Skip to Main Content

Abstract

We analyzed detrital zircons in seven samples of metasedimentary rock from north–central Idaho, U.S.A., to test the previous assignment of these rocks to the Mesoproterozoic Belt–Purcell Supergroup. Correlating these rocks with known sedimentary units through field observations is difficult if not impossible due to the high metamorphic grade (amphibolite facies) and intensity of deformation. Zircon analysis by laser–ablation inductively coupled plasma mass spectrometry (LA–ICPMS) reveals that five of the seven samples contain multiple zircon populations between 1700 and 1400 Ma and a scatter of Paleoproterozoic and Archean ages, similar to results reported from the Belt Supergroup to the north and east. These results indicate that the likely protoliths of most high–grade metamorphic rocks northwest of the Idaho batholith were upper strata of the Belt Supergroup. In contrast, a quartzite at Bertha Hill north of Pierce lacks grains younger than 1600 Ma and thus is distinctly unlike the Ravalli Group of the Belt Supergroup, with which it was previously correlated. Possible correlatives that contain similarly old populations of zircons and feldspar–poor quartzite include the Neihart Formation (lowermost Belt Supergroup in Montana), Neoproterozoic quartzite (Syringa metamorphic sequence), and Cambrian quartzite. A sample from the North Fork of the Clearwater River yielded a large number of zircons with concordant Neoproterozoic ages, all of which had low Th/U ratios that suggest either a Neoproterozoic metamorphic event or the transport and deposition of zircons that were metamorphosed in the Neoproterozoic. SHRIMP (sensitive high–resolution ion microprobe) dating of a granite (now augen gneiss) that intruded sedimentary rocks west of Pierce, Idaho, yields an age of 1379 ± 12 Ma based on seven of fourteen analyses; this provides a lower age limit for sediment deposition of some rocks mapped as metamorphosed Belt Supergroup, and which had detrital zircon populations in the 1700 to 1400 Ma range. Additional analyses of three zircon rims yield an age range of 87–82 Ma, which is similar to the youngest ages from the North Fork sample. We interpret these ages to reflect the time of zircon overgrowth synchronous with the emplacement of the Cretaceous Idaho Batholith. None of the metasedimentary rocks dated can be older than Mesoproterozoic, and, with the exception of the Bertha Hill quartzite, none can be older than the Belt–Purcell Supergroup.

Introduction

The Mesoproterozoic Belt–Purcell Supergroup is exposed throughout much of northern Idaho, western Montana, and northeastern Washington (Fig. 1). To understand its original extent and history, the metamorphosed and deformed rocks must be accurately correlated with their more pristine Belt–Purcell equivalents. Most amphibolite–facies metasedimentary rocks northwest of the Bitterroot lobe of the Idaho batholith have been correlated with formations at lower grade within the Belt Supergroup exposed in northern Idaho and western Montana (Hietanen, 1962, 1963b; Reid et al., 1973). Exceptions include the rocks near Boehls Butte (Fig. 1), which several workers have anorthosite in the Boehls Butte area (Doughty and Chamberlain, this volume) confirms the presence of pre–Belt rocks in the region. Metasedimentary rocks that postdate the Belt are also present. Low–metamorphic–grade rocks of the Neoproterozoic Windermere Supergroup are exposed in northeastern Washington and northwestern Idaho (Fig. 1; Miller, 1994). Higher–grade rocks in central Idaho have been dated and correlated with these (Lund et al., 2003), and recent detrital zircon dating shows that Neoproterozoic rocks exist in the Lowell area as well (Fig. 1; Lund et al., 2005; Lewis et al. 2005).

Fig. 1.—

Simplified geologic map of northern Idaho and surrounding area. Localities discussed in text: BB, Boehls Butte; KB, Kamiak Butte; SB, Steptoe Butte; SH, Smoot Hill. Other uppercase-letter pairs are sample localities listed in Table 1.

Fig. 1.—

Simplified geologic map of northern Idaho and surrounding area. Localities discussed in text: BB, Boehls Butte; KB, Kamiak Butte; SB, Steptoe Butte; SH, Smoot Hill. Other uppercase-letter pairs are sample localities listed in Table 1.

Table 1.—

Sample location and age summary.

Sample nameSample no.LatitudeLongitudeLithologyUnit207Pb/206Pb age results
HP (Hoodoo Pass)02MDM21946.9746N115.0235Wcalcareous quartziteWallace Fm.primarily 1900-1450 Ma
BC (Beaver Cr.)03RL14746.7672N115.6752Wcalc-silicate gneissmeta-Beltprimarily 1800-1400 Ma
SC (Snake Cr.)03RL14546.6023N115.9058Wbiotite quartzitemeta-Beltprimarily 1800-1400 Ma
LC (Leopold Cr.)04RL15046.6482N116.4426Wbiotite quartzitemeta-Beltprimarily 1750-1370 Ma; some metamorphic
DB (Dent bridge)04RL15146.6069N116.1712Wbiotite quartzitemeta-Beltprimarily 1450-1400 and 1800-1600 Ma
BH (Bertha Hill)03RL14646.7773N115.7410Wquartziteuncertainprimarily 1900-1700 and 2700-2400 Ma
NF (North Fork)03RL14846.8388N115.5742Wcalc-silicate gneissuncertainprimarily 1700-800 Ma; many metamorphic
WD (west of Dent)00RL59546.6339N116.2320Waugen gneissaugen gneiss1379 ± 12 Ma cores; 82-87 Ma rims
Sample nameSample no.LatitudeLongitudeLithologyUnit207Pb/206Pb age results
HP (Hoodoo Pass)02MDM21946.9746N115.0235Wcalcareous quartziteWallace Fm.primarily 1900-1450 Ma
BC (Beaver Cr.)03RL14746.7672N115.6752Wcalc-silicate gneissmeta-Beltprimarily 1800-1400 Ma
SC (Snake Cr.)03RL14546.6023N115.9058Wbiotite quartzitemeta-Beltprimarily 1800-1400 Ma
LC (Leopold Cr.)04RL15046.6482N116.4426Wbiotite quartzitemeta-Beltprimarily 1750-1370 Ma; some metamorphic
DB (Dent bridge)04RL15146.6069N116.1712Wbiotite quartzitemeta-Beltprimarily 1450-1400 and 1800-1600 Ma
BH (Bertha Hill)03RL14646.7773N115.7410Wquartziteuncertainprimarily 1900-1700 and 2700-2400 Ma
NF (North Fork)03RL14846.8388N115.5742Wcalc-silicate gneissuncertainprimarily 1700-800 Ma; many metamorphic
WD (west of Dent)00RL59546.6339N116.2320Waugen gneissaugen gneiss1379 ± 12 Ma cores; 82-87 Ma rims

The sedimentary structures and stratigraphic order used to correlate Belt strata elsewhere have been obliterated in north–central Idaho by the metamorphism and deformation. Conse quently, we are using spectra of detrital–zircon age in conjunction with protolith estimates to test correlations. Pioneering work by Ross et al. (1992) and Ross and Villeneuve (2003) established that zircon populations in eastern Belt–Purcell rocks are characterized by prominent age groupings at 2700 Ma and 1800 Ma (Fig. 2A) and lack grains with ages 1610 to 1490 Ma, corresponding to the “North American magmatic gap”. Many western Belt units have zircons with ages in this gap, presumably derived from non–North American (non–Laurentian) sources (Fig. 2B). To obtain age spectra for comparison with this previous work, we sampled southwestward from low–grade Belt Supergroup rocks along the Montana border at Hoodoo Pass (HP in Fig. 1) to higher–grade rocks west–northwest of Pierce, Idaho. U–Pb ages for zircons were obtained by laser–ablation inductively coupled plasma mass spectrometry (LA–ICPMS). In addition, a granitic augen gneiss sill in the metasedimentary rocks near the southwest extent of the sampling transect was dated using a sensitive high–resolution ion microprobe (SHRIMP) to establish a minimum age of host sediment deposition at that locality (WD in Fig. 1). Metamorphism affected some of the zircons, and although this complicated interpretation it allowed a metamorphic event to be dated.

Fig. 2.—

Age-density probability distribution diagram for the lower Belt through middle Belt carbonate from Ross and Villeneuve (2003). The gray bands correspond to the North American magmatic gap (1610-1490 Ma). A) Laurentian Belt sands from eastern Belt facies, B) non-Laurentian Belt sands from western facies.

Fig. 2.—

Age-density probability distribution diagram for the lower Belt through middle Belt carbonate from Ross and Villeneuve (2003). The gray bands correspond to the North American magmatic gap (1610-1490 Ma). A) Laurentian Belt sands from eastern Belt facies, B) non-Laurentian Belt sands from western facies.

Regional Geology

Low–Grade Belt Rocks

Lower–greenschist–facies metasedimentary rocks of the Belt Supergroup are exposed over much of northern Idaho and western Montana (Fig. 1). Correlative rocks in Canada are assigned to the Purcell Supergroup. The stratigraphically lowest exposed rocks in western Montana and the Idaho panhandle are deep– water turbidites of the Prichard Formation (Cressman, 1989), which were intruded by penecontemporaneous mafic sills dated at about 1470 Ma (Anderson and Davis, 1995; Sears et al., 1998). On the northeast side of the basin in Canada and central Montana, the lowermost rocks are nearshore, shallow–water strata that include carbonate and fluvial–deltaic deposits (Höy et al., 2000). The Prichard Formation is overlain by shallow–water clastic rocks of the Ravalli Group, which are characterized by finegrained feldspathic quartzite (Winston, 1986). The Ravalli Group, in turn, is overlain by carbonate rocks of the Wallace and Helena formations. These consist of dolomitic and calcitic siltite and quartzite, and, toward the southwest, carbonate–poor black argillite. A bentonite near the top of the Helena Formation in Glacier National Park is dated at about 1454 Ma (Evans et al., 2000). Overlying these carbonate rocks are shallow–water clastic rocks of the Missoula Group. The youngest age determined for the Belt Supergroup (1401 Ma) is from a felsic tuff in the upper part of the Missoula Group in Montana (uppermost Bonner Formation; Evans et al., 2000).

Detrital–zircon results reported from low–grade Prichard Formation, Ravalli Group, and Wallace Formation in northern Idaho and western Montana include a significant number of grains in the 1610–1490 Ma range, indicating a western, non–Laurentian source (Fig. 2B). These contrast with detrital–zircon age populations from eastern facies of the lower and middle parts of the Belt, which have prominent ages groupings at about 2700 and 1800 Ma (Fig. 2A) and lack zircons with ages in the 1610–1490 Ma North American magmatic gap (Ross and Villeneuve, 2003).

High–Grade Belt Rocks

Formations within the Belt Supergroup can be traced 5–10 km south of the garnet isograd, in the area east–northeast of Moscow, despite increasing metamorphic grade (Fig. 1). Farther south of the garnet isograd, individual formations become increasingly difficult to identify. Calc–silicate gneiss, granofels, and mica schist east of Pierce are thought to be derived from the Wallace (or Helena) formations because they generally have carbonate–bearing protoliths and, where they are only slightly deformed, preserve sedimentary structures common to the Wallace Formation. Quartzite, most of which is feldspathic, appears to occur strati–graphically beneath the calc–silicate rocks and may correlate with the Ravalli Group. Calc–silicate gneiss and quartzitic rocks along the western edge of the Bitterroot lobe of the Idaho batholith southeast of Pierce (Fig. 1) are probably metamorphic equivalents of the Belt Supergroup, but correlation is highly uncertain. Similarly, metamorphic rocks near Elk City appear to have Belt protoliths (Lewis et al., 1998) but have not been studied in detail.

Syringa Metamorphic Sequence

Occurring with the units described above, which are interpreted to be high–grade Belt, is a quartzite–rich assemblage termed the Syringa metamorphic sequence (Lewis et al., 1992; Lewis et al., 1998). This sequence, previously correlated with Belt–Purcell strata (Prichard Formation and Ravalli Group) west of Lowell and north of Orofino (Hietanen, 1962; Reid et al., 1973), differs from Belt–Purcell strata in that the quartzite is clean (typically 2 percent or less feldspar) and is associated with calc–silicate rocks. These features are not common in the Ravalli Group or elsewhere in the Belt Supergroup, where quartzite generally contains 20 percent or more feldspar and is interlayered with argillite and siltite rather than carbonate rocks. Recent mapping by Lewis et al. (2005) has expanded the area of rocks assigned to the Syringa sequence from the region west of Lowell to areas north of Orofino and west to Smoot Hill northwest of Moscow (SH, Fig. 1). Similar rocks in the Gospel Hump Wilderness southeast of Grangeville are assigned to the Umbrella Butte Formation (Lund et al., 2003). All of these rocks are of sillimanite grade. Typical lithologies are coarsely crystalline quartzite, sillimanite–muscovite–biotite schist, and calc–silicate gneiss. Less common compositions include garnet– and sillimanite–bearing quartzite. Rocks at Smoot Hill are mostly highly recrystallized quartzite with sparse feldspar stringers. The boundary between the Syringa sequence and the meta–Belt Supergroup is poorly defined but is placed between quartzite–dominated Syringa rocks and calc–silicate and schist–dominated meta–Belt sequences. Detrital zircon dating by Lund et al. (2005) and Lewis et al. (2005) has shown that the Syringa quartzite near Lowell (quartzite of Wild Goose Camp) is Neoproterozoic and thus younger than the Belt Supergroup. Detrital zircons from a quartzite at Smoot Hill are mostly 1875 to 1725 Ma, with smaller populations between 2840 and 2460 Ma (Mueller et al., 2003). Absent are zircons with syn–Belt ages and ages in the North American magmatic gap. The Smoot Hill quartzite sample contains a single 1290 Ma concordant zircon (Mueller et al., 2003), which suggests that it is younger than 1290 Ma. Because of this age, and the lithologic similarity to quartzite near Syringa, we have tentatively assigned the Smoot Hill quartzite to the Neoproterozoic Syringa metamorphic sequence that is exposed along strike to the east in Idaho (Fig. 1).

Unnamed High–Grade Rocks in Bertha Hill Area

A large body of feldspar–poor quartzite is exposed north of Pierce, Idaho, at Bertha Hill (BH, Fig. 1). This quartzite was previously correlated with the Ravalli Group (Hietanen, 1963a), but it typically lacks the 15–20 percent feldspar that is characteristic of the Ravalli. More likely it correlates with another feldspar– poor quartzite such as from the younger Syringa sequence or the basal Belt Neihart Formation (Keefer, 1972) exposed at the east margin of the Belt basin. If it predates the Belt, possible correlatives include basement rocks in the Monashee complex of British Columbia (Armstrong et al., 1991; Parkinson, 1991; Crowley, 1997). Northeast of Bertha Hill and southeast of Boehls Butte (BB, Fig. 1) is a fault–bounded block of calc–silicate gneiss. This rock, presently unnamed, includes minor amounts of graphitic marble. Although once assigned to the Wallace Formation of the Belt–Purcell Supergroup (Hietanen, 1963a, 1963b), data presented below indicate that the carbonate–bearing protolith probably postdated Belt–Purcell deposition.

Quartzite of Southeastern Washington

Quartzite is exposed as erosional remnants rising above the Columbia River basalt flows at Kamiak Butte and Steptoe Butte, just west of the Idaho–Washington border northwest of Moscow and north of Smoot Hill (KB and SB, Fig. 1). Kamiak Butte is composed of clean (< 1 percent feldspar), recrystallized quartzite, locally with small quartz pebbles and relict cross beds (Savage, 1973). Steptoe Butte consists of strongly deformed and recrystallized muscovite–bearing quartzite. The abundance of approximately 1780 Ma zircons and lack of syn–Belt and North American magmatic gap ages in the Kamiak Butte and Steptoe Butte quartzites (Ellis et al., 2004) may indicate an older Laurentian source. Unique correlation is thwarted because several units in the region contain this older “Laurentian only” population, including the basal Neihart Formation (basal Belt), many of the eastern Belt formations, and the Aldridge Formation (Ross and Villeneuve, 2003, Mueller et al., 2003), as well as Neoproterozoic Windermere quartzites (Ross and Parrish, 1991) and Cambrian quartzites (Smith and Gehrels, 1991).

Proterozoic Intrusive Rocks

Granitic augen gneiss present between Pierce and Moscow as sill–like bodies (small triangles in Fig. 1) is most abundant near Dent Bridge (DB in Fig. 1). Most bodies are too small to show in Figure 1, or even at 1:100,000 map scale. This gneiss is granite to granodiorite in composition but now exists as coarse–grained garnet–muscovite–biotite augen gneiss with potassium feldspar augen typically 4–6 cm (locally as much as 12 cm) in length (Fig. 3). The bodies near Dent Bridge were first recognized by Kopp (1959), who noted two exposures that occupy hinges of tight folds. The augen gneiss near Dent Bridge closely resembles (in texture, mineralogy, and chemistry) augen gneiss near Elk City (Fig. 1) dated at about 1370 Ma (Evans and Fisher, 1986). The granitic augen gneiss west of Dent Bridge (sample WD, discussed below) has a similar age. Chemically, these Proterozoic augen gneisses are characterized by high K2O/Na2O ratios (1.3–2.0) and high Y concentrations (25–122 ppm) relative to the Idaho batholith. These characteristics, along with low Al2O3 concentrations, are typical of A–type “anorogenic” Proterozoic granites and granitic gneisses of similar age elsewhere in the western United States (Anderson, 1983).

Fig. 3.—

Granitic augen gneiss along the shore of Dworshak reservoir near Dent Bridge.

Fig. 3.—

Granitic augen gneiss along the shore of Dworshak reservoir near Dent Bridge.

Boehls Butte Rocks

The Boehls Butte anorthosite and its high–grade aluminous metasedimentary host rock appear to predate deposition of the Belt–Purcell Supergroup. Recent U–Pb dating of igneous zircons in the anorthosite yielded an age of about 1787 Ma (Doughty and Chamberlain, this volume), which confirms a previous pre–Belt age assignment by Reid et al. (1973). These basement rocks are structurally overlain by quartzite, sulfide–rich fine–grained granofels, calcite–rich granofels, marble, and calc–silicate rocks. The structure is a mylonitic fault zone (Jug Rock mylonite; Doughty and Buddington, 2002; Sha, 2004) that attenuated an unknown thickness of section. The overlying mylonitic quartzite may be the equivalent of the Neihart Formation (Freeman and Winston, 1987) and the overlying metasedimentary rocks equivalent to the Prichard Formation. Alternatively, the quartzite might be part of the Prichard Formation. The pyrrhotite content and planar layering in the siltite, the apparent soft–sediment deformation, and the association with mafic sills and dikes strongly suggest correlation with the Prichard Formation, most likely one of the lower members.

Methods LA–ICPMS

The detrital zircons in this study were analyzed by laser ablation–inductively coupled plasma mass spectrometry (LA–ICPMS) at Washington State University. The full description of this method is reported in Chang et al. (2006), so only a brief summary is presented here. Zircons were separated from 5–10 kg rock samples by standard gravimetric and magnetic techniques at the University of Idaho. Approximately 200 grains were randomly selected from each sample, mounted in epoxy along with zircon standards and two to three other unknown samples, and polished to expose the interiors of the grains. The U–Pb analyses were performed using a New Wave™ UP 213 Nd–YAG (213 nm) laser–ablation system coupled with a ThermoFinnigan Element2™ single collector, high–resolution magnetic sector ICP–MS. The laser was operated using a 10 Hz repetition rate and fluence of 10–11 J/cm2. Laser beam size was typically 30 μm in diameter, although on some zircons 40 μm spots were used. The typical pit depth was approximately 25 microns. Blanks were measured before each analysis with the laser off to determine background signals on the peaks of interest. The laser was then fired and, after a delay to allow for sample uptake, the signals were collected for ~ 35 seconds in 300 sweeps through the mass range. Signals of 202Hg, 204(Pb+Hg), 206Pb, 207Pb, 208Pb, 232Th, 235U, and 238U were measured. We calculated 206Pb/238U and 207Pb/206Pb ratios from the measured intensities and calculated 207Pb/235U ratios from the 206Pb/238U and 207Pb/206Pb ratios and using the natural abundance of U. Data were reduced off line using an Excel–based program (Chang et al., 2006) that plots time series of the analytical results (ratios and intensities for all measurements as a function of time) with programmed 2 filters to remove outliers. Cumulative probability, histogram, and concordia plots were generated using Isoplot/ Ex v. 2.49 (Ludwig, 2001a).

One of the major sources of uncertainty during U–Pb LA–ICPMS analyses is time–dependent, laser–induced U/Pb fractionation. Our approach, following the methods of George Gehrels (personal communication) and described by Sylvester and Ghaderi (1997) and Kosler et al. (2002), is to use the regression–line method. The assumption is that the Pb/U fractionation trend over the signal acquisition time (~ 35 seconds) is linear, which is demonstrated by the linear distribution of data points on a plot of Pb/U vs. time (Chang et al., 2006). The intercept at time zero is assumed to be free of laser–induced time–dependent fractionation (Sylvester and Ghaderi, 1997). The 206Pb/238U intercept is determined for each standard analysis and compared to the accepted 206Pb/238U value (determined by TIMS) for that standard in order to calculate a 206Pb/238U fractionation factor. A similar approach is used for the 207Pb/206Pb value except that an average (rather than intercept) value is used to determine the fractionation factor because the 207Pb/ 206Pb value is not measurably time–dependent over the duration of the analysis. The fractionation factors determined from the standards are then applied to the unknowns to calculate the corrected ratios. These factors are typically in the range of 0.75 to 0.90 for 206Pb/238U and 0.98–0.99 for 207Pb/206Pb. The reported 207Pb/235U values are derivative from the calculated 206Pb/238U and 207Pb/206Pb ratios. We also calculate 207Pb/235U ratios and ages directly from the measured 207Pb and 235U intensities as a check on the calculated 207Pb/235U values. These are useful when there is a problem with the measurement of the 206Pb/238U or 207Pb/ 206Pb ratios. Hg is present in our argon gases and produces a significant isobaric interference (204Hg) with 204Pb that cannot be resolved during zircon analysis. Owing to the persistent 204Hg peak, we were unable to determine excess 204Pb in the zircon analyses and, therefore, did not apply a correction for common lead in any of the reported zircon analyses. Because the 207Pb/206Pb ages are not corrected for common Pb, these ages should be regarded as maximum ages.

We used two primary standards during the course of this study: an in–house standard, Peixe (provided by George Gehrels, University of Arizona; Klepeis et al., 2004), with a TIMS age of 564 ± 4 Ma (George Gehrels, personal communication); and TEMORA 1, with a TIMS age of 416.8 ± 1.1 Ma (Black et al., 2003a). Our reported uncertainty is a combination of two sources combined quadratically: (1) uncertainty (2a) in the 206Pb/238U intercept and in the average 207Pb/206Pb ratio determined during each analysis and (2) variance in the 206Pb/238U and 207Pb/ 206Pb determinations of the standards that bracket the unknowns (reported as two standard deviations of the population). These in–run errors for individual determinations average 3 percent for the 206Pb/238U ages and 2 percent for the 207Pb/ 206Pb ages (2a). These errors do not take into account uncertainty in the TIMS values of the standards or any bias that might exist between the analysis of the standard and unknown zircon due to matrix or cumulative instrumental effects. In practice, we determined our fractionation factors using Peixe and checked the validity of these values by treating the TEMORA 1 standard as an unknown. Typically, we analyze both Peixe and Temora 2–3 times every 5–10 unknowns. This approach over the course of this work yielded a 206Pb/238U age of 414.6 ± 1.2 Ma, a 207Pb/235U age of 414.8 ± 1.5 Ma, and a 207Pb/235U age of 414.5 ± 4.5 Ma (two–standard–error weighted average, n = 93) for TEMORA 1, all of which are within error of its published TIMS age of 416.8 ± 1.1 Ma (Black et al., 2003). Our determinations represent laser analyses performed on several different days and using different normalization factors and, therefore, provide an estimate (albeit probably “best case”) of the reproducibility, precision, and accuracy of Pb/U and Pb/Pb analyses of a homogeneous zircon population.

In the cumulative probability plots, we included only those analyses that are less than 10 percent discordant based on the calculated 206Pb/238U and 207Pb/206Pb ages. All grains for which we determined 206Pb/238U and 207Pb/206Pb ages are shown in the Tera–Waserburg plots, with the > 10 percent discordant analyses shown with gray ellipses. Cumulative probability plots use 207Pb/ 206Pb ages only. Not shown on either the Tera–Waserburg or cumulative probability plots are analyses that produced 207Pb/ 206Pb ages but unreliable 206Pb/238U ages. This occurs when there are irregular Pb/U spectra during the analyses.

Shrimp

To establish additional controls on the age of Proterozoic intrusions in the study area, zircons were separated from a granitic orthogneiss sample and analyzed for U–Pb on the SHRIMP–RG (–reverse geometry) instrument at the Stanford–U.S. Geological Survey Microanalytical Center. Zircon grains were separated from a 2 kg sample by standard gravimetric and magnetic techniques at the University of Idaho. Grains were hand picked under alcohol on the basis of clarity and lack of inclusions and cracks. Selected grains were mounted in epoxy and polished to expose grain centers. Cathodoluminescence (CL) images of zircons were used to characterize the grains and select spots for analysis. Calibration concentrations and isotopic compositions were based on replicate analyses of zircon standards SL13 and R33 (419 Ma; Black et al., 2004). A 30–mm–diameter spot size was used for all analyses. The analytical routine followed Williams (1998), and data reduction used the SQUID program of Ludwig (2001b).

Results

Location, lithologic information, and results are summarized in Table 1. Individual analyses of detrital zircons, too numerous to include here, are available in digital form from Vervoort et al. (2006). SHRIMP data for the sample of augen gneiss are in Table 2.

Table 2.—

U-Pb geochronologic data and apparent ages for augen gneiss sample 00RL595 from west of Dent (WD; Figure 1).

SpotUThTh/U206Pb*af206Pbc*a238U/206Pb*b207Pb/206Pb*b206Pb/238U*c206Pb/238U*d207Pb/206Pb*d
(ppm)(ppm)(ppm)(Ma)(Ma)(Ma)
172798460.0231--78.16(0.5)0.0473(1.8)82.0 (0.4)82.0 (0.4)
34310790.02500.0674.32(0.3)0.0482(1.4)86.1 (0.3)86.2 (0.3)
122830810.03330.1673.68(0.4)0.0490(1.7)86.8 (0.4)87.0 (0.4)
421241050.053111.515.86(0.3)0.0852(0.4)1,016 (3)1,318 (8)
135741100.20116--4.25(0.4)0.0857(0.7)1,362 (5)1,323 (14)
11675910.14134--4.34(0.5)0.0854(2.5)1,338 (6)1,325 (48)
22182260.01431--4.34(0.2)0.0856(0.4)1,335 (3)1,327 (7)
15482610.1397--4.27(0.5)0.0864(0.8)1,356 (6)1,340 (15)
921651510.074320.024.30(0.2)0.0866(0.4)1,347 (3)1,350 (7)
124971320.05544--3.94(0.3)0.0868(0.3)1,457 (3)1,353 (6)
77301170.171490.014.22(0.4)0.0876(0.7)1,371 (5)1,371 (14)#
8669910.141350.074.25(0.4)0.0876(0.7)1,363 (5)1,374 (13)#
10133700.54270.234.33(0.9)0.0879(1.4)1,342 (11)1,381 (28)#
145581160.211050.674.57(0.5)0.0886(0.8)1,275 (5)1,382 (15)#
64841140.24100--4.16(0.5)0.0882(0.8)1,390 (6)1,382 (16)#
161631050.67340.094.14(0.9)0.0893(1.3)1,395 (11)1,385 (26)#
5341480.15700.124.21(0.7)0.0885(0.9)1,374 (8)1,387 (18)#
SpotUThTh/U206Pb*af206Pbc*a238U/206Pb*b207Pb/206Pb*b206Pb/238U*c206Pb/238U*d207Pb/206Pb*d
(ppm)(ppm)(ppm)(Ma)(Ma)(Ma)
172798460.0231--78.16(0.5)0.0473(1.8)82.0 (0.4)82.0 (0.4)
34310790.02500.0674.32(0.3)0.0482(1.4)86.1 (0.3)86.2 (0.3)
122830810.03330.1673.68(0.4)0.0490(1.7)86.8 (0.4)87.0 (0.4)
421241050.053111.515.86(0.3)0.0852(0.4)1,016 (3)1,318 (8)
135741100.20116--4.25(0.4)0.0857(0.7)1,362 (5)1,323 (14)
11675910.14134--4.34(0.5)0.0854(2.5)1,338 (6)1,325 (48)
22182260.01431--4.34(0.2)0.0856(0.4)1,335 (3)1,327 (7)
15482610.1397--4.27(0.5)0.0864(0.8)1,356 (6)1,340 (15)
921651510.074320.024.30(0.2)0.0866(0.4)1,347 (3)1,350 (7)
124971320.05544--3.94(0.3)0.0868(0.3)1,457 (3)1,353 (6)
77301170.171490.014.22(0.4)0.0876(0.7)1,371 (5)1,371 (14)#
8669910.141350.074.25(0.4)0.0876(0.7)1,363 (5)1,374 (13)#
10133700.54270.234.33(0.9)0.0879(1.4)1,342 (11)1,381 (28)#
145581160.211050.674.57(0.5)0.0886(0.8)1,275 (5)1,382 (15)#
64841140.24100--4.16(0.5)0.0882(0.8)1,390 (6)1,382 (16)#
161631050.67340.094.14(0.9)0.0893(1.3)1,395 (11)1,385 (26)#
5341480.15700.124.21(0.7)0.0885(0.9)1,374 (8)1,387 (18)#
Note: All analyses were performed on the SHRIMP-RG ion microprobe at the U.S. Geological Survey-Stanford Microanalytical Center at Stanford University. Calibration concentrations and isotopic compositions were based on replicate analyses of SL13 and R33 (419 Ma; Black et al., 2004). Analytical routine followed Williams (1998). Data reduction utilized SQUID program of Ludwig (2001b).
a

Pb* denotes radiogenic Pb; Pbc denotes common Pb; f206Pbc = 100*(206Pbc/206Pbtotai).

b

Reported ratios are not corrected for common Pb. Errors are reported in parentheses as percent at the 1 a level.

c

Ages calculated from ratios corrected for common Pb using 207Pb for the 206Pb/238U age. Uncertainties in millions of years reported as 1 a.

d

Ages calculated from ratios corrected for common Pb using 204Pb. Uncertainties in millions of years reported as 1 a. Ages denoted by # were used in calculation of weighted mean 207Pb/206Pb age.

Hoodoo Pass (HP)

The easternmost sample (02MDM219) was collected from Belt strata where formational assignment is well constrained. Sample HP is a carbonate–bearing, fine–grained, feldspathic quartzite from the lower part of the Wallace Formation at Hoodoo Pass along the Idaho–Montana border (HP, Fig. 1). These strata are interpreted to be lateral equivalents of the Helena Formation in Montana (Winston, this volume).

Most analyzed grains are concordant, or nearly so (Fig. 4A). The 207Pb/206Pb ages of the zircons from Hoodoo Pass are mostly 1900–1450 Ma (Fig. 4B) with single grains at 2723 ± 28 Ma, 2426 ± 29 Ma, and 2359 ± 29 Ma (2a). Five grains that are more than 10 percent discordant and three grains that yielded only 207Pb/ 206Pb ages are not used in the probability plot. Of the 83 grains that were 10 percent or less discordant, 13 have ages within the North American magmatic gap (1610–1490 Ma). The most common age is between 1750 and 1650 Ma (32 grains). These results are similar to those reported by Ross and Villeneuve (2003) for a sample of the Wallace Formation from about 22 km southeast of Sandpoint, Idaho (Fig. 1).

Fig. 4.—

Zircon age data from Hoodoo Pass (HP) sample 02MDM219. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray. Data-point error ellipses are 2a. B) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Fig. 4.—

Zircon age data from Hoodoo Pass (HP) sample 02MDM219. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray. Data-point error ellipses are 2a. B) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Beaver Creek (BC)

Sample BC (03RL147) is calc–silicate gneiss from a quarry along Beaver Creek north of Pierce, Idaho (BC, Fig. 1). Pegmatite veins 1 cm to 1 m in width are common in the outcrop, but we avoided these in sampling. In addition to quartz, sample BC contains abundant plagioclase and amphibole (Fig. 5A). Of 65 grains with 10 percent or less discordance (Fig. 6A), most are 1800 to 1400 Ma and 21 are in the range of the North American magmatic gap (Fig. 6B). The youngest is 1366 ± 25 Ma. Not included in the probability plot are 36 grains that were > 10 percent discordant and 31 that produced only 207Pb/206Pb ages. The age distribution is similar overall to that found in the Hoodoo Pass sample, although more grains are discordant and a higher proportion of ages are in the North American magmatic gap.

Fig. 5.—

Examples of metasedimentary rocks sampled for detrital zircons. Quartz is medium gray and plagioclase is light gray. A) Calc-silicate gneiss sample from Beaver Creek. B) Coarsely recrystallized feldspathic biotite quartzite sample from Snake Creek. C) Quartzite sample from Bertha Hill illustrating coarsely recrystallized texture.

Fig. 5.—

Examples of metasedimentary rocks sampled for detrital zircons. Quartz is medium gray and plagioclase is light gray. A) Calc-silicate gneiss sample from Beaver Creek. B) Coarsely recrystallized feldspathic biotite quartzite sample from Snake Creek. C) Quartzite sample from Bertha Hill illustrating coarsely recrystallized texture.

Fig. 6.—

Zircon age data from the Beaver Creek (BC) sample 03RL147. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray. Data-point error ellipses are 2σ. B) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Fig. 6.—

Zircon age data from the Beaver Creek (BC) sample 03RL147. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray. Data-point error ellipses are 2σ. B) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Snake Creek (SC)

Sample SC (03RL145) is coarsely recrystallized feldspathic biotite quartzite within a calc–silicate gneiss sequence along Snake Creek (SC, Fig. 1). Figure 5B illustrates the recrystallized nature of the Snake Creek quartzite. Of 78 grains with 10 percent or less discordance (Fig. 7A), most are 1850 to 1400 Ma and 28 are in the range of the North American magmatic gap (Fig. 7B). The youngest is 1419 ± 46 Ma. Not shown in the probability plot are 27 grains that were > 10 percent discordant and 23 that produced only 207Pb/206Pb ages. The age distribution is most similar to that found in the Beaver Creek sample.

Fig. 7.—

Zircon age data from the Snake Creek (SC) sample 03RL145. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray. Data-point error ellipses are 2σ. B) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Fig. 7.—

Zircon age data from the Snake Creek (SC) sample 03RL145. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray. Data-point error ellipses are 2σ. B) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Leopold Creek (LC)

Sample LC (04RL150) is coarsely recrystallized biotite quartzite from Leopold Creek (LC, Fig. 1). Of 70 grains with 10 percent or less discordance (Fig. 8A), 13 are in the range of the North American magmatic gap (Fig. 8B). The youngest is 1292 ± 34 Ma (grain 63, Fig. 9). Not used in the probability plot are 22 grains that were > 10 percent discordant and four that produced only 207Pb/ 206Pb ages. Aside from the youngest grain, the Leopold Creek quartzite sample has a detrital–age spectrum most similar to the Hoodoo Pass sample.

Fig. 8.—

Zircon age data from the Leopold Creek (LC) sample 04RL150. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray. Data-point error ellipses are 2σ. B) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Fig. 8.—

Zircon age data from the Leopold Creek (LC) sample 04RL150. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray. Data-point error ellipses are 2σ. B) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Fig. 9.—

Cathodoluminescence (CL) image of zircon 63 from Leopold Creek (LC) sample 04RL150. The 30 μm diameter pit shown in this image yielded a 207Pb/206Pb age of 1292 ± 34 Ma. The analysis for this pit has a low Th/U ratio (0.01) suggesting a metamorphic origin for this portion of the zircon.

Fig. 9.—

Cathodoluminescence (CL) image of zircon 63 from Leopold Creek (LC) sample 04RL150. The 30 μm diameter pit shown in this image yielded a 207Pb/206Pb age of 1292 ± 34 Ma. The analysis for this pit has a low Th/U ratio (0.01) suggesting a metamorphic origin for this portion of the zircon.

Dent Bridge (DB)

Sample DB (04RL151) is coarsely recrystallized biotite quartzite within a biotite–feldspar–quartz gneiss sequence just east of the Dent Bridge (DB, Fig. 1). Of 78 grains with 10 percent or less discordance (Fig. 10A), only five are in the range of the North American magmatic gap (Fig. 10B). Given the uncertainty of the analyses (25–35 Ma at 2a), there may be no ages in the gap. Omitted from the probability plot are 20 grains that were > 10 percent discordant and six that produced only 207Pb/206Pb ages. The youngest is 1392 ± 31 Ma. The bimodal distribution of ages with the major peak at about 1700 Ma and a secondary peak with syn–Belt ages (1450–1400 Ma), but with relatively few grains between 1600 and 1450 Ma (Fig. 10B), sets this sample apart from the previous four samples. The overall age range, however, is similar.

Fig. 10.—

Zircon age data from the Dent Bridge (DB) sample 04RL151. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray. Data-point error ellipses are 2a. B) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Fig. 10.—

Zircon age data from the Dent Bridge (DB) sample 04RL151. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray. Data-point error ellipses are 2a. B) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Bertha Hill (BH)

Sample BH (03RL146) is coarsely recrystallized quartzite from along Bingo Creek north of Pierce on the east side of Bertha Hill (BH, Fig. 1). This sample is atypical of most of the Bertha Hill quartzite in that it contains about seven percent feldspar (Fig. 5C); most Bertha Hill exposures contain 2 percent or less, although locally as much as 10 percent plagioclase is present. The Bertha Hill sample has a zircon population significantly different from the six other metasedimentary samples analyzed. Of the 90 grains that are less than 10 percent discordant, the youngest is 1614 ± 45 Ma (Fig. 11A), and the sample has large peaks at about 2650 and 1800 Ma (Fig. 11B). This sample differs from all the other samples analyzed in this study by the absence of syn–Belt (1470–1400 Ma) ages and the abundance of old (2700–2600 Ma) ages. Not used in the probability plot are 23 grains that were > 10 percent discordant and six that produced only 207Pb/206Pb ages. The Bertha Hill quartzite has a zircon population similar to the Laurentian–sourced sediments described by Ross and Villeneuve (2003) and contrasts strongly with the non–Laurentian populations in the Hoodoo Pass, Beaver Creek, and Snake Creek samples, as well as with a Ravalli Group sample analyzed by Ross and Villeneuve (2003).

Fig. 11.—

Zircon age data from the Bertha Hill (BH) sample 03RL146. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray. Data-point error ellipses are 2σ. B) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Fig. 11.—

Zircon age data from the Bertha Hill (BH) sample 03RL146. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray. Data-point error ellipses are 2σ. B) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

North Fork (NF)

Sample NF (03RL148) is calc–silicate gneiss from the North Fork of the Clearwater River 3.5 km east of Aquarius Campground (NF, Fig. 1). Pegmatite veins 1 cm to 2 m in width are common in the outcrop, but we attempted to avoid these in sampling. Because this calc–silicate gneiss had a carbonate protolith, it, like the Beaver Creek calc–silicate gneiss, was thought to be a high–grade equivalent of the Wallace Formation or a carbonate–bearing facies of the Prichard Formation. However, it differs from the nearby samples in that it contains a significant number of concordant zircons with ages younger than the Belt Supergroup (< 1400 Ma), including a population of late Cretaceous metamorphic zircons (Figs. 12A, B). Analytical results show that 16 of 65 zircons with discordance < 10 percent have 207Pb/206Pb ages between 1400 and 1350 Ma and 26 are < 1350 Ma (Fig. 12C). All of these have lower Th/U ratios (< 0.16) than zircons with older ages (Fig. 13), consistent with growth during a metamorphic event. The relatively young (1380–800 Ma) zircons in this sample indicate either (1) lead loss or zircon growth during a postdepositional event, or (2) a young protolith that postdates the Belt–Purcell Supergroup. In addition to the presence of young zircons, this sample also differs from the Hoodoo Pass, Beaver Creek, Snake Creek, and Leopold Creek samples in the near absence of 1750–1600 Ma grains.

Fig. 12.—

Zircon age data from the North Fork (NF) sample 03RL148. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray for concordia range between 2400 and 800 Ma. Data-point error ellipses are 2σ. B) Tera-Waserburg plot showing only 105-55 Ma portion of concordia. Data-point error ellipses are 2a. Data in black yield a weighted mean of 87.0 ± 1.3 Ma. C) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Fig. 12.—

Zircon age data from the North Fork (NF) sample 03RL148. A) Tera-Waserburg plot showing grains less than 10 percent discordant in black and those greater than 10 percent discordant in gray for concordia range between 2400 and 800 Ma. Data-point error ellipses are 2σ. B) Tera-Waserburg plot showing only 105-55 Ma portion of concordia. Data-point error ellipses are 2a. Data in black yield a weighted mean of 87.0 ± 1.3 Ma. C) Age-density probability distribution diagram and histogram for zircon grains less than 10 percent discordant.

Fig. 13.—

Plot of Th/U ratios versus 207Pb/206Pb age for detrital zircons between 700 and 2000 Ma. Note the correlation of low Th/U values with young ages in the Leopold Creek and North Fork samples.

Fig. 13.—

Plot of Th/U ratios versus 207Pb/206Pb age for detrital zircons between 700 and 2000 Ma. Note the correlation of low Th/U values with young ages in the Leopold Creek and North Fork samples.

Augen Gneiss West of Dent (WD)

Sample WD (00RL595) was collected from a granitic augen gneiss along the shoreline of Dworshak Reservoir west–north–west of Pierce and about 5.5 km northwest of the Dent Bridge detrital zircon sample (WD, Fig. 1). The sill–like body is in paragneiss tentatively assigned a Belt–Purcell protolith (same unit as that of DB sample). U–Pb zircon SHRIMP age determinations on 14 oscillatory–zoned zircon cores yielded discordant to concordant analyses with 204Pb–corrected 207Pb/206Pb ages that range from 1387 to 1318 Ma (Table 2, Fig. 14). The seven oldest concordant analyses from low–U–concentration cores (Table 2) yield an age of 1379 ± 12 Ma (Fig. 14B). The location of one of the seven analyses (analysis 5) is shown in a cathodoluminescence (CL) image in Figure 15. Discordance and younger Proterozoic 207Pb/206Pb ages observed for the remaining seven analyses are interpreted to reflect Pb loss or metamictization during subsequent Proterozoic (e.g., McClelland et al., 2005; Vervoort et al., 2005) or younger metamorphism, because these have low Th/U ratios (0.01–0.20). Three zircon rims were analyzed, one of which is shown in Figure 15 (analysis 12). Rims with U concentrations that range from 2300 to 4300 ppm and 207Pb–corrected 206Pb/238U ages ranging from 87 to 82 Ma are interpreted to reflect the time of zircon growth during Cretaceous metamorphism (Table 2, Fig. 14).

Fig. 14.—

Zircon age data from augen gneiss west of Dent (WD), sample 00RL595. A) Tera-Waserburg plot of all zircon ages; MSWD = mean square of weighted deviates. B) Tera-Waserburg plot showing only ages of cores.

Fig. 14.—

Zircon age data from augen gneiss west of Dent (WD), sample 00RL595. A) Tera-Waserburg plot of all zircon ages; MSWD = mean square of weighted deviates. B) Tera-Waserburg plot showing only ages of cores.

Fig. 15.—

Cathodoluminescence (CL) images of zircon grains in augen gneiss sample WD showing spots analyzed that yielded intrusive (spot 5) and metamorphic (spot 12) ages. Data in Table 2.

Fig. 15.—

Cathodoluminescence (CL) images of zircon grains in augen gneiss sample WD showing spots analyzed that yielded intrusive (spot 5) and metamorphic (spot 12) ages. Data in Table 2.

Discussion

The detrital–zircon ages provide useful information, but it is important to keep in mind that although the age of deposition cannot be older than the youngest detrital zircon it can be considerably younger. Also, sediment supply to any one place of deposition may vary with time, and detrital–zircon age spectra may vary within a unit. Nevertheless, detrital zircons with syn–Belt ages (1470–1400 Ma) definitely rule out correlation with pre–Belt units.

The one low–metamorphic–grade Belt sample (Hoodoo Pass) and three of the high–grade metasedimentary samples (Beaver Creek, Snake Creek, and Leopold Creek) contain zircon populations with ages within the North American magmatic gap (16101490 Ma) and overall are similar to zircon populations reported by Ross and Villeneuve (2003) for the western facies of the Belt–Purcell Supergroup (Fig. 16). The Beaver Creek, Snake Creek, and Leopold Creek samples contain a significant number of grains with 207Pb/206Pb age ages between 1450 and 1380 Ma (16 total). Thus their protoliths are probably younger than the Prichard Formation, which is known to be about 1470 Ma (Anderson and Davis, 1995; Sears et al., 1998). Some of the youngest 207Pb/206Pb ages are younger than Belt. The youngest Beaver Creek zircon age (1366 ± 25 Ma and 9.9 percent discordance) is within error (25 Ma) of acceptable Belt ages but the youngest from the Leopold Creek sample (207Pb/206Pb age of 1292 ± 34 Ma; 4.5 percent discordant) is not. However, the analyzed spot may have sampled recrystallized (metamorphic) material (relatively uniform gray areas in Fig. 9). This grain has a low Th/U ratio (0.01), as do the eight other grains in this sample that are less than 1400 Ma (all have Th/U < 0.04; Fig. 13). Zircons with Th/U ratios less than 0.1 are generally thought to be metamorphic in origin (Williams and Claesson, 1987; Hoskin and Black, 2000; Rubatto et al., 2001), and we believe that these nine grains are also metamorphic in origin. The similarity of the zircon age distributions from the Beaver Creek, Snake Creek, and Leopold Creek high–grade samples indicates that they were derived from Belt–Purcell units, and probably the upper Belt (middle Belt carbonate or above).

Fig. 16.—

Comparison of age-density probability-distribution diagrams from our study and Ross and Villeneuve (2003). The gray bands correspond to the North American magmatic gap (1610-1490 Ma).

Fig. 16.—

Comparison of age-density probability-distribution diagrams from our study and Ross and Villeneuve (2003). The gray bands correspond to the North American magmatic gap (1610-1490 Ma).

The Dent Bridge sample has a suite of 1450–1400 Ma (syn–Belt) zircons similar to the samples mentioned above, but only four zircons within the age range of the North American magmatic gap (Fig. 10B). Given the analytical uncertainty of about 25–35 Ma, the significance of these four zircons is unclear. However, the metasedimentary rocks at Dent Bridge must be older than the 1380 Ma augen gneiss (WD sample) that intrudes them. Sixteen zircons in the Dent Bridge samples are between 1390 and 1450 Ma (Th/U > 0.13). Thus, the protolith of the Dent Bridge sample, like that for the Beaver Creek, Snake Creek, and Leopold Creek, is probably upper Belt equivalent. The non–Laurentian zircons found in most samples are attributed to derivation from a western source area that was rifted from Laurentia in the Neoproterozoic (initial rifting at about 780 Ma; Stewart, 1972, Lund et al., 2003). These results indicate that the likely protoliths of most high– grade metamorphic rocks northwest of the idaho batholith were upper strata of the Belt–Purcell Supergroup. The 1470–1400 Ma ages rule out the possibility that these rocks are older than the Belt.

The only sample that lacks zircons with ages in, or younger than, the North American magmatic gap is the Bertha Hill quartzite (Figs. 11B, 16). This unit was tentatively assigned to the Revett Formation of the Ravalli Group by Hietanen (1963a) on the basis of field observations, but its detrital zircon signature is significantly different from a known Ravalli Group sample that has been analyzed. The sample from east of Coeur d’Alene analyzed by Ross and Villeneuve (2003) contains western Belt, non–Laurentian zircons, which are absent from the Bertha Hill quartzite (Fig. 16). Instead, the Bertha Hill quartzite appears to have been derived from an older Laurentian source region. In addition to the striking difference in detrital–zircon populations, the Bertha Hill quartzite also lacks the relatively high feldspar content that characterizes the Revett to the north. Correlation of the Bertha Hill quartzite with sequences of known age, however, is highly problematic. Other quartzite–bearing units in the region that have Laurentian detrital zircons include, in order of decreasing age, (1) Neihart Formation of central Montana, (2) Aldridge Formation of southern Canada, (3) Fort Steele Formation of southern Canada, (4) Missoula Group of the Belt Supergroup in Montana, (5) Neoproterozoic Windermere Supergroup, and (6) Cambrian quartzite of southern Canada.

The Neihart Formation, which forms the basal Belt unit along the north side of the Helena embayment in central Montana (Keefer, 1972; Schieber, 1989), is typically feldspar poor, as is most of the Bertha Hill quartzite. It contains an older Laurentian zircon age distribution (Ross and Villeneuve, 2003; Mueller et al., 2003) similar to the one from Bertha Hill. The Aldridge Formation (Prichard equivalent in Canada) locally contains quartz arenite with Laurentian zircons (Ross and Villenueve, 2003), but also feldspar–rich quartzite. The Fort Steele Formation of southern Canada (Höy, 1992) has a similar zircon age distribution, although it contains more zircons with 2800–2400 Ma ages than does the Neihart (Ross and Villenueve, 2003) or our Bertha Hill sample. Although it occupies a stratigraphic position similar to that of the Neihart in that it is low in the section, it is feldspar–rich (typically 20 percent or more). The Missoula Group also contains only older Laurentian zircons, but quartzite in the Missoula Group is feldspar–rich (typically 20 percent). Younger quartzite–bearing intervals in the region include the Neoproterozoic Windermere Supergroup (Lund et al., 2003) and Cambrian strata (Bush, 1989). The quartzite in the lower part of the Windermere is generally feldspathic, contains lenses of conglomerate, and is associated with thick carbonate intervals and volcanic rocks (Miller, 1994). In contrast, the upper part of the Windermere does contain intervals of clean quartzite, such as the Umbrella Butte Formation in the Gospel Hump Wilderness southeast of Grangeville, Idaho (Lund et al., 2003). Cambrian quartzites in the region are also similar to the Bertha Hill quartzite in that they both generally lack feldspars. Lithologically, the more likely correlatives to the Bertha Hill quartzite are the Neihart Formation, Cambrian strata, and, more locally, the quartzite of the Syringa metamorphic sequence. Because the Syringa now appears to be Neoproterozoic (Lund et al., 2005; Lewis et al., 2005), the Bertha Hill quartzite may also be this age. There are two implications of a potential Neoproterozoic age for the Bertha Hill quartzite. First, the lithostratigraphic packages of Neoproterozoic rocks in the area lack coarse, feldspathic clastics, diamictites, and igneous components as found in the Windermere to the northwest and in the Big Creek area to the south (Lund et al., 2003). Second, the sedimentary packages may lack diagnostically young zircons because of diverse sources, some of which may be similar to sources that fed basal Belt and basal Paleozoic successions. The southeastern Washington quartzites (Kamiak and Steptoe Buttes), which have detrital–zircon age spectra very similar to the Bertha Hill quartzite (Ellis et al., 2004; see Regional Geology above), have the same correlation possibilities as the Bertha Hill rocks.

Most samples we analyzed have zircons that do not appear to have been significantly affected by metamorphism, inasmuch as most data plot in a restricted age range on or near concordia. The two exceptions to this are the Leopold Creek and North Fork samples. The Leopold Creek sample appears to have a minor component of metamorphic zircon as mentioned above, but the North Fork sample has a strong metamorphic overprint resulting in a wide distribution of zircon ages, including some as young as Late Cretaceous. Eight zircons in the North Fork sample have 206Pb/238U ages between 90 and 61 Ma (Fig. 12B). All eight zircons have Th/U ratios < 0.05. Six of eight analyses (shown in black in Fig. 12B) yield a weighted average 206Pb/238U age of 87.0 ± 1.3 Ma (MSWD = 0.97). Zircons with Th/U ratios less than 0.1 are generally thought to be metamorphic in origin, and we believe that these eight grains are also formed by metamorphism. The age of the zircon rims within the augen gneiss sample from west of Dent (87–82 Ma; Fig. 12), as well as an 89.6 ± 2.6 Ma Lu–Hf age from a garnet amphibolite from the Dent Bridge area (Vervoort et al., 2005), corroborate the timing of this metamorphic event. Although Cretaceous metamorphism in the region has been long suspected (e.g., Hietanen, 1962), these data more definitively establish the age of at least one high–grade metamorphic event. More problematic than the Cretaceous zircon ages are the 24 grains between 1350 and 800 Ma (Fig. 12C). The relatively young zircons in this sample were previously thought to reflect a young protolith that postdated the Belt–Purcell Supergroup (Lewis et al., 2004). A closer look at the data indicates that the zircons with younger ages (less than about 1400 Ma) are characterized by Th/ U < 0.1, as shown in Figure 13, which plots Th/U as a function of 207Pb/206Pb age. These ratios are consistent with zircon growth during metamorphism, but the near concordance of most of these analyses makes it unlikely that lead loss or zircon growth during the Cretaceous is the cause of the young ages. More likely, a late Proterozoic event affected either the rock or the source of the detrital zircons. The protolith could be Belt, but the paucity of high Th/U analyses makes it difficult to compare the age distribution with Belt samples. If this sample is a Belt equivalent, it could have been affected by the 1.0 Ga event recorded in garnet in the Boehls Butte core complex to the northwest (Sha, 2004; McClelland et al., 2005; Vervoort et al., 2005), but this would not explain ages younger than 1 Ga. Alternatively, if the sediment source had been affected and the zircons later eroded, the North Fork sample is younger than the Belt. We cannot correlate the North Fork sample with other units currently exposed in the region, but we suspect that it is younger than the Belt–Purcell Supergroup.

The 1379 ± 12 Ma intrusive age we obtained for the granitic augen gneiss west of Dent is consistent with the age of texturally and compositionally similar augen gneisses to the south near Elk City (Evans and Fisher, 1986) and in east–central Idaho near Shoup (Evans and Zartman, 1990; Doughty and Chamberlain, 1996). The new date expands the known extent of anorogenic A–type Proterozoic intrusions in northern Idaho and places an important age constraint on the metasedimentary rocks near Dent. The original sediments were deposited after about 1400 Ma (15 detrital zircons were 1450–1400 Ma) not long before intrusion of the granite at about 1380 Ma.

Results of this study show that it is possible to obtain useful ages from detrital zircons in amphibolite–grade metasedimentary rocks and that the LA–ICPMS method is an effective way to do it. Some of the high–grade metasedimentary rocks northwest of the Idaho batholith contain zircons with ages similar to those in low– grade Belt–Purcell Supergroup rocks to the north, indicating that upper units of the Belt–Purcell basin probably extend at least as far southwest as Pierce. An exception is the quartzite at Bertha Hill, which has only an older (> 1614 Ma) Laurentian zircon population. The quartzite at Bertha Hill may be either older or younger than the Belt–Purcell Supergroup (Neihart equivalent, Windermere equivalent, or Cambrian), or an eastern–sourced Belt–Purcell unit (such as the Fort Steele Formation in Canada). We prefer the hypothesis that the Bertha Hill quartzite is younger than the Belt, given that detrital–zircon ages from nearby Beaver Creek and Snake Creek samples support only upper Belt–Purcell protoliths with western sources. Also, the lack of expected Prichard Formation lithologies (sulfide–rich schist, gneiss, and quartzite with amphibolite sills) favors a younger age for the Bertha Hill quartzite. Despite the presence of the Paleoproterozoic Boehls Butte anorthosite in the area, we found no support for presence of Belt basement south of it. The complexity of metasedimentary rocks in this region cannot be resolved with field mapping alone. Detrital–zircon studies help to provide important constraints on the geological relationships in this metamorphosed region of the North American Cordillera.

References

Anderson
,
J.L.
,
1983
,
Proterozoic anorogenic plutonism of North America
,
in
 
Medaris
,
L.G.
Jr.
Byers
,
C.W.
Mickelson
,
D.M.
Shanks
,
W.C.
, eds., Proterozoic Geology: Selected Papers from an International Symposium: Geological Society of America, Memoir 161, p.
133
154
.
Anderson
,
H.E.
Davis
,
D.W.
,
1995
,
U-Pb geochronology of the Moyie Sills, Purcell Supergroup, southeastern British Columbia: Implications for the Mesoproterozoic geological history of the Purcell (Belt) Basin: Canadian Journal of Earth Sciences
,
v. 32
, p.
1180
1193
.
Armstrong
,
R.L.
,
1975
,
Precambrian (1500 m.y. old) rocks of central Idaho—The Salmon River arch and its role in Cordilleran sedimentation and tectonics: American Journal of Science
,
v. 275-A
, p.
437
467
.
Armstrong
,
R.L.
Parrish
,
R.R.
van der Heyden
,
P.
Scott
,
K.
Runkle
,
D.
Brown
,
R.L.
,
1991
,
Early Proterozoic basement exposures in the southern Canadian Cordillera: Core gneiss of Frenchman Cap, Unit 1 of the Grand Forks Gneiss, and Vaseaux Formation: Canadian Journal of Earth Sciences
,
v. 28
, p.
1169
1201
.
Black
,
L.P.
Kamo
,
S.L.
Allen
,
C.M.
Aleinikoff
,
J.N.
Davis
,
D.W.
Korsch
,
R.J.
Foudoulis
,
C.
,
2003
,
TEMORA 1: A new zircon standard for Phanerozoic U-Pb geochronology: Chemical Geology
,
v. 200
, p.
155
170
.
Black
,
L.P.
Kamo
,
S.L.
Allen
,
C.M.
Davis
,
D.W.
Aleinkoff
,
J.N.
Valley
,
J.W.
Mundii
,
R.
Campbell
,
I.H.
Korscha
,
R.J.
Williams
,
I.S.
Foudoulisa
,
C.
,
2004
,
Improved 206Pb/238U microprobe geochronology by the monitoring of a trace element-related matrix effect: SHRiMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards: Chemical Geology
,
v. 205
, p.
115
140
.
Bush
,
J.H.
,
1989
,
The Cambrian system of northern Idaho and northwestern Montana
,
in
 
Chamberlain
,
V.E.
Breckenridge
,
R.M.
Bonnichsen
Bill
, eds., Guidebook to the Geology of Northern and Western idaho and Surrounding Areas: idaho Geological Survey, Bulletin 28, p.
103
121
.
Chang
,
Z.
Vervoort
,
J.D.
McClelland
,
W.C.
Knaack
,
C.
,
2006
,
U-Pb dating of zircon by LA-ICP-MS: Geochemistry, Geophysics, Geosystems
,
v. 7
,
no. 5
, p.
1
14
.
Cressman
,
E.R.
,
1989
,
Reconnaissance stratigraphy of the Prichard Formation (Middle Proterozoic) and the early development of the Belt Basin, Washington, Idaho, and Montana: U.S. Geological Survey, Professional Paper 1490
,
80
p.
Crowley
,
J.L.
,
1997
,
U-Pb geochronologic constraints on the cover sequence of the Monashee complex, Canadian Cordillera: Paleoproterozoic deposition on basement: Canadian Journal of Earth Sciences
,
v. 34
, p.
1008
1022
.
Doughty
,
P.T.
Buddington
,
A.M.
,
2002
,
Eocene structural evolution of the Boehls Butte anorthosite and Clearwater core complex, north central idaho: A basement-involved extensional strike-slip relay (abstract): Geological Society of America, Abstracts with Programs
,
v. 34
,
no. 6
, p.
332
.
Doughty
,
P.T.
Chamberlain
,
K.R.
,
1996
,
Salmon River arch revisited: New evidence for 1370 Ma rifting near the end of deposition in the Middle Proterozoic Belt basin: Canadian Journal of Earth Science
,
v. 33
, p.
1037
1052
.
Doughty
,
P.T.
Price
,
R.A.
Parrish
,
R.R.
,
1998
,
Geology and U-Pb geochronology of Archean basement and Proterozoic cover in the Priest River complex, northwestern United States, and their implications for Cordilleran structure and Precambrian continent reconstructions: Canadian Journal of Earth Sciences
,
v. 35
, p.
39
54
.
Ellis
,
J.R.
Pope
,
M.C.
McClelland
,
W.C.
Vervoort
,
J.D.
,
2004
,
Quartzite buttes in the Palouse region, SE Washington: Their relationship to the Belt basin on the basis of U-Pb LA-ICPMS detrital zircon data (abstract): Geological Society of America, Abstracts with Programs
,
v. 36
,
no. 4
, p.
7
.
Evans
,
K.V.
Aleinikoff
,
J.N.
Obradovich
,
J.D.
Fanning
,
C.M.
,
2000
,
SHRIMP U-Pb geochronology of volcanic rocks, Belt Supergroup, western Montana: Evidence for rapid deposition of sedimentary strata: Canadian Journal of Earth Sciences
,
v. 37
, p.
1287
1300
.
Evans
,
K.V.
Fischer
,
L.B.
,
1986
,
U-Pb geochronology of two augen gneiss terranes, idaho—new data and tectonic implications: Canadian Journal of Earth Sciences
,
v. 23
, p.
1919
1927
.
Evans
,
K.V.
Zartman
,
R.E.
,
1990
,
U-Th-Pb and Rb-Sr geochronology of Middle Proterozoic granite and augen gneiss, Salmon River Mountains, east-central Idaho: Geological Society of America, Bulletin
,
v. 102
, p.
63
73
.
Freeman
,
W.
Winston
,
D.
,
1987
,
A quartz arenite blanket at the base of, or below the Middle Proterozoic Belt Supergroup? Montana and Idaho (abstract): Geological Society of America, Abstracts with Programs
,
v. 19
,
no. 5
, p.
276
.
Hietanen
,
A.
,
1962
,
Metasomatic metamorphism in western Clearwater County, Idaho: U.S. Geological Survey, Professional Paper 344-A
,
113
p., scale 1:48,000.
Hietanen
,
A.
,
1963a
,
Anorthosite and associated rocks in the Boehls Butte quadrangle and vicinity, Idaho: U.S. Geological Survey, Professional Paper 344-B
,
78
p., scale 1:48,000.
Hietanen
,
A.
,
1963b
,
Idaho batholith near Pierce and Bungalow, Clearwater County, Idaho: U.S. Geological Survey, Professional Paper 344-D
,
42
p., scale 1:48,000.
Hietanen
,
A.
,
1984
,
Geology along the northwest border zone of the Idaho batholith: U.S. Geological Survey, Bulletin 1608
,
16
p.
Hoskin
,
P.W.O.
Black
,
L.P.
,
2000
,
Metamorphic zircon formation by solid-state recrystallization of protolith igneous zircon: Journal of Metamorphic Geology
,
v. 18
, p.
423
439
.
Höy
,
T.
,
1992
,
Geology of the Purcell Supergroup in the Fernie west-half map area, southeastern British Columbia: Victoria, British Columbia, Ministry of Energy, Mines and Petroleum Resources, Bulletin 84
,
157
p.
Höy
,
T.
Anderson
,
D.
Turner
,
R.J.W.
Leitch
,
C.H.B.
,
2000
,
Tectonic, magmatic, and metallogenic history of the early synrift phase of the Purcell Basin, southeastern British Columbia
,
in
 
Lydon
,
J.W.
Höy
,
T.
Slack
,
J.F.
Knapp
,
M.E.
, eds.,
The Geological Environment of the Sullivan Deposit, British Columbia: Geological Association of Canada, Mineral Deposits Division, Special Publication 1
, p.
32
60
.
Keefer
,
W.R.
,
1972
,
Geologic map of the west half of the Neihart 15- minute quadrangle, central Montana: U.S. Geological Survey, Miscellaneous Geologic Investigations Map
, I-0726.
Klepeis
,
K.A.
Clarke
,
G.L.
Dazko
,
N.
Gehrels
,
G.E.
Vervoort
,
J.D.
,
2004
,
Processes controlling vertical coupling and decoupling between the upper and lower crust of orogens: Results from Fjordland, New Zealand: Journal of Structural Geology
,
v. 26
, p.
765
791
.
Kopp
,
R.S.
,
1959
,
Petrology and structural analysis of the Orofino meta-morphic unit: University of Idaho
, M.S. thesis,
73
p.
Kosler
,
J.
Fonneland
,
H.
Sylvester
,
P.
Tubrett
,
M.
Pedersen
,
R-B.
,
2002
U-Pb dating of detrital zircons for sediment provenance studies: a comparison of laser ablation ICPMS and SIMS techniques: Chemical Geology
,
v. 182
, p.
605
618
.
Lewis
,
R.S.
Burmester
,
R.F.
Bennett
,
E.H.
,
1998
,
Metasedimentary rocks between the Bitterroot and Atlanta lobes of the of the Idaho batholith and their relationship to the Belt Supergroup
,
in
 
Berg
,
R.B.
, ed., Belt Symposium III: Montana Bureau of Mines and Geology, Special Publication 112, p.
130
144
.
Lewis
,
R.S.
Burmester
,
R.F.
Reynolds
,
R.W.
Bennett
,
E.H.
Myers
,
P.E.
Reid
,
R.R.
,
1992
,
Geologic map of the Lochsa River area, northern Idaho: Idaho Geological Survey, Geological Map Series GM-19
, scale 1:100,000.
Lewis
,
R.S.
Burmester
,
R.F.
Oswald
,
P.
Vervoort
,
J.D.
,
2005
,
Detrital zircon constraints on Neoproterozoic sediment distribution and tectonic elements near the Clearwater River, idaho (abstract): Geological Society of America, Abstracts with Programs
,
v. 37
,
no. 7
, p.
218
.
Lewis
,
R.S.
Bush
,
J.H.
Burmester
,
R.F.
Kauffman
,
J.D.
Garwood
,
D.L.
Myers
,
P.E.
Othberg
,
K.L.
,
2005
,
Geologic map of the Potlatch 30’ x 60’ quadrangle, Idaho: Idaho Geological Survey, Geological Map 41
, scale 1:100,000.
Lewis
,
R.S.
Vervoort
,
J.D.
McClelland
,
W.C.
Chang
,
Z.
,
2004
,
Age constraints on metasedimentary rocks northwest of the idaho batholith based on detrital zircons and intrusive sills (abstract): Geological Society of America, Abstracts with Programs
,
v. 36
,
no. 4
, p.
87
.
Ludwig
,
K.R.
,
2001a
,
Isoplot/EX rev. 2.49: A geochronological toolkit for Microsoft Excel: Berkeley Geochronology Center, Special Publication 1a
, p.
1
58
.
Ludwig
,
K.R.
,
2001b
,
Squid version 1.02: A users manual: Berkeley Geochronology Center, Special Publication 2
, p.
1
22
.
Lund
,
K.
Aleinikoff
,
J.N.
Evans
,
K.V.
Fanning
,
C.M.
,
2003
,
SHRIMP U-Pb geochronology of Neoproterozoic Windermere Supergroup, central idaho: implications for rifting of western Laurentia and synchroneity of Sturtian glacial deposits: Geological Society of America, Bulletin
,
v. 115
, p.
349
372
.
Lund
,
K.I.
Aleinikoff
,
J.N.
Unruh
,
D.M.
Yacob
,
E.Y.
Fanning
,
C.M.
,
2005
,
Evolution of the Salmon River suture and continental delamination in the Syringa embayment (abstract): 15th Annual V.M. Goldschmidt Conference Abstracts, Special Supplement to Geochimica et Cosmochimica Acta
, p.
A246
.
McClelland
,
W.C.
Vervoort
,
J.D.
Oldow
,
J.S.
Watkinson
,
A.J.
Shaw
,
G.S.
,
2005
,
Grenville-age metamorphism on the western margin of Laurentia, northern Idaho: Evidence from Lu-Hf garnet geochronology (abstract): 15th Annual V.M. Goldschmidt Conference Abstracts, Special Supplement to Geochimica et Cosmochimica Acta
, p.
A305
.
Miller
,
F.K.
,
1994
,
The Windermere Group and Late Proterozoic tectonics in Northeastern Washington and Northern idaho: Regional Geology of Washington State, Washington Department of Natural Resources, Bulletin 80
, p.
1
19
.
Mueller
,
P.
Foster
,
D.
Wooden
,
J.
Mogk
,
D.
Lewis
,
R.
,
2003
,
Archean and Proterozoic sources for basal quartzites from the eastern and western margins of the Belt basin: Northwest Geology
,
v. 32
, p.
215
216
.
Parkinson
,
D.
,
1991
,
Age and isotopic character of Early Proterozoic basement gneisses in the southern Monashee complex, southeastern British Columbia: Canadian Journal of Earth Sciences
,
v. 28
, p.
1159
1168
.
Reid
,
R.R.
Morrison
,
D.A.
Greenwood
,
W.R.
,
1973
,
The Clearwater Orogenic zone: A relict of Proterozoic orogeny in central and northern Idaho: Belt Symposium I
,
v. 1
, Idaho Bureau of Mines and Geology, p.
10
56
.
Ross
,
G.M.
Parrish
,
R.R.
,
1991
,
Detrital zircon geochronology of metasedimentary rocks in the southern omineca Belt, Canadian Cordillera: Canadian Journal of Earth Sciences
,
v. 28
, p.
1254
1270
.
Ross
,
G.M.
Villeneuve
,
M.
,
2003
,
Provenance of the Mesoproterozoic (1.45 Ga) Belt basin (western North America): another piece in the pre-Rodinia paleogeographic puzzle: Geological Society of America, Bulletin
,
v. 115
, p.
1191
1217
.
Ross
,
G.M.
Parrish
,
R.R.
Winston
,
D.
,
1992
,
Provenance and U-Pb geochronology of the Mesoproterozoic Belt Supergroup (north-western United States): implications for age of deposition and pre-Panthalassa plate reconstructions: Earth and Planetary Science Letters
,
v. 113
, p.
57
76
.
Rubatto
,
D.
Williams
,
I.S.
Buick
,
I.S.
,
2001
,
Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia: Contributions to Mineralogy and Petrology
,
v. 140
, p.
458
468
.
Savage
,
C.N.
,
1973
,
A geological field trip in Benewah and Whitman counties, Idaho and Washington, respectively: Belt Symposium I
,
v. 1
,
University of Idaho
,
Moscow
, p.
253
322
.
Schieber
,
J.
,
1989
,
The origin of the Neihart Quartzite, a basal deposit of the mid-Proterozoic Belt Supergroup, Montana, U.S.A.: Geological Magazine
,
v. 126
, p.
271
281
.
Sears
,
J.W.
Chamberlain
,
K.R.
Buckley
,
S.N.
,
1998
,
Structural and U-Pb geochronological evidence for 1.47 Ga rifting in the Belt basin, western Montana, Canadian Journal of Earth Sciences
,
v. 35
, p.
467
475
.
Sha
,
G.S.
,
2004
,
The tectonic evolution of the Boehls Butte-Clearwater core complex, north-central idaho: Washington State University
, M.S. thesis,
143
p.
Smith
,
M.T.
Gehrels
,
G.E.
,
1991
,
Detrital zircon chronology of Upper Proterozoic to lower Paleozoic continental margin strata of the Kootenay arc: implications for the early Paleozoic tectonic development of the eastern Canadian Cordillera: Canadian Journal of Earth Sciences
,
v. 28
, p.
1271
1284
.
Stewart
,
J.H.
,
1972
,
Initial deposits in the Cordilleran geosyncline: Evidence of a Late Precambrian (< 850 m.y.) continental separation: Geological Society of America, Bulletin
,
v. 83
, p.
1345
1360
.
Sylvester
,
P.J
Ghaderi
,
M.
,
1997
,
Trace element analysis of scheelite by excimer laser ablation-inductively coupled plasma-mass spectrometry (ELA-ICP-MS) using a synthetic silicate glass standard: Chemical Geology
,
v. 141
, p.
49
65
.
Vervoort
,
J.D.
McClelland
,
W.C.
Oldow
,
J.S.
Watkinson
,
A.J.
Sha
,
G.S.
,
2005
,
Grenville-age metamorphism on the western margin of Laurentia, northern Idaho: evidence from Lu-Hf garnet geochronology (abstract): Geological Society of America, Abstracts with Programs
,
v. 37
,
no. 7
, p.
89
.
Vervoort
,
J.D.
Lewis
,
R.S.
Chang
,
Z.
,
2006
,
U-Pb detrital zircon analyses of metasedimentary rocks in the Pierce area, north-central Idaho: Idaho Geological Survey Digital Analytical Data 3, Excel spreadsheet
.
Williams
,
I.S.
,
1998
,
U-Pb by ion microprobe
,
in
 
McKibben
,
M.A.
Shanks
,
W.C.
Ridley
,
W.I.
, eds.,
Applications of Microanalytical Techniques to Understanding Mineralizing Processes: Society of Economic Geologists, Reviews in Economic Geology
,
v. 7
, p.
1
35
.
Williams
,
I.S.
Claesson
,
S.
,
1987
,
Isotopic evidence for the Precam-brian provenance and Caledonian metamorphism of high-grade paragneisses from the Seve Nappes, Scandinavian Caledonides, II. ion microprobe zircon U-Th-Pb: Contributions to Mineralogy and Petrology
,
v. 97
, p.
205
217
.
Winston
,
D.
,
1986
,
Sedimentology of the Ravalli Group, middle Belt carbonate, and Missoula Group, Middle Proterozoic Belt Super-group, Montana, Idaho and Washington
,
in
 
Roberts
,
S.M.
, ed., Belt Supergroup: A Guide to Proterozoic Rocks of Western Montana and Adjacent Areas: Montana Bureau of Mines and Geology, Special Publication 94, p.
85
124
.

Acknowledgments

Funding for our work has come from several sources, including the U.S. Geological Survey STATEMAP Program and National Science Foundation grants 0CE–0137365 and EAR–0230145 to Vervoort. We are grateful to George Gehrels for his help in establishing the LA–ICPMS technique at Washington State University. Charles Knaack of WSU was instrumental in establishing the optimum operating parameters for zircon analysis using the Element2. Discussions with Don Winston, Paul Link, Ted Doughty, Karen Lund, Gerry Ross, Tom Frost, Trygve Höy, and David Foster have helped considerably in our mapping efforts and our understanding of Proterozoic rocks in the region. Technical reviews by John Aleinikoff and Paul Mueller are gratefully acknowledged.

Figures & Tables

Table 1.—

Sample location and age summary.

Sample nameSample no.LatitudeLongitudeLithologyUnit207Pb/206Pb age results
HP (Hoodoo Pass)02MDM21946.9746N115.0235Wcalcareous quartziteWallace Fm.primarily 1900-1450 Ma
BC (Beaver Cr.)03RL14746.7672N115.6752Wcalc-silicate gneissmeta-Beltprimarily 1800-1400 Ma
SC (Snake Cr.)03RL14546.6023N115.9058Wbiotite quartzitemeta-Beltprimarily 1800-1400 Ma
LC (Leopold Cr.)04RL15046.6482N116.4426Wbiotite quartzitemeta-Beltprimarily 1750-1370 Ma; some metamorphic
DB (Dent bridge)04RL15146.6069N116.1712Wbiotite quartzitemeta-Beltprimarily 1450-1400 and 1800-1600 Ma
BH (Bertha Hill)03RL14646.7773N115.7410Wquartziteuncertainprimarily 1900-1700 and 2700-2400 Ma
NF (North Fork)03RL14846.8388N115.5742Wcalc-silicate gneissuncertainprimarily 1700-800 Ma; many metamorphic
WD (west of Dent)00RL59546.6339N116.2320Waugen gneissaugen gneiss1379 ± 12 Ma cores; 82-87 Ma rims
Sample nameSample no.LatitudeLongitudeLithologyUnit207Pb/206Pb age results
HP (Hoodoo Pass)02MDM21946.9746N115.0235Wcalcareous quartziteWallace Fm.primarily 1900-1450 Ma
BC (Beaver Cr.)03RL14746.7672N115.6752Wcalc-silicate gneissmeta-Beltprimarily 1800-1400 Ma
SC (Snake Cr.)03RL14546.6023N115.9058Wbiotite quartzitemeta-Beltprimarily 1800-1400 Ma
LC (Leopold Cr.)04RL15046.6482N116.4426Wbiotite quartzitemeta-Beltprimarily 1750-1370 Ma; some metamorphic
DB (Dent bridge)04RL15146.6069N116.1712Wbiotite quartzitemeta-Beltprimarily 1450-1400 and 1800-1600 Ma
BH (Bertha Hill)03RL14646.7773N115.7410Wquartziteuncertainprimarily 1900-1700 and 2700-2400 Ma
NF (North Fork)03RL14846.8388N115.5742Wcalc-silicate gneissuncertainprimarily 1700-800 Ma; many metamorphic
WD (west of Dent)00RL59546.6339N116.2320Waugen gneissaugen gneiss1379 ± 12 Ma cores; 82-87 Ma rims
Table 2.—

U-Pb geochronologic data and apparent ages for augen gneiss sample 00RL595 from west of Dent (WD; Figure 1).

SpotUThTh/U206Pb*af206Pbc*a238U/206Pb*b207Pb/206Pb*b206Pb/238U*c206Pb/238U*d207Pb/206Pb*d
(ppm)(ppm)(ppm)(Ma)(Ma)(Ma)
172798460.0231--78.16(0.5)0.0473(1.8)82.0 (0.4)82.0 (0.4)
34310790.02500.0674.32(0.3)0.0482(1.4)86.1 (0.3)86.2 (0.3)
122830810.03330.1673.68(0.4)0.0490(1.7)86.8 (0.4)87.0 (0.4)
421241050.053111.515.86(0.3)0.0852(0.4)1,016 (3)1,318 (8)
135741100.20116--4.25(0.4)0.0857(0.7)1,362 (5)1,323 (14)
11675910.14134--4.34(0.5)0.0854(2.5)1,338 (6)1,325 (48)
22182260.01431--4.34(0.2)0.0856(0.4)1,335 (3)1,327 (7)
15482610.1397--4.27(0.5)0.0864(0.8)1,356 (6)1,340 (15)
921651510.074320.024.30(0.2)0.0866(0.4)1,347 (3)1,350 (7)
124971320.05544--3.94(0.3)0.0868(0.3)1,457 (3)1,353 (6)
77301170.171490.014.22(0.4)0.0876(0.7)1,371 (5)1,371 (14)#
8669910.141350.074.25(0.4)0.0876(0.7)1,363 (5)1,374 (13)#
10133700.54270.234.33(0.9)0.0879(1.4)1,342 (11)1,381 (28)#
145581160.211050.674.57(0.5)0.0886(0.8)1,275 (5)1,382 (15)#
64841140.24100--4.16(0.5)0.0882(0.8)1,390 (6)1,382 (16)#
161631050.67340.094.14(0.9)0.0893(1.3)1,395 (11)1,385 (26)#
5341480.15700.124.21(0.7)0.0885(0.9)1,374 (8)1,387 (18)#
SpotUThTh/U206Pb*af206Pbc*a238U/206Pb*b207Pb/206Pb*b206Pb/238U*c206Pb/238U*d207Pb/206Pb*d
(ppm)(ppm)(ppm)(Ma)(Ma)(Ma)
172798460.0231--78.16(0.5)0.0473(1.8)82.0 (0.4)82.0 (0.4)
34310790.02500.0674.32(0.3)0.0482(1.4)86.1 (0.3)86.2 (0.3)
122830810.03330.1673.68(0.4)0.0490(1.7)86.8 (0.4)87.0 (0.4)
421241050.053111.515.86(0.3)0.0852(0.4)1,016 (3)1,318 (8)
135741100.20116--4.25(0.4)0.0857(0.7)1,362 (5)1,323 (14)
11675910.14134--4.34(0.5)0.0854(2.5)1,338 (6)1,325 (48)
22182260.01431--4.34(0.2)0.0856(0.4)1,335 (3)1,327 (7)
15482610.1397--4.27(0.5)0.0864(0.8)1,356 (6)1,340 (15)
921651510.074320.024.30(0.2)0.0866(0.4)1,347 (3)1,350 (7)
124971320.05544--3.94(0.3)0.0868(0.3)1,457 (3)1,353 (6)
77301170.171490.014.22(0.4)0.0876(0.7)1,371 (5)1,371 (14)#
8669910.141350.074.25(0.4)0.0876(0.7)1,363 (5)1,374 (13)#
10133700.54270.234.33(0.9)0.0879(1.4)1,342 (11)1,381 (28)#
145581160.211050.674.57(0.5)0.0886(0.8)1,275 (5)1,382 (15)#
64841140.24100--4.16(0.5)0.0882(0.8)1,390 (6)1,382 (16)#
161631050.67340.094.14(0.9)0.0893(1.3)1,395 (11)1,385 (26)#
5341480.15700.124.21(0.7)0.0885(0.9)1,374 (8)1,387 (18)#
Note: All analyses were performed on the SHRIMP-RG ion microprobe at the U.S. Geological Survey-Stanford Microanalytical Center at Stanford University. Calibration concentrations and isotopic compositions were based on replicate analyses of SL13 and R33 (419 Ma; Black et al., 2004). Analytical routine followed Williams (1998). Data reduction utilized SQUID program of Ludwig (2001b).
a

Pb* denotes radiogenic Pb; Pbc denotes common Pb; f206Pbc = 100*(206Pbc/206Pbtotai).

b

Reported ratios are not corrected for common Pb. Errors are reported in parentheses as percent at the 1 a level.

c

Ages calculated from ratios corrected for common Pb using 207Pb for the 206Pb/238U age. Uncertainties in millions of years reported as 1 a.

d

Ages calculated from ratios corrected for common Pb using 204Pb. Uncertainties in millions of years reported as 1 a. Ages denoted by # were used in calculation of weighted mean 207Pb/206Pb age.

Contents

GeoRef

References

References

Anderson
,
J.L.
,
1983
,
Proterozoic anorogenic plutonism of North America
,
in
 
Medaris
,
L.G.
Jr.
Byers
,
C.W.
Mickelson
,
D.M.
Shanks
,
W.C.
, eds., Proterozoic Geology: Selected Papers from an International Symposium: Geological Society of America, Memoir 161, p.
133
154
.
Anderson
,
H.E.
Davis
,
D.W.
,
1995
,
U-Pb geochronology of the Moyie Sills, Purcell Supergroup, southeastern British Columbia: Implications for the Mesoproterozoic geological history of the Purcell (Belt) Basin: Canadian Journal of Earth Sciences
,
v. 32
, p.
1180
1193
.
Armstrong
,
R.L.
,
1975
,
Precambrian (1500 m.y. old) rocks of central Idaho—The Salmon River arch and its role in Cordilleran sedimentation and tectonics: American Journal of Science
,
v. 275-A
, p.
437
467
.
Armstrong
,
R.L.
Parrish
,
R.R.
van der Heyden
,
P.
Scott
,
K.
Runkle
,
D.
Brown
,
R.L.
,
1991
,
Early Proterozoic basement exposures in the southern Canadian Cordillera: Core gneiss of Frenchman Cap, Unit 1 of the Grand Forks Gneiss, and Vaseaux Formation: Canadian Journal of Earth Sciences
,
v. 28
, p.
1169
1201
.
Black
,
L.P.
Kamo
,
S.L.
Allen
,
C.M.
Aleinikoff
,
J.N.
Davis
,
D.W.
Korsch
,
R.J.
Foudoulis
,
C.
,
2003
,
TEMORA 1: A new zircon standard for Phanerozoic U-Pb geochronology: Chemical Geology
,
v. 200
, p.
155
170
.
Black
,
L.P.
Kamo
,
S.L.
Allen
,
C.M.
Davis
,
D.W.
Aleinkoff
,
J.N.
Valley
,
J.W.
Mundii
,
R.
Campbell
,
I.H.
Korscha
,
R.J.
Williams
,
I.S.
Foudoulisa
,
C.
,
2004
,
Improved 206Pb/238U microprobe geochronology by the monitoring of a trace element-related matrix effect: SHRiMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards: Chemical Geology
,
v. 205
, p.
115
140
.
Bush
,
J.H.
,
1989
,
The Cambrian system of northern Idaho and northwestern Montana
,
in
 
Chamberlain
,
V.E.
Breckenridge
,
R.M.
Bonnichsen
Bill
, eds., Guidebook to the Geology of Northern and Western idaho and Surrounding Areas: idaho Geological Survey, Bulletin 28, p.
103
121
.
Chang
,
Z.
Vervoort
,
J.D.
McClelland
,
W.C.
Knaack
,
C.
,
2006
,
U-Pb dating of zircon by LA-ICP-MS: Geochemistry, Geophysics, Geosystems
,
v. 7
,
no. 5
, p.
1
14
.
Cressman
,
E.R.
,
1989
,
Reconnaissance stratigraphy of the Prichard Formation (Middle Proterozoic) and the early development of the Belt Basin, Washington, Idaho, and Montana: U.S. Geological Survey, Professional Paper 1490
,
80
p.
Crowley
,
J.L.
,
1997
,
U-Pb geochronologic constraints on the cover sequence of the Monashee complex, Canadian Cordillera: Paleoproterozoic deposition on basement: Canadian Journal of Earth Sciences
,
v. 34
, p.
1008
1022
.
Doughty
,
P.T.
Buddington
,
A.M.
,
2002
,
Eocene structural evolution of the Boehls Butte anorthosite and Clearwater core complex, north central idaho: A basement-involved extensional strike-slip relay (abstract): Geological Society of America, Abstracts with Programs
,
v. 34
,
no. 6
, p.
332
.
Doughty
,
P.T.
Chamberlain
,
K.R.
,
1996
,
Salmon River arch revisited: New evidence for 1370 Ma rifting near the end of deposition in the Middle Proterozoic Belt basin: Canadian Journal of Earth Science
,
v. 33
, p.
1037
1052
.
Doughty
,
P.T.
Price
,
R.A.
Parrish
,
R.R.
,
1998
,
Geology and U-Pb geochronology of Archean basement and Proterozoic cover in the Priest River complex, northwestern United States, and their implications for Cordilleran structure and Precambrian continent reconstructions: Canadian Journal of Earth Sciences
,
v. 35
, p.
39
54
.
Ellis
,
J.R.
Pope
,
M.C.
McClelland
,
W.C.
Vervoort
,
J.D.
,
2004
,
Quartzite buttes in the Palouse region, SE Washington: Their relationship to the Belt basin on the basis of U-Pb LA-ICPMS detrital zircon data (abstract): Geological Society of America, Abstracts with Programs
,
v. 36
,
no. 4
, p.
7
.
Evans
,
K.V.
Aleinikoff
,
J.N.
Obradovich
,
J.D.
Fanning
,
C.M.
,
2000
,
SHRIMP U-Pb geochronology of volcanic rocks, Belt Supergroup, western Montana: Evidence for rapid deposition of sedimentary strata: Canadian Journal of Earth Sciences
,
v. 37
, p.
1287
1300
.
Evans
,
K.V.
Fischer
,
L.B.
,
1986
,
U-Pb geochronology of two augen gneiss terranes, idaho—new data and tectonic implications: Canadian Journal of Earth Sciences
,
v. 23
, p.
1919
1927
.
Evans
,
K.V.
Zartman
,
R.E.
,
1990
,
U-Th-Pb and Rb-Sr geochronology of Middle Proterozoic granite and augen gneiss, Salmon River Mountains, east-central Idaho: Geological Society of America, Bulletin
,
v. 102
, p.
63
73
.
Freeman
,
W.
Winston
,
D.
,
1987
,
A quartz arenite blanket at the base of, or below the Middle Proterozoic Belt Supergroup? Montana and Idaho (abstract): Geological Society of America, Abstracts with Programs
,
v. 19
,
no. 5
, p.
276
.
Hietanen
,
A.
,
1962
,
Metasomatic metamorphism in western Clearwater County, Idaho: U.S. Geological Survey, Professional Paper 344-A
,
113
p., scale 1:48,000.
Hietanen
,
A.
,
1963a
,
Anorthosite and associated rocks in the Boehls Butte quadrangle and vicinity, Idaho: U.S. Geological Survey, Professional Paper 344-B
,
78
p., scale 1:48,000.
Hietanen
,
A.
,
1963b
,
Idaho batholith near Pierce and Bungalow, Clearwater County, Idaho: U.S. Geological Survey, Professional Paper 344-D
,
42
p., scale 1:48,000.
Hietanen
,
A.
,
1984
,
Geology along the northwest border zone of the Idaho batholith: U.S. Geological Survey, Bulletin 1608
,
16
p.
Hoskin
,
P.W.O.
Black
,
L.P.
,
2000
,
Metamorphic zircon formation by solid-state recrystallization of protolith igneous zircon: Journal of Metamorphic Geology
,
v. 18
, p.
423
439
.
Höy
,
T.
,
1992
,
Geology of the Purcell Supergroup in the Fernie west-half map area, southeastern British Columbia: Victoria, British Columbia, Ministry of Energy, Mines and Petroleum Resources, Bulletin 84
,
157
p.
Höy
,
T.
Anderson
,
D.
Turner
,
R.J.W.
Leitch
,
C.H.B.
,
2000
,
Tectonic, magmatic, and metallogenic history of the early synrift phase of the Purcell Basin, southeastern British Columbia
,
in
 
Lydon
,
J.W.
Höy
,
T.
Slack
,
J.F.
Knapp
,
M.E.
, eds.,
The Geological Environment of the Sullivan Deposit, British Columbia: Geological Association of Canada, Mineral Deposits Division, Special Publication 1
, p.
32
60
.
Keefer
,
W.R.
,
1972
,
Geologic map of the west half of the Neihart 15- minute quadrangle, central Montana: U.S. Geological Survey, Miscellaneous Geologic Investigations Map
, I-0726.
Klepeis
,
K.A.
Clarke
,
G.L.
Dazko
,
N.
Gehrels
,
G.E.
Vervoort
,
J.D.
,
2004
,
Processes controlling vertical coupling and decoupling between the upper and lower crust of orogens: Results from Fjordland, New Zealand: Journal of Structural Geology
,
v. 26
, p.
765
791
.
Kopp
,
R.S.
,
1959
,
Petrology and structural analysis of the Orofino meta-morphic unit: University of Idaho
, M.S. thesis,
73
p.
Kosler
,
J.
Fonneland
,
H.
Sylvester
,
P.
Tubrett
,
M.
Pedersen
,
R-B.
,
2002
U-Pb dating of detrital zircons for sediment provenance studies: a comparison of laser ablation ICPMS and SIMS techniques: Chemical Geology
,
v. 182
, p.
605
618
.
Lewis
,
R.S.
Burmester
,
R.F.
Bennett
,
E.H.
,
1998
,
Metasedimentary rocks between the Bitterroot and Atlanta lobes of the of the Idaho batholith and their relationship to the Belt Supergroup
,
in
 
Berg
,
R.B.
, ed., Belt Symposium III: Montana Bureau of Mines and Geology, Special Publication 112, p.
130
144
.
Lewis
,
R.S.
Burmester
,
R.F.
Reynolds
,
R.W.
Bennett
,
E.H.
Myers
,
P.E.
Reid
,
R.R.
,
1992
,
Geologic map of the Lochsa River area, northern Idaho: Idaho Geological Survey, Geological Map Series GM-19
, scale 1:100,000.
Lewis
,
R.S.
Burmester
,
R.F.
Oswald
,
P.
Vervoort
,
J.D.
,
2005
,
Detrital zircon constraints on Neoproterozoic sediment distribution and tectonic elements near the Clearwater River, idaho (abstract): Geological Society of America, Abstracts with Programs
,
v. 37
,
no. 7
, p.
218
.
Lewis
,
R.S.
Bush
,
J.H.
Burmester
,
R.F.
Kauffman
,
J.D.
Garwood
,
D.L.
Myers
,
P.E.
Othberg
,
K.L.
,
2005
,
Geologic map of the Potlatch 30’ x 60’ quadrangle, Idaho: Idaho Geological Survey, Geological Map 41
, scale 1:100,000.
Lewis
,
R.S.
Vervoort
,
J.D.
McClelland
,
W.C.
Chang
,
Z.
,
2004
,
Age constraints on metasedimentary rocks northwest of the idaho batholith based on detrital zircons and intrusive sills (abstract): Geological Society of America, Abstracts with Programs
,
v. 36
,
no. 4
, p.
87
.
Ludwig
,
K.R.
,
2001a
,
Isoplot/EX rev. 2.49: A geochronological toolkit for Microsoft Excel: Berkeley Geochronology Center, Special Publication 1a
, p.
1
58
.
Ludwig
,
K.R.
,
2001b
,
Squid version 1.02: A users manual: Berkeley Geochronology Center, Special Publication 2
, p.
1
22
.
Lund
,
K.
Aleinikoff
,
J.N.
Evans
,
K.V.
Fanning
,
C.M.
,
2003
,
SHRIMP U-Pb geochronology of Neoproterozoic Windermere Supergroup, central idaho: implications for rifting of western Laurentia and synchroneity of Sturtian glacial deposits: Geological Society of America, Bulletin
,
v. 115
, p.
349
372
.
Lund
,
K.I.
Aleinikoff
,
J.N.
Unruh
,
D.M.
Yacob
,
E.Y.
Fanning
,
C.M.
,
2005
,
Evolution of the Salmon River suture and continental delamination in the Syringa embayment (abstract): 15th Annual V.M. Goldschmidt Conference Abstracts, Special Supplement to Geochimica et Cosmochimica Acta
, p.
A246
.
McClelland
,
W.C.
Vervoort
,
J.D.
Oldow
,
J.S.
Watkinson
,
A.J.
Shaw
,
G.S.
,
2005
,
Grenville-age metamorphism on the western margin of Laurentia, northern Idaho: Evidence from Lu-Hf garnet geochronology (abstract): 15th Annual V.M. Goldschmidt Conference Abstracts, Special Supplement to Geochimica et Cosmochimica Acta
, p.
A305
.
Miller
,
F.K.
,
1994
,
The Windermere Group and Late Proterozoic tectonics in Northeastern Washington and Northern idaho: Regional Geology of Washington State, Washington Department of Natural Resources, Bulletin 80
, p.
1
19
.
Mueller
,
P.
Foster
,
D.
Wooden
,
J.
Mogk
,
D.
Lewis
,
R.
,
2003
,
Archean and Proterozoic sources for basal quartzites from the eastern and western margins of the Belt basin: Northwest Geology
,
v. 32
, p.
215
216
.
Parkinson
,
D.
,
1991
,
Age and isotopic character of Early Proterozoic basement gneisses in the southern Monashee complex, southeastern British Columbia: Canadian Journal of Earth Sciences
,
v. 28
, p.
1159
1168
.
Reid
,
R.R.
Morrison
,
D.A.
Greenwood
,
W.R.
,
1973
,
The Clearwater Orogenic zone: A relict of Proterozoic orogeny in central and northern Idaho: Belt Symposium I
,
v. 1
, Idaho Bureau of Mines and Geology, p.
10
56
.
Ross
,
G.M.
Parrish
,
R.R.
,
1991
,
Detrital zircon geochronology of metasedimentary rocks in the southern omineca Belt, Canadian Cordillera: Canadian Journal of Earth Sciences
,
v. 28
, p.
1254
1270
.
Ross
,
G.M.
Villeneuve
,
M.
,
2003
,
Provenance of the Mesoproterozoic (1.45 Ga) Belt basin (western North America): another piece in the pre-Rodinia paleogeographic puzzle: Geological Society of America, Bulletin
,
v. 115
, p.
1191
1217
.
Ross
,
G.M.
Parrish
,
R.R.
Winston
,
D.
,
1992
,
Provenance and U-Pb geochronology of the Mesoproterozoic Belt Supergroup (north-western United States): implications for age of deposition and pre-Panthalassa plate reconstructions: Earth and Planetary Science Letters
,
v. 113
, p.
57
76
.
Rubatto
,
D.
Williams
,
I.S.
Buick
,
I.S.
,
2001
,
Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia: Contributions to Mineralogy and Petrology
,
v. 140
, p.
458
468
.
Savage
,
C.N.
,
1973
,
A geological field trip in Benewah and Whitman counties, Idaho and Washington, respectively: Belt Symposium I
,
v. 1
,
University of Idaho
,
Moscow
, p.
253
322
.
Schieber
,
J.
,
1989
,
The origin of the Neihart Quartzite, a basal deposit of the mid-Proterozoic Belt Supergroup, Montana, U.S.A.: Geological Magazine
,
v. 126
, p.
271
281
.
Sears
,
J.W.
Chamberlain
,
K.R.
Buckley
,
S.N.
,
1998
,
Structural and U-Pb geochronological evidence for 1.47 Ga rifting in the Belt basin, western Montana, Canadian Journal of Earth Sciences
,
v. 35
, p.
467
475
.
Sha
,
G.S.
,
2004
,
The tectonic evolution of the Boehls Butte-Clearwater core complex, north-central idaho: Washington State University
, M.S. thesis,
143
p.
Smith
,
M.T.
Gehrels
,
G.E.
,
1991
,
Detrital zircon chronology of Upper Proterozoic to lower Paleozoic continental margin strata of the Kootenay arc: implications for the early Paleozoic tectonic development of the eastern Canadian Cordillera: Canadian Journal of Earth Sciences
,
v. 28
, p.
1271
1284
.
Stewart
,
J.H.
,
1972
,
Initial deposits in the Cordilleran geosyncline: Evidence of a Late Precambrian (< 850 m.y.) continental separation: Geological Society of America, Bulletin
,
v. 83
, p.
1345
1360
.
Sylvester
,
P.J
Ghaderi
,
M.
,
1997
,
Trace element analysis of scheelite by excimer laser ablation-inductively coupled plasma-mass spectrometry (ELA-ICP-MS) using a synthetic silicate glass standard: Chemical Geology
,
v. 141
, p.
49
65
.
Vervoort
,
J.D.
McClelland
,
W.C.
Oldow
,
J.S.
Watkinson
,
A.J.
Sha
,
G.S.
,
2005
,
Grenville-age metamorphism on the western margin of Laurentia, northern Idaho: evidence from Lu-Hf garnet geochronology (abstract): Geological Society of America, Abstracts with Programs
,
v. 37
,
no. 7
, p.
89
.
Vervoort
,
J.D.
Lewis
,
R.S.
Chang
,
Z.
,
2006
,
U-Pb detrital zircon analyses of metasedimentary rocks in the Pierce area, north-central Idaho: Idaho Geological Survey Digital Analytical Data 3, Excel spreadsheet
.
Williams
,
I.S.
,
1998
,
U-Pb by ion microprobe
,
in
 
McKibben
,
M.A.
Shanks
,
W.C.
Ridley
,
W.I.
, eds.,
Applications of Microanalytical Techniques to Understanding Mineralizing Processes: Society of Economic Geologists, Reviews in Economic Geology
,
v. 7
, p.
1
35
.
Williams
,
I.S.
Claesson
,
S.
,
1987
,
Isotopic evidence for the Precam-brian provenance and Caledonian metamorphism of high-grade paragneisses from the Seve Nappes, Scandinavian Caledonides, II. ion microprobe zircon U-Th-Pb: Contributions to Mineralogy and Petrology
,
v. 97
, p.
205
217
.
Winston
,
D.
,
1986
,
Sedimentology of the Ravalli Group, middle Belt carbonate, and Missoula Group, Middle Proterozoic Belt Super-group, Montana, Idaho and Washington
,
in
 
Roberts
,
S.M.
, ed., Belt Supergroup: A Guide to Proterozoic Rocks of Western Montana and Adjacent Areas: Montana Bureau of Mines and Geology, Special Publication 94, p.
85
124
.

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal