Geology and Geochronology of

Granitoid and Metamorphic

Rocks of Late Archean Age

in Northwestern Wisconsin

Geology and Geochronology of 

Granitoid and Metamorphic 

Rocks of Late Archean Age 

in Northwestern Wisconsin

By P. K. SIMS, Z. E. PETERMAN, R. E. ZARTMAN, and F. C. BENEDICT 

CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1292-C

A description of Archean rocks 

in the Puritan batholith

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1985
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Geology and geochronology of granitoid and metamorphic rocks of late Archean age in northwestern Wisconsin.

(Contributions to the geology of the Lake Superior Region)(Geological Survey Professional Paper; 1292-C)

iii, 17 p.

Bibliography: p. 15-17

Supt. of Docs. No.: I 19.16:1292-C

1. Granite Wisconsin. 2. Rocks, Metamorphic. 3. Geological time. 4. Geology, Stratigraphic Archean. 

I. Sims, P. K. (Paul Kibler), 1918-. II. Series. III. Series: Geological Survey professional paper; 1292-C. 

QE462.G7G43 1985 552'.3 84-600033

For sale by the Branch of Distribution

U.S. Geological Survey

604 South Pickett Street

Alexandria, VA 22304
CONTENTS

Abstract ............................................. Cl

Introduction ........................................... 1

Geologic setting ......................................... 2

Puritan batholith ......................................... 5

Rock units .......................................... 5

Granitoid rocks ..................................... 5

Biotite gneiss and amphibolite ............................ 5

Mafic dikes and late granite ............................. 9

Chemical compositions ................................... 10

Geochronology ........................................ 11

Correlation .......................................... 12

Structure and metamorphism ............................... 12

Discussion ............................................ 14

References cited ......................................... 15

ILLUSTRATIONS

Page

FIGURE 1. Generalized geologic map of part of northern Wisconsin and adjacent Michigan ...................... C3

2. Concordia diagram of zircon data for sample RL-2, northwestern Wisconsin ....................... 4

3. Rubidium-strontium isochron plot for samples of Early Proterozoic schist and Late Archean gneiss from northwestern

Wisconsin ............................................................ 4

4. Geologic map of Precambrian rocks in part of northwestern Wisconsin ........................... 7

5. Geologic sketch map of Morse area, northwestern Wisconsin, showing sample localities ................. 8

6. K20-Na20-CaO diagram for granitoid rocks in Puritan batholith, northwestern Wisconsin and Michigan ........ 11

7. Rubidium-strontium isochron diagram for rocks and minerals, Puritan batholith ...................... 13

8. Uranium-lead concordia diagram for zircons from rocks in Puritan batholith ........................ 15

9. Generalized geologic map of the Lake Superior region showing relationship of elongate batholiths in the Archean greenstonegranite terrane to Archean boundary (Great Lakes tectonic zone) ............................ 16

TABLES

TABLE 1. Approximate modes of representative samples of Archean rocks in the Puritan batholith, northwestern Wisconsin . . C9

2. Chemical composition of representative samples of Archean rocks in the Puritan batholith, northwestern Wisconsin . . 10

3. Rubidium and strontium data for whole-rock samples and biotite separates, Puritan batholith, and for miscellaneous samples

from northwestern Wisconsin ................................................. 11

4. Potassium and argon data for biotite separates from rocks in the Puritan batholith .................... 12

5. Uranium, thorium, and lead data for zircons .......................................... 14

in

CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

GEOLOGY AND GEOCHRONOLOGY OF GRANITOID AND

METAMORPHIC ROCKS OF LATE ARCHEAN AGE

IN NORTHWESTERN WISCONSIN

By P. K. SIMS, Z. E. PETERMAN, R. E. ZARTMAN, and F. C. BENEDICT

ABSTRACT

Granitoid rocks of the Puritan Quartz Monzonite and associated 

biotite gneiss and amphibolite in northwestern Wisconsin compose 

the southwestern part of the Puritan batholith of Late Archean age. 

They differ from rocks in the Michigan segment of the batholith in 

having been deformed by brittle-ductile deformation and partly recrystallized during shearing accompanying development of the midcontinent rift system of Keweenawan (Middle Proterozoic) age.

Granitoid rocks ranging in composition from granite to tonalite are 

dominant in the Wisconsin part of the batholith. To the north of the 

Mineral Lake fault zone, they are massive to weakly foliated and 

dominantly of granite composition, whereas south of the fault zone 

they are more strongly foliated and mainly of tonalite composition. 

Massive granite, leucogranite, and granite pegmatite cut the dominant granitoid rocks. Intercalated with the granitoid rocks in small 

to large conformable bodies are biotite gneiss, amphibolite, and local 

tonalite gneiss. Metagabbro dikes of probable Early Proterozoic age 

as much as 15 m thick cut the Archean rocks.

Rubidium-strontium whole-rock data indicate a Late Archean age 

for the granitoids and gneisses, but data points are scattered and 

do not define a single isochron. Zircon from two samples of tonalitic 

gneiss for uranium-thorium-lead dating define a single chord on a 

concordia diagram, establishing an age of 2,735 ±16 m.y. The lower 

intercept age of 1,052 ±70 m.y. is in close agreement with rubidiumstrontium and potassium-argon biotite ages from the gneisses.

Two episodes of deformation and metamorphism are recorded in 

the Archean rocks. Deformation during the Late Archean produced 

a steep west-northwest-oriented foliation and gently plunging fold 

axes and was accompanied by low amphibolite-facies metamorphism 

of the bedded rocks. A younger deformation resulting from largely 

brittle fracture was accompanied by retrogressive metamorphism; 

this deformation is most evident adjacent to the Mineral Lake fault 

and took place during Keweenawan rifting about 1,050 m.y. ago. The 

Mineral Lake fault is one of several northwest-trending faults in the 

Lake Superior region that originated in the Late Archean and were 

reactivated intermittently during the Proterozoic, including 

Keweenawan time. The faults dominantly have right-lateral displacements.

The Archean rocks of the Puritan batholith exposed in northwestern Wisconsin compose part of the greenstone-granite terrane, as 

defined in the Lake Superior region. These rocks were formed 2,750- 

2,600 m.y. ago. The long dimension of the Puritan batholith as well 

as that of several batholiths in adjacent Minnesota are oriented subparallel to the boundary between the greenstone-granite terrane and 

the older gneiss terrane, to the south. This conformity in trend is 

interpreted as indicating that the granite probably was emplaced 

after the two basement crustal segments had been joined.

INTRODUCTION

As a part of regional studies of the boundary between the two Archean crustal segments recognized in 

the Lake Superior region (Morey and Sims, 1976; Sims, 

1980), geologic reconnaissance and isotopic dating were 

carried out in an area south of Mellen, in Ashland and 

Bayfield Counties, northwestern Wisconsin. Exposures 

of granite, granitic gneiss, and amphibolite had previously been reported in this area (Button and Bradley, 

1970; Hotchkiss, 1915; Hotchkiss and Bean, 1929), but 

the rocks had not previously been systematically 

examined.

Our studies have shown that the granitoid and associated metamorphic rocks in northwestern Wisconsin 

are grossly correlative with those described in the 

western part of northern Michigan (Schmidt, 1976), and 

accordingly should be included in the large batholith 

of Archean granitoid rocks extending from the vicinity 

of Lake Gogebic, Mich., southwestward into northwestern Wisconsin (fig. 1), which we have called the Puritan 

batholith (Sims and others, 1984). The Archean rocks 

in northwestern Wisconsin differ from those in the 

Michigan segment of the batholith, however, in having 

been deformed by brittle-ductile deformation and partly 

recrystallized during shearing accompanying development of the midcontinent rift system of Keweenawan 

(Middle Proterozoic) age. K-Ar (potassium-argon) biotite ages of 1,050 m.y. are given by deformed rocks 

as much as 15 km from the Keweenawan mafic volcanic 

and intrusive rocks that occupy a large part of the rift 

system. So far as we know, this is the first observation 

in the Lake Superior region of severe disturbance of

ci
C2 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

isotopic systems by brittle-ductile deformation in preKeweenawan rocks at a moderately great distance from 

the Keweenawan rift system. Low-grade metamorphism of country rocks was noted at some 15 km from 

the Mellen Intrusive Complex of Keweenawan (Middle 

Proterozoic) age (Schmidt, 1976), in the Ironwood, 

Mich., area; and isotopic disturbance by thermal effects 

has been noted in Minnesota (Goldich and others, 1961).

Our studies in northwestern Wisconsin support the 

conclusion reached from studies in the Marenisco, 

Mich., area (Sims and others, 1984) that the Archean 

layered rocks associated with the batholith probably 

were deformed in Late Archean time by lateral compression related to impingement of the Archean gneiss 

terrane against the more stable Archean greenstonegranite terrane. In other parts of the greenstone-granite terrane, deformation of the layered greenstone generally is attributed to the diapiric emplacement of Late 

Archean granite into the volcanic pile (Schwerdtner and 

others, 1979).

So far as we know, the Early Proterozoic Penokean 

tectonothermal event had no appreciable effect on the 

Archean rocks in the Puritan batholith. Neither penetrative structures nor isotopic disturbances that can be 

attributed to Penokean tectonism have been definitely 

recognized in the Archean rocks. The apparent absence 

of a Penokean structural overprint on the Archean 

rocks contrasts with observations at the northeast end 

of the batholith, in Michigan, where conspicuous penetrative structures were superposed on Archean structures during the Penokean event (Sims and others, 

1984).

The reconnaissance mapping was done at intervals 

during the summers of 1975, 1976, and 1978. M. G. 

Mudrey, Jr., K. J. Schulz, and J. K. Greenberg assisted in the fieldwork. Mudrey also assisted in collecting 

samples for isotopic analyses, at times under arduous 

conditions. Personnel of the Soo Line assisted in transporting samples collected along the railway.

GEOLOGIC SETTING

The Puritan batholith, as designated herein, is a body 

of Archean granitoid rocks of the Puritan Quartz Monzonite and associated metamorphic rocks that is about 

125 km long and has an exposed width of generally 

more than 15 km (fig. 1). Its actual width is not known 

because the Archean rocks are unconformably overlain 

on the northwest and southeast by Proterozoic rocks. 

Thus, the batholith is similar in size at the present erosional level to the Giants Range batholith in northern 

Minnesota (Sims and Viswanathan, 1972), one of the 

larger known granitoid bodies in the Archean 

greenstone-granite terrane of the Lake Superior region 

and adjacent areas (Morey and others, 1982).

In the western part of northern Michigan, the 

batholith consists mainly of granitic rocks but includes 

bodies of migmatite that have a biotite schist or amphibolite paleosome and a granite or pegmatite neosome. In this area, the major rock type is a pink, equigranular to slightly porphyritic, generally massive 

leucocratic granite that has been named (Schmidt, 1976) 

the Puritan Quartz Monzonite. The granite intrudes 

metavolcanic rocks, called the Ramsay Formation, east 

of Ironwood, and biotite schist at the northeastern extremity of the batholith. In the Ironwood area, Schmidt 

(1976) delineated a few bodies of rather mafic foliated 

rocks. One such body is a foliated hornblende tonalite 

named the Whiskers Creek Gneiss, which is either an 

early intrusive phase or a hybrid rock resulting from 

contamination of granitic magma by older mafic volcanic rocks. On the basis of Rb-Sr (rubidium-strontium) 

data, the age of the Whiskers Creek Gneiss is indistinguishable from that of the Puritan Quartz Monzonite 

(2,650 ±140 m.y.; Sims and others, 1977). The granitoid 

rocks that have been dated have an initial ^Sr/^Sr 

ratio of 0.7015 ±0.0017, which is interpreted as indicating a lower-crust or mantle source.

As described in chapter A , a belt of gneiss, amphibolite, and granitoid rocks, about 5 to 10 km wide, lies 

southeast of the batholith proper in the Marenisco area 

and is separated from the batholith by a 5- to 10-kmwide belt of infolded bedded rocks of Early Proterozoic 

age (Sims and others, 1984). The Archean bedded rocks 

are intruded by small bodies of granite and pegmatite 

that are considered as satellitic bodies from the main 

Puritan batholith. The bedded rocks were deformed 

and metamorphosed in Late Archean time, and together with the granite were deformed and 

metamorphosed again during the Early Proterozoic, 

about 1,800 m.y. ago.

The rocks of the Puritan batholith are overlapped on 

the north by steeply dipping homoclinal sedimentary 

rocks (Marquette Range Supergroup) of the Gogebic 

Range (fig. 1), and on the south by folded and 

metamorphosed volcanic rocks (Xiv unit, fig. 1) that 

probably are grossly correlative with those of the 

Gogebic Range. Southwest of the Gogebic Range, volcanic rocks of the Keweenawan (Middle Proterozoic) 

midcontinent rift system directly overlie the Archean 

rocks. Schmidt (1980) has described the Proterozoic 

strata in the Gogebic Range; we have described the 

Proterozoic rocks on the south flank of the batholith 

(Sims and others, 1984), which also were previously discussed by Alien and Barrett (1915).

The previously noted Archean layered and granitoid 

rocks are part of the greenstone-granite terrane, as delineated in the area east of the midcontinent gravity 

high (Sims, 1980). The southwestern termination of the
GRANITOID AND METAMORPHIC ROCKS OF LATE ARCHEAN AGE, WISCONSIN C3

greenstone-granite terrane is tentatively interpreted as 

a right-lateral fault of large displacement. (See Sims 

and Peterman, 1983.)

The position of the boundary between the 

greenstone-granite terrane and the gneiss terrane is

not known accurately in northwestern Wisconsin, but 

is inferred to lie just southeast of the northeast-trending belt of gneiss, amphibolite, and granitoid rocks 

(Wga unit, fig. 1) that is southeast of the Puritan 

batholith. Presumably the boundary underlies the

EXPLANATION

PALEOZOIC

Sandstone and siltstone 

MIDDLE PROTEROZOIC (800-1,600 M.Y.)

Gabbroic rocks

Lava, layered gabbroic rocks, and sedimentary rocks of midcontinent 

rift system

MIDDLE AND EARLY PROTEROZOIC

Platform quartzite 

EARLY PROTEROZOIC (1,600-2,500 M.Y.)

Granite (age, 1,760 m.y.)

Marquette Range Supergroup

Menominee and Chocolay Groups in Gogebic Range

Dominantly metasedimentary rocks; includes Michigamme 

Formation in Michigan

Dominantly metavolcanic rocks; includes Blair Creek Formation in Michigan

W241.

Mafic to felsic metavolcanic rocks 

Gneiss, amphibolite, and foliated granitoid rocks 

LATE ARCHEAN (2,500 M.Y. AND OLDER)

Granitoid rocks of Puritan Quartz Monzonite (age, about 2,650 m.y.)

Gneiss, amphibolite, and granitoid rocks; includes metagraywacke in

area southeast of Lake Gogebic 

Metavolcanic rocks; includes Ramsay Formation

Biotite schist and migmatite 

Gneiss; may include older rocks 

LATE TO EARLY ARCHEAN

Gneiss, amphibolite, and granitoid rocks (age, 2,600-3,550 m.y.)

CONTACT Approximately located

FAULT -Arrows show relative horizontal movement, where known.

Bar and ball on downthrown side, where known 

LOCALITY OF AGE-DATED SAMPLE

FIGURE L Generalized geologic map of part of northern Wisconsin and adjacent Michigan. (Modified from Morey and others, 1982.)
C4 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

Early Proterozoic metavolcanic rocks (Xiv unit, fig. 1) 

that lie immediately southeast of the gneiss belt previously discussed. The elliptical domes of gneiss, amphibolite, and granitoid rocks (Wgn unit, fig. 1) are 

components of the Archean gneiss terrane. These rocks 

have been described from the dome north of 

Watersmeet, Mich. (fig. 1), and are in part as old as 

3,550 m.y. (Sims and others, 1984).

Recently completed U-Pb (uranium-lead) zircon dating (W. R. Van Schmus, oral commun., 1982) of rocks 

to the southeast of the Puritan batholith (fig. 1) has 

disclosed that layered gneisses at Fifield, Wis., 6.5 km 

south of Park Falls, Wis., and interlayered biotite 

gneiss and amphibolite, 20 km east-northeast of Butternut, Wis. (fig. 4), which is 10 km north of Park Falls, 

are Late Archean. Associated with these rocks are 

biotitic schists, such as those exposed near Powell, Wis. 

(about 50 km east of Butternut) and near Park Falls, 

that are Early Proterozoic in age. Two zircon size fractions from a sample (RL-2) of biotite schist from the 

north end of Blockhouse Lake, Wis., which is 10 km 

northeast of Park Falls, give a discordia intercept age 

of 1,853 m.y. (fig. 2). The Rb-Sr whole-rock isochron 

age for this sample and for two other biotite schist samples in the area (fig. 3), including a kyanite-staurolitegarnet biotite schist from near Powell, is indistinguishable from the zircon age of sample RL-2. These 

radiometric age data are interpreted as indicating that 

Archean and Early Proterozoic rocks in this area are 

intercalated, probably as a result of northeast-trending 

faulting that has juxtaposed rocks of the two ages. The 

deformation was accompanied, or preceded, by 

metamorphism to amphibolite grade, with the local de0.4

0.3

0.2

0.1

0.0

4

207pb/235U

FIGURE 2. Concordia diagram of zircon data for sample RL-2, biotite schist, from vicinity of Blockhouse Lake, northwestern Wisconsin. Intercepts defined by two size fractions are 1,852 ±6 m.y. and 

5 ±29 m.y. Numbers in parentheses designate size fractions.

velopment of the high-pressure mineral kyanite. These 

Archean and Early Proterozoic rocks are interpreted 

as being integral parts of the Archean gneiss terrane; 

they could be underlain by older gneisses, perhaps of 

Middle Archean age.

The rocks southeast of Park Falls and vicinity differ 

in both age and lithology from those previously described. They are metavolcanic rocks (Xv unit, fig. 1) 

and granitoid rocks, including gneiss and amphibolite 

(Xgg unit, fig. 1); the granitoid rocks are exposed in 

the cores of domes and the gneiss and amphibolite 

stratigraphically underlie the metavolcanic rocks. Scattered samples of the granitoid rocks from the area have 

been dated by both the U-Pb zircon method (Van 

Schmus, 1980) and the Rb-Sr whole-rock isochron 

method (Sims and Peterman, 1980) at about 1,850 m.y. 

Inasmuch as outcrops are poor in the area, our current 

interpretation of the relationship between the granitoid 

and metavolcanic rocks is based largely on analogy with 

the geology of northeastern Wisconsin, where Early 

Proterozoic gneiss and amphibolite demonstrably underlie metavolcanic rocks assigned to the Quinnesec 

Formation (Sims and others, 1982), and both the gneiss 

and the Quinnessec are intruded by granitoid rocks. Because of the differences in the age and lithology of these 

rocks with respect to those near Park Falls and areas 

to the northwest and north, a major fault or shear zone 

is inferred to separate the two lithostructural domains, 

as shown on figure 1.

0,90

0.86

0.82

0.78

0.74

0.70

79-7A

79-7B f

2 3

87Rb/86Sr

FIGURE 3. Rubidium-strontium isochron plot for samples of Early 

Proterozoic schist (RL-2, RL-1-1, and W241) and Late Archean 

gneiss (79-109, 79-53, 79-7B, and 79-7A) from northwestern Wisconsin. Data for Archean samples scatter about a reference isochron 

of 2,700 m.y. For the Early Proterozoic samples, data points for 

RL-2 and W241 lie near a reference isochron of 1,820 m.y. for 

metagraywacke from the Marenisco, Mich., area; sample RL-1-1 is 

disturbed.
GRANITOID AND METAMORPHIC ROCKS OF LATE ARCHEAN AGE, WISCONSIN C5

PURITAN BATHOLITH

In northwestern Wisconsin, granitoid rocks and associated metamorphic rocks in the Puritan batholith are 

exposed sporadically over an area of about 1,500 km2 

(fig. 4). Most of the exposures are along the Bad River 

in the vicinity of Cayuga and Morse (see fig. 5), and 

these together with those along the Marengo River and 

its tributaries in extreme eastern Bayfield County (lat 

46°15' N., long 91°00' E.; fig. 4) were examined during 

our study. Because of poor exposures south of these 

localities, the general outline of the southern extremity 

of the Puritan batholith is delineated by widely scattered outcrops and drill-hole data in published reports 

(Hotchkiss, 1915; Hotchkiss and Bean, 1929) and unpublished reports of the Wisconsin Geological and Natural History Survey. The area of granitoid rocks in the 

southwestern part of the batholith includes two pronounced negative Bouguer gravity anomalies (Ervin 

and Hammer, 1974), as shown on figure 4. Electromagnetic data suggest that granite underlies the 

strong negative gravity anomalies, and migmatite or 

gneiss underlies intervening areas (M. G. Mudrey, Jr., 

written commun., 1982).

ROCK UNITS

Granitoid rocks ranging in composition from granite 

to tonalite are the dominant rocks exposed in the southwestern part of the Puritan batholith. Biotite gneiss 

and amphibolite are intercalated with the granitic rocks 

in a broad zone south of the Mineral Lake fault, and 

compose local mappable units of mixed rocks (fig. 5). 

In the Morse-Cayuga area, a tonalite gneiss of probable 

intrusive origin occupies a narrow belt between the two 

metamorphic rock units. The biotite gneiss and amphibolite and the tonalite gneiss are included with granite and tonalite in the Puritan batholith.

GRANITOID ROCKS

The granitoid rocks vary in structure as well as in 

composition. The rocks exposed north of the Mineral 

Lake fault zone (fig. 4) are dominantly massive to 

weakly foliated, and the rocks are physically similar to 

those in northern Michigan (Schmidt, 1976; Sims and 

others, 1977); whereas those south of the fault zone are 

moderately to weakly foliated and at places have a 

gneissic structure. The gneissic structure has mainly 

resulted from ductile deformation; the biotite and felsic 

components are streaked out and an augen structure 

is developed locally.

Three principal varieties of granitoid rocks occur in 

this segment of the batholith. The dominant granitoid 

rocks are pinkish gray or gray, fine to medium grained, 

and equigranular to slightly porphyritic. Both microcline and plagioclase compose the phenocrysts in inequigranular varieties. Modal compositions range from 

granite to tonalite (table 1). Textures are typically 

hypidiomorphic granular except where cataclastic structures mortar structures and shears and accompanying recrystallization have obliterated the primary texture. Plagioclase (An2o-25) commonly has a weak concentric zoning and is myrmekitic adjacent to microcline. 

Microcline occurs both as phenocrysts and as interstitial 

material, and it is both twinned and untwinned. Except 

locally, quartz commonly is highly strained or broken 

and recrystallized. Biotite is brown and forms two generations in many rocks an older, typically frayed and 

partly altered one and a younger, fine-grained, fresh 

brown variety. Moderate alteration is widespread. 

Biotite characteristically is partly altered to chlorite 

and epidote/clinozoisite, and plagioclase is partly altered to sericite and epidote/clinozoisite.

Pink to gray, massive granite and leucogranite and 

coarse granite pegmatite commonly cut the dominant 

granitoid rocks, mainly as dikes as much as a few meters thick. Porphyritic varieties of the younger granite 

are also common. Sample W61-1B (table 1) is typical 

of these rocks. To the south of the Mineral Lake fault, 

these rocks are cataclastically deformed to varying degrees.

The third variety, tonalitic gneiss, is a medium- to 

dark-gray, medium- to coarse-grained, irregularly 

layered gneiss, which at places is cut by massive granite dikes. Thin sections show a moderate to strong mortar structure somewhat annealed by recrystallization 

(W61, table 1). Plagioclase and biotite are altered similarly to that in other granitoid rocks, and two generations of biotite are common.

BIOTITE GNEISS AND AMPHIBOLITE

Biotite gneiss and amphibolite are intercalated at 

various scales with the granitoid rocks and form mappable units in the vicinity of Morse and Cayuga (fig. 5).

Biotite gneiss is dominantly a medium-gray, finegrained, thinly layered rock but includes coarser 

grained biotitic gneisses and migmatitic gneisses. 

Hornblende is a minor constituent in many samples. 

The rocks have granoblastic textures. Plagioclase 

(about An30) is anhedral, typically twinned, and locally 

myrmekitic. Quartz occurs both as discrete grains and 

as aggregates. Biotite generally has two orientations, 

with the younger generation being associated with 

cataclastic textures and recrystallization. Sphene, 

opaque oxides, and zircon are principal minor minerals.

Amphibolite and associated hornblende gneiss are 

dark-gray, massive to layered rocks that contain some 

quartz and biotite. Not uncommonly they have thin 

layers of granite or granite pegmatite (leucosome) and
C6 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

CORRELATION OF MAP UNITS

PALEOZOIC

MIDDLE 

PROTEROZOIC

MIDDLE AND EARLY 

PROTEROZOIC

EARLY 

PROTEROZOIC

46°15'l

25 

^ 

65

DESCRIPTION OF MAP UNITS

PALEOZOIC

Sandstone and siltstone 

MIDDLE PROTEROZOIC

Sedimentary rocks of Oronto Group

Chengwatana Volcanic Group of Hall (1900)

Portage Lake Volcanics

Gabbroic rocks

Mellen Intrusive Complex (age, about 1,000 m.y.)

Sedimentary rocks

Mafic lavas 

MIDDLE AND EARLY PROTEROZOIC

Quartzite

EARLY PROTEROZOIC MARQUETTE RANGE SUPERGROUP 

Tyler Formation 46°00' 

Unnamed biotite schist containing garnet, staurolite, and kyanite 

Ironwood Iron-formation and Palms Formation

Unnamed metavolcanic rocks and lesser sedimentary rocks,

including Iron-formation 

ARCHEAN

Dominantly granite and tonalite of Puritan Quartz Monzonite

Biotite schist and amphibolite 

Metavolcanic rocks

Biotite gneiss, in part migmatitic; amphibolite; and foliated tonalite 

and granite

CONTACT Approximately located

FAULT Arrows show relative horizontal movement, where

known. Bar and ball on downthrown side, where known 

BEARING AND PLUNGE OF MINOR FOLDS

STRIKE AND DIP OF BEDS

91°00'

STRIKE AND DIP OF FOLIATION

STRIKE OF VERTICAL FOLIATION

STRIKE AND DIP OF FOLIATION AND PLUNGE OF LINEATION

ABANDONED IRON MINE OR PROSPECT

OUTCROP

DRILLHOLE

BOUGUER GRAVITY CONTOUR In milligals. Hachures indicate 

closed gravity lows
GRANITOID AND METAMORPHIC ROCKS OF LATE ARCHEAN AGE, WISCONSIN C7

91°00'

46°15'

46°00'

FIGURE 4. Geologic map of Precambrian rocks in part of northwestern Wisconsin. Principal rock types reported in outcrops (x) and drillholes 

( ) (see text for sources) are: gr, granite; gn, gneiss; grgn, granite gneiss; am, amphibolite; and d, diabase. Gravity contours are 

from Ervin and Hammer (1974). (Modified from Sims and others, 1978.)
CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

EXPLANATION

89°35'

MIDDLE AND EARLY PROTEROZOIC

Granite

Sedimentary rocks of Gogebic Range 

LATE ARCHEAN

Granite-tonalite, granite gneiss, and amphibolite

Metavolcanic rocks

Tonalite gneiss

Dominantly biotite- gneiss and granite-tonalite

Dominantly amphibolite and granite-tonalite

50

45

W59

CONTACT Approximately located

APPROXIMATE CONTACT BETWEEN LITHO-TECTONIC ZONES

FAULT Approximately located

BEARING AND PLUNGE OF MINOR FOLDS

STRIKE AND DIP OF BEDS

STRIKE AND DIP OF FOLIATION

STRIKE OF VERTICAL FOLIATION

STRIKE AND DIP OF FOLIATION AND PLUNGE OF LINEATION

SAMPLE LOCALITY

FIGURE 5. Geologic sketch map of Morse area, northwestern Wisconsin, showing sample localities for radiometric age determinations.

Geology by P. K. Sims and M. G. Mudrey, Jr., 1976.
GRANITOID AND METAMORPHIC ROCKS OF LATE ARCHEAN AGE, WISCONSIN C9

TABLE 1. Approximate modes, in volume percent, of representative samples ofArchean rocks in the Puritan batholith,

northwestern Wisconsin

[Tr, trace; leaders (-), not determined]

Sample No. W68 W71 W72B W60 W90Y W95A W99 W105B W61-1B W57 W105A W59A W61 W88 W104A W90X W95B W95C

Plagioclase-- 49.5 54.3 

Quartz 38.5 34

Potassium 

feldspar 9 3

Rint-ito--- _ 7 t; ft

60 

24

6 

in

30 

37

23 

in

29

32

t;

40 

29

21

7 H

54 

19

17

£ R

32

33

9

37 

44

7.5 

1 1 t:

53

3

R R

56

Oft C

7 

ft

63 

27

0 

in

52

0 

is

48 

39

0

7

54 

27

0 

1ft

42 

5 5

0 

1 R

50 

20

0 

?

57 

12.5

0 

? R

Hornblende 

Epidote 

Opaque oxides 

Accessory 

minerals---

0 

0 

Tr

Tr

) 0

) 0

) 0

.7 Tr

0

0

0

Tr

0

0

Tr

.5 

.5

0

1.5

1

1.5 1

0

0

Tr

Tr

0

0

0

Tr

0 

0 

Tr

.5

) 0

) Tr

) Tr

.5 Tr

0

Tr 

Tr

Tr

0

Tr 

Tr

36 

14 

Tr

27 

Tr 

Tr

21.5

1

3

2.5

Plagioclase 

composition ^25 An on An 25 An 25 An 25 An 25 An 25 An 28 An 32 An 29 An 25 An 25

SAMPLE DESCRIPTIONS AND LOCALITIES

W68 SW1/4SWI/4 sec. 23, T. 44 N., R. 3 W. Pinkish-gray, medium-grained tonalite.

W71 Ne1/4SW1/4 sec. 23, T. 44 N., R. 3 W. Porphyritic tonalite.

W72B SE1/4NW1/4 sec. 23, T. 44 N., R. 3 W. Porphyritic tonalite; contains cataclastic streaks.

W60 NWViSEVi sec. 8, T. 43 N., R. 2 W. Foliated porphyritic granite.

W90Y SWV4 sec. 36, T. 44 N., R. 5 W. Porphyritic granite.

W95A SEViSEVi sec. 7, T. 44 N., R. 2 W. Gray foliated granite.

W99 NE1/4NE1/4 sec. 17, T. 43 N., R. 2 W. Foliated granodiorite.

W105B NEViNWV^ sec. 36, T. 44 N., R. 3 W. Pink, fine-grained granite.

W61-1B SW1/4NW1/4 sec. 6, T. 43 N., R. 2 W. Pinkish-gray, fine-grained tonalite dike.

W57 NE'ASWVi sec. 16, T. 43 N., R. 2 W. Weakly foliated tonalite; has mortar texture.

W105A Same locality as W105B. Foliated tonalite.

W59A NWV4NEV4 sec. 9, T. 43 N., R. 2 W. Layered tonalite gneiss.

W61 SEViNWVi sec. 6, T. 43 N., R. 2 W. Gray, medium- to coarse-grained layered tonalite gneiss.

W88 NEViNEV* sec. 35, T. 44 N., R. 5 W. Layered biotite gneiss; has some mortar texture.

W104A SE1/4NW1/4 sec. 36, T. 44 N., R. 3 W. Fine-grained, layered biotite gneiss.

W90X Same locality as W90Y. Layered amphibolite.

W95B Same locality as W95A. Weakly foliated hornblende gneiss.

W95C Same locality as W95A. Hornblende gneiss.

have a conspicuously banded appearance. The 

hornblende is green or bluish green and commonly elongated, giving a prominent mineral lineation. Opaque 

oxides, sphene, biotite, and chlorite are minor minerals. 

Generally, the rock is moderately altered; sericite has 

replaced plagioclase and epidote forms rims on 

hornblende.

MAFIC DIKES AND LATE GRANITE

Metagabbro dikes as much as 15 m thick cut the Archean rocks. They seem to lack any systematic pattern 

of orientation and instead occupy diversely oriented 

fractures. At places the dikes are deformed, mainly by

mild folding or by breaking into boudins. Despite the 

deformation, however, primary ophitic textures commonly are retained, although plagioclase in the rocks 

may be comminuted and recrystallized in fine-grained 

aggregates. The cataclastically deformed rocks also 

have crosscutting veinlets of epidote/clinozoisite. The 

dikes are mainly massive, and consist of pale-green 

hornblende and actinolitic hornblende, albite or, at 

places, zoned plagioclase, and minor amounts of sphene, 

leucoxene, opaque oxides, epidote/clinozoisite, chlorite, 

and sericite.

In addition to the late-stage metagabbro dikes, a 

small body of epizonal granite was observed along the
CIO CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

TABLE 2. Chemical composition of representative samples ofArchean rocks in the Puritan batholith, northwestern Wisconsin

[Rubidium and strontium analyses given in table 3. Chemical analyses of samples W99, W108, and W59A by Hezekiah Smith, samples W61, W104A, and W105A by Z. A. Hamlin, 

and samples W105BB, W61-1B, W61-1A, W104X, W105X, and W95C by Z. A. Hamlin and Nancy Skinner. Uranium and thorium analyses by H. T. Millard, Jr., and others. 

Leaders (-), not determined]

Major oxides (in weight percent)

Sample No. W99 W105BB W61-1B W105A W59A W61 W61-1A W104A W104X W105X W108 W95C

SiQ2 -

A1 20 3 - 

FeoOo -

L- O

FeO -

MgO -

CaO -

Na 2 0 

v n

u n+

H

n~

Ti02 - 

D P 2°5 n ' 

MnO -

m

 68.7

ICO

 1.1

 2.2

QQ

2 n

3 7

 4.2

.69

.14

11

05

.06

. QQ

73 2

JL O . L.

QQ

1.2

3

1.2

*j * *J

4.8

C O

.04

1 P

.05

 03\J+J

.02

QQ

70.9

14.1

1.1

2.2

Q1

1.7

4.4

2.2

QQ

.06

 35*J-J

.12

.04

.02

QQ

CO O

1C O

.6

1.0

.79 

3 0 *J m \J

4.8

2.1 

.7

.09 

.16

no

.02 

.12

QQ

\J J * *J

JL *J £_

70

2

£

1.1

O «J

4.2

1.7 

55

 JL J

 O *J

.11

.05

no

mn

CQ O

1C O

2.0

1 3 A » *

3 1 ** . A

3 Q

1.6 

.9

.1

3

.09

.04 

.01

QQ

CO "7

16.6

1.7

3 7

2.1

4.8

3 9 ** . y 

1 P

1.1

.07

.57

1 P

.09

.02

QQ

67.6

1 C O

.71

2.0

1.6

3 7 v » /

4.2

1.4

.75

 A 13**

op -

.17

.04

.02

QQ

61 5

1 C Q

2.4

4 5

2

C

6.6

3 9 ** ?

.66

QC

03

CO

.20

np

.06

inn

74.7

11.9

1.2

1.7

.46

2 5

3.7

1.6

.64

.04

.21

.07

.01

.07

QQ

60.5

17.5

1.3

4.8

2.4

4.7

4 1 " j. 

1 P.

1.2

.16

.59

.21

np

05

QQ

en P

16.0

4.1

7.0

4 5

8.4

4.0

1.1

1.1

.04

1.4

oc

.16

.04

QQ

Trace elements (in parts per million)

U-- 

Th1.1 

35.0

1.1 

5.3

0.7 

11.5

0.5 

1.6

3.2 0.6 

2.5

SAMPLE DESCRIPTIONS AND LOCALITIES

Modes and sample localities for W99, W61-1B, W105A, W59A, W61, W109A, and W95C are given in table 1.

W105-BB NEVMWV* sec. 36, T. 44 N., R. 3 W. Pink, fine-grained granite.

W61-1A SWy4NWy4 sec. 6, T. 43 N., R. 2 W. Gray, medium-grained tonalite gneiss.

W104X SE^NW^ sec. 36, T. 44 N., R. 3 W. Layered biotite gneiss.

W105X NE1/4NW1/4 sec. 36, T. 44 N., R. 3 W. Medium-grained foliated tonalite; contains 42 percent plagioclase, 46 percent quartz, 8

percent potassium feldspar, and 4 percent biotite. 

W108 SEV4NWV4 sec. 6, T. 43 N., R. 2 W. Tonalite gneiss.

railway tracks adjacent to the Bad River, 4 km north 

of Cayuga (fig. 5). The granite is a pale-red, finegrained, slightly porphyritic massive rock. In thin section, it is seen to be partly altered but, except for undulose extinction of feldspars and quartz, it is not visibly deformed. The rock contains slightly perthitic 

microcline, concentrically zoned plagioclase, darkbrown biotite, and traces of green hornblende. The 

plagioclase is mildly altered to sericite, clinozoisite, and 

zeolite(?) minerals. Biotite is partly altered to epidote/ 

clinozoisite, chlorite, and quartz. Sphene, zoned zircon, 

opaque oxides, allanite, and calcite are minor minerals. 

A Rb-Sr determination indicates that the granite is 

Keweenawan.

CHEMICAL COMPOSITIONS

Analyses of selected samples of Archean rocks in the 

area are listed in table 2. Samples W99 and W105BB 

fall within the range of granite, as defined by Streckeisen (1976). The other felsic samples are either 

granodiorite or tonalite in composition on the basis of 

their chemistry. Sample W95C, the hornblende gneiss, 

has the composition of a tholeiitic basalt.

Samples W99 and W105BB are similar in mineralogy, 

major elements, and rubidium and strontium contents 

to the main granitic phase of the Puritan Quartz Monzonite in Michigan. The other samples, including biotite 

gneiss and amphibolite, form a well-defined sodic trend
GRANITOID AND METAMORPHIC ROCKS OF LATE ARCHEAN AGE, WISCONSIN Cll

on a K20-Na20-CaO diagram (fig. 6) and show little 

evidence for potassium enrichment with decreasing calcium, which would indicate a compositional linkage with 

the granitic rocks.

The tonalitic and granodioritic rocks are also characterized by high strontium and low rubidium contents, 

and have rubidium to strontium ratios in the range of 

0.09 to 0.21 features that also distinguish these from 

the granitic rocks.

The compositional features shown by these limited 

data suggest that the tonalitic and granodioritic rocks 

probably were derived from different sources than 

those of the main granitic phase of the batholith.

GEOCHRONOLOGY

Rb-Sr analyses were made of selected whole-rock 

samples and biotite, and K-Ar ages were determined 

for the biotites. U-Th-Pb (uranium-thorium-lead) 

analyses were made for zircons from two samples. 

Isotopic and decay constants used in all of the age calculations are those recommended by the IUGS Subcommission on Geochronology (Steiger and Jager, 1977). 

Analytical uncertainties are the same as those reported 

in Peterman and others (1980).

Rb-Sr data (table 3) for the tonalitic gneisses and 

associated rocks in the Puritan batholith generally indicate a Late Archean age, but do not define a single 

isochron, because of appreciable scatter of data points 

that exceed analytical errors (fig. 7). Biotite separates 

from two samples (W104A and W61-1A, table 3, fig. 

5) have Rb-Sr ages of about 900 m.y., and K-Ar ages 

are slightly greater at 1,053 m.y. and 1,043 m.y., respectively (table 4). We interpret these ages as reflectCaO

GRANITOID ROCKS, 

NORTHWESTERN WISCONSIN

GRANITOID ROCKS, 

MICHIGAN-Reported by 

Sims and others, 1984

K-,0 Na-,0

FIGURE 6. K^-Ns^O-CaO diagram for granitoid rocks in Puritan 

batholith, northwestern Wisconsin and Michigan.

ing brittle-ductile deformation and recrystallization during the Keweenawan.

As previously described, sample W67 is an undeformed and only slightly altered granite from a small

TABLE 3. Rubidium and strontium data for whole-rock samples and 

biotite separates, Puritan batholith, and for miscellaneous samples 

from northwestern Wisconsin

[Analyst: Kiyoto Futa. Suffix letters: WR, whole rock; Bi, biotite]

Sample No. Rb Sr 87Rb/ 86 Sr 87 Sr/ 86Sr

(ppm)

Puritan batholith

W59(WR)

W61(WR)

W61-1A(WR)

W61-lA(Bi)

W61-1B(WR)

W95C(WR)

W99(WR)

W104(WR)

W104(Bi)

W104X(WR)

W105A(WR)

W105X(WR)

W105BB(WR)

W108(WR)

59.2

33.4

39.6

209.0

43.1

16.8

107.1

47.3

367.0

14.4

65.1

42.8

147.4

38.2

277.4

260.2

336.6

13.85

204.3

286.1

453.3

717.0

13.83

243.4

763.6

294.7

184.3

370.5

0.619

.372

.341

46.31

.611

.170

.685

.191

84.77

.171

.247

.421

2.333

.298

0.7277

.7174

.7157

1.3216

.7284

.7074

.7260

.7104

1.7650

.7073

.7112

.7174

.7848

.7148

Miscellaneous samples

W67(WR)

RL-l-l(WR)

RL-2(WR)

W241(WR)

79-7A(WR)

79-7B(WR)

79-53(WR)

79-109(WR)

188.7

64.6

96.0

101.6

126.2

100.9

86.0

20.0

151.9

159.2

574.3

174.7

89.8

107.7

289.9

390.2

3.617

1.179

.484

1.689

4.130

2.741

.862

.148

0.7639

.7412

.7155

.7488

.8597

.8196

.7337

.7094

SAMPLE DESCRIPTIONS AND LOCALITIES

W59 through W108 Rock names and localities are given in tables

1 and 2. 

W67 Pink, medium-grained biotite granite. SWVi sec. 23, T. 44 N.,

R. 3W. 

RL-1-1 Pinkish-gray, fine-grained, garnetiferous biotite schist.

SWViSWi sec. 11, T. 40 N., R. 1 E. 

RL-2 Gray, medium-grained garnetiferous biotite schist.

NWV4SEV4 sec. 11, T. 40 N., R. 1 E.

W241 Gray, coarse-grained, kyanite-, staurolite-, and garnet-bearing biotite schist. NE1/4NW1/4 sec. 33, T. 42 N., R. 4 E. 

79-7A Gray, fine-grained biotite gneiss from abandoned railway in

Fifield. SE1/4SW1/4 sec. 6, T. 39 N., R. 1 E. 

79-7B Light-gray, medium-grained biotite gneiss; interlayered with

79-7A. Same location as 79-7A. 

79-53 Gray, fine-grained, layered garnet-biotite gneiss, north side

of Flambeau River. SWV4NEV4 sec. 4, T. 41 N., R. 2 E. 

79-109 Dark-gray, weakly layered amphibolite. SE1/4SE1/4 sec. 3, T.

41 N., R. 1 E. Collected from Pete's landing.
C12 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

TABLE 4. Potassium and argon data for biotite separates from 

rocks in Puritan batholith

[Analyses: Harald Mehnert (Ar) and Paul Klock (K20); Ar*, mean radiogenic Ar; sample 

localities given in tables 1 and 2]

Sample No.

W61-1A

W104A

K20 

(pet)

8.69

8.84

8.90

40Ar* 

(moles/g)

1.766 x 1CT 8

1.827 x 10' 8

40Ar* 

(pet)

99.3

99.0

Age 

(m.y.)

1,043±17

1,053±17

plug adjacent to the Mineral Lake fault (fig. 5). A 

model Rb-Sr age is 1,180 m.y., based on a mantle-like 

initial ^Sr/^Sr ratio of 0.7027. If the rock has remained 

a closed system with respect to rubidium and strontium 

since crystallization, this model age is a maximum 

value. Higher initial ratios would lower the model age, 

for example, to 1,138 m.y. with an initial ratio of 0.705, 

and 1,042 m.y. with an initial ratio of 0.710. Even with 

these uncertainties, the model calculations clearly establish the granite as Keweenawan. The lack of significant deformation indicates that it was emplaced after 

the major movement along the Mineral Lake fault.

The excessive scatter in the whole-rock data for the 

Archean samples probably results from open-system 

behavior during cataclasis and recrystallization. In addition, the systems would have been highly susceptible 

to surficial alteration because of the internal discordances of the minerals as indicated by the Rb-Sr biotite 

ages.

Zircon separated from two samples of tonalitic gneiss 

for U-Th-Pb dating (table 5) establishes a Late Archean 

age for these units. Zircon from W105X is brown, 

somewhat rounded, highly fractured, and iron stained. 

Its morphology is simple prism and pyramidal faces 

with length-to-width ratios of 2:1 to 3:1. As might be 

expected from much lower uranium and thorium contents (table 5), zircons from W61-1A are much less degraded physically than those from W105X. A large 

proportion of the W61-1A population is clear and unfractured with a gradation to darker and somewhat 

fractured grains. Sharp face intersections are typically 

well preserved. The grains are more elongate, on the 

average, than those from W105X, and the terminations 

are much steeper pyramids.

Two hand-picked size fractions of each sample define 

a single chord on the concordia diagram (fig. 8) with 

an upper intercept of 2,735 ±16 m.y. The regression 

is that of Ludwig (1982), and the fit of the chord is 

well within analytical uncertainty as indicated by a 

MSWD (mean square of the weighted deviates) of 0.58. 

(A value of less than 1 indicates a fit within the assigned analytical errors.) Accordingly, we interpret the 

upper intercept as the primary age of the tonalitic 

rocks. The lower intercept age of 1,052 ±70 m.y. is of 

particular interest in that it agrees with the Rb-Sr and 

K-Ar biotite ages. The well-defined chord is best interpreted as resulting from discordance (lead loss) imposed 

at the time indicated by the lower intercept. This age 

lends further support to the effects of a Keweenawan 

disturbance on these rocks.

The age of 2,735 ±16 m.y. is somewhat greater than 

the Rb-Sr age of 2,650 m.y. for the main granitic phase 

of the Puritan Quartz Monzonite in Michigan, but the 

latter has a large uncertainty of ±140 m.y. Within 

error, the zircon age is identical to the 2,750 m.y. age 

obtained for zircon from a tonalitic gneiss interlayered 

with amphibolite southeast of Marenisco (Peterman and 

others, 1980). It is also similar to a zircon age of 

2,718 ±67 m.y. for a tonalitic phase of the Giants Range 

batholith in Minnesota (USGS, unpub. data).

On the Rb-Sr diagram (fig. 7), two reference isochrons corresponding to the zircon age are constructed 

through the respective samples. Data for the other 

whole-rock samples generally scatter about these lines. 

Sample W105BB, a granite, has a much higher Rb/Sr 

ratio than do the tonalites, and the model age of 2,480 

m.y. indicates some disturbance.

CORRELATION

The massive and foliated varieties of granitoid rocks 

in the area are considered correlative with the Puritan 

Quartz Monzonite as delineated by Schmidt (1976) in 

the western part of northern Michigan and described 

by Sims and others (1977) for a broader area in Michigan. The proportion of tonalite apparently is higher in 

northwestern Wisconsin than in northern Michigan, 

however, and in this regard the granitoid rocks in the 

Wisconsin area are similar to the satellitic bodies of 

Puritan Quartz Monzonite in the Marenisco area described by us in chapter A (Sims and others, 1984). 

The tonalite gneiss has no known correlative in northern Michigan, but it could be comparable to the Whiskers Creek Gneiss. The biotite gneisses and amphibolite are grossly correlative with the metavolcanic rocks 

in Michigan that have been formally named the Ramsay 

Formation. The biotite gneisses had a dacitic or volcanogenic graywacke protolith and the amphibolite had 

a mafic lava or pyroclastic protolith.

STRUCTURE AND METAMORPHISM

Two major episodes of deformation and metamorphism are recorded in the Archean rocks in this area, 

an older one that produced west-northwest-oriented 

layering and foliation and was accompanied by low am-
GRANITOID AND METAMORPHIC ROCKS OF LATE ARCHEAN AGE, WISCONSIN C13

phibolite-facies metamorphism of the bedded rocks, and 

a younger deformation resulting from brittle-ductile deformation that was accompanied by retrogressive 

metamorphism. The former occurred during the Late 

Archean and the latter during Keweenawan (Middle 

Proterozoic) rifting.

The Archean deformation produced penetrative, 

steeply dipping structures and minor folds that plunge 

moderately to either west or east. The structures were 

mainly formed by ductile deformation, as indicated by 

alined platy minerals in foliation planes and elongated 

minerals oriented parallel to fold axes. The foliation in 

the granitoid rocks is interpreted as having resulted 

from intrusion of magma into the bedded rocks during 

the Late Archean deformation; apparently deformation 

ceased before the granite crystallized entirely.

The younger deformation produced a west-northwest-trending cataclastic foliation that locally obliterated the older structures in the Archean rocks. Commonly, however, two or more S-surfaces are visible in 

outcrop, both being defined mainly by oriented biotite 

and hornblende. This deformation was not pervasive; 

instead, it was intense along and immediately south of 

the Mineral Lake fault zone and locally intense in areas 

farther south.

Metamorphism of the Archean gneisses and amphibolite during the folding produced andesine and a green 

hornblende in the mafic rocks and oligoclase-andesine 

and, locally, almandine garnet in the biotitic rocks. Although the granitoid intrusive rocks were foliated in 

part during this tectonism, they were not appreciably 

metamorphosed. The younger metamorphism that ac0.790

W105BB.J,

0.780 

0.770

0.760

0.750

' T model =2,480 m.y.

c

t/5

W61-1A

0.740

0.730

0.720

0.710

XX

X 2,736 m.y

X^

920 m.y.

W61 - 1 V^/?X

WM>>oW99

W104A 

870 m.y.

XX

W104X

0.700

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

FIGURE 7. Rubidium-strontium isochron diagram for rocks and minerals, Puritan batholith. Solid lines are isochrons drawn through data 

points for biotite and their respective whole-rock samples. Dashed lines are reference isochrons based on the uranium-lead zircon ages 

for samples W105X and W61-1A. Dash-dot line through sample point W105BB corresponds to the model age of this sample.
C14 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

TABLE 5. Uranium, thorium, and lead data for zircons

[Isotopic composition of initial lead is assumed to be ^Pb^Pb^Pb^Pb = 1:12.12:13.77:32.05 for samples W61-1A and W105X and = 1:15.47:15.26:35.10 for sample RL-2. Numbers

in parentheses below atom ratios, ages in millions of years. Analyst: Loretta Kwak]

Concentration

Sample No.*

W61-1A(100-150)

W61-1A(200-270)

W105X(50-100)

W105X(-270)

RL-2(200-250)

RL-2(-325)

U

190.5

190.1

762.4

773.2

804.3

747.4

(ppm)

Th

49.1

50.4

312.3

394.2

514.2

532.8

Pb

91.1

96.4

351.3

347.9

243.2

184.1

Isotopic composition of lead

204pb

0.015

.020

.018

.031

.026

.012

(atom

206 pb

79.44

78.47

76.53

75.47

75.38

74.68

pet)

207 pb

14.33

14.41

13.39

13.14

8.88

8.61

208pb

6.21

7.10

10.07

11.37

15.71

16.69

206 pb

238u

0.4405

(2,353)

.4612

(2,445)

.4086

(2,209)

.3926

(2,135)

.2634

(1,507)

.2140

(1,250)

Atom ratios

207p b

235u

10.8089

(2,507)

11.4755

(2,563)

9.6922

(2,406)

9.1539

(2,354)

4.1114

(1,657)

3.3396

(1,490)

207 Pb

206 pb

0.1782

(2,636)

.1807

(2,659)

.1722

(2,579)

.1693

(2,551)

.1132

(1,852)

.1132

(1,851)

208Pb

232-pn

0.1196

(2,284)

.1388

(2,627)

.1202

(2,294)

.1031

(1,984)

.0788

(1,533)

.0635

(1,244)

Numbers in parentheses after sample numbers are screen sizes of the zircons.

companied the cataclastic deformation was retrogressive. Assemblages containing sericite, epidote/clinozoisite, biotite, and chlorite were developed at this time. 

This greenschist facies metamorphism is well shown by 

mineral assemblages in the metagabbro dikes in the 

area. They contain pale-green hornblende and actinolite, albite, and varying amounts of leucoxene, sphene, 

opaque oxides, chlorite, sericite, epidote, and zeolite(?) 

minerals, indicative of greenschist facies metamorphism.

The retrogressive alteration apparently caused a partial breakdown of biotite to potassium feldspar, for barrel-shaped lenses of potassium feldspar commonly occur 

in biotites in granitoid rocks. This alteration is similar 

to that described in chapter A in Archean rocks of the 

Marenisco area (Sims and others, 1984).

Of the numerous known and inferred faults in the 

area, the Mineral Lake fault is particularly worthy of 

discussion. It belongs to a family of northwest-trending 

faults in the Lake Superior region that originated in 

the Late Archean and were reactivated intermittently 

during the Early and Middle Proterozoic (Sims and 

Peterman, 1982). The faults dominantly have had rightlateral displacements. As can be seen on figure 2, the 

Mineral Lake fault cuts and displaces the Early Proterozoic iron-formation and associated strata on the 

west end of the Gogebic Range, and has an apparent 

horizontal displacement of about 6 km; other faults of 

this trend to the south displace the same rocks lesser 

distances. The iron-formation terminates to the south 

against a fault of this trend. Clearly, the Mineral Lake

fault was the locus of major movement during the 

Keweenawan rift event. Although it is not exposed in 

this area, and its location is inferred mainly by offsets 

of lithologic units and by a lineament on magnetic 

maps, subsidiary mylonitic fracture zones are abundant 

in the vicinity of the fault. We interpret the faulting 

and related cataclasis as having culminated about 1,050 

m.y. ago, at the time when K-Ar biotite ages were 

reset in some rocks and when the U-Pb systems in the 

zircon record a lead-loss event.

DISCUSSION

The Archean rocks that compose the Puritan 

batholith are definitely part of the greenstone-granite 

terrane, as defined in the Lake Superior region (Sims, 

1980). Previously, we had tentatively considered the 

gneisses south of the Mineral Lake fault as being a part 

of the Archean gneiss terrane (Sims and others, 1978; 

Sims, 1980). The ages obtained on rocks from northwestern Wisconsin are indistinguishable from those in 

other parts of the greenstone-granite terrane in the region (Peterman, 1979), and it is becoming increasingly 

certain that all the rocks in this terrane within the region were formed 2,750-2,600 m.y. ago.

The long dimension of the Puritan batholith, as defined herein, and internal structures within it are subparallel to the boundary between the two Archean terranes identified in the region, as shown on figure 9. 

In the same way, the elongate Giants Range batholith 

and adjacent granitic bodies are oriented roughly parallel to the boundary, as is the northern complex of the
GRANITOID AND METAMORPHIC ROCKS OF LATE ARCHEAN AGE, WISCONSIN C15

Marquette Range. The Archean structures in the 

greenstone-granite terrane seem to reflect the sigmoidal structure of the boundary zone, which in part has 

resulted from the northwest-trending faults that displace the boundary. This conformity of major granitoid 

bodies in the greenstone terrane with the basement 

boundary possibly could have several explanations, but 

we prefer a model whereby the granite was emplaced 

after the two basement crustal segments had been 

joined, and one in which the strain field in the vicinity 

of the boundary was controlled by interaction of the 

two Archean crustal segments. As discussed here and 

in our report concerning the Marenisco area (Sims and 

others, 1984), the Archean metavolcanic-metasedimentary rocks adjacent to the boundary were deformed 

on axes subparallel to it, and granitoid bodies were 

emplaced near the end of or after the deformation, and 

accordingly generally escaped deformation. Possibly the 

granitoid bodies were emplaced during periods of extension oriented approximately at right angles to the 

boundary.

The parallelism of Archean and younger Proterozoic 

structures (Sims and Peterman, 1984) with the basement boundary supports our previous contention (Sims 

and Peterman, 1981; Sims and others, 1980) that the 

feature is a major crustal structure in the upper midcontinent region. It has been named the Great Lakes 

tectonic zone (Sims and others, 1980). The boundary 

between the two crustal segments was a zone of weakness that was repeatedly reactivated, and to a large 

extent younger Precambrian tectonic features 

mimicked the older, primary structural grain.

0.54 

0.50 

0.46 

0.42

0.38 -

0.34

W61-1A(200-270) 

W61-1A(1OO-150)

W105X(50-100) 

W105X(-270)

11

207pb/235u

FIGURE 8. Uranium-lead concordia diagram for zircons from samples 

W105X and W61-1A, Puritan batholith. Intercepts are 2,735±16 

m.y. and 1,052±70 m.y. Numbers in parentheses designate size 

fractions. The ellipses correspond to assigned anaty jcal error.

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Alien, R. C., and Barrett, L. P., 1915, Contributions to the preCambrian geology of northern Michigan and Wisconsin: Michigan 

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Dutton, C. E., and Bradley, Reta, 1970, Lithologic, geophysical, and 

mineral commodity maps of Precambrian rocks in Wisconsin: 

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Map 1-631, scale 1:500,000.

Ervin, C. P., and Hammer, S. H., 1974, Bouguer anomaly gravity 

map of Wisconsin: Wisconsin Geological and Natural History Survey, scale 1:500,000.

Goldich, S. S., Nier, A. 0., Baadsgaard, Halfdan, Hoffman, J. H.,and 

Krueger, H. W., 1961, The Precambrian geology and geochronology of Minnesota: Minnesota Geological Survey Bulletin 

41, 193 p.

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Hotchkiss, W. 0., 1915, Mineral land classification: Wisconsin Geological and Natural History Survey Bulletin 44, Economic Series 

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Hotchkiss, W. 0., and Bean, E. F., 1929, Mineral lands of part of 

northern Wisconsin: Wisconsin Geological and Natural History 

Survey Bulletin 46, Economic Series 21, 212 p.

Ludwig, K. R., 1982, Programs for filing and plotting U-Pb isotope 

data for concordia diagrams, using an HP-9830 computer and 

HP-9832 plotter: U.S. Geological Survey Open-File Report 82- 

386, 22 p.

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. 87, p. 141-152.

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 

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Peterman, Z. E., 1979, Geochronology and the Archean of the United 

States: Economic Geology, v. 74, p. 1544-1562.

Peterman, Z. E., Zartman, R. E., and Sims, P. K., 1980, Tonalitic 

gneiss of early Archean age from northern Michigan, in Morey, 

G. B., and Hanson, G. N., eds., Selected studies of Archean 

and lower Proterozoic rocks, southern Canadian Shield: Geological Society of America Special Paper 182, p. 125-134.

Schmidt, R. G., 1976, Geology of the Precambrian W (lower Precambrian) rocks in western Gogebic County, Michigan: U.S. 

Geological Survey Bulletin 1407, 40 p.

 1980, The Marquette Range Supergroup in the Gogebic iron 

district, Michigan and Wisconsin: U.S. Geological Survey Bulletin 1460, 96 p.

Schwerdtner, W. M., Stone, D., Osadetz, K., Morgan, J., and Stott, 

G. M., 1979, Granitoid complexes and the Archean tectonic record in the southern part of northwestern Ontario: Canadian 

Journal of Earth Sciences, v. 16, p. 1965-1977.

Sims, P. K., 1980, Boundary between Archean greenstone and gneiss 

terranes in northern Wisconsin and Michigan, in Morey, G. B., 

and Hanson, G. N., eds., Selected studies of Archean and lower 

Proterozoic rocks, southern Canadian Shield: Geological Society 

of America Special Paper 182, p. 113-124.

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U.S. Geological Survey Open-File Report 78-318, scale 

1:250,000.
C16 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

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GRANITOID AND METAMORPHIC ROCKS OF LATE ARCHEAN AGE, WISCONSIN C17

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, Part 

I, v. 91, p. 690-698.

Sims, P. K., and Peterman, Z. E., 1980, Geology and Rb-Sr age 

of lower Proterozoic granitic rocks, northern Wisconsin, in 

Morey, G. B., and Hanson, G. N., eds., Selected studies of Archean and lower Proterozoic rocks, southern Canadian Shield: 

Geological Society of America Special Paper 182, p. 139-146.

 1981, Archean rocks in the southern part of the Canadian 

Shield A review, in Glover, J. E., and Groves, D. I., eds., 

Archaean geology Second International Symposium, Perth, 

1980: Geological Society of Australia Special Publication 7, p. 85- 

98. 

-1983, Evolution of Penokean foldbelt, Lake Superior region,

and its tectonic environment, in Medaris, L. G., Jr., ed., Early 

Proterozoic geology of the Great Lakes region: Geological Society 

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Sims, P. K., Peterman, Z. E., and Prinz, W. C., 1977, Geology and 

. Rb-Sr age of Precambrian W Puritan Quartz Monzonite, northern Michigan: U.S. Geological Survey Journal of Research, v. 

5, p. 185-192.

Sims, P. K., Peterman, Z. E., Prinz, W. C., and Benedict, F. C., 

1984, Geology, geochemistry, and age of Archean and Early 

Proterozoic rocks in the Marenisco-Watersmeet area, northern 

Michigan: U.S. Geological Survey Professional Paper 1292-A, p. 

A1-A41.

Sims, P. K., Peterman, Z. E., and Schulz, K. J., 1982, Dunbar gneiss 

dome and implications for the Proterozoic stratigraphy of northern Wisconsin [abs.]: Annual Institute on Lake Superior Geology, 28th, International Falls, Minnesota, Proceedings, p. 44.

Sims, P. K., and Viswanathan, S., 1972, Giants Range batholith, in 

Sims, P. K., and Morey, G. B., eds., Geology of Minnesota A 

centennial volume: Minnesota Geological Survey, p. 120-139.

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359-362.

Streckeisen, A. L., 1976, To each plutonic rock its proper name: 

Earth Science Reviews, v. 12, p. 1-33.

Van Schmus, W. R., 1980, Chronology of igneous rocks associated 

with the Penokean orogeny in Wisconsin, in Morey, G. B., and 

Hanson, G. N., eds., Selected studies of Archean and lower Proterozoic rocks, southern Canadian Shield: Geological Society of 

America Special Paper 182, p. 159-168.

U. S. GOVERNMENT PRINTING OFFICE 1985/576-049/20,027
Contributions to the

Geology of the

Lake Superior Region

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1292

This volume was published as 

chapters A and B, and 

chapter C

CONTENTS

[Letters designate the chapters]

(A) Geology, geochemistry, and age of Archean and Early Proterozoic rocks in the 

Marenisco-Watersmeet area, northern Michigan, by P. K. Sims, Z. E. Peterman, 

W. C. Prinz, and F. C. Benedict.

(B) Geologic interpretation of gravity data, Marenisco-Watersmeet area, northern Michigan, 

by J. S. Klasner and P. K. Sims.

(C) Geology and geochronology of granitoid and metamorphic rocks of Late Archean age 

in northwestern Wisconsin, by P. K. Sims, Z. E. Peterman, R. E. Zartman, and 

F. C. Benedict.