Evidence for Continental Crustal Assimilation in the 

Hemlock Formation Flood Basalts of the 

Early Proterozoic Penokean Orogen, 

Lake Superior Region

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Ave.
Chapter I

Evidence for Continental Crystal Assimilation in the 

Hemlock Formation Flood Basalts of the 

Early Proterozoic Penokean Orogen, 

Lake Superior Region

By WARREN BECK and V. RAMA MURTHY

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 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government.

UNITED STATES GOVERNMENT PRINTING OFFICE: 1991

For sale by the

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Library of Congress Cataloging-in-Publication Data

Beck, J. Warren.

Evidence for continental crustal assimilation in the hemlock formation flood 

basalts of the Early Proterozoic Penokean orogen, Lake Superior region / by 

Warren Beck and V. Rama Murthy.

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

ch. !) (U.S. Geological Survey bulletin ; 1904-1)

Includes bibliographical references.

1. Rock deformation Superior, Lake, Region. 2. Geology Stratigraphic 

Proterozoic. 3. Basalt Superior, Lake, Region. 4. Geology, Structural 

Superior, Lake, Region. I. Murthy, V. Rama. II. Title . III. Series. 

IV. Series: U.S. Geological Survey bulletin ; 1904-1. 

QE75.B9 no. 1904-1. 

[QE604] 

557.3 s dc20

[551.7'15'097749] 91-9934

CIP
CONTENTS

Abstract II

Introduction II

Acknowledgments 14

Analytical methods 14

Geochemistry of the Hemlock and Quinnesec Formations 15

Isotopic and trace element data 15

Mixing relationships 19

Assimilation-fractional crystallization model calculations compared with observed

chemistries 114 

Conclusions 118 

References cited 119

FIGURES

1. Simplified geologic map of the Lake Superior region and of segments of the 

Penokean orogen 12

2. Plots comparing chondrite-normalized REE patterns of the Quinnesec and 

Hemlock Formations with typical volcanic rock types from various modem 

and ancient sources 16

3. Rb/Sr errorchron for the Hemlock volcanic rocks 17

4. Rb/Sr errorchron for the Quinnesec volcanic rocks 17

5. Diagram showing present-day Sm/Nd isotopic systematics for the Quinnesec 

and Hemlock volcanic rocks 18

6. 143Nd/144Nd isotopic evolution diagram, relating Quinnesec to MORE and 

Bulk Earth (CHUR) 18

7. Plot of present-day 143Nd/144Nd versus 147Sm/144Nd for the Hemlock volcanic 

rocks 18

8. Plot of Sm/Nd isotopic data for the Quinnesec and Hemlock volcanic rocks at 

approximately 1.87 Ga 18

9. Plots of initial £Nd (approximately 1.87 Ga) versus Mg*, Hf, La, Cr, Th, 

Rb/Sr, and K2O 110

10. Plot comparing chondrite-normalized REE patterns of the Hemlock basaltic 

component and MORE 112

11. Plot comparing chondrite-normalized HFS elements and REE patterns of the 

Hemlock crustal component and several crustal reservoirs 115

12. Plot comparing chondrite-normalized REE patterns of the Hemlock crustal 

component and granites of the Isle of Skye 116

13. Plot comparing Cr and Ni composition of the Hemlock Formation basaltic

and crustal components and several crustal and mantle reservoirs 116 

14-20. Plots comparing chemical compositions calculated from assimilationfractional crystallization model calculations with those of the Hemlock 

Formation:

14. Mg* plotted against V for six configurations of TRACE.FOR 117

15. Mg* plotted against K2O for six configurations of TRACE.FOR 118

16. Mg* plotted against Nb for six configurations of TRACE.FOR 119

Contents III
17. La plotted against £Nd for six configurations of TRACE.FOR/ 

ISOTOPE 120

18. Mg* plotted against £Nd for six configurations of TRACE.FOR/ 

ISOTOPE 121

19. SiO2 plotted against eNd for six configurations of TRACE.FOR/ 

ISOTOPE 122

20. K2O plotted against eNd for six configurations of TRACE.FOR/ 

ISOTOPE 123

TABLES

1. Rb-Sr data for the Hemlock and Quinnesec Formations 17

2. Sm-Nd data for the Hemlock and Quinnesec Formations 19

3. Geochemical comparison among the theoretical basaltic end-member, MORE, 

Keweenawan tholeiites, island arc basalts, and ocean island basalts 113

4. Geochemical comparison between the theoretical crustal end-member and several 

representative crustal compositions 114

IV Contents
CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

Evidence for Continental Crustal Assimilation in the 

Hemlock Formation Flood Basalts of the 

Early Proterozoic Penokean Orogen, 

Lake Superior Region

By Warren Beck1 and V. Rama Murthy1

Abstract

Two major suites of mafic volcanic and intrusive rocks 

are juxtaposed across the Niagara fault zone within the central 

part of the Early Proterozoic Penokean orogen, in the Lake 

Superior region. North of the fault zone, the volcanic rocks 

overlie Archean crust, and possibly were erupted during a 

major Early Proterozoic (rifting) event =2.0 Ga. The volcanic 

and intrusive rocks south of the fault zone, however, are 

largely juvenile crust emplaced during the Early Proterozoic. 

This area of crust comprises the Wisconsin magmatic terranes, 

and taken together it resembles a modern volcanic arc.

Neodymium isotopic systematics from Quinnesec Formation tholeiites in the northern part of the Wisconsin 

magmatic terranes yield an isochron age of 1.87±0.05 Ca, and 

an eNd=+4.2. Such a large positive eNc| is indicative of 

derivation from a mantle with long-term depletion in light rare 

earth elements, and is similar to values predicted by some 

models for the source regions for mid-ocean ridge basalts at 

1.9 Ca. This characteristic, combined with the general geochemical characteristics and tectonic setting of these magmatic terrane tholeiites, suggests that these rocks may have 

been erupted onto oceanic crust and derived from an Early 

Proterozoic depleted mantle reservoir. The overall tectonic 

affinity of the Wisconsin magmatic terranes, however, appears 

to be arc related.

In contrast, the neodymium isotopic data of Hemlock 

Formation continental tholeiites from the continental margin 

terrane (north of the fault zone) do not form an isochron; 

rather, they indicate mixing of a juvenile basaltic component 

with a continental-crustal light rare earth element-enriched

Manuscript approved for publication January 28, 1991. 

Department of Geology and Geophysics, University of Minnesota, 

Minneapolis, MN 55455.

reservoir. Two-component assimilation-fractional crystallization models suggest that the source of the basaltic endmember of this mixing suite was the same as that which 

generated the light rare earth element-depleted basalts of the 

large magmatic belt south of the fault zone, for example, an 

Early Proterozoic depleted mantle mid-ocean ridge basalt 

reservoir. The continental crustal component of these basalts 

appears to have been enriched in many incompatible 

elements, and thus probably was not a lower crustal granulite, 

but rather may have been higher crustal level granite or 

sediments.

INTRODUCTION

The Penokean orogen in the Lake Superior region 

contains several major sequences of Early Proterozoic 

rocks. These include the Marquette Range Supergroup, the 

Animikie basin strata, and the Penokean magmatic terranes 

of Wisconsin and Minnesota (fig. 1). The Marquette Range 

Supergroup and Animikie basin rocks are temporally and 

stratigraphically similar, and are typified by southwardthickening packages of sedimentary rocks, interfingered 

with locally extensive continental flood basalt sequences. 

Although age control on the deposition of the three packages of rocks is poor, all were apparently deposited on or 

near the south margin of the Superior craton between 2.5 

and 1.86 Ga (Van Schmus, 1976; Beck, 1988).

Recent tectonic models of the Early Proterozoic 

evolution of the Lake Superior region have been posed in 

terms of one of the following processes: (1) plate margin 

sedimentation at and near the south margin of the Superior 

province, followed by the development of an active subduction zone and arc-related magmatism along this boundary 

(Van Schmus, 1976); or (2) either one period of continental

Hemlock Formation Flood Basalts, Lake Superior Region 11
92° 82°

EASTERN SEGMENT: 

HURONIAN SUPERGROUP480

WESTERN SEGMENT: 

ANIMIKIE'BASIN

NORTH

SOUTH 

DAKOTA

,_g

42°

MINNESOTA

EXPLANATION

Wisconsin magmatic terranes

Animikie Group, Mille Lacs Group, 

and Marquette Range Supergroup

Pre 2.3 Ga (Huronian)

CENTRAL SEGMENT: 

MARQUETTE GRANGE 

SUPERGROUP 

ANDWVISCONSIN 

MAGMATIC'TERRANES

200

I

400 KILOMETERS

_I

EXPLANATION

MIDDLE PROTEROZOIC 

Jacobsville Sandstone EARLY PROTEROZOIC

WISCONSIN MAGMATIC TERRANES 

Granitoid rocksGneiss and granitoid rocks 

Felsic metavolcanic rocks 

Mafic metavolcanic rocks

CONTINENTAL-MARGIN ASSEMBLAGE

Sedimentary and volcanic rocks of 

Marquette Range Supergroup

Gneiss (Archean)

Margeson 

Creek 

Gneiss

(Archean)

45 C 

45'

45° 

40'

R32W R31 W

87°55' 87°50'

D MICHIGAN

DICKINSON CO 

MENOMINEE CO

5 KILOMETERS

J
rifting (Cambray, 1977, 1978) or several (Larue, 1983; 

Larue and Sloss, 1980; Ueng and others, 1988; LaBerge and 

others, 1984; Schulz and others, in press), leading to the 

development of a passive margin and an oceanic basin, 

followed by ocean closure and the collision of island arc 

complexes with the Superior province during the Penokean 

orogeny. Other recent models interpret the Animikie basin 

strata and the upper strata of the Marquette Range Supergroup as having been deposited in a migrating foredeep 

basin related to the late-stage docking of the Penokean arc 

complex at the south margin of the Superior province 

(Hoffman, 1987; Southwick and others, 1988; Barovich and 

others, 1989).

The central part of the Lake Superior region can be 

divided into two distinctive Early Proterozoic terranes 

(fig. IB), separated by a major east-trending boundary 

the Niagara fault zone. This fault zone was a major crustal 

boundary during the Early Proterozoic (Larue, 1983; Greenberg and Brown, 1983; LaBerge and others, 1984; Larue 

and Ueng, 1985; Sims and others, 1985; Ueng and Larue, 

1988), and was the principal axis of deformation during the 

Penokean orogeny, representing the proposed frontal overthrust zone of the Penokean magmatic terranes (Sims and 

others, 1989; Klasner, 1984; LaBerge and others, 1984; 

Klasner and others, 1988).

North of the Niagara fault zone, in Michigan and 

Wisconsin, the Marquette Range Supergroup was deposited 

on Archean crust near the south margin of the Superior 

province and consists of supracrustal sedimentary rocks and 

continental flood basalts. A second, 250-km-wide terrane 

the Wisconsin magmatic terranes lies to the south of the 

Niagara fault zone; these magmatic terranes consist 

principally of submarine tholeiites, rhyolites, and calcalkaline volcanic rocks, together with extensive tonalitic, 

trondhjemitic, and granitic intrusive rocks (Van Schmus, 

1976; Sims and others, 1989). A few sedimentary rocks are 

also present in this southern terrane, but they appear to 

represent a minor component. The southern terrane strongly 

resembles a modern volcanic arc complex (Van Schmus, 

1976, 1980; Cudzillo, 1978; LaBerge and others, 1984; 

Sims and others, 1989), whereas the northern terrane 

resembles a continental rift, subsiding margin, and foredeep 

basin assemblage.

Figure 1 (facing page). Views of the Early Proterozoic Penokean orogen, Lake Superior region. A, Simplified geologic 

map showing locations of the Animikie and Mille Lacs Groups 

of Animikie basin (western segment of orogen), Marquette 

Range Supergroup (central segment), and Wisconsin magmatic 

terranes. Modified from Morey and others (1982). Also shown 

is the Huronian Supergroup (eastern segment). B, Enlarged 

view of central segment; boxes indicate locations of views C 

and D. C, Location of West Kiernan sill and adjacent rocks, 

showing sample localities of the Hemlock Formation. 

D, Sample localities for the Quinnesec Formation. Data for 

numbered sample localities are in tables 1 and 2.

Significant mafic volcanism in the central segment of 

the Lake Superior region began to develop about 1.90 Ga 

(Banks and Rebello, 1969; Banks and Van Schmus, 1971; 

Van Schmus, 1976; Beck, 1984). Major flood basalt 

sequences were erupted in the Marquette Range Supergroup 

during the Menominee Group period of deposition (Cannon, 

1986), but the most extensive volcanic sequences developed 

in the Wisconsin magmatic terranes south of the Niagara 

fault zone.

Sequences of related mafic volcanic and intrusive 

rocks of the northern terrane include the Hemlock 

Formation, Clarksburg Volcanics Member of the Michigamme Formation, Badwater Greenstone, and the Emperor 

Volcanic Complex, all of the Marquette Range Supergroup. 

Although the original extent of these metavolcanic complexes is difficult to estimate from the sketchy geologic 

record, taken together they probably formed a major Early 

Proterozoic continental flood basalt province. Each of these 

rock bodies is locally as much as 5-10 km thick, but they 

lack great lateral extent. Whether these metavolcanic rocks 

are the erosional remnants of a much greater expanse of 

flood basalts or were deposited as local accumulations is not 

clear. Numerous large layered mafic intrusions occur within 

the Hemlock Formation, which are thought to be genetically 

related to the volcanic rocks (Ueng and others, 1988; Gair 

and Wier, 1956; Bayley, 1959). One of the largest of 

these the West Kiernan sill (fig. 1C) has well-developed 

igneous layering (Gair and Wier, 1956; Wier, 1967) 

throughout its approximately 2 km thickness. For this 

report, the West Kiernan sill, together with the Hemlock 

metabasalt, was sampled for geochemical analyses representative of the northern terrane.

The Early Proterozoic metavolcanic rocks south of 

the Niagara fault zone underlie a large area of northern 

Wisconsin (Bayley and others, 1966; Jenkins, 1973; Greenberg and Brown, 1983; Sims and others, 1989). These rocks 

form the southern magmatic belt, comprising the Wisconsin 

magmatic terranes. A widespread volcanic body, the 

Quinnesec Formation in eastern Florence and northern 

Marinette Counties, Wisconsin (fig. ID), consists primarily 

of metamorphosed pillowed basalt flows with minor pyroclastic rocks; it spans a broad compositional range from 

tholeiitic through calc-alkaline to rhyolitic. Rare occurrences of metasedimentary rocks occur within the Quinnesec Formation, generally as thin lenses of graphitic slate, 

iron-formation, marble, quartzite, and metagray wacke (Button, 1971; Cummings, 1978; Greenberg and Brown, 1983; 

Sims, 1990). Overall thicknesses of these rocks, though 

difficult to estimate, probably exceed 3 km (Bayley and 

others, 1966). Structural thickening of these units, however, 

undoubtedly occurred during the Penokean orogeny.

The rocks of the Quinnesec Formation have been 

metamorphosed to greenschist facies, except adjacent to the 

Penokean plutons, where metamorphic grade may exceed

Hemlock Formation Flood Basalts, Lake Superior Region 13
hornblende-oligoclase or hornblende-andesine amphibolite 

(Bayley and others, 1966). Large areas of the northern 

continental-margin terrane have also been metamorphosed 

to lower or middle greenschist facies; however, rocks in the 

cores of the several large metamorphic nodes, such as the 

Republic or Watersmeet nodes, may reach sillimanite or 

kyanite grade (James, 1955).

Based on their chemical and isotopic compositions, 

most of the igneous rocks in the Wisconsin magmatic 

terranes appear to be juvenile crust erupted during the Early 

Proterozoic, although evidence exists for interaction with 

older Archean basement in the southernmost of these 

terranes, in central Wisconsin (Anderson and Cullers, 1987; 

Van Schmus and Anderson, 1977). The Quinnesec is the 

southern terrane formation sampled in this study to determine the nature and source of volcanic rocks in the 

Wisconsin magmatic terranes. Neodymium and strontium 

isotopic evidence from both mafic and felsic igneous rocks 

of the magmatic terranes supports these general conclusions; the data show depleted-mantle source signatures near 

the Niagara fault zone (Beck, 1984; Barovich, Patchett, and 

Peterman, 1987; Barovich and others, 1989) and a progressive southward decrease in £Nd values along a northsouth trend (Barovich and others, 1989).

In contrast, substantial chemical and isotopic 

evidence indicates involvement between the Early Proterozoic igneous rocks of the northern continental-margin terrane and older Archean crust. Initial Nd isotopic data from 

continental tholeiites of this terrane are indicative of mixing 

with Archean continental crust, whereas two-component 

AFC (assimilation-fractional crystallization) models (modified from Nielson, 1985, 1988; and DePaolo, 1981) suggest 

that the source of the basaltic end-member of this mixing 

suite was the same as that which generated the LREEdepleted basalts of the southern magmatic belt, for example, 

an Early Proterozoic depleted mantle (MORB) reservoir. 

The continental crustal component of these basalts appears 

to have been enriched in many incompatible elements, and 

thus was probably not a lower crustal granulite; instead, it 

may have been higher level granitic crust or sediments. The 

observation that these continental tholeiites appear to have 

been generated via crustal contamination of magmas 

derived from the depleted mantle is similar to that observed 

in several modern flood basalt provinces (for example, 

Carter and others, 1978; Carlson and others, 198la, b; 

Dickin, 1981; Mahoney and others, 1984), and thus indicates some long-term uniformity in the processes generating 

this type of igneous activity. The main objective of this 

report is to compare the geochemical signatures of the mafic 

volcanic and intrusive igneous rocks found in these two 

Early Proterozoic terranes, and to evaluate the differences 

in their genesis and history, emphasizing the Hemlock 

Formation.

ACKNOWLEDGMENTS

Financial support derived from the National Science 

Foundation grant # NSF EAR 7919590, the Minnesota 

Geological Survey, the University of Minnesota Department of Geology and Geophysics, and the Stanford 

University Department of Geology is all gratefully acknowledged. Special thanks go to David Southwick and G.B. 

Morey of the Minnesota Geological Survey; David Larue 

and Charles Ueng of Stanford University; Paul Sims, Klaus 

Schulz, Zell Peterman, and Sam Goldich of the U.S. 

Geological Survey; and Tom Fox and John Wilban of 

Michigan State University.

ANALYTICAL METHODS

Major element analyses for the Hemlock tholeiites 

and West Kiernan sill intrusive rocks were obtained by 

induced couple plasma (ICP) at Barringer Magenta 

Company Ltd., Toronto, Ontario; REE, Th, Ta, and Hf 

analyses were obtained by neutron activation at Michigan 

State University (Fox, 1983). Other trace elements, 

including Ba, Zr, Y, V, Cr, and Ni, were measured using 

X-ray fluorescence at Stanford University (Ueng and others, 

1988). Samples of the Quinnesec volcanic rocks and some 

trace element analyses were provided by K.J. Schulz, U.S. 

Geological Survey.

Neodymium and strontium isotopic measurements 

were made at the University of Minnesota Department of 

Geology and Geophysics, using a 10-cm double-focusing 

Mattuch-Hertzog design MINMASS solid source mass 

spectrometer (Nier and Schlutter, 1985; Nier and others, 

1983), and on a 12-in. NBS design Nier-type singlefocusing mass spectrometer. Elemental concentrations were 

determined by isotope dilution using 87Rb, 84Sr, and a 

combined 145Nd-149Sm spike tracers. All Sm and Nd 

measurements were obtained as a metal ion emission using 

a double Ta-Re filament geometry, with Nd and Sm loaded 

as a chloride on the Ta side filament. Both Sr and Rb were 

run as chlorides on single Re filaments, with the Sr 

filaments first being coated with a layer of Ta2O5 powder. 

Strontium isotopic composition analyses were corrected for 

in-run mass fractionation assuming a 86Sr/88Sr ratio of 

0.1194, whereas Nd isotopic composition analyses were 

corrected utilizing a 146Nd/144Nd ratio of 0.7219. Strontium 

analyses were normalized against NBS987 assuming a 

87Sr/86Sr ratio=0.71020, whereas Nd analyses were normalized against LJNd standard, assuming a 143Nd/144Nd 

ratio=0.511860. Mean 2a errors are reported at ± 0.000050 

for the Sr isotopic composition analyses and at ± 0.000030 

for the Nd isotopic analyses. All errors are reported with a 

2a confidence level. Decay constants of X=1.42xlO~ny" 1 

and 6.54xlO"12y"1 were used as for 87Rb and 147Sm, 

respectively.

14 Precambrian Geology of Lake Superior Region
Sample locations for rocks of the Hemlock and 

Quinnesec Formations are shown in figure 1C and ID, 

respectively. More detailed descriptions have been given by 

Beck (1988), Fox (1983), and Ueng and others (1988).

GEOCHEMISTRY OF THE HEMLOCK AND 

QUINNESEC FORMATIONS

The major and trace element characteristics of the 

volcanic rocks on opposite sides of the Niagara fault zone 

have been described by Bayley (1959), Cudzillo (1978), 

Greenberg and Brown (1983), Fox (1983), Schulz (1984), 

and Ueng and others (1988). The Hemlock volcanic rocks 

are almost exclusively bimodal basalts and rhyolites that 

contain several large consanguineous gabbroic intrusive 

bodies. In general, these volcanic rocks have tholeiitic to 

subalkaline AFM trends and relatively high concentrations 

of the LIL (large ion lithophile) elements. Compositionally, 

the Hemlock volcanics appear to be indistinguishable from 

other volcanic suites in the Marquette Range Supergroup, 

such as the Badwater Greenstone or Clarksburg Volcanics 

Member of the Michigamme Formation (Cudzillo, 1978).

The volcanic rocks of the southern magmatic terrane, 

in contrast, span a large compositional range from basaltic 

through andesite to rhyolite, but are dominantly calcalkaline in composition. The Quinnesec Formation is 

distinctive in this regard in that it is predominantly tholeiitic 

in composition and is strongly depleted in the LIL elements 

and HFS (high field strength) elements. These characteristics distinguish the Quinnesec volcanic rocks from the 

Hemlock Formation, and are consistent with the general 

findings of Cudzillo (1978), who demonstrated that overall, 

the basalts in the northern terrane show significantly higher 

concentrations of TiO2 and Zr as compared to those south of 

the Niagara fault zone. In Cenozoic basalts, these elements, 

together with the other HFS elements (Nb, Hf, and Ta), have 

proved to be useful geochemical discriminates for determining basalt tectonic provenance, particularly for discriminating between basalts erupted in convergent plate 

environments versus those found in divergent settings 

(Pearce and Cann, 1973; Pearce and Norry, 1979; Pearce, 

1982; Arculus, 1987). Using this modern analog, basalts 

erupted along convergent plate margins characteristically 

have lower TiO2 and Zr concentrations than basalts from 

divergent and intraplate settings (Pearce and Cann, 1973). 

In general, the concentrations of these elements in the 

Hemlock volcanic rocks are consistent with an intraplate 

origin, whereas the concentrations found in the Quinnesec 

volcanic rocks are consistent with a convergent margin 

origin (Cudzillo, 1978).

Among the most revealing tectonic discriminates 

between the two contrasting suites of volcanic rocks is their 

REE (rare earth element) patterns (fig. 2). The REE data 

from the Quinnesec basalts (Sims and others, 1989) have

average rockxhondrite abundances of about 10, flat heavy 

REE patterns, and no appreciable Eu anomalies; and they 

reveal depletion in the light REE. A comparison with REE 

patterns from various modern and ancient tectonic 

environments shows that the Quinnesec volcanics have 

patterns similar to modern island arc tholeiites or modern 

MORB, and patterns similar as well to those of some 

Archean basaltic komatiites and Archean magnesian basalts 

of Munro Township, Ontario. They are probably most 

similar to the average modern MORB pattern, or to those of 

abyssal shield tholeiites generated during the early stages of 

island arc genesis, which are known to have REE patterns 

strongly resembling MORB (Kay and Senechal, 1976; 

Pallister and Knight, 1981). The Quinnesec REE patterns 

are, however, distinctly dissimilar to those of any continental flood basalt suite, modern or ancient (fig. 2).

In contrast to the Quinnesec basalts, the Hemlock 

volcanic rocks have high average rockxhondrite REE 

abundances (20-50) and are typically light REE enriched. 

Their REE patterns are strikingly similar to those of 

intraplate volcanic rocks, particularly those of typical 

continental flood basalts, such as the Keweenawan basalts 

or the Deccan flood basalts (fig. 2). Thus, these REE 

profiles serve to distinguish between the Quinnesec and 

Hemlock Formations, as well as to characterize the 

Hemlock as being probably of continental flood basalt 

affinity.

ISOTOPIC AND TRACE ELEMENT DATA

Many studies have utilized Sr and Nd isotopic tracers 

in concert to help determine the petrogenesis of continental 

volcanic rocks (for example, Mahoney, 1984; Mahoney and 

others, 1981; Hawkesworth and Vollmer, 1979; Dosso, 

1984; Carlson and others, 1981a, b; Dickin, 1981; Carter 

and others, 1978; Moorbath and Thompson, 1980; Thompson and others, 1982; McDougall, 1976). For young 

volcanic rocks, such covariations often yield information 

about the nature of the source regions or about magma 

contamination processes. Unfortunately in our study, as is 

the case in many ancient terranes, the volcanic rocks of the 

northern continental-margin terrane have been subjected to 

postemplacement regional metamorphism. The bulk of the 

rocks in this belt have been metamorphosed to at least 

greenschist facies, and some parts have been metamorphosed to sillimanite or kyanite grade. As a result, the 

Sr whole-rock isotopic systematics have been disturbed or 

reset, and are probably useful only in a general way for 

determining petrogenesis.

The whole-rock Sr isotopic data from the Hemlock 

and Quinnesec Formations (figs. 3, 4; table 1) define trends 

which might be considered errorchrons, yielding ages of 

about 1.5-1.6 Ga. Other geologic and geochronologic 

evidence from this region clearly indicates that these cannot 

be true crystallization ages; however, such ages are in the

Hemlock Formation Flood Basalts, Lake Superior Region 15
1000

E 100

Q

Z 

O

o

o 

o

DC

10

1000

- D

100

10

+2o

-2o

La Ce Nd Sm Eu Tb Ho Yb Lu La Ce Nd Sm Eu Tb Ho Yb Lu

1000

E 100

Q

z

O

I

O

^

O 

O

DC

10

B

1000

100

10

- £

La Ce Nd Sm Eu Tb Ho Yb Lu La Ce Nd Sm Eu Tb Ho Yb Lu

1000

= c

E 100

O

O 

<2 

o 

O

DC

10

1000

- F

100

10

EXPLANATION 

Deccan basalts 

Columbia River basalts 

Keweenawan basalts reference suite

La Ce Nd Sm Eu Tb Ho Yb Lu La Ce Nd Sm Eu Tb Ho Yb Lu

RARE EARTH ELEMENTS

16 Precambrian Geology of Lake Superior Region
0.8200

0.8000

0.7800

0.7600

0.7400

0.7200

0.7000

i | i | i | r 

Metamorphic age = 1.55±0.2 Ga

O~t OC

Initial Sr/Sr(meta.) = 0.70572±14 (2o)

87Rb/ 86Sr

Figure 3. Rb/Sr errorchron for the Hemlock Formation 

volcanic rocks, showing metamorphic age for these rocks.

0.7500

0.7400

0.7300

0.7200

0.7100

I ' I ' 

Rb/Sr age = 1,573+113 Ma 

- 87Sr/ 86Sr (Initial) = 0.70258±8 (2o)

0.7000

0.0 0.5 1.0 

87Rb/ 86Sr

1.5 2.0

Figure 4. Rb/Sr errorchron for the Quinnesec Formation 

basalts, showing metamorphic age for these rocks.

range of other Sr metamorphic ages reported from northern 

Wisconsin and upper Michigan (1.65-1.55 Ga; Sims and 

Peterman, 1980), and therefore may record true metamorphic ages. Such an interpretation of these data would, 

however, require large-scale reequilibration of the Sr 

isotopic composition. As no igneous intrusives of this age 

have yet been found in this region, the nature of such 

Middle Proterozoic metamorphic events remains poorly 

understood. Because of the metamorphic overprinting on 

the whole-rock Sr isotopic systematics, the primary initial 

87Sr/86Sr isotopic compositions cannot be calculated

Figure 2 (facing page). Chondrite-normalized REE patterns 

for A, the Quinnesec and B, the Hemlock Formations, 

compared with C, island arc tholeiites (Gill, 1981); D, MORB 

(mid-ocean ridge basalts; Basaltic Volcanism Study Project, 

1981); E, Archean magnesian basalts and basaltic komatiites of 

the Abitibi Group (Basaltic Volcanism Study Project, 1981); 

and F, continental flood basalts of the Deccan (Mahoney, 

1984), Keweenawan, and Columbia River basalt provinces 

(Basaltic Volcanism Study Project, 1981). Quinnesec data are 

from K.J. Schulz, U.S. Geological Survey; Hemlock data are 

from Ueng and others (1988).

Table 1. Rb-Sr data for the Hemlock and Quinnesec 

Formations

[Sample numbers refer to locations given in figure IC-D; only first two- or 

three-digit number for Quinnesec samples appears in fig. ID]

Sample No. Rb (ppm) Sr (ppm)

Hemlock Formation

5A

29F

29D

5B

5C

VIA

29E

34G

15.178

10.06

23.51

55.24

55.46

35.08

5.88

9.45

350.4

537.1

318.8

68.42

34.76

256.2

177.8

167.1

0.70695±26

.70533±13

.708657±65

.75114±7

.80808±8

.714058±80

.70994±13

.708237±94

0.1259

.0541

.2132

2.3285

4.6604

.3959

.1443

.1634

Quinnesec Formation

62-1

203-31-1

463-86-1

491-90-1

565-101-1

105-4-1

1.767

52.8

69.79

11.79

1.810

1.115

63.1

153.66

119.2

85.06

82.91

78.45

0.70482±9

.72442±9

.74038±12

.712050±50

.70365±7

.703499±64

0.0810

.9934

1.6926

.4007

.0631

.0411

unambiguously, and thus they probably yield no useful 

information about the source regions of these two suites of 

rocks.

Unlike the Sr isotopic compositions, the Nd isotopes 

do not appear to have been disturbed by metamorphism 

subsequent to emplacement of the rocks. The Quinnesec 

data form an isochron that yields an age of 1.870+ 0.056 Ga, 

and an initial £Nd of +4.2 (fig. 5; table 2). This age is 

interpreted as a crystallization age, and is coeval with the 

1.87 (+0.030-0.010) Ga Pb/Pb concordia intercept age 

obtained for several felsic intrusive rocks in the southern 

magmatic terrane (Banks and Rebello, 1969; Banks and 

Van Schmus, 1971). The initial £Nd is that of a very 

LREE-depleted reservoir, and indicates that a mantle 

reservoir with long-term LIL element depletion had existed 

for at least several hundred million years prior to the 

eruption of the Quinnesec basalts. This initial £Nd falls 

along the trajectory for the MORB mantle reservoir on a Nd 

isotopic evolution model (fig. 6), suggesting that the 

Quinnesec basalts may have been derived from the Proterozoic precursor of the modern MORB mantle reservoir.

In contrast to the coherent nature of the Quinnesec Nd 

data, the Hemlock data do not define a Nd isochron. A 

linear regression through the Hemlock data produces a line 

that could be interpreted as an errorchron, with an age of 

2.7+0.5 Ga (fig. 7; table 2). Although this is the age of large 

parts of the Superior granite-greenstone terrane, which 

underlies parts of the region, an Archean age for the 

Hemlock is unlikely, inasmuch as other geochronologic 

evidence suggests that the Hemlock cannot be older than 

2.1 Ga and probably is -1.9 Ga (Banks and Van Schmus,

Hemlock Formation Flood Basalts, Lake Superior Region 17
Figure 5. Present-day Sm/Nd isotopic systematics for the 

Quinnesec (solid squares) and Hemlock (open squares) 

volcanic rocks. Quinnesec Formation has an age of 

1.870±0.056 Ga and an initial eNd of +4.2. Hemlock data do 

not define a Nd isochron.

0.5140

0.5130

0.5120 -

2 0.5110 -

0.5100 -

0.5090

Region for MORE 

(assuming continuous 

evolution since 4.6 Ga)

3.0 2.0 1.0 Present 

TIME BEFORE PRESENT, IN BILLIONS OF YEARS

Figure 6. 143 Nd/144 Nd isotopic evolution diagram, showing 

location of Quinnesec initial ratio and age (with 2a errors) 

relative to the trajectories for MORB and Bulk Earth (CHUR).

1971; Banks and Rebello, 1969; Aldrich and others, 1965; 

Hoffman, 1987). Several other explanations of the quasilinear nature of the Hemlock data are possible, however. For 

example, they might be interpreted as a so-called "mantle 

isochron," or alternatively, they could represent some sort 

of mixing array. The possibility that this array represents a 

mantle isochron will be considered briefly following, but 

because of other relationships within the isotopic, trace 

element, and major element systematics, it is considered 

most likely that the Hemlock array represents a mixing 

array. Evidence for this assertion may be derived from a plot 

of the initial Nd isotopic compositions for both the Hemlock 

and Quinnesec Formations (fig. 8).

Calculation of initial isotopic ratios requires that 

some estimate be made of the age of deposition of the 

Hemlock tholeiites. The best estimate of this age comes 

from the 1,950 Ma Pb/Pb concordia intercept age obtained

0.5140

0.5130

2 0.5120

0.5110

Hemlock Formation 

Errorchron age = 2.7 + 0.5 Ga

0.12 0.14 0.16

147

0.18 0.20 0.22

Sm/'^Nd 144.

Figure 7. Plot of present-day 143 Nd/144 Nd versus 

14 Sm/144 Nd for the Hemlock volcanic rocks. If interpreted as 

an errorchron, these data suggest an age of 2.7+0.5 Ga (2a). 

The preferred interpretation, however, is that this array 

represents a two-component mixing line.

-10

-12

\

Quinnesec 

n D volcanic rocks R rn

°° ~~~~~~~~Hemlock

D volcanic rocks

n

n

Average 

Archean crust 

at 1.87 Ga

H i i i I

,/

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 

U7Sm/ 144Nd

Figure 8. Sm/Nd isotopic data for the Quinnesec (solid 

squares) and Hemlock (open squares) volcanic rocks, 

calculated for approximately 1.87 Ga. Also shown is a box for 

8Nd of average Archean crust, specifically Amitsoq gneiss of 

southwestern Greenland at 1.87 Ga (from Baadsgaard and 

others, 1986), relative to the average 147Sm/144 Nd of crustal 

granitoids (from Othman and others, 1984).

from a rhyolite of the Hemlock Formation (Banks and Van 

Schmus, 1971; Van Schmus, 1976 (old constants)). Using 

the new constants (Steiger and Jager, 1977), this age would 

be approximately 1.90 Ga. Other geologic and geochronologic evidence supports this as a geologically reasonable 

age (Van Schmus, 1976, 1980; Bickford and others, 1986). 

Such an age is slightly older than that of the Quinnesec 

Formation, which certainly fits with the overall tectonics of 

the region. If a depleted-mantle model age is calculated for 

the least contaminated (most LREE depleted) Hemlock 

basalt, we obtain an age of about 1.87 Ga, which again 

corroborates the assertion that the Hemlock volcanics are 

approximately the same age as the Quinnesec volcanics. If 

we accept this as the crystallization age for the volcanics of 

the Hemlock Formation, then we can plot their initial Nd 

isotopic ratios together with those of the Quinnesec basalts

18 Precambrian Geology of Lake Superior Region
Table 2. Sm-Nd data for the Hemlock and Quinnesec Formations

[Sample numbers refer to locations given in fig. IC-D; only first two- or three-digit number for 

Quinnesec samples appears in fig. ID]

Sample No. Sm (ppm) Nd (ppm) i43Nd/i44Nd 147Sm/l44Nd ( 143Nd/144Nd)i

Hemlock Formation isotopic data

A6I

5J-2

A27F

5A

29F

29D

5B

5C

71A

29B

29E

34G

A2A

31D

A5J

A9D

29C

68A

A5G

3.8853

3.075

3.635

2.48

.893

6.012

17.64

18.5

8.897

1.862

5.894

3.438

5.837

4.25

.9724

1.1216

2.774

2.560

1.25

16.42

13.334

15.1313

10.16

4.016

27.3

76.00

88.76

41.426

5.3716

26.84

13.236

25.02

20.696

3.826

4.3871

11.722

9.470

4.844

0.511992117

.511903120

.511973154

.512011127

.511823143

.511911117

.511722141

.511495151

.511671124

.513028131

.511786+27

.512312114

.511886+21

.511713131

.512185188

.512302150

.512017146

.512263123

.512273125

0.1431

.1395

.1453

.1476

.1345

.1332

.1403

.1261

.1299

.2097

.1328

.1571

.1411

.1242

.1537

.1546

.1431

.1635

.1561

0.510231

.510190

.510194

.510195

.510168

.510271

.509996

.509943

.510072

.510448

.510152

.510379

.510159

.510185

.510294

.510399

.510256

.510251

.510249

Quinnesec Formation isotopic data

62-1

198-28-1

203-31-1

463-86-1

491-90-1

105-4-1

100-2

1.905

1.935

2.654

1.664

.976

.766

.3637

7.423

4.323

5.381

2.748

1.656

2.024

.8462

0.512337140

.513766151

.514068117

.514941130

.514855150

.513273138

.513668+26

0.1552

.2707

.2983

.3662

.3560

.2289

.2600

0.510432

.510432

.510432

.510432

.510432

.510432

.510432

relative to their corresponding initial 147Sm/144Nd ratios 

(fig. 8). As seen in this illustration, the most radiogenic 

representatives of the Hemlock Formation have initial 8Nd 

values equivalent to those of the Quinnesec Formation, 

whereas the remainder of the values define an array pointing 

towards the region of the box identified as Archean 

continental crust. This box spans the range in 147Sm/144Nd 

ratios for both average crustal granulites and average 

granitoids worldwide (Othman and others, 1984), whereas 

the 8Nd range for this box represents the Archean Amitsoq 

gneisses of southwestern Greenland recalculated for 1.87 

Ga (modified from Baadsgaard and others, 1986). Such data 

are often associated with some type of mixing process, and 

suggest that the Hemlock volcanics may have been derived 

from a LREE-depleted source with an 8Nd value similar to 

that of the Quinnesec volcanics, but were subsequently 

contaminated by older continental crust. Alternatively, the 

Hemlock volcanic rocks could potentially have been 

derived from two distinct mantle sources, or from a mantle 

source with variable Nd isotopic composition, some 

portions of which experienced a long-term LIL enrichment

whereas others experienced a long-term LIL depletion. This 

second alternative would give rise to a so-called mantle 

isochron, and necessitates that the magma giving rise to the 

Hemlock volcanics remained unhomogenized during 

magma genesis. In either case, the isotopic data seem to 

imply mixing between two discrete sources that have 

distinctive Nd isotopic compositions. This in turn implies 

long-term differences in the REE compositions for these 

respective reservoirs.

MIXING RELATIONSHIPS

It is difficult to determine whether the Hemlock 

Formation was derived from a mixed mantle source or 

experienced continental crustal contamination during 

passage through or emplacement in the crust. One way to 

address this question is to look for sympathetic variations 

between the initial Nd isotopic compositions and the major 

and trace element compositions of the Hemlock basalts and 

associated rocks. If sufficiently good data correlations can

Hemlock Formation Flood Basalts, Lake Superior Region 19
be defined, then end-member compositions can sometimes 

be characterized, which then can be compared with compositions of known crustal and mantle potential source 

rocks. Pure two-component mixing should lead to 

hyperbolic mixing relationships on some types of variation 

diagrams, and to linear plots on other variation diagrams 

(Langmuir, Bender, and others, 1977; Langmuir, Vocke, 

and others, 1977; Vollmer, 1976). Most commonly, however, when mixing occurs, it occurs in combination with 

fractional crystallization. This more complicated case may 

lead to more complex curve geometries with lines and 

hyperbolas as end-member cases, as discussed by DePaolo 

(1981). In such cases, the rock compositions may need to be 

compared with the results of more complex magma genesis 

models to determine whether magma mixing or contamination has occurred.

When the data of the Hemlock volcanic rocks are 

plotted on discriminate diagrams, data correlations indicative of both mixing and fractional crystallization are found. 

Many of the major and trace elements indeed correlate with 

the initial Nd isotopic compositions of these basalts. These 

include Th, Ta, REE, Zr, Nb, Hf, and Y, as well as a number 

of the major elements and oxides, including K2O, MgO, and 

Mg*. Because of space constraints, only a few of these are 

shown here (fig. 9), but they include representatives from 

the Hemlock basalts as well as gabbros, ultramafics, and 

granophyres from the related West Kiernan sill. On each of 

these plots, the data from the basalts and granophyres form 

an arcuate trend with the samples always appearing in the 

same relative order. The ultramafics and gabbros of the 

West Kiernan sill, however, form their own distinct trend on 

some plots, indicating that a second process may have 

operated in the West Kiernan sill magma chamber.

On each plot in figure 9 a best-fit right-rectangular 

hyperbola has been fitted to the basalt-granophyre data 

using a nonlinear least-squares regression technique (Beck, 

1988). As mentioned, this type of hyperbolic relationship is 

in some cases taken as evidence of simple two-component 

mixing. Proceeding on this assumption, compositional 

information about the end-members may at times be derived 

from the hyperbolic approach of the data to asymptotic 

values. In this case, additional constraints on the Nd isotopic 

composition of the end-members may be obtained from 

figure 8, which can then be used in figure 9 to infer 

something about other compositional parameters of the 

end-members. Figure 8 indicates that the mantle which 

generated the Quinnesec volcanics had an 8Nd of about 

+4.2. This range of 8Nd has been plotted in figure IOA for 

comparison with the Hemlock volcanics. One end of the 

basalt-granophyre trend is located in the range of highly 

depleted 8Nd-high Mg* values typical of basalts derived 

from the source regions for MORB at approximately 2.0 Ga, 

whereas the other end falls at the low Mg*-low 8Nd end of 

this diagram, corresponding to compositions typical of 

Archean continental crust at approximately 2.0 Ga. For

0 10 20 30 40 50 60 70 80 90 100 

A Mg*

Figure 9 (above and facing page). Plots of initial eNd 

(approximately 1.87 Ga) versus A, Mg*; B, Hf; C, La; D, Cr; E, 

Th; F, Rb/Sr; C, K2O for the basalt-granophyre trend of the 

Hemlock data. A best-fit right-rectangular nonlinear leastsquares hyperbola is shown on each plot. On any plot, solid 

circle, average of most depleted MORB basalts (Basaltic 

Volcanism Study Project, 1981); solid square, representative 

granodiorite from the Rainy Lake district, northwestern 

Minnesota (from Shirey and Hanson, 1987). In A, vertical line, 

range of initial eNd for Quinnesec Formation; cross, range of 

initial 8Nd corresponding to Mg* numbers in the range 68-72 

typical of basalts derived from the source regions for MORB 

(Frey and Prinz, 1978; Frey and others, 1978). Basaltgranophyre trend implies mixing between Archean crust and 

MORB.

modern MORB, most primitive basaltic melts have Mg* 

numbers in the range 68-72 (Frey and Prinz, 1978; Frey and 

others, 1978; Basaltic Volcanism Study Project, 1981). An 

error bar in figure 9A shows the range in eNd that 

corresponds to this range in Mg* numbers for the Hemlock 

volcanics. If the Hemlock volcanics were derived from the 

convecting mantle (which is also the source region for 

MORB), then the most primitive Hemlock magmas should 

also fall in this range of Mg* numbers. This is indeed the 

case, and as shown on this plot (fig. 9A), the eNd value 

corresponding to this range of Mg* numbers for the 

Hemlock volcanics is for all intents and purposes identical 

to the 8Nd initial value for the Quinnesec volcanics. The 

implication of this equivalence is that the Hemlock 

continental flood basalts were apparently derived from the 

same highly depleted mantle region from which the 

Quinnesec volcanics were derived, which as already noted 

corresponds to the range in isotopic characteristics expected 

from the Early Proterozoic precursor of the MORB 

reservoir. That is, both the Quinnesec and Hemlock volcanics appear to have been derived from the Early Proterozoic 

convecting mantle (MORB) reservoir.

The other end of the Hemlock basalt-granophyre 

trend shown in figure 9A falls at the low Mg*-low 8Nd of 

the diagram, corresponding to compositions typical of 

Archean continental crust at =2.0 Ga. This coincidence

110 Precambrian Geology of Lake Superior Region
TJ 

Z 

00

0 5 10 15 20 25 

Hf, IN PARTS PER MILLION

: c

i i i

5 10 15 

Th, IN PARTS PER MILLION

20 40 60 80 100 

La, IN PARTS PER MILLION

120 0.0

D

1.0 1.5 

Rb/Sr

2.0 2.5

TJ 

Z 

00

-4

-6

D

D

D

200 400 600 

Cr, IN PARTS PER MILLION

800 0 1234 

K20, WEIGHT PERCENT

Hemlock Formation Flood Basalts, Lake Superior Region 111
supports the contention that the Hemlock magma was 

contaminated by Archean continental crust. We know that 

the Hemlock volcanics were erupted through and onto 

2.7-3.6 Ga Archean crust (Van Schmus, 1976; Futa, 1981). 

Because granodiorite is among the most common rock 

compositions of upper crustal plutonic rocks throughout the 

Canadian Shield (Taylor and McLennan, 1985), granodiorite seems an appropriate composition to select as a 

crustal contaminant. The square box in figure 9A depicts an 

average Mg* value for six representative 2.7 Ga Archean 

granodiorites from the Superior province, together with 

their corresponding mean 8Nd values calculated for 1.87 Ga 

(from Shirey and Hanson, 1987). This data area fits 

satisfactorily on the mixing trajectory for the basaltgranophyre data trend, plotting as an extreme point at the 

low 8Nd end of this curve. Thus, figure 9A seems to provide 

us two suitable and quite reasonable end-members for a 

two-component mixing array. If we use these compositional 

data as a priori known values for the eNd of our two 

end-members of the mixing array, then we can approximate 

other compositional parameters for the end-members. We 

can then compare these bulk compositions with those of 

known actual rock compositions, as a test of the mixing 

hypothesis.

Each view of figure 9B-F shows the basalt-granophyre trend for the Hemlock volcanic rocks, as well as data 

points representing compositions for an average of most 

depleted MORE, and of representative granodiorites from 

the Rainy Lake area, modified from Shirey and Hanson 

(1987). The composition of the hypothetical basaltic endmember can be determined from these plots by noting the 

compositions of the points of intersection between the 

mixing hyperbolas and the line defined by 8Nd=+4.2. Table 

3 summarizes these results and compares the data with 

compositional data representative of mid-ocean ridge tholeiites, island arc tholeiites, and ocean island tholeiites from 

Hawaii, as well as representative Keweenawan rift olivine 

tholeiites. Although the results are not absolutely conclusive, the basaltic end-member determined from this 

analysis seems to bear the strongest similarity to that of type 

I MORE. This similarity is perhaps most strongly revealed 

by a comparison of the REE patterns for the hypothetical 

basaltic component plotted relative to average patterns for 

MORE (fig. 10), which clearly shows that the Hemlock 

hypothetical basaltic component is similar in REE 

concentrations and REE pattern to that of MORB. Thus, the 

composition of the Hemlock basaltic component is consistent with its derivation from a mantle reservoir with a 

long-term LREE depletion.

In like manner, a footprint of the crustal component of 

the Hemlock volcanic rocks can be defined, as presented in 

table 4 and figures 11 and 12. Table 4 shows a compilation 

of mixing curve data for the hypothetical Hemlock crustal 

contaminant, together with compositional data from average 

felsic upper crust (Taylor and McLennan, 1985), Archean

100

<r Q z. 

o

q 10 

<2.

o 

O 

cc.

\ \ \

+2o

-2o

La Ce Sm Eu Tb 

RARE EARTH ELEMENTS

Yb Lu

Figure 10. Chondrite-normalized REE patterns showing a 

comparison between the Hemlock basaltic component and 

that of MORB (from Basaltic Volcanism Study Project, 1981).

granodiorite (Shirey and Hanson, 1987), quartz-rich graywacke (Taylor and McLennan, 1985; Bhatia, 1981), and 

average Huronian mudstone (Taylor and McLennan, 1985; 

McLennan, 1979). A fairly good match can be made 

between the Hemlock crustal component and the Archean 

granodiorite, except that the hypothetical crustal contaminant appears to be preferentially enriched in the LIL 

elements and HFS elements (fig. 11). All patterns except the 

average lower continental crust show elevated K2O and Th 

contents, whereas all exhibit negative Nb anomalies characteristic of continental crust, as well as LREE enrichment, 

and high overall concentrations of the REE and HFS 

elements. Although the overall elemental patterns among 

these composite rock types are similar, the hypothetical 

crustal component is clearly much more enriched in many 

of these elements than the average crustal sources. Nevertheless, the hypothetical crustal component is not 

unreasonably enriched in these elements, and could potentially represent a small percentage partial melt of a crustal 

source already enriched in these elements. For example, 

figure 12 shows the REE pattern of the crustal contaminant 

plotted relative to the granites of the Isle of Skye (Thorp and 

others, 1977; Thompson, 1982; Thompson and others, 

1982). The pattern of LREE enrichment and the overall 

elevated REE concentrations exhibited by the crustal 

component are similar to those of the granites of the Isle of 

Skye, but are typically enriched in these elements by about 

a factor of two. Although such enrichments are uncommon, 

Hanson (1980) has shown that factor of 2-3 enrichments of 

the REE can be obtained by small percentages (3-5 percent) 

of batch partial melting of a granitic source at 800 °C and 4 

kb (fig. 12). Partial melts of a granite into a basaltic melt at

112 Precambrian Geology of Lake Superior Region
Table 3. Geochemical comparison among the theoretical basaltic end-member, 

MORB, Keweenawan tholeiites, island arc basalts, and ocean island basalts

[All major elements in weight percent; all trace elements in parts per million. Data from type I 

MORE, island arc tholeiites, ocean island tholeiites, and Keweenawan olivine tholeiites from 

Basaltic Volcanism Study Project (1981). Range in eNd values for Keweenawan tholeiites from 

Dosso, 1984. Estimated £Nd for island arc tholeiites from Albarede and Brouxel, 1987. Leaders 

( ), no data]

Element, 

oxide, 

ratio

SiO2

Ti02

A1203

FeOt

MnO

MgO

CaO

Na2O

K2O

P205

Hf

Ta

Zr

Nb

Y

Th

Yb

Cr

Ni

La

Ce

Nd

Sm

Eu

Lu

Mg*

Rb/Sr

^Nd

Basaltic 

end-member 

(1 .87 Ga)

48

1.0

13.5

0.18

10

10

...

0.5

0.06

1.7

0.5

70

13

20

1.2

2.5

600

200

3.5

10

7

2.5

0.1

0.4

70

0.01

+4.2

Type 1 

MORB 

(1.87Ga)

48^9

0.6-0.62

12.1-16.5

8.8-9.0

0.12-0.15

10.3-17.81

11. .2-12.4

1.3-1.92

0.03-0.07

0.06

1-8

0.2-1.0

32

1-5

0.2-2.3

270-700

300

1.2

6.4

4.7

1.54

0.6

0.3

68-72

0.01

+4.2

Keweenawan 

Rift olivine 

tholeiites

46-50

1.2-1.8

14.5-17.5

0.15-0.22

6.2-8.4

6.3-10.5

2.4-3.8

0.3-1.3

0.14-0.20

2.3

0.5

40-140

6

20-76

0.9

3-7

40-270

35-200

10

20

12

3

1.1

0.3

43-60

0.03

-2.6 to +3.2

(at 1.1 Ga)

Island 

arc 

tholeiites

50-54

0.5-1.1

15.7-19

6.3-8.5

0.14-0.24

5.7-9.0

10.5-12.9

1.5-3.0

0.18-0.50

0.07-0.19

2.0

45-130

1-6

20-30

0.5-6.5

2.0

40-110

10-40

4

10

8.5

2.1

0.8

0.3

51-71

0.015

4.0?

Ocean 

island 

tholeiites

48-52

1.4-2.6

9.0-13.8

0.16-0.17

7.2-14

7-11.2

1.5-2.3

0.27-0.5

0.17-0.3

3-7

120-250

13-24

17-24

1.0-1.65

2.0

300-700

15

38

6.8

2.0

0.3

57-78

0.03

...

=1,100 °C could potentially result in even greater enrichments of the melt in the REE and HFS elements, although 

such an assertion is speculative because of the notably poor 

constraints on distribution coefficients between granitic 

mineral phases and high-temperature basaltic melts. Alternatively, additional enrichments of these elements could 

have occurred as a result of differentiation subsequent to the 

incorporation of the crustal component into the basaltic 

melt. Thus, REE patterns similar to that of the Hemlock 

crustal component do not appear out of line if the crustal 

component represents a small percentage partial melt of a 

granite similar in composition to the granites of the Isle of 

Skye.

The Cr and Ni concentrations of the crustal and 

basaltic end-members are both plotted in figure 13 together

with the average compositions for oceanic crust, upper 

continental crust, lower continental crust, total continental 

crust, erogenic andesites, and primitive mantle (from Taylor 

and McLennan, 1985). The composition of the Hemlock 

basaltic component is close to that of modern oceanic crust, 

whereas the Hemlock crustal component is similar to the 

composition of average upper crust, and considerably 

different from that of the lower crust. Whereas the depleted 

nature of Cr and Ni in the Hemlock crustal component may 

well result from fractionation occurring subsequent to its 

incorporation into the basaltic melt, its composition may 

also be related to the source composition of this component. 

Thus the Cr/Ni relations of the crustal and basaltic 

components are consistent with their apparent derivation 

from the upper continental crust and MORB reservoirs,

Hemlock Formation Flood Basalts, Lake Superior Region 113
Table 4. Geochemical comparison between the theoretical end-member 

and several representative crustal compositions

[Chemical compositions of the hypothetical Hemlock crustal contaminant, together with 

comparisons with compositions of average felsic upper crust (Taylor and McLennan, 

1985), Archean granodiorite (Shirey and Hanson, 1987), quartz-rich graywacke (Taylor and 

McLennan, 1985; Bhatia, 1981), and average Huronian mudstone (Taylor and McLennan, 

1985; McLennan, 1979). Major elements in weight percent; trace elements in parts per 

million. Leaders ( ), no data]

Element, 

oxide, 

ratio

SiO2

TiO2

A1203

FeO*

MnO

MgO

CaO

NasO

K2O

P205

Hf

Ta

Zr

Nb

Y

Th

Yb

Cr

Ni

La

Ce

Nd

Sm

Eu

Lu

Mg*

Rb/Sr

Average 

felsic upper 

crust

69.7

.5

15.8

.15

1.4

2.9

4.3

2.2

4

200

14

.22

30

5

37

75

33

5.7

1.6

.22

.21

Archean 

granodiorite

74.8

.22

15.0

.06

.21

1.46

4.07

2.94

.04

76-665

6-27

1-15

1-17

4-12

10-40

20-130

10-75

1.5-18

0.4-3.5

11-30

Crustal 

end-member

80

10.5

 _

.2

3.5^.5

20-25

4.5-5

800-900

50

150-200

16-20

16

5

10

100-120

175-225

100

25-40

5-8

1.2

10

2-2.5

Quartz-rich 

graywacke

81.1

.62

10,1

2.76

.03

1.44

.26

1.69

1.93

.14

10.1

384

11

32

16.4

2.9

51

19

43

83

42

7.1

1.1

2.9

2.0

Average 

Huronian 

mudstone

71.69

.41

15.96

3.36

.02

1.67

.26

1.35

5.51

.12

52

3.8

56

14

49

115

51

9.3

1.8

8.04

respectively. Hence the assertion that the geochemical 

signatures of the Hemlock volcanic rocks may have been 

generated by crustal contamination of a basaltic melt from 

the depleted mantle appears to be corroborated by the 

isotopic, trace element, and major element signatures of 

these rocks.

ASSIMILATION-FRACTIONAL 

CRYSTALLIZATION MODEL 

CALCULATIONS COMPARED WITH 

OBSERVED CHEMISTRIES

The preceding discussion of element composition and 

possible sources and processes in the Hemlock Formation 

may be compared with the results of combined AFC

(assimilation-fractional crystallization) models, as a further 

test of the assertion that the Hemlock basalts resulted from 

mixing between a MORB-like basaltic source and a 

continental crustal contaminant. Two such AFC models 

were coupled together for this purpose. One of these 

models, TRACE.FOR (Nielson, 1985; 1988), is an incremental crystallization model used to determine the major 

and trace element concentrations during differentiation of a 

mafic magma. The model Nd isotopic compositions, on the 

other hand, were determined using the output from TRACE. 

FOR in combination with a second AFC model, based 

principally upon one developed by DePaolo (1981). 

DePaolo's AFC model was not used directly to predict the 

isotopic compositions of melt fractions resulting from 

TRACE.FOR, because DePaolo's model calculates melt 

isotopic compositions assuming a constant bulk distribution

114 Precambrian Geology of Lake Superior Region
500

400

300

200

100

EXPLANATION

Crustal component

Average bulk crust 

Average lower crust 

Average Canadian Shield 

Quartz-rich graywacke

K,0 Th Nb La Ce Nd Zr Hf Sm 

ELEMENT, OXIDE, OR RATIO

Yb Rb/Sr Cr Ni

Figure 11. Spider diagram showing chondrite-normalized concentrations of the HFS elements, some REE, and several 

incompatible elements for the Hemlock crustal component, average bulk crust, average lower continental crust (modified from 

Taylor and McLennan, 1985), average Canadian Shield, and average quartz-rich graywacke.

coefficient for the fractionating phases. In nature (and in 

TRACE.FOR), the modal proportions of the crystallizing 

phases often change as crystallization progresses, whereas 

individual mineral/melt distribution coefficients may also 

change during the fractionation process owing to variations 

in temperature. Since such effects generally result in 

variations of the bulk distribution coefficient, DePaolo's 

model had to be adjusted to allow for such conditions. 

Another difficulty with the DePaolo model is that it does not 

have a provision allowing for periodic recharge and (or) 

eruption to occur concurrently with assimilation or fractional crystallization. TRACE.FOR, on the other hand, does 

have provisions which allow for periodic recharge and (or) 

eruption. As such, DePaolo's model has been modified to 

utilize the output of TRACE.FOR to calculate the Nd 

elemental and isotopic composition of the melt, as discussed 

by Beck (1988).

An average MORE parental melt (table 3, from 

Basaltic Volcanism Study Project, 1981) was used as a 

starting composition for the basaltic melt input into 

TRACE.FOR. For those runs utilizing a crustal contaminant, the average granite composition from Taylor and 

McLennan (1985, tables 9.2, 3.1) was used (see table 4). 

TRACE.FOR was run in a variety of configurations

utilizing fractional crystallization ±magma recharge±assimilation of granitic crustfcsimultaneous eruption. The amount 

of recharge, assimilation, and eruption occurring during 

each program run can be represented in a shorthand form 

(X/Y/Z); X, Y, and Z can be any non-negative numbers, 

generally less than 1, where X represents the amount of 

recharge, Y the amount of assimilation, and Z the amount of 

eruptive material. A value of 1 would represent an amount 

of recharge, assimilation, or eruption equivalent to the 

amount removed from the melt system by crystallization; 

0.5 would represent an amount of recharge, assimilation, or 

eruption equivalent to one-half of the mass removed from 

the magma chamber by crystallization, and so on. For 

example, (X/Y/Z)=( 1.0/0.3/1.0) represents a run in which for 

every increment of melt removed from the magma by 

crystallization, an equivalent increment of melt is added to 

magma chamber by recharge, 0.3x such increment is added 

to the magma from a crustal assimilant, and 1.0 equivalent 

increment is removed from the system by eruption. 

Obviously, this case represents a non-steady-state case in 

which the total volume of magma will decrease with time, 

with 2.0 magma masses leaving the system for every 1.3 

masses added.

Hemlock Formation Flood Basalts, Lake Superior Region 115
1000

Hemlock Formation 

crustal component

La Ce Nd Sm Eu Gd Tb Tm Yb Lu 

RARE EARTH ELEMENTS

EXPLANATION

REE distribution patterns for 

granites of Isle of Skye

REE pattern for crustal component 

from least squares analysis

REE pattern for granitic parent and aplite 

differentiate (from Hanson, 1980)

Figure 12. Comparison of chondrite-normalized REE patterns 

of the Hemlock crustal component, granites of the Isle of Skye 

(from Thompson and others, 1982; Thorp and others, 1977), as 

well as granitic parent and theoretical small-percentage (1-2 

percent) partial melt of this parent (differentiate) (from Hanson, 

1980).

Some of the results of these AFC models are presented in figures 14 20. Six different configurations of 

recharge, assimilation, and eruption are shown in these 

figures, including (0.0/0.3/0.0), (0.0/0.3/0.5), (0.5/0.3/0.5), 

(1.0/0.3/0.0), as well as (0.5/0.0/0.5), and (1.0/0.0/0.5). The 

first four options include a term for assimilation of continental crust, whereas the last two do not. All runs included 

fractional crystallization.

Figures 14-16 show several major and trace elements 

plotted relative to the fractionation index Mg*, whereas the 

remaining figures (17-20) show several major and trace 

element concentrations plotted relative to their initial eNd . 

Each graph shows the results of the TRACE.FOR/ 

ISOTOPE model compared with compositions of the basalts 

and granophyres of the Hemlock Formation.

Plots of REE versus Mg* (not shown) reveal that all 

permutations of the model provide a reasonable match to the 

Hemlock data, and in general do not discriminate among the 

various potential processes. Likewise, the same may be said

1000

100 Orogenic 

andesites

Oceanic crust

Lower crust

Total crust

Hemlock Formation 

basaltic component

Upper crust

Hemlock Formation crustal component

i i i i i i i i i i i i i i

100 1000 

Ni, IN PARTS PER MILLION

Figure 13. Plot of Cr and Ni showing composition of the 

Hemlock Formation basaltic and crustal components relative 

to composition of upper continental crust, erogenic andesites, 

total continental crust, lower continental crust, oceanic crust, 

and primitive mantle (modified from Taylor and McLennan, 

1985).

for the plots of Zr versus Mg* (not shown), except perhaps 

for the model (0.0/0.3/0.5), whose poor match for low Mg* 

values may indicate that magma recharge±assimilation may 

be required to enrich the melt in the HFS elements such as 

Zr.

Plots of Ni (not shown) and V (fig. 14) versus Mg* 

likewise generally reflect fairly good matches within the 

scatter of the data between the Hemlock volcanic rocks and 

the results of the models, indicating that olivine and 

clinopyroxene fractionations probably were indeed important controls on melt chemistry during genesis of the 

Hemlock volcanics. As was shown for the REE, these Ni 

and V trends do not discriminate among the various 

potential processes, except that the models involving 

assimilation may provide a slightly better fit to the Hemlock 

data for V plotted against Mg* than those models without 

assimilation.

The plots of K2O (weight percent) versus Mg* 

(fig. 15) reveal excellent matches for the four cases 

involving crustal assimilation, but only poor correlations 

between the Hemlock volcanics and the model data for the 

two cases not involving crustal assimilation [(1.0/0.0/0.5) 

and (0.5/0.0/0.5)]. If this potassic enrichment is not a 

product of subsequent regional metamorphism (which is 

unlikely to have occurred on this scale), then the Hemlock 

magmas must have tapped some source of potassium 

enrichment during the genesis of its melts. These models 

show that crustal assimilation is a suitable potential source 

of such an enrichment. Alternatively, this type of potassic

116 Precambrian Geology of Lake Superior Region
800

600 -

400 -

200 -

1.0/0.3/0.0 1.0/0.0/0.5 0.0/0.3/0.5

. 1

1 + ~_

~

- » -

° °*n * -

n n

 n nnn 

n

, 4f ,* 1 , 1 , 1 , 1 ,

1UUU

800

600

400

200

n

' l ' l ' I ' l ' I ' n

DDn

- °* \ ~

n n 

« n

* DC|-I ^ ~

A LL^

- <V

_

- ,*, ,*l , I , I , I , "

8UU

600

400

200

n

' I ' ' ' I ' I '

4

"

- ** ? ~

D A

4 n * -

 ^ n _

~n ~

A 

i ^l i" i i i i

10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70

0.0/0.3/0.0 0.5/0.3/0.5 0.5/0.0/0.5

ouu

600

400

200

1 1 ' 1 ' 1 ' 1 ' 1 '

-

-

- -

-

n

 ^ n 

n * n n* -

n n ^

~ ~

, « ,* l , l , l , l ,

8UU

600

400

200

n

1 1

-

n

 i i 

n

^

n

n

n 4t ^

n

 n

* nn[

*

*

* ~

"

suu

600

400

200

n

I I I I I I I I I I I

n

n

- * -

n

n ^ n

- 4 n -

* °n -

D*

, * , i , i , i , i ,

10 20 30 40 50 60 70 "o 20 40 60 80 "lO 20 30 40 50 60 7

Mg* Mg* Mg*

Figure 14. Plots of V (ppm) versus Mg* for the results of six different configurations of the assimilation-fractional crystallization 

model TRACE.FOR (open squares, from Nielson, 1988), compared with compositions of the Hemlock Formation (filled diamonds).

enrichment could equally well have been caused by mantle 

metasomatism just prior to the eruption of the Hemlock 

magmas.

As with K2O, the plots of Nb versus Mg* (fig. 16) 

indicate that models without assimilation cannot generate a 

Nb trend similar to that of the Hemlock volcanics. In fact, 

those models with both assimilation and magma recharge 

seem to provide the best fits to the Hemlock data. These 

plots suggest that both magma recharge and assimilation 

result in Nb enrichments of the melt, but that crustal 

assimilation is the more important of the two processes.

Figures 17-20 show some of the Nd isotopic results 

of the TRACE.FOR/ISOTOPE models together with the 

data from the Hemlock volcanics. As noted before, and as 

shown by each of these plots, the large spread in eNd values 

in the Hemlock data either necessitates assimilation of old 

continental crust or requires the Hemlock lavas to have been 

generated from a mixed mantle source one source with a 

highly depleted Nd isotopic composition and another highly 

enriched in Nd. All these plots show that those models 

lacking a term for crustal assimilation [such as (1.0/0.0/0.5) 

and (0.5/0.0/0.5)] cannot generate a range of eNd values 

such as seen in the Hemlock data. The remaining four

models in each figure all contain a term for crustal assimilation, and (as shown) some of these do generate appropriate 

ranges of 8Nd approximating that seen in the Hemlock data.

In the plot of La versus eNd (fig. 17), all models 

containing an assimilation term generate patterns similar to 

that of the Hemlock data, except for model (1.0/0.3/0.0), 

which has terms for recharge, assimilation, and fractional 

crystallization, but no term for volcanism. For this model, 

the 8Nd values plateau out at a level above that for the most 

evolved members of the Hemlock volcanic rocks, suggesting that for this model, the eNd values are buffered 

(1) by the large influx of Nd with depleted-mantle signature 

from the magma recharge term, and (2) by enrichment of Nd 

in the residual melt because it is an incompatible element. 

All other models containing an assimilation term generate 

suitable ranges of 8Nd, but do not discriminate among 

themselves enough to allow selection of a preferred model. 

This same pattern is repeated in plots of all the REE versus 

8Nd (not shown).

Virtually the same results are shown by the plots of 

eNd versus Zr, Ni, Mg* (fig. 18), SiO2 (fig. 19), and K2O 

(fig. 20), all of which show reasonable matches for the

Hemlock Formation Flood Basalts, Lake Superior Region 117
1.0/0.3/0.0

n ' i ' i ' r

n

0.0/0.3/0.5 1.0/0.0/0.5

n

n

I , p m

n

10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 

0.0/0.3/0.0 0.5/0.3/0.5 0.5/0.0/0.5

n

n * _

* n

* n

* D +

, I , I , I , I , I Rn

I ' I ' I 

D

D*

n

* n

* p *

I , I , l D n,

' I ' I ' I ' I ' I '

*

*

*

* * *

, | , D | P |D , DID DDnmrt

0 20 30 40 50 60 70 0 20 40 60 80 10 20 30 40 50 60 7

Mg* Mg* Mg*

Figure 15. Plots of K2O (weight percent) versus Mg* for the results of six different configurations of the assimilation-fractional 

crystallization model TRACE.FOR (open squares, from Nielson, 1988), compared with compositions of the Hemlock Formation 

(filled diamonds).

models containing an assimilation term, except for model 

(1.0/0.3/0.0), which again does not provide sufficiently 

negative eNd values for the most evolved magmas.

The general conclusion that may be drawn from these 

mixing and AFC calculations is that some combination of 

fractional crystallization, magma recharge, and assimilation 

of Archean upper crust adequately explains the data 

observed in the Hemlock volcanic rocks, and that the data 

are fit best by a model that involves all three of these 

processes. Although it is possible that these chemical and 

isotopic systematics could have been generated by mixing 

of two mantle sources, this is considered an unlikely 

explanation because the low eNd component appears to have 

had a highly evolved rock composition, unlike any potential 

mantle source, LIL enriched or otherwise.

CONCLUSIONS

We conclude that the Niagara fault zone is a major 

crustal boundary that juxtaposes two Early Proterozoic

terranes of different character. This dichotomy is quite clear 

cut with respect to the contrasting composition of volcanic 

rocks on either side of the fault zone. The Hemlock volcanic 

and related intrusive rocks to the north of the shear zone 

have many of the compositional characteristics of continental flood basalts, whereas the Quinnesec volcanic and 

related intrusive rocks to the south have compositions 

indicating a tectonic affinity with island arc or ocean floor 

volcanic regimes. Other geologic evidence, however, 

suggests that the Quinnesec and associated volcanics are 

most probably of island arc or back arc basin affinity (Sims 

and others, 1989).

The Nd and Sr isotopic compositions of the 

Quinnesec volcanic rocks suggest that they were derived 

from a mantle source with a long-term LREE depletion, and 

a eNd similar to that anticipated for the source region for 

MORE at =1.9 Ga. The Nd and Sr isotopic compositions of 

the Hemlock volcanics are typical of those in continental 

flood basalt provinces. These compositions, when coupled 

with their trace and major element compositions, suggest 

that the Hemlock volcanics may have been generated by a

118 Precambrian Geology of Lake Superior Region
1.0/0.3/0.0

~r~ti ' I ' I ' T 

n

n

* n»

« °

D

D -

50

40

30

20

10

1.0/0.0/0.5

50

40

30

20

10

0.0/0.3/0.5

D

0 ' ' ' ' ' ' ' ' ' I I I I Q I I I I I I I I I I i M-II 'UJ LJ I g

10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70

0.0/0.3/0.0 0.5/0.0/0.5 0.5/0.3/0.5

DU

40

30

20

10

«

, | , | ,

+

h D 4 J

n *

4 * 4 -

n

n

n *

n 

n

i i i l i l i , n

J>U

40

30

20

10

n

' I ' I ' I ' I ' I '

-4^

h *

4

+ * 4 -

 ^

° D n n n i I i I i I i j i I ' ' lii I I

50

40

30

20

10

r\

I I I < I I ^ -n *

L n + J

n *

_ n * _

n

 rj 4 -

l , l , Pn,

10 20 30 40 50 60 70 "lO 20 30 40 50 60 70 "o 20 40 60 8

Mg* Mg* Mg*

Figure 16. Plots of Nb (ppm) versus Mg* for the results of six different configurations for the assimilation-fractional crystallization 

model TRACE.FOR (open squares, from Nielson, 1988), compared with compositions of the Hemlock Formation (filled diamonds).

two-component mixing process between a mantle 

component similar in composition to the source regions for 

the Quinnesec volcanics and a second LIL-elementenriched component which was either Archean upper 

continental crust or an old "metasomatized" mantle component. The results of combined mixing fractional crystallization models suggest that the most likely candidate for 

this second component was Archean granodioritic crust or 

sediments derived from this source.

REFERENCES CITED

Albarede, F., and Brouxel, M., 1987, The Sm/Nd secular evolution 

of the continental crust and the depleted mantle: Earth and 

Planetary Science Letters, v. 82, p. 25-35.

Aldrich, L.T., Davis, G.L., and James, H.L., 1965, Ages of 

minerals from metamorphic and igneous rocks near Iron 

Mountain, Michigan: Journal of Petrology, v. 6, p. 445^172.

Anderson, J.L., and Cullers, R.L., 1987, Crust-enriched mantlederived tonalites in the Early Proterozoic Penokean orogen 

of Wisconsin: Journal of Geology, v. 95, p. 139-154.

Arculus, R.J., 1987, The significance of source versus process in 

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Baadsgaard, H., Nutman, A.P., and Bridgwater, D., 1986, Geochronology and isotopic variation of the early Archean 

Amitsoq gneisses of the Isukasia area, southern West 

Greenland: Geochemica et Cosmochimica Acta, v. 50, 

p. 2173-2183.

Banks, P.O., and Rebello, D.P., 1969, Zircon age of a Precambrian 

rhyolite, north-eastern Wisconsin: Geological Society of 

America Bulletin, v. 80, p. 907-910.

Banks, P.O., and Van Schmus, W.R., 1971, Chronology of 

Precambrian rocks of Iron and Dickinson counties, 

Michigan: 17th Annual Institute on Lake Superior Geology, 

Duluth, Minnesota, Abstracts and Field Guides, p. 9-10.

Barovich, K.M., Patchett, P.J., and Peterman, Z.E., 1987, Origin of 

the 1.9-1.7 Ga Penokean continental crust of the Lake 

Superior region [abs.]: Eos, v. 68, p. 1547.

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.

Hemlock Formation Flood Basalts, Lake Superior Region 119
-4

-6

-D

.n

1.0/0.3/0.0 1.0/0.0/0.5 0.5/0.3/0.5

50 100 150 200 0

- ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' '-

* 111 nun 111 ii ii 111 ii ii ii ii n n i i i ii

- 4

: \ -

r 47 

i 1 1 1 1 1 1 1 1 i 1 1 1 1 1 1 1

_ ' ' ' ' ' ' ' ' I ' ' I ' ' I ' '_ 

k

I I

n

: 4 :

- 4n

%

D 4

n

- n n A ~ 

D T: n n -

ii ii ii i i I i i T i i I i i

0 20 40 60 80 100 120 0 20 40 60 80 100 120 14

-2

-4

-6

0.0/0.3/0.0 0.5/0.0/0.5

V

n

n 4

?. n nun n ii in n n

i i i i i ii

0.0/0.3/0.5

n

n

4a

50 100

La

150 0 20 40 60 80 100 120 0 

La

50 100 

La

150 200

Figure 17. Plots of La (ppm) versus £Nd (1.87 Ga) for the results of six different configurations of the assimilation-fractional 

crystallization model TRACE.FOR/ISOTOPE (Beck, 1988; modified from Nielson, 1988, and DePaolo, 1981) (open squares), 

compared with compositions of the Hemlock volcanic rocks (filled diamonds).

Basaltic Volcanism Study Project, 1981, Basaltic volcanism on the 

terrestrial planets: New York, Pergamon Press, 1286 p.

Bayley, R.W., 1959, Geology of the Lake Mary quadrangle, Iron 

County, Michigan: U.S. Geological Survey Bulletin 1077, 

112 p.

Bayley, R.W., Button, C.E., Lamey, C.A., and Treves, S.B., 1966, 

Geology of the Menominee iron-bearing district, Dickinson 

County, Michigan and Florence and Marinette Counties, 

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of some flysch deposits: Canberra, Australia, Australian 

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I-1360-B.

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Crustal influence in the generation of continental flood 

basalts: Nature, v. 289, p. 160-162.

120 Precambrian Geology of Lake Superior Region
6 

-2

-4

-6

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-6

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, , , , , 10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 

1.0/0.0/0.5 0.5/0.0/0.5 0.0/0.3/0.0

TT1 i i i i i i i i naff n a a a n nnnni

i \ i i D

a

10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 

Mg* Mg* Mg*

Figure 18. Plots of Mg* versus £Nd (1.87 Ga) for the results of six different configurations of the assimilation-fractional 

crystallization model TRACE.FOR/ISOTOPE (Beck, 1988; modified from Nielson, 1988, and DePaolo, 1981) (open squares), 

compared with compositions of the Hemlock volcanic rocks (filled diamonds).

____1981b, Columbia River volcanism The question of 

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202.

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Hemlock Formation Flood Basalts, Lake Superior Region 121
- 1.

-6

1.0/0.3/0.0 1.0/0.0/0.5 0.0/0.3/0.5

n 

n

n

, , , ,

n

n

n

n

40 50 60 70 80 90 40 50 60 70 80 40 50 60 70 80

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- 

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, i , i , f ,

0 50 60 70 80 40 50 60 70 80 40 50 60 70 8

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Figure 19. Plots of SiO2 (weight percent) versus £Nd (1.87 Ga) for the results of six different configurations of the 

assimilation-fractional crystallization model TRACE.FOR/ISOTOPE (Beck, 1988; modified from Nielson, 1988, and DePaolo, 

1981) (open squares), compared with compositions of the Hemlock volcanic rocks (filled diamonds).

Futa, Kiyoto, 1981, Sm-Nd systematics of a tonalitic augen gneiss 

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pluton in northeastern Wisconsin: 30th Annual Institute on 

Lake Superior Geology, Wausau, Wisconsin, Abstracts, 

p. 24.

122 Precambrian Geology of Lake Superior Region
1.0/0.3/0.0 1.0/0.0/0.5 0.5/0.3/0.5

o

4

2

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-4

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Figure 20. Plots of K2O (weight percent) versus £Nd (1.87 Ga) for the results of six different configurations of the 

assimilation-fractional crystallization model TRACE.FOR/ISOTOPE (Beck, 1988; modified from Nielson, 1988, and DePaolo, 

1981) (open squares), compared with compositions of the Hemlock volcanic rocks (filled diamonds).

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Mahoney, J.J., Macdougall, J.D., and Lugmair, G.W., 1984, Origin 

of contemporaneous tholeiitic and K-rich alkalic lavas A 

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Sankar Das, M., and Gopalan, K., 1981, Origin of the

Hemlock Formation Flood Basalts, Lake Superior Region 123
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Morey, G.B., Sims, P.K., Cannon, W.F., Mudrey, M.G., Jr., and 

Southwick, D.L., 1982, Geologic map of the Lake Superior 

region Minnesota, Wisconsin, and northern Michigan: 

Minnesota Geological Survey State Map Series S-13, scale 

1:1,000,000.

Nielson, R.L., 1985, EQUIL.FOR A program for modelling low 

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____ 1988, TRACE.FOR A program for the calculation of 

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Ti, Zr, Y, and Nb variations in volcanic rocks: Contributions 

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the Lake Superior region Geochemistry and tectonic 

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Hemlock Formation Flood Basalts, Lake Superior Region 125

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