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Geology and Geochemistry of 

Early Proterozoic Rocks in the 

Dunbar Area, Northeastern Wisconsin

By P.K. SIMS, KJ. SCHULZ, and Z.E. PETERMAN

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1517

Field relations, lithology, isotope ages, and tectonic 

implications of volcanic and granitoid rocks

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1992
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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

descriptive purposes only and does not imply endorsement by the

U.S. Government

Library of Congress Cataloging in Publication Data

Sims, P.K. (Paul Kibler), 1918-

Geology and geochemistry of Early Proterozoic rocks in the Dunbar area, northeastern Wisconsin / by P.K. Sims, 

K.J. Schulz, and Z.E. Peterman.

p. cm. (U.S. Geological Survey professional paper ; 1517) 

Includes bibliographical references. 

Supt. of Docs, no.: I 19.16:1517

1. Geology, Stratigraphic Proterozoic. 2. Geology Wisconsin Dunbar Region. 3. Geochemistry Wisconsin 

Dunbar Region. I. Schulz, K.J. II. Peterman, Zell E. III. Title. IV. Series. 

QE653.5.S55 1992

551.7'15'097753 dc20 90-14105

CIP

For sale by Book and Open-File Report Sales, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225
CONTENTS

Page

Abstract ......................................................................................... 1

Introduction .................................................................................. 2

Acknowledgments ......................................................................... 2

Rock units ..................................................................................... 4

Rock texture terminology ...................................................... 5

Dunbar Gneiss ....................................................................... 5

General features ............................................................. 5

Description ...................................................................... 6

Geochemistry .................................................................. 7

Petrogenesis .................................................................. 10

Marinette Quartz Diorite .................................................... 13

General features ........................................................... 13

Description .................................................................... 15

Geochemistry ................................................................ 19

Petrogenesis .................................................................. 20

Mafic rocks (SiO2<65 percent) ............................. 20

Silicic rocks (SiO2>65 percent) ............................. 21

Hoskin Lake Granite ........................................................... 23

General features ........................................................... 23

Description .................................................................... 26

Spikehorn Creek Granite .................................................... 27

General features ........................................................... 27

Description .................................................................... 27

Geochemistry ................................................................ 28

Petrogenesis of Hoskin Lake and Spikehorn Creek

Granites ............................................................................ 28

Bush Lake Granite .............................................................. 29

General features ........................................................... 29

Description .................................................................... 31

Geochemistry ................................................................ 31

Granitoid dikes, pegmatite, and aplite .............................. 31

General features and description ................................ 31

Geochemistry of garnet-bearing aplite dikes ............. 33

Rock units Continued

Petrogenesis of Bush Lake Granite and late aplite dikes. 

Metasedimentary rocks ......................................................

Quinnesec Formation .........................................................

General features ..........................................................

Geochemistry ...............................................................

Newingham Tonalite ..........................................................

General features ..........................................................

Description ...................................................................

Geochemistry ...............................................................

Petrogenesis .................................................................

Megacrystic facies of Newingham Tonalite ......................

General features and description ...............................

Geochemistry and petrogenesis ..................................

Twelve Foot Falls Quartz Diorite .....................................

Granitoid dikes in supracrustal rocks ..............................

Diabase dikes ......................................................................

Tectonic implications of granitoid rock geochemistry .............

Geochronology ............................................................................

Structure ....................................................................................

Small-scale structures ........................................................

D! structures ...............................................................

D2 structures ...............................................................

D3 structures ...............................................................

D4 structures ...............................................................

Large-scale structure .........................................................

Shear zones .........................................................................

Metamorphism ...........................................................................

Evolution of dome ......................................................................

Post-doming events ....................................................................

Mineral resources ......................................................................

Discussion ..................................................................................

References cited ..........................................................................

33

35

35

35

36

36

36

37

40

41

41

41

42

43

44

46

47

48

49

50

50

50

51

51

52

54

57

58

59

59

60

61

ILLUSTRATIONS

FIGURE 1- Generalized geologic-tectonic map of northern Michigan and northern Wisconsin..........

2. Geologic map of Dunbar area, northeastern Wisconsin.......................................................

3. Photographs of outcrops showing fine-scale and large-scale layering in Dunbar Gneiss

4. Quartz alkali feldspar plagioclase diagram for Dunbar Gneiss........................................

5. Modified Peacock diagram showing rocks of Dunbar area.................................................

ill
IV CONTENTS

Page

FIGURE 6. Chondrite-normalized rare earth element plots for samples of Dunbar Gneiss ....................................................................... 10

7. Minor element diagrams for Dunbar Gneiss................................................................................................................................ 11

8. Chondrite-normalized rare earth element plots for amphibolites in Dunbar Gneiss............................................................. 12

9. Diagram of La:Yb versus Yb, showing partial melting model for Dunbar Gneiss.................................................................... 12

10. Quartz-alkali feldspar-plagioclase diagram for Marinette Quartz Diorite ............................................................................. 13

11. Photograph showing compositional layering in Marinette Quartz Diorite................................................................................ 19

12. Chondrite-normalized rare earth element plots for samples of Marinette Quartz Diorite...................................................... 20

13. Chondrite-normalized rare earth element plots for samples W511 and W511B of Marinette Quartz Diorite....................... 20

14. Plot of modal clinopyroxene+hornblende+sphene versus Sc content in mafic samples of Marinette Quartz Diorite............ 21

15. FeOT-MgO-(Na2O+K2O) diagram for mafic samples of Marinette Quartz Diorite................................................................... 21

16. Normalized trace-element diagram for mafic samples of Marinette Quartz Diorite, lamprophyre dike east of

Dunbar dome, and diorite, lamprophyre, and a minette dike from other locations for comparison.............................. 22

17. Photograph showing replacement of Marinette Quartz Diorite by Hoskin Lake Granite ....................................................... 23

18. Quartz-alkali feldspar plagioclase diagram for Hoskin Lake, Spikehorn Creek, and Bush Lake Granites ......................... 26

19. Chondrite-normalized rare earth element plots for Hoskin Lake and Spikehorn Creek Granites......................................... 28

20. Variation diagrams of selected components versus FeOT for Marinette Quartz Diorite and Hoskin Lake and

Spikehorn Creek Granites.................................................................................................................................................... 30

21. Chondrite-normalized rare earth element plots for Bush Lake Granite................................................................................... 31

22. Chondrite-normalized rare earth element plots for three garnet-bearing aplite dikes............................................................ 33

23. Graph showing enrichments and depletions in sample W411 and average aplite relative to sample W411F....................... 34

24. Chondrite-normalized rare earth element plots for Quinnesec Formation ............................................................................... 36

25. Quartz-alkali feldspar-plagioclase diagram for Newingham Tonalite and megacrystic facies of Newingham Tonalite...... 37

26. Chondrite-normalized rare earth element plots for samples of Newingham Tonalite............................................................. 40

27. Plot of Nb versus Rb contents for Newingham Tonalite, megacrystic facies of Newingham Tonalite, and

Hoskin Lake Granite and related dikes .............................................................................................................................. 44

28. Plot of Rb versus Sr contents for Newingham Tonalite, megacrystic facies of Newingham Tonalite, and

Hoskin Lake Granite............................................................................................................................................................. 45

29. Chondrite-normalized rare earth element plot for sample of Twelve Foot Falls Quartz Diorite and other rocks................. 46

30. Rb versus (Yb+Ta) diagram for granitoid rocks of the Dunbar area, plotted with respect to fields for

volcanic-arc granites, syn-collisional granites, within-plate granites, and ocean-ridge granites ................................... 49

31. Ta versus Yb diagram for granitoid rocks of the Dunbar area.................................................................................................. 50

32. Rb-Hf-Ta diagram for granitoid rocks of the Dunbar area......................................................................................................... 51

33. Ta:Yb versus K:Yb diagram for recent volcanic and intrusive rocks and for rocks of the Dunbar area and

fields for Early Proterozoic basalts from the epicratonic continental-margin sequence in Upper Michigan and 

east-central Minnesota and rocks of the Wisconsin magmatic terranes .......................................................................... 52

34. U-Pb concordia plot of zircon data for rocks of the Dunbar area............................................................................................... 53

35. Summary diagram of selected isotopic ages for rocks of the Dunbar area and environs......................................................... 54

36. Structure map of the Dunbar dome.............................................................................................................................................. 55

37. Equal-area projection of structural elements in the Dunbar Gneiss and Marinette Quartz Diorite for southwestern

part of the Dunbar dome ...................................................................................................................................................... 56

38. Equal-area projection of L4 stretching lineation in core-cover boundary and in Quinnesec Formation, northeast of

Bush Lake lobe...................................................................................................................................................................... 56

39. Whole-rock Rb-Sr isochron for samples of all units within Dunbar dome ................................................................................ 60

40. Map showing Rb-Sr biotite ages for Archean and Early Proterozoic rocks in northeastern Wisconsin and

adjacent northern Michigan ................................................................................................................................................. 61

TABLES

TABLE 1. Approximate modes and chemical analyses of representative samples of Dunbar Gneiss........................................................ 8

2. Approximate modes and chemical analyses of selected samples of Marinette Quartz Diorite.................................................. 14

3. Approximate modes and minor element analyses of selected samples of Marinette Quartz Diorite........................................ 18

4. Approximate modes and chemical analyses of representative samples of Hoskin Lake Granite,

Spikehorn Creek Granite, and Bush Lake Granite............................................................................................................. 24
CONTENTS V

Page

TABLE 5. Mass balance calculation using samples W522A and W523 and mineral compositions from

Bender and others (1984)...................................................................................................................................................... 29

6. Approximate modes and chemical analyses of representative samples of aplite dikes in the Dunbar Gneiss ...................... 32

7. Approximate modes and chemical analyses of representative samples of Newingham Tonalite and associated dikes ...... 38

8. Approximate modes and minor element analyses of selected samples of megacrystic facies of Newingham Tonalite ........ 42

9. Approximate modes and chemical analyses of selected samples of megacrystic facies of Newingham Tonalite................... 43

10. Chemical analysis of Twelve Foot Falls Quartz Diorite ............................................................................................................. 46

11. Approximate modes and minor element analyses of selected samples of granitoid dikes in the supracrustal rocks............ 47

12. Chemical analyses of Middle Proterozoic diabase dike............................................................................................................... 48

13. Structural sequence, Dunbar dome .............................................................................................................................................. 51

14. Stratigraphic-tectonic evolution of Dunbar dome........................................................................................................................ 58

GEOLOGY AND GEOCHEMISTRY OF EARLY PROTEROZOIC ROCKS IN THE 

DUNBAR AREA, NORTHEASTERN WISCONSIN

By P.K. SIMS, KJ. SCHULZ, and Z.E. PETERMAN

ABSTRACT

The Dunbar area, in northeastern Wisconsin, is immediately south 

of the Niagara fault zone, the tectonic boundary between the Early 

Proterozoic continental margin to the north and the Wisconsin 

magmatic terranes of the Penokean orogen to the south. The geology 

appears to be typical of the northern part of the Wisconsin magmatic 

terranes, an oceanic arc complex of volcanic and granitoid rocks that 

was accreted to the North American continent about 1,850 million 

years ago. East of the Dunbar area, the Niagara fault zone contains 

a dismembered ophiolite and may represent a paleosuture zone.

The complex structure of the Dunbar area has resulted from one or 

more northward-verging thrust faults that juxtaposed volcanic and 

granitoid arc rocks against sedimentary rocks and Archean basement of the continental margin. The present structure is an irregular 

dome, composed of several discrete bodies of granitoid rocks and a 

large surrounding area comprising volcanic rocks assigned to the 

Quinnesec Formation and a comagmatic volcanic-arc intrusion, the 

Newingham Tonalite. On the west margin, metasedimentary rocks 

having continental-margin affinities separate granitoid rocks of the 

dome and the Quinnesec Formation.

The granitoid rocks within the dome consist of, from oldest to 

youngest, the Dunbar Gneiss, the Marinette Quartz Diorite, the 

Hoskin Lake Granite, and the Bush Lake and Spikehorn Creek 

Granites; the latter two are possibly coeval. The Dunbar and the 

Marinette constitute the core of the dome. The Bush Lake and 

Spikehorn Creek Granites compose lateral protuberances (lobes) 

from the core. The Newingham Tonalite composes the southern part 

of the core and a prominent protuberance.

The Dunbar Gneiss, which constitutes a substantial part of the 

core, consists mainly of layered biotite gneisses that are predominantly granodiorite in composition and calc-alkaline in character. Its gneissic fabric probably resulted from deformation of an 

incompletely crystallized magma. The Marinette Quartz Diorite is a 

layered mafic intrusion of alkaline-calcic to calc-alkaline composition 

that was intruded as a generally concordant body along the contact 

between the Dunbar Gneiss and the Quinnesec Formation; its 

composition was partly controlled by crystal accumulation. The 

granite bodies intrude the Marinette and a narrow fringe of the 

Quinnesec Formation adjacent to the core. The Newingham Tonalite 

is a homogeneous body that intrudes the volcanic rocks of the 

Quinnesec Formation and within the core is in turn intruded by the

Manuscript approved for publication August 6, 1990.

Marinette Quartz Diorite. Crystalline rocks in the core of the dome 

were variably metasomatized by a source like the Hoskin Lake 

Granite, resulting in the addition of potassium feldspar and largeion-lithophile elements.

The Newingham Tonalite has the compositional characteristics of 

the high Al2O3-type trondhjemite-tonalite, consistent with volcanicarc intrusions found in subduction-related settings. In contrast, the 

Dunbar Gneiss, Marinette Quartz Diorite, and the three granite 

bodies in the dome show compositional affinities with syn- to 

post-collisional granites present in Holocene orogenic belts. Their 

chemical composition, particularly high Ta and Nb concentrations, 

suggests magma derivation from and (or) magma interaction with 

within-plate-type basaltic melts possibly generated from continental 

lithosphere during the collisional event (Penokean orogeny).

Uranium-lead zircon ages of these rocks range from 1,865 to 1,835 

Ma, which interval presumably represents the time span of major 

rock-forming and structural events in the Dunbar area. The older 

rocks, the Dunbar Gneiss, Marinette Quartz Diorite, and Quinnesec 

Formation, about 1,865 Ma, are variably foliated and deformed, 

whereas the youngest dated rock, the Spikehorn Creek Granite, is 

virtually undeformed. Rubidium-strontium whole-rock and biotite 

ages are variably reset and are always 100 million years or more 

younger than zircon ages.

The Dunbar dome is about 200 square kilometers in area and is 

characterized by a consistent parallelism of structures in the cover 

rocks and in the margins of the crystalline core, and by intensely 

foliated and lineated rocks, indicative of high strain, along its north 

margin. The dome is interpreted as a large-scale fold-interference 

structure resulting from crossfolding; it was modified by intrusion of 

granitoid rocks.

A zone of prograde metamorphism extends beyond the north 

margin of the dome. An amphibolite-facies inner zone, ranging from 

less than 0.5 to at least 8 kilometers wide, occurs in the Quinnesec 

volcanic rocks and metasedimentary rocks adjacent to the core and 

has resulted from prograde metamorphism of regional greenschistfacies rocks. The amphibolite zone crosses the Niagara fault (suture) 

zone, passing into rocks of the continental-margin assemblage, 

indicating that the culmination of this thermal metamorphism 

postdated collision. The crystalline rocks in the northern part of the 

core of the dome were metamorphosed to amphibolite facies and 

metasomatized during this thermal event. The source of the heat and 

metasomatism was the Hoskin Lake Granite or a related deeper 

granitic body.
EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

Our geologic mapping and related studies have shown that the 

boundary zone between the Wisconsin magmatic terranes and the 

epicratonic continental-margin assemblage both is complex and is a 

tectonic collage of lithologies having affinities with continental as 

well as oceanic arc terranes. Presumably, rocks of the two disparate 

terranes were juxtaposed by northward-directed thrusting during 

the Penokean orogeny, about 1,850 Ma.

INTRODUCTION

The Dunbar area, in northeastern Wisconsin, is 

immediately south of the juncture between the two 

major crustal segments in the Early Proterozoic 

Penokean orogen in the eastern part of the Lake 

Superior region (fig. 1): (1) a continental-margin epicratonic assemblage to the north, and (2) a complex 

island-arc sequence to the south. The continentalmargin assemblage, formally designated as the Marquette Range Supergroup (Cannon and Gair, 1970), is 

a generally southward thickening, few thousandmeters-thick accumulation of sedimentary and 

bimodal volcanic rocks that lies on an Archean 

crystalline basement (Morey and others, 1982). The 

island-arc sequence, termed the Pembine-Wausau terrane of the Wisconsin magmatic terranes (Sims and 

others, 1989), consists of mafic to felsic volcanic rocks, 

subordinate sedimentary rocks, and granitoid intrusive rocks largely of calc-alkaline affinity. The two 

major terranes are juxtaposed along the Niagara fault 

(Sims and others, 1985), a major ductile deformation 

zone interpreted as a paleosuture (Sedlock and Larue, 

1985).

The Dunbar area provides a key to understanding 

the stratigraphic and tectonic evolution of Early 

Proterozoic rocks in the northern part of the Wisconsin 

magmatic terranes. These rocks are fairly well 

exposed, compared to other areas in northern Wisconsin, and include a moderate-sized gneiss unit that 

intrudes a widespread, thick succession of metavolcanic rocks. Granitoid rocks that generally are 

foliated intrude the gneiss and surrounding volcanic 

rocks. These rocks taken together appear to be roughly 

correlative with other bodies of less well exposed gneiss 

and granitoid rocks in northern Wisconsin (Morey and 

others, 1982), which have comparable isotopic ages of 

about 1,850 Ma (Sims and Peterman, 1980; Sims and 

others, 1989) as well as chemical similarities (Schulz, 

1983) to rocks in the Dunbar area.

The Dunbar area lies a short distance south of the 

former iron ore-producing Menominee range, which 

was mapped previously by Bayley and others (1966) 

and Dutton (1971). Also, Cain (1964) made an earlier 

reconnaissance geologic map of the Dunbar area. As a 

consequence of this mapping, most of the major rock

units were named and described prior to our study. 

Prinz (1959; 1965) named the Marinette Quartz 

Diorite and the Hoskin Lake Granite, important units 

in the northern part of the area, and Cain (1964) 

named the Dunbar Gneiss and the Newingham Granodiorite, major rock units to the south. The volcanic 

unit, the Quinnesec Formation, was named "Quinnesec 

schists" by Van Hise and Bayley (1900).

Geologic mapping of the rocks in the central part of 

the Dunbar dome has been difficult, because the Newingham Tonalite and the Marinette Quartz Diorite 

were partially metasomatized subsequent to their 

solidification, producing rocks that are similar mineralogically to the Dunbar Gneiss. This convergence in 

modal mineralogy of the three major lithotypes in the 

region and the lack of continuous outcrops led to 

uncertainties in rock designations, which were not 

resolved satisfactorily until abundant data on minor 

element chemistry became available. Accordingly, 

figure 2 of this report differs in some minor respects 

from earlier maps (Sims and others, 1985; Sims and 

Schulz, 1988).

The main purpose of this study was to gain a better 

understanding of the geologic evolution of this part of 

the Wisconsin magmatic terranes and its relationship 

to the adjacent continental-margin assemblage in 

Michigan. As part of the study, we mapped in detailed 

reconnaissance fashion the Dunbar and Dunbar NE 

7V2-minute quadrangles (Sims and Schulz, 1988) 

mainly during the summers 1980-1983, and the 

northern part of the Pembine 15-minute quadrangle 

(Schulz, unpub. data, 1990). Samples of representative 

rocks were collected from these areas as well as from 

previously mapped areas, for petrographic and 

chemical studies and isotopic age determinations. The 

locations of outcrops discussed in this report are shown 

on the geologic map of the Dunbar and Dunbar NE 

quadrangles.

This report describes the field relations, structure, 

geochemistry, and ages of major rock units in the 

Dunbar area. The geochemistry of the granitoid rocks 

and their relation to Early Proterozoic tectonic regimes 

are emphasized. We conclude that the major rock bodies 

in the Dunbar area were formed in two disparate 

tectonic environments that later were tectonically 

juxtaposed, presumably by large-scale thrusting.

ACKNOWLEDGMENTS

We acknowledge the assistance of M.G. Mudrey, Jr., 

particularly during early reconnaissance mapping, and 

G.L. LaBerge, who mapped a small area northwest of 

the Dunbar dome during 1982. In addition to analysts
ACKNOWLEDGMENTS

EXPLANATION

WISCONSIN MAGMATIC TERRANES 

EARLY PROTEROZOIC

Metavolcanic and subordinate metasedimentary rocks

Gneiss and granitoid rocks (1,890-1,835 Ma)

CONTINENTAL-MARGIN ASSEMBLAGE 

EARLY PROTEROZOIC

Metasedimentary and metavolcanic rocks of Marquette 

Range Supergroup (2,100-1,850 Ma) GLTZ

ARCHEAN

Greenstone-granite complexes (2,75Q-2,600Ma)

Gneiss terrane (3,550-2,600 Ma)

Approximate contact 

Thrust fault Sawteeth on upper plate 

Fault Relative movement not known 

Great Lakes tectonic zone

FIGURE 1. Generalized geologic-tectonic map of northern Michigan and northern Wisconsin.
EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

R21 F

FIGURE 2 (above and facing page). Geologic map of Dunbar area, northeastern Wisconsin. Compiled by P.K. Sims, 1988. Base from

Iron Mountain, Mich.-Wis. 1:100,000 quadrangle.

credited in the text, Ross Yeoman provided X-ray 

fluorescence analyses of minor elements. Robert L. 

Jenkins and W.C. Prinz introduced us to the geology of 

the area in 1975. In this report, Sims is responsible for 

the geologic mapping, petrographic studies, and 

structural interpretation; Schulz is responsible for the 

geochemical data; and Peterman is responsible for the 

isotopic age data. Edward du Bray, Russell Tysdal, 

M.G. Mudrey, Jr., and H.J. Stein made many helpful 

comments on drafts of the manuscript.

ROCK UNITS

Two major groups of rocks are exposed in the Dunbar 

area. These are (1) Dunbar Gneiss, Marinette Quartz

Diorite, and three granite bodies Hoskin Lake, Spikehorn Creek, and Bush Lake Granites which compose 

a complex domal structure, the Dunbar dome, and 

(2) the Quinnesec Formation and associated intrusive 

bodies the Newingham Tonalite, the Twelve Foot 

Falls Quartz Diorite, and unnamed metagabbro 

which surround the crystalline core (fig. 2). All these 

rocks were formed during the relatively short interval 

1,865 1,835 Ma. Distinct chemical differences between 

the two groups of rocks suggest that they were formed 

in different tectonic environments. The Quinnesec 

Formation and associated intrusive bodies appear to 

have formed in an oceanic island-arc. Subsequent 

collision of the arc terrane with the continentalmargin assemblage contributed to the generation and
ROCK UNITS

// Xhl

Xd

1 Xmn

}Xq|

:.".Xms /

EXPLANATION

MIDDLE PROTEROZOIC (1,600-900 Ma) 

Diabase

EARLY PROTEROZOIC (2,500-1,600 Ma) 

Spikehorn Creek Granite 

Bush Lake Granite 

Hoskin Lake Granite 

Marinette Quartz Diorite 

Dunbar Gneiss 

Megacrystic facies of Newingham Tonalite

Newingham Tonalite

Twelve Foot Falls Quartz Diorite

Metagabbro sills

Quinnesec Formation

Metasedimentary rocks

Area of strong to moderate potassium metasomatism

Approximate contact 

Gradational contact 

Fault Relative movement not known 

Thrust fault Teeth on upper plate

88°15' 88°00'

45°

45'

4b°

30"

DUB,

P.K. Sims

and

K.J. Schulz

(reconnaissance)

(1971)

Bayley and others (1 966)' -^^

Sims and Schulz (1988)

K.J. Schulz.

work in

progress

INDEX TO MAPPING

juxtaposition of the gneiss and associated granitoids in 

the Dunbar dome.

The stratigraphic nomenclature used by previous 

authors is retained in this report with minor modifications. The formal terms Dunbar Gneiss, Marinette 

Quartz Diorite, Quinnesec Formation, and Twelve Foot 

Falls Quartz Diorite are retained. The formal term 

Hoskin Lake Granite is retained for the granite body 

in the type area, but the other two granite bodies 

previously assigned to the Hoskin Lake Granite 

(Dutton and Linebaugh, 1967; Dutton, 1971) have 

been renamed the Spikehorn Creek Granite and the 

Bush Lake Granite, respectively (Sims and Schulz, 

1988). Also, the Newingham Granodiorite of Cain 

(1964) has been redescribed as the Newingham Tonalite (Sims and Schulz, 1988), because our mapping 

indicates that the original tonalite composition was 

subsequently modified in the southern part of the dome 

by potassium metasomatism. We have designated the 

hybrid tonalite informally as the megacrystic facies of 

the Newingham Tonalite; it is primarily granodiorite 

in composition.

ROCK TEXTURE TERMINOLOGY

The crystalline rocks in the Dunbar area have been 

variably deformed by ductile deformation, resulting in 

grain-size reduction by syntectonic recrystallization; 

this grain-size reduction was accompanied in places by 

growth of potassium feldspar porphyroblasts. According to the terminology of Wise and others (1984), many 

of the rocks are protomylonites or orthomylonites. 

These rocks have a mylonitic foliation, and generally 

contain variable percentages of concentrically zoned 

plagioclase phenocrysts that have survived destruction 

through crystal-plastic grain-size reduction. The resultant microstructure is a predominantly mortar structure or core and mantle structure (White, 1976) in 

which new, smaller grains occur adjacent to older host 

grain boundaries and within zones of localized high 

intragranular strain such as isolated shears. Hanmer 

(1982) has termed this type of new grain aggregates as 

type IP and type IM aggregates. Type IP aggregates 

result from recrystallization of the host plagioclase, 

and type IM aggregates result from recrystallization of 

the host potassium feldspar. A less common microstructure existing in the rocks and resulting from 

relatively lower bulk strains has been termed type IIP 

new grain aggregates (Hanmer, 1982). This type of 

aggregate is characterized by an intragranular distribution of new grains independent of host grain 

boundaries and shears, and results from microfracturing and rotation of domains within a host plagioclase 

grain. Bent plagioclase grains and undulose extinction 

in plagioclase, microcline, and quartz are other 

textural characteristics observed in the rocks.

DUNBAR GNEISS

GENERAL FEATURES

The Dunbar Gneiss is a succession of interlayered 

biotite gneisses and subordinate amphibolite exposed 

in the vicinity of Dunbar (Sims and Schulz, 1988).
EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

FIGURE 3. Dunbar Gneiss outcrops. A, Thinly banded gneiss surrounded by late, coarse-grained pegmatite (sec. 26, T. 37 N., R. 18 E.). Lens 

cap (5 cm) for scale. B, Relatively massive (lower part of photo) to thickly banded gneiss. Rock face is about 5 m high (sec. 24, T. 37 N., 

R. 18 E.).

Some exposures in sees. 13, 14, 15, and 21, T. 37 N., 

R. 18 E. are large and spectacular. Other good exposures, including freshly blasted rock, are along Marinette County Highway U in sees. 13 and 14. Granite 

pegmatite and aplite intrude the Dunbar in the 

western part of the dome and not uncommonly compose 50 percent or more of outcrops. A few outcrops of 

the unit occur in sec. 36, T. 38 N., R. 18 E., near the 

north margin of the dome.

The gneiss is folded on northwest-trending axes (F2). 

Foliation (S^ is subparallel to layering, and an axialplanar foliation is developed only rarely.

The gneiss is intruded locally by dikes of the Marinette Quartz Diorite, but the contact between the two 

subconformable bodies is not exposed. In the western 

part of the dome, the gneiss is in contact with metasedimentary rocks (fig. 2); this contact could be tectonic. The contact of the gneiss with the surrounding

Quinnesec Formation has been largely obliterated by 

the intrusive Marinette Quartz Diorite, which commonly intervenes between the two rock types.

DESCRIPTION

The Dunbar Gneiss consists predominantly of gray 

biotite gneiss units, which are layered at scales ranging from a few centimeters to several meters (fig. 3). 

The layering generally is expressed by differences in 

the amount and kind of the major minerals but also by 

differences in grain size. Typically, the rocks range 

from fine grained to the coarse side of medium grained 

and are inequigranular; layers containing megacrysts 

of plagioclase and (or) potassium feldspar as much as 

2 cm in length are moderately common. Rarely, amphibolite layers are intercalated with the biotite gneiss. 

Leucogranite dikes cut the gneiss at several localities.
ROCK UNITS

Quartz

Plagioclase

Granite \ Granodiorite* \Tonalit

50 65 90

FIGURE 4. Quartz alkali feldspar plagioclase diagram showing 

composition of representative rocks in the Dunbar Gneiss. Rock 

classification modified from Streckeisen (1976). Modes determined 

by 600 to 1,000 point counts on standard thin sections.

The biotite gneisses are mylonitic rocks. They have a 

dominant xenomorphic granular (granoblastic) texture 

and a penetrative foliation expressed by oriented biotite and, less commonly, by elongate and flattened 

aggregates of quartz and plagioclase that are generally 

subparallel to compositional layering. The ductile flow 

textures in these rocks are superposed on and at places 

obliterate original magmatic textures expressed 

mainly by large, concentrically zoned plagioclase 

phenocrysts and by potassium feldspar phenocrysts.

The biotite gneisses have a moderate range in 

composition from layer to layer; their average and 

modal composition is granodiorite (table 1; fig. 4). 

Plagioclase (calcic oligoclase-andesine) and quartz are 

the principal minerals, potassium feldspar varies from 

0 to about 30 percent, and biotite generally composes 

from 10 to 20 percent of the rock. Myrmekite and 

myrmekitic plagioclase are abundant. Dusky green 

hornblende is a local varietal mineral. Sphene 

(titanite) is the principal accessory mineral and composes as much as 2 percent of the rock. Plagioclase 

phenocrysts, as much as 2 cm in diameter, are anhedral to subhedral, and have a weak to moderate 

concentric zoning ranging from about An30 to An25. 

Small patches of microcline (antiperthite) occur locally 

in the grains. The plagioclase grains have combined 

carlsbad-albite twins and lesser pericline twins and are 

commonly weakly altered to sericite and epidoteclinozoisite.

+ 0.6

+ 0.4

+0.2

-0.2

-0.6

-1.0

-1.2 -

-1.6 -

-1.8

Field of Holocene calc-alkaline rocks

EXPLANATION

Aplite

Bush Lake Granite

Hoskin Lake Granite and 

Spikehorn Creek Granite

Marinette Quartz Diorite 

Newingham Tonalite 

Dunbar Gneiss 

Field of W511 and W511B

50 60 70 

SiO2 , IN WEIGHT PERCENT

80

FIGURE 5. Modified Peacock diagram showing rocks of Dunbar 

area. Field for Holocene calc-alkaline rocks (modified from Brown, 

1982), included for comparison. Samples numbered W511 and 

W511B, high-silica, minimum-melt granites, discussed in text, 

p. 19.

Potassium feldspar is dominantly microcline, which 

is fresh and either has grid twinning or is untwinned 

microperthite. Most potassium feldspar occurs as small 

interstitial grains, but larger poikiloblasts with myrmekitic plagioclase, biotite, and quartz inclusions are 

present locally. Biotite is dark reddish brown, generally fresh, with locally ragged outlines, and is weakly 

altered to epidote-clinozoisite and opaque oxides or to 

chlorite. Quartz occurs mainly in recrystallized aggregates showing undulose extinction; some aggregates 

(ribbon quartz) are elongate parallel to biotite.

GEOCHEMISTRY

Samples of Dunbar Gneiss (table 1) generally have 

calc-alkaline composition (fig. 5), are weakly peraluminous and corundum normative, and show approximately curvilinear compositional trends of decreasing
TABLE 1. Approximate modes and chemical analyses of representative samples ofDunbar Gneiss

[Leaders (-»), not determined; Tr, trace; <, less than. Major oxide analyses by Z.A. Harnlin, J.S. Wahlberg, A.J. Bartel, J. Taggart, and J. Baker. FeO, H2O, and CO2 analyses by Z.A. Hamlin, H.G. Neiman

elements by X-ray fluorescence analysis by J. Storey, S. Donahey, B. Vaughn, and M. Coughlin; samples W43C, W602, W587, and W617 analyzed by Z.E. Peterman. Rb and Sr analyses for samples W

dilution method by K. Futa. Minor elements by instrumental neutron activation analysis by J.S. Mee and K.J. Schulz]

Sample No... W43A W43C W144 W145 W143B W143A W679A W679B W140 W602 W587 W495A W42 W588 W613 W

Modal analyses (volume percent)

PlagioPotassium 

feldspar . 

Biotite. ......

Hornblende.... 

Sphene...... 

Apatite...... 

Opaque 

oxides .... 

Allanite..... 

Other 

minerals.

37 

29

28 

6

0 

Tr 

Tr

Tr 

Tr

Tr

45.5 38 

29.5 30

11.5 19.5 

12.5 11.5

0 0 

Tr 0.5 

Tr Tr

0.5 0 

Tr Tr

0.5 0.5

50 

20.5

1

27

0 

1 

Tr

Tr 

Tr

0.5

51.5 47 28 43 52 51.5 

17 22 33 30 17 25.5

8.5 13 29 13 7 6.2 

19 14 9.5 13 20.5 16.3

3.5 1.1 0 0 1.5 0 

0.2 0.8 Tr 0.5 1.9 0.3 

Tr 0.2 Tr 0.3 Tr Tr

0 0 Tr 0.2 0 0 

0.1 0.1 0.4 Tr 0.1 Tr

0.2 1.8 0.1 Tr Tr 0.2

49 

31.5

6 

13

0 

0 

Tr

0.5 

Tr

Tr

52 45.8 

29 22

5 5.2 

12 23.6

0 0 

0.5 1.8 

Tr Tr

0 0 

Tr Tr

1.5 1.6

46 

22

18.4 

13

0 

0.4 

Tr

Tr 

Tr

0.2

40.8 

32.8

13.4 

12.6

0 

0.4 

Tr

Tr 

Tr

Tr

Major oxides (weight percent) by X-ray fluorescence analysis

SiO

Fe2O3 . .......

FeO...........

MgO ........ ..

CaO. ........ ..

Na2O ....... ..

KaO ...........

H2O-.. .......

TiO

P2O5 ..........

MnO ..........

CO2 ...........

67.0 

15.8 

0.6 

3.39 

1.23 

2.36 

3.87 

3.26 

0.48 

0.02 

0.71 

0.16 

0.08 

<0.01 

...

63.6 65.9 71.6 67.7 61.7 

16.3 16 14.0 15.9 17.5 

0.88 0.95 0.40 0.51 1.02 

4.07 2.8 2.18 2.99 4.41 

1.64 1.2 0.44 1.0 1.64 

3.87 3 1.40 2.74 3.20 

4.21 3.9 2.75 3.55 4.22 

2.42 3.3 5.12 3.66 3.16 

0.76 0.65 0.43 0.55 0.85 

0.06 0.07 0.14 0.14 0.02 

0.82 0.53 0.38 0.59 0.82 

0.26 0.16 0.09 0.16 0.18 

0.09 0.06 <0.02 0.04 0.09 

0.03 0.02 0.02 0.01 0.03

68.2 

16.1 

0.62 

2.3 

1.01 

3.14 

3.90 

2.37 

0.60 

0.04 

0.40 

0.06 

0.04 

<0.01 

...

...

Minor elements (parts per million) by X-ray fluorescence analysis

Rb. .............

Sr...............

Y................

Zr...... .........

Nb .............

...

153 

300 

9 

92 

24

162 

265 

10 

237 

17

112 105 176 178 137 93 

488 458 297 423 438 573 

14 15 11 10 

282 297 230 120 211 

24 33 47 22

202 

457 

8 

213 

22

92 

549

218 

18

...

1

3

1
Minor elements (parts per million) by instrumental neutron activation analysis

U..............

Th............

Ta ....... .....

Hf ............

Cr.............

Co............

Sc.............

Zn............

Ba ............

Cs ............

La ....... .....

Ce............

Nd............

Sm ...........

Eu............

Gd............

Tb ............

Yb............

Lu ............

3.1

10.3

3.95

7.6

25

9.9

8

810

4.9

63

119

34

7.13

1.76

0.94

1.55

0.21

5 

22.6 

2.95 

8 

18

6.9

5.7

1,200 

3.7

76 

137 

40 

6.92 

1.41

0.71 

1.07 

0.16

1.77 

38.1

7.93 

12

2.82 

2.65 

45 

1,760 

4.0

118 

191 

69 

8.93 

1.43 

5.8 

0.45 

0.33 

0.06

2.91 

21.7 

2.57 

6.46 

23

6.05 

4.58 

61 

1,320 

13.9

65.3 

107 

39 

5.37 

1.22 

3.9

1.08 

0.12

3.99 

9.79 

3.78 

6.75 

7.8

10.2 

5.13 

74 

883 

5.65

35.1 

61.8 

27 

5.1 

1.17 

4.4 

0.66 

1.16 

0.20

1.45 

9.96 

2.30 

7.30 

15

7.33 

6.01 

67 

1,380 

5.95

89.8 

144 

48 

7.1 

1.45

5.2

1.22 

0.14

SAMPLE DESCRIPTIONS AND LOCALITIES

W43A NW4SW^t sec. 24, T. 37 N., R. 18 E. Fine-grained layer from W140

banded gneiss. 

W43C Same locality as W43A. Layer containing conspicuous potassium- W602

feldspar megacrysts. W587 

W144 NW^4SW^4 sec. 13, T. 37 N., R. 18 E. Inequigranular layer in W495A

banded gneiss. W42 

W145 NW4SEMt sec. 27, T. 37 N., R. 18 E. W588 

W143B NEVtSEV* sec. 14, T. 37 N., R. 18 E. Equigranular gneiss. W613 

W143A Same locality as W143B. Contains plagioclase megacrysts. 

W679A SW^tSWMt sec. 21, T. 37 N., R. 18 E. Medium-grained layer W617

from banded gneiss. W491 

W679B Same locality as W679A. W742

sec. 15, T. 37 N., R. 18 E. Layered biotitehornblende gneiss.

NWV4NWV4 sec. 14, T. 37 N., R. 18 E. Contains plagiocla

NW4NWV4 sec. 13, T. 37 N., R. 18 E. Contains plagiocla

NW4NE^4 see. 15, T. 37 N., R. 18 E. Layered gneiss. 

SEV^EV4 sec. 26, T. 37 N., R. 18 E. Layered gneiss. 

NW4NWV4 sec. 13, T. 37 N., R. 18 E.

NEViNWVi sec. 24, T. 37 N., R. 18 E. Contains plagioclas

megacrysts.

sec. 26, T. 37 N., R. 18 E. 

sec. 36, T. 38 N., R. 18 E. Augen gneiss. 

SE^tSEVi sec. 19, T. 37 N., R. 18 E.
10 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

A1203, FeO, MnO, MgO, CaO, Na2O, TiO2, and P2O5, 

and increasing K^O with increasing Si02 content. They 

have intermediate SiO2 contents, ranging from 61.7 to 

71.6 wt. percent, moderately high A12O3 (14 to 17.5), 

and high K^O (2.4 to 5.1 wt. percent). Most trace 

element abundances are relatively high in the Dunbar 

samples, particularly the abundances of Rb, Ba, Th, 

and the HFS (high field strength) elements. However, 

the trace elements show only a weak or no correlation 

with SiO2 content. Steep negative slopes reflecting 

high light-REE contents ([La]=106 to 358 times chondrite) and relatively small negative Eu anomalies 

characterize rare earth element (REE) patterns (fig. 6). 

The sample with the highest SiO2 content (W679A) has 

the highest light REE content and largest negative Eu 

anomaly but is strongly depleted in heavy REE (table 

1, fig. 6); the depletion in heavy REE may reflect 

fractionation of a phase like zircon.

Sample W742 is included with the Dunbar Gneiss 

(table 1), but it is compositionally distinct from other 

samples in having lower A12O3 and HFS element 

contents and relatively enriched heavy REE. These 

data suggest that this sample probably is not cogenetic 

with other Dunbar samples. The sample is similar in 

composition to the Twelve Foot Falls Quartz Diorite.

Compared to the Newingham Tonalite (see table 7), 

the Dunbar Gneiss has higher abundances of Ba, Rb, 

KaO, high valence cations (U, Th, Ta, Nb, Zr, Hf), and 

REE. This difference in composition, particularly in 

the higher Ta and Nb concentrations, suggests that 

the two units were derived from different sources 

(Thompson and others, 1984; Pearce and others, 1984). 

In contrast to the Newingham Tonalite, the other 

igneous units of the Dunbar dome are also characterized by relatively high abundances of Ba, Rb, K^O, 

and the high valence cations.

PETROGENESIS

The overall characteristics of the Dunbar Gneiss, 

including its medium grain size, presence of relict 

zoned plagioclase, and the existence of deformed 

pegmatite-aplite sheets and dikes, suggest that its 

protolith was plutonic. Widespread myrmekite adjacent to potassium feldspar grains is interpreted as a 

product of late-stage crystallization of hydrous magma 

(Hibbard, 1979). Accordingly, we interpret the gneissic 

fabric as resulting from deformation of an incompletely 

crystallized magma, as proposed elsewhere by Hibbard 

(1987). We conclude that the Dunbar Gneiss was 

deformed simultaneously with D2, in a relatively high 

temperature environment wherein pressure was also 

relatively high and the rocks were subjected to slowstrain-rate penetrative deformation.

500

La Ce Sm Eu Gd Tb

RARE EARTH ELEMENT

Yb Lu

FIGURE 6. Chondrite-normalized rare earth element plots for 

samples of Dunbar Gneiss. Chondrite-normalization values from 

Haskin and others (1968). Note that except for sample W679A, 

which is anomalously depleted in heavy REE, the samples have 

relatively constant heavy-REE abundances but variable light-REE 

contents and crossing patterns.

The approximately curvilinear compositional trends 

of decreasing A12O3, CaO, MgO, and FeO with increasing SiO2 content (64-72 wt. percent) could indicate 

that these samples of Dunbar Gneiss are related by 

fractional crystallization. However, such a relationship is not supported by trace element variations. For 

example, fractionation of the dominant minerals 

observed in the gneiss plagioclase, potassium feldspar, quartz, and biotite with minor hornblende 

cannot account for the observed enrichment trends in 

Rb and Ba (fig. 7A). Increasing the amount of hornblende fractionated improves the fit for the Rb trend 

but does not account for the Ba enrichment (fig. IE). 

Also, samples W143A and W143B (table 1), which come 

from adjacent bands in the gneiss, have similar Rb and
ROCK UNITS 11

Sr contents but crossing REE patterns and Th contents 

that vary by a factor of two; this variation also cannot 

be accounted for by simple crystal-liquid fractionation.

Probably metamorphism affected the abundance of 

some elements in the Dunbar Gneiss, particularly the 

alkali elements, which are most prone to secondary 

mobilization. Alteration could also account for some of 

the variation in Rb, Ba, and K2O, but it is unlikely to 

have significantly effected variations in Th, Ta, and 

REE, which generally remain relatively immobile even 

during granulite-facies metamorphism (Jahn and 

Zhang, 1984; Weaver and Tarney, 1981). Differences in 

mineral abundance possibly contributed to the variation of these elements. Recent studies have shown that 

granitoid rocks may not represent actual liquid compositions: instead, these rocks may consist of crystals 

with some proportion of trapped liquid (that is, are 

partly cumulate; McCarthy and Hasty, 1976; Zen, 

1986). Further, studies have shown that the trace 

element budget of granitoid rocks in particular may be 

controlled largely by the abundance of minor and trace 

phases like sphene, zircon, and allanite (Gromet and 

Silver, 1983; Sawka, 1988). The higher Co, Sc, and 

heavy-REE content of sample W143B may reflect the 

higher abundance of hornblende relative to W143A, 

whereas the higher Th and light REE of sample W143A 

could reflect slightly greater amounts of allanite. Such 

variations could be the result of the original igneous 

protolith or could have occurred during later metamorphism and deformation (Hibbard, 1987).

In the simplest end member models, the igneous 

protolith of the Dunbar Gneiss could have been derived 

by fractional crystallization from a more primitive 

magma, or alternatively, the parental magma(s) could 

have been derived directly by partial melting of a more 

mafic source followed by some degree of crystal fractionation. For either case, the generally high abundance of LIL (large-ion-lithophile) elements in Dunbar 

Gneiss samples suggests derivation of the parental 

magma(s) from a relatively enriched source. Also, the 

high content of HFS elements in the gneiss, particularly Ta and Nb, suggests a "within-plate" source 

component, as defined by Brown and others (1984) and 

Pearce (1983).

FIGURE 7 (facing column). Minor elements in Dunbar Gneiss. A, Rb 

versus Sr; B, Ba versus Sr, showing Dunbar Gneiss samples, 

fractionation vectors for hornblende, plagioclase, biotite, and potassium feldspar, and fractionation vectors for sample W140 with the 

observed mineral proportions and calculating all biotite as hornblende. Note that fractionation of the observed or recalculated 

mineral assemblage of a relatively unevolved sample like W140 

cannot account for the observed compositional variation in Dunbar 

Gneiss. Mineral-melt distribution coefficients fromHanson (1978).

Derivation of the igneous protolith of the Dunbar 

Gneiss by crystal fractionation from a more primitive, 

probably mantle derived melt cannot be totally

100

50

y 1,000

< 500

100

\ I I I

Hornblende

I____I III

,W140 with all

Plagioclase

Hornblende

Biotite

B

100 500 1,000 

Sr, IN PARTS PER MILLION

2,000
12 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

300

100

50

10

La Ce Nd Sm Eu Tb

RARE EARTH ELEMENT

Yb Lu

FIGURE 8. Chondrite-normalized rare earth element plots for 

amphibolite xenoliths in Dunbar Gneiss. Also shown are fields for 

Quinnesec basalts and Dunbar Gneiss. Note that REE patterns for 

amphibolite xenoliths are generally similar to those of the 

Quinnesec basalts except for anomalously enriched La and Ce, 

probably resulting from contamination of amphibolite by Dunbar 

Gneiss.

discounted, but no samples more primitive than about 

62 percent SiO2 (table 1) have been recognized as 

clearly cogenetic with the Dunbar Gneiss. Except for 

somewhat elevated light-REE contents, possibly resulting from contamination with the gneiss, amphibolite 

layers within the gneiss have compositions similar to 

the Quinnesec tholeiitic basalt (fig. 8) and could be 

inclusions. These amphibolites are too depleted in LIL 

and HFS elements to have been parental to the Dunbar 

compositions. If the protolith was derived from a more 

mafic parent melt by fractional crystallization, fractionation would have had to occur in a separate, deeper 

chamber.

Several experimental studies have shown that partial melting of mafic compositions under both relatively 

low pressure hydrous conditions (amphibolite) and 

high pressure anhydrous conditions (granulite to eclogite) can produce melts of intermediate (tonalitic500

100

50

0.5

0.1

-Quartz 

eclogite

Garnet 

amphibolite

Vx Dunbar 

'\ Gneiss

Amphibolite

Early Proterozoic

rift basalt 

(Upper Michigan)

Quinnesec

S\ x ' / basalt 

v^- ' (Wisconsin)

0.01 0.5 1 5 

Yb, IN PARTS PER MILLION

10 30

FIGURE 9. La:Yb versus Yb diagram showing Dunbar Gneiss, Early 

Proterozoic rift basalts of Upper Michigan, Quinnesec basalt, and 

partial melting curves for quartz eclogite, garnet amphibolite, and 

amphibolite mineral assemblages. Partial melting curves 

calculated using mineral proportions of Jahn and Zhang (1984) and 

mineral-melt distribution coefficients of Hanson (1978). Origin of 

upper curves was arbitrarily taken near limit of compositional field 

of Early Proterozoic rift basalt; origin for lower curves near average 

Quinnesec basalt. Arrowhead on curve, theoretical limiting value 

for 0 percent melting; tick mark, 50 percent melting. Partial 

melting of amphibolite of average rift basalt composition gives the 

best fit to the Dunbar Gneiss data. Dots, individual samples of 

Dunbar Gneiss.

granodioritic) composition (Green and Ringwood, 1968; 

Stern and Wyllie, 1978). As shown in figure 9 (see also 

fig. 24ZJ), partial melting of LIL element-enriched 

amphibolite could produce melts with the general REE 

characteristics of the Dunbar Gneiss. Such a model is 

supported by the weakly peraluminous composition of 

even the most mafic of the samples and their low 

abundances of Sc, Cr, Co, and Yb, which suggest 

equilibration with a mafic metaluminous phase like 

hornblende (Noyes and others, 1983; Zen, 1986).
ROCK UNITS 13

A possible mafic source for the parent magma(s) 

of the Dunbar Gneiss may be the basalt in the 

continental-margin assemblage, exposed in Upper 

Michigan. This basalt, emplaced during the rifting 

stage of evolution of the passive margin (Schulz and 

others, in press), has relatively enriched LIL and HFS 

element compositions (Schulz, 1984a; Ueng and others, 

1988); it probably underplated the continental crust, as 

has been suggested for basaltic magmatism related to 

Holocene continental margins (White and others, 

1987). Partial melting of such underplated basalt could 

have occurred in response to thermal perturbations 

resulting from the collision and overthrusting of the 

Pembine-Wausau arc terrane over the Superior craton 

margin.

The Nd isotope data for samples of Dunbar Gneiss 

(this report, page 49, and Barovich and others, 1989) 

provide further constraints on the petrogenesis of the 

Dunbar protolith. The calculated Sm-Nd model ages of 

2.28 to 2.06 Ga, which are significantly older than the 

determined crystallization age, indicate derivation 

from or interaction with a source relatively enriched in 

isotopes. Barovich and others (1989) suggested that 

this source was produced by mixing between Early 

Proterozoic depleted mantle and Archean crustal 

material, probably through subduction of Archean 

sediments. Although such a model can explain the 

range in Nd isotopes in samples from the northern part 

of the Wisconsin magmatic terranes, it would not 

directly account for the high abundance of HFS 

elements in units like the Dunbar Gneiss because both 

the depleted mantle (and the magmas derived from it) 

and typical Archean sediments are relatively depleted 

in HFS elements (Taylor and McLennan, 1986) and 

are unlikely to produce a "within-plate" signature 

(Thompson and others, 1984). Alternatively, if the 

Dunbar protolith was derived by partial melting of a 

mafic source similar in composition to the basalts 

found in the continental-margin assemblage, then both 

the enriched Nd-isotope and HFS element characteristics of the Dunbar Gneiss could directly reflect 

those characteristics of the source. With this scenario, 

the older Sm-Nd model ages could represent the 

approximate time of separation of the mafic source 

from a depleted mantle reservoir (McCulloch, 1987). 

Alternatively, the older model ages could also reflect 

some degree of contamination with Archean crust of 

the mafic source.

In summary, the Dunbar Gneiss is interpreted to 

have had an igneous, probably intrusive protolith. 

Although petrogenesis of that protolith remains 

somewhat equivocal, a model involving partial melting 

of a LIL-enriched mafic source such as the Early 

Proterozoic rift basalts of the continental margin

Quartz

50 65 90

FIGURE 10. Quartz-alkali feldspar-plagioclase diagram for Marinette Quartz Diorite.

appears most compatible with present data. Potential 

underplated equivalents to these basalts could have 

undergone partial melting during the overthrusting of 

the arc terrane onto the continental margin during the 

Penokean orogeny.

MARINETTE QUARTZ DIORITE

GENERAL FEATURES

The Marinette Quartz Diorite is a large, layered, 

sill-like intrusive body that was emplaced in the 

contact zone between the Dunbar Gneiss and the 

Quinnesec Formation and associated Newingham 

Tonalite (fig. 2). The rocks are dominantly quartz 

diorite or diorite in composition (fig. 10), and can be 

distinguished from the Dunbar Gneiss and the Newingham Tonalite by their moderately low quartz 

content and moderately high biotite, hornblende, and 

sphene contents (tables 2 and 3). Because of abundant 

sphene, the Marinette Quartz Diorite can also be 

distinguished from the generally finer grained, sphenepoor volcanic rocks of the Quinnesec Formation.

In the north-central part of the Dunbar dome, the 

layered rocks contain variable amounts of potassium 

feldspar. These are interpreted as hybrid rocks: the 

primary quartz diorite was modified by the addition of 

post-crystallization potassium feldspar. At a few places 

in the east-central part of the dome, intrusive rocks of
14 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

TABLE 2. Approximate modes and chemical analyses

[Leaders ( ), not determined; Tr, trace; <, less than. For samples W522A, W523, W520, W527, and W406A, major oxide analyses by J.S. Wahlberg, A. Bartel, J. Taggart, and J. Baker; 

by instrumental neutron activation analysis by J.S. Mee. For samples SIM-2-1 through SPB-959, major oxide analyses by Z.A. Brown, F.W. Brown, and H. Smith; minor elements 

by X-ray fluorescence analysis by Z.E. Peterman (slabs)]

.................... W659-1A W522A W524A W519 W520 W521 W406A W36A W36B W527 W527-1

Modal analyses

Quartz ..........................................

Potassium feldspar......................

Biotite. ..........................................

Hornblende ..................................

Opaque oxides.............................

Epidote-clinozoisite ....................

Accessory minerals......................

.................. 64.5

.................. 11

.................. 0

.................. 22

.................. 0

.................. 0

.................. Tr

.................. Tr

.................. Tr

.................. 0.5

50 

18 

14 

10

7 

0 

1 

Tr 

Tr 

Tr

42.5 

13.5 

22.5 

9 

12 

Tr 

Tr 

Tr 

Tr 

0.5

50 

19 

5 

19 

4 

0 

2.5 

Tr 

Tr 

0.5

51 

6 

0 

24.5 

14.5 

0 

4 

Tr 

Tr 

Tr

45 

23 

0.7 

29 

0 

0 

1.5 

Tr 

Tr 

0.8

37 

8.6 

0.2 

17 

27 

0 

4.3 

Tr 

0.2 

5.7

41 

11 

9 

28.5 

4.7 

0 

5.8 

Tr 

Tr 

Tr

49 

5 

Tr 

32 

3 

0 

6 

Tr 

5 

Tr

38 

14 

4 

25 

10 

0 

4.3 

0.2 

3 

1.5

45 

9 

7 

21 

14.5 

0 

3 

Tr 

Tr 

0.5

Major oxides (weight percent)

SiO2 ..............................................

A1203 ............................................

Fe2O3 ............................................

FeO...............................................

MgO ..............................................

CaO...............................................

Na2O.............................................

KjO. .............................................

H2O+ ............................................

H-jO' ..............................................

TiO2 ..............................................

P2O5 ..............................................

MnO..............................................

CO2 ...............................................

65.0 

15.9 

0.64 

3.01 

1.56 

3.20 

4.30 

3.69 

0.44 

0.02 

0.79 

0.18 

0.07 

0.04

__

51.8 

16.8 

1.97 

7.77 

3.50 

6.18 

3.88 

2.62 

1.23 

0.07 

2.42 

0.64 

0.15 

<0.01

...

51.1 

16.2 

1.42 

7.36 

5.48 

7.67 

3.16 

2.32 

1.59 

0.14 

2.24 

0.63 

0.15 

<0.01

... ___

54.1 

17.0 

1.45 

6.84 

3.09 

5.55 

3.54 

3.43 

1.13 

0.14 

1.77 

0.59 

0.13 

0.10

Minor elements (parts per million)

Rb ................................................

Sr ..................................................

Y...................................................

Zr .................................................

Nb. ..................................................................

118 

459 

13 

150 

46

84 

727 

25 

208 

51

117 

727 

20 

231 

55

85.9 

696 

23 

235

44

1Sample descriptions are on p. 16-17.

granodioritic composition (as for example sample 

W667B, table 3) contain inclusions of mafic rocks 

typical of the Marinette; these intrusive rocks are 

included in the Marinette as mapped (fig. 2), but their 

extent and nature are obscure because of their sparse 

exposure.

The northern part of the Marinette Quartz Diorite, 

mapped previously by W.C. Prinz (Bayley and others, 

1966, pis. 1 and 2), is moderately well exposed in small 

hills and cuts along the Chicago, Milwaukee, St. Paul, 

and Pacific Railway. Except for an area of about 2 km2 

west of Fischer Lake (Sims and Schulz, 1988), exposures elsewhere in this part of the dome are rather 

sparse. Another moderately good outcrop area occurs 

in the southwestern part of the dome, south of Dunbar. 

In this area as well as a smaller area east of Dunbar 

(sec. 20, T. 37 N., R. 19 E.), white to pink granite

pegmatite and aplite similar to that cutting the Dunbar Gneiss profusely intrude the Marinette Quartz 

Diorite.

The Marinette Quartz Diorite has a moderately well 

developed foliation expressed mainly by oriented 

biotite that generally is subparallel to, but locally 

transects, compositional layering. The rocks were 

deformed together with the Dunbar Gneiss and the 

Quinnesec volcanic rocks on northwest-oriented fold 

axes (F2).

The quartz diorite in the elongate body north of the 

Niagara lobe (fig. 2), which is separated from the main 

body by the intrusive Spikehorn Creek Granite, shows 

sharp intrusive contacts against the Quinnesec 

Formation; a lit-par-lit intrusive zone about 50 m wide 

is exposed in the east half of sec. 22, T. 38 N., R. 20 E. 

In the central part of the dome, the Marinette intrudes
ROCK UNITS 15

of selected samples ofMarinette Quartz Diorite 1

FeO, H20, and C02 analyses by H. Neiman, J. Ryder, and G. Mason; minor elements by X-ray fluorescence analysis by J. Storey, S. Donahey, B. Vaughn, and M. Coughlin; minor elements 

by X-ray fluorescence analysis by R. Johnson, K. Dennen, and B. Scott; minor elements by instrumental neutron activation analysis by L.J. Schwarz. For sample W404, minor elements

W688A W511A W511B W511 SIM-2-1 SIM-6-82 SDNE SDNE SDNE SPB

100-82 104-1-82 104-2-82 959 W507 W704 W664B W404 W465-1A

(volume percent)

48

7

0

23

18

0

3

Tr

0

1

53

23

0

21

0

0

2.5

0

Tr

0.5

25

35.5

35.5

4

0

0

Tr

0

Tr

Tr

55.7

13.9

2.3

18

3

0

3.3

0.4

0.8

2.6

54

7.6

1.6

25.4

3.7

0

3.4

0.9

Tr

3.3

42.5

4.4

0

12.7

31.1

0

1.1

4.7

0.2

3.3

49.8

2.4

0

23.6

19.7

0

5

0.5

Tr

Tr

60.2

2.3

0

27.2

2.3

0

3.4

0.4

Tr

4.2

29.4

8.8

21

27.9

7.8

0

3

0.8

Tr

1.3

52

11

11.5

11.5

11

0

3

Tr

Tr

Tr

53

4

Tr

27

11

0

4.5

Tr

Tr

0.5

56

13

2.5

22.5

0

0

4

Tr

Tr

2

56

5.5

0

25

11

0

2.5

0

Tr

Tr

47

11.5

0

27

9

0

5

0

Tr

0.5

by X-ray fluorescence analysis

_ 63.0

16.7

0.81

4.34

1.81

3.13

4.17

2.79

0.76

0.02

1.06

0.13

0.12

0.01

74.2

13.9

0.14

0.80

0.29

1.11

2.94

5.16

0.25

<0.01

0.09

<0.05

<0.02

0.01

75.0

13.5

0.08

1.22

0.22

1.25

3.17

4.51

0.24

0.10

0.14

<0.05

<0.02

0.03

58.7

16.0

2.0

5.6

2.7

3.9

3.7

2.7

2.5

0.26

1.5

0.5

0.13

0.23

55.3

19.3

1.6

5.2

2.6

5.0

4.8

3.1

1.2

0.20

1.5

0.42

0.09

0.08

49.5

16.4

2.3

9.6

4.2

7.3

3.7

1.8

1.5

0.18

2.4

1.0

0.17

0.17

49.7

18.1

2.4

8.2

3.6

6.5

3.9

2.6

1.5

0.18

2.3

0.55

0.15

0.24

55.9

20.3

1.7

3.8

2.0

4.9

5.7

2.0

1.3

0.25

0.93

0.32

0.07

0.48

57.8

17.6 -

^ 7

4>

OK

t-t.lj

4 g

49

9 Q tj.u

1 I

0.18 

12,

Q.33 

0.12 

0.06 

_ _

by X-ray fluorescence analysis

213 237

236

7.8

68

14.5

51

497

26

104

50

109

703

21

579

60

65

692

28

176

42

91

655

26

301

48

85

715

14

302

22

-IRQ JLtJ«7

502

21

977 * * i t

70

60

1,153

16

247

41

...

the megacrystic fades of the Newingham Tonalite, but 

details of their relationship are not known. Also, dikes 

of the Marinette intrude both the Quinnesec Formation and the Dunbar Gneiss, but major contacts 

against the Dunbar Gneiss are not exposed.

The Hoskin Lake Granite has intruded the north 

margin of the Marinette Quartz Diorite and has 

produced a contact zone of mixed rocks as much as 4 

km wide. As described by Bayley and others (1966), 

this broad contact zone contains many rock types 

gradational between the two end members, quartz 

diorite and granite, and broad areas composed predominantly of one or the other of the members. In this 

zone, fine-grained leucogranite dikes cut both the 

quartz diorite and the granite. The Spikehorn Creek 

Granite in the Niagara lobe (fig. 2), on the other hand, 

has moderately sharp contacts with the quartz diorite,

and at places crosscuts previously foliated Marinette 

Quartz Diorite.

DESCRIPTION

The Marinette Quartz Diorite ranges in composition 

from diorite or gabbro to tonalite and granodiorite and 

rarely, granite (fig. 10). Quartz diorite is the predominant rock type, as noted by Prinz (1959; 1965). 

Subsequent to crystallization, the rocks were differentially metamorphosed and metasomatized, as well as 

deformed. Prograde metamorphism of the rocks in the 

north-central part of the core (fig. 2) was accompanied 

by the metasomatism.

In the relatively unmetamorphosed eastern part of 

the body, the Marinette Quartz Diorite is a dark-gray 

to black, medium-grained, generally massive rock that
16 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

TABLE 2. Approximate modes and chemical analyses

Sample No.................... W659-1A W522A W524A W519 W520 W521 W406A W36A W36B W527 W527-1 W688A

Minor elements (parts per million) by

u .....................

Th....................

Ta....................

Hf ....................

Cr ....................

Co....................

Sc ....................

Zn....................

Ba........... .........

Cs ....................

La....................

Ce....................

Nd. ..................

Sm..... ..............

Eu ...................

Gd. ..................

Tb....................

Yb....................

Lu...... ..............

......... -- 2.8

......... 23

......... 3.50

......... 4.63

......... 17

......... 9.67

......... 6.35

......... 58

......... --- 1,050

......... 3.42

......... 54.3

......... 83.1

......... 28

......... 4.21

......... 1.12

......... 3.3

......... 0.34

......... 1.08

......... 0.15

? 1

19.4

3.85

5.87

6.4

28.5

13.1

94

701

4.36

57.2

118

60

10.2

2.50

8.6

1.01

2.20

0.32

1.79

5.9

3.46

5.6

92

29 7

20.2

107

964

14.9

67.7

124

56

8.92

2.33

6.9

0.80

1.73

0.26

2.4

22.4

2.58

7.70

9.6

22.7

13.6

97

1,205

2.56

114

195

75

11.5

2.59

9.3

0.94

2.16

0.29

	SAMPLE DESCRIPTIONS

W659-1A NE^SEVi sec. 22, T. 38 N., R. 20 E. Dike that cuts Quinnesec Formation.

W522A NWV4 sec. 22, T. 38 N., R. 20 E. Shows retrogressive metamorphism.

W524A SEViNEVi sec. 22, T. 38 N., R. 20 E. Contains relict grains of clinopyroxene.

W519 Railway cut, NWV4 sec. 10, T. 38 N., R. 20 E. Hypidiomorphic granular texture.

W520 Near locality W519. Granoblastic texture.

W521 SWV4SWV4 sec. 21, T. 38 N., R. 20 E. Hypidiomorphic granular texture.

W406A NW^SE^ sec. 35, T. 37 N., R. 18 E. Mafic layer in banded rock.

W36A Railway cut, SWViNEVi sec. 18, T. 38 N., R. 20 E. Granoblastic texture.

W36B Same locality as W36A. Granoblastic texture.

W527 Railway cut 90 m south of locality W36A. Granoblastic texture.

W527-1 Railway cut, NE^SE^ sec. 18, T. 38 N., R. 20 E. Hypidiomorphic granular texture.

W688A SWJ/4NW1/4 sec. 34, T. 38 N., R. 19 E.

W511A Small prospect pit, NE^NW^ sec. 33, T. 38 N., R. 19 E. Granoblastic texture.

is hypidiomorphic granular but porphyroblastic near 

the Hoskin Lake Granite. The rocks have a well- to 

ill-defined layering and generally lack a penetrative 

foliation. Plagioclase, the most abundant mineral in 

the rocks, occurs as tabular, normally zoned crystals as 

much as 5 mm long that display combined carlsbad and 

albite twins, and less commonly pericline twins. 

Oscillatory zoning was noted in a few thin sections. 

The plagioclase is dominantly calcic oligoclase, but has 

cores as calcic as An40 . The plagioclase tends to be 

weakly to moderately altered to epidote-clinozoisite 

and lesser sericite. Small patches of microcline occur in 

some grains. Microfractures (Hanmer, 1982; type IIP) 

occur locally in the plagioclase. Potassium feldspar 

occurs in variable amounts (tables 2 and 3) as fresh, 

twinned microcline or microperthite, and generally has 

inclusions of myrmekitic plagioclase, quartz, biotite, 

and hornblende. Quartz commonly is clear or shows

strong undulatory extinction and generally has been 

recrystallized as aggregates that are finer grained than 

the original crystals. It tends to be a late, interstitial 

mineral. Clinopyroxene occurs as rare relict grains in 

the cores of some hornblende crystals. Hornblende is 

grayish olive or pale green and moderately pleochroic. 

Generally it is weakly altered along grain margins and 

cleavages to a medium-green or bluish-green hornblende, which pseudomorphically replaces the older 

generation. In some of the more mafic rocks, hornblende is absent and a green chlorite having dark-blue 

interference colors is the dominant mafic silicate 

mineral. This chlorite is associated with calcite, 

sphene, opaque oxides, and epidote-clinozoisite. Biotite 

is grayish red and commonly has frayed crystal 

outlines. In the elongate body north of the Niagara 

lobe, hornblende appears to be unstable where in 

contact with microcline and may be partly replaced by
ROCK UNITS 17

of selected samples ofMarinette Quartz Diorite Continued

511A W511B W511 SIM-2-1 SIM-6-82 SOME 

100-82

SDNE 

104-1-82

SDNE 

104-2-82

SPB 

959 W507 W704 664B W404 W465-1A

instrumental neutron activation analysis

4.24

14.8

2.49

14.8

9 ^J7 i.O /

1.3

1.01

0.88

24

1,680

8.11

23.5

39.2

16.0

2.28

0.66

2.2

0.32

0.05

2.28

11.5

2.58

2

0Q .OC7

17

1.34

1.05

26

1,620

10.9

26.1

42.1

15.9

2.47

0.67

1.4

0.52

0.06

3.3

24.1

3.36

9 9 £t.£.t

4.0

18.8

8.46

86

855

0.7

57

107

8.8

1.97

0.83

1.3

0.25

3.6

23.0

3.22

16.4

1.7

18.4

8.45

76

902

3.7

88

137

7.9

2.08

0.71

1.7

0.30

1.3

6.5

3.19

4.7

12.0

34.4

13.7

109

666

2.4

71

126

9.8

2.52

0.85

2.1

0.29

1.2

5.9

3.6

a K D.O

6.6

31.3

12.5

121

1,020

2.8

67

123

9.7

2.40

0.84

2.4

0.33

2.6

12.4

1.16

7 ^t / .O

18.0

11.9

5.78

71

884

3.2

66

96

28

3.8

2.06

0.27

0.5

0.15

1.75

18.9

5.54

5. 77i i 

lg

14.95

10.33

860

71.9

108

6.42

1.54

0.65

1 47

0.22

AND LOCALITIES

W511B

W511

SIM-2-1

SIM-6-82

SDNE-100-82

SDNE-104-1-82

SDNE-104-2-82

SPB-959

W507

W704

W664B

W404

W465-1A

Same locality as W511A.

Same locality as W511B. Layered rock.

Railway cut, NWVi sec. 20, T. 38 N., R. 20 E. Hypidiomorphic granular texture.

Railway cut, SWVUSTEVi sec. 18, T. 38 N., R. 20 E. Granoblastic texture.

SEV4SWY4 sec. 33, T. 38 N., R. 20 E. Hypidiomorphic granular texture.

NWY4SWV4 sec. 33, T. 38 N., R. 20 E. Granoblastic texture.

Same locality as SDNE-104-1-82. Has plagioclase phenocrysts.

NEViSEVi sec. 21, T. 38 N., R. 20 E. Hypidiomorphic granular texture.

NEV4NWY4 sec. 26, T. 38 N., R. 19 E. Contains radiating hornblende crystals.

SEV4SEV4 sec. 23, T. 38 N., R. 19 E.

Railway cut, NWViSEVi sec. 33, T. 38 N., R. 20 E.

SW^NEVi sec. 36, T. 37 N., R. 18 E.

NWV4SWV4 sec. 20, T. 37 N., R. 19 E. Granoblastic texture.

microcline. Sphene and opaque oxides, either separately or as grains having opaque oxide cores and 

sphene rims, constitute about 5 percent of the rocks. 

They are closely associated with hornblende and biotite, as is epidote-clinozoisite. Epidote in the more 

mafic rocks is yellow. The common accessory minerals, 

in order of decreasing abundance, are apatite, allanite, 

and zircon. Typically, allanite has narrow rims of 

epidote-clinozoisite. Much of the zircon is zoned.

Many of the rocks in the easternmost part of the 

dome have retrograde mineral assemblages indicative 

of the following partial reactions: clinopyroxene > 

light-green hornblende+quartz >medium-green hornblende >biotite+opaque oxides+sphene. At places, the 

light-green hornblende alters directly to epidoteclinozoisite.

The most southerly exposures of the Marinette 

Quartz Diorite, in the vicinity of Dunbar, are mediumto dark-gray mesocratic, layered quartz diorite and

diorite (fig. 11) that are cut by abundant pegmatite and 

aplite; they are generally equigranular rocks but 

include porphyritic varieties containing plagioclase 

as much as 1 cm in diameter. Layering is mainly 

expressed by compositional differences; some layers 

are clinopyroxene-bearing gabbro. Plagioclase 

(An30_50) occurs as generally equant, anhedral grains 

1 mm or less in diameter in which albite and pericline 

twins are moderately well developed. The plagioclase 

grains locally have numerous microfractures type IIP 

structures of Hanmer (1982). Pale-olive or light-olive, 

but locally bluish green hornblende forms subhedral 

crystals that tend to elongate in the direction of 

foliation. At places, hornblende is partly altered to 

biotite+opaque oxides+sphene. The biotite is reddish 

brown and generally associated with a few percent of 

sphene, some of which has opaque oxide cores. Epidoteclinozoisite is the main alteration mineral, of both
TABLE 3. Approximate modes and minor element analyses of selected samples ofMarinette Quartz Diorite

[Leaders ( ), not determined; Tr, trace. Minor element analyses of rock slabs by Z.E. Peterman]

Sample No... W671 W710B W669A W672B W710A W469 W667A W667B W623 W624 W626 W651 W408A W409A W357 W652 W707 W515 

Modal analyses (volume percent)

Plagio

clase. ......

Quartz.......

Potassium

feldspar .

Biotite

Hornblende....

Sphene......

Apatite......

Opaque

oxides ....

Allanite.....

Accessory

minerals.

59.5

10.5

2.5

24.5

1

2

Tr

Tr

Tr

Tr

53.3

18

15.5

12.5

0

0.5

Tr

Tr

Tr

0.2

49

20

13

17.5

0

0.5

Tr

0

Tr

Tr

52

28

10

10

0

Tr

Tr

0

0

Tr

56

20

6.5

17

0

0.5

Tr

0

Tr

Tr

52

6.5

0

25.5

9.5

6

Tr

0

Tr

0.5

47.5

0.5

0

29.5

15.5

5

Tr

2

Tr

Tr

56.5

20.5

12

9.5

0

1

Tr

Tr

Tr

0.5

57

10

0

16

14

1

0.5

0.5

Tr

Tr

51

21

4

20

3.5

0.5

Tr

0

Tr

Tr

60

2.5

0

15.5

12.5

5

Tr

Tr

0

4.5

57

6

0

19

14

2.5

Tr

Tr

Tr

1.5

51

9

15

19

4

2

Tr

Tr

Tr

Tr

38

12

18

27

3

2

Tr

Tr

Tr

Tr

43

3

0

14

37.5

1.5

0.5

0.3

0.2

Tr

54

9

Tr

21.5

13.5

1.5

Tr

0.5

Tr

Tr

54.5

10

0

22

10

3

Tr

0

Tr

Tr

53

20

6

14.5

4.5

1.5

Tr

Tr

0.5

Tr

Minor elements (parts per million) by X-ray fluorescence

Rb.........

ov

Y...........

Zr..........

Nb ........

1,109

19

1 Q£

07

££

KQ4.

C

177

11

QC

C£o

1 3

9QQ

3Q

n c

KK-I

1 cn

9C

A CO

R71

99

occ

40

£Q

841

on

964

45

QQ

A*3R

q

990

oc

47

1,159

10

oon

31

110

777

7

1 OC

11

141

799

10

55

9Q

SAMPLE DESCRIPTIONS AND LOCALITIES

W671 NW4NE1/4 sec 9, T. 37 N., R. 19 E. Foliated megacrystic W651

granitic rock. Granoblastic texture. 

W710B NEV4SW4 sec. 34, T. 38 N., R. 19 E. Foliated megacrystic W408A

granitic rock. Protomylonite. W409A 

W669A NEV4NEV4 sec. 1, T. 37 N., R. 19 E. Foliated granitic rock.

Protomylonite. W357 

W672B SWV4SE1/4 sec. 4, T. 37 N., R. 19 E. Prominently layered W652

granitic rock.

W710A Same locality as W710B. Foliated granitic rock. Protomylonite. W707 

W469 NW4SEV4 sec. 10, T. 37 N., R. 19 E. Granoblastic texture. 

W667A SWV4NW4 sec. 32, T. 38 N., R. 20 E. Mafic phase that occurs W515

as inclusions in W667B. Has weak retrogressive metamorphism. 

W667B Same locality as W667A. Foliated granitic rock that contains W471

inclusions of more mafic phase of Marinette Quartz Diorite. W708C 

W623 SWV4SEV4 sec. 27, T. 37 N., R. 18 E. Foliated megacrystic rock. 

W624 SWV4NE1/4 sec. 34, T. 37 N., R. 18 E. Similar to W623. W493C 

W626 SEV4SWV4 sec. 25, T. 37 N., R. 18 E. Sheared and moderately

altered.

SWV4NW4 sec. 20, T. 37 N., R. 19 E. Foliated mega

migmatitic.

NEV4NEV4 sec. 34, T. 38 N., R. 19 E. Foliated mega

SEY4SEY4 sec. 28, T. 38 N., R. 19 E. Foliated granit

conspicuous potassium feldspar megacrysts. Gran

SWV4SE1/4 sec. 21, T. 37 N., R. 18 E. Strongly lineat

SEV4NW4 sec. 20, T. 37 N., R. 19 E. Foliated grani

contains mafic inclusions. 

NW4SWV4 sec. 34, T. 38 N., R. 19 E. Foliated rock 

succession. Granoblastic texture. 

SWV4NEV4 sec. 33, T. 38 N., R. 19 E. Foliated mega

Granoblastic texture.

NWV4SWV4 sec. 36, T. 37 N., R. 18 E. Foliated mega

NEV4SE1/4 sec. 33, T. 38 N., R. 19 E. Foliated megac

in layered succession. 

SWV4NWV4 sec. 31, T. 38 N., R. 19 E. Dike that tran

Dunbar Gneiss.
ROCK UNITS 19

FIGURE 11. Primary compositional layering in Marinette Quartz 

Diorite (sec. 36, T. 37 N., R. 18 E.) defined by variations in grain 

size and mineral proportions. Hammer (30 cm) for scale.

andesine and hornblende. Sericite is a rare alteration 

product of plagioclase. Minor minerals are sphene and 

opaque oxides, and accessory minerals are apatite, 

allanite, and zircon.

The northern and northwestern parts of the Marinette Quartz Diorite, adjacent to the Hoskin Lake 

Granite, have been metamorphosed to gneisses under 

amphibolite-facies conditions. These are dominantly 

dark mesocratic rocks that are layered and have a 

well-developed foliation and lineation. The layering is 

expressed by subtle differences in composition and 

texture, and individual layers range in thickness from 

less than a meter to a few tens of meters. The foliation 

is subparallel to the layering and is expressed mainly 

by oriented biotite plates and more locally by hornblende and plagioclase.

The metamorphosed quartz diorite is a nearly equigranular gneiss that has xenomorphic granular 

textures. Except for relict normally zoned plagioclase 

phenocrysts(?), probably all the mineral constituents 

have been recrystallized. The gneisses contain 40-60 

percent plagioclase, 0-20 percent potassium feldspar, 

10-45 percent combined hornblende and biotite, and as 

much as 6 percent sphene. Opaque oxides occur rarely 

as cores of grains rimmed by sphene. Epidoteclinozoisite is sparse, mainly forming narrow rims on 

altered allanite grains. Apatite and zircon are the 

other accessory minerals.

Plagioclase occurs in two forms: (1) as somewhat 

tabular, relict, zoned crystals (andesine) as much as 

5 mm long having slightly more calcic cores, and (2) as 

anhedral unzoned, smaller recrystallized grains. In 

part, the latter occur as polycrystalline aggregates 

surrounding the older, zoned grains, forming a mortar 

structure or a core and mantle structure (White, 1976).

Carlsbad combined with albite and pericline twins are 

prevalent. Adjacent to potassium feldspar, myrmekite 

commonly is present. The plagioclase is weakly altered 

to epidote-clinozoisite or sericite. The potassium feldspar is microcline, which forms interstitial grains and 

poikilitic porphyroblasts containing plagioclase, 

myrmekite, biotite, and quartz. The hornblende is 

fresh, dark green or dark blue green, and contains 

poikilitic quartz inclusions. Most hornblende has 

oriented exsolution (hornblende?) textures and, locally, 

narrow rims of blue-green hornblende. At places, 

biotite appears to be an alteration product of hornblende, but more commonly it is intergrown with 

hornblende and appears to have crystallized together 

with it. The biotite is fresh, olive gray or grayish red, 

and commonly has inclusions of quartz. It is intergrown with sphene and opaque oxides with sphene 

rims. Quartz occurs mainly as polycrystalline aggregates, in places having undulatory extinction. It also 

occurs as elongate aggregates (ribbon quartz). Sphene 

forms well-crystallized subhedral grains that are associated with biotite and hornblende.

GEOCHEMISTRY

The lithologic diversity of the Marinette Quartz 

Diorite is reflected in the chemistry presented in tables 

2 and 3. Compositions range from mafic with high KgO 

and REE contents to granite. Samples are characterized by high abundances of A12O3, KgO, total 

alkalies, TiO2, P2O5, Ta, Nb, Zr, Rb, and REE and are 

alkali-calcic in character (fig. 5). Chondrite-normalized 

REE patterns (fig. 12) show steep negative slopes with 

enriched light REE ([La]=170-340) and generally 

small negative to no Eu anomalies; sample 104-2-82, 

which has a large positive Eu anomaly and very steep 

REE pattern, contains abundant plagioclase megacrysts. Major elements show a general inverse 

correlation with increasing SiO2 content, except for 

KgO which increases slightly. Trace elements generally 

show a poor correlation with SiO2 content and are 

probably controlled by the abundance of minor and 

trace phases such as titanite, apatite, and zircon (see 

Noyes and others, 1983).

Field relations and petrography clearly show that 

the mafic to intermediate rocks are part of a layered, 

differentiated complex and that individual samples 

represent crystal cumulates to varying degrees.

Two samples (W511 and W511B, table 2) taken 

adjacent to the Hoskin Lake Granite are high-silica 

minimum-melt granites. They have low rare earth 

element abundances, and low Zr and Y, and they lack 

prominent Eu anomalies (fig. 13). The granites from
20 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

500

100

50

10

Sample 104-2-82

La Ce Nd Sm Eu Gd Tb

RARE EARTH ELEMENT

Yb Lu

FIGURE 12. Chondrite-normalized rare earth element plots for mafic 

samples of Marinette Quartz Diorite. Chondrite-normalization 

values from Haskin and others (1968).

which these samples came are finely interlayered with 

tonalite (sample 511A, table 2). Whether these granites 

are related to the Marinette or Hoskin Lake intrusions 

is uncertain.

PETROGENESIS

MAFIC ROCKS (SiO2<65 PERCENT)

The mafic rocks of the Marinette Quartz Diorite 

(SiO2<65 percent) show field and petrographic evidence of crystal accumulation and probably do not 

represent liquid compositions. Figure 14 shows the Sc 

content of mafic rocks plotted against the combined 

modal abundance of clinopyroxene, hornblende, and 

sphene, phases in which Sc is a compatible element. 

The strong positive correlation demonstrates that the 

Sc abundance is a function of the modal mineralogy 

and that these samples are at least in part cumulate in

500

100

50

10

W511B

\

La Ce Nd Sm Eu Gd Tb

RARE EARTH ELEMENT

Yb Lu

FIGURE 13. Chondrite-normalized rare earth element plots for felsic 

samples (W511 and W511B) from Marinette Quartz Diorite. 

Chondrite-normalization values from Haskin and others (1968).

nature. The cumulate nature is further demonstrated 

by the P2O5 content, which is highest in samples with 

high modal apatite. Also, the REE pattern of sample 

104-2-82 (table 2), which has abundant plagioclase 

megacrysts, exhibits a strong positive Eu anomaly 

(fig. 12).

The cumulate nature of the mafic rocks may be 

further supported by their near-constant FeOT: 

(FeOT+MgO) ratio (fig. 15), which is unlikely to result 

from a magma crystallizing mafic phases over a 

compositional range of 50 to 65 percent SiO2 (Barker, 

1978). Instead, the near-constant FeO:MgO ratio 

would be compatible with mixing (or unmixing) of 

mafic phases of about constant composition and 

variable proportions of liquid. From the variation in Sc, 

Yb, Ti, and other elements, it appears that variable 

hornblende content is a major factor controlling the 

composition of these samples.
ROCK UNITS 21

LU

Q

CQ

O > 

CCLU >-z.

QLLJJ 

OX

20

1K 15

O 

_i 

<

Q 

O

10

W522A

6 8 10 12 H 16 18 

Sc, IN PARTS PER MILLION, IN WHOLE ROCK

20

FIGURE 14. Plot of modal clinopyroxene+hornblende+sphene versus 

whole rock Sc content from mafic samples of Marinette Quartz 

Diorite. General positive correlation between Sc content and modal 

mineralogy suggests that samples are at least in part cumulates.

Although the mafic rocks cannot be taken as directly 

reflecting liquid compositions, they do provide 

constraints on the nature of the magma from which 

they crystallized. The general compositional characteristics of high total alkalies and high TiO2, P2O5, Zr, 

Nb, Ta, Hf, and light-REE contents, together with the 

hydrous mineral phases hornblende and biotite, 

suggest crystallization from a hydrous magma with 

alkaline affinities (Bender and others, 1984). The 

existence of such a magma is further supported by the 

similarity of the REE patterns of the Marinette Quartz 

Diorite mafic rocks to those of diorites and lamprophyres having alkaline affinities (fig. 16; also Thompson and others, 1984).

A chilled-margin for the Marinette Quartz Diorite 

has not been recognized, but lamprophyre dikes that 

cut the volcanic rocks outside the Dunbar dome have 

chondrite-normalized trace-element patterns similar 

to those of the Marinette but with lower elemental 

abundances (fig. 16). These dikes attest to the presence 

of late hydrous alkaline magmas in the area of the 

Dunbar dome.

SILICIC ROCKS (SiO2>65 PERCENT)

The most silicic rocks in the Marinette Quartz 

Diorite, represented by samples W511 and W511B 

(table 2), have nearly equal modal amounts of quartz,

FeO

Na2O+K?O MgO

FIGURE 15. FeOT-MgO-(Na2O+K2O) diagram for mafic samples 

(SiO2<65 percent) of Marinette Quartz Diorite. Note the constant 

FeOrMgO.

plagioclase, and potassium feldspar, high SiO2 and 

K2O contents, low HFS element contents (Zr, Nb, and 

Y), and variable Rb:Sr. They occur within 25 m of the 

contact with the Hoskin Lake Granite.

Samples W511 and W511B are compositionally distinct from the Hoskin Lake and Spikehorn Creek 

Granites and do not lie along the same compositional 

trends. Of particular importance is their low total REE 

content, lack of a Eu anomaly, low HFS element 

contents, and variable RbiSr ratios. They represent 

highly evolved, near-minimum-melt granite compositions.

Highly evolved granitic rocks like these samples can 

form from silicate melts or aqueous solutions (Hanson, 

1980; Nabelek, 1986). In the case of formation from 

aqueous fluids derived from melts or from crystalline 

granitic rocks, the resulting product would have REE 

concentrations significantly lower than that of the 

parent (Hanson, 1980) and would tend to have 

relatively low abundances of HFS elements but high 

Rb, Ba, and Sr (Nabelek, 1986). We propose that these 

granites represent the crystallized products from a 

fluid phase exsolved from either the Marinette Quartz 

Diorite or the Hoskin Lake Granite late in its 

crystallization history. Exsolution of such an alkalienriched fluid from the Hoskin Lake is supported by 

the occurrence of late potassium feldspar crystals that 

transect quartz veins in the granite. Such a fluid phase 

could account for the late molybdenum mineralization 

that is associated with these evolved granites.
22 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

1,000

100

10

EXPLANATION

 Minette dike, Arizona

0 Spanish Peaks, Colorado

0 Cortlandt Complex lamprophyre, New York

A Northeastern Wisconsin lamprophyre

Ba Rb Th K Nb Ta La Ce Sr Nd P

TRACE ELEMENTS

Sm Zr Hf Ti Tb Yb

FIGURE 16. Normalized trace-element diagram for mafic samples of Marinette Quartz Diorite (shaded field) and a lamprophyre dike from 

east of the Dunbar dome (K.J. Schulz, unpub. data, 1989). Diorite from the middle Tertiary intrusive complex at Spanish Peaks, Colo. 

(Lipman, 1987), lamprophyre from the Ordovician Cortlandt Complex, N.Y. (Bender and others, 1984), and a minette dike (Oligocene), 

Arizona (Thompson and others, 1984) are shown for comparison. Note that all samples have similar trace element patterns with relative 

enrichments of Th and light REE and depletion of Sr. Normalization values from Thompson and others (1984).
ROCK UNITS 23

FIGURE 17. Marinette Quartz Diorite (dark layers) intruded and metasomatized by Hoskin Lake Granite (light layers) 

(sec. 33, T. 38 N., R. 19 E). Note potassium feldspar megacrysts in diorite layers. Hammer (40.6 cm) for scale.

HOSKIN LAKE GRANITE

GENERAL FEATURES

The arcuate, convex-northward body of granite along 

the north margin of the dome and that in the Niagara 

lobe (fig. 2) were named the Hoskin Lake Granite by 

Prinz (1959, 1965), and the granite that composes the 

Bush Lake lobe was given the same name by Button 

(1971). However, because of structural and compositional differences, we have delineated the granite 

bodies separately and renamed the granites in the 

Niagara lobe (Spikehorn Creek Granite) and the Bush 

Lake lobe (Bush Lake Granite). The Spikehorn Creek 

Granite seems to grade into the Hoskin Lake Granite 

in the area south of Niagara (fig. 2). The Bush Lake 

Granite appears to be an isolated body.

The Hoskin Lake Granite is characterized by

(1) large, oriented, tabular potassium feldspar crystals,

(2) abundant inclusions of mafic-intermediate rocks, 

and (3) late, euhedral crystals of potassium feldspar 

that lie athwart an older foliation and, at least locally, 

transect centimeter-thin quartz veins. The rock has

been deformed and metamorphosed. Excellent descriptions of the granite appear in Bayley and others (1966). 

The Hoskin Lake Granite is well exposed in the Iron 

Mountain 7V2-minute quadrangle (Bayley and others, 

1966); it intrudes the Quinnesec Formation and the 

Marinette Quartz Diorite. The contact between the 

granite and the Quinnesec is sharp, and generally is a 

ductile shear zone. In a broad area near the contact, 

the granite contains small to large inclusions of 

Quinnesec volcanic rocks, the largest (in sec. 18, T. 38 

N., R. 20 E.) being nearly 1 km long. A fault-bounded 

inclusion, 15 m wide, exposed along the Chicago, 

Milwaukee, St. Paul and Pacific Railway in SEV4SWV4 

sec. 7, T. 38 N., R. 20 E., appears to be a tectonic wedge. 

Shear zones within the Hoskin Lake in this general 

area are common, and are oriented subparallel to the 

contact with the Quinnesec. The granite is reddened 

within the shear zones. A shear zone in the NE14NE14 

sec. 18, same township, contains substantial purple 

fluorite and associated milky quartz lenses about 0.5 m 

wide. The shear zone has been prospected, apparently 

as a possible source of sulfide minerals. The contact 

with the Marinette Quartz Diorite, on the other hand,
TABLE 4. Approximate modes and chemical analyses of representative samples ofHoskin Lake Granite, Spikehorn Creek Granite, and Bush Lake Granite

[Leaders (---), not determined; Tr, trace; <, less than. For samples W38, W506, W711, W155, W411, and W411F, major oxide analyses by J.S. Wahlberg, A. Bartel, J. Taggart, and J. Baker; FeO, H20, and C02 analyses by H. Neiman, 

G. Mason, and F. Newman; minor elements by X-ray fluorescence analysis by J. Storey, S. Donahey, B. Vaughn, and M. Coughlin; minor elements by instrumental neutron activation analysis by K.J. Schulz and J.S. Mee. For samples 3817-11-2 and SPB 120-10, major oxide analyses by Z.A. Hamlin; H20 and C02 analyses by R. Somers; minor elements by X-ray fluorescence analysis by R. Johnson, K. Dennen, and B.Scott; minor elements by instrumental neutron activation analyses by K.J. Schulz]

bO

Hoskin Lake Granite

Sample No... W38 W528 W508 W509 W506

Spikehorn Creek Granite

W711 W523 W155 W158 W162 W407 SPB 120-10 W411

Bush Lake Granite

W411F 3817- .. 11-2 W744

Modal analyses (volume percent)

Plagioclase.......Potassium 

feldspar .

Muscovite . 

Sphene ...... Opaque 

oxides .... 

Apatite...... Allanite..... Epidote- clinozoisite.... 

Chlorite..... Carbonate . 

Accessory 

minerals.

23.5 

19

48 

8.5 

Tr TrTr 

Tr 

TrTr 

0 

Tr

1

54 

1816.3 10 

Tr 

Tr

Tr Tr 

0

Tr 

0 

Tr

1.7

50 

26

14.8 

9.2 

Tr Tr

Tr 

Tr TrTr 

0 0

Tr

33.7 36

27 

3 0 

Tr

Tr 

Tr 

0

Tr 

0 

00.3

28 

42

22 

5.5 

0.8 0.8

0 

0.1 0.4

0.4 0.7 

1.8

Tr

37 49 29 38 33 26 24 38 25.4 28

19 15 24 28 32 

8.9 12 4.5 8 2 5.1 0 0.4 Tr Tr 

1 Tr 0.4 Tr 0

Tr Tr 0.4 0 0 

0 Tr 000 0 Tr 0 Tr Tr

0.6 Tr 1.6 Tr 3 

0.1 0 1.7 Tr Tr 

0 0 Tr 0 Tr

Tr Tr Tr Tr 2

48 

20

27 

4.5 

Tr Tr

0 0 0

Tr Tr 

0

0.5

...

30 34

26 

9 0.3 0.1

0.1 

0.3 

0.3

Tr 

0.1 

0

Tr

24 30

37 

7.7 

Tr 

0.1

0 

Tr 

TrTr 

0.1 0

Tr

...

Major oxides (weight percent) by X-ray fluorescence analysis

SiO2 . ..........

A12O3 . ........

Fe203 ........

FeO...........MgO ..........

CaO. ......... .

Na20. ........KaO . ..........H20+ .........

H20" ..........Ti02 ...........P n 2U5 .. ....... .MnO. .........

C02 ...........

Total......

70.8 

14.2 

0.48 

1.87 

0.72 

1.55 

3.24 5.15 0.60 

<0.01 

0.39 0.08 

0.04 

0.08

99.2

...

... _... _...

69.1 

15.0 

0.56 2.67 

0.91 

1.91 

3.45 

4.69 

0.63 

0.13 0.51 

0.11 

0.05 

0.08

99.8

70.1 69.6 73.0 

14.6 15.2 13.7 

0.46 0.63 0.31 2.22 1.80 1.78 0.83 1.02 0.41 

1.64 1.71 1.06 

3.72 3.71 3.51 

4.60 4.59 4.63 

0.58 0.41 LOI= 

0.10 <0.01 0.44 0.42 0.46 0.18 0.10 0.10 <0.05 0.04 0.04 0.05 0.52 0.07

99 9

...

...

71.4 

15.2 

0.79 

1.8 

0.92 

1.9 

4.1 4.0 0.64 

0.12 

0.35 

0.15 0.06 

0.08

...

74.6 

13.7 

0.23 0.96 

0.15 

0.65 3.43 5.06 

LOI= 

0.28 0.03 

<0.05 

0.03

...

72.7 

13.6 

0.27 2.07 0.40 

1.19 

2.74 5.48 0.39 

0.12 0.27 0.09 0.04 

0.02

99.4

74.1 

14.4 

0.34 

1.4 

0.38 

1.2 

3.0 5.5 0.56 

0.09 0.18 0.12 0.04 0.17

...

Minor elements (parts per million) by X-ray fluorescence analysis

Rb..............

Sr...............

Y.. ..............

Zr...............

Nb .............

... ...

...

153 

279 

15 

163 

39

180 175 244 186 

223 249 117 119 

14 13 --- 12 

135 173 137 115 

42 48 19

...

150 

245 

18 

182 

30

329

58

57

270 

143 

31 

154 

32

249 285 

155 120 

39 13 

167 144 

35 44
Minor elements (parts per million) by instrumental neutron activation analysis

u.............

Th. ...........

Ta. ...........

Hf. ......... ..

Cr ............

Co. ......... ..

Sc.............

Zn. ......... ..

Ba. ...........

Cs. ...........

La............

Ce. ...........

Nd ...........

Sm ...........

Eu............

Gd............

Tb.....

Yb. ...........

Lu............

4.5

33.7

4.61

5.91

22

5.75

3.31

44

705

4.21

56.3

85.6

28

4.43

0.89

3.3

0.39

1.38

0.19

4.9 5.4

33.2 41.5

4.86

5.76

14

5.33

3.86

44

714

4.75

50.6

77.2

26

4.10

0.83

0.30

0.18

11

25.4

5.50

4.9

23

2.41

1.70

466

7.1

32.0

59

18

3.22

0.54

0.52

1.14

0.19

4.2

23.1

4.17

5.58

10

5.06

3.33

639

5.4

53.9

89.4

27.5

4.62

0.86

0.38

1.18

0.18

15

24

5.78

2.80

22

0.74

3.20

184

12.8

19.6

42

16

4.05

0.29

0.99

3.51

0.54

11

46

3

5

18

2

4

47

787

5

69

121

48

8

0

8

0

2

0

SAMPLE DESCRIPTIONS AND LOCALITIES

Hoskin 

W38

W528 

W508

W509 

W506 

W711 

W523

Lake Granite:

Railway cut in SWViSEVi sec. 7, T. 38 N., R. 20 E. Coarse-grained 

granite containing tabular plagioclase porphyroclasts and 

subrectangular potassium feldspar porphyroblasts.

Same locality as W38. Sphene is partly converted to opaque oxides.

SEV4SEY4 sec. 28. T. 38 N., R. 19 E. Same lithology as W38. 

Potassium feldspar porphyroblasts are unoriented and 

transect a cataclastic foliation. Contains sparse fluorite.

SW^SEVi sec. 28, T. 38 N., R. 19 E. Fine-grained, slightly 

inequigranular granite dike that cuts Hoskin Lake Granite.

SWJ/4NW4 sec. 13, T. 38 N., R. 19 E. Contains tabular porphyroblasts of potassium feldspar; has mortar structure.

SE^NEVi sec. 16, T. 39 N., R. 20 E. Coarse-grained granite 

containing bluish-gray quartz grains.

Center sec. 22, T. 38 N., R. 20 E. Hypidiomorphic granular texture.

Spikehorn Creek Granite:

W155 Road cut, U.S. Highway 141-8 in NE^NW^ 

R. 20 E.

W158 NE^NWVi sec. 1, T. 37 N., R. 20 E. Has cat

W162 NEJ/4SW1/4 sec. 1, T. 37 N., R. 20 E. 

W407 NW^NEVi sec. 28, T. 38 N., R. 20 E. Rock c

fractures coated by muscovite. 

SPB-120-10 Road cut, old U.S. Highway 141-8 in NW^N

R. 20 E.

Bush Lake Granite:

W411 NW4NW4 sec, 13, T. 38 N., R. 17 E. 

W411F Same locality as W411. Relationship to W41

W3817-11-2 Dike cutting metasediments, drill core, NW^

R. 17 E. 

W744 SEV4SEV4 sec. 18, T. 38 N., R. 18 E. Medium

biotite granite.
26 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

is gradational, and is marked by a zone of mixed rock 

many meters wide, as discussed earlier. An excellent 

exposure showing the relations with the Marinette is 

in the small hill in SEV&SEV^ sec. 28, T. 38 N., R. 19 E. 

At this locality, Hoskin Lake lithology is finely intercalated stratigraphically with equigranular layered 

rocks, and appears to have resulted from partial 

replacement of certain layers in the gneiss by 

potassium-rich fluids (fig. 17).

The Hoskin Lake Granite is sill-like in form and 

convex northward, and it ranges in exposed width from 

about 0.6 km to 3.2 km. Toward the east, it grades 

transitionally into the little-deformed Spikehorn Creek 

Granite in the Niagara lobe. Toward the west, the 

granite is covered, and its western limit (fig. 2) is 

uncertain. The granite is cut by numerous small dikelike bodies of aplite and granite pegmatite and by 

minute ramifying veinlets of quartz and black tourmaline, some of which also contain pyrite and arsenopyrite. Aplite dikes that have been intensely deformed 

by brittle-ductile deformation are common in the 

adjacent Quinnesec Formation as much as 0.1 km 

away from the contact. At places in the vicinity of the 

contact the dikes are boudined.

Many inclusions of biotite and hornblende schist in 

the granite apparently derived from the Marinette 

Quartz Diorite protolith were folded prior to emplacement of the Hoskin Lake. In an excellent exposure in a 

rock knob in SEViSEVi sec. 13, T. 38 N., R. 19 E., 

adjacent to Florence County Highway N, the granite 

has a foliation expressed by biotite alignment that 

postdates the folds in the inclusions, and this foliation 

in turn is transected by relatively undeformed tabular 

potassium feldspar grains 2-4 cm long. Such tabular 

feldspars that lie athwart the foliation in the granite 

are common, and in SEViSEVi sec. 28, T. 38 N., R. 19 E., 

some transect a centimeter-wide quartz vein.

DESCRIPTION

The Hoskin Lake Granite is a pink or gray, coarsegrained inequigranular rock that contains large 

tabular feldspar megacrysts. At most places, it has a 

foliation and lineation given by the alignment of the 

feldspars and biotite plates. Large tabular to subrectangular megacrysts of potassium feldspar and plagioclase (oligoclase) as much as 7 cm long dominate the 

texture. These are set in a matrix of oligoclase, quartz, 

potassium feldspar, and biotite. The potassium feldspar, quartz, and oligoclase are present in about equal 

amounts (fig. 18; table 4), although the proportion of 

potassium feldspar to plagioclase varies considerably, 

even within one and the same outcrop. Biotite and its 

alteration product chlorite compose 5-10 percent of the

EXPLANATION 

 Spikehorn Creek Granite 

X Bush Lake Granite 

O Hoskin Lake Granite

Quartz

50 65 90

FIGURE 18. Quartz-alkali feldspar-plagioclase diagram showing 

composition of representative samples of Hoskin Lake Granite, 

Spikehorn Creek Granite, and Bush Lake Granite.

rock. The accessory minerals are allanite, sphene, 

apatite, monazite, and zircon.

The granite is a mylonitic rock in which plagioclase, 

quartz, biotite and, at places, potassium feldspar have 

been partly or entirely recrystallized to a finer grain 

size, commonly resulting in a mortar structure. The 

deformation was dominantly ductile, but late shear 

zones indicative of brittle deformation are present 

locally, particularly along the north margin of the 

body. The ductile deformation was accompanied by the 

development of potassium feldspar porphyroblasts, 

and the brittle deformation by local retrogressive 

metamorphism.

Plagioclase occurs as (1) tabular or lath-shaped 

anhedral porphyroclasts that are twinned according to 

the combined carlsbad-albite law and, rarely, the pericline twin law, and as (2) polycrystalline aggregates of 

smaller anhedral grains derived by the recrystallization of the older, tabular grains (type IP structure). The 

older grains have a moderate normal zoning and, at 

places, oscillatory zoning. They are variably embayed 

and corroded by potassium feldspar and locally contain 

small patches of potassium feldspar. The recrystallized 

plagioclase generally is unaltered and untwinned and 

contains vermicules of quartz where in contact with 

potassium feldspar. All the plagioclase seems to be 

oligoclase, but some of the recrystallized plagioclase 

could be more sodic. The plagioclase in granite from the 

transitional zone with the Spikehorn Creek Granite in 

the Niagara lobe (locality W711, table 4) lacks both a
ROCK UNITS 27

distinct mortar structure and myrmekite, although 

potassium feldspar is present in the rock. Potassium 

feldspar has two modes of occurrence: (1) large 

porphyroblasts of microcline microperthite that 

includes poikilitic plagioclase, myrmekite, quartz, and 

biotite, and (2) smaller, anhedral grains interstitial to 

plagioclase that are associated with polycrystalline 

aggregates of plagioclase, quartz, and biotite. Some 

large porphyroblasts contain minute remnants of 

myrmekitic plagioclase and abundant tiny quartz 

globules. The potassium feldspar typically has undulatory extinction. Quartz is recrystallized as finer 

grained aggregates of sutured grains, and at places is 

drawn out into leaves or lenses. It has a ubiquitous 

undulatory extinction. The primary quartz grains 

seem to have been interstitial to plagioclase grains. 

Biotite is reddish brown where fresh, and generally 

occurs in clots or as trains of oriented plates associated 

with moderately fine grained recrystallized plagioclase 

and quartz. Accordingly, biotite tends to wrap around 

plagioclase grains and to occur at plagioclase-quartz 

grain boundaries. Associated coarse sphene locally is 

partly altered to opaque oxides, and less commonly to 

epidote-clinozoisite. Dark-yellowish-green hornblende 

is a minor constituent that occurs in the cores of biotite 

aggregates. Retrograde alteration is moderately weak 

except in the vicinity of the late, brittle fractures, 

where the granite is reddened because of the presence 

of hematite, which is associated with chlorite. Alteration products of plagioclase are epidote-clinozoisite, 

muscovite (sericite), and less commonly, calcite. Biotite 

is variably altered to muscovite and (or) chlorite. 

Locally, it has sheaths along its cleavage of a zeolite(?) 

mineral.

SPIKEHORN CREEK GRANITE

GENERAL FEATURES

The Spikehorn Creek Granite is a gray to pinkishgray, fine- to medium-grained rock containing scattered anhedral potassium feldspar grains as much as 

2 cm in diameter. It generally is massive, but locally 

(especially near the margins of the body), it bears a 

mylonitic foliation expressed mainly by recrystallized 

quartz leaves and oriented biotite. This foliation tends 

to be subparallel to the contacts, but at places is 

oriented northeastward, at a large angle to the trend of 

the contact.

The granite has sharp intrusive contacts against the 

Quinnesec volcanic rocks and the Marinette Quartz 

Diorite, and small ramifying dikes intrude these rocks 

for distances of as much as 400 m from the contact. The 

granite in the dikes mainly has an aplitic texture and

is leucocratic. Where observed, as in sec. 1, T. 37 N., 

R. 20 E., along U.S. Highway 141-8, the contact 

against the Quinnesec is highly sheared and marked 

both by brittle and ductile deformation and by dikes of 

granite in the adjacent volcanic rocks. The granite in 

the contact zone is altered and contains abundant gray 

quartz pods and scattered almond-shaped inclusions of 

the volcanic rocks. A large xenolith of the metavolcanic 

rocks about 0.15 km wide and perhaps a kilometer long 

is present about 0.5 km north of this contact zone. The 

contact of the granite with this xenolith also is marked 

by shears and a cataclastic foliation and by thin 

ramifying aplite dikes. The contacts of the granite 

against the Marinette Quartz Diorite, where observed 

along the power line in NWV4 sec. 28, T. 38 N., R. 20 E., 

crosscut layering in the quartz diorite. Certain layers 

in the quartz diorite contain large potassium feldspar 

megacrysts, indicating that, at least here, they were 

formed before the Marinette was intruded by the 

Spikehorn Creek Granite.

DESCRIPTION

The Spikehorn Creek is a granite as defined by 

Streckeisen (1976) (fig. 18; table 4). All thin sections 

examined show some mylonitic or cataclastic textures, 

mainly expressed by recrystallized quartz but locally 

expressed by recrystallized biotite and by narrow 

shears filled mainly with sericite. Plagioclase (sodic 

oligoclase) has weak normal zoning, and is mainly 

twinned according to the albite and pericline twin 

laws. In addition to occurring as single zoned crystals, 

the plagioclase also forms glomeroporphyritic aggregates of small zoned crystals as much as 5 mm across. 

The grains locally contain small patches of potassium 

feldspar, and are weakly altered to epidote-clinozoisite 

and sericite. Potassium feldspar is microcline microperthite of both string and bleb types and, less 

commonly, grid-twinned microcline. The microcline 

embays plagioclase, and its principal form is as 

crystals as much as 6 mm in diameter that contain 

inclusions of plagioclase with albite rims, quartz, and 

biotite. Quartz has undulose extinction and is locally 

recrystallized as new grains that are aggregated and 

locally aligned, to give a mylonitic foliation. Biotite is 

dark reddish brown and typically has ragged outlines. 

Most sphene and opaque oxides occur at grain margins 

and at the junction between unaltered biotite and 

marginal zones that contain wormy intergrowths of 

quartz. Sheaths of a fibrous mineral, possibly a zeolite 

mineral, occur at places parallel to the cleavage, and 

epidote-clinozoisite is a common alteration mineral 

along grain margins. Chlorite is another common 

alteration mineral after biotite. In some sections, 

opaque oxides occur in late fractures in the rock.
28 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

500

100

50

Hoskin Lake 

Granite

Spikehorn Creek' \* 

Granite ^

La Ce Nd Sm Eu Gd Tb

RARE EARTH ELEMENTS

Yb Lu

FIGURE 19. Chondrite-normalized rare earth element plots for 

Hoskin Lake and Spikehorn Creek Granites. Chondritenormalization values from Haskin and others (1968).

Accessory minerals are zircon, allanite, apatite (rare), 

and fluorite (rare).

GEOCHEMISTRY

The Spikehorn Creek Granite (of the Niagara lobe) 

and the Hoskin Lake Granite are compositionally 

similar except that the Hoskin Lake has a slightly 

higher K2O content. Rb:Sr ratios range from about 0.55 

to 2.0 and show a positive correlation with increasing 

SiO2 content; K:Rb ratios range from about 250 to 155 

and show a negative correlation with increasing SiO2 

content. The samples show light-REE enrichment, 

small to moderate negative Eu anomalies, and decreasing light-REE abundance with increasing SiO2 ; they 

also show slightly fractionated heavy-REE (fig. 19).

PETROGENESIS OF HOSKIN LAKE AND 

SPIKEHORN CREEK GRANITES

The granitic rocks in the dome have had a complex 

crystallization and post-crystallization history. The 

Spikehorn Creek Granite in the Niagara lobe clearly 

was injected as a magma, as indicated by its internal 

homogeneity and the sharp-walled inclusions of the 

Quinnesec Formation. Although its contact with the

Quinnesec is mainly tectonic, local crosscutting 

intrusive relations with the Quinnesec are indicative of 

magmatic injection. As noted later, this body is 

interpreted to have been emplaced diapirically. In 

accordance with earlier field observations by Prinz 

(1959) and Bayley and others (1966), no evidence exists 

that the granite was produced by granitization of 

Quinnesec basalt, as suggested by Lyons (Emmons and 

others, 1953, p. 108).

Both field and petrographic observations indicate 

that the Hoskin Lake Granite also formed as an 

igneous intrusion with local selective replacement of 

preexisting rocks of the Marinette Quartz Diorite. 

Subsequent to or late in the crystallization of the 

intrusion, the rock was modified by neocrystallization 

of potassium feldspar porphyroblasts. Apparently 

much of the porphyroblastic potassium feldspar formed 

during ductile deformation, as indicated by oriented 

tabular feldspar grains in many outcrops; but at least 

some of it formed after the deformation, as indicated by 

disoriented crystals that lie across the foliation and by 

rare crystals that transect thin quartz veins in the 

granite.

A possible mechanism for the origin of the potassium 

feldspar porphyroblasts is partial replacement of 

plagioclase via potassium-rich fluids released late in 

the crystallization history of the Hoskin Lake Granite 

and fluid movement through cataclastic and mylonitic 

zones in the host rocks. The replacement mechanism 

frees sodium that moves to adjacent, altered plagioclase grains, which are recrystallized as more sodic 

species. Myrmekite is formed where more silica is 

trapped ahead of advancing potassium replacement 

than can be incorporated in plagioclase relative to its 

anorthite content. Where silica is not excessive, it is 

incorporated in potassium feldspar, as was noted in 

several thin sections.

Judged from the distribution of amphibolite-facies 

mineral assemblages in rocks both within and outside 

the core of the Dunbar dome, the Hoskin Lake Granite 

generally coincides with the zone of highest temperature attained during the doming process. This 

zone also was the locus of maximum potassium metasomatism of preexisting rocks.

The general field, petrographic, and compositional 

characteristics of the Hoskin Lake and Spikehorn 

Creek Granites are similar and suggest that the two 

granites may be related or at least derived by comparable processes. Also, their spatial association and 

similarity in mineralogy and composition with the 

Marinette Quartz Diorite suggest that the granites 

could be directly related to the diorite. The lack of 

straight-line compositional trends between the 

granites and mafic Marinette Quartz Diorite samples 

precludes a simple mixing or unmixing relationship
ROCK UNITS 29

TABLE 5. Mass balance calculation using samples W522A and W523 and mineral compositions from Bender and

others (1984)

The parent is W522A:

Coefficient

0.090

0.010

0.146

0.001

0.005

0.748

Percent

0.355 AMPH2 

0.040 BIOT2 

0.579 PLAG2 

0.006 SPH2 

0.032 APATITE

W523 is the daughter

SiO9 TiO, A12O3 FeO MnO MgO CaO Na2O PO

W523

OBS

CALC

DIF

Sum

70.45

66.14

66.22

-0.03

0.47

0.80

0.88

-0.08

15.39

16.18

15.96

0.11

2.40

3.65

3.64

0.01

0.04

0.07

0.06

0.01

1.

1.

1.

-0.

03

59

70

11

1.73

3.26

3.16

0.10

3.76

4.38

4.45

-0.07

4.65

3.75

3.69

0.06

0.10

0.18

0.31

-0.13

of squares of residuals =0.067

between them. As shown in figure 20, the granite 

samples plot along trends offset from the mafic 

samples but extending back through sample W522A. 

Sample W522Ais intermediate in composition between 

the mafic Marinette Quartz Diorite and the Hoskin 

Lake and Spikehorn Creek Granite samples but has a 

REE pattern similar to the granites except for the lack 

of a Eu anomaly. This suggests that crystal fractionation of the Marinette Quartz Diorite, or a compositionally similar magma, may have given rise to the 

Hoskin Lake and Spikehorn Creek Granites. Mass 

balance calculations using sample W522A as a parent 

and mineral compositions from the literature for compositionally similar rocks (Bender and others, 1984) 

suggest that the granites could be derived by about 25 

percent fractional crystallization involving plagioclase+amphibole+biotite+apatite+sphene. (See table 

5.) These phases are compatible with those observed in 

the Marinette rocks (table 2). The slightly peraluminous composition of the granites, suggesting 

equilibration with metaluminous phases like amphibole, is also compatible with this model.

We tentatively conclude that the Hoskin Lake and 

Spikehorn Creek Granites could have been derived by 

crystal fractionation from the Marinette Quartz Diorite or a similar magma at depth. It is likely that the 

actual fractionation process was more complex than 

simple fractional crystallization and may have 

involved segregation of phases by sidewall crystallization as described by Noyes and others (1983) 

and Sawka (1988) for plutons of the Sierra Nevada 

batholith.

It should be noted that sample W155 (table 4), the 

most evolved of the Spikehorn Creek granitic rocks, 

shows a relative depletion in light REE, Th, Ba, Sr, and 

Eu. Preferential depletion of light-REE content in 

evolving high SiO2 systems has been shown to be a 

common feature (Miller and Mittlefehldt, 1984) variously attributed to (1) fractionation of minor phases 

such as allanite or monazite, which have partition 

coefficients much greater than one for the light REE 

(Mittlefehldt and Miller, 1983), (2) liquid-state diffusion processes in zoned magma chambers (Hildreth, 

1979), and (3) transport in a fluid phase due to complexing of light REE (Hudson and Arth, 1983). In the 

case of the Spikehorn Creek Granite, the combined 

depletion of both the light REE and Th in sample W155 

would be compatible with fractionation of a phase like 

monazite (Mittlefehldt and Miller, 1983) or allanite 

(Brooks and others, 1981). The depletion in Ba, Sr, and 

Eu suggests further that potassium feldspar (±biotite) 

was a major fractionating phase. This is in agreement 

with the observed presence of megacrystic potassium 

feldspar in the Hoskin Lake and Spikehorn Creek 

Granites.

BUSH LAKE GRANITE

GENERAL FEATURES

The granite in the Bush Lake lobe is poorly exposed, 

and the outline of the lobe as shown in figure 2 is based 

largely on aeromagnetic data and scattered exposures 

mapped earlier by Dutton (1971). Contacts with adjacent rocks have not been observed, but granite dikes 

presumably related to the main body cut the country
30 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

70 -

I 60

20

£ 18

16

14

12

« 4

EXPLANATION

Hoskin Lake and Spikehorn

Creek Granites 

Marinette Quartz Diorite

i r

J04-2

W522A

468 

FeOT , IN WEIGHT PERCENT

10

1 

2.5

2.0

1.5

1.0

0.5

12

,104-2

.W522A

104-2*

468 

FeO T, IN WEIGHT PERCENT

10 12

FIGURE 20. Variation of CaO, A12O3, SiO2, Rb:Sr, and Sm:Eu versus FeOT for Marinette Quartz Diorite and Hoskin Lake and Spikehorn 

Creek Granites. Note that the data define two trends that project through sample W522A, the most evolved diorite sample. Sample 

SDNE-104-2-82, the plagioclase cumulate sample, is also identified by reference (104-2).
ROCK UNITS 31

rocks on the west and north sides of the lobe. The 

following description is partly taken from Button's 

(1971) report.

DESCRIPTION

The granite is a gray, buff, or orange-pink, coarsegrained, slightly porphyritic rock containing feldspar 

phenocrysts that commonly range from 1 to 4 cm in 

length. Where observed, it has been deformed and has 

a weak to moderate secondary foliation expressed 

mainly by oriented biotite and fine-grained aggregates 

of recrystallized quartz. Potassium feldspar generally 

is more abundant than plagioclase, but in some rocks 

the two are nearly equal in amount (table 4). Plagioclase is strongly inequigranular and is mainly calcic 

oligoclase or sodic andesine. Larger, primary crystals 

as much as 0.5 mm in size are anhedral and are 

twinned according to combined carlsbad-albite and 

pericline twin laws. Typically, the primary grains are 

partly surrounded by finer grained polycrystalline 

aggregates (type IP of Hanmer, 1982). These aggregates are intergrown with fine-grained recrystallized 

quartz, biotite, and potassium feldspar. Not uncommonly, the polycrystalline aggregates occupy shears. 

Potassium feldspar is dominantly microcline microperthite, which forms anhedral or subhedral porphyroblasts as much as 4 cm long; they are twinned 

according to the carlsbad law. The larger grains are 

strongly poikilitic and include plagioclase, quartz, and 

biotite. The potassium feldspar embays and replaces 

plagioclase. Smaller grains associated with new plagioclase aggregates tend to have grid twinning, and the 

adjacent plagioclase has extensive vermicular intergrowths of quartz. Quartz is gray and has wavy 

extinction. It embays plagioclase, and is recrystallized 

as aggregates of sutured grains, some of which are 

lensoid. Biotite is the only mafic mineral. It is dark 

reddish brown and typically occurs as ragged crystals. 

It has mainly been recrystallized and is associated 

with new grains of plagioclase and quartz; as a result, 

it commonly forms partial rims around primary plagioclase crystals. The accessory minerals, apatite, allanite 

(altered), and zircon, are associated closely with 

biotite. Zircon, a common mineral, forms zoned prismatic grains. The rocks show weak retrogressive 

alteration, mainly expressed by the alteration of biotite 

to sphene and muscovite or to chlorite.

GEOCHEMISTRY

The Bush Lake Granite is compositionally distinct 

from the Hoskin Lake and Spikehorn Creek Granites;

300

100

5°

i i i i i i r i r

La Ce Nd Sm Eu Gd Tb

RARE EARTH ELEMENTS

Yb Lu

FIGURE 21. Chondrite-normalized rare earth element plots for 

samples of Bush Lake Granite. Chondrite-normalization values 

from Haskin and others (1968).

it has slightly higher SiO2 and K^O contents and 

higher K2O:Na2O, U:Th, and RbrSr ratios. The REE 

patterns are also distinctive (fig. 21), exhibiting large 

negative Eu anomalies, relatively flat heavy-REE 

slope, and significant depletion in the light REE with 

increasing SiO2 content.

GRANITOID DIKES, PEGMATITE, AND APLITE

GENERAL FEATURES AND DESCRIPTION

Small granitoid dikes, mainly granodiorite and 

tonalite, intrude all the rocks in the core of the Dunbar 

dome and a narrow fringe of metasedimentary and 

metavolcanic rocks. These dikes range from less than 

1 m to several meters wide. Pegmatite forms sheetlike 

bodies that are as much as hundreds of meters thick, 

mainly in the Dunbar Gneiss in the southwestern part 

of the dome, and aplite-pegmatite composite dikes are 

common. Except for a late garnet-bearing aplite and 

associated pegmatite, all these rocks are foliated, indicating that they were deformed (D2) together with 

their host rocks subsequent to emplacement.
32 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

TABLE 6. Approximate modes and chemical analyses of representative samples ofaplite dikes in the Dunbar Gneiss

[Leaders (--), not determined; Tr, trace; <, less than. Major oxide analyses (W143C) by J.S.

elements by X-ray fluorescence analysis (W143C) by J. Storey, S. Donahey, B. Vaughn, 

and M. Coughlin, and (W495, W679C) by R. Johnson, K. Dennen, and B. Scott; minor

elements by instrumental neutron activation analysis by L.J. Schwarz]

Sample No. ....................... W143C W495 W679C

Plagioclase......... .......... 46 36 24

Quartz.......................... 29 35 27

Potassium feldspar..... 23 27 46 

Biotite .......................... 1.4 0.1 0.8

Muscovite .................... 0 0.7 0.8

Garnet......................... Tr 0.2 1

Zircon........................... Tr Tr 0

Apatite......................... 0 0 0.1

Tourmaline.................. 0 0 0.1

Allnnitfi TV TV ft

Monazite.............. ........ Tr Tr 0

Chlorite........................ Tr Tr Tr

*5firir>itf> TV ft TV

SiO2.............................. 74.2 76.3 72.9

A12O3 ............................ 14.5 13.3 14.4

Fe2O3 ........................... 0 0.13 0.14

FeO .............................. 0.65 0.58 0.74

MgO. ............................ 0.12 0.06 0.06

CaO.............................. 0.43 0.39 0.29

Na2O ............................ 5.21 4.4 3.0

K2O ................... ........... 3.90 4.6 7.8

H2O+ ............. ............... LOI- 0.86 0.67

H2O~.... ........... ............. 0.17 0.14 0.14

TiO2 ............................. <0.02 0.02 0.03

P2O5 ................ ............. <0.05 0.04 0.05

MnO............................. <0.02 0.14 0.23

CO2 .............................. 0.02 0.02

Granitoid dikes are locally common in the Dunbar 

Gneiss. They are mainly light gray or pinkish-gray, 

fine-grained foliated rocks dominantly tonalite or 

granodiorite and generally leucocratic. Plagioclase 

commonly has a weak concentric zoning and welldeveloped albite twinning, and is myrmekitic adjacent 

to potassium feldspar. Where present, potassium feldspar is generally interstitial to plagioclase and quartz. 

A brown biotite is the dominant mafic mineral; it 

is associated with sphene, apatite, and epidoteclinozoisite. Green hornblende is a local mafic mineral. 

Zircon, apatite, and allanite are common accessory 

minerals. 

Pegmatite is abundant in much of the Dunbar 

Gneiss; its volume locally exceeds that of the country 

rock. The pegmatite is white and very coarse grained, 

and consists of bleb-type microcline microperthite, 

quartz, and plagioclase with distinct albite rims. 

Muscovite is an abundant alteration mineral, and tiny 

red garnet crystals are present in some finer grained 

parts. The pegmatite generally has an ill-defined foliaTABLE 6. Approximate modes and chemical analyses of representative samples ofaplite dikes in the Dunbar Gneiss Continued

Sample No. ....................... W143C W495 W679C

Minor elements (parts per million) by X-ray fluorescence analysis

Rb................................. 454 429 666

Sr ................................. 9.75 38 47

................................... 9o o4 do 

Zr ................... .............. 35 39 43

Nb ................................ 60 55 66

Minor elements (parts per million) by instrumental neutron

activation analysis

U... ............................... 7.6 4.9 4.6

Th................................. 6.6 8.6 6.7

Ta................................. 23.2 23.0 16.2

Hf................................. 2.9 2.5 2.6

Cr................................. 28 20 14

Co................................. 0.6 0.4 0.6

Sc ................................. 18.10 3.84 2.79

Zn................................. <6 7 23

Ba................................. 65 68 138

Cs................................. 1.5 2.1 4.9

La................................. 5 15 8

Ce................................. 11 38 21

Nd ........................... .....

Sm. ............................... 3.9 5.3 5.7

Eu ................................ 0.03 0.03 0.15

Gd ................................ 6.7

Tb................................. 1.44 0.88 0.84

Yb................................. 11.1 8.9 3.5

Lu................................. 1.56 1.23 0.46

SAMPLE LOCALITIES

W143C NEV4SEV4 sec. 14, T. 37 N., R. 18 E.

W495 NWViNEVi sec. 15, T. 37 N., R. 18 E.

W679C SWViSW^ sec. 21, T. 37 N., R. 18 E.

tion expressed by highly strained and sutured elongate 

aggregates of quartz. Brown or pink aplite of two 

generations also is common; an older generation is 

deformed, whereas a younger garnet-bearing generation, which is largely confined to the western part of 

the Dunbar Gneiss, is not deformed. Commonly, this 

latter aplite is associated with pegmatite in zoned 

dike-like bodies, but it also occurs as discrete dikes. 

Modes of three selected samples of garnet-bearing 

aplite are given in table 6. The plagioclase is albite, and 

in one section (W143C) the feldspar has a fine-scale 

rapakivi texture. 

Granitoid dikes that cut the Newingham Tonalite 

are medium-gray, locally slightly porphyritic tonalite 

and granodiorite, and presumably are cogenetic with 

the Newingham Tonalite. They are distinctly finer 

grained than the host rock and are foliated. Plagioclase 

has a moderate concentric zoning, and biotite is the 

principal mafic mineral. Locally, sphene is associated 

with the biotite; other accessory minerals are zircon 

and allanite.
ROCK UNITS 33

300

100-

:; io1 -

0.5-

La Ce Nd Sm Eu Gd Tb

RARE EARTH ELEMENTS

Yb Lu

FIGURE 22. Chondrite-normalized rare earth element plots for three 

garnet-bearing aplite dikes. Shaded field shows rare earth 

patterns for Bush Lake Granite. Chondrite-normalization values 

from Haskin and others (1968).

Granite dikes are also common in the Hoskin Lake 

Granite, at least near the south margin of the body. 

They are medium gray or pinkish gray, equigranular, 

and much finer grained than the Hoskin Lake Granite. 

A mode of a typical dike (W509) is given in table 4. The 

granite has elongate aggregates of quartz and the 

biotite is strongly oriented, indicating that it was 

deformed and metamorphosed subsequent to its 

emplacement. Plagioclase occurs as anhedral grains 

that have good albite twinning; myrmekite is 

developed adjacent to potassium feldspar. Potassium 

feldspar is slightly perthitic and includes poikilitic 

quartz and plagioclase. Biotite is reddish brown and 

subhedral, and contains tiny rutile needles. Zircon 

occurs as zoned crystals as much as 0.1 mm long.

GEOCHEMISTRY OF GARNET-BEARING APLTTE DIKES

Garnet-bearing aplite (table 6) has highly evolved 

compositions with high SiO2 and total alkali contents. 

Relative to the granite of the Dunbar dome, the aplite 

is strongly depleted in FeOT, MgO, CaO, TiO2, P2O5, 

MnO, Sr, Zr, Ba, Eu, and light REE, but is enriched in 

SiO2, total alkalies, Y, Ta, Nb, Rb, and the heavy REE. 

The REE patterns are particularly distinctive, with 

flat to positive slopes and very large negative Eu 

anomalies (fig. 22). The aplites also have very low 

ZriHf (<17) and Nb:Ta (<5) ratios. The aplite probably 

represents an evolved differentiate from the Bush Lake 

Granite, as discussed following.

PETROGENESIS OF BUSH LAKE GRANITE AND 

LATE APLITE DIKES

The Bush Lake Granite is compositionally distinct 

from the Hoskin Lake and Spikehorn Creek Granites, 

as discussed previously. Further, examination of the 

compositional characteristics of the Bush Lake Granite 

reveals that it is probably not genetically related to the 

Marinette Quartz Diorite-Hoskin Lake-Spikehorn 

Creek suite. For example, for a similar SiO2 content, 

the Bush Lake Granite has higher KgO, Rb, Th, U, and 

REE contents than the Hoskin Lake and Spikehorn 

Creek Granites as well as a distinctive chondritenormalized REE pattern. Such differing abundances 

are incompatible with the observed compositional 

trends shown by the Hoskin Lake and Spikehorn Creek 

Granites.

On compositional variation diagrams, the garnetbearing aplite dikes plot on extensions of the trends 

defined by the Bush Lake Granite, suggesting that the 

aplites (and related pegmatites) may represent highly 

evolved differentiates of this granite. As shown in 

figure 23, relative to the least evolved Bush Lake 

Granite sample (W411F), the relatively evolved sample 

W411 is enriched in Na, Cr, Rb, Ta, and heavy REE 

(Tb-Lu) and depleted in all other components. Further, 

the figure shows that the aplites are depleted and 

enriched in the same components as W411, but the 

magnitudes of difference are greater than in sample 

W411, the only exception being Sc, which is depleted in 

sample W411 but enriched in the aplites.

Miller and Mittlefehldt (1984) have shown that a 

general similarity exists in compositional trends (that 

is, enrichments and depletions) among extremely 

fractionated felsic magmas. They have also demonstrated that these trends are at least qualitatively like 

those anticipated from fractional crystallization. Alternatively, Hildreth (1981) has argued for noncrystal-
34 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

10

EXPLANATION

Average aplite: W411 F

W411: W411F

Ta

Yb

Rb

Sr

Nb

Zr Ba La Ce Nd Sm Eu

Lu

Hf Th

0.5

0.1

FIGURE 23. Elemental enrichments and depletions in sample W411, the most evolved Bush Lake Granite sample, and in the average aplite 

composition relative to the least evolved Bush Lake Granite sample, W411F. Values plotted are concentration of element in sample W411 

and average aplite divided by the concentration in sample W411F. Note that except for Sc, the average aplite composition has a similar 

enrichment and depletion pattern to that of sample W411.

liquid processes like thermogravitational diffusion to 

control evolution in high SiO2 magmatic systems.

Comparison of the compositional trends between 

samples of the Bush Lake Granite and the garnetbearing aplite dikes in the Dunbar Gneiss with those 

discussed by Miller and Mittlefehldt (1984) for other 

high SiO2 suites confirms the remarkable similarity 

between these trends, including: (1) only slight 

variation of Si and Al, (2) slight increase of Na and 

depletion of K, (3) large depletion of Mg, Ti, Ba, Sr, Zr, 

and light REE, and (4) large enrichments of Rb, Y, and 

heavy REE. As discussed by Miller and Mittlefehldt 

(1984; see also Michael, 1984), this enrichment-anddepletion pattern is compatible with fractionation of 

biotite (Fe, Mg, Ti, Mn, and Ba depletion), potassium 

feldspar (K, Sr, Ba, and Eu depletion), plagioclase (Ca, 

Sr, and Eu depletion), and accessory phases like 

allanite and monazite (light REE and Th depletion).

An interesting compositional characteristic of the 

aplites is their anomalously low Nb:Ta (<5) and Zr:Hf 

(<17) ratios. These low ratios result from the preferential enrichment of Ta to Nb and depletion of Zr to 

Hf (fig. 23). In contrast, typical igneous suites show 

Zr:Hf ratios decreasing from 60 in gabbroic rocks to 

about 40-30 in diorite and granitic rocks, and near 

constant Nb:Ta ratios of about 17-11 (Cerny and 

others, 1985). However, low ratios similar to those of 

these aplites are observed in some other highly 

evolved, often rare-metal granitic pegmatites and 

aplites (Cerny and others, 1985).

The cause of the low ZrrHf and NbrTa ratios remains 

uncertain. The absolute depletion of Zr and Hf in the 

aplites is compatible with fractionation of zircon in a 

F-dominated felsic system, in contrast to an enrichment of these elements observed in Cl-rich systems 

(Hildreth, 1981). It has been suggested that differences 

in the stabilities and mobilities of Zr- and Hf-bearing 

complexes may enhance the effects of crystal-melt 

fractionation in highly evolved granitic systems and 

result in decreasing ZrrHf ratios (Cerny and others, 

1985). The same relationship may apply to Nb and Ta. 

Thus, fluid complexing may also have contributed to
ROCK UNITS 35

the fractionation of some element ratios in the highly 

evolved Bush Lake granite-aplite system.

In conclusion, the Bush Lake Granite and garnetbearing aplites from the Dunbar dome appear to 

represent the products of a fractionated high SiO2 

magmatic system. Compositional trends are generally 

compatible with fractionation of observed crystal 

phases, although late-stage fluid complexing may have 

contributed to enhanced fractionation of Zr:Hf, Nb:Ta, 

and LREEiHREE ratios.

The Bush Lake Granite is compositionally similar to 

many other high SiO2 felsic systems. Hildreth (1981) 

and Thompson and others (1984) stated that graniterhyolite systems having such characteristic extreme 

depletions of Ba, Sr, P, and Ti appear only in continental settings. Examples include the late erogenic 

granites from the Variscan and Caledonian belts of 

England (Watson and others, 1984), the Pikes Peak 

pluton (Colorado) and its associated Redskin cupola 

(Ludington, 1981), and the high- and low-temperature 

parts of the Western United States Bishop Tuff 

(Hildreth, 1979). Following Hildreth (1981) and 

Thompson and others (1984), we conclude that the 

Bush Lake Granite was probably derived largely from 

a crustally derived melt possibly generated and 

modified by basalt additions to the crust.

METASEDIMENTARY ROCKS

Metasedimentary rocks are exposed sparsely on the 

west side of the Dunbar dome and adjacent to and 

northwest of the Bush Lake lobe, in the extreme 

northwestern part of the map area (fig. 2). They were 

included by Button (1971) in the Quinnesec Formation, 

but subsequent mapping, geophysical data, and core 

drilling by a private company show that these strata 

underlie the Quinnesec (Schulz and Sims, 1982). The 

thickness of the succession is not known because of 

folding, but on the west side of the Bush Lake lobe 

proprietary aeromagnetic, electromagnetic, and geologic data suggest a thickness at places of a kilometer 

or more. Metasedimentary rocks seem to be absent or 

at least thinner on the east side of the Bush Lake lobe. 

The belt of metasedimentary rocks extending northnorthwest from the end of the Bush Lake lobe has an 

estimated width of about 0.5 km. We interpret these 

rocks as lying above a north-northwest-trending buried 

extension of the granite in the Bush Lake lobe, because 

of their amphibolite grade and the scattered small 

bodies of granitoid rocks that cut them and the 

adjacent metavolcanic rocks (Button, 1971, pi. 1).

The exposed metasedimentary rocks are mainly 

quartz rich schist, impure marble, calc-silicate rocks, 

and biotite schist. The electromagnetic data and

drilling indicate that a graphitic schist lies along the 

west side of the Bush Lake lobe, near the granite in the 

Bush Lake lobe. It is apparently stratigraphically 

below the exposed metasedimentary rocks.

A large exposure of metasedimentary rocks in the 

SEV* sec. 7, T. 37 N., R. 18 E. consists of interbedded 

calc-silicate rocks and biotite schist that are cut by 

granite pegmatite and aplite, identical to that exposed 

within the western part of the Bunbar dome. On the 

northwest margin of the Bush Lake lobe, a succession 

at least 100 m thick of marble, calc-silicate rocks, and 

thin interbeds of biotite schist and ferruginous quartzite is exposed on a low hill in the NWViSWV^ sec. 11, 

T. 38 N., R. 17 E. The marble has structures suggestive 

of stromatolites. The principal exposures northwest of 

the Bush Lake lobe are in and adjacent to the Popple 

River (Button, 1971, pi. 1). At Popple Rapids, in the 

NWV4 sec. 27, T. 39 N., R. 17 E., a succession at least 

300 m thick of thin- to medium-bedded quartz-rich 

schist is exposed. It is pink and resembles quartzite, 

and has interbedded siliceous biotite schist and calcsilicate rocks. Similar schist occurs in the SWVi of sec. 

27 and in the SWVi of sec. 34, same township. These 

rocks are cut by thin dikes of leucogranite. All the 

observed metasedimentary rocks have assemblages 

characteristic of amphibolite-facies metamorphism 

(Nielsen, 1984; 1985). The presence of stromatolitic 

marble suggests that the metasedimentary succession 

was deposited in shallow water.

The metasedimentary rocks on the west margin of 

the dome resemble in many respects those of the 

Chocolay Group of the Marquette Range Supergroup 

(Early Proterozoic), as exposed in the Menominee iron 

range to the north (Bayley and others, 1966). Probably, 

the rocks compose a tectonic slice of continentalmargin rocks that is interleaved with granitoid rocks of 

the Bunbar dome and volcanic rocks of the Quinnesec 

Formation.

QUINNESEC FORMATION

GENERAL FEATURES

The principal supracrustal rocks in the area are 

assigned to the Quinnesec Formation, a predominantly 

volcanic succession that has thin interbeds of metasedimentary rocks biotite schist, quartz-magnetitehornblende iron-formation, and impure quartzite and 

thick mafic layered sills (Bayley and others, 1966, 

p. 17). The Quinnesec Formation was redescribed by 

James (1958). Earlier the rocks were called "Quinnesec 

schists" (Van Hise and Bayley, 1900) and "Quinnesec 

greenstone" (Leith and others, 1935). Bayley and 

others (1966) considered the metagabbro sills as being 

younger intrusive rocks, but we consider them as
36 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

coeval with the volcanic rocks of the Quinnesec Formation. Schulz (1984b) has subdivided the Quinnesec 

on the east side of the Dunbar dome into four separate 

stratigraphic units; these units are being delineated on 

map compilations of this area. Bayley and others 

(1966) have given a complete description of the 

Quinnesec rocks from the northeastern part of the map 

area (fig. 2), and Button (1971) has described those 

from the northwestern part.

GEOCHEMISTRY

The Quinnesec Formation consists of basalt, andesite, dacite, and rhyolite flows and pyroclastic rocks 

(Schulz, 1984b) that have been altered regionally to 

greenschist-facies mineral assemblages. An older 

succession consists of tholeiitic basalt and basaltic 

andesite. These rocks show limited FeO enrichment, 

low TiO2 , and are relatively depleted in HFS (high field 

strength) elements (Sims and others, 1989). Rare earth 

element patterns show moderate to extreme light-REE 

depletion (fig. 24A). Gabbro sills within the basalt have 

major and trace element characteristics similar to the 

basalt, and probably are consanguineous with it. Plagiorhyolite is locally interlayered with the basalt; it has 

low K^O and REE abundances and a flat REE pattern 

(fig. 24A). The association of plagiorhyolite with 

serpentinite and sheeted dikes within the tholeiitic 

basalt (Schulz, 1987) suggests that these rocks are a 

dismembered ophiolite. The basalts are compositionally similar to some basalts of the Troodos ophiolite 

(Kay and Senechal, 1976) and the Oman ophiolite 

(Pearce and others, 1981), and suggest formation in a 

supra-subduction zone environment.

A calc-alkaline suite, ranging in composition from 

andesite to rhyolite, locally overlies the tholeiitic basalt 

succession. Major and trace element characteristics of 

the suite are similar to normal-K2O calc-alkaline andesite suites. They have moderately enriched light REE 

with relatively flat heavy-REE patterns, and increasingly negative Eu anomalies with increasing SiO2 

content (fig. 24B). The calc-alkaline volcanic rocks plot 

in fields for calc-alkalic subduction-related magma 

suites (Wood, 1980) and are geochemically similar to 

calc-alkaline volcanic suites from oceanic island-arcs 

such as the New Hebrides.

NEWINGHAM TONALTTE

GENERAL FEATURES

The Newingham Tonalite is a homogeneous intrusive body that composes the Pembine lobe and extends 

into the core of the Dunbar dome. Within the core, the

50

10

1

100

50

10

i i i i i r

/Plagiorhyolite

Andesite

B

J__I J_L

La Ce Nd SmEu Tb

RARE EARTH ELEMENTS

Yb Lu

FIGURE 24. Chondrite-normalized rare earth element plots for 

A, basalt and plagiorhyolite, and B, calc-alkaline andesite and 

rhyolite from the Quinnesec Formation. Chondrite-nonnalization 

values from Haskin and others (1968).

tonalite has been largely converted to granodiorite or 

granite by post-crystallization addition of potassium 

feldspar. This hybrid facies of the Newingham, informally called herein the megacrystic facies of the Newingham Tonalite, is discussed separately on page 41.

The Newingham Tonalite forms a large body about 

75 km2 in area that intrudes the volcanic rocks of the 

Quinnesec Formation (fig. 2) with sharp contacts. 

A contact is well exposed in NEV4 sec. 5, T. 36 N., 

R. 20 E., north of U.S. Highway 8. The contact zone is 

at least 100 m wide, and consists of an interlayering of 

tonalite tongues and amphibolite (metabasalt) that 

takes place within a few meters to a few tens of meters, 

and of small anastomosing tonalite dikes that transect 

the layering within the metabasalt. The larger tonalite 

bodies are essentially conformable with the southeastdipping foliation in the metabasalt. Other good exposures of the contact zone are in sees. 3 and 4, T. 36 N., 

R. 19 E. In this area, the Newingham contains moderately abundant, generally angular inclusions of the
ROCK UNITS 37

metavolcanic rocks that retain the orientation of the 

foliation and lineation that was developed during D2 

deformation. (See section, "Structure.") Although the 

Newingham Tonalite was deformed together with the 

Quinnesec during D2 deformation, in part it has a 

distinct, younger, northeast-trending mylonitic foliation. This foliation crosscuts the intrusive contacts.

The Newingham is well exposed in scattered knobs 

and low hills and in road cuts along U.S. Highway 8, 

west of Pembine (Sims and Schulz, 1988). The 

principal foliation in the western part of the intrusion 

is a mylonitic foliation expressed by oriented biotite 

plates and quartz leaves or ribbons. This is superposed 

on, and partly destroys, a weak older foliation (S2) that 

trends N. 45°-55° E., dips steeply southeastward, and 

postdates porphyritic tonalite dikes that intrude the 

main intrusive phase. A weak east- or northwesttrending foliation is present in the eastern part of the 

body. An exception is in sees. 30 and 31, T. 37 N.,R. 20 

E., near Belgian Lake, where a pronounced northwesttrending mylonitic foliation largely obscures the older 

foliation. The age of the mylonitic foliations is 

discussed in a later section.

DESCRIPTION

The Newingham Tonalite is a uniform light-gray, 

medium-grained, slightly porphyritic rock that generally has a good secondary foliation expressed mainly 

by oriented biotite, but locally in the eastern portion 

lacks any penetrative structure. It is dominantly 

tonalite but is granodiorite at places in the eastern 

part of the body (fig. 25). Dikes of slightly porphyritic 

tonalite and, locally, granite pegmatite cut the tonalite. 

Throughout, it is weakly to moderately altered by 

retrogressive metamorphism.

The tonalite has a hypidiomorphic granular texture, 

but at many places this fabric has been modified by 

recrystallization of new biotite and quartz and, locally, 

of polycrystalline aggregates of new plagioclase, 

forming a core and mantle structure or type IP structure. The primary texture dominantly consists of 

plagioclase that occurs mainly as glomero-porphyritic 

aggregates of equant or lathlike normally zoned 

crystals with interstitial quartz.

The rock contains an average of about 55 percent 

plagioclase, 29 percent quartz, 2.5 percent microcline, 

10 percent biotite, and a fraction of a percent of 

hornblende and sphene (table 7). Epidote-clinozoisite, 

muscovite and, locally, chlorite are minor minerals. 

The accessory minerals are apatite, allanite, and 

zircon.

Plagioclase mainly occurs as subhedral, normally 

zoned crystals of calcic oligocase-sodic andesine

EXPLANATION

' Newingham

Tonalite 

3 Megacrystic facies

of Newingham

Tonalite

Quartz

65 90

FIGURE 25. Quartz-alkali feldspar-plagioclase diagram showing 

composition of representative rocks in Newingham Tonalite and 

megacrystic facies of Newingham Tonalite.

composition, but rarely it has oscillatory zoning. 

Typically, it is twinned according to the combined 

carlsbad-albite twin laws. Generally, the zoning was 

not destroyed by deformation and recrystallization. 

New plagioclase grain aggregates occur at places 

adjacent to the primary grain boundaries and within 

zones of high intergranular strain such as isolated 

shears. Quartz is clear or has strong undulatory 

extinction, and mainly has been recrystallized as grain 

aggregates as a result of high strain. Some aggregates 

are elongate and are oriented parallel to the foliation 

expressed by biotite. In least deformed rocks, the 

quartz clearly is interstitial to aggregates of plagioclase grains. Potassium feldspar (microcline) is mainly 

fresh and interstitial to plagioclase and quartz; it 

occurs at places as patches in plagioclase. Grid 

twinning is poorly developed; some grains are microperthite. Typically, the microcline embays and corrodes plagioclase, and adjacent plagioclase has 

myrmekitic intergrowths of quartz. Biotite occurs as 

grayish-red, typically ragged grains that always have 

associated sphene. The sphene commonly occurs as 

beads at biotite grain boundaries. Biotite is generally 

oriented, and in part forms sheaths around plagioclase 

grains, where it clearly has recrystallized. The ubiquitous presence of sphene with biotite suggests that it 

possibly recrystallized in part from alteration of 

original hornblende or an older generation of biotite. 

Epidote-clinozoisite is a common alteration mineral of 

biotite, being most abundant at biotite grain
38 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

TABLE 7. Approximate modes and chemical analyses of representative

[Leaders ( ), not determined; Tr, trace; <, less than. Major oxide analyses (W122A, W134, W132, W152A, W167A, W126B, W129B, W170A) by J.S. Wahlberg, A.J. Bartel, J. Taggart, and 

elements by X-ray fluorescence analysis by J. Storey, S. Donahey, B. Vaughn, and M. Coughlin; samples W122A, W134, and W757 through W723-4 analyzed by Z.E. Peterman. 

analyses (W132, W128, and SPB 357-66) by K. J. Schulz; (W152A, W167A) by J.S. Mee; (SDNE 39-82) by L.J. Schwarz]

W122A W127 W134 W175 W497A W497B W131A W131B W132 W123 W729 W152A W573 W167A W126B W129B

Modal analyses

Plagioclase ................

Quartz.......................

Potassium feldspar .. 

Biotite .......................

Muscovite. .................

Epidote-clinozoisite .. 

Hornblende ...............

57.3 51 

27.4 36.7 

1.3 0 

10.5 11.1 

Tr Tr 

3.5 1.2 

0 0 

Tr Tr

54.2 62.3 

31.5 26.2 

2.6 0 

8.8 8.8 

Tr Tr 

2.9 2.3 

0 0 

Tr Tr

53 53.2 63.3 56.3 

38.7 36 25 29.5 

0 2.4 0.6 5.8 

8.3 8.4 10 6.2 

Tr Tr Tr 1.4 

Tr Tr 0.6 0.8 

00 0.3 0 

Tr Tr 0.2 Tr

45 46 

33 34 

0.1 1 

14 7.5 

1 1 

5 5.5 

0 0 

1.9 5

55.5 

30 

0.5 

10.5 

Tr 

3 

0 

0.5

57 57 

28 30.5 

2 1.5 

11 9 

Tr Tr 

2 1.5 

0 0 

Tr 0.5

57 60 59.5 

25.5 30 28.5 

3.5 0 Tr 

11 9 11 

Tr Tr Tr 

Tr 1 0.5 

300 

Tr Tr 0.5

Major oxides (weight percent)

SiO2 ...........................

A12O3 . ........................

Fe2O3 .........................

FeO............................

MgO ..........................

CaO ...........................

Na2O .........................

KaO ...........................

H20+ ..........................

H20...........................

Ti02 ...........................

P o

MnO ..........................

C02 ............................

67.1 

16.6 

0.80 

1.62 

1.29 

3.68 

3.86 

1.93 

1.10 

0.01 

0.27 

0.10 

0.04 

0.12 

67.8 

16.4 

0.52 

1.98 

1.23 

3.58 

3.93 -- 

2.25 ~ 

0.79 

<0.01 

0.34 

0.10 

0.03 

0.08 

...

68.0 

15.7 

0.51 

2.64 

1.38 

3.53 

3.88 

2.15 

LOI= 

0.51 

0.37 

0.13 

0.04 

 -

69.1 

16.2 

0.57 

1.49 

0.97 

3.34 

4.20 

1.97 

0.72 

0.03 

0.24 

0.09 

0.03 

0.03 

66.8 68.8 67.3 

16.9 16.4 16.9 

0.66 0.60 0.55 

2.04 1.84 2.0 

1.48 1.07 1.30 

3.53 4.06 4.01 

3.99 3.47 4.24 

2.37 1.58 1.54 

0.69 1.07 0.53 

0.05 0.04 0.07 

0.33 0.22 0.32 

0.09 0.08 0.10 

0.03 0.03 0.03 

<0.0 0.22 0.02

Minor elements (parts per million)

Rb..............................

Sr...............................

Y................................

Zr...............................

Nb..............................

29 

732 

5 

120 

17

44

806 

4

114 

6

QK O*J

O J.o -- «-

A K

85 

c

66.9 39.3 

656 744.2 

7

124 

14

51

584 

8 

97 

14

48 

607

14 

128 

17

52

Minor elements (parts per million) by

U................................

Th ..............................

Ta

Hf ..............................

Cr ..............................

Co..............................

Sc...............................

Zn ..............................

Ba..............................

Cs ..............................

La ...

Ce ..............................

Nd..............................

Sm.............................

Eu... ...........................

Gd..............................

Tb ..............................

Yb ..............................

Lu..............................

2.07 

9.36 

...

___

2.24 

7.67 

...

 

...

~

1.8 

6.0 

1.75 

3.4 

42 

6.97 

4.5

961 

6.1 

27.9 

52

17 

2.67 -- 

0.77 

0.26 

0.42 

0.06 

...

1.78 

q ai _

0.71 

2.94 

9 

4.28 

3.85 

47 

450 

1.13 

33.5 

51 

Y]

2.16 

0.61 

1.9 

0.44 

0.06 

0.8 1.55 1.52 

6.41 5.01 6.19 

1.12 

g 44 _

16 

7.11 

5.23 

55 

797

1 4/1

39.6 

64.8 

22 

3.11 

0.84 

2.6 

0.44 

0.06 

1Sample descriptions and localities appear on p. 40.
ROCK UNITS 39

samples ofNewingham Tonalite and associated dikes1

J. Baker, (W128, SPB 357-66) by Z.A. Hamlin, (SDNE 39-82) by Z.A. Brown and F.W. Brown. FeO, H2O, and CO2 by H. Neiman, J. Ryder, and R. Somers. Minor 

Sample W498A analyzed by R. Yeoman. Rb and Sr analyses for samples W132, W123, W170, and W739 by isotope dilution method by K. Futa. Neutron activation

W128 ~fv Ec(. ?DI1? W170A W170 W730 W817

OO 1 OO OS OA

(volume percent)

58 50 58 54 

27 26 29.5 32 

5.5 614 

7.5 9.5 9 9 

... Tr 3 Tr Tr 

 Tr 41 0.5 

0 100 

Tr 0.5 1.5 0.5

by X-ray fluorescence analysis

££ £ 71 7 7A 3 £7 Q

18.0 16.0 16.1 16.4 

1.1 0.65 0.85 0.66 

2.4 1.4 1.3 1.88 

1.6 0.96 0.93 1.24 

A K o a Q a o CA

4.1 4.8 4.0 4.02 

1.7 1.4 1.6 2.16 

0.82 0.87 1.1 0.91 

010 Oil 014 0 04

n 37 n 03 n 03 n 30

0.18 0.14 0.09 0.11 

0.03 0.02 0.04 0.03 

0.05 0.47 0.02 0.01 

by X-ray fluorescence analysis

54 38 42 43 42.2 

724 604 630 620 662.8 

9 10 10 7 

100 130 103 114 

8 7 12 10 

instrumental neutron activation analysis

n QR o o 13 i oo

2.6 5.91 4.5 7.44 

0 44 0^7 0 Q3

2.0 4.0 2.4

14 11 5.5 

8.77 4.71 4.4 

6.74 4.0 3.72 

54

570 640 620 

0.81 0.93 1.2 

12.5 22.9 20 

99 R 40 0 31

79 13 11

1 97 9 94 18

0 44 0 ^3 0 4Q

0.11 0.16 0.14 

018 0 9Q

0.03 0.05 

W757 W739 W765B W680A W498A W474

57 58.5 58 54 57 58.5 

30 27 29 28 23.5 28 

2 2.5 Tr 0 0 0.5 

9 10.5 11 14 18 13 

Tr Tr 0.5 0 Tr Tr 

1 1.5 0.5 Tr 1 Tr 

000 000 

1 Tr 1 4 0.5 Tr

_ _ _ _ _ _

 

29 75.3 64 26 75 60 

743 634.5 623 884 604 772 

553 564 

97 107 96 121 148 135 

12 13 10 11 14 8

... ... ... ... 704

... ...

W466

67 

18 

0.5 

13 

0 

Tr 

0.5 

1

/2O Q

1Q 7

0.19 

2.27 

1.01

A CO

4.89 

1.50 

0.52 

0.02

n 39

0.06

0.03

0.02

57 

928 

10 

159 

13

1.07

16.4

...

...

W742S W717

59 53.5 

30.5 32 

1.5 4 

9 10 

0 Tr 

0 Tr 

0 0 

Tr 0.5

 - 

...

32 41 

658 338 

2 2 

56 92

8 7

...

W725B

62.5 

23 

3.5 

10 

Tr 

Tr 

0 

1.0

...

_

45 

702 

6 

171 

19

...

...

W723-4

55 

27 

5.5 

12 

Tr 

Tr

0.5

 

...

63 

606 

7 

112 

23

...
40 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

SAMPLE DESCRIPTIONS AND LOCALITIES

W122A Cut on U.S. Highway 8 in NWV4 sec. 5, T. 36 N., R. 20 E.

Rock has two conspicuous foliations expressed by biotite

laths.

W127 SWViSWVi sec. 35, T. 37 N., R. 19 E. 

W134 South-center sec. 22, T. 37 N., R. 19 E. Plagioclase has

microcline patches and quartz blebs.

W175 Rock knob in SWVi sec. 20, T. 37 N., R. 20 E. 

W497A Low outcrop east of Walton road in NWVi sec. 24, T. 37 N.,

R. 19 E. 

W497B Same locality as W497A. Dike that cuts rock sample

W497A. 

W131A NEViSEVi sec. 34, T. 37 N., R. 19 E. Medium-grained

tonalite; contains cataclastic zones.

W131B Same locality as W131A. Fine-grained tonalite. 

W132 Rock cut on Threemile road in SWViSWVi sec. 26,

T. 37 N., R. 19 E. 

W123 Rock knob on north side of U.S. Highway 8 in NEViNEVi

sec. 6, T. 37 N., R. 20 E.

W729 Knob, 0.16 km north-northwest of W730. 

W152A Rock knob on south side of Marinette County Highway 00

in NWViSWVi sec. 33, T. 37 N., R. 20 E. 

W573 Rock knob on east side of Chicago, Milwaukee, St. Paul,

and Pacific Railway in SWVi sec. 22, T. 37 N., R. 20 E. 

W167A SEViSEVi sec. 14, T. 38 N., R. 20 E. Pinkish-gray, weakly

foliated tonalite. 

W126B Rock cut on U.S. Highway 8 in NWViNWVi sec. 2,

T. 36 N., R. 19 E. Fine-grained, possible dike. 

W129B NEViNWVi sec. 3, T. 36 N., R. 19 E. Fine-grained tonalite

dike that cuts tonalite.

W128 SWViSEVi sec. 34, T. 37 N., R. 19 E. 

SPB 357-66 Railway cut, NWViNE1/4 sec. 27, T. 37 N., R. 20 E.

Porphyritic tonalite. 

SDNE 39-82 Road cut, U.S. Highway 8, NW1/4NWVi sec. 2, T. 36 N.,

R. 19 E. Porphyritic tonalite dike. 

W170A NE'/4 sec. 26, T. 37 N., R. 20 E. 

W170 Same as W170A. 

W730 NWViNEVi sec. 19, T. 37 N., R. 20 E. 

W817 Rock knob on southwest side of lake in SWViSEVi sec. 30,

T. 37 N., R. 20 E.

W757 SW1/4SE1/4 sec. 16, T. 37 N., R. 20 E. 

W739 SEViSWVi sec. 13, T. 37 N., R. 19 E. Intrudes Quinnesec

Formation. 

W765B NE 1/4NW1/4 sec. 23, T. 37 N., R. 19 E. Intrudes Quinnesec

Formation.

W680A SEV4NEK sec. 16, T. 37 N., R. 20 E. 

W498A SE1/4SW1/4SW1/4 sec. 13, T. 37 N., R. 19 E. Intrudes

Quinnesec Formation. 

W474 SWViSEVi sec. 34, T. 37 N., R. 18 E. Apparent isolated

body.

W466 SEViSEVi sec. 9, T. 37 N., R. 19 E. Granoblastic texture. 

W742S SW1/4NW1/4 sec. 11, T. 37 N., R. 19 E. 

W717 SEViNWVi sec. 7, T. 37 N., R. 20 E. Contains amphibolite

inclusions.

W725B SWV4NWV4 sec. 6, T. 37 N., R. 20 E. 

W723-4 NWViSWVi sec. 12, T. 37 N., R. 19 E.

boundaries. Hornblende is a rare primary mineral in 

the body. Where enclosed by quartz, it is fresh, but in 

other associations it is partly altered to biotite and 

epidote-clinozoisite.

500

100

50

10

La Ce Nd Sm Eu Gd Tb

RARE EARTH ELEMENTS

Yb Lu

FIGURE 26. Chondrite-normalized rare earth element plots for 

samples of Newingham Tonalite. Chondrite-normalization values 

from Haskin and others (1968).

Mild retrogressive metamorphism is ubiquitous. The 

cores of plagioclase grains are weakly to moderately 

altered to epidote-clinozoisite, muscovite, rare calcite, 

and possibly, zeolite minerals. Biotite is partly altered 

to epidote-clinozoisite, zeolite(?), and locally to green 

chlorite; at places it alters directly to muscovite plus 

quartz. The zeolite(?) occurs as small lenses along 

biotite cleavages. As noted above, the ubiquitous 

association of sphene with biotite is interpreted also to 

indicate retrogressive metamorphism. Allanite nearly 

always has epidote-clinozoisite rims.

GEOCHEMISTRY

The Newingham Tonalite is calcic in character 

(fig. 5) and relatively homogeneous (table 7). Two 

samples from a slightly porphyritic phase of the 

tonalite have higher SiO2 contents and are trondhjemitic. All samples have steep, near-linear REE 

patterns with slight positive or no Eu anomaly (fig. 26).
ROCK UNITS 41

Samples show a positive correlation between increasing Rb:Sr, increasing total REE abundance, and 

decreasing magnitude of the Eu anomaly that suggests 

fractionation of feldspar. Relative to the Dunbar 

Gneiss, the Newingham Tonalite has higher A12O3, 

CaO, Na2O, and Sr contents, lower Rb:Sr ratio, lower 

REE contents, and much lower abundances of Ta, Nb, 

Zr, Hf, U, Th, Rb, and

PETROGENESIS

The Newingham Tonalite has the compositional 

characteristics of the high-Al2O3 type trondhjemitictonalitic suite as defined by Arth (1979) and Barker 

and Arth (1976). These characteristics include high 

A12O3 (>15 percent) and Sr content, low Rb content, 

and moderately enriched light REE but depleted 

heavy-REE patterns with either no or positive Eu 

anomalies.

Barker and Arth (1976) have attributed petrogenesis 

of high- Al trondhjemitic-tonalitic magmas to fractional 

crystallization of wet basalt parent melts or to partial 

melting of a basaltic parent leaving a residue with 

little or no plagioclase but residual amphibole and (or) 

garnet. Gromet and Silver (1987) have also suggested 

melting of eclogite for the petrogenesis of tonalites 

from the Peninsular Range batholith of southern 

California and the Baja Peninsula. For the Newingham Tonalite, the partial melting model is preferred 

because of the relatively uniform composition of the 

tonalite and the absence of more mafic lithologies with 

suitable compositions to represent more primitive 

melts. The near-linear chondrite-normalized REE 

patterns with strongly depleted heavy REE and 

relatively enriched light REE (fig. 26) suggest that the 

source for the Newingham Tonalite magma contained 

garnet as a residual phase (for example, Hanson, 1981) 

and was not light REE depleted. However, it should be 

noted that the basaltic source required for the Newingham Tonalite is compositionally distinct from that 

required for the Dunbar Gneiss. Specifically, the Newingham Tonalite lacks strong "within-plate" source 

characteristics and must have been derived from a 

relatively HFS-element-depleted basaltic source. The 

calc-alkaline basalts of the Wisconsin magmatic terranes, with flat to slightly enriched light-REE and 

depleted HFS element compositions, could have been 

the source for the melt that formed the Newingham 

Tonalite.

MEGACRYSTIC FACIES OF NEWINGHAM TONAIITE

GENERAL FEATURES AND DESCRIPTION

The megacrystic facies of the Newingham is a hybrid 

rock of granodiorite or granite composition (fig. 25)

that has resulted from partial replacement of the 

primary tonalite by post-crystallization potassium 

feldspar. The facies is delineated separately from the 

unaltered Newingham Tonalite (fig. 2), but it contains 

relicts of virtually unaltered tonalite, as for example 

samples W717, W725B, and W723-4 (table 7). In 

addition, the megacrystic facies rocks contain inclusions of amphibolite and biotite schist, derived from 

the Quinnesec Formation, and tongues of Marinette 

Quartz Diorite, which presumably cut the megacrystic 

facies. The contact between the Newingham Tonalite 

and the megacrystic facies is gradational (fig. 2), as is 

to be expected from the nature of the replacement 

mineralogy in the megacrystic facies.

The megacrystic facies of the Newingham is a heterogeneous, medium- to dark-gray, generally medium 

grained igneous rock that commonly contains porphyroclasts of plagioclase as much as 1 cm in diameter, 

moderate amounts of biotite, and variable, but appreciable amounts of potassium feldspar. It has a 

pervasive, wavy, mylonitic foliation expressed mainly 

by oriented biotite but also by elongate and flattened 

quartz aggregates. It also forms the leucosome of 

migmatite in contact zones with biotite gneisses. A 

light-gray or pink, medium- to coarse-grained, equigranular leucogranite is associated with the unit 

locally.

The rock ranges in composition from tonalite to 

granite (fig. 25). It has a xenomorphic granular texture 

modified by later partial recrystallization to finer 

grained mineral aggregates. Except for the plagioclase 

phenocrysts(?), probably all the major minerals were 

recrystallized during the deformation that imposed the 

northeast foliation. The plagioclase phenocrysts have 

been partly modified by recrystallization and embayment and replacement by potassium feldspar.

Plagioclase is the dominant mineral, composing 

between 30 and 50 percent of the rock. It has a bimodal 

texture consisting of (1) relict, mainly anhedral phenocrysts(?) as much as 1 cm in diameter and having weak 

normal zoning, and (2) smaller (generally less than 0.5 

mm in diameter) recrystallized anhedral plagioclase 

crystals that generally are unzoned. The larger grains 

typically have combined carlsbad-albite twins and, less 

commonly, pericline twins. The overall texture 

expressed by the plagioclase is a core-mantle structure 

or type IP structure. The plagioclase has a composition 

of calcic oligoclase, generally An.25_30. Patches of microcline are common in the plagioclase, and where in 

contact with potassium feldspar it has a fringe of 

myrmekite. The plagioclase typically is weakly altered 

to epidote-clinozoisite, sericite, and possibly, zeolite 

minerals. Quartz generally composes 20-30 percent of 

the rock; it is highly strained and generally recrystallized to aggregates of sutured grains. Biotite is
42 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

TABLE 8. Approximate modes and minor element analyses of selected samples of megacrystic fades of

Newingham Tonalite

[Leaders ( ), not determined; Tr, trace. Minor element analyses of rock slabs by Z.E. Peterman]

Sample No. ..................... W504A W504 W726S W677 W715 W719 W743S W566 W724 W499B W760A W465A

Modal analyses (volume percent)

Plagioclase.................

Quartz........................

Potassium feldspar ... 

Biotite ........................

Apatite..... ............... ...

Allanite ......................

Accessory minerals ...

43 

21.5 

26.5 

9 

0 

Tr 

Tr 

Tr 

Tr 

Tr

33 

40 

17 

7 

0 

0.5 

Tr 

0 

Tr 

2.5

30.5 

25.5 

35 

7.5 

0 

0.5 

Tr 

0 

Tr 

1.0

50 50.5 

30 30 

9.5 8 

10.5 10 

0 0 

Tr 1 

Tr Tr 

Tr Tr 

Tr Tr 

Tr 0.5

37.5 

28 

27.5 

7 

0 

Tr 

Tr 

0 

Tr 

Tr

50 

28.5 

12.5 

9 

0 

Tr 

Tr 

0 

Tr 

Tr

52.5 

25 

13 

9 

0 

0.3 

Tr 

0 

Tr 

0.2

48 

31 

10.5 

10 

0 

Tr 

Tr 

Tr 

Tr 

0.5

49 

26 

16 

9 

0 

Tr 

Tr 

0 

Tr 

Tr

46 

26.5 

21 

6.5 

0 

0 

Tr 

0 

Tr 

Tr

34.5 

30.5 

25.5 

9 

0 

Tr 

Tr 

0 

0 

0.5

Minor elements (parts per million) by X-ray fluorescence analysis

Rb...............................

Sr................................

Y.................................

Zr................................

Nb...............................

95 

262 

2 

81 

13

95 

289 

9 

101 

21

71 

478 

7 

104 

11

58 56 

498 496 

8 5 

146 104 

21 13

103 

344 

5 

71 

16

85 

343 

9 

145 

22

85 

395 

5 

88 

20

83 

434 

7 

138 

25

68 

648 

2 

93 

8

SAMPLE DESCRIPTIONS AND LOCALITIES

W504A SEWSEVi sec. 2, T. 37 N., R. 19 E. Foliated W743S 

granitic rock that contains mafic inclusions. W566

W504 Same locality as W504A. Foliated granitic rock. W724

W726S NE'ANE'A sec. 13, T. 37 N., R. 19 E. Contains W499B 

mylonitic shears.

W677 NWy4NE'/4 sec. 10, T. 37 N., R. 19 E. W760A 

Protomylonite. W465A

W715 SW/4NEV4 sec. 7, T. 37 N., R. 20 E. Protomylonite.

W719 SE'/4NWV4 sec. 7, T. 37 N., R. 20 E. Protomylonite.

sec. 10, T. 37 N., R. 19 E. Protomylonite.

c. 22, T. 37 N., R. 19 E. 

SWJ/4SWy4 sec. 6, T. 37 N., R. 20 E. 

SEy4SEy4 sec. 12, T. 37 N., R. 19 E. Contains 

inclusions of amphibolite and biotite schist, 

sec. 23, T. 37 N., R. 19 E.

sec. 20, T. 37 N., R. 19 E. Granoblastic 

texture.

moderate brown to reddish brown and fresh or weakly 

altered to epidote-clinozoisite, chlorite, and zeolite(?) 

minerals. The minerals believed to be zeolite occur as 

rare barrel-shaped, fibrous masses along cleavage 

planes in the biotite. Typically, the biotite has two 

intersecting orientations in a single thin section, one of 

which is expressed by biotite oriented along plagioclase 

grain margins or along plagioclase-quartz interfaces. It 

is associated with minor amounts of sphene, opaque 

oxides, apatite, allanite, and epidote-clinozoisite; some 

of the sphene occurs as small beads along biotite grain 

margins. Potassium feldspar ranges in amount from 

about 5 to 35 percent, and is mainly grid-twinned 

microcline and microcline microperthite. It is irregularly distributed, occurring mainly as interstitial 

grains intergrown with recrystallized plagioclase, 

quartz, and biotite but also as poikiloblasts as much as 

5 mm in diameter containing inclusions of plagioclase, 

myrmekite, quartz, and biotite. Some inclusions of 

plagioclase have albitic rims. The larger grains embay 

and replace plagioclase. Accessory minerals are 

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

Zircon is a moderately abundant accessory, and it 

commonly forms acicular, zoned crystals.

GEOCHEMISTRY AND PETROGENESIS

The megacrystic facies differs chemically from the 

unaltered Newingham Tonalite in having higher SiO2, 

K2O, and Nb contents and Rb:Sr ratios, and lower 

A12O3 , FeOT, MgO, CaO, and Na2O (compare table 7 

with tables 8 and 9), reflecting its more granitic modal 

composition (fig. 25).

The potassium metasomatism indicated by replacement textures of potassium feldspar in the megacrystic 

rocks apparently followed a late, post-crystallization 

ductile deformation, which produced type IP structures. These mylonitic textures could have provided 

relatively permeable channelways for circulation of 

potassic solutions. The restriction of the potassium 

metasomatism to the northern part of the Newingham 

Tonalite body suggests a northerly source for the 

circulating solutions, perhaps the Hoskin Lake Granite 

or its equivalent, which extensively hybridized the 

adjacent Marinette Quartz Diorite, as discussed 

earlier. That a source like the Hoskin Lake Granite 

probably caused the potassium metasomatism is 

suggested by a plot of Nb versus Rb for the Newingham 

Tonalite, the Hoskin Lake Granite and related dikes,
ROCK UNITS 43

TABLE 9. Approximate modes and chemical analyses of

selected samples of megacrystic fades ofNewingham

Tonalite

[Leaders ( ), not determined; Tr, trace; <, less than. Major oxide analyses by J.S. 

Wahlberg, A. Bartel, J. Taggart, and J. Baker; FeO, H20, and C02 analyses by 

H. Neiman, J. Ryder, and G. Mason; minor elements by X-ray fluorescence 

analysis by J. Storey, S. Donahey, B. Vaughn, and M. Coughlin]

Sample No......................................................... W503A W465-1B

Modal analyses (volume percent)

Plagioclase............................................... 30 38

Quartz...................................................... 30 36

Potassium feldspar................................. 34 19.5

Biotite...................................................... 5.5 6.5

Sphene..................................................... 0.5 Tr

Opaque oxides......................................... Tr 0

Accessory minerals................................. Tr Tr

Major oxides (weight percent) by X-ray fluorescence analysis

SiO2 .......................................................... 72.9 71.5

A12O3 ........................................................ 14.1 14.6

Fe2O3 ........................................................ 0.34

FeO........................................................... 1.42 1.41

MgO......................................................... 0.62 0.81

CaO.......................................................... 1.55 1.47

Na2O........................................................ 3.16 3.52

K2O.......................................................... 4.25 3.73

H2O+ ......................................................... 0.34 0.71

H2O"......................................................... 0.03 0.06

TiO2 .......................................................... 0.22 0.15

P2O5 ......................................................... <0.05 <0.05

MnO......................................................... 0.02 0.04

CO2 ........................................................... <0.01 <0.01

Minor elements (parts per million) by X-ray fluorescence 

analysis

U............................................................... 1.03 1.9

Th............................................................. 6.98 3.8

Rb............................................................. 99 113

Sr.............................................................. 350 312

Y............................................................... 6 7

Zr.............................................................. 60 47

Nb............................................................. 9_____20

and the megacrystic facies of the Newingham (fig. 27). 

With respect to the Newingham Tonalite, the megacrystic facies is enriched in both Nb and Rb, and these 

values fall between the fields for the Newingham and 

the Hoskin Lake Granite and the dikes, strongly 

suggesting their introduction from the granite or a 

similar magmatic source. In the same way, a plot of Rb 

versus Sr for the Newingham and the megacrystic 

facies (fig. 28) shows a higher Rb:Sr ratio for the 

megacrystic facies that is intermediate to ratios in the 

Newingham and the Hoskin Lake Granite and granitic 

dikes. These data suggest that both Rb and Nb were 

introduced together with potassium from the Hoskin 

Lake Granite or a comparable magmatic source. This 

interpretation is consistent with the previous conclusion that potassic metasomatism from the Hoskin Lake

Granite or a similar buried granite also modified the 

adjacent Marinette Quartz Diorite body.

TWELVE FOOT FALLS QUARTZ DIORITE

The Twelve Foot Falls Quartz Diorite, named by 

Cain (1964), is a poorly exposed body on the south side 

of the Dunbar dome that occupies an area of about 80 

km2. It intrudes and locally contains inclusions of 

metavolcanic rocks of the Quinnesec Formation. The 

quartz diorite is massive in the eastern part of the 

main body, but becomes foliated toward the west. The 

foliation strikes west-northwest and dips steeply. At 

the type locality at Twelvefoot Falls on the North 

Branch of the Pike River (NWY4 sec. 32, T. 36 N., R. 19 

E.), the quartz diorite has been intensely sheared, has 

a pronounced foliation, and is mainly a mylonitic 

gneiss. This zone of mylonitic gneiss extends northward at least to Eighteenfoot Falls, a distance of 0.8 

km. Foliation within the gneiss in the shear zone 

strikes N. 70° W. and dips steeply to either north or 

south; a steep (stretching) lineation occurs in the plane 

of the foliation. The gneiss within the zone is strongly 

altered to retrograde assemblages; hornblende is 

partly to completely altered to epidote and chlorite, 

and plagioclase is saussuritized. Quartz is extremely 

comminuted. The mylonitic rocks lack any primary 

mafic mineral and are fine-grained aggregates of 

plagioclase, quartz, epidote, sericite, and chlorite. 

Garnet and opaque oxides are local accessory minerals. 

The shear zone partly coincides with a northwesttrending positive magnetic anomaly, apparently 

because of a high content of magnetite developed in the 

quartz diorite in the shear zone.

Outside the shear zone, the quartz diorite is medium 

to coarse grained and is characterized by subhedral 

plagioclase (sodic andesine) crystals as much as 1 cm 

long, smaller subhedral hornblende crystals, in part 

pseudomorphic after pyroxene, and anhedral crystals 

of blue quartz as much as 1 cm in diameter. Microcline 

locally occurs as a late interstitial mineral. The primary texture is hypidiomorphic granular, but finer 

grained secondary textures are superposed on it at 

many places. Characteristically, the rock is considerably retrograded: plagioclase is partly to largely 

altered to epidote and albite, and hornblende is partly 

altered to biotite, epidote, and chlorite. Other alteration minerals are sphene, opaque oxides, and calcite. 

In some sections, quartz is a late interstitial mineral 

that has sharp crystal faces against plagioclase and 

hornblende, suggestive of a granophyric texture.

Its chemical composition, especially its low Ta and 

Nb concentrations (table 10) and REE pattern (fig. 29), 

suggests that the Twelve Foot Falls Quartz Diorite is a 

subvolcanic intrusion comagmatic with calc-alkaline 

rocks of the Quinnesec Formation.
44 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

60

50

40

30

20

EXPLANATION

A Newingham Tonalite

 Megacrystic fades of 

Newingham Tonalite

 * Hoskin Lake Granite 

D Dikes in supracrustal rocks

-AA

i Newingham 

Tonalite t

A

\ A 

\ A

A , A A

1 * /

A A ^/

A __ '

30 50 70 90 110 130 

Rb, IN PARTS PER MILLION

150 170 190 210

FIGURE 27. Plot of Nb versus Rb contents for Newingham Tonalite, megacrystic facies of Newingham Tonalite, and Hoskin Lake Granite and

related dikes.

GRANITOID DIKES IN SUPRACRUSTAL ROCKS

In addition to the Marinette Quartz Diorite, Newingham Tonalite, and Spikehorn Creek Granite, which are 

known to intrude the supracrustal rocks in their 

contact zones, granitic dikes of uncertain affinity are 

common in the volcanic and sedimentary rocks adjacent to the crystalline core. On the north side of the 

dome, southwest of Niagara (fig. 2), granitic dikes 

extend outward from the boundary for a distance of at 

least 0.3 km, and on the northwest margin of the Bush 

Lake lobe for a distance of about 0.8 km. Elsewhere, 

the dike-bearing zone in the supracrustal rocks is 

narrower. In general, the dikes increase in abundance 

as the boundary is approached. The dikes are as much 

as 30 m wide, and generally are discordant. Button 

(1971, pi. 1) has shown that granitic dikes occur 

sporadically in the Quimiesec Formation and rnetasedimentary rocks northwest of the northwest margin 

of the Bush Lake lobe; the dikes are known to be 

present as much as 5 km from the dome margin, and 

they probably extend even farther from it.

Pegmatite is rare in the supracrustal rocks, but a 

body about 85 m wide crosscuts metasedimentary 

rocks at a small angle adjacent to Macintire Creek

(sec. 7, T. 38 N., R. 18 E.) on the west margin of the dome. 

The pegmatite and associated aplite are megascopically identical to that cutting the Dunbar Gneiss in the 

vicinity of Dunbar.

The dikes are pinkish-gray, fine- to medium-grained, 

partly inequigranular, leucocratic rocks that range in 

composition from granite to tonalite (table 11). They 

are mostly metamorphosed and deformed, and 

generally have a xenomorphic granular texture and a 

superposed mylonitic texture.

The granitic dikes on the north margin of the Dunbar dome are in part inequigranular; they contain 

poikilitic microcline grains as much as 1 cm long in a 

finer grained matrix of plagioclase, microcline, biotite, 

and rarely, green hornblende. Microcline is late paragenetically; it embays and replaces plagioclase and not 

uncommonly contains inclusions of plagioclase having 

albite rims and myrmekitic intergrowths of quartz. 

Where subjected to strong ductile deformation, patch 

antiperthite and bleb microperthite are developed. 

Biotite was recrystallized in deformation zones. 

Accessory minerals are rare; those that occur include 

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

mild retrogressive metamorphism was superposed on
ROCK UNITS 45

iUU

180

160

140

z

0

^ 120

cc

LU 

Q_

g<

^ 100

1

80

60

40

20 

AD |DD*| I I I I I I

 \ D \^ Hoskin Lake

\ <\ Granite

i i

v EXPLANATION

\ i A Newingham Tonalite

\ Megacrystic facies of 

 ^ 1 Newingham Tonalite 

^ 1 ° Hoskin Lake Granite

D Dikes in supracrustal

rocks

X

_ 

 V '

1 

 

^~ -~ ~~~ "A "~~" ft.

/ * """' "^-/

\A A A / 

\ AA A

-

-

/ Newingham

/ Tonalite

^A^ _

\ A A ^\

\ A

V AA A \» A A

V A A A A 

\_ ^A A"

i i i i i i i

0 100 200 300 400 500 600 700

(

. A 4 _

AA '

/

1 1

800 900 1,0

Sr, IN PARTS PER MILLION

FIGURE 28. Plot of Rb versus Sr contents for Newingham Tonalite, megacrystic facies of Newingham Tonalite, and Hoskin Lake Granite and 

related dikes. Note that as in figure 27, values for the megacrystic facies are intermediate between those for the Newingham Tonalite 

and the Hoskin Lake Granite and related dikes. Arrows associated with dike symbols show that values exceed scale given.

the primary textures. Rare hornblende is partly 

altered to epidote, and plagioclase is altered to unidentified low-birefringence minerals.

On the southwest margin of the Dunbar dome, 

garnet is a local component of the tonalitic (aplite) 

dikes, and muscovite and sericite are common alteration minerals. Albite twinning in plagioclase is 

wedgelike. Possibly the aplite is that described previously from the Dunbar area.

In the reentrant between the Niagara and Pembine 

lobes, dikes in the metavolcanic rocks are tonalite in

composition and contain plagioclase with oscillatory 

zoning, indicating a definite magmatic origin. The 

dikes have been deformed and now are mainly protomylonite. The composition and porphyritic texture of 

the dikes make them most closely resemble the Newingham Tonalite.

The granitoid dikes occur in the inner margin of the 

supracrustal rocks, adjacent to the core, in rocks metamorphosed to amphibolite facies. The greater abundance of dikes on the north margin of the dome, and 

their dominant granite composition, support the
46 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

TABLE 10. Chemical analysis of Twelve Foot Falls 

Quartz Diorite

[M^jor oxide analysis by rapid rock ICP by ZA. Brown and F.W. 

Brown; minor elements by X-ray fluorescence analysis by R. 

Johnson, K. Dennen, and B. Scott; minor elements by instrumental 

neutron activation analysis by L.J. Schwarz]

Sample No................................................................................. STF-4-1

Major oxides (weight percent) by X-ray fluorescence analysis

SiO2 ................................................................... 56.9

ALjOs................................................................. 15.5

Fe2O3 ................................................................. 4.2

FeO.................................................................... 5.1

MgO.................................................................. 4.1

CaO................................................................... 7.7

Na2O................................................................. 2.3

K2O................................................................... 0.84

H2O+.................................................................. 1.8

H2O................................................................... 0.15

TiO2 ................................................................... 0.44

P2O5 .................................................................. 0.16

MnO.................................................................. 0.20

CO2 ................................................................... 0.17

Minor elements (parts per million) by X-ray fluorescence analysis

Rb...................................................................... 21

Sr....................................................................... 475

Y........................................................................ 15

Zr....................................................................... 57

Nb...................................................................... 6

Minor elements (parts per million) by instrumental neutron 

activation analysis

U........................................................................ 0.3

Th...................................................................... 2.0

Ta...................................................................... 0.09

Hf...................................................................... 1.3

Cr....................................................................... 18

Co...................................................................... 22.2

Sc....................................................................... 33.6

Zn...................................................................... 83

Ba...................................................................... 258

Cs...................................................................... 0.8

La...................................................................... 18

Ce...................................................................... 36

Nd...................................................................... 19

Sm..................................................................... 3.8

Eu...................................................................... 0.94

Gd...................................................................... 2.8

Tb...................................................................... 0.37

Yb...................................................................... 1.6

Lu...................................................................... 0.28

conclusion based on the width of the amphibolite-facies 

zone and the intensity of metamorphism here that this 

margin was the hotter, perhaps deeper, part of the 

dome. Diopsidic pyroxene developed during culmination of the metamorphism in the andesine amphibolite 

facies zone, adjacent to the core.

As a group, the dikes are characterized by high Rb:Sr 

ratios, low Zr and Y contents, and variable Nb con100

50-

10

EXPLANATION

 Sample STF-4-1 (table 10) 

X Sample W7 42 (table 1)

Field for Quinnesec 

calc-alkaline andesite

La Ce Nd Sm Eu Gd Tb

RARE EARTH ELEMENTS

Yb Lu

FIGUBE 29. Chondrite-normalized rare earth element plot for a 

sample of Twelve Foot Falls Quartz Diorite. Also shown is sample 

W742 from the Dunbar Gneiss and the field for Quinnesec andesite. Note that sample W742 has a rare earth element pattern very 

similar to that of the Twelve Foot Falls Quartz Diorite. Chondritenormalization values from Haskin and others (1968).

tents. Sample W412C is modally and compositionallylike other dikes around the Bush Lake Granite (table 6, 

sample W679C) and is probably related to them. The 

remaining dikes listed in table 11, all of which intrude 

the Quinnesec Formation, are similar both modally 

and compositionally to the Hoskin Lake Granite and 

are probably related to it.

DIABASE DIKES

Two diabase dikes are associated spatially with the 

dome (fig. 2), one of which is partly exposed. The 

diabase is presumed to be Middle Proterozoic in age 

(Sims and Schulz, 1988), correlative with 1,100 Ma 

Keweenawan magmatism in the Midcontinent rift 

system, but this is uncertain because of the diabase's 

retrograde alteration. A diabase dike that cuts the 

Newingham Tonalite has an ophitic texture and is 

moderately altered to greenschist-facies mineral 

assemblages. The mafic minerals are now actinolite, 

chlorite, and opaque oxides. The opaques compose as 

much as 15 percent of the altered rock, and account for 

the moderate positive magnetic anomaly over the dike.

The compositions of a chilled-margin and interior 

sample of the dike cutting the Newingham Tonalite are 

given in table 12. The dike is a high-iron, quartznormative tholeiite with a light-REE enriched pattern 

and is similar compositionally to some Keweenawan 

dikes in the region (Green and others, 1988).
TECTONIC IMPLICATIONS OF GRANITOID ROCK GEOCHEMISTRY 47

TABLE 11. Approximate modes and minor element analyses of selected samples of granitoid

dikes in the supracrustal rocks

[Leaders ( ), not determined; Tr, trace. Minor elements by X-ray fluorescence analysis by Z.E. Peterman]

W477B W569B W569-1 W412C W490A W660A

Modal analyses (volume percent)

Biotite.............................

............... 60.3

............... 34

............... 2.5

............... 0

............... 3

............... 0.1

............... 0

............... 0.1

54.5 

33.5 

10.5 

1.5 

Tr 

0 

0 

Tr

37 

31 

30 

2 

Tr 

0 

0 

Tr

21.5 

21 

53.5 

3 

Tr 

0 

Tr 

1

25 

32 

40 

2 

0 

0 

1 

Tr

55 

31 

7 

7 

Tr 

0 

0 

Tr

Minor elements (parts per million)

Rb ...................................

Sr....................................

(Rb/Sr .............................

Zr.............................. ......

Y.....................................

Nb...................................

............... 246

............... 24

............... 10.25

............... 4

............... 4

............... 36

27 

28 

0.97 

40 

4 

24

209 

67 

3.12 

31 

5 

4

313 

89 

3.52 

21

179 

202 

0.89 

71 

1 

12

 )

SAMPLE DESCRIPTIONS AND LOCALITIES

W477B NE ViNWVi sec. 3, T. 36 N., R. 18 E. Pink aplite dike in Quinnesec Formation.

W569B NWV4NEV4 sec. 16, T. 38 N., R. 20 E. Leucocratic granodiorite dike in Quinnesec

	Formation, 200 m north of contact with Hoskin Lake Granite.

W569-1 60 m south of locality W569B. Leucogranite dike in Quinnesec Formation.

W412C NWViSWVi sec. 11, T. 38 N., R. 17 E. Leucogranite dike in marble.

W490A SWViSEVi sec. 25, T. 38 N., R. 18 E. Leucogranite dike in Quinnesec Formation.

W660A SEV4NEV4 sec. 4, T. 37 N., R. 20 E. Tonalite dike in Quinnesec Formation.

TECTONIC IMPLICATIONS OF GRANITOID 

ROCK GEOCHEMISTRY

Igneous suites with differing compositional characteristics, such as mid-ocean ridge basalts, calc-alkaline 

rocks, continental tholeiites, and kimberlites, appear 

to be related to particular tectonic settings. Traceelement discrimination diagrams have been used as a 

means of fingerprinting the tectonic setting of several 

igneous suites, using particularly the basaltic compositions (for example, Pearce and Cann, 1973; Floyd 

and Winchester, 1975; Shervais, 1982; Thompson and 

others, 1984), and more recently, felsic (graniticrhyolitic) compositions (Pearce and others, 1984; 

Harris and others, 1986; Brown and others, 1984; 

Thompson and others, 1984; Whalen and others, 1987). 

Although the use of these diagrams as discriminants of 

tectonic setting can be inconclusive, the diagrams do 

provide useful constraints on possible tectonic 

environments.

As discussed previously (Sims and others, 1985), the 

lithologic units in the Dunbar area range from calcic 

(Newingham Tonalite) to calc-alkaline (Dunbar 

Gneiss) to alkali-calcic (Marinette Quartz Diorite) in 

terms of the alkali-lime (Peacock) index (fig. 5), and

they are generally similar to rocks formed along 

convergent plate margins. However, further evaluation 

of the relevant data in light of recent reports on the 

tectonic characterization of granitoid rocks suggests 

some modification of earlier conclusions.

Except for the Newingham Tonalite, the granitoid 

rocks of the Dunbar area are characterized by relatively high K, Rb, Ba, Th, Nb, Ta, and light-REE 

abundances. Their high Nb and Ta concentrations are 

particularly distinctive, and contrast markedly with 

the lower values that characterize most other Penokean igneous rocks in the Wisconsin magmatic terranes (see fig. 33). This distinctive compositional 

characteristic of the granitoid rocks in the Dunbar area 

suggests that they were derived from a different source 

and probably formed in a tectonic setting different 

from that of the volcanic arc-related rocks in the 

Wisconsin magmatic terranes.

The tectonic characterization of the Dunbar area 

rocks is shown in figures 30, 31, and 32. Samples of the 

Newingham Tonalite plot well within the field of 

volcanic arc granites in figure 30, but overlap the fields 

of volcanic arc and syn-collisional granites in figures 31 

and 32. The overlap into the syn-collisional field 

reflects the somewhat high Ta values in some samples
48 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

TABLE 12. Chemical analyses of Middle Proterozoic diabase dike

[Major oxide analyses by rapid rock TCP by H. Smith and F.W. Brown; minor elements by 

X-ray fluorescence analysis by R. Johnson and K. Dennen; minor elements by 

instrumental neutron activation analysis by J.S. Mee]

Sample No............................................ SDNE-113-1-83 SDNE-115-83

Major oxides (weight percent) by X-ray fluorescence analysis

SiO2 ............................................... 49.5 50.2

A12O3 ............................................. 13.4 13.2

Fe2O3 ............................................. 4.8 4.2

FeO................................................ 10.5 11.1

MgO............................................... 4.7 4.8

CaO............................................... 8.1 8.4

Na2O.............................................. 2.8 2.8

KjO................................................ 1.2 1.1

H20+ .............................................. 1.4 1.4

HaCT.............................................. 0.35 0.35

TiO2 ............................................... 2.0 2.0

P2O5 ............................................... 0.50 0.49

MnO.............................................. 0.22 0.23

CO2 ................................................ 0.15 0.17

Minor elements (parts per million) by X-ray fluorescence analysis

Rb.................................................. 45 39

Sr................................................... 502 538

Y.................................................... 34 35

Zr................................................... 155 149

Nb.................................................. 11 10

Minor elements (parts per million) by instrumental neutron 

activation analysis

U.................................................... <2 1.1

Th.................................................. 2.88 2.87

Ta.................................................. 0.46 0.55

Hf................................................... 3.78 3.85

Cr................................................... 26.3 27.8

Co.................................................. 43 44.5

Sc................................................... 40 40.7

Zn.................................................. 150 146

Ba.................................................. 645 617

Cs................................................... 2.88 2.28

La.................................................. 27.1 26.8

Ce.................................................. 54 49

Nd.................................................. 33 35

Sm................................................. 8.1 6.97

Eu.................................................. 1.81 1.84

Gd.................................................. 6.0 5.2

Tb.................................................. 1.0 1.0

Yb.................................................. 3.6 3.6

Lu..................................................______0.55_______0.52

SAMPLE DESCRIPTIONS AND LOCALITIES

SDNE-113-1-83 

SDNE-115-83

Chill margin 

Diabase

of the tonalite, possibly resulting from metasomatism 

by Hoskin Lake-type granite. The other granitoid rocks 

of the Dunbar dome are distinct from the Newingham 

Tonalite and plot with volcanic arc and syn-collisional 

granites in figure 30 and with syn-collisional and

within-plate granites in figures 31 and 32. Although 

some ambiguity remains in their classification, the 

granitoid rocks of the Dunbar area are considered from 

their overall chemistry to be most similar to syn- to 

post-collisional granites of some recent orogens (Harris 

and others, 1986). This classification agrees with the 

structural relationships of the granitoids as syntectonic to posttectonic.

Harris and others (1986) and Thompson and others 

(1984) have attributed the compositional characteristics of some syn- to post-collision-related granitoids to derivation from parental mantle-derived 

basaltic magmas having within-plate compositional 

characteristics derived from continental lithosphere. 

These basaltic melts undergo differentiation and 

possible mixing with crustal-derived melts during 

their ascent through the thickened crust of the collisional orogen. The contribution of a within-plate 

component to the Dunbar dome granitoids is emphasized in figure 33 where, with the exception of the 

Newingham Tonalite, the granitoid rocks plot close to 

within-plate magma types (see inset). It should be 

noted that basalts from the Early Proterozoic epicratonic sequence in Michigan and Minnesota also plot 

with recent within-plate basalts, consistent with their 

rift-related origin (Schulz, 1984a).

We conclude that, except for the Newingham Tonalite, granitoid rocks of the Dunbar area show compositional affinities with syn- to post-collisional 

granites of recent erogenic belts. Further, their compositions suggest derivation from and (or) mixing with 

within-plate mafic magmas rather than derivation 

from the subduction-related volcanic-arc assemblages 

that characterize the Wisconsin magmatic terranes. 

These collision-related magmas were probably generated by melting of continental lithosphere during the 

Penokean orogeny. Overthrusting of the Wisconsin 

magmatic terranes onto the margin of the Superior 

craton and subsequent decompression resulting from 

uplift (Harris and others, 1986) could have contributed 

to the generation of the magmas.

GEOCHRONOLOGY

Uranium-lead zircon ages of the rocks in and 

adjacent to the Dunbar dome (fig. 34) are clustered in 

the range 1,865 Ma to 1,835 Ma, but the data are not 

sufficiently precise to establish the timing of specific 

intrusive and tectonic events within this range. The 

Dunbar Gneiss has a concordia upper intercept age of 

1,862±5 Ma, which is interpreted as the age of the 

igneous protolith for the layered succession. The Marinette Quartz Diorite, which intrudes the Dunbar 

Gneiss, has a comparable age, 1,862±15 Ma. A dacite
STRUCTURE 49

1,000 I I I T i r i i i r

500 -

EXPLANATION

Aplite 

~~ Bush Lake Granite

T Hoskin Lake Granite and 

Spikehorn Creek Granite

 Marinette Quartz Diorite

^ Newingham Tonalite

O Dunbar Gneiss

Syn-collisional 

granites

r / i \ i i i i

100 

50

Newingham Tonalite

Volcanic-arc granites

10

Within-plate granites

Ocean-ridge granites

0.5 1.0 5 10 

Yb+Ta, IN PARTS PER MILLION

50 100

FIGURE 30. Rb versus (Yb+Ta) diagram (modified from Pearce and others, 1984) for granitoid rocks of the Dunbar area with fields for 

volcanic-arc granites, syn-collisional granites, within-plate granites, and ocean-ridge granites.

from the Quinnesec Formation has an age of 1,8<36±39 

Ma, and the volcanic rocks probably were deposited in 

the interval 1,870-1,860 Ma. The Newingham Tonalite, which intrudes the Quinnesec, has an age of 

1,861±40 Ma; and the youngest rock unit, the Spikehorn Creek Granite, has an age of 1,836±6 Ma. These 

ages on the intrusive rocks are generally consistent 

with the geologic relationships. Rubidium-strontium 

whole-rock and biotite ages are consistently reset and 

are 100 m.y. or more younger than the zircon ages 

(Peterman and others, 1985).

Samarium-neodymium model ages of 2,130 and 

2,280 Ma for two samples of Dunbar Gneiss (fig. 35) are 

substantially older than the crystallization ages

defined by the zircon data. Other Early Proterozoic 

igneous rocks in northern Wisconsin have yielded 

similar "old" Sm-Nd ages (Nelson and DePaolo, 1985; 

Barovich and others, 1989). The Sm-Nd ages, together 

with Pb-isotope data (Afifi and others, 1984), indicate a 

major involvement of Archean crustal material in the 

genesis of some Early Proterozoic volcanic and plutonic 

rocks and syngenetic mineralization.

STRUCTURE

The principal structural feature in the Dunbar area, 

the Dunbar dome, is an irregular entity about 200 km2 

in area composed of a core of gneiss and foliated
50 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

 Syn-collisional granites

EXPLANATION

Aplite

Bush Lake Granite

Hoskin Lake Granite and 

Spikehom Creek Granite

Marinette Quartz Diorite 

Newingham Tonalite 

Dunbar Gneiss

0.1

0.5 1 5 10 

Yb, IN PARTS PER MILLION

50 100

FIGURE 31. Ta versus Yb diagram for granitoid rocks of the Dunbar area. Fields (which are the same as in fig. 30) from Pearce and others 

(1984). Note that with exception of the Newingham Tonalite, the granitoid rocks plot in the fields of recent syn-collisional and 

within-plate granites.

granitoid rocks mantled by steeply dipping metavolcanic rocks. Except for three conspicuous protuberances of the core (fig. 2), named the Bush Lake lobe, 

the Niagara lobe, and the Pembine lobe (Sims and 

others, 1985), the margins of the dome tend to be 

smooth. The outline of the dome is interpreted as 

resulting from polydeformation and emplacement of 

the granitoid rocks.

SMALL-SCALE STRUCTURES

Small-scale structures examined in the field indicate 

four principal deformational events in and adjacent to 

the Dunbar dome (table 13) and two younger episodes 

of shearing that produced local mylonitic foliations.

D! STRUCTURES

The oldest recognized structure is a pervasive 

foliation (Sj) that is parallel to compositional layering 

in the Dunbar Gneiss. It is defined mainly by a

preferred orientation of biotite and hornblende and by 

quartz leaves. A lineation related to Sj has not been 

recognized. The lack of an Lj linear fabric suggests a 

strong component of flattening strain normal to S^

The other major rock type in the core of the dome, the 

Marinette Quartz Diorite, also has a foliation that is 

parallel to igneous layering and is expressed by a 

preferred orientation of biotite and hornblende. It is 

designated Sr.

D2 STRUCTURES

The major structures in the region are F2 folds. They 

deform the Dunbar Gneiss, Quinnesec Formation, 

Newingham Tonalite, and Marinette Quartz Diorite, 

as well as an older generation of granite pegmatite and 

aplite associated with the Dunbar Gneiss. An antiform 

oriented N. 60° W. and plunging 35°-45° SE. in nearvertical axial surfaces has been delineated in the 

southwestern part of the Dunbar dome, and secondorder folds are common on the limbs. The folds 

are upright, slightly asymmetrical, open to closed
STRUCTURE 51

TABLE 13. Structural sequence, Dunbar dome

Foliation parallel to layering in Dunbar Gneiss 

D! (Si). Foliation parallel to igneous layering in

Marinette Quartz Diorite (Sr). 

D2 Northwest-oriented folds in Dunbar Gneiss,

Quinnesec Formation, Newingham Tonalite,

and Marinette Quartz Diorite (F2). Foliation

(S2) parallel to axial planes of folds. Local.

Lineation (L2) parallel to fold axes. Local. 

D3 Asymmetrical S- and Z-type folds along

northwest and southeast margins of core.

Lineation (L3) parallel to fold axes (F3). 

D4 Mylonitic foliation S4 and stretching lineation

(L4) along north margin of dome and in

adjacent Quinnesec Formation.

structures. Except locally, the folds do not transpose 

the older foliation (Sj) and layering (S0). An axial-plane 

foliation (S2) is best developed in relatively massive 

tonalitic Dunbar Gneiss, where it is defined by oriented 

tabular feldspars and biotite. A lineation (L2) that is 

parallel to fold axes (F2) is best developed in mica- and 

hornblende-rich schists of the Marinette Quartz 

Diorite and is expressed by elongate minerals and 

mineral aggregates in Sj planar structures. The existence of a southeast-plunging synform in the covered 

area to the north is suggested by the orientation of 

layering in exposed Marinette Quartz Diorite (fig. 36). 

The F2 antiform in the vicinity of Dunbar is defined 

by outward-dipping Sj and S0 planar structures on 

opposite limbs of the fold (fig. 37). The girdle expressing folding of Si and S0 surfaces has a beta fold axis 

plunging 63° S. 75° E. The axial orientations of F2 fold 

hinges and L2 lineations, plotted in figure 37, show 

considerable variation, being steeper on the southwest 

limb than on the northeast limb, but they generally 

plunge less steeply than the beta fold axis. The fold 

hinges that plunge gently N. 90° E. probably represent 

a slightly older set of upright, tight to isoclinal intrafolial folds in the SWV4 sec. 13, T. 37 N., R. 18 E. 

Probably, they were formed during an early stage of D2 

deformation.

D3 STRUCTURES

Following deformation D2, the rocks adjacent to the 

southeast and northwest margins of the core were 

refolded. Along the southeast margin (loc. B, fig. 36), 

Z-type asymmetrical folds (F3) were locally superposed 

on the previously deformed Marinette Quartz Diorite 

and Quinnesec Formation; hinge lines plunge 30°-50° 

S. 5° W. and axial surfaces dip southeastward, subparallel to the core-cover boundary. Along the northwest 

margin (loc. C, fig. 36), a steep, close-spaced foliation 

that parallels the core-cover boundary and an associated southwest-plunging stretching lineation were

EXPLANATION

Aplite

Bush Lake Granite

Hoskin Lake Granite 

and Spikehorn Creek 

Granite

 Marinette Quartz 

Diorite

A Newingham 

Tonalite

Rb/30

Dunbar 

Gneiss

FIGURE 32. Rb-Hf-Ta diagram (modified from Harris and others, 

1986) showing granitoid rocks of the Dunbar area. Except for 

collision-related granites, fields are the same as in figure 30.

developed in the Dunbar Gneiss, and S-shaped asymmetrical folds that plunge at a moderate angle 

southwest were formed in the adjacent volcanic rocks 

of the Quinnesec Formation. Amphibolite-facies metamorphism accompanied or followed D3 on the northwest margin. Possibly the gentle east-northeasttrending folds in the Marinette Quartz Diorite at 

locality E (fig. 36) and the south-southwest-plunging 

folds in it at locality A were formed at this time also, 

under amphibolite-facies metamorphic conditions.

D4 STRUCTURES

D4 structures, which are characterized by a stretching lineation (L4) defined by elongate clasts, mullions, slickenside striae, and rare folds, and a mylonitic 

foliation, are predominant in the Quinnesec Formation 

and in the core-cover boundary on the north side of the 

dome. Similar structures extend to and beyond the 

Niagara fault zone (Sedlock and Larue, 1985). On the 

north margin of the dome (loc. D, fig. 36), the foliation 

dips 70°-80° S. and the lineation plunges uniformly 

60°-70° SW. The zone of mylonitic foliation is as much 

as 500 m wide; the foliation at this locality increases in 

intensity toward the boundary, indicating that the 

deformation here is strongly controlled by the corecover boundary.

Similar northwest-striking foliations (S4) and 

southwest-plunging stretching lineations (L4) are 

present in the Quinnesec Formation on the northeast 

side of the Bush Lake lobe. Lineations measured in this
52 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

10

0.5

0.1

0.5

0.1

MORB

DUNBAR 

DOME /Newingham 

Tonalite

0.1 0.5 1 5 10 

(K : Yb) x 10 3

50 100

EXPLANATION

Bush Lake Granite

Hoskin Lake Granite and 

Spikehorn Creek Granite

Marinette Quartz Diorite 

Newingham Tonalite 

Dunbar Gneiss

EARLY PROTEROZOIC BASALTS, 

MICHIGAN AND MINNESOTA

WISCONSIN 

MAGMATIC 

TERRANES

0.1 0.5 1.0 10 50 100

(K : Yb) x

FIGURE 33. Ta:Yb versus K:Yb diagram (modified from Rogers and 

others, 1987) showing rocks of the Dunbar area, and fields for 

Early Proterozoic rift basalts from the epicratonic continentalmargin sequence in Upper Michigan and east-central Minnesota 

(K.J. Schulz, unpub. data, 1989), and rocks of the Wisconsin 

magmatic terranes (K.J. Schulz, unpub. data, 1989). Shown in 

inset are fields for recent volcanic and intrusive rocks: data for 

MORB (Mid-Oceanic Ridge basalt) from leg 33 DSDP (Jackson and

area mainly plunge 50°-60° S. 40°-70° W. (fig. 38). D4 

structures in this area are most intense near the 

margin of the Bush Lake lobe, where they obliterate 

older structures related to D2 deformation. The 

structures are the predominant ones in this area and 

compose a unique belt of highly strained rocks about 10 

km wide that straddles the Niagara fault (Sedlock and 

Larue, 1985).

LARGE-SCALE STRUCTURE

The Dunbar dome is interpreted as a foldinterference structure resulting from superposition of 

F2 and F3 folds modified by diapirism and the 

emplacement of granitoid intrusive rocks. Many of the 

criteria indicative of diapirism, as listed by Brun and

others, 1976), Iceland from Wood (1978), Hawaii from Frey and 

Claque (1984) and Roden and others (1984), Azores from White 

and others (1979), African rift from DeMulder and others (1986), 

and volcanic arcs from Leo and others (1980), McBirney and others 

(1987), Lopez-Escobar and others (1977), Deruelle (1982), Innocenti and others (1981), Barker and others (1986), and Noyes and 

others (1983).

others (1981), are observed in the dome: (1) cleavage 

parallel to dome boundaries, (2) steeply plunging 

lineation in dome boundaries, and (3) higher strain 

intensities located on dome boundaries.

The outline of the core of the dome (fig. 36) is mainly 

the result of superposed F2 and F3 folding. The 

southwest margin is the southwest limb of the major 

northwest-trending F2 antiform cored by Dunbar 

Gneiss. Small-scale structures indicate that the antiform plunges southeast. Possibly, the antiform is 

doubly plunging, to account for the westward closure of 

the dome, but this was not confirmed because of the 

absence of exposures in the extreme western part of the 

dome. The steeply dipping cover rocks along the west 

margin and fabrics indicative of high strain indicate 

that diapirism was intense in this boundary zone.
STRUCTURE 53

0.38

0.34

0.30

0.26

0.22

0.18

3 0.14

8

OJ.. 0.10

£ 0.38

CO

o

CM

0.34

0.30

0.26

0.22

0.18

0.14

0.10

1.5

1,800 Ma

1,400 Ma

Intercepts: 1,862±4Ma 

471 ±23 Ma

J______I

1,800 Ma

1,400 Ma

Intercepts: 1,835±6 Ma 

204±10Ma

1,800 Ma

1,400 Ma

Intercepts: 1,861 ±40 Ma 

358±204Ma

1,800 Ma

1,400 Ma

Intercepts: 1,866±39 Ma 

337±259 Ma

2.5 3.5 4.5 5.5 1.5 

207 pb; 235

2.5 3.5 4.5 5.5

FIGURE 34. U-Pb concordia plot of zircon data for rocks of the 

Dunbar area. Filled circles are unpublished U.S. Geological 

Survey data. Open circles are from Banks and Cain (1969), Banks 

and Rebello (1969), and Aldrich and others (1965). For A, Dunbar 

Gneiss, regression includes only the USGS data. Array for 

B, Newingham Tonalite, includes one data point for the Marinette

Quartz Diorite. Regression for C, Spikehorn Creek Granite, is 

based on the three USGS data points from an undeformed phase of 

the granite. Inclusion of all the data shown in C does not 

appreciably change upper intercept (1,842±59 Ma). D, Quinnesec 

Formation.

The northwest and southeast margins of the core of 

the dome are subparallel to small-scale D3 structures, 

and are interpreted as the limbs of a major northeastoriented antiform. The asymmetry of the F3 flattening 

folds on these margins suggests that this episode of 

deformation resulted from a shear couple oriented 

northeastward, with sinistral shear on the northwest 

margin and dextral shear on the southeast margin. 

The folds on both margins are slightly overturned to 

the northwest.

On the north, overturned margin of the core, the 

contact between the Hoskin Lake Granite and the 

Quinnesec Formation was the site of intense D4 

deformation, which almost completely obliterated older 

structures.

The Niagara lobe, which is composed of nearly undeformed Spikehorn Creek Granite, transects at nearly 

right angles the outer (east) margin of the central core,

composed of Marinette Quartz Diorite, and it is 

interpreted as a second-order diapir in the first-order 

one, of the type described by Schwerdtner and others 

(1979). The granite in the Niagara lobe has a steep 

foliation near its walls, and the contact is sheared, 

indicating that in part at least it is a tectonic contact. 

The volcanic rocks of the Quinnesec Formation are 

molded around the margin of the dome. We conclude 

that the lobe of granite flowed diapirically upward and 

outward in a plastic state during a late stage of dome 

inflation. As a consequence of the second-order diapir, 

a "cleavage triple point" (Bran and others, 1981) 

developed in the supracrustal volcanic rocks at the 

intersection of the Niagara lobe and the east margin of 

the core (loc. F, fig. 36).

The geology and isotopic ages of rocks in the Dunbar 

dome indicate that the dome was developed during 

regional deformation within the Penokean orogen. The
54 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

Major events 

Rb-Sr ages

Biotite 

Aplite dikes 

WR isochron (22 samples) 

U-Pb zircon ages

Amberg Granite of Cain (1964) 

Athelstane Quartz Monzonite 

Spikehorn Creek Granite 

Newingham Tonalite 

Dunbar Gneiss 

5

 CSa^l

O

O O 

GOOD oo

4

)O O O 

- O O

2 1

O O

O

Quinnesec Formation 

Sm-Nd ages

Dunbar Gneiss 

O

-O

-O

 O

1.0 1.2 1.4 1.6 

BILLIONS OF YEARS AGO -

1.8 2.0 2.2

FIGURE 35. Summary of isotopic ages for rocks of the Dunbar area and environs. Events shown are: 1, main interval of Penokean igneous 

activity; 2, post-Penokean 1.76 Ga igneous event; 3, 1.60±0.05 Ga event that disturbed isotopic systems throughout much of the 

Precambrian of Wisconsin; 4, emplacement of the Wolf River batholith at about 1.47 Ga; and 5, Keweenawan igneous activity at about 

1.1 Ga. Isotopic ages shown are from Aldrich and others (1965), Banks and Cain (1969), Banks and Rebello (1969), Van Schmus (1980), 

and Z.E. Peterman (unpub. data, 1989, Rb-Sr, U-Pb, and Sm-Nd ages). Each circle represents an age.

regional (northwest-trending) structural fabric and perhaps also the persistent southwest-plunging stretching 

lineation in the core-cover boundary of the dome (L4) 

could have resulted from subhorizontal compression 

oriented north-northeastward during collision along 

the arc-continent margin (Niagara fault zone). The 

northeast elongation of the core appears to be related 

to more local forces, perhaps thermal perturbations 

within the core of the dome, because D3 structures are 

virtually confined to the dome.

Sedlock and Larue (1985) concluded from a study of 

the highly strained rocks along the Niagara fault zone, 

including those on the north side of the Dunbar dome 

(D4 structure of this report), that the zone of ductile 

shear is as much as 10 km wide and the axes of the 

finite strain ellipsoid during this deformation were 

oriented: X, subvertical; Y, subhorizontal, west-northwest-east-southeast; and Z, subhorizontal, northnortheast-south-southwest. Accordingly, the steep, 

southwest-plunging stretching lineation (L4) records 

the X axis of the strain ellipsoid.

Whether the core-mantle boundary of the Dunbar 

dome is a stratigraphic or a tectonic contact was not 

determined during the mapping because (1) the contact itself is nowhere exposed and (2) high strain 

was imposed on the boundary during D3 and D4

deformations. Support for the core-cover boundary's 

being a tectonic surface is provided by the aeromagnetic map of the region (King and others, 1966) 

and by two-dimensional density modeling of the 

Dunbar dome by Klasner and Osterfeld (1984). The 

aeromagnetic map clearly shows that the strong 

anomaly associated with the iron-formation exposed on 

the Menominee range extends southward beneath the 

body of Spikehorn Creek Granite (fig. 2), suggesting 

that the rocks of the Dunbar area have overridden the 

continental-margin assemblage (see fig. 1). Klasner 

and Osterfeld concluded that the granitoid rocks of the 

Dunbar dome, including the Dunbar Gneiss, extend to 

a depth of only about 1.6 km. This depth is much 

shallower than many large granitic plutons (Bott and 

Smithson, 1967), and accordingly, Klasner and Osterfeld suggested that the Dunbar dome possibly is 

truncated at depth by a south-dipping, low-angle 

thrust fault, technically related to the Niagara fault. 

Therefore, the dome itself could be allochthonous with 

respect to at least a part of the surrounding rocks.

SHEAR ZONES

Two steep zones of shearing that postdate the 

structural development of the Dunbar dome were
STRUCTURE 55

45°45' 

ROCKS OF MARQUETTE RANGE SUPERGROUP

ELQRENCECOUNTY

MARINETTE

21 E

EXPLANATION

J

40 * F2 antiform Showing plunge 

f~ * F2 synform Showing direction of plunge, inferred "-

50'

55

 20 L2 lineation Showing plunge

' F3 asymmetrical fold Showing plunge

 10 F3 symmetrical fold Showing plunge

S3 , S« foliation Showing dip 

65 L« stretch lineation

~- Trend of Si, Sv foliation

Si, Si- foliation Showing dip

Si, S-T foliation, vertical

®

Post-1,835 Ma mylonitic foliation 

Mylonite in shear zone

Metamorphic isograd bi, biotite; 

gar, garnet. Modified from Dutton (1971)

Metamorphic isofacies Queried where extent 

uncertain; gs, greenschist facies; am, 

amphibolite facies. Modified from Bayley 

and others (1966)

Thrust fault Sawteeth on upthrust block 

Fault Relative movement not known 

Location referred to in text

FIGURE 36. Structure map of the Dunbar dome. Dome (shaded) is surrounded by metavolcanic rocks of the Quinnesec Formation and 

associated subvolcanic rocks. Annular foliation around Niagara lobe is result of diapirism. Compiled by P.K. Sims, 1988.
56 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN 

N N

FIGURE 37. Equal-area projection of structural elements in the 

Dunbar Gneiss and Marinette Quartz Diorite for southwestern 

part of the Dunbar dome. Poles to Si and Sr foliations are 

contoured in percent (1, 2, 3, and 5; 146 points). F2 fold hinges and 

lineations (48 points) are represented by dots. X, Beta fold axis.

recognized during our mapping: (1) the northwesttrending Twelvefoot Falls shear zone, which lies on the 

south side of the dome, and (2) an unnamed northeasttrending zone, also on the south side, that is interpreted to pass through the western part of the Newingham Tonalite pluton (fig. 36).

The Twelvefoot Falls shear zone is a broad, illdefined ductile deformation zone that strikes about 

N. 70° W., dips steeply, and so far as known, ranges in 

width from about 1 km in the vicinity of Twelvefoot 

Falls on the North Pike River (sec. 21, T. 36 N., 

R. 19 E.) to about 15 km in the vicinity of Amberg, 

about 15 km to the southeast. We have traced it 

discontinuously along strike for a distance of about 35 

km. The type area for the deformation is at Twelvefoot 

Falls. The shear zone is superposed on the volcanic 

rocks of the Quinnesec Formation (age -1,860 Ma) and 

the Twelve Foot Falls Quartz Diorite and, to the 

southeast of the map area (fig. 1), on the Athelstane 

Quartz Monzonite (age -1,835 Ma) and the Amberg 

Quartz Monzonite (age 1,760 Ma). Rocks within the 

shear zone have a weak to strong secondary foliation 

and, typically, a steep lineation. In the mafic rocks, the 

foliation is penetrative. In greenschist-facies rocks, it is 

expressed by finely comminuted plagioclase and quartz

FIGURE 38. Equal-area projection of L4 stretching lineation (20 

points) in core-cover boundary and in Quinnesec Formation, 

northeast of Bush Lake lobe.

(mylonite) and oriented chlorite and epidote-clinozoisite aggregates; in amphibolite-facies rocks, it is 

expressed by recrystallization of the original minerals 

to medium or coarse grain size through penetrative 

ductile flow (terminology of Wise and others, 1984). In 

both metamorphic grades, the lineation generally is 

more conspicuous than the foliation. In the granitic 

rocks, the foliation is not penetrative; instead, it is a 

spaced fracture or flow cleavage commonly marked by 

crudely aligned micas and granulated quartz lenses.

The unnamed, northeast-trending shear zone was 

first recognized from small outcrops of the Quinnesec 

Formation in the northern part of sec. 22, T. 36 N., R. 

18 E. (Peterman and others, 1985). At this locality it 

contains a pronounced vertical mineral lineation. This 

shear projects on a N. 60° E. bearing into the western 

part of the Newingham Tonalite body (fig. 36), where it 

is expressed by an ill-defined, broad zone of mylonitic 

foliation that strikes N. 60°-80° E. and dips steeply 

southeast. The shearing was not observed in other 

adjacent, more mafic and finer grained rocks. However, 

a cataclastic foliation striking N. 60°-80° E. and 

dipping about 60° SE. can be seen locally in the 

Spikehorn Creek Granite along U.S. Highway 141-8, 

in the northern part of sec. 1, T. 37 N., R. 20 E. (fig. 2); 

probably this structure has resulted from shearing
METAMORPHISM 57

within this northeast-trending zone. Thin sections of 

some samples of the granite show cataclastic textures. 

Field relations indicate that the shearing in the 

Twelvefoot Falls shear zone postdates intrusion of the 

Amberg Quartz Monzonite, which has a U-Pb zircon 

age of 1,760 Ma (Van Schmus, 1980), but analysis of 

disparate Rb-Sr biotite ages (Peterman and others, 

1985), discussed later, indicate still younger movement 

in this zone as well as on the northeast-trending shear 

zone.

METAMORPHISM

The north margin of the Dunbar dome has a narrow 

metamorphic aureole that increases in intensity 

inward, from low greenschist facies away from the 

dome to amphibolite facies adjacent to and within part 

of the core. The pattern is similar to but more 

asymmetrical than the nodal distribution pattern of 

metamorphism surrounding some domes cored by 

Archean rocks in northern Michigan (James, 1955), 

such as the Watersmeet dome (Sims and others, 1984).

Along the north margin of the dome, southeast of 

Niagara (fig. 36), the amphibolite-facies zone is about 1 

km wide, and includes a narrow inner zone of andesine 

amphibolite grade, as defined by Bayley and others 

(1966). Westward, the amphibolite zone widens, 

diverging markedly from the core-cover boundary. It 

intersects the Niagara fault east of Aurora and follows 

the fault for about 5 km to the west (Bayley and others, 

1966). Farther west, Button (1971, pi. 5) mapped a 

garnet isograd that crosses the fault and then extends 

northwestward subparallel to it. According to Button 

(1971, p. 41), the mafic rocks of the Quinnesec 

Formation and associated mafic sills on the northwest 

side of the dome are metamorphosed mainly to the 

andesine amphibolite facies, and thus equate with this 

facies mapped by Bayley and others (1966) to the east. 

Adjacent to the northwest margin of the core of the 

dome (loc. C, fig. 36), some mafic metavolcanic rocks 

are diopside bearing, and thus probably equivalent to 

the hornblende-hornfels facies of Fyfe and others 

(1958). This assemblage occurs adjacent to the Bunbar 

Gneiss as well as adjacent to the Hoskin Lake Granite, 

as was noted previously by Bayley and others (1966). A 

thin section from SWViSEVi sec. 25, T. 38 N., R. 18 E. 

in Florence County (fig. 2), in mafic metavolcanic rock 

adjacent to the gneiss, shows that diopside occurs as 

anhedral, poikilitic grains that formed later than 

mylonitization of the rock and recrystallization of 

plagioclase to a mosaic texture. Subsequently, the 

diopside was partly altered to minerals stable at lower 

temperatures: hornblende, biotite, and epidote.

As shown by Button (1971, pi. 5), all the rocks 

adjacent to the Bush Lake lobe of the dome are garnet 

grade, and equivalent to amphibolite facies. The 

metasediments adjacent to the west end of the lobe 

locally contain garnet, andalusite, and cordierite. 

Common mineral assemblages in calc-silicate rocks 

within the amphibolite zone are: diopside-oligoclase or 

andesine-(quartz); garnet-hornblende-andesine; oligo- 

 clase or andesine-biotite-hornblende-garnet-quartz; 

and hornblende-oligoclase or andesine-calcite-(microcline). Pelitic rocks contain the assemblage biotitegarnet-oligoclase-quartz±cordierite. This metamorphic 

zone extends northwestward, on the southwest side of 

the Niagara fault, at least to Nelma, Wis., a distance of 

10 km from the northwest margin of the Bush Lake 

lobe. Nielsen (1984) has described a late-stage 

retrograde overprinting of the rocks in this area. The 

most prevalent alteration includes: cordierite-»pinite+ 

sericite; garnet »biotite+chlorite; and hornblende » 

actinolite+biotite±chlorite.

The mineral assemblage in the mafic rocks within 

the amphibolite zone is oligoclase-epidote-hornblende, 

andesine-hornblende (blue green or brownish green), 

and andesine-hornblende-diopside; the assemblage in 

the greenschist-facies zone is albite-chlorite-biotiteactinolitic amphibole-epidote (Bayley and others, 

1966). The mineral assemblages are consistent with a 

low to moderate temperature-pressure environment. 

The diopside- and cordierite-bearing assemblages are 

indicative of a moderately low pressure environment.

Within the core, the northern parts of the Marinette 

Quartz Biorite and the Hoskin Lake Granite are metamorphosed to amphibolite facies. The width of this 

zone in core rocks along the north margin is as much as 

4 km. Metamorphism of the northern part of the 

Marinette Quartz Biorite and the Hoskin Lake Granite 

followed deformation B2 and probably continued until 

deformation B4, when the Spikehorn Creek Granite 

flowed diapirically outward into the Niagara lobe. It 

postdated suturing of the magmatic terrane and the 

continental-margin assemblage. At least the latter 

part of this metamorphism was mainly thermal, as 

indicated by unoriented hornblende needles in the 

Marinette Quartz Biorite within the zone; these were 

observed both in sec. 26, T. 38 N., R. 19 E., and in sec. 

22, T. 38 N., R. 20 E., near the Hoskin Lake Granite.

The locus of intense metamorphism along the north 

margin of the core is spatially associated with the 

Hoskin Lake Granite. Probably, emplacement of the 

magma was one phase of a continuum of thermal 

events during later stages of the doming that included 

diapirism of the granite in the Niagara lobe and 

mobility of potassium and other lithophile elements. 

The mobility of potassium is most evident in the
58 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

TABLE 14. Stratigraphic-tectonic evolution ofDunbar dome

Age 

in Ma

Deformation Event

Quartz-tourmaline veinlets and fluorite in brittle

fractures.

1,835 Spikehorn Creek Granite and, possibly, Bush Lake

Granite emplaced as diapirs. Aplite and pegmatite

intruded into Dunbar Gneiss and Marinette Quartz

Diorite in western part.

D4 Intense deformation along and adjacent to north margin

of dome.

Metamorphism of rocks in northern part of core and 

adjacent cover rocks resulting from rise in isotherms 

(overlapped D3 deformation); accompanied by 

potassium metasomatism.

D3 Northeast-trending folds on dome margins resulting 

from local shear couple.

D2 Deformation of Dunbar Gneiss, Marinette Quartz 

Diorite, Hoskin Lake Granite, Quinnesec Formation, 

and Newingham Tonalite on northwest-oriented fold 

axes (regional structure) resulting from continued 

arc-continent collision.

Dj Development of layering and foliation (Sj) in Dunbar 

Gneiss and Marinette Quartz Diorite and intrusion of 

granite pegmatite and aplite during early stages of 

regional deformation.

=1,860 Formation of Quinnesec volcanic rocks and Newingham 

Tonalite in an oceanic arc environment followed by 

collision with continental margin and formation of 

collision-zone granitoid rocks that now compose the 

Dunbar dome.

northern part of the core, mainly in the Marinette 

Quartz Diorite and Hoskin Lake Granite; but as noted 

earlier, the primary composition of the Newingham 

Tonalite in the central core was also considerably 

altered by the addition of potassium feldspar porphyroblasts, as were certain layers in the Dunbar Gneiss. 

The potassium metasomatism was accompanied 

particularly by the addition of Rb and Nb.

The timing and cause of the retrogressive metamorphism that is so widespread, particularly in the 

Marinette Quartz Diorite in the eastern part of the 

dome, are not known; but most likely the metamorphism was associated with post-doming events, as 

recorded by discordant Rb-Sr whole-rock ages and 

Rb-Sr and K-Ar mineral ages that affected the region.

EVOLUTION OF DOME

The stratigraphic-tectonic evolution of the Dunbar 

dome spanned a relatively short time of about 30 m.y. 

during the Early Proterozoic, from about 1,862 Ma to 

1,835 Ma (table 14).

The contrasting chemistry of the granitoid rocks 

within and outside the dome indicates that they 

formed from different sources, and presumably in

different tectonic environments, and subsequently 

were tectonically superposed. The Newingham Tonalite and associated Quinnesec volcanic rocks were 

generated in a volcanic arc during subduction, and 

represent new mantle-derived crust. The intrusions 

within the dome have characteristics of syncollisional 

and post-collisional plutons, as defined by Pearce and 

others (1984), and presumably were generated from 

melting of continental lithosphere during collision of 

the arc with the continental margin.

The Dunbar Gneiss and the Marinette Quartz 

Diorite were intruded during regional deformation. We 

interpret the gneissic fabric (Dx ) in the Dunbar Gneiss 

as resulting from deformation of an incompletely 

crystallized magma during the regional deformation 

(D2). A compositional and textural layering as well as 

ductile flow textures resulted from this penetrative 

deformation.

The Marinette Quartz Diorite was emplaced approximately along the contact between the Dunbar Gneiss 

and the Quinnesec volcanic rocks. Somewhat later, the 

Hoskin Lake Granite was emplaced. This igneous 

event was followed by major regional deformation (D2), 

which produced northwest-trending folds in both the 

core and cover rocks and produced regional 

greenschist-facies metamorphism. The fold axes are 

subparallel to the Niagara fault, implying that the 

folding in this area resulted from northeast-oriented 

compression during collision of the arc and continental 

margin. The regional folding was followed by local 

shear deformation (D3) along the northwest and 

southeast margins of the dome, perhaps as a result of 

short-lived oblique collision. A rise of the isotherms in 

the northern part of the dome in the area of the 

Hoskin Lake Granite resulted in amphibolite-facies 

metamorphism of the Marinette in the northern part of 

the dome and the adjacent Quinnesec to the north. The 

culmination of deformation (D4) occurred during peak 

metamorphic temperatures and was accompanied by 

inflation of the northern part of the dome. A steep 

lineation (L4) and a mylonitic foliation (S4) formed in 

rocks within the core-cover boundary and in the cover 

rocks to the north. This event is interpreted as the 

culmination of the collision of the Wisconsin magmatic 

terranes with the continental margin. Following 

suturing of the two terranes, isotherms within the 

northern part of the dome continued to rise, and the 

thermal aureole advanced northward across the Niagara fault (fig. 36), developing lower amphibolite 

mineral assemblages in the Michigamme Formation of 

the Marquette Range Supergroup. This increase in the 

thermal regime presumably led to emplacement of the 

Spikehorn Creek and Bush Lake Granites as diapirs. 

Later cooling of the rocks was accompanied by
MINERAL RESOURCES 59

development of quartz-tourmaline veinlets and fluorite 

locally in brittle fractures both within and near the 

north margin of the dome.

POST-DOMING EVENTS

Post-doming events have severely perturbed Rb-Sr 

whole-rock and mineral ages. Twenty-two whole-rock 

samples (5-10 kg each), representing both massive and 

foliated units within the Dunbar dome, define a Rb-Sr 

isochron of 1,697±28 Ma, with an initial 87Sr:86Sr ratio 

of 0.7037±0.0014 (fig. 39). We attribute the disturbance 

of the Rb-Sr system at the whole-rock scale to opensystem behavior related to cataclasis that variably 

affected all the units in the dome. Recrystallization of 

biotite (and microcline where present) and sericitization and epidotization of plagioclase increased the 

mobility of Rb and Sr. Fluids undoubtedly played a 

major role in the migration of Rb and Sr as well as 

other elements. A relation between rock composition 

and degree of resetting is suggested by an isochron age 

of 1,733±43 Ma obtained by regressing only those 

samples with 87Rb:86Sr ratios <3. This separation 

roughly divides the data according to rock type, with 

the granites (in the strict sense) having 87Rb:86Sr 

ratios >3 and the tonalites and granodiorites having 

ratios <3. This correlation between rock composition 

and degree of resetting of the Rb-Sr system is probably 

related to differences in physical properties of the 

rocks. The granites, being less biotitic and more quartz 

rich than the tonalites and granodiorites, probably 

deformed in a more brittle fashion, which led to a 

higher permeability and thus a greater opportunity for 

interaction with a fluid phase. Some of the units, 

although open systems on the sample-size scale (tens of 

centimeters), appear to have remained closed at larger 

scales. For example, average 87Rb: 86Sr and 87Sr:86Sr 

values calculated for the Dunbar Gneiss (11 samples) 

by weighing each sample by its Sr content, are used to 

calculate a model age of 1,833±50 Ma, using an initial 

Sr ratio of 0.7020. Although the uncertainty is large, 

the model age is indistinguishable from the 

crystallization age given by the U-Pb zircon data.

Rubidium-strontium biotite ages of rocks within the 

Dunbar dome decrease from east to west (fig. 40). This 

variation is part of a regional pattern of Rb-Sr biotite 

ages that extends north to the Marquette trough 

(Peterman and others, 1985; Peterman and Sims, 

1988). Within this area, 54 biotite ages define a 

tripartite distribution with well-defined modes at 

1,580±70 Ma, 1,320±50 Ma, and 1,140+30 Ma. The 

older group is a composite that contains the tightly 

clustered 1,630±30 Ma ages for Archean rocks of the

southern complex in northern Michigan (Van Schmus 

and Woolsey, 1975) and slightly younger ages from 

areas to the south. Van Schmus and Woolsey (1975) 

correlated the 1.63 Ga ages with a cryptic event that 

has affected Precambrian rocks over much of 

Wisconsin.

The younger group of ages, 1,140±30 Ma, in the 

western part of the dome, inspired us to undertake a 

comprehensive regional study of biotite ages in 

Wisconsin and northern Michigan (Peterman and 

Sims, 1988). This study delineated a large elliptical 

area approximately 100-150 km long and 50-100 km 

wide in northern Wisconsin where biotite from Early 

Proterozoic crystalline rocks yields Keweenawan ages 

(mean of 1,128±20 Ma). The biotite ages were interpreted to record uplift and cooling of this area, named 

the Goodman swell, during Keweenawan rifting and 

igneous activity. The west end of the Dunbar dome lies 

at the east end of the Goodman swell. The uplift is 

interpreted as a crustal forebulge that formed in 

response to depression of the crust along the axis of the 

rift by loading with heavy basaltic rocks. Tensional 

stresses developed in the upper part of the elastic plate 

along the crest of the forebulge, and these stresses 

facilitated the vertical reactivation of faults in and 

around the Dunbar dome.

The intermediate group of ages, 1,320±50 Ma, does 

not correlate with known events in the region (fig. 35). 

Aldrich and others (1965) suggested a thermal event at 

this time but did not elaborate on a cause. Possibly, the 

surface now characterized by the 1,320-Ma age group 

was uplifted and cooled during the Keweenawan from 

a depth at which the biotite systems were only 

partially reset.

MINERAL RESOURCES

The Early Proterozoic volcanic rocks in northern 

Wisconsin, which presumably are roughly equivalent 

to the Quinnesec Formation (Sims and others, 1989), 

contain scattered volcanic-hosted Cu-Zn-type massive 

sulfide deposits of future economic significance. At 

least four significant deposits are known (Afifi and 

others, 1984; Sims, 1987), the largest of which is the 

Crandon deposit (May and Schmidt, 1982), near Rhinelander, about 100 km west of Dunbar. The Crandon 

deposit contains estimated reserves of 83 million tons 

grading at 5 percent Zn, 1.1 percent Cu, 0.4 percent Pb, 

and 0.03 oz Au per ton (Mudrey, 1979). The other 

known deposits are smaller.

A small pyrrhotite-rich, massive sulfide lens that 

contains only trace amounts of base and precious 

metals has been drilled by the Duval Corporation in
60 EARLY PROTEROZOIC ROCKS, DUNBAR AREA, NORTHEASTERN WISCONSIN

1.2

DUNBAR DOME (ALL SAMPLES) 

T=1,697±28Ma 

IR=0.7037±0.0014

1.1

1.0

tf)

tf)

1=1,733 + 43 Ma 

IR=0.7032± 0.0008

0.9

FIGURE 39. Whole-rock Rb-Sr isochron for samples of all units within Dunbar dome. The isochron of 1,697±28 Ma is based on all the samples 

(22). Inset shows age and initial ratios (IR) for samples with 87Rb:86Sr ratios <3 (mainly tonalites and granodiorites).

sees. 2 and 3, T. 35 N., R. 18 E. and sec. 28, T. 36 N., R. 

18 E. (Hollister and Cummings, 1982). The deposit is 

associated with an iron-formation unit and occurs in 

the interval between an upper marine clastic sedimentary succession and a lower submarine volcanic 

and iron-formation succession.

Presumably, other parts of the volcanic complex in 

northern Wisconsin are favorable for the occurrence of 

additional massive sulfide deposits. Also, the volcanic 

rocks merit consideration as exploration targets for 

gold and silver deposits.

A minor occurrence of molybdenite has been 

observed in the southwestern part of the Hoskin Lake 

Granite exposure area, in sec. 33, T. 38 N., R. 19 E. 

(fig. 2). The molybdenite occurs with pegmatite and 

aplite dikes and associated quartz veins. Several test 

pits were dug in this area in the 1950's, but commercial

concentrations of molybdenite were not found (Fisher, 

1957; Greenberg, 1983).

DISCUSSION

Prior to this study, it was generally accepted that the 

Niagara fault separated the Wisconsin magmatic terranes from the epicratonic continental-margin assemblage to the north. (See Sims and others, 1989, for 

discussion.) This conclusion was based mainly on the 

marked contrast in the geology, the volume of magmatism, and the geochemistry of the Early Proterozoic 

igneous rocks in the two areas (Sims and others, 1985). 

The volcanic rocks in the epicratonic assemblage are 

largely bimodal with abundant tholeiitic basalt and 

minor high-K2O rhyolite. The basalts show strong iron
REFERENCES CITED 61

01.66 

01.69 

jl.60 SOUTHERN 1.65

1 ' 63 COMPLEX

FIGURE 40. Rb-Sr biotite ages (Ga) for Archean and Early Proterozoic rocks in northeastern Wisconsin and adjacent northern 

Michigan. Data are from Van Schmus and Woolsey (1975) for the 

southern complex, Aldrich and others (1965) for the Felch trough 

area, and Peterman and others (1985) for the Dunbar dome and 

vicinity.

enrichment and high TiO2 and incompatible element 

contents (Fox, 1983); they are compositionally similar 

to continental rift basalts, such as those of the 

Keweenawan in Minnesota (Green, 1983). In contrast, 

the voluminous volcanic rocks in Wisconsin range from 

basalt through andesite to rhyolite, lack strong iron 

enrichment, and have volcanic-arc-derived characteristics (Greenberg and Brown, 1983; Schulz, 1984a,

b). Van Schmus (1976), Cambray (1978), LaBerge and 

others (1984), Larue and Sloss (1980), and Schulz and 

others (1984) have proposed various plate tectonic 

models to account for the differences in the Early 

Proterozoic geology in Wisconsin and Michigan.

This study has shown some of the complexities of the 

boundary zone between the Wisconsin magmatic terranes and the continental-margin assemblage to the 

north. The boundary is not a simple suture that 

sharply separates the two contrasting terranes; 

instead, it is a complex tectonic zone of intricately 

mixed rocks that were formed in different tectonic 

environments and subsequently were juxtaposed, 

mainly by thrusting. The granitoid rocks are a key to 

the paleoenviromnents. As discussed earlier, the Newingham Tonalite is typical of volcanic-arc granitoid 

bodies (Pearce and others, 1984), generated by subduction during closure of the ocean that intervened 

between arc complexes of the Wisconsin magmatic 

terranes and the continental margin. The remainder of 

the granitoid rocks in the Dunbar area show affinities 

with syn- to post-collisional granites of recent orogenic 

belts. Apparently these were generated by melting of 

continental lithosphere and crust during collision an 

event unique to the collision zone.

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Zen, E-an, 1986, Aluminum enrichment in silicate melts by

fractional crystallization Some mineralogic and petrographic

constraints: Journal of Petrology, v. 27, p. 1095-1117.

. GOVERNMENT PRINTING OFFICE: 1992-673-049/46045

SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports 

of wide and lasting interest and importance to professional scientists and 

engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of 

lasting scientific interest but are generally more limited in scope or geographic coverage than Professional Papers. They include the results of 

resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrology, availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific information of wide popular interest in a format designed for distribution at no cost to the public. Information is usually of short-term interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at depositories. They are a nonpermanent form of publication that maybe cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on topographic bases in 7 1/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering 

geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric bases at various scales, they show results of surveys using geophysical techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 7 1/2-minute quadrangle photogeologic maps on planimetric 

bases which show geology as interpreted from aerial photographs. The 

series also includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, 

stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology in relation to specific mining or mineral-deposit problems; post-1971 maps 

are primarily black-and-white maps on various subjects such as environmental studies or wilderness mineral investigations.

' Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; the principal 

scale is 1:24,000, and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive 

listings of U.S. Geological Survey publications are available under the 

conditions indicated below from the U.S. Geological Survey, Books and 

Open-File Reports Sales, Box 25425, Denver, CO 80225. (See latest 

Price and Availability List.)

"Publications of the Geological Survey, 1879-1%1" may be purchased by mail and over the counter in paperback book form and as a set 

microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a set 

of microfiche.

"Publications of the US. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982, 1983, 1984, 1985, 1986, and for subsequent years since the last permanent catalog may be purchased by mail 

and over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and Water-Supply Reports and Maps For (State)," may be purchased by mail and 

over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet 

form only.

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" is available free of charge by mail or may be 

obtained over the counter in paperback booklet form only. Those wishing 

a free subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 National Center, Reston, VA 22092.

Note.-Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the prices 

charged may differ from the prices in catalogs, announcements, and publications.