Petrography and Geochemistry of 

Early Proterozoic Granitoid Rocks in 

Wisconsin Magmatic Terranes of 

Penokean Orogen, Northern Wisconsin 

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Chapter J 

Petrography and Geochemistry of 

Early Proterozoic Granitoid Rocks in 

Wisconsin Magmatic Terranes of 

Penokean Orogen, Northern Wisconsin 

By P.K. SIMS, K.j. SCHULZ, ED DEWITI, and 

BRUCE BRASAEMLE 

A reconnaissance study carried out in 

conjunction with regional geologic mapping 

U.S. GEOLOGICAL SURVEY BULLETIN 1904 

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION 

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

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Robert M. Hirsch, Acting Director 

Manuscript approved for publication April 21, 1993 

Published in the Central Region, Denver, Colorado 

Photocomposition by Shelly A. Fields 

Graphics by Wayne Hawkins 

Edited by Lorna Carter 

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

does not imply endorsement by the U. S. Government. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1993 

For sale by 

U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center 

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Library of Congress Cataloging-in-Publication Data 

Petrography and geochemistry of Early Proterozoic granitoid rocks in Wisconsin 

magmatic terranes of Penokean orogen, northern Wisconsin I by P.K. Sims ... [et 

al.]. 

p. em.- (U.S. Geological Survey bulletin : 1904-J) 

"A reconnaissance study carried out in conjunction with regional geologic 

mapping." 

(Contributions to Precambrian geology of Lake Superior region ; ch. J) 

Includes bibliographical references. 

Supt. of Docs. no.: 119.3: 1904-J 

1. Granodiorites-Wisconsin. 2. Granite-Wisconsin. 3. Geology, 

Stratigraphic- Proterozoic. 4. Geochemistry-Wisconsin. 5. GeologyWisconsin. I. Sims, P.K. (Paul Kibler), 1918- . II. Series. Ill. Series: 

Contributions to Precambrian geology of Lake Superior region, ch. J. 

QE75.B9 no. 1904-J 

[QE462.675] 93-25264 

552'.3-dc20 CIP 
I 

CONTENTS 

Abstract Jl 

Introduction J2 

Geologic setting J2 

Wisconsin magmatic terranes J2 

Pembine-Wausau terrane J6 

Marshfield terrane J6 

Granitoid rocks J6 

Pembine-Wausau terrane J6 

Syntectonic granodiorite suite J7 

Post-tectonic suite J7 

Athelstane Quartz Monzonite J7 

1,835 Ma alkali-feldspar granite J9 

1,760 Ma granodiorite-granite group J13 

Marshfield terrane JtS 

Syntectonic tonalite-granodiorite suite J16 

Gneissic granite near Neillsville J23 

1,835 Ma alkali-feldspar granite J24 

Summary and conclusions J24 

References cited J27 

Appendix A. Petrographic descriptions of rock samples listed in table 1 J29 

Appendix B. Petrographic descriptions of rock samples listed in table 2 J30 

FIGURES 

1. Geologic map of eastern part of Lake Superior region showing relationships of rocks 

within the Penokean orogen J3 

2. Geologic map of Wisconsin magmatic terranes and adjacent areas J4 

3. Quartz-alkali feldspar-plagioclase diagram showing composition of representative 

rocks in the Pembine-Wausau and Marshfield terranes J7 

4. Diagram showing classification of granitoid rocks of Pembine-Wausau and Marshfield terranes J12 

5. Si02 versus (Fe0+0.89 Fe203):(Fe0+0.89 Fe20j+MgO) diagram for plutonic rocks 

in Pembine-Wausau and Marshfield terranes J13 

6. Si02 versus K20:(K20+Na20) diagram for plutonic rocks in Pembine-Wausau and 

Marshfield terranes J13 

7. FeO:(FeO+MgO) versus Si02 diagram for the syntectonic granodiorite suite and 

rocks of the post-tectonic Athelstane Quartz Monzonite, Pembine-Wausau 

terrane J13 

8. Chondrite-normalized REE plots for syntectonic granodiorite suite, PembineWausau terrane J14 

9. Chondrite-normalized REE plots for samples of Athelstane Quartz Monzonite, 

Pembine-Wausau terrane JtS 

10. Nb versus Y diagram for granitoid rocks in the Pembine-Wausau terrane J16 

11. Chondrite-normalized REE plots for samples of 1,760 Ma granodiorite-granite 

group J17 

12. Chondrite-normalized REE plots for samples of syntectonic granodiorite suite, 

Marshfield terrane J23 

13. Rb versus Y +Nb diagram showing granitoid rocks of the Marshfield terrane J24 

Contents Ill 
14. Chondrite-normalized REE plots for samples of red alkali-feldspar granite, Marshfield terrane J25 

15. Chondrite-normalized REE plots for samples of diorite and lamprophyre associated 

with alkali-feldspar granite, Marshfield terrane J26 

16. Nb versus Y diagram showing granitoid rocks of the Marshfield terrane J26 

17. Si02 versus logw (K20:MgO) diagram showing distinction between calc-alkalic and 

alkali granite suites J26 

18. Si02 versus K20:(K20+Na20) diagram showing comparison of compositions of 

plutonic rocks in Pembine-Wausau and Marshfield terranes with those in Proterozoic terranes in western United States J27 

TABLES 

IV Contents . 

1. Modes and chemical analyses of selected granitoid rocks in Pembine-Wausau terrane 

(exclusive of Dunbar area) J8 

2. Modes and chemical analyses of selected Early Proterozoic granitoid rocks in 

Marshfield terrane J18 
CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION 

Petrography and Geochemistry of Early Proterozoic 

Granitoid Rocks in Wisconsin Magmatic Terranes of 

Penokean Orogen, Northern Wisconsin 

By P.K. Sims, K.J. Schulz, Ed DeWitt, and Bruce Brasaemle 

Abstract 

Granitoid rocks in the Pembine-Wausau and Marshfield 

terranes (Wisconsin magmatic terranes) of the Early Proterozoic Penokean orogen compose two distinct and contrasting 

suites, calc-alkalic rocks that are broadly syntectonic and alkali 

granites that are post-tectonic. Rocks of the syntectonic suites 

are spatially associated with coeval tholeiitic and calc-alkalic 

volcanic rocks. Composition of the rocks in each suite is similar in both terranes. 

In the Pembine-Wausau terrane, a syntectonic granodiorite suite (1,870-1,837 Ma) intrudes tholeiitic and calc-alkalic 

volcanic rocks (1,889-1,840 Ma). The syntectonic suite is calcic to calc-alkalic, average to rich in magnesium content, and 

sodic to very sodic; it has intermediate Si02 concentrations, 

intermediate to high Al 20 3 (12.4-16.4 percent), high strontium 

concentrations, and low rubidium, niobium, and yttrium concentrations. The rocks are similar in composition to the Newingham Tonalite in the Dunbar, Wisconsin, area, which is 

interpreted as a volcanic-arc granite. The post-tectonic suite in 

the Pembine-Wausau terrane consists of three distinctive rock 

groups: the =1,836 Ma Athelstane Quartz Monzonite, a 1,835 

Ma alkali-feldspar granite, and a =1,760 Ma granodiorite-granite group. The Athelstane Quartz Monzonite is mildly peraluminous; its Si02 content is high (68-77 percent), Al 20 3 

content is intermediate (12-15 percent), and it exhibits 

Na:K<1. The rocks are strongly enriched in iron and have distinctive rare-earth element patterns, exhibiting large negative 

europium anomalies. The Athelstane Quartz Monzonite is 

compositionally similar to younger Middle Proterozoic granites of the Wolf River batholith in Wisconsin. The 1,835 Ma 

alkali-feldspar granite is mildly peraluminous, alkali-calcic, 

and iron-rich; it contains very low strontium and high uranium 

concentrations and exhibits large negative europium anomalies. These rocks plot as within-plate granites. The ::::1,760 Ma 

granodiorite-granite group is coeval with the 1,760 Ma anorogenic rhyolite-granite terrane in south-central Wisconsin, but 

differs from these anorogenic rocks in being more variable in 

composition and in having somewhat lower FeO:(FeO+MgO) 

and higher strontium content. The granodiorite-granite group 

contains the most rubidium- and thorium-rich rocks in the 

area. 

In the Marshfield terrane, a syntectonic tonalite-granodiorite suite contains a range of rocks from hornblende-biotite 

diorite to biotite trondhjemite; the suite is calc-alkalic to alkalicalcic. The rocks are mostly meta luminous, rich to very rich in 

magnesium, and sodic. The tonalite-granodiorite suite has 

been compared to Holocene orogenic andesite-dacite suites, 

and it is associated with tholeiitic and calc-alkalic volcanic 

rocks, tentatively dated at 1,860 Ma, which overlie an Archean 

basement. Available U-Pb zircon ages on the tonalite-granodiorite suite range from ""1,840 Ma to 1,892 Ma, suggesting 

that the associated volcanic rocks are in part at least older than 

1,860 Ma. A unit of limited areal extent, gneissic granite near 

Neillsville (1,871 ±5 Ma), has a more highly evolved chemical 

composition than the tonalite-granodiorite suite. The 1,835 Ma 

post-tectonic alkali-feldspar granite in the Marshfield terrane is 

petrographically and chemically identical to the alkali-feldspar 

granite in the Pembine-Wausau terrane. 

The older rocks (1,870-1,852 Ma) of the granodiorite suite 

in the Pembine-Wausau terrane formed during volcanism initiated during southward subduction of oceanic crust prior to collision (1,852 Ma) of the Pembine-Wausau arc terrane and the 

continental margin along the Niagara fault zone. Younger 

(1,853-1,837 Ma) rocks in the granodiorite suite presumably 

formed during later north-dipping subduction along the Eau 

Pleine shear zone(= 1,840 Ma) that terminated with collision of 

the Marshfield minicontinent with the Pembine-Wausau terrane. The anorogenic Athelstane Quartz Monzonite was generated by partial melting of lower crust 5-10 million years after 

termination of orogenic episodes. At about the same time, the 

1,835 Ma alkali-feldspar granite was generated by crustal melting and emplaced astride the Eau Pleine suture zone. The final 

product of crustal melting, the 1,760 Ma granodiorite-granite 

group, is variable in composition, probably indicating partial 

melting of different source materials. 

Early Proterozoic Granitoid Rocks, Northern Wisconsin Jl 
The granitoid rocks in the arc terranes show a temporal 

progression from magnesium-rich granodiorite or trondhjemite 

to iron-rich granodiorite-granite. The syntectonic granodiorite 

suites in both terranes are primitive, magnesium-rich rocks typical of immature island or continental arcs. Iron content of plutonic rocks younger than ""1,850 Ma ranges from average to 

rich to very rich. The post-tectonic Athelstane Quartz Monzonite and the 1,835 Ma red alkali-feldspar granite are uncommonly enriched in zirconium for evolved granites. Compared 

with other Proterozoic orogenic terranes in the western United 

States, the plutonic rocks of the Wisconsin magmatic terranes 

are notably sodic. 

INTRODUCTION 

Reconnaissance sampling of granitoid rocks in northern 

Wisconsin during regional geologic mapping (Sims, I992) 

provides a means of characterizing plutonic rock bodies in 

the Early Proterozoic Wisconsin magmatic terranes (Sims 

and others, 1989), previously a poorly known part of the 

Penokean orogen (Sims and Peterman, 1983). Concurrently, 

a systematic, detailed mapping and sampling program was 

conducted in the Dunbar area in northeastern Wisconsin 

(Sims and others, 1985; Sims and others, I992). 

Reconnaissance geologic mapping and sampling in the 

area began in 1975 and continued intermittently through 

1987. Geologic maps of the Eau Claire-Green Bay (Sims, 

1990a), Rice Lake (Sims, 1989), and Iron MountainEscanaba (Sims, I990b) 1°X2° quadrangles have been published. A regional geologic map (scale 1 :500,000) of Precambrian rocks of northern Michigan and Wisconsin also 

has been published (Sims, 1992) in cooperation with the 

Wisconsin Geological and Natural History Survey. 

Most rock names used in this report are based on the 

chemical classification of DeLaRoche and others (1980). In 

the sample descriptions accompanying tables I and 2, rock 

names based on the lUGS (Streckeisen, 1976) classification 

are also listed for those samples that differ from the 

DeLaRoche classification. The DeLaRoche classification 

together with A/CNK values (molecular ratio of AI203: 

(CaO+Na20+K20) is useful in identifying altered samples. 

We thank M.G. Mudrey, Jr., B.A. Brown, and J.K. 

Greenberg of the Wisconsin Geological and Natural History 

Survey, R.S. Maass of the University of Wisconsin, 

Madison, G.L. LaBerge of the University of Wisconsin, 

Oshkosh, and P.E. Myers of the University of Wisconsin, 

Eau Claire, for guidance during much of our early reconnaissance mapping. Published maps of Marathon (LaBerge 

and Myers, 1983), Portage (Greenberg and Brown, I986), 

and Wood (Brown and Greenberg, I986) Counties (scale 

1:100,000) were helpful in the map compilation (Sims, 

1992). W.R. Van Schmus of the University of Kansas provided most of the isotopic ages, which were reported in Sims 

and others (1989). We have benefited from discussions with 

Zell E. Peterman and from constructive comments on the 

manuscript by Holly J. Stein. 

)2 Precambrian Geology of Lake Superior Region 

GEOLOGIC SETTING 

The 1.90--I.83 Ga Penokean orogen consists of two 

distinct lithologic domains (fig. I), a northern deformed 

continental margin prism overlying an Archean basement, 

and a southern assemblage of island arcs and (or) closed 

back-arc basins, known as the Wisconsin magmatic terranes, which for the most part lack an Archean basement 

(Sims and others, 1989). The northern domain includes the 

Marquette Range Supergroup in Michigan and Wisconsin 

(Cannon and Gair, 1970), which consists of a rifted passive-margin sequence overstepped northward by a synorogenic foredeep sequence (Hoffman, 1987; Barovich and 

others, 1989). Early Proterozoic deformation involved 

north-directed thrusting and folding of the supracrustal 

rocks and, at least locaJly, the Archean basement rocks 

(Gregg, I993; Klasner and Sims, 1993) on the continental 

margin. Crustal thickening resulted in metamorphism and 

development of local basement gneiss domes (Sims and 

Peterman, 1983; Klasner and others, I988; Holm and 

others, 1988; Attoh and Klasner, 1989). 

The southern, magmatic terranes are composed of Early 

Proterozoic calc-alkalic and tholeiitic volcanic rocks and 

calc-alkalic and alkali granite plutonic rocks (Sims and others, 1989). The southernmost of the magmatic terranes, the 

Marshfield terrane, is at least partly underlain by Archean 

basement rocks. This Archean basement, in central Wisconsin, differs from the Archean gneiss basement of the continental margin in lacking rocks of Early Archean (3.6 Ga) 

age and in Jacking granulite-facies gneisses. Therefore, the 

central Wisconsin Archean basement is not considered to be 

a rifted fragment from the nearby continental margin. 

The Niagara fault zone, which separates the south-facing 

continental margin from the arc terranes in Wisconsin and 

Michigan, is a steep, south-dipping structure at the surface 

(Larue and Ueng, 1985; Sedlock and Larue, I985), but it 

may flatten at depth to a shallow-dipping thrust. The fault 

zone contains a dismembered ophiolite and has been interpreted as a paleosuture (Schulz, I987) along which the two 

terranes were juxtaposed at 1,860--1 ,850 Ma. 

WISCONSIN MAGMATIC TERRANES 

The Wisconsin magmatic terranes comprise two distinct 

assemblages that are distinguished on the basis of rock types 

and structure: (1) a northern terrane, caiJed Pembine-Wausau 

and (2) a southern terrane, called Marshfield (figs. 1, 2). The 

two terranes are juxtaposed along the Eau Pleine shear zone, 

a presumed south-verging paleosuture zone (Cannon and 

Figure 1 (facing page). Geologic map of eastern part of Lake 

Superior region showing relationships of rocks within the Penokean orogen. M, Marquette; I, Ironwood; E, Eau Claire; W, 

Wausau; S, Stevens Point; H, Holcombe; P, Pembine. Precambrian Y, Middle Proterozoic (900-1,600 Ma); Precambrian X, 

Early Proterozoic (1,600-2,500 Ma); A, Archean (2,500-3,800 

Ma) in the stratigraphic column. Modified from Sims and 

others, 1989. 
E3 

[L] 

\ ,'1'•,, rll 

<,.1 .~ 

Yi 

... .~. 

' r •' • '~,1' . 

. . : .• · .. ·.!· ~' · .. ~ - .··. ·.·· • 'lo •, ,; :~~ •• ': • o • .'' It,·.~·~.· 

:. • •. •, • I"': :' .• • ,. o ',.f ~. ; • '· .. ~ • o~., ;' • • o • ·' 

EXPlANATION 

Sedimentary rocks of Paleozoic age 

Mafic igneous and sedimentary rocks of Midcontinent 

rift system (1,000-1,200 Ma) 

Anorogenic Igneous rocks (1,470-1,500 Ma) 

Barron Quartzite (Early Proterozoic) 

Alkali-feldspar granite ( = 1,835 Ma) 

Tonalite-granodiorite-granite ( 1, 7 60-1,8 7 0 Ma) 

Gneiss and granitoid rocks (1,835-1,865 Ma) 

Volcanic and lesser sedimentary rocks (1,840-1,880 Ma) 

Gneiss and schist (2,800-3,000 Ma); includes 

tonalite (1,890 Ma) 

Marquette Range Supergroup (=1,850-2,100 Ma) 

Granite and greenstone (2,600-2, 750 Ma) 

Gneiss (2,700-3,550 Ma) 

WOLF RIVER 

BATHOLITH 

50 100 KILOMETERS 

---- Contact 

---- High-angle fault 

--==:;;__ Transcurrent fault-Barbs show direction of 

relative movement; queried where extent 

uncertain 

-.--..... r- Thrust fault-Dashed where concealed; queried 

where extent uncertain; sawteeth on upper 

plate 

---,.-~ Shear zone containing mylonite-Dotted 

where concealed 

EP 

J 

A 

MT 

0 

Foliation trend 

Shear zones 

Eau Pleine (paleosuture) 

Jump River 

Athens 

Mountain 

Locality referred to in text 

Early Proterozoic Granitoid Rocks, Northern Wisconsin )3 
:.;:::::-

45° 

91° 

... · .·: . 

. . ~: . 

·.·.·.·.·· 

.. 

:. : ......... :/:.·~· 

.·:··_,.·.: .. : .· 

· .. :·. 

.~ 

Pzu 

. . . .. .. •' 

· . ·. :: ·::·: .-:.' ~ .. Xv 

· Wgn · 

. ·.-::.'·<.· . B · ..... 

·::·. 

·.· 

··.: ... : .. · ... ·.··. 

.- ·: .~ . 

. ·. ··· .. • • • 0 •• ~· • 

·-·. ·.- .. 

,'• .. 

91° 

0 • • • ·: ··~ • 

.. ··/ .. :->·>:·-::·' .. 

.)" 

t 

"9 

_:'1, 

~ ·: : ~ , II 

• t ' It ~ ,, II .. 

~ ~ ~ ~' t ., J o, 

l ~ 9 

h '\' \\ 

' ~ 

'(' \\ '"i f ~ I 

~'' : II '•' ~- l' \I~ 

- . '-', 

MARATHON 1 <.;' ~COUN~ _j ",, 

~ I I 

Yw 

11 11 

I/)' \\ ~~ 

~ "11 

Figure 2 (above and facing page). Geologic map of Wisconsin magmatic terranes and adjacent areas. Modified from Sims (1992). Br, Black 

River Falls; Ch, Cherokee; E, Eau Claire; Mo, Monico; Ma, Marshfield; Ne, Neillsville; D, Dunbar; L, Lugerville; P, Pembine; R, Rhinelander; 

S, Stevens Point; W, Wausau; Wr, Wisconsin Rapids. Circled letters designate localities discussed in text. 

J4 Precambrian Geology of Lake Superior Region 
~Xga 

~ . ' 

.. ... ~: 

~ :,.-

i" 

,, 

I' 

'· .._.,. 

I. 

Yw 

It' 

I. 

~ 

......... ~1,. 

I, ;.- • \\, 

... . . 

11;· 

Pzu 

Pzu 

, II 

I::- I:.· 1,' ~ •• .: •• •• :· •• • ••• • 

' . •. ·. ~ '\: :. ~.,''I I:·, . • .. • •. • 

·:. -~· . ·. :~·· . 

0 ••• • •• 

. : 

25 50 KILOMETERS 

---- Contact-Approximately' located 

---- High-angle fault-Sense of movement not known; 

dashed where concealed 

---,r--- High-angle fault-Bar and ball on downthrown side; 

dashed where concealed 

---- Transcurrent fault-Barbs show relative horizontal 

movement 

""T'"""T""-:--~ High-angle reverse fault of Middle Proterozoic 

(Keweenawan) age-Hachures on downthrown 

side 

4 • • High-angle thrust fault of Early Proterozoic 

(Penokean) age- Dashed where concealed: 

sawteeth on upthrown side 

~ ""'rv""' Mylonite 

EXPLANATION 

r--~----, PHANEROZOIC 

Pzu J Paleozoic rocks, undivided 

! : I M:~:.::~:::::::::~;::~:::·~:~:o~= (1,470 Ma) 

5B 

f Ys J 

f~H:.~~::;·:/,. 

45° , ,·x99 · ~ 

~ 

~ 

Rhyolite associated with Wolf River batholith 

Anorogenic granitoid rocks of Wausau pluton ( = 1,515 

Ma) 

Anorogenic granitoid rocks of Stettin syenite pluton 

(""1,520 Ma) 

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

Pembine-Wausau terrane 

Barron Quartzite 

Granitoid rocks (1,760 Ma) 

Hines Quartz Diorite (1,812.7±3.6 Ma) 

Baldwin Conglomerate-Adjacent to Mountain shear 

zone 

Alkali-feldspar granite (1,835 Ma) 

Rhyolite and welded tuff 

Spikehorn Creek (1,835±6 Ma) and Bush Lake 

Granites 

Granite near Cherokee (1,853±21 Ma) 

Athelstane Quartz Monzonite (1,836±15 Ma) 

Felsic metavolcanic rocks (""' 1,835-1,845 Ma) 

Granodiorite and tonalite (1,837-1,845 Ma) 

Quartzite 

Granite to quartz diorite and granitoid gneiss 

(1,855-1,885 Ma) 

Gneiss and schist in Mountain shear zone 

Metamorphosed volcanic and sedimentary rocks 

(1,860-1,889 Ma); some rocks may be younger 

Marshfield terrane 

Rhyolite of 1, 760 Ma age group 

Alkali feldspar granite (1,835 Ma) 

Rhyolite and welded tuff 

Granitic rocks, undivided 

Foliated tonalite-granodiorite (1,840-1,892 Ma) 

Milladore Volcanic Complex ( = 1,860 Ma) 

Metavolcanic rocks in Eau Claire River valley 

(=1,860 Ma) 

Gneissic granite near Neillsville (1,871±5 Ma) 

,......,...,..,.,.,...,.,..., EARLY PROTEROZOIC AND ARCHEAN 

Fault rocks-Mylonite in Eau Pleine shear zone 

Early Proterozoic tonalite and Archean gneiss (1,840-

1,892 Ma) 

~~,.------,LATE ARCHEAN (2,500-2,800 Ma) 

t: w~n ] Gneiss, migmatite, and amphibolite- Includes small 

bodies of Early Proterozoic rocks 

'"=",..,.,...,.,..,-,-.,ROCKS NORTH OF NIAGARA FAULT 

XAr j Metamorphosed sedimentary and volcanic rocks of 

Early Proterozoic Marquette Range Supergroup 

and Archean basement gneiss (2,600-3,650 Ma) 

and granitoid rocks (2,650-2,750 Ma) 

Early Proterozoic Granitoid Rocks, Northern Wisconsin )5 
others, 1991 ). After amalgamation of the two terranes at 

==1,840 Ma, plutons of alkali-feldspar granite intruded and 

cogenetic silicic rhyolite erupted astride the Eau Pleine shear 

zone at 1,835 Ma. Controls on the age of the Eau Pleine 

shear zone are not tight. The shear zone is cut by a body of 

red alkali-feldspar granite dated at== 1,835 Ma (unit Xgr, fig. 

2), and therefore is older. Apparently also the shear zone is 

cut by the granite near Cherokee, which has an age of 

1,853±21 Ma (Sims and others, 1989). Because of the large 

uncertainty in the age of the Cherokee and its probable 

correlation with rocks of the Athelstane batholith ( 1 ,836± 15 

Ma), we have assumed an age of== 1,840 Ma for culmination 

of movement on the Eau Pleine shear zone. 

Pembine-Wausau Terrane 

The Pembine-Wausau terrane has an exposed strike 

length of 275 km and a maximum width of 150 km (fig. 2); 

it is covered at both ends by Paleozoic rocks. Width of the 

terrane decreases to the west because of convergence of the 

Eau Pleine shear zone and the Niagara fault zone. The Pembine-Wausau terrane is composed mainly of a thick sequence 

of volcanic rocks (unit Xv, fig. 1; unit Xvs, fig. 2) deposited 

between about 1,889 and 1,860 Ma (Sims and others, 1989). 

These volcanic rocks are dominantly metamorphosed 

(mainly greenschist facies) tholeiitic basalt and basaltic 

andesite, but include calc-alkalic dacite, and rhyolite flows 

and pyroclastic rocks (Sims and others, 1989; Sims and 

others, 1992). A bimodal suite of high-aluminum basalt and 

basaltic andesite intercalated with dacite to rhyolite contains 

associated copper-zinc massive sulfide deposits (Mudrey, 

1979; Sims, 1987 and references therein) in north-central 

Wisconsin (near Mo, fig. 2). This volcanic sequence is 

intruded by granitoid rocks (units Xgg, Xgt, Xgat, Xgc, Xgrs, 

Xgr, Xhg, and Xga, fig. 2) ranging in age from 1,870 Ma to 

1,760 Ma. In Marathon County and vicinity (fig. 2), a calcalkalic volcanic succession presumed to have formed in the 

interval 1,845-1,835 Ma apparently overlies mafic volcanic 

rocks interpreted to belong to the 1,889-1,860 Ma sequence 

(Sims, 1990a). This calc-alkalic volcanic sequence has associated subvolcanic granodioritic sills(?) (LaBerge and 

Myers, 1983) but lacks exposed deeper seated plutons of 

approximately the same age. Younger, 1,835 Ma alkalifeldspar granite (unit Xgr, fig. 2) forms abundant plutons in 

Marathon County and vicinity, in the southern part of the 

exposed magmatic terranes. 

Marshfield Terrane 

The Marshfield terrane differs from the Pembine-Wausau terrane with respect to rock types and structure. 

Archean gneisses constitute more than 50 percent of the 

exposed rocks and probably underlie the Early Proterozoic 

volcanic rocks in much of the terrane (Sims, 1990a); the 

Pembine-Wausau terrane, in contrast, lacks an Archean 

basement. A steep foliation, lineation, and mylonitic fabric 

)6 Precambrian Geology of Lake Superior Region 

are superposed on older structures in Archean and Early 

Proterozoic rocks throughout most of the Marshfield terrane. This mylonitic structure developed before emplacement of the 1,835 Ma alkali-feldspar granite suite (unit 

Xgr, fig. 2), but after 1 ,860 Ma, the presumed age of the 

supracrustal volcanic rocks (units Xv, Xmc, fig. 2). 

Because of this structural overprint coupled with generally 

sparse exposures, knowledge of these pre-1 ,835 Ma Proterozoic rocks is rather meager. 

Early Proterozoic volcanic rocks of mainly upper greenschist metamorphic grade crop out in two areas, and probably represent erosional remnants of a once-extensive 

volcanic cover. The larger area (locality A, figs. 1, 2) contains a sequence of interlayered felsic to mafic volcanic 

rocks, dacite porphyry, and sedimentary rocks-impure 

quartzite, ferruginous chert, conglomerate, and carbonaceous argillite (unit Xmc, fig. 2). The sedimentary rocks 

were assigned to the Baraboo interval of Dott (1983)--that 

is, an age between about 1,750 and 1,450 Ma-by Greenberg and Brown (1984; 1986) and Brown and Greenberg 

(1986). However, with the possible exception of ferruginous chert, the sedimentary rocks are deformed (have a 

steep mylonitic lineation) in the same way as the volcanic 

rocks, indicating that they are older than 1 ,835 Ma, and 

probably approximately contemporaneous with the volcanism at ==1,860 Ma (Sims, 1990a; 1990c). The interlayered 

volcanic and sedimentary sequence is intruded by small 

bodies of comagmatic(?) metagabbro and metadiorite 

(Sims, 1990a) and by a large (35 km2) pluton of foliated 

tonalite (unit Xft, fig. 2). Volcanic rocks exposed in the 

second area, the Eau Claire River valley (locality B, figs. 1, 

2), consist of rhyolite and andesite as well as impure 

quartzite and conglomerate containing clasts of Archean 

granite gneiss (Sims, 1990a). Two rocks in these volcanic 

sequences have been dated. A felsic volcanic rock in the 

Eau Claire River valley (part of unit Xv, fig. 2) has a U-Pb 

zircon upper intercept age of 1 ,858±5 Ma, and a subvolcanic tonalite within the Eau Pleine shear zone has a U-Pb 

zircon upper intercept age of I ,860±7 Ma (Sims and others, 

1989). The possibility exists, however, that the volcanic 

rocks are in part older, because foliated diorite (W283B, 

table 2 and appendix B) from the Stevens Point (S, fig. 2) 

area, which has a U-Pb upper intercept zircon age of 

1 ,892±9 Ma (Sims and others, 1989), is chemically similar 

to dacite porphyry (sample W270A, table 2 and appendix 

B) in the volcanic sequence (locality A, fig. 1). Possibly, 

the 1,892 Ma tonalite is an early subvolcanic equivalent of 

the volcanic rocks. 

GRANITOID ROCKS 

Pembine-Wausau Terrane 

Granitoid rocks in the Pembine-Wausau terrane are 

mainly granodiorite _but range from tonalite and gabbro to 
alkaJi granite (table 1; figs. 3, 4). Outcrops of these rocks 

are most abundant in the northernmost part of the terrane, 

near the Niagara fault zone (fig. 2). The granitoid rocks 

can be grouped into two broad classes: (1) a syntectonic 

granodiorite suite and (2) a post-tectonic suite consisting 

of the =I ,836 Ma Athelstane Quartz Monzonite, 1 ,835 Ma 

red alkali-feldspar granite, and a I ,760 Ma granodioritegranite group. 

Syntectonic Granodiorite Suite 

The syntectonic granodiorite suite is spatially associated 

with the major ( 1 ,889-1,860 Ma) sequence of volcanic rocks 

(unit Xvs, fig. 2) in the Pembine-Wausau terrane. The rocks 

in the syntectonic suite are foliated and mainly compose unit 

Xgg in figure 2. The granodiorite suite is calcic to calcalkaJic (fig. 4), average to rich in magnesium content (fig. 

5), and sodic to very sodic (fig. 6); it has intermediate Si02 

concentrations, intermediate to high Al203 ( 12.4-I6.4 percent), high Sr concentrations, and low Rb, Nb, and Y concentrations. These rocks are compositionally similar to the 

Newingham Tonalite in the Dunbar, Wis., area (Sims and 

others, 1992), which is a normal volcanic-arc granite. On a 

FeO:(FeO+MgO) versus Si02 diagram (fig. 7), rocks of the 

syntectonic granodiorite suite plot mainly in the caJc-alkaJic 

(MgO-rich) field and lie within the synorogenic field of 

Anderson ( I983) for Penokean granitoid rocks in Wisconsin. 

The synorogenic field is virtually distinct from the field for 

the Athelstane batholith and the I ,835 Ma alkali-feldspar 

granite, indicating distinctly different magmas for these syntectonic and post-tectonic rocks. The granodiorite suite has 

moderately enriched light REE (rare earth elements) accompanied by depleted heavy REE patterns generally showing 

no or positive europium anomalies (fig. 8). Four samples 

from this suite are extremely depleted in HREE. One sample 

has a negative europium anomaJy. 

Tonalite and granodiorite (unit Xgt, fig. 2) exposed in 

central Wisconsin are included in the granodiorite suite, but 

they are distinctly younger than most of the granitoids 

exposed farther north. Two analyzed samples have U-Pb 

zircon upper intercept ages of I ,846±II Ma (W317, analysis 

in table I) and 1 ,838±6 Ma (W3I4, analysis in table 1 ), 

respectively. These rocks are calc-alkalic to alkalic calcic, 

have high Fe:Mg ratios, and have Rb, Sr, Y, Nb, and Zr 

concentrations indicative of subduction-related magmas. 

Post-Tectonic Suite 

The post-tectonic suite consists of the Athelstane Quartz 

Monzonite, which has an age of 1,836±15 Ma, 1,835 Ma 

aJkali-feldspar granite, and =1 ,760 Ma granodiorite-granite. 

Athelstane Quartz Monzonite 

The Athelstane Quartz Monzonite comprises a batholith 

of about 900 km2 extent in northeastern Wisconsin (unit 

EXPlANATION 

Pembine-Wausau terrane 

X 1,760 Ma granite-granodiorite suite 

+ 1,836 Ma Athelstane Quartz Monzonite 

6. Uncertain classification 

o Syntectonic granodiorite suite 

Marshfield terrane 

• 1,835 Ma alkali-feldspar granite 

.& Uncertain classification 

• Gneissic granite near Neillsville 

• Tonalite-granodiorite suite 

Figure 3. Quartz-alkali feldspar-plagioclase diagram showing composition of representative rocks in the PembineWausau and Marshfield terranes. 

Xgat, fig. 2). It is a coarse-grained granite to granodiorite 

containing nearly equal amounts of microcline microperthite, plagioclase, and quartz and 5-IO percent biotite and 

(or) hornblende. Typically, the mafic minerals are interstitial 

and give the rock a clotty appearance. The rocks are massive 

except along the north margin of the batholith, where they 

have a steep ductile stretching lineation. The stretching lineation probably formed during uplift of the batholith subsequent to crystallization of the magma. Granite from a large 

quarry in the southern part of the batholith has a U-Pb zircon 

age of 1,836±15 Ma (Sims, 1990b). Granite near Cherokee 

(unit Xgc, fig. 2), southwest of Wausau, is similar compositionally to rocks in the Athelstane batholith, and it is tentatively considered correlative with these rocks. A sample 

of the granite near Cherokee has a U-Pb zircon upper intercept age of 1,853±21 Ma (Sims, 1990a), which because of 

the large uncertainty is virtually indistinguishable from the 

1,836±15 Ma age on the Athelstane sample. The Athelstane 

Quartz Monzonite and the granite near Cherokee are mildly 

peraluminous, and have high Si02 (68-77 percent), intermediate Al203 (12-15 percent), and Na:K <1. These rocks have 

notably higher concentrations of Y, Zr, and Nb and are more 

highly evolved than the syntectonic granodiorite suite. The 

rocks are strongly enriched in iron and plot in the anorogenic 

Early Proterozoic Granitoid Rocks, Northern Wisconsin J7 
Table 1. Modes and chemical analyses of selected granitoid rocks in Pembine-Wausau terrane (exclusive of Dunbar area) 

[Leaders ( --),not determined. Tr, trace; X, present. Modal analyses by Bruce Brasaemle. Major oxide analyses by J.S. Wahlberg, A. Bartel, J. Taggart, and J. Baker. 

FeO, H20, and C02 by H. Neiman and G. Mason. Minor element analyses by X-ray fluorescence by Ross Yeoman. Minor element analyses by instrumental neutron 

activation by K.J. Schulz, J.S. Mee, and L.J. Schwarz] 

Syntectonic granodiorite suite Uncertain 

Sample No.--- IM12 IM16 W319A W325 W66 W249B W249A W251 W118 W205B W317 W314 W318 SWAU 

6-83 

Modal analyses (volume percent) 

Plagioclase 62.5 54.5 48 34 46 42 54 61 61 26 47.5 54 39 

Quartz 26 32 29.5 24.5 28 27 28 28 23 35.5 30 31 27 

K-feldspar 5 5 17.5 31 14 22 4 0.5 4.5 28 2.5 11 21.5 

Biotite 4 7 3.5 8.5 10 6.5 12 5.5 10 9 11 14.5 8 

Muscovite 1.5 0 0.5 0 0 Tr Tr 0 Tr 0 0.5 0 0.5 

Hornblende 0 0 0 0 2 Tr 0 5 1 0.5 0 0.5 0 

Epidote/ 

clinozoisite 1 1 1 1 Tr 2 2 0 Tr 0.5 2 3 1 

Sphene 0 0 0 0 0 Tr 0 Tr Tr Tr 0 0 0 

Chlorite 0 0 0 Tr 0 0 0 Tr 0 0 0 0 Tr 

Carbonate 0 Tr 0 0 0 0 0 0 0 0 0 0 0 

Accessory 

minerals Tr 0.5 Tr 1 Tr Tr Tr Tr 0.5 0.5 Tr 0.5 0.5 

Allanite 0 X X 0 0 X X X X X 0 X X 

Apatite 0 X X X 0 X X X X X X X X 

Opaque oxides X X X X 0 X X 0 X X 0 X X 

Zircon X X X X X X X X X X X X X 

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

Si02 72.6 69.8 73.4 67.0 68.5 71.2 

Al20 3 14.9 16.0 14.4 15.0 14.6 14.6 

Fe20 3 0.39 0.55 0.52 1.50 1.0 0.62 

FeO 1.76 2.05 0.61 2.47 2.6 2.3 

MgO 0.43 0.79 0.38 1.07 1.3 0.92 

CaO 3.13 3.88 2.02 2.89 2.2 2.5 

Na20 3.92 3.43 3.49 2.62 3.1 3.1 

~0 1.30 2.32 3.69 5.23 4.2 3.8 

H2

o+ 0.52 0.53 0.39 0.96 0.29 0.64 

H2o· <0.01 <0.01 0.03 <0.01 0.18 0.10 

Ti02 0.09 0.20 0.11 0.32 0.27 0.30 

P20s <0.05 0.06 <0.05 0.14 0.12 0.10 

MnO 0.07 0.04 0.06 0.05 0.07 0.06 

C02 <0.01 0.03 <0.01 <0.01 0.16 0.06 

Sum 99.17 99.69 99.16 99.27 98.59 100.3 

A/CNK 1.10 1.05 1.08 0.99 1.07 1.06 

field in the FeO:(FeO+MgO) versus Si02 diagram (fig. 7). 

Although the rocks are evolved compared to the syntectonic 

granodiorite suite, they are not potassic, as are most highly 

evolved granites. The granite overlaps the upper part of the 

field for the younger Middle Proterozoic granites of the Wolf 

River batholith (Anderson, 1983, fig. 10c). The REE patterns 

are distinctive (fig. 9), exhibiting large negative Eu 

)8 Precambrian Geology of Lake Superior Region 

69.3 65.7 69.7 71.7 69.3 68.9 71.8 72.3 

15.2 16.4 15.3 13.7 15.5 14.2 14.8 13.8 

0.85 0.53 1.2 0.97 1.12 1.2 0.56 1.0 

2.6 3.2 1.6 1.92 2.18 3.1 1.41 1.6 

0.87 1.6 0.89 0.57 0.95 0.96 0.67 0.99 

3.3 4.3 3.4 1.79 3.76 3.4 2.09 2.2 

3.4 4.0 3.8 2.70 3.91 3.5 3.45 2.6 

2.2 1.7 2.0 4.94 1.56 3.0 3.52 4.6 

0.61 0.76 0.48 0.59 0.73 0.77 0.64 0.81 

0.15 0.13 0.08 0.04 <0.01 0.01 0.01 0.38 

0.31 0.35 0.22 0.29 0.30 0.29 0.21 0.29 

0.15 0.09 0.10 0.07 0.08 0.11 0.08 0.08 

0.04 0.07 0.03 0.03 0.05 0.08 0.02 0.05 

0.03 0.08 0.02 <0.01 <0.01 0.05 <0.01 0.08 

99.01 98.91 98.82 99.32 99.46 99.57 99.27 100.78 

1.09 1.01 1.05 1.05 1.04 0.94 1.12 1.04 

anomalies, relatively flat heavy-REE slope, and depletion in 

the light REE. In a Nb versus Y diagram (fig. 1 0), the 

Athelstane Quartz Monzonite plots with other post-tectonic 

granitoids in the region astride the fields for within-plate 

granites and volcanic-arc and syncollisional granites. 

Although the Athelstane Quartz Monzonite makes up 

most of the Athelstane batholith, younger (::::1,760 Ma) 
Table 1. Modes and chemical analyses of selected granitoid rocks in Pembine-Wausau terrane (exclusive of Dunbar 

area)-Conti nued 

1,835 Ma 

Athelstane Quartz Monzonite alkali-feldspar 1,760 Ma granodiorite-granite group 

granite 

W213 WI-1 W188 W549 W533 W556 W555 W384 W12B SPB W54 W55 824-7 W252 

232-39 

Modal analyses (volume percent) 

20.5 26.5 34.5 28 26 16.5 13 41.5 27.5 36 41 28.5 36.5 

38.5 32.5 24.5 32.5 33.5 44 31.5 23 27 25 26 30.5 28 

38 27.5 27 35.5 38 35 49.5 46 36 27 28 37.5 7.5 

1.5 7.5 7 2 2 3.5 5 2 9.5 9 5 3.5 21 

0 0 0 0 0 0 0 0 0 1.5 Tr 0 0 

1 5.5 6.5 1.5 0 0 0.5 0 0 0 0 0 5 

Tr 0 Tr Tr Tr Tr Tr 0 Tr 1 Tr Tr Tr 

0 Tr Tr Tr 0 0 0 0 Tr 0 Tr 0 1 

0 0 0 0 Tr 0 0 0 Tr Tr 0 Tr 0 

0 0 0 Tr 0 Tr 0 0 0 0 0 0 0 

0.5 0.5 0.5 0.5 0.5 1 0.5 0.5 Tr Tr 0.5 Tr 1 

X 0 X X X X X 0 0 0 X 0 X 

X X X X 0 X X 0 0 0 X X X 

X 0 X X X X X X 0 0 X 0 X 

X X X X X X X 0 X X X X X 

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

76.7 71.4 68.1 75.0 75.0 77.1 76.5 75.7 76.5 70.0 71.0 67.7 

11.9 12.4 15.2 12.3 12.3 12.0 12.2 12.7 13.5 14.5 15.1 15.8 

0.90 1.17 0.77 0.56 0.44 0.28 0.54 0.35 0.54 1.3 0.62 1.5 

0.91 2.96 2.87 1.83 1.99 1.38 1.52 0.55 0.80 1.7 1.5 2.9 

0.11 0.44 0.75 0.37 0.22 0.25 0.20 0.11 0.23 0.68 0.44 1.6 

0.43 1.64 2.11 1.20 0.83 0.46 0.72 0.43 0.85 1.9 2.0 3.6 

3.40 2.92 3.35 2.51 3.30 2.32 2.23 4.09 3.0 3.0 4.1 3.3 

4.61 4.73 4.96 5.26 4.44 5.81 6.0 4.56 5.2 4.4 3.9 3.1 

0.41 0.50 0.55 0.34 0.37 0.36 0.22 0.45 0.64 0.47 0.86 

<0.01 0.04 <0.01 <0.01 0.05 <0.01 <0.01 0.10 0.07 0.15 0.13 

0.14 0.45 0.54 0.26 0.12 0.20 0.21 0.05 0.07 0.27 0.25 0.58 

<0.05 0.10 0.10 <0.05 <0.05 <0.05 <0.05 <0.05 0.07 0.09 0.09 0.29 

<0.02 0.06 0.04 0.03 0.03 <0.02 <0.02 <0.02 0.04 0.04 0.02 0.05 

<0.01 0.01 0.01 0.06 0.05 0.01 0.09 0.03 0.08 0 

99.58 98.82 99.36 99.77 100.33 100.56 98.92 101.38 98.59 99.72 101.40 

1.05 0.96 1.03 1.03 1.05 1.10 1.07 1.02 1.12 1.10 1.04 1.03 

bodies called Amberg Granite compose 25-30 percent of the (analysis in table 1 ), from the Granite Heights area, northeast 

batholith. of Wausau (unit Xgr, fig. 2) (LaBerge and Myers, 1983), is 

representative of these red alkali-feldspar granites. These 

1,835 Ma Alkali-Feldspar Granite 

rocks form generally small, separate plutons, some of which 

have been quarried for building stone. The plutons lack a 

penetrative foliation but contain abundant brittle fractures 

Known post-tectonic granitoid rocks of the 1,835 Ma and exhibit extensive retrogressive metamorphism. Feldgroup are confined to Marathon County and adjacent areas, spars are reddened by hematite, and biotite is altered mainly 

and truncate the Eau Pleine shear zone. Sample W384 to chlorite, probably as a result of deuteric alteration. 

Early Proterozoic Granitoid Rocks, Northern Wisconsin J9 
Table 1. Modes and chemical analyses of selected granitoid rocks in Pembine-Wausau terrane (exclusive of Dunbar area)-Continued 

Syntectonic granodiorite suite Uncertain 

Sample No.--- IM12 IM16 W319A W325 W66 W249B W249A W251 W118 W205B W317 W314 W318 SW AU 

6-83 

Minor elements (parts per million) by X-ray fluorescence analysis and INAA for Ta, Hf, Sc, Cs, Th, and U 

Rb 45 60 66 134 74 105 53 74 40 154 35 60 83 82 

Sr 656 695 447 293 505 319 411 459 472 177 385 348 259 214 

y 3 1 0 9 7 13 3 24 

Zr 37 64 50 88 115 91 141 154 

Nb 5 2 1 6 3 4 7 13 

Ba 1,317 733 1,503 1,742 1,306 800 838 175 930 1,483 859 960 1,226 1,040 

Cs 

Ta 

Hf 

Th 

u 

Sc 

Zn 

La 

Ce 

Nd 

Sm 

Eu 

Tb 

Yb 

Lu 

0.50 0.52 1.38 1.01 1.05 2.96 0.56 1.35 

0.22 0.27 1.41 1.21 0.56 3.05 0.43 0.42 

1.2 2.15 2.21 4.14 4.26 2.46 3.2 3.4 

0.71 3.2 <15 16 8.22 18 9.1 11.6 5.4 23 <15 7.01 19 

0.15 0.8 <15 <15 8.4 2 0.81 2.7 0.9 <15 <15 2.7 <15 

1.9 3 8.68 5.5 3.86 8.71 4.3 9.18 

50 40 35 

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

9.31 16.1 24.2 38.7 44.9 18.9 26.3 26.9 

17.3 29.6 51.2 73 87.2 39.3 47 53.3 

7 9 21 28 28 15 14 19 

1.15 1.42 4.06 3.76 3.58 3.41 2.25 3.55 

0.37 0.49 0.9 0.77 0.85 0.67 0.63 0.8 

0.10 0.4 0.37 0.21 0.58 0.18 0.44 

0.44 0.22 1.15 0.98 0.33 1.48 0.33 1.96 

0.08 0.04 0.17 0.15 0.05 0.23 0.05 0.34 

SAMPLE DESCRIPTIONS AND LOCALITIES 

Most rock names are based on chemical classification of DcLaRoche and others, 1980. Rock names bac;ed on Streckeisen (1976) classification given in parentheses where they differ. Rocks clao;sed as alkali granites are identified. 

IM12 

IM16 

W319A 

W325 

W66 

W249B 

W249A 

W251 

W118 

W205B 

W317 

W314 

W318 

SWAU 6-83 

SYNTECTONIC GRANODIORITE SUITE 

Granodiorite (tonalite), sec. 24, T. 38 N., R. 11 E. 

Granodiorite (tonalite), sec. 21, T. 37 N., R. 11 E. 

Granodiorite, sec. 21, T. 37 N., R. 11 E. Age 1,870±7 Ma. 

Granodiorite (granite), sec. 28, T. 34 N., R. 2 W. 

Hornblende-biotite granodiorite (tonalite), sec. 19, T. 34 N., R. 6 W. 

Biotite granodiorite, sec. 27, T. 37 N., R. 3 W. 

Granodiorite (tonalite), sec. 27, T. 37 N., R. 3 W. 

Foliated tonalite, sec. 20, T. 38 N., R. 5 W. 

Biotite granodiorite (tonalite), sec. 22, T. 36 N., R. 6 W. 

Granodiorite, Grandmother Dam, sec. 10, T. 33 N., R. 6 E. 

Granodiorite (tonalite) near Dudley, sec. 23, T. 33 N., R. 8 E. Age 1,846±11 Ma 

Granodiorite, Grandfather Dam, sec. 30, T. 33 N., R. 6 E. Age 1,838±6 Ma. 

Granodiorite, sec. 13, T. 32 N., R. 7 E. 

UNCERTAIN 

Granite (alkali granite), sec. 34, T. 35 N., R. 20 E. 

Jl 0 Precambrian Geology of Lake Superior Region 

0.74 

0.40 

4.37 

8.67 

1.5 

5.24 

39 

43.1 

71.4 

29 

5.24 

0.94 

0.60 

1.49 

0.23 
Table 1. Modes and chemical analyses of selected granitoid rocks in Pembine-Wausau terrane (exclusive of Dunbar 

area)-Conti nued 

W213 

136 

15 

32 

152 

23 

204 

18 

<15 

65 

1,835 Ma 

Athelstane Quartz Monzonite alkali-feldspar 1,760 Ma granodiorite-granite group 

granite 

WI-1 W188 W549 W533 W556 W555 W384 W12B SPB W54 W55 824-7 W252 

232-39 

Minor elements (parts per million) by X-ray fluorescence analysis and INAA for Ta, Hf, Sc, Cs, Th, and U 

141 

114 

50 

292 

50 

1,020 

1.5 

2.48 

13.2 

37.8 

2.2 

4.68 

103 

201 

63 

W213 

WI-I 

W188 

W549 

W533 

W556 

W555 

W384 

12.6 

1.71 

1.99 

5.78 

0.82 

Wl2B 

SPB-232-39 

W54 

W55 

824-7 

W252 

97 94 119 80 88 77 163 131 118 119 

185 96 85.4 88.1 69 29 83 346 603 473 

26 32 38 24 25 27 24 

234 285 164 154 257 107 110 245 170 233 

15 10 13 7 11 17 27 

1,824 1,354 1,113 830 1,404 1,106 870 1,934 1,305 1,326 

2.1 1.6 4.5 0.65 3.33 1.88 

1.0 0.87 1.76 0.59 1.55 2.30 

6.35 10.1 4.57 7.35 5.35 6.45 

<15 <15 13.6 8.67 <15 <15 28.8 39.6 25.4 30.8 

<15 <15 1.5 1.5 <15 17 10 2.2 4.5 3.10 

3.20 4.0 3.03 3.96 2.21 9.62 

74 

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

44.1 161 41.5 105 

99.6 311 74.0 185 

39 94 22.5 52 

9.65 14.68 6.0 6.68 

1.18 1.07 0.42 1.02 

1.45 1.48 0.75 0.40 

5.23 3.93 2.88 0.60 

0.84 0.61 0.53 0.11 

ATHELSTANE QUARTZ MONZONITE 

Granite near Cherokee (alkali granite), NE1/4 sec. 23, T. 28 N., 

R. 2 E. Age 1,853±21 Ma. Considered correlative with rocks of Athelstane batholith. 

Granite, sec. 23, T. 39 N., R. 6 W. Similar chemically to Athelstane Quartz Monzonite. 

Athelstane Quartz Monzonite (alkali granite), NE1/4 sec. 15, T. 34 N., R. 19 E. 

Athelstane Quartz Monzonite (alkali granite), NE1/.t sec. 12, T. 35 N., R. 20 E. 

58.3 

110 

25 

4.08 

0.91 

0.29 

0.49 

0.06 

Athelstane Quartz Monzonite (alkali granite), quarry at Mt. Tom, sec. 19, T. 33 N., R. 20 E. Age 1,836±15 

Ma(?). 

Athelstane Quartz Monzonite (alkali granite), SW1/.t sec. 13, T. 35 N., R. 20 E. 

Athelstane Quartz Monzonite (alkali granite), SWI/4 sec. 14, T. 35 N., R. 20 E. 

1,835 MA ALKALI-FELDSPAR GRANITE 

Alkali-feldspar granite (alkali granite), sec. 31, T. 30 N., R. 8 E. 

1,760 MA GRANODIORITE-GRANITE GROUP 

Granite near Amberg, NEI/4SEI/4 sec. 9, T. 35 N., R. 20 E. Age 1,752±8 Ma. 

Granite near Amberg, NWI/4SWI/4 sec. 31, T. 36 N., R. 21 E. 

Granodiorite (granite) near Lugerville, SWI/.t sec. 17, T. 38 N., R. I W. Age 1,773±8 Ma. 

Biotite granodiorite (granite), sec. 6, T. 38 N., R. 3 E. 

Granite near Monico, SWI/4 sec. 6, T. 35 N., R. 11 E. Age 1,739±8 Ma. 

Granodiorite near Radisson, NE1/4 sec. 22, T. 38 N., R. 7 W. Age 1,765±6 Ma. 

91.5 

164 

50 

7.29 

1.37 

0.56 

1.55 

0.22 

Early Proterozoic Granitoid Rocks, Northern Wisconsin J11 
1500 

1000 

500 

QSy 

0 ~ 

slightly altered; not / 

representative of fresh rocks Alk Gr 

1000 1500 2000 2500 3000 

R1 = 4,000Si- ll,OOO(Na + K)- 2,000(Fe + Ti) 

EXPLANATION 

Marshfield terrane 

~ 1,835 Ma alkali-feldspar granite 

• Syntectonic granodiorite 

+ Uncertain affinity 

RI 

Pembine-Wausau terrane 

6 Athelstane Quartz Monzonite 

o Syntectonic granodiorite 

x Uncertain affmity 

o 1,760 Ma granodiorite-granite 

o 1,835 Ma alkali-feldspar granite 

Figure 4. Classification of granitoid rocks, Pembine-Wausau and Marshfield terranes (modified from DelaRoche and others, 1980). Classification of plutonic rocks: Gb-No, gabbronorite; 01 Gb, olivine gabbro; Alk Gb, alkali gabbro; Thr, theralite; Mel, melteigite; Gb-Di, 

gabbro-diorite; Mz Gb, monzogabbro; Sy Gb, syenogabbro; Ess, essexite; ljo, ijolite; Di, diorite; Mz Di, monzodiorite; Mz, monzonite; Sy Di, syenodiorite; Gd, granodiorite; Gr, granite; Alk Gr, alkali granite; Q M, quartz monzonite; Q Sy, quartz syenite; Sye, syenite; N Sye, 

nepheline syenite. 

)12 Precambrian Geology of Lake Superior Region 
... 

z 

w 

0 

0:: 

w 

Q.. 

!i: 

C) 

~ ~ 

0 "' 

V5 

so~----~------~--~--.~ ----~-.~,----~ 

Very Mg rich ; Mg nch 1 .& 11\ 

I ,' 0/w.~U/".,. A 

i I + '6 v,-6, 

/ ~ / / IX 0 I Q, / 1\ ' • '_Q_ (r,t ' w. 

/ n p:;!J o<tJ / / 

./ 0'/ X ,'4 

• /0 ,/ ,/ 

-

70 -

-

-

- .& ./ / / 1. I I 

/'• ;// 1/1 

60 - I I -

..... ·~ :,,// ,/'// 

.... ... ,,~'" + ,,""' -

/ Average /Fe rich Very Fe rich 

so~-·~/--·~~-/~I~A='~/-'~···~1·------~'------~~----~ 0.5 0.6 0.7 0.8 0.9 

(FcO + 0.89F~~):(Fc0 + 0.89F~~ + MgO) 

EXPLANATION 

Marshfield terrane 

.& 1,835 Ma alkali-feldspar granite 

• Syntectonic granodiorite 

+ Uncertain affinity 

Pembine-Wausau terrane 

6 Athelstane Quartz Monzonite 

o Syntectonic granodiorite 

x Uncertain affinity 

O 1,760 Ma granodiorite-granite 

<> 1,835 Ma alkali-feldspar granite 

I 0 

Figure 5. Si02 versus (Fe0+0.89 Fe20 3):(Fe0+0.89 

Fe20 3+Mg0) diagram (modified from DeWitt, 1989) showing 

compositions of plutonic rocks in Pembine-Wausau and Marshfield terranes. 

The alkali-feldspar granite is mildly peraluminous, 

alkali-calcic, Fe rich, and sodic, and has very low Sr, and 

high U (Sims and others, 1989, fig. 9), high FeO:FeO+MgO 

(>0.80), Na20:K20 ( <1.0), Ba:Sr (>3.0), and K:Rb (>300), 

and large negative Eu anomalies (Sims and others, 1989, fig. 

10), similar to the Athelstane batholith. On a Ta-Yb diagram 

(Sims and others, 1989, fig. 11), these granite bodies plot as 

within-plate granite. 

1,760 Ma Granodiorite-Granite Group 

The 1,760 Ma granodiorite-granite group crops out as 

scattered small plutons (unit Xga, fig. 2) across northern 

Wisconsin (see appendix A). Except for the pluton south of 

Monico (Mo, fig. 2), these rocks intrude older granitic rocks. 

The principal cluster of these young intrusions is in the 

Athelstane batholith, where three bodies of Amberg Granite 

(1 ,752±8 Ma) intrude the Athelstane Quartz Monzonite 

(Sims, 1990b). The 1,760 Ma rocks lack a penetrative foliation but are cut by ductile-brittle shear zones, such as the 

Twelvefoot Falls shear zone in northeastern Wisconsin. Zircon ages average approximately 1,760 Ma but range from 

EXPLANATION 

Marshfield terrane 

.& 1,835 Ma alkali-feldspar granite 

• Syntectonic granodiorite 

+ Uncertain affinity 

Pembine-Wausau terrane 

6 Athelstane Quartz Monzonite 

0 Syntectonic granodiorite 

x Uncertain affinity 

o 1,760 Ma granodiorite-granite 

<> 1,835 Ma alkali-feldspar granite 

Figure 6. Si02 versus K20:(K20+Na20) diagram (modified 

from DeWitt, 1989) showing compositions of plutonic rocks in 

Pembine-Wausau and Marshfield terranes. 

.... 

z 

w 

u 

a: 

w 

~ 

.... 

J: 

(.!) 

jjj 

?; 

~ 

e-1 

0 

;n 

Calc-alkalic 

(MgO rich) 

Tholeiitic 

(FeO rich) 

FeO: (FeO+MgO) 

Figure 7. FeO:(FeO+MgO) versus Si02 diagram for the syntectonic granodiorite suite (open circles) and rocks of the 

post-tectonic Athelstane Quartz Monzonite (solid dots), Pembine-Wausau terrane. Synorogenic field of Anderson (1983) 

for Penokean granitoid rocks in Wisconsin shown for 

comparison. 

Early Proterozoic Granitoid Rocks, Northern Wisconsin J13 
w 

!::: 

a: 

c 

z 

0 

:I: 

u 

w 

..J 

a.. 

:iE 

<( 

en 10 

Figure 8. Chondrite-normalized REE plots for syntectonic granodiorite suite, Pembine-Wausau terrane. Analyses given in table 

1. Gd, no data. 

1,773±8 Ma to 1,739±8 Ma (Sims and others, 1989). The 

plutons are coeval with the anorogenic rhyolite-granite terrane in south-central Wisconsin (Anderson and others, 1980; 

Smith, 1983; Sims, 1990d), which probably overlies buried 

rocks of the Wisconsin magmatic terranes, but they differ 

from these anorogenic rocks in being more variable in composition and in having somewhat lower FeO:(FeO+MgO) 

and higher Sr. 

)14 Precambrian Geology of Lake Superior Region 

The rocks have variable REE patterns with enriched 

light-REE concentrations (fig. 11 ). The four samples for 

which REE have been determined include two from near 

Lugerville (L, fig. 2) in northwestern Wisconsin (W54, 

W55) that display steep REE patterns. The sample of 

Amberg Granite (SPB-232-39) from northeastern Wisconsin and the sample of granite from near Radisson (W252) in 

northwestern Wisconsin, in contrast, have less depleted 
500 

100 

~ 

a: 

0 

z 

0 

:I: 

u 

w 

...J 

0... 

W556' 

'\. 

'\ 

\ 

\ 

\ 

\ " \ ' \ \ \ 

73-8 '\ \ 

W553 ~\ '\.'\. 

" \ ", ~~ \ '\. 

', ,, ~ 

~ """ \ ' \ ~ <( ' \ C/) 

'',,..,, \ 

I \\ 

\ ~/ r .... ':._'-- ----- 1 I -._ -- 1 II I ::::::.;::--_ -------

1 I I ............ --- -.... I \ v / ............ ---

............................. ------..__ 

10 

I r I ..................... ... I I ...... ..._ I I ................ 

\ I ......... _ ..... 

I / ~ 

I I 

I I I I 

I I 

\ I 

w 

Yb Lu 

Figure 9. Chondrite-normalized REE plots for samples of Athelstane Quartz Monzonite, Pembine-Wausau terrane. Analyses given in table 1. Gd, no data. 

heavy-REE and small (Radisson) to large (Amberg) negative 

Eu anomalies. The variability in composition of the I ,760 

Ma granodiorite-granite group from the Pembine-Wausau 

terrane probably reflects differences in the source materials 

that were melted. The variability of these sources appears to 

have been greater than that in sources for the I ,760 Ma 

anorogenic rhyolite-granite group in south-central Wisconsin (Smith, 1983). 

Marshfield Terrane 

Granitoid rocks of Early Proterozoic age in the Marshfield terrane are mainly tonalite-granodiorite and alkali-feldspar granite, but range from gabbro-diorite to alkali granite 

(fig. 3). In general, tonalite and granodiorite (unit Xft, fig. 

2) are more abundant in the eastern part of the terrane, 

between Marshfield and Stevens Point, where they intrude 

Archean basement (fig. 2). A body of gneissic granite (granite near Neillsville (Ne); unit Xggr, fig. 2) at least 30 km 

long (east-west) and 3-5 km wide is exposed in the Black 

River valley and its tributaries. The gneissic granite intrudes 

previously deformed Archean gneisses and itself has a 

pronounced mylonitic foliation and lineation. Red alkalifeldspar granite of the I ,835 Ma group (unit Xgr, fig. 2) is 

widespread in the southern part of the terrane, west of Wisconsin Rapids (Wr, fig. 2). Other granitoid rocks are exposed 

discontinuously, but they are too poorly known to be 

characterized either petrographically or chemically; they are 

assigned to unit Xg in figure 2. The granitoid rocks can be 

grouped on the basis of composition, structure, and age into 

Early Proterozoic Granitoid Rocks, Northern Wisconsin J15 
z 

3100 

...J 

~ 

a:: 

w 

ll. 

~ ~ 

~ 

.D 

z 

VAG+ 

Syn-COLG 

0 0 ORG 0 

0 

,L-~--~~~ll,--~~_L~~,o~o--~_L~~~,ooo 

Y, IN PARTS PER MILLION 

EXPLANATION * 1,760 Ma granodiorite-granite group 

o 1,835 Ma alkali-feldspar granite 

• Athelstane Quartz Monzonite 

o Syntectonic granodiorite suite 

WPG Within-plate granite 

ORG Ocean ridge granite 

VAG Volcanic-arc granite 

Syn-COLG Syncolllsional granite 

Figure 10. Nb versus Y diagram (modified from Pearce and 

others, 1984) for granitoid rocks in the Pembine-Wausau terrane. Excludes Dunbar area (Sims and others, 1992). 

two classes: (1) a syntectonic tonalite-granodiorite suite and 

gneissic granite near Neillsville, and (2) post-tectonic alkalifeldspar granite of the I ,835 Ma group. 

Syntectonic Tonalite-Granodiorite Suite 

The tonalite-granodiorite suite is exposed intermittently 

in the Wisconsin River valley and its tributaries west of 

Stevens Point (S, fig. 2), predominantly in unit XAgt. Unit 

XAgt consists of Archean gneiss intruded by numerous, generally small bodies of Early Proterozoic tonalite-granodiorite. Excellent exposures at Conants Rapids, south of Stevens 

Point, have been mapped in detail and described by Maass 

and others (1980). Earlier, Anderson (1972) described the 

field relations and the petrology of tonalitic bodies in Mill 

Creek and in other tributaries west of the Wisconsin River. 

Anderson and Cullers ( 1987) described the geochemistry of 

the tonalitic-granodioritic rocks. 

The granodiorite-tonalite in the Stevens Point area crops 

out as small irregular bodies and dikes, and is divided into 

three textural-structural groups. From oldest to youngest 

these groups are: (1) foliated biotite-hornblende granodiorite 

to diorite, (2) lineated granodiorite to granite, and 

J16 Precambrian Geology of Lake Superior Region 

(3) massive granodiorite to tonalite. Anderson (1972) and 

Anderson and Cullers ( 1987) referred to each of these three 

rock groups as tonalites, as did Sims (1990c). The textural 

groups are believed to represent decreasing ductile deformation intensity during granodiorite-diorite emplacement 

(Sims, 1990c). 

The foliated granodiorite to diorite is a gray to palereddish-brown, medium-grained gneiss. A foliation 

expressed by biotite, with or without hornblende, that wraps 

around plagioclase (oligoclase-andesine) defines a "tensepod" fabric (Anderson, 1972). The granodiorite typically 

forms small irregular bodies and dikes ( 1-3 m thick) that 

have sharp contacts with Archean gneiss. Although contacts 

with the gneiss are discordant, foliation in the two units is 

parallel. The foliated granodiorite has a xenomorphic 

inequigranular assemblage of blocky plagioclase porphyroclasts (2-4 mm) surrounded by granular, finer grained plagioclase and quartz. Aligned biotite and hornblende in the 

groundmass yield a prominent foliation. The texture is typical IP mantle (or mortar) structure (Hanmer, 1982) characteristic of mylonitic rocks. Except for the relict plagioclase 

porphyroclasts, and perhaps part of the hornblende, the 

major rock constituents are recrystallized, commonly to 

finer grained aggregates, as a consequence of ductile deformation. The rocks can be classed as protomylonite or orthomylonite (Wise and others, 1984 ). The hornblendeandesine (An26-32) assemblage indicates metamorphism at 

the amphibolite facies. Typical modes (W283B, W260, and 

W254B) are given in table 2. According to the chemical data 

reported in Anderson and Cullers (1987), the foliated granodiorite to diorite ranges from calc-alkalic granodiorite to 

alkali-calcic to calc-alkalic tonalite-granodiorite. 

The lineated granodiorite to granite is gray to pale reddish brown, medium to fine grained ( 1-3 mm), and finer 

grained than associated foliated granodiorite. These lineated 

rocks form small irregular bodies and dikes (3 cm-3 m) that 

sharply cut older rocks. In contrast to the Archean gneiss 

and foliated granodiorite, the lineated tonalite is generally 

not folded. The lineation, defined by elongate, oriented 

grains of minor hornblende and biotite and elongate aggregates of these minerals with quartz, plunges steeply, generally to the southeast, and is parallel to that in the foliated 

granodiorite. A photograph showing this penetrative lineation is reproduced in Maass and others (1980, fig. 7). The 

lineated granodiorite to granite consists of a xenomorphic 

granular assemblage of oligoclase (An2o-2s) that may be 

weakly zoned, microcline, biotite, and minor hornblende. 

Because of the lack of distinctive, large porphyroclasts, the 

degree to which the rocks were recrystallized during ductile 

deformation is equivocal. Typically, the rocks are bimodal 

in texture and consist of aligned streaks of fine-grained, 

oriented biotite, quartz, and plagioclase in a coarser grained 

assemblage of the same minerals; they can be classified as 

either protomylonite or orthomylonite. A mode for a typical 

lineated granodiorite (W283A) is listed in table 2. 
w 

1:: 

a: 

0 

z 

0 

J: 

u 

~ 

u 

~ 

100 

10 

W54, granite 

near Lugerville 

W252, 

granodiorite \ 

near Radisson \ 

\ 

W55, \.\ 

biotite granite '. "\ 

' \ ', \ 

SPB 232-39, ' \ 

Amberg Granite \ '\ \ 

\ \ \ 

~\ ~ ,, 

~ 

" '' \""' \ ' ~ ~ 

\, ""-, ~ 

',',,,1\ 

', I \ 

' \ \ '\~ ~ 

I\ ''~--~\ ----- ---- 1 , I ----...-

\ '\j \ _____ __ 

\ ,, ---- 1 \ \ ----- 'v' ',, \ ----~ ~ 

\ '-

'-....... ....... ....... 

' ' ....... 

' ' ....... ....... 

' ' ' ....... ....... 

' ' ' ....... ....... ....... ....... 

' ' ' ' ' '. 

Figure 11. Chondrite-normalized REE plots for samples of 1,760 Ma granodiorite-granite group. Gd, no data. 

The third group, massive granodiorite to tonalite, Jacks 

penetrative structures and cuts deformed granodiorite bodies. Where observed in sees. 1 and 12, T. 23 N., R. 7 E., 

adjacent to Rocky Run, and southwest of Stevens Point, 

granodiorite is gray, medium grained (==3 mm), and has a 

hypidiomorphic granular texture modi tied by weak 

Early Proterozoic Granitoid Rocks, Northern Wisconsin J17 
--co 

""'0 Table 2. Modes and chemical analyses of selected Early Proterozoic granitoid rocks in Marshfield terrane .. 

I'D 

~ [Leaders ( --),not detennined. Tr, trace; X, present.<, less than. Modal analyses by Bruce Brasaemle. Major oxide analyses by J.S. Wahlberg, A. Bartel, J. Taggart. and J. Baker. FeO, H20, and COz analyses 

3 by H. Neiman and G. Mason. Minor element analyses by X-ray fluorescence by Ross Yeoman. Minor element analyses by instrumental neutron activation by K.J. Schulz, J.S. Mee, and L.J. Schwarz} f:T 

.. 

;;· 

:I 

~ Smtectonic tonalite-granodiorite suite Granitoid rocks of uncertain classification Q Granite near N ellisville 0 

OQ 

-< 

Q Sample No.- W283B W214-2 W270A W260 W254B W237 W283A W214-1 W309C W390 21-84 W284 W285-1 W288 W285 W305 26-84 W306 W859 -,... 

AI 

'II:" 

I'D 

V) Modal analyses (volume percent) c 

"'CC 

I'D 

.. 

s· Plagioclase 57 -- - 52 63 45.5 56 - 36 26.5 21 43 35.5 32 30 35.5 32 24 .. Quartz 20 -- -- 28 28 33 30.5 43 41.5 42.5 33 41 36 34 36 33 32 :II' - I'D K-feldspar 1 0 0 Tr 3 18 26.5 32.5 18 20.5 30.5 31.5 19.5 27.5 38.5 o.:9. - -- - Q 

:I Biotite 11.5 - -- 11 7 17.5 10 - 1 4.5 3.5 5.5 Tr 1 0.5 3 6 4.5 

Muscovite 0 - - 0 0 Tr 0 - 1 0.5 Tr Tr Tr 0 Tr 0 0.5 0 

Hornblende 9 - - 8 0 0 0 - 0 0 0 0 0 0 0 5.5 0 0 

Epidote/ 

clinozoisite Tr - - Tr 1.5 1 0 - Tr Tr 0 Tr 1 Tr 2 Tr 0.5 Tr 

Sphene 1 -- - 0.5 Tr 0.5 0 - Tr 0.5 Tr 0 0 0 0 Tr 0 Tr 

Chlorite 0 - - 0 0 0 0 - 0 Tr 0 0 0 Tr 0 0 Tr 0 

Carbonate 0 -- - 0 0 1.5 0 - 0 0 0 0 0 0 0 0 0 0 

Accessory 

minerals 0.5 - - 0.5 0.5 1 0.5 - 1 Tr 0.5 0.5 2 0.5 2 0.5 0.5 1 

Allanite X X X 0 X X X 0 0 0 X 0 X X X 

Apatite X X 0 X X X X 0 X X 0 X X X X 

Fluorite 0 0 X 0 0 X X X 0 0 0 0 0 0 0 

Opaque oxides X X X X X 0 X X 0 X X X X 0 X 

Zircon X X X X X X X X X 0 0 0 X X X 

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

Si02 61.8 66.6 64.9 - - - 70.5 74.2 - - - - - - - - - 75.2 53.6 

Al20 3 17.5 15.7 16.2 - - - 15.4 13.8 - -- --- - - - - - - 13.1 15.9 

Fe20 3 1.18 1.1 0.83 - - -- 0.75 0.68 - - -- - - - - - - ~:~~ } 10.6 FeO 3.72 2.5 2.98 - - - 1.74 0.84 - - - -- - - - - - MgO 2.40 1.8 2.25 - - - 0.88 0.52 - - - - - - - - - 0.33 4.63 

CaO 4.98 3.7 5.02 - - - 2.36 1.7 - - - - - - - - - 1.50 7.90 

NazO 4.35 4.1 3.86 - - - 4.33 3.4 - - - - - - - - - 3.43 2.90 

KzO 2.18 2.8 1.54 - - - 2.70 4.2 - - - - - - - - - 3.76 1.78 

H20+ 0.78 0.57 1.18 - - - 0.41 0.60 - - - - - - - - - 0.18 } 1.42 

H2

o- 0.02 0.19 - - - 0.03 0.23 - - - - - - - - - <0.01 

Ti02 0.55 0.48 0.42 - - - 0.32 0.16 - - - - - - - - - 0.18 1.04 

PzOs 0.20 0.14 0.14 - - - 0.1 0.05 - - - - - - - - - <0.05 0.23 
l"r'' 

~ 

-< 

., 

0 

;;-

0 

s 

;:;· 

~ AI 

;:, 

5 

a: 

:Ia 

Q 

1"1 

f." 

z 

Q 

:::l 

.. 

~ 

;:, 

~ c;;· 

a 

;:, 

(I> 

;· 

-... 

\D 

MnO 

C02 

Sum 

A/CNK 

Rb 

Sr 

y 

Zr 

Nb 

Ba 

Cs 

Ta 

Hf 

Th 

u 

Sc 

Zn 

La 

Ce 

Nd 

Sm 

Eu 

Tb 

Yb 

Lu 

0.08 

0.04 

99.78 

0.94 

71 

663 

10 

113 

6 

707 

41 

<15 

78 

-

-

-

-

-

-

-

-

0.07 

0.03 

99.78 

0.95 

80 

639 

15 

134 

<5 

747 

1.02 

0.55 

3.5~ 

7.5 

2.6 

8.04 

57 

32.4 

55.2 

28.0 

4.07 

0.92 

0.38 

1.17 

0.17 

0.04 

9936 

0.95 

0.04 0.04 

0.02 

99.57 100.44 

1.08 1.05 

Minor elements. (parts per million) by X-ray fluorescence analysis and INAA for Ta, Hf, Sc, Cs, Th, and U 

39 - - 63 87 84 55 228 - - - - - 809 - - 400 359 316 72 45 - - - - - <10 - - 6 9 16 24 8 - - - - - 112 - - 123 101 116 116 31 - - - - - <5 - - 4 8 13 15 8 - - - - - 616 - - - 669 1300 - - - - - - - 1.19 - - - - 0.91 - - - - - - - 033 - - - - 1.15 - - - - - - - 2.44 - - - - 3.23 - - - - - - - 6.02 - - 8 15 8.69 11 7 - - - - - 1.11 - - 15 <15 4.0 24 17 - - - - - 6.88 - - - - 2.15 - - - - - - - 61 - - 68 47 30 44 44 - - - - -

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

35.0 - - - - 18.0 - - - - - - - 53.1 - - - - 30.4 - - - - - - - 15.1 - - - - 15 - - - - - - - 2.56 - - - - 2.55 - - - - - - - 0.85 - - - - 0.50 - - - - - - - 0.27 - - - - 0.28 - - - - - - - 0.51 - - - - 1.27 - - - - - - - 0.09 - - - - 0.18 - - - - - - -

0.02 0.15 

0.02 

9932 100.15 

1.06 0.76 

- - 82 38 

- - 201 443 

- - 8 20 

- - 114 112 

- - 6 8 

- - 896 470 

- - - 0.59 

- - - 0.59 

- - - 3.22 

- - <15 3.67 

- - <15 1.22 

- - - 28.2 

- - - 97 

- - - 26.8 

- - - 54.4 

- - - 25.0 

- - - 4.95 

- - - 138 

- - - 0.58 

- - - 1.87 

- - - 0.27 
~ Table 2. Modes and chemical analyses of selected Early Proterozoic granitoid rocks in Marshfield terrane-Continued 

= 

... 

""0 1.835 Ma al.kali-feldsoar lrranite I'D 

n 

Ill 

3 SWI =- ... Sample No.- W307B W307A W398 W394 wm W849 W299 W851-2 8-86 W851-1 W851-3 W851-4 ii" 

~ 

" I'D 

Q Modal analyses (volume percent) sOQ 

"< 

Q Plagioclase 33 27.5 29.5 19 12 

- Quartz 33 37.5 32.5 39 34.5 ,.... 

Ill 

~ K-feldspar 30 29 32 39.5 52 V) 

c:: Biotite 1.5 5 5.5 1.5 0.5 "C 

~ Muscovite 0.5 0.5 0 0 0 ~- Hornblende 0 0 0 0 0 ~ 

I'D Epidote/ OQ 

s· clinozoisite 0.5 Tr 0 0 0 ~ Sphene Tr Tr 0 0 0 

Chlorite Tr 0 Tr Tr 0.5 

Carbonate 0 0 0 0 0 

Accessory 

minerals 1.5 0.5 0.5 1 0.5 

Allanite X X X 

Apatite 0 X X X 

Fluorite 0 0 0 X 

Opaque oxides X X X X 

Zircon X X X X 

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

Si02 76.3 77.5 - - 76.5 75.9 75.6 76.3 76.5 63.5 50.8 58.5 

Al203 12.0 11.6 - - 12.2 11.9 11.8 11.7 12.2 15.8 14.9 14.9 

Fe20 3 1.31 0.59 - - 0.87 0.39 0.43 0.34 1.11 1.03 1.8 1.53 

FeO 0.73 0.88 - - 0.40 1.40 1.58 1.24 3.72 6.48 5.54 

MgO 0.13 0.35 - - 0.16 0.11 0.13 0.19 0.11 2.75 4.71 4.46 

CaO 0.59 0.57 - - 0.31 0.53 0.66 0.77 0.34 5.26 6.97 6.06 

N~O 3.38 3.46 - - 3.06 3.79 3.76 3.55 3.45 3.43 2.97 3.12 

~0 4.37 3.71 - - 5.64 4.53 4.26 4.75 5.03 2.74 4.9 2.28 

H2

o+ 0.22 0.31 - - 0.23 0.35 0.60 0.33 0.50 0.29 1.29 1.15 H2

o- 0.02 <0.01 - - 0.03 

Ti02 0.19 0.16 - - 0.18 0.12 0.15 0.14 <0.08 0.58 0.91 0.99 

P20S <0.05 <0.05 - - <0.05 <0.05 <0.05 <0.05 <0.05 0.22 0.96 0.49 

MnO <0.02 <0.02 - - <0.02 <0.02 <0.02 <0.02 <0.02 0.07 0.14 0.1 

C02 0.02 0.02 - - 0.01 

Sum 99.32 99.22 99.65 99.08 99.03 99.37 99.38 99.72 96.83 99.12 

A/CNK 1.06 1.08 1.04 0.99 0.99 0.95 1.04 0.87 0.65 0.80 
Minor elements (parts per million) by X-ray fluorescence analysis and INAA for Ta, Hf, Sc, Cs, Th, and U 

Rb 93 89 - - 150 94 70 66 120 73.1 111 42.4 

Sr 60 62 - - 17 32 41 46 25 601 3,820 710 

y 71 78 - - 29 54 54 55 30 13 

Zr 291 281 - - 141 267 395 333 125 149 184 230 

Nb 25 29 - - 17 28 27 27 18 9 

Ba 1,482 793 - - 256 1,236 1,390 1,294 631 1,370 6,170 1,120 

Cs - - - - - 032 0.2 0.15 135 0.95 

Ta - - - - - 2.48 2.18 23 1.51 0.93 0.54 1.08 

Hf - - - - - 9.69 10.4 9.11 4.45 4.1 5.28 4.98 

Th 27 22 - - - 16.7 15.6 153 21.0 11.7 8.63 6.7 

u <15 <15 - - - 4.29 3.66 3.68 5.70 333 2.24 1.8 

Sc - - - - - 0.78 1.04 1.16 3.22 13.2 2.24 1.8 

Zn - - - - - 28 19 33 35 48 194 100 

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

La 81.9 101 103 49.8 52.7 74.7 61.6 

Ce 159 194 197 97.9 91 144 115 

Nd 62 73.9 77 37.1 30.8 62.8 44.5 

Sm 12.8 14.45 14.77 7.67 5.28 11.08 7.99 

Eu 1.61 2.19 2.05 0.47 1.21 3.04 1.87 

Tb 1.61 1.69 1.72 0.82 0.45 0.94 0.70 

Yb 6.76 6.48 6.70 3.06 1.27 2.09 1.92 m 

~ Lu 0.94 0.93 0.92 0.45 0.17 032 0.29 

-< 

.., 

Q 

;;-

Q 

N 

Q 

;::;· 

11.1 

~ 

~ 

§'" 

c: 

::11::1 

Q 

n 

71:" 

.!!' 

z 

~ 

~ ~ 

~ 

:::s 

~ ;;;· 

n 

Q 

:::s 

"' :r 

B 
SAMPLE DESCRIPTIONS AND LOCALITIES (TABLE 2) 

SYNTECTONIC TONALITE-GRANODIORITE SUITE 

W283B 

W214-2 

W270A 

W260 

W254B 

W237 

W283A 

W214-1 

W309C 

W390 

21-84 

W284 

W285-1 

W288 

W285 

W305 

26-84 

W306 

W859 

W307B 

W307A 

W398 

W394 

W286 

W849 

W299 

W851-2 

SWI-8-86 

W851-1 

W851-3 

W851-4 

Foliated diorite (tonalite) at Conants Rapids, sec. 8, T. 23 

N ., R. 8 E. Age 1 ,892±9 Ma. 

Foliated tonalite at Conants Rapids. 

Dacite porphyry in Milladore Volcanic Complex (Sims 

and others, 1989), sec. 18, T. 25 N., R. 6 E. 

Foliated tonalite, sec. 11, T. 23 N., R. 7 E. 

Foliated tonalite, sec. 1, T. 23 N., R. 6 E. 

Foliated tonalite, sec. 8, T. 25 N., R. 5 E. 

Lineated granodiorite (tonalite) at Conants'Rapids, sec. 

8, T. 23 N., R. 8 E. 

Same as W283A. 

Granite gneiss (granodiorite) near Neillsville, sec. 15, T. 

24 N., R. 2 W. Contains 12 ppm Cu. 

Granite gneiss near Neillsville, sec. 17, T. 24 N., R. 2 W. 

Age 1 ,871 ±5 Ma. Contains 14 ppm Cu. 

Granite gneiss near Neillsville, sec. 2, T. 24 N., R. 2 W. 

GRANITOID ROCKS OF UNCERTAIN 

CLASSIFICATION 

Mylonitic granodiorite, Biron Dam, sec. 33, T. 23 N., R. 

6 E. Cuts tonalite that has age of I ,841±10 Ma. 

Leucogranite near Marshfield, sec. 9, T. 25 N., R. 2 E. 

Leucogranite, sec. 10, T. 23 N., R. 3 E. 

Leucogranite, sec. 9, T. 25 N., R. 2 E. 

Lineated granite, sec. 33, T. 27 N., R. 2 W. 

Granite, sec. 21, T. 26 N., R. 2 W. 

Leucogranite, sec. 3, T. 26 N., R. 2 W. 

Diorite, Hemlock Creek, SEI/4NWI/4 sec. 23, T. 23 N., 

R.4E. 

ALKALI-FELDSPAR GRANITE 

Biotite granite (alkali granite), sec. 27, T. 26 N., R. 2 W. 

Biotite granite (alkali granite), sec. 27, T. 26 N., R. 2 W. 

Red granite at Black River Falls, sec. 14, T. 21 N., R. 4 

W. Age 1,835±6 Ma. 

Red granite, Granite Heights, sec. 19, T. 23 N., R. 2 W. 

Red granite (alkali granite), sec. 21, T. 24 N., R. 3 E. 

Red granite, quarry, SWI/4 sec. 25, T. 24 N., R. 2 E. 

Age 1 ,833±4 Ma. 

Red granophyric granite (alkali granite), Cary Mound, 

SWI/4 sec. 34, T. 24 N., R. 2 E. 

Red granophyric granite (alkali granite), NWI/4 sec. 1, T. 

23 N., R. 2 E. 

Red alkali-feldspar granite (alkali granite), Vesper, 

SWI/4NEI/4 sec. 2, T. 23 N., R. 4 E. 

Diorite (tonalite), fine-grained, same locality as 

W851-2. 

Lamprophyre(?), same locality as W851-2. 

Diorite, medium-grained, same locality as W851-2. 

mylonitization (protomylonite). Within restricted narrow 

shears, plagioclase, quartz, hornblende, and biotite are 

recrystallized into aligned aggregates of finer grain size. 

Hornblende within the recrystallized shears is distinctly blue 

green, whereas outside of shears, it is dark green; sphene is 

associated with blue-green hornblende and biotite and its 

presence suggests slight retrograding during the ductile 

deformation. 

)22 Precambrian Geology of Lake Superior Region 

A body of foliated granodiorite (unit Xft, fig. 2) similar 

to that in the Stevens Point area, 16 km long and 5 km wide, 

cuts volcanic rocks of the Milladore Volcanic Complex 

(Xmc) in an area northeast of Marshfield. The granodiorite 

is medium gray, medium grained, and equigranular, and it 

contains widely scattered xenoliths of mafic and felsic volcanic rocks of the Milladore Volcanic Complex. The rock 

is variably foliated as a result of ductile deformation and is 

now protomylonite to orthomylonite. Type IP mantle aggregates (Hanmer, 1982) are common. A mode (W237) is given 

in table 2. 

The relationship of the granodiorite-diorite bodies 

exposed near Stevens Point (unit XAgt, fig. 2) to the volcanic 

rocks of the Milladore Volcanic Complex is uncertain. If the 

foliated tonalite (W237, unit Xft, fig. 2; table 2) that cuts 

rocks of the Milladore Volcanic Complex is correlative with 

the tonalite-granodiorite suite in the Stevens Point area, then 

all tonalitic rocks are essentially coeval with the volcanic 

rocks. Such an interpretation is supported by major- and 

minor-element compositions of both the granodiorite-diorite 

and the Milladore Volcanic Complex. The chemistry of a 

sample (W270A, table 2) of dacite porphyry from the Milladore Volcanic Complex is very similar to that of a sample 

of foliated diorite from Conants Rapids (W283B, table 2). 

The foliated diorite at Conants Rapids has a U-Pb zircon 

upper intercept age of 1 ,892±9 Ma, however, which is substantially older than ages determined directly from volcanic 

rocks within the Milladore Volcanic Complex (approximately 1,860 Ma). A preliminary age of 1 ,842±10 Ma was 

reported earlier for this same sample (W283B) of foliated 

diorite, and an age of 1 ,824±25 Ma was reported for a 

lineated tonalite at Conants Rapids (Van Schmus, 1980, p. 

162). Maass and others (1980) interpreted these ages as 

representing the time span of Penokean deformation, a 

premise no longer tenable (Sims and others, 1989). With 

present data, deformation in this terrane can only be bracketed as younger than 1 ,860 Ma, the apparent age of the 

Milladore Volcanic Complex, and older than 1 ,835 Ma, the 

age of the post-tectonic granite. 

Foliated granodiorite at Biron Dam, north of Wisconsin 

Rapids (fig. 2), has a U-Pb zircon age of 1 ,841±1 0 Ma (Sims 

and others, 1989). All the rocks exposed here, including 

amphibolite dikes in the foliated granodiorite, have the same 

deformational style (Maass and others, 1980, fig. 3). 

The granodiorite to tonalite group in the Stevens Point 

area ranges from hornblende-biotite diorite to biotite 

trondhjemite and is calc-alkalic to alkali-calcic (fig. 4 and 

Anderson and Cullers, 1987). The rocks are mostly metaluminous but include peraluminous varieties for silica values 

in excess of 65 weight percent. The rocks range from alkalicalcic to calc-alkalic tonalite to granite, are Mg rich to very 

Mg rich, metaluminous to mildly peraluminous, and sodic; 

and they contain high Sr and low Rb and Zr. Anderson and 

Cullers (1987) showed that these plutons are analogs of 

modern orogenic andesite-dacite suites; REE patterns are 
w 

.... 

a: 

0 

z 

0 

:X: 

u 

100 

10 

W270A \ W214·2 ~ 

W859 ~ 

' 

W214-1 -_ 

\ 

\ 

' ' \ 

\ ..._ 

' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' " " ~ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ \-- __ ... 

Figure 12. Chondrite-normalized REE plots for samples of syntectonic granodiorite suite, Marshfield terrane. Analyses given in 

table 2. Gd, no data. 

similar to those of high-potassium andesites, with similar 

negatively sloping light-REE to heavy-REE patterns and 

minor or nonexistent Eu anomalies (fig. 12). A small amount 

of Archean material may have been incorporated during 

tonalite emplacement (Anderson and Cullers, 1987). Anderson and Cullers interpreted these tonalitic magmas as being 

derived by partial melting of an eclogitized, altered tholeiitic 

source, possibly a subducted oceanic slab. Their interpretation is consistent with the location of tonalite-granodiorite 

samples in the volcanic-arc granite field on a diagram of Rb 

versus Y +Nb (fig. 13). 

Gneissic Granite Near Neillsville 

The gneissic granite near Neillsville (Ne; unit Xggr, fig. 

2) is moderate red to pale red, and medium to fine grained; 

it has a mylonitic foliation and a conspicuous steep mylonitic 

lineation. The granite intrudes layered and previously 

deformed Late Archean gneiss and amphibolite, as noted on 

the east bank of the Black River at the west edge of Neillsville, and has a U-Pb zircon upper intercept age of 1 ,871±5 

Ma (Sims and others, 1989). Texture varies according to the 

degree of mylonitization and recrystallization (grain-size 

reduction). In a quarry 4 km west of Neillsville, a felsic dike 

(W390, table 2) that cuts the granite has the same structural 

fabric as the granite. The gneissic granite has the composition of leucogranite, ranging locally to granodiorite (table 

2). Potassium feldspar is microperthitic (as much as 5 mm 

in diameter); typically, adjacent plagioclase (oligoclase) is 

mynnekite. Quartz is present as sutured, recrystallized 

grains. Muscovite forms coarse porphyroblasts. The granite 

has a high quartz content (>40 volume percent) and 

Early Proterozoic Granitoid Rocks, Northern Wisconsin J23 
10 100 

Y+Nb, IN PARTS PER MILLION 

EXPlANATION 

o Tonalite-granodiorite suite 

X Gneissic granite near Neillsville 

• 1,835 Ma alkali-feldspar granite 

WPG Within-plate granite 

ORG Ocean ridge granite 

VAG Volcanic-arc granite 

Syn-COLG Syncolllsional granite 

1000 

Figure 13. Rb versus Y +Nb diagram (modified from Pearce 

and others, 1984) showing granitoid rocks of the Marshfield 

terrane. Field of Newingham Tonalite modified from Sims and 

others (1992). 

commonly contains accessory fluorite. On a Rb versus 

Y +Nb diagram (fig. 13), this gneissic granite plots mainly 

with volcanic-arc granites, but overlaps the fields of syncollisional granites and within-plate granites, which indicates a 

much more highly evolved chemical composition than that 

of the tonalite-granodiorite suite. The granite near Neillsville 

has a much more highly evolved composition than would be 

expected for its age group. 

1,835 Ma Alkali-Feldspar Granite 

The post-tectonic red alkali-feldspar granite in the 

Marshfield terrane is petrographically and compositionally 

similar to the red granite in the Pembine-Wausau terrane. 

All are mildly peraluminous and alkali-calcic, range from 

alkali granite to normal granite, and are similar to but 

slightly more alkali-enriched than the Athelstane batholith. 

A granite from Rock Township (north of Cary Mound; locality C, fig. 2) has a U-Pb zircon upper intercept age of 

1,833±4 Ma, and a sample from Black River Falls (Br, fig. 

2) has an upper intercept age of 1 ,835±6 Ma (Sims and 

others, 1989). The close similarity in these ages, as wen as 

)24 Precambrian Geology of Lake Superior Region 

compositional similarities of the red granites, suggests that 

all red granites in this and the adjacent Pembine-Wausau 

terrane are coeval. The REE patterns are distinctive (fig. I4), 

exhibiting large negative Eu anomalies and a relatively flat 

heavy-REE slope. On aRb versus Y+Nb diagram (fig. I3), 

five samples of alkali-feldspar granite from the Marshfield 

terrane plot in the within-plate field and four samples plot 

in the field of volcanic-arc granites. Three dioritic samples 

(W85I-I, W851-3, W85I-4; table 2) associated with the 

red granite at Cary Mound have lamprophyric affinities with 

most having high Ba and Sr (table 2) and steep REE patterns 

(fig. 15). That these dioritic rocks are related, in a narrow 

sense, to the 1,835 Ma alkali-feldspar granite is doubtful; if 

they were, they should have alkali-calcic affinities. 

Two samples of alkali-feldspar granite (W307 A, 

W307B) of I ,835 Ma group listed in table 2 are deformed 

ductilely, but they are included in this group because they 

chemically resemble undeformed red granites of the 1,835 

Ma group. The deformation of these rocks is attributed to 

shearing in the Athens shear zone at about 1,815 Ma, the 

age determined for the Mountain shear zone (Sims and others, 1989; Sims and others, I990). On a Nb versus Y diagram, the alkali-feldspar granite plots mainly in the withinplate granite field (fig. I6). 

SUMMARY AND CONCLUSIONS 

The granitoid rocks in the Pembine-Wausau and Marshfield terranes compose two distinct suites, calc-alkalic rocks 

such as those that commonly are emplaced within primitive 

island or Andean arcs (for example, Sierra Nevada 

batholith), and alkali granite suites (fig. 17). The rocks in 

each class are remarkably similar in composition in both 

terranes. 

The syntectonic granodiorite suite in the Pembine-Wausau terrane is the product of the two collisional events recognized in the Wisconsin magmatic terranes: (1) continentarc collision along the Niagara fault zone at =I ,852 Ma and 

(2) arc-minicontinent collision along the Eau Pleine shear 

zone at about 1,840 Ma (Sims and others, 1989). The syntectonic granodiorite suite in the Pembine-Wausau terrane 

is mainly coeval with major tholeiitic and calc-alkalic arc 

volcanism (1,889-1,860 Ma) and is calc-alkalic to calcic 

(Sims and others, 1992, fig. 5; fig. 4). Brown (1982) has 

related such compositional changes to progressive arc evolution, spanning immature island arcs, mature arcs and continental margins (calc-alkalic rocks), and continental 

collision zones. The Pembine-Wausau terrane granitoids are 

mainly volcanic-arc granodiorites, as represented by the 

Newingham Tonalite in the Dunbar area, which developed 

during southward subduction of oceanic crust prior to collision of the Pembine-Wausau terrane with the continental 

margin to the north. In the Dunbar dome of northeastern 

Wisconsin, adjacent to the Niagara fault zone, granitoid 
w 

t:: 

a: 

0 

z 

0 

:I: 

u 

w 

..J 

Q.. 

~ 

~ 

100 

10 

W851-2 

W299 

Figur·e 14. Chondrite-normalized REE plots for samples of red alkali-feldspar granite, Marshfield terrane. Gd, no data. 

rocks are generally more evolved than the Newingham 

Tonalite (Sims and others, 1992), which suggests that they 

were derived from different sources and, potentially, formed 

in a different tectonic setting. The Dunbar granitoid rocks 

are characterized by relatively high K, Rb, Ba, Th, Nb, Ta, 

and light-REE. High Ta and Nb are particularly diagnostic. 

These potassium-enriched rocks cross the fields for volcanic-arc granite into syncollisional granites (see Sims and 

others, 1992, fig. 30). Accordingly, we conclude that granites adjacent to the Niagara fault zone are syn- to postcollisional granites, as defined by Harris and others (1986). 

Barovich and others ( 1989) presented Nd isotopic data 

for selected granitoid rocks from the Pembine-Wausau terrane, and found a wide range of £Nd(T) values, from -4.5 to 

+4.0; the authors noted that the more negative £Nd(T) values 

are in samples near the Niagara fault zone, the suture 

between the Pembine-Wausau terrane and Archean crust to 

the north. Their model explained these results as due to 

mixing of variable amounts of new crustal material and 

recycled Archean detritus, with the proportion of added 

Archean detritus being greatest closest to the collisional 

boundary. Although this model accounts for the observed 

isotopic variations, it does not explain enrichment of highfield-strength elements (HFSE), particularly Ta and Nb, for 

many of the granitoid rocks along the Niagara fault zone 

(Sims and others, 1992). Both new crustal material of the 

Pembine-Wausau terrane (the volcanic and related intrusive 

rocks) and the detritus derived from Archean crust show 

HFSE depletions. Mixing between and partial melting of 

such sources cannot account for the HFSE enrichments 

observed in some of the terrane's granitoids (Thompson and 

others, 1984). 

Early Proterozoic Granitoid Rocks, Northern Wisconsin J2S 
100 

w 

1-

a: 

c 

z 

0 

:I: 

u 

.. 

w 

..J 

a.. 

:IE 

< (/) 

10 

5 LaCe Nd Yb Lu 

Figure 15. Chondrite-normalized REE plots for samples of diorite and lamprophyre associated with alkali-feldspar granite, 

Marshfield terrane. Gd, no data. 

100 

z 

0 

:::::i 

..J 

~ 

a: 

w 

a.. 

(/) 

1-

a: 

< a.. 

~ 

.D 

z 

1 1 

0 

VAG+ Syn-COLG 

o:J • 

• o ORG 

0 ,P ~ 

10 100 

Y, IN PARTS PER MILLION 

EXPlANATION 

o Syntectonic tonalitegranodiorite suite 

• Granitoid rocks of uncertain 

classlflcatlon 

' o 1,835 Ma alkali-feldspar 

granite 

WPG Within-plate granite 

ORG Ocean ridge granite 

VAG Volcanic-arc granite 

Syn·COLG Syncolllslonal granite 

1000 

Figure 16. Nb versus Y diagram (modified from Pearce and 

others, 1984) showing granitoid rocks of the Marshfield terrane. 

Arrows indicate less than values shown. 

As a modification of the mixing model of Barovich and 

others (1989), Sims and others (1992) suggested that granitoids near the Niagara fault zone enriched in HFSE (for 

example, sample W251, table 1) may have been derived 

through partial melting of mantle-derived basalts having 

)26 Precambrian Geology of Lake Superior Region 

1-

z 

w 

u 

a: 

w 

a.. 

1-

::I: 

(!) 

w 

3: 

~ 

N 

0 

(j) 

Log (K20:Mg0) 

Figure 17. Si02 versus log10 (K20:Mg0) diagram showing 

distinction between calc-alkalic and alkali granite suites in 

Wisconsin magmatic terranes (modified from Greenberg, 

1980). Includes samples from the Dunbar, Wis., area (Sims and 

others, 1992). 

within-plate compositions (for example, Early Proterozoic 

basalt of the Marquette Range Supergroup). In this model, 

the negative ENd(T) values for these samples would at least 

in part reflect that of the basaltic source. Partial melting 

would have occurred during collision and overthrusting of 

the Pembine-Wausau terrane onto the continental margin. 

For those granitoids with negative ENd(T) and depleted 

HFSE, some crustal material and Archean detritus probably 

were incorporated in the source prior to collision. 

Younger, late-tectonic granodiorite-tonalite bodies in the 

Marathon County area (unit Xgt, fig. 2) presumably formed 

during north-dipping subduction of oceanic crust that terminated with collision of the Marshfield (minicontinent) terrane with the Pembine-Wausau terrane. The resulting 

granite-granodiorite suite is calc-alkalic to alkali calcic. The 

suite appears more evolved than the Newingham Tonalite 

and other older volcanic-arc granodioritic rocks. 

The Athelstane Quartz Monzonite has a composition 

similar to that of granitic rocks of the Middle Proterozoic 

Wolf River batholith (Anderson and Cullers, 1978; Anderson, 1983, fig. 10C) and other Middle Proterozoic anorogenic complexes in North America. Rare earth element 

abundances are also similar to those of the Wolf River 

batholith (see Anderson, 1983, fig. 11). Anderson and Cullers (1978) have argued for an origin of the Wolf River 

batholith through partial melting of a quartz diorite to granodioritic crustal source, such as the syntectonic granodiorite suite discussed in this report. A similar source is proposed 

for the Athelstane Quartz Monzonite. Crustal melting followed orogenic episodes in the region by a few million 

years. Possibly the heat necessary for crustal melting was 

generated by the earlier northward subduction of the Marshfield terrane beneath the Pembine-Wausau terrane. 
80~----~~-------~,------~,~--~~------~ 

Verysodic ~'~~ ~/ 1

.' 

1-

70 1-

1-

0 

::~~~ .' ' -

.~ .~/ --~.:;.$-' 11.7 Gaplutons, 

11-, • ~;-·if;_··.;·:(~.:...,'~~;; . . ,' Colorado 

0 0 ~ 'I ~.-~ ~- ····I ' - .... [:~_(' .~::-y ' \) · · • /' .';'V X : 1 7 Ga syntectomc 

.. · .P ,./ .' '"~::~· / p:utons, -

,./ .A .'f" / / southeastern ••. ~ • ~~·".1: 1 California 60 ~'-'.................. ,." ·'"'; ' -

Sodlc _//// .& ;:;) ...... _./ •·•·•· 1 4 Ga plutons, 

- • southwestern U.S. f- ,.,""" ,/'" -

i'",, Average ,.ft~;otassic Ver;t potassic 

so~----~~--~~"_,._~-~~------~'"~----~'------~ 0 0.2 0.4 0.6 0.8 I 0 

EXPLANATION 

Marshfield terrane 

.& I ,835 Ma alkali-feldspar granite 

• Syntectonic granodiorite 

+ Uncertain affinity 

Pembine-Wausau terrane 

6 Athelstane Quartz Monzonite 

o Syntectonic granodiorite 

x Uncertain affinity 

o 1, 760 Ma granodiorite-granite 

o 1,835 Ma alkali-feldspar granite 

Figure ~ 8. Si02 versus K20:(K20+Na20) diagram showing 

companson of compositions of plutonic rocks in PembineWausau and Marshfield terranes with those in Proterozoic terranes in Colorado, southeastern California, and southwestern 

United States. Plutonic rocks in the Wisconsin terranes are markedly more sodic than those in western parts of the United States. 

Com pi led by Ed DeWitt, 1992. 

Approximately 5-10 m.y. after suturing of the PembineWausau and Marshfield terranes along the Eau Pleine shear 

zone, red alkali-feldspar granite and cogenetic subaerial rhyolite were emplaced in the vicinity of the shear zone; they 

postdate collision. The alkali-feldspar granites are mainly 

alkalic and share characteristics of within-plate and volcanic-arc granites (fig. 16). 

Syntectonic granitoid rocks within the Marshfield terrane 

are mainly calc-alkalic granodiorites; they are similar in 

composition to the syntectonic granodiorite suite of the Pembine-Wausau terrane. The tonalite-granodiorite suite overlaps the compositional field of the Newingham Tonalite. 

Possibly the tonalite-granodiorite rocks were generated by 

a southward-dipping subduction zone beneath the Marshfield terrane prior to collision along the Eau Pleine shear 

zone. The granite near Neillsville also could have been generated in this manner, but it is more highly evolved chemically than the tonalite-granodiorite suite. 

Plutonic rocks of the Pembine-Wausau and Marshfield 

terranes record changing types of magmatism over time. 

Prior to == 1,850 Ma, intrusive rocks are calcic to calc-alkalic, magnesium rich, sodic to very sodic, high-strontium 

granodiorite to trondhjemite typical of immature island or 

continental arcs. Rocks younger than == 1,850 Ma are characteristically alkali-calcic, iron rich to very iron rich, sodic 

to average granodiorite to granite typical of more mature 

island arcs and somewhat evolved continental arcs. The 

post-tectonic 1,760 Ma plutons are the most chemically 

evolved, having the highest Rb and Th concentrations, but 

they do not differ much from 1,835 Ma rocks, as they are 

alkali-calcic, iron rich to average, sodic to average granodiorite to granite. 

Distinctive and as yet unexplained features of the igneous rocks of the Pembine-Wausau and Marshfield terranes 

include: (1) Zr concentrations (300-400 ppm) of the 

Athelstane Quartz Monzonite and the possibly related red 

alkali-feldspar granite are anomalously high. Evolved 

granites such as the Athelstane typically contain only 

150-200 ppm. (2) In some pre-1,850 Ma granodiorite bodies in the Pembine-Wausau terrane, HREE values are 

anomalously low (only 2-3 times greater than chondrite 

abundances). (3) Magmatism from =1,890 to =1,500 Ma 

in northern Wisconsin exhibits an overall sodic nature (fig. 

18). Other orogenic terranes in the western United States 

typically evolved to potassic or even highly potassic magmatism over time, but the Wisconsin terranes remained 

sodic to average. Source areas beneath both Wisconsin 

magmatic terranes certainly had a major role in controlling 

the temporal chemical variations. Perhaps further studies 

will result in explanations for some of the anomalous features revealed by this investigation. 

REFERENCES CITED 

Anderson, J.L., 1972, Petrologic study of a migmatitc-gneiss 

terrain in central Wisconsin and the effect of biotitemagnetite equilibria on partial melts in the granite system: 

Madison, Wis., University of Wisconsin M.S. thesis, 103 p. 

_____ 1983, Proterozoic anorogenic granite plutonism of North 

America, in Medaris, L.G., Jr., Byers. C.W., Mickelson, 

D.M., and Shanks, W.C., eds.. Proterozoic 

geology-Selected papers from an international Proterozoic 

symposium: Geological Society of America Memoir 161, 

p. 133-154. 

Anderson, J.L., and Cullers, R.L., 1978, Geochemistry and 

evolution of the Wolf River batholith, a late Precambrian 

rapakivi massif in North Wisconsin, U.S.A.: Precambrian 

Research, v. 7, p. 287-324. 

_____ 1987, Crust-enriched mantle-derived tonalites in the Early 

Proterozoic Penokean orogen of Wisconsin: Journal of 

Geology, v. 95, p. 139-154. 

Anderson. J.L., Cullers, R.L., and Van Schmus. W.R., 1980, 

Anorogenic metaluminous and peraluminous granite 

plutonism in the mid-Proterozoic of Wisconsin. U.S.A.: 

Contributions to Mineralogy and Petrology. v. 74, 

p. 311-328. 

Attoh, Kodjopa, and Klasner, J.S., 1989, Tectonic implications of 

metamorphism and gravity field in the Penokean orogen of 

northern Michigan: Tectonics. v. 8. p. 911-933. 

Early Proterozoic Granitoid Rocks, Northern Wisconsin J27 
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Natural History Survey Map 86-4, scale 1:100,000. 

Brown, G.C., 1982, Calc-alkaline intrusive rocks-Their diversity, 

evolution, and relation to volcanic arcs, in Thorpe, R.S., ed., 

Andesites: New York, John Wiley, p. 437~61. 

Cannon, W.F., and Gair, J.E., 1970, A revision of stratigraphic 

nomenclature of middle Precambrian rocks in northern 

Michigan: Geological Society of America Bulletin, v. 8, 

p. 2843-2846. 

Cannon, W.F., Lee, M.W., Hinze, W.J., Schulz, K.J., and Green, 

A.G., 1991, Deep crustal structure of Precambrian basement 

beneath northern Lake Michigan, midcontinent North 

America: Geology, v. 19, p. 207-210. 

DeLaRoche, H., Letterrier, J., Grandclaude, P., and Marchal, M., 

1980, A classification of volcanic and plutonic rocks using 

R1Rrdiagram and major-element analyses-Its 

relationships with current nomenclature: Chemical Geology, 

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DeWitt, Ed, 1989, Geochemistry and tectonic polarity of Early 

Proterozoic (1700-17 50 Ma) plutonic rocks, north-central 

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evolution of Arizona: Tucson, Ariz., Arizona Geological 

Society Digest 17, p. 149-163. 

Dott, R.H., Jr., 1983, The Proterozoic red quartzite enigma in the 

north-central United States-Resolved by plate collision?, in 

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

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v. 92, pt. 1, p. 224-232; pt. 2, p. 749-840. 

Greenberg, J.K., and Brown, B.A., 1984, Cratonic sedimentation 

during the Proterozoic-An anorogenic connection in 

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___ 1986, Bedrock geology of Portage County, Wisconsin: 

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allochthonous terranes of the Penokean orogeny in Upper 

Michigan-Comparisons with northern Appalachian 

tectonics: U.S. Geological Survey Bulletin 1904-Q, 28 p. 

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Harris, N.B.W., Pearce, J.A., and Tindle, A.G .. 1986, Geochemical 

characteristics of collision-zone magmatism, in Coward, 

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magmatism, and Superior-type iron-formations of the 

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evolution: Geodynamics Series, v. 17, American 

Geophysical Union, p. 85-98. 

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footwall deformation and imbrication-A model for the 

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Society of America Bulletin, v. 100, p. 1811-1818. 

)28 Precambrian Geology of Lake Superior Region 

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Early Proterozoic accretionary complex, Lake Superior 

region, U.S.A.: Geological Society of America Bulletin, v. 

96, p. 1179-1187. 

Maass, R.S., Medaris, L.G., Jr., and Van Schmus, W.R., 1980, 

Penokean deformation in central Wisconsin, in Morey, G.B., 

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

and lower Proterozoic rocks, southern Canadian Shield: 

Geological Society of America Special Paper 182, 

p. 147-157. 

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Wisconsin: Wisconsin Geological and Natural History 

Survey Paper 79-2, 20 p. 

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element discrimination diagrams for the tectonic 

interpretation of granitic rocks: Journal of Petrology, v. 25, 

p. 956-983. 

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Orogen: Geological Association of Canada Program and 

Abstracts, v. 12, p. 87. 

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regional plunge and Proterozoic terrane accretion in the 

southern Lake Superior region: Precambrian Research, v. 30, 

p. 249-262. 

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

in the Great Lakes region-A brief overview: U.S. 

Geological Survey Bulletin 1694-E. p. 55-74. 

___ 1989, Geologic map of Precambrian rocks of Rice Lake 

1 °x2° quadrangle, northern Wisconsin: U.S. Geological 

Survey Miscellaneous Investigations Series Map 1-1924, 

scale 1:250,000. 

___ 1990a, Geologic map of Precambrian rocks of Eau Claire 

and Green Bay 1 °X2° quadrangles. central Wisconsin: U.S. 

Geological Survey Miscellaneous Investigations Series Map 

1-1925, scale 1 :250,000. 

___ 1990b, Geologic map of Iron Mountain and Escanaba 1 °x2° 

quadrangles, northeastern Wisconsin and northwestern 

Michigan: U.S. Geological Survey Miscellaneous 

Investigations Series Map 1-2056, scale 1:250,000. 

___ 1990c, Geologic map of Precambrian rocks of NeillsvilleStevens Point area, central Wisconsin: U.S. Geological 

Survey Miscellaneous Investigations Series Map 1-1926, 

scale 1: 100,000. 

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an Olympic Dam-type deposit?: U.S. Geological Survey 

Bulletin 1932, p. 40-47. 

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Superior region, Wisconsin and northern Michigan: U.S. 
Geological Survey Miscellaneous Investigations Series Map 

1-2185, scale 1 :500,000. 

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shear zone, northeastern Wisconsin, U.S.A.-A ductile 

deformation zone within the Early Proterozoic Penokean 

orogen: U.S. Geological Survey Bulletin 1904-A, 15 p. 

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foldbelt, Lake Superior region, and its tectonic environment. 

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Great Lakes region: Geological Society of America Memoir 

160, p. 3-14. 

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Gneiss-granitoid dome-Implications for early Proterozoic 

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geochemistry of Early Proterozoic rocks in the Dunbar area, 

northeastern Wisconsin: U.S. Geological Survey 

Professional Paper 1517. 65 p. 

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

1989, Tectonostratigraphic evolution of the Early 

Proterozoic Wisconsin magmatic terranes of the Penokean 

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p. 2145-2158. 

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Proterozoic, post-Penokean rhyolites, granites, and related 

rocks of south-central Wisconsin, U.S.A., in Medaris, L.G., 

Jr .• ed., Early Proterozoic geology of the Great Lakes region: 

Geological Society of America Memoir 160, p. 113-128. 

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

Thompson, R.N., Morrison, M.A., Hendry, G.L., and Parry, S.J., 

1984, An assessment of the relative roles of crust and mantle 

in magma genesis-An elemental approach: Philosophical 

Transactions of the Royal Society of London, v. A310, 

p. 549-590. 

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

and Hanson, G.N., eds .• Selected studies of Archean gneisses 

and lower Proterozoic rocks, southern Canadian Shield: 

Geological Society of America Special Paper 182, 

p. 159-168. 

Wise, D.U., and others, 1984, Fault-related rocks-Suggestions for 

terminology: Geology, v. 12, p. 391-394. 

APPENDIX A. PETROGRAPHIC 

DESCRIPTIONS OF ROCK SAMPLES 

LISTED IN TABLE 1 

SYNTECTONIC GRANODIORITE SUITE 

IM12 Light-gray, medium- to coarse-grained, 

weakly foliated granodiorite (tonalite) containing irregular clots of brown biotite. Has 

steep northeast-trending mylonitic foliation. Plagioclase has weak concentric zoning. Quartz has strain shadows. Microcline 

is interstitial. Tonalite is cut by a 

IM16 

W319A 

W325 

W66 

W249B 

W249A 

W251 

W118 

W205B 

W317 

leucogranite dike with cataclastic foliation 

expressed by oriented muscovite. 

Gray, foliated granodiorite (tonalite) with 

weak mylonitic foliation. Plagioclase has 

weak concentric zoning, quartz has strain 

shadows, and biotite (greenish brown) is 

oriented in mylonitic zones. Contains euhedral epidote/clinozoisite. 

Gray, medium- to coarse-grained, foliated 

granodiorite that cuts migmatitic amphibolite and is cut by fine-grained tonalitic 

dikes. Has weak northwest-trending mylonitic foliation. Plagioclase has weak concentric zoning and quartz has strain 

shadows. 

Pink, massive, medium-grained, altered granodiorite (granite), sheared in northern part 

of outcrop. Plagioclase is altered to epidote/ 

clinozoisite, sericite, and stilpnomelane and 

biotite is chloritized. 

From drill core HT -4, depth 15 m, of Bear 

Creek Mining Co. Reddish-brown, medium-grained, equigranular, hornblende-biotite granodiorite (tonalite). Plagioclase has 

concentric zoning. Has strong foliation and 

lineation. 

Pinkish-gray, medium- to coarse-grained, foliated granodiorite. Plagioclase (An20-25) 

has weak normal concentric zoning and is 

myrmekitic adjacent to potassium feldspar. 

Light-olive-green, medium-grained granodiorite (tonalite). Plagioclase has strong 

concentric zoning. Potassium feldspar is interstitial. Epidote has replaced biotite. 

Medium-gray, medium-grained, foliated tonalite. Plagioclase has weak concentric 

zoning. Biotite is grayish yellow to grayish 

red and slightly altered to chlorite and iron 

oxides. 

Medium- to light-gray, medium-grained, foliated biotite granodiorite (tonalite). Plagioclase (An22-3o) has conspicuous concentric 

zoning. Biotite is moderate yellow to olive 

gray and fresh. Hornblende is dusky blue 

green. 

Pinkish-gray, medium-grained, weakly foliated biotite granodiorite. Plagioclase 

(An38-42) has weak concentric zoning, 

quartz has strain shadows, biotite is brown 

to grayish brown, and hornblende is green 

to blue green. 

Gray foliated granodiorite (tonalite). Plagioclase (An26-34) has strong concentric oscillatory zoning, quartz has strain shadows, 

Early Proterozoic Granitoid Rocks, Northern Wisconsin J29 
W314 

W318 

and biotite is brown to reddish brown. Rock 

is protomylonite. 

Medium-gray, medium-grained foliated granodiorite. Plagioclase has weak concentric 

zoning. Rock is protomylonite. 

Gray, medium-grained foliated granodiorite. 

Plagioclase has weak concentric zoning, is 

myrmekitic, and is moderately sericitized. 

Biotite is altered to chlorite, epidote/clinozoisite, and muscovite. Rock is protomylonite. 

UNCERTAIN 

SW AU 6-83 Coarse-grained granitoid similar to Wl88. 

ATHELSTANE QUARTZ MONZONITE AND 

COEVAL ROCKS 

W213 

WI-1 

Wl88 

W549 

W533 

W556 

W555 

W384 

Granite near Cherokee. Pink to red, coarsegrained granite (alkali granite) with clots of 

biotite. Has a mylonitic foliation. Plagioclase (Ant4-t8) is fractured. Potassium 

feldspar occurs as large irregular grains of 

microperthite. Quartz has strain shadows. 

Coarse-grained granite. Plagioclase has weak 

concentric zoning; myrmekite present adjacent to potassium feldspar. 

Athelstane Quartz Monzonite. Pinkish-gray, 

coarse-grained, massive granite (alkali 

granite). Plagioclase and potassium feldspar (microcline and microperthite) occur 

as large rectangular grains. Hornblende is 

olive green and altered to biotite and 

opaque oxides. Biotite is brown to reddish 

brown. 

Same as Wl88. 

Same as Wl88. 

Same as W 188. Zircon is common. 

Same as W188. 

1,835 MA ALKALI-FELDSPAR GRANITE 

Red, medium- to coarse-grained, massive 

leucogranite (alkali granite). Plagioclase 

(Ans-to)is fractured and potassium feldspar 

is dominantly microperthitic. Biotite is dark 

olive green. 

1,760 MA GRANODIORITE-GRANITE GROUP 

Wl2B Amberg Granite. Gray, fine-grained mylonitic granite gneiss. Plagioclase (An2s-27) 

has moderate concentric zoning, potassium 

feldspar is microperthitic, and biotite is 

)30 Precambrian Geology of Lake Superior Region 

frayed. Lies within Twelve-foot Falls shear 

zone (Sims, 1990b). 

SPB 232-39 Amberg Granite, NW1/4SW1/4 sec. 31, T. 36 

N., R. 21 E. 

W54 

W55 

824-7 

W252 

Granodiorite (granite) near Lugerville. Pinkish-gray, medium-grained, weakly foliated, 

equigranular biotite granite. Hypidiomorphic granular texture. Plagioclase (An24-3o) 

has weak concentric zoning and albite rims. 

Potassium feldspar is microperthite. Biotite 

is grayish olive, moderately frayed, and 

partly chloritized. 

Light-gray, medium-grained, equigranular, 

weakly foliated biotite granodiorite (granite). Plagioclase has normal concentric zoning and locally is myrmekitic. Biotite is 

olive gray. 

Granite near Monico. Pink to grayish-pink, 

coarse-grained, slightly inequigranular 

leucogranite. Plagioclase (An21-24) has 

moderate concentric zoning and narrow albite rims adjacent to microperthite. Biotite 

is moderately altered to chlorite, epidote, 

and zeolite(?). 

Grayish-pink, coarse-grained foliated granodiorite. Hypidiomorphic-granular texture. 

Plagioclase has concentric zoning; potassium feldspar is microperthitic. 

APPENDIX B. PETROGRAPHIC 

DESCRIPTIONS OF ROCK SAMPLES 

LISTED IN TABLE 2 

SYNTECTONIC TONALITE-GRANODIORITE SUITE 

W283B 

W214-2 

W270A 

W260 

Gray to pale-reddish-brown medium-grained 

foliated diorite (tonalite) containing plagioclase porphyroclasts with strong concentric 

zoning and a groundmass of fine-grained 

granular plagioclase, potassium feldspar, 

quartz, biotite, and hornblende. Some 

myrmekite. Mode of potassium feldspar 

may be underestimated. A prominent mylonitic foliation is produced by aligned 

minerals. 

Foliated tonalite at Conants Rapids. 

Gray, inequigranular, mylonitic, lineated dacite porphyry. Relict phenocrysts of concentrically zoned plagioclase in a finer grained 

finely comminuted groundmass of quartz, 

plagioclase, biotite, and sparse hornblende. 

Gray, medium-grained foliated tonalite that 

crosscuts a foliated tonalite. Rock is 
W254B 

W237 

W283A 

W214-1 

W309C 

W390 

21-84 

protomylonite. Plagioclase has weak 

concentric zoning. 

Gray, medium-grained foliated tonalite. Cuts 

coarser grained foliated tonalite. Plagioclase (An26-28) has weak concentric zoning. 

Quartz is strained. Rock is protomylonite. 

Medium-gray, mottled, medium-grained, 

nearly equigranular foliated tonalite. Moderate alteration. Plagioclase has concentric 

zoning. Rock has mylonitic foliation. 

Gray to pale-reddish-brown, medium- to finegrained (1-3 mm) granodiorite (tonalite). A 

mylonitic lineation is expressed by elongate 

grains of biotite and aggregates of biotite 

and quartz. Plagioclase has weak concentric zoning. Cuts foliated tonalite. Mode of 

potassium feldspar may be underestimated. 

Same as W283A. 

GRANITE NEAR NEILLSVILLE 

Same as W390, but granodiorite in cpmposition. 

Moderate-red to pale-red, medium- to finegrained mylonitic leucogranite gneiss. Plagioclase (Ans-to) has weak concentric zoning. Potassium feldspar is microperthite 

and forms megacrysts as much as 5 mm in 

diameter; strong, steep lineation expressed 

by elongate clots of biotite and quartz rods. 

Same as W390, but entirely recrystallized to 

average grain size of about 0.5 mm. 

GRANITOID ROCKS OF UNCERTAIN 

CLASSIFICATION 

W284 Gray, fine- to medium-grained lineated granodiorite with bimodal texture; is protomylonite. Cuts foliated tonalite that has an age 

of 1,841±10 Ma. 

W285-1 Pink, medium-grained equigranular leucogranite. Cut by brittle shears, but lacks penetrative foliation. Plagioclase has weak 

concentric zoning and is moderately altered 

to sericite and epidote/clinozoisite. 

W288 Pinkish-red, medium-grained, massive 

leucogranite. 

W285 Pink, medium-grained, inequigranular, massive leucogranite. Cut by brittle fractures, 

but lacks penetrative structures. 

~·u.S. ~PRINTING OFFICE: 1994-573-038/86019 

W305 Pinkish-gray, medium-grained granite gneiss 

with a strong stretching lineation expressed 

by biotite-hornblende streaks. Sample is 

near age-sample locality 16, "Foliated tonalite, Greenwood," 1 ,831±7 Ma (Sims, 

1990a). 

26-84 Gray, fine- to medium-grained lineated granite; contains graphic intergrowths in microcline. Muscovite is an alteration product. 

W306 Gray, fine- to medium-grained, foliated and 

strongly lineated leucogranite. Rock is protomylonite. Rock is near the 1,831 ±7 Ma 

age locality. 

W859 Diorite, Hemlock Creek. 

1,835 MA ALKALI-FELDSPAR GRANITE 

W307B Gray, medium-grained foliated biotite granite 

(alkali granite) that cuts foliated granite 

(W307 A). Plagioclase is An13-18· Rock is 

protomylonite. 

W307 A Gray, medium-grained foliated biotite granite 

(alkali granite) cut by both ductile and brittle fractures. 

W398 Red, medium-grained massive granite containing microperthite and concentrically 

zoned plagioclase (An8). Rock is protomylonite. 

W394 Red, medium-grained alkali-feldspar granite. 

W286 

W849 

W299 

W851-2 

SWI-8-86 

W851-1 

W851-3 

W851-4 

Contains microperthite and oligoclase 

(Anw-12). Quartz has strain shadows and 

brown biotite is partly altered to chlorite. 

Red, medium-grained alkali-feldspar granite 

(alkali granite). Microperthite is dominant 

feldspar. Biotite is altered to chlorite and 

opaque oxides. 

Red alkali-feldspar granite containing alkalifeldspar and finely comminuted quartz. 

Rock is fractured and biotite is largely altered to opaque oxide minerals plus chlorite. Age is 1 ,833±4 Ma. 

Red, coarse-grained graphic alkali-feldspar 

granite. Somewhat fractured. 

Red granophyric granite (alkali granite). 

Red alkali-feldspar granite. 

Diorite (tonalite), Cary Mound. 

Larnprophyre(?), Cary Mound. 

Diorite, Cary Mound. 

Early Proterozoic Granitoid Rocks, Northern Wisconsin )31 

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