Geology, Geochemistry, and Age of 

Archean and Early Proterozoic 

Rocks in the Marenisco-Watersmeet 

Area, Northern Michigan

and

Geologic Interpretation 

of Gravity Data, 

Marenisco-Watersmeet Area, 

Northern Michigan

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1292-A, B

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

and

Geologic Interpretation 

of Gravity Data, 

Marenisco-Watersmeet Area, 

Northern Michigan

By J. S. KLASNER and P. K. SIMS

CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1292-A,B

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1984
UNITED STATES DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Main entry under title:

Geology, geochemistry, and age of Archean and early Proterozoic rocks in the Marenisco-Watersmeet area, 

Northern Michigan.

(Contributions to the geology of the Lake Superior Region)

(Geological Survey Professional Paper ; 1292A-B)

Bibliography: P1292-A, 41 p.; P1292-B, 13 p.

Supt. of Docs. No.: 119.16:1292-A & B

1. Geology, Stratigraphic Pre-Cambrian. 2. Geology Michigan Gogebic County. 3. Gravity Michigan Gogebic County.

I. Sims, P. K. (Paul Kibler), 1918- II. Klasner, J. S. Geologic interpretation of gravity data, MareniscoWatersmeet area. 1983. III. Series. IV. Series: Geological Survey professional paper ; 1292A-B.

QE653.G477 1983 551.7T09774983 83-600384

For sale by the Branch of Distribution

U.S. Geological Survey

604 South Pickett Street

Alexandria, VA 22304
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

CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1292-A

A study of some of the

oldest rocks in North America

and their geologic setting

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1984

CONTENTS

Abstract ..........................

Introduction ........................

Acknowledgments .....................

Stratigraphy ........................

Archean rocks .....................

Rocks in the Watersmeet dome ........

Tonalitic augen gneiss ...........

Biotite gneiss succession ..........

Interlayered amphibolite and biotite gneiss 

Biotite leucogranite .............

Gneiss and amphibolite .............

Metagraywacke ..................

Puritan Quartz Monzonite ............

Age and correlation ...............

Early Proterozoic rocks ...............

Blair Creek Formation .............

Copps and Michigamme Formations ......

Metadiabase dikes ................

Page 

Al

2

4

5

10

13

13

15

15

18

18

21

22

24

25

	Page

Geochemistry .......................... A26

Characteristic features .................. 26

Genetic significance .................... 29

Structure ............................ 30

Archean deformation ................... 31

Penokean deformation .................. 31

Faults ........................... 33

Northwest fault set ................. 33

East-northeast fault set ............... 33

Northeast fault set ................. 35

Metamorphism ......................... 35

Archean metamorphism ................. 35

Early Proterozoic (Penokean) metamorphism ..... 36

Discussion ......................... 37

Geologic history ........................ 37

Occurrence of fluorite and copper in rocks within the

Watersmeet dome ...................... 39

References cited ........................ 39

ILLUSTRATIONS

FIGURE 1. Map of Lake Superior region showing location of study area relative to boundary between Archean basement terranes A3

2. Generalized geologic map of the Marenisco-Watersmeet area, northern Michigan .................... 4

3. Geologic section showing gross stratigraphy and structure of Archean and Early Proterozoic rocks in the MareniscoWatersmeet area ...................................................... ?

4. Geologic map of part (T. 45 N., R. 39 W.) of northern margin of Watersmeet dome ................. 9

5. Photographs of hand specimens of Early Archean gneisses from the Watersmeet dome ................ 12

6. Rb-Sr plot for Archean and Early Proterozoic metagraywacke .............................. 18

7. K/Rb ratios of Archean and Early Proterozoic metagraywacke .............................. 18

8. Quartz-K-feldspar-plagioclase diagram showing modal composition of Late Archean granitoid rocks .......... 19

9. Concordia plot of zircon data for samples of granitoid rocks in the Watersmeet dome ................. 21

10. Rb-Sr isochron plot for samples of gneiss, schist, and leucogranite from the Watersmeet dome ........... 22

11. Rb-Sr isochron plot for samples of Puritan Quartz Monzonite and biotite gneiss from Archean gneiss belt southeast

of Marenisco ........................................................ 23

12. Concordia plot of zircon data for samples of biotite gneiss and associated Puritan Quartz Monzonite from gneiss belt

southeast of Marenisco ................................................... 23

13. Rb-Sr isochron plot for samples of Archean and Early Proterozoic metagraywackes and associated rocks ...... 24

14-18. Diagrams of data for selected Archean and Early Proterozoic rocks in the Marenisco-Watersmeet area:

14. K20-Na20-CaO diagram ............................................... 27

15. AFM diagram ..................................................... 28

16. Ti02-P205 plot ..................................................... 29

17. Th-U plot ....................................................... 29

18. Rb-Sr plot ....................................................... 29

19. Contour plots of poles of foliation related to Penokean folds, Marenisco area ...................... 32

20. Map showing traces of faults, Marenisco-Watersmeet area, Michigan ........................... 34

AV
AVI CONTENTS

TABLES

TABLE 1. Correlation of pre-Keweenawan rock units used in this report with those of Fritts (1969) ................ A8

2. Approximate modes and chemical analyses of tonalitic augen gneiss from the Watersmeet dome ........... 10

3. Approximate modes and chemical analyses of representative samples of the biotite gneiss succession in the Watersmeet

	dome ............................................................. 11

4. Approximate modes, in volume percent, of representative samples of the interlayered amphibolite and biotite gneiss

	succession in the Watersmeet dome ............................................ 14

5. Approximate modes and chemical analyses of biotite leucogranite in the Watersmeet dome .............. 14

6. Approximate modes and chemical analyses of selected rocks in the gneiss and amphibolite unit ............ 16

7. Chemical analyses of Archean and Proterozoic metagraywacke, Marenisco-Watersmeet area .............. 17

8. Approximate modes and chemical analyses of Puritan Quartz Monzonite from bodies in the gneiss and amphibolite unit . 19

9. Approximate modes and chemical analyses of Puritan Quartz Monzonite in the Puritan batholith ........... 20

10. Analytical data for samples dated by the Rb-Sr method ................................. 23

11. Chemical analyses of lavas from the Blair Creek Formation ................................ 25

12. Average compositions of Early Archean augen gneisses from northern Michigan, Greenland, and Labrador ..... 30

13. Precambrian stratigraphic-tectonic evolution, western Upper Michigan ........................... 38
CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

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

ABSTRACT

The Marenisco-Watersmeet area lies astride the boundary between 

the two Archean basement terranes recognized in the Lake Superior 

region. Rocks characteristic of the gneiss terrane are exposed 

sparsely in a gneiss dome at Watersmeet. They consist of a tonalitic 

augen gneiss (~3,560 m.y. old), which is unconformably overlain by 

a biotite gneiss succession of about the same age and a younger succession of interlayered amphibolite and biotite gneiss (~2,640 m.y. 

old). These rock types were intruded by biotite leucogranite 2,590 

m.y. old. Greenstone-granite complexes, which are characteristic of 

the Archean greenstone-granite terrane, are exposed in the northwest part of the area. They consist of metavolcanic and metasedimentary rocks that are intruded by the Puritan Quartz Monzonite ( 2,650 

m.y. old). The major body of Puritan Quartz Monzonite composes 

the northeast part of the Puritan batholith; satellitic bodies of the 

granite cut similar, but more highly deformed and metamorphosed 

bedded rocks in a northeast-trending belt to the southeast of the 

batholith.

The Archean rocks of both terranes are overlain unconformably 

by a succession of metavolcanic and metasedimentary rocks assigned 

to the Marquette Range Supergroup. A basal volcanic unit of the 

Marquette Range Supergroup, named here the Blair Creek Formation, is overlain by metagraywacke and meta-argillite of the Copps 

and Michigamme Formations of the Baraga Group. Locally, quartzite 

and siliceous dolomite of the Chocolay Group are preserved beneath 

the Blair Creek Formation. The Blair Creek Formation thickens 

southeastward and is estimated to have a maximum thickness in the 

area of about 2,000 m. The Blair Creek and the older Archean rocks 

are intruded by numerous dikes of metadiabase (Early Proterozoic) 

and diabase (Keweenawan).

Major and trace element compositions reflect both primary chemical attributes and modifications attendant with the Early Proterozoic 

tectonism. The Early Archean tonalitic augen gneiss has a relatively 

high Ti02 and P205 content for its silica content, a Na^O/K^O ratio 

near unity, a Rb/Sr ratio greater than unity, and a low K/Rb(<200) 

ratio. Leucogranite in the Watersmeet dome also has a Rb/Sr ratio 

greater than unity. The Late paragenesis of K-feldspar, its poikilitic 

habit, and its close association with cataclastic and recrystallized fabrics in the augen gneiss suggest that potassium was introduced during Penokean tectonism. Probably either rubidium was added or 

strontium was removed diffentially concurrently with the migration 

of potassium. The late Archean Puritan Quartz Monzonite has a high 

FeO*/(FeO* + MgO) ratio, and many samples have uncommonly high 

amounts of thorium and (or) uranium. Similarly, the leucogranite in 

the gneiss dome has high thorium and uranium contents. The Archean metagraywacke is distinguished chemically from the Proterozoic

graywacke in having lower CaO and higher MgO contents and a lower 

Rb/Sr ratio. The basalt in the Proterozoic Blair Creek Formation 

has a high Na20/K20 ratio and shows an iron-enrichment trend in 

the tholeiite field. The protoliths of the Early Archean augen gneiss 

and the biotite gneisses of both terranes possibly were dacitic volcanic rocks; the amphibolites of both terranes also are of volcanic derivation. The composition of the Puritan Quartz Monzonite is consistent 

with its derivation by partial melting of short-lived graywacke at 

crustal depths.

Deformation during the Penokean orogeny produced open to tight 

folds overturned to the northwest or north in the Proterozoic bedded 

rocks. A southeast-dipping axial plane foliation accompanied the folding and was superposed on previously deformed rocks of Late Archean age along the southern margin of the greenstone terrane. The 

intensity of the deformation decreased northward, and the width of 

the tectonically deformed zone is about 25 km. The folding was followed shortly by a low-pressure metamorphism having a nodal distribution pattern centered on the Watersmeet dome.

The rocks within the core of the dome were metamorphosed to 

amphibolite facies; surrounding the core is a zone of epidote amphibolite facies about 15 km wide, which passes outward into greenschist 

facies. The metamorphism resulted from heat that accompanied reactivation and uplift of the Archean gneiss in the core of the dome. 

The deformation and metamorphism of the Archean rocks during the 

Penokean orogeny produced open systems that allowed substantial 

migration of strontium and probably also rubidium. The apparent 

isotopic homogenization was local and resulted from cataclasis and 

recrystallization. The resulting secondary whole-rock and mineral 

isochrons give ages of 1,750 to 1,800 m.y., which are thought to represent the approximate time span of the Penokean event in this area. 

Additionally, one sample of intensely sheared and recrystallized 

gneiss gave a U-Pb zircon age of 1,760 m.y. We interpret this zircon 

as a reconstituted mineral in which the U-(Th)-Pb system was completely reset as a consequence of the intense metamorphism. Isotopic 

systems in the Archean gneisses and granite in the deformed and 

metamorphosed belt southeast of the Puritan batholith also were 

highly disturbed during this event, whereas those in the Puritan 

Quartz Monzonite in the batholith were only moderately disturbed.

Faulting materially affected the distribution and structure of the 

Archean and Early Proterozoic rocks. A northwest-trending set, 

which probably was initiated in the Archean, had both strike-slip 

and dip-slip movements. An east-northeast set was initiated in Early 

Proterozoic time, as indicated by alined metadiabase dikes, but its 

principal movement was in Late Proterozoic time, when the Archean 

and Proterozoic rocks of the Gogebic Range were tilted steeply northward. A younger set trending northeast cuts and offsets the lower 

Keweenawan lavas in the area.

Al
A2 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

INTRODUCTION

The Marenisco-Watersmeet area, in the western part 

of the Upper Peninsula of Michigan, lies astride the 

boundary between the two major crustal segments that 

have been delineated in the Lake Superior region 

(Morey and Sims, 1976): a Late Archean greenstonegranite terrane and an older Archean gneiss terrane 

(fig. 1). The greenstone-granite terrane is composed of 

greenstone- granite complexes 2,600-2,750 m.y. in age 

that are typical of most of the Superior province of the 

Canadian Shield. The gneiss terrane is composed of 

migmatitic gneisses and amphibolite that are in part 

3,560 m.y. old or older. The two basement terranes 

have been so designated because they appear to have 

evolved largely as separate crustal entities (Sims and 

Peterman, 1981) and had vastly different tectonic 

stabilities, especially during the Early Proterozoic 

(Sims and others, 1981). They strongly influenced crustal evolution in the Upper Midwest during Precambrian 

time. The boundary between the two basement terranes is marked by a distinctive zone of tectonism that 

has been named the Great Lakes tectonic zone (Sims 

and others, 1980).

The area is contiguous to the eastern end of the 

Gogebic Range, and because of its potential as a source 

of iron ore has been studied intermittently for nearly 

a century. Alien and Barrett (1915) summarized the results of iron ore exploration and presented reconnaissance geologic maps of parts of the area. Later, Fritts 

(1969) mapped the area in moderate detail, as a part 

of the U.S. Geological Survey cooperative program 

with the Geological Survey Division of the Michigan 

Department of Conservation. The area adjoining that 

mapped by Fritts on the west the east end of the 

Gogebic Range was mapped subsequently by Trent 

(1973), also as a part of the Federal-State cooperative 

program. The geologic structure of this part of the 

Gogebic Range was described earlier in an unpublished 

report by Hendrix (1960).

Since publication of the earliest reports concerning 

the region, interpretations of the geology have differed 

and, in part, have been contradictory. This confusion 

has resulted from the complexity of the geology, the 

inadequacy of bedrock exposures in critical areas, and 

a lack of radiometric data needed to distinguish the 

ages of various rock bodies. Without this powerful tool, 

interpretation of the geology was highly subjective.

The main purpose of our study, begun in 1974, was 

to determine critical geologic relationships in the tectonically disturbed zone at the juncture of the two 

Archean basement terranes. At an early stage in our 

reconnaissance mapping and radiometric studies we 

recognized that several aspects of the geology reported

in the literature appeared to be erroneous. Accordingly, a regional reconnaissance map (scale 1:125,000) 

of a much larger area was prepared by Prinz (1981), 

and selected parts of the Marenisco, Thayer, and 

Watersmeet 15-minute quadrangles, previously 

mapped by Fritts (1969) and Trent (1973), were remapped. A generalized version of the revised map is 

given on figure 2. Concurrently, samples of granitic 

rocks, gneisses, and metagraywacke were collected for 

age determinations. Because of intense isotopic disturbance in the crystalline rocks of the area, the U-Pb 

zircon method has been required to date many of the 

rocks. Rb-Sr data on whole-rock and mineral samples 

are very informative, however, especially for dating 

isotopic disturbances recorded in the rocks. Some of 

the results of the geochronology study have been summarized by Sims and Peterman (1976), Sims and others 

(1977), and Peterman and others (1980). All ages are 

calculated using the isotopic and decay constants recommended by the IUGS Subcommission on GeochronolandJager, 1977).

The following principal conclusions of our study differ 

from those of earlier studies: (1) The bedded rocks in 

the Marenisco-Watersmeet area are complexly folded 

and are of both Late Archean and Early Proterozoic 

ages. In previous publications, all the bedded rocks 

were considered to be of Proterozoic age. (2) Except 

for the Gogebic Range, the Marquette Range Supergroup consists essentially of two formations, a lower, 

dominantly volcanic unit, containing lenses of iron-formation, conglomerate, and wacke, herein named the 

Blair Creek Formation; and an upper graywacke-argillite unit, the Copps Formation in the west and its 

equivalent in the east, the Michigamme Formation. 

Previously (Fritts, 1969), the successions in the area 

were interpreted as a homoclinal, conformable sequence 

of Early Proterozoic rocks, younging eastward, having 

a stratigraphic thickness of at least 40,000 feet. (3) The 

Wolf Lake Granite of Alien and Barrett (1915) is a composite unit of gneiss, metavolcanic rocks, and granite 

of Archean age. It composes a gneiss dome that was 

reactivated during the Penokean event concurrently 

with metamorphism of the overlying bedded rocks of 

the Marquette Range Supergroup (Sims and Peterman, 

1976). Both Fritts (1969) and Alien and Barrett (1915) 

interpreted the Wolf Lake Granite as being younger 

than the Early Proterozoic bedded rocks. (4) The granite and gneiss east of Marenisco, called the granite near 

Thayer by Fritts (1969), are Late Archean in age, and 

the granite is equivalent to the Puritan Quartz Monzonite exposed in the Puritan batholith west of Lake 

Gogebic. Fritts (1969) considered the granite near 

Thayer to be younger than the bedded rocks of the 

Marquette Range Supergroup, whereas Alien and Bar-
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA

96° 92°

A3

44° 

FIGURE 1. Map of Lake Superior region showing location of study area relative to boundary between Archean 

basement terranes. Rocks of midcontinent rift system shown by diagonal rule pattern.

rett (1915) correlated it on the basis of similar lithology 

with the Puritan Quartz Monzonite (Presque Isle Granite of Alien and Barrett, 1915).

In addition, a local problem of long standing in the 

eastern Gogebic Range appears to have been resolved. 

Alien and Barrett (1915) and, later, Trent (1973) concluded that biotite schist and amphibolite in the southern part of T. 47 N., R. 43 W., are metamorphosed 

phases of the Palms Formation that were developed 

adjacent to the Puritan Quartz Monzonite (Presque Isle 

Granite of Alien and Barrett, 1915). Instead, we conclude that a major fault (Presque Isle fault, fig. 2) separates typical Palms, on the north, from the schist and 

amphibolite, and that the schist and amphibolite are 

Archean bedded rocks that were intruded by the 

granite.

On a regional scale, the study has produced a number 

of new discoveries about stratigraphic and tectonic patterns in the boundary zone between the two Archean 

basement terranes: (1) Metavolcanic rocks of Archean 

age were deposited unconformably on Early Archean 

gneiss, and these rocks were intruded by Late Archean 

granite. Thus the depositional history of the gneiss terrane is more complex than previously thought (Sims, 

1980). (2) The Late Archean bedded rocks of the 

greenstone terrane adjacent to the boundary were deformed on axes subparallel to the boundary in Late Archean time. Elsewhere in the greenstone terrane, folding was mainly caused by the diapiric rise of Late Archean magmas, in a manner described by Schwerdtner 

and others (1979). (3) The boundary zone was tectonically active intermittently during the Early Proterozo-
A4 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

89" 3D'

T. 47 N.

T. 46 N.

T. 45 N.

46° 15'

R. 6E. R. 43 W. R. 42 W.

10 KILOMETERS

R. 41 W.

FIGURE 2. Generalized geologic map of the Marenisco-Watersmeet area, northern Michigan. See page A6 for explanation.

ic, as indicated by the nondeposition or erosion of 

Chocolay Group strata of the Marquette Range Supergroup, the occurrence of locally derived conglomerate 

and grit at the base of the Proterozoic section, the extrusion of mafic volcanic rocks approximately contemporaneously with deposition of the major iron-formation 

in the area (the Ironwood Iron-formation), and the existence of overturned folds and a penetrative cleavage 

in rocks within the zone. The folds are overturned toward the northwest, apparently as a result of impingement of the Archean gneiss block against the more 

rigid greenstone- granite block.

ACKNOWLEDGMENTS

This report is based mainly on our reconnaissance 

geologic mapping and radiometric studies during the 

past five years. The published geologic maps of Fritts
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA

89" 15'

A5

46° 30'

T. 47 N.

T. 46 N.

T. 45 N.

.1. 40 W. R. 39 W. R. 38 W.

46° 15'

-V. A. Trent 119731 

C. E. Fritts 119691

Geology by C. E. Fritts 119691 and

V. A. Trent (19731; revised by

P. K. Sims and W. C. Prinz, 1976-78.

(1969) and Trent (1973) were extremely useful. The 

field and related studies were the responsibility of Sims 

and Prinz, Peterman was responsible for the geochronology, and Benedict assisted with the petrographic studies. Klaus J. Schulz and Steven F. Olson 

assisted in the fieldwork. The chemical analyses reported herein were done by the single-solution procedure described by Shapiro (1967). Analysts are credited 

specifically in the tables.

STRATIGRAPHY

It is useful to discuss the stratigraphy of the area 

with respect to a generalized section across the strike 

of the major units (fig. 3). The oldest known rocks are
A6 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION 

CORRELATION OF MAP UNITS

MIDDLE 

PROTEROZOIC

EARLY 

PROTEROZOIC

ARCHEAN

Xbu

Xbi

Xbs

LIST OF MAP UNITS

MIDDLE PROTEROZOIC

Jacobsville Sandstone

Siemens Creek Formation (basaltic lava flows) and

Bessemer Quartzite (local basal quartzite) 

Diabase Inversely polarized dikes 

EARLY PROTEROZOIC 

Copps Formation

Michigamme Formation 

Ironwood Iron-formation 

Emperor Volcanic Complex

Blair Creek Formation 

Undivided

Unexposed magnetic unit

Dominantly volcanic rocks

Lean iron-formation

Mafic volcanic rocks Includes porphyritic lava

Basal conglomerate, arkose, wacke, and felsite

XmdAgt

EARLY PROTEROZOIC-continued

Unexposed magnetic unit

Metadiabase 

Palms Formation 

Bad River Dolomite

Chocolay and Menominee Groups, undivided Sedimentary rocks known mainly from drill cores

ARCHEAN

Puritan Quartz Monzonite of Puritan batholith 

Approximately 2,650 m.y. old 

Biotite schist Locally migmatized; contains some

tonalite and amphibolite 

Metagraywacke Includes lenses of dacite tuff

Dacitic volcanic rocks 

Metabasalt In part pillowed

Gneiss and amphibolite Approximately 2,750 m.y. old.

Includes local bodies of Puritan Quartz Monzonite 

Metavolcanic, metasedimentary, and sparse granitoid rocks

Gneiss and granite, undivided Possibly includes some rocks

of Early Proterozoic age 

Unexposed magnetic rocks In part may be of Early

Proterozoic age

CONTACT Approximately located

 I HIGH-ANGLE DIP-SLIP FAULT-Bar and ball on 

downthrown side

 ^ HIGH-ANGLE FAULT Arrows show relative movement

 ( SYNCLINE

 y OVERTURNED SYNCLINE 

STRIKE AND DIP

40 ' Bedding

d Overturned bedding

55 * Foliation

 » BEARING AND PLUNGE OF LINEATION

 ^^ METAMORPHIC FACIES BOUNDARY-Queried where 

location is uncertain

 B BIOTITE ISOGRAD

............... APPROXIMATE SOUTHERN LIMIT OF OUTCROPS

FIGURE 2. Generalized geologic map of the Marenisco-Watersmeet area, northern Michigan Explanation.

Early Archean gneisses in the core of the gneiss dome 

at Watersmeet. Younger basement rocks in the western part, Late Archean in age, consist of metavolcanic

and metagraywacke units that are intruded by granite 

of Late Archean age. Unconformably overlying the Archean basement are bedded metavolcanic and
NORTHWEST SOUTHEAST

Gogebic 

Range

Yj Ysb Xc Xi / Abs

PURITAN 

BATHOLITH

Ap

CORPS 

SYNCLINE

\ 

Xc Surface \Xb\Aga Aga Agw Xb Xm

WATERSMEET 

DOME _ Agn

10 KILOMETERS

LIST OF MAP UNITS

MIDDLE PROTEROZOIC-900 to 1,600 m.y. 

Jacobsville Sandstone

Siemens Creek Formation and Bessemer Quartette

EARLY PROTEROZOIC; MARQUETTE RANGE SUPERGROUP-1,600 to 2,500 m.y. 

Michigamme Formation

Copps FormationBlair Creek FormationIronwood Iron-formation and associated bedded rocks

ARCHEAN-2,500 m.y. and older 

Puritan Quartz MonzoniteMetagraywacke and minor dacitic tuffGneiss and amphiboliteDominantly biotite schistTonalitic gneiss and overlying gneiss and schist

APPROXIMATE CONTACTFAULT Arrows show direction of relative movement

CLEAVAGEFOLIATION

O 

H 

O 

f 

O 

O

O 

H 

O O

wH

Si

3O > 

O 

H 

OO O

03CO O

cc

FIGURE 3. Gross stratigraphy and structure of Archean and Early Proterozoic rocks in the Marenisco-Watersmeet area.
A8 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

metasedimentary rocks of the Early Proterozoic Marquette Range Supergroup. In the MareniscoWatersmeet area, these rocks consist of a lower, dominantly volcanic unit, herein named the Blair Creek 

Formation, and an upper metagraywacke-argillite unit 

called the Copps Formation in the west and the 

Michigamme Formation in the east. The Copps Formation directly overlies the Ironwood Iron-formation and 

associated volcanic rocks (Emperor Volcanic Complex) 

at the east end of the Gogebic Range, as shown on 

figure 2. Both the Archean and the Early Proterozoic 

rocks are overlain unconformably along the north edge 

of the area (fig. 2) by lava and sandstone of Keweenawan (Middle Proterozoic) age. These younger Proterozoic rocks have been described by Hubbard (1975) 

and are not discussed in this report.

The stratigraphic nomenclature used in this report 

is compared with that of Fritts (1969) in table 1. As 

mentioned previously, Fritts considered the stratigraphic succession to be homoclinal and conformable, 

younging eastward. He arranged the bedded rocks, 

from oldest to youngest, as follows: strata near Cup 

Lake, strata near Banner Lake, strata near Blair Lake, 

and strata near Paulding. He assigned these rocks to 

the Animikie Series. Later Cannon and Gair (1970) 

named the Marquette Range Supergroup designating 

it as a replacement term for the Animikie in Michigan. 

It should be noted that the rock units on the east end 

of the Gogebic Range are omitted from table 1. Their 

stratigraphic relationship to those listed in table 1 can 

be seen by reference to the explanation for figure 2.

ARCHEAN ROCKS

Archean rocks belonging to both basement terranes 

crop out in the area. Rocks that represent the gneiss 

terrane are exposed in the Watersmeet gneiss dome, 

in the southeast part, and rocks that represent the 

greenstone-granite terrane crop out in the cores of anticlinal blocks in the northwest part. Except for the Puritan Quartz Monzonite, Archean rocks associated with 

the Puritan batholith, in the extreme northwest part 

of figure 2, are not described here. They resemble 

rocks mapped in areas to the west, which have been 

discussed by Schmidt (1976).

ROCKS IN THE WATERSMEET DOME

Three successions of Archean rocks are exposed in 

the Watersmeet dome. An older tonalitic augen gneiss, 

informally called the gneiss at Watersmeet (Peterman 

and others, 1980), is unconformably overlain in the 

northeast part of the dome by a succession of layered 

biotite gneisses. The third succession, composed of interlayered amphibolite and biotite gneiss and schist,

TABLE 1. Correlation ofpre-Keweenawan rock units used in this 

report with those of Fritts (1969) 1

This report Fritts (1969)

Early Proterozoic: 2,500-1,600 m.y.

Michigamme Formation Strata near Paulding. 

Copps Formation Copps Formation.

Blair Creek Formation Strata near Blair Lake,

strata of the Marenisco 

range (part), strata near 

Cup Lake (part).

Iron-formation, schist, Strata near Banner Lake, 

dolomite, and quartzite.

Archean: >2,500 m.y.

Puritan Quartz Monzonite Granite near Nelson Creek,

granite near Thayer.

Biotite schist Gneiss near Mount Kimberly.

Metagraywacke Strata near Banner Lake.

Amphibolite Strata near Cup Lake (part).

Migmatitic gneisses Wolf Lake Granite of Alien

and Barrett (1915).

iJable omits Archean metavolcanic rocks and 

Proterozoic Palms Formation, Ironwood Iron-formation, 

and Emperor Volcanic Complex of the east Gogebic Range.

crops out sporadically in the southern part of this area 

and apparently overlies the gneiss of the Watersmeet 

dome. All the layered rocks are cut by small bodies 

and dikes of biotite leucogranite. Exposures are limited 

to the northern part of the dome, and even here they 

are too widely separated to allow accurate mapping of 

the different rock successions. Accordingly, on the 

geologic map (fig. 2) all the rocks in the dome are included under a single map unit, gneiss and granite undivided (Agn); an exception is the unit of unexposed 

magnetic rocks (Avs), which is delineated separately. 

For the main area of outcrops in the northeast part 

of the dome, a more detailed map (fig. 4) was prepared 

in order to show areas underlain by each of the three 

rock successions. This map is essentially an outcrop 

map. The locations of specific outcrops are shown on 

the earlier published map (Fritts, 1969).

TONALITIC AUGEN GNEISS

Tonalitic augen gneiss is exposed sporadically in the 

northern part of the Watersmeet dome. The principal 

exposures are in a small window about 1 km2 in area 

in sees. 4 and 5, T. 45 N., R. 39 W. (fig. 4). A small 

outcrop largely overlain by amphibolite is present in 

SWV4 SEVfc sec. 5, T. 45 N., R. 39 W.

The tonalitic gneiss, as exposed in the window (fig. 

4), is a medium-gray, medium- to coarse-grained,
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A9

T. 46 N.

T. 45 N.

TO WATERSMEET

EXPLANATION

EARLY PROTEROZOIC

Michigamme Formation 

. , ARCHEAN: 

Aa Interlayered amphibolite and biotite gneiss

Biotite gneiss 

Tonalitic augen gneiss

60

FAULT Approximately located

APPROXIMATE LIMIT OF ROCK SUCCESSION

BEARING AND PLUNGE OF FOLD AXIS

STRIKE AND DIP OF FOLIATION

BEARING AND PLUNGE OF LINEATION

ISOLATED OUTCROP OF TONALITIC AUGEN GNEISS

ABUNDANT ACCESSORY FLUORITE

FIGURE 4. Geologic map of part (T. 45 N., R. 39 W.) of northern margin of Watersmeet dome showing distribution and structure of

Precambrian rocks.

irregularly layered rock containing sparse plagioclase 

augen. It contains rare layers and lenses of amphibolite 

at most a few meters thick. The layering is given by 

alternating feldspar-quartz and biotite-rich zones about 

a centimeter thick. The fabric of the gneiss as seen 

in thin sections is highly irregular. Generally, coarse 

plagioclase (An25_33) and amoeboid aggregates of sutured quartz grains are crossed by irregular seams and 

braided zones of finer grained quartz, plagioclase, and 

biotite, resulting from recrystallization of finely granulated material. The plagioclase (as much as 2 mm in 

diameter) is anhedral, is in part weakly zoned, and is 

weakly to moderately altered to sericite, calcite, and 

epidote/clinozoisite. Biotite is recrystallized within the 

cataclastic zones and accordingly generally has two preferred orientations. It is olive brown and either clustered or strung out in foliation planes. Abundant (~1 

percent) sphene and associated opaque oxides and lesser epidote/clinozoisite are associated with the biotite. 

Microcline is sparse to common, interstitial, slightly

poikilitic, and commonly confined to the cataclastic and 

recrystallized zones. Characteristic accessory minerals 

are zircon, allanite, apatite, sphene, and opaque oxides.

The changes in fabric of the tonalite gneiss are interpreted as indicating progressively greater cataclasis 

and recrystallization of an originally coarse-grained 

rock of tonalitic composition. Petrographic studies 

suggest that potassium was mobile in these rocks subsequent to primary recrystallization. Representative 

modes and chemical analyses of the tonalitic gneiss are 

listed in table 2.

Amphibolite is interlayered with the tonalitic gneiss 

at places in sec. 4, T. 45 N., R. 39 W. The amphibolite 

occurs as lenses as much as a meter thick. It is a darkgray, medium- to coarse-grained hornblende-plagioclase 

rock with a typical salt-and-pepper appearance. It has 

a moderate to strong foliation and lineation. A sample 

from the south-central part of sec. 4 has the following 

mode, in volume percent: hornblende, 68.5; plagioclase 

(An25-3o), 24; biotite, 5; other minerals, 2.5. The rock
A10 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

TABLE 2. Approximate modes and chemical analyses oftonalitic 

augen gneiss from the Watersmeet dome

[Leaders ( ) indicate not determined. Tr, trace. Chemical analyses of sample M45L by 

Samuel Botts; samples D1042, 1042G, and 10421 by N. Skinner; and sample M83 by 

J. S. Wahlberg, J. Taggart, and J. Baker. U and Th analyses by H. T. Millard, 

Jr., and others]

Constituent D1042 M45L M83 D1042I D1042G

Modal composition (volume percent)

PI agiocl ase 

Quartz 

K-feldspar 

Biotite 

Accessory minerals

55

35

0

5

3

2

44 5

35

IS 5

Tu«

1.5

48

30

1-3

Tr.

1

Major oxides (weight percent)

Si02 

A1263 

Fe203 

Fe6-- 

MgQ 

CaO 

Na20 

K26 

H?0+ 

H20~ 

Ti02 

p Q -

MnO 

co2 

Loss on ignition

70.8

13.7

.63

2.9

.97

1.8

3.6

3.5

1.2

.13

.48

.14

.00

1.0

mi

68.6

15.2

1.1

3.2

1.7

2.4

4.1

2.7

.87

.02

.74

.17

.05

.08

mi

66.8

14.2

.79

4.75

2.0

2.46

3.4

2.71

.90

<.2

<.08

.88

mn

72.1

13.7

.64

1.8

.81

1.4

3.6

4.3

.78

.00

.43

.12

.02

.66

mn

69.7

13.9

.83

3.5

1.4

2.0

3.5

3.7

.84

.05

.58

.17

.02

.31

mi

Trace elements (parts per million)

Rb 

Sr 

U 

Th 

 149

_______ 1-30

184

194

1 ft

8 7

O1 £

150

147

124

1Q-3

160

D1042.

M45L.

M83.

D1042G and 

D1042I.

SAMPLE DESCRIPTIONS AND LOCALITIES

Roadcuts along Michigan Highway 45. NWV4SEV4 

sec. 4, T. 45 N., R. 39 W. Irregular aggregates of piagioclase and quartz and alined 

biotite define a foliation. Plagioclase is 

partly granulated and recrystallized and is 

poorly twinned, partly because of strain; it 

is moderately altered to muscovite, epidote/ 

clinozoisite, and calcite. Quartz forms 

amoeboid aggregates of sutured, recrystallized grains. Biotite is olive brown and 

weakly chloritized. Accessory minerals are 

zircon, apatite, sphene, opaque oxides, and 

allanite. Allanite is partly rimmed by 

epidote/clinozoisite.

Same locality as D1042. Microcline is interstitial to piagioclase and quartz. About 1 

percent sphene is associated with biotite.

NWV^SW 1/* sec. 4, T. 45 N., R. 39 W.

Same locality as D1042.

contains a trace of quartz and microcline, and small 

amounts of epidote, sphene, and opaque oxides. The 

hornblende is a dark-bluish-green variety. It is not 

known whether the protolith of the amphibolite was extrusive basalt or a dike. The amphibolite can be distinguished from the Early Proterozoic metadiabase and 

metagabbro dikes in the Watersmeet dome because it 

has (1) darker hornblende, (2) better twinned plagioclase that has a weak concentric zoning, (3) mosaic texture, and (4) partial alteration of plagioclase to epidote/ 

clinozoisite.

BIOTITE GNEISS SUCCESSION

Unconformably overlying the tonalitic gneiss in the 

Watersmeet dome is a succession of layered biotite 

gneiss and schist. The unconformity is well exposed in 

small outcrops in the NEV4 SEV4 sec. 5, T. 45 N., R. 

39 W. At this locality, folded biotite schist overlies 

steeply dipping tonalite augen gneiss. Both rock types 

are intruded by small bodies of biotite leucogranite.

The bedded rocks compose a folded, supracrustal succession that strikes generally northeastward and dips 

moderately northward toward the north margin of the 

gneiss dome. They are generally finer grained and more 

massive than the tonalitic gneiss. In the principal exposed area (fig. 4), the rocks are mainly interlayered, 

banded, and massive biotite-quartz-feldspar gneisses 

containing scattered layers and lenses of biotite schist. 

Some of the biotite schist is fine grained and massive. 

Interlayered cataclastic gneiss and fine-grained biotite 

schist that are exposed along Gogebic County Road 206 

in sees. 3 and 4, T. 45 N., R. 40 W., are interpreted 

as belonging to this supracrustal succession.

The gneiss and schist have highly variable fabrics as 

a result of multiple deformations and recrystallization, 

and have a wide range in composition. Two foliations 

can be seen in thin sections of most rocks, and these 

are commonly cut by local fractures coated by biotite. 

Each of the foliations is marked by oriented biotite and 

elongate aggregates of quartz and feldspar. As a result 

of the multiple episodes of recrystallization, textures 

vary considerably even within a thin section, although 

the mineralogy developed during each event was similar. An exception was a late shearing, which is expressed by some retrograde metamorphism, including 

chloritization of biotite and sericitization of plagioclase.

The major rock type in the succession is a light-gray 

to medium-gray, medium-grained gneiss that megascopically resembles granite gneiss. It varies from a massive 

to a layered rock, and the two varieties are interlayered 

on scales of a few meters or a few tens of meters.
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA All

TABLE 3. Approximate modes and chemical analyses of representative samples of the biotite gneiss succession in the Watersmeet dome

[Leaders ( ) indicate not determined. Tr, trace. Chemical analyses of samples D1394, D1395, D1396, D2438, and D2439 by Z. A. Hamlin. sample M48L by S. Botts, and samples 

M95B.M109, M-93, andM199byJ. S. Wahlberg, J. Taggart, and J. Baker. U and Th analyses by H. T. Millard, Jr., and others]

Constituent 

Plagioclase 

Quartz 

K-feldspar 

Biotite 

Other minerals 

M95B

17 

25 

30 

22 

6

M109

38 

32 

15 

10 

5

D1394

42 

26 

26 

6 

Tr.

Biotite gneiss

D1395 D1396 M93

Modal

65 

24 

5 

6 

Tr

analyses

52 

32 

11 

5 

Tr

M193A M321A M48L D2439

Biotite schist

D2438 M199

(volume percent)

36 

30 

29.5 

2.5 

2

36 

28 

30 

5

1

42 

30 

17 

11 

Tr.

40 

11 

22 

20 

7

~

Major oxides (weight percent)

SiOo Ai 263 

Fe20 3 

Fe6--~- - 

MgO 

CaO 

Na 0 

K 26 

H ?0+ 

H2QTi02 

PoOc 

MnO 

co2 

Loss on ignition

69.3

13.0

5.86

1.8

.60

3.3

4.31

.45

<.l

.07

.51

69.5

15.0

3.66

1.2

2.25

3.4

3.39

.41

.1

.03

1.0

71.2

13.7

.29

2.2

.56

.65

4.8

4.0

.86

.18

.06

.05

.04

.08

70.7

14.7

.42

1.9

.91

1.5

4.6

2.6

.86

.28

.08

.05

.03

.08

75.5

12.9

.13

1.8

.54

1.3

4.8

1.8

.62

.14

.00

.05

.03

.13

73.4

13.6

1.79

.73

.54

4.2

4.38

~

.13

<.l

<.02

.41

62.

16.

6.

1.

1.

5.

4.

~

2

3

30

0

3

9

3

1

65

02

58

10

15

52

72.3

12.7

.27

2.2

.5

2.3

3.6

3.5

0.74

0.03

.38

.08

.08

1.1

--

69.3

14.7

.79

2.6

.69

2.4

4.3

2.7

.65

.08

.25

.10

.05

.08

61.4

17.8

6.58

.97

2.11

6.5

2.55

.35

<.l

.08

.96

Sum 99 100 99 99 100 99 99 100 99 99

Trace elements (parts per million)

Rb -

Sr -

U .

Th -

- 116

 30.6

- 12.2

104

170

2.0

no

2.4

1 P C

PQ

240

1.4

20.4

57

IIP

14 3

29.9

105

Pn

3 .9

.4

96

65

6 p

35 o

P7

52

5

0

IRQ

314 i in

6.6

30 9

98

oc

6.9

45 7

SAMPLE DESCRIPTIONS AND LOCALITIES

M95B.

M109.

D1394.

D1395.

D1396.

M93.

M193A.

M321A. 

M48L.

D2439.

D2438. 

M199.

NW 1/4 NE 1/4 sec. 5, T. 45 N., R. 39 W.

contains 6 percent muscovite. 

NE 1/4 NE 1/4 sec. 5, T. 45 N., R. 39 W. 

NE 1/4 sec. 5, T. 45 N., R. 39 W. 

Same locality as D1394. 

Same locality as D1394. 

SW 1/4 NE 1/4 sec. 5, T. 45 N., R. 39 W.

Gray, fine-grained biotite gneiss with seams and clots of biotite;

Gray, fine- to medium-grained biotite gneiss. 

Light-gray, fine- to medium-grained, weakly layered gneiss. 

Similar to D1394 but coarser grained and distinctly inequigranular.

Light-gray, fine- to medium-grained, distinctly layered gneiss.

Layering result of brittle deformation and recrystallization of biotite and cataclastic zones. 

NE 1/4 NE 1/4 sec. 5, T. 45 N., R. 39 W. Gray, fine- to medium-grained, foliated rock. Biotite clots occur

in foliation planes.

NE 1/4 NE 1/4 sec. 5, T. 45 N., R. 39 W. Gray, medium-grained layered biotite gneiss. 

Same locality as D2438. Interlayered with biotite schist (D2438). Strongly cataclasized and

recrystallized. Bimodal grain size, with larger plagioclase (An2 5_30) and poikilitic microcline in a finer

grained matrix. Contains 3 percent muscovite and 4 percent calcite. 

NE 1/4 NW 1/4 sec. 4, T. 45 N., R. 40 W. Dark-gray, fine-grained biotite schist. Interlayered(?) with a

coarser grained cataclastic biotite gneiss (M48L). Contains several percent of microcline and sparse

garnet.

Same locality as D2439. 

NE 1/4 SE 1/4 sec. 5, T. 45 N., R. 39 W. Gray, fine-grained biotite schist. Contains about 15 percent

biotite. Plagioclase is untwinned.
A12 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

t "»as%-'i3^^,vv-;-.-: ->*

M321A

M83C

M194A

FIGURE 5. Hand specimens of Early Archean gneisses from the Watersmeet dome, shown actual size. M321A and M194A are thinly layered 

phases in the biotite gneiss succession. Thin banding is sharply defined and continuous. M83C is the tonalitic augen gneiss showing 

the discontinuous layering imparted by feldspar-quartz- and biotite-rich zones.

Modes and chemical analyses of representative samples 

are given in table 3. Layering in the rock reflects both 

compositional differences and structural deformation. 

Rocks having a compositional layering (fig. 5, sample

M194A) characteristically have a younger schistosity 

given mainly by alined biotite. Layering produced by 

deformation has resulted from cataclasis and recrystallization and is particularly shown by streaks of biotite
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A13

and (or) chlorite. Textures in all the rocks are granoblastic and typically inequigranular. Biotite is brown, 

plagioclase has variably developed polysynthetic twinning and is oligoclase-andesine, and microcline tends to 

be poikilitic. Accessory minerals are apatite, allanite, 

opaque oxides, zircon, and, locally, fluorite. The zircon 

occurs as subrounded or subhedral crystals within biotite or scattered throughout the rock; most grains are 

less than 0.01 mm in diameter, but in some of the thinly 

banded gneiss the grains are somewhat larger. The fluorite is disseminated through the rocks but seems to 

be more abundant in coarser grained recrystallized 

zones. Quite commonly, the rocks have a pronounced 

schistosity marked by discontinuous lenses and clots of 

shiny biotite, which is somewhat coarser than the biotite in the remainder of the rock. We interpret this 

generation of biotite as having crystallized during the 

Penokean deformation.

Another common rock type is a fine-grained ( 0.10- 

mm-diameter) biotite schist that megascopically resembles the graywacke or argillite of the Proterozoic 

Michigamme Formation and has been mistaken for it 

in the past (Fritts, 1969). This rock generally appears 

massive in outcrops, but it typically has a fine layering 

and a foliation given mainly by oriented biotite. It contains quartz, poorly twinned oligoclase, microcline, biotite, and, locally, small amounts of hornblende (table 

3). Minor minerals are garnet, muscovite or sericite, 

epidote/clinozoisite, opaque oxides, apatite, zircon, and 

fluorite.

The cataclastic gneiss exposed in the northwest part 

of the Watersmeet dome is a medium-gray, mediumgrained gneiss characterized by clots of strongly 

oriented, shiny biotite. Larger anhedral crystals of plagioclase (An25_30) and microcline are scattered through 

a finer grained matrix of plagioclase, quartz, microcline, and biotite that was formed by cataclasis and recrystallization. Microcline commonly embays and replaces the larger plagioclase grains; it also occurs as 

interstitial, poikilitic grains. Sphene is locally associated with the biotite, which is grayish yellow to 

olive brown. Common accessory minerals are apatite, 

allanite, zircon, and fluorite. Analyses of a typical sample (M48L) are given in table 3.

INTERLAYERED AMPHIBOLITE AND BIOTITE GNEISS

The youngest recognized succession is exposed in the 

southern part of the outcrop area (fig. 4) and along the 

northern margin of the dome, mainly in the vicinity 

of Brush Lake. It consists dominantly of amphibolite 

but includes interlayers of biotite gneiss and schist and 

hornblende-biotite gneiss. The biotite gneiss layers 

generally are less than a meter or only a few meters 

thick. These rocks are structurally simpler than those

of the two older successions; they strike nearly eastward and have a moderate or shallow northward-dipping foliation, as shown in figure 4. The deformation 

is interpreted as being Penokean, for the structures in 

these rocks are comparable to those observed in the 

Early Proterozoic metagabbro dikes.

The mafic bedded rocks range from relatively massive amphibolite to distinctly layered hornblende-plagioclase-quartz gneiss. The latter is mainly confined to 

the northwestern part of sec. 6, T. 45 N., R. 39 W., 

west of Brush Lake; it superficially resembles the tonalitic gneiss in fabric and structure but is considered 

to be a part of the younger succession of bedded rocks. 

The amphibolite is medium grained and has the typical 

salt-and-pepper appearance of this rock type. The plagioclase (oligoclase and andesine) and quartz (less than 

5 percent) are variably granulated as a result of cataclasis and recrystallization at smaller grain sizes. The 

hornblende is green or bluish green and forms subhedral grains. Apatite, sphene, and opaque oxides are 

minor minerals. The layered gneiss has irregular, alternating quartz-plagioclase-biotite and hornblende-biotite-plagioclase layers. In outcrop, a foliation given 

mainly by oriented hornblende and biotite and rarely 

by quartz-feldspar lenses is seen to cross the compositional layering. The biotite in these rocks is a reddishbrown variety. The accessory minerals are the same 

as those in amphibolite, but garnet occurs rarely in 

biotitic zones. Modes of representative rocks and the 

chemical composition of one sample (M261) are given 

in table 4; this amphibolite contains more K20 than the 

Archean amphibolite in the gneiss and amphibolite unit 

(table 6) in the Marenisco area.

The felsic gneiss and schist interlayered with the amphibolitic rocks are dominantly gray or pinkish-gray, 

fine- to medium-grained biotite gneisses and schists. 

Textures vary considerably, mainly because of sporadic 

cataclasis and recrystallization. The quartz-bearing 

rocks in the Brush Lake area (fig. 4) are in part at 

least rather sodic, for they locally contain riebeckite; 

they lack K-feldspar. Those in sees. 4 and 5 (fig. 4) 

typically contain considerable K-feldspar (table 4). Zircon in these rocks tends to be elongate, unzoned, and 

clear.

The younger succession of supracrustal rocks is interpreted as representing metamorphosed mafic and felsic 

volcaniclastic rocks deposited in a subaqueous environment. The mafic rocks are dominantly metamorphosed 

mafic and intermediate pyroclastic deposits, although 

some of the more massive amphibolite could have been 

basaltic lava.

BIOTITE LEUCOGRANITE

Biotite leucogranite, which intruded the tonalitic 

gneiss and the supracrustal bedded rocks, forms widely
A14 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

TABLE 4. Approximate modes, in volume percent, of representative 

samples of the interlayered amphibolite and biotite gneiss succession in the Watersmeet dome

[Tr., trace]

Constituent M111A M261 M202A M113 M202B M203B M260

Plagioclase 40.5 32 33 24 52.5 41 35

Quartz Tr. 0 25 30 29.5 30 33 

K-feldspar 0 0 0 0 0 24 24

Biotite 12 Tr. 21 2 15.5 5 8

Hornblende 42.5 63 17 41 Tr. 0 0 Sphene Tr. 5 Tr. 0 Tr. Tr. 0

Opaque oxides 3 0 0 Tr. 0 0 0 

Other minerals 2 Tr. 4 3 2.5 Tr. Tr.

SAMPLE DESCRIPTIONS AND LOCALITIES

M111A. SWV4NWV4 sec. 6, T. 45 N., R. 39 W. Massive, 

medium- to coarse-grained biotitic amphibolite. 

Apatite is uncommonly abundant. 

M261. SEV4SEV4 sec. 5, T. 45 N., R. 39 W. Foliated, 

medium-grained amphibolite. Small grains of brown 

biotite occur sparsely in plagioclase. Chemical 

analysis: Si02 , 50.5; A1 2 0 3 , 14.4; total Fe as 

Fe 20 3 , 13.4; MgO, 5.8; CaO, 10.0; Na 2 0, 2.3; KoO, 

1.28; Ti02 , 1.66; P 20c, <0.2; MnO, 0.26; loss on 

ignition, 0.76; sum, 100; Rb, 46 ppm; Sr, 255 ppm. 

Analysts: J. S. Wahlberg, J. Taggart, and J. 

Baker. 

M202A. SWV4SWV4 sec. 4, T. 45 N., R. 39 W. Hornblendebiotite schist. 

M113. NWV4NWV4 sec. 6, T. 45 N., R. 39 W. Garnetiferous 

hornblende schist. Opaque oxides have sphene 

rims. 

M202B. SWV4SW1/4 sec. 4, T. 45 N., R. 39 W. Gray, mediumgrained biotite gneiss. 

M203B. SEV4SWV4 sec. 4, T. 45 N., R. 39 W. Pinkish-gray, 

medium-grained biotite gneiss. 

M260. SEV4SEV4 sec. 5, T. 45 N., R. 39 W. Pink, fineto medium-grained biotite gneiss.

scattered, small bodies. The bodies range in size from 

dikes a meter or less thick to dikes several meters 

thick. They generally crosscut the older rocks and were 

emplaced after the bedded rocks were deformed. They 

range in structure from nearly massive to foliated rocks 

and in texture from nearly equigranular, fine- to 

medium- grained rocks to inequigranular, dominantly 

fine-grained rocks. The variable fabric has resulted 

from recrystallization under different stress environments; the more equigranular rocks crystallized in a 

virtually static stress field and the strongly foliated 

rocks crystallized in a high-stress environment. Cataclasis followed by recrystallization at a finer grain size 

was the dominant mechanism for development of the 

foliation in the rocks. 

The biotite leucogranite is a light-gray rock containing roughly equal amounts of oligoclase (An25-28)> 

quartz, and K-feldspar (table 5). The plagioclase is 

bimodal, forming anhedral, rarely zoned crystals a millimeter or more in diameter and smaller crystals within 

cataclasized and recrystallized zones. The larger crystals commonly have a distinct mortar structure. The

TABLE 5. Approximate modes and chemical analyses of biotite 

leucogranite in the Watersmeet dome

 [Leaders ( ) indicate not determined. Tr., trace. Chemical analyses of sample M45H by 

Samuel Botts, samples D1042B, D1042H, and D1042J by N. Skinner. U and Th 

analyses by H. T. Millard, Jr., and others]

Constituent M45H M45C D1042B D1042H D1042J

Modal analyses (volume percent)

Plagioclase 33.5 32 

Quartz 28 30

K-feldspar 32 33 

Biotite 5 5

Other minerals 1.5 Tr.

Major oxides (weight percent)

SiQ? 74.1 75.0 74.6 74.2 

Al ? 6o 14.2 13.6 12.8 13.1

Fe~0o .30 .7 .31 .18

FeS.f .76 - .98 1.4 1.1

MgO .59 .8 .59 .44

CaO .46 .45 1.1 .83

Na ?0 3.6 3.8 3.6 3.6 

I/ ft _ ______ 5? _ 50 44 46

H^0+ .64 .45 .66 1.0

H^O" .05 .05 .00 .20

TJ n 01 10 ta no

p Or .06 .10 .11 .06

MnO- .01 - .00 .00 .02

C02 .07 .07 .47 .02

Sum 100 101 100 99

Trace elements (parts per million)

Rb 136 168 143 176

Sr 46.6 -- 57.7 56.8 94.6

U 17.6 1.7

Th 67.4 10.1

SAMPLE DESCRIPTIONS AND LOCALITIES

M45H. Roadcut on Michigan Highway 45 in NW \ SE !/4 

sec. 4, T. 45 N., R. 39 W. Light-gray, 

fine- to medium-grained, weakly foliated 

granite. Rock traversed by local, irregular fractures coated by biotite. Cuts 

tonalite gneiss. 

M45C. Thin dike that cuts tonalite gneiss. Same 

locality as M45H. 

D1042B. Same locality as M45H. 

D1042H. Same locality as M45H. 

D1042J. Same locality as M45H.

quartz tends to form elongate aggregates or scattered 

anhedral crystals and commonly has strain shadows. 

The biotite is yellowish gray to yellowish brown and 

generally fine grained; it occurs in clusters as unoriented grains and in thin streaks within cataclastic 

zones. The K-feldspar is well-twinned microcline, which 

is poikilitic. It includes quartz, plagioclase, and, locally,
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A15

biotite, and it undoubtedly crystallized late paragenetically. Alteration is variable; muscovite or sericite and 

calcite are associated with the plagioclase, and the biotite is partly altered to chlorite and rutile(?). Epidote/ 

clinozoisite is a local alteration product of biotite. Accessory minerals are relatively sparse and include zircon, allanite, apatite, and sphene.

GNEISS AND AMPHIBOLITE

Interlayered gneiss and amphibolite compose a northeast-trending belt about 2.5 km wide southeast of 

Marenisco (fig. 2, Aga). The rocks are intruded sporadically by relatively small bodies of Puritan Quartz Monzonite, which are not shown separately on figure 2. The 

gneiss and amphibolite appear to grade laterally southeastward into metagraywacke, but probably are mainly 

older than it. Presumably correlative metavolcanic 

rocks metabasalt and biotite schist (Av and Abs, fig. 

2) on the north side of the Puritan batholith have a 

somewhat lower metamorphic grade, and pillows are 

preserved locally in the metabasalt. The gneiss and amphibolite were included by Fritts (1969) in his map unit 

"strata near Cup Lake"; he also included rocks that we 

assign to the Blair Creek Formation in this unit. He 

assigned the granite that cuts the bedded rocks to an 

informal unit called "granite near Thayer."

The gneiss and amphibolite are interlayered on scales 

ranging from about 3 meters to several hundred 

meters, and contacts generally are sharp. Exposures 

are not adequate to map the larger bodies of each rock 

type separately. Small bodies of Puritan Quartz Monzonite cut the bedded rocks in the Cup Lake area (fig. 

2).

The gneiss and amphibolite were deformed twice. 

During an earlier deformation, the rocks were folded 

on northeast-trending axes, as indicated by folded 

layering in the rocks and the existence of an older foliation. Later, a nearly pervasive east-northeast-trending 

foliation that did not destroy the older foliation was 

superposed on the layered rocks. This foliation is 

marked by oriented biotite and associated fine-grained 

granulated quartz and plagioclase. A pronounced lineation that plunges steeply south or southwest was produced by superposition of the younger foliation on an 

older compositional layering and foliation.

The gneiss is a pinkish-gray to medium-gray, 

medium-grained, generally inequigranular biotite 

gneiss that ranges in composition from granite to tonalite (table 6). Neither the distribution nor the relationship between rocks of the two compositions is known, 

although tonalite seems to be dominant. The gneisses 

range from massive rocks having a cataclastic foliation 

to conspicuously layered rocks, which also have cataclastic textures. The cataclastic foliation is given by

oriented biotite and aggregates of quartz and plagioclase, which are finer grained than the remainder of 

the rock. Differences in the intensity and nature of the 

cataclastic structures can be observed in the blasted 

outcrop along the U.S. Forest Service road in sec. 36, 

T. 46 N., R. 43 W. Plagioclase (An25_3o) commonly occurs as megacrysts, which are granulated to different 

degrees; the biotite in the rocks is a reddish-brown variety. Alteration minerals that accompany the granulation and recrystallization are muscovite or sericite, epidote/clinozoisite, and chlorite. Zircon is a common 

accessory mineral. Other accessory minerals are allanite (commonly rimmed by epidote), apatite, and 

opaque oxides. Garnet and staurolite are rare metamorphic minerals. A fine-grained biotite schist is intercalated with the coarser grained gneisses in the Cup Lake 

area.

The amphibolite in this unit is a fine- to mediumgrained, layered to massive rock composed of 

hornblende, plagioclase, and lesser quartz. At places 

it contains subrounded clasts of dacite or dacite porphyry as much as 4 cm in diameter. It is strongly 

foliated and lineated. The hornblende is a green or 

bluish-green variety. Clinopyroxene is intergrown with 

hornblende in one sample (M71D, table 6) from the Cup 

Lake area. Chemical compositions of selected samples 

are given in table 6.

The gneiss and amphibolite are interpreted as 

metamorphosed felsic- intermediate pyroclastic deposits 

and mafic volcanic rocks, respectively. Evidence of primary textures has been destroyed by the multiple deformations and metamorphism, but the interlayering of 

the two contrasting rock types and their mineral and 

chemical compositions support a volcanic derivation. 

Also, they appear to grade eastward into metagraywacke and associated tuffs, which have preserved some 

primary bedding features indicative of turbidites.

METAGRAYWACKE

Metagraywacke is a major rock unit, composing a 

northeast-trending belt as much as 3 km wide north 

of Thayer (fig. 2). The unit contains thin intermediate 

tuff beds and lenses one of which is mapped on figure 

2 and rare, thin iron-formation. The metagraywacke 

is folded on tight north- northeast-trending axes; its 

stratigraphic thickness probably does not exceed 1 km. 

The rock grades into gneiss and amphibolite to the west 

through a broad zone of intercalated amphibolite and 

metagraywacke, and the contact between these two 

units is rather arbitrarily located. It appears to be overlain unconformably to the east by the Early Proterozoic Blair Creek Formation. The metagraywacke was 

mapped by Fritts (1969) as "strata near Banner Lake."

The metagraywacke is a medium-gray, fine-grained,
A16 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

TABLE 6. Approximate modes and chemical analyses of selected rocks in the gneiss and amphibolite unit

[Leaders ( ) indicate not determined. Tr., trace. Chemical analyses of sample M68 by Samuel Botts, samples M147-1A, M147-1B, M147-2, and M150, by H. Smith, sample M89X 

by N. Skinner and D. Kobilis, sample M324A by J. S. Wahlberg, J. Taggart, and J. Baker, and sample D1392 by Z. A. Hamlin. U and Th analyses by H. T. Millard, C. 

M. Ellis, and V. C. Smith]

Constituent M68 D1043 M69 M150 M147-1A M147-1B M147-2 M89X M71D D1392 M324A

Modal analyses (volume percent)

Plagioclase 

Quartz 

K-feldspar 

R-intite---- ____

22

22

8 R

37

32

20

a

42

50

Tr.

A R

64

0

 *

29

0

m

70

0

8

Tr.

0

0

Tr

41

0

0

n

ClinopyroxeneAlteration minerals

Accessory minerals0

1.5

1

0

2

1

0

1

1

0

1

Tr.

0

.5

1.5

0

Tr.

.5

3

38.5

Tr.

0

Tr.

4

Major oxides (weight percent)

Si02 

Al 2 0o 

Fe 2 0 3 - 

FeO 

MgQ 

CaO 

Na 2 0 

K2 o 

H2Q+ 

H 2 0~ 

Ti02 

P 2 o 5 

MnO 

co 2 

Loss on ignition 

Sum70.6

13.1

1.1

3.2

.30

1.7

3.0

4.4

.75

.02

.56

.12

.04

.27

__

99

72.6

15.2

.09

1.5

.79

2.6

6.8

.54

.52

.11

.18

.06

.03

.07

__

67.8

15.1

.35

3.2

2.2

2.5

4.2

1.8

1.2

.04

.41

.13

.06

.05

99

67.7

15.8

.92

3.0

1.6

2.6

5.6

1.3

0.65

0.18

0.41

0.13

0.06

0.07

100

72.35

14.2

.31

2.0

.95

2.

5.5

.92

.53

.14

.22

.08

.05

.28

--

99

50.9

13.2

2.9

10.6

5.8

9.6

2.0

.71 --

1.6

.11 --

1.2

.18 

.23 -

.14 

--

99

47.1

21.3

2.0

6.9

3.6

10.1

2.9

1.9

1.9

.14

.57

.05

.12

.18

99

52.5

14.1

12.4

5.77

8.70

3.1

.93

1.20

.2

.21

~

.95

100

Trace elements (parts per million)

Rb 

Sr 

U 

TU

 102

 136

 4.1

r\c o

118 15

119 275

.6

 40t .u

1.1

OQ a

1.2

13.5

17.4

190

.5

13 2

.6

2.7

91 16

243 197

5

.0

M68.

D1043. 

M69.

M150. 

M147-1A,

M147-1B, 

M147-2. 

M89X. 

M71D.

D1392. 

M324A.

SAMPLE DESCRIPTIONS AND LOCALITIES

NE V4 NE 1/4 sec. 36, T. 46 N., R. 43 W. Pinkish-gray, medium-grained flaser gneiss.

streaks of mafic minerals, which wrap around feldspar augen. 

Same locality as M68. Mode shows difference in mineralogy from layer to layer.

Foliation given by

NWV4SWV4 sec. 30, T. 46 N., R. 42 W. 

mafic minerals.

Gray tonalitic gneiss. Foliation given by irregular streaks of 

East-center sec. 30 T. 46 N., R. 42 W. Pinkish-gray, medium-grained, slightly inequigranular, weakly

foliated gneiss. 

SEV4SW1/4 sec. 21, T. 46 N., R. 42 W. Medium-gray, medium-grained, layered tonalite gneiss. Layering

given by alternating biotite-rich and quartz- and plagioclase-rich bands. Biotite is alined in

cataclastic zones.

Same locality as M147-1A. Similar in composition and structure to M147-1A. 

Same locality as M147-1A. 

NEV4NW1/4 sec. 27, T. 46 N., R. 42 W. 

SW1/4SW1/4 sec. 17, T. 46 N., R. 41 W.

amphibolite; has streaks of feldspar.

Thinly layered metabasalt with pronounced lineation. 

Collected from cut along abandoned railway. Dark-gray foliated 

Epidote (23 percent) and muscovite (12 percent) are major

alteration minerals. Amphibolite is included in foliated granite. 

Same locality as M71D. 

NW 1/4 SW1/4 sec. 30, T. 46 N., R. 42 W. Fine-grained amphibolite; contains 4 percent sphene.
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A17

thin- to thick-bedded rock having a nearly pervasive 

northeast-trending schistosity. Graded beds are common; they indicate the existence of folds having widths 

on the order of 100-200 m. The graywacke consists 

mainly of plagioclase, quartz, and biotite but also contains chlorite, muscovite, and, locally, garnet. Zircon 

and opaque oxides are sparse accessory minerals. Epidote is a sparse secondary mineral. Two generations 

of biotite are present: an older, generally finer- grained 

variety and a younger, generally coarser variety. The 

older generation defines an older schistosity (Si), which 

is nearly parallel to bedding, and the younger generation defines a younger schistosity (S2). Both generations of biotite are weakly altered to chlorite. The 

plagioclase forms small anhedral, generally untwinned 

grains that are difficult to distinguish from quartz, and 

it is weakly altered to muscovite.

Although the metagraywackes of both Archean and 

Early Proterozoic ages are similar in composition, they 

are clearly distinguished by subtle differences in some 

of the major elements and larger differences in 

rubidium and strontium (table 7). The Archean rocks 

are slightly lower in CaO and higher in MgO than the 

Proterozoic rocks and give consistently higher MgO/ 

CaO ratios. The substantially lower rubidium in the Archean metagraywackes (fig. 6) results in generally

TABLE 7. Chemical analyses ofArchean and Proterozoic metagraywacke, Marenisco-Watersmeet area

[Chemical analyses by Z. A. Hamlin; U and Th analyses by H. T. Millard, C. M. Ellis, and V. C. Smith]

Constituent

Archean

M91 M97

metagraywacke unit

M99 M115 M98-1 M124

Michigamme Formation

D2433 D2434 D2435 D2436 D2437 D2440

Copps 

Fm.

M108

m an y

Creek

Fm. 

M106

Major oxides (weight percent)

Si02 

Ai263 

FeS-- 

 _ _/\

TaO

NaoO 

v h _____

H20~ 

Ti02 

P 205 -

Mnfi

re\

62.0 63.1

15.8 16.0

.83 1.1 

6.2 4.8

4.0 3.3

1 ft 17

3.6 3.5

2 a o i

1.8 1.7

.18 .09

.59 .42

.2 .18

.06 .04

.00 .02

60.0

16.7

.54 

6.3

3.8

1 Q

5.1

0.92

2.7

.08

1.4

.17

.05

.01

62.1

16.1

.76 

4.8

3.1

2 3

3.4

3.1

2.0

.09

.52

.19

.07

.00

63.8

15.7

.95 

4.1

2.9

1.3

6.4

1.0

1.6

.11

.48

.19

.03

.04

61.9

19.4

.64 

1.8

1.7

4

f\

5.4

2 5

1.2

.08

.32

.19

.02

.83

63.2 58.5 67.6

16.4 16.4 14.6

1.2 .33 .61 

3.7 9.3 6.0

1.6 3.0 2.4

3 Q "> (\ 1 ft

4.4 3.1 3.4

2 A 1 A 1C

.98 2.8 1.8

.08 .05 .05

.70 .87 .70

.24 .14 .13

.06 .06 .04

.02 .08 .02

60.1

16.0

2.0

7.1

3.0

2.9

2.8

1.9

1.7

.07

.85

.13

.09

.08

63.3

16.8

1.3 

5.9

2.7

1.9

3.4

2

C

1.9

.06

.79

.19

.03

.01

66.4

14.9

.82 

5.4

2.3

1.7

4.6

.51

2.0

.07

.70

.12

.03

.02

66.0

14.7

.76 

5.2

2.2

1.4

3.8

1.9

1.8

.03

.56

.13

.07

.00

53.9

13.5

8.0 

5.8

3.5

4.1

4.0

1.9

1.2

.04

2.1

.49

.09

.10

Sum 100 99 100 99 99 100 99 99 101 99 101 100 99 99

Trace elements (parts per million)

Rb -

U -

Th 

 58.5 66.7

 337 450

2 Q to

 8.6 9.5

23 

306 

2.1 

6.3

83 

520 

2.4

7.1

17.6 

166 

2.5 

9.0

46 

646 

1.4 

4.5

114 

271 

2.5 

6.5

95 

280 

2.9 

5.8

76 

341 

2.2 

5.3

98 

211 

2.5 

6.9

115 

274 

3.5 

4.9

20 

316 

2.5 

5.9

91 

277 

2.1 

4.0

26.5 

173 

1.8 

5.9

	SAMPLE DESCRIPTIONS AND LOCALITIES

M91. SEV4SWV4 sec. 30, T. 46 N., R. 41 W.

M97. SEV4NEV4 sec. 31, T. 46 N., R. 41 W.

M99. SEV4SWV4 sec. 30, T. 46 N., R. 41 W

M115. SEV4 sec. 29, T. 46 N., R. 41 W.

M98-1. West-center sec. 32, T. 46 N., R. 41 W. Intermediate tuff containing 1 mm plagioclase crystals. Adjacent to

	thin iron-formation.

M124. SEV4NW1/4 sec. 28, T. 46 N., R. 41 W.. Dacitic tuff.

D2433. SW1/4SW1/4 sec. 34, T. 46 N., R. 39 W.

D2434. SEV4SE1/4 sec. 21, T. 46 N., R. 39 W.

D2435. NEV4 sec. 33, T. 46 N., R. 39 W.

D2436. SEV4NEV4 sec. 29, T. 46 N., R. 39 W.

D2437. NEV4 sec. 34, T. 46 N., R. 40 W.

D2440. SEV4NEV4 sec. 32, T. 46 N., R. 40 W.

M108. SWV4NW1/4 sec . 18, T. 46 N., R. 42 W.

M106. NWV4SEV4 sec. 26, T. 46 N., R. 41 W.. Iron-rich metasediment.
A18 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

g150

£100

50

O Early Proterozoic 

 Archean

100 200 300 400 500 

STRONTIUM, IN PARTS PER MILLION

600 700

FIGURE 6. Rb-Sr plot for Archean and Early Proterozoic metagraywacke. For the Proterozoic samples, letters designate the 

formation: M, Michigamme; C, Copps; and B, Fe-rich metasediment in the Blair Creek. The Archean sample with the highest 

Sr content is a metavolcanic rock (M-124) associated with the 

metagraywackes.

lower Rb/Sr ratios. A striking difference is seen in the 

K/Rb ratios of the two rocks of different ages (fig. 7). 

Over a range of K20 from 0.5 to 2.6 percent, K/Rb 

ratios of the Early Proterozoic metagraywackes are unusually low, varying little from their average value of 

174. Within a similar range in K20, K/Rb ratios of the 

Archean metagraywackes are nearly double this figure, 

their average being 330.

These geochemical differences are considered to be 

primary features related to the source material and the 

primary mineralogic composition. The lower K/Rb 

ratios in the Early Proterozoic metagraywacke may reflect a greater proportion of original clay matrix in 

these rocks, perhaps related to more intense weathering of the source rocks.

PURITAN QUARTZ MONZONITE

Rocks that are similar in appearance and composition 

to the Puritan Quartz Monzonite in the Puritan 

batholith (fig. 2, Ap) and presumably are equivalent 

occur in the belt of Archean gneiss and amphibolite 

southeast of Marenisco. Contact relations are poorly exposed, and the extent of the granitic rocks is not known 

accurately; accordingly, these outlying bodies of the 

Puritan are;i not mapped separately on figure 2. The 

rocks have been deformed and have widespread cataclastic textures, especially a mortar structure; the deformation is Penokean in age.

The principal bodies of Puritan Quartz Monzonite 

within the gneiss belt are in the Cup Lake area (fig. 

2), mainly in sees. 16 and 17, T. 46 N., R. 41 W. They 

consist of pinkish-gray, medium- to coarse-grained, 

somewhat inequigranular, massive to weakly foliated 

leucocratic biotite granite and coarse-grained 

leucogranite. Much of the coarse-grained granite grades 

into syenitic or granitic pegmatite. A less common

40

O Early Proterozoic 

 Archean

40 60 80 

RUBIDIUM, IN PARTS PER MILLION

100 120

FIGURE 7. K/Rb ratios of Archean and Early Proterozoic metagraywacke. The Archean sample at 2.5 percent K20 and 46 ppm 

Rb is a metavolcanic rock (M-124) and is not used in calculating 

the average K/Rb ratio.

facies is fine to medium grained and has a moderate 

to strong foliation given by alined biotite. The coarse 

granite and pegmatite are widely scattered in sees. 16 

and 17, as noted above, and are abundant in the eastern 

part of sec. 23, T. 46 N., R. 42 W. Samples M72, M78, 

and M85 (table 8) are representative of this facies. Possibly, some of these coarse-grained rocks are not intrusive, but instead have a volcanic protolith and should 

be grouped with the felsic gneiss.

The granitic rocks contain variable amounts of sodic 

plagioclase, quartz, and microcline (table 8), but in general their compositions are within the limits for granite 

defined by Streckeisen (1976). Biotite is sparse and 

tends to occur as frayed aggregates of small brown 

grains. Some of the biotite contains tiny barrel-shaped 

blebs of K-feldspar.

As can be seen from figure 8, the modal mineralogy 

of the bodies in the gneiss belt is similar to that of 

the Puritan Quartz Monzonite in the Puritan batholith. 

The modes and chemical analyses of the Puritan from 

the batholith, given in table 9, are of samples dated 

previously by the Rb-Sr isochron method (Sims and 

others, 1977).

The occurrence of Puritan Quartz Monzonite in the 

gneiss belt that is virtually identical compositionally to 

that in the Puritan batholith suggests that the bodies 

in the gneiss belt are satellitic bodies related to the 

main batholithic mass, as shown diagrammatically on 

figure 3.

AGE AND CORRELATION

The Archean rocks exposed in the area are representative of the two tectono-stratigraphic environments 

recognized in the Lake Superior region, as shown on 

figure 1.
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A19

TABLE 8. Approximate modes and chemical analyses of Puritan 

Quartz Monzonite from bodies in the gneiss and amphibolite unit

[Leaders ( ) indicate not determined. Tr., trace. Chemical analyses of samples D1389 and 

D1391 by Z. A. Hamlin, sample M85 by H. Smith, and sample M71 by Samuel Botts. 

U and Th analyses by H. T. Millard, Jr., and others]

Constituent M71A M71C D1389 D1390 D1391 M78 M85 M72 

Modal analyses (weight percent)

Plagioclase

Quartz 

K-feldsparBiotite 

Alteration

minerals

Accessory

minerals

30

39

28.5

2.5

Tr.

Tr.

38.5

31.5

23 '.5

5.5

Tr.

Tr.

24

O/T £O

45

5

Tr.

Tr.

20

30

47

3

Tr.

Tr.

46

34

16

4

Tr.

Tr.

27

33

33

7

Tr.

Tr.

48

6

44

0.2

Tr.

Tr.

40

22

33

5

Tr.

Tr.

Major oxides (weight percent)

SiO? 

A1 203 

Fe 03 

FeO-- 

Mgo 

CaO 

Na20 

K.Q _____

H2o+ 

T10 _______

p Q ....

MnO- - 

co2 

Sum 

72.4

14.6

.40 

.76 

.16 -

1.4

3.5

4.8

.50 

.02 

.11 

.04 -

.00 

.08 

99

71.7

14.0

.27

1.9

.21

.81

3.2

5.8

.56 

.18

.00

.04

.01

.06

99

 72.3

 13.8

.00

2.2

.35

1.0

3.7

4.1

.74 

.19

.05

.03

.02

.06

 99

 76.1

 13.2

.19

 1.2

.09

.73

 4.3

 3.3

.32

.08

.02

.05

.12

.04

 100

__

--

__

_

--

Trace elements (parts per million)

RbU . 

Th -

101

94

20.5

28.9

 135

 Ill

~ 10.2

 48.7

99

2.0 

5.2

93

61.7

17.4

5.0

SAMPLE DESCRIPTIONS AND LOCALITIES

M71A. SWV4SW1/4 sec. 17, T. 46 N., R. 41 W. Collected 

from cut along abandoned railway. Pinkish-gray, 

medium- to coarse-grained, inequigranular, weakly 

foliated granite. Plagioclase has weak concentric 

zoning.

M71C. Same locality as M71A. Light-gray, fine- to mediumgrained foliated granite. Intercalated with 

M71A. Foliation given by alined biotite and 

recrystallized quartz aggregates.

D1389. Same locality as M71A. Pinkish-gray, coarse-grained, 

weakly foliated granite similar to M71A.

D1390. Same locality as M71A. Pink, medium-grained, 

inequigranular, weakly foliated qranite.

D1391. Same locality as M71A. Pinkish-gray, fine-grained,

foliated granite. Foliation given by thin zones of 

oriented biotite, similar to M71C.

M78. NEV4NWV4 sec. 17, T. 46 N., R. 41 W. Pink, coarsegrained massive qranite.

M85. NWV4SWV4 sec. 23, T. 46 N., R. 42 W. Pinkish-gray, 

coarse-grained, inequigranular, massive syenite. 

Contains sparse garnet.

M72. SWV4SW1/4 sec. 17, T. 46 N., R. 41 W. Pinkish-gray, 

coarse-grained, weakly foliated granite.

The rocks exposed in the Watersmeet dome, which 

belong to the gneiss terrane, were formed during three 

rock-forming episodes. Analyses of two samples of the

QUARTZ EXPLANATION

o Puritan batholith 

+ Cup Lake area

 Biotite leucogranite in 

Watersmeet dome

10 

K-FELDSPAR

90 

PLAGIOCLASE

FIGURE 8. Quartz-K-feldspar-plagioclase diagram showing modal 

composition of Late Archean granitoid rocks. Except for the biotite leucogranite in the Watersmeet dome, all are assigned to 

the Puritan Quartz Monzonite. Lines are compositional boundaries established by Streckeisen (1976).

tonalitic gneiss give a firm minimum U-Th-Pb age of 

3,400 m.y. and a concordia-intercept age of about 3,560 

m.y. (fig. 9; Peterman and others, 1980). Analyses of 

zircon from one sample (M321) of the supracrustal biotite gneiss in the dome indicate an age virtually identical to that of the tonalitic gneiss (U-Pb zircon method). 

The third succession, interlayered amphibolite and biotite gneiss, has an apparent age of about 2,640 m.y. 

(fig. 9); this is based on zircon from sample M203. 

These rocks are cut by dikes of leucogranite having a 

U-Pb zircon age of about 2,590 m.y. (fig. 9). All the 

Archean rocks in the Watersmeet dome were strongly 

deformed and metamorphosed during the Penokean 

event. As a consequence, the Rb-Sr whole rocks and 

mineral systems were reset about 1,750 m.y. ago, as 

shown by the secondary isochrons on figure 10. The 

analytical data for these and other dated samples are 

given in table 10. Biotite gneiss whole-rock and mineral 

samples from the M-48 locality (including M-48, D2438, 

and D2439) define a relatively precise isochron of 1,750 

m.y. Similarly, the array of points for biotite leucogranite at the D1042 locality, including microcline and 

plagioclase, is consistent with an isochron of the same 

slope. The other samples do not define linear arrays 

but plot between these two isochrons, indicating 

isotopic homogenization on a limited scale. Reference 

isochrons at 3,500 and 2,700 m.y. are shown for comparison (fig. 10). Whole-rock samples of the tonalitic 

augen gneiss and of the leucogranite plot to the right 

of the 3,500 m.y. and 2,700 m.y. isochrons, respectively, suggesting that the discordances observed may
A20 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

TABLE 9. Approximate modes and chemical analyses of Puritan 

Quartz Monzonite in the Puritan batholith

[Tr., trace. Chemical analyses by Z. A. Hamlin. U and Th analyses by H. T. Millard, 

Jr., C. M. Ellis, and V. C. Smith]

Constituent D1729 D1730 D1732 D2492 D2493

Modal analyses (volume percent)

Plagioclase 

Quartz -----

K- feldspar 

Biotite 

Other minerals

43 

31

22 

4

Tr.

40 

31

26 

3

Tr.

31 

31

34 

3

1

36 

30

28 

5

1

52 

20

23 

5

Tr.

Major oxides (weight percent)

SiOo 

A1 2 6 3 

FeoOo 

Fe6-- 

MnO _

 71.0

--- 13 R

1.1

_ 3 ?

___ n Rn

70.7

13.6

0.79

2.9

O.A.

71.4

13.7

0.34

2 15

as

71.2

U

o

0.42

2.0

A.A.

CO C

16.0

0.50

1 5

AH

CaO

H 20'-

Ti02 -

P2°5- 

MnOC02 -

1.9

4.3

4.0 

.84 

.07

1.3

4.1

4.0 

.51 

.04

.49

4.3

4.8 

.42 

.08

.71

3.6

5.0 

.88 

.21

1.7

5.3

3.6 

.67 

.16

.09

.06

.11

.12

.15

.04

.05

.08

.03

.03

.04

.16

.13

.06

.03

.10

.05

.05

.03

.08

Sum- 101 99 99 99 99

Trace elements (parts per million)

DK __ ____ _

Sr - 

n __ __ _

Th 

 119

1 on

 7.4

___ 32 4

1 1 Q

174

6.0

29.7

54 4

79.7

7.7

CO 1

233

1 KQ

2 5

OC O

104

640

1.2

3 4

SAMPLE LOCALITIES

D1729. Sec. 7, T. 46 N., R. 43 VI.

D1730. Sec. 7, T. 46 N., R. 43 W.

D1732. Sec. 9, T. 46 N., R. 43 W.

D2492. Sec. 24, T. 47 N., R. 47 W.

D2493. Sec. 5, T. 46 N., R. 45 W.

be related to increases in the ^Rb/^Sr ratios rather 

than simple redistribution of radiogenic Sr at 1,750 

m.y. ago. The radiometric data indicate that the tonalitic augen gneiss and the overlying biotite gneiss in the 

Watersmeet dome are approximately the same age as 

the Early Archean gneisses in the Minnesota River valley in southwestern Minnesota (Goldich and others, 

1980; Goldich and Wooden, 1980), which has been designated as the type area for the Archean gneiss terrane 

in the Lake Superior region (Sims, 1980; Sims and 

Peterman, 1982). The 2,590-m.y.-old leucogranite that 

cuts the supracrustal rocks in the dome is similar 

lithologically and chemically to the granite facies of the

Puritan Quartz Monzonite; possibly, the two granites 

are comagmatic.

The rocks in the area that are representative of the 

greenstone-granite terrane are all Late Archean in age. 

The gneiss and amphibolite unit in the belt southeast 

of Marenisco is considered approximately correlative 

with the metabasalt (Ramsay Formation) and biotite 

schist associated with the Puritan batholith; it differs 

from the rocks associated with the batholith mainly in 

having been deformed and metamorphosed twice, once 

in the Late Archean and again during the Penokean 

event. The Puritan Quartz Monzonite that intrudes the 

gneiss and amphibolite also differs from that in the 

batholith in having been tectonically disturbed during 

the Penokean event. The Puritan in the batholith has 

been dated by the Rb-Sr isochron method as being 

2,650 ± 140 m.y. old (Sims and others, 1977). Rb-Sr data 

for samples of Puritan Quartz Monzonite in small bodies 

that cut gneiss and amphibolite show considerable scatter, but are consistent with the 2,650-m.y. isochron for 

the main batholith (fig. 11). Rb-Sr data on samples of 

gneiss that are cut by the Puritan indicate a severe 

disturbance and define an isochron of 1,750 m.y. with 

a high initial 87Sr/86Sr of 0.7101. This array includes 

biotite from sample M-68. The isotopic disturbance of 

these systems accords with the strong ductile deformation and recrystallization seen in samples of the 

analyzed gneiss.

Zircons from two samples of gneiss and one of the 

Puritan Quartz Monzonite were dated by the U-Th-Pb 

method (fig. 12). Three size fractions of zircon from one 

sample (M147) give a concordia-intercept age of 2,750 

m.y. Zircons from sample M68, a flaser gneiss, and 

from M71, Puritan Quartz Monzonite from the Cup 

Lake area, have unusually high U contents, and their 

isotopic ages are extremely discordant. Both plot near 

the lower end of the discordia defined by M147, indicating an intercept at about 650 m.y. Presumably, these 

zircons have become metamict as a consequence of their 

high U contents and have suffered substantial Pb loss. 

The Late Archean ages determined for gneiss in the 

belt southeast of Marenisco and for the Puritan Quartz 

Monzonite are indistinguishable from ages obtained by 

both Rb-Sr and U-Pb methods on metavolcanic and 

associated intrusive rocks from greenstone belts in 

northern Minnesota (Peterman, 1979).

Rb-Sr whole-rock analyses of samples of Archean 

metagraywacke have not provided unique age information because of their low Rb/Sr ratios and the excessive 

scatter of data points (fig. 13). However, relative to 

the isochron defined by the Early Proterozoic metagraywacke, three of the Archean samples have significantly higher 87Sr/86Sr ratios. A reference isochron of 

2,750 m.y. bisects the field defined by data points for
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A21

0.6

rO.4

0.2

3,562 m.y.

M 203(100-

M83(100-150)NM-ABR 

M83(200-270)NM

A M83(100-150)NM 

01042(60-80)^ A-^042(100_ 150)

2,643 m.y. .4^M321 (200-250)NM 

a M203(200-250)NM^^ 01042(200-270)

XM203(200-250)

<XCA M83(200-270)M 

%/!45H<50-100)^ VM321 (200-250)

M321(200-250)FLUX 

M32K-400)

763 m.y 

633 m.y.

EXPLANATION

Biotite leucogranite

Interlayered amphibolite and biotite gneiss

Biotite gneiss

Tonalitic augen gneiss

Grain-size ranges, in mesh size, shown in 

parentheses. Suffix letters: M magnetic, 

NM =nonmagnetic, ABR =abraded, and FLUX: 

flux decomposition.

0 10 20 30

207Pb/235U

FIGURE 9. Concordia plot of zircon data for samples of granitoid rocks in the Watersmeet dome. Upper and lower intercepts, respectively, 

are 3,562 m.y. and 1,251 m.y. for augen gneiss and biotite gneiss (M83(200-270)NM excluded from regression); 2,643 m.y. and 633 

m.y. for the interlayered amphibolite and biotite gneiss (M112 excluded from the regression); and 2,585 m.y. and 763 m.y. for the 

biotite leucogranite. Data for M321 and M203 from Peterman and others (in press).

the Archean rocks. As shown by petrographic data, the 

Archean metagraywacke was subjected to a pre-Penokean metamorphism of at least garnet grade. The scatter shown by the Rb-Sr data probably reflects the 

second metamorphism imposed on the earlier mineral 

systems. Even though a precise age is not defined by 

the Rb-Sr results, the data strongly suggest that the 

graywacke is older than that of the Michigamme and 

Copps Formations, and thus are consistent with other 

evidence indicating an Archean age for these rocks.

EARLY PROTEROZOIC ROCKS

The classic sedimentary sequence (Marquette Range 

Supergroup) of the main part of the Gogebic Range 

(Schmidt, 1980; Cannon and Gair, 1970; this report, fig. 

3) gives way at the east end to a grossly correlative 

succession containing a substantial thickness of volcanic 

rocks (Emperor Volcanic Complex) and cogenetic(?) 

sills and dikes. The change in character of the rocks 

from west to east reflects a transition from a shallowwater platform environment to deeper water conditions 

in a less stable tectonic environment. At the east end 

of the Gogebic Range, the Emperor Complex is interbedded with and overlies the Ironwood Iron-formation 

(Trent, 1973), which in this area is composed mainly 

of tuffaceous rocks and, at least locally, of relatively 

nonmagnetic black slate (Hendrix, 1960). As shown on 

figure 2, the Ironwood locally overlies the Palms Formation. The Ironwood and Emperor are overlain, apparently unconformably, by the Copps Formation. The 

Early Proterozoic rocks in the eastern part of the 

Gogebic Range have been described by Van Hise and 

Leith (1911) and by Schmidt (1980) and are not described here.

East of the Gogebic Range, the Ironwood Iron-formation is not recognized as a discrete stratigraphic 

unit, but magnetic iron-formation occurs at approximately the same stratigraphic position within the lowermost Proterozoic unit distinguished in this area, the 

Blair Creek Formation. The iron-formation is magnetic,
A22 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

1.000

0.900

0.800 

0.700

Reference 

3,50C

M-45H 

Isochrons D1042B °-

M-95B 

D1042J(Mi) /'

M-48(Bi) 

(270, 7.723L

^

EXPLAIMATIOIM

A Tonalitic augen gneiss

O Leucogranite

O Biotite gneiss and schist

D Interlayered amphibolite 

and biotite gneiss

0 10 11 12

FIGURE 10. Rb-Sr isochron plot for samples of gneiss, schist, and leucogranite from the Watersmeet dome. Mineral analyses are designated 

by suffix abbreviations in parentheses: PI, plagioclase; Mi, microcline; and Bi, biotite. The remaining points represent whole-rock analyses. 

The 1,750-m.y. isochron is derived from whole-rock and mineral data for the M-48 locality. A regression of these points yields an 

age of 1,750 ±90 m.y. with an initial 87Sr/86Sr of 0.7217 ±0.0019 (uncertainties at the 95-percent confidence level). A parallel reference 

isochron is drawn through the array of points for the leucogranite. The 2,700- and 3,500-m.y. isochrons are shown for comparison.

thin, and apparently discontinuous. The Blair Creek is 

overlain by graywacke-slate called Copps Formation in 

the west and Michigamme Formation in the east (fig. 

2). These rocks unconformably overlie Archean basement rocks.

The local occurrence of rocks typical of the Chocolay 

Group of the Marquette Range Supergroup suggests 

that these strata underlie the Blair Creek Formation, 

at least at places, in the area. Drilling at two localities 

in T. 45 N., R. 43 W., in the southwest part of the 

map area (fig. 2), has disclosed thick quartzite and dolomite successions overlain by iron-formation and slate 

(Alien and Barrett, 1915). These rocks are interpreted 

as being correlative with the lowermost exposed rocks 

of the Marquette Range Supergroup in the Gogebic 

Range. Also, interlayered quartzite and tremolitic dolomite are exposed as an isolated body in a roadcut on 

Gogebic County Highway 206 in sec. 4, T. 45 N., R. 

40 W. These rocks are tentatively considered to be 

equivalent to the Bad River Dolomite; they probably

lie directly on Archean rocks along the northwestern 

margin of the Watersmeet dome.

Mafic dikes of two ages cut all the rocks in the map 

area (fig. 2) except the Copps and Michigamme Formations. The older set is Early Proterozoic; these dikes 

are composed of metadiabase and metagabbro and in 

the northwest part of the region are mainly oriented 

northeastward, subparallel to the structural grain in 

the Archean and Proterozoic rocks. Sparse exposures 

in the Watersmeet dome suggest that here they trend 

northwestward. (See Fritts, 1969.) The younger dike 

set is presumed to be Keweenawan in age. It consists 

of tholeiitic gabbro that is neither deformed nor 

metamorphosed. The dikes trend subparallel to the 

older metamorphosed dikes. Only the Early Proterozoic 

metadiabasic dikes are described in this section.

BLAIR CREEK FORMATION

The Blair Creek Formation is named herein to include the succession of mafic volcanic rocks and lesser
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A23

TABLE 10. Analytical data for samples dated by the Rb-Sr 

method

[Data for metagraywackes and M83, M93, M95B, M109, and M199 are new analyses. 

Other data have been reported previously in Sims and Peterman (1976), Sims 

and others (1977), and Peterman and others (1980). Abbreviations in parentheses 

denote minerals: PI, plagioelase; Mi, mieroeline; Bi, biotite]

Sample No. Rock type Rb Sr 

(ppm) (ppm)

87 Rb 87 Sr 

86 Sr 86 Sr

Watersmeet dome

01042 

M45L 

M83 

M48 

M48 (PI) 

M48 (Mi) 

M48 (Bi) 

02439 

01394 

01395 

01396 

M93 

M1D9 

M203 

M112 

M261 

02438 

M958 

M199 

D1Q42 

D1042J (PI)

D1042J (Mi)

D1042H 

D1042B 

M45H 

Augen gneiss 

 do 

 do 

Biotite gneiss 

 do 

 do 

 do 

 do 

 do 

 do 

 do 

 do 

 do 

 do 

Hornblendebiotite gneissAmphibol ite 

Biotite schist---

 do 

 do 

Leucogranite -

 do 

 do 

 do 

 do 

 do 

149

184

216

96.3

18.4

89.1

356

87.2

95.2

89.1

57.3

105

104

104

33.0

45.6

314

116

98.2

176

25.9

369

167

168

136

132

194

150

64.8

91.2

56.9

6.43

52.3

98.2

240

118

79.5

170

284

218

262

130

30.6

84.5

94.6

32.7

115

69.5

57.7

46.6

3.32

2.78

4.22

4.36

.585

4.59

270

4.89

2.81

1.08

1.42

3.87

1.78

1.07

.439

.505

7.10

11.3

3.411

5.46

2.29

9.26

7.07

8.55

8.65

0.8478

.8256

.8605

.8303

.7365

.8305

7.723

.8466

.8139

.7560

.8087

.8380

.7686

.7548

.7224

.7377

.9044

.8535

1.0249

.9096

.8363

1.0022

.9430

.9901

.9986

Gneiss belt

01043 

M68 

M68 (Bi) 

M147-1A 

M147-1B 

M150 

01392 

M324A 

Flaser gneiss 

 do 

 do 

Tonal ite gneiss--

 do 

Biotite gneiss 

Amphibol ite

inclusion 

Amphibol ite 

118

102

474

51.4

28.6

14.8

91.0

15.0

119

136

26.4

304

239

275

243

203

2.88

2.19

59.4

.491

.346

.156

1.09

.214

.7829

.7659

2.1655

.7226

.7197

.7128

.7413

.7118

Puritan Quartz Monzonite

01389 - 

D1390 

01391 - 

M71 - 

M712 (Bi)- 

M85 -

Biotite granite 

 do 

 do 

 do 

 do 

Syenite 

135

126

99.

101

144

93.

Ill 

88.

164 

94, 

38, 

61,

3.50

4.10

1.75

3.15

11.05

4.45

Archean metagraywacke

Proterozoic metagraywacke

.8391

.8538

.7723

.8312

1.0012

.8867

Mfl 1

MO 7

MQQ 1

M99 

Ml 1 C

M124 

 dO 

 dO 

CO C

66.7

17.6

22.9

QO 1

45.9

337

450

166

one

646

.503

.430

.307

ooc

.463

one

.7191

.7137

.7136

71 7Q

.7092

02433 

02434 

D2435 

02436 

D2437 

D2440 

M106 

M108 

 Metagraywacke 

 dO 

 dO 

__ do 

._ do 

 dO 

 Iron rich 

-- Metagraywacke 

114

95.1

76.0

98.1

115

20.3

26.5

91.1

271

280

341

211

274

316

173

277

1.22

.985

.646

1.35

1.22

.186

.443

.956

.7438

.7325

.7213

.7414

.7369

.7098

.7169

.7302

0.950

0.900 

Reference Isochron 2,650 m.y.

3.0

87Rb/86Sr

FIGURE 11. Rb-Sr isochron plot for samples of Puritan Quartz Monzonite (circles) and biotite gneiss (squares) from the Archean 

gneiss belt southeast of Marenisco. An isochron through points 

representing the biotite gneiss, including biotite from M-68, 

gives an age of 1,750 ±35 m.y. with an initial ^Sr/^Sr of 

0.7101 ±0.0005 (uncertainties at 95-percent confidence limits). A 

reference isochron with this slope is drawn through granite samples M-71, D1389, and D1390. The 2,650-m.y. isochron corresponds in slope and intercept to that defined by samples from 

the main body of Puritan Quartz Monzonite north of Marenisco 

(Sims and others, 1977).

0.6

0.5

0.4

0.3

0.2

0.1

M-147(50-150)

M-147(200-270)

^650 m.y.

6 8

207Pb/235U

10 12 14 16

FIGURE 12. Concordia plot of zircon data for samples of biotite gneiss 

(squares) and associated Puritan Quartz Monzonite (circle) from 

gneiss belt southeast of Marensico. Intercepts defined by three 

fractions of zircon from biotite gneiss sample M-147 are 2,750 and 

650 m.y. Zircons M-68 (flaser gneiss) and M-71 (Puritan Quartz 

Monzonite) are extremely discordant, probably as a result of 

metamictization due to their high uranium contents.
A24 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

u. /ou

0.750

0.740

0.730

0.720

0.710

0.700C

I I

Early Proterozoic

I I I I

graywackes

_ T =1,820 ±50 m.v. . _

R i =0.7050±O.OOO6

-

Reference Isochron 2

_

M-91

 M-97^

M-990 £/' IV 

^2440^-^ M -98- 1 

^X^ M-124

/

I I

) 0.2 0.4

D243>_r

,750 m.y. D2434n x^X^ D2433

V O 

/ ^s'

*/ s^

''^X^°D2435 

M-115

1106 EXPLANATION

O Early Proterozoic 

 Archean

till

0.6 0.8 1.0 1.2 1.4

87Rb/86Sr

FIGURE 13. Rb-Sr isochron plot for samples of Archean and Early 

Proterozoic metagraywackes and associated rocks. The sample 

of metavolcanic rock, M-124, is included with the Archean 

metagraywackes.

conglomerate, wacke, and iron-formation that compose 

the lower part of the Marquette Range Supergroup in 

the region southeast of the Gogebic Range (fig. 2). The 

type area is the area of sporadic exposures in the vicinity of Blair Creek in T. 46 N., R. 40-41 W. Additional 

exposures of the formation are present in sees. 1-6, 

T. 45 N., R. 42 W., east of Barb Lake; in the vicinity 

of Marenisco; and in the narrow belt extending from 

sec. 30, T. 46 N., R. 42 W. northwestward to sec. 7, 

T. 46 N., R. 41 W. The rocks within this narrow belt 

are on the overturned, southeast limb of the Copps 

syncline (fig. 3); they unconformably overlie and truncate Archean gneiss.

The Blair Creek is divided into five map units on 

figure 2, but because of poor exposures only three of 

the units represent known lithotypes. The basal unit 

(Xbs, fig. 2) is exposed on the southeast limb of the 

Copps syncline. In sec. 21, T. 46 N., R. 42 W., it is 

composed of arkosic grit, conglomerate, wacke, and 

local iron-formation and is a maximum of 200 m thick. 

The arkose and conglomerate were derived locally from 

underlying granitoid rocks. In sees. 9, 13, 14, and 22, 

T. 46 N., R. 42 W., the basal unit consists dominantly 

of granite-pebble conglomerate of local derivation, 

quartz-pebble conglomerate, arkosic grit, and iron-formation. Overlying the basal unit on the southeast limb 

of the overturned syncline is a mafic volcanic rock unit 

(Xbv, fig. 2) as much as 400 m thick composed of massive metabasalt and porphyritic metabasalt. On the 

southeast side of the belt of Archean rocks southeast 

of the Copps syncline, the Blair Creek Formation is 

much thicker than to the west. On the south side of

the Barb Lake fault, the Blair Creek is exposed across 

a width of about 1.5 km. Fritts (1969) noted a quartzbearing conglomerate at the base, and we assign this 

bed to the basal unit. It is overlain stratigraphically 

by about 1,500 m of massive and porphyritic 

metabasalt. Equivalent rocks in the mafic volcanic rock 

unit on the north side of the Barb Lake fault, extending 

northeastward from the vicinity of Thayer, are described by Fritts (1969) as consisting mainly of porphyritic lava and massive or pillowed lavas. He delineated 

quartz-conglomerate beds near the base of the formation, but it is uncertain whether or not these are its 

basal beds. Unless additional northeast-trending faults 

have repeated the succession, it seems probable that 

the conglomerate beds are higher in the section, intercalated with lavas. The lean iron-formation unit (Xbi, 

fig. 2) in the approximate middle of the formation in 

the Barb Lake and Thayer areas was mapped mainly 

from aeromagnetic anomalies, but the iron-formation 

unit is exposed along Blair Creek adjacent to the 

Gogebic-Ontonagon County line (Fritts, 1969). The 

upper part of the formation, mapped as dominantly volcanic rocks (Xbvu unit on fig. 2), is poorly exposed, 

but outcrops along Blair Creek are described by Fritts 

as consisting mainly of layered metatuff and tuffaceous 

metagraywacke. The unexposed magnetic unit (Xbm, 

fig. 2) at the top of the formation, presumably contains 

thin beds of iron-formation. A similar magnetic unit 

(Xam) outlines most of the gneiss dome at Watersmeet; 

probably it is equivalent to a part of the Blair Creek 

Formation, but it is not necessarily correlative with the 

unexposed magnetic unit at the top of the formation 

near Blair Lake. The formation thickens southeastward 

from 0 to an estimated 2,000 m.

In thin section, weakly metamorphosed parts of the 

porphyritic lava have an ophitic texture. This texture 

was destroyed during higher grade metamorphism, and 

the amphibole (hornblende or actinolitic hornblende) 

throughout the rock was conspicuously lineated. Biotite 

is commonly associated with the hornblende. Some of 

the porphyritic lava has visible pillow structures, as do 

some of the more massive lavas. Chemical analyses of 

representative samples of the porphyritic lava and associated lavas (table 11) indicate that the volcanics have 

the composition of tholeiitic basalt.

COPPS AND MICHIGAMME FORMATIONS

The Copps and Michigamme Formations are considered correlative, for they both are composed of metagraywacke and slate and directly overlie the Blair 

Creek Formation. Presumably the contact between the 

Copps and Blair Creek units is subconformable. The 

Copps Formation occupies the keel of an overturned 

syncline east of Marenisco, forms a narrow belt of
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A25

TABLE 11. Chemical analyses of lavas from the Blair Creek 

Formation

[Chemical analyses by N.Skinner and D. Kobilis. U and Th analyses by H. T. Millard, 

Jr., and others]

Lab. No.- 0216361 D216362 D216364 0216365

Field No. M96X M96Y M179 M179-1

Major oxides (weight percent)

SiOo 

A1 20 3 

FeO 

MgO -

CaO 

Na20 

K20- 

H ?0 

H2(T 

Ti02 

p o 5 - 

MnO 

co2 .

47.8

14.2

2.1

9.0

6.9

11.4

2.7

.49

1.4

.12

1.6

.27

.16

.88

47.6

14.0

2.6

11.2

5.5

9.4

3.2

.36

1.5

.27

2.3

.74

.23

.52

51.1

13.9

1.7

9.5

5.9

9.4

2.7

.63

1.6

.13

1.6

.19

.23

.79

49.3

18.3

1.5

6.0

6.0

12.2

2.2

.46

1.2

.09

.65

.07

.14

.48

Sum 99 99 99 99

Trace elements (parts per million)

u .

Th -

O n

2

1

1.0

5.6

O ne

3.5

0.2

1.5

SAMPLE DESCRIPTIONS AND LOCALITIES

M96X. SEV4SWV4 sec. 32, T. 46 N., R. 41 W.

Collected adjacent to U.S. Hwy. 2. Darkgray, medium-grained basalt with conspicuous foliation and lineation.

M96Y. Same locality as M96X. Pillow basalt.

M179. West-center sec. 21, T. 46 N., R. 42 W. 

Probably pillow basalt.

M179-1. Same locality as M179. Porphyritic basalt. 

Has ophitic texture.

steeply dipping strata adjacent to the Puritan batholith 

and associated biotite schist, and overlies somewhat 

folded strata of the Ironwood Iron-formation and the 

associated Emperor Volcanic Complex at the eastern 

end of the Gogebic Range (Hendrix, 1960; Trent, 1973). 

It stratigraphically overlies the Blair Creek 

Formation throughout most of the syncline, but to the 

northwest it lies directly on the Archean Puritan 

batholith; further west, it overlies strata on the East 

Gogebic Range that are approximately correlative with 

the Blair Creek Formation. Where it overlies the Archean batholithic rocks, it has a local basal conglomerate derived from the underlying basement rocks (Alien 

and Barrett, 1915). The Copps thins westward because 

of erosion, and it pinches out beneath Keweenawan 

(Middle Proterozoic) lavas near Wakefield, Mich., 15

km west of the map area. The formation is composed 

dominantly of thick-bedded graywacke and argillite, the 

beds typically being 1 to 3 m thick. Graded beds are 

common and are distinguished most readily by diffraction of cleavage as it passes from the coarser to finer 

parts of a bed. This relationship is well illustrated in 

outcrops on Ice House Bay, at the southern end of 

Lake Gogebic. Carbonate lenses, generally rotated into 

the cleavage, are common. Mineralogically, the Copps 

consists mainly of grains of plagioclase, quartz, and biotite.

The Michigamme Formation is exposed sporadically 

in the area north and northeast of the gneiss dome at 

Watersmeet, where it occupies the keel of a syncline 

between the gneiss dome and the south-dipping beds 

of the Blair Creek Formation (fig. 2). The Michigamme 

resembles the Copps Formation. It is both thick and 

thin bedded, has weakly graded beds, and contains impure carbonate lenses. It is composed dominantly of 

quartz, plagioclase, and biotite; garnet is present adjacent to the gneiss dome.

As noted earlier, the Copps and Michigamme differ 

in major element chemistry from the Archean metagraywacke in having higher CaO and slightly lower 

MgO and Na20 (table 7). Chemical differences are particularly indicated by rubidium and strontium contents 

and by K/Rb ratios; the Copps and Michigamme have 

much higher rubidium contents and generally lower 

strontium contents (fig. 6) and higher K/Rb ratios (fig. 

7) than the Archean metagraywacke. Rb-Sr data for 

samples of the Michigamme and Copps Formations and 

a single sample of an iron-rich sedimentary rock from 

the Blair Creek Formation scatter somewhat on an 

isochron diagram (fig. 13) but define a slope corresponding to an age of 1,820 ±50 m.y. and an intercept 

^Sr/^Sr value of 0.7050 ±0.0006. Within this array, 

only two of the samples (D2433 and D2434) depart from 

the isochron by an amount significantly greater than 

analytical error. These variations could reflect either 

open-system behavior, variation in initial strontiumisotope ratios, or both. Exclusion of these points does 

not alter the slope of the isochron appreciably, although 

petrographic evidence does not indicate that these samples are different than the others. Based on previous 

studies dealing with the Rb-Sr systems in 

metasedimentary rocks, we consider the isochron age 

as a close approximation of the time of metamorphism.

METADIABASE DIKES

Metadiabase dikes of Early Proterozoic age are abundant in the northwest part of the area (fig. 2), especially in the Archean rocks belonging to the greenstonegranite terrane. Most of the dikes shown on figure 2 

were mapped by Fritts (1969) and Trent (1973). The
A26 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

dikes are dominantly oriented northeastward, subparallel to the structural fabric of the country rock and, as 

noted earlier, subparallel to the boundary between the 

two basement terranes (Sims, 1980, fig. 5). Most are 

only a few meters thick, but some are several tens of 

meters thick. We estimate that they make up as much 

as 5 percent by volume of the rocks near the boundary 

zone.

The dikes have a general basaltic composition, and 

have been metamorphosed and deformed differently, 

depending on their geographic distribution. The mafic 

dikes in the gneiss belt in the northwest part of the 

area (fig. 2) locally have chilled margins and relict ophitic textures; they are dark gray and fine to medium 

grained. Modally, they consist mainly of an amphibole 

(mainly hornblende), lesser plagioclase, and a few percent of biotite and quartz. Sphene, opaque oxides, epidote, and calcite are conspicuous alteration minerals. 

The color of the hornblende and the composition of the 

plagioclase depend on the grade of metamorphism. The 

dikes have a weak to moderate foliation and lineation 

but are not folded, so far as known.

Within the Watersmeet dome, the mafic dikes are 

deformed as well as metamorphosed. They are dark 

gray to nearly black and coarser grained than those 

to the west. Relict ophitic textures are rare. The 

hornblende is bluish green or green, strongly poikilitic, 

and associated with a few percent of sphene and opaque 

oxides. The plagioclase (An25_35) is poorly twinned, typically contains subrounded quartz blebs, and is partly 

replaced by biotite, epidote, and calcite. Much of it has 

been granulated and recrystallized on a fine scale. Biotite composes 5 to 15 percent of the rock. Garnet is 

a local metamorphic mineral. At several places, smallscale folds that have northward-dipping axial surfaces 

and a north- to northeast-plunging lineation can be 

seen. The folds resulted from the deformation associated with the rise of the gneiss dome during the 

Penokean event. Exposures of such folds are present 

in the NEV4 sec. 5, T. 45 N., R. 39 W.

The mineralogy of the metadiabase dikes is similar 

to that of the mafic lavas in the Blair Creek Formation; 

possibly the dikes and lavas are cogenetic.

GEOCHEMISTRY 

CHARACTERISTIC FEATURES

Because of the long time interval between the ages 

of the Early Archean gneisses in the Watersmeet dome 

and the Late Archean intrusive and metamorphic 

rocks, compositional differences between these rocks 

are to be expected. Major- and trace-element analyses 

given in the previous tables show that most of the units

have certain characteristic compositional features. 

These are discussed below.

Variations in major elements are illustrated on K20- 

Na20-CaO and AFM diagrams (figs. 14 and 15). The 

range in composition of the intrusive rocks and gneisses 

from trondhjemitic and tonalitic to granitic is shown 

on the K20-Na20-CaO diagram (fig. 14). Samples of the 

Puritan Quartz Monzonite from the batholith and from 

the Cup Lake area are granitic, both modally (fig. 8) 

and chemically (table 8). Data for the four samples of 

biotite leucogranite from the Watersmeet dome cluster 

closely; they have Na20/K20 ranging from 0.69 to 0.82 

and (Na20 + K20)/CaO ranging from 7.3 to 20. They are 

contained in the broader field defined by samples of 

Puritan Quartz Monzonite. Modally, the Early Archean 

augen gneiss is tonalitic and contains more biotite than 

samples of the Puritan Quartz Monzonite (table 2). 

These differences are not pronounced on the K20-Na20- 

CaO plot, which shows that the Early Archean gneiss 

is relatively potassic and similar in Na20/K20 ratios to 

many of the samples of Late Archean granite. Much 

of the K20 in the augen gneiss is in biotite and sericite 

rather than microcline, and this difference can account 

for the modal differences and chemical similarities between the two rock types. Biotite gneisses and schists 

both within and outside of the Watersmeet dome derived from volcanic rocks vary widely in Na20/K20 

ratios and in the proportion of total alkalies to CaO. 

Within the dome, potassium may have been introduced 

into these rocks after crystallization. Three samples of 

amphibolite from the gneiss belt and one from the dome 

plot near the CaO apex. Rocks intermediate in composition between the amphibolite and felsic gneisses have 

not been recognized, suggesting that collectively the assemblage may be bimodal in composition, a common 

volcanic association in many Archean terranes (Barker 

and Peterman, 1974).

The metagraywackes of Early Proterozoic and Late 

Archean ages are only partially separated on the alkalilime plot (fig. 14). They have the characteristic features 

of most graywackes; in specific, their Na20/K20, MgO/ 

CaO, and FeO/Fe203 ratios are greater than unity (Pettijohn and others, 1973). Metabasalt of the Blair Creek 

Formation is characterized by high Na20/K20 ratios, 

which is consistent with its tholeiitic character; the 

samples plot near the CaO apex in accord with their 

basaltic compositions.

The Archean granitic and gneissic rocks plot closest 

to the alkali apex of the AFM diagram (fig. 15) because 

of their felsic nature. Significant differences among the 

units are evident, notably in the proportions of FeO* 

(total iron expressed as FeO) and MgO. Samples of the 

Puritan Quartz Monzonite from the batholith and from 

the smaller bodies near Cup Lake have similar and un-
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA

CaO CaO

A27

K9O Na,O Na 9O

EXPLANATION

Archean:

Watersmeet Dome: 

Tonalitic augen gneiss

Biotite gneiss succession 

Amphibolite and biotite gneiss 

Leucogranite

Archean - Continued:

Southeast of Marenisco: 

tfl Gneiss and amphibolite

* Metagraywacke

* Puritan Quartz Monzonite

Early Proterozoic: 

6 Metagraywacke

+ Blair Creek Formation 

FIGURE 14. K20-Na20-CaO diagram for selected Archean and Early Proterozoic rocks in the Marenisco-Watersmeet area.

commonly high FeO*/(FeO* MgO) ratios, averaging 

0.89 and 0.90, respectively. The augen gneiss samples 

from the Watersmeet dome have significantly lower 

FeO*/(FeO* MgO) ratios, their mean being 0.75. In this 

respect, they are similar to the Late Archean leucogranite dikes, but the latter are lower in FeO* and MgO 

relative to total alkalies and generally are more siliceous. The biotite gneisses from both tectonic environments are highly variable in FeO*/(FeO* +MgO).

The Early Proterozoic and Late Archean graywackes 

are clearly separated on the AFM diagram (fig. 15). 

The Early Proterozoic rocks are uniformly higher in 

FeO*/(FeO*+MgO) than the Archean rocks. The nearlinear trends of decreasing FeO*+MgO with increasing 

alkalies are similar to those defined by differentiated 

rock series. If the graywackes are dominantly volcanogenic in origin, as we suspect, these trends may

reflect the nature of the volcanic series from which they 

were derived. The Early Proterozoic graywackes 

suggest an iron-enrichment trend of tholeiitic character, whereas the trend defined by the Archean 

graywackes is more calc-alkaline.

The four samples of metabasalt from the Blair Creek 

Formation are alined along an iron-enrichment trend 

in the tholeiitic field. The Early Proterozoic graywacke 

that has the highest relative FeO* content is an ironrich sample intercalated with metabasalt in the Blair 

Creek Formation. Collectively, the Early Proterozoic 

graywacke and basalt are strongly suggestive of a 

tholeiitic series that has a moderate iron enrichment.

Significant differences among some of the units are 

shown by oxides of the minor elements titanium and 

phosphorus (fig. 16). Samples of the Puritan Quartz 

Monzonite are low in both elements and define a field
A28 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

FeO

Alkali Alkali MgO 

FIGURE 15. AFM diagram for selected Archean and Early Proterozoic rocks in the area. Symbols are the same as on figure 14.

fairly distinct from the other units. Relative to these 

rocks, the tonalitic augen gneiss contains substantially 

more Ti02 and P205 and has a limited range of values. 

The leucogranite dikes are intermediate between these 

two groups. Graywackes of both ages are relatively 

high in Ti02 and P205, and although there is some overlap, the Archean graywacke tends to have lower Ti02/ 

P205 ratios than the Early Proterozoic graywacke. The 

iron-rich metasediment in the Blair Creek Formation 

has contents of these elements that are indistinguishable from those in the associated basalt.

Thorium and uranium vary considerably within and 

among units (fig. 17). Some of the scatter may be a 

consequence of surficial processes, for mobility of uranium in crystalline rocks through leaching is well established. The most coherent groups of data are formed 

by the values for the graywackes. Uranium content in 

the two groups is approximately the same, but the Late 

Archean rocks have higher thorium values, resulting 

in higher Th/U ratios. (Averages are 3.0 for the Archean rocks and 2.4 for the Proterozoic rocks.) Several 

samples are significantly enriched in both uranium and 

thorium above values considered to be average for 

granitic rocks (4 ppm U and 18 ppm Th; Rogers and 

Adams, 1969). One sample of leucogranite has high uranium and thorium contents of 17.6 and 67.4 ppm, respectively. Field measurements with a scintillometer 

indicate that these dikes are commonly above average 

in their total radioactivity. Values for samples of the 

Puritan Quartz Monzonite scatter widely, but several 

show uncommonly high contents of thorium and (or) 

uranium.

Although rubidium and strontium contents also vary 

widely, the intrusive rocks and gneisses collectively 

show the common pattern of increasing Rb/Sr ratios 

with increasing K20 as the compositions trend from tonalitic or trondhjemitic to granitic (fig. 18). Samples 

of the leucogranite dikes tend to have high Rb/Sr 

ratios, mainly as a result of their lower strontium contents. Rb/Sr ratios of the tonalitic augen gneiss are limited to a range between about 1.0 and 1.5. Biotite 

gneiss and schist within the Watersmeet dome illustrate an unusual trend in that their rubidium content 

is roughly constant and their strontium content ranges 

by an order of magnitude. This is not a common differentiation trend, for increasing rubidium is usually associated with decreasing strontium. This feature probably reflects unusual mobility of rubidium and strontium 

in these fine-grained rocks during the Penokean 

metamorphism. The graywackes are among the samples having lower Rb/Sr ratios and show some differences related to age as noted earlier. Samples of the
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A29

3.0

1.0

0.1

0.01

~\ i i i i rr I I I I I TTT

+

e

 P 0A <

ee A e

o<

0°

* o<

o 

* *

0.01 0.1

P2O5, PERCENT

1.0

FIGURE 16. Ti02-P205 log-log plot for selected Archean and Early 

Proterozoic rocks in the Marenisco-Watersmeet area. Symbols 

are the same as on figure 14. The less-than symbols (<) adjacent 

to certain points indicate that the P205 values are equal to or 

less than the values indicated.

100

0.1 1.0 10.0 

URANIUM, IN PARTS PER MILLION

30.0

FIGURE 17. Th-U log-log plot for selected Archean and Early Proterozoic rocks in the Marenisco-Watersmeet area. Symbols are 

the same as on figure 14. Lines representing Th/U ratios of 1, 

4, and 20 are shown for reference.

Puritan Quartz Monzonite scatter considerably and define no particular trend or pattern. Samples of the 

Early Archean tonalitic augen gneiss have unusually

500

100

1010 100 

STRONTIUM, IN PARTS PER MILLION

1000

FIGURE 18. Rb-Sr log-log plot for selected Archean and Early Proterozoic rocks in the Marenisco-Watersmeet area. Symbols are 

the same as on figure 14.

low K/Rb ratios (less than 200; table 2). With few exceptions, K/Rb ratios of the other granitic and gneissic 

units are between 200 and 400.

GENETIC SIGNIFICANCE

Limited variations within the major granitic and 

gneissic units and the possibility of metamorphic effects 

make genetic deductions highly speculative. Nevertheless, some observations, based largely on comparison 

with units of other regions, seem appropriate.

The Puritan Quartz Monzonite is similar in age, composition, and geologic setting to the Late Archean 

granitic batholiths of northern Minnesota, where detailed geochemical studies have been completed. Arth 

and Hanson (1975) concluded that granites of the Vermilion Granitic Complex and the Giants Range Granite 

(a batholith) originated through partial melting of 

short-lived graywacke at crustal depths. Although our 

chemical data are not as complete as those of Arth and 

Hanson, the general compatibility of features is consistent with a similar origin for the Puritan Quartz Monzonite. The predominance of granitic rocks in the Puritan 

Quartz Monzonite and their uncommonly high FeO*/ 

(FeO*+MgO) ratios do not favor an origin based on 

fractional crystallization of a more mafic parental 

magma. Three of the four known Early Archean terranes in the North American shield Greenland, Labrador, and northern Michigan contain intermediate to 

felsic augen gneisses as major components. The significance of these units in the evolutionary history of these 

terranes is not clearly established, but some similarities
A30 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

TABLE 12. Average compositions of Early Archean augen gneisses 

from northern Michigan, Greenland, and Labrador

[Compositions normalized to 100 percent on a C02- and H20-free basis. Total iron expressed 

as FeO*. Data for major oxides in percent]

Constituent Watersmeet Gothab, Saglek, 

dome 1 Greenland2 Labrador 3

Si02 

A1 2 0 3 - 

FeO* 

MgQ 

CaO --

Na-O 

K2 0 

Ti02 

P 2 o 5 ~ 

MnO 

Rb, ppmSr, ppm 

Rb/Sr 

K/Rb 

Alkalies/CaO 

FeO*/(FeO*+MgO)

70.4

14.3

3.99

1.39

2.03

3.68

3.42

.63

.13

.03

176

152

1.16

161

3.5

.75

66.4

14.6

5.86

1.11

3.64

3.54

3.43

1.00

.31

.09

86

220

.39

328

1.9

.84

68.7

13.9

5.26

1.20

2.78

3.72

3.34

.78

.22

.09

155

299

.52

176

2.5

.81

Average for samples 01042, M-45L, D1042G, M83, 

and D1042I.

2Average for Amitsoq augen gneiss, from McGregor 

(1979).

3 Average for Uivak II augen gneisses, from 

Collerson and Bridgwater (1979).

in composition warrant a brief discussion. Compositional attributes of the augen gneisses in Greenland and 

Labrador have been discussed in detail (McGregor, 

1979; Collerson and Bridgwater, 1979). Average compositions for the augen gneisses are shown in table 12. 

Whereas the limited sampling of the augen gneiss in 

the Watersmeet dome has not identified any compositional variations attributable to wide ranges in primary 

composition, the Amitsoq and Uivak II augen gneisses 

from Greenland and Labrador, respectively, do show 

significant variations in both major and trace elements. 

These variations are not apparent in table 12, however, 

as it only shows averages.

Although there are significant differences among the 

average compositions of the augen gneisses, certain features are in common. The augen gneisses are relatively 

high in Ti02 and P205 for their Si02 contents. All of 

the units have average Na20/K20 ratios near unity. K/ 

Rb ratios are exceptionally low in the Uivak II gneisses 

and the augen gneiss at Watersmeet. Collerson and 

Bridgwater (1979) attribute the low ratio for the Uivak 

II gneisses to metamorphism. We suggest also that differential mobility of lithophile elements during the 

Penokean event can account for the low K/Rb ratio in 

the tonalitic augen gneiss at Watersmeet.

The protolith of the augen gneisses is problematic. 

McGregor (1979) and Collerson and Bridgwater (1979) 

draw analogies with anorogenic granitic rocks commonly associated with gabbros and anorthosites. Collerson and Bridgwater (1979) suggest that the Uivak II 

gneisses were formed by partial melting of tonalite and 

trondhjemite at granulite fades conditions followed by 

some fractional crystallization. McGregor (1979) appears to favor a similar origin for the Amitsoq augen 

gneisses. The Uivak II gneisses and the Amitsoq augen 

gneisses are considered to be intrusive igneous rocks. 

With respect to the augen gneiss in the Watersmeet 

area, evidence for establishing either the origin or the 

early crustal history of the augen gneiss is lacking. 

However, the presence of relatively large plagioclase 

crystals in a generally finer grained matrix is compatible with a volcanic protolith of intermediate composition.

However, consideration of the augen gneiss protolith 

as a volcanic rock requires explanation of some of its 

chemical features. For the most part, the major element composition can be matched with that of some 

calc-alkaline rhyodacites, although these rocks rarely 

attain such high Rb/Sr ratios. However, more extreme 

differentiates are typically lower in iron, magnesium, 

and calcium. The low K/Rb ratios, as noted by Collerson and Bridgwater (1979), are suggestive of differential mobility of these elements, probably during 

metamorphism. The rubidium content of the average 

augen gneiss (table 2) would have to be reduced to 

about 100 ppm to form a normal K/Rb ratio of 300. 

Addition of a potassium-free, rubidium-rich phase 

seems unlikely, but modification of K/Rb ratios could 

also occur by mobilization and removal of a K-feldsparrich phase, leaving a biotite-enriched restite, because 

these two minerals effectively fractionate K/Rb ratios 

(Goldich and others, 1970). If this process occurred, the 

present composition of the augen gneiss has been severely modified from the composition of the protolith, 

and this possibility greatly complicates the task of determining the original rock type.

The limited data available for the Early Proterozoic 

rocks are compatible with a rift tectonic setting, as proposed earlier (Sims and Peterman, 1983). Metabasalts 

from the Blair Creek Formation are clearly tholeiitic 

in character, a feature that is common in more modern 

regimes developed in rift zones.

STRUCTURE

Deformation in the area was complex and involved 

at least three and probably four episodes of folding in 

the gneiss terrane and two episodes in the greenstonegranite terrane: one during the Archean and another
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A31

during the Early Proterozoic Penokean event. 

Moreover, in both terranes, faulting began in the Archean and continued intermittently until well into 

Proterozoic Y (Keweenawan) time.

ARCHEAN DEFORMATION

Deformation in the gneiss terrane, as recorded by 

the gneisses and associated rocks in the Watersmeet 

dome, was more involved and prolonged than that in 

the greenstone-granite terrane. The oldest recognized 

deformation involved the development of a generally 

steep foliation in the tonalitic augen gneiss and an accompanying metamorphism, apparently at least to amphibolite grade. This deformation preceded deposition 

of the supracrustal biotite gneiss succession, which subsequently was deformed on northeast-trending axes, 

again under amphibolite-facies conditions. Apparently, 

a third episode of folding followed deposition of the 

Late Archean succession of interlayered amphibolite 

and biotite gneiss and schist, for these rocks contain 

two generations of biotite and associated planar features, which are indicative of polydeformation. Inasmuch as the Late Archean leucogranite shows evidence 

for only a single deformation, of Penokean age, the 

older foliation in the amphibolite-biotite gneiss succession must be Archean. The attitude and pattern of this 

deformation is poorly known, but folding must have 

been on roughly east-west axes and under amphibolitegrade conditions. Further detailed studies are warranted. The oldest deformation took place about 3,560 

m.y. ago, if the U-Pb zircon age of the tonalitic augen 

gneiss is indicative of its primary age. The second 

episode of folding occurred between about 3,560 m.y. 

ago and 2,700 m.y. ago. The youngest deformation 

must have taken place shortly before emplacement of 

the 2,590-m.y.-old leucogranite; it could have been concurrent with the Late Archean episode in the 

greenstone terrane. All the rocks in the dome, including the Early Proterozoic metadiabase, were strongly 

deformed and metamorphosed during the Penokean 

event, and this episode partly obscured the record of 

the older events.

The Precambrian rocks to the northwest of the gneiss 

dome, which belong to the Archean greenstone-granite 

terrane, were deformed during a Late Archean event, 

about 2,700 m.y. ago. The biotite gneiss and amphibolite in the Archean belt southeast of Marenisco were 

deformed into steep, moderately tight, north-northeastto east-trending folds. In the same way, the Archean 

metagraywacke was deformed on steep north-northeast-trending fold axes. The axial planes of these folds 

commonly have quartz lenses. Judged from reversals 

in stratigraphic facing directions in the graywacke, the 

fold crests are approximately 200 m apart. The structure in the biotite schist unit adjacent to the Puritan 

batholith (fig. 2) was not studied in detail, but according to Fritts (1969), the foliation in these rocks strikes 

nearly east and is steep. West of the Prospector Creek 

fault (fig. 2), the foliation in these rocks and the apparently intertonguing metabasalt trends about N. 60° E. 

and dips uniformly about 60° SE. In both areas of Archean bedded rocks, the intrusive rocks of the Puritan 

Quartz Monzonite were not deformed during the Late 

Archean tectonism. The foliation and fold axes in the 

belt southeast of Marenisco are subparallel to the 

boundary between the two Archean terranes; presumably, they resulted from compression caused by the impingement of the Archean gneiss segment against the 

greenstone-granite segment.

PENOKEAN DEFORMATION

The Archean and Early Proterozoic rocks in the 

Marenisco-Watersmeet area were deformed to different 

degrees in Early Proterozoic time by a deformation assigned to the Penokean orogeny. The major tectonic 

elements were folding of the Proterozoic rocks on 

northeast- to east-trending axes and the development 

of a strong to weak foliation in both Early Proterozoic 

and Archean rocks. The foliation is axial planar to the 

folds. This deformation decreased in intensity northwestward. A slightly younger, more local deformation, 

also assigned to the Penokean, resulted from the uplift 

of the gneiss dome at Watersmeet; it produced a rim 

syncline in the previously deformed overlying Proterozoic rocks and folds and a penetrative foliation in the 

Precambrian rocks in the core of the dome. The approximate trace of the axis of the rim syncline is shown 

on figure 2.

The style and nature of the larger scale regional folds 

in the Early Proterozoic rocks are shown on figure 3. 

The northeast-trending folds, as exemplified by the 

Copps syncline, just east of Marenisco, are overturned 

toward the northwest, have an axial-plane S2 (Penokean) cleavage dipping 45°-70° SE., and plunge gently 

or moderately northeast. The subjacent Archean basement rocks have a penetrative schistosity that is subparallel to the cleavage in the younger supracrustal 

rocks. The east-trending folds in the Michigamme Formation on the north side of the Watersmeet dome, 

which presumably formed at the same time as the 

northeast-trending folds to the west, similarly are overturned to the north and have a steeply dipping penetrative S2 cleavage. Observed folds in the Michigamme are 

small in scale; they plunge about 15° E.

The regional foliation related to the Penokean folding 

in both Archean and Early Proterozoic rocks is shown 

by the Schmidt net plots on figure 19. The foliation 

varies slightly in both strike and dip from one area to
A32 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

FIGURE 19. Contour plots (in percent) of poles to foliation related to Penokean folds in the Marenisco area; upper hemisphere projection. 

A, Foliation in Copps and Blair Creek Formations and in Archean amphibolite and metagraywacke units in the northeast-trending 

belt east of Marenisco. B, Foliation in Archean granitic rocks in Puritan batholith. (Data from Trent, 1973.) C, Foliation in Archean 

metavolcanic unit on northwest side of Puritan batholith. (Data from Trent, 1973.) D, Axial-plane cleavage in Emperor Volcanic Complex, 

sections 14, 15, 22, and 23, T. 47 N., R. 43 W. (after Hendrix, 1960).
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A33

another and from one rock type to another but does 

not differ greatly in orientation. The intensity of the 

foliation decreases to the northwest, but according to 

Hendrix (1960) the foliation persists to the center of 

T. 47 N., R. 44 W., which is about 20 km west of Lake 

Gogebic.

In contrast, in the Archean rocks and Early Proterozoic metagabbro dikes in the core of the Watersmeet 

dome, the foliation trends easterly and dips moderately 

northward. This foliation is axial planar to recumbent 

folds, apparently of small scale, that can be observed 

in the bedded rocks and the metagabbro. These folds 

plunge northward or northeastward where observed 

within the area of figure 4. The metagraywacke and 

slate of the Michigamme Formation on the north margin of the dome also have a gently north-dipping schistosity, which is axial planar to recumbent folds. These 

structures largely obliterate the older (S2) foliation in 

the area south of the rim syncline. Thus, uplift of the 

gneiss dome was accompanied by recumbent folding of 

the Late Archean and Early Proterozoic bedded rocks 

that unconformably overlie the Archean basement 

gneiss. This mode of deformation was restricted to the 

gneiss dome itself and reflects inhomogeneous deformation of the basement rocks. These data suggest that 

changes in the direction of dip of foliation throughout 

the region can be related to different strain patterns 

within separate basement crustal blocks, resulting from 

both vertical and horizontal movements.

The vergence of the regional Penokean folds in the 

Proterozoic rocks and the related cleavage and schistosity indicate maximum compressive stresses in a northwest-southeast direction and, in the area north of the 

Watersmeet dome, stresses in a north-south direction. 

Presumably the axis of maximum finite shortening was 

subhorizontal and was directed northwestward and 

northward. Judged from the radiometric data, the principal deformation and accompanying metamorphic recrystallization in this area took place 1,800-1,750 m.y. 

ago.

The Penokean-age foliation in the Archean rocks of 

the greenstone-granite terrane, so far as known, is restricted to its southern margin adjacent to the gneiss 

terrane. In this area, the width of this tectonized zone 

is about 25 km.

FAULTS

Three main sets of high-angle faults of different ages 

have been distinguished in the Marenisco-Watersmeet 

area (fig. 20). These are (1) a northwest-trending set, 

represented by the Banner Lake fault; (2) an eastnortheast-trending set, represented by the Presque

Isle fault; and (3) a northeast-trending set, represented 

by the Slate River fault. A fourth set of northwesttrending faults in the eastern part of the map area appears to cut and offset lower Keweenawan lavas (Ysb 

unit, fig. 2).

NORTHWEST FAULT SET

The principal northwest-trending fault, here named 

the Banner Lake fault, extends from the western margin of the map area (fig. 2) to the south side of the 

gneiss dome at Watersmeet. Evidence for the fault in 

this area is mainly the right-lateral apparent offset of 

magnetic anomalies given by the Blair Creek Formation. West of the map area, in T. 46-47 N., R. 44 W., 

the fault separates Archean metavolcanic rocks of the 

Ramsay Formation from the Puritan Quartz Monzonite; 

it probably connects with the Sunday Lake fault in the 

Wakefield area (Prinz and Hubbard, 1975). Movement 

on the fault seems to have been both strike-slip and 

dip-slip, and probably took place intermittently from 

late Archean to at least Proterozoic Y time. The Sunday Lake fault does not cut the Proterozoic Y 

(Keweenawan) lavas in the Wakefield area (Prinz and 

Hubbard, 1975).

An apparent eastward branch from the Banner Lake 

fault, the Barb Lake fault (fig. 10), trends eastward 

from the vicinity of Banner Lake and appears to form 

the north, faulted edge of the Watersmeet gneiss dome. 

This fault has an apparent right-lateral displacement 

of about 6.5 km as indicated by offset of the Blair Creek 

Formation. (See fig. 2.)

The northwest-trending faults probably belong to a 

family of faults that includes many faults in northern 

Wisconsin (Sims and others, 1978). The major fault in 

this system, the Mineral Lake fault (Sims, 1980), has 

had an apparent right-lateral displacement of about 16 

km.

EAST-NORTHEAST FAULT SET

East-northeast-trending faults have had a major effect on the distribution of rock units in the area. The 

principal fault, named the Presque Isle fault by Hendrix 

(1960) and called the Presque Isle wrench fault by Trent 

(1973), forms a conspicuous N. 70° E. lineament in T. 

46 N., R. 44 W., southeast of Wakefield; in the map 

area (fig. 2), it separates downdropped, steeply northdipping Early and Late Proterozoic rocks of the 

Gogebic Range and vicinity from Archean rocks to the 

south. It is inferred that the Presque Isle fault continues along the same strike to the southwest for a distance of at least 22 km and that it closely follows the 

boundary between amphibolite-facies metamorphic
A34 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

89° 45'

46° 15'

5 10 KILOMETERS 

I i i i i I

EXPLANATION

Northeast-trending fault (known or inferred to have formed 

in late Keweenawan time)

East-northeast-trending fault Bar and ball on downthrown 

side, (known or inferred to have formed in Early Proterozoic 

time and to have been rejuvenated in late Keweenawan 

time)

FIGURE 20. Traces of faults in the Marenisco-Watersmeet area, northern Michigan.

Northwest-trending fault (known or inferred to have

formed in Archean time) 

Fault of uncertain age (probably formed in Keweenawan

time)

rocks on the south and greenschist-facies rocks in the 

Ironwood, Mich., area, as mapped by Schmidt (1976). 

The apparent movement on the fault in the map area 

is dip-slip, the north side having moved downward. The 

possibly related Whitney Creek-Palms Creek lineament, mapped by Schmidt, is subparallel to the Presque 

Isle fault and 3.3 km north of it. Another possible fault 

in this set, the Judson Falls fault, is inferred from regional geologic relationships to be present about 10 km 

south of the Presque Isle fault. It is inferred to truncate 

the Keweenawan lavas east of Lake Gogebic, and accordingly is interpreted as having dominant dip-slip 

movement, with the north side having moved relatively 

upward. Under this interpretation, lower Keweenawan 

lavas in the horst between the Presque Isle and Judson 

Falls faults either were completely removed by erosion, 

as suggested by the map pattern (fig. 2), or were 

eroded downdip to the northeast and now are covered 

by the Jacobsville Sandstone. Possibly the broad magnetic anomaly in the center of T. 47 N., R. 42 W. (U.S. 

Geological Survey, 1972) reflects the buried downdip 

erosional remnant of the lavas. Other faults in this set 

were mapped by Fritts (1969) from displacements visible in outcrops just west of the south end of Lake 

Gogebic, and three of these are shown on figure 2.

Movement on fractures in this fault set apparently 

began during, or before, Early Proterozoic time, for 

abundant metadiabase dikes trend parallel to the fault 

set and probably occupy fractures related to it. The 

dikes cut rocks as young as the Ironwood Iron-formation and the Emperor Volcanic Complex in the eastern 

part of the Gogebic Range and the Blair Creek Formation to the southeast. The last movement recorded by 

the rocks in the map area (fig. 2) was after deposition 

of the basaltic lava flows of Middle Precambrian 

(Keweenawan) age. The faults of this set, and the Presque Isle fault in particular, appear to have been the 

principal structures responsible for the regional north-
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A35

ward tilting of Proterozoic rocks along the Gogebic 

Range. Presumably the Jacobsville Sandstone covers 

the Presque Isle fault and is younger.

NORTHEAST FAULT SET

Abundant faults that trend N. 50°-55° E. and cut and 

displace rocks as young as the lower Keweenawan lavas 

(fig. 2) are known and inferred in the MareniscoWatersmeet area. This fault set was first recognized 

(Sims and others, 1978) during interpretation of the 

aeromagnetic map of northern Wisconsin (Zietz and 

others, 1977). Some of the faults have been extended 

into the Marenisco-Watersmeet area, mainly on the 

basis of magnetic lineaments, apparent displacement of 

magnetic anomalies, and displacement of the lower 

Keweenawan lavas (Ysb unit, fig. 2). The pattern of 

northeast-trending faults shown on figure 2 is a modification of the fault pattern interpreted previously by 

Fritts (1969). The faults of this set offset the Keweenawan lavas, and one of them, herein named the Slate 

River fault, clearly offsets and repeats the Blair Creek 

Formation in the vicinity of Slate Creek, in sec. 22, 

T. 46 N., R. 42 W. A steep cataclastic foliation on the 

west bank of the river supports the interpretation of 

a fault. Judged from the displacement of the Blair 

Creek Formation at the Slate River locality, noted 

above, and of the gently north-dipping Keweenawan 

lavas east of Lake Gogebic (fig. 2), movement on the 

faults was mainly dip-slip, the southeast side having 

moved relatively upward. Similar northeast-trending 

faults that displace the lower Keweenawan lavas have 

been mapped west of the map area (fig. 2) and extend 

westward to the vicinity of Wakefield (Trent, 1973; 

Prinz and Hubbard, 1975). The fault set is interpreted 

as being late Keweenawan in age, although some faults 

of this trend may have been initated in Early Proterozoic time.

METAMORPHISM

A major metamorphism was approximately synchronous with the deformation (Penokean) that affected all 

the Early Proterozoic (formerly called Proterozoic X) 

and most of the Archean rocks in the area. This 

metamorphism was nodal in distribution and was centered on the gneiss dome at Watersmeet, which James 

(1955) called the Watersmeet metamorphic node. The 

metamorphic intensity decreased outward from the amphibolite facies (staurolite zone) to the chlorite zone of 

the greenschist facies. Except within the core of the 

gneiss dome, the metamorphism that was superposed 

on the previously metamorphosed Archean rocks was 

retrogressive.

Because mafic rocks are at least as abundant as argillaceous rocks in the area, the metamorphic facies classification is used here rather than the metamorphic isograds of James (1955). The approximate boundaries 

between amphibolite, epidote amphibolite, and greenschist facies are shown on figure 2. In addition, the 

greenschist facies is divided into biotite and chlorite 

zones. The boundaries shown on the map (fig. 2) are 

only approximately located because of the sparsity of 

outcrops in many critical areas.

The data from the area are consistent with the mineral assemblages listed by James (1955, table 2) for the 

metamorphic zones in northern Michigan. The amphibolite facies approximately equates with the staurolite 

zone, the epidote amphibolite facies with the upper part 

of the biotite zone and the lower part of the garnet 

zone, and the greenschist facies with the lower part 

of the biotite zone and the chlorite zone. Subgreenschist 

facies have not been recognized.

All the mafic rocks in the area, regardless of origin, 

have a similar metamorphic mineralogy. Within the 

chlorite zone of the greenschist facies, they contain the 

assemblage chlorite, pale-green hornblende (actinolitic 

hornblende), actinolite, epidote/clinozoisite, and albite; 

less abundant constituents are leucoxene, calcite, 

opaque oxides, and quartz. Except in areas of great 

deformation, primary textures are largely preserved. 

In the biotite zone of the greenschist facies, the appearance of a greenish-brown biotite distinguishes this 

metamorphic facies from the chlorite zone. Primary labradorite occurs in some of the mafic lavas of the Blair 

Creek Formation. In the epidote amphibolite facies, the 

mafic rocks have characteristic assemblages containing 

bluish-green hornblende, albite or sodic oligoclase, 

brown biotite, epidote, and sphene, as well as many 

minerals characteristic of the greenschist facies. In the 

amphibolite facies, the mafic rocks contain a bluishgreen or green hornblende and andesine or calcic oligoclase. Brown or reddish-brown biotite, epidote, and 

sphene are common minerals, as are opaque oxides and 

quartz; almandine garnet occurs in some bedded rocks 

and in Early Proterozoic metagabbro.

Late, retrograde alteration characterized by chloritization of biotite, sericitization of plagioclase, and the 

development of calcite is widespread and apparently related to late Penokean or younger fracturing.

ARCHEAN METAMORPHISM

All the Archean rocks except the Puritan Quartz 

Monzonite and the leucogranite in the Watersmeet 

dome were metamorphosed in Archean time. The tonalitic augen gneiss and overlying gneiss and schist 

were metamorphosed at least to amphibolite facies, apparently during several episodes of deformation.

The Late Archean volcanic-sedimentary rocks in the
A36 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

northwest part of the map area (fig. 2), within the 

greenstone-granite terrane, were metamorphosed concurrently with their deformation in Late Archean time. 

The metavolcanic rocks in the gneiss and amphibolite 

unit within the Archean block southeast of Marenisco 

were mainly metamorphosed to the amphibolite facies, 

as indicated by the occurrence of assemblages containing a green or greenish-brown hornblende, 

clinopyroxene (local), staurolite, almandine garnet, oligoclase or andesine, and brown or reddish-brown biotite. The associated graywacke to the east was metamorphosed at least to garnet grade, as indicated by 

the widespread presence of relict garnet in these rocks. 

A Late Archean age for this garnet is indicated by its 

breakdown to epidote and chlorite during the recrystallization of biotite associated with the S2 (Penokean) 

cleavage. The garnet is preserved where it was armored by quartz. The Late Archean volcanic and sedimentary rocks associated with the Puritan batholith, 

west of Lake Gogebic (fig. 2), also were metamorphosed to the amphibolite or epidote amphibolite 

facies during a Late Archean event. To the west, probable equivalent bedded rocks of the Ramsay Formation 

show an increase in metamorphic grade toward bodies 

of Puritan Quartz Monzonite and contain amphibolitefacies assemblages adjacent to these bodies (Schmidt, 

1976; Sims and others, 1977).

The Archean rocks were affected to different degrees 

by the younger, Penokean metamorphism. This metamorphism decreased in intensity away from the 

Watersmeet node and did not materially affect the Archean rocks associated with the Puritan batholith.

EARLY PROTEROZOIC (PENOKEAN) 

METAMORPHISM

The metamorphism during the Early Proterozoic followed deposition of the Early Proterozoic bedded rocks 

and emplacement of the metadiabase dikes and sills, 

and apparently took place during two closely spaced 

stages. This is indicated by the wide distribution of 

greenschist-facies metamorphsm, as noted earlier by 

James (1955), and by the more local, definitely nodal 

pattern superposed on the regional low-grade pattern. 

In this area, the regional greenschist-facies metamorphism apparently accompanied development of S2 

schistosity, for biotite and chlorite are alined on these 

surfaces. In the area north of the Watersmeet dome, 

garnet is seen to have formed after the S2 schistosity, 

and the schistosity, as well as the bedding, was deformed during the rise of the gneiss dome. Quite probably, the northeast-trending S2 foliation in the rocks 

northwest of the gneiss dome formed more or less contemporaneously with the regional schistosity in the 

area to the east, but we have not been able to discern

two distinct stages of Proterozoic metamorphism in the 

rocks of the northwestern area.

The metamorphism associated with the Watersmeet 

node is characterized by roughly concentric metamorphic zones (James, 1955) delineated by metamorphic changes in the Proterozoic bedded rocks and 

metadiabase and to a lesser degree by changes in the 

Archean rocks. Amphibolite-facies metamorphic rocks 

in the core of the Watersmeet dome are surrounded 

by successive envelopes of epidote amphibolite and 

greenschist facies. The width of the epidote amphibolite 

zone is about 15 km.

Amphibolite-facies metamorphism is confined to the 

core of the Watersmeet dome. Here, Proterozoic metagabbro and metadiabase in the form of dikes contain 

a green or bluish-green hornblende and andesine. A 

dike in the north-central part of sec. 5, T. 45 N., R. 

39 W., mapped by Fritts (1969) but not shown on figure 

4, contains garnet in addition to the above minerals. 

The Archean rocks contain mineral assemblages compatible with those in the Proterozoic mafic dikes. At 

least two episodes of recrystallization are indicated in 

most of these rocks by oriented biotite, hornblende, and 

quartz-feldspar aggregates. The minerals associated 

with the two foliations, one of which is Penokean, are 

similar. In the tonalitic gneiss, microcline is typically 

interstitial, confined to granulated and recrystallized 

zones, and has a poikilitic texture, containing inclusions 

of plagioclase, quartz, and biotite. It is probable that 

it was introduced or, alternatively, was developed by 

a metamorphic reaction during the recrystallization accompanying either an Archean event or the Penokean 

event. Garnet is common in the gneiss and biotite 

schist, and riebeckite occurs in porphyroblasts within 

a biotite-garnet schist interlayered with dominant amphibolitic rocks in the Brush Lake area. The associated 

amphibolite contains a green or bluish-green 

hornblende and oligoclase or andesine.

A broad zone of epidote-amphibolite metamorphism, 

reflected by mineral assemblages in bedded rocks of 

the Michigamme and Blair Creek Formations and the 

metadiabase in dikes and sills, surrounds the amphibolite-grade node. The argillaceous rocks of the 

Michigamme Formation have a well-developed mosaic 

texture and lack evidence of appreciable cataclasis. 

Aside from samples obtained near the margins of the 

Watersmeet dome, where the regional schistosity was 

deformed by diapiricrise of the dome, the rocks have 

a single schistosity. Near the dome, two schistosities 

are given by alinement of biotite. Typical assemblages 

are almandine garnet, brown biotite, albite or oligoclase, and quartz. At places, muscovite is intergrown 

with biotite, and chlorite, opaque oxides, clinozoisite, 

and rutile are present. In the same way as the
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A37

Michigamme Formation, the rocks of the Blair Creek 

Formation have simple mosaic textures and lack evidence of multiple metamorphism. Mafic volcanic rocks 

contain bluish-green hornblende, albite or oligoclase, 

epidote/clinozoisite, brown biotite, and minor quartz 

and opaque oxides. Ophitic textures are preserved in 

the porphyritic lavas. Iron-formation within the Blair 

Creek contains the assemblage quartz-magnetitebrown biotite-green hornblende-epidote (local). 

Proterozoic metadiabase dikes contain bluish-green 

hornblende, albite or oligoclase, quartz, brown biotite, 

epidote, sphene, and opaque oxides.

The Archean rocks within the zone of Penokean epidote-amphibolite metamorphism have retrograde 

metamorphic assemblages. Conspicuous retrograde 

metamorphism is indicated in Archean metagraywacke 

by the breakdown of garnet to chlorite+muscovite+lesser epidote and opaque oxides. The breakdown is most 

evident in streaks of coarse-grained brown biotite, 

which was developed along S2 (Penokean) cleavage surfaces. Garnet persisted through the metamorphism 

where it was sheathed by quartz. Grunerite is a local 

mineral that was formed in iron-rich rocks during the 

Penokean metamorphism. It should be noted that 

Fritts (1969) implied that the garnet in the graywacke 

(strata near Banner Lake unit of Fritts) was Penokean 

in age. Rocks within the Archean gneiss and amphibolite unit, east of Marenisco, were cataclastically deformed to different degrees and partly refoliated as a 

result of the Penokean deformation. These deformed 

zones characteristically contain muscovite, chlorite, and 

some epidote/clinozoisite as well as biotite. Also, primary plagioclase is slightly to moderately altered to 

sericite and commonly has albite rims. Calcite is a local 

alteration mineral. In one sample, staurolite is largely 

altered to chlorite as a result of the retrograde 

metamorphism.

An outer zone of greenschist metamorphism is delineated from mineral assemblages in the mafic lavas 

of the Blair Creek Formation, the metadiabase dikes 

(Proterozoic X), and the Copps Formation. Within the 

biotite zone of the greenschist facies, the lavas typically 

contain a pale-green hornblende, minor brown biotite, 

epidote, albitic plagioclase, and minor quartz. Primary 

labradorite locally is preserved. Within the chlorite 

zone of this facies, the lavas consist dominantly of actinolite, chlorite, and albite.

DISCUSSION

The metamorphic zonation associated with the 

Watersmeet node is similar to that in other metamorphic nodes in northern Michigan (James, 1955). James 

ascribed the nodal pattern to heat derived from subjacent bodies of magma, by means of which heat acquired

at a greater depth was transferred by mass movement 

to higher levels in the crust. The later recognition of 

the Watersmeet dome as a mantled gneiss dome and 

isotopic dating of the rocks within the core and the supracrustal cover led Sims and Peterman (1976) to propose that the metamorphism in the Watersmeet node 

was related to heat transfer during remobilization and 

rise of the Archean gneiss in the core of the dome. 

Metamorphic aureoles of comparable shape and intensity are characteristic of mantled gneiss domes, as for 

example, the Proterozoic Y Baltimore Gneiss domes in 

Maryland (Southwick, 1969).

With respect to temperature-pressure conditions during the metamorphism, the mineralogy of the highest 

grade rocks (amphibolite facies) indicates that they 

were formed under moderate temperatures and 

pressures.

GEOLOGIC HISTORY

The stratigraphic-tectonic evolution of the MareniscoWatersmeet area can be discussed in the context of the 

regional geologic framework, inasmuch as the area lies 

astride the boundary between the two Archean crustal 

segments that have been delineated in the Lake 

Superior region. The two crustal segments have had 

different histories, both prior to and subsequent to 

their being joined together into a single continental 

mass near the end of the Archean (table 13).

The area (fig. 1) spans the northern margin of the 

gneiss terrane and the deformed margin of the juxtaposed greenstone-granite terrane. This belt of tectonized Archean and Early Proterozoic rocks has been 

named the Great Lakes tectonic zone (Sims and others, 

1980). That part of the greenstone-granite terrane discussed in the left column of table 13 characterizes the 

area along and adjacent to the central or main part 

of the Gogebic Range, west of Wakefield, Mich.

The Late Archean greenstone-granite complexes in 

the area were formed as a part of the vast tectonicigneous episode that yielded much of the Archean crust 

now composing the core of the North American craton. 

The tectonic environment in which these rocks were 

formed is little understood, but from regional geologic 

relations it seems probable that they accumulated, at 

least in this area, adjacent to the older sialic crustal 

block, which 2,700 m.y. ago had an area of at least 

150,000 km2 (Sims and Peterman, 1981). Possibly, the 

Late Archean supracrustal metavolcanic rocks in the 

Watersmeet dome were deposited at about the same 

time on at least the northern fringe of the sialic gneissic 

crust. The environment may have been analogous to 

modern continental borderlands or island arcs. Following this volcanism, the volcanic rocks were intruded by
A38 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

TABLE 13. Precambrian stratigraphic-tectonic evolution, western Upper Michigan

Approx. 

age

Greenstone-granite terrane

Main body Tectonized zone 

(Great Lakes tectonic zone; width 25 km)

Gneiss terrane

1,100

1,750

1,900

2,100(?)

2,600

2,650

2,750

Uncertain

3,500

-ErosionContinental rifting across both basement terranes, with deposition of mafic volcanic rocks, emplacement of qabbroic layered bodies, and 

deposition of epiclastic sedimentary rocks.

Weak deformation and metamorphism.

Deposition of quartzite, carbonate rocks, 

iron-formation, graywacke and shale in 

shallow-water environment.

Crust is stable.

Uplift of Puritan batholith and associated 

Archean bedded rocks, with resultant 

doming of younger supracrustal rocks.

Development of northward-facing folds and a 

penetrative cleavage; brittle deformation 

of Archean greenstone-granite complexes. 

Greenschi st-amphi bolite metamorphi sm.

Deposition of quartzite, carbonate rocks, 

iron-formation, basaltic lavas and 

pyroclastics, graywacke and shale in 

an unstable environment.

Initiation of early Proterozoic structural 

basin over boundary between greenstone 

and gneiss terranes by riftinqf?).

-Penokean orogenyDiapiric rise of granitoid plutons accompanied by intense deformation and thermal 

metamorphism (steep gradients) of volcanogenic rocks.

-Intrusion of Puritan Quartz MonzoniteSubmarine eruption of tholeiitic basalt in orogenic ensimatic environment, 

followed by eruption of felsic-intermediate pyroclastics that formed 

graywacke.

Diapiric rise of gneiss, forming mantled aneiss 

domes, and attendant thermal nodal metamorphism 

of supracrustal and basement rocks; differential 

mobility of lithophile elements.

Development of regional steep folds and a 

penetrative cleavage; greenschist-facies 

metamorphism.

Deposition of volcanic rocks, graywacke and shale 

in an unstable environment.

Intrusion of anorogenic leucogranite dikes.

Ductile deformation and, probably, amphibolitefacies metamorphism.

Submarine eruption of basalt and pyroclastics.

Ductile deformation and amphibolite-facies 

metamorphism.

Submarine eruption of felsic and intermediatecomposition rocks in ensialic environment.

Development of tonal i tic gneiss; orotolith 

probably of igneous origin.

voluminous granitoid bodies, which rose diapirically upward into the pile, deforming the bedded rocks into 

tight, steep folds and metamorphosing them to lower 

amphibolite grade, in a manner similar to that described by Schwerdtner and others (1979). At about the 

same time in the gneiss terrane, sparse anorogenic 

granite in the form of dikes was emplaced into the older 

Archean gneisses, at least in the area now composing 

the Watersmeet dome. The lack of gravity-induced tectonics in the gneiss terrane at this time suggests that 

this segment of the crust was stronger and more stable 

than that beneath the greenstone belts to the north 

during Late Archean time.

In the Early Proterozoic (ca. 2,000 m.y. ago), the 

structural basin that now contains the sedimentary-volcanic sequence assigned to the Marquette Range Supergroup was formed over and along the zone of weakness 

between the two Archean crustal segments (Sims, 1976; 

Sims and others, 1981). Presumably, the basin was initiated by rift-faulting (Sims and Peterman, 1983), which

was concentrated along the boundary zone and produced local troughs within a broad platform (Larue and 

Sloss, 1980). The faulting was controlled to a substantial degree by Archean basement structures. The fault 

troughs and other irregularities in basin topography, 

together with partial erosion of some of the supracrustal rocks, can account for the lenticularity that 

exists in the stratigraphy of the lower part of the Marquette Range Supergroup in the area of this report. 

It is clear that the Archean antiformal blocks were topographic highs during part of the time of deposition 

of the sequence, for the lower part of the sequence is 

missing in most of this area, and conglomerates of local 

derivation occur locally at the base of the Blair Creek 

and Copps Formations. Also, it is clear that the Puritan 

batholith and associated Archean bedded rocks rose as 

a diapiric dome after deposition of the Copps, as recorded by the steep dips in the rocks adjacent to the 

Archean core. Evidently, tectonic reconstitution of this 

granitoid body took place under essentially nonmeta-
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A39

morphic conditions (Sims and Peterman, 1976), for internal cataclasis is minimal, there is no thermal aureole 

surrounding the body, and the whole-rock Rb-Sr system in the granitic rocks was not highly disturbed 

(Sims and others, 1977). Possibly this doming occurred 

approximately contemporaneously with the formation of 

the gneiss dome at Watersmeet, which occurred under 

much more intense temperature-pressure conditions, as 

a result of the impingement of the gneiss crustal block 

against the greenstone-granite block. The supracrustal 

strata adjacent to the gneiss dome were metamorphosed to staurolite grade, and the Rb-Sr and K-Ar 

whole-rock and mineral systems in the Archean rocks 

within the core were reset at the time of the doming, 

about 1,800-1,750 m.y. ago. The reactivation of the 

gneiss dome during the Penokean event was accompanied by differential mobility of potassium and other 

lithophile elements, as indicated by the petrology and 

chemistry of the Archean rocks within the core.

Most faulting in the Marenisco-Watersmeet area resulted from the midcontinent rifting during the 

Keweenawan, although the Banner Lake fault and 

others in the same fault set probably were initiated late 

in Archean time and were recurrently active thereafter. The Presque Isle fault and related east-northeasttrending faults appear to have resulted from extension 

in approximately a N. 20° W.-S. 20° E. direction. Apparently the faulting was part of a tectonic continuum 

that began in the Early Proterozoic and continued into 

Keweenawan time, for abundant Proterozoic X metadiabase dikes having the same trend occur within the 

zone of faulting, and the faults themselves cut the 

lower Keweenawan lavas (fig. 2). The Presque Isle 

fault not only separates Archean from Early Proterozoic rocks in the map area (fig. 2), but apparently also 

was a major structure related to the steep, northwest 

tilting of the Proterozoic rocks in the Gogebic Range 

area in Keweenawan time. The Keweenawan lavas east 

of the other major fault in this set, the Judson Falls 

fault, dip only 5°-15°N., indicating that the hinge line 

for the tilting was northwest of this fault, possibly coinciding with the Presque Isle fault.

The northeast-trending faults apparently are somewhat younger than the east-northeast set, but this conclusion is equivocal. These faults also cut and displace 

the lower Keweenawan lavas, and mainly had dip-slip 

movements.

OCCURRENCE OF FLUORITE AND

COPPER IN ROCKS WITHIN THE

WATERSMEET DOME

Fluorite and apatite are uncommonly abundant accessory minerals in the Archean supracrustal rocks in the 

core of the Watersmeet dome, and concentrations of

copper occur in a quartzite layer within a small outcrop 

of probable Bad River Dolomite along the northwest 

edge of the dome.

The principal occurrences of fluorite and apatite 

noted in the Archean metavolcanic rocks during studies 

of thin sections are shown on figure 4. At these 

localities, fluorite and apatite compose as much as 5 

percent by volume of the rocks. Fluorite also was noted 

as a common accessory mineral in a fine-grained 

metavolcanic rock, which is cut by leucogranite, in road 

cuts along Gogebic County Highway 206, north-central 

part of sec. 4, T. 45 N., R. 40 W. At this locality, 

galena and sphalerite locally occur as coatings on late 

fractures that cut the Archean rocks. The fluorite is 

colorless or purple and forms irregular, disseminated 

grains. It is most common in coarser grained, recrystallized zones seen in the thin sections. The apatite is associated mainly with biotite. Both minerals also are locally common in the leucogranite that cuts the 

metamorphosed volcanic rocks. The association of fluorite and apatite with both metavolcanic and granitic 

rocks possibly indicates a volatile-rich primary environment for these rocks. Alternatively, the fluorine in the 

fluorite and apatite could have been released from biotite in the host rocks during the recrystallization, in 

the presence of water, that accompanied the Penokean 

metamorphism.

The copper occurrence has been described by Cannon 

(1980). Visible chalcopyrite and malachite are present 

in two beds of vitreous quartzite between 0.25 and 0.5 

m thick that are interlayered with tremolite-bearing 

marble. The copper is clearly of the stratabound type. 

Because exposures are scarce in much of the region, 

the true extent of these rock units is not known; they 

could be more widespread than now thought and should 

be considered as a potential host for sedimentary copper deposits.

REFERENCES CITED

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

Geological Survey Publication 18, Geologic Series 15, p. 13-164.

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

the early Precambrian crust of northeastern Minnesota: 

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

Barker, Fred, and Peterman, Z. E., 1974, Bimodal tholeiitic-dacitic 

magmatism and the early Precambrian crust: Precambrian Research, v. 1, p. 1-12.

Cannon, W. F., 1980, Copper-bearing quartzite near Watersmeet, 

Michigan: U.S. Geological Survey Open-File Report 80-390, 6

PCannon, W. F., and Gair, J. E., 1970, A revision of stratigraphic 

nomenclature for middle Precambrian rocks in northern Michigan: Geological Society of America Bulletin, v. 81, p. 2843-2846.
A40 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

Collerson, K. D., and Bridgwater, D., 1979, Metamorphic development of early Archean tonalitic and trondhjemitic gneisses, Saglek area, Labrador, in Barker, F., ed., Trondhjemites, dacites, 

and related rocks: Amsterdam-Oxford-New York, Elsevier Scientific Publishing Company, p. 205-273.

Fritts, C. E., 1969, Bedrock geologic map of the MareniscoWatersmeet area, Gogebic and Ontonagon Counties, Michigan: 

U.S. Geological Survey Miscellaneous Geologic Investigations 

Map 1-576, scale 1:48,000.

Goldich, S. S., Hedge, C. E., and Stern, T. W., 1970, Age of the 

Morton and Montevideo Gneisses and related rocks, southwestern Minnesota: Geological Society of America Bulletin, v. 81, p. 

3671-3696.

Goldich, S. S., Hedge, C. E., Stern, T. W., Wooden, J. L., Bodkin, 

J. B., and North, R. M., 1980, Archean rocks of the Granite 

Falls area, southwestern Minnesota, in Morey, G. B., and Hanson, G. N., eds., Selected studies of Archean gneisses and lower 

Proterozoic rocks, southern Canadian Shield: Geological Society 

of America Special Paper 182, p. 19-43.

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

southwestern Minnesota, Part III, Geochronology, in Morey, G. 

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

Geological Society of America Special Paper 182, p. 77-94.

Hendrix, T. E., 1960, Structural history of the East Gogebic iron 

range: University of Wisconsin, Madison, Ph. D. thesis, 84 p.

Hubbard, H. A., 1975, Lower Keweenawan volcanic rocks of Michigan and Wisconsin: U.S. Geological Survey Journal of Research, 

v. 3, p. 529-541.

James, H. L., 1955, Zones of regional metamorphism in the Precambrian of northern Michigan: Geological Society of America Bulletin, v. 66, p. 1455- 1487.

Lame, D. K., and Sloss, L. L., 1980, Early Proterozoic sedimentary 

basins of the Lake Superior region Summary: Geological Society of America Bulletin, pt. 1, v. 91, p. 450-452.

McGregor, V. R., 1979, Archean gray gneisses and the origin of the 

continental crust Evidence from the Godthab region, west 

Greenland, in Barker, F., ed., Trondhjemites, dacites, and related rocks: Amsterdam-Oxford-New York, Elsevier Scientific 

Publishing Company, p. 169-204.

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.

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, Early 

Archean tonalitic gneiss from northern Michigan USA, in Morey, 

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

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

 1984, A protracted Archean history in the Watersmeet gneiss 

dome, northern Michigan in Peterman, Z. E., ed., Shorter contributions to isotope research: U.S. Geological Survey Professional Paper.

Pettijohn, F. J., Potter, P. E., and Siever, Raymond, 1973, Sand 

and sandstone: New York-Heidelberg-Berlin, Springer-Verlag, 

618 p.

Prinz, W. C., 1981, Geologic map of the Gogebic Range-Watersmeet 

area, Gogebic and Ontonagon Counties, Michigan: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1365, scale 

1:125,000.

Prinz, W. C., and Hubbard, H. A., 1975, Preliminary geologic map

of the Wakefield quadrangle, Gogebic County, Michigan: U.S. 

Geological Survey Open-File Report 75-119, scale 1:24,000.

Rogers, J. J. W., and Adams, J. A. S., 1969, Abundances in common 

igneous rocks, in Uranium, chap. 92 of Wedepohl, K. H., ed., 

Handbook of geochemistry: Berlin, Springer-Verlag, p. 92-E-l 

to 92-E-8.

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. (includes 1:48,000 scale map).

 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.

Shapiro, Leonard, 1967, Rapid analysis of rocks and minerals by a 

single-solution method, in Geological Survey research 1967: U.S. 

Geological Survey Professional Paper 575-B, p. B187-B191.

Sims, P. K., 1976, Precambrian tectonics and mineral deposits, Lake 

Superior region: Economic Geology, v. 71, p. 1092-1118.

 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.

Sims, P. K., Cannon, W. F., and Mudrey, M. G., Jr., 1978, Preliminary geologic map of Precambrian rocks in part of northern Wisconsin: U.S. Geological Survey Open-File Report 78-318, scale 

1:250,000.

Sims, P. K., Card, K. D., and Lumbers, S. B., 1981, Evolution of 

early Proterozoic basins of the Great Lakes region, in Campbell, 

F. H. A., ed., Proterozoic basins of Canada: Geological Survey 

of Canada Paper 81-10, p. 379-^397.

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, pt. 

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

Sims, P. K., and Peterman, Z. E., 1976, Geology and Rb-Sr ages 

of reactivated Precambrian gneiss and granite in the MareniscoWatersmeet area, northern Michigan: U.S. Geological Survey 

Journal of Research, v. 4., p. 405-414.

 -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 No. 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 

of America Memoir 160, p. 3-14.

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.

Southwick, D. L., 1969, Crystalline rocks of Harford County, in The 

geology of Harford County, Maryland: Baltimore, Md., Maryland 

Geological Survey, p. 1-76.

Steiger, R. H., and Jager, E., 1977, Subcommission on Geochronology Convention on the use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, p. 

359^362.
GEOLOGY, GEOCHEMISTRY, AND AGE OF ROCKS, MARENISCO-WATERSMEET AREA A41

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

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

Trent, V. A., 1973, Geologic map of the Marenisco and Wakefield 

NE quadrangles, Gogebic County, Michigan: U.S. Geological 

Survey open-file report, scale 1:20,000.

U.S. Geological Survey, 1972, Aeromagnetic map of northwestern

Ontonagon County, Michigan: U.S. Geological Survey Geophysical Investigations Map GP-834, scale 1:62,500.

Van Hise, C. R., and Leith, C. K., 1911, The geology of the Lake 

Superior region: U.S. Geological Survey Monograph 52, 641 p.

Zietz, Isidore, Karl, J. H., and Ostrom, M. E., 1977, Preliminary 

aeromagnetic map covering most of the exposed Precambrian terrane in Wisconsin: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-888, scale 1:250,000.

Geologic Interpretation 

of Gravity Data, 

Marenisco-Watersmeet Area, 

Northern Michigan

By J. S. KLASNER and P. K. SIMS

CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1292-B

A study of crustal structure 

in the boundary zone between 

two Archean basement terranes

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1984

CONTENTS

Page

Abstract ............................................ Bl

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

Geology ............................................. 1

Gravity map .......................................... 6

Geologic interpretation of gravity anomalies ........................ 6

Gravity model ......................................... 9

Relation of tectonic zone to regional gravity ........................ 11

Tectonic environment ..................................... 12

References cited ........................................ 12

ILLUSTRATIONS

FIGURE 1. Map of Lake Superior region showing distribution of Archean basement terranes .................... B2

2. Generalized geologic map of the Marenisco-Watersmeet area, northern Michigan .................... 4

3. Simple Bouguer gravity anomaly map of the Marenisco, Thayer, and Watersmeet 15-minute quadrangles ...... 7

4. Two-dimensional gravity model and interpretive geologic section along a composite gravity profile .......... 8

5. Regional gravity profile, Lake Superior region, showing relation to Archean basement terranes ........... 12

TABLE

TABLE 1. Measured densities for selected rock units of the Marenisco-Watersmeet area, northern Michigan ........... BIO

Bin

CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

GEOLOGIC INTERPRETATION OF GRAVITY DATA, 

MARENISCO-WATERSMEET AREA, NORTHERN MICHIGAN

By J. S. KLASNER and P. K. SIMS

ABSTRACT

Gravity measurements have been made in the western part of 

northern Michigan to aid in determining the crustal structure of the 

tectonic zone marking the boundary between the two Archean crustal 

segments recognized in the Lake Superior region: a greenstone-granite terrane on the north and a gneiss terrane on the south. The gravity anomalies generally correspond closely with mapped geologic 

units. Pronounced lows overlie the Puritan batholith, composed 

mainly of Late Archean granitic rocks, and a gneiss dome at 

Watersmeet, which has a core of Archean rocks. Most positive 

anomalies are associated with synclinal basins of Early Proterozoic 

rocks, and mainly reflect dense volcanic rocks and local iron-formation 

in units of the Marquette Range Supergroup. A north-sloping gravity 

gradient of about 5 milligals per kilometer coincides with the position 

of the boundary zone between the two Archean basement terranes 

and reflects density increases from north to south in rocks of both 

the upper and lower crusts.

The gravity data support the earlier interpretation that, during 

the Early Proterozoic, the boundary zone between the two Archean 

crustal segments was an intracratonic feature which localized the deposition of thick sedimentary and volcanic rocks now assigned to the 

Marquette Range Supergroup.

INTRODUCTION

A major crustal feature in the Lake Superior region 

is the boundary between the terrane of Late Archean 

greenstone-granite complexes, which compose the 

southern part of the Superior province, and an older 

Archean gneiss terrane to the south of this province 

(fig. 1). These two basement terranes have been delineated on both the west (Morey and Sims, 1976) and 

east (Sims, 1980) sides of the midcontinent rift system. 

Because of the great length (> 1,200 km) of the boundary zone and the distinctive tectonic deformation of 

both the Archean and Early Proterozoic rocks within 

it, the feature has been named the Great Lakes tectonic 

zone (Sims and others, 1980).

Rocks within the boundary zone are rather well exposed in the Marenisco-Watersmeet area in northern 

Michigan, and accordingly this area has been studied 

in moderate detail to determine their stratigraphy,

structure, and geologic history. Details of the general 

geology are described by Sims and others (this volume). 

Earlier reports presented data on the radiometric ages 

of the rocks (Sims and others, 1977; Peterman and 

others, 1980) and on the tectonic evolution of Archean 

rocks in the cores of gneiss domes in the area that were 

rejuvenated in the Early Proterozoic (Sims and Peterman, 1976).

To help understand the crustal structure, a gravity 

survey of the area was made in 1978, and detailed gravity profiles were measured along three traverses across 

the Archean boundary zone. The gravity study was 

part of a broader survey of the Iron River I°x2° quadrangle (Klasner and Jones, 1979).

The main purposes of this paper are to relate observed gravity anomalies to the geology and to interpret the structure of the upper crust in the boundary 

zone from combined geological and gravity data. We 

also briefly discuss the significance of the regional gravity with respect to the two Archean crustal segments.

Klasner is responsible for the gravity data obtained 

in the region, and Sims is responsible for the geology. 

Both of us were involved in interpreting the gravity 

data. S. A. Jankowski assisted in the gravity modeling 

while she was a student at Western Illinois University. 

A. E. Grosz and P. J. Gerasi assisted in making gravity 

observations along the detailed profiles, and E. L. 

Coward, Christopher French, S. A. Jankowski, and W. 

A. O'Niell assisted in gathering the regional gravity 

data.

GEOLOGY

The Marenisco-Watersmeet area lies astride the 

boundary between the two Archean crustal segments 

that have been delineated in the Lake Superior region. 

In the northwestern part of the area (fig. 2), 

greenstone-granite complexes of Late Archean age 

( 2,700 m.y. old; Sims and others, 1977; Peterman and 

others, 1980) compose the basement; and in the southeastern part, the basement consists of tonalitic augen

Bl
B2 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

96° 92°

GNEISS TERRANE 

MICHIGAN

FIGURE 1. Distribution of Archean basement terranes in the Lake Superior region. Younger rocks omitted, except 

for the lava and gabbro of the midcontinent rift system (Proterozoic Y), shown by ruled pattern.

gneiss of Early Archean age (3,560 m.y. old) and supracrustal gneiss and amphibolite that were intruded by 

2,590-m.y.-old granite. The Archean basement rocks of 

both terranes are overlain in part by infolded and 

faulted sedimentary and volcanic rocks of the Early 

Proterozoic Marquette Range Supergroup (Sims and 

others, this volume; Schmidt, 1980). Except for the 

youngest units of this supergroup, the equivalent 

Copps and Michigamme Formations (western and eastern parts of the area, respectively), the bedded 

Proterozoic rocks and older basement rocks are intruded by abundant Early Proterozoic metadiabase 

dikes. In addition, all these rocks are intruded by Middle Proterozoic diabase dikes, most of which are 

oriented subparallel to the buried boundary between 

the two basement terranes. Mafic lavas and younger

sandstone of Middle Proterozoic (Keweenawan) age, deposited in the midcontinent rift system, overlap the 

north side of the older Proterozoic and Archean rocks. 

The Archean and Early Proterozoic rocks are described 

by Sims and others (this volume), and accordingly are 

discussed only briefly here.

The Late Archean basement rocks in the northwest 

part of the map area consist of metabasalt and metagraywacke and equivalent amphibolite and biotite 

schist (Abs, fig. 2) that are intruded by granitoid rocks 

of the Puritan Quartz Monzonite (Ap, fig. 2). The granitic rocks compose the eastern part of the Puritan 

batholith, which is an elongate body as much as 20 km 

wide that extends southwest for a distance of more 

than 100 km, to the vicinity of Mellen, Wis. Small 

granitic bodies, not shown on figure 2, which are be-
GEOLOGIC INTERPRETATION OF GRAVITY DATA, MARENISCO-WATERSMEET AREA B3

lieved to be connected at depth with the granitic rocks 

of the Puritan batholith, intrude the gneiss and amphibolite in the northeast-trending belt east of 

Marenisco. The metamorphosed basaltic lavas associated with the Puritan batholith are correlative with 

the Ramsay Formation (Schmidt, 1976) as delineated 

in areas to the west in northern Michigan. Metagraywacke and equivalent biotite schist are moderately 

abundant within the map area (fig. 2) but are sparse 

to the west. The granitoid rocks in the Puritan 

batholith are dominantly granite, as defined by Streckeisen (1976).

Archean basement rocks that are characteristic of the 

gneiss terrane are exposed in the core of the 

Watersmeet dome, along its north margin. The oldest 

unit, a tonalitic augen gneiss, crops out within an area 

of about a square kilometer along and near Michigan 

Highway 45 (sees. 4 and 5, T. 45 N., R. 39 W.). It 

is overlain unconformably by bedded biotite gneiss, biotite schist, and amphibolite that dip moderately to the 

north. These bedded rocks and the tonalitic gneiss are 

both intruded by dikes of biotite leucogranite that has 

a U-Pb zircon age of 2,590 m.y. (Peterman and others, 

1980).

The bedded rocks of the Proterozoic Marquette 

Range Supergroup mainly consist of a basal metavolcanic unit that increases in thickness eastward the 

Blair Creek Formation (Sims and others, this volume) and a conformably(?) overlying unit of metagraywacke and meta-argillite, assigned to the Copps and 

Michigamme Formations. Older rocks of the Chocolay 

Group occur locally in the Gogebic Range, in a largely 

buried body in the southwest part of the map area, 

and at one locality along the north margin of the 

Watersmeet dome, about a kilometer east of the north 

end of Beatons Lake (fig. 2).

The structure of the rocks is complex. The Early 

Proterozoic rocks were folded during the Early 

Proterozoic Penokean orogeny. Also, within the area, 

an accompanying Penokean penetrative foliation was 

superposed on the previously deformed Archean basement rocks.

Within the gneiss terrane, at least two and probably 

three Archean deformations have been delineated in 

rocks in the Watersmeet dome. The oldest recognized 

deformation, about 3,500 m.y. ago, produced a steep 

foliation in the tonalitic augen gneiss and probably was 

accompanied by amphibolite-facies metamorphism. A 

second deformation followed deposition of the supracrustal biotite gneisses and, probably, a third, younger 

deformation followed deposition of a Late Archean succession of interlayered amphibolite and biotite gneiss. 

The age of the second deformation has not been determined, but it occurred during the interval between

about 3,500 m.y. ago and deposition of the younger amphibolite and biotite gneiss, which has been dated by 

the U-Pb zircon method at about 2,640 m.y. The probable third deformation took place before emplacement 

of the 2,590-m.y.-old biotite leucogranite. The two 

younger deformations were also accompanied by amphibolite-facies metamorphism.

The rocks in the greenstone terrane, as reflected by 

structures in the Archean rocks in the northwest part 

of the map area, were deformed during a Late Archean 

(~2,700-m.y.-old) tectonic event. This event produced 

folds in the bedded rocks with steep axial surfaces and 

was accompanied by upper greenschist- to lower amphibolite-facies metamorphism. Presumably, the folding 

was accomplished by diapiric rise of the Late Archean 

granitoid rocks, a mechanism generally favored to account for deformation of the Archean bedded rocks 

within the Superior province. (See Schwerdtner and 

others, 1979.) Also at this time, the Archean paragneiss 

and amphibolite unit and metagraywacke unit (Aga and 

Agw, fig. 2) in the belt east of Marenisco were deformed. These rocks were folded on steep northeasttrending axial surfaces, which are subparallel to the 

boundary between the two Archean terranes. Presumably, this deformation resulted from impingement of the 

gneiss terrane against the greenstone terrane; it preceded emplacement of the Late Archean batholithic 

rocks, for the Puritan Quartz Monzonite in the belt of 

gneiss and amphibolite east of Marenisco was not deformed at this time.

The Early Proterozoic deformation affected all the 

Early Proterozoic and Archean rocks in the area and 

apparently involved two separate episodes of tectonism 

assigned to the Penokean orogeny (Cannon, 1973). The 

major and most widespread tectonic episode involved 

folding of the Early Proterozoic bedded rocks on axial 

surfaces overturned to the northwest and, in the area 

north of the Watersmeet dome, to the north. The folding was accompanied by development of a steep, southward-dipping, axial-planar foliation in the Early Proterozoic rocks and a similar foliation in the Archean 

rocks that was superposed on older Late Archean structures. The folding and the development of cleavage decrease in intensity northwestward from the center of 

the map area (fig. 2) but persist to the northernmost 

outcrops of the Copps Formation at the east end of 

the Gogebic Range. Probably this deformation took 

place under upper greenschist metamorphic conditions. 

Later, during reactivation of the Archean rocks in the 

Watersmeet dome, the overlying Early Proterozoic 

bedded rocks were deformed, producing a rim syncline 

in the supracrustal rocks about 1.5-2 km outside of the 

core-mantle boundary. During this uplift, the Archean 

supracrustal rocks overlying the tonalitic gneiss in the
B4 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION
CORRELATION OF MAP UNITS

Xam

^'''': $! f:- Ar;

., r '* * 

MIDDLE 

PROTEROZOIC

EARLY 

PROTEROZOIC

ARCHEAN

LIST OF MAP UNITS

MIDDLE PROTEROZOIC

Jacobsville Sandstone

Siemens Creek Formation and Bessemer 

Quartzite Basaltic lava flows and local basal quartzite 

EARLY PROTEROZOIC

Copps Formation

Michigamme Formation Blair Creek Formation 

Unexposed magnetic unitChocolay and Menominee Groups undivided Sedimentary and volcanic rocks

ARCHEAN

Puritan Quartz Monzonite

Biotite schist, metabasalt, and migmatite

j 

ARCHEAN-Continued

Paragneiss and amphibolite

Unexposed magnetic rocks

Metavolcanic, metasedimentary, and 

granitoid rocks, undivided

Biotite gneiss, amphibolite, biotite 

schist, granite, and older undivided tonalitic gneiss

CONTACT Approximately located 

FAULT Bar and ball on downthrown side 

SYNCLINE 

OVERTURNED SYNCLINE

GENERALIZED STRIKE AND DIP 

Bedding

Overturned bedding Foliation

O 

H 

Or1 

o o

HH

O

FIGURE 2. Generalized geology of the Marenisco-Watersmeet area, northern Michigan.

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zo^

oo

CO 

O

i

bd
B6 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

core of the Watersmeet dome were deformed, mainly 

by recumbent folding and development of a northwarddipping axial-planar foliation. During this tectonism, all 

the Archean rocks in the dome were cataclastically deformed and recrystallized, and both the Archean rocks 

and the supracrustal mantling bedded rocks were subjected to amphibolite-grade metamorphism. Radiometric studies indicate that this event took place about 

1,800 m.y. ago (Peterman and others, 1980).

Three major fault sets have been recognized in the 

area: a northwest-trending set, which probably was initiated in the Archean and later rejuvenated, an eastnortheast-trending set, and a northeast-trending set 

(Sims and others, this volume, fig. 19). The main northwest-trending fault is the Banner Lake fault. A major 

related fault, the Barb Lake fault, has an apparent 

right-lateral displacement of about 6.5 km, as measured 

by lateral offset of the Blair Creek Formation (fig. 2). 

Eastward, the Barb Lake fault appears to truncate the 

northern margin of the Watersmeet dome; its vertical 

displacement is not known.

The east-northeast-trending fault set had an important effect on the distribution of the Keweenawan or 

Middle Proterozoic basaltic lavas and the structure of 

rocks in the Gogebic Range. The northernmost known 

fault in this set, the Presque Isle fault, separates 

steeply north-dipping rocks of the east Gogebic Range 

and the overlying Keweenawan lavas from Archean 

rocks on the south. The fault appears to have been 

mainly responsible for the tilting, which occurred after 

extrusion of the Keweenawan lavas and before deposition of the Middle Proterozoic Jacobsville Sandstone.

The north-northeast-trending fault set also is late 

Keweenawan in age; it is inferred to be younger than 

the east-northeast set and, unlike the east-northeast 

set, had little effect on the distribution of rock units, 

for it mainly displaced the Keweenawan lavas, probably 

by dominantly dip-slip movements. One fault in this 

set, however the Slate River fault repeated the 

Blair Creek Formation in the vicinity of U.S. Highway 

2, south of Lake Gogebic (fig. 2).

A minor set of faults in the eastern part of the map 

area trends northwest and appears to displace the 

Keweenawan lavas. Probably the faults are late 

Keweenawan in age.

GRAVITY MAP

The Bouguer gravity anomaly map prepared for this 

study (fig. 3) was compiled from approximately 310 

gravity stations. Data for about 180 of these stations 

were obtained during an earlier regional gravity survey 

of the Upper Peninsula of Michigan (Bacon, 1957); the

remainder were obtained recently by the U.S. Geological Survey to fill gaps in the coverage and to construct 

profiles across the geologic structure of this area. Stations are spaced about 3 km apart on the average, but 

are as much as 6.5 km apart in some areas of poor 

accessibility. Most of the new regional gravity data 

were obtained as part of a regional gravity survey of 

the Iron River I°x2° quadrangle (Klasner and Jones, 

1979). About 85 additional gravity readings were taken 

at 300-m intervals along roads. The stations were located by chaining, and altitudes were determined by 

multiple-loop altimeter surveys from U.S. Geological 

Survey bench marks. The altitude readings have an estimated accuracy of 5 feet or better. The gravity data 

were tied to or recalculated to conform to the 1971 International Gravity Standardization Net (Morelli, 1974). 

Data were reduced to sea level datum using the 1967 

Geodetic Reference System (International Association 

of Geodesy, 1971) and using a density of 2.67 g/cm3 

for rock between the ground surface and datum. Theestimated accuracy of the gravity data is ±0.3 milligal 

according to a code developed by Robbins and others 

(1974) for land-based gravity data in California.

GEOLOGIC INTERPRETATION OF 

GRAVITY ANOMALIES

Comparison of the Bouguer gravity anomaly map 

with the geologic map (fig. 2) shows a close correspondence of gravity anomalies and surface geology. A gravity low deeper than -50 milligals at A (fig. 3) lies above 

a basin in which the low-density Jacobsville Sandstone 

is probably thicker than that in other areas within figure 3. In contrast, moderately high gravity values at 

B (fig. 3) are associated with dense lower Keweenawan 

lavas in the northwest corner of the area where, according to a gravity model by Klasner and Jones (1979), 

the Jacobsville Sandstone is thin and the underlying 

lavas have moderately steep northward dips. A gravity 

plateau (-30 milligals) at C overlies exposed lower 

Keweenawan lavas; elsewhere in the northeast part of 

the map area these lavas may be too thin and discontinuous to provide positive gravity anomalies, or 

perhaps the gravity stations are too widely spaced to 

recognize them. A pronounced elongate gravity low of 

nearly -50 milligals (D, fig. 3) coincides closely with 

the Puritan batholith, which is composed mainly of 

granite; the width and amplitude of this anomaly 

suggest that the batholith is deep-rooted, a characteristic of many of the larger linear granitic plutons in the 

Archean greenstone terrane of Minnesota (Sims, 1972). 

A lesser gravity low (E) overlies Archean granitic 

gneisses and associated pegmatite, which is cogenetic 

with the Puritan batholith. Another gravity low (F)
89° 00'

46° 15

0 10 KILOMETERS

A

EXPLANATION

Gravity contourGravity station Detailed gravity profile

Composite gravity profile represented in figure 4

 ~" "" ~" Boundary of gravity anomaly

B Label for gravity anomaly discussed in text

FIGURE 3. Simple Bouguer gravity anomaly map of the Marenisco, Thayer, and Watersmeet 15-minute quadrangles, Michigan.

Contour interval 2 milligals.

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2! 

O *}

§

H 

2!

hH

03 O

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B8 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

NORTH

A\-

SOUTH

HC

10

5

0

c/j -5

<

tD

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z

-.-,.

<

-20

£-26

13 -30

-35

-40

-45

> 3000

H 

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K | 2500 Z ^

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ID 

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B\- \B

RESIDUAL GRAVITY

BOUGUER GRAVITY

AEROMAGNETIC DATA

COMPOSITE GRAVITY MODEL 

Density (g/cm3) shown in parentheses
GEOLOGIC INTERPRETATION OF GRAVITY DATA, MARENISCO-WATERSMEET AREA B9

coincides with the Watersmeet gneiss dome, in the 

southestern part of the map area.

Most positive gravity anomalies in the area underlain 

by pre-Keweenawan rocks are associated with exposed 

Early Proterozoic rocks of the Marquette Range Supergroup. The broad gravity high at the west end of the 

Watersmeet dome (Gi, fig. 3) corresponds in part with 

a sharp, positive magnetic anomaly that encircles the 

western and southern margins of the dome (U.S. 

Geological Survey, 1972a). This unit is mapped on figure 2 as an unexposed magnetic unit; a drill hole on 

the anomaly south of Watersmeet penetrated biotite 

schist (C. E. Button, oral commun., 1976). The extension of anomaly G! to the northeast (designated G2 , fig. 

3) overlies the Michigamme Formation, but undoubtedly reflects underlying denser Proterozoic rocks 

probably the Blair Creek Formation, which has a density of about 0.25 g/cm3 greater than the Michigamme. 

Dense dolomite of Early Proterozoic age (Xcm, fig. 2), 

which is exposed at one place along the county highway 

about a kilometer east of Beatons Lake and is known 

to contain some sulfide minerals, could also be associated with this positive anomaly. The gravity high 

extends both to the east and west (G3) but is more

EXPLANATION FOR COMPOSITE GEOLOGIC MODEL

MIDDLE PROTEROZOIC

Siemens Creek Formation and Bessemer Quartzite 

Basaltic lava flows and local basal quartzite 

EARLY PROTEROZOIC 

Copps Formation

Michigamme Formation 

Blair Creek Formation

ARCHEAN

Puritan Quartz Monzonite

Biotite schist, metabasalt, and migmatite

Metagraywacke

Paragneiss and amphibolite

Tonalitic gneiss and younger amphibolite, biotite 

gneiss, biotite schist, and granite, undivided

CONTACT

FAULT

5 KILOMETERS

i i i i

FIGURE 4. Two-dimensional gravity model and interpretive geologic 

section along a composite gravity profile constructed from profiles A-A', B-B', and C-C'. (See fig. 3.) Note the projections 

onto the line shown on figure 3. The model has the same vertical 

and horizontal scales.

subdued in these areas, probably because of deeper 

burial of the relatively dense rocks or because of thinning of these rocks. The positive gravity anomaly designated as G4 coincides with volcanic rocks and lean ironformation of the Blair Creek Formation (Xb, fig. 2) 

along the northwest and north margins of outcrops of 

the Michigamme Formation. A slight gravity high (H, 

fig. 3) occurs above the volcanic rocks of the Blair 

Creek Formation along the southwest, overturned limb 

of the Copps syncline. This anomaly is widest along 

U.S. Highway 2, about 12 km east of Marenisco, where 

the Blair Creek is repeated by a north-northeast-trending fault. The amplitude of the anomaly suggests that 

dense Archean amphibolite, which underlies the Blair 

Creek at this locality, contributes to the positive anomaly.

GRAVITY MODEL

A two-dimensional gravity model (fig. 4) was constructed from a composite gravity profile to aid in determining shallow crustal structure across the Great 

Lakes tectonic zone. The composite gravity profile was 

constructed by projecting segments A-A', B-B', and 

C-C' roughly along gravitational strike onto the line 

shown on figure 3. The three segments were positioned 

to take advantage of detailed gravity measurements 

along roads and are located in areas where the surface 

geology is moderately well known. The principal uncertainties in knowledge of the geology occur in segments 

B-B' and C-C', for the structure in the basement rocks 

and the exact nature of the discontinuity between the 

Archean gneiss in the Watersmeet dome and the rocks 

in the Archean greenstone block to the northwest are 

not known. The geology of this area is further complicated by the fault along the north margin of the 

Watersmeet dome.

Regional gravity on figure 4 was estimated by a 

graphical cross-profile interpretation of the Bouguer 

gravity map. The residual, obtained by subtracting regional from Bouguer gravity values, has an abrupt increase of about 20 milligals from north to south, with 

the lowest values (D, fig. 3) being above the Puritan 

batholith and the highest being above exposed Blair 

Creek Formation (G4, fig. 3). As mentioned above, a 

lesser gravity high (H) also overlies Early Proterozoic 

rocks in the south, overturned limb of the Copps 

syncline.

Aeromagnetic data (U.S. Geological Survey, 1972a, 

b) are plotted on the gravity profile and were useful 

in constructing the gravity model. Sharp magnetic 

highs correspond with gravity highs over some parts 

of areas underlain by Proterozoic metavolcanic and 

metasedimentary rocks, apparently indicating the positions of iron-formation. Magnetic lows occur above the
BIO CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

Puritan batholith and the Watersmeet dome. Smallamplitude highs within the magnetic low over the dome 

probably mark the position of Late Archean metavolcanic and metasedimentary supracrustal rocks that 

overlie the older tonalite gneiss in the core of the dome 

(Sims and others, this volume).

Density measurements were made on rocks that crop 

out along or near the position of the composite gravity 

profile (table 1). These 26 determinations and some additional values taken from the literature for certain 

rock types provide a reasonably accurate estimate of 

the range in the densities of the rock units at the surface. Densities used in the model for units 4 and 7 correspond closely with the median density of these units 

(table 1).

Unit 1 Although the north end of the gravity profile 

represents an area underlain by Jacobsville Sandstone, 

as shown on the geologic map (fig. 2), earlier gravity 

studies in this area (Klasner and Jones, 1979) indicated 

that the Jacobsville is thin and is underlain by 

Keweenawan lavas at shallow depths. For this reason, 

the Jacobsville Sandstone was not included in the 

model.

Unit 2 Four samples of the Copps and Michigamme 

Formations have a median density of 2.72 g/cm3. However, measurement of many more samples of the 

Michigamme Formation from the western Marquette 

district, about 100 km to the east, indicate that a density of 2.70 g/cm3 is a more appropriate figure (Cannon 

and Klasner, 1976), and this figure is used in the model.

Unit 3 Although the Blair Creek Formation consists 

mainly of mafic volcanic rocks having densities in the 

range 2.92 to 2.99 g/cm3 , the formation also contains 

lenses of other rock types, such as iron-formation,

metaconglomerate, metagraywacke, and felsic volcanic 

rocks, whose distributions are poorly known, especially 

at depth. A density of 2.95 g/cm3 was used in the 

model.

Unit 5 Geologic mapping shows that this unit is 

composed of biotitic schist and gneiss, migmatite, and 

lesser amphibolite, but the proportions of these rock 

types in the segment along the composite gravity profile are uncertain. The 2.83 g/cm3 density used in the 

calculations is based on an estimate that the rock contains 85 percent tonalite or equivalent rocks having a 

density of 2.8 g/cm3 and 15 percent amphibolite having 

a density of 2.98 g/cm3.

Unit 6 The Archean paragneiss and amphibolite unit 

is a mixed unit containing felsic gneisses, amphibolite, 

and a few small bodies of granite; accordingly, it is difficult to accurately estimate its density. The 2.83 g/cm3 

density (table 1) is based on an estimate that the unit 

contains 40 percent amphibolite having a density of 3.00 

g/cm3 and 60 percent intermediate volcanic rocks with 

a density of 2.72 g/cm3.

Unit 8 The rocks in the core of the Watersmeet 

dome are poorly exposed, but judged from geologic 

mapping (Sims and others, this volume) and aeromagnetic data they consist of Early Archean tonalitic gneiss 

(density approximately 2.70 g/cm3) overlain by supracrustal biotite gneiss, biotite schist, and amphibolite 

that are intruded locally by Late Archean granite. The 

selected density of 2.75 in the model is an estimate that 

falls well within the range of measured densities in 

table 1.

An additional uncertainty, perhaps of greater significance to gravity modeling than uncertainties in the 

mean density of separate rock units, is the distribution

TABLE 1. Measured densities for selected rock units of the Marenisco-Watersmeet area, northern Michigan

Unit

No.

1 

2 

3

4

5 

6 

7 

8

Unit name and (or) rock type

Copps and Michigamme Formations: metagraywacke 

Blair Creek Formation; basaltic lava, graywacke,

Puritan Quartz Monzonite in batholith; granite

Tonalitic gneiss, amphibolite, and sparse granite

M.G C3.^1TcLyWcLCK.6 eUTlCl _I_G S S GIT t U.I t

Tonalite gneiss, biotite gneiss and schist,

Map 

symbol 

(fig. 2)

Ysb

Xc, Xm

Xb

Ap 

Abs 

Aga

Agw

Agn

Number of 

measurements

C 1 )

4 

4

3 

( 2 ) 

8 

4

3

Density 

range 

(g/cm3 )

2 CC "^ 1

2.71-2.74 

2.92-2.99 

2.60-2.69

( 2 ) 

2.60-3.07

2.70-2.81

Median 

density 

(g/cm3 )

2.98 

2.72

2.95

2.63 

2.83 

2.83 

2.70

2.76

iprom Klasner and Jones (1979); number of measurements not reported.

2 From Daly and others (1966); number of measurements and density range not reported.
GEOLOGIC INTERPRETATION OF GRAVITY DATA, MARENISCO-WATERSMEET AREA Bll

of the rock units in the subsurface. In contrast to the 

stratiform nature of the Proterozoic bedded rocks, the 

Archean rocks are composite bodies composed of lenticular rock units having a complex internal structure. 

In the same way, the nature and attitude of the boundary between rocks of the Archean greenstone terrane 

and those of the Archean gneiss terrane, represented 

at the surface by the Watersmeet dome (fig. 2), is uncertain. In this area, the boundary is further complicated by the presence of high-angle faults. Thus, gravity modeling of the boundary zone is subjective, and 

the model presented here represents just one of several 

possible models.

Despite the uncertainties imposed by limitations in 

our knowledge of mean densities of rock bodies and of 

the detailed subsurface structure, the gravity data provide useful information about the gross stucture of the 

upper crust in the vicinity of the Archean basement 

boundary, and they support earlier contentions that 

this juncture is an important crustal feature in the 

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

1976; Sims and others, 1980).

First, the gravity model suggests that the density 

of the crust in the gneiss terrane is higher than that 

of the greenstone-granite terrane, on the north. A density difference of approximately 0.1 g/cm3 is necessary 

to explain the 20-milligal increase in gravity from the 

strongly negative anomaly (D, fig. 3) over the Puritan 

batholith to the positive anomaly (G2) over Proterozoic 

rocks on the north flank of the Watersmeet dome. This 

model involves only the upper 6 km of the crust. There 

is an additional north-sloping regional gravity gradient 

that also spans the boundary zone; this regional gradient has been interpreted by Klasner and Bomke 

(1977) as resulting from density variations in the deeper 

crust.

Secondly, the gravity data suggest that the Proterozoic structural basins in the area are somewhat deeper 

over Archean gneiss basement rocks than over Archean 

greenstone-granite complexes. As shown in the 

geologic model (fig. 4), the Copps syncline is interpreted as being about 2 km deep, whereas the 

Michigamme basin (G2, fig. 3) is at least half a kilometer deeper. The Michigamme basin widens eastward, 

and the Michigamme Formation covers a vast region 

to the east of the map area.

Finally, the gravity data and the model support a 

sharp discontinuity between the two juxtaposed Archean crustal segments. However, seismic studies such 

as that carried out recently by COCORP (the Consortium for Continental Reflection Profiling) across this 

tectonic zone in Minnesota are needed to determine 

more precisely the attitude and nature of the juncture 

between the two basement crustal blocks.

RELATION OF TECTONIC ZONE TO 

REGIONAL GRAVITY

East of the midcontinent rift system, the Great 

Lakes tectonic zone lies along the northern edge of a 

generally east-trending gravity high that has a 

wavelength of about 80 km. In northern Michigan 

(Klasner and others, 1979) and northern Wisconsin 

(Ervin and Hammer, 1974) the gravity high is a 

gradual, convex northward arc that extends from the 

eastern end of the Upper Peninsula of Michigan to 

northwestern Wisconsin. Near the west end of Lake 

Superior, the gravity high is offset across the midcontinent rift system by a northwest-trending fault (Sims 

and others, 1980), and west of the rift system it occurs 

in central Minnesota. Thus, the gravity expression 

across the Great Lakes tectonic zone is comparable 

throughout the Lake Superior region.

To further show the regional gravity expression of 

the Great Lakes tectonic zone, we have constructed a 

profile (fig. 5), taken from regional gravity maps, that 

extends from just north of the Minnesota-Canada border across western Lake Superior, along the detailed 

gravity profile of this study, into north-central Wisconsin. The smoothed regional profile approximates the 

gravity field if the effects of the Keweenawan volcanic 

and gabbroic rocks are removed. Regional gravity on 

the north end of the profile approximates the -60-mGal 

gravity field over Archean rocks near the Canadian 

border; the regional gravity in Michigan and Wisconsin 

is closely controlled by observed gravity. The high-frequency anomalies about the gravity field in Michigan 

and Wisconsin reflect near-surface geology, such as 

Early Proterozoic basins and reactivated gneiss domes.

On the profile (fig. 5), the Great Lakes tectonic zone 

lies at the north edge of a broad gravity high; the gravity gradient slopes gently northward from the tectonic 

zone over the greenstone-granite terrane (Superior province). The gravity model of the detailed profile (fig. 

4) suggests that the steep north-sloping gradient results from an increase from north to south in the density of near-surface rocks. It is also likely that the 

broad regional high is caused by southward density increases in the upper crust, but as mentioned above, 

previous regional models (Klasner and Bomke, 1977) 

suggested that the broad gravity high results from 

changes in density of the deep crust, perhaps at a depth 

of about 16 km. Thus, it is reasonable to conclude that 

the broad high results from the combined effect of 

changes in density of near-surface rocks and the deeper 

crust, thereby indicating that the discontinuity between 

the two Archean basement segments is a fundamental 

crustal feature.
B12 CONTRIBUTIONS TO THE GEOLOGY OF THE LAKE SUPERIOR REGION

40i 

MINNESOTA WISCONSIN

Middle

Proterozoic

Duluth Complex

-Greenstone-granite terrane

Great Lakes 

tectonic zone

100 KILOMETERS

FIGURE 5. Regional gravity profile, Lake Superior region, showing relation to Archean basement terranes.

TECTONIC ENVIRONMENT

The boundary between the two Archean basement 

terranes in the Lake Superior region has been interpreted as resulting from the joining together of two ancient crustal segments of contrasting lithologies and 

ages in Late Archean time (Sims and others, 1980). 

Geologic data strongly indicate that the two terranes 

were tightly juxtaposed in Early Proterozoic time, 

when the sedimentary-volcanic sequence of the Marquette Range Supergroup accumulated, but that extensive movement of both extensional and compressional 

regimes took place along the boundary during this time 

interval. The contrasting tectonic behavior of the two 

basement terranes in Early Proterozoic time has been 

attributed primarily to differences in heat flux (Sims 

and others, 1981). The greenstone-granite terrane 

stabilized near the end of the Archean, probably as a 

result of depletion of lithophile elements during the extensive igneous episodes 2,700 m.y. ago; whereas the 

gneiss terrane retained much of its lithophile element 

content and remained somewhat mobile until about 

1,600 m.y. ago.

The gravity data as interpreted above support the 

earlier interpretation that the boundary between the 

two Archean crustal segments was an intracratonic feature during the Early Proterozoic (Sims and others, 

1981). The residual gravity increases rather abruptly 

across the boundary from north to south. Some of the 

increase results from lateral changes in the density of 

Archean rocks in the upper crust, indicating that crustal rocks in the gneiss segment are denser than those 

in the greenstone segment. Much of it can be accounted 

for by dense Early Proterozoic rocks that clearly lie 

unconformably on the basement gneises. Some of these 

supracrustal rocks span the boundary and thus overlie

both basement segments. The absence of a major positive anomaly over the boundary zone shows clearly that 

extension across the boundary was not sufficient to 

cause separation and formation of oceanic crust.

REFERENCES CITED

Bacon, L. 0., 1957, Relationship of gravity to geological structure 

in Michigan's Upper Peninsula, in Snelgrove, A. K., ed., Geological exploration Institute on Lake Superior Geology: Houghton, 

Mich., Michigan College of Mining and Technology, p. 54-58.

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

in Young, G. M., ed., Huronian stratigraphy and sedimentation: 

Geological Association of Canada Special Paper 12, p. 251-271.

Cannon, W. F., and Klasner, J. S., 1976, Geologic map and geophysical interpretation of the Witch Lake quadrangle, Marquette, 

Iron, and Baraga Counties, Michigan: U.S. Geological Survey 

Miscellaneous Investigations Map 1-987, scale 1:62,500.

Daly, R. A., Manger, G. E., and Clark, S. P., Jr., 1966, Density 

of rocks, in Clark, S. P., Jr., ed., Handbook of physical constants: Geological Society of America Memoir 97, p. 19-26.

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

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

International Association of Geodesy, 1971, Geodetic reference system, 1967: Special Publication 3, 115 p.

Klasner, J. S., and Bomke, D., 1977, A regional gravity anomaly 

in northern Michigan and Wisconsin, possible extension around 

the Superior province, associated foldbelts [abs.]: EOS (American Geophysical Union Transactions), v. 59, no. 4, p. 226.

Klasner, J. S., and Jones, W. J., 1979, Simple Bouguer gravity anomaly map and geologic interpretation, Iron River I°x2° quadrangle, northern Michigan and Wisconsin: U.S. Geological Survey Open-File Report 79-1564, 22 p., 1 pi.

Klasner, J. S., Wold, R. J., Hinze, W. J., Bacon, L. 0., O'Hara, 

N. W., and Berkson, J. M., 1979, Bouguer gravity anomaly map 

of the northern Michigan-Lake Superior region: U.S. Geological 

Survey Geophysical Investigations Map GP-310, scale 

1:1,000,000.

Morelli, Carlo, 1974, The international gravity standardization net, 

1971: International Association of Geodesy Special Publication 4, 

194 p.
GEOLOGIC INTERPRETATION OF GRAVITY DATA, MARENISCO-WATERSMEET AREA B13

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.

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

Archean tonalitic gneiss from northern Michigan, U.S.A., 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.

Robbins, S. L., Oliver, H. W., Holden, K. D., and Farewell, R. 

C., 1974, Principal facts, accuracies, and sources for 3,046 gravity stations on the San Jose I°x2° quadrangle, California: 

Springfield, Va., U.S. Department of Commerce, National Technical Information Service, Report PB-232 729/As, 106 p.

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., 1972, Regional gravity field, in Sims, P. K., and Morey, 

G. B., eds., Geology of Minnesota, A centennial volume: Minneapolis, Minn., Minnesota Geological Survey, p. 581-584.

 1976, Precambrian tectonics and mineral deposits, Lake 

Superior region: Economic Geology, v. 71, p. 1092-1118.

 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 rock, southern Canadian Shield: Geological Society 

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

Sims, P. K., Card, K. D., and Lumbers, S. B., 1981, Evolution of 

early Proterozoic basins of the Great Lakes region, in Campbell, 

F. H. A., ed., Proterozoic basins of Canada: Geological Survey 

of Canada Paper 81-10, p. 379-397.

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, pt. 

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

Sims, P. K., and Peterman, Z. E., 1976, Geology and Rb-Sr ages 

of reactivated Precambrian gneisses and granite in the 

Marenisco-Watersmeet area, northern Michigan: U.S. Geological 

Survey Journal of Research, v. 4, no. 4, p. 405-414.

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, no. 2, p. 185-192.

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

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

U.S. Geological Survey, 1972a, Aeromagnetic map of the Watersmeet 

region, Ontonogan and Gogebic Counties, Michigan, and Vilas 

County, Wisconsin: U.S. Geological Survey Geophysical Map 

GP-831, scale 1:62,500.

 1972b, Aeromagnetic map of central Gogebic County, Michigan, and northern Vilas County, Wisconsin: U.S. Geological Survey Geophysical Map GP-828, scale 1:62,500.

a- U. S. Government Printing Office 1984-781-242