Provenance analysis of lower Paleozoic cratonic quartz...

ABSTRACT

Lower Paleozoic supermature quartz aren-ites in Wisconsin and Michigan were derivedfrom several Proterozoic and Archean ter-ranes. Single-grain, detrital zircon populationsfrom the Galesville and St. Peter Sandstones inWisconsin yield very similar, closely concor-dant age distributions (Cambrian GalesvilleSandstone: 1.1 Ga [n = 4], 1.4 Ga [n = 2], 1.8 Ga[n = 1], and 2.7 Ga [n = 2]; Ordovician St. PeterSandstone: 1.1 Ga [n = 2], 1.8 Ga [n = 1], and2.7 Ga [n = 6]). In contrast, most of the nine de-trital zircons that were analyzed from the St.Peter Sandstone in the Michigan basin havehigh U contents (450–2500 ppm) and arestrongly discordant (60%–90%). Six zirconsfrom the Michigan basin that have ca. 1.0 Ga207Pb*/206Pb* ages define a regression line thathas an upper intercept of about 1100 Ma and alower intercept of 15 Ma; one zircon has a 2.7Ga 207Pb*/206Pb* age. The zircon data indicatethat although a number of different terranescontributed detrital material to the Paleozoicquartz arenites in Wisconsin, 1.1 and 2.7 Gaterranes were the dominant sources, and notthe local basement, which primarily consists ofthe ≥2.7 Ga Marshfield terrane, the 1.8 GaPenokean orogen, and the 1.4 Ga Wolf Riverbatholith. A terrane that has a 1.1 Ga age isprobably the main source for the St. PeterSandstone in the Michigan basin.

Quartz separates were also analyzed for Pb-Pb and Sm-Nd isotope variations, and the datado not indicate significant source differencesbetween the heavy mineral fraction (zircons)and the quartz framework grains. Pb-Pbisochrons and Sm-Nd isotope data for quartz

separates reflect mixing of the age groups andapproximate relative proportions that areidentified from the single zircon results.

All isotope data on quartz separates and U-Pb zircon data indicate that the detrital con-stituents (zircons and quartz frameworkgrains) that compose the lower Paleozoicquartz arenites in Wisconsin were primarilyderived from the 2.7 Ga granite-greenstoneterrane of the southern Superior Province anda 1.1 Ga terrane. The latter terrane is eitherthe silicic volcanic rocks associated with theMidcontinent rift system, or, more likely, thevoluminous granitic rocks that are associatedwith the Grenville Province on the easternmargin of North America. The Middle Prot-erozoic Grenville Province was the most im-portant ultimate source of quartz and zirconsto the St. Peter Sandstone in the Michiganbasin; lesser amounts of material were con-tributed from the Archean Superior Province.

INTRODUCTION

Supermature quartz arenites are common inthe sedimentary record, particularly in the Prot-erozoic and lower Paleozoic sections of stablecratons, and usually are composed of >97%quartz and <1% heavy minerals (e.g., Tyler,1936; Dott et al., 1986). However, conventionalpetrographic (e.g., Dickinson and Suczek, 1979;Folk, 1980) and bulk geochemical (e.g., Bhatiaand Crook, 1986; Basu et al., 1990) approachesto provenance analysis of supermature quartzarenites generally yield few insights into thesehomogeneous rocks. The source ages of heavymineral components, such as zircons, have provento be an important aspect of provenance studies(e.g., Ross and Parrish, 1991; Zhao et al., 1992;McLennan et al., 1993; Smith and Gehrels,

1994), but few isotopic studies have focused onthe quartz grains.

The stratigraphy and physical sedimentologyof lower Paleozoic quartz arenites in the north-ern midcontinent region of North America havebeen intensively studied; the provenance andmany aspects concerning the depositional his-tory of these extensive cratonic sheet sand-stones have been the subject of considerabledebate since the early 1900s (Dake, 1921; Dottand Byers, 1981; Dott et al., 1986). On the ba-sis of directional indicators of sediment trans-port (Potter and Pryor, 1961; Dott et al., 1986)and the composition of heavy minerals (Tyler,1936), it has been suggested that detrital grainsthat compose the cratonic sandstones in Wis-consin were ultimately derived from Precam-brian felsic plutonic rocks that are exposed inthe Lake Superior region. Precambrian bedrockof the northern midcontinent region of NorthAmerica includes rocks that span >2.5 b.y. inage (Fig. 1; e.g., Hoffman, 1989; Sims et al.,1989), reflecting a diverse range of potentialsources for the Paleozoic cratonic sandstones inWisconsin and Michigan. These sources in-clude the >3.0 Ga southern gneiss terrane of theSuperior Province; the ca. 2.7 Ga granite-greenstone terrane of the Superior Province;the ≥2.7 Ga Archean inlier of the Marshfieldgneiss terrane in central Wisconsin; the ca.1.8–1.7 Ga Penokean orogen and postorogenicrocks; the ca. 1.4 Ga anorogenic granites of theWolf River batholith, eastern granite-rhyoliteprovince, and parts of the Grenville Province;and the ca. 1.1 Ga Keweenawan rocks of theMidcontinent rift system and the majority ofthe Grenville Province (Fig. 1).

In this contribution we report the results of arare earth element (REE), Sm-Nd, Pb-Pb, and U-Pb isotope study of quartz grains, as well as U-Pb

1723

Provenance analysis of lower Paleozoic cratonic quartz arenites of the NorthAmerican midcontinent region: U-Pb and Sm-Nd isotope geochemistry

Clark M. Johnson* Department of Geology and Geophysics, University of Wisconsin,Bryce L. Winter 1215 West Dayton Street, Madison, Wisconsin 53706

GSA Bulletin; November 1999; v. 111; no. 11; p. 1723–1738; 11 figures; 6 tables.

*E-mail: [email protected].

Data Repository item 9990 contains additional material related to this article.

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geochronology on single detrital zircons, fromtwo lower Paleozoic supermature quartz sand-stone units in south-central Wisconsin (CambrianGalesville Sandstone and Ordovician St. PeterSandstone), and one unit in the east-centralMichigan basin (Ordovician St. Peter Sandstone)(Figs. 1 and 2). Both zircons and quartz were an-alyzed in an attempt to address possible differ-ences in the sources of zircons and the primaryquartz framework grains. The data highlight thegreat antiquity of most quartz and heavy mineralsin the cratonic quartz arenites, and identifysource terranes and possible sediment transportpathways.

GEOLOGIC SETTING

The lower Paleozoic stratigraphy of the NorthAmerican midcontinent is characterized by 60–90 m.y. (i.e., second order) cycles that consist ofa basal quartz arenite and an upper carbonateunit, the former of which was deposited on wide-spread unconformities. These well-known, inten-sively studied strata were the basis for the devel-opment of the term orthoquartzite-carbonatesuite (Pettijohn, 1957), and the concept of uncon-formity-bounded lithologic sequences (Sloss,1963). These sandstones, which have exceptionalcompositional and textural maturity, are com-posed of >97% unstrained, monocrystallinequartz, and there is a distinct paucity of shale.They have a sheet-like geometry, and typicallyhave a thickness of 40–50 m over thousands ofsquare kilometers (Dott et al., 1986).

Most current sedimentologic models agreethat the sheet geometry is largely the result of eo-lian and fluvial processes that, following ex-tended periods of subaerial erosion, distributedsand across large areas of the craton (Dott et al.,1986). Nonmarine deposition was followed byreworking and further deposition of sand in ashallow-marine environment. Paleocurrent indi-cators from the subaerial facies of Cambrian andOrdovician quartz arenites are southwest di-rected, consistent with inferred deposition in thepaleo-trade wind belt. Paleocurrent indicatorsfrom the subaqueous facies are consistent withother stratigraphic evidence that the Precambriancraton of the Great Lakes region was a subtletopographic high, and therefore an ultimatesource of sediment throughout Paleozoic time(Potter and Pryor, 1961).

Wisconsin Sandstones

The Cambrian Galesville Sandstone and the Or-dovician St. Peter Sandstone in Wisconsin (Fig. 2)are composed of quartz that is medium to coarsegrained, well sorted, and well rounded (Dott et al.,1986). The quartz grains are very poorly ce-

mented, and typically have only incipient quartzovergrowths (<2 µm; Odom et al., 1976). Our en-ergy-dispersive electron-microprobe analysis ofmineral inclusions in 38 quartz grains from thesetwo formations supports the detailed study ofTyler (1936), who physically separated andcounted a statistically significant number of min-eral inclusions. Tyler’s results from the St. PeterSandstone indicate that zircon and apatite aredominant (70%–90% of heavy mineral inclu-sions), and titanite, rutile, ilmenite, garnet, andpyrite are present in most samples. Biotite andhornblende are important inclusions locally, andfluorite and kyanite are rarely present as mineralinclusions in quartz. The predominance of zirconand apatite as inclusions suggests that most of thequartz grains were derived from a terrane thatconsisted largely of intermediate- to silicic-com-position plutonic rocks, whereas the presence ofonly small quantities of kyanite and garnet inclu-sions suggests that pelitic metamorphic rocks

were a minor component in the ultimate sourceregion(s) (Tyler, 1936).

The predominance of only the most durabletype of quartz (i.e., unstrained, monocrystalline)and composition of the framework heavy miner-als in the detrital suites led Tyler (1936) to sug-gest that the detrital grains have been throughmultiple sedimentary cycles, and that the imme-diate source was a sedimentary terrane, ratherthan an igneous or metamorphic source. Possiblesedimentary sources for the Galesville Sandstoneand St. Peter Sandstone in Wisconsin, based onTyler’s work, include the Cambrian Mt. SimonSandstone (Wisconsin), the Upper Keweenawan(ca. 1.1 Ga) Hinkley Sandstone (Minnesota) andBayfield Group sandstones (Wisconsin), and sev-eral of the Middle Proterozoic quartzites of theBaraboo interval (i.e., Sioux-Minnesota, Barron-Wisconsin, Flambeau-Wisconsin). However, themajority of Middle Proterozoic quartzites in Wis-consin, including the Baraboo, McCaslin, and

JOHNSON AND WINTER

1724 Geological Society of America Bulletin, November 1999

Figure 1. Precambrian geologic basement map of the Great Lakes region (adapted from Van Schmus, 1992). A major crustal boundary, the Great Lakes tectonic zone (GLTZ), dividesthe southern Superior Province into a southern gneiss terrane (GT) of age 3.0–3.6 Ga and anorthern granite-greenstone terrane (GGT) of age 2.7 Ga. The Niagara fault zone (NFZ) is thesouthern boundary of the Superior Province and represents the suture zone with the magmaticarcs of the ca. 1.8 Ga Penokean orogen. An inlier of 2.7 to >3.2 Ga Archean rocks in the Penokeanorogen is defined as the Marshfield terrane (MT). Anorogenic plutons emplaced ca. 1.4 Ga per-forate the Early Proterozoic crust throughout the midcontinent region; the Wolf River batholith(WRB) in northeastern Wisconsin is a typical example. South of the Penokean orogen is the ca.1.4 Ga eastern granite-rhyolite province (EGRP). The ca. 1.1 Ga Midcontinent rift system(MCRS) consists of an axial portion that is dominated by mafic igneous rocks (black), and postriftflanking sedimentary basins (diagonal rules). The ca. 1.1 Ga Grenville Province (GP) comprisesthe eastern margin of the North American craton. Additional data are from Sims et al. (1989).

Waterloo Quartzites, are unlikely to be importantsand contributors because of the strained natureof the quartz grains and the absence of tourma-line (e.g., Sims et al., 1993). The Cambrian Jor-dan Sandstone (Wisconsin) and the KeweenawanFreda Sandstone (Oronto Group, Wisconsin) arealso deemed unlikely direct sources because oftheir relatively high garnet content (Tyler, 1936).

Michigan Basin Units

The lithology of the St. Peter Sandstone in theMichigan basin is more variable, vertically and lat-erally, than it is in Wisconsin (e.g., Winter et al.,1995). Samples for this study were obtained froma depth of ~3600 m in the east-central Michiganbasin, just south of Saginaw Bay (Fig. 2). In thispart of the Michigan basin (Fig. 2), the St. PeterSandstone is ~200 m thick and is composed of ma-ture quartz arenite that is interstratified with sevencarbonate units (1–6 m thick). The sandstone isprincipally composed of quartz that is medium tocoarse grained, well sorted, well rounded, andmoderately to tightly cemented by quartz over-growths (e.g., Graham et al., 1996). Energy-dispersive electron-microprobe analyses of min-eral inclusions in 21 quartz grains from the Michi-gan basin identify the presence of zircon, rutile, il-menite, and apatite. In addition, it is important tonote that 9 of the 21 samples analyzed contain rel-atively large domains (50–200 µm across) of

potassium feldspar that are usually present at theedge of the mounted grain, indicating that feldsparoccurs as diagenetic overgrowths.

ANALYTICAL TECHNIQUES

Detrital Zircons

We collected ~70 kg of rock from outcrops ofthe Ordovician St. Peter Sandstone near Madi-son, Wisconsin, and from the CambrianGalesville Sandstone in the vicinity of the Wis-consin Dells (Fig. 2; see Table DR-11). In addi-tion, ~20 kg of the St. Peter Sandstone was ob-tained from a core drilled in the east-centralportion of the Michigan basin (Fig. 2; Table DR-2 [see footnote 1]). Samples were crushed andzircons separated using standard shaker table,heavy liquid, and magnetic techniques. The non-magnetic zircons were separated into size frac-tions using disposable nylon screens. After airabrading the largest size fractions with pyrite for~20 hr (Krogh, 1982), the zircons were cleanedrepeatedly in warm ~5 M HNO3. On the basis ofour experience with detrital zircons in Protero-

zoic quartzites in Wisconsin, these procedureshave the greatest likelihood of producing concor-dant U/Pb analyses (Van Wyck, 1995). Single zir-cons were then hand-picked for analysis on thebasis of clarity and the absence of cracks and in-clusions in an attempt to analyze only those zir-cons that would give concordant ages. In order toanalyze zircons reflecting all of the possiblesource terranes, the complete range of colors andmorphological types was selected from eachsample. Nonetheless, it remains possible that oursampling is biased, because only the largest zir-cons were analyzed in order to ensure sufficientradiogenic lead. Each zircon was cleaned twotimes with warm ~7 M HNO3 for ~30 min, fol-lowed by rinsing two times with H2O. After spik-ing with a 205Pb–235U tracer, each zircon was dis-solved with HF in Teflon microcapsules within aTeflon-lined acid-digestion bomb, evaporated todryness, and then redissolved in HCl (Parrish,1987). U and Pb were separated using the meth-ods of Parrish et al. (1987), and then analyzed ona Micromass Sector 54 mass spectrometer usingsingle-collector analysis and a Daly detector.

Quartz Grains

Samples for U-Pb analysis of quartz were col-lected from: (1) five different outcrops of theGalesville Sandstone near Lone Rock, Wiscon-sin, and the Wisconsin Dells (one sample fromeach locality), (2) six different outcrops of the St.Peter Sandstone in the vicinity of Madison, Wis-consin (nine individual samples), and (3) twocores taken at ~3600 m depth in the east-centralMichigan basin (19 individual samples) (Fig. 2).The sand grains were fully disaggregated in aclean mortar and pestle and then sieved to obtainsingle, medium sand-sized (300–500 µm)grains. This process produced samples of virtu-ally 100% pure quartz with minimal detritalheavy mineral contamination, because the traceconstituents (e.g., feldspar and heavy minerals)in these sandstones are generally <150 µm (e.g.,Odom et al., 1976). The quartz grains analyzedfrom Wisconsin were very well rounded andcontained no quartz overgrowths visible under abinocular microscope, whereas many samplesfrom the Michigan basin contained visiblequartz overgrowths.

For U/Pb or Pb-Pb isotope analysis, quartzgrains were ultrasonically cleaned in ~5 M HNO3for ~2 hr, rinsed with H2O, leached in warm 5 MHNO3 for ~10 hr, and then vigorously rinsedmultiple times with H2O. Quartz grains that wereanomalously colored or contained visible inclu-sions under a binocular microscope were subse-quently removed by hand-picking. After a finalrinse with warm HNO3 and multiple H2O rinses,quartz grains were dissolved in Teflon vials with

LOWER PALEOZOIC CRATONIC QUARTZ ARENITES, NORTH AMERICAN MIDCONTINENT

Geological Society of America Bulletin, November 1999 1725

Figure 2. Generalized map (adapted from Dott et al., 1986) showing the outcrop areas of theGalesville Sandstone and the St. Peter Sandstone in Wisconsin, and the isopachs (contour inter-val = 100 m) of the St. Peter Sandstone in the Michigan basin. Stars mark general localities in thisstudy. Precise locations for sample collection sites are given in Data Repository Tables DR-1 andDR-2.1 The outcrop in the northern peninsula of Michigan is that of the Munising Formation,which is presumed to be correlative with the Galesville Sandstone. The generalized lower Paleo-zoic stratigraphy of Wisconsin is also shown.

1GSA Data Repository item 9990, sample loca-tion data tables, is available on the Web athttp://www.geosociety.org/pubs/drpint.htm. Re-quests may also be sent to Documents Secretary,GSA, P.O. Box 9140, Boulder, CO 80301; e-mail:[email protected].

JOHNSON AND WINTER


concentrated HF on a hot plate for ~12 hr. Analiquot of some samples was spiked with a mixed208Pb–235U tracer to determine concentrations.HF dissolution was followed by treatment withhot 6M HCl, to ensure that all inclusions wouldbe dissolved; no solid residue was visible. Pb andU were separated using a small volume of anionexchange resin in HBr and HNO3, respectively,then analyzed on a Micromass Sector 54 massspectrometer using static multicollection.

For REE and Sm-Nd isotope analysis, largesamples (~500 mg) of cleaned (as in preceding)quartz from the Galesville Sandstone and St. PeterSandstone in Wisconsin and Michigan were dis-solved in Teflon vials with concentrated HF on ahot plate for ~12 hr. HF dissolution was followedby treatment with hot 6M HCl (see preceding),and no solid residue was observed. The solutionswere analyzed for REE contents (by isotope dilu-tion) and Nd isotope ratios. The analytical meth-ods and standard Nd isotope ratios were discussedin detail in Johnson and Thompson (1991) andVanWyck (1995).

INTERPRETATION OF DATA FOR SINGLE DETRITAL ZIRCONS

Detrital zircon U-Pb isotope data indicate thata variety of sources contributed to the sandstones,ranging from 2.7 to 1.1 Ga in age. The single zir-con data provide important constraints for inter-preting the Pb-Pb and Sm-Nd data of the bulkquartz samples discussed later.

Wisconsin

Nine zircons that have different colors andmorphology from the Cambrian Galesville Sand-stone in Wisconsin yield nearly concordant agesof ca. 1050 Ma (n = 4), ca. 1400 Ma (n = 2), ca.1800 Ma (n = 1), and ca. 2700 Ma (n = 2) (Table1; Fig. 3). Eight of nine zircons representing var-ious populations from the Ordovician St. PeterSandstone in Wisconsin yield nearly concordantages of ca. 1050 Ma (n = 2), ca. 1800 Ma (n = 1),and ca. 2700 Ma (n = 6) (Table 1; Fig. 3). Ura-nium contents for the zircons from both of these

sandstones are relatively low (~20–140 ppm;Table 1), and are within the range measured forzircons from Archean intrusive rocks of thesouthwestern Superior Province (e.g., Davis andEdwards, 1986; Corfu, 1988), or the MiddleProterozoic Grenville Province (e.g., McLellandet al., 1993; Owens et al., 1994). The concor-dance of most of the analyses indicates that the207Pb*/206Pb* ages can be interpreted as crystal-lization ages with a high degree of confidence.Most of the zircons were naturally well roundedand contained few facets. All zircons that had apurple tint are Archean in age, although not allArchean zircons are purple; in general, there islittle correlation between the physical characterof the zircons and their ages.

The distribution of ages for detrital zirconsfrom the two Wisconsin sandstones is remark-ably similar, although ca. 1.4 Ga zircons are notrepresented in the nine grains analyzed from theSt. Peter Sandstone in Wisconsin. Tyler’s (1936)interpretation that the St. Peter Sandstone in Wis-consin was derived largely from the erosional re-

TABLE 1. U-Pb DATA FOR SINGLE DETRITAL ZIRCONS FROM LOWER PALEOZOIC QUARTZ ARENITES, NORTHERN MIDCONTINENT

Sample† Mass§ U 206Pb* 206Pb/204Pb 206Pb*/238U 207Pb*/235U Correlation 207Pb*/206Pb* 207Pb/206Pb Percent(µg) (ppm) (ppm) raw coefficient age discordancy

(Ma) number

St. Peter Sandstone, Michigan basin (field sample MI-SP, locality 13)MI-SP-A c 8 2508 35 1306 0.0161 ± 0.50% 0.1604 ± 0.51% 0.96 0.0724 ± 0.14% 998.3 ± 5.7 90MI-SP-B spi 32 594 11 1850 0.0209 ± 0.40% 0.2590 ± 0.40% 0.97 0.0898 ± 0.09% 1420.9 ± 3.5 92MI-SP-C spi 103 56 11 4660 0.2371 ± 0.15% 2.4478 ± 0.15% 0.95 0.0749 ± 0.05% 1065.5 ± 2.0 –32MI-SP-D sy 14 662 125 4699 0.2190 ± 0.17% 5.2705 ± 0.18% 0.98 0.1745 ± 0.04% 2601.7 ± 1.2 56MI-SP-E c 103 52 10 2407 0.2154 ± 0.28% 2.3010 ± 0.29% 0.97 0.0775 ± 0.07% 1133.3 ± 3.0 –12MI-SP-F dp 20 2144 13 821 0.0072 ± 1.02% 0.1814 ± 1.01% 1.00 0.1827 ± 0.09% 2677.4 ± 3.0 99MI-SP-G spi 40 444 5 416 0.0137 ± 1.23% 0.1361 ± 1.28% 0.95 0.0723 ± 0.39% 995 ± 16 92MI-SP-H spi 294 38 2 1496 0.0466 ± 0.59% 0.5119 ± 0.59% 0.97 0.0797 ± 0.14% 1190.2 ± 5.3 77MI-SP-I c 30 500 21 2242 0.0498 ± 0.41% 0.4954 ± 0.42% 0.97 0.0722 ± 0.10% 990.8 ± 4.0 70

St. Peter Sandstone, Wisconsin (field sample WI-SP, locality 3)WI-SP-A db 17 124 46 643 0.4238 ± 0.55% 11.0989 ± 0.55% 0.99 0.1900 ± 0.09% 2741.8 ± 3.0 20WI-SP-B mb 23 38 6 132 0.1668 ± 2.88% 1.6939 ± 3.22% 0.90 0.0737 ± 1.38% 1032 ± 56 4WI-SP-C c 14 86 36 879 0.4805 ± 0.84% 12.0206 ± 0.83% 1.00 0.1814 ± 0.08% 2666.0 ± 2.6 6WI-SP-D c 38 75 21 514 0.3154 ± 0.52% 4.8532 ± 0.57% 0.93 0.1116 ± 0.21% 1825.5 ± 7.5 4WI-SP-E c 27 17 8 144 0.4826 ± 2.07% 12.2801 ± 2.08% 0.98 0.1846 ± 0.41% 2694 ± 14 7WI-SP-F db 20 127 56 1025 0.5054 ± 0.39% 13.1463 ± 0.39% 0.99 0.1887 ± 0.06% 2730.6 ± 1.9 4WI-SP-G sb 20 40 7 142 0.1744 ± 3.03% 1.8014 ± 3.25% 0.93 0.0749 ± 1.21% 1066 ± 48 3WI-SP-H dp 23 49 22 302 0.4922 ± 0.87% 12.6348 ± 0.89% 0.98 0.1862 ± 0.19% 2708.7 ± 6.3 6WI-SP-I sp 14 58 27 250 0.5129 ± 1.15% 12.8624 ± 1.16% 0.98 0.1819 ± 0.24% 2670.0 ± 7.8 0

Galesville Sandstone, Wisconsin (field sample WI-G, locality 8)WI-G2 my 18 25 3 146 0.1578 ± 6.35% 1.5867 ± 6.30% 0.97 0.0729 ± 1.47% 1012 ± 59 7WI-G3 sp 20 42 18 347 0.4802 ± 1.94% 11.6912 ± 1.91% 1.00 0.1766 ± 0.17% 2621.1 ± 5.8 4WI-G4 sp/b 21 45 18 385 0.4652 ± 1.80% 11.8213 ± 1.77% 1.00 0.1843 ± 0.16% 2692.1 ± 5.4 10WI-G5 c 21 138 21 1327 0.1780 ± 0.61% 1.8440 ± 0.62% 0.97 0.0751 ± 0.15% 1072.2 ± 5.9 2WI-G6 c 28 87 17 794 0.2222 ± 0.86% 2.6175 ± 0.86% 0.98 0.0854 ± 0.18% 1325.2 ± 7.0 3WI-G7 c 15 119 25 767 0.2433 ± 1.23% 3.0691 ± 1.24% 0.98 0.0915 ± 0.25% 1456.6 ± 9.6 4WI-G8 my 32 41 6 447 0.1750 ± 2.34% 1.8030 ± 1.99% 0.97 0.0747 ± 0.53% 1062 ± 21 2WI-G9 my 11 100 15 400 0.1763 ± 2.34% 1.8436 ± 2.34% 0.98 0.0758 ± 0.52% 1090 ± 21 4WI-G10 c 24 49 13 817 0.3194 ± 0.65% 4.8777 ± 0.66% 0.98 0.1108 ± 0.13% 1812.1 ± 4.7 2

Notes: Isotope compositions have been adjusted for the following: (1) blank Pb (15–50 pg, 206Pb/204Pb = 18.8 ± 0.2, 207Pb/204Pb = 15.4 ± 0.2, 208Pb/204Pb = 37.6 ± 0.5) andU (<9 pg); (2) fractionation of +0.12% ± 0.05% per amu for Pb (based on 14 analyses of NBS-981 on Daly detector) and +0.05% ± 0.04% per amu for U (based on 7 analysesof U500 on Daly detector); (3) Pb contribution from 205Pb-235U spike; (4) initial Pb isotope composition from Stacey and Kramers model (1975), assuming uncertainties of 1.5%for 206Pb/204Pb, 0.4% for 207Pb/204Pb, and 2.0% for 206Pb/204Pb. Uncertainties for isotope ratios reported above are 1σ (SD) in percent; uncertainties for ages are 2σ (SE) inMa. All data computations were made using the programs of Ludwig (1991a, 1991b). Locations of sample collection sites are given in Tables DR-1 and DR-2 (see text footnote1).

*Radiogenic.†Zircon appearance: s—slight, m—medium, d—dark, c—colorless, b—brown, p—purple, pi—pink, y—yellow.§Mass calculated from measured dimensions, assuming density of 4.69 g/cm3.#Percent discordancy is calculated using the 206Pb*/238U age relative to the 207Pb*/206Pb* age.

working of the Galesville Sandstone, based onvirtually identical heavy mineral suites, is sup-ported by our results, which indicate similar agedistributions of the detrital zircons in the twounits. The ages determined for detrital zirconsfrom both Wisconsin sandstones include most allof the ages known for Precambrian basementrocks exposed in the Great Lakes region (Fig. 3),although there is a marked clustering of ages at1.1 Ga (typical of volcanic rocks of the Midcon-tinent rift system and the Grenville Province) and2.7 Ga (typical of southern Superior granite-greenstone terrane). Zircons of ca. 1800 Ma ageare surprisingly rare, given the extensive expo-sures of Penokean orogenic rocks in central andnorthern Wisconsin (Fig. 1) and the large abun-dance of 1.8 Ga detrital zircons in Baraboo inter-val quartzites (Van Wyck, 1995; Medaris et al.,1996). Early Archean zircons from the 3.0–3.6Ga Superior gneiss terranes, as well as olderrocks from the nearby Marshfield terrane(2.7–>3.2 Ga) are absent from the detrital zirconpopulation (Figs. 1 and 3).

Michigan Basin

All nine zircons analyzed from the St. PeterSandstone in the Michigan basin are highly dis-

cordant (Table 1; Fig. 3, inset), which is instriking contrast to the U-Pb systematics of zir-cons from this formation in Wisconsin. Manyof the zircons from the Michigan basin havemarkedly higher U contents (all but three haveU contents of ~450–2500 ppm) than those fromWisconsin, and the most discordant zircons aregenerally those that have the highest U con-tents. The contrast in concordancy in the zir-cons from Wisconsin and Michigan is not dueto analytical procedures, because both suiteswere analyzed simultaneously, and results ofzircon standard analyses were consistentthroughout the study (Table 1). Zircons thathave comparatively high U contents are foundin some highly evolved granitic rocks of theGrenville Province (e.g., Chiarenzelli andMcLelland, 1993). Although highly discordantzircons make it difficult to confidently deter-mine the crystallization ages, regression of sixzircons that have 207Pb*/206Pb* ages of ca. 1 Ga,including the two that are reversely discordant,yields an upper intercept of 1104 ± 118 Ma anda lower intercept of 14 ± 24 Ma (MSWD [meansquare of weighted deviates] = 8.7). One zirconhas a 207Pb*/206Pb* age of ca. 2600 Ma and is~60% discordant (MI-SP5), and is certainly anArchean zircon. We interpret these data to indi-

cate that most of the zircons from the Michiganbasin reflect derivation from a ca. 1100 Masource terrane, which included distinctive high-U zircons.

INTERPRETATION OF DATA FOR DETRITAL QUARTZ GRAINS

The age and sources of quartz grains in sedi-mentary rocks has been a long-standing problem.In the following we show how U-Pb and Sm-Ndisotope systematics of bulk quartz separates can beexplained through sedimentary mixing processes,using the age constraints provided by the detritalzircons.

Results of U-Pb and REE Analyses

As shown by previous studies, natural quartzcan contain significant U, Pb, and REE contents(Götze and Lewis, 1994; Hemming et al., 1994).Lead and U contents of 11 quartz samples ana-lyzed in this study range from 0.10 to 1.1 ppmand 0.065 to 0.26 ppm, respectively (Tables 2and 3). The quartz samples from the OrdovicianSt. Peter Sandstone (n = 11) and the CambrianGalesville Sandstone (n = 12) in Wisconsin havesimilarly radiogenic but variable Pb isotope ra-tios (206Pb/204Pb = 19.9–35.2 and 21.3–58.9, re-spectively), whereas quartz from the Michiganbasin (n = 21) has relatively nonradiogenic andmore restricted Pb isotope ratios (206Pb/204Pb =16.8–32.3; all but two are between 16.8 and20.7) (Table 2; Fig. 4). The 206Pb/204Pb ratios forall quartz samples analyzed in this study are lessradiogenic than those measured for the EarlyProterozoic Pokegama Quartzite (Minnesota)studied by Hemming et al. (1994), and we inter-pret this to reflect a larger proportion of nonradi-ogenic mineral inclusions or overgrowths in thequartz grains studied here. REE contents of thethree quartz samples from this study (Nd =0.7–2.1 ppm; Table 4) overlap those measuredfor whole-rock samples of the PokegamaQuartzite (Hemming et al., 1994; Fig. 5A).

Location of U, Pb, and REEs in Quartz Grains

208Pb/204Pb–206Pb/204Pb Systematics.Our Pbisotope data and REE patterns for quartz grains arebest explained by microinclusions of common ac-cessory and/or heavy minerals such as zircon, ti-tanite, apatite, and monazite. Zircon, titanite, andapatite are the most likely U- and REE-rich acces-sory (heavy) minerals of those identified by Tyler(1936) as inclusions in quartz grains. However,208Pb/204Pb–206Pb/204Pb isotope variations of thequartz grains require that a mineral that has a highTh/U ratio, such as monazite, is present as a minor



Figure 3. U-Pb concordia diagram for single detrital zircons from lower Paleozoic quartzarenites of the northern midcontinent region of North America. Likely source terranes for thezircon age groups are indicated (see Fig. 1). Most U-Pb ages from Wisconsin are nearly concor-dant, in marked contrast to the generally discordant ages from the Michigan basin. Most of thehighly discordant analyses from the Michigan basin have 207Pb*/206Pb* ages of ca. 1.1 Ga (seeTable 1). Error ellipses are much smaller than the plotted symbols. Inset shows detail for dis-cordant ca. 1.1 Ga zircons from the Michigan basin.


TABLE 2. U AND Pb DATA FOR DETRITAL QUARTZ FROM LOWER PALEOZOIC QUARTZ ARENITES, NORTHERN MIDCONTINENT

Sample* Locality Mass U Pb 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb(mg) (ppm) (ppm)

St. Peter Sandstone, WisconsinWI-SP-1 1 92.2 0.154 0.948 27.468 17.084 41.524WI-SP-2 1 103.2 0.065 0.102 29.058 17.245 40.876WI-SP-3 2 106.6 0.164 0.948 22.292 16.110 40.314WI-SP-4 2 127.3 0.200 0.230 35.221 18.269 45.071WI-SP-5 3 123.9 0.196 0.842 24.933 16.590 42.057WI-SP-6 3 93.1 0.158 0.218 33.839 18.170 48.269WI-SP-7, M-2.5 4 66.3 N.D. N.D. 19.886 15.802 39.595WI-SP-7, NM-1 4 81.3 N.D. N.D. 23.183 16.430 43.447WI-SP-10, NM-1 5 65.1 N.D. N.D. 21.538 16.083 41.491WI-SP-10, M-1 5 72.9 N.D. N.D. 21.383 16.122 41.439WI-SP-11 6 100.6 N.D. N.D. 30.217 17.768 42.207

Galesville Sandstone, WisconsinGALE-D1, NM-1 7 89.3 N.D. N.D. 26.767 17.082 44.839GALE-D1, M-2.5 7 94.0 N.D. N.D. 23.573 16.409 43.659GALE-D1, M-5 7 77.7 N.D. N.D. 23.702 16.384 41.604GALE-D6, NM-1 8 75.6 N.D. N.D. 41.360 19.676 49.536GALE-D6, M-2.5 8 97.2 N.D. N.D. 21.308 16.048 39.772GALE-D4 9 97.3 N.D. N.D. 25.559 16.694 43.868GALE-D2, NM-1 10 99.2 N.D. N.D. 22.611 16.305 40.471GALE-D2, M-5 10 46.9 N.D. N.D. 23.731 16.565 39.687GALE-LR1, NM-1 11 99.3 N.D. N.D. 29.857 17.446 45.720GALE-LR1, M-1 11 83.0 N.D. N.D. 42.030 19.127 55.398GALE-LR1, M-2.5 11 88.7 N.D. N.D. 47.937 20.988 57.582GALE-LR1, M-5 11 49.5 N.D. N.D. 58.945 21.957 55.137

St. Peter Sandstone, Michigan basinWI0944.3A 12 120.8 N.D. N.D. 17.826 15.583 36.672W10944.3B 12 118.5 N.D. N.D. 17.877 15.558 36.985W10950.2 12 153.2 N.D. N.D. 27.052 16.904 46.664W10951.3 12 94.3 N.D. N.D. 18.106 15.600 37.226W10951.7 12 114.0 N.D. N.D. 20.699 16.048 39.944W10991.2 12 336.8 0.272 1.11 19.737 15.820 38.903W11002.9 12 207.9 N.D. N.D. 17.816 15.540 37.470SW10985.7 13 106.5 N.D. N.D. 19.185 15.759 39.154SW10994.5 13 112.9 N.D. N.D. 19.939 15.944 41.071SW10999.5 13 91.4 N.D. N.D. 17.885 15.597 37.839SW11000 13 97.2 N.D. N.D. 19.100 15.607 38.117SW11022.5A 13 111.6 N.D. N.D. 17.917 15.631 38.728SW11022.5B 13 81.8 N.D. N.D. 17.671 15.534 36.958SW11048.5A 13 114.8 N.D. N.D. 16.831 15.434 36.560SW11048.5B 13 108.8 N.D. N.D. 16.751 15.429 36.515SW11054.4 13 131.7 N.D. N.D. 17.912 15.567 38.885SW11060.1 13 122.1 N.D. N.D. 17.895 15.581 37.376SW11063.4 13 144.2 N.D. N.D. 32.362 17.901 58.189

Notes: Total procedural blanks ranged from 70 to 200 pg Pb (206Pb/204Pb = 18.5 ± 0.5, 207Pb/204Pb = 15.1 ± 0.4, 208Pb/204Pb = 37.1 ± 0.5) and <25pg U, which resulted in negligible blank corrections. Mass fractionation correction for Pb was +0.10% ± 0.03% per amu as determined by 14analyses of NBS-981 on Faraday collectors. Mass fractionation corrections for U were not applied for Faraday analyses, based on eight analyses ofthe U500 standard, which produced the accepted ratio ( 235U/238U = 0.9997). In run, 2σ uncertainties were <0.1%, but a minimum 0.2% error was assigned in isochron regression calculations to be conservative. Precise localities of sample collection sites are given in Tables DR-1 and DR-2 (seetext footnote 1). Note that separate samples at localities 3, 8, and 13 were used for single zircon analyses (Table 1). Data for sample SW11068(Shell Whyte core, Michigan basin), which was used for three-step dissolution experiment, are reported in Table 3. N.D. = not determined.

*Samples separated on the basis of magnetic susceptibility are indicated: M = magnetic fraction. NM = non-magnetic fraction. The number indicates the tilt angle in degrees.

TABLE 3. U AND Pb DATA FOR QUARTZ SAMPLE SW11068 (LOCALITY 13), ST. PETER SANDSTONE, MICHIGAN BASIN

Sample Mass % of U U % of total Pb Pb % of total 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb(mg) total mass (ppm) (ng) U (ppm) (ng) Pb

One-step, total dissolutionA 415.0 N.D. 0.260 108 N.D. 0.511 212 N.D. 24.18 ± 0.06 16.480 ± 0.09 39.15 ± 0.12B 20.7 N.D. 0.141 2.92 N.D. 1.05 21.7 N.D. 18.89 ± 0.13 15.741 ± 0.16 40.14 ± 0.20C 55.9 N.D. 0.200 11.2 N.D. 0.308 17.2 N.D. 19.79 ± 0.21 15.79 ± 0.22 38.64 ± 0.19

Three-step, sequential dissolutionD1 34.8 13 0.232 8.08 16 1.95 67.8 74 17.24 ± 0.07 15.464 ± 0.10 36.53 ± 0.13D2 127.3 48 0.150 19.1 39 0.132 16.8 18 33.01 ± 0.69 17.317 ± 0.30 41.35 ± 0.26D3 101.3 39 0.216 21.9 45 0.071 7.19 8 98.7 ± 6.0 26.24 ± 3.3 149.9 ± 2.1Total 263.4 100 0.186 49.1 100 0.349 91.8 100 26.47 16.63 46.22

E1 36.2 14 0.167 6.05 26 1.86 67.2 76 16.95 ± 0.08 15.420 ± 0.11 36.49 ± 0.13E2 112.6 43 0.090 10.1 44 0.101 11.4 13 32.2 ± 1.2 17.515 ± 0.61 58.8 ± 1.2E3 115.0 43 0.060 6.9 30 0.085 9.78 11 60.5 ± 4.3 22.94 ± 2.4 136.5 ± 4.5Total 263.8 100 0.087 23.1 100 0.335 88.4 100 23.78 16.54 50.51

Note: In-run uncertainties are 2σ (in percent). See Table 2 for analytical details. Total values for three-step, sequential dissolution reflect mass-weighted sumsof the individual steps. N.D. = not determined.



component in the inclusion mineral assemblage(Fig. 4). It is striking that 208Pb/204Pb–206Pb/204Pbvariations for quartz from the Cambrian and Or-dovician Wisconsin sandstones indicate a rela-tively consistent proportion of monazite (2%–5%of the accessory mineral population; Fig. 4),whereas the 208Pb/204Pb–206Pb/204Pb variations forthe Michigan basin samples indicate a higher pro-portion of monazite (~10% of accessory minerals;Fig. 4), suggestive of derivation from distinctsource terranes. Although monazite was not ob-served in our electron microprobe analysis, or byTyler’s (1936) petrographic work, the small

amounts that are required to explain the Pb isotopedata may be missed by these observations. Themost likely terranes to contribute monazite are ca.1.4 Ga anorogenic granites of the Wolf Riverbatholith (e.g.,Anderson and Cullers, 1978), or theevolved, high-alkali granites of the GrenvilleProvince (e.g., McLelland et al., 1993).

REE Systematics.Chondrite-normalized REEpatterns of the quartz grains broadly reflect thoseof potential Archean and Proterozoic source ter-ranes, although REE contents of the quartz are10–100 times lower than those of bulk igneousrocks from these terranes (Fig. 5A). The light

REE enrichment (CeN/SmN=2.5 to 4.0; N refers tochondrite normalization) and negative Eu anom-alies (Eu/Eu*= EuN /[SmN*GdN]0.5= 0.5–0.7) ofthe quartz grains are consistent with derivationfrom granitic or rhyolitic rocks, rather than frommore mafic (plutonic), quartz-bearing lithologies.The REE patterns are not consistent with zirconbeing the sole mineral inclusion in the quartzgrains, nor with a simple mixture of, e.g., apatiteand zircon. Permissible combinations include zir-con ± apatite and major contributions from titanite(~50% of the inclusion population) and/or minorcontributions from monazite (~5% of the inclu-sions) (Fig. 5B), the latter of which is indicated bythe208Pb/204Pb–206Pb/204Pb variations (see pre-ceding; Fig. 4). The absolute REE contents of thequartz grains are well explained by ~1 part in1000 concentration of accessory (heavy) mineralsas microinclusions (Fig. 5B), which is consistentwith the U and Pb abundances of the quartz grainsrelative to those expected for the accessory miner-als. Although garnet inclusions may in part ex-plain the relatively high Th/U ratios required bythe 208Pb/204Pb–206Pb/204Pb variations (e.g.,Mezger et al., 1989), garnet cannot be a signifi-cant inclusion based on the REE patterns, becausegarnet has extreme enrichments of heavy REEs.The REE patterns also confirm the similarity ofthe two Wisconsin sandstones (CeN/YbN =5.2–6.7) in contrast to the St. Peter Sandstonefrom the Michigan basin (CeN/YbN = 3.5), whichfurther substantiates different sources of quartzfor these two geographic regions.

Leaching Experiments. Sequential HF-leaching experiments on a Michigan basin sam-ple were undertaken to isolate the radiogenic Pbcomponent in the relatively nonradiogenic bulkquartz separates (Tables 2 and 3). The leachingexperiments indicate that the Pb isotope compo-sitions of the bulk quartz separates most likely re-flect a mixture between high U/Pb (high-µ) ac-cessory mineral inclusions and low-µ inclusionsor overgrowths; in the Michigan basin samples,the nonradiogenic component is likely to be K-feldspar, consistent with petrographic observa-

Figure 4. 208Pb/204Pb–206Pb/204Pb variations for quartz grains from the Galesville and St. PeterSandstones, which constrain the Th/U ratios of the accessory (heavy) mineral inclusions in detri-tal quartz. Relative Th/U ratios for common accessory minerals are also shown, as calculatedfrom U-Pb studies that analyzed multiple coexisting minerals (Williams et al., 1983; Davis andEdwards, 1986; Corfu, 1988). The 208Pb/204Pb–206Pb/204Pb variations indicate that zircon cannotbe the primary accessory (heavy) mineral inclusion in the quartz grains, and that variableamounts of monazite (mon) are required (percentages of accessory mineral population areshown). Error ellipses are much smaller than the plotted symbols.

TABLE 4. RARE EARTH ELEMENT AND Sm-Nd ISOTOPE DATA FOR QUARTZ GRAINS

wt Ce Nd Sm Eu Gd Dy Er Yb 147Sm/144Nd 143Nd/144Nd εNd (0)* εNd (t)§ TDM#

(mg)

WI-SP-3(locality 2) 575.4 2.46 0.900 0.145 0.023 0.111 0.120 0.085 0.094 0.09699 0.511379 ± 9 –24.6 –18.6 2.24GALE-D2(locality 10) 496.2 4.17 2.13 0.382 0.061 0.346 0.319 0.207 0.205 0.10823 0.511433 ± 8 –23.5 –17.7 2.40SW11068(locality 13) 516.1 1.93 0.706 0.115 0.026 N.D. 0.153 0.130 0.140 0.09799 0.511689 ± 6 –18.5 –12.7 1.87

Notes: WI-SP-3 and SW11068 are from the St. Peter Sandstone in Wisconsin and the Michigan basin, respectively. GALE-D2 is from the Galesville Sandstone in Wisconsin.Sample locations are given in Tables DR-1 and DR-2 (see text footnote 1). REE concentrations in ppm. Uncertainties of isotope ratios are 2σ errors. N.D. = not determined.

*Present-day εNd (0) were calculated using 0.512635 for the 143Nd/144Nd ratio of present-day CHUR, which reflects the averages of eight analyses of the BCR-1 standardduring the study (±0.000020 2SD); the Nd isotope composition of the BCR-1 standard has been shown to be equal to that of CHUR (Wasserburg et al., 1981).

§εNd values at the time of deposition [εNd(t)] were calculated using 515 Ma for the Galesville Sandstone and 465 Ma for the St. Peter Sandstone depositional ages.#Depleted mantle model ages (TDM) were calculated by assuming that the mantle evolved linearly to its present-day composition of (143Nd/144Nd)DM = 0.51315 and

(147Sm/144Nd)DM = 0.217.

JOHNSON AND WINTER


tions. Two large (~260 mg) aliquots (D and E,Table 3) of quartz grains from a single Michiganbasin quartz sample (SW11068) were subjectedto a sequential three-step leaching in warm HF;~14% of the total mass of sample material wasdissolved during the first stage of dissolution and~43% of the total mass was dissolved in both thesecond and third stages (Table 3). Total (one step)dissolution of three aliquots of SW11068 (A, B,and C in Table 3) yielded U and Pb contents rang-ing from 0.14 to 0.26 ppm and 1.05 to 0.31 ppm,respectively (Table 3); reintegration of the U andPb contents for each leach stage of aliquots D andE yields nearly identical total concentrations(Table 3). Of the total Pb in quartz samples D andE, ~75% is contained in the first leachates (~1.9ppm; Table 3), even though this stage dissolvedonly 14% of the sample by mass. The second andthird stage leachates each contain ~13% of the to-tal Pb (~0.1 ppm, Table 3). In summary, the mostsoluble fractions of the samples have the least ra-diogenic Pb isotope ratios and highest Pb con-tents, whereas successive leachates have progres-sively more radiogenic Pb isotope ratios andlower Pb contents.

The leaching experiments suggest that potas-sium feldspar, which was identified as mi-croovergrowths from energy-dispersive electronmicroprobe analysis (see preceding), is the likelysource of nonradiogenic Pb in the most solublefraction. Lead in the second- and third-stageleachates was progressively derived from phasesthat have relatively high-µ values and low Pbcontents, such as zircon or other accessory(heavy) minerals that are comparatively resistantto HF dissolution. A high-µ component that has~0.1 ppm Pb is consistent with the proportion ofaccessory mineral inclusions in quartz that is sug-gested by the REE data (~1 part in 1000), as wellas measured Pb contents of single detrital zir-cons. Regression of the entire data set yields awell-correlated 206Pb/204Pb–ppm Pb trend of [Pb]= [20*log(206Pb/204Pb)–26.5]–1; this correlationwill be used in constraining mixing models forthe Pb-Pb isochrons for the quartz grains, dis-cussed in the following.

Pb-Pb Isotope Systematics of Quartz Grains

Wisconsin Sandstones.Analyses of quartz (n = 11) from the Ordovician St. Peter Sandstonethat is exposed in Wisconsin define a linear arraythat yields a Pb-Pb age of 2539 ± 140 Ma(MSWD = 62), and 12 analyses of quartz fromthe Galesville Sandstone in Wisconsin define anidentical Pb-Pb age of 2512 ± 140 Ma (MSWD =324). The similarity in ages supports the interpre-tation that the Galesville was the source for theSt. Peter Sandstone, or that they were derivedfrom similar sources. Regression of the data from

Figure 5. Chondrite-normalized rare earth element (REE) patterns for quartz grains of theGalesville and St. Peter Sandstones. (A) Comparison of quartz grains with possible source ter-ranes that may be represented by the Archean southern Superior Province (Shirey and Hanson,1986), the Proterozoic Pokegama Quartzite (Hemming et al., 1994), and Proterozoic orogenicrocks from the Penokean orogen and post-Penokean and anorogenic Middle Proterozoic rocksof the northern midcontinent (Anderson and Cullers, 1978; Anderson et al., 1980; Van Wyckand Johnson, 1997). Note the order of magnitude scale change from the left (data for quartzgrains and Pokegama Quartzite) to the right (Archean and Proterozoic rocks). (B) Comparisonof quartz grains with possible accessory (heavy) mineral assemblages. Total inclusion density isassumed to be 0.1%. The lower boundary of the shaded fields represents the REE pattern forquartz, if 100% of the inclusions are zircon. The upper boundary of the diagonally ruled fieldrepresents an inclusion assemblage of 50% zircon and 50% apatite. The upper boundary of thestippled field represents an inclusion assemblage of 50% zircon and 50% titanite. The upper-most boundary of the fields shown represents an inclusion assemblage of 95% zircon and 5%monazite. Inclusions of zircon ± apatite with 50% titanite and/or 5% monazite can explain thelight REE/heavy REE enrichment that is observed in the quartz grains. Note that the importantproperty of the REEs for quartz is the relative REE patterns, rather than the absolute values,which will vary depending upon inclusion density. Note the two order of magnitude scale changebetween the left (data for quartz grains) and right (model accessory/heavy mineral assemblage)axes. Relative REE partitioning data were taken from granites that have REE data for coexist-ing minerals (Gromet and Silver, 1983; Fujimaki, 1986; Sawka, 1988; Sawka and Chappell,1988; Paterson et al., 1992; Kingsbury et al., 1993; Wark and Miller, 1993).

both formations together (n = 33) yields a Pb-Pbage of 2519 ± 85 Ma (MSWD = 486), which in-tersects the average crustal Pb curve (Stacey andKramers, 1975) at 4 and 2521 Ma (Fig. 6). Thegreater scatter about the regression line by thefour quartz samples that have the highest206Pb/204Pb ratios (Figure 6) does not add signif-icant uncertainty to the Pb-Pb age determination.

Michigan Basin.Analyses of quartz (n = 27)from the St. Peter Sandstone in the Michiganbasin define a linear Pb-Pb array that yields anage of 2242 ± 93 Ma (MSWD = 446, Fig. 7).Elimination of the leachate samples (n = 21)yields a Pb-Pb age of 2396 ± 78 Ma (MSWD =18). Unlike the data for the Wisconsin samples,the regressed Pb-Pb data for Michigan do not in-tersect the zero point on the Stacey-Kramersgrowth curve. Assuming that the first leach com-positions closely approximate the initial isotopecompositions for the isochron, these composi-tions are significantly more radiogenic than theStacey-Kramers curve at 2.2 Ga.

Interpreting Quartz Pb-Pb Ages

Hemming et al. (1994) showed that the Pb iso-tope composition of quartz grains directly sepa-rated from Archean plutonic rocks is sufficientlyvariable and radiogenic to yield reasonably pre-cise (±100 Ma) Pb-Pb ages that are consistentwith crystallization ages determined by more con-ventional means. Hemming et al. (1994) analyzedquartz grains from three different plutons repre-senting a variety of chemical compositions anddifferent origins (i.e., mantle- vs. crustal-derived),but all from the same major crust-formation inter-val within the 2.7 Ga Superior Province. Usingsample sizes generally between 5 and 40 mg,Hemming et al. (1994) demonstrated that quartzgrains from different plutons have different rangesof 206Pb/204Pb ratios, and together they define aPb-Pb age of 2632 ± 64 Ma, which is in excellentagreement with the known crystallization ages ofthese rocks.

Well-constrained Pb-Pb isochrons may also bedetermined on detrital quartz grains that were de-rived from source terranes of restricted age range.Hemming et al. (1994) showed that clear (igneous)detrital quartz in the Early Proterozoic PokegamaQuartzite (Minnesota) yields a Pb-Pb age of 2647± 16, which is identical to that of known igneoussource terranes. In addition, separation of milky(vein) quartz produced slightly younger207Pb*/206Pb* ages of ca. 2.4 Ga (Hemming et al.,1994), suggesting a different source. However, Pb-Pb isochrons are expected to be more complicatedfor sandstones having depositional ages that aremuch younger than those of their source terranes.Unlike the Pokegama Quartzite, zircon data for theWisconsin and Michigan cratonic sandstones (see



Figure 6. 206Pb/204Pb–207Pb/204Pb variations for quartz from the Ordovician St. Peter Sand-stone and the Cambrian Galesville Sandstone in Wisconsin (see Fig. 2 for localities). Dashedlines show the 95% confidence limit error envelope for the data. The Pb isotope growth curvefor average continental crust (S-K; Stacey and Kramers, 1975) is marked at 500 m.y. intervals.Error ellipses are much smaller than the plotted symbols.

Figure 7. 206Pb/204Pb–207Pb/204Pb variations for quartz from the Ordovician St. Peter Sandstoneat ~3600 m depth in the Michigan basin (see Fig. 2 for locality). Dashed lines show the 95% confi-dence limit error envelope for the data. Error ellipses for samples processed by total dissolutionare much smaller than the plotted symbols. Data obtained from three-step sequential HF leachingare plotted with true error ellipses (sample SW11068). The Pb isotope growth curve for averagecontinental crust shown in the inset (S-K; Stacey and Kramers, 1975) is marked at 250 m.y. inter-vals. Third stage HF leaches are interpreted to largely record the isotopic compositions of includedaccessory minerals (ACC MINS).

preceding) indicate that detrital constituents werederived from multiple igneous terranes, spanningan age range of 1600 m.y. In the following wemodel the effects of mixing on Pb-Pb regressions,which are generally applicable to multicyclic sed-imentary rocks that involve source terranes ofwidely different ages.

Two-Component Mixing Models for Quartz Pb-Pb Isotope Data

Single detrital zircons provide the first-orderconstraints for the range of ages of source ter-ranes to multicyclic sediments. However, it is im-practical to use single zircons to rigorously deter-mine the relative proportions of source terranes,which would require hundreds of analyses persample to be statistically significant. Instead, weuse the range of source ages from the zircon dataas a primary constraint, and calculate the relativeproportions of source terranes using the Pb-Pbisotope compositions of large bulk samples.

The 207Pb/204Pb–206Pb/204Pb regression agescalculated for quartz separates from Wisconsinand the Michigan basin fall between those of thelikely end-member source ages of 2.7 and 1.1 Ga(as inferred by the detrital zircon ages). The largesample sizes we have used should be sufficient torepresent the relative proportions of the differentsources and the effects of multiple mixing events.

We have modeled the regression ages that areproduced through two-component mixing in-volving 2.7 Ga and 1.1 Ga sources; the relativelyminor amount of 1.8–1.4 Ga zircons in the rocksindicates that these components can be ignored inthe mixing calculations. The source isochrons areassumed to have initial Pb isotope compositionson the Stacey-Kramers average crust curve(Stacey and Kramers, 1975), and range to a max-imum 206Pb/204Pb ratio of 80. Stage 1 of mixing(Table 5) involves 1000 random samplings fromalong each of the source isochrons, and calcula-tion of the resulting Pb isotope compositions ofthe 1000 mixtures. In addition, the relative frac-tion of 2.7 Ga and 1.1 Ga components is allowedto randomly vary between set limits. We have al-lowed the concentration ratio of the two compo-nents, (Pb)2.7Ga/(Pb)1.1Ga,to vary from 1 to 2, ac-counting for the generally greater radiogenic Pbcontents of the older component. In addition, Pbcontents of the end members are varied as a func-tion of the 206Pb/204Pb ratio of the randomlygenerated end member, matching the observed206Pb/204Pb–ppm Pb variations measured in thequartz grains (see above); this approach realisti-cally reflects the higher Pb contents that are mea-sured (and expected) in the relatively nonradi-ogenic quartz samples.

In the stage 1 mixing calculations, whichmight approximate first-cycle sedimentation, an

average mixture of 25% 1.1 Ga and 75% 2.7 Gasources (obtained through random variation be-tween 0% and 50% 1.1 Ga component) producesthe 2500–2600 Ma regression age for the Wis-consin quartz arenites (Table 5A), although theaverage 206Pb/204Pb ratio and spread along themixture isochron exceeds that measured in thesamples (Table 5A; Fig. 8A). A greater propor-tion of a 1.1 Ga component is required to producethe younger age of the Michigan basin sandstone,and a ca. 2200 Ma regression age may be ob-tained using an average mixture of 50% 1.1 Gaand 50% 2.7 Ga sources (obtained through ran-dom variations between 0% and 100% 1.1 Gacomponent), as shown in Table 5B and Figure8C. The interplay between scaling of Pb contentsalong the source isochrons and the relative pro-portions derived from the respective end-membersources is shown by comparison of Figure 8, Aand C, which differ only in the relative proportionof quartz derived from 1.1 Ga and 2.7 Ga sourceterranes in the mixtures. If the mixture isochronsare calculated using fixed relative proportions ofthe sources, rather than random variation withinthe limits noted here, the calculated data cluster

impossibly tightly and do not reflect the spread inthe measured data.

The supermature quartz arenites studied hererepresent multicyclic sedimentation, as argued bymany other workers. We can estimate the effectof multicyclic sedimentation using a secondstage (stage 2 of Table 5,A and B) of mixing, ob-tained by taking 100 10-point averages of themixtures calculated in stage 1, weighted for theirPb contents. These results more closely matchthe measured data, and illustrate the collapse ofthe mixtures to a greater number of nonradio-genic isotope compositions (Fig. 8B for Wiscon-sin and Fig. 8D for the Michigan basin). Relativeto stage 1 mixing, second-stage mixing magnifiesthe effect of scaling the Pb concentration as afunction of the 206Pb/204Pb ratio of the source endmembers, and illustrates how the large number ofmeasured nonradiogenic compositions for thequartz grain samples can be generated (cf. Fig. 6and Fig. 8, B and D). Regression of the first 25compositions calculated for stage 2 mixing, tomore closely match the number of measuredsamples in Figures 6 and 7, results in excellentagreement with the age and average 206Pb/204Pb

JOHNSON AND WINTER


TABLE 5A. MIXING MODELS FOR QUARTZ Pb-Pb ISOCHRONS FOR WISCONSIN SAMPLES

Measured Stage 1 Stage 2data First cycle seds Second cycle seds

(1000 points) (100 points) (First 25 points)(Fig. 8A) (Fig. 8B)

Age (Ma): 2519 2527 2508 2522Error: 85 16 20 41Y Int:: 12.55 12.42 12.49 12.45Error: 0.19 0.42 0.06 0.09

Fraction of 1.1 Ga component: 0 to 0.5 0 to 0.5 0 to 0.5

Avg 206Pb/204Pb: 29.4 43.1 28.81 SD: 9.6 17.4 5.5

Notes: Mixing calculated assuming random sampling along 2.7 Ga and 1.1 Ga isochrons that are concordantwith the Stacey-Kramers average crust curve (Stacey and Kramers, 1975). Maximum 206Pb/204Pb for theendmember isochrons set at 80. Intermediate components such as 1.8–1.9 Ga Penokean crust are not used because single zircon ages indicate that these are minor components. For the fraction of 1.1 Ga component random values within the noted interval were calculated. Scaling of Pb contents follows regression of measureddata: ppm Pb = 1/[20*log(206Pb/204Pb) – 26.5); r2 = 0.8. K-feldspar overgrowths assumed to contain 10 ppm Pb(e.g., Patterson and Tatsumoto, 1964), 206Pb/204Pb = 16.751, 207Pb/204Pb = 15.429 (equal to least radiogenic sam-ple, SW11048.5B).

TABLE 5B. MIXING MODELS FOR QUARTZ Pb-Pb ISOCHRONS FOR MICHIGAN SAMPLES

Measured Stage 1 Stage 2 Stage 3data First cycle seds Second cycle seds Addition of

(1000 points) (100 points) (First 25 points) feldspar overgrowths(Fig. 8C) (Fig. 8D) (100 points) (First 25 points)

(Fig. 8E)

Age (Ma): 2242 2193 2229 2265 2213 2188Error: 93 51 52 96 60 118Y Int:: 13.10 13.05 12.99 12.91 13.07 13.11Error: 0.32 0.99 0.11 0.16 0.07 0.07

Fraction of 1.1 Ga component: 0 to 1 0 to 1 0 to 1

Percent feldspar overgrowths: 0 to 5 0 to 5

Avg 206Pb/204Pb: 25.0 42.9 29.5 24.41 SD: 17.0 17.0 5.5 3.3

Note: See footnote for Table 5A for details of models.



ratio measured for the Wisconsin samples, aswell as in a reasonable match for the errors inslope and intercept of the mixture isochron(Table 5A).

The petrographic observation of K-feldsparovergrowths in the Michigan basin samples leadsus to a third-stage model to explain the signifi-cantly less radiogenic Pb isotope ratios of thesesamples; this stage involves addition of a nonradi-ogenic diagenetic component. The presence of

such a component is supported by the leachingexperiments noted here (first-stage leach), and wehave modeled this effect by adding a K-feldsparcomponent to the results of the stage 2 mixing byrandomly varying the percentage of K-feldsparcomponent between 0% and 5% by mass (Table5B). These calculations show how further col-lapse of the mixtures to nonradiogenic Pb isotopecompositions can occur (Fig. 8E). The age, aver-age 206Pb/204Pb ratio, and errors in the regression

for the Michigan basin samples are closely ap-proximated by this model (Table 5B). An impor-tant aspect of this model is that the initial Pb iso-tope compositions of the mixture isochron nolonger intersects the zero point on the Stacey-Kramers curve (despite the fact that the 2.7 Gaand 1.1 Ga components are concordant with theStacey-Kramers curve), because the K-feldsparcomponent is off the Stacey-Kramers curve, as in-dicated by the first-stage leaches (Fig. 7).

Michigan samples (Table 5B). Regression of the 1000 calculated values yields an age of 2193 ±± 51 Ma. The maximum frequency value is 42. (D) Second-stage mixing of C (see Table 5B) for Michigan samples, which represents 100, 10-point, concentration-weighted averages of the first-stage mixing. Regression of the 100 calculated values yields an age of 2229 ±± 52 Ma. The maximum frequency value is 18. (E) Third-stage mix-ing for Michigan samples, where 0%–5% K-feldspar overgrowths are added to the compositions of second-stage mixing shown in D (see Table5B). Regression of the 100 calculated values yields an age of 2213 ±± 60 Ma. Maximum frequency value is 30.

Figure 8. Three-dimensional histograms for Pb-Pb isochron mixing(see summary in Table 5) between 2.7 Ga and 1.1 Ga source terranes(source isochrons are shown where visible). The Z scale (frequency)varies for each plot and is not shown for clarity. (A) 1000 point stage 1,first-cycle mixing for Wisconsin samples (Table 5A). Regression of1000 calculated values yields an age of 2527 ±± 16 Ma. Maximum fre-quency value is 40. (B) Second-stage mixing of A (see Table 5A) forWisconsin samples, which represents 100, 10-point, concentration-weighted averages of the first-stage mixing. Regression of the 100 cal-culated values yields an age of 2508 ±± 20 Ma. The maximum fre-quency value is 20. (C) 1000 point stage 1, first-cycle mixing for

JOHNSON AND WINTER


Sm-Nd Isotope Systematics of Quartz Grains

The general mixing proportions that are indi-cated by the detrital zircon U-Pb and quartz Pb-Pbdata are supported by the Sm-Nd isotope data ob-tained on quartz grains from Wisconsin and theMichigan basin. Although the εNd-time evolutiontrends for the quartz data might suggest a domi-nantly ca. 1.8 Ga source, such as the Penokeanorogenic rocks in Wisconsin, the near lack of ca.1.8 Ga detrital zircons instead indicates that theSm-Nd isotope data are best interpreted as reflect-ing a mixture of old (ca. 2.7 Ga) and young (ca.1.1 Ga) sources (Fig. 9); variable contributionsfrom intermediate-age components are likelyvariants from a simple two-component mixingmodel. The Sm-Nd isotope data for quartz fromthe Galesville Sandstone and St. Peter Sandstonein Wisconsin are similar (Fig. 9), supporting thehypothesis that they were derived from similarsources. Assuming relatively equal Nd contentsbetween the two source regions, the εNd values ofthe two Wisconsin samples can be produced by

70:30 mixing between 2.7 and 1.1 Ga sources, re-markably consistent with the mixing proportionssuggested by the Pb-Pb isochron for the quartzgrains. A dominant contribution from 2.7 Gasources is also indicated by the detrital zirconpopulation.

The Sm-Nd isotope data for the Michiganbasin sample indicate a greater proportion of ayounger component, and the comparatively highεNd value can be explained by ~35:65 mixing be-tween 2.7 and 1.1 Ga sources. This proportion isalso consistent with the mixing proportions esti-mated by the Pb-Pb isochron for the quartz grains.

PROVENANCE OF LOWER PALEOZOICCRATONIC SANDSTONES

All of the isotope data indicate that the quartzand heavy minerals were ultimately derived fromPrecambrian basement terranes. The granite-greenstone terrane of the Superior Province is theultimate source of the major 2.7 Ga detrital com-ponents, whereas the 1.1 Ga components are ulti-

mately derived from either the proximal Ke-weenawan volcanic rocks that are associated withthe Midcontinent rift system or from synorogenicintrusions of the Grenville Province. Derivationof significant volumes of quartz from Ke-weenawan rhyolites, which contain 5%–15%phenocrysts of quartz and accessory zircon,would require significant concentration in pre-early Paleozoic basins, inasmuch as rhyolites rep-resent only ~10% of the exposed Keweenawanvolcanic rocks (Nicholson, 1992; Green andFritz, 1993); most of the estimated 1.5 x 106 km3

of volcanic rocks associated with the Midconti-nent rift system are basaltic. In contrast, theGrenville Province of North America is an ~500-km-wide orogenic belt, extending from the coastof Labrador to northern Mexico, that consists ofvoluminous granitic plutons ranging in age from1.0 to 1.2 Ga (e.g., Rainbird et al., 1997), and thisseems to be the most likely ultimate source for1.1 Ga constituents in the lower Paleozoic quartzarenites in Wisconsin and Michigan.

The comparatively minor amount of Middle

Figure 9. εNd-age evolution trends for quartz grains (shown in heavy lines) from the Cambrian Galesville Sandstone in Wisconsin (G-WI),St. Peter Sandstone in Wisconsin (SP-WI), and the St. Peter Sandstone in the Michigan basin (SP-MI). Nd isotope evolution trends for possi-ble source terranes are shown in various patterns. The Nd isotope compositions for quartz grains can be obtained by mixing average 1.1 GaGrenville crust with average 2.7 Ga crust from the southern Superior Province at the time of deposition. The comparatively high εNd value ofthe St. Peter Sandstone from the Michigan basin can be explained by 65:35 mixing of 1.1 Ga and 2.7 Ga sources, whereas the significantlylower εNd values of quartz from the Galesville and St. Peter Sandstones in Wisconsin suggest a 30:70 mixing of 1.1 Ga and 2.7 Ga sources. TheNd isotope composition of the 2.7 Ga source is likely to be similar to that of the Pokegama Quartzite from Minnesota (Hemming et al., 1994).Additional Nd isotope data from Ashwal and Wooden (1983), Ashwal et al. (1986), Shirey and Hanson (1986), Barovich et al. (1989), Marcan-tonio et al. (1990), Nicholson and Shirey (1990), Daly and McLelland (1991), Nelson (1991), Dickin and Higgins (1992), Emslie and Hegner(1993), McLelland et al. (1993), Owens et al. (1994), and Van Wyck and Johnson (1997).

Proterozoic (ca. 1.4 Ga) zircons (and possiblyquartz grains) in cratonic sandstones from Wis-consin were probably ultimately derived from theWolf River batholith; generally south-directed pa-leocurrent indicators throughout the Paleozoic se-quence (Hamblin, 1961; Potter and Pryor, 1961)are less consistent with derivation from the Mid-dle Proterozoic eastern granite-rhyolite provinceto the south (Fig. 1). The striking paucity of zir-cons derived from the major local basement (i.e.,

the ca. 1.8 Ga Penokean orogenic belt and the 2.7to >3.2 Ga Marshfield terrane; Fig. 1) providesthe best evidence that the detrital componentshave been transported a considerable distancefrom their ultimate basement source terrane.

Sedimentary Source Units

Proterozoic quartz-rich sandstones, which arecommon in the Lake Superior region (Table 6),

are likely to be the immediate sources for thelower Paleozoic quartz arenites of this study. Thepostvolcanic Keweenawan Bayfield Group innorthern Wisconsin is the most probable directsource of Keweenawan or Grenville age (1.1 Ga)detrital quartz and zircons to the CambrianGalesville and Ordovician St. Peter Sandstones inWisconsin. The Bayfield Group (Table 6) in-cludes texturally and compositionally superma-ture quartz arenites, which have been interpretedto have been derived from uplifted Archeangranitic basement (i.e., the Superior Province) thatwas exposed after erosion of the Keweenawanvolcanic sequence (Morey and Ojakangas, 1982;Ojakangas and Morey, 1982a). In contrast, quart-zose units (Oronto Group; Table 6) of the lowerpart of the postvolcanic Keweenawan sequenceare relatively immature in terms of texture andcomposition, and reflect erosion of Midcontinentrift–related volcanic rocks (Ojakangas andMorey, 1982a); these rocks are unlikely contribu-tors to the Galesville and St. Peter Sandstones.The 1.1 Ga component in the St. Peter Sandstoneof the Michigan basin could be directly derivedfrom Grenville basement (Fig. 10), or from post-Keweenawan quartzose sediments, such as the Ja-cobsville Sandstone (Table 6).

The immediate source for the 2.7 Ga compo-nent in the Galesville and St. Peter Sandstonesremains unclear. Prevolcanic Keweenawanquartzose sandstones in Minnesota, Wisconsin,and Michigan (Table 6) could have contributed2.7 Ga detrital components, on the basis ofgrain compositions and paleocurrent indicators(Ojakangas and Morey, 1982b). However, thewidespread Middle Proterozoic quartzites thatare exposed throughout Wisconsin and south-west Minnesota, such as those belonging to theBaraboo interval of Dott (1983) (Table 6), can-not have contributed substantial detrital compo-nents to the Galesville and St. Peter Sandstonesbecause the quartzites contain major 1.8 GaPenokean components that are virtually absentfrom the Cambrian and Ordovician sandstones(Fig. 11). This conclusion is similar to thatdrawn by Tyler (1936), based on the contrast inheavy mineral assemblages of the quartzites andthe lower Paleozoic sandstones. Early Protero-zoic quartzites (Table 6) that were depositedprior to the Penokean orogeny probably contain2.7 Ga components (Grout et al., 1951; Morey1973; Sims et al., 1981), and it is possible thatthese were reworked and eventually incorpo-rated in the lower Paleozoic sandstones.

Rainbird et al. (1997) envisioned that a verylarge river system emanated from the Grenvilleorogenic highlands and delivered sediment to thenorthwest margin of the continent in Late Prot-erozoic time. Proximal and medial portions ofsuch a river system could have deposited detrital



Figure 10. Interpretive map of the northern midcontinent region of North America showingsediment transport pathways for quartz grains and zircons analyzed in the Galesville Sandstone(Cambrian) and the St. Peter Sandstone (Ordovician). Sample localities in Wisconsin andMichigan are marked with large stars. Major transport pathways are shown in heavy arrows.Sources and ages (in Ga) for quartz and zircons are shown in zircon shae symbols. Map patternsfor various terranes are from Figure 1. Shown below the map are proportions of 1.1, 1.4, 1.8, and2.7 Ga components for the Wisconsin and Michigan basin localities based on the age distribu-tion of single zircons, as well as the proportions of 1.1 and 2.7 Ga components that are inferredfrom mixing models estimated from Pb-Pb and Sm-Nd isotope data for quartz grains (see textfor discussion).

material from the Grenville Province in LateProterozoic basins (rift or foreland) anywhere inthe midcontinent region. It is possible, therefore,that late Keweenawan quartz-rich sandstoneswere the direct sources for both 2.7 Ga and 1.1Ga detrital components to the Galesville and St.Peter Sandstones (Fig. 10).

CONCLUSIONS

Detrital zircon U-Pb ages and Sm-Nd and Pb-Pb isotope data on quartz separates from quartzarenites of the Paleozoic northern midcontinentregion demonstrate the importance of Archeanand Late Proterozoic terranes as the ultimatesources for the detrital constituents. The data pre-sented here demonstrate the small amount of de-trital constituents in the Wisconsin Paleozoicquartz arenites that was derived from the localbasement, including the lack of >2.7 Ga material,Penokean age rocks (ca. 1.8 Ga), or Middle Prot-erozoic quartzites (Baraboo-interval), which pro-vides strong support for a multicyclic origin forthe Galesville and St. Peter Sandstones. This is incontrast to the detrital components in the MiddleProterozoic quartzites, which are dominated bylocal basement components (Fig. 11).

The dominant sedimentary pathways for thedetrital constituents of the St. Peter Sandstone inWisconsin are as follows.

1. There was deposition of 2.7 Ga quartz de-rived from the southern Superior Province intolate Keweenawan (ca. 1.1 Ga) basins, which alsoreceived 1.1 Ga material, either from local Ke-weenawan silicic volcanic rocks, or, more likely,from Grenville rocks to the east (Fig. 10).

2. Keweenawan quartz-rich sandstones wereeroded and reworked during Cambrian time; thismaterial was transported southward and de-posited as the Galesville Sandstone in Wisconsin(Fig. 10).

3. The Ordovician St. Peter Sandstone in Wis-consin may have been largely derived through re-working of the Cambrian Galesville Sandstone,which has a closely similar detrital zircon popula-tion, as well as Pb-Pb and Sm-Nd systematics, andheavy mineral suites. In contrast, the St. PeterSandstone in the Michigan basin received a muchgreater proportion of 1.1 Ga detrital zircons andquartz from the nearby and extensive GrenvilleProvince (Fig. 10), as indicated by the U-Pb zirconand Pb-Pb and Sm-Nd data on quartz separates.

Mixing models for Pb-Pb isochron arrays, aswell as Sm-Nd data, determined on quartz sepa-rates, indicates that the relative proportions ofold and young quartz grains approximately re-flects those of the single detrital zircon popula-tions. This observation suggests that there waslittle relative fractionation of quartz and zirconduring multiple sedimentary cycles, and hence

JOHNSON AND WINTER


Figure 11. Histograms comparing thedistribution of ages of single detrital zirconpopulations from the Ordovician St. PeterSandstone (Wisconsin and Michiganbasin), Cambrian Galesville Sandstone(Wisconsin), and Baraboo Interval MiddleProterozoic quartzites (Wisconsin; datafrom Medaris et al., 1996; Van Wyck, 1995).MCR—Midcontinent rift; GP—GrenvilleProvince; PEN—Penokean; SP—Granite-greenstone terrane (GGT) of the SuperiorProvince; WR—Wolf River batholith;GT—Gneiss terrane of the southern Supe-rior Province. The Middle ProterozoicBaraboo Interval quartzites have a verylarge local basement (Penokean) compo-nent, whereas the Cambrian and Ordovi-cian rocks have only minor components thatwere derived from the local basement (in-cluding the Middle Proterozoic quartzites).

TABLE 6. PRECAMBRIAN QUARTZ-RICH SEDIMENTARY UNITS OF THE LAKE SUPERIOR REGION

Northwest Minnesota Wisconsin Michigan Southeast region region

Postvolcanic Bayfield GroupKeweenawan Hinkley (150 m) Chequamegon JacobsvilleSandstone (1) Fond du Lac (150 m) (900 m)

(600 m) Devils Island(90 m)Orienta (570 m)

Oronto GroupSolor Church Freda(600–1000 m) (1500–4000 m)

Prevolcanic Puckwunge Bessemer BessemerKeweenawan (~100 m) (~100 m) (~100 m)Sandstone (2) Nopeming

(~100 m)

Mid-Proterozoic Sioux Barabooquartzites (3) Barron

FlambeauMcCaslinWaterloo

Early-Proterozoic Baragaquartzites (4) Group

Goodrich

Animikie MenomineeGroup Group

Pokegama AjibikPalms

Mille Lacs Choclay Group Group

Sunday MesnardSturgeon

Note: (1) Daniels (1982); Kalliokoski (1982); Morey and Ojakangas (1982); Ojakangas and Morey (1982a); (2)Ojakangas and Morey (1982b); (3) Dott (1983); Sims et al. (1993); (4) Grout et al. (1951); Hemming et al. (1994);Morey (1973); Sims et al. (1981, 1993).

that little age bias exists between the frameworkgrains and the heavy mineral fraction. Becausethe statistical applicability of our single detritalzircon data set is limited, the Pb-Pb isochronsdetermined on bulk quartz separates better re-flect the average source mixing proportions,when the end-member ages are constrained us-ing single-zircon U-Pb data.

ACKNOWLEDGMENTS

We thank Scott McLennan for motivating usto conduct single-detrital zircon analyses. Wethank colleague Bob Dott for reviewing an earlyversion of the manuscript and helpful discus-sions. Journal reviews were provided by ScottMcLennan, Sydney Hemming, and GeraldRoss. Nicholas Van Wyck and Karin Barovichare thanked for assistance in the laboratory. Thisresearch was supported by a grant from theDonors of the Petroleum Research Fund admin-istered by the American Chemical Society(PRF-25677AC8) and by National ScienceFoundation grants EAR-910566 and EAR-9304455 to Johnson.

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