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Abstract

Mafic intrusions within the Mesoproterozoic Belt-Purcell basin of the Northern Rocky Mountains of Montana record important Mesoproterozoic and Neoproterozoic rifting episodes. Previous studies established four major Mesoproterozoic igneous intrusive events. This project focuses on sills and dikes that represent a fifth, Neoproterozoic intrusive event and establishes their regional extent and relationship to the Gunbarrel magmatic event and late Proterozoic Windermere rift activity.

The sills and dikes of this study intruded Archean basement rock and the Mesoproterozoic formations of the Belt-Purcell Supergroup and are unconformably overlain by the Cambrian Flathead Sandstone. When restored on a palinspastic map, the thickest and deepest segments of the sills, along with dike swarms in western Montana and northern Wyoming, are colinear with the Belt-Purcell basin axis. The sills and dikes fall on the eastern margin of Windermere rock exposures along the margins of the Laurentian craton.

The sills and dikes are tholeiitic diabase and have locally undergone low-grade metamorphism and alteration. Granophyre is associated with some sills, and Belt-Purcell xenoliths found in the granophyre show some signs of interaction with the magma. Geochemical analysis is compatible with a single intrusive event.

A U-Pb zircon date of 777.5 ± 2.5 Ma obtained from granophyre in the Holland Lake sill supports argon dates of previous studies. Discordant U-Pb data from two other sills, exposed at Turah and near Rogers Pass, are consistent with this date, although it is also possible that these sills were emplaced at different times. The 777.5 ± 2.5 Ma date from the Holland Lake sill establishes the emplacement of this sill during the Gunbarrel magmatic event (ca. 780 Ma) and may reflect earliest Windermere rift activity.

Introduction

The purpose of this study is to determine the age, extent, and tectonic significance of a set of mafic sills within the Belt-Purcell basin of northwestern Montana. The Mesoproterozoic Belt-Purcell basin dominates the Northern Rocky Mountains of Montana (Fig. 1). It has been the focus of many geologic studies because of its economic ore deposits as well as its significance for the tectonic history of the Northern Rocky Mountains in the United States and Canada (e.g. Hoy, 1989; Doughty and Chamberlain, 1996; Sears et al., 1998; Anderson and Parrish, 2000; Evans et al., 2000; Hoy et al., 2000). The stratigraphy, structures, and areal configuration of the basin record important Mesoproterozoic and Neoproterozoic rifting events. Mesozoic and Cenozoic orogenic events have rotated and thrust the basin, exposing the Belt-Purcell Supergroup over 130,000 square kilometers (Obradovich and Peterman, 1968; Lyons et al., 2000; Price and Sears, 2000).

Continental crust on the north, east, and south sides of the basin (Winston and Link, 1993) is thought to have been a part of a Mesoproterozoic supercontinent. To the west was an unknown “western craton”. The Belt-Purcell Supergroup crops out east of the initial strontium 0.706 isopleth, which marks the western edge of thick Precambrian crust (Ross and Villeneuve, 2003). After Belt-Purcell deposition ceased, during the opening of the

Paleo-Pacific Ocean, tectonic processes removed the western sediment source for the basin (Ross et al., 1992; Ross and Villeneuve, 2003). The identification of this source area has become the subject of great interest and debate, because its identification would lay the foundation for determining plate positions in the Mesoproterozoic and the reconstruction of Rodinia. Current theories variously place Australia (Jefferson, 1978; Ross et al., 1992; Karlstrom et al., 2001), Siberia (Sears and Price, 1978, 2003), and an Antarctica-Australia combination (Jefferson, 1978; Dalziel, 1991; Hoffman, 1991; Moores, 1991) against the western edge of the basin.

Examination of Proterozoic igneous intrusions within the Belt-Purcell basin provides additional information about its history. This information may also provide data for determining piercing points created from igneous bodies split during continent separation. These are an important tool for plate reconstructions (cf. Wingate et al., 1998).

Mesoproterozoic Belt-Purcell Basin Intrusive Episodes

The Belt-Purcell basin contains abundant mafic intrusions that intruded from the earliest stages of basin development. The geometry of the developing basin seems to have controlled emplacement of these early sill structures (Burtis et al., 2003a, 2003b). These intrusions cut Belt-Purcell rock from the lower-Belt Prichard Formation to the Garnet Range Formation (Winston and Link, 1993), and occurred during discrete episodes, most likely related to extensional tectonics within the basin (Chamberlain et al., 2003). Sills are the dominant mafic intrusive form in eastern Washington, northern Idaho, and northwestern Montana (Winston and Link, 1993). They comprise mostly basalt, and some contain granophyric regions or caps (Bishop, 1973; Winston and Link, 1993; Poage, 1997; Poage et al., 2000). Based on high precision geochronology, there appear to be four mappable Mesoproterozoic sill complexes (Sears et al., 1998; Price and Sears, 2000, Chamberlain et al., 2003) and one Neoproterozoic sill complex within the Belt-Purcell basin. The Mesoproterozoic complexes are the 1469 Ma event (Anderson and Davis, 1995; Sears et al., 1998), the 1450 Ma event (Sears et al, 1998; Chamberlain et al., 2003), the 1443 Ma Purcell-Nichol Creek lavas of Glacier/Waterton National Park and vicinity (Evans et al., 2000), and the 1379 Ma intrusions along the Salmon River (Doughty and Chamberlain, 1996).

Fig. 1.

.—The locations of Belt-Purcell exposures in the western U.S. (after Winston, 1990).

Fig. 1.

.—The locations of Belt-Purcell exposures in the western U.S. (after Winston, 1990).

Neoproterozoic Sills and Dikes

This project focuses on Neoproterozoic diabase intrusions that represent a fifth, or youngest, sill complex that intruded the Belt-Purcell basin. These intrusions are found mapped as Pre- cambrian diabase (“Zd” or “ZYd”) on several U. S. Geological Survey and Montana Bureau of Mines and Geology geologic maps (Nelson and Dobell, 1961; Wells, 1974; Mudge et al., 1982; Harrison et al., 1986; Whipple et al., 1987; Harrison et al., 1992; Lewis, 1998).

Previous projects focused on individual sill and dike outcrops of the Neoproterozoic intrusive system (Daly, 1912; Calkins and Emmons, 1915; Fenton and Fenton, 1937; Deiss, 1943; Mertie et al., 1951; Hunt, 1962; Obradovich and Peterman, 1968; Mejstrick, 1975; McGimsey, 1985). Correlation of these individual exposures for various mapping projects relied principally on rock descriptions and sparse dates (Nelson and Dobell, 1961; Wells, 1974; Mudge et al., 1982; Harrison et al., 1986; Whipple et al., 1987; Harrison et al., 1992; Lewis, 1998).

Harlan et al. (2003) and Harlan et al. (2005) documented the Gunbarrel magmatic event (ca. 780 Ma) from Wyoming to northwest Canada. The sills and dikes of this study are likely part of the Gunbarrel system of mafic dikes to the southeast in the Beartooth and Teton ranges of Wyoming, which intrude mostly Archean basement rock (Harlan et al., 1997; Harlan et al., 2003).

This study examined seven sites and used previously published data for four other locations to analyze the structure, geochemistry, and geochronology of the Neoproterozoic sills and dikes. Combined, these sites represent the majority of Neoproterozoic diabase sill exposures within Belt-Purcell basin rocks in Montana. The primary sampling sites in Montana are located at Alberton, Milltown, Turah, Rogers Pass, McDonald Reservoir, Holland Lake, and the Kootenai National Forest (Fig. 2, Table 1). Previously studied locations included Milltown, Montana (Eisenbeis, 1958), Wolf Creek, Montana (Schmidt, 1978), Glacier National Park, Montana (Mejstrick, 1975), and the Teton and Beartooth Mountains in Montana and Wyoming (Mueller, 1971; Mueller and Rogers, 1973; Harlan et al., 1997; Harlan et al., 2003; Harlan et al., 2005; Ernst and Buchan 2000). Samples for U- Pb zircon dating were collected at Rogers Pass, Turah, and Holland Lake because these sites expose a significant amount of granophyre. All sites provided structural and geochemical data.

Structure And Field Relationships

The sills and dikes of this study intrude Mesoproterozoic formations of the Belt-Purcell Supergroup in Montana. All sill contacts observed in this study are sharp with no disruption of the country rock, indicating that the sills intruded after lithifica- tion of Belt-Purcell sediments. A dike swarm intrudes the younger part of the supergroup (Garnet Range Formation) in the Alberton area, but not the Cambrian Flathead Sandstone, which uncon- formably overlies Belt-Purcell rocks (Wells, 1974; Kruger, 1988). One of the sills in Glacier National Park crosscuts the 1443 Ma Purcell lava (McGimsey, 1985; Evans et al., 2000). The apparent intrusion into lithified sediments, the relationship to the Purcell Lavas (as observed by McGimsey (1985), and truncation by the sub-Cambrian unconformity collectively demonstrate that this system is younger than the Purcell lava and older than the Flathead Sandstone.

Although the sill and dike outcrops have been dismembered and repositioned by Jurassic through Paleocene thrusting and Cenozoic extension, one aspect of this study was to determine whether or not they could represent a single large sill and related intrusions. If that is true, the structure of the individual Neoproterozoic intrusions should be congruent with an overall large-scale structure representing emplacement pattern. If it is not true, the structure of individual outcrops should berandom and show little congruity.

Fig. 2.

—Sample sites locations (black circles) in Montana. The sills are exposed along a NW trend (sloped black line). Sites were chosen along and across (N-S and E-W black lines) the exposure trend. Additional data was obtained from previous work in the Beartooth, Teton, and Tobacco Root mountain ranges (marked by arrows). Stars identify major cities.

Fig. 2.

—Sample sites locations (black circles) in Montana. The sills are exposed along a NW trend (sloped black line). Sites were chosen along and across (N-S and E-W black lines) the exposure trend. Additional data was obtained from previous work in the Beartooth, Teton, and Tobacco Root mountain ranges (marked by arrows). Stars identify major cities.

Generally, when plotted on a map the Neoproterozoic sill and dike exposures follow a northwest trend in Montana (Fig. 2; Burtis, 2003). This northwest trend is due in part to tectonic events that exposed Belt-Purcell rocks during Phanerozoic time. The sample sites selected for this study create an east-west section across and a northwest-southeast section along this trend of sill and dike exposures (Fig. 2). This provided structural data for analysis across the basin.

Table 1.

—Sample identifications, locations, and coordinates for this study. Coordinates were obtained using a hand-held GPS unit. Degree of alteration was estimated from hand sample and thin section.

Palinspastic maps provide views of the Neoproterozoic sills and dikes as they existed at the time of emplacement within the Belt-Purcell basin (Figs. 3, Fig. 4). Because the sills and dikes are relatively thin in Montana, this study calculated reconstructed sill thickness and depth (Table 2) from the Cambrian unconformity using field data collected during this project, as well as Eisenbeis (1958), Mejstrick (1975), Mudge et al. (1968), Mudge et al. (1982)), and Watson (1984). Restored cross sections created by Mudge et al. (1982)), Harrison et al. (1986), Harrison et al. (1992), Price and Sears (2000), Sears et al. (2002), and Sears (unpublished), which crosscut the Belt-Purcell basin from east to west, provided data on unit thickness and depth for several sample sites as well as for sills not selected for sampling in this study. The zero contour represents the boundary of Neoproterozoic sill and dike exposures found on area geologic maps and is the inferred limit of intrusion. Figure 3 shows the reconstructed unit thickness of the sills on a palinspastic map, which removes the effects of Jurassic through Paleocene thrusting and Cenozoic extension. At the eastern margin of the basin, the Cambrian Flathead Sandstone unconformably overlies the sills. Figure 4 shows the depth from the base of the Flathead Sandstone (Cambrian unconformity) to the top of the sills. The deepest and thickest segments of the sills parallel the northwest trend of the Belt-Purcell basin and intruded the lower parts of the Belt-Purcell Supergroup. The dike swarms in northern Wyoming, southwestern Montana (Harlan et al., 1997), and near Alberton, Montana, are also aligned with the northwest basin trend. Towards the basin margins the sills systematically intrude younger strata to both the northeast and southwest across the width of the basin, defining a bowl shape locally in Montana (Fig. 5) (Burtis, 2003). Because Belt-Purcell basin geometry controlled Mesoproterozoic sill intrusion, this bowl geometry is interpreted as the result of basin structure controlling Neoproterozoic intrusion geometry. The Neoproterozoic sill geometry is significant in that it demonstrates sill congruity, because independent intrusions are more likely to show random thickness throughout the basin and unsystematically intrude Belt-Purcell strata. As the magma rose, it encountered horizontal strata of the Belt-Purcell basin, which facilitated sill formation as magma followed easily parted bedding planes instead of creating new fractures through very competent Belt-Purcell rocks. As the magma progressed laterally through the basin, it may have encountered existing fractures. Neoproterozoic dikes are rare in Belt-Purcell rock and are interpreted as magma migrating between strata along these fractures (Fig. 5) (Burtis, 2003).

Relationship to Windermere Supergroup

Windermere-related volcanic and sedimentary rocks are found in British Columbia, Washington, and Idaho, but they are absent in Montana. This could be due a lack of sediment accommodation space, shifts in rift axes, or erosion due to Mesozoic and Cenozoic orogenic events. The sills and dikes are apparently the only representatives of Windermere rifting preserved in Montana. Figure 6 shows the location of the intrusive system of this study (shaded area) with respect to Windermere exposures in the U.S. and Canada.

Dates inferred for rift activity and deposition of Windermere rocks vary between 780 Ma and 550 Ma (Jefferson and Parrish, 1989; Lund et al., 2003; Bond et al., 1984). This range is most likely due to episodic progression of rift activity along a westward- shifting axis over this time span, with final continent separation occurring around 550 Ma (Bond et al., 1984; Lund et al., 2003). The dates for early Windermere rocks range from ca. 780 to 740 Ma (Roots and Parrish, 1988; Jefferson and Parrish, 1989; Parrish and Scammell, 1988; Devlin et al., 1988) and overlap the dates for the sills and dikes of this study produced by earlier workers (Obradovich and Peterman, 1968; Condie et al., 1969; Mueller and Rogers, 1973; Harlan et al., 1997; Harlan et al., 2003; Harlan et al., 2005).

Fig. 3.

—The minimum total reconstructed thickness of the Neoproterozoic sills and dikes on a palinspastic map. Data were obtained during this study at locations indicated in Table 2. Additional data from Sears, unpublished data. (after Price and Sears, 2000).

Fig. 3.

—The minimum total reconstructed thickness of the Neoproterozoic sills and dikes on a palinspastic map. Data were obtained during this study at locations indicated in Table 2. Additional data from Sears, unpublished data. (after Price and Sears, 2000).

Fig. 4.

—The minimum reconstructed depth to the top of the Neoproterozoic sills from the base of the Cambrian Flathead Sandstone on a palinspastic map. Data were obtained during this study at locations indicated in Table 2. Additional data from Sears, unpublished data. (after Price and Sears, 2000).

Fig. 4.

—The minimum reconstructed depth to the top of the Neoproterozoic sills from the base of the Cambrian Flathead Sandstone on a palinspastic map. Data were obtained during this study at locations indicated in Table 2. Additional data from Sears, unpublished data. (after Price and Sears, 2000).

Rift-related volcanic rocks have been recently U-Pb dated as 685 Ma from central Idaho (Lund et al., 2003) and 667-717 Ma from southern Idaho (Fanning and Link, 2004), and we suggest that this is further evidence of episodic rift axis progression southwest along the eventual margin of Laurentia. Devlin et al.(1988) obtained a Sm-Nd date of 762 ± 44 Ma for Windermere volcanic Huckleberry Formation in northeastern Washington, which is along trend of the sills and dikes of this study (Fig.6). The age for Huckleberry rocks correlates well with the ages determined on the dikes and sills of this study, suggesting that they are coeval.

Methods—U-Pb Geochronology

Methods for U-Pb geochronology followed standard procedures and are described in the notes to Table 3. Magmatic zircons were recovered from all three samples and were characterized by skeletal, striated morphologies that are typical of magmatic zircon in mafic rocks (e.g., Krogh et al., 1982; Heaman et al., 1992). The grains were particularly fragile due to their morphology and did not survive air abrasion to any significant extent. Consequently, all the data are from non-abraded grains. Results are presented in Tables 3 and 4.

Table 2.

—Estimated unit thickness and depth from the base of the Flathead Sandstone (Cambrian unconformity) used for Figures 3 and 4. Additional information was obtained using restored cross sections by Mudge et al. (1982)), Harrison et al. (1986), Harrison et al. (1992), Price and Sears (2000), and Sears et al. (2002) at exposures without recognizable geographic reference points and by Sears, unpublished data.

Fig. 5.

—A schematic representation of a cross section of the Belt-Purcell basin. Sill intrudes along the basin axis and rises outward towards the margins. As shown here, the sill does not necessarily intrude the youngest strata at the basin margins. East is to the right.

Fig. 5.

—A schematic representation of a cross section of the Belt-Purcell basin. Sill intrudes along the basin axis and rises outward towards the margins. As shown here, the sill does not necessarily intrude the youngest strata at the basin margins. East is to the right.

Methods—Geochemical

Sampling methods at each site followed standards designed to minimize the effects of grain size and metamorphism during geochemical analysis. A detailed description of preparation methods used for the geochemical analyses conducted during this study are presented in Burtis (2003). Major elements were analyzed by XRF methods and trace elements by ICP-MS. Johnson et al. (1999) and Knaack et al. (1994) describe the XRF and ICP-MS procedures, respectively, used in this study. Table 5 contains geochemical data collected during this study and Table 6 contains data collected during previous studies.

U-Pb Results

Holland Lake Sill (MTHL-12, MTHL-13)

Two samples of granophyre were collected from a single outcrop of the Holland Lake sill, approximately 35 meters apart (MTHL-12 and MTHL-13). Both samples yielded skeletal, striated zircons that are interpreted as magmatic. U-Pb data from four multi-grain fractions (3-5 grains each) of zircon from MTHL-12 have relatively high, blank-corrected 206Pb/204Pb values of 423-2669, are only 1.5 to 11% discordant, and have 207Pb/ 206Pb ages that range from 763 to 777 Ma (Table 3, Fig. 7). Linear regression of these four analyses is statistically significant (MSWD of 0.55) and yields an upper intercept date of 777.5 ± 2.5 Ma. Three analyses of zircon from the second sample, MTHL- 13, including two single grains, are more variable than the data from 12 and generally yielded older 207Pb/206Pb ages, as old as 807 Ma (Table 3, Fig. 7). The data from MTHL-13 are interpreted to reflect minor inheritance of older zircon in a 778 Ma sill. The presence of partially digested country-rock inclusions in the sill supports this interpretation. The upper intercept of MTHL-12 (777.5 ± 2.5 Ma) is interpreted as the crystallization age of the Holland Lake sill.

Turah sill (MTTU-1)

Skeletal, striated zircon grains were also recovered from a granophyre from the Turah sill (MTTU-1), but most of the analyses had high initial Pb relative to radiogenic Pb (> 80% common Pb and low 206Pb/204Pb values; Table 4). U-Pb concordia coordinates and ages from these analyses are too model dependent to be interpreted reliably. Two other morphologies of zircon exist in this sample: equant euhedral grains that also have high common Pb, and scarce clear, thin, anhedral zircon flakes with relatively low common Pb (Table 4). Three analyses of the flake morphology have enough radiogenic Pb to be interpreted on a concordia diagram (Fig. 8). One of these yielded a radiogenic 207Pb/206Pb date of 904 ± 5 Ma and is interpreted to reflect minor inheritance. A chord through the other two points overlaps the Holland Lake age but has high uncertainty of ± 43 Myr (Table 4, Fig. 8).

Table 3.

—U-Pb zircon data for Holland Lake, Turah, and Rogers Pass sample sites in Montana.

Notes: zircon fraction: nm_, m_ represent either amperage (A) or angles of paramagnetic susceptibility on a barrier-style Frantz separator; sk = skeletal; pk = pick; cl fl = clear flakes; str = striated; aa = air abraded. Number of grains (g) in each fraction indicated.
a

Pbc common Pb (ppm) corrected for laboratory blank,

b

ratio corrected for blank and mass discrimination

c

radiogenic Pb in ratios; all values corrected for blank and mass discrimination

d

values in parentheses are 2σ errors in percent

e

206Pb/238U vs 207Pb/235U error correlation coefficient

f

percent discordant

Zircon dissolution and chemistry were adapted from methods developed by Krogh (1973) and Parrish et al. (1987). Aliquots of dissolved sample were spiked with a mixed 208pb/235u tracer. Pb and U samples were loaded onto single rhenium filaments with silica gel and graphite, respectively; isotopic compositions were measured in multicollector, static mode on a VG Sector mass spectrometer at the University of Wyoming. Mass discrimination factors of 0.08 ± 0.04%/amu for Pb and 0.0 ± 0.06%/amu for U were determined by replicate analyses of NIST SRM 981 and U-500, respectively. Procedural blank improved from ~ 10 pg to 2 pg Pb over the course of the study.U blanks were consistently 0.4 pg. Concordia coordinates, intercepts, and uncertainties were calculated using PBDAT and ISOPLOT programs (Ludwig, 1988, 1991); initial Pb compositions for the Holland Lake and Turah sills were estimated by Zartman and Doe (1981) model mantle for 800 Ma; Rogers Pass reductions used Stacey and Kramers (1975) model for 750 Ma. The decay constants used by PBDAT are those recommended by the I.U.G.S. Subcommission on Geochronology (Steiger and Jäger, 1977): 0.155125 x 10-9/yr for 238U, 0.98485 x 10-9/yr for 235U and present-day 238U/235U = 137.88.
Table 4.

—Total Pb data for the Turah sill in Montana.

Notes: zircon fraction: d_,nm_ represent angles of diamagnetic and paramagnetic susceptibility on a barrier style Frantz separator; sk = skeletal; eq = equant;anh pi = anhedral plates; str = striated; cl fl = clear flakes. Number of grains (g) indicated for each fraction.
*

excluded from isochron

%

common Pb (cPb) estimated by assuming Stacey and Kramers (1975) model isotopic compositions at 750 Ma for the common component Atomic ratios corrected for blank and mass discrimination Values in parentheses are 2σ errors in percent

Rho = error correlation coefficientsDissolution, chemical processing, mass spectrometry and data reduction were the same as described in Table 2.
Fig. 6.

—Map of the western U.S. and Canada showing an en echelon pattern trending approximately N20° W in Windermere-related volcanic and sedimentary rocks. The sills and dikes of this study (shaded) lie at the eastern edge. (Map after Ross et al., 1995).

Fig. 6.

—Map of the western U.S. and Canada showing an en echelon pattern trending approximately N20° W in Windermere-related volcanic and sedimentary rocks. The sills and dikes of this study (shaded) lie at the eastern edge. (Map after Ross et al., 1995).

The best estimate provided by this study for the age of the Turah sill comes from interpretation of blank-corrected 206Pb/ 204Pb vs. 207Pb/204Pb data (Table 4, Fig. 9). The simplest interpretation of linearity on a 206Pb/204Pb vs. 207Pb/204Pb plot is that the data are mixtures of single reservoirs of radiogenic and common Pb, and that any chemical disturbance of the U-Pb system involved recent loss of Pb (e.g., Geyh and Schleicher, 1990). In this case, the slope gives the age of crystallization. Linear regression of data from 9 of the 11 analyses have good statistics (MSWD = 0.76, probability of fit of 62%) and a date of 772 ± 10 Ma (Fig. 9). Data from one fraction of distinct, equant, euhedral grains lie off the line and were excluded from the regression, along with the data from the most radiogenic analysis, nm4 cl fl 4gr. This point falls on the line because of large errors in these coordinates, but U-Pb analysis indicated an inherited component. The equant, euhedral zircons are either of a different age or formed with a different initial Pb isotope composition. Based on the Pb/Pb isochron date from the remaining nine analyses, the Turah sill could have been part of the same magmatic event as the Holland Lake sill.

Rogers Pass Sill (RPKRC02-2)

U-Pb data from zircons separated from the Rogers Pass sill (RPKRC02-2) are nearly as complex as the data from the Turah sill. The data are extremely discordant (33 to 78%, Table 3) and are nonlinear on both concordia and Pb/Pb plots. One fraction (nm 0.5a) has a 207Pb/206Pb age of 916 Ma (Table 3) and is interpreted to include an inherited zircon component. Clasts of partially digested country rock were observed in the outcrop 20 meters from the collection site. The zircons display relatively high common Pb concentrations (up to 40 ppm; Table 3) and radiogenic U- Pb data are strongly dependent on choice of initial Pb isotopic composition. A two-point chord of data reduced with Stacey and Kramers (1975) model Pb values yields an upper intercept within error of the age of the Holland Lake sill (Table 3, Fig. 10). It must be stressed, however, that the Rogers Pass data are too discordant and complex to produce a precise date.

Summary of Age Results

In summary, the Holland Lake sill data yielded a robust U- Pb crystallization age of 777.5 ± 2.5 Ma. Pb/Pb data from the Turah sill overlaps this date at 772 ± 10 Ma, and the data from the Rogers Pass sills are consistent with these ages of emplacement. All three sills have evidence for inherited zircon components, consistent with field evidence for contamination during emplacement. At the simplest interpretation, the data support an interpretation that all three sills are part of the ca. 780 Ma Gunbarrel magmatic event.

Geochemical Results

The Neoproterozoic intrusive rocks are tholeiitic diabase basalt in composition (Table 5), dominated by plagioclase, augite,and pigeonite. With the exception of the Kootenai National Forest samples, the average content of SiO2, MnO, CaO, MgO, K2O, and Na2O falls within the subalkalic range (Fig. 11). Tectonic events have fractured and metamorphosed the intrusive rocks to low- grade greenschist facies locally, while alteration has affected some exposures. Geochemical data, including data from Milltown (Eisenbeis, 1958), the Wyoming dike swarm (Mueller, 1971), and Wolf Creek (Schmidt, 1978), also indicate possible alteration.

Table 5.

—Non-normalized element analysis for samples of this study.

MT ALE 01MT ALE 02MT KF 01MT IF 02MT KF 03MT MR 01MT MR 02MT RPE 04MT RPE 21MT RPM 01 |MT RPM 02Average
Date9-M-039-Jul-039-M-039-Jul-039-Jul-0310-Jul-0310-JuI-0310-JuI-0310-Jul-0310-JuI-0310-JuI-03
Non-normalized Major Elements (Weight %):
SiO250.0052.4248.4348.1648.0549.6750.4150.0350.6150.1950.0349.82
Al2O313.4813.3815.3615.4715.0412.2312.2512.4012.9212.3512.3513.38
TiO22.4982.0192.9282.5152.8352.8592.5552.8722.2452.8722.8442.64
FeO14.6111.1714.2113.6613.7316.0015.3315.9714.2216.0715.7914.62
MnO0.2160.2150.1900.1920.2070.2490.2370.2310.2230.2320.2350.22
CaO9.747.256.708.088.779.489.789.219.989.328.998.85
MgO4.986.954.094.554.295.005.504.855.824.894.855.07
K2O0.731.581.902.301.340.760.770.700.650.720.831.12
Na2O2.452.892.582.793.262.332.242.332.332.292.372.53
P2O50.2240.2100.5140.4710.5310.2560.2250.2520.2120.2550.2520.31
Total98.9398.0896.9098.1998.0698.8399.3098.8499.2199.1998.5498.55
Non-normalized Trace Elements (ppm):
Ni50137272831525554625150
Cr7029944373567101651086663
Sc4026292333484242404241
V422308222183209480467470417463470
Ba133631769929603155173197109144236
Rb2545556040252628202834
Sr153202306368410160149153158151162
Zr167162256236255185180186156189199
Y4228434042464347414747
Nb11.312.926.524.327.212.611.912.910.413.812.2
Ga2620262529222623242423
Cu† 303123172072† 386† 241† 356† 275† 315† 366
Zn142169133140149149127144125138142
Pb554601666448
La153138463887642118735
Ce5548887476403339245225
Th25634626252
MT ALE 01MT ALE 02MT KF 01MT IF 02MT KF 03MT MR 01MT MR 02MT RPE 04MT RPE 21MT RPM 01 |MT RPM 02Average
Date9-M-039-Jul-039-M-039-Jul-039-Jul-0310-Jul-0310-JuI-0310-JuI-0310-Jul-0310-JuI-0310-JuI-03
Non-normalized Major Elements (Weight %):
SiO250.0052.4248.4348.1648.0549.6750.4150.0350.6150.1950.0349.82
Al2O313.4813.3815.3615.4715.0412.2312.2512.4012.9212.3512.3513.38
TiO22.4982.0192.9282.5152.8352.8592.5552.8722.2452.8722.8442.64
FeO14.6111.1714.2113.6613.7316.0015.3315.9714.2216.0715.7914.62
MnO0.2160.2150.1900.1920.2070.2490.2370.2310.2230.2320.2350.22
CaO9.747.256.708.088.779.489.789.219.989.328.998.85
MgO4.986.954.094.554.295.005.504.855.824.894.855.07
K2O0.731.581.902.301.340.760.770.700.650.720.831.12
Na2O2.452.892.582.793.262.332.242.332.332.292.372.53
P2O50.2240.2100.5140.4710.5310.2560.2250.2520.2120.2550.2520.31
Total98.9398.0896.9098.1998.0698.8399.3098.8499.2199.1998.5498.55
Non-normalized Trace Elements (ppm):
Ni50137272831525554625150
Cr7029944373567101651086663
Sc4026292333484242404241
V422308222183209480467470417463470
Ba133631769929603155173197109144236
Rb2545556040252628202834
Sr153202306368410160149153158151162
Zr167162256236255185180186156189199
Y4228434042464347414747
Nb11.312.926.524.327.212.611.912.910.413.812.2
Ga2620262529222623242423
Cu† 303123172072† 386† 241† 356† 275† 315† 366
Zn142169133140149149127144125138142
Pb554601666448
La153138463887642118735
Ce5548887476403339245225
Th25634626252
Major elements are normalized on a volatile-free basis, with total Fe expressed as FeO.

“†” denotes values > 120% of our highest standard.

Table 6.

—Non-normalized element analysis for samples of previous studies.

WC65-3HE-1MBT-3MBT-5MBT-22MBT-23MBT-24MBT-30MBT-51MBT-81MBT-109
Non-normalized major elements (weight %):
SiO249.3045.4451.3452.2851.8752.5252.3252.4252.8552.1451.79
AI2O313.9013.6911.8912.4311.8412.3912.4612.5412.6012.7013.50
TiO22.702.432.852.902.862.852.862.872.852.642.87
Fe2O314.606.06NDNDNDNDNDNDNDNDND
FeO*2.4015.1216.1916.9416.4516.0116.7416.8719.3117.3217.57
MnO0.250.21NDNDNDNDNDNDNDNDND
CaO7.506.157.828.248.338.388.378.448.618.849.16
MgO3.007.604.464.504.594.514.474.614.904.754.80
K2O0.650.701.191.141.041.121.071.010.950.970.88
Na2O2.602.202.603.242.482.562.502.663.192.492.49
P2O50.400.01NDNDNDNDNDNDNDNDND
WC65-3HE-1MBT-3MBT-5MBT-22MBT-23MBT-24MBT-30MBT-51MBT-81MBT-109
Non-normalized major elements (weight %):
SiO249.3045.4451.3452.2851.8752.5252.3252.4252.8552.1451.79
AI2O313.9013.6911.8912.4311.8412.3912.4612.5412.6012.7013.50
TiO22.702.432.852.902.862.852.862.872.852.642.87
Fe2O314.606.06NDNDNDNDNDNDNDNDND
FeO*2.4015.1216.1916.9416.4516.0116.7416.8719.3117.3217.57
MnO0.250.21NDNDNDNDNDNDNDNDND
CaO7.506.157.828.248.338.388.378.448.618.849.16
MgO3.007.604.464.504.594.514.474.614.904.754.80
K2O0.650.701.191.141.041.121.071.010.950.970.88
Na2O2.602.202.603.242.482.562.502.663.192.492.49
P2O50.400.01NDNDNDNDNDNDNDNDND

The Neoproterozoic sills and dikes have a high concentration of TiO2 (average 2.69%), which is more typical of alkalic basalts than of continental tholeiites (average 2.0%) (Hyndman, 1985). As will be shown below, the average TiO2 content is unique to this group of intrusions when compared to older mafic intrusions within the Belt-Purcell basin and is a possible identifying characteristic for the Montana portion of the Gunbarrel magmatic event.

The REE patterns of the sills are shown in Figure 12. Each sample plots with similar trends, with the exception of the highly altered LREE enriched Kootenai National Forest, Montana (MT-KF) samples. It is possible that this enrichment is due to alteration found in thin sections of MT-KF samples and as seen in hand samples from this location. The HREE compositions of the MT- KF samples are similar to those of the samples from other locations. However, it is possible that the Kootenai sills are unrelated to the other Neoproterozoic sills examined during this project. A sample obtained from Alberton, Montana (MT-AL-02) shows lower concentrations of less mobile HREE, which could be due to alteration as seen in thin section. Trends on a large ion lithophile (LIL) and high field strength (HFS) diagram (Fig. 13) show a similar pattern, with the more-altered samples concentrating the more-mobile elements (Sr, K, Rb, and Ba). Elevated concentrations of the more-mobile elements are most likely the result of alteration, which has affected all samples to some degree. These data, with the possible exception of the Kootenai National Forest sill, imply that the individual outcrops are similar in composition and a product of one magma type and are compatible with a single intrusive event.

Fig. 7.

.—U-Pb concordia plot of data from the Holland Lake sill. Both samples were collected from the same outcrop. Data from MTHL-12 (open symbols) are interpreted to be the best estimate of the age of sill emplacement. Data from MTHL-13 (filled symbols) has some inherited zircon components.

Fig. 7.

.—U-Pb concordia plot of data from the Holland Lake sill. Both samples were collected from the same outcrop. Data from MTHL-12 (open symbols) are interpreted to be the best estimate of the age of sill emplacement. Data from MTHL-13 (filled symbols) has some inherited zircon components.

Fig. 8.

.—Concordia plot of high 206Pb/204Pb data from MTTU-1, Turah sill. One analysis clearly includes an inherited component, and it is possible the others do as well. Data are consistent with the age of the Holland Lake sill, but a precise date cannot be determined for the Turah sample from this plot.

Fig. 8.

.—Concordia plot of high 206Pb/204Pb data from MTTU-1, Turah sill. One analysis clearly includes an inherited component, and it is possible the others do as well. Data are consistent with the age of the Holland Lake sill, but a precise date cannot be determined for the Turah sample from this plot.

Fig. 9.

—Pb/Pb data from all analyses of the Turah sample, MTTU-1. Linear regression of 9 of the 11 points yields a Pb/ Pb age that overlaps the crystallization age of the Holland Lake sill. The filled data point was excluded from the regression and comes from a fraction of distinct, euhedral, equant zircons. The second excluded point, nm4 cl fl 4g, plots on this chord due to high errors in these coordinates (high 206Pb/ 204Pb) but displays inheritance on a concordia plot, Figure 8.

Fig. 9.

—Pb/Pb data from all analyses of the Turah sample, MTTU-1. Linear regression of 9 of the 11 points yields a Pb/ Pb age that overlaps the crystallization age of the Holland Lake sill. The filled data point was excluded from the regression and comes from a fraction of distinct, euhedral, equant zircons. The second excluded point, nm4 cl fl 4g, plots on this chord due to high errors in these coordinates (high 206Pb/ 204Pb) but displays inheritance on a concordia plot, Figure 8.

Fig. 10.

—Concordia plot of data from Rogers Pass sill, RPKRC02- 2. Data are extremely discordant, and some analyses include inheritance. Data are consistent with the age of the Holland Lake sill, but a precise date cannot be determined for the Rogers Pass sample. The filled symbols were used for a two- point regression.

Fig. 10.

—Concordia plot of data from Rogers Pass sill, RPKRC02- 2. Data are extremely discordant, and some analyses include inheritance. Data are consistent with the age of the Holland Lake sill, but a precise date cannot be determined for the Rogers Pass sample. The filled symbols were used for a two- point regression.

When Mueller (1971) and Mueller and Rogers (1973) grouped the Wyoming dikes based on age, they also discovered that these groups corresponded to specific TiO2 content. Their youngest dike set (740 ± 32 Ma) has an average TiO2 content (2.84%) very similar to the Montana sills and dikes (2.69%). Based on these averages and ages, it is reasonable to suggest that the Montana intrusions might also be grouped with the Wyoming dikes. In Figure 14, TiO2 data for samples of this study and previous studies (Mueller, 1971; Schmidt, 1978) are plotted against silica. The samples plot in two groups, a sill group and dike group. Silica content is variable, and the amount of TiO2 remains constant between these two groups. The TiO2 content of the sills and dikes is higher than older diabase intrusions within the Belt-Purcell basin (Plains sill) and, as stated above, could be used as an identifier for these intrusions. Although the Wyoming dikes have different silica content, their TiO2 content falls within the range of the intrusions of this study. As will be shown below, the geochemical similarities between these two groups are compatible with a distinct intrusive event. In addition, titanium is not a very mobile element, making it a good element for comparison of the sills and dikes.

Fig. 11.

—Graph of total alkalis vs. silica with the Mesoproterozoic Plains sill included. (WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971); samples TLP and CC from Poage (1997)).

Fig. 11.

—Graph of total alkalis vs. silica with the Mesoproterozoic Plains sill included. (WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971); samples TLP and CC from Poage (1997)).

Fig. 12.

—REE spidergram for sill samples from this study. With the exception of the Kootenai National Forest samples (MT-KF prefix), the sills follow a similar trend. MT-KF-01; 02; 03 have increased amounts of more-mobile LREE because they are significantly altered. Their concentrations of immobile HREE is similar to those of the less altered samples.

Fig. 12.

—REE spidergram for sill samples from this study. With the exception of the Kootenai National Forest samples (MT-KF prefix), the sills follow a similar trend. MT-KF-01; 02; 03 have increased amounts of more-mobile LREE because they are significantly altered. Their concentrations of immobile HREE is similar to those of the less altered samples.

Figure 15 plots TiO2 against Al2O3, and Figure 16 plots TiO2 against MgO. In these figures, both the sills and dikes plot in one group, indicating similar composition. It is important to note that MgO can be considered an early-crystallizing constituent and is not very mobile during alteration. This is significant when considering that both the sills and the dikes have similar MgO and TiO2 contents. The similar MgO, Al2O3, CaO, and TiO2 contents between the sills and dikes are interpreted to suggest a single parental magma.

Fig. 16.

—Graph of TiO2 vs. MgO. WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971); samples TLP and CC and from Poage (1997).

Fig. 16.

—Graph of TiO2 vs. MgO. WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971); samples TLP and CC and from Poage (1997).

Importantly, the sills and dikes sampled during this study differ chemically from Mesoproterozoic tholeiitic basalt sills in the Belt-Purcell Supergroup. Several samples from the much older 1469 Ma Paradise sill (Poage, 1997) were plotted with the Neoproterozoic sills and dikes (Fig. 11). It is significant that the Paradise sill plots as a separate data set with a lower overall alkali content that distinguishes this sill from those of this study. TiO2 content also differs, being higher in the Neoproterozoic intrusions (Figs. 14, 16), and supports the use of the TiO2 as an identifying characteristic.

Fig. 13.

—Trace-element spidergram for sill samples from this study. (Chondrite data from Sun and McDonough, 1989; Anders and Grevesse, 1989).

Fig. 13.

—Trace-element spidergram for sill samples from this study. (Chondrite data from Sun and McDonough, 1989; Anders and Grevesse, 1989).

Discussion And Conclusions

Gunbarrel igneous rocks are regionally extensive, ranging from northern Wyoming to the Montana-Canada border. Structurally, the dikes intruded Archean basement rocks in Wyoming, whereas the sills intruded the formations of the Belt-Purcell Supergroup in Montana after lithification of those sediments. When plotted on a palinspastic map, the thickest and deepest segments of the Montana sill complex follow the N20° W axis of the Belt-Purcell basin, which is colinear with the dike swarms in Alberton, Montana, and Wyoming. Given that the dike swarms intrude Archean basement rock, the colinear relationship may represent initial intrusion that fed the sills, which developed as the magma followed the horizontal bedding planes of the BeltPurcell Supergroup. The sills rise and thin from the inferred axis towards the basin margins, creating a regional bowl-like shape that conforms to the overall structure of the basin and demonstrates sill congruity.

Zircon U-Pb dates determined from the granophyre associated with these sills correlate well, within error, with the argon dates determined by previous workers for sills near Alberton (760 Ma) and Wolf Creek (779 Ma), Montana, as well as with dates determined for dikes in the Teton (783 Ma) and Beartooth (777-779 Ma) ranges (Table 7). Harlan et al. (1997), Harlan et al. (2003), and Harlan et al. (2005) associate the dikes in Wyoming with the Wolf Creek sill using age data. The Wolf Creek sill can be traced on the Choteau 1° x 2° geologic map (Mudge et al., 1982) northwest to the same outcrop at Rogers Pass that was analyzed and dated in this study (samples MT-RPE and RP- KRC). Ernst and Buchan (2000) interpreted the Wolf Creek sill as being related to the 780 Ma Mackenzie dikes in northwestern Canada and part of a large igneous province. Harlan et al. (2003) also correlate the sills and dikes in Montana and northern Wyoming with intrusions in northwestern Canada and suggest that they are part of the ca. 780 Ma magmatic event (Gunbarrel magmatic event) that may be related to the commencement of early Windermere mafic volcanism, i.e., the Huckleberry Formation in northeastern Washington.

Table 7.

—Names, dates, and methods for Gunbarrel age rocks. The sills dated during this study are included.

Letters in parenthesis are abbreviations used by Harlan et al. (2003) and Harlan et al. (2005).

The high TiO2 content (2.69%) of the sills and dikes is uncommon for Belt-Purcell mafic intrusions. Furthermore, when compared to TiO2 averages for continental tholeiites only some individual flood-basalt flows and ocean-island tholeiites have comparable averages (Hyndman, 1985; Winters, 2001). This study interprets the high titanium content as compatible with the hypothesis that the Neoproterozoic sills and dikes represent a distinct intrusive event in Montana and northern Wyoming.

The western margin of the Laurentian craton truncated both the Neoproterozoic sills and dikes and the four Mesoproterozoic intrusive rock suites (Hoy et al., 2000). This truncation is significant when reconstructing Precambrian plate margins, inasmuch as the intrusions are also likely to be found on the conjugate plate. If the Gunbarrel magmatic event, with the Neoproterozoic intrusions as a segment, is considered to be a large igneous province that extended beyond Laurentia into the now detached conjugate craton, a significant piercing point could be established in conjunction with the four Mesoproterozoic intrusive events. Because the likelihood of five igneous events with identical ages and compositions in more than one geographic location is statistically low, finding similar igneous provinces within Belt-type rocks on different cratons would suggest that the cratons were attached prior to Rodinia breakup. This relationship would help determine which craton split off from the western margins of Laurentia by comparing ages of similar igneous provinces on the Antarctic, Australian, China, or Siberian cratons.

The spatial relationship of the Neoproterozoic intrusions in Montana and Wyoming, similar ages, alignment of the bowl geometry of the sills (defined by the sill structure) with the northwest trend of the Wyoming and Alberton, Montana dike swarms, and the average (2.69%) TiO2 content all argue for a single intrusive event, the Gunbarrel magmatic event. In addition, the geographic relationship and structural position with Windermere-related rock exposures, and similar age of the Neoproterozoic intrusions with early Windermere rocks, support a relationship with earliest Windermere rift activity.

Fig. 14.

—Graph of TiO2 vs. silica. Sills and dikes plot in separate groups (shaded) that show a decrease in SiO2 from dikes to sills. TiO2 contents remain relatively constant. The Mesoproterozoic Plains sill plots in a separate group from the dike group and the sill group. The TiO2 content of the Plains sill is clearly not as high as in the Windermere intrusions. (WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971); samples tlp TLP and CC from Poage (1997).

Fig. 14.

—Graph of TiO2 vs. silica. Sills and dikes plot in separate groups (shaded) that show a decrease in SiO2 from dikes to sills. TiO2 contents remain relatively constant. The Mesoproterozoic Plains sill plots in a separate group from the dike group and the sill group. The TiO2 content of the Plains sill is clearly not as high as in the Windermere intrusions. (WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971); samples tlp TLP and CC from Poage (1997).

Fig. 15.

—Graph of TiO2 vs. Al2O3. Sills and dikes plot in one group, indicating similar TiO2 and Al2O3 amounts. Because neither titanium nor aluminum are very mobile, the separate grouping of Kootenai National Forest samples may indicate that this sill represents an unrelated intrusion. (WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971)).

Fig. 15.

—Graph of TiO2 vs. Al2O3. Sills and dikes plot in one group, indicating similar TiO2 and Al2O3 amounts. Because neither titanium nor aluminum are very mobile, the separate grouping of Kootenai National Forest samples may indicate that this sill represents an unrelated intrusion. (WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971)).

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Acknowledgements

This Project was undertaken as part of EWB’s Master’s thesis. Fieldwork was supported by National Science Foundation grant EAR017024 to JWS and the Harrison Field Scholarship to EWB awarded by the Tobacco Root Geological Society. U-Pb geochronology work was supported by National Science Foundation grants EAR0107088 and EAR0310149 to KRC. Peter Valley assisted the geochronology. Brian Collins assisted with GIS. Manuscript benefited from discussions with and reviews by Don Hyndman, Reed Lewis, Paul Link, Bill McClelland, Jim Mortensen, and Don Winston.

Figures & Tables

Fig. 1.

.—The locations of Belt-Purcell exposures in the western U.S. (after Winston, 1990).

Fig. 1.

.—The locations of Belt-Purcell exposures in the western U.S. (after Winston, 1990).

Fig. 2.

—Sample sites locations (black circles) in Montana. The sills are exposed along a NW trend (sloped black line). Sites were chosen along and across (N-S and E-W black lines) the exposure trend. Additional data was obtained from previous work in the Beartooth, Teton, and Tobacco Root mountain ranges (marked by arrows). Stars identify major cities.

Fig. 2.

—Sample sites locations (black circles) in Montana. The sills are exposed along a NW trend (sloped black line). Sites were chosen along and across (N-S and E-W black lines) the exposure trend. Additional data was obtained from previous work in the Beartooth, Teton, and Tobacco Root mountain ranges (marked by arrows). Stars identify major cities.

Fig. 3.

—The minimum total reconstructed thickness of the Neoproterozoic sills and dikes on a palinspastic map. Data were obtained during this study at locations indicated in Table 2. Additional data from Sears, unpublished data. (after Price and Sears, 2000).

Fig. 3.

—The minimum total reconstructed thickness of the Neoproterozoic sills and dikes on a palinspastic map. Data were obtained during this study at locations indicated in Table 2. Additional data from Sears, unpublished data. (after Price and Sears, 2000).

Fig. 4.

—The minimum reconstructed depth to the top of the Neoproterozoic sills from the base of the Cambrian Flathead Sandstone on a palinspastic map. Data were obtained during this study at locations indicated in Table 2. Additional data from Sears, unpublished data. (after Price and Sears, 2000).

Fig. 4.

—The minimum reconstructed depth to the top of the Neoproterozoic sills from the base of the Cambrian Flathead Sandstone on a palinspastic map. Data were obtained during this study at locations indicated in Table 2. Additional data from Sears, unpublished data. (after Price and Sears, 2000).

Fig. 5.

—A schematic representation of a cross section of the Belt-Purcell basin. Sill intrudes along the basin axis and rises outward towards the margins. As shown here, the sill does not necessarily intrude the youngest strata at the basin margins. East is to the right.

Fig. 5.

—A schematic representation of a cross section of the Belt-Purcell basin. Sill intrudes along the basin axis and rises outward towards the margins. As shown here, the sill does not necessarily intrude the youngest strata at the basin margins. East is to the right.

Fig. 6.

—Map of the western U.S. and Canada showing an en echelon pattern trending approximately N20° W in Windermere-related volcanic and sedimentary rocks. The sills and dikes of this study (shaded) lie at the eastern edge. (Map after Ross et al., 1995).

Fig. 6.

—Map of the western U.S. and Canada showing an en echelon pattern trending approximately N20° W in Windermere-related volcanic and sedimentary rocks. The sills and dikes of this study (shaded) lie at the eastern edge. (Map after Ross et al., 1995).

Fig. 7.

.—U-Pb concordia plot of data from the Holland Lake sill. Both samples were collected from the same outcrop. Data from MTHL-12 (open symbols) are interpreted to be the best estimate of the age of sill emplacement. Data from MTHL-13 (filled symbols) has some inherited zircon components.

Fig. 7.

.—U-Pb concordia plot of data from the Holland Lake sill. Both samples were collected from the same outcrop. Data from MTHL-12 (open symbols) are interpreted to be the best estimate of the age of sill emplacement. Data from MTHL-13 (filled symbols) has some inherited zircon components.

Fig. 8.

.—Concordia plot of high 206Pb/204Pb data from MTTU-1, Turah sill. One analysis clearly includes an inherited component, and it is possible the others do as well. Data are consistent with the age of the Holland Lake sill, but a precise date cannot be determined for the Turah sample from this plot.

Fig. 8.

.—Concordia plot of high 206Pb/204Pb data from MTTU-1, Turah sill. One analysis clearly includes an inherited component, and it is possible the others do as well. Data are consistent with the age of the Holland Lake sill, but a precise date cannot be determined for the Turah sample from this plot.

Fig. 9.

—Pb/Pb data from all analyses of the Turah sample, MTTU-1. Linear regression of 9 of the 11 points yields a Pb/ Pb age that overlaps the crystallization age of the Holland Lake sill. The filled data point was excluded from the regression and comes from a fraction of distinct, euhedral, equant zircons. The second excluded point, nm4 cl fl 4g, plots on this chord due to high errors in these coordinates (high 206Pb/ 204Pb) but displays inheritance on a concordia plot, Figure 8.

Fig. 9.

—Pb/Pb data from all analyses of the Turah sample, MTTU-1. Linear regression of 9 of the 11 points yields a Pb/ Pb age that overlaps the crystallization age of the Holland Lake sill. The filled data point was excluded from the regression and comes from a fraction of distinct, euhedral, equant zircons. The second excluded point, nm4 cl fl 4g, plots on this chord due to high errors in these coordinates (high 206Pb/ 204Pb) but displays inheritance on a concordia plot, Figure 8.

Fig. 10.

—Concordia plot of data from Rogers Pass sill, RPKRC02- 2. Data are extremely discordant, and some analyses include inheritance. Data are consistent with the age of the Holland Lake sill, but a precise date cannot be determined for the Rogers Pass sample. The filled symbols were used for a two- point regression.

Fig. 10.

—Concordia plot of data from Rogers Pass sill, RPKRC02- 2. Data are extremely discordant, and some analyses include inheritance. Data are consistent with the age of the Holland Lake sill, but a precise date cannot be determined for the Rogers Pass sample. The filled symbols were used for a two- point regression.

Fig. 11.

—Graph of total alkalis vs. silica with the Mesoproterozoic Plains sill included. (WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971); samples TLP and CC from Poage (1997)).

Fig. 11.

—Graph of total alkalis vs. silica with the Mesoproterozoic Plains sill included. (WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971); samples TLP and CC from Poage (1997)).

Fig. 12.

—REE spidergram for sill samples from this study. With the exception of the Kootenai National Forest samples (MT-KF prefix), the sills follow a similar trend. MT-KF-01; 02; 03 have increased amounts of more-mobile LREE because they are significantly altered. Their concentrations of immobile HREE is similar to those of the less altered samples.

Fig. 12.

—REE spidergram for sill samples from this study. With the exception of the Kootenai National Forest samples (MT-KF prefix), the sills follow a similar trend. MT-KF-01; 02; 03 have increased amounts of more-mobile LREE because they are significantly altered. Their concentrations of immobile HREE is similar to those of the less altered samples.

Fig. 16.

—Graph of TiO2 vs. MgO. WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971); samples TLP and CC and from Poage (1997).

Fig. 16.

—Graph of TiO2 vs. MgO. WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971); samples TLP and CC and from Poage (1997).

Fig. 13.

—Trace-element spidergram for sill samples from this study. (Chondrite data from Sun and McDonough, 1989; Anders and Grevesse, 1989).

Fig. 13.

—Trace-element spidergram for sill samples from this study. (Chondrite data from Sun and McDonough, 1989; Anders and Grevesse, 1989).

Fig. 14.

—Graph of TiO2 vs. silica. Sills and dikes plot in separate groups (shaded) that show a decrease in SiO2 from dikes to sills. TiO2 contents remain relatively constant. The Mesoproterozoic Plains sill plots in a separate group from the dike group and the sill group. The TiO2 content of the Plains sill is clearly not as high as in the Windermere intrusions. (WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971); samples tlp TLP and CC from Poage (1997).

Fig. 14.

—Graph of TiO2 vs. silica. Sills and dikes plot in separate groups (shaded) that show a decrease in SiO2 from dikes to sills. TiO2 contents remain relatively constant. The Mesoproterozoic Plains sill plots in a separate group from the dike group and the sill group. The TiO2 content of the Plains sill is clearly not as high as in the Windermere intrusions. (WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971); samples tlp TLP and CC from Poage (1997).

Fig. 15.

—Graph of TiO2 vs. Al2O3. Sills and dikes plot in one group, indicating similar TiO2 and Al2O3 amounts. Because neither titanium nor aluminum are very mobile, the separate grouping of Kootenai National Forest samples may indicate that this sill represents an unrelated intrusion. (WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971)).

Fig. 15.

—Graph of TiO2 vs. Al2O3. Sills and dikes plot in one group, indicating similar TiO2 and Al2O3 amounts. Because neither titanium nor aluminum are very mobile, the separate grouping of Kootenai National Forest samples may indicate that this sill represents an unrelated intrusion. (WC65-3 from Schmidt (1978); HE-1 from Eisenbeis (1958); MBT samples from Mueller (1971)).

Table 1.

—Sample identifications, locations, and coordinates for this study. Coordinates were obtained using a hand-held GPS unit. Degree of alteration was estimated from hand sample and thin section.

Table 2.

—Estimated unit thickness and depth from the base of the Flathead Sandstone (Cambrian unconformity) used for Figures 3 and 4. Additional information was obtained using restored cross sections by Mudge et al. (1982)), Harrison et al. (1986), Harrison et al. (1992), Price and Sears (2000), and Sears et al. (2002) at exposures without recognizable geographic reference points and by Sears, unpublished data.

Table 3.

—U-Pb zircon data for Holland Lake, Turah, and Rogers Pass sample sites in Montana.

Notes: zircon fraction: nm_, m_ represent either amperage (A) or angles of paramagnetic susceptibility on a barrier-style Frantz separator; sk = skeletal; pk = pick; cl fl = clear flakes; str = striated; aa = air abraded. Number of grains (g) in each fraction indicated.
a

Pbc common Pb (ppm) corrected for laboratory blank,

b

ratio corrected for blank and mass discrimination

c

radiogenic Pb in ratios; all values corrected for blank and mass discrimination

d

values in parentheses are 2σ errors in percent

e

206Pb/238U vs 207Pb/235U error correlation coefficient

f

percent discordant

Zircon dissolution and chemistry were adapted from methods developed by Krogh (1973) and Parrish et al. (1987). Aliquots of dissolved sample were spiked with a mixed 208pb/235u tracer. Pb and U samples were loaded onto single rhenium filaments with silica gel and graphite, respectively; isotopic compositions were measured in multicollector, static mode on a VG Sector mass spectrometer at the University of Wyoming. Mass discrimination factors of 0.08 ± 0.04%/amu for Pb and 0.0 ± 0.06%/amu for U were determined by replicate analyses of NIST SRM 981 and U-500, respectively. Procedural blank improved from ~ 10 pg to 2 pg Pb over the course of the study.U blanks were consistently 0.4 pg. Concordia coordinates, intercepts, and uncertainties were calculated using PBDAT and ISOPLOT programs (Ludwig, 1988, 1991); initial Pb compositions for the Holland Lake and Turah sills were estimated by Zartman and Doe (1981) model mantle for 800 Ma; Rogers Pass reductions used Stacey and Kramers (1975) model for 750 Ma. The decay constants used by PBDAT are those recommended by the I.U.G.S. Subcommission on Geochronology (Steiger and Jäger, 1977): 0.155125 x 10-9/yr for 238U, 0.98485 x 10-9/yr for 235U and present-day 238U/235U = 137.88.
Table 4.

—Total Pb data for the Turah sill in Montana.

Notes: zircon fraction: d_,nm_ represent angles of diamagnetic and paramagnetic susceptibility on a barrier style Frantz separator; sk = skeletal; eq = equant;anh pi = anhedral plates; str = striated; cl fl = clear flakes. Number of grains (g) indicated for each fraction.
*

excluded from isochron

%

common Pb (cPb) estimated by assuming Stacey and Kramers (1975) model isotopic compositions at 750 Ma for the common component Atomic ratios corrected for blank and mass discrimination Values in parentheses are 2σ errors in percent

Rho = error correlation coefficientsDissolution, chemical processing, mass spectrometry and data reduction were the same as described in Table 2.
Table 5.

—Non-normalized element analysis for samples of this study.

MT ALE 01MT ALE 02MT KF 01MT IF 02MT KF 03MT MR 01MT MR 02MT RPE 04MT RPE 21MT RPM 01 |MT RPM 02Average
Date9-M-039-Jul-039-M-039-Jul-039-Jul-0310-Jul-0310-JuI-0310-JuI-0310-Jul-0310-JuI-0310-JuI-03
Non-normalized Major Elements (Weight %):
SiO250.0052.4248.4348.1648.0549.6750.4150.0350.6150.1950.0349.82
Al2O313.4813.3815.3615.4715.0412.2312.2512.4012.9212.3512.3513.38
TiO22.4982.0192.9282.5152.8352.8592.5552.8722.2452.8722.8442.64
FeO14.6111.1714.2113.6613.7316.0015.3315.9714.2216.0715.7914.62
MnO0.2160.2150.1900.1920.2070.2490.2370.2310.2230.2320.2350.22
CaO9.747.256.708.088.779.489.789.219.989.328.998.85
MgO4.986.954.094.554.295.005.504.855.824.894.855.07
K2O0.731.581.902.301.340.760.770.700.650.720.831.12
Na2O2.452.892.582.793.262.332.242.332.332.292.372.53
P2O50.2240.2100.5140.4710.5310.2560.2250.2520.2120.2550.2520.31
Total98.9398.0896.9098.1998.0698.8399.3098.8499.2199.1998.5498.55
Non-normalized Trace Elements (ppm):
Ni50137272831525554625150
Cr7029944373567101651086663
Sc4026292333484242404241
V422308222183209480467470417463470
Ba133631769929603155173197109144236
Rb2545556040252628202834
Sr153202306368410160149153158151162
Zr167162256236255185180186156189199
Y4228434042464347414747
Nb11.312.926.524.327.212.611.912.910.413.812.2
Ga2620262529222623242423
Cu† 303123172072† 386† 241† 356† 275† 315† 366
Zn142169133140149149127144125138142
Pb554601666448
La153138463887642118735
Ce5548887476403339245225
Th25634626252
MT ALE 01MT ALE 02MT KF 01MT IF 02MT KF 03MT MR 01MT MR 02MT RPE 04MT RPE 21MT RPM 01 |MT RPM 02Average
Date9-M-039-Jul-039-M-039-Jul-039-Jul-0310-Jul-0310-JuI-0310-JuI-0310-Jul-0310-JuI-0310-JuI-03
Non-normalized Major Elements (Weight %):
SiO250.0052.4248.4348.1648.0549.6750.4150.0350.6150.1950.0349.82
Al2O313.4813.3815.3615.4715.0412.2312.2512.4012.9212.3512.3513.38
TiO22.4982.0192.9282.5152.8352.8592.5552.8722.2452.8722.8442.64
FeO14.6111.1714.2113.6613.7316.0015.3315.9714.2216.0715.7914.62
MnO0.2160.2150.1900.1920.2070.2490.2370.2310.2230.2320.2350.22
CaO9.747.256.708.088.779.489.789.219.989.328.998.85
MgO4.986.954.094.554.295.005.504.855.824.894.855.07
K2O0.731.581.902.301.340.760.770.700.650.720.831.12
Na2O2.452.892.582.793.262.332.242.332.332.292.372.53
P2O50.2240.2100.5140.4710.5310.2560.2250.2520.2120.2550.2520.31
Total98.9398.0896.9098.1998.0698.8399.3098.8499.2199.1998.5498.55
Non-normalized Trace Elements (ppm):
Ni50137272831525554625150
Cr7029944373567101651086663
Sc4026292333484242404241
V422308222183209480467470417463470
Ba133631769929603155173197109144236
Rb2545556040252628202834
Sr153202306368410160149153158151162
Zr167162256236255185180186156189199
Y4228434042464347414747
Nb11.312.926.524.327.212.611.912.910.413.812.2
Ga2620262529222623242423
Cu† 303123172072† 386† 241† 356† 275† 315† 366
Zn142169133140149149127144125138142
Pb554601666448
La153138463887642118735
Ce5548887476403339245225
Th25634626252
Major elements are normalized on a volatile-free basis, with total Fe expressed as FeO.

“†” denotes values > 120% of our highest standard.

Table 6.

—Non-normalized element analysis for samples of previous studies.

WC65-3HE-1MBT-3MBT-5MBT-22MBT-23MBT-24MBT-30MBT-51MBT-81MBT-109
Non-normalized major elements (weight %):
SiO249.3045.4451.3452.2851.8752.5252.3252.4252.8552.1451.79
AI2O313.9013.6911.8912.4311.8412.3912.4612.5412.6012.7013.50
TiO22.702.432.852.902.862.852.862.872.852.642.87
Fe2O314.606.06NDNDNDNDNDNDNDNDND
FeO*2.4015.1216.1916.9416.4516.0116.7416.8719.3117.3217.57
MnO0.250.21NDNDNDNDNDNDNDNDND
CaO7.506.157.828.248.338.388.378.448.618.849.16
MgO3.007.604.464.504.594.514.474.614.904.754.80
K2O0.650.701.191.141.041.121.071.010.950.970.88
Na2O2.602.202.603.242.482.562.502.663.192.492.49
P2O50.400.01NDNDNDNDNDNDNDNDND
WC65-3HE-1MBT-3MBT-5MBT-22MBT-23MBT-24MBT-30MBT-51MBT-81MBT-109
Non-normalized major elements (weight %):
SiO249.3045.4451.3452.2851.8752.5252.3252.4252.8552.1451.79
AI2O313.9013.6911.8912.4311.8412.3912.4612.5412.6012.7013.50
TiO22.702.432.852.902.862.852.862.872.852.642.87
Fe2O314.606.06NDNDNDNDNDNDNDNDND
FeO*2.4015.1216.1916.9416.4516.0116.7416.8719.3117.3217.57
MnO0.250.21NDNDNDNDNDNDNDNDND
CaO7.506.157.828.248.338.388.378.448.618.849.16
MgO3.007.604.464.504.594.514.474.614.904.754.80
K2O0.650.701.191.141.041.121.071.010.950.970.88
Na2O2.602.202.603.242.482.562.502.663.192.492.49
P2O50.400.01NDNDNDNDNDNDNDNDND
Table 7.

—Names, dates, and methods for Gunbarrel age rocks. The sills dated during this study are included.

Letters in parenthesis are abbreviations used by Harlan et al. (2003) and Harlan et al. (2005).

Contents

GeoRef

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