A RT I C L E S
Late Paleoproterozoic Climate, Tectonics, and Metamorphism in the
Southern Lake Superior Region and Proto–North America:
Evidence from Baraboo Interval Quartzites
L. G. Medaris, Jr., B. S. Singer, R. H. Dott, Jr., A. Naymark,
C. M. Johnson, and R. C. Schott1
Department of Geology and Geophysics, 1215 West Dayton Street, University of Wisconsin—Madison,
Madison, Wisconsin 53706, U.S.A.
(e-mail: [email protected])
ABSTRACT
Red, supermature quartzites of the Baraboo interval of the Lake Superior region contain detrital zircon that ranges
in age from 1782 to 1712 Ma. Deposition clearly occurred after the geon 18 Penokean orogeny. These late Paleoproterozoic sedimentary rocks consist largely of quartz, kaolinite or pyrophyllite, and hematite; detrital feldspar and
muscovite are rare or absent. Their Chemical Index of Alteration ranges from 96.8 to 98.6, among the most chemically
mature clastic sediments in the geological record. The quartzites are underlain by mature, feldspar-free paleosols,
accounting for the absence of feldspar in the overlying sediments and indicating the presence of first-cycle quartzose
detritus. Such physical and chemical characteristics imply that late Paleoproterozoic deposition in the Lake Superior
region occurred in a stable tectonic setting with subdued topographic relief in a warm, humid climate. Folding and
low-grade metamorphism of the quartzites is thought to reflect ∼1630 Ma foreland deformation related to the Mazatzal
orogeny. Younger hydrothermal alteration is widespread in the Baraboo and Sioux quartzites and, based on 40Ar/39Ar
dating of low-temperature minerals, is attributed to the migration of fluids along permeable channels in response to
the thermal effects of magmatism associated with the 1465 Ma Wolf River batholith. Much of the Paleo- and Mesoproterozoic crust of Proto–North America may have been affected by areally extensive, but stratigraphically restricted, hydrothermal alteration related to the influence of geon 14 transcontinental A-type granitic magmatism.
Introduction
The Baraboo and six correlative red quartzites (the
Sioux, Barron, Flambeau, McCaslin, Waterloo, and
probably Rib Mountain; fig. 1), which are included
in the Baraboo interval (1450–1750 Ma; Dott 1983),
have long been recognized as important and distinctive stratigraphic elements in the Proterozoic
evolution of the southern Lake Superior region
(Dott 1983; Ojakangas and Weber 1984; Southwick
et al. 1986). These seven quartzites are remnants
of a once southward-thickening wedge of clastic
strata, which apparently covered much of the
southern margin of the Superior Province (fig. 1).
Manuscript received April 11, 2002; accepted September 30,
2002.
1
Department of Geology and Physics, Lake Superior State
University, Sault Sainte Marie, Michigan 49783, U.S.A.
These supermature quartz arenites are among the
world’s oldest redbeds and imply deposition on a
stable craton of subdued topographic relief in a
warm, humid climate under the influence of an
oxidizing atmosphere. Based on sedimentary structures in all seven of the quartzites, deposition was
largely by braided fluvial systems (Henry 1975;
Dott 1983; Ojakangas and Weber 1984; Southwick
et al. 1986), although reactivation surfaces and
symmetrical ripples in the upper portion of the Baraboo Quartzite indicate tidal and wave processes
that reflect marine transgression of a passive
protocontinental margin. Overlying banded iron
formation and black shale (now slate) indicate
deeper marine deposition following the initial
transgression.
The quartzites of the southern Lake Superior re-
[The Journal of Geology, 2003, volume 111, p. 243–257] 䉷 2003 by The University of Chicago. All rights reserved. 0022-1376/2003/11103-0001$15.00
243
244
L. G. MEDARIS, JR., ET AL.
Figure 1. Distribution of Baraboo and correlative quartzites in the Lake Superior region, including average paleocurrent directions, thicknesses in meters, critical mineral assemblages, and summary of post-Penokean detrital zircon
207
Pb/206Pb ages (upright numbers, conventional analyses [this investigation; Van Wyck 1995]; italicized numbers, ion
probe determination [Holm et al. 1998]; number of analyzed grains in parentheses). Stars, paleosol localities. Heavy
line is the 1630 Ma thermal and tectonic front, based on 40Ar/39Ar cooling ages of basement minerals (Holm et al.
1998; Romano et al. 2000). Mineral assemblages: qtz-kln, unmetamorphosed; qtz-prl, ∼1630 Ma; ms-bearing assemblages, ∼1465 Ma.
gion rest nonconformably on Archean basement,
geon 18 Penokean basement, or geon 17 granite and
rhyolite. The Waterloo and McCaslin quartzites are
intruded by granitic rocks associated with the 1465
Ma Wolf River batholith (Aldrich et al. 1959; Anderson and Cullers 1978), which underlies an area
of ∼9,300 km2 in northeastern Wisconsin and is the
local manifestation of a geon 14 transcontinental
magmatic event in North America (Anderson 1983;
Bickford and Anderson 1993). Rb/Sr and 40Ar/39Ar
cooling ages of mica and amphibole in basement
rocks define a sharp thermal front in northern Wisconsin (fig. 1) that separates post-Penokean cooling
ages of 1750–1700 Ma to the north from younger,
reset ages of ≤1630 Ma to the south (Holm et al.
1998). Spatial coincidence of the 1630 Ma thermal
front with an apparent deformational front in the
quartzites, which is delineated by significant folding in the Flambeau, McCaslin, and Baraboo
quartzites in contrast to the largely flat-lying Barron and Sioux quartzites, implies that folding was
contemporaneous with isotopic resetting at ∼1630
Ma. Thus, deposition of this distinctive suite of
quartz arenites is constrained to an interlude of cratonic stability between 1750 and 1630 Ma.
For many years, there has been some controversy
about the age and correlation of the Baraboo
Quartzite in south central Wisconsin. This arose
first from an interpretation that the quartzite was
older than the Baxter Hollow Granite (Gates 1942)
and from the later suggestion of a pre-Penokean age
for the Baraboo Quartzite (LaBerge et al. 1991). This
investigation was undertaken to resolve the lingering uncertainty about the age of the Baraboo
Journal of Geology
Table 1.
PALEOPROTEROZOIC BARABOO QUARTZITES
245
U-Pb Analyses of Detrital Zircon Grains, Baraboo Quartzite
Grain
206
204
A, 3
B, 4
C, 5
D, 6
E, 7
F, 8
G, 9
2424
2222
1745
1677
1846
696
172
Weight
U
(mg)
(ppm)
.013
.019
.013
.013
.035
.013
.016
864
416
293
320
147
308
169
206∗
238
Error
(%)
.246233 .685
.266062 .889
.272764 .900
.278290 .809
.227178 .810
.156848 1.46
.269969 2.36
Age
1419.0
1520.8
1554.8
1582.7
1319.7
939.24
1540.7
207∗
235
Error
(%)
3.52071 .679
4.18590 .879
3.94447 .899
4.13306 .804
3.40676 .810
2.30208 1.47
3.91046 2.38
Quartzite by reexamining field relations between
the quartzite and Baxter Hollow Granite and by
determining U-Pb ages of detrital zircon grains in
the basal part of the quartzite. Subsequently, the
investigation was expanded to include the larger
quartzite suite, which led to recognition and analysis of two widely separated paleosols, quantitative
evaluation of the degree of chemical maturity of
the sedimentary rocks, and determination of metamorphic mineral assemblages and conditions on a
regional scale. This study provides new insight into
the characteristics of late Paleoproterozoic weathering, sedimentation, and metamorphism in the
southern Lake Superior region and, by comparison
with contemporaneous supermature red quartzites
in northwestern Canada and the southwestern
United States, over much of Proto–North America.
Age
207
206
Error
(%)
Age
1531.9
1671.3
1622.9
1660.9
1506.0
1212.9
1615.8
.103701
.114105
.104882
.107714
.108761
.106448
.105050
.124
.149
.192
.156
.176
.351
.641
1691.4
1865.8
1712.2
1761.1
1778.8
1739.5
1715.2
207
206
Discordance
age error
(%)
2.3
2.7
3.5
2.9
3.2
6.4
12.0
12.8
14.5
7.2
7.9
20.6
38.1
8.0
Age of the Baraboo Quartzite
investigators have interpreted the Baraboo Quartzite to lie nonconformably on the underlying igneous rocks because of the absence of any intrusive
features and the rare occurrence of rhyolite clasts
in the quartzite. To test this interpretation, seven
euhedral detrital zircon grains from a single stratum near the base of the quartzite were analyzed
for U and Pb isotopes by conventional dissolution
and mass spectrometry (table 1; fig. 2), using a
mixed 235U-205Pb spike and following the methods
described by Johnson and Winter (1999). Five
slightly discordant grains yield 207Pb/206Pb ages of
1866 Ⳳ 3, 1779 Ⳳ 3, 1761 Ⳳ 3, 1740 Ⳳ 6, and
1712 Ⳳ 4 Ma, and one U-rich grain may be as young
as 1691 Ⳳ 2 Ma. Such results demonstrate the
post–1710 Ma depositional age of the Baraboo
Quartzite, and similar results from other members
of the distinctive red quartzite suite in the southern
Lake Superior region confirm the long-standing correlation of the Baraboo, McCaslin, Flambeau, Bar-
The Baraboo Quartzite, which is exposed in a major, east-west trending, doubly-plunging syncline,
is underlain by the Baxter Hollow Granite and the
Denzer Diorite beneath the south limb of the syncline and by rhyolite lavas and tuffs beneath the
north and south limbs of the syncline at its east
end (Dalziel and Dott 1970). Both the Baxter Hollow Granite and the rhyolite were correlated on
lithologic grounds with granites and rhyolites of
the Montello Batholith, exposed to the northeast
of the Baraboo Range in east central Wisconsin
(Dalziel and Dott 1970; Anderson et al. 1980). Such
a correlation has been confirmed by U/Pb zircon
ages for both the Baxter Hollow granite and the
rhyolite in the Baraboo Range, which are indistinguishable at 1749 Ⳳ 12 Ma (Van Wyck 1995), and
for granite (1746 Ⳳ 3 Ma) and rhyolite (1759 Ⳳ 2
Ma) in the Montello Batholith (Van Schmus et al.
2001). The precision of these and all other radioisotopic ages reported herein is given at the 2j level.
Although the contacts between quartzite and underlying granite and rhyolite are nowhere fully exposed (the closest outcrops being ∼5 m apart), most
Figure 2. U-Pb concordia plot for seven detrital zircon
grains from a single stratum near the base of the Baraboo
Quartzite. 207Pb/206Pb ages and errors (2j, in parentheses)
are indicated for each grain.
246
L. G. MEDARIS, JR., ET AL.
ron, and Sioux quartzites (fig. 1; Van Wyck 1995;
Holm et al. 1998). Van Wyck (1995) analyzed seven
rounded detrital zircon grains from a stratigraphic
level ∼1250 m above the base of the Baraboo
Quartzite and obtained 207Pb/206Pb ages ranging
from 2588 to 1844 Ma. The apparent absence of
geon 17 detrital zircon from high in the quartzite
section is not surprising because at this stage of
deposition the local 1750 Ma basement was long
buried and detritus was derived from more distant
northerly and more deeply eroded sources of Penokean and Archean basement.
to do so, the Barron and Baraboo paleosols have
been investigated in detail, including chemical
analysis of protoliths and their weathered products
(table 2).
The Barron paleosol represents the archetype for
late Paleoproterozoic paleosols in the region because it lies north of the 1630 Ma thermal and tectonic front (fig. 1) and is unaffected by postweathering metamorphism and metasomatism. Red
saprolite, exposed in a large outcrop beneath the
quartzite at its eastern extent, was derived by
weathering of Penokean metatonalite (1848 Ma;
Sims et al. 1989), which occurs in scattered outcrops in the vicinity (Routledge et al. 1981). The
metatonalite texture has been preserved in saprolite, in which large relict quartz grains occur in a
matrix consisting of fine-grained quartz, kaoliniterich domains (after feldspar), hematite-rich domains (including both filamentous and subhedral
hematite after hornblende), traces of sericite, and
tiny, euhedral crystals of crandallite-florencite (aluminophosphate minerals).
The advanced chemical maturity of the Barron
saprolite is revealed by the virtual absence of
Na, Ca, Mg, and Mn and by low concentrations
of K, Rb, Sr, and Ba (table 2). Its Chemical Index
of Alteration (CIA), defined as 100 # molar
Al 2 O 3 /(Al 2 O 3 ⫹ K 2 O ⫹ Na 2 O ⫹ CaO), is exceptionally high, with a value of 95.7. Assessment of
chemical changes associated with weathering of
Barron and Baraboo Paleosols
An important aspect of the Baraboo story is the
existence of well-developed paleosols beneath the
Sioux, Barron, and Baraboo quartzites (fig. 1; Routledge et al. 1981; Southwick and Mossler 1984; Medaris et al. 1997). All the paleosols share a common
attribute in that plagioclase and alkali feldspar of
the igneous and metamorphic protoliths are completely replaced by kaolinite or sericite, which explains the absence of feldspar in the overlying supermature quartzites and indicates that firstcycle quartzose detritus is a significant component
of these late Paleoproterozoic quartz arenites. Although the paleosols clearly reflect a high level of
mineralogical maturity, their degree of chemical
maturity has not been previously assessed. In order
Table 2.
Average Chemical Analyses of Late Paleoproterozoic Saprolites and Protoliths
Barron
wt%:
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Sum
CIA
ppm:
Rb
Sr
Ba
Zr
99BN3A,
tonalite
(n p 2)
67.85
.24
14.00
5.63
.07
2.41
4.08
3.68
1.17
.06
.70
j
j
.05
.00
.00
.00
.00
.01
.00
.02
.01
.00
.00
70.25
.29
16.15
7.05
.00
.09
.04
.01
.58
.05
5.53
.15
.01
.05
.10
.00
.04
.03
.01
.01
.01
.02
99.88
14
321
251
80
Baraboo
99BN4A,
saprolite
(n p 2)
100.03
95.7
1
1
1
1
8
90
31
91
96BH1C,D,
granite
(n p 2)
69.25
.25
15.15
3.46
.09
.89
1.55
4.40
3.14
.09
1.53
j
.85
.06
.35
.23
.01
.19
.10
.04
.55
.02
.53
99.80
2
5
4
4
79
271
1365
260
Core 613,
saprolite
(n p 11)
j
70.56
.34
17.08
2.72
.00
.04
.13
.35
4.73
.12
2.45
3.12
.08
1.61
.73
.00
.10
.07
.06
.48
.05
.24
98.52
74.2
9
71
75
5
142
130
366
254
18
55
59
25
Note. XRF analyses provided by XRAL Laboratories, Ontario. CIA, Chemical Index of Alteration. 74.2, CIA modified by K
metasomatism.
Journal of Geology
PALEOPROTEROZOIC BARABOO QUARTZITES
the Barron metatonalite can be made through application of the isocon diagram (Grant 1986), a simple graphical method for evaluating changes in concentrations during some type of alteration process,
such as weathering. On a plot of concentrations in
the weathered rock against those in the protolith,
components whose relative concentrations have remained constant during weathering will lie on a
straight line emanating from the origin, the “isocon,” the slope of which is proportional to the
change in mass resulting from alteration. The deviation of a data point from the isocon defines the
concentration change for that component. Application of the isocon method to the Barron saprolite
and metatonalite protolith (fig. 3A) indicates that
the relative concentrations of Al2O3, TiO2, and Zr
remained constant during weathering and were accompanied by a 17% decrease in mass of the protolith, as calculated from the slope (1.15) of the
best-fit line to the values for Al2O3, TiO2, and Zr.
On an elemental basis (fig. 4), weathering resulted
in effective removal of Na, Ca, Mg, and Mn; substantial reductions in Si (∼10%), K (∼55%), Rb
(∼50%), Ba (∼90%), and Sr (∼75%); and an apparent
increase in Fe of ∼10%. The chemical composition
of Barron saprolite relative to its protolith is similar
to that for a present-day, mature saprolite that was
derived by weathering of granite in a warm, humid
climate in the Amazon region (fig. 4; Lovering
1959), except that the present-day saprolite exhibits
larger reductions in Si (∼50%), K (∼85%), and Fe
(∼45%).
Paleosol crops out in the Baraboo Range beneath
the south limb of the quartzite, where saprolite had
formed from Baxter Hollow Granite, and beneath
the north limb of the syncline at the east end of
the range, where saprolite had formed from rhyolite
(Medaris et al. 1997; Medaris and Dott 2001). Both
occurrences of saprolite preserve the textures of
their respective protoliths, and both types consist
of relict quartz, hematite, and sericite after feldspar.
The saprolite zones are ∼10 m thick on the south
limb of the syncline and ∼20 m thick on the north
limb. In addition, eight holes drilled in Baxter Hollow in 1959 by the Army Corps of Engineers penetrated the quartzite-granite contact. Material from
the contact was recovered in one of these, in which
overlying pebbly quartzite was separated from
underlying granitic saprolite by a 75-cm-thick
reddish-purple pedogenic zone, consisting of finegrained hematite, quartz, and sericite. The pedogene was cut by quartz veins in its upper part and
is separated from the underlying saprolite by a 3cm-thick zone of sheared saprolite.
Chemically, the Baraboo saprolite is similar to
247
Figure 3. Isocon plots for Barron tonalite protolith and
saprolite (A) and Baraboo granite protolith and metasaprolite (B). Oxides in wt% and trace elements in ppm;
selected values are scaled for convenience in plotting.
the Barron saprolite, except for a higher concentration of K2O and Rb and lower concentration of
Fe2O3 (table 2). Application of the isocon method
to Baxter Hollow Granite and saprolite (fig. 3B) suggests that the relative concentrations of Al2O3,
TiO2, and perhaps Zr remained constant during
weathering, recognizing that slight variations may
have existed in the distribution of zircon grains in
the granite protolith. A line fit through the origin
248
L. G. MEDARIS, JR., ET AL.
Huronian paleosols on granite in Ontario (Gay and
Grandstaff 1980) and Quebec (Rainbird et al. 1990)
and are interpreted to result from K metasomatism
during diagenesis or low-grade metamorphism of
preexisting weathering profiles.
Chemical and Mineralogical Composition of
the Sedimentary Rocks
Figure 4. Percent change in elemental compositions of
saprolites, relative to their respective protoliths. Elements are arranged in order of decreasing ionic radii. Circles, Baraboo; triangles, Barron; stars, present-day saprolite from a warm, humid climate (major elements only;
Lovering 1959).
of the isocon plot and Al2O3, which is taken as the
best monitor of mass transfer during weathering
because of its relatively immobile behavior and
high concentration, yields a slope of 1.14, corresponding to a 13% decrease in mass of the protolith
during weathering. Compared to the original granite, Na, Ca, Mg, and Mn were effectively removed,
Ba and Sr were substantially reduced, and Fe and
Si decreased by 30% and 10%, respectively. In contrast to the Barron saprolite, K and Rb in the Baraboo saprolite increased by 35% and 58%, respectively (fig. 4), which is reflected mineralogically in
the predominance of sericite over kaolinite.
Taking the unaltered Barron saprolite as a model
for weathering and considering the absence of detrital feldspar and muscovite in the Baraboo quartzite, it is likely that the high concentration of K (and
Rb) and occurrence of sericite in the Baraboo paleosol is due to K metasomatism of an original
kaolinite-bearing weathering profile. Metasomatism was probably localized by fluid flow along a
channel provided by the sub-Baraboo nonconformity. Support for this interpretation is provided by
an apparent age of 1336 Ⳳ 75 Ma from an Rb-Sr
whole-rock isochron for nine samples of Baraboo
saprolite and pedogene (table 3; fig. 5). This apparent age is consistent with introduction of Rb (and
K) substantially later than formation of the paleosol
at ca. 1700 Ma and is approximately concordant
with a 40Ar/39Ar plateau age of 1456 Ma for muscovite from metasaprolite, as described below. Similar chemical characteristics are found in sub-
It has long been known that the Baraboo and correlative quartzites are supermature sedimentary
rocks, based on the absence of feldspar, rarity of
detrital muscovite or illite, common occurrence of
pyrophyllite or kaolinite, predominance of zircon,
tourmaline, rutile, magnetite, and hematite in the
heavy minerals, and presence of vein quartz,
quartzite, chert, and iron-formation clasts in conglomeratic horizons (Dott 1983; Ojakangas and Weber 1984; Southwick et al. 1986). Such a supermature constitution originated by derivation of the
sediments from a deeply weathered and chemically
leached basement from which feldspar was removed, as exhibited by the Barron, Baraboo, and
Sioux paleosols. Maturity would have been further
enhanced by additional weathering during fluvial
transport in a warm, humid climate, as seen in the
Orinoco Basin today (Johnsson et al. 1988), and by
preferential destruction of labile minerals and lithic
fragments in the high-energy, fluvial, and shallow
marine environments (Odom et al. 1976).
Although the mineralogical maturity of the Baraboo and related quartzites is well established,
quantitative estimates of chemical maturity are
lacking because of the paucity of published chemical analyses. Accordingly, chemical analyses have
been obtained of siltstone and claystone, which,
although volumetrically subordinate, are petrologically significant members of the quartzite sequences and are more informative geochemically
than are the quartzites.
Eleven samples of siltstone and claystone (or
their metamorphosed equivalents) from the Baraboo, Barron, and Sioux quartzites consist almost
entirely of SiO2, TiO2, Al2O3, Fe2O3, and H2O (table
4), reflecting a mineralogy dominated by quartz,
kaolinite or pyrophyllite, hematite, and rutile. Mineral assemblages for these fine-grained sedimentary
rocks were determined by X-ray diffraction methods, and modes were calculated by mass balance
from the bulk chemical compositions, assuming
end-member compositions for the constituent minerals (table 4). The predominant aluminous phase
in the unmetamorphosed Barron Quartzite is kaolinite and in the metamorphosed Baraboo and
Sioux quartzites is pyrophyllite. Hematite is abun-
Journal of Geology
Table 3.
PALEOPROTEROZOIC BARABOO QUARTZITES
249
Rb-Sr Isotope Data for Baraboo Paleosol
Sample
Lithology
Meters below
quartzite
Measured
87
Sr/86Sr
2j
Rb
(ppm)
Sr
(ppm)
1-613-1
2-613-2
3-613-3
4-614-4
6-613-6
7-613-7
8-613-8
9-613-9
16-613-16
Regolith
Regolith
Regolith
Regolith
Regolith
Saprolite
Saprolite
Saprolite
Saprolite
.11
.27
.48
.66
.75
.78
.85
.95
2.48
.717732
.753276
.750569
.717270
.735434
.748273
.775427
.731465
.838121
.000017
.000018
.000018
.000015
.000020
.000018
.000017
.000021
.000018
19.7
122.0
145.9
109.8
123.0
119.9
108.0
115.7
132.1
116.9
149.4
192.3
654.1
232.2
158.5
91.6
294.7
58.2
dant in 10 of the 11 samples, ranging from 3.7 to
8.0 wt% and imparting the red color that is so characteristic of the Baraboo interval sediments. The
low contents of K2O in the samples, 0.07–0.39 wt%,
are accommodated by trace amounts of muscovite,
on the order of 1%–2%.
CIA values for these fine-grained sedimentary
rocks range from 96.8 to 98.8, with a mean of 97.9
(table 4). For comparison, CIA values for average
shales of Archean, Proterozoic, Paleozoic, and
Mesozoic-Cenozoic ages range from 50.4 to 65.4,
and values for fine-grained particulates from the
present-day Amazon and Congo rivers are 72.5 and
83.9, respectively (Taylor and McLennan 1985).
The remarkable chemical maturity of the finegrained Baraboo and related sediments is illustrated
in figure 6, in which the samples are normalized
to the average composition of upper continental
crust (Taylor and McLennan 1985) and compared
to the averages of shales of various ages, also normalized. The Baraboo, Barron, and Sioux samples
are extremely reduced in K, Na, Ca, and Mg compared to average shales and contain even smaller
amounts of Na, Ca, and Mg than do Phanerozoic
transported kaolinite clays (not shown). We attribute the extreme composition of the Baraboo and
related sedimentary rocks to an episode of unusually intense chemical weathering in late Paleoproterozoic time in the Lake Superior region.
87
Rb/86Sr
.488
2.373
2.205
.486
1.537
2.198
3.434
1.139
6.652
2j
.010
.049
.045
.010
.032
.045
.071
.023
.137
on a scale of 10–50 mm to irregular domains of end
member microcline and albite. Van Schmus et al.
(1975) previously recognized widespread resetting
of Rb-Sr systems in the Lake Superior region at
∼1650 Ma, and recalculation of a whole-rock RbSr isochron for 21 samples of 1750 Ma igneous
rocks from the Baraboo Range and Montello batholith yields an apparent age of 1635 Ⳳ 33 Ma (original data from Dott and Dalziel [1972] and Van
Schmus et al. [1975], using l p 1.42 # 10⫺11 yr⫺1).
The MSWD for the isochron fit is 8.1, which is an
acceptable value in this instance, considering that
most of the analyzed samples are not comagmatic.
We suggest that this apparent Rb-Sr isochron age
records the time of low-grade recrystallization of
the Baraboo basement and quartzite and note that
it is consistent with Ar cooling ages of basement
south of the 1630 Ma thermal and tectonic front
(Romano et al. 2000).
The supermature composition of the six quartzite sequences restricts their mineral assemblages to
Metamorphism
Although the diorite, granite, and rhyolite beneath
the Baraboo Quartzite have retained their igneous
textures on a hand specimen scale, they are seen
in thin section to be extensively recrystallized to
low-grade mineral assemblages (Medaris and Dott
2001). Hornblende has been replaced by a mixture
of chlorite, actinolite, and cummingtonite, biotite
by chlorite, and plagioclase by albite and finegrained epidote. Sericite occurs locally, and alkali
feldspar of intermediate composition has exsolved
Figure 5. Rb-Sr whole-rock isochron for the metamorphosed Baraboo paleosol, including regolith (R) and saprolite (S). The isochron has been fit to all nine samples.
Table 4.
Chemical and Modal Analyses of Late Paleoproterozoic Sedimentary Rocks
Baraboo
Barron
Sioux
BQ2A
BQ2C
BQ3B
BRN2A
BRN2D BRNPS
PP48
GEM07
GEM08
GEM14
GEM15
(argillite) (metapelite) (metapelite) (siltstone) (siltstone) (pelite) (argillite) (metapelite) (metapelite) (metapelite) (argillite) Meana
Oxides (wt%):
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Sum
CIA
Calculated modes (wt%):
Qtz
Hem
Rt
Kln
Prl
Ms
76.20
.58
15.80
4.41
.01
.01
.01
.05
.11
.07
3.00
59.70
1.03
25.40
8.04
.01
.04
.03
.04
.21
.12
4.70
68.80
.80
20.50
4.79
.01
.01
.01
.02
.28
.08
3.80
67.70
.81
17.30
5.51
.01
.01
.07
.01
.39
.08
6.15
82.20
.24
9.49
3.73
.01
.01
.07
.01
.07
.09
3.70
50.50
.61
31.40
5.52
.00
.03
.14
.02
.08
.08
11.60
72.90
.55
15.30
5.71
.01
.01
.07
.04
.16
.11
3.35
65.20
.80
23.30
5.34
.01
.07
.09
.11
.28
.14
4.20
64.30
.79
24.10
5.87
.01
.04
.04
.11
.28
.13
4.45
72.70
.81
21.40
.40
.01
.07
.10
.08
.28
.10
4.00
77.00
.43
14.50
4.91
.01
.06
.05
.05
.13
.08
2.80
68.28
.64
18.95
4.18
.01
.02
.05
.04
.18
.1
4.33
100.25
98.6
99.32
98.6
99.10
98.3
98.03
96.8
99.61
97.7
99.98
98.8
98.21
97.7
99.54
97.3
100.12
97.7
99.95
97.2
100.02
97.9
99.46
97.9
40.9
4.4
.6
3.3
49.9
.9
2.3
8.0
1.0
3.7
83.1
1.8
20.2
4.8
.8
…
71.8
2.4
47.9
5.6
.8
42.4
…
3.3
71.4
3.7
.2
24.2
…
.5
12.8
5.5
.6
80.4
…
.7
42.8
5.8
.6
9.9
39.5
1.4
10.9
5.3
.8
…
80.6
2.4
6.7
5.9
.8
…
84.2
2.4
22.0
.4
.8
…
74.4
2.4
45.2
4.9
.4
3.8
44.6
1.1
Note. XRF analyses provided by XRAL Laboratories, Ontario. CIA, Chemical Index of Alteration.
a
n p 11.
j
8.45
.21
5.85
1.77
.00
.02
.04
.03
.1
.02
2.35
Journal of Geology
PALEOPROTEROZOIC BARABOO QUARTZITES
251
ious quartzite localities, the highest-grade assemblage, quartz ⫹ muscovite ⫹ andalusite, is widely
developed in the Waterloo Quartzite, where muscovite from pegmatite that cuts the quartzite yields
an apparent Rb-Sr age of 1440 Ma (Aldrich et al.
1959) and muscovite from the quartzite yields a KAr age of 1410 Ma (Goldich et al. 1966).
Metamorphic conditions for the post–1750 Ma
quartzites can be evaluated from chemographic relations in the system, K2O-Al2O3-SiO2-H2O (KASH),
in which four univariant dehydration reactions are
sufficient to constrain the observed mineral assemblages (fig. 7):
Al 4 Si 4 O10 (OH)8 (kln) ⫹ 4SiO2 (qtz) p
Al 4 Si 8O20 (OH) 4 (prl) ⫹ 2H 2 O (V),
Figure 6. Major element compositions of Baraboo, Barron, and Sioux fine-grained sedimentary and metasedimentary rocks, and the averages of Precambrian and
Phanerozoic shales, all normalized to the average composition of upper continental crust (Taylor and McLennan 1985). CIA p Chemical Index of Alteration.
(1)
2Al 4 Si 4 O10 (OH)8 (kln) p Al 4 Si 8O20 (OH) 4 (prl)
⫹ 4AlO(OH) (dsp) ⫹ 4H 2 O (V),
(2)
Al 4 Si 8O20 (OH) 4 (prl) ⫹ 12AlO(OH) (dsp) p
8Al 2 SiO5 (and) ⫹ 8H 2 O (V),
(3)
Al 4 Si 8O20 (OH) 4 (prl) p 6SiO2 (qtz)
some combination of quartz, kaolinite, pyrophyllite, one of the Al2SiO5 polymorphs, and diaspore,
as summarized in figure 1. Muscovite occurs in
those samples that were modified by younger K
metasomatism, such as the Baraboo paleosol, and
Fe2O3 and TiO2 in the quartzites are accommodated
by accessory hematite and rutile. The Barron
Quartzite, located north of the 1630 Ma thermal
front, has not been metamorphosed and contains
quartz and kaolinite, which commonly occurs in
distinctive vermicular books. In contrast, the
folded Flambeau Quartzite, south of the thermal
front, has been metamorphosed and contains pyrophyllite rather than kaolinite. Farther south in
the Baraboo Range, metasedimentary rocks contain
quartz and pyrophyllite, metasomatized saprolite
contains quartz and muscovite, and thin hydrothermal veins that locally cut the basal part of the
quartzite just above the nonconformity are composed of muscovite, pyrophyllite, and diaspore. Although kaolinite is present in the Sioux Quartzite
(Southwick et al. 1986), pyrophyllite pseudomorphs
after vermicular kaolinite occur in many finegrained samples. The classic Sioux pipestone (a.k.a.
catlinite), which has been quarried for at least 400
yr by Native Americans for ceremonial pipes, contains an assemblage of muscovite, pyrophyllite, and
diaspore, which formed by K metasomatism and
recrystallization of fine-grained sedimentary rocks
(Morey 1983; Medaris et al. 1999). Among the var-
⫹ 2Al 2 SiO5 (and) ⫹ 2H 2 O (V).
(4)
Equilibrium temperatures of 285⬚C, 305⬚C, 345⬚C,
and 360⬚C for reactions (1), (2), (3), and (4), respectively, were calculated at a pressure of 1 kbar and
Figure 7. Stable reactions in the system, KASH, calculated for a(H2O) p 1, and equilibrium mineral assemblages in the Barron, Baraboo, Sioux, and Waterloo
quartzites. The metamorphic assemblage qtz ⫹ prl is
∼1.63 Ga in age, and the ms-bearing metamorphic assemblages are younger, at ∼1.46 Ga.
252
L. G. MEDARIS, JR., ET AL.
unit H2O activity by means of the GeoCalc software and associated thermodynamic database
(Brown et al. 1989). Muscovite, which is an additional phase in K-bearing samples, is stable
throughout the temperature range of interest. As
summarized in figure 7, the assemblage quartz ⫹
kaolinite (Barron) is stable below 285⬚C (reaction
[1]), the association quartz ⫹ pyrophyllite (Baraboo,
Flambeau, Sioux) lies between 285⬚ and 360⬚C (reactions [1], [4]), the assemblage pyrophyllite ⫹
diaspore ⫹ muscovite (Baraboo, Sioux) is more
tightly constrained between 305⬚ and 345⬚C (reactions [2], [3]), and quartz is stable with andalusite
and muscovite (Waterloo) at temperatures above
360⬚C (reaction [4]). The stable association of
chloritoid with quartz, andalusite, and muscovite
in some Waterloo samples (Geiger et al. 1981)
places an upper limit of ∼500⬚C for this assemblage.
Note that the small amount of kaolinite reported
for some of the Baraboo and Sioux samples in table
4 is retrograde in origin, replacing pyrophyllite.
Because of the extreme chemical maturity and
mineralogical simplicity of the quartzite sequences,
the 40Ar/39Ar radioisotopic chronometer can be applied only to the younger muscovite-bearing assemblages. Step heating, using a defocused CO2 laser and
the methods described by Singer and Brown (2002),
of a single muscovite grain (ca. 0.001 mg) from the
Waterloo quartz-muscovite-andalusite schist yields
an almost concordant age spectrum with a plateau
age of 1452 Ⳳ 7 Ma (fig. 8; sample WAMU, app. 1,
available from The Journal of Geology Data Depository). Such an age, combined with a relatively high
equilibration temperature of 360⬚–500⬚C and intrusion by ∼1440 Ma pegmatite, is consistent with contact metamorphism of the Waterloo Quartzite by
granitic rocks associated with geon 14 Wolf River
magmatism.
For Baraboo samples, a muscovite grain (0.001–
0.01 mg in size) from one of the muscovite-pyrophyllite-diaspore hydrothermal veins yielded a discordant spectrum with a well-defined plateau at
1467 Ⳳ 11 Ma (fig. 8; app. 1, sample 00BOW1). A
muscovite grain (0.001–0.01 mg in size) from metasaprolite also yielded a discordant spectrum with
a well-defined plateau at 1456 Ⳳ 11 Ma (fig. 8; app.
1, sample 96BH1), which is within error of that for
the hydrothermal vein. Considering the relatively
low equilibration temperature for hydrothermal assemblages in the Baraboo Range, ∼325⬚C, it is likely
that the 40Ar/39Ar plateau ages approximate the
time since muscovite growth, which is indistinguishable from the 1465 Ma age of the Wolf River
batholith. This coincidence suggests that Wolf
River igneous activity provided the thermal flux
Figure 8.
Age spectra of muscovite from Waterloo
muscovite-quartz-andalusite-hematite schist, Baraboo
muscovite-pyrophyllite-diaspore vein and Baraboo
muscovite-quartz-hematite metasaprolite, and whole
rock from two samples of Sioux muscovite-pyrophyllitediaspore pipestone. Length of arrows indicates the steps
included in the plateau age calculation for each sample.
necessary for regional-scale fluid flow along permeable channel ways, such as unconformities, and
promoted localized hydrothermal alteration in the
Baraboo Range.
Separation of muscovite from Sioux pipestone is
impractical because of its extremely fine grain size
of 5–15 mm in radius. Step heating of a pipestone
whole-rock sample (ca. 0.01 mg), consisting of muscovite (40%), pyrophyllite (56%), hematite (3.6%),
and small amounts of diaspore and rutile, yielded
a discordant spectrum with a relatively welldefined plateau age of 1370 Ⳳ 10 Ma (fig. 8; app. 1,
sample 00PNM01). A second, compositionally similar whole-rock sample (36% muscovite) also
yielded a discordant spectrum, but with a younger
plateau age of 1280 Ⳳ 13 Ma (fig. 8; app. 1, sample
00PNM03).
Because the 40Ar/39Ar plateau ages are between
Journal of Geology
PALEOPROTEROZOIC BARABOO QUARTZITES
1467 Ⳳ 11 Ma and 1280 Ⳳ 13 Ma, we infer that the
muscovite either grew or cooled below its closure
temperature, during or after intrusion of the Wolf
River batholith at ca. 1465 Ma. The range of plateau
ages is roughly correlated with the sizes of muscovite crystals in the dated samples. Muscovite
grains in the Sioux pipestone have 5–15-mm radii,
up to an order of magnitude smaller than grains in
the Baraboo samples. We suggest that the small
crystal radii in the pipestones, and even smaller
intracrystalline diffusion domains, led to closure at
substantially lower temperatures and may have allowed leakage of radiogenic argon at higher rates
compared to coarser mica in the Baraboo samples.
Thus the age spectra reveal simple-looking, diffusive loss-type profiles comprising plateaus much
younger in apparent age than the larger crystals (fig.
8). Regardless of the interpretation of the 40Ar/39Ar
age spectra for Sioux pipestone, it is clear that
development of the assemblage, muscovite ⫹
pyrophyllite ⫹ diaspore, in hydrothermally altered
parts of the Sioux and Baraboo quartzites is younger
than, and unrelated to, the 1630 Ma thermal and
tectonic front in northern Wisconsin.
Implications for Late Paleoproterozoic
Weathering, Sedimentation, and Tectonism
in the Lake Superior Region
The term “Baraboo interval” was introduced by
Dott (1983) to encompass the sequence of sedimentation, deformation, and metamorphism in the
southern Lake Superior region between 1450 and
1750 Ma. Previously, there was no generally recognized name for this interval, which included important Proterozoic events. In retrospect, however,
it would be preferable to restrict this term to the
1630–1750 Ma episode of weathering and sedimentation in the southern Lake Superior region,
thereby redefining the Baraboo interval as a stratigraphic term and excluding disparate tectonometamorphic events. The distinctive red supermature
quartzites of the Baraboo interval have long been
interpreted to represent passive margin sedimentation in a tectonically stable region undergoing
extensive chemical weathering. The present investigation reinforces the previous view of Baraboo interval sedimentation and reveals the remarkable
degree of chemical maturity attained by the sedimentary rocks, whose extreme CIA values of
96.8–98.8 place them among the most chemically
mature clastic sediments in the geological record.
That such chemically extreme sediments were
derived from a deeply weathered terrane is suggested by the existence of well-developed, mature
253
paleosols beneath the Baraboo interval sediments.
The feldspar-free nature of the paleosols raises the
possibility that substantial quantities of quartz, hematite, and rutile in these sediments are first cycle
in origin. However, the vast quantity of detrital
quartz in the Baraboo interval sediments cannot be
accounted for by first-cycle quartz alone, and the
presence of quartzite pebbles and quartz grains with
abraded silica-cement overgrowths (Ojakangas and
Weber 1984; Southwick and Mossler 1984) testify
to the contribution of second-cycle and multicycle
debris.
How can we reconcile the extreme compositional maturity of both the coarse and fine sediments in the Baraboo Quartzite and its correlatives? The dominantly sandy sediment required
braided streams with sufficient gradients to transport large volumes of sand-size material and rare
pebbles up to 2 cm in diameter, which implies substantial flow velocities of as much as 200–300 cm
s⫺1. Stabilization of portions of the same landscape
to allow the degree of chemical maturation of the
soil indicated by our analyses of the pelitic sediments presents a seeming paradox. Dott (1983) suggested a tectonically stable landscape with little
topographic relief under the influence of a warm,
humid climate, but more seems required to produce
such extreme chemical weathering. We believe that
microbiotic crusts or mats, such as those that characterize so-called cryptogamic soils in arid regions
today, provide the most plausible mechanism for
physically binding surfaces between active fluvial
channels and also contribute a biochemical component to the weathering process.
Biologists have long argued that cyanobacteria
and green algae followed by fungi and lichens must
have been the first organisms to conquer the land
(see, e.g., Campbell 1979; Schwartzman and Volk
1989; Gray and Shear 1992). Whereas the biologic
probability of this scenario is unchallenged, the
timing has long been in doubt. Marine cyanobacterial stromatolites as old as 3500 Ma are well
known, and freshwater stromatolites 12000 Ma old
have also been reported (Buck 1980). Although it
has been suggested that cyanobacteria soon spread
from ponds to subaerial land surfaces, only two localities with Precambrian soil microbe fossils have
been reported so far. These have microscopic filaments preserved in 800 Ma and 1200 Ma paleokarsts in California and Arizona, respectively (Horodyski and Knauth 1994). There is, however, more
indirect evidence that implies many biotic soil cappings. Precambrian paleosols are being recognized
increasingly (see Retallack 1988), and geochemistry
(especially oxygen isotope ratios) support the in-
254
L. G. MEDARIS, JR., ET AL.
terpretation of certain carbonaceous zones as ancient microbial soil crusts as old as 2600 Ma (Watanabe et al. 2000).
The spatial coincidence of an apparent deformational boundary in the Baraboo interval quartzites with the 1630 Ma thermal front in northern
Wisconsin, based on Rb/Sr and 40Ar/39Ar cooling
ages of mica and amphibole in basement rocks, suggests that folding of the quartzites was contemporaneous with isotopic resetting at ∼1630 Ma
(Holm et al. 1998). Such folding, isotopic resetting,
and low-grade metamorphism in the Lake Superior
region are thought to be the result of foreland deformation related to the Mazatzal Orogeny (Dott
1983; Van Schmus et al. 1993; Holm et al. 1998;
Romano et al. 2000), well documented in Arizona
and New Mexico (Karlstrom et al. 1997). Folding
of the Baraboo Quartzite is also attributed to this
1630 Ma event, and Rb/Sr isotopic resetting at 1635
Ma of granite and rhyolite beneath the quartzite is
consistent with such an interpretation. If correct,
then the prevalent quartz-pyrophyllite mineral assemblage, which is related to folding in the quartzite, is also ∼1630 Ma.
We were surprised to find 40Ar/39Ar plateau ages
of ∼1460 Ma for muscovite-bearing samples from
hydrothermal veins and metasaprolite in the Baraboo Quartzite, having expected values of ∼1630
Ma, the time of folding of the quartzite. However,
such a result is not surprising when considered in
the context of the well-known geon 14 transcontinental igneous event (Anderson 1983; Bickford
and Anderson 1993), during which numerous Atype granitic plutons, including the 1465 Ma Wolf
River batholith in Wisconsin, were emplaced in a
broad belt extending from Labrador to southern
California. Such voluminous introduction of magmas into the crust would be accompanied by a major heat flux, thereby providing a thermal pulse sufficient for generating regional-scale fluid flow along
permeable channels, such as unconformities, and
promoting laterally extensive, but stratigraphically
localized, hydrothermal alteration. Thus, the influence of geon 14 granitic magmatism was apparently
far more extensive in the Lake Superior region than
previously recognized, promoting widespread hydrothermal alteration not only in the Baraboo
Quartzite, which is located ∼100 km from the Wolf
River batholith, but also in the Sioux Quartzite,
which is even more distant, at ∼400 km.
Implications for Proto–North America
Red, supermature quartz arenites of late Paleoproterozoic age are not confined to the Lake Superior
region. Quartz arenites of similar age and characteristics occur in widely separated localities across
the North American continent, including the Mazatzal Quartzite in Arizona (Trevena 1979), the Ortega Quartzite in New Mexico (Soegaard and Eriksson 1989), and the Athabasca, Thelon, and
Hornby Bay basins in northwestern Canada (Ramaekers 1981). The Athabasca Basin is also underlain by a well-developed, mature paleosol, with CIA
values of 94.5–95.2 and local modification by K and
Mg metasomatism (Macdonald 1980). Such extensive distribution of coeval, primary supermature
quartz arenites indicates that profound chemical
weathering, so well demonstrated in the Lake Superior region, was continental in scale, affecting
much of the Proto–North American craton in late
Paleoproterozoic time. Where sedimentation was
associated with silicic volcanism, as in the lower
part of the Mazatzal Quartzite in Arizona, the original sediments were feldspar bearing, and the present supermature composition is secondary in origin, feldspar having been subsequently removed by
intrastratal solution (Cox and Comstock 1998).
The relatively thick accumulation, on the order
of 1000 m, of Baraboo-type quartz arenites deposited in fluviatile to shallow marine environments
is attributed to passive margin sedimentation,
which culminated in marine transgression (Dott
1983). The transcontinental distribution of such deposits may be related to a late Paleoproterozoic eustatic rise in sea level, as suggested by Soegaard and
Eriksson (1989). The collective chemical and petrological characteristics of the late Paleoproterozoic supermature quartzites imply deposition in a
stable cratonic environment, which was attained
on a continental scale sometime between ∼1750
Ma, the youngest age of underlying basement, and
∼1630 Ma, when the quartzites were folded during
the Mazatzal orogeny (Dott 1983; Van Schmus et
al. 1993; Holm et al. 1998; Romano et al. 2000).
Following the accretion of 1.6–1.8 Ga orogenic
provinces along the eastern and southern margins
of Laurentia, the accreted provinces were invaded
by numerous A-type granites of the geon 14 transcontinental magmatic belt (Bickford and Anderson
1993). The epizonal to mesozonal granite plutons
were typically accompanied by contact metamorphism, although regional metamorphism and deformation occurred in Arizona, New Mexico, and
Colorado (Karlstrom et al. 1997), and far-reaching
hydrothermal alteration is now recognized in the
Baraboo and Sioux quartzites of the southern Lake
Superior region. It is intriguing that the Athabasca
basin, which occurs ∼2,000 km northwest of the
transcontinental belt, was affected by several Pro-
Journal of Geology
PALEOPROTEROZOIC BARABOO QUARTZITES
terozoic high-temperature (200⬚C) hydrothermal
events, including one at 1477 Ⳳ 57 Ma, as indicated
by a Rb-Sr isochron for diagenetic illite that formed
from high-temperature basin fluids (Kotzer et al.
1992). It thus appears that Late Paleoproterozoic
sedimentary sequences far from the belt of transcontinental intrusion were modified by hydrothermal fluids, which were propelled for long distances
along permeable channel ways by the thermal effects of geon 14 magmatism.
Such regionally extensive fluid migration is not
unique to the Proterozoic Eon, however. Paleozoic
sedimentary basins and the continental platform
throughout much of North America were modified
during Pennsylvanian-Permian time by brines,
which originated in the forelands of the Alleghenian and Ouachita orogenies and migrated for hundreds of kilometers (Bethke and Marshak 1990).
The connection between tectonism and brine migration is poorly understood, but tectonic compression and thrusting, sediment compaction, and
topographic uplift are all thought to play a role,
with topographic uplift probably being the most
significant (Bethke and Marshak 1990). Appreciable
topographic uplift was likely associated with the
geon 14 transcontinental magmatic belt as well,
considering the large quantity of heat that must
have been advected to the crust by the numerous
255
granitic intrusions. Thus, topographic uplift, in addition to thermal input, may have been important
in promoting extensive fluid migration in North
America at 1.4–1.5 Ga.
ACKNOWLEDGMENTS
Acquisition of the U-Pb and Rb-Sr data was made
possible by support from National Science Foundation (NSF) grant EAR-9628549. The University
of Wisconsin (UW)—Madison Rare Gas Geochronology Laboratory was constructed with support
from NSF grant EAR-9972851, the UW—Madison
Graduate School, the Lewis G. Weeks and Albert
and Alice Weeks Foundations, Shell Oil Company,
and Henry F. Nelson. Brian Jicha helped with 40Ar/
39
Ar data reduction. A. Naymark thanks the
UW—Madison College of Letters and Science Honors Program and the Geological Society of America
for financial support. We are indebted to Bruce
Brown for providing access to the Baxter Hollow
drill cores, to Mike Mudrey for informing us of the
Barron paleosol, and to Kristin Legg and Alice Erickson of the U.S. National Park Service for permission to sample material from Pipestone National Monument. We thank Daniel Holm, Dave
Southwick, and Mike Williams for their thoughtful
and constructive reviews of the manuscript.
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