Precambrian Crustal Blocks in Minnesota 

Neodymium Isotope Evidence from Basement 

and Metasedimentary Rocks

U.S. GEOLOGICAL SURVEY BULLETIN 1904-U
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Chapter U

Precambrian Crustal Blocks in Minnesota 

Neodymium Isotope Evidence from Basement 

and Metasedimentary Rocks

By S.R. HEMMING, G.N. HANSON, and S.M. McLENNAN

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

P.K. SIMS and L.M.H. CARTER, Editors
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

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Library of Congress Cataloging-in-Publication Data

Precambrian crustal blocks in Minnesota neodymium isotope evidence

from basement and metasedimentary rocks/by S.R. Hemming, G.N. Hanson, and

S.M. McLennan.

p. cm. (Contributions to Precambrian geology of Lake Superior region ; ch. U) 

(U.S. Geological Survey bulletin ; 1904)

Includes bibliographical references (p. - ).

Supt. of Docs, no.: I 19.3:1904U

1. Geology, Stratigraphic Precambrian. 2. Samarium-neodymium dating 

Minnesota. 3. Geology Minnesota. I. Hemming, S.R. II. Hanson, Gilbert 

N. III. McLennan, Scott M. IV. Series. V. Series: U.S. Geological Survey 

bulletin ; 1904. 

QE75.B9no. 1904-U 

[QE653] 

557.3 s dc20

[551.7'1'09776] 95-649

CIP
CONTENTS

Abstract Ul 

Introduction Ul 

Acknowledgments U3 

Geologic setting U3

Superior province U3

Minnesota River Valley gneiss terranes U3

Early Proterozoic geology U4 

Sample selection and analysis U6

Analytical data U6

Results U7 

Discussion U9

Archean foreland U9

Pre-Animikie metasedimentary rocks U9

Plutons east of the Malmo discontinuity Ull 

Summary: Crustal blocks of the Penokean orogen U12 

References cited U13

FIGURES

1. Precambrian basement map of midcontinent region. United States, showing location 

of Penokean orogen U2

2. Map of Precambrian rocks in a part of western Minnesota U4

3. Map of Penokean orogen in central Minnesota US

4. Generalized/Sm/Nd versus eNd plot from Shirey (1984) U9

5. Plots of /Sm/Nd versus eNd for samples discussed in this report U10

6. Diagram of depleted mantle model ages for Nd in structural-stratigraphic 

units U12

TABLES

1. Nd isotopic data from igneous and metaigneous rocks in central and western 

Minnesota U7

2. Nd isotopic data from Early Proterozoic metasedimentary rocks in central 

Minnesota U8

Contents III

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

Precambrian Crustal Blocks in Minnesota 

Neodymium Isotope Evidence from Basement and 

Metasedimentary Rocks

ByS.R. Hemming, 1 '2 G.N. Hanson, 1 andS.M. McLennan 1

Abstract

A Sm-Nd survey of Precambrian granitic and metasedimentary rocks collected from several geophysically defined 

blocks in central Minnesota confirms the presence of some fundamental geologic boundaries previously mapped on the basis 

of structure. The Malmo discontinuity an Early Proterozoic 

structure separates the Archean Minnesota River Valley terranes from an Early Proterozoic (Penokean magmatic) terrane 

and the McGrath Gneiss dome of east-central Minnesota 

North of this boundary, the Penokean fold-and-thrust belt can 

be divided into fault-bounded terranes that were assembled 

during the Penokean orogeny. Each terrane within the study 

area has distinct crust formation ages. These terranes and ages, 

from south to north, are as follows: (1) Early Proterozoic crust 

of the Penokean magmatic terrane; (2) McCrath dome and 

Benson block, including the McGrath Gneiss and Ortonville 

Granite (TpM (depleted mantle) ages from 3.0 to 3.3 Ga), other 

granites within the Benson block (T^M of 2.7-2.9 Ga), and 

metasedimentary rocks of the Mille Lacs Group that overlie the 

McGrath Gneiss (T^M of 2.6-3.1 Ga); (3) metasedimentary and 

metavolcanic rocks of the Moose Lake-Glen Township terrane 

(TDM °f 2.1 Ga); (4) metasedimentary rocks of the Cuyuna 

North range (TDM °f 2.5-3.0 Ga); and (5) the main bowl of the 

Animikie basin, which records a progression from Archean 

sources in the lower units to Early Proterozoic sources in the 

younger (and more southerly) units.

Department of Earth and Space Sciences, State University of New 

York at Stony Brook, Stony Brook, NY 11794-2100.

Present address: Lamont-Doherty Earth Observatory, Palisades, NY 

10964.

INTRODUCTION

The Great Lakes region of North America lies at the 

transition from the Archean Superior craton of the Canadian Shield into Early Proterozoic erogenic complexes that 

form the basement of the midcontinent (Van Schmus, 

1976; Nelson and DePaolo, 1985; Sims, 1990). The 

Penokean and Trans-Hudson orogens, bordering the Superior craton (fig. 1), include large volumes of newly formed 

Early Proterozoic continental crust (Chauvel and others, 

1987; Horan and others, 1987; Spencer, 1987; Barovich 

and others, 1989). Continuing crustal accretion of the 

North American continent during latter stages of the Early 

Proterozoic is recorded in extensive terranes extending 

from the Central Plains throughout the southwestern 

United States (Nelson and DePaolo, 1985; Bennett and 

DePaolo, 1987; McLennan and others, in press).

Minnesota lies astride the boundary between 

Archean rocks of the Superior province and Early Proterozoic rocks of the Penokean orogen (Sims and others, 

1993). The boundary in Minnesota is named the Malmo 

structural discontinuity (Southwick and others, 1988), and 

both it and adjacent Archean (Southwick and others, 1989) 

and Proterozoic rocks (Southwick and others, 1988) are 

poorly exposed; delineation of tectono-stratigraphic 

terranes has been based mainly on aeromagnetic and gravity maps combined with selective test-drilling (Southwick 

and Morey, 1991). Because of the utility of Nd isotope 

data in delineating structural blocks having different lithologies and histories (Nelson and DePaolo, 1985; Bennett 

and DePaolo, 1987), a systematic Sm-Nd isotope study 

was carried out to test and supplement the geologic framework established by the mapping. In general, the Nd data 

presented here support the earlier conclusions based on

Precambrian Crustal Blocks, Minnesota; Neodymium Isotope Evidence U1
104° 100

49°

45°

41° -

37°

UNITED STATES

ND 

SUPERIOR PROVINCE

100

I

200 300 400 500 KILOMETERS

EXPLANATION

MIIHlj Middle Proterozoic rocks 

EARLY PROTEROZOIC OROGENS 

Penokean 

Trans-Hudson 

Central Plains

ARCHEAN TERRANES 

Superior province 

Archean gneisses

Generalized contact 

Fault

Figure 1. Precambrian basement map of midcontinent region, United States, showing location of Penokean orogen relative to 

Superior province and Early Proterozoic orogens that make up the midcontinental United States. GLTZ, Great Lakes tectonic 

zone, is interpreted by Sims and others (1980) to be the boundary between volcano-plutonic complexes of Superior province 

and older gneiss terranes of Minnesota River Valley (MRV). Modified from Sims and Peterman (1986) and Sims (1990).

U2 Precambrian Geology of Lake Superior Region
mapping, but they also reveal some problems that require 

additional isotopic studies.

Stratigraphic names of Early Proterozoic strata are 

used extensively in this report. As a guide to these names 

the reader is referred to figure 7 in Morey and Van 

Schmus (1988) and figure 1 in Ojakangas (1994).

ACKNOWLEDGMENTS

This research was supported by National Science 

Foundation Grants EAR-8816386 and EAR-8957784 and 

by the Donors of the Petroleum Research Fund, administered by the American Chemical Society to S.M. 

McLennan.

The project would not have been possible without 

the help and advice of D.L. Southwick and G.B. Morey of 

the Minnesota Geological Survey, and we appreciate the 

many discussions with them. We thank Barbara Bock, 

Dan Davis, Bob Folk, G.B. Morey, Bill Myers, Troy Rasbury, and D.L. Southwick for reading various versions of 

this manuscript. Thanks go to P.K. Sims and Mary Horan 

for their thorough and constructive reviews.

GEOLOGIC SETTING 

Superior Province

The Superior province of the Canadian Shield (fig. 

1) is interpreted to consist of a series of arc terranes that 

were accreted from north to south during the interval -3.0 

to 2.6 Ga (Krogh and Davis, 1971; Card and Ciesielski, 

1986; Hoffman, 1989). Morey and Sims (1976) recognized two distinct Archean terranes in Minnesota that differ in age, rock assemblages, metamorphic grade, and 

structural style. Greenstone-granite terrane (Wawa subprovince, Card, 1990) rocks of the southern Superior province are characterized by mantle-like initial radiogenic 

isotopic compositions (for example, Shirey and Hanson, 

1986; Stern and Hanson, 1992; Gariepy and Allegre, 1985; 

Arth and Hanson, 1975). In Minnesota, arc-like rocks are 

inferred to be in contact with mainly older, high grade 

gneisses of the Minnesota River Valley terranes along the 

Morris fault (G.B. Morey, oral commun., 1994). Sims and 

others (1980) and Sims and Day (1993) interpreted the 

Morris fault to be a segment of the Great Lakes tectonic 

zone, a major Late Archean crustal boundary between 

Late Archean greenstone-granite terrane (on the north) and 

Middle Archean gneiss throughout the Lake Superior and 

Lake Huron regions. In central Minnesota, the Morris 

fault segment of the Great Lakes tectonic zone (figs. 1, 2, 

3) has been interpreted to be a seismic reflector that dips 

northward at 30° to mid-crustal level (Gibbs and others,

1984). More recently, it has been suggested that the 

shallow-dipping reflectors are actually recumbent and 

gently inclined folds, and the Morris fault has a steep 

southeast dip that is transparent in seismic profiles (Sims 

and Day, 1993).

Minnesota River Valley Gneiss Terranes

Recent geophysical work and scientific drilling by 

the Minnesota Geological Survey (Southwick and others, 

1989, 1990; Southwick, 1993) have provided evidence of 

multiple terranes within the Minnesota River Valley 

region, and the isotope systematics of the region are complicated. Despite the complexity, gneisses that crop out 

along the Minnesota River Valley in the vicinity of Morton and Montevideo (fig. 2) are known to have Middle 

Archean heritage (McCulloch and Wasserburg, 1976; 

McCulloch, 1980; Goldich and others, 1970) and are interpreted to have experienced high-grade metamorphism and 

crustal melting in the Late Archean (-3.0 Ga and =2.6 Ga, 

Goldich and Wooden, 1980). Late Archean high-grade 

metamorphism and crustal melting (such as in the Sacred 

Heart Granite, Doe and Delevaux, 1980) in the Minnesota 

River Valley are attributed to continental collision of the 

Minnesota River Valley gneisses with the Superior province rocks along the Great Lakes tectonic zone (Sims and 

others, 1980; Hoffman, 1988) at about 2.68 Ga. Although 

the -1.8 Ga granite of Section 28 has Pb isotopic compositions that suggest it was derived from crustal melting 

(Doe and Delevaux, 1980), and zircon lower intercept ages 

in the Morton area are Early Proterozoic (Goldich and 

Wooden, 1980), Early Proterozoic metamorphism is not 

generally regarded as profoundly overprinting older metamorphism in this area. (See, among others, Doe and Delevaux, 1980; Goldich and Wooden, 1980.)

Gneisses having a structural fabric similar to that in 

the Minnesota River Valley crop out sporadically between 

exposures in the river valley and the trace of the Great 

Lakes tectonic zone (Sims and others, 1980). The gneiss 

terranes are truncated on the east (fig. 2) by the Early Proterozoic Malmo discontinuity (Southwick and Morey, 

1991). Some of the gneisses, located east of the Malmo 

discontinuity in the zone previously considered to be Middle Archean gneisses (Morey and Sims, 1976), have Early 

Proterozoic Nd depleted mantle model ages (Horan and 

others, 1987). Thus, the extent of Archean crust in this 

region is uncertain.

Within the Minnesota River Valley gneiss terranes 

(fig. 2), south of the Great Lakes tectonic zone, are several tectonic blocks that have been identified on the basis 

of geophysical lineaments and geologic mapping (Southwick and others, 1989). From north to south, these are 

(Southwick and others, 1989, 1990; Southwick, 1993):

Precambrian Crustal Blocks, Minnesota; Neodymium Isotope Evidence U3
95°00'

45°30' -

EXPLANATION

t tl Penokean magmatic terrane

; B

M

Superior province 

Benson block 

Montevideo block

Figure 2. Precambrian rocks in a part of western Minnesota. GLTZ, Great Lakes tectonic zone (Sims and others, 1980). AGL, 

Appleton geophysical lineament (Southwick and others, 1989). MD, Malmo discontinuity (Southwick and Morey, 1991). Sample localities are marked with asterisks; cores (BPL and APL designations) are from the Minnesota Geological Survey test-drilling 

program, and sample localities are from Southwick and others (1990). OG, Ortonville Granite. Modified from Southwick and 

others (1990).

(1) Benson block, (2) Montevideo block, (3) Morton 

block, and (4) Jeffers block (D.L. Southwick, written 

commun., 1994). The Morton and Jeffers blocks are 

south of the area of figure 2. The Benson block (fig. 2) 

is a poorly exposed terrane composed of gneiss and abundant granite. The Appleton geophysical lineament (fig. 2) 

separates it from the Montevideo block, which is composed primarily of gneissic rocks such as those exposed 

along the Minnesota River Valley from Sacred Heart to 

Granite Falls (-15 km south of the area of fig. 2). The 

rock types in the Benson block are sufficiently different 

from those in the Montevideo and Morton blocks (Goldich and others, 1970, among others) for Southwick and 

others (1989) to question whether the =3.5 Ga age component and the granulite facies metamorphism are represented in the Benson block. Our results do not 

unambiguously resolve this question, although we have 

found no depleted mantle model ages greater than 3.1 Ga 

in the Benson block.

Early Proterozoic Geology

The Early Proterozoic Penokean orogen (fig. 1) is a 

northwest-verging tectonic system along the south margin 

of the Superior craton that is exposed in Minnesota, Wisconsin, and Michigan (Goldich and others, 1961; Cannon, 

1973; Sims and Peterman, 1983), and may have extended 

to the east beyond the Grenville front of Ontario (Dickin 

and McNutt, 1989, among others). The Penokean orogen 

is interpreted to consist of a deformed continental margin 

prism and associated foreland basin, the Animikie basin, 

overlying an Archean craton (fig. 3) (Hoffman, 1987, 

1988; Southwick and others, 1988), and a southeastern 

assemblage of arc terranes, termed the Wisconsin magmatic terranes (Sims, 1987). Two terranes with extensive 

Early Proterozoic magmatism have been identified in Wisconsin (LaBerge and Myers, 1984; Sims and others, 1989; 

Brown and Maass, 1992; Maass and Brown, 1993): the

U4 Precambrian Geology of Lake Superior Region
EXPLANATION

Keweenawan rocks (Middle Proterozoic) 

EARLY PROTEROZOIC TERRANES

Animikie basin

Cuyuna North range

Cuyuna South range

Moose Lake-Glen Township 

Archean foreland and McGrath Gneiss

Generalized contact 

Fault

Figure 3. Map of Penokean orogen in central Minnesota. GLTZ, Great Lakes tectonic zone (Sims and others, 1980); MALMO, 

Malmo discontinuity; GR, Giants Range Granite. Within the North range of the Cuyuna district, A, H, M, and N represent Arko, 

Hillcrest, Merrit, and Northland cores, respectively (Grout and Wolf, 1955). LF, McG, HM, LP, NO represent Little Falls Formation, McGrath Gneiss, Hillman Migmatite, Long Prairie basin, and Nimrod outlier, respectively. The numbers are SRH sample designations in tables 1 and 2. Modified from Southwick and others (1988).

Pembine-Wausau terrane in the north and the Marshfield 

terrane in the south (Sims and others, 1989).

Within the continental margin prism in Wisconsin 

and Michigan, Barovich and others (1989) found that the 

Nd isotopic character of the lower stratigraphic units of

the Marquette Range Supergroup is consistent with derivation from Late Archean sources in the Superior province. 

However, the Nd isotope character of Michigamme Formation, the uppermost unit of the Marquette Range Supergroup, is more complicated. Samples in the northern part

Precambrian Crustal Blocks, Minnesota; Neodymium Isotope Evidence U5
of Barovich and others' study area, near exposed Archean 

basement, have Late Archean sources and lack evidence 

for an Early Proterozoic contribution (TDM (depleted mantle age) = 2.70-3.11 Ga), whereas samples in the southern 

part have more primitive Nd isotopic characteristics 

(TrjM-2-04-2.29 Ga), comparable to those of the Wisconsin magmatic terranes.

In central Minnesota, the rocks of the Early Proterozoic sedimentary and volcanic prism are metamorphosed 

and strongly folded and faulted (Southwick and others, 

1988; Southwick and Morey, 1991). Rocks assigned to 

the Mille Lacs Group occur within the Penokean fold-andthrust belt where they have been tectonically dismembered 

into several slices that are bounded by structural discontinuities, and they probably do not represent a coherent 

stratigraphic package (Southwick and others, 1988). 

Southwick and others (1988) divided the fold-and-thrust 

belt into three parts (fig. 3): (1) external zone, North 

range of Cuyuna district and Thomson Formation outcrops; (2) medial zone, South range of Cuyuna district and 

Moose Lake-Glen Township terrane; and (3) internal 

zone, McGrath-Little Falls terrane.

Within the external zone, the North range of the 

Cuyuna district consists of the Mille Lacs Group overlain 

by the North Range Group (Schmidt, 1963). The Mille 

Lacs Group consists of quartz-rich sedimentary rocks 

together with iron-formation and assorted volcanic rocks 

in the lower part and quartz wacke together with minor 

dolomite and volcanic material in the upper part (Morey, 

1978). The North Range Group (Morey and Southwick, 

1993) consists, in ascending order, of (1) the Mahnomen 

Formation, a quartz-rich sandstone-siltstone sequence; (2) 

the Trommald Formation, an iron-rich and locally manganese rich sequence; and (3) the Rabbit Lake Formation, a 

black shale-graywacke sequence.

The medial zone of the fold-and-thrust belt is composed dominantly of volcanic and related rocks of the 

Mille Lacs Group, although some unnamed fine-grained 

sedimentary sequences exist in the structurally higher 

zones (Southwick and Morey, 1991; Morey and Southwick, 1993). Beck (1987) determined a whole-rock SmNd isochron age of 2,197±39 Ma for the Glen Township 

Formation basalts of the Mille Lacs Group. Furthermore, 

the Nd isotopic composition of these basalts (eNd(2.2 

Ga)=+2.8) is consistent with derivation from a depleted 

mantle source at that time.

The internal zone or McGrath-Little Falls terrane is 

composed of several types of gneisses, amphibolite-facies 

metasedimentary schist, and a variety of granitoid intrusions (Southwick and others, 1988). Spencer (1987) and 

Horan and others (1987) concluded that the Early Proterozoic granitoid intrusions and the spatially associated Early 

Proterozoic mafic dikes of this belt near St. Cloud are 

products of an arc system. Nd and Pb isotopic compositions are identical within both mafic and granitic rocks in

the St. Cloud area and indicate a substantial Early Proterozoic mantle contribution (Horan and others, 1987; Spencer, 1987). It is likely that any extensive volcano-plutonic 

arc terrane equivalent to the Wisconsin magmatic terranes 

was mostly displaced by the Middle Proterozoic Midcontinent rift into southeastern Iowa (fig. 1).

Although rocks of the Animikie Group are deformed 

in the southern part of the Animikie basin, they lie with 

angular unconformity on the older sequences of the foldand-thrust belt (Southwick and others, 1988; Southwick 

and Morey, 1991; Morey and Southwick, 1993). Southwick and others (1988) and Southwick and Morey (1991) 

interpreted the Animikie basin of central Minnesota to be 

a foreland basin to the Penokean orogen, in which case the 

uppermost units are flysch deposits derived from the 

south. Hemming and others (1992; 1995) found that the 

Thomson and Virginia Formations (uppermost Animikie 

Group, fig. 3) have primitive Nd isotopic ratios consistent 

with their having been derived solely from Early Proterozoic sources similar to the Wisconsin magmatic terranes. 

The Rove Formation of the Gunflint Range, near the 

northeastern Minnesota-Ontario border, thought to be 

stratigraphically equivalent to the Virginia Formation 

(Morey, 1967, among others), has Nd model ages of 

2.0-2.6, suggesting mixing of Archean and Early Proterozoic sources (Hemming and others, 1995).

SAMPLE SELECTION AND ANALYSIS

Our samples fall into three groups: (1) basement 

plutons and gneissic plutons of the Archean foreland (figs. 

2 and 3) and McGrath-Little Falls terrane (fig. 3); (2) preAnimikie Group metasedimentary rocks of the fold-andthrust belt (figs. 2 and 3); and (3) plutons east of the 

Malmo discontinuity (fig. 2) (Southwick and others, 

1990).

Analytical Data

Approximately 20 g of each rock was powdered in 

an agate shatter box; 200 nig of this powder was fused 

with a lithium metaborate flux at 1,200°C for 10 minutes 

and dissolved in 30 mL of IN HNO3. Five mL aliquots 

were spiked with a mixed REE (rare earth element) spike. 

Rare earth elements were separated as a group on columns 

with 50WX8 cation exchange resin, with elution by HC1 

followed by HNO3. The rare earth elements were then 

separated into smaller groups on 50WX8 cation exchange 

columns using methylactic acid (see Stem and Hanson, 

1991). For concentration data, Sm and Nd were collected 

and analyzed together on a double, degassed Re filament 

assembly by isotope dilution on an NBS design 15 cm

U6 Precambrian Geology of Lake Superior Region
Table 1. Nd isotopic data from igneous and metaigneous rocks in central and western Minnesota

represents deviation in parts per 104 of the 143 Nd/l44Nd ratio from average chondritic meteorites at any specific 

time T (here, in billions of years). ^m/iMd represents fractional deviation of the 147Sm/144 Nd ratio from average chondritic meteorites. TOM, depleted mantle age. Depleted mantle values from Taylor and McLennan (1985). Basement 

cores are described in Southwick and others (1990). Giants Range Granite sample from Arth and Hanson (1975). Blank, 

no data]

Sample

No.

Sm 

(ppm)

Nd 

(ppm)

147Sm/144Nd 143Nd/ 144Nd ENdO £Ndl.8 TDM 

(Ga)

/Sm/Nd

Depleted mantle 0.2170 0.510696 10.0 5.3 0.103

Basement cores

BPL2a

BPL2b

BPL4a

BPL4b

BPL6

BPL11

BPL12

BPL13

APL 20-343

18.6

3.79

17.0

4.83

18.5

1.72

1.88

2.33

138

36.4

9.01

22.8

130

45.2

12.2

12.6

0.0813

0.0629

0.1135

0.1283

0.0863

0.0936

0.0932

0.1119

0.510696

0.510312

0.511220

0.511439

0.512675

0.511182

0.511212

0.510751

0.511331

-37.9

-45.4

-21.1

-23.4

0.7

-28.4

-27.8

-36.8

-25.5

-11.3

-14.5

-8.5

-7.6

-2.9

-4.0

-13.0

-5.9

2.74

2.79

2.83

2.92

2.28

2.38

2.93

2.62

-0.587

-0.680

-0.423

-0.348

-0.561

-0.524

-0.526

-0.431

Ortonville Granite

OG14b

OGllb

OB 37

1.40

4.87

6.57

11.1

43.8

38.6

0.0768

0.0672

0.1028

0.510363

0.510141

0.510794

-44.4

-48.7

-36.0

-16.8

-18.9

-14.4

3.01

3.04

3.12

-0.610

-0.659

-0.477

McGrath Gneiss

SRH7d

SRH8

SRH8b

2.71

3.83

5.71

18.4

16.8

34.0

0.0891

0.1376

0.1014

0.510520

0.511423

0.510741

-41.3

-23.7

-37.0

-16.5

-10.1

-15.1

3.11

3.29

3.15

-0.547

-0.300

-0.484

Giants Range Granite

DL7 4.28 32.5 0.0796 0.510612 -39.5 -12.5 2.80 -0.595

radius-of-curvature mass spectrometer. For isotopic composition, Nd was completely separated from Sm and Ce. 

Nd isotopic composition was measured in static mode on a 

Finnigan MAT 262 using a degassed double Re filament 

assembly. Samples were corrected assuming linear mass 

fractionation and 146Nd/ 144Nd=0.7219, and accepted runs 

had in-run precision better than ±0.000010 (2 sigma 

mean). Nd isotopic data were accepted only if the measured 148Nd/144Nd was in the range 0.241620±0.000020. 

The external reproducibility of Nd standard solutions 

using this filter is better than 0.3 epsilon units. During 

data collection for this study, less than 5 percent of measurements failed to meet this criterion. The average

143Nd/144Nd value measured for La Jolla Nd standard 

solution was 0.511820 during the period of these analyses, 

and the ratios were adjusted to a value of 0.511865.

Results

Results from plutonic rocks are presented in table 1, 

and results from metasedimentary rocks are presented in 

table 2. Plots of /sm/Nd versus eNd(time) (Shirey and Hanson, 1986) allow an evaluation of Nd isotope systematics 

at a given time that makes it conceptually simple to compare data from different REE reservoirs.

Precambrian Crustal Blocks, Minnesota; Neodymium Isotope Evidence U7
Table 2. Nd isotopic data from Early Proterozoic metasedimentary rocks in central Minnesota

[ENd(T) represents deviation in parts per 104 of the 143 Nd/144Nd ratio from average chondritic meteorites at any specific time T (here, in billions of years). /^m/Nd represents fractional deviation of the l47Sm/144Nd ratio from average 

chondritic meteorites. TDM, depleted mantle age]

Sample Sm Nd 

No. 1 (ppm) (ppm)

147Sm/ 144Nd 143Nd/144Nd ENdO eNd 1.8 TDM 

(Ga)

/Sm/Nd

Thomson Formation

Sandstone 4.53

Shale 5.02

24.4

25.5

0.1139

0.1191

0.511679

0.511768

-18.7

-17.0

0.4

1.0

2.17

2.14

-0.42

-0.39

North Range Group

Rabbit Lake Formation:

Hillcrest 80.2 2.03

Hillcrest 136.7 1.20

Hillcrest 140.7 3.50

Arko box 150 3.24

Mahnomen Formation:

Merrit 7 18 1.65

Merril 753.8 1.95

Merrit 771 2.43

Northland 369.4 3.87

Northland 774.8 2.79

11.7

5.74

21.8

18.8

9.85

9.69

12.8

25.4

15.9

0.1052

0.1262

0.0970

0.1042

0.1148

0.1215

0.1144

0.0922

0.1060

0.511005

0.511339

0.510913

0.511332

0.511315

0.511272

0.511134

0.510924

0.511228

-31.9

-25.3

-33.6

-25.5

-25.8

-26.6

-29.3

-33.4

-27.5

-10.8

-9.1

-10.7

-4.1

-6.9

-9.3

-10.4

-9.3

-6.6

2.91

3.02

2.82

2.45

2.72

2.98

2.98

2.70

2.62

-0.465

-0.358

-0.507

-0.470

-0.416

-0.382

-0.418

-0.531

-0.461

Mille Lacs Group

Little Falls Formation:

SRH 1 5.76

SRH la 5.97

SRH 2 5.17

SRH 3 2.10

SRH4b 5.42

SRH 4e 5.88

BPL 10a2 6.16

BPL 10b2 4.74

Denham Formation:

29.8

30.0

26.7

8.29

27.5

29.0

32.6

29.0

0.1170

0.1201

0.1169

0.1532

0.1190

0.1221

0.1143

0.0988

0.511744

0.511794

0.511711

0.511948

0.511769

0.511797

0.511620

0.511309

-17.4

-16.5

-18.1

-13.5

-17.0

-16.4

-19.9

-25.9

1.0

1.2

0.3

-3.4

1.0

0.7

-0.8

-3.3

2.13

2.13

2.18

2.85

2.14

2.18

2.26

2.36

-0.405

-0.389

-0.406

-0.221

-0.395

-0.376

-0.419

-0.497

Moose Lake-Glen Township panel

SRH 5a 7.47

McGrath-Little Falls panel

SRH 7b 2.38

(amphibolite)

SRH7f 1.22

(quartzite)

SRH 7j 3.04

(calc. silicate)

36.4

10.1

7.51

12.3

0.1238

0.1429

0.0981

0.1494

0.511878

0.511845

0.510700

0.512004

-14.8

-15.5

-37.8

-12.4

2.0

-3.1

-15.1

-1.4

2.07

2.67

3.12

2.57

-0.370

-0.273

-0.501

-0.241

Numbers after Hillcrest, Merrit, and Northland are depth in feet in cores. 

2Little Falls(?) Formation BPL samples are described in Southwick and others (1990).

Figure 4 is a schematic /Sm/Nd versus £Nd(T) plot to 

show how different reservoirs and processes plot. Data 

from central Minnesota are presented in figure 5. Data 

from different terranes of Southwick and others (1988), 

Southwick and Morey (1991), Southwick and others 

(1990), and Morey and Southwick (1993) are presented on 

separate plots for clarity (fig. 5). Core samples in eastern

parts of the Benson block (fig. 5A, table 1) have Late 

Archean Nd model ages. These samples could be juvenile 

Late Archean granites or they could be Early Proterozoic 

granites with compositions representing mixing between 

older Archean crustal sources and newly derived granites 

such as found east of the Malmo discontinuity. Ortonville 

Granite samples have a large variation in/sm/Nd but a small

U8 Precambrian Geology of Lake Superior Region
variation in Nd model age (fig. 5A, table 1), implying that 

the Nd model age is a reasonable estimate of the mantle 

extraction age. For reference and discussion, published 

data from igneous rocks of the Wisconsin magmatic terranes from Wisconsin and Minnesota (Horan and others, 

1987; Spencer, 1987; Barovich and others, 1989) are plotted in figure 5D, and data from sedimentary rocks (Miller 

and O'Nions, 1985; Stille and Clauer, 1986; Jacobsen and 

Pimentel-Klose, 1988; Barovich and others, 1989; Hemming and others, 1995) of the Animikie basin are plotted 

in figure 5F. One sample of the Rabbit Lake Formation 

(fig. 5E, table 2) has a substantial Early Proterozoic component, opening the possibility that all samples could be 

mixtures between Early Proterozoic and Archean sources 

with crustal histories older than the -2.9 Ga segments of 

the Superior province. Regardless, these samples have a 

large Archean source contribution, and are distinct from 

nearby upper Animikie Group sedimentary rocks (fig. 5F), 

from most of the Little Falls Formation, and from the 

Denham Formation sample from the Moose Lake-Glen 

Township terrane.

DISCUSSION 

Archean Foreland

Although the McGrath Gneiss is probably allochthonous, having been transported northward an unknown distance on the south-dipping Malmo structure (fig. 6; written 

commun. from D.L. Southwick concerning magnetotelluric data from R. Wunderman), it has similar Nd

Figure 4. Generalized fsm/Nd versus e^d plot from Shirey 

(1984). Axes with arrows show the types of trajectories rocks 

will follow at their time of crystallization from a homogeneous 

source with £[sid=0 (vertical), and direction of change in e^ 

with time in light rare earth element enriched and depleted 

reservoirs (horizontal). Radiating lines show the magnitude of 

evolution of e^d with time.

systematics to the Ortonville Granite, and both make up 

part of the Archean foreland (fig. 5A, B) on which the 

Penokean orogen developed (Southwick and others, 1988; 

D.L. Southwick, written commun., 1994). The Ortonville 

Granite and BPL cores 2, 4, and 6 are all in the Benson 

block (fig. 2); however, in its Nd isotopic systematics (fig. 

5A) the Ortonville Granite, whose TDM values range from 

3.0 to 3.1 Ga, is distinct from the other granites, in which 

TDM ranges from 2.7 to 2.9 Ga.

Although three samples of the Ortonville Granite 

(fig. 2) and three samples of the McGrath Gneiss (TDM °f 

3.0-3.3 Ga; table 1) have Nd isotopic compositions that 

are slightly more evolved than most Superior province 

rocks (fig. 5A), their eNd (1.8 Ga) values are not nearly so 

negative as they would be if they had acquired their light 

rare earth element enriched character at 3.5 Ga. Additionally, one of the more evolved samples of the Giants Range 

Granite (fig. 3) has identical Nd systematics (table 1; fig. 

5A). Thus, the Benson block is not clearly distinguishable 

from the Superior province on the basis of Nd isotopic 

composition alone. Our limited results are consistent with 

the petrologic observations by Southwick and others 

(1989) that the 3.5 Ga rocks found in other parts of the 

Minnesota River Valley may not be present in the Benson 

block.

Granitic rocks from cores BPL 2, 4, and 6 within 

the Benson block (fig. 2) have Nd isotopic compositions 

within the field of normal Superior province compositions 

(fig. 5A). If geochronology of the granites in these cores 

confirms Late Archean crystallization ages, they represent 

Late Archean crustal additions like much of the Superior 

province. Alternatively, given their geographic location 

between older Archean continental crust represented by 

the Ortonville Granite and Early Proterozoic continental 

crust across the Malmo discontinuity (fig. 2; fig. 5A, D), 

these granites could have crystallized in the Early Proterozoic, and their Nd systematics could be a result of mixing 

between these two end members.

Pre-Animikie Metasedimentary Rocks

Quartzofeldspathic units of the Mille Lacs Group, 

which overlies the McGrath Gneiss (fig. 3; fig. 5C, samples SRH 7, table 2), have Nd systematics indistinguishable from those of the McGrath Gneiss itself. Within the 

same series of outcrops, calc-silicate and amphibolitic 

rocks have high /sm/Nd and also nave Archean Nd model 

ages of 2.57 and 2.67 Ga. In contrast to samples from the 

eastern part of the McGrath-Little Falls terrane (terrane 

designation of Southwick and Morey, 1991; location 7, 

fig. 3; table 2), a phyllite within the Moose Lake-Glen 

Township terrane (location 5, fig. 3; table 2), also assigned 

to the Mille Lacs Group, has a TDM °f 2.07 Ga.

Precambrian Crustal Blocks, Minnesota; Neodymium Isotope Evidence U9
0.1

0

-0.1

-0.2

 u

1 -0.3

W>

^

-0.4

-0.5

-0.6

-0.7

_ EXPLANATION

 BPL cores in Benson block

^ Ortonville Granite 

O McGrath Gneiss

O APL core in Montevideo block

 A Giants Range Granite

O8 ..,,

gjBPtMb

 j|BPL;4a

_ 8bO^G37 , CAPL20

 OG14^ l<BPL2a

~ *BPL2b

II

" 

!

0.1

0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

_ EXPLANARON

 Minnesota

H Wisconsin

~

~D

jt 

if

m

m -

Bl 

 !1BEP

m

r BJT m

--

,

-25 -20 -15 -10 -5 0 5 10 -25 -20 -15 -10 -5051

0.1

0

-0.1

-0.2

Z -0.3

y^"*%5 -0.4

-0.5

-0.6

-0.7

1 1 1

_

 O BPL OBP

B \ *1 1

'

_

2 

L11

_

I

0.1

0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

_ EXPLANATION

D Rabbit Lake

 Formation ; 

 Mahnomen "

 Formation

_

Jl r

Kiiii

M7?t|):; ,v, 71 o

M753.8JP Jt N774.8 

_ Hfip:S " CWko

~E

'

»

 

 

 

__

 

~

__

|

-25 -20 -15 -10 -50 5 10 -25 -20 -15 -10 -5051

0.1

0

-0.1

-0.2

2 -O-3

\

g

+^ ~°- 4

-0.5

-0.6

-0.7

1 1

_ EXPLANATION

0 Little Falls Formation

H st-gt-bi schist, BPL 10

 Denham Formation

3,j~J ^

'

" ;; mjr' n

WL10a/gP 5a

- j r Jmi,2,4 -

~~ " (2,197±39Ma) ~

BPLlOb

c T i

0.1

0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

\ _ EXPLANATION

 Upper Animikie or

 Baragci Group 

O Iron-formation :

^ Lower Animikie or *' ;

 Menominee Group

_

~F

1

r

V

_

* -y~\ ^-^1 ril

J^^^O^CrB ~~ "^ ^<fm

| -

-25 -20 -15 -10 -5 0 

eNd1.8Ga

10 -25 -20 -15 -10 -5

£Nd1.8Ga

05 10

LJ10 Precambrian Geology of Lake Superior Region
Figure 5 (facing page). Plots of /Sm/Nd versus e^d for samples discussed in this report. Shaded area, field of Superior 

province data from southern Ontario and northern Minnesota 

(Shirey and Hanson, 1986; Stern and Hanson, 1992). A, Plutonic samples from the Archean foreland and McCrath Gneiss 

(figs. 2, 3). Samples of the McGrath Gneiss have similar Nd 

isotopic systematics to the Ortonville Granite. (Numbers without prefixes, see SRH group, table 1.) B, Plutons east of Malmo 

discontinuity have Early Proterozoic model ages. C, Mille Lacs 

Group. Amphibolite and calc-silicate samples collected near 

the McGrath Gneiss have Late Archean model ages, and plot 

within Superior province field. Samples of the Little Falls Formation, including core samples from BPL 10, are dominated 

by Early Proterozoic sources, except one sample (3) that has 

systematics similar to the calc-silicates and amphibolite samples of the Denham area (SRH 7, table 2). (Numbers without 

prefixes, see SRH group, table 2.) Also shown for reference is a 

line (black) representing the data of Beck (1987) from the Glen 

Township Formation basalts (GTB (2,197±39 Ma)); st-gt-bi 

schist, staurolite-garnet-biotite schist. D, Published data from 

Wisconsin magmatic terranes in Wisconsin (Barovich and others, 1989) and Minnesota (Horan and others, 1987; Spencer, 

1987). E, Data from the North Range Group of the Cuyuna district. These samples mostly have compositions consistent with 

a solely Superior province (or Benson block) derivation. F, 

Published data from the main bowl of the Animikie basin. 

Data from the Marquette Range Supergroup are from Barovich 

and others (1989); Pokegama Quartzite from Hemming and 

others (1994); Biwabik Iron-formation from Miller and 

O'Nions (1985) and Hemming and others (1995). Data from 

the Gunflint Iron-formation are from Stille and Clauer (1986), 

Jacobsen and Pimentel-Klose (1988), and Hemming and others 

(1992); Negaunee Iron-formation from Miller and O'Nions 

(1985); Rove, Tyler, and Virginia Formations from Hemming 

and others (1992, 1995).

Additionally, phyllitic samples from the Little Falls Formation at several locations in the western parts of the 

McGrath-Little Falls terrane (locations ]-4, fig. 3; table 

2), with TDM values of 2.14 to 2.18 Ga (one is 2.85, with 

very high 147Sm/144Nd), were derived from a dominantly 

Early Proterozoic source. To the southwest of Little Falls 

(fig. 2), two samples of staurolite schist from core BPL 

10, TDM=2.26 and 2.36 Ga, also lie within the range of 

Early Proterozoic rocks of the Wisconsin magmatic terranes (fig. 5C, D), and may record mixing between Early 

Proterozoic and Archean sources.

Metasedimentary rocks from the Early Proterozoic 

North Range Group of the Cuyuna North range (figs. 3, 

5E) record a substantial Archean component in their 

sources. The data permit that these sedimentary rocks are 

dominated by a ~2.9 Ga mantle-derived source; however, 

one sample of the Rabbit Lake Formation (TrjM-2-45 Ga) 

probably records mixing of an Early Proterozoic source. 

Thus, mixing between Early Proterozoic and Middle

Archean sources is also allowed by the data. Additionally, 

evidence of Early Proterozoic volcanic and hydrothermal 

fumarolic activity is recorded in the Trommald Formation 

(McSwiggen and others, 1992; Morey and others, 1992). 

Regardless of which scenario is correct, the rocks of the 

North Range Group record a substantially different source 

characteristic than do rocks of the upper Animikie Group 

(figs. 3, 5F). The Mahnomen and Rabbit Lake Formations, TDM=2-6-3.0 Ga, have Nd isotope character similar 

to those of the Pokegama Quartzite (Hemming and others, 

1994) and lower parts of the Marquette Range Supergroup 

(Barovich and others, 1989). These data are consistent 

with the new stratigraphic/structural hierarchy proposed by 

Southwick and others (1988) and Southwick and Morey 

(1991).

Plutons East of the Malmo Discontinuity

As mapped by Southwick and Morey (1991), the 

Malmo discontinuity (figs. 2, 3) traces a sharp southward 

bend in the Penokean orogen, and separates basement of 

Archean age in the Minnesota River Valley region from 

the McGrath Gneiss dome and the Early Proterozoic province of central Minnesota, which is most likely related to 

the Wisconsin magmatic terranes. Our results from SmNd analysis of core samples are consistent with this mapping (fig. 5B). Granite samples from localities immediately west of the Malmo discontinuity (fig. 2, BPL 2, BPL 

4) have TDM values of 2.62-2.93 Ga. Two granites (BPL 

11, BPL 12) and two samples of metasedimentary rocks 

(BPL lOa and b) from east of the discontinuity have TDM 

from 2.28 to 2.38 Ga, similar to results from granitic and 

mafic igneous rocks in the St. Cloud area (Horan and others, 1987; Spencer, 1987). One sample (BPL 13), which 

was collected approximately on the boundary, has a TFJM 

of 2.93 Ga. Thus, there appears to be a rather abrupt 

change in the crust formation age that coincides closely 

with this geophysically mapped boundary. In the area of 

figure 3, the McGrath Gneiss, which is inferred to have 

been part of the Archean foreland, has TDM °f 3.11-3.29 

Ga, identical to those from the Ortonville Granite. However, a large embayment of Early Proterozoic crust lies 

between the McGrath Gneiss dome and the Benson block, 

which includes the granitic and mafic igneous rocks of the 

St. Cloud area (Horan and others, 1987; Spencer, 1987), 

core samples from just east of the Malmo discontinuity 

(fig. 2), and possibly the Hillman Migmatite (unpublished 

U-Pb zircon age from S.S. Goldich).

The granitic rocks in the eastern part of the Benson 

block (BPL 2, BPL 4, fig. 2) could be Late Archean

Precambrian Crustal Blocks, Minnesota; Neodymium Isotope Evidence U11
granites with mantle-like Nd isotopic compositions at their 

time of crystallization, such as seen for much of the Superior province. Alternatively, they could be Early Proterozoic granites with a large Archean crustal component in 

their sources. They are close to the Malmo discontinuity, 

and they have a younger range of Nd model ages than 

recorded in the Ortonville Granite or McGrath Gneiss 

(TDM=2.74-2.92 versus 3.01-3.29 Ga). Full understanding of the nature of interaction between the Archean crust 

of the Minnesota River Valley terranes and the Penokean 

orogen must be preceded by dating of these granitic rocks.

SUMMARY: CRUSTAL BLOCKS OF THE 

PENOKEAN OROGEN

Neodymium model ages are shown on a schematic 

north-south representation of the Penokean orogen, modified from Morey and Southwick (1993) (fig. 6). The Little Falls Formation of the McGrath-Little Falls terrane of 

Southwick and Morey (1991) is grouped with the 

Penokean magmatic terrane rather than with the McGrath 

Gneiss. This grouping is supported by the Nd model ages 

in the Little Falls Formation (mostly Early Proterozoic; 

table 2) and by aeromagnetic patterns.

More samples should be analyzed to better understand the complicated framework of the Penokean orogen. 

Specific targets should be samples of (1) Hillman Migmatite (H, fig. 3), and (2) unnamed metasedimentary rocks of

the Cuyuna South range terrane, Mille Lacs Group rocks 

of the Cuyuna North range terrane, and metasedimentary 

rocks of the Emily district. Knowing the isotopic compositions of some of the potentially correlative units within 

the Long Prairie basin and Nimrod outlier, interpreted to 

be isolated remnants of strata in the Animikie basin 

(Southwick and others, 1988) (fig. 3), would also be valuable.

We can make some general conclusions concerning 

the relationships between structural boundaries and crustal 

ages in central Minnesota. The Penokean orogen (fig. 6) 

from south to north consists of:

1. An Early Proterozoic magmatic terrane that was 

derived from a depleted mantle source with some Archean 

crustal contamination (Horan and others, 1987; Spencer, 

1987). Metasedimentary rocks within this terrane have 

dominantly Early Proterozoic sources.

2. A block or blocks of Archean crust represented 

by the McGrath Gneiss and the Benson block. Metasedimentary rocks within this terrane have Archean model 

ages.

3. A foreland fold-and-thrust belt. Metasedimentary (SRH 5a, this study) and metavolcanic (Beck, 1987) 

rocks within the Moose Lake-Glen Township terrane of 

the fold-and-thrust belt have Early Proterozoic Nd model 

ages, whereas metasedimentary rocks in the North Range 

Group of the Cuyuna district have Archean model ages.

Within the main bowl of the Animikie basin, a progression in sources is recorded, from lower to higher 

stratigraphic levels and from north to south (Hemming and

 PENOKEAN OROGEN

-ANIMIKIE BASIN

Mesabi Emily

[Virginia Fm. (1.86-2.32 Ga)

Biwabik Iron-fm. (2.5-2.8 Ga) 

[Pokegama Qtz. (2.9 Ga) -FOLD-AND-THRUST BELTSuperior province 

(2.8-3.0 Ga)

Cuyuna North 

range

| Rabbit Lake Fm. (2.5-3.0 Ga) 

Trommald Fm. and equiv. 

iMahnomen Fm. (2.6-3.0 Ga)qj^l z^o

Serpent Lake discont.

VAv'WvVvVWv'VWvAv'x

>Cuyuna South range>

 /;. Moose Lake-Glen Twn.:;-v

|-jn unnnamed 

f f» argillite

rg unnamed 

" -r-r-r graphite sc

Glen Township Fm. 

(-2.2 Ga, Beck, 1987)

Denham Fm. (2.1 Ga)

hist"

M"

-71 %

m = ^

-OROGENICCOREArchean foreland V//////////////////////A

Denham Fm. 

(2.6-3.1 Ga)

EXPLANATION

Unconformity

Thrust fault that bounds 

lithologic unit

Archean rocks of 

cratonic foreland basement unknown

'^mmmmmzm

^ McGrath Gneiss and ^

Ortonville Granite

(3.0-3.3 Ga)

Penokean 

imagmatic terrane

Little Falls Fm. b^ -£ g 

(2.1-2.3 Ga) ^ =5

Ric 

Gneisses and c,.~...^~

St. Cloud area 

|<2.1-2.3 Ga. Spencer. 19871|

Figure 6. Depleted mantle model ages for Nd in structural-stratigraphic units. Modified from Morey and Southwick (1993). 

U12 Precambrian Geology of Lake Superior Region
others, 1995). Stratigraphically lower units within the 

Animikie basin, including the Pokegama Quartzite (Hemming and others, 1994, 1995) and the lower Marquette 

Range Supergroup (in Michigan) (Barovich and others, 

1989) have Archean model ages. Iron-formations, located 

at intermediate stratigraphic positions, have a range of Nd 

model ages indicating mixing between Archean and Early 

Proterozoic Nd sources. Most of the younger units record 

Early Proterozoic model ages (Hemming and others, 1995) 

including Virginia, Thomson, Tyler, Rove, and Michigamme Formations. Exceptions can be found. (1) Some 

samples of the Rove Formation, which is located in the 

northern part of the presently preserved Animikie basin, 

have a TDM of 2.55 Ga (Hemming and others, 1995); and 

(2) some northern samples of the Michigamme Formation 

in Michigan have TDM from 2.70 to 3.09 Ga (Barovich 

and others, 1989).

The isotopic data presented here, gleaned from 

diverse blocks and assemblages, are consistent with current geology- and geophysics-based interpretations of central and southwestern Minnesota. The coincidence of the 

geophysical boundaries with rocks having substantial differences in Nd isotopic characteristics provides important 

support for both approaches, and supports the presence of 

important discontinuities between some of these crustal 

blocks. The Nd isotopic compositions and model ages add 

useful constraints on the crustal history of the various 

structurally and Stratigraphically defined units. Many data 

are necessary in such a complex region, and more Nd isotopic mapping in conjunction with additional geologic and 

geophysical mapping could permit close comparisons of 

this region with specific aspects of the Early Proterozoic 

evolution of the midcontinent of North America.

REFERENCES CITED

Arth, J.G., and Hanson, G.N., 1975, Geochemistry and origin of the 

early Precambrian crust of northeastern Minnesota: 

Geochimica et Cosmochimica Acta, v. 39, p. 325-362.

Barovich, K.M., Patchett. P.J., Peterman, Z.E., and Sims, P.K., 

1989, Nd isotopes and the origin of 1.9-1.7 Ga Penokean 

continental crust of the Lake Superior region: Geological 

Society of America Bulletin, v. 101, p. 333-338.

Beck, J.W., 1987, Implications for Early Proterozoic tectonics and 

the origin of continental flood basalts, based on combined 

trace element and neodymium/strontium isotopic studies of 

mafic igneous rocks of the Penokean Lake Superior belt, 

Minnesota, Wisconsin and Michigan: Minneapolis, Minn., 

University of Minnesota Ph. D. thesis, 262 p.

Bennett, V.C., and DePaolo, D.J.. 1987, Proterozoic crustal history 

of the western United States as determined by neodymium 

isotopic mapping: Geological Society of America Bulletin, 

v. 99, p. 674-685.

Brown, B.A., and Maass, R.S., 1992, Early Proterozoic magmatism 

and tectonism related to southward-dipping subduction and

microcontinental accretion in central Wisconsin: Geological 

Society of America Abstracts with Programs, v. 24, p. 178.

Cannon, W.F., 1973, The Penokean orogeny in northern Michigan, 

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Goldich, S.S., and Wooden, J.L., 1980, Origin of the Morton 

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 1995, Geochemical and Nd/Pb isotopic evidence for the 

provenance of the Early Proterozoic Virginia Formation,

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 1988, United plates of America, the birth of a craton Early 

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Koran, M.F., Hanson, G.N., and Spencer, K.J, 1987, Pb and Nd isotope and trace element constraints on the origin of basic 

rocks in an early Proterozoic igneous complex, Minnesota: 

Precambrian Research, v. 37, p. 323-342.

Jacobsen, S.B., and Pimentel-Klose, M.R., 1988, Nd isotopic variations in Precambrian banded iron formations: Geophysical 

Research Letters, v. 15, p. 393-396.

Krogh, T.E., and Davis, G.L., 1971, Zircon U-Pb ages of Archean 

metavolcanic rocks in the Canadian Shield: Carnegie Institute of Washington Yearbook 73, p. 560-563.

LaBerge, G.L., and Myers, P.E., 1984, Two Early Proterozoic successions in central Wisconsin and their tectonic significance: 

Geological Society of America Bulletin, v. 95, p. 246-253.

Maass, R.S., and Brown, B.A., 1993, Early Proterozoic magmatism 

and tectonism related to southward-dipping subduction and 

microcontinental accretion in central Wisconsin [abs.]: 39th 

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McCulloch, M.T., 1980, Part I, Samarium-neodymium and rubidium-strontium chronology of crustal formation; Part II, Barium, neodymium and samarium: Berkeley, Calif., California 

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McCulloch, M.T., and Wasserburg, G.J., 1976, Sm-Nd and Rb-Sr 

chronology of continental crust formation: Science, v. 200, 

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McLennan, S.M., Hemming, S.R., Taylor, S.R., and Eriksson, 

K.A., in press, Early Proterozoic crustal evolution Geochemical and Nd-Pb isotopic evidence from 

metasedimentary rocks, southwestern North America: 

Geochimica et Cosmochimica Acta.

McSwiggen, P.L., Morey, G.B., and Cleland, J.M., 1992, Acmite in 

the Trommald Formation of the Cuyuna Iron Range Revisited [abs.]: 38th Annual Institute on Lake Superior Geology, 

Hurley, Wisconsin, Proceedings, v. 38, pt. 1, p. 61-62.

Miller, R.G., and O'Nions, R.K., 1985, Source of Precambrian 

chemical and clastic sediments: Nature, v. 314, p. 325-329.

Morey, G.B., 1967, Stratigraphy and sedimentology of the middle 

Precambrian Rove Formation in northeastern Minnesota: 

Journal of Sedimentary Petrology, v. 37, p. 1154-1162.

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Morey, G.B., McSwiggen, P.L., and Cleland, J.M., 1992, Evidence 

of an exhalative contribution to the manganese mineralogy of 

the Trommald Formation, Cuyuna Iron Range, east-central

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Morey, G.B., and Sims, P.K., 1976, Boundary between two Precambrian W terranes in Minnesota and its geologic significance: Geological Society of America Bulletin, v. 84, p. 

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Morey, G.B., and Southwick, D.L., 1993, Stratigraphic and sedimentological factors controlling the distribution of epigenetic manganese deposits in iron-formation of the Emily district, 

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Geology, v. 88, p. 104-122.

Morey, G.B., and Van Schmus, W.R., 1988, Correlation of Precambrian rocks of the Lake Superior region, United States: U.S. 

Geological Survey Professional Paper 1241-F, 31 p.

Nelson, B.K., and DePaolo, D.J., 1985, Rapid production of continental crust 1.7 to 1.9 b.y. ago Nd isotopic evidence from 

the basement of the North American mid-continent: Geological Society of America Bulletin, v. 96, p. 746-754.

Schmidt, R.G., 1963, Geology and ore deposits of the Cuyuna 

North range, Minnesota: U.S. Geological Survey Professional Paper 407, 96 p.

Shirey, S.B., 1984, The origin of Archean crust in the Rainy Lake 

area, Ontario: Stony Brook, N.Y., State University of New 

York at Stony Brook Ph. D. thesis, 393 p.

Shirey, S.B., and Hanson, G.N., 1986, Mantle heterogeneity and 

crustal recycling in Archean granite-greenstone belts Evidence from Nd isotopes and trace elements in the Rainy Lake 

area, Superior Province, Ontario, Canada: Geochimica et 

Cosmochimica Acta, v. 50, p. 2631-2651.

Sims, P.K., 1987, Metallogeny of Archean and Proterozoic terranes 

in the Great Lakes region A brief overview: U.S. Geological Survey Bulletin 1694, p. 56-74.

 1990, Precambrian basement map of the northern midcontinent, U.S.A.: U.S. Geological Survey Miscellaneous Investigations Series Map I-1853-A, scale 1:1,000,000, with 

pamphlet, 9 p.

Sims, P.K., Card, K.D., Morey, G.B., and Peterman, Z.E., 1980, 

The Great Lakes tectonic zone A major crustal structure in 

central North America: Geological Society of America Bulletin, v. 91, p. 690-698.

Sims, P.K., and Day, W.C., 1993, The Great Lakes tectonic 

zone Revisited: U.S. Geological Survey Bulletin 1904-S, 

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Sims, P.K., and Peterman, Z.E., 1983, Evolution of Penokean foldbelt, Lake Superior region, and its tectonic environment, in 

Medaris, L.G., Jr., ed., Early Proterozoic geology of the 

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 1986, The Early Proterozoic Central Plains orogen A 

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Sims, P.K., Van Schmus, W.R., Schulz, K.J., and Peterman, Z.E., 

1989, Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean orogen: 

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U14 Precambrian Geology of Lake Superior Region
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Southwick, D.L., 1993, Possible suture origin for a low-grade mafic 

belt within the Minnesota River Valley subprovince 

(Archean), SW Minnesota: Geological Society of America 

Abstracts with Programs, v. 25, p. 235.

Southwick, D.L., and Morey, G.B., 1991, Tectonic imbrication and 

foredeep development in the Penokean orogen, east-central 

Minnesota An interpretation based on regional geophysics 

and the results of test-drilling: U.S. Geological Survey Bulletin 1904-C, 17 p.

Southwick, D.L., Morey, G.B., and McSwiggen, P., 1988, Geological map (scale 1:250,000) of the Penokean orogen, central 

and eastern Minnesota, and accompanying text: Minnesota 

Geological Survey Report of Investigations RI-37.

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Archean terranes in SW Minnesota The old gray gneiss she 

ain't what she used to be [abs.]: 35th Annual Institute on 

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lithologic and stratigraphic results, 1987-1988, and some

preliminary geological conclusions: Minnesota Geological 

Survey Information Circular 31, 98 p.

Spencer, K.J., 1987, Isotopic, major and trace element constraints 

on the sources of granites in an 1800 Ma igneous complex 

near St. Cloud, Minnesota: Stony Brook, N.Y., State University of New York at Stony Brook Ph. D. thesis, 249 p.

Stern, R.A., and Hanson, G.N., 1991, Archean high-Mg granodiorite A derivative of light rare earth-enriched monzodiorite 

of mantle origin: Journal of Petrology, v. 32, p. 201-238.

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Province, Canada Rare earth element and Nd-Sr isotopic 

evidence: Contributions to Mineralogy and Petrology, 

v.l 11, p. 515-526.

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Published in the Central Region, Denver Colorado 

Manuscript approved for publication April 28, 1995 

Graphics and Photocomposition by Mari L. Kauffmann

Precambrian Crustal Blocks, Minnesota; Neodymium Isotope Evidence U15

U.S. GOVERNMENT PRINTING OFFICE: 1995 - 673-046 / 20002 REGION NO.

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