Geology of Volcanic Rocks in the South Half 

of the Ishpeming Greenstone Belt, Michigan

U.S. GEOLOGICAL SURVEY BULLETIN 1904-P
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Chapter P

Geology of Volcanic Rocks in the South Half 

of the Ishpeming Greenstone Belt, Michigan

By THEODORE J. BORNHORST and RODNEY C. JOHNSON

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 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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does not imply endorsement by the U. S. Government

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Library of Congress Cataloging-in-Publication Data

Bornhorst, Theodore J.

Geology of volcanic rocks in the south half of the Ishpeming greenstone belt, 

Michigan / by Theodore J. Bornhorst and Rodney C. Johnson.

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

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

Includes bibliographical references. 

Supt. of Docs, no.: 119.3:81904P

1. Volcanic ash, tuff, etc. Michigan. 2. Geology, Stratigraphic Archaean. 

3. Geology Michigan. 4. Kitchi Formation. 5. Mona Formation. 

I. Johnson, R.C. (Rodney C.) II. Title. III. Title: Ishpeming greenstone belt, 

Michigan. IV. Series. V. Series: U.S. Geological Survey bulletin ; 1904-P. 

QE75.B9 no. 1904-P 

[QE461] 

557.3 s dc20

I552'.2'09774] 92-27841

CIP
CONTENTS

Abstract PI 

Introduction PI 

Acknowledgments PI 

Volcanic terminology P3 

Structural framework P4 

Volcanic stratigraphy P4 

Kitchi Formation P4 

Definition P4 

Description PS

Basalt flow unit P6 

Tuff unit P6 

Lahar unit P7 

Mona Formation P7 

Definition P7 

Description P8

Basalt flow unit P8 

Lapilli-tuffunit P9

Upper tuff unit (formerly Nealy Creek Member) P9 

Related intrusive rocks P10 

Deer Lake Peridotite P10 

Gabbro P10 

Geochemical character P10 

Environmental interpretation P12 

Volcanic history P12 

References cited P13

FIGURES

1. Geologic map of west half of the Archean Superior province showing location of 

	Ishpeming greenstone belt P2

2. Geologic map of Ishpeming greenstone belt, Michigan P3

3. Geologic map of southwest block of Ishpeming greenstone belt P5

4. Generalized stratigraphic column for the Kitchi Formation P6

5. Geologic map of southeast block of Ishpeming greenstone belt P8

6. Generalized stratigraphic column for the Mona Formation P9

7. Jensen cation plots for rocks of south half of Ishpeming greenstone belt Pll

Contents III

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

Geology of Volcanic Rocks in the South Half of the 

Ishpeming Greenstone Belt, Michigan

By Theodore J. Bornhorst1 and Rodney C. Johnson 1 '2

Abstract

The south half of the Ishpeming greenstone belt is dominated by a thick succession of subaqueously erupted volcanic 

rocks. A regional structural interpretation of the belt provides a 

framework for placing this succession in a stratigraphic order. 

The south half of the Ishpeming greenstone belt preserves the 

remnants of overlapping subaqueous volcanic complexes 

within an arc system.

The Kitchi Formation, as redefined herein, comprises the 

lower half of the volcanic succession and is subdivided into 

three informal units. The basal basalt flow unit is composed of 

pillowed, tholeiitic, basalt flows and minor banded iron-formation and dacitic tuff. It is overlain by the second and third units, 

which are tuff and lahar (volcanic debris flow) units that interfinger with one another. These units range in composition from 

calc-alkalic basalt to dacite. The Kitchi Formation represents 

an evolving tholeiitic, mafic shield to calc-alkalic, mafic to felsic subaqueous volcano.

The Mona Formation, which is redefined herein, is in 

structural contact with the Kitchi Formation, but is likely 

younger. It is subdivided into three informal units. The base 

consists of a basalt flow unit, which is composed of pillowed, 

tholeiitic to magnesium-rich basalt flows and minor amounts 

of banded iron-formation and tuff. The basalt is overlain by a 

calc-alkalic dacite to rhyolite lapilli-tuff unit and a laterally 

continuous upper tuff unit of calc-alkalic basalt to andesite. 

The Mona Formation represents an evolving tholeiitic, mafic 

shield to mafic-felsic calc-alkalic, mafic to felsic subaqueous 

volcano. The Deer Lake Peridotite, an ultramafic sill complex 

that intrudes the Kitchi Formation, is speculatively interpreted 

as a subvolcanic ultramafic komatiite base of the Mona 

Formation.

INTRODUCTION

The Ishpeming greenstone belt occupies the southeast 

corner of the northern complex of the Marquette district 

(figs. 1, 2). The northern complex comprises the Archean 

crustal rocks north of the Great Lakes tectonic zone, an 

Archean continent-continent suture (Sims and others, 

1980; Sims, 1991). Although the complex is composed of 

a wide variety of Archean igneous, sedimentary, and 

metamorphic rocks, it is best characterized as a greenstone-granite terrane. The greenstone part occupies about 

an eighth of the surface area of the complex. These rocks 

are part of the Wawa subprovince of the Canadian Shield 

as defined by Card and Ciesielski (1986), and may be the 

southernmost Archean greenstone belt in the Superior 

province.

The Ishpeming greenstone belt is a 13-km-thick succession of subaqueous mafic flows, intermediate pyroclastics, and volcaniclastic sedimentary rocks. These rocks are 

intruded by mafic and felsic dikes, granitoid plutons, and 

two peridotite bodies. The rocks are Archean in age, 

dating from probably about 2,700 Ma or before (Morgan 

and DeCristoforo, 1980; Trow, 1979; Hammond, 1978). 

The Archean rocks are unconformably overlain by and in 

fault contact with Early Proterozoic sedimentary rocks, 

and they are cut by diabase dikes of Early and Middle 

Proterozoic ages.

The objective of this report is to present the volcanic 

stratigraphy of the south half of the Ishpeming greenstone 

belt Volcanic stratigraphy and related intrusive rocks are 

analyzed in terms of volcanic environment.

'Department of Geological Engineering, Geology, and Geophysics, 

Michigan Technological University, Houghton, MI 49931.

2Westem Mining Corporation (USA), Marquette, MI 49855.

ACKNOWLEDGMENTS

Geologic studies in the Ishpeming greenstone belt have 

been financially supported by the Michigan Geological

Volcanic Rocks, South Half of Ishpeming Greenstone Belt, Michigan PI
50° 

PIKWITONEI

 ffff:fttlK-;i:matfy^V--f-fffA

UNITED STATES

LAKE SUPERIOR

Ishpeming

GLTZ

MINNESOTA 

RIVER VALLEY

500 KILOMETERS

EXPLANATION

Archean subprovince type 

Plutonic rocks 

Volcanic-plutonic rocks 

Metasedimentary rocks 

High-grade gneiss

Superior province boundary 

Subprovince boundary

Figure 1 . Geologic map of west half of the Archean Superior province showing location of Ishpeming greenstone belt in the eastern part of the northern complex of the Marquette district, Michigan. The belt is north of the Great Lakes tectonic zone (GLTZ). 

The high-grade gneiss in Minnesota is part of the Archean gneiss terrane. Unpatterned areas, post-Archean rocks. Modified from 

Card (1990).

P2 Precambrian Geology of Lake Superior Region
87°30'

46°30'

Bp RIVER FALLS SHEAR ZONE

R28W R27W R26W R25 W

10 KILOMETERS

EXPLANATION

Middle Proterozoic (900-1,600 Ma)

Jacobsville Sandstone (Midcontinent

rift system) 

Early Proterozoic (1,600-2,500 Ma)

Sedimentary rocks of Marquette Range

Supergroup 

Archean

Granitoid rocks

Metavolcanic rocks

Contact

r^i r^> r^i Shear zone

-4 Approximate trace of axial plane of 

plunging anticline

- -$ Approximate trace of axial plane of 

plunging syncline

Figure 2. Geologic map of Ishpeming greenstone belt, Michigan, showing the major lithostratigraphic blocks, shear zones (SZ), 

and fold axes.

Survey, the COGEOMAP program of the U.S. Geological 

Survey, the Mineral Institute Program administered by the 

U.S. Bureau of Mines, Callahan Mining Corporation, 

Western Mining Corporation (USA), and Michigan Technological University. Our understanding of the geology 

has benefited from discussions with geologists of Callahan 

Mining Corporation, Western Mining Corporation, and 

Kerr-McGee Corporation, as well as several present and

former graduate students at Michigan Tech. Critical 

reviews by P.K. Sims, W.F. Cannon, and G.W. Johns 

improved the manuscript.

VOLCANIC TERMINOLOGY

In this report we follow the terminology for volcanic 

rocks presented by Easton and Johns (1986), which

Volcanic Rocks, South Half of Ishpeming Greenstone Belt, Michigan P3
commonly has been used to describe the volcanic rocks of 

the Superior province. In general, volcanic rocks are subdivided into extrusive (lava) and fragmental (explosive) 

rocks that are emplaced in either a subaerial or subaqueous environment. In the Ishpeming greenstone belt textural 

evidence suggests that all the volcanic rocks were 

emplaced in a subaqueous environment.

The distinction between pyroclastic and epiclastic 

materials can be difficult to determine in a subaqueous 

environment. Subaqueous pyroclastic rocks, transported at 

least in part in a water medium (Cas and Wright, 1987; 

Cas and others, 1990), will, when identifiable, be called 

herein by the pyroclastic rock names of Easton and Johns 

(1986). When genesis is unknown or obscure, a nongenetic grain-size classification (pyroclastic breccia, >64 

mm; lapilli-tuff, 2-64 mm; tuff, <2 mm) is used. No corresponding classification scheme exists for clearly redeposited fragmental (epiclastic) but still volcanic-origin 

rocks. Some epiclastic rocks, such as many lahars (volcanic debris flows), may form by direct volcanic action. In 

the case of fine-grained schistose rocks, we distinguish 

pyroclastic from epiclastic protolith (Easton and Johns, 

1986) using criteria of Ayres (1969). A volcanic deposit is 

considered pyroclastic (Schmid, 1981) if it is composed of 

less than 25 percent epiclasts+chemical sediment clasts. 

Pyroclastic names are given to rocks composed of 

volcanic fragments if evidence for an epiclastic origin is 

lacking.

The terminology for volcanic rock types such as basalt, 

andesite, dacite, and rhyolite is based on the Jensen cation 

plot (Grunsky, 1981). This system is commonly applied 

elsewhere to rocks of the Superior province.

STRUCTURAL FRAMEWORK

The rocks of the Ishpeming greenstone belt have been 

strongly deformed by at least two major events. The oldest 

recognized event (D,) was recumbent folding. This was 

followed by upright folding about east-west-oriented axes 

(D2) (Johnson and Bornhorst, 1991). Shear zones are interpreted to have formed during and later than D2 and may 

have been reactivated during the Early Proterozoic 

Penokean deformation. The structural history of the belt as 

determined by us is a critical component of stratigraphic 

reconstruction.

The Ishpeming greenstone belt consists of three lithostratigraphic blocks (fig. 2) (Bornhorst, 1988). Each block 

has an internal stratigraphic continuity, but the stratigraphic relationships among separate blocks are less certain because of intervening shear zones. No evidence 

exists for stratigraphic repetition within blocks by thrust 

faults. The northern block of the belt, composed of the 

Lighthouse Point Basalt (Johnson and Bornhorst, 1991), is

dominated by pillowed, tholeiitic, basalt flows with minor 

massive flows. Subordinate interflow pyroclastic units and 

an iron-formation provide stratigraphic continuity within 

the block and are the basis for determining the structural 

history. The northern block is separated from the southern 

blocks by the Dead River shear zone (fig. 2). This ductile 

to brittle-ductile shear has north-side-up and right-lateral 

kinematics.

Regional structural studies by one of us (Johnson) suggest that the steeply dipping Archean volcanic rocks of the 

southeast block are part of a large isoclinal D, fold (fig. 

2). The strata and Sj foliation of the southwest block 

form a vertically plunging (closed) anticline.

The southeast and southwest blocks are separated by a 

shear zone, first recognized by Clark and others (1975), 

that follows the Carp River for a considerable distance. 

This shear zone (Carp River shear zone) is poorly exposed 

and its significance is uncertain. Recent interpretations of 

the stratigraphic framework of the belt by Bornhorst 

(1988) and Brozdowski (1989) used published geologic 

maps which suggest that rocks to the southwest of the 

Carp River shear zone (southwest block) face to the southeast and those to the northeast (southeast block) face 

north. Thus, these authors interpreted the Carp River 

shear zone as a major shear separating two distinct lithostratigraphic blocks that had unknown stratigraphic relationships. New field data gathered by us suggest, 

however, that the rocks directly south of this zone actually 

face north in the same way as those to the north of the 

zone; thus, stratigraphic continuity across the shear zone is 

a reasonable possibility. Within the southwest block 

stratigraphic continuity obtains about a D2 fold axis.

Rock units in all blocks are also cut by minor shear 

zones. These high-strain zones are characterized by rocks 

that are more intensely foliated than the surrounding 

rocks. In that these foliated rocks are a result of secondary processes and not original stratigraphy, they are not 

described further.

The stratigraphy for the south half of the Ishpeming 

greenstone belt can be constructed based on the structural 

framework. The oldest rocks (Kitchi Formation) are those 

that compose the southwest block (fig. 2). The rocks in the 

southeast block (Mona Formation) are younger than those 

of the southwest block.

VOLCANIC STRATIGRAPHY 

Kitchi Formation

Definition

The Kitchi Schist was named by Van Rise and Bayley 

(1895) for the volcanic rock in the Kitchi Hills (sec. 33, T. 

48 N., R. 27 W.). (Note that the name Kitchi Hills does 

not appear on modern topographic maps.) Puffett (1974)

P4 Precambrian Geology of Lake Superior Region
87°45' 87°40'

46°32'30"

 * ^* «<Vj \ S , V -^ \ * " >" A

:;.'.~.'fw.i-»xv --o-, 

^ :-.V->::.->^-(-)iAO

t^fe^^ 

sm^^fem^E

EXPLANATION

Early Proterozoic

Undifferentiated metasedimentary rocks 

Archean

Undifferentiated rocks

Metagabbro

Deer Lake Peridotite

Kitchi Formation 

Lahar unit

65

Tuff unit 

Basalt flow unit

Contact

Strike and dip of beds

Shear zone

 -f- Approximate trace of axial plane of anticline 

Note: All rocks are metamorphosed

Figure 3. Geologic map of southwest block of Ishpeming greenstone belt. Lettered localities are discussed in text. Modified from 

Brozdowski (1989), Cannon and Klasner (1977), Clark and others (1975), and Puffett (1974), with additions by the authors.

and Clark and others (1975) placed andesitic to dacitic 

volcaniclastic and pyroclastic rocks in the Negaunee and 

Negaunee SW llk minute quadrangles, respectively, in the 

Kitchi Schist. Clark and others (1975) subdivided the Kitchi Schist into three informal map units: agglomerate; 

complex of dacitic tuff, flows, and probable hypabyssal 

intrusive rocks; and metabasalt.

Because the rocks assigned to the Kitchi Schist are heterogeneous and have recognizable parent lithologies, we 

propose that rocks previously known as Kitchi Schist be 

redefined the Kitchi Formation. The type area is the southwest block of the Ishpeming greenstone belt in the vicinity 

of Deer Lake as shown in figure 3. Further, we propose to 

subdivide the Kitchi Formation into three informal rock 

units: (1) basalt flow unit (metabasalt and metagabbro of 

Clark and others, 1975, and the mafic-ultramafic complex 

of Cannon and Klasner, 1977); (2) tuff unit (complex of 

dacitic tuff, flows, and probable hypabyssal intrusive rocks 

within the Kitchi Schist of Clark and others, 1975); and 

(3) lahar unit (agglomerate within Kitchi Schist of Clark 

and others, 1975), which occurs as interlayers in the tuff 

unit (fig. 4).

Clark and others (1975) recognized and mapped an 

Archean rock unit within the Negaunee SW llk minute 

quadrangle that they termed metagabbro. These rocks continue to the southwest into the Diorite llk minute quadrangle, where Cannon and Klasner (1977) called them the 

mafic-ultramafic complex. Detailed mapping and diamond 

drill coring by geologists with Callahan Mining Corporation (Brozdowski, 1989) show, however, that this unit is 

composed of basaltic flows that interfinger with the overlying rocks. Thus, we assign these rocks in the westernmost part of the southwest block to the basalt flow unit of 

the redefined Kitchi Formation.

Description

The Kitchi Formation is composed of three informal 

units, from oldest to youngest, basalt flow, tuff, and lahar 

units (fig. 3). The basalt flow unit is overlain by and interfingers with the tuff unit (fig. 4). The lahar unit occurs as 

layers and lenses within the thicker tuff unit. Rocks 

assigned to the Kitchi Formation underlie an area of

Volcanic Rocks, South Half of Ishpeming Greenstone Belt, Michigan P5
Kitchi Formation

METERS 

On

500-

100CH

Lahar unit

Lahar unit

Basalt flow 

unit

Banded iron-formation.,

^Gloraerophyric flow

Tuff unit

Figure 4. Generalized stratigraphic column for the Kitchi 

Formation.

approximately 29 km2 and extend from 1 km north of 

Negaunee to east of Gold Mine Lake. The eastern limit of 

the formation is the Carp River shear zone, the western 

limit is Archean granitoid intrusive rocks, the northern 

limit is Early Proterozoic sedimentary rocks, and the 

southern limit is the Carp River Falls shear zone and Early 

Proterozoic sedimentary rocks. The total thickness of the 

formation is estimated to be about 6,700 m.

Basalt Flow Unit

The lower part of the Kitchi Formation is a thick succession of pillowed and massive tholeiitic basalt flows. 

These flows crop out in an 8-km2-wide area east of Gold 

Mine Lake (fig. 3), in the extreme western part of this

block. The basalt flow unit has a maximum map width of 

between 1,700 and 1,800 m within the Diorite quadrangle. 

The typical dip of pillowed flows and interbedded sediments is 75° SE. to vertical (Brozdowski, 1989). The continuity of interflow units and the consistent southeastfacing direction suggest that the rocks in the unit are not 

structurally repeated. The exposed thickness is approximately 1,700 m, but inasmuch as the base of the unit is 

truncated by Archean granitoid intrusive rocks, this is a 

minimum thickness.

Brozdowski (1989) provided a detailed geologic map 

and a description of the basalt flow unit, as summarized 

following. The unit consists of pillowed basalt flows with 

lesser and relatively thin massive flows. The mineralogy 

of the basalt consists of plagioclase, hornblende, clinozoisite, actinolite, and chlorite as well as a variety of minor 

minerals, all of which are consistent with upper greenschist facies metamorphism. Several gabbro sills occur in 

the basalt flow unit. Some of these so-called sills may be 

massive interiors to pillowed flows rather than intrusive 

rocks.

Several thin zones of chert and dacitic to rhyolitic tuff 

occur near the middle of the exposed rocks of the basalt 

flow unit. A lens of chlorite-magnetite rock up to 65 m 

thick occurs in the upper part of the unit; the lens is interpreted as a basaltic tuff, but it might be a fine-grained 

hyaloclastite. The latter is overlain by a 30-m-thick glomerophyric basalt flow (fig. 4). This flow is characterized 

by plagioclase phenocrysts as much as 2 cm in diameter. 

As a whole, the basalt flow unit consists of subaqueously 

deposited basalt flows containing minor interbedded ironformation and felsic tuffs, which were deposited during a 

hiatus in mafic volcanism.

Tuff Unit

The upper part of the Kitchi Formation is a succession 

of tuffs, which crop out in a 21 km2 area surrounding Deer 

Lake. The stratigraphic thickness of the tuff unit is poorly 

known, and is complicated by the intrusive Deer Lake 

Peridotite and by folding (fig. 3). The maximum exposed 

stratigraphic thickness of the tuff unit is about 5,000 m 

(fig. 4).

The rocks at the base of the tuff unit interfinger with 

the top of the basalt flow unit. As described by 

Brozdowski (1989), the lower part of the unit (northwest 

of the Deer Lake Peridotite) is dominated by dacite, crystal to lithic tuff, which now is a quartz-sericite-plagioclase 

schist. At least two massive dacite lenses, containing plagioclase phenocrysts set in a quartz-sericite matrix, have 

been recognized. One lens has a maximum thickness of 

about 100 m. Brozdowski (1989) has interpreted these 

lenses as flows. The stratigraphic section in the vicinity of 

the Ropes gold mine includes a few thin lapilli-tuff lenses

P6 Precambrian Geology of Lake Superior Region
as much as 5 m thick, and several fine-grained quartz-rich 

graywacke and siltstone lenses, one of which has a maximum thickness of about 60 m. In the same part of the 

stratigraphic section, several lenses of thin banded ironformation are present; typical thickness of these is less 

than 5 m and maximum thickness is 20 m. The ironformation is quartz-magnetite-chlorite rock with centimeter-wide bands. Pyrite is secondary and occurs as layers 

that are both discordant and concordant to banding and 

(or) bedding.

To the southeast of the Deer Lake Peridotite, the tuff 

unit consists of quartz-feldspar-sericite-chlorite schist. 

Feldspar phenocrysts are common. This unit contains crystal, vitric, and lithic tuffs and lapilli tuffs. In sec. 21, T. 48 

N., R. 27 W. (locality A, fig. 3), a repeated succession of 

lapilli tuff with pumice fragments in an ash and crystal 

matrix is overlain by laminated vitric tuff which fines and 

contains fewer crystals upwards. These features closely 

match those expected for a subaqueous pyroclastic flow. 

In sec. 35, T. 48 N., R. 27 W. (locality B, fig. 3), 20-cmthick crystal tuff beds overlain by 3 cm of vitric tuff are 

repeated several times. Cas and others (1990) interpreted 

stratified crystal tuffs as multiple pyroclastic eruptions of 

ash hydraulically sorted during emplacement on the flank 

of the volcano by grainflows or turbidity currents. In Slk 

sec. 25, T. 48 N., R. 27 W. (locality C, fig. 3), several 

parallel-laminated and ripple cross-laminated tuff beds are 

exposed. They are a light-gray porcellaneous rock composed of sericite and very fine grained quartz with altered 

phenocrysts of plagioclase and mafic minerals (Puffett, 

1974). Grain size and textures match those expected for 

turbiditic ash deposits.

Puffett (1974) and Clark and others (1975) described 

areas of more massive porphyritic rock with phenocrysts 

like those found in the tuffs. These rocks may be either 

flows or dikes. Their origin and distribution are poorly 

known.

Lahars are interlayered within the tuff unit (fig. 4). The 

more prominent zones of lahars were mapped by Clark 

and others (1975) as a separate unit (discussed following), 

but the tuff unit includes subordinate lahars too thin to be 

mapped separately.

The existence of pillowed basalt flows that interfinger 

with the tuff unit and turbiditic volcanic deposits near the 

top of the unit supports a subaqueous deposition for the 

tuff. Although the extent of redeposition is uncertain, the 

tuff unit is interpreted as having been originally emplaced 

by volcanic action such as subaqueous pyroclastic flow 

and fall deposits.

Lahar Unit

Mappable lenses and beds of lahar are interlayered 

with tuffs of the tuff unit (fig. 4), and crop out in a 4 km2 

area near Deer Lake (fig. 3). The maximum exposed

thickness of the lenses and beds within the stratigraphic 

section is approximately 1,000 m.

The lahar unit consists of angular to subrounded clasts 

of andesite to dacite. Although delicately shaped bombs 

are lacking, all fragments are volcanic in origin. The 

clasts range in size from sand to boulder; a maximum 

cross-sectional width is 1.3 by 0.6 m. Clasts 20 30 cm in 

diameter are quite common. The clasts are matrix supported, and their abundance ranges from a few percent to 

about 60 percent of the rock. Zones within the lahar unit, 

as much as 1 m thick, contain relatively larger clasts. 

Most clasts are quartz or quartz+feldspar phenocrystic 

andesite; some are aphyric and laminated. The groundmass consists of schistose sericite, chlorite, feldspar, and 

quartz. The mineralogic composition of the matrix is the 

same as that of the tuffs of the tuff unit. The lahars were 

subaqueously emplaced as is indicated by the presence of 

a pillowed basalt layer situated between two lahar zones 

and by turbidites occurring higher in the stratigraphic section within the tuff unit (fig. 4). The lahar unit is interpreted as subaqueous volcanic debris related to volcanic 

action.

Mona Formation

Definition

The formal name Mona Schist was first used by Van 

Hise and Bayley (1895) for basalt exposed in the Mona 

Hills (sees. 27 and 28, T. 48 N., R. 25 W.). (Note that the 

name Mona Hills does not appear on modern topographic 

maps.) Within the Marquette 7lk minute quadrangle, Gair 

and Thaden (1968) subdivided the Mona Schist into the 

lower member and the Lighthouse Point Member. Puffett 

(1974) extended the name Mona Schist to metavolcanic 

rocks in the Negaunee quadrangle to the west, and he subdivided it into four members: lower member, Nealy Creek 

Member, sheared rhyolite tuff member, and Lighthouse 

Point Member. The Lighthouse Point Member of the 

Mona Schist has been revised and redefined as the Lighthouse Point Basalt by Johnson and Bornhorst (1991) and 

is part of the northern block (fig. 2). As revised by 

Johnson and Bornhorst (1991), the Mona Schist consists 

of two informal members (the lower and the sheared rhyolite tuff members) and one formal member (Nealy Creek 

Member).

Because many rocks within the Mona Schist are not 

schists and because they have recognizable parent lithologies and are heterogeneous, we propose that rocks previously known as Mona Schist be redefined the Mona 

Formation. Further, we propose that all members be informal. The lower member of the Mona Schist is herein 

called the basalt flow unit of the Mona Formation, the 

sheared rhyolite tuff member of the Mona Schist is called 

the lapilli tuff of the Mona Formation, and the Nealy

Volcanic Rocks, South Half of Ishpeming Greenstone Belt, Michigan P7
46° 35

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5 10 KILOMETERS

EXPLANATION

Early Proterozoic

Undifferentiated metasedimentary rocks

Archean

Undifferentiated rocks 

Metagabbro 

Mona Formation 

Upper tuff unit

Lapilli-tuff unit 

Basalt flow unit

- Contact Dotted where concealed

 Shear zone

 -± Approximate trace of axial plane of 

plunging syncline

Note: All rocks are metamorphosed

Figure 5. Geologic map of southeast block of Ishpeming greenstone belt. Lettered localities are discussed in text. Modified from 

Brozdowski (1989), Clark and others (1975), Puffett (1974), and Cair and Thaden (1968), with additions by the authors.

Creek Member of the Mona Schist is called the upper tuff 

of the Mona Formation. The type area is the same as that 

previously designated for the Mona Schist.

Description

The Mona Formation comprises three lithologically 

distinct and mappable informal units, from oldest to 

youngest, basalt flow, lapilli-tuff, and tuff units (fig. 5). 

The basalt flow unit is overlain by the lapilli-tuff unit, 

which in turn interfingers with and is overlain by the tuff 

unit (fig. 6). Rocks assigned to the Mona Formation 

underlie an area of approximately 65 km2 and extend 

from the city of Marquette, to the east, to about 5 km 

north of the city of Ishpeming, to the west. The eastern 

limit of the formation is Lake Superior, the western limit 

is the Carp River shear zone, the northern limit is the 

Dead River shear zone, and the southern limit is the Carp 

River Falls shear zone and Early Proterozoic sedimentary 

rocks (fig. 2). The estimated thickness of the formation is 

about 6,400 m.

Basalt Flow Unit

The lower part of the Mona Formation is a thick succession of metamorphosed tholeiitic pillow basalt flows. 

These flows crop out in an area of 54 km2, which extends 

from south of Marquette to 3 km north of Ishpeming (fig. 

5). The pillows and interbedded sedimentary zones dip 

between 70° SE. and vertical (Gair and Thaden, 1968). 

The consistent stratigraphic facing direction and a lack of 

evidence supporting shearing associated with recumbent 

folding support a continuous stratigraphic succession, 

without significant repetition resulting from faulting or 

folding. The lack of stratigraphic marker zones makes 

thickness estimates difficult. The maximum exposed thickness is approximately 5,500 m. The original thickness of 

the unit is unknown, as it is truncated on the south by the 

Carp River Falls shear zone.

Pillows are conspicuous throughout the basalt flow 

unit. They are bulbous or bun shaped, typically 0.3 to 1 m 

thick and 0.3 to 1 m long. Rarely, pillows are as much as 

3.5 m thick. Vesicles are present in some places in the 

outer margins of pillows, and radial fractures occur in the

P8 Precambrian Geology of Lake Superior Region
Mona Formation

Lapilli-tuff 

unit

METERS 

On

500-

1000 J

Upper 

tuff unit

Basalt 

)> flow 

unit

Figure 6. Generalized stratigraphic column for the Mona 

Formation.

outer parts of pillows. Cusps on pillows can be found in 

many exposures; they consistently indicate stratigraphic 

tops to the north. Interpillow void spaces are filled with 

various minerals, including quartz, carbonate, and epidote.

The proportion of massive basalt flows in the basalt 

flow unit is not well known, but is estimated to be considerably less than pillowed basalt. Massive basalt is green 

on fresh surfaces; its composition is a fine- to coarsegrained mixture of plagioclase, chlorite, actinolite, epidote, 

and sericite, with minor amounts of magnetite, quartz, leucoxene, sphene, and carbonate. This mineralogy is consistent with greenschist-facies metamorphism. Some bodies 

of gabbro within this unit mapped as metadiabase by Gair 

and Thaden (1968) and Puffett (1974) could represent 

massive interiors of flows rather than intrusions.

The basalt flow unit contains a zone of several thin 

iron-formation lenses and tuffaceous beds (not shown in 

fig. 6). Textures and field relations suggest that the tuffaceous beds are neither hyaloclastites nor sheared rocks. 

At the north end of the Pine Hill quarry (SE1ANW1 /4 sec. 

25, T. 48 N., R. 26 W.; locality A, fig. 5), several banded 

quartz-magnetite iron-formations having a maximum 

thickness of a few meters are exposed. Also, a few 10- to 

15-cm-thick layers of red jasper banded iron-formation are 

present. The iron-formation units are interlayered with pillowed basalt and basaltic tuff. In sec. 28, T. 48 N., R. 25 

W. (locality B, fig. 5), Brozdowski (1989) and Gair and 

Thaden (1968) described several lenses of basaltic and 

dacitic tuffs. Just south of the city of Marquette along U.S.

Highway 41 (S'A sec. 23, T. 48 N., R. 25 W.; locality C, 

fig. 5) several red jasperoid and quartz-magnetite cherty 

iron-formation zones between 5 and 15 cm thick are 

exposed. The layering in these rocks ranges from a few 

millimeters to several centimeters in thickness. At this 

locality, the iron-formation units are interlayered with 

basaltic lapilli-ash tuff, tuff, and pillowed basalt. In sec. 

21, T. 48 N., R. 25 W. (locality D, fig. 5), tuffaceous 

rocks and a graphitic zone crop out. The stratigraphic relationship among these localities is uncertain.

The basalt flow unit of the Mona Formation comprises 

a succession of subaqueously deposited flows. The thin 

tuff and iron-formation units represent more explosive volcanism in the volcanic cycle during quiescent intervals in 

lava eruption.

Lapilli-tuff Unit

The basalt flow unit of the Mona Formation is overlain by the lapilli-tuff and upper tuff units, respectively 

(fig. 6). The lapilli-tuff unit crops out in a 3 km2 area at 

the east end of the Dead River storage basin (fig. 5). Its 

maximum map width, about 490 m, probably indicates the 

approximate stratigraphic thickness. The rocks in this 

unit are strongly foliated rocks because they are adjacent 

to the Dead River shear zone (fig. 2).

The lapilli-tuff unit is dominated by lapilli clasts set in 

a fine-grained quartz, sericite, and chlorite matrix. Elongate clasts as much as about 20 cm long are present in 

some intervals. Quartz phenocrysts as much as 2 mm in 

diameter are common in the clasts and the matrix. Less 

abundant feldspar phenocrysts are of similar size. The 

composition of both the clasts and the matrix is rhyolitic. 

Puffett (1974) described green altered lapilli composed of 

sericite and iron-rich chlorite. These lapilli tuffs are interpreted as subaqueous pyroclastic deposits.

Upper Tuff Unit (Formerly Nealy Creek Member)

The lapilli-tuff unit grades upward and laterally into 

the upper tuff unit, which directly overlies both the lapillituff unit and the basalt flow unit (fig. 6). It crops out in an 

8 km2 area in the vicinity of the Dead River storage basin 

(fig. 5). Its maximum map width, about 900 m, approximates the stratigraphic thickness. Rocks that Gair and 

Thaden (1968) mapped as chloritic and felsitic schists and 

slates, Morgan and DeCristoforo (1980) and Brozdowski 

(1989) correlated with the upper tuff unit

The upper tuff unit consists of quartz-feldspar-sericitechlorite schist. The proportion of these minerals varies; an 

average proportion is 3 [quartz]: 4 [feldspar and sericite]: 

3 [chlorite and biotite] (Puffett, 1974). Most schist has

Volcanic Rocks, South Half of Ishpeming Greenstone Belt, Michigan P9
abundant fine-grained matrix containing scattered angular 

to subhedral quartz and feldspar grains. Possible primary 

layering from 1 mm to 60 cm thick is present in some 

exposures (Puffett, 1974). Based on sand-size, lenticular 

volcanic fragments, and lack of sand-size quartz and feldspar in an intact framework, most of these schist beds are 

interpreted as tuff. However, some rocks within this unit 

contain sand-size quartz and feldspar in an intact framework, and these are interpreted as volcanic-derived 

graywacke. Sericitic slate within this unit may represent 

redeposited pyroclastic material based on compositional 

similarity with other rocks. The Nealy Creek Member 

comprises metamorphosed subaqueous pyroclastic and 

redeposited pyroclastic deposits.

RELATED INTRUSIVE ROCKS

The volcanic rocks of the Ishpeming greenstone belt 

are cut by a variety of intrusive rocks. Peridotite and gabbro intrusions are interpreted as resulting from the magmatic activity that caused the eruption of the volcanic 

rocks. In addition, margins of the volcanic belt are 

intruded by a tonalite to granite batholith, which has an 

arc geochemical signature (Wilkin and Bornhorst, 1990). 

Internally, the greenstone belt volcanic rocks are cut by 

porphyritic rhyolite dikes and stocks of coarse-grained 

granitoid rocks (Johnson and Bornhorst, 1991; Puffett, 

1974).

Deer Lake Peridotite

Morgan and DeCristoforo (1980) proposed the name 

Deer Lake Peridotite for a 6 km2 body of serpentinized 

peridotite in vicinity of Deer Lake (fig. 3). Most of the 

peridotite is dark gray to green and composed of finegrained serpentine with minor opaque oxides. The serpentine is a mixture of chrysotile and (or) lizardite with lesser 

amounts of antigorite (Bornhorst and others, 1986). In 

hand samples and thin sections, 1- to 5-mm serpentine 

pseudomorphs after olivine or pyroxene have been identified; primary olivine or pyroxene is absent. Cumulate textures indicate an intrusive origin. Textures indicative of 

extrusive komatiite, such as spinifex textures, flow contacts, or polysuturing, have not been found. Based on 

CIPW normative minerals, olivine to pyroxene ratios 

range from 0.28 to 1.93, and all the calculated norms fall 

in or near the harzburgite field of Coleman and Keith 

(1971). Since calcium losses can occur during serpentinization, the ultramafic protolith may have had more clinopyroxene than indicated by the norms. The peridotite 

has high MgO, more typical of intrusive ultramafics than

extrusive komatiites (Bornhorst and others, 1986). Various 

altered varieties of peridotite occur in the vicinity of the 

Ropes gold mine (Bornhorst and others, 1986; 

Brozdowski, 1989).

Based on presumed discordant contacts, Clark and others (1975) suggested that the Deer Lake Peridotite is an 

intrusive body. Bornhorst and others (1986) interpreted the 

body as a structurally emplaced Alpine-type peridotite. 

However, drilling and detailed surface mapping show that 

the contacts are conformable (Brozdowski, 1989). Morgan 

and DeCristoforo (1980) proposed a komatiitic flow origin 

for the peridotite because of pillow-like structures, that is, 

elongate pods of relatively unveined peridotite outlined by 

anastomosing crossfiber serpentine. These structures were 

later interpreted by Bornhorst and others (1986) as shearpolyhedra. They might also represent jointing in a shallowly emplaced sill. The ore host rock at the Ropes gold 

mine probably represents an interdigitated wedge of tuff 

unit of the Kitchi Formation (Brozdowski, 1989). Conformable contacts and intrusive textures support 

Brozdowski's conclusion that the Deer Lake Peridotite is a 

sill complex (Brozdowski, 1989).

Gabbro

The volcanic rocks of the belt contain numerous bodies 

of gabbro, some of which are clearly discordant to the surrounding pillowed basalt flows; these bodies and sills of 

gabbro that cut the felsic volcanic rocks are definitely 

intrusive. However, most gabbroic bodies are conformable 

with the basalt flows and an intrusive relationship is 

uncertain. Although it is likely that some of these bodies 

are interiors of flows, most are interpreted as sills. The 

mineralogic composition of the gabbro bodies is similar to 

that of the basalts, but the gabbro bodies are coarser 

grained. In some massive interiors of basalt flows, grain 

sizes are intermediate between typical basalt and gabbro. 

Most gabbro bodies are interpreted as subvolcanic sills.

GEOCHEMICAL CHARACTER

The volcanic and related intrusive rocks of the south 

half of the Ishpeming greenstone belt range in chemical 

composition from peridotite to rhyolite (fig. 7). The basalt 

flow unit of the Kitchi Formation is tholeiitic to iron-rich 

tholeiitic basalt. Several flows of calc-alkalic andesite 

occur near the top of the basalt flow unit. Thin interbedded tuffs are calc-alkalic dacite to rhyolite. The tuff and 

lahar units of the Kitchi Formation range from calc-alkalic 

basalt to dacite with andesite as the dominant rock type.

PI 0 Precambrian Geology of Lake Superior Region
Kitchi Formation

basalt flow

unit

Kitchi Formation

tuff and lahar

units

A1203 MgO A1 203 MgO

Mona Formation

basalt flow

unit

Mona Formation

EXPLANATION 

 Upper tuff unit 

+ Lapilli-tuff unit

A1203 MgO A1203 MgO

Deer Lake 

Peridotite

A1203 MgO A1 203 MgO

Figure 7. Jensen cation plots for rocks of south half of Ishpeming greenstone belt. Rock type fields after Grunsky (1981). Data 

from Brozdowski (1989), Puffett (1974), Gair and Thaden (1968), and Bornhorst (unpub. data, 1992). Suspected altered rocks are 

excluded.

Volcanic Rocks, South Half of Ishpeming Greenstone Belt, Michigan P11
The stratigraphically higher basalt flow unit of the 

Mona Formation ranges from magnesium-rich to iron-rich 

tholeiitic basalt but is dominated by tholeiitic to magnesium-rich tholeiitic basalt. Limited data suggest that the 

lapilli-tuff unit of the Mona Formation is calc-alkalic 

dacite to rhyolite with intercalated tholeiitic basalt. The 

overlying and interfingering upper tuff unit is calc-alkalic 

basalt to andesite in composition.

The chemical composition of the Deer Lake Peridotite 

is within the range expected for komatiites (Arndt and 

Nisbet, 1982). However, field data indicate that it is not 

extrusive, but instead is a sill complex. The age relationship between the Deer Lake Peridotite and other rock units 

is unknown. The peridotite intrudes the Kitchi Formation, 

which is older than the Mona Formation. The peridotite 

magmas may represent the ultramafic komatiite (fig. 7) 

subvolcanic base of a volcanic cycle that progressed with 

eruption of the basalt flow unit of the Mona Formation. 

Other interpretations are that (1) the peridotite may represent the residue of a basaltic magma chamber, or (2) the 

peridotite may represent part of a tectonically dismembered Alpine-type intrusion.

The chemical composition of gabbro intrusions is similar to that of the basalts (fig. 7) and hence, the gabbros 

and basalts have been interpreted as comagmatic (Bornhorst and Baxter, 1987).

In summary, the geochemical evolution of the south 

half of the Ishpeming greenstone belt was as follows: (1) 

tholeiitic basalt and calc-alkalic basalt to dacite (Kitchi 

Formation) followed by (2) tholeiitic to magnesium-rich 

tholeiitic basalt and calc-alkalic basalt, andesite, and rhyolite (Mona Formation). Perhaps the base of (2) was peridotitic komatiite (Deer Lake Peridotite). Studies of other 

greenstone belts suggest that the silicic magmas in these 

volcanic piles may have no fractionation relationship with 

the basalts (Thurston and Chivers, 1990; Thurston and 

others, 1985). Further geochemical study of the volcanic 

rocks will aid in understanding magma genesis and in 

reconstructing the volcanic history of the belt.

ENVIRONMENTAL INTERPRETATION

Volcanic environments can be subdivided into four 

characteristic facies with respect to increasing distance 

from the volcanic vent: central or vent facies, proximal 

facies, distal facies, and epiclastic facies (Easton and 

Johns, 1986). The types of rocks that are characteristic of 

the various facies depend on the style of volcanic activity, 

for example, mafic plain volcanism versus mafic shield 

volcano versus intermediate to felsic composite volcano or 

caldera. Our environmental interpretation of the volcanic 

rocks of the Ishpeming greenstone belt is based on 

descriptions provided here and in references cited and on 

extensive unpublished field study of representative and

critical outcrops. We expect that future studies will provide more details but not significantly modify our environmental interpretation.

The basalt flow units of both the Kitchi and Mona Formations are characterized by a large proportion of pillowed flows, relatively thin massive flows, and sparse 

interlayered calc-alkalic intermediate to felsic tuff zones. 

Although some flows can be traced for distances of as 

much as several kilometers, none have been documented 

with extensive lateral continuity. Rare vesicles, lack of 

hyaloclastic breccias, and interbedded oxide-facies ironformation are indicative of intermediate to deep-water 

paleoenvironment. Both basalt flow units are capped by 

calc-alkalic intermediate to felsic units. These features 

suggest that neither basalt flow unit represents mafic plain 

volcanism as described by Thurston and Chivers (1990). 

Rather, we interpret the basalt flow units as products of a 

submarine mafic shield volcano and part of mafic to felsic 

cyclical volcanism.

The interbedded tuff and lahar units of the Kitchi Formation are interpreted as being proximal facies based on 

dominance of tuffs, volcanic debris flows, and lava flows, 

and lack of sedimentary rocks. The abundance of lahar in 

the lower part of the stratigraphic column (fig. 4) and to 

the south contrasts with turbiditic rocks near the top and 

suggests deposition of the latter farther from the vent. We 

interpret these rocks as having been deposited in a submarine fan on the flanks of the volcano.

The lapilli-tuff and upper tuff units of the Mona Formation are interpreted as representing a more distal environment than the interbedded tuff and lahar units of the 

Kitchi Formation, based on the much higher proportion of 

sedimentary rocks, graywacke, and mudstone. We suggest 

that the environment is near the transition between proximal and distal (distal is more than 5-15 km from the vent) 

on the flank of the volcano.

The style of volcano for the intermediate to felsic units 

of both the Kitchi and Mona Formations may have been 

either submarine caldera or composite volcano and dome 

complex. Based on present data, the simplest interpretation 

is that the Kitchi and Mona Formations each represent a 

single overlapping volcano. Felsic zones interbedded within 

the basalt flow unit may be products of adjacent distal volcanoes not otherwise exposed in the Ishpeming greenstone 

belt. Although the intermediate to felsic units at the top of 

each formation were deposited in a subaqueous environment on the flank of a volcano, determining whether the 

vent itself was subaqueous or subaerial is not possible.

VOLCANIC HISTORY

The oldest Archean volcanic event recognized in the 

south half of the Ishpeming greenstone belt was the 

subaqueous eruption of tholeiitic basalt flows from a mafic

P12 Precambrian Geology of Lake Superior Region
shield volcano (basalt flow unit of the Kitchi Formation), 

probably before 2,700 Ma. As the volcano evolved, calcalkalic andesite to dacite was erupted (tuff and lahar units 

of the Kitchi Formation). Later the Kitchi volcanic complex was buried by subaqueous eruptions of tholeiitic 

basalt (basalt flow unit of the Mona Formation) derived 

from another mafic shield volcano. We speculate that initial peridotite magmas never erupted but rather were 

emplaced as subvolcanic sills (Deer Lake Peridotite). As 

this volcano evolved, calc-alkalic rhyolite and basalt were 

erupted and buried the tholeiitic basalt flows (lapilli-tuff 

and upper tuff units of the Mona Formation). During development of the mafic shield volcanoes, comagmatic gabbro 

sills were intruded as subvolcanic bodies into the volcanic 

pile. These volcanic rocks were part of an arc developed in 

response to an inferred north-directed subduction of oceanic crust (Card, 1990). This magmatic arc system ceased 

with the initiation of deformation, which produced recumbent folding. The deformation may have been produced by 

collision along the Great Lakes tectonic zone.

REFERENCES CITED

Ayers, L.D., 1969, Geology of the Muskrat Dam Lake area: Ontario 

Department of Mines, Geological Report 74,74 p.

Arndt, N.T., and Nisbet, E.G., 1982, Komatiites: Boston, Mass., 

Alien and Unwin, 526 p.

Bornhorst, T.J., 1988, Geological overview of the Marquette 

greenstone belt, Michigan: 34th Institute on Lake Superior 

Geology, Marquette, Michigan, Field Trip Guidebook, v. 34, 

pt. 2, p. A1-A18.

Bornhorst, T.J., and Baxter, D.A., 1987, Geochemical character of 

Archean rocks from the east half of the northern complex, 

Upper Peninsula, Michigan: 33rd Institute on Lake Superior 

Geology, Wawa, Ontario, Proceedings and Abstracts, v. 33, 

pt. 1, p. 12.

Bornhorst, T.J., Shepeck, A.W., and Rossell, D.M., 1986, The 

Ropes gold mine, Marquette County, Michigan, U.S.A. An 

Archean hosted lode gold deposit, in Macdonald, A.J., ed., 

Proceedings of Gold '86, an International Symposium on the 

Geology of Gold, Toronto, p. 213-227.

Brozdowski, R.A., 1989, Ropes pyritic gold deposit in a dilational 

bend, Marquette greenstone belt, Michigan: London, 

Ontario, University of Western Ontario Ph. D. thesis, 592 p.

Cannon, W.F., and Klasner, J.S., 1977, Bedrock geologic map of 

the southern part of the Diorite and Champion 7'/i-minute 

quadrangles, Marquette County, Michigan: U.S. Geological 

Survey Miscellaneous Investigations Series Map 1-1058, 

scale 1:24,000.

Card, K.D., 1990, A review of the Superior Province of the 

Canadian Shield, a product of Archean accretion: 

Precambrian Research, v. 48, p. 99-156.

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.

Cas, R.A.F., Alien, R.L., Bull, S.W., Clifford, B.A., and Wright, 

J.V., 1990, Subaqueous, rhyolitic dome-top tuff cones A 

model based on the Devonian Bunga Beds, southeastern 

Australia and a modern analogue: Bulletin of Volcanology, 

v. 52, p. 159-174.

Cas, R.A.F., and Wright, J.V., 1987, Volcanic successions, modern 

and ancient: London, Alien and Unwin, 528 p.

Clark, L.D., Cannon, W.F., and Klasner, J.S., 1975, Bedrock 

geologic map of the Negaunee SW Quadrangle, Marquette 

County, Michigan: U.S. Geological Survey Geologic 

Quadrangle Map GQ-1226, scale 1:24,000.

Coleman, R.G., and Keith, T.E., 1971, A chemical study of 

serpentinization Burro Mountain, California: Journal of 

Petrology, v. 12, p. 311-328.

Easton, R.M., and Johns, G.W., 1986, Volcanology and mineral 

exploration The application of physical volcanology and 

facies studies: Ontario Geological Survey Miscellaneous 

Paper 129, p. l^M).

Gair, J.E., and Thaden, R.E., 1968, Geology of the Marquette and 

Sands Quadrangles, Marquette County, Michigan: U.S. 

Geological Survey Professional Paper 397, 77 p., scale 

1:24,000.

Grunsky, E.G., 1981, No. 16 An algorithm for the classification of subalkalic volcanic rocks using the Jensen cation 

plot: Ontario Geological Survey Miscellaneous Paper 100, 

p. 61-65.

Hammond, R.D., 1978, Geochronology and origin of Archean 

rocks in Marquette County, Upper Michigan: Lawrence, 

Kans., University of Kansas M.S. thesis, 69 p.

Johnson, R.C., and Bornhorst, T.J., 1991, Archean geology of the 

northern block of the Ishpeming greenstone belt, Marquette 

County, Michigan: U.S. Geological Survey Bulletin 1904-F, 

20 p.

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of the Ishpeming Greenstone Belt, Michigan, U.S.A.: 

Precambrian Research, v.ll, p. 23-41.

Puffett, W.P., 1974, Geology of the Negaunee Quadrangle, 

Marquette County, Michigan: U.S. Geological Survey 

Professional Paper 788, 53 p., scale 1:24,000.

Schmid, R., 1981, Descriptive nomenclature and classification of 

pyroclastic deposits and fragments Recommendations of 

the IUGS Subcommission on the systematics of igneous 

rocks: Geology, v. 9, p. 41-43.

Sims, P.K., 1991, Great Lakes tectonic zone in Marquette area, 

Michigan Implications for Archean tectonics in northcentral United States: U.S. Geological Survey Bulletin 1904- 

E, 17 p.

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, v. 91, pt. 1, p. 690-698.

Thurston, P.C., Ayres, L.D., Edwards, G.R., Gelinas, L., Ludden, J.N., and Verpaelst, P., 1985, Archean bimodal volcanism: Geological Association of Canada Special Paper 28, 

p. 7-21.

Thurston, P.C., and Chivers, K.M., 1990, Secular variation in 

greenstone sequence development emphasizing Superior 

Province, Canada: Precambrian Research, v. 46, p. 21-58.

Trow, J., 1979, Final report of diamond drilling for geologic 

information in the Middle Precambrian basins in the western 

portion of Northern Michigan: Michigan Geological Survey, 

Department of Natural Resources, Open File Report UDOE 

OFRGJBX-162(79),44p.

Van Hise, C.R., and Bayley, W.S., 1895, Preliminary report on the 

Marquette iron-bearing district of Michigan: U.S. Geological 

Survey, 15th Annual Report, p. 485-650.

Wilkin, R.T., and Bornhorst, T.J., 1990, Geochemistry of Archean 

granitoids of the south-central Superior Province: Geological Society of America Abstracts with Programs, v. 22, no. 7, 

p. A244.

AU.S. GOVERNMENT PRINTING OFFICE: 1993-774-049/66040 Volcanic Rocks, South Half of Ishpeming Greenstone Belt, Michigan P13



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

investigations in all phases of hydrology, including hydrology, availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific information of wide popular interest in a format designed for distribution at no cost to the public. Information is usually of short-term interest.

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

publications, and they are also available for public inspection at depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at depositories. They are a nonpermanent form of publication that maybe cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on topographic bases in 7 1/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering 

geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric bases at various scales, they show results of surveys using geophysical techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

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

present a wide variety of format and subject matter. The series also includes 7 1/2-minute quadrangle photogeologic maps on planimetric 

bases which show geology as interpreted from aerial photographs. The 

series also includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, 

stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology in relation to specific mining or mineral-deposit problems; post-1971 maps 

are primarily black-and-white maps on various subjects such as environmental studies or wilderness mineral investigations.

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

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

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

Catalogs

Permanent catalogs, as well as some others, giving comprehensive 

listings of U.S. Geological Survey publications are available under the 

conditions indicated below from USGS Map Distribution, Box 25286, 

Building 810, Denver Federal Center, 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 

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" is available free of charge by mail or may be 

obtained over the counter in paperback booklet form only. Those wishing 

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

Geological Survey" should write to the U.S. Geological Survey, 582 National Center, Reston, VA 22092.

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

announcements, and publications may be incorrect. Therefore, the prices 

charged may differ from the prices in catalogs, announcements, and publications.