzal quartzite (Arizona), and Uncompahgre Formation (Colorado) as well as numerous smaller, more localized exposures having similar lithologies and outcrop characteristics. Existing age constraints suggest that sedimentation occurred generally after the Yavapai orogeny and before the Mazatzal orogeny (Jones and Connelly, 2006; Jessup et al., 2006). Quartzite successions are commonly nearly pure (>95%) quartz with minor muscovite, Al-silicates, hematite, zircon, and monazite. Primary sedimentary structures are locally well preserved, including common cross-stratification. Depositional facies are similar from bottom to top and region to region and indicate shallow-marine (<10 m water depth) or fluvial environments (Harris and Eriksson, 1990; Soegaard and Eriksson, 1985, 1989; Trevena, 1979). Across much of the southwestern United States, quartzites directly overlie thick successions of voluminous, high-silica rhyolite (Fig. 1). The contact between rhyolite and quartzite is generally inter-layered to gradational and is commonly marked by a distinctive, Mn-rich contact interval (Bauer and Williams, 1989). In quartzite successions exposed in New Mexico and Arizona, U-Pb ages of interlayered rhyolite locally constrain the onset of quartzite deposition at 1701 ± 2 Ma (e.g., Cox et al., 2002b). In southern Colorado, however, rhyolite is notably absent. Quartzites are typically underlain by pebble to cobble conglomerates up to a few meters thick, and these conglomerates contain a variety of clast compositions, including vein quartz, quartzite, jasper, chert, and, locally, granite (Barker, 1969; Reuss, 1974). Although clast compositions do not always reflect the local makeup of underlying basement assemblages, the uppermost part of the basement is locally marked by a zone of deep weathering (regolith), which is interpreted to have developed prior to the onset of quartzite sedimentation.
Basement rock assemblages underlying quartzite-rhyolite successions regionally are typically characterized by metamorphosed mafic volcanic rocks and volcanogenic marine metasedimentary rocks (Bauer and Williams, 1989; Jessup et al., 2005). Examples of these older (1.80–1.72 Ga; Condie, 1982) assemblages include the Moppin Complex in northern New Mexico (Bauer and Williams, 1989) and the Dubois and Cochetopa successions in southern Colorado (Bickford and Boardman, 1984). Basement rocks commonly contain evidence for multiple episodes of deformation and/or metamorphism that are not recognized in the overlying quartzite (Gibson and Harris, 1992). Thus, the contact between basement assemblages and quartzite successions has been variably interpreted as an unconformable depositional contact, a sheared unconformity, or a fault contact.
The quartzite-rhyolite successions and underlying basement assemblages have been deformed by folding and thrust imbrication. Across Colorado, quartzite exposures presently occur as tight, upright synclinal "keels," which are interpreted to be the roots of larger, now-eroded folds (Fig. 2; Reuss, 1974; Wells et al., 1964). In the Needle Mountains of southwestern Colorado, the Tusas Mountains of northern New Mexico, and the Mazatzal Mountains of Arizona, 1–2-km-thick sections of quartzite define tight to open, large-wavelength (kilometer scale) folds consistent with some fold-and-thrust-belt geometries (Harris, 1990; Williams, 1991; Williams et al., 1999). The quartzites were buried to depths up to 10–15 km and subsequently resided at these mid-crustal depths until at least ca. 1.4 Ga, when they were widely intruded by coarse-grained granitic plutons. Whereas published absolute age constraints from quartzite successions exposed in the Yavapai and Mazatzal Provinces indicate that sedimentation occurred during the Paleoproterozoic (Bauer and Williams, 1989; Cox et al., 2002b; Shastri, 1993), the minimum depositional age is poorly constrained because crosscutting igneous bodies generally yield ages of ca. 1.4 Ga and, hence, do not provide sufficient restrictions on the age of deposition or of deformation (Harris et al., 1987).
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BLUE RIDGE, COLORADO, QUARTZITE SUCCESSION |
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TOPABSTRACTINTRODUCTIONGEOLOGIC BACKGROUNDBLUE RIDGE, COLORADO, QUARTZITE...QUARTZITE DETRITAL ZIRCON...DISCUSSIONTECTONIC IMPLICATIONSTHE QUARTZITE COMPOSITIONAL...CONCLUSIONSREFERENCES CITED |
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U-Pb Geochronology
Four samples were collected for new U-Pb geochronology to constrain the age of quartzite deposition and deformation. Samples included foliated granodiorite exposed north of the quartzite succession, strongly deformed granodiorite from the southern, sheared contact, and two pegmatite dikes that crosscut the folded succession and sheared granodiorite (see Fig. 2A for locations). Basal conglomerate and quartzite located 10 m higher in the stratigraphic section were also sampled as part of the detrital zircon study described here. U-Pb isotopic data and sample location coordinates are available from the GSA Data Repository (Table DR1, see footnote 1), and concordia diagrams are presented in Figure 3. Analytical methods followed those of Jones and Connelly (2006). Zircon fractions were handpicked, examined using a petrographic microscope, characterized by cathodoluminescence, extensively abraded (Krogh, 1982), and then subjected to a final optical reevaluation before analysis.
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Sheared Granodiorite (K00-BR-25)
Strongly foliated to locally mylonitic, coarse-grained granodiorite was sampled from the southern contact of the Gooseberry Gulch syncline (Fig. 2A). Although the nature of the original contact between the quartzite succession and granodiorite is ambiguous at this location due to shear-zone development, this sample was collected to correlate the age of basement granitoids on both sides of the folded quartzite succession. This sample yielded a single population of colorless to light tan, equant, euhedral to subhedral prismatic grains typical of an igneous origin. The four fractions analyzed all plot near concordia and have 207Pb/206Pb ages ranging from 1708 to 1693 Ma (Table DR1, see footnote 1). Three fractions (Z2–Z4) plot along a reference line with intercepts of 1698 Ma and 0 Ma (Fig. 3B). The upper intercept of this line corresponds to the average 207Pb/206Pb age of 1698 ± 4 Ma for the three fractions and is interpreted to represent the crystallization age of the granodiorite. The fourth fraction (Z1) plots slightly below the reference line and is interpreted to contain inherited zircon. The age of this sample overlaps within error the age of granodiorite north of Blue Ridge (sample J01-BR3) and with published ages for the Twin Mountain and Crampton Mountain batholiths, suggesting that the southern contact of the quartzite succession might have originally been an unconformable depositional contact.
Crosscutting Pegmatite Dikes (K00-BR-26 and J03-BR4)
Two pegmatite dikes that crosscut the southern limb of the folded quartzite succession and the sheared granodiorite were sampled to constrain the age of deformation locally. Sample K00-BR-26 was collected from of a suite of sub-planar, steeply dipping to vertical intrusions that are commonly thin (<0.5 m) but can be, locally, up to a few meters thick. Sample J03-BR4 was collected from a pink to red, thin (30 cm) pegmatite dike that cuts the sheared granodiorite and quartzite fabric but also locally deflects foliation in quartzose schist. Both samples yielded a single population of light pink to colorless, euhedral to subhedral prismatic zircon typical of an igneous origin. Some zircon fractions have 207Pb/206Pb ages of ca. 1700 Ma or older (Fig. 3C; Table DR1 [see footnote 1]), which are attributed to inheritance from host-rock granodiorite and/or quartzite. Two fractions, one of which was concordant, have 207Pb/206Pb ages of ca. 1436 Ma (Fig. 3C; Table DR1 [see footnote 1]). This age is interpreted as the crystallization age of the pegmatites, indicating that they formed coeval with a well-documented regional suite of granites and related pegmatite dikes emplaced ca. 1440–1430 Ma (Reed et al., 1993; Karlstrom et al., 2004).
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QUARTZITE DETRITAL ZIRCON GEOCHRONOLOGY |
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TOPABSTRACTINTRODUCTIONGEOLOGIC BACKGROUNDBLUE RIDGE, COLORADO, QUARTZITE...QUARTZITE DETRITAL ZIRCON...DISCUSSIONTECTONIC IMPLICATIONSTHE QUARTZITE COMPOSITIONAL...CONCLUSIONSREFERENCES CITED |
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Sample preparation techniques and analytical methods followed those described in Jones and Connelly (2006). Zircon from the least magnetic fraction was handpicked to include grain populations representing each of the various morphologies, sizes, and colors that were recognized. Approximately 100 grains from each sample were analyzed, and age distribution plots and U-Pb concordia diagrams are shown in Figure 4. Detrital zircon ages are summarized in Table 1, and complete data tables are available in the GSA Data Repository (Table DR3, see footnote 1). The estimated uncertainty (i.e., external reproducibility) of each individual analysis was ~50 m.y. (~3%). Peak and minimum detrital ages were calculated using only grains that were determined to be <3% discordant. The minimum age reported for each sample (Table 1) represents the weighted average of the youngest cluster of concordant grains with overlapping ages.
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The detrital zircon populations do not appear to change significantly in both cases where samples were collected from different stratigraphic horizons within the same succession. In two samples from exposures along Blue Ridge, Colorado, basal conglomerate is characterized by a relatively restricted Paleoproterozoic detrital age population (1815–1681 Ma; Fig. 4O), whereas quartzite 10 m upsection is characterized by a broader age range, including Archean material (3079–1696 Ma; Fig. 4M). The peak detrital zircon ages for the basal conglomerate and quartzite were 1722 Ma and 1734 Ma, respectively (Table 1). In four samples from the Needle Mountains, Colorado, there is noticeable change in the detrital zircon population from the Vallecito Conglomerate to the basal conglomerate and Q4 quartzite of the Uncompahgre Formation (Fig. 5). Samples from higher in the stratigraphic section yielded a larger range of detrital zircon ages, and the upper quartzite (Q4) contains a distinct population of ca. 1880 Ma detrital grains not recognized in the other samples. All of the samples are dominated by a single population of Paleoproterozoic-aged detritus, though, and the peak ages do not vary significantly.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGIC BACKGROUNDBLUE RIDGE, COLORADO, QUARTZITE...QUARTZITE DETRITAL ZIRCON...DISCUSSIONTECTONIC IMPLICATIONSTHE QUARTZITE COMPOSITIONAL...CONCLUSIONSREFERENCES CITED |
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Quartzite Depositional Age
New geochronology from exposures along Blue Ridge, Colorado, reveals that granitoid basement underlying the metasedimentary succession at this locality crystallized at 1706 ± 5 Ma (Fig. 3). The presence of granodiorite "regolith" just beneath the metasedimentary succession is interpreted to represent rapid unroofing and weathering of the plutonic rocks prior to the onset of sedimentation. The gradational transition from regolith to phyllitic conglomerate that defines the base of the succession is interpreted to represent an unconformable depositional contact. Thus, 1706 ± 5 Ma is interpreted to represent a robust maximum age for the deposition of quartzite locally. The youngest detrital zircon from basal conglomerate and quartzite higher in the succession exposed along Blue Ridge yielded averaged ages of 1701 ± 16 Ma and 1707 ± 18 Ma, respectively (Table 1), which also provide maximum ages for deposition. Our best estimate for the maximum quartzite depositional age based on detrital zircon data is 1701 ± 3 Ma, the minimum age of detrital zircon from quartzite exposed in Phantom Canyon (see previous). This age is supported by detrital zircon data from exposures throughout the region, and slightly older minimum ages for a few samples are interpreted to represent the youngest material present in the source area rather than an older depositional age for the quartzite successions.
The minimum age of quartzite deposition has been more difficult to constrain. In many areas, the only firm minimum ages are provided by crosscutting 1.4 Ga granites. At Blue Ridge, for example, quartzites are intruded by multiple ca. 1436 Ma crosscutting pegmatite dikes (Fig. 2). These dikes cut the folded quartzite succession, requiring deposition of the sedimentary protoliths and two episodes of deformation and metamorphism (D1/M1 and D2/M2) prior to dike emplacement. One of the pegmatite dikes (sample J03-BR4) was observed to deflect foliation in host-rock quartzite and schist, suggesting that some degree of deformation, perhaps involving local fabric reactivation, might have accompanied magmatism at 1436 Ma. However, the dikes are otherwise strongly discordant with respect to the kilometer-scale syncline (F2) that dominates exposures along Blue Ridge, and they are interpreted to postdate deformation and metamorphism. Similar crosscutting relationships are well documented across southern Colorado. In the Needle Mountains (Fig. 1), folded quartzite and schist of the Uncompahgre Formation are sharply truncated by the coarse-grained 1442 ± 3 Ma Eolus Granite (Gibson and Harris, 1992; Gonzales and Van Schmus, 2007). In the Sangre de Cristo Mountains (Fig. 1), quartzite is intruded by the 1434 ± 2 Ma Music Pass pluton (Jones and Connelly, 2006). These relationships not only require that quartzites were deposited prior to granitic magmatism, but they additionally require that the successions were multiply deformed and buried to granitic emplacement depths (~8 km for the Eolus Granite; Dean, 2004) prior to intrusion.
The best minimum age constraints for the timing of deposition come from apparent intrusive relationships, where ca. 1700 Ma granites are in contact with quartzite of the Ortega Formation in the Cimarron Mountains, northern New Mexico (Pedrick et al., 1998). Where the contact is observed, there are domains that contain quartz pods and inclusions within granite that are interpreted to be partly digested quartzite xenoliths. Based on these relationships, deposition of the Ortega Formation would be constrained at ca. 1700 Ma by the underlying rhyolite (1700 ± 25 Ma; Silver, 1984) and intrusive granite (1703 ± 10 Ma; Pedrick et al., 1998). This tight bracketing may be explained by graniterhyolite associations where shallow-level plutons fed caldera-related rhyolites mainly before, but also overlapping in time with, quartzite deposition at 1700 Ma. This is our preferred interpretation as it is in agreement with data from the Mazatzal Group of Arizona where caldera-related rhyolites intrude quartzites (Brady, 1987) and 1701 ± 2 Ma rhyolites are interlayered with quartzite (Cox et al., 2002b).
Other minimum ages come from preliminary analyses of monazite grains in the Ortega Mountains of the northern Tusas Mountains, New Mexico (Kopera, 2003; Kopera et al., 2002). Here, detrital monazite cores with ages that range from 1853 to 1725 Ma have metamorphic overgrowths with ages of 1689 ± 8 Ma and 1670 ± 3 Ma that are interpreted to reflect early metamorphism during burial of quartzites by thrusting during the Mazatzal orogeny. These preliminary constraints place a minimum age for the Ortega Formation deposition at 1689–1670 Ma (Kopera, 2003).
Sedimentary Provenance
The bulk of the detrital zircon from all samples is Paleoproterozoic in age and defines a relatively narrow range of ages (1800–1700 Ma). Peak detrital ages vary somewhat but similarly display a narrow age range (1762–1700 Ma), and systematic shifts correspond to the age of the underlying basement. Detrital zircon morphologies are consistent with minimal transport and derivation from dominantly igneous source rocks. The range of detrital zircon ages closely corresponds with the general age range of juvenile basement assemblages of the Yavapai Province exposed across the region, and the peak detrital ages agree well with published U-Pb zircon ages of underlying basement assemblages locally (Figs. 5–7; Reed et al., 1993; Karlstrom et al., 2004). These results and observations suggest that the quartzite successions formed from first-cycle sediments characterized by a relatively juvenile population of detrital zircon derived almost exclusively from local source regions, presumably mainly from granitoids. The first-cycle character is also suggested by the general textural immaturity of the quartzites and the local abundance of volcanic phenocrystic quartz (Cox et al., 2002b). Minor amounts of Archean detrital zircon (up to 5%) are interpreted to reflect limited (and in many cases completely absent) sedimentary input from the Wyoming Province to the north (Fig. 1) but might also reflect reworked sedimentary rocks or unrecognized, Archean-aged crustal fragments that were possibly incorporated into the Paleoproterozoic accretionary orogen (Hill and Bickford, 2001). Mazatzal-aged (ca. 1650–1600 Ma) detritus was not recognized in any of the quartzites sampled, and the minimum detrital ages described previously closely coincide with the culmination of the Yavapai orogeny (1710–1700 Ma; Karlstrom and Bowring, 1988). Although the absence of Mazatzal-aged detrital zircon could simply represent depositional systems that derived material exclusively from northern, older (>1700 Ma) source terranes, our preferred interpretation is that quartzites were deposited ca. 1700–1680 Ma, prior to accretion of the Mazatzal Province to the south. Regional observations and constraints supporting this interpretation are discussed later.
In the Uncompahgre Formation of southwestern Colorado, the basal conglomerate and Q4 units are separated stratigraphically by more than 2 km (Harris and Eriksson, 1990), yet the detrital zircon age spectra are similar, and the peak ages do not vary significantly (Fig. 5; Table 1). Similar relationships are documented in basal conglomerate and quartzite exposed along Blue Ridge (Figs. 7B and 7C). The broadening of detrital zircon age spectra upward through the stratigraphic section suggests that the depositional systems evolved through time to reach older Proterozoic source terranes to the north. However, the relatively small shift in peak ages indicates that the depositional systems were still dominated by locally derived Paleoproterozoic-aged detritus throughout the depositional history of even the thickest (>2 km) metasedimentary successions.
Depositional Environment
Detrital zircon characteristics, quartzite textures, and crosscutting relationships of metased-imentary successions exposed throughout Colorado and northern New Mexico indicate that sediment was derived from local volcanic and/or plutonic sources. Quartzites in the southern Yavapai Province are underlain by voluminous rhyolite, whereas quartzites to the north are not directly associated with rhyolite but were deposited during prolonged granitic magmatism throughout the same region. In thicker, more complete successions where sedimentological data are available, there is a facies progression from alluvial to shallow marine in the lower units, and the middle and upper units are dominated by shallow-water marine deposits (Harris and Eriksson, 1990; Soegaard and Eriksson, 1985). Interlayering of fine-grained units and quartzite layers (Fig. 5) is interpreted to represent shoaling cycles attributed to eustatic oscillations and/or changing subsidence rates due to tectonism at the basin margins (Harris and Eriksson, 1990). Paleocurrent data and the nature of cross-bedding indicate a change from more linear transport of sediment in the lower parts of the successions to more distributed transport and/or reworking in the shallow-marine environment (Fig. 5; Harris and Eriksson, 1990). The spatial distribution of facies and paleocurrent patterns indicate that depositional basins were deeper to the south and that sediment was dominantly derived from the north (Figs. 2 and 5; Harris and Eriksson, 1990; Reuss, 1974). Similarities between detrital zircon age spectra and peak ages of some samples suggest that multiple successions were likely derived from the same source rocks and deposited within a single basin (Fig. 7). However, based on the facies distribution patterns, paleocurrent data, variable rhyolite/granite association, and outcrop extents of different successions, it is difficult to envision deposition occurring in a single regional basin. Instead, we envision multiple basins or subbasins that were oriented NE-SW, parallel to the southern Laurentian margin at the time of deposition (see Fig. 1 for approximate extents).
Cox et al. (2002b) argued for deposition of the Mazatzal Group in Arizona in an intra-arc basin based on (1) the compositional characteristics of the sediment; (2) the spatial and temporal association with felsic volcanic rocks; (3) the timing of sedimentation with respect to volcanism; and (4) a progressive change in upsection sediment provenance. An intra-arc setting helps to explain the predominantly felsic character of volcanism preceding sedimentation, and the temporal progression from volcanism to sedimentation is interpreted to represent the transition from an active arc to a relatively stable continental setting (Cox et al., 2002b). The relatively shallow nature of intra-arc basins helps to explain the prevalence of shallow-marine facies throughout the depositional history of the successions, where changes in water depth were controlled by eustatic effects and/or local subsidence rates. The Mazatzal Group is most similar to the Ortega Formation in New Mexico, especially in terms of the rhyolite association, and the two successions occur approximately along strike parallel to the Yavapai-Mazatzal Province boundary (Fig. 1). Thus, it seems reasonable that these two successions were deposited in similar basins within an active arc along the southern margin of the Yavapai Province ca. 1700 Ma. Quartzite successions to the north that lack underlying rhyolite were more likely deposited in basins developed along the continental margin away from the active arc. Quartzite successions exposed even farther to the north (i.e., Coal Creek and Park Range localities) might have been deposited in epicontinental basins, because these localities are hundreds of kilometers from the southern margin of the Yavapai Province, and there is not an apparent spatial association with coeval magmatism. However, the ages of these successions are not well established.
Regional Constraints on Quartzite Deposition
Zircon from Vadito Group rhyolite underlying quartzite of the Ortega Formation in the Tusas Mountains, New Mexico (Fig. 2), yielded U-Pb ages of ca. 1700 Ma (Bauer and Williams, 1989). Rhyolite layers locally grade into quartz-rich metasedimentary rocks, which include trough-bedded quartzites and, thus, are interpreted to represent part of a continuous stratigraphic succession. Detrital zircon characteristics of quartzite from the Ortega Formation are nearly identical to all of the quartzites exposed to the north in southern Colorado (Fig. 4), suggesting that all of the successions are broadly correlative. Outcrop characteristics, crosscut-ting relationships, and/or limited detrital zircon data from other quartzites exposed in central and northern Colorado (e.g., Coal Creek quartzite; Aleinikoff et al., 1993) suggest that additional correlative successions exist in the northern Yavapai Province (Fig. 1). However, additional work is needed to test these correlations.
Regional correlations are strengthened by the consistent structural style, orientation, and magnitude of crustal shortening exhibited by quartzite successions exposed throughout the southern Yavapai Province. Williams (1991) described deformation in the Tusas Mountains, northern New Mexico, that involved north-directed kilometer-scale folding and thrusting of the Ortega and Vadito Group quartziterhyolite succession and underlying basement assemblages. Quartzites of the Ortega Formation record a minimum of 50% shortening, which was accommodated primarily by reverse-slip ductile shearing, ductile thrusting, and imbrication (Williams, 1991). In the Needle Mountains, southern Colorado, folded quartzite and schist of the Uncompahgre Formation underwent early thin-skinned, north-directed thrust faulting followed by upright folding into a large (>10 km), complex cusp-shaped fold (Harris et al., 1987; Harris, 1990). Localized quartzite exposures across central and southern Colorado are commonly exposed in tight, upright synclinal "keels" that formed during regional sub-horizontal, northwest-southeast crustal shortening (e.g., Gooseberry Gulch syncline; Reuss, 1974), and they are interpreted to represent the preserved roots of much larger folds.
The structural styles and deformation described here are all consistent with Mazatzal-aged deformation documented in exposures throughout the southwestern United States (Karlstrom and Bowring, 1993). Deformation associated with this orogenic episode primarily affected Arizona and New Mexico but also propagated northward into Colorado. The "Mazatzal deformation front" represents the approximate northern extent of these effects (Fig. 1; Karl-strom and Bowring, 1993; Karlstrom and Daniel, 1993), and all of the quartzites sampled for this study occur south of this deformation front. The metamorphic monazite ages described from Ortega Formation quartzite in the Tusas Mountains of northern New Mexico agree well with the age of progressive D1 and D2 deformation of the underlying Moppin Complex (ca. 1690–1630 Ma) and Vadito Group in the same range (Davis, 2002). Similarly, Read et al. (1999) argued that deformation (progressive development of S1 and S2) and metamorphism of the Ortega Formation in the Rincon Range, New Mexico, was broadly synchronous with emplacement of the 1682 ± 7 Ma Guadalupita pluton. Structures that formed during the Mazatzal orogeny in Arizona are characterized by foreland thrust belt geometries, and estimates of shortening related to northwest-directed thrusting between 1692 and 1630 Ma range from 35% to greater than 50% (>18 km; Puls, 1986; Doe and Karlstrom, 1991; Karlstrom and Bowring, 1993). These age estimates agree well with minimum age constraints described here and confirm that quartzite deposition in the southern Yavapai Province predated the Mazatzal orogeny.
Possible Laurentian Correlative Sequences
Paleoproterozoic metasedimentary successions containing abundant orthoquartzite occur elsewhere throughout southern Laurentia, most notably in the upper part of the Midwestern United States (Fig. 1). Baraboo interval quartzites occur in seven geographically separate but lithologically and stratigraphically similar successions (Fig. 1; Dott, 1983; Dott and Dalziel, 1972). The red, supermature quartz arenites are underlain by ca. 1750 Ma potassic rhyolite and epizonal granite (Smith, 1983; Van Schmus, 1978), and they were deposited at 1730–1650 Ma (Holm et al., 1998b) on polyd-formed, Penokean-aged basement to the north (1870–1820 Ma; Van Schmus et al., 1993) and ca. 1800–1760 Ma crust to the south, which has recently been interpreted to represent the northeastern extension of the Yavapai Province (NICE Working Group, 2007). Deposition is thought to have occurred in a tectonically stable passive-margin setting (Dott, 1983), and sedimentary characteristics suggest two general depositional environments: an early braided fluvial system (Henry, 1975; Dott, 1983) and a later tidally influenced shelf environment (Davis, 2006). Recent recognition of feldspar-free paleosols and evidence for extreme chemical alteration of quartzites suggests that unusually intense chemical weathering accompanied deposition (Medaris et al., 2003).
The Baraboo Quartzite, which is exposed in a megascopic east-west–trending, doubly plunging syncline in southern Wisconsin, is characterized by detrital zircon with an age range of 1866–1712 Ma (Medaris et al., 2003), and similarities in detrital zircon ages among Baraboo interval quartzites are interpreted to confirm the long-standing correlation of the various successions (Medaris et al., 2003; Van Wyck and Norman, 2004; Holm et al., 1998b). Absolute minimum age constraints on quartzite deposition are restricted to localized crosscutting igneous bodies emplaced during Mesoproterozoic regional magmatism. However, a minimum depositional age of 1630 Ma is inferred based on a sharp break in basement cooling ages that corresponds well with the northern limit of quartzite deformation (Romano et al., 2000; Holm et al., 1998b). The close spatial coincidence of the deformation front in quartzites and the ca. 1630 Ma thermal front in underlying basement assemblages is interpreted to represent thin-skinned fold-and-thrust deformation accompanied by low-grade regional metamorphism during Mazatzal-related tectonism across the Lake Superior region (Dott, 1983; Holm et al., 1998b).
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TOPABSTRACTINTRODUCTIONGEOLOGIC BACKGROUNDBLUE RIDGE, COLORADO, QUARTZITE...QUARTZITE DETRITAL ZIRCON...DISCUSSIONTECTONIC IMPLICATIONSTHE QUARTZITE COMPOSITIONAL...CONCLUSIONSREFERENCES CITED |
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THE QUARTZITE COMPOSITIONAL PARADOX |
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TOPABSTRACTINTRODUCTIONGEOLOGIC BACKGROUNDBLUE RIDGE, COLORADO, QUARTZITE...QUARTZITE DETRITAL ZIRCON...DISCUSSIONTECTONIC IMPLICATIONSTHE QUARTZITE COMPOSITIONAL...CONCLUSIONSREFERENCES CITED |
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It is widely recognized that a significant oxidation event occurred on Earth's surface around 2.0 Ga (Holland, 1984; Holland and Beukes, 1990) that was likely driven by an increased input of oxygen to the atmosphere due to increased sedimentary burial of organic matter between 2.3 and 2.0 Ga (Karhu and Holland, 1996). Progressive oxygenation of Earth's atmosphere is thought to have resulted in increased sulfide oxidation during continental weathering, which, in turn, caused a corresponding increase in marine sulfate concentration (Canfield et al., 2000). Shallow ocean waters were likely oxygenated during this time (Shen et al., 2003), but complete aeration of the oceans did not occur until a second major oxygenation event during the Neoproterozoic (0.80–0.54 Ga; Canfield and Teske, 1996). Instead, geochemical and isotopic data suggest that the deep ocean remained anoxic and became highly sulfidic due to increased rates of sulfate reduction (Shen et al., 2003; Canfield, 1998). The sulfidic transition is thought to have occurred at 1.84 Ga (Poulton et al., 2004), thus preceding Paleoproterozoic orogenesis and quartzite deposition in southern Laurentia. Pavlov et al. (2003) argued that methane fluxes resulting from sulfidic oceans and a low (but nonzero) oxygen atmosphere could have been 10–20 times the modern value, producing sustained Proterozoic atmospheric greenhouse conditions ca. 2.30–0.75 Ga. Chemical weathering would indeed have been extreme due to increasing oxygen levels, sulfide oxidation, and the resultant production of sulfuric acid. Weathering was further enhanced by prolonged greenhouse conditions that were unique to the transitional atmosphere and ocean chemistry of the Proterozoic. These various phenomena might explain the compositional maturity of Paleoproterozoic quartzite successions and also help to explain the lack of modern analogs in tectonically active environments.
More detailed petrologic and geochemical work is required to adequately evaluate the relative roles of diagenetic effects and/or extreme weathering in producing the compositional maturity that is observed in Paleoproterozoic quartzite successions throughout the southwestern United States. Modern examples of first-cycle quartz arenites occur in the Orinoco River basin within the Andean foreland of Venezuela and Colombia (Johnsson et al., 1988; 1991), where tropical weathering is extreme. However, there are no Phanerozoic analogs for thick, homogeneous successions of shallow-water, synorogenic quartz arenite in association with high-silica rhyolite. During the Paleoproterozoic, lithosphere was thick and stable enough to form large supercontinents but thin and hot enough that syntectonic basins were rapidly formed and deformed within wide accretionary orogens. Orthoquartzite successions represent a unique tectonic marker in the polyphase tectonic history of rocks in the southwestern United States, and they help us to better understand the nature, timing, extent, and symmetry of post–1.8 Ga events at the regional and global scale. Moreover, similarities among quartzites exposed throughout the region and along the Laurentian margin suggest that they also represent a widespread, and perhaps global, episode of sedimentation involving a distinctive syntectonic setting and unique climatic conditions, a combination that might make these units a signature lithology for Paleoproterozoic time.
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TOPABSTRACTINTRODUCTIONGEOLOGIC BACKGROUNDBLUE RIDGE, COLORADO, QUARTZITE...QUARTZITE DETRITAL ZIRCON...DISCUSSIONTECTONIC IMPLICATIONSTHE QUARTZITE COMPOSITIONAL...CONCLUSIONSREFERENCES CITED |
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONGEOLOGIC BACKGROUNDBLUE RIDGE, COLORADO, QUARTZITE...QUARTZITE DETRITAL ZIRCON...DISCUSSIONTECTONIC IMPLICATIONSTHE QUARTZITE COMPOSITIONAL...CONCLUSIONSREFERENCES CITED |
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RECEIVED FOR PUBLICATION October 23, 2007
REVISED MANUSCRIPT RECEIVED June 7, 2008
MANUSCRIPT ACCEPTED July 1, 2008
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3%) Paleoproterozoic grains only. For complete data set, see GSA Data Repository Table DR3 (see text 
