Neodymium Isotopic Evidence for Early 

Proterozoic Units in the Watersmeet 

Gneiss Dome, Northern Michigan

Temperature-Pressure Estimates of 

Dynamically Recrystallized Rocks in the 

Early Proterozoic Mountain Shear Zone, 

Northeastern Wisconsin

U.S. GEOLOGICAL SURVEY BULLETIN 1904-G,H
AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY

Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the current-year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey publications released prior to the current year are listed in the most recent annual "Price and Availability List." Publications 

that are listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and 

Availability List" are no longer available.

Prices of reports released to the open files are given in the listing "U.S. Geological Survey Open-File Reports," updated monthly, which is for sale in microfiche from the U.S. Geological Survey, Books and Open-File Reports Section, Federal Center, Box 

25425, Denver, CO 80225. Reports released through the NTIS may be obtained by writing to the National Technical Information 

Service, U.S. Department of Commerce, Springfield, VA 22161; please include NTIS report number with inquiry.

Order U.S. Geological Survey publications by mail or over the counter from the offices given below.

BY MAIL

OVER THE COUNTER 

Books

Books Professional Papers, Bulletins, Water-Supply Papers, Techniques

of Water-Resources Investigations, Circulars, publications of general in- Books of the U.S. Geological Survey are available over the

terest (such as leaflets, pamphlets, booklets), single copies of Earthquakes counter at the following Geological Survey Public Inquiries Offices, all

& Volcanoes, Preliminary Determination of Epicenters, and some mis- of which are authorized agents of the Superintendent of Documents:

cellaneous reports, including some of the foregoing series that have gone

out of print at the Superintendent of Documents, are obtainable by mail

from WASHINGTON, D.C.-Main Interior Bldg., 2600 corridor,

18th and CSts.,NW.

U.S. Geological Survey, Books and Open-File Reports DENVER, Colorado-Federal Bldg., Rm. 169,1961 Stout St. 

Federal Center, Box 25425 LOS ANGELES, California-Federal Bldg., Rm. 7638,300 N. 

Denver, CO 80225 Los Angeles St.

 MENLO PARK, California-Bldg. 3 (Stop 533), Rm. 3128,

Subscriptions to periodicals (Earthquakes & Volcanoes and 345 Middlefield Rd.

Preliminary Determination of Epicenters) can be obtained ONLY from RESTON, Virginia-503 National Center, Rm. 1C402,12201 

the Sunrise Valley Dr.

 SALT LAKE CITY, Utah-Federal Bldg., Rm. 8105,125 

Superintendent of Documents South State St.

Government Printing Office SAN FRANCISCO, California-Customhouse, Rm. 504,555 

Washington, D.C. 20402 Battery St.

 SPOKANE, Washington~U.S. Courthouse, Rm. 678, West

(Check or money order must be payable to Superintendent of Docu- 920 Riverside Ave.. 

ments.) ANCHORAGE, Alaska-Rm. 101,4230 University Dr.

 ANCHORAGE, Alaska-Federal Bldg, Rm. E-146,701 C St.

Maps

For maps, address mail orders to

U.S. Geological Survey, Map Distribution Maps

Federal Center, Box 25286 Maps may ^ purchased over the counter at the U.S. GeologiDenver, CO 80225 caj Su^gy offices where books are sold (all addresses in above list) and

at the following Geological Survey offices: 

Residents of Alaska may order maps from

 ROLLA, Missouri-1400 Independence Rd. 

Alaska Distribution Section, U.S. Geological Survey, . DENVER, Colorado-Map Distribution, Bldg. 810, Federal

New Federal Building - Box 12 Center 

101 Twelfth Ave., Fairbanks, AK 99701 . FAIRBANKS, Alaska-New Federal Bldg., 101 Twelfth Ave.
Neodymium Isotopic Evidence for Early 

Proterozoic Units in the Watersmeet 

Gneiss Dome, Northern Michigan

By KARIN M. BAROVICH, P.JONATHAN PATCHETT, 

ZELL E. PETERMAN, and P.K. SIMS

Temperature-Pressure Estimates of 

Dynamically Recrystallized Rocks in the 

Early Proterozoic Mountain Shear Zone, 

Northeastern Wisconsin

By WARREN C. DAY, P.K. SIMS, and ZELL E. PETERMAN

Chapters G and H are issued as a single volume and are not available separately

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

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

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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

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

UNITED STATES GOVERNMENT PRINTING OFFICE: 1991

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Neodymium isotopic evidence for Early Proterozoic units in the Watersmeet 

Gneiss Dome, northern Michigan / by Karin M. Barovich ... [et al]. 

Temperature-pressure estimates of dynamically recrystallized rocks in the Early 

Proterozoic Mountain Shear Zone, northeastern Wisconsin / by Warren C. Day, 

P.K. Sims, and Zell E. Peterman.

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

ch. G-H) (U.S. Geological Survey bulletin ; 1904-G-H)

"Chapters G and H issued as a single volume and are not available 

separately."

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:1904-G-H

1. Geology, Stratigraphic Proterozoic. 2. Geology, Stratigraphic 

Archaean. 3. Gneiss Michigan. 4. Radioactive dating Michigan. 

5. Geology Michigan. 6. Rock deformation Wisconsin. 7. Geology 

Wisconsin. I. Barovich, Karin M. II. Day, Warren C. III. Sims, P.K. 

(Paul Kibler), 1918- . IV. Peterman, Zell E . V. Title: Watersmeet Gneiss 

Dome, northern Michigan. VI. Title: Mountain Shear Zone, northeastern 

Wisconsin. VII. Series. VIII. Series: U.S. Geological Survey bulletin ; 

1904-G-H.

QE75.B9 no. 1904-G-H 

[QE6535] 

557.3 s dc20

[551.7'5'09774] 90-24807

CIP
Chapter G

Neodymium Isotopic Evidence for Early 

Proterozoic Units in the Watersmeet 

Gneiss Dome, Northern Michigan

By KARIN M. BAROVICH, P. JONATHAN PATCHETT, 

ZELL E. PETERMAN, and P.K. SIMS

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

P.K. SIMS and L.M.H. CARTER, Editors
CONTENTS

Abstract Gl

Introduction Gl

Geology G3

Geochronology and geochemistry G3

Analytical methods G3

Results G5

Discussion G6

Conclusions G7

References cited G7

FIGURES

1. Geologic map of the northern part of Watersmeet gneiss dome, northern 

Michigan G2

2. AFM diagram for rocks from Watersmeet gneiss dome G3

3. Graph showing Nd isotopic data for rocks from Watersmeet gneiss dome G5

TABLE

1. Sm-Nd data for rocks from Watersmeet gneiss dome G4

IV Contents
CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

Neodymium Isotopic Evidence for

Early Proterozoic Units in the

Watersmeet Gneiss Dome, Northern Michigan

By Karin M. Barovich, 1 P. Jonathan Patchett, 1 Zell E. Peterman, and P.K. Sims

Abstract

In the Watersmeet gneiss dome, Upper Peninsula of 

Michigan, neodymium isotopic data for three gneiss units 

previously thought to be entirely Archean in age demonstrate 

that one unit, biotite gneiss and schist, must be primarily Early 

Proterozoic in origin. TCHUR (chondrite uniform reservoir) ages 

for six of the gneiss samples range from 3.72 to 3.54 Ca, 

confirming the Early Archean origin of the majority of the 

gneisses in the dome. TDM (depleted-mantle) ages for four 

samples from a biotite gneiss and schist unit from the dome, 

however, range from 2.37 to 2.25 Ca, and eNd(1.8) values 

range from -3.1 to -2.3, indicating a mixture of an Early 

Proterozoic source with some recycled Archean material. The 

biotite gneiss and schist unit has uncommonly high samarium 

and neodymium concentrations relative to the other gneisses 

in the dome, between 13 and 26 parts per million and 71 and 

142 parts per million, respectively. These and other high 

incompatible element concentrations suggest that the protolith 

of the Early Proterozoic unit possibly was volcanic, 

representing quite a strongly chemically differentiated magma. 

The presence of disseminated fluorite in the Early Proterozoic 

rocks suggests that they formed in a volatile-rich environment.

INTRODUCTION

Two Archean terranes are recognized in the Lake 

Superior region: (1) a Late Archean greenstone-granite 

terrane and (2) an Early to Late Archean gneiss terrane. The 

gneiss terrane lies south of the greenstone-granite terrane 

and is separated from it by the Great Lakes tectonic zone

Manuscript approved for publication, September 21, 1990. 

'Department of Geosciences, University of Arizona, Tucson, AZ

85721.

(GLTZ), a presumed paleosuture of subcontinental length 

(Sims and others, 1980). The greenstone-granite terrane is 

the southernmost part of the Superior province, and is 

termed the Wawa subprovince (Card and Ciesielski, 1986).

The Watersmeet gneiss dome, in the Upper Peninsula 

of Michigan, is one of several gneiss domes south of the 

GLTZ (Morey and others, 1982) that are cored by gneiss, 

migmatite, and amphibolite and are partly overlain by Early 

Proterozoic metasedimentary and metavolcanic rocks of the 

Marquette Range Supergroup (Sims, 1990). They owe their 

origin to Early Proterozoic (Penokean) deformation (Sims 

and others, 1984). Of the domes that have been studied, the 

Watersmeet dome formed at a higher pressure (7 kbar) than 

the others and at temperatures in the range of 600-650 °C 

(Attoh and Klasner, 1989). As a consequence, rocks in the 

core of the Watersmeet dome were metamorphosed to 

amphibolite facies and were complexly folded and tectonically interleaved and mylonitized during the doming. Sims 

and others (1984) concluded from the geochronological data 

available that all the gneisses in the core probably are 

Archean in age. A subsequent Sm-Nd whole-rock isotopic 

study of representative samples of the gneisses, reported 

herein, however, has shown that some of the gneisses are 

Early Proterozoic.

The purpose of this report is to (1) describe the 

principal types of gneisses in the dome, (2) present the 

Sm-Nd data, and (3) point out the utility of Nd isotopic 

work in distinguishing crustal formation ages in highly 

complex, metamorphosed terranes. Subsequent to recognition of the Early Proterozoic gneisses, two of us (Sims and 

Peterman) reexamined the field relations, collected 

additional samples, and studied additional thin sections. 

Figure 1, revised from the earlier published geologic map 

(Sims and others, 1984, fig. 2), is based on these new 

studies combined with the Nd isotopic data.

Neodymium Isotopic Evidence, Watersmeet Gneiss Dome, Michigan G1
46°20' ___________ ^^^v^^^^fe;

/148, D2439

EXPLANATION

EARLY PROTEROZOIC 

Michigamme Formation

Unexposed magnetic unit Blair Creek Formation Bad River Dolomite

Tonalitic augen gneiss 

Late to Early Archean 

Gneiss, amphibolite, and schist

Biotite gneiss and schist

EARLY PROTEROZOIC AND ARCHEAN

Unexposed magnetic unit

Gneiss and amphibolite 

Early Archean 

Biotite gneiss

Approximate contact Dashed where inferred;

dotted where concealed 

Normal fault Bar and ball on downthrown side;

dotted where concealed

Strike and dip of bedding Strike and dip of foliation Bearing and plunge of lineation 

Sample locality

Figure 1. Geologic map of the northern part of Watersmeet gneiss dome, northern Michigan (modified from Sims, 1990). No contact line between units indicates 

position of contact not known.
GEOLOGY

Three major gneiss units were delineated during 

mapping in the area of the Watersmeet dome (Sims and 

others, 1984). These units are (1) tonalitic augen gneiss, 

exposed mainly in the northern part of the dome (fig. 1); 

(2) a complexly interlayered biotite gneiss succession, 

which overlies the tonalitic augen gneiss, and (3) an 

interlayered bimodal amphibolite and biotite gneiss succession, which crops out along the northern part of the 

dome. Our Sm-Nd study shows that part of the biotite gneiss 

succession (biotite gneiss and schist) is Early Proterozoic in 

age.

The tonalitic augen gneiss, the oldest unit (Utg on 

fig. 1), is a medium-gray, medium- to coarse-grained 

irregularly layered rock that is invariably mylonitized and 

recrystallized to finer grain sizes.

The biotite gneiss succession consists of two major 

rock types, which are intercalated: (1) a medium-grained 

biotite gneiss (Ubg, fig 1) that varies from massive to 

layered, on a scale of meters, and (2) a fine- to mediumgrained biotite gneiss and schist (Xgs, fig. 1). Both rock 

types show multiple deformation and recrystallization, 

indicated by wide variations in texture and fabric. Some 

facies of the finer grained gneiss and schist resemble 

garnetiferous schist of the Early Proterozoic Michigamme 

Formation (Fritts, 1969). The latter (Xgs, fig. 1) is 

apparently the dominant unit within the biotite gneiss 

succession. It differs from the older biotite gneisses in 

having as much as 6 percent muscovite and as much as 1 

percent fluorite.

The third unit (Wga, fig. 1), interlayered amphibolite 

and biotite gneiss, is a distinctive bimodal volcanic 

succession that occurs both inside and outside the dome 

(Sims, 1990).

GEOCHRONOLOGY AND 

GEOCHEMISTRY

Fe as FeO

EXPLANATION

+ Tonalitic augen gneiss

 Biotite gneiss

O Biotite gneiss and schist

jf Calc-alkaline

Op-" trend

v

Na 2 O + K 2 O MgO

Figure 2. AFM diagram for rocks from Watersmeet gneiss 

dome. Calc-alkaline trend from Ringwood (1974). Data from 

Sims and others (1984).

deformation and metamorphism this unit underwent during 

the Penokean orogeny, Sims and others (1984) concluded 

that the Rb-Sr isochron was a secondary isochron, and 

interpreted the 1,755 Ma zircon age as resulting from 

recrystallized zircon formed during the 1.9-1.7 Ga Penokean orogeny. As discussed herein, however, the analyzed 

sample (M48) represents a definite Early Proterozoic rock. 

Sims and others (1984) presented some major element geochemistry for all the three major rock bodies 

delineated in the Watersmeet gneiss dome. The range of 

compositions for samples from the tonalitic augen gneiss 

and the biotite gneiss succession is narrow, as shown in 

figure 2; the samples plot as calc-alkaline on this diagram. 

Accordingly, on the basis of major element geochemistry 

alone, there is no indication of differences in the age of any 

of the analyzed rocks of the tonalitic augen gneiss and the 

biotite gneiss succession.

Peterman and others (1986) determined a U-Pb zircon 

age of 3,562±39 Ma for the tonalitic augen gneiss of the 

Watersmeet dome. Zircons from one unit of the mediumgrained biotite gneiss within the biotite gneiss succession 

also plotted on a chord with an upper intercept of 3.56 Ga. 

Rubidium-strontium whole-rock and mineral analyses on 

these units showed that these systems were reset, and 

indicated ages of about 1,750 Ma. The interlayered amphibolite and biotite gneiss unit has an age of about 2,640 Ma 

(Sims and others, 1984).

One sample (M48, fig. 1) of biotite gneiss and schist 

of the biotite gneiss succession gave a concordant U-Pb 

zircon age of 1,755 Ma, which is virtually indistinguishable 

from the whole-rock and mineral Rb-Sr age of 1,750±90 Ma 

on the rocks (Sims and others, 1984). Because of the intense

ANALYTICAL METHODS

The analytical techniques used for Sm-Nd for crustal 

rocks are summarized here. Details are given by Patchett 

and Ruiz (1987).

1. Whole-rock powders were placed in open Teflon 

bombs with a HF-HNO3 solution on hot plates to dissolve 

the major minerals, then sealed in the Teflon bombs with 

fresh HF-HNO3 for 7 days.

2. Concentrations and 147Sm: 144Nd ratios were 

determined with a 149Sm-150Nd combined spike.

3. Two duplicate analyses are reported (table 1, 

M45L and D1042). They show that for a rock powder, 

147Sm: 144Nd can be reproduced to within 0.5 percent.

Neodymium Isotopic Evidence, Watersmeet Gneiss Dome, Michigan G3
Table 1 . Sm-Nd data for rocks from Watersmeet gneiss dome 

[Leaders («), not determined]

Sample 

No.

Sm 

(ppm)

Nd 

(ppm)

147Sm: 

144Nd

143 Nd: TCHUR age 

(Ga)

TDM age 

(Ga)

Tonalitic augen gneiss

M45L

M45L

M83

D1042

D1042

4.58

4.63

10.92

5.62

5.88

28.43

28.61

67.17

35.46

36.16

0.09737

0.09768

0.09821

0.09847

0.09833

0.510216±9

0.510223±9

0.510262±9

0.510251±13

0.510268±7

3.69

3.69

3.65

3.64

3.64

«

-

-

--

Biotite gneiss succession

Biotite gneiss:

M93

M321A

M321B

4.10

2.56

1.82

23.90

15.11

10.96

0.10423

0.10220

0.10012

0.510387±13

0.510315±7

0.510376±10

3.68

3.72

3.54

-

Biotite gneiss and schist:

M95B

M199

M48

D2439

26.33

23.77

19.70

13.01

142.27

139.63

102.91

70.94

0.11189

0.10287

0.11570

0.11088

0.511502±7

0.511405±9

0.511519±8

0.511508±3

2.04

2.00

2.10

2.00

2.30

2.25

2.37

2.27

* Errors are ±2 standard errors of the mean.

SAMPLE DESCRIPTIONS

Tonalitic augen gneiss (3.56 Ga):

M45L Medium-grained irregularly layered gneiss. NWV4SEV4 sec. 4,

T. 45 N., R. 39 W. 

M83 Similar to M45L. Coarser layering. NWV4SWV4 sec. 4, T. 45 N.,

R. 39 W. 

D1042 Similar to M45L. NWViSEVi sec. 4, T. 45 N., R. 39 W.

(Same locality as M45L.)

Biotite gneiss (3.56 Ga):

M93 Light-gray fine- to medium-grained layered gneiss. SWV4NEV4 sec. 5,

T. 45 N., R. 39 W. 

M321A Medium-grained distinctly layered biotite gneiss. NEVi sec. 5,

T. 45 N., R. 39 W. 

M321B Similar to M321A, but finer layering. NE1A sec. 5, T. 45 N., R. 39 W.

(Same locality as M321A.)

Biotite gneiss and schist (Early Proterozoic):

M95B Gray fine-grained biotite gneiss. NWViNEVi sec. 5, T. 45 N.,

R. 39 W.

Ml99 Gray fine-grained biotite schist. NEViSEVi sec. 5, T. 45 N., R. 39 W. 

M48 Strongly cataclasized biotite schist with porphyritic plagioclase.

NEV4NWV4 sec. 4, T. 45 N., R. 40 W. 

D2439 Gray fine-grained biotite schist. NEV4NWV4 sec. 4, T. 45 N., R. 40 W.

(Same locality as M48.)

G4 Precambrian Geology of Lake Superior Region
-12

-16

3.6 3.2 2.8 2.4

BILLIONS OF YEARS AGO

1.6

Figure 3. Nd isotopic data for rocks from Watersmeet gneiss dome, plotted as eNd versus time. CHUR, the chondrite uniform 

reservoir or bulk Earth isotopic evolution; DM, depleted mantle (DePaolo, 1981). Epsilon, the deviation in parts per ten thousand 

of the 143 Nd: 144Nd ratio of the sample from the chondrite value of the same age. Cross, Early Archean tonalitic augen gneiss; 

open circle, Early Archean biotite gneiss; open rectangle, range of eNd(1.8) values for the Early Proterozoic samples.

4. Throughout the period of this study, the La Jolla 

Nd standard gave l43Nd: 144Nd=0.511862±7 (2o of the 

mean); 2-sigma errors reported correspond to the last two 

significant figures.

RESULTS

We analyzed 10 whole rock samples, 3 from the 

tonalitic augen gneiss, 3 from the biotite gneiss unit, and 4 

from the biotite gneiss and schist unit for Nd isotopic 

composition and Sm and Nd concentrations. Descriptions 

and locations of the samples are given in table 1. 

Approximate sample localities are shown in figure 1. 

Neodymium isotopic data are presented in table 1, and 

isotopic evolution, e values, and TCHUR and TDM model 

ages2 are shown in figure 3. TDM ages were calculated only

2Ages derived using chondrite uniform reservoir or bulk Earth 

isotopic evolution, and depleted-mantle method.

for those samples that were determined to have an Early 

Proterozoic crustal origin. The soundness of the depletedmantle model used (DePaolo, 1981) has been well documented for the Early Proterozoic in this region (Nelson and 

DePaolo, 1984; Patchett and Arndt, 1986).

The data fall into two groups, according to their 

model ages. The three tonalitic augen gneiss samples 

(M45L, M83, and D1042) and the three biotite gneiss 

samples (M321A, M321B, and M93) form the first group. 

This group yields Early Archean TCHUR ages between 3.72 

and 3.54 Ga. These model ages are in agreement with the 

U-Pb upper intercept age of 3.56 Ga determined for the 

tonalitic augen gneiss and one layer of the biotite gneiss unit 

(Peterman and others, 1980). The evolution through time of 

these Early Archean gneisses is plotted for reference. At the 

U-Pb crystallization age of 3.56 Ga, eNd values for these 

samples range from -2.0 to +0.2. Sm and Nd concentrations 

and 147Sm: 144Nd ratios generally are typical for Early 

Archean felsic gneisses. (See Taylor and McLennan, 1985, 

for summary.) The second group includes the biotite gneiss 

and-schist samples (M48, D2439, M199, and M95B). These

Neodymium Isotopic Evidence, Watersmeet Gneiss Dome, Michigan G5
samples have a narrow range of Early Proterozoic TDM ages 

between 2.37 and 2.25 Ga (fig. 3). Because the crystallization ages of these gneisses and schists are unknown, we 

have plotted their evolution back to their intersection with 

the depleted mantle curve of DePaolo (1981). Whereas the 

147Sm: 144Nd ratios of these samples are representative of 

typical felsic upper crust, the Sm and Nd concentrations are 

anomalously high: Sm values range between 13 and 26 ppm 

and Nd values between 71 and 142 ppm. These values are 

as much as 4.5 times higher than average upper crustal felsic 

material.

DISCUSSION

The TCHUR model ages and eNd values for the first 

group of samples (the tonalitic augen gneiss and the biotite 

gneiss samples) show that these rocks represent new crustal 

additions approximately 3.60 Ga. Their model ages are also 

in agreement with Nd model ages for samples of the 

tonalitic augen gneiss of the Watersmeet dome analyzed by 

McCulloch and Wasserburg (1980) and Futa (1981).

The second group of samples has a narrow range of 

Early Proterozoic TDM ages, varying by only 0.12 b.y. (from 

2.37 to 2.25 Ga). It is theoretically possible that this unit is 

also Early Archean in age, and that its whole-rock system 

was reset during a later metamorphic episode. That these 

rocks contain fluorine (in the form of disseminated fluorite) 

suggests that they may have been exposed to a volatile-rich 

environment at some point in their history. But if they are 

Early Archean in age, it would be necessary to invoke largescale REE (rare earth element) mobility to explain their 

Early Proterozoic model ages. An implausible scenario such 

as massive flooding of the rocks by extremely REE enriched 

hydrothermal fluids with an Early Proterozoic isotopic 

signature during a major metasomatic event would be 

needed. To achieve the consistent Early Proterozoic model 

ages, it would be necessary to replace all the original Sm 

and Nd in the rock, and completely delete any Archean 

isotopic signature. We find this unlikely for two reasons: 

(1) These rocks show no evidence of the severe and 

pervasive hydrothermal alteration that would have to result 

from the circulation of such large volumes of REE-enriched 

fluids. Megascopically they do not show any evidence of 

hydrothermal alteration, and microscopically they are 

unusual only in the presence of fluorite as a common 

accessory mineral. If the fluorite was introduced by metasomatic fluids, then its occurrence as disseminated grains 

throughout the unit (not in confined veinlets) would 

strongly suggest pervasive as opposed to selective hydrothermal alteration. Yet no other minerals record a pervasive 

alteration event. (2) Given the complicated structural 

relationships whereby the Early Archean tonalitic augen 

gneiss and biotite gneiss are interleaved with the biotite 

gneiss and schist, it is impossible to see how this REE 

replacement during massive fluid circulation could have

been so selective as long as we assume that the rock types 

were in their current structural positions relative to each 

other before the proposed alteration event. We suggest 

instead that the biotite gneiss and schist unit, represented by 

the samples with Early Proterozoic model ages, was added 

to the crust around 1.80 Ga, probably during an early phase 

of the 1.9-1.7 Ga Penokean orogeny, and that it was then 

structurally interleaved with the Archean gneiss units of the 

Watersmeet dome during deformation associated with the 

Penokean event. Barovich and others (1989) have already 

demonstrated that the Penokean orogeny was a time of 

major growth of crust from the mantle. The =2.30 Ga TDM 

ages of the biotite gneiss and schist samples do not correlate 

with any known igneous or volcanic activity. Instead, we 

suggest that mixing of two components, depleted mantle 

and a small percentage of preexisting Archean crustal 

material, produced the precursors of these rocks, around 

1.80 Ga. Thus the TDM model ages of -2.30 Ga are older 

than the proposed crystallization age of 1.80 Ga, and 

represent a weighted average of the two components, 

primitive mantle and recycled Archean crustal material. 

Existence of a depleted mantle in this area and others in 

North America has been well documented (Nelson and 

DePaolo, 1984; Barovich and others, 1989). That some 

Archean material was incorporated during formation of the 

gneiss protoliths is reasonable to assume, because the 

terrane from which these samples were taken contains 

considerable amounts of Archean basement.

As a result of subsequent field and laboratory work, 

we have distinguished the Early Proterozoic biotite gneiss 

and schist from the Early Archean units within the Watersmeet dome. In thin section, the biotite gneiss and schist unit 

is characterized by 3-6 percent modal muscovite and 

accessory fluorite. In addition, trace element analyses by 

KJ. Schulz (unpub. data, 1990) have shown that the rocks 

are uncommonly enriched in incompatible elements, 

including U, Th, Ta, Hf, and Zr.

The elevated REE and incompatible element 

concentrations of these Early Proterozoic rocks are uncommon. The minerals that host the REE budget of the whole 

rock have not been identified. The presence of fluorite as an 

accessory mineral disseminated throughout the rocks 

suggests a volatile-rich environment for the precursor of this 

unit. Although little experimental evidence exists to gauge 

the mobility of rare earth and other incompatible elements 

as fluorine complexes and explain the elevation of incompatible element concentrations in fluorine-rich rocks, much 

evidence of an empirical nature is available. Alderton and 

others (1980) attributed REE loss in an altered granite in 

southwest England to REE complexing by fluorine in the 

altering fluids. Christiansen and others (1986) cited 

characteristic enrichments in incompatible lithophile 

elements in Cenozoic fluorine-rich topaz rhyolites from the 

Western United States.

G6 Precambrian Geology of Lake Superior Region
CONCLUSIONS

Although most of the rock units in the Watersmeet 

gneiss dome are Early Archean in age, as confirmed by our 

Nd isotopic data for the tonalitic augen gneiss and the 

biotite gneiss, our data have shown that the biotite gneiss 

and schist unit within the dome is Early Proterozoic. Trace 

element concentrations, both from this study and unpublished data by KJ. Schulz, show that, geochemically, the 

Early Proterozoic gneiss and schist unit is quite distinct 

from the Early Archean gneisses of the Watersmeet dome. 

Further work is needed to determine the origin of this Early 

Proterozoic unit and the manner and timing in which these 

rocks were so complexly interleaved with the Early Archean 

units of the gneiss dome.

REFERENCES CITED

Alderton, D.H.M., Pearce, J.A., and Potts, P.J., 1980, Rare earth 

element mobility during granite alteration Evidence from 

southwest England: Earth and Planetary Science Letters, 

v. 49, p. 149-165.

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.

Barovich, K.M., Patchett, P.J., Peterman, Z.E., and Sims, P.K., 

1989, Nd isotopes and the origin of 1.9-1.7 Ga Penokean 

continental crust of the Lake Superior region: Geological 

Society of America Bulletin, v. 101, p. 333-338.

Card, K.D., and Ciesielski, Andre, 1986, DNAG No. 1 Subdivision 

of the Superior Province of the Canadian Shield: Geoscience Canada, v. 13, no. 1, p. 5-13.

Christiansen, E.H., Sheridan, M.F., and Burt, D.M., 1986, The 

geology and geochemistry of Cenozoic topaz rhyolites from 

the western United States: Geological Society of America 

Special Paper 205, 82 p.

DePaolo, D.J., 1981, Neodymium isotopes in the Colorado Front 

Range and crust-mantle evolution in the Proterozoic: 

Nature, v. 291, p. 193-196.

Fritts, C.E., 1969, Bedrock geologic map of the MareniscoWatersmeet area, Gogebic and Ontonagon Counties, 

Michigan: U.S. Geological Survey Miscellaneous Geologic 

Investigations Map 1-576, scale 1:48,000.

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

and its constituent minerals from northern Michigan: 

Geochimica et Cosmochimica Acta, v. 45, p. 1245-1249.

McCuIloch, M.T., and Wasserburg, G.J., 1980, Early Archean 

Sm-Nd model ages from a tonalitic gneiss, northern

Michigan, in Morey, G.B., and Hanson, G.N., eds., Selected 

studies of Archean and lower Proterozoic rocks, northern 

Canadian Shield: Geological Society of America Special 

Paper 182, p. 135-138.

Morey, G.B., Sims, P.K., Cannon, W.F., Mudrey, M.G., Jr., and 

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

region, Minnesota, Wisconsin, and northern Michigan: 

Minnesota Geological Survey State Map Series 5-13, scale 

1:1,000,000.

Nelson, B.K., and DePaolo, D.J., 1984, 1,700 Ma greenstone 

volcanic successions in southwestern North America and 

isotopic evolution of Proterozoic mantle: Nature, v. 312, 

p. 143-146.

Patchett, P.J., and Amdt, N.T., 1986, Nd isotopes and tectonics of 

1.9-1.7 Ga crustal genesis: Earth and Planetary Science 

Letters, v. 78, p. 329-338.

Patchett, P.J., and Ruiz, J., 1987, Nd isotopic ages of crust 

formation and metamorphism in the Precambrian of eastern 

and southern Mexico: Contributions to Mineralogy and 

Petrology, v. 87, p. 329-338.

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

Archean tonalitic gneiss from northern Michigan, U.S.A., in 

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

Archean and lower Proterozoic rocks, southern Canadian 

Shield: Geological Society America Special Paper 182, 

p. 125-134.

____1986, A protracted Archean history in the Watersmeet 

gneiss dome, northern Michigan, in Peterman, Z.E., and 

Schnabel, D.C., eds., Shorter contributions to isotope 

research: U.S. Geological Survey Bulletin 1622, p. 51-64.

Ringwood, A., 1974, The petrological evolution of island arc 

systems: Geological Society of London Quarterly Journal, 

v. 130, p. 183-204.

Sims, P.K., 1990, Geologic map of Precambrian rocks, Marenisco, 

Thayer, and Watersmeet 15-minute quadrangles, Gogebic 

and Ontonagon Counties, Michigan, and Vilas County, 

Wisconsin: U.S. Geological Survey Miscellaneous Investigations Series Map 1-2093, scale 1:62,500.

Sims, P.K., Card, K.D., Morey, G.B., and Peterman, Z.E., 1980, 

The Great Lakes tectonic zone A major crustal structure in 

central North America: Geological Society of America 

Bulletin, pt. I, p. 690-698.

Sims, P.K., Peterman, Z.E., Prinz, W.C., and Benedict, F.C., 1984, 

Geology, geochemistry, and age of Archean and Early 

Proterozoic rocks in the Marenisco-Watersmeet area, 

northern Michigan: U.S. Geological Survey Professional 

Paper 1292-A,42p.

Taylor, S.R., and McLennan, S.M., 1985, The continental 

crust Its composition and evolution: Oxford, England, 

Blackwell Scientific Publications, 312 p.

Neodymium Isotopic Evidence, Watersmeet Gneiss Dome, Michigan G7

Chapter H

Temperature-Pressure Estimates of 

Dynamically Recrystallized Rocks in the 

Early Proterozoic Mountain Shear Zone, 

Northeastern Wisconsin

By WARREN C. DAY, P.K. SIMS, and ZELL E. PETERMAN

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

P.K. SIMS and LM.H. CARTER, Editors
CONTENTS

Abstract HI 

Introduction HI

Regional setting HI

Local geologic setting H2

Analytical methods H2 

Texture and mineral composition of rocks within the Mountain shear zone H2

M2 mineralogy and texture H2

Mineralogic compositions H5 

Geobarometry H8 

Geothermometry H10 

Conclusions HIS 

References cited H16

FIGURES

1. Regional geologic map of northern Wisconsin and adjacent Michigan H3

2. Geologic map of the Mountain shear zone, showing sample localities H6

3. Quartz-alkali feldspar-plagioclase ternary diagram of granitoid rocks H8

4. Photograph showing mosaic texture of quartz and feldspar in a granodiorite 

(129-85) inside the shear zone H8

5. Photograph showing a small M2 veinlet formed along margin of primary minerals 

in a granodiorite (165-85) outside the shear zone H9

6. Plot of hornblende composition, IVA1 versus Na+K H9

7. Histogram of An content of plagioclase in rocks from the Mountain shear zone 

area H9

8. Plot of plagioclase and alkali feldspar compositions for rocks from the Mountain 

shear zone Hll

9. Photograph showing biotite and (Mt) garnet in a fine-grained felsic volcanic rock 

(32-85) outside the shear zone HIS

10. Photograph showing biotite and (M2) garnet in a granitoid rock (46-85) inside 

the shear zone HIS

11. Garnet-biotite and two-feldspar geothermometry diagram H16

TABLES

1. Modal mineralogy for rocks of the Mountain area, northeastern Wisconsin H4

2. Whole-rock geochemistry and normative minerals of rocks from the Mountain 

shear zone H7

3. Summary of hornblende microprobe data, and pressure estimates for rocks of the 

Mountain shear zone H10

4. Summary of mean and standard deviation for feldspar microprobe data and 

temperature estimates for rocks in the Mountain shear zone area H12

5. Summary microprobe data for garnet and biotite, and temperature estimates for 

rocks of the Mountain shear zone H14
CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

Temperature-Pressure Estimates of Dynamically 

Recrystallized Rocks in the Early Proterozoic 

Mountain Shear Zone, Northeastern Wisconsin

By Warren C. Day, P.K. Sims, and Zell E. Peterman

Abstract

The Mountain shear zone, in northeastern Wisconsin, is 

a discrete structural belt 2 kilometers wide and 12 kilometers 

long that transects older (1,889 to 1,860 Ma) supracrustal and 

granitoid rocks of the Pembine-Wausau terrane of the Early 

Proterozoic Wisconsin magmatic terranes. The shear zone is 

intruded by the 1,812 Ma Mines Quartz Diorite, which provides a minimum age of deformation. Dextral transpressive 

second-phase (D2 ) ductile deformation within the zone reoriented original regional (DJ deformational fabric and caused 

dynamic recrystallization, which is recorded in the secondphase (M2) mineral assemblages. The weak, nearly beddingparallel regional (S,) foliation preserved outside the shear zone 

is overprinted within it by a strong second-phase (S2 ) 

schistosity. Protomylonite and orthomylonite, as well as S-C 

tectonite, are developed within the shear zone.

Hornblende geobarometry of granitoid rocks and metabasalt indicates that the regional M^ pressure of about 4.5±1 

kilobars was elevated during the second metamorphic episode 

(M2) both within and adjacent to the shear zone to approximately 6.0±1 kbar. Carnet-biotite geothermometry indicates 

that M, ranged from 585 °C to 615 °C, but that M2 equilibrium 

temperatures may have increased to as much as 654 °C. 

However, garnet is a rare mineral both outside and inside the 

shear zone, and hence, the peak M2 thermal conditions are 

only an estimate. Two-feldspar geothermometry indicates that 

inside the shear zone the feldspars underwent continuous 

subsolidus reequilibration from peak M2 conditions (574 °C at 

6.0 kilobars) down to very low temperatures (280 °C at 6.0 

kilobars). These data indicate that shear-induced dynamic M2 

recrystallization effectively increased pressure and temperature and produced significant alteration in rock and structures 

during its operation in the Mountain shear zone. Maximum 

temperature-pressure conditions in the shear zone were 

followed by cooling to very low temperatures, equivalent to 

that outside the shear zone.

INTRODUCTION

This study consisting of field and geochemical 

analysis documents the mineralogical changes and associated temperatures and pressures attendant to deformation 

within the Mountain shear zone, northeastern Wisconsin. 

Early regional Dl deformation textures and associated M! 

metamorphic minerals were overprinted in and adjacent to 

the shear zone by shear-induced D2 fabrics and M2 metamorphic mineral assemblages. Geobarometry and geothermometry applied to rocks of similar bulk composition from 

within and outside the shear zone provide estimates of the 

temperature and pressure changes produced by the ductile 

deformation.

Regional Setting

The Mountain shear zone is a discrete zone of ductile 

deformation in metavolcanic and granitoid rocks of the 

Pembine-Wausau terrane, within the Early Proterozoic 

Penokean orogen (Sims, 1987; Sims and others, 1989). The 

Penokean orogen lies along the south margin of the Superior 

province of the Canadian Shield, and in northern Wisconsin 

and Michigan consists of two lithostratigraphic assemblages 

(fig. 1). To the north is a continental-margin assemblage of 

metasedimentary rocks of the Early Proterozoic Marquette 

Range Supergroup, which overlies an Archean basement. 

To the south is an assemblage of volcanic and granitoid 

rocks of the Wisconsin magmatic terranes. The two assemblages are separated by the east-west-trending Niagara fault 

zone, which is thought to be an Early Proterozoic paleosuture (Sims and others, 1989). The supracrustal rocks of the 

Mountain shear zone were generated in an island arc and 

(or) back-arc basin environment in the interval 1,889-1,860 

Ma, and were intruded by granitoid rocks in the interval

Temperature-Pressure Estimates, Mountain Shear Zone, Northeastern Wisconsin H1
between 1,870 and 1,760 Ma. The Hines Quartz Diorite 

intruded the Mountain shear zone at about 1,812 Ma, thus 

providing a minimum age for the shear zone. For a more 

comprehensive regional geologic review the reader is 

referred to reports by Sims and others (1989; 1990) and 

Sims (1987).

Local Geologic Setting

The Mountain shear zone is a northeast-trending zone 

of ductile deformation that is 2 km wide and exposed for a 

length of 12 km (fig. 2). Early Proterozoic metavolcanic 

rocks of the Waupee Volcanics as well as granodiorite occur 

along the southeast margin of the shear zone. The eastern 

part of the shear zone is covered by Quaternary glacial 

deposits. The shear zone is intruded by the 1.47 Ga Middle 

Proterozoic anorogenic rocks of the Wolf River batholith 

along the northern and western periphery (Sims, 1989; 

fig. 2).

Outside the shear zone, the Waupee Volcanics are 

massive to thinly bedded, moderately foliated dacitic to 

rhyolitic tuff and tuff breccia and minor volcanogenic 

metasedimentary rocks (unit Xwf, fig. 2; tables 1 and 2) 

containing Mj mineral assemblages. Interbedded with these 

felsic rocks are minor layers of massive to thinly layered 

mafic metavolcanic rocks. Along the southeast margin the 

Waupee Volcanics are olivine-normative metabasaltic rocks 

with low Mg-numbers (cation ratio of 100xMg:(Mg+0.85 

Fetotal)) that form massive and pillow lava flows and thinly 

layered mafic tuffs (table 2). Inside the shear zone, the 

Waupee volcanic rocks are schistose and comprise M2 

mineral assemblages. In the western part of the shear zone, 

they consist of interlayered biotite schist, interpreted to have 

had a felsic volcanic and volcanogenic protolith, and minor 

amphibole schist. The eastern part of the shear zone is 

composed dominantly of amphibole schist and quartz 

diorite schist. The protolith for the amphibole schist was 

mafic volcanic rocks of the Waupee Volcanics, similar to 

those outside the shear zone (unit Xwm, fig. 2).

An Early Proterozoic peraluminous granitoid rock 

intrudes the Waupee Volcanics both outside and inside the 

shear zone (fig. 2; tables 1 and 2). Outside, the rock is a 

medium-grained, moderately foliated hornblende granodiorite (fig. 3) with a porphyritic texture resulting from 

plagioclase and potassium-feldspar phenocrysts; the rock 

has regional M, mineral assemblages. Inside the shear zone, 

the granitoid is a mylonitic biotite-hornblende granodiorite 

to tonalite gneiss with M2 mineral assemblages. The unit 

contains porphyroclasts of plagioclase and microcline and 

lensoid recrystallized quartz aggregates in a fine-grained 

recrystallized groundmass of plagioclase, alkali feldspar, 

quartz, and minor biotite and amphibole. Using the 

terminology of Wise and others (1984), the unit is 

dominantly a protomylonite with zones of orthomylonite.

The protolith of the unit is the granodiorite exposed outside 

the shear zone (unit Xg, fig. 2).

Analytical Methods

Representative samples (5-10 kg) for geochemical 

analysis (table 2) were selected from outcrops that exhibited 

the least alteration. The samples were crushed, split, and 

powdered to less than 100 mesh. All analyses were done in 

U.S. Geological Survey laboratories in Denver, Colo. 

Major-oxide sample splits were fused and analyzed by 

X-ray fluorescence using the method outlined by Taggart 

and others (1987). Determinations for FeO followed 

procedures outlined by Peck (1964), for total H2O by 

Jackson and others (1985), and for CO2 by those reviewed 

by Jackson and others (1987). Analytical precision and 

accuracy for the major oxides are less than 3 percent of the 

reported value. The trace elements Rb, Sr, Y, Zr, and Nb 

were determined by X-ray fluorescence using an energydispersive spectrometer with a 109Cd source. Estimated 

analytical error for this technique is less than 10 percent.

The mineral compositions reported in tables 3-5 were 

obtained using an analytical research laboratory electron 

microprobe with six movable spectrometers. Mineral standards chosen were those that are well characterized and had 

a mean atomic weight similar to that of the unknown. The 

samples were analyzed with an electron beam current of 10 

nanoamps and an accelerator voltage of 15 kev. The data 

were reduced using the MAGIC4 matrix correction 

program.

TEXTURE AND MINERAL 

COMPOSITION OF ROCKS WITHIN 

THE MOUNTAIN SHEAR ZONE

The rocks of the Mountain shear zone have undergone a regional Mt metamorphism, which is overprinted 

within the shear zone by the shear-induced M2 metamorphism. The regional Ml metamorphism was of lower 

amphibolite facies. Outside the shear zone, M1 metamorphism of the felsic volcanic and volcaniclastic rocks 

produced weakly foliated equigranular, medium-grained 

biotite-garnet-muscovite schists. Regional M1 metamorphism of the basaltic rocks produced hornblendeplagioclase amphibolite (table 1). Muscovite, epidote, and 

chlorite are Mt alteration minerals of the granodiorite both 

inside and outside the shear zone.

M2 Mineralogy and Texture

Ductile D2 deformation caused dynamic recrystallization (M2) both within and adjacent to the shear zone. 

Adjacent to the shear zone, the felsic volcanic rocks have

H2 Precambrian Geology of Lake Superior Region
91°

45° .

EXPLANATION

50 KILOMETERS

Sedimentary rocks of Paleozoic age

Middle Proterozoic (Keu/eenau/an) mafic igneous 

and sedimentary rocks of Midcontinent rift 

system (1,200-1,000 Ma)

Anorogenic igneous rocks (1,510-1,470 Ma)

WISCONSIN MAGMATIC TERRANES 

Metavolcanic and granitoid rocks in PembineWausau terrane (1,880-1,760 Ma)

Metavolcanic and granitoid rocks in Marshfield 

terrane (1,890-1,835 Ma)

Gneiss in Marshfield terrane (2,800 Ma)

M

CONTINENTAL-MARGIN ASSEMBLAGE

Marquette Range Supergroup (2,100-1,820 Ma) and 

gneiss, granite, and greenstone (3,550-2,600 Ma)

Contact Dashed where inferred 

High-angle fault

1 Thrust fault Sawteeth on overthrust block 

Shear zone

Mountain shear zone

Figure 1. Regional geology of northern Wisconsin and adjacent Michigan showing relationship of continental-margin 

assemblage rocks to north and Wisconsin magmatic terranes to south of Niagara fault zone. Study area is near Mountain, Wis. 

(M). Modified from Sims and others (1990).

Temperature-Pressure Estimates, Mountain Shear Zone, Northeastern Wisconsin H3
Table 1. Modal mineralogy for rocks of the Mountain area, northeastern Wisconsin

[Values in volume percent. Tr, trace; blank, not determined; amphibole in sample 91-85 is a clinoamphibole, probably 

cummingtonite]

Outside the shear zone

Waupee Volcanics and 

Inside the

Column No. 

Sample No. 

Rock type

Plagioclase

Quartz

Microcline

Biotite

Hornblende

Muscovite

Garnet

Epidote

Sphene

Chlorite

Calcite

Magnetite

Accessory

1 

23-85 

Basalt

47.2

1.0

51.4

0.2

0.2

2 

32-85 

Dacite

55.2

8.0

Tr

33.6

Tr

Tr

2.0

1.2

3 

32B-85 

Dacite

42.0

10.0

27.5

2.5

15.5

2.5

4 

18-85 

Basalt

25.0

1.0

68.5

3.0

2.0

Tr

0.3

0.2

Tr

5 

164-85 

Basalt

32.0

Tr

64.0

2.0

Tr

Tr

Tr

2.0

Tr

6 

20-85 

Basalt

40.8

18.6

35.6

4.0

1.0

7 

1 34-85 

Quartz 

diorite

65.6

9.7

5.6

18.4

0.7

8 

1 72-85 

Quartz 

diorite1

48.8

22.6

12.2

15.8

0.6

9

144-85 

Mafic 

schist

42.0

41.5

13.0

Tr

Tr

3.5

Tr

Tr

10 

91-85 

Mafic 

schist

27.5

31.0

3.0

7.0

Tr

Tr

Tr

Tr

Tr

Granodiorite 

Outside the shear zone

Mines Quartz Diorite 

Inside the shear zone

Column No. 22 23 24 25 26 27 28 29 30 

Sample No. 101-85 60B-85 35-85 112-85 165-85 34A-85 Average Standard 147-85 150-85 88-85

deviation

Plagioclase

Quartz

Microcline

Biotite

Hornblende

Accessory

Alteration

minerals

42.8

18.2

24.6

8.8

1.8

0.5

3.3

49.5

15.0

17.5

10.0

7.5

0.5

Tr

40.7

37.6

15.4

5.4

0.8

0.1

44.8

14.6

20.6

10.6

7.3

2.1

Tr

51.8

18.2

11.0

9.0

9.8

0.2

Tr

55.2

21.6

5.7

7.0

9.0

1.5

Tr

47.5

20.9

15.8

8.5

7.1

0.9

0.6

5.1

7.8

6.2

1.8

2.8

0.7

1.2

47.7

32.6

12.4

6.3

0.9

0.1

54.2

2.0

0.9

241.7

1.2

Tr

41.0

37.3

10.0

8.5

2.5

Tr

Tr

'Unit XWQ.

2Includes relicts of clinopyroxene.

minor fine-grained intercrystalline M2 mineral phases. 

Phenocrysts of feldspar and quartz locally have interlocking 

sutured grain boundaries. The interstitial fine-grained 

minerals (quartz, feldspar, mica, and opaque minerals) are 

randomly distributed around the margins of the larger 

phenocrysts. The felsic volcanic rocks inside the shear zone 

are strongly foliated and medium grained with ubiquitous 

fine-grained granular intercrystalline M2 minerals. The 

primary medium-grained quartz and feldspar crystals form a 

well-developed mosaic with distinct interlocking sutured 

crystal boundaries. The intercrystalline M2 mineral matrix 

forms an interconnected groundmass of fine-grained equigranular "bleb-shaped" quartz, feldspar, mica, and opaque 

minerals that defines the strong foliation.

Similar textural variations are observed in the mafic 

volcanic rocks. Outside the shear zone the unit (Xwm, 

fig. 2) has a weak foliation (Sj), with medium- to coarsegrained plagioclase phenocrysts within fine-grained hornblende aggregates. Fine-grained, granular M2 mineral 

phases (hornblende, feldspar, and minor quartz) form along 

the interstices of the M. 1 mineral grain boundaries with no

preferred orientation. Inside the shear zone, the foliation 

within the mafic volcanic rocks (units Xwa and Xwi) is 

intense, and is defined by a matrix of interlocking elongated 

fine-grained hornblende crystals and quartz-feldspar blebs 

and stringers (fig. 4). Both inside and outside the shear zone, 

the plagioclase phenocrysts are commonly altered to sericite, which is thought to be an Ml phase. However, inside 

the shear zone the fine-grained M2 plagioclase essentially 

lacks sericite alteration.

Crystal-plastic (D2) deformation of the granitoid 

rocks (unit Xgn, fig. 2) within the shear zone has caused M2 

recrystallization of the coarse-grained quartz crystals, 

producing large (centimeter-scale) multicrystalline quartz 

stringers in which the individual crystals have sutured 

boundaries. The crystal boundaries of the coarse-grained 

phenocrysts of plagioclase and potassium feldspar differ in 

being more angular and less recrystallized than the quartz. 

Biotite is generally restricted to the fine-grained interstitial 

M2 material. Small 50-150 micrometer-sized blebs of fresh 

M2 plagioclase and quartz occur within the primary plagioclase phenocrysts (fig. 4).

H4 Precambrian Geology of Lake Superior Region
associated quartz diorite 

shear zone

11

13-85

Dacite

50.8 

41.2

2.0

4.9

0.8

Tr

Tr

Tr

0.2

12

1 99-85

Dacite

49.1 

3.5

31.5

2.4

0.1

44.2

0.1

Tr

0.0

13

76-85

Dacite

53.3 

37.3

0.6

5.8

Tr

0.5

Tr

Tr

1.0

Tr

14

141-1-85

Dacite

42.0 

41.5

2.1

13.0

Tr

Tr

3.5

Tr

Tr

15

1 67-85

Dacite

31.0 

29.0

28.0

5.0

Tr

1.0

Tr

6.0

16

1 68-85

Dacite

50.5 

37.5

11.5

Tr

Tr

0.3

Tr

Tr

Tr

0.2

17

1 70-85

Dacite

40.0 

21.5

37.0

Tr

1.0

Tr

Tr

Tr

0.5

18

18-85

Dacite

57.7 

30.7

10.7

0.9

19

64-85

Rhyolite

50.5 

38.7

8.3

1.8

0.3

0.2

Tr

Tr

0.2

20

1 73-85

Rhyolite

51.5 

22.5

10.0

15.0

Tr

Tr

Tr

Tr

Tr

21

79-85

Rhyolite

46.7 

23.6

21.7

7.2

0.8

Tr

Tr

0.2

Cranodiorite gneiss 

Inside the shear zone

31 32 33 34 35 36 37 38 39 40 41

39-85 46-85 50-85 129A-85 129B-85 130A-85 130B-85 55-85 40A-85 45B-85 44-85 Average Standard

deviation

44.2

39.0

10.6

6.0

Tr

49.4

41.2

8.4

0.7

48.2

40.4

1.5

8.4

1.0

29.5

37.1

30.9

1.1

0.4

54.0

32.2

1.0

11.0

1.0

0.7

51.0

35.3

5.0

8.2

0.4

47.0

37.0

8.0

6.5

1.0

46.7

38.3

9.4

3.4

1.0

Tr

46.1

34.4

1.3

15.0

0.7

0.5

44.4

30.5

19.0

5.4

Tr

48.4

36.1

8.7

6.5

Tr

46.3

36.5

9.5

7.3

0.9

0.4

6.0

3.1

8.8

3.5

0.1

0.4

0.3 0.5 1.0 0.1 0.1 0.5 1.2 2.0 0.7 0.3 0.6 0.6

Minerals produced by M2 are also developed 

(although in less abundance) outside the shear zone, 

indicating that the D2 deformation was not restricted to the 

shear zone itself. Small M2 blebs of quartz and feldspar are 

present locally in the granitoid rocks, and thin veinlets (0.1 

mm to 0.5 cm) of fine-grained, comminuted quartz and 

feldspar occur along the margins of the primary minerals 

(fig. 5).

Mylonitic fabric, similar to the type II S-C mylonite 

fabric of Lister and Snoke (1984), is developed within the 

granitoid rocks inside the shear zone. The matrix material is 

oriented parallel to the dominant foliation and represents 

zones of high-strain (C-surface). Fine-grained mica within 

the matrix material is commonly oriented at an acute angle 

of 20°-40° to the dominant foliation, forming a secondary 

foliation (S-surface) that is compatible with a dextral sense 

of shearing. Locally, muscovite and (or) biotite "fish," 

common in S-C mylonites, are developed in the interstitial 

M2 material and indicate a dextral sense of shear. Sims and

others (1990) have described both the regional and local 

effects of the Dj and D2 deformation events.

Mineralogic Compositions

Recrystallization (M2) resulting from the D2 

deformation within the Mountain shear zone produced 

distinct changes in mineral compositions. The hornblende 

composition changed slightly as a result of reequilibration. 

Figure 6 shows the microprobe data (table 3) for hornblende 

from basaltic and granodioritic rocks from the study area. In 

general, in the same sample the early M^ hornblende is 

slightly lower in IVA1 and Na+K abundances than the M2 

hornblende. The lower calculated IVA1 content for the Ml 

hornblende is a result of relatively higher silicon content; 

however, the Ml hornblende also has lower total aluminum 

content.

The plagioclase composition of rocks within the shear 

zone changed dramatically as a result of M2 dynamic 

recrystallization. In general, plagioclase became more

Temperature-Pressure Estimates, Mountain Shear Zone, Northeastern Wisconsin H5
45° 

12' 

30"

45° 

10'

88°27'30" 

I

25' 88°22'30" 

I

Ywh

Ywb

1 KILOMETER 

I

SHEAR ZONE

OUTSIDE 

SHEAR ZONE

Xwq

EXPLANATION

MIDDLE PROTEROZOIC (1,600-900 Ma) 

Belongia Granite of Wolf River batholith ( = 1,480 Ma)

Hager Formation

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

Mines Quartz Diorite

Baldwin Conglomerate

Granite gneiss Deformed and metamorphosed granodiorite

Biotite schist and amphibolite schist Interlayered

Quartz diorite schist

Amphibole schist with thin layers of felsic schist

Granodiorite Equivalent to unit Xgn in shear zone

Waupee Volcanics 

Dominantly felsic volcanic rocks 

Mafic lava flows

Contact Dashed where approximate 

Transcurrent fault 

Trace of Si foliation 

Trace of S% foliation

-i t i t f t Southeastern boundary of shear zone 

 ' Sample locality

H6 Precambrian Geology of Lake Superior Region
Table 2. Whole-rock geochemistry and normative minerals of rocks from the Mountain shear zone, 

northeastern Wisconsin

[Major oxide analyses by J. Taggart, A.J. Bartel, and K. Stewart; FeO, H2O, and CO2 by R. Brandt and S. Root; minor 

elements Rb and Sr by isotope dilution; chemistry by K. Futa; mass spectrometry by K. Barovich and T. Ball; Y, 

Zr, and Nb (EOXRF) by K. Barovich; Mg-number=MgxlOO/(Mg+Fe+2), Fe+2 calculated as 0.85xFe total. 

A:KNC=Al2O3/(K2O+Na2O+CaO), molar ratio. Blank, not determined]

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

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

FP re2O u3 ...............

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

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

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

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

K2O...................

Ti02 ....... ...........

P205 ..................

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

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

H20..................

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

Total..............

Mg-number .......

A:KNC..............

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

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

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

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

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

K:Rb.................

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

Corundum.........

Orthoclase.. .......

Anorthite...........

Wollastonite...... 

Enstatite ...........

Ferrosilite.........

Fosterite ...........

Fayalite ............

Magnetite..........

Ilmenite. ...........

Apatite .............

Calcite ..............

Magnesite.........

Outside

1

T3 OC

Basalt

49 0

189

2.00

954

3 54

109

3.40

1 09

0.66

0.1

0.16

1.28

<0.01

0.19

. 100.8

22.10

0.71

17

. 299

12

35

2

. 541

6.39

32 84

1.77

7.93 

2.98

5.09

4.05

7.62

2.88

1.24

0.24

0.43

2.14

Relation to shear zone

Inside Outside

20 

23456

85 153 85 27-85 35 85 39 85

Basalt Dacite Rhyolite Grano- Granite 

diorite gneiss

Major oxides, in weight percent

51.8 63.1 72.4 69.3 72.7

17.70 17.40 14.50 14.20 13.20

2.69 0.21 0.31 0.98 0.89

730 289 2 19 3 14 3 18

4.77 1.64 1.69 1.00 0.50

7.35 7.52 2.19 1.47 2.22

3.29 2.68 3.44 2.79 3.60

2 28 2 97 1 33 5 52 2 50

0.73 0.65 0.29 0.23 0.19

0.25 0.23 0.05 0.09 <0.05

0.16 0.05 <0.02 0.04 0.05

1.48 0.40 1.15 0.68 0.58

0.04 0.02 0.02 <0.01 0.04

0.03 <0.01 <0.01 0.04 0.15

99.9 99.8 99.6 99.5 99.9

30.86 32.67 38.43 18.41 10.25

0.83 0.82 1.31 1.07 1.04

Trace elements, in parts per million

48 35 22 70 47

555 208 168 431 175

21 15 22 8 27

92 91 165 115 93

97826

396 711 511 658 440

Normative minerals of rocks

18.37 38.90 25.68 35.07

3.58 1.28 1.00

13.49 17.59 7.89 32.79 14.80

26.83 26.74 10.52 6.49 9.75

3.28 3.79 

9.64 4.09 4.23 2.50 1.25

8.34 4.16 3.34 4.68 4.89

1.58

1.51

3.91 0.31 0.45 1.43 1.29

1.39 1.23 0.55 0.44 0.36

0.60 0.55 0.12 0.21 0.12

0.07 0.02 0.02 0.09 0.34

2.88 0.99 1.02 0.60 0.30

Ins

7

130-85

Granite 

gneiss

73 2

13.00

0.83

3.10

0.55

2.21

3.62

2.34

0.18

<0.05

0.04

0.45

0.03

0.09

99.7

11.49

1.04

48

195

18

97

4

402

35.95

0.83

13.87

10.10

1.37

4.80

1.21

0.34

0.12

0.21

0.33

ide

8

129 85

Granite 

gneiss

75.6

12.40

0.61

1 ^9

0.22

0.41

2.39

6.48

0.02

<0.05

<0.02

0.29

0.06

0.02

100.1

8.80

1.06

148

54

22

42

7

363

34.81

0.87

38.26

1.58

0.55

2.30

0.88

0.04

0.12

0.05

0.13

9

150-85

Hines 

Quartz 

Diorite

50.1

19.00

0.90

10.80

4.57

9.68

2.64

0.64

0.70

0.17

0.19

0.99

0.02

0.05

100.5

26.35

0.84

14

346

9

27

1

377

20.20

9.62

15.40

1.19

2.11

1.30

1.32

0.40

0.11

2.76

Figure 2 (facing page). Geology of the Mountain shear zone, 

northeastern Wisconsin, showing sample localities. Numbers 

refer to column numbers in tables 1 and 2. Modified from Sims 

and others (1990).

albitic for rocks of equivalent composition (table 4). Plotted 

in figure 7 are the combined core and rim analyses for 

plagioclase for the various rock compositions. A significant 

shift to more albitic plagioclase compositions occurred in 

each rock type. The plagioclase composition in the felsic

Temperature-Pressure Estimates, Mountain Shear Zone, Northeastern Wisconsin H7
Figure 3. Quartz-alkali feldspar-plagioclase diagram for 

granodiorite outside the shear zone (dots), granodioritic gneiss 

inside the shear zone (triangles), and the Mines Quartz Diorite 

(stars), Mountain area, northeastern Wisconsin. Fields for 

diagram: I, quartz alkali syenite; II, quartz syenite; III, quartz 

monzonite; IV, quartz monzodiorite; V, quartz diorite; VI, 

alkali granite; VII, granite; VIII, granodiorite; IX, quartz diorite 

(tonal ite).

volcanic rocks (fig. 8) changed from oligoclase (An24_3i) to 

albite (An10_13). Inside the shear zone the medium-grained 

plagioclase phenocrysts have essentially equivalent compositions to the recrystallized M2 matrix plagioclase, 

suggesting that the larger crystals equilibrated with the 

matrix during D2 shearing.

A similar shift in plagioclase composition is recorded 

in the granodioritic rocks. Outside the shear zone (unit Xg, 

fig. 2) the cores of the plagioclase phenocrysts are dominantly andesine (An27_^0) w'm rims °f oligoclase (An17). 

Inside the shear zone (unit Xgn, fig. 2), however, the 

composition becomes distinctly more albitic in the phenocryst cores (An16_22), rims (An23_14), and within the recrystallized fine-grained matrix (An16_22), again indicating that 

the plagioclase reequilibrated to more albitic compositions 

as a result of shear-induced M2 recrystallization.

GEOBAROMETRY

The mineral assemblages of the rocks within the 

Mountain shear zone are simple, and, therefore, allow 

application of only one geobarometer (table 3). Recently, 

Hammerstrom and Zen (1986) presented an empirical geobarometer in which they calibrated the aluminum content of 

hornblende observed in calc-alkaline plutons as a function 

of depth of crystallization (pressure). They estimated 

pressure from the metamorphic mineral assemblages of the 

associated country rocks. Hammerstrom and Zen (1986)

Figure 4. Photograph showing mosaic texture of quartz and 

feldspar in a granodiorite (129-85) inside the shear zone.

noted that the ratio of total Al (Al1) to tetrahedral Al (IVA1) 

in hornblende varies linearly, and that the Al1 content is 

principally a function of pressure. Subsequent work by 

Hollister and others (1987) and Rutter and others (1989) has 

verified the hornblende geobarometer for use in calcalkaline plutonic rocks. Hollister and others (1989) 

independently confirmed the approach of Hammerstrom 

and Zen (1986) with additional data for hornblendes from 

calc-alkalic plutons from nine localities in the Central 

Gneiss Complex of British Columbia. They refined the 

equation for determining pressure, which resulted in 

decreasing the estimated error from ±3 kbar to ±1 kbar. 

Rutter and others (1989) experimentally calibrated the 

geobarometer at higher pressures (10 kbar) and confirmed 

its application to specific multimineral assemblages in 

which hornblende is in equilibrium with plagioclase, biotite, 

potassium feldspar, quartz, sphene, and magnetite or 

ilmenite+epidote. These phases are present in the granodiorite both inside and outside the Mountain shear zone.

Microprobe data and calculated pressures for hornblende from the granodiorite and metamorphosed basalt 

inside and outside the Mountain shear zone are presented in

H8 Precambrian Geology of Lake Superior Region
Figure 5. Photograph showing a small M2 veinlet formed 

along margin of primary minerals in a granodiorite (165-85) 

outside the shear zone.

table 3. The basalt does not strictly meet the multimineral 

assemblage criteria in that it lacks potassium feldspar. 

However, the hornblende compositions in the basalt are 

very similar to those in the granodiorite. Outside the shear 

zone the estimated equilibrium pressures using the method 

of Hollister and others (1987) for the primary Ml hornblende range from 4.0 kbar for the granodiorite to 4.7 kbar 

for the basalt, both of which are within the estimated error 

for the geobarometer of ±1 kbar, suggesting that the 

geobarometer yields a reasonable estimate for the metabasalt. The secondary M2 hornblende in the basalt, which 

occurs along the rims of the Ml hornblende, yields an 

estimated pressure of 5.8 kbar. This is significantly higher 

than the MI hornblende outside the zone.

Inside the shear zone the estimated pressure for the 

primary Ml hornblende is 4.3 kbar for the basalt and 5.5 

kbar for the granodiorite gneiss, similar to Ml outside the 

shear zone. However, the calculated pressure of 

equilibration of the secondary M2 hornblende from the 

granodiorite gneiss is elevated (6.1 kbar), as is the M2 

hornblende from outside the zone. These data suggest that

0.5 1.0 

(Na+K) ATOMS

1.5

Figure 6. Hornblende composition data for rocks of the 

Mountain shear zone, northeastern Wisconsin. M, hornblende (diamond) contains less Na+K and IVAI compared 

to the younger M2 hornblende (triangle). Data listed in 

table 3; analytical error approximately the symbol size.

30

20

10

1/3

LU 

</)

< O 1

LJ_ f

0 30

A

EXPLANATION 

Mafic volcanic rock 

Felsic volcanic rock 

Granitoid rock

10 20 30 40 50

20

10

B

10 20 30 40

ANORTHITE CONTENT IN PLAGIOCLASE,

IN PERCENT

50

Figure 7. Histogram of plagioclase anorthite (An) content of 

rocks from A, inside, and B, outside the Mountain shear zone.

the M! generation of hornblende equilibrated at similar 

pressures both inside and outside the shear zone at about 

4.5+1 kbar. Subsequent deformation (D2) associated with 

the development of the shear zone, as recorded in the M2

Temperature-Pressure Estimates, Mountain Shear Zone, Northeastern Wisconsin H9
Table 3. Summary of microprobe data for hornblende and pressure estimates 

for rocks of the Mountain shear zone

Outside shear zone

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

A12O3 ...................

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

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

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

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

K2O ......................

Ti02 .....................

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

Total (wt. pet.) 

No. analyses.........

23-85 

Basalt

Primary 

M,

44.05 

9.33 

19.95 

8.68 

11.62 

1.15 

0.98 

0.86 

0.40

97.02 

6

Secondary 

M2

43.20 

10.31 

20.07 

8.16 

11.80 

1.19 

1.10 

0.81 

0.38

97.02 

6

Number of cations on

Si..........................

IVA1 ......................

Al VI ....................

Fe+2 .......... ............

Mg........................

Ca.........................

Na.........................

K ..........................

Ti..........................

Mn........................

Al total.................

6.752 

1.248 

0.438 

2.558 

1.983 

1.909

0.342 

0.192 

0.099 

0.052 

1.686

6.641 

1.359 

0.509 

2.580 

1.869 

1.944

0.355 

0.216 

0.094 

0.049 

1.868

165-85 

Granodiorite

Primary 

M,

43.55 

8.45 

20.30 

8.40 

11.24 

1.01 

1.01 

1.15 

0.68

95.79 

3

the basis

6.790 

1.210 

0.343 

2.647 

1.952 

1.878

0.305 

0.201 

0.135 

0.090 

1.553

Inside shear zone

20-85 

Basalt

Primary 

M,

45.48 

9.11 

16.78 

11.04 

11.93 

1.06 

0.64 

0.59 

0.48

97.11 

10

129-85 

Granodiorite 

gneiss

Primary 

M,

43.14 

10.04 

22.49 

7.34 

11.49 

0.90 

1.06 

0.52 

0.49

97.66 

6

Secondary 

M2

42.66 

10.56 

22.79 

7.02 

11.65 

1.00 

1.18 

0.54 

0.52

97.92 

7

of 23 oxygens

6.834 

1.166

0.448 

2.109 

2.472 

1.921

0.309 

0.123 

0.067 

0.061 

1.614

6.659 

1.341 

0.486 

2.903 

1.689 

1.901

0.269 

0.209 

0.060 

0.064 

1.827

6.586 

1.414 

0.508 

2.943 

1.615 

1.927

0.299 

0.232 

0.063 

0.068 

1.922

Pressure estimates (kbar)

Hammers trom 

and Zen (1986) 

(error ±3 kbar)

Hollister and 

others (1987) 

(error ±1 kbar)

4.6

4.7

5.5 

5.8

3.9

4.0

4.2 

4.3

5.3 

5.5

5.7 

6.1

hornblende, occurred at elevated pressures of approximately 

6.0+1 kbar. Therefore, the dynamic recrystallization 

recorded in the shear zone was accompanied by a relative 

increase of approximately 1.5 kbar pressure.

GEOTHERMOMETRY

Garnet-biotite geothermometry was applied to measure the thermal peaks of the Ml and M2 mineral 

assemblages, whereas a two-feldspar geothermometer was 

used to unravel the effects on low-temperature subsolidus 

recrystallization (post-M2) observed in the Mountain shear 

zone. The garnet-biotite geothermometers utilize the 

Mg:(Fe-Mg±Mn) ratio of coexisting garnet and biotite, 

and, therefore, the calculated temperatures should be 

internally consistent, allowing a comparison of the relative 

temperatures calculated for the Ml and M2 mineral 

assemblages. Although rare, garnet does coexist in textural

equilibrium with biotite in the metamorphosed felsic volcanic rocks (unit Xwf, fig. 2) outside the shear zone (fig. 9) 

(samples 32-85 and 32B-85; tables 1 and 5). An extensive 

effort was made to find garnet within the shear zone; 

however, it is not present in the felsic volcanic rock (table 

1). Garnet was noted in only one sample inside the shear 

zone, which was granodiorite gneiss (46-85). The garnet is 

an M! metamorphic mineral, crosscutting the original 

igneous minerals (fig. 10). The garnet in this sample is 

compositionally zoned, having a more Fe-rich overgrowth 

rim (M2-stage).

The calculated temperatures using the biotite-garnet 

geothermometer of Ferry and Spear (1978), Hodges and 

Spear (1982), and Perchuck and Lavrent'eva (1983) are 

listed in table 5. These temperatures assume a pressure of 

4.5 kbar for the Ml metamorphism (garnet cores) and 6.0 

kbar for the M2 metamorphism (garnet rims) as calculated 

from the hornblende geobarometer (table 3). The effect of 

pressure variation on the temperature calculations is

H10 Precambrian Geology of Lake Superior Region
20-85

C

R 

RX

Mafic volcanic rocks

Inside

23-85

C -

R - 

RX

C

R

RX

-13-85

64-85

Outside

Felsic volcanic rocks

Inside

C 

R

RX

C 

R 

RX

39-85

>129-85

129B-85

Outside

Granodiorite gneiss

c»

60B-85

^165-85 

35-85

20

PLAGIOCLASE 

ANORTHITE, IN PERCENT

40

Outside

70 90

ALKALI FELDSPAR 

ORTHOCLASE, IN PERCENT

Figure 8. Plot of plagioclase anorthite (An) and alkali feldspar orthoclase (Or) content for rocks of various bulk compositions 

from inside and outside the Mountain shear zone. Error bars are one standard deviation from the mean (dot) for a sample. C, core; 

R, rim; RX, recrystallized M2 feldspar.

minimal (3 °C/kbar using Hodges and Spear, 1982), which The Ferry and Spear (1978) geothermometer yields

implies that any error in the calculated pressure is negligible an estimated temperature of 595 °C for Ml metamorphism

when comparing the relative temperature variation. (4.5 kbar) and 591 °C for M2 metamorphism (6.0 kbar) for

Temperature-Pressure Estimates, Mountain Shear Zone, Northeastern Wisconsin H11
Table 4. Summary of mean and standard deviation for feldspar microprobe data and 

temperature estimates using the two-feldspar geothermometer of Haselton and others (1983) 

for rocks in the Mountain shear zone area

[(No.), number of analyses; blank, not determined]

(No.) An

Outside the

23-85 Basalt: 

Plagioclase Core (7) 

Rim (10)

Ab

shear zone

40.3±4.6 59.2±4.6 

46.1±1.8 52.9±2.0

Or

0.4±0.1 

1.011.0

T(°C) 

4.5 kbar 6.0 kbar

32-85 Biotite schist (felsic meta volcanic):

Plagioclase Core (2) 24.2±4.0 74.8±2.7 1.0±0.3

Alkali-feldspar Core (3) 0.010.0 3.5±1.0 96.5±1.1

32B-85 Biotite schist (felsic meta volcanic):

Plagioclase Core (4) 31.3±1.7 68.3±2.9 3.8±0.9

35-85 Granodiorite porphyry: 

Plagioclase Core

Rim 

Alkali-feldspar Core

Rim

60B-85 Granodiorite:

Plagioclase Core

Alkali-feldspar Core

Myrmekite

Plagioclase Host

Alkali-feldspar Bleb

(7) 39.9±1.2 58.2±1.1 1.9±0.3

(6) 0.0 6.2±2.4 93.8±2.4

(4) 26.112.0 73.5+0.8 0.410.1

(1) 0.2 4.8 95.0

Intergranular recrystallized feldspar (with sericite alteration): 

Plagioclase Host (5) 31.610.2 67.713.6 0.6+0.2 

New (6) 36.511.8 62.911.6 0.6+0.1

165-85 Granodiorite:

Plagioclase Core (5) 26.712.3 83.611.6 0.910.1

Alkali-feldspar Core (3) 1.211.0 11.0+5.7 87.8113.8

297 312

(9)

(3)

(5)

(1)

27.112.0

17.517.6

0.110.2

1.3

72.212.0

69.514.6

3.710.3

3.0

0.610.4

13.019.2

96.110.9

95.7

310

278

325

293

420

340

439

356

441 458

dacite outside the shear zone. These temperatures are 

identical within the estimated error of the technique 

(±50 °C). Inside the shear zone, the estimated Ml 

temperature is relatively elevated to 645 °C, whereas the M2 

temperature of 612 °C (at 6.0 kbar) was essentially the same 

as that recorded outside the shear zone (table 5).

The Ferry and Spear (1978) geothermometer is based 

on ideal solid solution model for Fe-Mg cation exchange 

between garnet and biotite. Hodges and Spear (1982) 

recalibrated the geothermometer by incorporating manganese cation exchange between garnet and biotite and 

calcium exchange with plagioclase. This method yields 

temperature estimates outside the shear zone of about 

610-615 °C for Mj and a slightly lower 601 °C for M2, all 

of which are within the estimated error of ±50 °C for the

technique. However, inside the shear zone the calculated 

temperatures are elevated to 661 °C for Mj and 654 °C for 

M2.

Perchuck and Lavrent'eva (1983) presented another 

geothermometer that is based on the Fe-Mg-Mn cation 

exchange between garnet and biotite. In a recent review, 

Chipera and Perkins (1988) compared garnet-biotite geothermometers and suggested that the Perchuck and 

Lavrent'eva (1983) method yields the most precise and 

accurate results. However, as Indares and Martignole (1985) 

noted, the temperatures estimated from the Perchuck and 

Lavrent'eva (1983) geothermometer generally are lower 

for garnet with elevated manganese content. Outside the 

shear zone, the manganese content of the garnet is relatively 

low (about 3.5 wt. percent; table 5), and temperatures

H12 Precambrian Geology of Lake Superior Region
Table 4. Summary of mean and standard deviation for feldspar microprobe data and 

temperature estimates using the two-feldspar geothermometer of Haselton and others (1983) for 

rocks in the Mountain shear zone area Continued

[(No.), number of analyses; blank, not determined]

(No.) An Ab Or T <°C) 

4.5 kbar 6.0 kbar

Inside the shear zone

20-85 Basalt: 

Plagioclase Core 

Rim 

Groundmass

(7) 38.1±6.2 60.8±5.6 

37.3±0.9 61.810.8 

37.6±1.7 61.811.6

1.111.0 

0.910.5 

0.610.1

13-85 Biotite schist (felsic volcanic): 

Plagioclase Core (5) 9.8+1.2 

Rim (4) 9.9+1.4 

Groundmass (5) 11.8+1.1

89.3+1.9 

87.8+3.9 

87.2+1.0

0.9±0.5 

2.212.0 

1.0+0.4

64-85 Felsic metavolcanic: 

Plagioclase Core

Rim

Groundmass 

Alkali-feldspar Groundmass

39-85 Granodiorite gneiss: 

Plagioclase Core

Rim

Groundmass 

Alkali-feldspar Core

Groundmss

129 85 Granite gneiss: 

Plagioclase Core

Rim 

Alkali-feldspar Core

Groundmass

129B-85 Granite gneiss: 

Plagioclase Core

Rim

Groundmass 

Alkali-feldspar Core

Groundmass

(8) 12.6+1.1

(3) 13.8+0.5

(5) 13.1+2.0

(4) 0.3+0.2

(6) 

(3) 

(2) 

(10) 

(2)

21.5±2.0

17.8+1.4

22.0+1.3

0.2±0.1

0.1+0.01

83.6+5.4 3.8+4.8

85.1+2.1 1.1+1.1

85.2±1.5 2.8+2.3

3.3±0.3 96.4+1.1

75.9±1.3 2.512.0

76.1+5.5 6.115.0

73.011.5 4.910.7

3.5+1.0 96.411.5

3.7+0.4 96.210.5

(5)

(3)

(4)

(2)

20.112.7

22.812.7

2.8+2.0

1.0+0.2

79.212.0

76.512.2

13.3+6.6

16.6+0.8

0.710.2

0.610.1

83.9110.5

82.4+0.8

(7)

(8) 

(5) 

(4) 

(2)

15.812.9

13.512.7

16.410.4

3.311.0

1.0+0.2

83.6+2.0 

83.513.7 

82.8+1.2 

13.316.0 

16.611.3

0.610.5

3.013.1

0.810.4

83.4+0.8

82.410.8

Crosscutting the shear zone

268

266

266

294

293

306

469

554

443

513

519

282

280

280

308

307

321

487

574

460

532

538

147-85 Hines Quartz Diorite:

Plagioclase Core

Rim

(5)

(5)

42.310.7

41.5+2.0

57.1+0.4

58.0+1.1

0.310.1

0.510.5

calculated are similar to those of the other two geothermometers. The estimated temperatures for Ml of about 590 °C 

and of about 576 °C for M2 metamorphism are essentially 

equivalent to those calculated by the other geothermometers 

(table 5). However, inside the shear zone the garnet has 

appreciable Mn content (about 11 wt. percent; table 5) and 

the temperatures calculated for both Ml (565 °C) and for M2 

(543 °C) are lower. The calculated M2 temperature is

approximately 100 °C cooler than that estimated by either 

the Ferry and Spear (1978) or the Hodges and Spear (1983) 

method and results from the effect in the calculation of 

elevated Mn content in the garnet.

Two-feldspar geothermometry indicates that the feldspars underwent subsolidus reequilibration during the 

waning stage of D2 deformation. Plotted in figure 11 are 

calculated temperatures of feldspar equilibration using the

Temperature-Pressure Estimates, Mountain Shear Zone, Northeastern Wisconsin H13
Table 5. Summary microprobe data for garnet and biotite, and temperature estimates for rocks of 

the Mountain shear zone, northern Wisconsin

[Major oxides in weight percent; cation proportions in terms of number of oxygens; na, not analyzed]

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

A12O3 ................

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

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

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

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

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

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

F........................

Cl ......................

-0=C1 ...............

Total..............

No. analyses ..... 

No. oxygens......

Si.......................

A1vi

Fe ......................

Mg.....................

Ca......................

Na......................

K.......................

Ti.......................

Mn.....................

F........................

Cl ......................

Outside shear zone 

32-85 32B-85 

Dacite Dacite

Garnet 

core

37.43 

20.71 

34.39 

2.00 

1.71 

0.06 

na 

0.06 

3.60 

na 

na

99.96

3 

12

2.983 

0.017

1.929 

2.293 

0.238 

0.146

0.010 

na 

0.006 

0.216 

na 

na

Biotite

33.84 

19.42 

22.86 

7.12 

0.04 

0.14 

9.68 

1.54 

0.08 

0.31 

0.07

95.10 

0.13 

0.02

94.95

5 

24

5.759 

2.241

1.655 

3.253 

1.806 

0.007

0.047 

2.101 

0.197 

0.012 

0.166 

0.020

Garnet 

core

37.04 

21.41 

35.19 

1.85 

1.24 

0.07 

na 

0.05 

3.22 

na 

na

100.07

4 

12

2.995 

0.005

2.025 

2.380 

0.222 

0.107

0.012 

na 

0.003 

0.220 

na 

na

Garnet 

rim

36.57 

20.97 

35.12 

1.78 

1.38 

0.06 

na 

0.05 

3.15 

na 

na

99.08

4 

12

2.994 

0.006

2.019 

2.404 

0.218 

0.121

0.009 

na 

0.003 

0.219 

na 

na

Biotite

33.49 

19.39 

23.00 

6.42 

0.03 

0.10 

9.32 

1.59 

0.18 

0.24 

0.11

93.87 

0.10 

0.02

93.75

6

24

5.776 

2.224

1.718 

3.317 

1.649 

0.005

0.035 

2.052 

0.206 

0.027 

0.133 

0.032

Inside shear zone 

46-85 

Granodiorite gneiss

Garnet 

core

37.82 

20.97 

25.60 

2.29 

1.37 

0.01 

na 

0.09 

11.36 

na 

na

99.51

6 

12

3.050 

0.000

1.994 

1.727 

0.276 

0.118

0.002 

na 

0.005 

0.776 

na 

na

Garnet 

rim

37.54 

20.81 

26.61 

2.32 

1.08 

0.02 

na 

0.05 

10.78 

na 

na

99.21

8 

12

3.045 

0.000

1.989 

1.805 

0.260 

0.094

0.003 

na 

0.003 

0.741 

na 

na

Biotite

34.80 

17.93 

20.86 

8.71 

0.05 

0.11 

9.90 

1.90 

0.35 

0.33 

0.08

95.02 

0.14 

0.02

94.86

8

24

5.884 

2.116

1.456 

2.950 

2.195 

0.009

0.037 

2.136 

0.242 

0.050 

0.177 

0.024

Temperature estimates (°C)

Ferry and Spear 

(1978): 

4.5 kbar 594 

6.0 kbar 600

Hodges and Spear 

(1982): 

4.5 kbar 615 

6.0 kbar 620

Perchuck and Lavrent'eva 

(1983): 

4.5 kbar 589 

6.0 kbar 579

595 

601

610 

616

591 

581

585 

591

601 

606

586 

576

645 

650

661

667

565 

556

607 

612

648 

654

552 

543

H14 Precambrian Geology of Lake Superior Region
Figure 9. Photograph showing biotite and (M,) garnet 

(arrows) in a fine-grained felsic volcanic rock (32-85) outside 

the shear zone.

Figure 10. Photograph showing biotite and (M2) garnet 

(arrows) in a granitoid rock (46-85) inside the shear zone.

method of Haselton and others (1983). Although deformation and recrystallization were concentrated within the 

Mountain shear zone, each rock type sampled in the study 

area has fine-grained recrystallized M2 quartz-feldspar 

matrix material. Both inside and outside the shear zone, the 

temperatures of feldspar reequilibration range down to 

about 275 °C (table 3), which is well below the peak M2 

thermal conditions of about 650-670 °C recorded in the 

garnet-biotite data (table 5). The feldspar within the shear 

zone records a continuum in temperature from just slightly 

below the peak M2 (574 °C at 6.0 kbar) to the low similar 

to that observed outside the shear zone (about 293 °C at 6.0 

kbar). Outside the zone occurs a distinct temperature gap 

between the M2 temperatures measured in garnet-biotite 

(575-600 °C) and those recorded in the feldspars (275-450 

°C).

The retrograde metamorphism may have caused 

reequilibration of the Fe-Mg cation exchange within the 

biotite and garnet pairs analyzed to establish the peak M2

metamorphic conditions. Therefore, the peak M2 conditions 

calculated are minimum estimates. The effects of the 

retrograde metamorphism should not drastically alter the 

relative peak M2 temperatures calculated, however, because 

the feldspar data indicate that the retrograde overprint is 

equivalent both inside and outside the shear zone.

CONCLUSIONS

Dynamic M2 recrystallization within the Mountain 

shear zone overprinted the regional Dl deformation fabric. 

Although the D2 deformation was localized within the shear 

zone, the effects can be observed in rocks immediately 

adjacent to the south. Hornblende geobarometry indicates a 

relative increase in pressure of about 1.5 kbar as a result of 

D2 deformation. Garnet-biotite geothermometry suggests a 

relative increase in temperature of as much as 60 °C. 

However, the geothermometer developed by Perchuck and 

Lavrent'eva (1983), points to a possible slight relative

Temperature-Pressure Estimates, Mountain Shear Zone, Northeastern Wisconsin H15
r F&S

46-85 < H&S

[ P&L 04

 M 

r F&S z*

*3 O D QC J LJCiC O^.D~OO "S nCTO i

0 4

(4

Inside

Outside

^A

I P&L AA

r F&S n

32-85^ H&S

L P&L D

1 , 1 , 1

D

, 200 300 400 500 600 

TEMPERATURE, IN DEGREES CELSIUS

700

Figure 11. Comparison of the temperature estimates (at 4.5 

kbar) from inside and outside the Mountain shear zone. 

Temperature estimates use the garnet-biotite geothermometers 

F&S (Ferry and Spear, 1978), H&S (Hodges and Spear, 1982), 

and P&L (Perchuck and Lavrent'eva, 1983). Solid symbols, 

core compositions; open symbols, rim. Sample 46-85 

(diamond), 32B-85 (triangle), and 32-85 (square). Temperature estimates using the two-feldspar geothermometer indicate 

(solid dots) that both inside and outside the shear zone, 

feldspar postpeak M2 reequilibrated down to about 275 °C.

decrease amounting to about 30 °C during D2. The lower 

estimated temperature of the Perchuck and Lavrent'eva 

(1983) geothermometer results in part from elevated 

manganese content in the garnet inside the shear zone. 

However, garnet is found only rarely both inside and outside 

the shear zone; therefore, these temperatures are only an 

estimate for the peak M2 thermal conditions. Reequilibration of the feldspars extended well below subsolidus 

temperatures. Only within the shear zone does a twofeldspar geothermometer yield temperatures similar to those 

of the garnet-biotite geothermometry.

REFERENCES CITED

Chipera, S.J., and Perkins, Dexter, 1988, Evaluation of biotitegarnet geothermometers Application to the English River 

subprovince, Ontario: Contributions to Mineralogy and 

Petrology, v. 98, p. 40-48.

Ferry, J.M., and Spear, F.S., 1978, Experimental calibration of the 

partitioning of Fe and Mg between biotite and garnet: 

Contributions to Mineralogy and Petrology, v. 66, 

p. 113-117.

Hammerstrom, I.E., and Zen, E-an, 1986, Aluminum in 

hornblende An empirical igneous geobarometer: 

American Mineralogist, v. 71, p. 1297-1313.

Haselton, H.T., Jr., Hovis, G.L., Hemingway, B.S., and Robie, 

R.A., 1983, Calorimetric investigations of the excess 

entropy of mixing in analbite-sanidine solid solutions 

H16 Precambrian Geology of Lake Superior Region

Lack of evidence for Na, K short-range order and 

implications for two-feldspar thermometry: American 

Mineralogist, v. 68, p. 398-^13.

Hodges, K.V., and Spear, F.S., 1982, Geothermometry, geobarometry and the A12O3 triple point at Mt. Moosilauke, New 

Hampshire: American Mineralogist, v. 67, p. 1118-1134.

Hollister, L.S., Grissom, G.C., Peters, E.K., Stowell, H.H., and 

Sisson, V.B., 1987, Confirmation of the empirical correlation of Al in hornblende with pressure of solidification 

of calc-alkaline plutons: American Mineralogist, v. 72, 

p. 231-239.

Indares, A., and Martignole, J., 1985, Biotite-garnet geothermometry in the granulite facies The influence of Ti and Al in 

biotite: American Mineralogist, v. 70, p. 272-278.

Jackson, L.L., Brown, F.W., and Neil, ST., 1987, Major and minor 

elements requiring individual determination, classical 

whole rock analysis, and rapid rock analysis, in Baedecker, 

P.A., ed., Methods of geochemical analysis: U.S. 

Geological Survey Bulletin 1770, p. G1-G23.

Jackson, L.L., Taggart, J.E., and Foord, E.E., 1985, Quantitative 

determination of water in small material samples: Abstract, 

Pittsburgh Conference, New Orleans, La., p. 1191.

Lister, G.S., and Snoke, A.W., 1984, S-C mylonites: Journal of 

Structural Geology, v. 6, p. 617-638.

Peck, L.C., 1964, Systematic analysis of silicates: U.S. Geological 

Survey Bulletin 1170, 89 p.

Perchuck, L.L., and Lavrent'eva, I.V., 1983, Experimental investigations of exchange equilibria in the system cordieritegarnet-biotite, in Saxena, S.K., ed., Kinetics and equilibrium in mineral reactions: New York, Springer-Verlag, 

p. 199-239.

Rutter, M.J., Van der Laan, S.R., and Wyllie, P.J., 1989, Experimental data for a proposed igneous geobarometer Aluminum in hornblende at 10 kbar pressure: Geology, v. 17, 

p. 897-900.

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. 55-80.

____ 1989, Geologic map of Proterozoic rocks near Mountain, 

Oconto County, Wisconsin: U.S. Geological Survey 

Miscellaneous Investigations Series Map 1-1903, scale 

1:24,000.

Sims, P.K., Klasner, J.S., and Peterman, Z.E., 1990, The Mountain 

shear zone, northeastern Wisconsin A discrete ductile 

deformation zone within the Early Proterozoic Penokean 

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

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

1989, Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean orogen: 

Canadian Journal of Earth Sciences, v. 26, p. 2145-2158.

Taggart, J.E., Lindsay, J.R., Scott, B.A., Vivit, D.V., Bartel, A.J., 

and Stewart, K.C., 1987, Analysis of geologic materials by 

wavelength-dispersive X-ray fluorescence spectrometry, in 

Baedecker, P.A., ed., Methods of geochemical analysis: 

U.S. Geological Survey Bulletin 1770, p. El-El 9.

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

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

*U.S. GOVERNMENT PRINTING OFFICE: 1991-573-049/46000


SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports of 

wide and lasting interest and importance to professional scientists and engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of lasting scientific interest but are generally more limited in scope or 

geographic coverage than Professional Papers. They include the results 

of resource studies and of geologic andtopographicinvestigations; as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrogeology, 

availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific 

information of wide popular interest in a format designed for distribution 

at no cost to the public. Information is usually of short-term interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at 

depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

depositories. They are a nonpermanent form of publication that may be 

cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 71/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric 

bases at various scales; they show results of surveys using geophysical 

techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 71/2-minute quadrangle photogeologic maps on planimetric bases 

which show geology as interpreted from aerial photographs. Series also 

includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information 

for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; principal scale 

is 1:24,000 and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, Books 

and Open-File Reports Section, Federal Center, Box 25425, Denver, 

CO 80225. (See latest Price and Availability List)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982,1983,1984,1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet form only.

Selected copies of a monthly catalog "New Publications of the U.S. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklet form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

National Center, Reston, V A 22092.

Note.-Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.