Revisions of Stratigraphic Nomenclature 

within the Keweenawan Supergroup of 

Northern Michigan

Geochemistry, Petrography, and Volcanology 

of Rhyolites of the Portage Lake Volcanics, 

Keweenaw Peninsula, Michigan

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Revisions of Stratigraphic Nomenclature 

within the Keweenawan Supergroup of 

Northern Michigan

By WILLIAM F. CANNON and SUZANNE W. NICHOLSON

Geochemistry, Petrography, and Volcanology 

of Rhyolites of the Portage Lake Volcanics, 

Keweenaw Peninsula, Michigan

By SUZANNE W. NICHOLSON

Chapters A and B are issued as a single volume 

and are not available separately

U.S. GEOLOGICAL SURVEY BULLETIN 1970-A,B

CONTRIBUTIONS TO THE GEOLOGY AND MINERAL RESOURCES OF THE 

MIDCONTINENT RIFT SYSTEM
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: 1992

For sale by

Book and Open-File Report Sales 

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Library of Congress Cataloging in Publication Data

Cannon, William F.

Revisions of stratigraphic nomenclature within the Keweenawan Supergroup 

of northern Michigan / by William F. Cannon and Suzanne W. Nicholson. 

Geochemistry, petrography, and volcanology of rhyolites of the Portage Lake 

Volcanics, Keweenaw Peninsula, Michigan / by Suzanne W. Nicholson.

p. cm. (Contributions to the geology and mineral resources of the 

Midcontinent rift system ; ch. A-B) (U.S. Geological Survey bulletin ; 1970 A-B) 

Includes bibliographical references. 

Supt. of Docs, no.: I 19.3:1970 A-B

I. Geology, Stratigraphic Proterozoic. 2. Volcanic ash, tuff, etc. Michigan. 3. Rhyolite Michigan Keweenaw Peninsula. 4. Porcupine Volcanics 

(Mich.) 5. Bergland Group (Mich.) 6. Copper Harbor Conglomerate 

(Mich.) 7. Portage Lake Volcanics (Mich.) I. Nicholson, Suzanne, W.

II. Nicholson, Suzanne W. Geochemistry, petrography, and volcanology of 

rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan. 

1992. III. Title. IV. Series. V. Series: U.S. Geological Survey bulletin ; 1970 

A-B.

QE75.B9 no. 1970 A-B 

[QE653.5] 557.3 s-dc20

[551.7'15'09774] 91^7744

CIP
Chapter A

Revisions of Stratigraphic Nomenclature 

within the Keweenawan Supergroup of 

Northern Michigan

By WILLIAM F. CANNON and SUZANNE W. NICHOLSON

A new formation name (Porcupine Volcanics) and a new group 

name (Bergland Group) are proposed for Middle Proterozoic 

volcanic units in northern Michigan, and a sedimentary unit 

(Copper Harbor Conglomerate) is recorrelated

U.S. GEOLOGICAL SURVEY BULLETIN 1970

CONTRIBUTIONS TO THE GEOLOGY AND MINERAL RESOURCES OF THE 

MIDCONTINENT RIFT SYSTEM

CONTENTS

Abstract Al 

Introduction Al 

Porcupine Volcanics Al

Historical Background Al

Stratigraphic Name and Type Area A2

Distribution and Thickness A4

Lithology A4

Stratigraphic Boundaries and Relations to Adjacent Formations A5

Age A6

Bergland Group A6

Correlation of the Copper Harbor Conglomerate A6 

Conclusions A7 

References Cited A8

FIGURES

1. Stratigraphic columns of Keweenawan Supergroup rocks in northern Michigan A2

2. Geologic map showing the distribution of units within the Keweenawan Supergroup 

in the western part of the northern peninsula of Michigan A3

3. Schematic diagram showing the relation of the Porcupine Volcanics to surrounding 

units and showing radiometric ages A5

CONVERSION FACTORS

Multiply By To obtain

millimeter (mm) 0.03937 inch (in.)

centimeter (cm) .3937 inch (in.)

meter (m) 3.281 foot (ft)

kilometer (km) .6214 mile (mi)

Contents III

Revisions of Stratigraphic Nomenclature within the 

Keweenawan Supergroup of Northern Michigan

William F. Cannon and Suzanne W. Nicholson

Abstract

The Porcupine Volcanics is here defined as a formation consisting of rhyolite, andesite, basalt, and minor 

interflow sedimentary rocks. It conformably overlies the 

Portage Lake Volcanics, and the two formations together 

are defined as the Bergland Group of the Keweenawan 

Supergroup. The Porcupine Volcanics has been recognized as a distinct unit for more than 20 years but has not 

been formally named; it was referred to previously as the 

"unnamed formation."

Also, we change the Stratigraphic correlation of the 

Copper Harbor Conglomerate from "middle Keweenawan" of old usage to the Oronto Group. This correlation is almost universally accepted by current workers in 

the region.

INTRODUCTION

The Keweenawan Supergroup in the Lake Superior 

region of the United States and Canada is composed of a 

very thick sequence of subaerially deposited volcanic rocks, 

mostly flood basalt, and clastic sedimentary rocks. The 

supergroup was deposited in and near the Midcontinent rift 

during a period of Middle Proterozoic continental rifting. 

Most units of the Keweenawan Supergroup have been 

formally named, and a correlation of units between geographically separated areas has been generally agreed upon 

for many years. Some of the most recent Stratigraphic 

summaries are presented by Green (1982), Morey and 

Green (1982), Ojakangas and Morey (1982), and Bergstrom 

and Morey (1985). In this paper, we make three revisions of 

U.S. Geological Survey (USGS) nomenclature for Keweenawan rocks in northern Michigan by naming a previously 

recognized but unnamed volcanic unit, by introducing a 

new group name for part of the volcanic succession, and by 

modifying the correlation of a long-recognized sedimentary 

unit to agree with the stratigraphy generally accepted by 

most workers in the region. Figure 1 illustrates the stratigraphic nomenclature for Keweenawan rocks as put forth by 

White (1972) and Hubbard (1975) and our revised nomenclature.

Manuscript approved for publication November 6, 1991.

PORCUPINE VOLCANICS

We here define the Porcupine Volcanics as a lithostratigraphic unit of formational rank. The name comes 

from the Porcupine Mountains of northern Michigan. The 

Porcupine Volcanics consists of a sequence of rhyolites, 

andesites, basalts, and minor sedimentary rocks that form a 

distinctive Stratigraphic unit in the western part of the 

northern peninsula of Michigan (fig. 2).

Historical Background

The Keweenawan volcanic rocks north of the 

Keweenaw fault in Michigan were not given formal stratigraphic names until the past few decades. Nevertheless, a 

detailed internal stratigraphy was recognized, and units, 

commonly consisting of individual flows or conglomerates, 

were informally named and traced over large regions as 

early as the late 1800's as a result of the importance of 

stratigraphy in the exploration for the native copper deposits 

for which these rocks are famous (see for instance, Lane, 

1911; Butler and Burbank, 1929). The volcanic rocks on the 

Keweenaw Peninsula were formally named Portage Lake 

Volcanics by White and others (1953) for exposures near 

Portage Lake in Houghton County, Mich. (fig. 2). In that 

area, the Portage Lake Volcanics is overlain by the Copper 

Harbor Conglomerate. As systematic detailed mapping by 

the USGS proceeded to the west, it became apparent that a 

lenticular, westward-thickening unit of andesite and rhyolite, distinct from the basalt flows of the Portage Lake 

Volcanics, was a regionally significant unit beginning in the 

Greenland quadrangle. These rocks were first shown as a 

map unit in the Matchwood quadrangle by Johnson and 

White (1969). Their mapping clearly showed that this unit 

is stratigraphically above the Portage Lake Volcanics and 

beneath the Copper Harbor Conglomerate. They referred to 

the unit simply as the "unnamed formation." Hubbard 

(1975) correlated the volcanic rocks exposed in the Porcupine Mountains with the unnamed formation. Although the 

intervening area has not been mapped in detail, regional 

mapping and compilation (for instance, by Cannon, 1986) 

indicate that Hubbard's correlation is correct.

Revisions of Stratigraphic Nomenclature within the Keweenawan Supergroup of Northern Michigan A1
White (1972) 

Hubbard (1975)

This paper

Cambrian rocks

 - ^. ^ 

Keweenawan Supergroup

Keweenawan" "upper 

"middle Keweenawan"

"lower Keweenawan"

Jacc

^-~- 

*t 

2 2 

ov

Dbsville Sandstone

Freda Sandstone

Nonesuch Shale

Copper Harbor 

Conglomerate

unnamed ^\ 

formation I

Portage Lake Volcanics

Mill Powder

Group

Kallander Creek 

Formation

Siemens Creek 

Formation

Bessemer Quartzite

 - _ ^ ^- 

pre-Keweenawan rocks

Cambrian rocks

^- _ _ ^- 

Supergroup Keweenawan 

J

^ 

Q.

3 

O

5

o I-"

c 

o

6

Bergland Group

Powder Mill

Group

acobsville Sandstone

Freda Sandstone

Nonesuch Shale

Copper Harbor 

Conglomerate

Porcupine 

Volcanics

Portage Lake 

Volcanics

Kallander Creek 

Formation

Siemens Creek 

Formation

Bessemer Quartzite

_-- - . -- 

pre-Keweenawan rocks

Figure 1. Comparison of the stratigraphic column of Keweenawan Supergroup rocks in northern 

Michigan as most recently defined by White (1972) and Hubbard (1975) and the revised lithostratigraphic 

column as proposed in this paper.

Since Johnson and White's (1969) recognition of this 

unit, the term "unnamed formation" has been widely used, 

and no formal designation has been proposed in the intervening years. The informal name "Oak Bluff Formation" 

has been used infrequently for these rocks (for instance, by 

Kopydlowski, 1983; Reed, 1987; Reed and Daniels, 1987). 

That name is derived from a topographic feature near where 

Johnson and White first recognized the unit. However, at 

Oak Bluff, exposures are sparse, consisting of only a few 

outcrops of andesite and minor rhyolite at the very top of the 

unit.

Stratigraphic Name and Type Area

The rocks here designated the Porcupine Volcanics 

have been recognized as a distinctive lithostratigraphic unit 

for more than 20 years (Johnson and White, 1969; Hubbard, 

1975), but the unit has not been formally named. It has 

commonly been referred to as the "unnamed formation." 

We propose the name Porcupine Volcanics for this unit and 

define the type area as the Porcupine Mountains in T. 50 

N., Rs. 43 and 44 W., and T. 51 N., Rs. 42 and 43 W., 

in Ontonagon County, Mich. (fig. 2). Although only the

A2 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
I

5!

s.

C/J

2.I 3

II

2S"

47° 30'

'Ybp.'i

PM

XAu

__________90°00'_______________________

EXPLANATION

Keweenawan Supergroup (Middle Proterozoic) 

Jacobsville Sandstone 

Oronto Group

Freda SandstoneNonesuch Shale

Copper Harbor Conglomerate 

Bergland Group

Porcupine Volcanics

Portage Lake Volcanics 

Powder Mill Group undivided

Lower Proterozoic and Archean 

rocks undivided

89°00' 88°00'

0 10 20 30 MILES

0 10 20 30 40 KILOMETERS

Eagle Ha

^^-^Mwitoul

^J-^ TieweenawPt

Contact 

Fault

Figure 2. Distribution of units within the Keweenawan Supergroup in the western part of the northern peninsula of Michigan. Geology generalized from Morey 

and others (1982).
upper part of this very thick formation is exposed in that 

area, all lithologies are well represented, and exposures are 

abundant. Specific localities where exceptionally good 

exposures are present are described in the following section.

Distribution and Thickness

The Porcupine Volcanics is exposed for a strike 

length of nearly 100 km along the main trend of the 

Keweenawan monocline (fig. 2) from near the town of 

Greenland in eastern Ontonagon County to near the Wisconsin border in western Gogebic County. The rocks are 

also exposed in a structural repetition of the strata in the 

arcuate highlands of the Porcupine Mountains. The downdip extent north of the outcrop belt is not known. The unit 

is as much as 2,500 m thick southeast of the Porcupine 

Mountains along the Keweenawan monocline. The unit is 

about 4,000 m thick south of the Porcupine Mountains 

where its total thickness is preserved. Its thickness in the 

Porcupine Mountains is not known because the base is not 

exposed. A minimum thickness of 750 m was estimated by 

Reed (1987) for the exposed units, but the total thickness is 

probably several times that figure.

Lithology

The Porcupine Volcanics is a sequence of subaerially 

deposited basalt, andesite, felsite, and quartz-porphyry lava 

flows, and minor interbedded volcaniclastic lithic sandstone, siltstone, and conglomerate (Hubbard, 1975). The 

abundance of felsic rocks (about 25 percent) and the 

predominance of andesite clearly distinguish the Porcupine 

Volcanics from the underlying Portage Lake Volcanics, 

which is dominated by flood basalt flows and contains 

minor (<1 percent) felsic volcanic rocks.

The following description of the Porcupine Volcanics 

is based on the reports of Johnson and White (1969), 

Hubbard (1975), and Kopydlowski (1983) and our own 

observations. The mafic volcanic rocks of the Porcupine 

Volcanics are mostly andesite flows that are dark gray to 

grayish red or grayish purple, are fine grained, and average 

about 8 m in thickness. In contrast, the Portage Lake 

Volcanics is dominantly ophitic basalt flows, averaging 

about 11-13 m in thickness (Johnson and White, 1969; 

Paces, 1988). Most flows in the Porcupine Volcanics 

contain sparse andesine phenocrysts less than 2 mm long. 

Flow tops are typically pahoehoe, but some are fragmented. 

Flow tops generally contain sparse vesicles filled with 

chlorite, epidote, quartz, and calcite. Andesite flows are 

also typically characterized by discontinuous, closely 

spaced banding and (or) jointing. Good exposures of typical 

andesite flows occur near the western edge of sec. 24, T. 50 

N., R. 41 W., the area where the unit was first recognized 

by Johnson and White (1969).

Felsite bodies compose about a quarter of the Porcupine Volcanics, occurring as subaerially deposited domes or 

flows, mostly near the top of the formation. Locally, felsite 

bodies lie at the contact between the Porcupine Volcanics 

and Portage Lake Volcanics. Individual felsite bodies range 

from aphyric to sparsely porphyritic with feldspar and 

quartz phenocrysts. A particularly good exposure of multiple rhyolite flows, rhyolite breccias and debris flows, and 

interspersed andesite and basalt flows is near the top of the 

prominent hill in the SWV* sec. 11, T. 50 N., R. 44 W., just 

south of the Beaver Creek Trail from Summit Peak to Little 

Carp River Trail in the Porcupine Mountains Wilderness 

State Park.

The eastern highlands of the Porcupine Mountains are 

underlain by a massive aphanitic rhyolite flow. The flow 

may be no more than about 150 m thick, but it occurs in an 

outcrop belt about 4 km wide because of its shallow dip and 

structural repetition across the crest of an anticline. The 

rhyolite may represent more than one flow, but no complete 

section is exposed through the body. Hubbard (1975) 

distinguished the massive lower part of the flow, exposed in 

the southern part of the park, especially along Lost Creek 

(sec. 31, T. 51 N., R. 42 W.), from the more vesicular 

upper part of the flow, which is exposed in many places. 

The massive lower part of the flow is generally pale purple 

to grayish purple to moderate reddish purple, whereas the 

vesicular part is grayish pink. Vesicular rhyolite is more 

abundant near the top of the flow and occurs as irregular 

masses or narrow layers within the more massive rhyolite, 

producing a banded appearance. Folds defined by the 

banding have variable orientations, which commonly are 

not consistent with regional tectonic folds. A good exposure 

of the layered rhyolite is on Inlet Creek near Trap Falls 

(NWV4 sec. 30, T. 51 N., R. 44 W.).

In the vicinity of Porcupine Peak, at least one rhyolite 

flow underlies the uplands and has an outcrop length of 

more than 10 km. The thickness is unknown. The palebrown to grayish-orange-pink rhyolite contains numerous 

rounded and embayed quartz phenocrysts up to 1.5 mm in 

diameter and strongly altered subhedral to euhedral feldspar 

phenocrysts up to 3 mm long. Some phenocrysts are 

broken. Accessory phases include zircon and, locally, 

fluorite. Good exposures of this rhyolite can be found on 

Porcupine Peak (sees. 29 and 31, T. 50 N., R. 44 W.) and 

along the Little Carp River (sec. 21, T. 50 N., R. 44 W.).

Three small outcrop areas of pale-red to moderatereddish-brown quartz latite (near Trap Falls in sec. 24, T. 

51 N., R. 43 W.; sec. 32, T. 51 N., R. 43 W.; and sec. 2, 

T. 50 N., R. 43 W.) lie within the park (Hubbard, 1975; 

Kopydlowski, 1983); an additional outcrop area is south of 

the park in the Thomaston 7.5-min quadrangle (Hubbard, 

1975). The quartz latite is massive but locally contains 

compressed pumice fragments. The unit also contains 

sparse feldspar and quartz phenocrysts.

A4 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
sw NE

Nonesuch Shale

basalt flow

Copper Harbor Conglomerate

"Lake Shore traps"

1,087.2 ±1.6 Ma

Portage Lake Volcanics

Greenstone flow 1,094 ±1.5 Ma

Figure 3. Schematic diagram showing the relation of the Porcupine Volcanics to surrounding units and 

showing radiometric ages of units determined by the U-Pb method on zircon (Davis and Paces, 1990).

Sedimentary rocks make up only a small percentage 

of the Porcupine Volcanics. In the Porcupine Mountains 

area, most of the sedimentary rocks are sandy conglomerate 

units, similar to the rocks of the overlying Copper Harbor 

Conglomerate. According to Hubbard (1975), sedimentary 

rocks of the Porcupine Volcanics contain more volcanic 

detritus and have a larger matrix component than the 

Copper Harbor Conglomerate. Pebbles and cobbles are 

generally less than 10 cm in diameter, but, locally, boulders 

as much as 1 m in diameter are exposed near Union Spring 

(sec. 22, T. 51 N., R. 42 W.). South of the Porcupine 

Mountains, sedimentary units are scarce and typically are 

siltstone (Butler and Burbank, 1929).

Stratigraphic Boundaries and Relations to 

Adjacent Formations

The Porcupine Volcanics is underlain by basalts and 

minor interflow sedimentary rocks of the Portage Lake 

Volcanics and probably interfmgers, in places, with the 

overlying fluvial conglomerate and sandstone of the Copper 

Harbor Conglomerate. The lower contact is placed between 

the dominantly rhyolite-andesite volcanic sequence here 

defined as the Porcupine Volcanics and the dominantly 

basaltic volcanic sequence of the Portage Lake Volcanics. 

In general, the lowermost units of the Porcupine Volcanics 

are andesite rather than rhyolite, and so the contact generally separates basalt from andesite along most of its surface 

trace. Although there are no exposures of the lower contact, 

it appears to be essentially conformable on the basis of the

regional map pattern. No structural discordance is detectable between the two formations, and any erosional discontinuity that might be present along the contact does not 

appear to be greater than that expected within a sequence of 

intermittently erupted lava flows.

The contact with the overlying Copper Harbor Conglomerate is placed either at the abrupt change from 

dominantly volcanic rocks of the Porcupine Volcanics to the 

entirely sedimentary sequence of the Copper Harbor or, 

where the Copper Harbor contains some interbedded volcanic rocks, at the base of the lowermost thick (more than 

about 100 m) interflow conglomerate unit. Although this 

contact is somewhat complex, no substantial hiatus is 

indicated between the units. There is an inverse relation 

between the thickness of the two units such that the Copper 

Harbor Conglomerate becomes thinner along strike as the 

Porcupine Volcanics becomes thicker (Johnson and White, 

1969); this relation suggests that the two are in part 

synchronous and that the Porcupine Volcanics formed a 

broad volcanic shield within the otherwise sedimentdominated basin into which the Copper Harbor was being 

deposited (fig. 3). It is likely that flows of the Porcupine 

Volcanics are in places interbedded with sedimentary strata 

of the Copper Harbor Conglomerate, but that relationship 

has not been definitively demonstrated in this rather poorly 

exposed region. In a few areas, an angular discordance 

appears to exist between the volcanic rocks of the Porcupine 

Volcanics and nearby sedimentary units probably belonging 

to the Copper Harbor Conglomerate. Such relations are 

believed to result from essentially synvolcanic erosion of

Revisions of Stratigraphic Nomenclature within the Keweenawan Supergroup of Northern Michigan A5
the subaerial volcanic shield of the Porcupine Volcanics 

followed soon after by deposition of the sediments of the 

Copper Harbor Conglomerate. The areas of discordance do 

not represent a major erosional interval.

Age

The absolute age of the Porcupine Volcanics is 

constrained by precise U-Pb zircon ages of nearby rock 

units (Davis and Paces, 1990). The relation of the Porcupine Volcanics to nearby dated units is shown schematically 

in figure 3. The age of the Greenstone flow, near the top of 

the Portage Lake Volcanics on the Keweenaw Peninsula, 

provides a maximum age for the Porcupine Volcanics of 

1,094.0±1.5 Ma. An age of 1,087.2±1.6 Ma has been 

determined for the "Lake Shore traps," a series of basalt 

flows about 1,000 m above the base of the Copper Harbor 

Conglomerate near the tip of the Keweenaw Peninsula. This 

age is believed to approximate the time of eruption of the 

Porcupine Volcanics, which, like the "Lake Shore traps," 

postdates the final flood basalt eruptions of the Portage 

Lake Volcanics and is equivalent in age to the lower part of 

the Copper Harbor Conglomerate.

BERGLAND GROUP

For many years, the Keweenawan Supergroup in 

Michigan was informally divided into lower, middle, and 

upper parts (fig. 1). The "upper Keweenawan" included the 

dominantly red sedimentary rocks of the Oronto Group and 

younger units, whereas the "lower and middle Keweenawan" comprised the dominantly volcanic rocks of the 

Portage Lake Volcanics and Powder Mill Group. The 

boundary between the "lower Keweenawan" and "middle 

Keweenawan" is a horizon recording a magnetic reversal, 

but that boundary is everywhere covered by the younger 

Jacobsville Sandstone in northern Michigan. Elsewhere in 

the Lake Superior region, the magnetic reversal is generally 

exposed within the volcanic section; units such as the North 

Shore Volcanic Group in Minnesota and the Osier Volcanic 

Group and Mamainse Point Formation in Ontario span the 

"lower Keweenawan"-"middle Keweenawan" contact. 

Thus, the tripartite subdivision of the Keweenawan Supergroup as used in Michigan generally is not useful elsewhere 

in the region. Rather, thick volcanic successions typically 

have been given group rank, and the magnetic reversal has 

been used as a time marker to correlate groups rather than as 

a boundary between units. In concert with usage in other 

parts of the region, we no longer use lower, middle, and 

upper subdivisions of the Keweenawan Supergroup. 

Instead, we prefer to consider units as parts of formally 

named groups, which can be placed in their proper stratigraphic positions relative to other groups of the Keweenawan Supergroup of the Lake Superior region.

Therefore, we propose the term "Bergland Group" for 

the combined Portage Lake Volcanics and Porcupine Volcanics. The group is named after the town of Bergland, in 

Ontonagon County, Mich. (fig. 2), which is near both the 

formations contained in the group. A group designation for 

this thick volcanic succession is consistent with generally 

accepted stratigraphic usage in other parts of the Lake 

Superior region, where comparably thick units have been 

assigned group rank. (See Bergstrom and Morey (1985) for 

the most recent stratigraphic summary of Keweenawan 

Supergroup rocks in the region.) All rocks in the Bergland 

Group have normal magnetic polarity (Books, 1972).

The type area for the group is north and northeast of 

Bergland, where good exposures of both the Portage Lake 

Volcanics and Porcupine Volcanics are abundant in T. 49 

N., R. 42 W. In the type area, the group is exposed in a 

north-facing monoclinal section about 9,000 m thick. The 

upper contact is the depositional surface at the base of the 

Copper Harbor Conglomerate described above. The basal 

contact is tectonic throughout the exposed length of the 

group. The Keweenaw fault, a thrust to high-angle reverse 

fault, forms the south margin of the outcrop belt of the 

Portage Lake Volcanics. The fault juxtaposes the Portage 

Lake Volcanics on the north and the younger Jacobsville 

Sandstone on the south.

CORRELATION OF THE COPPER HARBOR 

CONGLOMERATE

The Copper Harbor Conglomerate is dominantly red 

fluvial conglomerate and sandstone. It is the stratigraphically lowest unit in the thick postvolcanic sedimentary 

section that forms the upper part of the Keweenawan 

Supergroup. The lower part of the Copper Harbor records 

the transition from a dominantly volcanic regime to a 

dominantly sedimentary regime. Basalt flows occur intermittently in the lowest 1,500 m of the unit in some places, 

including the type area near the tip of the Keweenaw 

Peninsula (White and others, 1953), but they are volumetrically subordinate to the clastic rocks. Because of the 

gradual nature of the change from volcanic to sedimentary 

conditions in some areas, the placement of the lower contact 

of the Copper Harbor is somewhat arbitrary and has resulted 

in inconsistent stratigraphic usage over the years.

Early usage, for instance by Irving (1883), placed the 

base of the "upper Keweenawan" (Oronto Group of present 

usage) at the top of the stratigraphically highest lava flow, 

the "Lake Shore traps" of the Keweenaw Peninsula. The 

conglomerate stratigraphically above this was called the 

"Outer Conglomerate." Thus, some very thick conglomerates below the uppermost lava flows were included in the 

"middle Keweenawan." This terminology was logical and 

provided a readily definable contact between "upper

A6 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
Keweenawan" (Oronto Group) and "middle Keweenawan" 

(Bergland Group) rocks on the Keweenaw Peninsula, but 

ultimately it created a serious Stratigraphic problem as the 

rock sequence was traced westward into areas where the 

stratigraphically high flows pinch out. There the contact 

was left about 1,500 m up in the fluvial sedimentary section 

and was not marked by any distinctive unit or lithologic 

break. Lane and Seaman (1907) recognized this problem 

and proposed the name "Copper Harbor Conglomerate" to 

include all the thick conglomerates and interlayered flows 

stratigraphically between the underlying volcanic sequence 

(Portage Lake Volcanics of present usage) and the overlying 

Nonesuch Shale, but this term was not widely accepted for 

many years. White and others (1953) were apparently the 

first of the modern workers to use the name Copper Harbor 

Conglomerate on the Keweenaw Peninsula essentially as it 

had been defined earlier by Lane and Seaman. White and 

others also considered the Copper Harbor Conglomerate to 

be a part of the Oronto Group. This usage was followed by 

subsequent USGS mappers in the region and virtually all 

other workers.

Although the adoption by White and others (1953) of 

the Copper Harbor Conglomerate as a formational unit that 

contains all the major conglomerates above the Portage 

Lake Volcanics provided a workable and generally accepted 

location for the base of the unit, it caused difficulty in 

assigning the Copper Harbor Conglomerate to a group. 

Thwaites (1912) originally defined the Oronto Group from 

exposures near Oronto Bay in northern Wisconsin to 

include, from the top down, the Freda Sandstone, the 

Nonesuch Shale, and a lower conglomerate. Thwaites 

followed Irving's 1883 nomenclature and called the conglomerate at the base of the Oronto Group the Outer 

Conglomerate as defined on the Keweenaw Peninsula. 

However, in Michigan, the Outer Conglomerate correlates 

only with the upper conglomeratic unit of the Copper 

Harbor as it is now defined. Hence the base of the Oronto 

Group as defined by Thwaites in Wisconsin was at a medial 

position in the redefined Copper Harbor Conglomerate 

in Michigan. A group boundary may not fall within a 

formation.

As a solution, White (1972) proposed that the Copper 

Harbor be removed from the Oronto Group and be considered part of the "middle Keweenawan" sequence. He 

suggested that a more easily definable and relatively unambiguous contact for the base of the Oronto Group was the 

base of the Nonesuch Shale where the gray to black shales 

of the Nonesuch are in sharp contact with the mostly reddish 

sandstones of the upper part of the Copper Harbor. White 

felt that this usage was preferable because any ambiguity in 

placement of the basal contact of the Copper Harbor would 

involve only a formational contact, not the contact between 

groups. Although this usage was adopted by the USGS, it is 

a substantial departure from previously widely used terminology and has not been adopted by most of the geologic

community; most geologists continue to consider the Copper Harbor Conglomerate as the basal unit of the Oronto 

Group. The evolution of usage was summarized by Morey 

and Green (1982), and the current generally accepted usage 

was defined by Bergstrom and Morey (1985).

We propose that USGS Stratigraphic terminology be 

modified so that the Copper Harbor Conglomerate as 

defined by White and others (1953) is considered the basal 

formation of the Oronto Group rather than the upper 

formation of the volcanic section (Bergland Group) as it 

now stands. The basal contact of the Oronto Group by our 

new definition is at the base of the stratigraphically lowest 

thick conglomerate overlying the dominantly basaltic flows 

of the Portage Lake Volcanics, the position originally 

specified by Lane and Seaman (1907) and White and others 

(1953). Although in places, particularly on the Keweenaw 

Peninsula, the lower 1,500 m of the Copper Harbor contains 

some mafic flows, the Copper Harbor is, in general, much 

more akin to the overlying sedimentary rocks of the Oronto 

Group than it is to the underlying volcanic rocks.

Since White's (1972) inclusion of the Copper Harbor 

Conglomerate in the "middle Keweenawan," additional 

mapping and work still in progress have shown that no 

substantial difficulties are encountered in identifying a basal 

contact for the Copper Harbor, even in areas where flows 

and sedimentary rocks are interlayered, and the placement 

must be somewhat arbitrary. Thus, White's principal concern that substantial difficulties would be encountered in the 

regional correlation of the group boundary, his main reason 

for placing the Copper Harbor in the "middle Keweenawan," has not materialized. We now revert to usage prior to 

1972 and once again include the Copper Harbor Conglomerate in the Oronto Group.

CONCLUSIONS

We propose three changes to Stratigraphic terminology for rocks of the Keweenawan Supergroup in northern 

Michigan.

(1) The formational name Porcupine Volcanics is introduced for a sequence of andesite, rhyolite, and basalt 

flows that overlies the basalt flows of the Portage Lake 

Volcanics. This unit, recognized and mapped for more 

than two decades, heretofore has not been formally 

named. Rather, it has been commonly referred to as the 

"unnamed formation."

(2) The name Bergland Group is introduced to comprise 

the Porcupine Volcanics and underlying Portage Lake 

Volcanics.

(3) The Copper Harbor Conglomerate is reassigned from 

the "middle Keweenawan" of old usage to the Oronto 

Group, a position almost universally accepted by workers in the region.

Revisions of Stratigraphic Nomenclature within the Keweenawan Supergroup of Northern Michigan A7
REFERENCES CITED

Bergstrom, D.J., and Morey, G.B., 1985, Correlation of Stratigraphic Units of North America (COSUNA) Project Northern mid-continent region: Tulsa, Okla., American Association of Petroleum Geologists, 1 sheet.

Books, K.G., 1972, Paleomagnetism of some Lake Superior 

Keweenawan rocks: U.S. Geological Survey Professional 

Paper 760, 42 p.

Butler, B.S., and Burbank, W.S., 1929, The copper deposits of 

Michigan: U.S. Geological Survey Professional Paper 144, 

238 p.

Cannon, W.F., 1986, Bedrock geologic map of the Iron River 

1°X2° quadrangle, Michigan and Wisconsin: U.S. Geological Survey Miscellaneous Investigations Map I-1360-B, 

scale 1:250,000.

Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic 

events on the Keweenaw Peninsula and implications for 

development of the Midcontinent rift system: Earth and 

Planetary Science Letters, v. 97, p. 54-64.

Green, J.C., 1982, Geology of Keweenawan extrusive rocks, in 

Wold, R.J., and Hinze, W.J., eds., Geology and tectonics of 

the Lake Superior basin: Geological Society of America 

Memoir 156, p. 47-56.

Hubbard, H.A., 1975, Geology of Porcupine Mountains in Carp 

River and White Pine quadrangles, Michigan: U.S. Geological Survey Journal of Research, v. 3, no. 5, p. 519-528.

Irving, R.D., 1883, The copper-bearing rocks of Lake Superior: 

U.S. Geological Survey Monograph 5, 464 p.

Johnson, R.F., and White, W.S., 1969, Preliminary report on the 

bedrock geology and copper deposits of the Matchwood 

quadrangle, Ontonagon County, Michigan: U.S. Geological 

Survey open-file report, 31 p., 1 map, scale 1:62,500.

Kopydlowski, P.J., 1983, The Oak Bluff Volcanics, a middle 

Keweenawan central volcano; Porcupine Mountains region, 

Michigan: Houghton, Mich., Michigan Technological University, M.S. thesis, 88 p.

Lane, A.C., 1911, The Keweenaw Series of Michigan: Michigan 

Geological and Biological Survey Publication 6, Geological 

Series 4, v. 1, 499 p.

Lane, A.C., and Seaman, A.E., 1907, Notes on the geological 

section of Michigan; Part I, The pre-Ordovician: Journal of 

Geology, v. 15, p. 680-695.

Morey, G.B., and Green, J.C., 1982, Status of the Keweenawan 

as a stratigraphic unit in the Lake Superior region, in Wold, 

R.J., and Hinze, W.J., eds., Geology and tectonics of the 

Lake Superior basin: Geological Society of America Memoir 

156, p. 15-26.

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 S-13, scale 

1:1,000,000.

Ojakangas, R.W., and Morey, G.B., 1982, Keweenawan sedimentary rocks of the Lake Superior region; a summary, in 

Wold, R.J., and Hinze, W.J., eds., Geology and tectonics of 

the Lake Superior basin: Geological Society of America 

Memoir 156, p. 157-164.

Paces, J.B., 1988, Magmatic processes, evolution and mantle 

source characteristics contributing to the petrogenesis of 

Midcontinent rift basalts: Portage Lake Volcanics, 

Keweenaw Peninsula, Michigan: Houghton, Mich., Michigan Technological University, Ph.D. dissertation, 413 p.

Reed, R.C., 1987, Porcupine Mountains Wilderness State Park, 

Michigan, in Geological Society of America, North-Central 

Section, Centennial Field Guide, Volume 3, ed. by D.L. 

Biggs: Boulder, Colo., p. 269-272.

Reed, R.C., and Daniels, J.D., 1987, Bedrock geology of 

northern Michigan: Lansing, Mich., Michigan Department of 

Natural Resources, Geological Survey Division, scale 

1:500,000.

Thwaites, F.T., 1912, Sandstones of the Wisconsin coast of Lake 

Superior: Wisconsin Geological and Natural History Survey 

Bulletin 25, 117 p.

White, W.S., 1972, The base of the upper Keweenawan, Michigan and Wisconsin: U.S. Geological Survey Bulletin 

1354-F, p. F1-F23.

White, W.S., Cornwall, H.R., and Swanson, R.W., 1953, 

Bedrock geology of the Ahmeek quadrangle, Michigan: U.S. 

Geological Survey Geologic Quadrangle Map GQ-27, scale 

1:24,000.

A8 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
Chapter B

Geochemistry, Petrography, and Volcanology 

of Rhyolites of the Portage Lake Volcanics, 

Keweenaw Peninsula, Michigan

By SUZANNE W. NICHOLSON

This description of some rhyolites related to the Midcontinent 

rift summarizes previous work and presents new data within a 

newly revised tectonic framework for the rift

U.S. GEOLOGICAL SURVEY BULLETIN 1970

CONTRIBUTIONS TO THE GEOLOGY AND MINERAL RESOURCES OF THE 

MIDCONTINENT RIFT SYSTEM

CONTENTS

Abstract Bl 

Introduction Bl

Acknowledgments B2 

Geological Setting B2

Midcontinent Rift B2 

Portage Lake Volcanics B4 

Sampling and Analytical Methods B4 

Descriptive Geochemistry B6

Classification and Assessment of Alteration B6 

Major Oxides and Trace Elements B9 

Basalt B9

Intermediate Rocks BIO 

Rhyolite B12 

Mineralogy and Petrography B13

Basalts and Intermediate Rocks B13 

Rhyolites B15

Type I Rhyolite B15 

Type II Rhyolite B15 

Hydration and Devitrification Textures B16 

Alteration around Vesicles and Fractures B17 

Field Relations of Rhyolites of Portage Lake Volcanics B18 

Stratigraphy B19 

Keystone Point B20 

Montreal River Area B21

Rhyolite of Montreal River B21

Pyroclastic Units and Associated Conglomerate of Montreal River B21 

Fish Cove Knob, Fish Cove Dike, and West Pond B21 

Bare Hill Area B22 

Mt. Houghton Area B22

Rhyolite of Mt. Houghton B22 

Pyroclastic Unit of Mt. Houghton B23 

Suffolk and Mt. Gratiot Areas B24 

Copper City Area B24 

Comparison of a Modern Rhyolite Lava Flow with Extrusive Rhyolites of

Portage Lake Volcanics B24

Comparison of Modern and Portage Lake Volcanics Pyroclastic Deposits B26 

Summary of Field Relations B26 

Discussion B27

Development of the Midcontinent Rift B27 

Rhyolitic Volcanism of Portage Lake Volcanics B28 

Midcontinent Rift Rhyolites B29 

Iceland: A Modern Volcanological Analog B29 

Comparison of Icelandic and Midcontinent Rift Rhyolites B30 

Implications for Mineral Resources B31 

Type I Rhyolite B32 

Type II Rhyolite B33

Contents III
Conclusions B34 

References Cited B35 

Appendixes

1 Sample locations and descriptions B42

2 Major-oxide and trace-element data for 70 samples of Portage Lake 

Volcanics B45

3 Major-oxide and CIPW normative compositions of Portage Lake 

Volcanics B52

FIGURES

1. Generalized map of Midcontinent rift of North America based on gravity and 

magnetic anomalies B2

2. Generalized geologic map of Lake Superior region showing location of major 

outcrops of Midcontinent rift-related volcanic and sedimentary rocks B3

3. Seismic-reflection profiles across Lake Superior B5

4. Stratigraphic correlation diagram of Middle Proterozoic rocks in western and 

central Lake Superior region B6

5. Map showing location of major felsic and intermediate extrusive and 

subvolcanic bodies within Portage Lake Volcanics, Keweenaw 

Peninsula B7 

6, 7. Plots showing:

6. Classification of Portage Lake Volcanics according to Na2O+K2O 

and SiO2 contents B8

7. K2O+Na2O and K2O/(K2O+Na2O) compositions of Portage Lake

Volcanics B8

8. Jensen cation diagram of Portage Lake Volcanics B8 

9, 10. Plots of:

9. Major oxides of rocks of Portage Lake Volcanics analyzed using 

SiO2 as a fractionation index BIO

10. Average trace-element composition of basalts of Portage Lake 

Volcanics and midocean ridge and ocean island basalts Bll 

11. Graph showing chondrite-normalized rare earth element patterns for average

major rock types in Portage Lake Volcanics Bll 

12, 13. Plots of:

12. Selected trace-element contents for two groups of rhyelites of 

Portage Lake Volcanics B13

13. Selected trace-element contents for two groups of rhyolites of 

Portage Lake Volcanics and other rhyolites of the Midcontinent 

rift B15

14. Photomicrographs of rhyolitic textures B17

15. Photograph and photomicrographs of en echelon fractures in rhyolite B18

16. Geologic map showing location of Type 1 rhyolite bodies on the eastern tip of 

Keweenaw Peninsula B19

17. Generalized Stratigraphic section of Portage Lake Volcanics in vicinity of 

rhyolite of Mt. Houghton B20

18. Photograph and photomicrograph of ashflow tuff that underlies Montreal 

River domes B21

19. Photographs of features in the rhyolite of Mt. Houghton B23

20. Plan view of rhyolite of Mt. Houghton showing location of diagnostic 

features, orientations of foliations, and adjacent rock units B23

21. Schematic cross section illustrating features of a typical rhyolite lava flow and 

rhyolite dome B24

22. Schematic interpretation of development of Midcontinent rift in Lake Superior 

region B27

IV Contents
23. Schematic representation of emplacement of rhyolites of Portage Lake 

Volcanics B28

24. Stratigraphic cross section of a central volcano in Iceland B30

25. Schematic cross section of a hypothetical flow-dome complex illustrating 

possible sites for mineralization B32

26. Map showing location of disseminated massive sulfide deposits on east end of 

Keweenaw Peninsula B34

TABLES

1. Average normalized major-oxide and raw trace-element compositions of major 

rock types in Portage Lake Volcanics, Keweenaw Peninsula, Mich. B9

2. Selected ratios of incompatible trace elements in basalts from Portage Lake 

Volcanics, Iceland, Hawaii, and midocean ridges Bll

3. Major-oxide and normative-mineral compositions and trace-element ratios of individual rhyolite bodies in Portage Lake Volcanics, Keweenaw Peninsula, 

Mich. B12

4. Major-oxide and trace-element compositions of selected Midcontinent rift 

rhyolites B14

5. Average major-oxide and trace-element compositions of rhyolites of Portage 

Lake Volcanics and representative bimodal, calc-alkalic, peralkalic, and topaz 

rhyolites B16

6. Chemical analyses of rhyolites of Portage Lake Volcanics and barren and 

productive Cenozoic topaz rhyolites in the Western United States B33

CONVERSION FACTORS

Multiply By To obtain

millimeter (mm) 0.03937 inch (in.)

centimeter (cm) .3937 inch (in.)

meter (m) 3.281 foot (ft)

kilometer (km) .6214 mile (mi)

Contents

Geochemistry, Petrography, and Volcanology 

of Rhyolites of the Portage Lake Volcanics, 

Keweenaw Peninsula, Michigan

By Suzanne W. Nicholson

Abstract

The Portage Lake Volcanics exposed on the 

Keweenaw Peninsula, Michigan, contains some of the 

youngest volcanic rocks (^1^95 Ma) erupted within the 

Midcontinent rift system (MRS). Recent geophysical studies in the Lake Superior region show that the Portage Lake 

Volcanics is the only exposed volcanic suite that was 

erupted at the edge of a deep (==20 kilometers), asymmetric central rift graben. Rhyolites make up less than 1 

percent of the Portage Lake section, although, overall, 

silicic volcanic rocks make up about 10 percent of the 

MRS-related volcanic rocks exposed in the Lake Superior 

region. Rhyolite bodies are restricted to a 0.5-kilometerthick interval near the faulted base of the Portage Lake 

Volcanics, an interval that also contains high-TiO2 basalts 

interlayered with intermediate rocks that range from 

basaltic andesite to dacite. The rocks above and below this 

evolved interval are dominantly low-TiO2 basalts.

Two groups of rhyolites are recognized in the Portage Lake Volcanics on the basis of mineralogy and geochemical composition. Type I rhyolite is typically dark red 

brown and aphyric or contains sparse feldspar microlites. 

This rhyolite is subalkalic, similar to rhyolites in other 

bimodal suites, including Icelandic rhyolites. Similarities 

include light rare earth element (REE) enrichment and a 

moderately negative europium anomaly. In contrast, Type 

II rhyolite is gray white and contains up to 10 percent 

quartz and feldspar phenocrysts. This rhyolite is chemically similar to Cenozoic topaz rhyolites in the Western 

United States. Like topaz rhyolites, Type II rhyolite is 

characterized by low REE abundances, a flat REE pattern, 

and enrichments in such elements as rubidium, thorium, 

uranium, niobium, and tantalum. All rhyolite bodies are 

Type I except the Copper City body, which is Type II.

The rhyolites occur as lensoidal extrusive flows and 

dome complexes that typically overlie thin rhyolitic pyroclastic units. Locally, carapace breccias are intact and 

provided debris for overlying conglomeratic units. Some 

delicate hydration and devitrification textures, including 

perlitic fractures and spherulites, are preserved. Most

Manuscript approved for publication May 3, 1991.

extrusive rhyolite bodies exhibit strong flow foliation. 

Irregular subvolcanic rhyolite bodies that clearly intrude 

surrounding mafic flows are present stratigraphically 

below the extrusive rhyolite horizon. The subvolcanic 

bodies are massive and locally exhibit mafic inclusions and 

minor flow foliation around the margins.

The association of intermediate and felsic rocks in a 

restricted stratigraphic interval surrounded by primitive 

basalt flows supports an analogy with central volcanoes in 

the active rift zones of Iceland. Comparisons of field 

relations of the Portage Lake Volcanics and within-rift 

volcanoes in Iceland indicate that these intermediate and 

felsic rocks of the MRS may represent the distal part of a 

central volcanic complex. This analogy also probably can 

be applied more broadly to other areas in the MRS where 

rhyolite is more abundant.

As with the Icelandic rhyolites, little economic mineral potential is indicated for Type I rhyolite. Type II 

rhyolite offers greater mineral potential; however, the 

only exposed Type II rhyolite body has not evolved 

enough to yield economic concentrations of lithophile 

elements. If other, more evolved Type II rhyolite bodies, 

extrusive or intrusive, are present, mineralization may 

occur.

INTRODUCTION

Recent results of seismic-reflection profiles 

(Behrendt and others, 1988; Cannon and others, 1989) 

collected in Lake Superior by the Great Lakes International Multidisciplinary Program on Crustal Evolution 

(GLIMPCE) have provided a tectonic framework in 

which to reconsider the development of the 1.1 billion-yearold (Ga) Midcontinent rift system as well as the distribution 

and magmatic evolution of its igneous rocks. In the Lake 

Superior region, a deep central rift graben contains volcanic 

rocks up to 15 km (kilometers) thick. Shallow basins up 

to 7 km deep that flank the deep central graben contain 

some of the earliest volcanic and sedimentary rocks associated with the Midcontinent rift system. The bimodal 

tholeiitic basalt-rhyolite volcanic suite has been remarkably

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B1
undeformed since rifting; thus, stratigraphic and structural 

relations have been preserved since 1.1 Ga. Subaerial 

eruption of most of IxlO6 km3 (cubic kilometers) of 

magma within less than 5 million years (m.y.) suggests that 

a mantle plume was the cause of the massive volume of 

magmatism (Cannon and others, 1989; Hutchinson and 

others, 1990).

Rhyolites make up a small but significant fraction of 

the bimodal tholeiitic basalt-rhyolite volcanic rocks associated with the rift. Most previous studies of the rift focused 

on basalt petrogenesis (Green, 1972, 1982; Basaltic Volcanism Study Project, 1981;Massey, 1983; Brannon, 1984; 

Berg and Klewin, 1988; Paces, 1988) and physical volcanology (Green, 1989). Recently, Fitz (1988) and Green and 

Fitz (1989) described the volcanological characteristics of 

several large rhyolite units in the North Shore Volcanic 

Group in Minnesota, rhyolites that lie within a flanking 

basin. Other workers (for example, Fritts, 1952; Papadakis 

and Raza, 1954; Browne and Haynes, 1956; Annells, 1973, 

1974; Mcllwaine and Wallace, 1976; Kopydlowski, 1983) 

have reported general descriptions of rhyolite occurrences 

elsewhere in the rift, but the recent geophysical models for 

the structure of the rift provide a newly refined tectonic 

framework in which to consider rhyolitic volcanism.

This report summarizes the geochemistry, petrography, and field relations of rhyolites of the Portage Lake 

Volcanics of the Keweenaw Peninsula, Mien., the only 

rhyolites that were erupted within the central rift graben in 

the central Lake Superior region. The volcanological characteristics of well-exposed rhyolite bodies are described and 

compared with those of modern rhyolites, as well as with 

those of other Midcontinent rift system and Icelandic 

rhyolites. Based on these comparisons, a model is presented 

for emplacement of the rhyolites within the Midcontinent 

rift system. Finally, the implications for mineralization 

associated with these rhyolites are addressed.

Acknowledgments

This study was conducted as part of my dissertation 

research at the University of Minnesota and was supported 

by the U.S. Geological Survey. I extend grateful appreciation to field assistants D. Jenkins and M. Reinhart, University of Minnesota. Thorough reviews by J.C. Green, B.C. 

Hearn, E. Ito, KJ. Schulz, P.W. Weiblen, and L.G. 

Woodruff substantially improved the manuscript and are 

gratefully acknowledged.

GEOLOGICAL SETTING 

Midcontinent Rift

The 1.1 billion-year-old Midcontinent rift system 

(MRS) extends for about 2,000 km from Kansas north40

EXPLANATION

Flanking gravity lows 

(Keweenawan clastic sedimentary rocks) 

Midcontinent gravity high 

(Keweenawan mafic igneous rocks)

0 200 MILES

J___I

\ I

200 KILOMETERS

Figure 1. Midcontinent rift of North America based on 

gravity and magnetic anomalies (after Green, 1983). Stippled areas define regions dominated by sedimentary 

rocks; ruled areas define regions dominated by volcanic 

rocks.

northeast to Lake Superior and then southeast through 

Michigan (fig. 1); its position is deduced from gravity and 

magnetic anomalies (Lyons, 1950; Black, 1955; Thiel, 

1956; Hinze and others, 1982; Van Schmus and Hinze, 

1985). The only exposures of rift-related rocks are around 

the margin of Lake Superior (fig. 2). Rift-related rocks 

south of Lake Superior are obscured by Paleozoic sedimentary rocks; samples are available only from drill core. The 

oldest MRS rocks are exposed in several areas: in the Osier 

Group and Mamainse Point Formation in Ontario, at the 

base of the North Shore Volcanic Group in Minnesota, and 

in the Powder Mill Group in Michigan. Each of these basal 

MRS sections either conformably overlies early-rift thin 

quartzites or unconformably overlies Archean or Early 

Proterozoic basement (fig. 2). Despite some complications 

in the Powder Mill and Mamainse Point sequences, most of 

the earliest MRS rocks have reversed magnetic polarity. 

This characteristic has been used for stratigraphic correlation where precise ages do not exist. All igneous rocks from 

the Portage Lake Volcanics, as well as volcanic rocks in the 

Porcupine Mountains and on Michipicoten Island in Lake 

Superior, have normal magnetic polarity.

U-Pb zircon dating of the basal Osier Group and the 

related Logan diabase sills in Ontario indicates that riftB2 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
92°

90

Paleozoic sedimentary 

rocks

47°-i

100 200 KILOMETERS

EXPLANATION

Bayfield Group and Jacobsville Sandstone, undivided 

(Middle Proterozoic) sedimentary rocks

Oronto Group (Middle Proterozoic) sedimentary rocks

Basalt flows and interflow sedimentary rocks 

(Middle Proterozoic)

Gabbroic intrusive rocks (Middle Proterozoic)

Basalt flows and subordinate underlying sedimentary rocks (where present) 

(Middle Proterozoic)

l> o 0 ° Sibley Group (Middle Proterozoic?)

|0 O nl

r ^ ' ^ "J Crystalline rocks (Early Proterozoic and Archean)

Figure 2. Location of major outcrops of Midcontinent rift-related volcanic and sedimentary rocks in Lake Superior region. 

Location of two seismic-reflection profiles (GLIMPCE lines A and C) and shot points along these profiles are shown 

(Cannon and others, 1989).

related magmatism began about 1,108.8+47-2 Ma during Sutcliffe, 1985; Davis and Paces, 1990). Rift-related maga period of reversed magnetic polarity (Davis and Sutcliffe, matism ceased shortly after the intrusion on Michipicoten

1985). The magnetic polarity shifted from reverse to normal Island of a normally polarized rhyolite porphyry dated at

between 1,097.6±3.7 Ma and 1,096.2± 1.8 Ma (Davis and 1,086.5 + 1.37-3.0 Ma (Palmer and Davis, 1987).

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B3
Although volcanism was active for about 22 m.y. (million 

years), most of the activity was restricted to the 3- to 5-m.y. 

interval shortly after 1,098 Ma (Davis and Paces, 1990).

Seismic-reflection profiles (figs. 2, 3) collected by 

GLIMPCE in Lake Superior show that the Keweenaw fault 

and Isle Royale fault acted as major normal growth faults 

during crustal extension (Behrendt and others, 1988; Cannon and others, 1989). As extension continued, a deep 

central graben developed and ultimately accommodated 

more than 15 km of volcanic rocks and 7 km of sedimentary 

rocks (Behrendt and others, 1988). Outside the central 

graben, broad shallow basins about 5-7 km deep preserve 

some of the earliest rift-related volcanic and sedimentary 

rocks. Mild postrift compression reactivated the large normal growth faults as reverse faults, resulting in uplift of the 

Portage Lake Volcanics along the Keweenaw fault at the 

southern edge of a central graben segment (fig. 3). The 

distribution of rhyolite varies within the Lake Superior 

region. Flanking basins (for example, the North Shore 

Volcanic Group) contain up to 10-25 percent rhyolite, 

whereas, within the deep central grabens (for example, the 

Portage Lake Volcanics), rhyolites are a minor (< 1 percent) 

component.

Portage Lake Volcanics

The Portage Lake Volcanics, located on the 

Keweenaw Peninsula in northern Michigan, contains some 

of the youngest flood basalts associated with the 1.1 

billion-year-old Midcontinent rift system (fig. 4). The 

exposed part of the Portage Lake Volcanics consists of 3-5 

km of subaerially deposited volcanic rocks. The original 

thickness is unknown, because the volcanic unit thickens 

toward the axis of the rift (Cannon and others, 1989) and is 

truncated at the surface by the high-angle reverse 

Keweenaw fault (fig. 3). Zircons from pegmatoidal zones in 

two large basalt flows in the Portage Lake Volcanics yield 

an average age of 1,095.1 ±1.9 Ma (Paces, 1988; Paces and 

Davis, 1988). On the Keweenaw Peninsula, the Portage 

Lake Volcanics is overlain by the basal member of the 

Oronto Group, the 2-km-thick Copper Harbor Conglomerate, a prograding alluvial fan complex that grades up into 

fluvial sandstones (Daniels, 1982; Cannon and Nicholson, 

this volume). To the west in the Porcupine Mountains area, 

the Porcupine Volcanics overlies the Portage Lake Volcanics (fig. 4).

The stratigraphy of the Portage Lake Volcanics on the 

Keweenaw Peninsula has been well known for more than 

100 years as a result of exploration for and extensive mining 

of native copper deposits (see Butler and Burbank, 1929). 

Tholeiitic basalts make up about 95 percent of the Portage 

Lake Volcanics (Paces, 1988), and intermediate and felsic 

rocks make up less than 2 percent. Conglomerates make up 

the remaining 3 percent of the formation (Merk and Jirsa,

1982) and become increasingly common toward the top of 

the section. Laterally extensive sandstones and conglomerates were used as marker horizons by the miners. In the 

main native copper district of the Keweenaw Peninsula, 22 

conglomerates were identified by number. Several of the 

most widespread conglomerates were also given informal 

names taken from local geographic or topographic features. 

These include the Allouez (No. 15), Calumet & Hecla (No. 

13), Bohemia (No. 8), and St. Louis (No. 6) conglomerates 

(see, for example, Butler and Burbank, 1929; Cornwall, 

1954a,b, 1955). These informal names remain in common 

usage today.

This report focuses on rhyolites of the Portage Lake 

Volcanics east of Calumet, Mich., and on the limited 

stratigraphic interval in which they occur (fig. 5). Hubbard 

(1898) summarized the early observations of rhyolites (or 

"acid rocks"), which were recognized by geologists as early 

as 1846, and also reported the results of his own detailed 

studies of felsite bodies on the Keweenaw Peninsula. 

Hubbard (1898) and subsequent workers identified about a 

dozen small intermediate to felsic bodies northeast of 

Houghton, Mich. Mapping during the 1950's by H.R. 

Cornwall, J.C. Wright, and W.S. White of the U.S. 

Geological Survey (USGS) and by students from Michigan 

Technological University resulted in detailed geologic maps 

for the Keweenaw Peninsula (White and others, 1953; 

Cornwall, 1954a,b,c, 1955; Cornwall and Wright, 1954, 

1956a,b; Wright and Cornwall, 1954; Cornwall and White, 

1955; Davidson and others, 1955; White, 1956; White and 

Wright, 1956) and detailed field and petrographic studies of 

several individual felsite bodies (Fritts, 1952; Papadakis 

and Raza, 1954; Browne and Haynes, 1956). Bornhorst 

(1975) examined the chemistry of the rhyolite at Fish Cove, 

and Grimes (1977) compiled geochemical compositions for 

many intermediate and felsic rocks of the Keweenaw 

Peninsula. On the basis of drill core that provided a vertical 

section through the Portage Lake Volcanics, Paces (1988) 

established the overall stratigraphic and geochemical characteristics of the mafic volcanic rocks on the Keweenaw 

Peninsula.

SAMPLING AND ANALYTICAL METHODS

More than 250 samples of rhyolites, basalts, intermediate rocks, and sedimentary rocks in the Portage Lake

Figure 3. Seismic-reflection profiles across Lake Superior, interpreted by Cannon and others (1989). A, 

Profile along line C showing structure beneath western 

Lake Superior. Inferred subsurface units are projected 

updip to their exposed extensions on land in northern 

Michigan and Minnesota. B, Profile along line A showing subsurface geology beneath central Lake Superior. 

Locations of lines A and C are shown in figure 2. 

Vertical scale is two-way travel time; no vertical exaggeration for average velocity of 6 kilometers per 

second.

B4 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
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Nonesuch 

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Chengwatana 

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North Shore 

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a U-Pb analyses of zircon and baddeleyite reported in Davis and Sutcliffe, 1985. 

U-Pb analyses of zircon and baddeleyite reported in Palmer and Davis, 1987. 

U-Pb analyses of zircon and baddeleyite reported in Davis and Paces, 1990.

Figure 4. Stratigraphic correlation of Middle Proterozoic rocks in western and central Lake Superior region.

Volcanics were collected from surface outcrops within and 

near rhyolite bodies within the lowest kilometer and a half 

of the Portage Lake Volcanics section. An effort was made 

to collect samples that had little to no visible alteration, 

veining, or extensive fractures. From this collection, 70 

volcanic rocks were chosen, on the basis of geographic 

distribution and minimal alteration, for major- and traceelement analysis. X-ray diffraction analyses by P.M. Okita, 

USGS, were conducted on eight whole-rock rhyolite samples. Sample locations and descriptions are given in appendix 1.

After the weathered surfaces were trimmed, the 70 

samples were split and reduced to pea size using a steel 

jaw-crusher. Final grinding to 200 mesh was done using 

an alumina ceramic shatterbox vessel. Major and trace 

elements were analyzed by the U.S. Geological Survey,

using inductively coupled plasma-atomic emission spectrometry, rapid rock, X-ray fluorescence, and instrumental 

neutron activation techniques, as described in U.S. Geological Survey Bulletin 1770 (Baedecker, 1987). Chemical 

analyses are given in appendix 2.

DESCRIPTIVE GEOCHEMISTRY 

Classification and Assessment of Alteration

Samples for this study were collected from rhyolite 

bodies and adjacent mafic flows within the lowest 1.5 km of 

the Portage Lake section. Rigorously classified according to 

SiO2 , Na2O, and K2O contents (Le Bas and others, 1986), 

this restricted suite of Portage Lake Volcanics has transiB6 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
Key to intermediate 

and rhyolite bodies

1 Keystone Point 

Montreal River 

West Pond 

Fish Cove 

Bare Hill 

Mt. Houghton 

Mt. Gratiot

8 Suffolk

9 Copper City

EXPLANATION 

Jacobsville Sandstone 

Freda Sandstone 

Nonesuch Shale 

Copper Harbor Conglomerate 

|_| Portage Lake Volcanics

10 MILES

I 

10 KILOMETERS

Figure 5. Location of major felsic and intermediate extrusive and subvolcanic bodies within Portage Lake Volcanics, 

Keweenaw Peninsula (after Grimes, 1977). Dashed line represents the stratigraphic horizon of the Bohemia conglomerate. Locations of intermediate and felsic bodies at the eastern end of the Keweenaw Peninsula are illustrated in greater 

detail in figure 16.

tional subalkalic to alkalic chemistry (fig. 6); mafic and 

intermediate rocks straddle the field boundary. However, 

drill core analyses chosen by Paces (1988) to reflect the 

freshest and most representative samples throughout the 

Portage Lake Volcanics lie within the subalkalic field. This 

suggests that the mafic volcanic rocks near the rhyolite 

bodies have been particularly affected by alkali metasomatism, resulting in enhanced concentration of Na2O+K2O. 

Comparison with a compilation of Na2O and K2O abundances for igneous rocks (Hughes, 1973) shows that this 

restricted suite of Portage Lake rocks has enhanced K2O 

abundances relative to other Portage Lake rocks and the 

igneous spectrum in general (fig. 7).

On a Jensen cation plot, which relies on elements that 

are generally stable during secondary alteration (Jensen, 

1976), basalts of the Portage Lake Volcanics in this study 

overlap with tholeiites reported by Brannon (1984) for the 

North Shore Volcanic Group in Minnesota and by Paces

(1988) for Portage Lake Volcanics (fig. 8). The few 

intermediate rocks lie close to the boundary between the 

tholeiitic and calc-alkalic fields and provide a transition 

from tholeiitic basalts to rhyolites.

Two corrections were applied to all major-element 

data to reduce the effects of postemplacement oxidation and 

hydration. Measured Fe2O3/FeO ratios for basalts to rhyolites range widely, from 0.60 to 55 (app. 2). Oxidation 

correction of Fe2O3/FeO ratios was performed according to 

the scheme of Middlemost (1989) in which Fe2O3/FeO 

increases from 0.15 in picrobasalts to 0.5 in rhyolites. 

These corrections were applied according to the location of 

samples in figure 6. However, a ratio of 0.15 was assigned 

to all samples that plotted in the basalt and trachybasalt 

fields in order to compare basalt analyses from this study 

with those from other MRS studies (Brannon, 1984; Paces, 

1988) in which the normalization factor of 0.15 of Brooks 

(1976) was used.

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan 87
45 50 55 60 65 70 

SiQ2 (weight percent)

75 80 85

Figure 6. Classification of Portage Lake Volcanics according to Na2O+K2O and SiO2 contents (Le Bas and others, 

1986). Most rocks lie within the subalkalic field, according 

to the field boundary of Irvine and Baragar (1971). Numbers within the fields are the Fe2O3/FeO ratios used to 

correct the measured iron analyses as recommended by 

Middlemost (1989), with the exception of basalts, for 

which Fe2O3/FeO=0.15 was used as recommended by 

Brooks (1976). Basalts from this study (solid dots; app. 3) 

have higher Na2O+K2O contents than representative 

basalts of the Portage Lake Volcanics (ruled area) analyzed 

by Paces (1988), suggesting that the restricted suite of 

basalts in the present study has been affected by metasomatism more alkalic than that affecting the rest of the 

Portage Lake section.

14

12

EXPLANATION

Basalts 

A Intermediate rocks

 Rhyolites

0.0 0.2 0.4 0.6 

K20/K20+N320 (weightpercent)

0.8 1.0

Figure 7. K2O+Na2O and K2O/(K2O+Na2O) compositions 

of Portage Lake Volcanics. Analyses of Portage Lake rocks 

sampled in this study are compared with analyses of 

representative basalts of the Portage Lake Volcanics (ruled 

field) reported by Paces (1988). Most unaltered igneous 

rocks lie between the two field boundaries (after Hughes, 

1973). This restricted suite of Portage Lake rocks shows 

greater K2O contents than most other basalts of the 

Portage Lake Volcanics and most igneous rocks in general.

FeO + Fe2 03 +Ti02

After the oxidation correction was made, major-oxide 

contents were normalized to 100 percent in order to reduce 

the effects of postemplacement hydration. All basalts collected in this study contain between 1.7 and 4.25 wt 

(weight) percent H2O+ ; thus, none can be considered fresh. 

However, normalization of these compositions allows a 

first-order comparison of major-oxide chemistry of MRS 

basalt compositions reported by Paces (1988) and Brannon 

(1984). A few rhyolites of the Portage Lake Volcanics 

contain up to 0.7 wt percent H2O+ , but for most of them, 

H2O+ is in the range of 0.2-0.4 wt percent. Four of the 

seventy samples in this study contained > 1 wt percent CO2 ; 

most of the samples contained <0.1 wt percent. CIPW 

norms were calculated on the basis of the normalized 

compositions reported in appendix 3.

Because of apparent alkali metasomatism, rock 

names used in this study are based on SiO2 weight percent 

in the normalized analyses. In this classification, SiO2 

content is <52 wt percent for basalt, 52-56 wt percent for 

basaltic andesite, 56-63 wt percent for andesite, 63-69 wt 

percent for dacite, and >69 wt percent for rhyolite. Rhyolite with SiO2>75 wt percent is classified as high-silica 

rhyolite. Average normalized compositions for major Portage Lake rock types are reported in table 1. Two rhyolite 

samples showed excessive silicification (>80 wt percent 

SiO2; app. 3) and were not included in calculating the 

average Type I rhyolite.

AI 203 50 

Cation Percent

Figure 8. Cation diagram (Jensen, 1976) of Portage Lake 

Volcanics. Basalts, intermediate rocks, and rhyolites from 

this study are represented by solid dots. The basalts of 

Portage Lake Volcanics lie within the tholeiitic field and 

overlap the representative suite of basalts of the Portage 

Lake Volcanics (ruled field) reported by Paces (1988) and 

those (field defined by dashed line) reported by Brannon 

(1984). Intermediate rocks lie close to the tholeiitic-calcalkalic boundary and are transitional between basalts and 

rhyolites.

B8 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
Table 1. Average normalized major-oxide and raw trace-element compositions of major rock types in Portage Lake 

Volcanics, Keweenaw Peninsula, Mich.

[Average major-oxide abundances (app. 3) and 1 sigma standard deviation are reported in weight percent on an anhydrous basis, after correcting Fe2O3/FeO 

ratios according to Middlemost (1989), except for basalts, for which Fe2O3/FeO=0.15 was used. Weight percentages may not total 100.00 owing to 

rounding. Trace elements are reported in parts per million as analyzed (app. 2). N= number of samples averaged. Mg#=Mg/(Fe+Mg+Mn) (mole 

percent).]

Major oxides:

Si02

Ti02

A1203

Fe2O3

FeO

MnO

MgO

CaO

Na2O

K->O

P2°5

Mg#

Trace elements:

Cr

Ni

Nb

Rb

Sr

Zr

Y

La

Sm

Tb

Yb

Hf

Ta

Th

Low-TiO2 

basalt

N=n

48.29+1.35

1.70+0.17

16.99±0.75

1.65±0.07

10.90±0.47

0.22+0.07

8.16±1.47

6.50±2.03

2.86+1.01

2.53±1.21

0.20+0.03

0.58

163+54

172±43

9±3

61±59

490 ±399

126±54

26±7.6

11.9±2.1

4.04±0.4

0.67±0.09

2.12±0.27

2.65+0.34

0.59±0.06

1.2+0.5

High-TiO2 

basalt 

N=8

49.37+1.62

2.68±0.33

15.21±0.40

1.84±0.14

12.13±0.94

0.24+0.06

6.05±0.87

7.32+1.79

3.01±0.94

1.81±1.23

0.36+0.10

0.47

76+60

88±35

16±3

38±17

291 ±187

200 ±37

44±11.9

20.9±4.7

6.88±1.16

1.16±0.21

3.88±0.9

5.00+0.9

1.01±0.24

2.56+0.93

Basaltic 

andesite

N=3

54.44±0.82

2.50+0.07

15.34±0.04

2.77±0.15

7.88±0.42

0.30±0.13

4.07±0.40

5.12±0.85

3.70±0.41

2.79±1.15

1.09+0.19

0.46

1.8+0.2

<80

25 ±2

77 ±32

276 ±23

354 ±6

74±10

46.3±1.6

13.22±0.64

2.03±0.07

6.39±0.23

8.36±0.34

1.9±0.2

5.2±0.27

Andesite 

N=4

61.10±1.40

1.39±0.18

15.72±0.53

2.38±0.26

5.24±0.51

0.17±0.04

2.68±0.26

2.53±0.94

4.72±0.64

3.54±0.76

0.56±0.12

0.38

6.0±3.11

<120

30±2

56 ±27

119±59

523±72

86±7

49.5±5.6

13. 25 ±0.48

2.14±0.05

7.48±0.33

11.8±0.96

2.24±0.15

7.06±0.29

Dacite

N=4

65. 5 ±1.34

0.63±0.17

15.83±0.25

1.7±0.11

3.58±0.30

0.12±0.05

1.41±0.61

2.12±2.09

4.52±0.65

4.41±2.41

0.2±0.07

0.38

<6

<70

33±2

56±22

90 ±39

743 ±55

90±9

59±6.0

13.93±0.3

2.21 ±0.04

8.85±0.18

16.14±0.98

2.58±0.18

9.52±0.79

Type 1 

rhyolite 

N-31

73.84±1.27

0.12±0.04

13.65±0.63

0.66±0.10

1.32±0.20

0.03±0.01

0.23±0.16

0.25±0.40

3.23±1.32

6.64±1.88

0.03±0.01

0.21

3.4±1.9

<50

41±8

112±30

55 ±30

240±41

56±12

30.3±15.2

6.57±2.75

1.30±0.27

5.55±0.95

7.53±0.84

2.91±0.54

15.09±1.12

Type II 

rhyolite

N=4

75.74±0.64

0.05±0.01

13.28±0.57

0.44±0.03

0.87±0.06

0.04±0.02

0.14±0.02

1.38±0.67

2.48±0.49

5.60±0.99

0.02±0.01

0.20

3.2±0.8

<40

51±8

465+30

28±2

144+4

74±22

12.0±5.8

6.9±0.96

1.85 ±0.39

7.68±1.35

6.3±0.64

4.9±0.86

63.0±5.9

Major Oxides and Trace Elements

Major-oxide compositions of rocks of the Portage 

Lake Volcanics in this study are illustrated in figure 9, 

where SiO2 is used as a fractionation index. In general, FeO* 

(total iron as FeO), MgO, and CaO decrease as SiO2 

increases. A12O3 remains relatively constant until quartz 

becomes a significant phase. The oxides of Na and K show 

much scatter, suggesting that they have been mobilized, but 

in general, K2O increases with increasing SiO2 and Na2O 

remains about the same. P2O5 rises with increasing SiO2 

until about 54 wt percent SiO2 and then decreases as SiO2 

increases further. TiO2 gradually decreases with increasing 

SiO2; however, there is a substantial range in TiO2 content 

among the basalts.

Despite the effects of alkali metasomatism as well as 

secondary oxidation and hydration, major and minor elements can be used in conjunction with systematic traceelement differences to distinguish among various basalt and

rhyolite groups. For general comparison, average analyses 

of major rock types are adequate for illustrating compositional variations.

Basalt

Two groups of basalts, low TiO2 and high TiO2 , are 

recognized near the rhyolite horizon. Low-TiO2 basalts 

contain less than 2.1 wt percent TiO2; compared with 

high-TiO2 basalts, they have lower average P2O5 and other 

incompatible trace element abundances, but higher A12O3 , 

MgO, K2O, and Cr, Ni, and Rb (table 1). Low-TiO2 basalts 

are olivine (to slightly nepheline) normative, and high-TiO2 

basalts are hypersthene normative (app. 3).

Comparisons of trace-element abundances for average compositions of the two basalt groups from the Portage 

Lake Volcanics show that high-TiO2 basalts have higher 

incompatible trace element abundances, but that ratios 

among trace elements are similar; thus, the patterns for the

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B9
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intermediate rocks were analyzed, it is not clear whether compositional gaps at SiO2 =55-59 weight percent and 68-71 

weight percent are real or whether rocks having these compositions simply were not sampled.

two groups yield similar slopes (fig. 10). Basalts from the 

Portage Lake Volcanics are enriched in large-ion-lithophile 

elements and other incompatible trace elements and, therefore, are distinctly different from normal midocean ridge 

basalts (N-MORB) and some Iceland basalts (fig. 10). 

Ratios of incompatible trace elements of basalts from the 

Portage Lake Volcanics resemble such ratios of enriched 

midocean ridge basalts (E-MORB) and some ocean island 

basalts from Iceland and Hawaii (table 2).

Tholeiitic basalts exhibiting a bimodal distribution of 

incompatible minor and trace elements, such as TiO2 , P2O5 , 

rare earth elements (REE), Hf, Ta, and Th, are common in 

both oceanic and continental environments. Among midocean ridge basalts, high-TiO2 basalts tend to be localized 

near propagating rift tips (Sinton and others, 1983), transform faults (Bryan, 1979; Bender and others, 1984), or 

hotspots (Brooks and Jakobsson, 1974; Wood, 1978; Juster 

and others, 1989), whereas "normal" low-TiO2 basalts are 

found at some distance from hotspots and transform faults. 

High- and low-TiO2 basalts have also been identified in the 

Parana and Karoo continental flood basalt provinces of 

South America and southern Africa, respectively (Cox,

1983; Bellieni and others, 1984; Marsh and Bales, 1984; 

Mantovani and others, 1985), as well as within the Mesozoic basins of the Eastern United States (Wiegand and 

Ragland, 1970; Smith and others, 1975). Although the 

processes thought to produce bimodal basalt suites and their 

bulk compositions vary from province to province, the 

presence of a bimodal basalt suite suggests that Keweenawan basaltic magmatism is not unique.

Intermediate Rocks

In the Portage Lake Volcanics, intermediate rocks 

occur only in the vicinity of the rhyolite bodies. Basaltic 

andesites and more evolved intermediate rocks are quartz 

normative; andesite and more felsic rocks are generally 

slightly corundum normative as well. REE concentrations 

for the intermediate rocks are strongly enriched compared 

with the basalts (fig. 11), but both basalts and intermediate 

rocks are enriched in light REE's, and their REE distribution patterns show similar slopes. Dacites have the highest 

REE, zirconium, and hafnium abundances of any Portage 

Lake rocks (table 1).

B10 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
1000

100

Cs Rb Ba Th U K Ta Nb La Ce Sr Nd P Hf Zr Sm Ti Tb Y

	EXPLANATION

 Low-TiC>2 basalt of the Portage Lake Volcanics

A High-Ti02 basalt of the Portage Lake Volcanics

+ Normal midocean ridge basalt (N-MORB)

° Enriched midocean ridge basalt (E-MORB)

= Iceland ISL 28

A Iceland 79

A Hawaii

Figure 10. Average trace-element composition of low- TiO2 

and high-TiO2 basalts of Portage Lake Volcanics and normal midocean ridge basalt (N-MORB), enriched midocean 

ridge basalt (E-MORB), and ocean island basalts. Basalts of 

the Portage Lake Volcanics are most similar to ocean 

island basalts and E-MORB. Data for N-MORB and Iceland 

from Wood and others (1979b); data for E-MORB from 

Wood and others (1979a), analysis 413-1; data for Hawaii 

from Thompson and others (1984). Primitive mantle normalizing values from Wood and others (1979b).

1000

100

10

La Ce Nd Sm Eu Tb 

Atomic Number

Yb Lu

EXPLANATION

 Low-TiO2 basalt

A High-TiC>2 basalt

D Basaltic andesite

EB Andesite

° Dacite

 Type I rhyolite

A Type II rhyolite

Figure 11. Chondrite-normalized rare earth element patterns for average major rock types in Portage Lake Volcanics. Normalization factors from Haskin and others (1968).

Table 2. Selected ratios of incompatible trace elements in basalts from Portage Lake Volcanics, Iceland, Hawaii, and

midocean ridges

[E-MORB, enriched midocean ridge basalt; N-MORB, normal midocean ridge basalt. Dashes indicate ratio not calculated]

Ratio

Portage Lake Volcanics1 

Low-TiO2 High-TiO2

Iceland2 Hawaii

ISL 79 ISL 28 RTH24 RTH 31

*Data from average analyses (table 1) for low-TiO2 basalts (N=ll) and high-TiO2 basalts (N=8).

2Data from Wood and others (1979b).

3Data from Thompson and others (1984).

4Data from Wood and others (1979a); analysis 413-1.

Midocean ridge

E-MORB4 N-MORB2

Y/Nb

Zr/Nb

Hf/Th

Zr/Y

La/Tb

La/Yb

La/Th

2.9

14.0

2.2

4.8

17.8

9.9

2.7

12.5

2.0

4.5

18.0

8.2

1.2

9.4

2.5

8.1

21.8

11.1

3.8

15.7

4.3

4.2

8.3

11.0

1.4

9.6

2.8

6.8

10.3

12.7

0.5

4.8

1.0

9.9

13.7

8.2

0.8

3.5

1.6

4.2

25.8

10.5

13.2

36.8

17.0

2.8

3.9

15.5

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B11
Table 3. Major-oxide and normative-mineral compositions and trace-element ratios of individual rhyolite bodies in 

Portage Lake Volcanics, Keweenaw Peninsula, Mien.

[Major-oxide abundances are reported in weight percent on an anhydrous basis, assuming Fe2O3 /FeO=0.5 (after Middlemost, 1989). Weight percentages 

may not total 100.00 owing to rounding. Trace-element ratios calculated for raw data reported in appendix 2. A/=number of samples averaged]

Type 1 rhyolite Type II rhyolite

Extrusive

Major oxides:

SiO2

TiO2

A1203

Fe203

FeO

MnO

MgO

CaO

Na2O

K2O

P205

CIPW norms:

Q

Or

Ab

An

C

Hy

En

Fs

Mt

11

Ap

Trace-element ratios:

Rb/Sr

Zr/Nb

Nb/Rb

Ba/Rb

Ba/Th

Zr/Hf

Ce/Yb

Mt. Houghton

N=6

74.89

.07

13.16

.56

1.10

.03

.11

.05

2.76

7.26

.03

30.19

42.94

23.32

.05

.72

1.77

.27

1.49

.81

.13

.07

6.30

4.00

.35

8.49

80.47

27.79

14.91

Montreal River

N=7

73.45

.13

13.64

.71

1.42

.04

.20

.31

3.11

6.93

.03

27.13

41.00

26.29

1.34

.51

2.38

.50

1.88

1.03

.25

.07

1.51

6.94

.37

10.45

75.87

33.17

13.33

Montreal River 

ashflow

N=1

73.47

.27

13.00

.70

1.40

.04

.52

.08

.11

10.38

.02

31.45

61.40

.93

.27

1.46

2.92

1.30

1.62

1.02

.51

.04

4.46

13.42

.16

8.61

62.05

37.50

27.61

Keystone Point

N=1

75.36

.17

13.41

.59

1.18

.03

.07

.20

4.13

4.84

.03

31.71

28.63

34.91

.80

1.07

1.63

.18

1.45

.86

.32

.07

1.00

8.88

.20

13.36

93.68

34.80

18.42

Bare Hill

N=9

74.06

.10

13.58

.61

1.22

.03

.21

.37

3.33

6.47

.03

28.18

38.27

28.14

1.64

.48

2.15

.52

1.62

.88

.19

.07

2.69

5.06

.47

7.56

51.63

30.90

13.53

Intrusive

West Pond 

N=3

73.28

.15

14.24

.73

1.46

.04

.43

.18

4.26

5.19

.03

26.86

30.70

36.01

.70

1.34

2.98

1.08

1.90

1.06

.29

.07

1.12

8.52

.42

8.13

59.49

34.67

12.75

Fish Cove knob 

N=3

73.28

.14

14.15

.77

1.53

.04

.31

.25

4.41

5.11

.01

26.22

30.23

37.27

1.18

.92

2.79

.78

2.01

1.12

.27

.02

1.45

7.14

.39

9.58

60.96

33.50

15.12

Intrusive (?)

Copper City

N=4

75.74

.05

13.28

.44

.87

.04

.14

1.38

2.48

5.60

.02

36.21

33.11

20.95

6.72

.66

1.57

.35

1.22

.64

.09

.04

16.66

2.88

.11

.87

6.23

23.01

5.89

Rhyolite

Average chemical and normative compositions of 

eight rhyolite bodies in the Portage Lake Volcanics are 

reported in table 3. In modern unaltered rhyolites, Na2O 

and K2O concentrations are generally subequal (Ewart, 

1979; Le Maitre, 1984). However, within individual rhyolites of the Portage Lake Volcanics, Na2O and K2O concentrations range from subequal (4 to 5 wt percent) to about 

10 wt percent K2O with minute quantities of Na2O, suggesting that the degree of alkali metasomatism varies 

locally. In these rocks, metasomatism is almost always in 

the direction of increasing K2O content and decreasing

content.

Despite the effect of alkali metasomatism, majoroxide and trace-element contents of the rhyolite at Copper 

City differ from those of all other rhyolite bodies at the east 

end of the Keweenaw Peninsula (table 3; fig. 12). For 

simplicity, the two compositional groups are distinguished 

as Type I, which includes all the rhyolite bodies except the 

rhyolite at Copper City, and Type II, recognized only at 

Copper City.

Silica content in individual Type I rhyolite samples 

ranges from about 71 to 76 wt percent on an anhydrous 

basis (app. 3), but analyses for individual bodies generally 

average SiO2 <75 wt percent (table 3). Type I rhyolite is 

mildly peraluminous to slightly metaluminous (A12O3/ 

(Na2O+K2O+CaO) = 0.8-1.14; normative corundum =

B12 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
70

c 60 

o

E 50 

oS

4°

a so

20

10

* Type I rhyolite 

A Type II rhyolite

M"

"

100 200 300 400 

Rb (parts per million)

500

B

100

CD

O.

10

10 100 

Rb (parts per million)

1000

Figure 12. Selected trace-element contents for two groups 

of rhyolites of Portage Lake Volcanics. A, Two groups of 

rhyolites of Portage Lake Volcanics can be distinguished 

on the basis of Th and Rb abundances. B, In Portage Lake 

Volcanics, Type II rhyolite is similar to topaz rhyolites; 

both are enriched in Nb and Rb compared with Type I 

rhyolite. Field for topaz rhyolites from Christiansen and 

others (1984).

0-1.8 percent). The peraluminous nature of the rhyolite 

may be due in part to alteration of volcanic glass as 

described by Chayes (1970). Generally K2O is greater than 

Na2O. Type I rhyolite exhibits slight enrichment in light 

REE's (LaN/Ybi^S) and a moderately negative Eu anomaly (fig. 11). Type I rhyolite has higher Ba/Rb («8-13.5) 

and Ba/Th (^50-95) ratios than Type II rhyolite (0.87 and 

6.23, respectively; table 3).

Type II rhyolite has SiO2 greater than 75 wt percent 

and is slightly peraluminous. Type II rhyolite can be clearly 

distinguished by high Th and Rb abundances compared with 

Type I rhyolites (fig. 12A). The Nb/Rb ratio for Type II 

rhyolite is 0.1, lower than that for Type I rhyolites but 

overlapping the range of such ratios for topaz rhyolites in 

the Western United States (fig. 125). Although the data are 

sparse, F concentrations are somewhat higher and Cl 

concentrations are somewhat lower than in Type I rhyolite. 

Type II rhyolite has a nearly flat REE pattern (LaN/ YbN 

^0.8) and a pronounced Eu depletion (fig. 11).

The two groups of rhyolites from the Portage Lake 

Volcanics are chemically similar to other MRS rhyolites 

(table 4). Selected trace-element data for rhyolites from 

other MRS suites are illustrated in figure 13. Type I rhyolite 

of the Portage Lake Volcanics is most similar to some 

rhyolites from the North Shore Volcanic Group or those 

from Michipicoten Island with respect to ratios of incompatible trace elements, but it has lower concentrations of all 

incompatible trace elements. Compared with a few rhyolites 

from the Mamainse Point Formation and the Porcupine 

Volcanics, the Type II rhyolite has a similar Ba/Th ratio 

(table 4) but a greater concentration of Th and a lower 

concentration of REE's.

Rhyolites of the Portage Lake Volcanics, and MRS 

rhyolites in general, occur in a bimodal basalt-rhyolite 

province, typical of extensional tectonic settings. Type I 

rhyolite is similar to moderate-silica rhyolites in other 

bimodal basalt-rhyolite provinces, such as Iceland, the 

Snake River province in Idaho, and Mono Craters in 

California (table 5). Type II rhyolite represents the highsilica end of the rhyolite spectrum in a bimodal province. 

Type II rhyolite of the Portage Lake Volcanics has traceelement characteristics that are most similar to those of a 

small subset of high-silica rhyolites called topaz rhyolites. 

Topaz rhyolites are typically enriched in Rb, Th, U, and Nb 

and depleted in Zr, compared with other high-silica rhyolites. This style of enrichment is characteristic of Fdominated rather than Cl-dominated high-silica systems 

(Hildreth, 1981).

MINERALOGY AND PETROGRAPHY 

Basalts and Intermediate Rocks

Paces (1988) described limited mineralogical variation among the tholeiitic olivine basalts of the Portage Lake 

Volcanics. His study sampled low-TiO2 basalts almost 

exclusively. The basalts are characterized by early crystallization of plagioclase (An85_90) and olivine (<Fo84), followed by augite and Fe-Ti oxides, and finally by minor 

late-stage, Ca-poor pyroxene (Paces, 1988). Most flows 

display fine-grained intergranular to coarsely ophitic texture. Rare phenocrysts of olivine and plagioclase occur. 

Most olivine grains have been altered to varying mixtures of 

chlorite, serpentine, talc, Fe-Ti oxides, and quartz. Locally, 

intersertal volcanic "glass" is present.

Although no mineral composition data were collected 

for basalts sampled in this study, textural differences allow 

a distinction between low- and high-TiO2 basalts near the 

tip of the Keweenaw Peninsula. Low-TiO2 basalts are 

commonly subophitic to coarsely ophitic, as described by 

Paces (1988). High-TiO2 basalts typically are finer grained, 

exhibiting trachytic to finely subophitic textures. This 

textural distinction was recognized in the field by previous 

workers who mapped a unit of "ophitic flows" overlain by

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B13
Table 4. Major-oxide and trace-element compositions of selected Midcontinent rift rhyolites

[Major oxides reported in weight percent on an anhydrous basis, after correcting Fe2O3/FeO ratios according to Middlemost (1989). Trace elements 

reported in parts per million as analyzed. Weight percentages may not total 100.00 owing to rounding]

Major oxides:

SiO2

Ti02

A1203

Fe2O3

FeO

MnO

MgO

CaO

Na2O

K2O

P2O5

North Shore

Volcanic Croup1

N=6

74.84

.35

11.64

1.39

2.77

.03

.39

.81

2.24

5.48

.06

Osier

Croup1

N=3

75.63

.38

12.80

.66

1.42

.05

.31

2.06

3.02

3.59

.08

Mamainse

Point

Formation1

N=3

76.78

.11

12.38

.52

1.03

.02

.21

2.03

1.56

5.32

.05

Michipicoten

Island

Formation1

N=2

76.79

.16

12.29

.49

.98

.01

.43

.27

2.06

6.51

.03

Porcupine

Volcanics1

N=8

75.9

.17

11.61

.88

1.77

.01

.28

.11

1.35

7.86

.06

Rhyolites of

Portage

Type 1

N=31

73.84

.12

13.65

.66

1.32

.03

.23

.25

3.23

6.64

.03

Lake Volcanics2

Type II

N=4

75.74

.05

13.28

.44

.87

.04

.14

1.38

2.48

5.60

.02

Trace elements:

Nb 35 27 130

Rb 145 60 328

Sr 57 219 38

Zr 641 559 342

Ba 1,509 573 83

Y 79 46 115

La 107 146 71.6

Sm r 17.58 19.3 17.3

Yb 7.16 3.63 15.3

Hf 15.88 12.9 11

Ta 2.72 1.73 9.09

Th 14.98 9.99 31.5

Trace-element ratios:

Ba/Rb 10.4 13.2 .3

Ba/Th 100.7 60.3 2.7

33

250

125

266

1,205

68

59

12.65

5.47

8.14

3.69

26.2

4. 

46

36

240

58

569

285

86

94.3

13.0

8.05

14.6

2.93

35.7

1.2

41

112

55

240

997

56

30.3

6.57

5.55

7.53

2.91

15.09

8.9

66

51

465

28

144

402

74

12.0

6.9

7.68

6.3

4.9

63

.9 

6.4

Unpublished data for samples collected by author.

2Major-oxide abundances are averages of normalized data reported in appendix 3. Trace-element abundances are averages of raw data reported in 

appendix 2.

"fine-grained melaphyres" (Cornwall, 1951, and references 

therein; Cornwall, 1955). The mapped boundary between 

these two textural types lies close to where the boundary 

would be drawn on the basis of chemical distinctions. In 

addition, similar textural variations have been recognized in 

basalts in the North Shore Volcanic Group in Minnesota 

(Basaltic Volcanism Study Project, 1981; Green, 1989).

The intermediate rocks, ranging from basaltic andesite to dacite, generally are dominated by plagioclase and 

contain lesser amounts of Fe-Ti oxide and clinopyroxene. 

Textures range from fine-grained subophitic to, more commonly, trachytic. Andesites and dacites typically contain a 

small percentage of feldspar phenocrysts. Browne and 

Haynes (1956) reported hornblende in the dacite of Mt. 

Gratiot; in one sample, they found pyroxene partly altered 

to chlorite. In most samples, however, mafic minerals have

been entirely altered, commonly to chlorite and opaque 

masses of Fe-Ti oxides. Interstitial quartz locally makes up 

as much as 20 percent of the dacite of Mt. Gratiot. 

Disseminated Fe-Ti oxides are abundant; apatite and rare 

zircon are accessory phases. Browne and Haynes (1956) 

reported pumpellyite, chlorite, carbonate, sericite, and 

epidote as secondary minerals.

The andesite near Suffolk (fig. 5) differs from other 

andesites in this study because it contains about 25 vol 

percent (volume percent) phenocrysts and glomerocrysts of 

coarse plagioclase, apatite, and Fe-Ti oxides in an aphanitic 

matrix and may be cumulate in part. Very fine grained 

disseminated Fe-Ti oxides give a reddish hue to the matrix. 

Plagioclase («=sAn35 ) phenocrysts show minor serialization. 

Any original mafic silicate phenocrysts have been altered to 

opaque masses of Fe-Ti oxides.

B14 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
1000

100

 1 10 'E

D Type I rhyolite 

 Type II rhyolite

Cs Rb Ba Th U K Ta Nb La Ce Sr Nd P Hf Zr Sm Ti Tb Y

B

1000

100

 5 10

E

E 1

CO 

CO

Cs Rb Ba Th U K Ta Nb La Ce Sr Nd P Hf Zr Sm Ti Tb Y

EXPLANATION

D Michipicoten Island Formation

A North Shore Volcanic Group

O Mamainse Point Formation

 Rhyolite of South Mirror Lake

* Rhyolite of Osier Group

Figure 13. Selected trace-element contents for two groups 

of rhyolites. A, Incompatible trace elements for average 

Type I and Type II rhyolites of Portage Lake Volcanics. B, 

Incompatible trace elements for average rhyolites from 

several localities within the Midcontinent rift. Normalization to primitive mantle using the normalizing values of 

Wood and others (1979b).

Rhyolites

The two types of rhyolites distinguished geochemically also can be distinguished macroscopically. Type I 

rhyolite samples are dark-red-brown aphyric rocks in which 

phenocrysts are absent or make up no more than 2 vol 

percent of the rock. Where present, the phenocrysts are 

typically small (<1.5 mm (millimeters)), euhedral to subhedral feldspars locally associated with very rare subhedral 

quartz phenocrysts. In comparison, the Type II rhyolite is 

gray white and distinctly porphyritic, containing up to 10 

vol percent phenocrysts of euhedral to anhedral quartz and 

euhedral to subhedral feldspar ranging in size from 0.5 to

2.5 mm. Detailed mineral composition data are not available for either rhyolite type.

Type I Rhyolite

In Type I rhyolite, both alkali and plagioclase feldspar are intergrown with quartz in a very fine grained 

devitrified matrix. The rhyolite of Mt. Houghton contains 

feldspar microlites of about 0.2 mm size that are generally 

aligned parallel to the direction of flow foliation and are 

slightly larger than the groundmass feldspar. All other Type 

I rhyolite bodies also have sparse andesine to oligoclase 

phenocrysts and glomerocrysts, in addition to the groundmass and microlite feldspars (Papadakis and Raza, 1954). 

Locally, plagioclase phenocrysts are mildly sericitized. 

Widespread, finely disseminated Fe-Ti oxides impart the 

characteristic red color to the Type I rhyolite. No mafic 

minerals are preserved, but some concentrations of Fe-Ti 

oxides may represent the sites of original mafic silicate 

minerals. Fe-Ti oxide microphenocrysts about 0.2-0.3 mm 

long are common. Whole-rock X-ray spectra collected 

during this study by P.M. Okita for several Type I rhyolite 

samples show the presence of quartz, orthoclase, and 

plagioclase, with local minor carbonate minerals in some 

samples.

Quartz is an essential groundmass constituent but 

rarely occurs as a phenocryst phase in Type I rhyolites. 

Where rare quartz phenocrysts are present (such as in the 

rhyolites of Montreal River and Bare Hill), the grains are 

anhedral, commonly embayed, and small (<1 mm). Secondary quartz fills vesicles, lithophysae, and small fractures 

and acts as a cement among breccia fragments. Zircon is a 

sparse but ubiquitous accessory mineral. Carbonate and 

epidote locally fill veins and cavities.

Type II Rhyolite

Feldspar phenocrysts in Type II rhyolite are both 

plagioclase and an alkali feldspar, most likely sanidine. The 

alkali feldspar phenocrysts are commonly altered and have 

been replaced by clay minerals, oxides, and (or) carbonate. 

Sparse albite phenocrysts show minor alteration to sericite. 

Quartz is a common phenocryst phase in the Type II 

rhyolite, typically making up about half the volume of 

phenocrysts. Quartz phenocrysts are subhedral to anhedral 

and show resorbed margins. No mafic silicate minerals are 

preserved in the Type II rhyolite, although, as is the case for 

the Type I rhyolites, it is possible that some Fe-Ti oxide 

concentrations represent sites of original mafic phases. 

Fe-Ti oxides also occur as sparse phenocrysts. Zircon is a 

common accessory phase, and, locally, carbonate has 

flooded the quartz-feldspar groundmass. Whole-rock X-ray 

spectra for Type II rhyolite samples show the presence of 

quartz, orthoclase, plagioclase, carbonate minerals, and 

illite; illite is not in the Type I rhyolite.

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B15
Table 5. Average major-oxide and trace-element compositions of rhyolites of Portage Lake Volcanics and representative 

bimodal, calc-alkalic, peralkalic, and topaz rhyolites

[Major oxides in weight percent; trace elements in parts per million. Empty cells, no data; n.d., not determined; N, number of samples in average analysis 

reported in table]

Bimodal basalt-rhyolite

Major oxides:

Si02

TiO2

A12O3

Fe203

FeO

MnO

MgO

CaO

Na2O

K2O

P2O5

Total

Trace elements:

Nb

Rb

Sr

Zr

Y

La *

Sm

Yb

Hf

Ta

Th

Iceland1

N=4

73.91

.20

12.79

.96

1.82

.05

.26

1.00

4.01

3.54

.59

99.13

71

90

82

451

140

77.2

20.74

1.82

14.8

5.35

11.2

Yellowstone 

and 

Snake River2

N=21/15

76.59

.16

12.36

1.08

.56

.03

.14

.54

3.47

5.04

.02

99.99

41

27

203

54

97

6.5

Mono Craters 

and 

Glass Mountain3

N=24/3-19

76.61

.06

12.77

.4

.56

.05

.08

.57

4.04

4.82

.04

100.00

17

185

121

111

25

39

3

Calc-Alkalic

Taupo4

N=25/35

74.22

.28

13.27

.88

.92

.05

.28

1.59

4.24

3.18

.05

98.96

5.6

108

125

160

27.5

28.4

5.5

3.55

4.5

n.d.

11.3

Peralkalic

Kenya Rift Valley5

/V=1

75.2

.17

12.11

.83

1.06

.04

.07

.44

4.59

4.73

n.d.

99.24

200

290

6.15

439

108

74

12.3

10.1

14.9

14.4

38.8

N=1

72.5

.23

10.44

1.9

2.21

.06

.02

.15

5.54

4.36

n.d.

97.44

590

637

1.25

2,018

296

153

36.8

34.6

60.5

49.4

124

Topaz rhyolite6

Spor 

Mountain

A/-11

74.2

.05

13.5

8 1.29

.06

.11

.61

3.95

4.86

0

98.63

109

1,010

6

110

116

38

6.5

8

6.7

26

67

Midcontinent rift 

Thomas rhyolites of 

Range Portage Lake Volcanics7

N=7

76.46

.13

12.53

.91

.24

.04

.18

.96

3.34

4.91

.02

99.72

53

423

28

129

58

59.5

15.6

15.6

5.5

5.6

54.8

Type 1 

N=31

73.84

.12

13.65

.66

1.32

.03

.23

.25

3.23

6.64

.03

100.00

41

112

55

240

56

30.3

6.57

5.55

7.53

2.91

15.09

Type II

N=4

75.74

.05

13.28

.44

.87

.04

.14

1.38

2.48

5.60

.02

100.04

51

465

28

144

74

12.0

6.9

7.68

6.3

4.9

63.0

'Wood (1978).

2Ewart (1979). N=2l for major oxides, 15 for trace elements.

3Ewart (1979). N=24 for major oxides, 3 for Yb, 10 for La, 18 for 

Rb, 19 for Nb, Sr, Zr, and Y.

4Ewart and others (1968). A/=25 for major oxides, 35 for trace 

elements.

Hydration and Devitrification Textures

Devitrification textures, developed in formerly glassy 

material, are common in silicic volcanic rocks and are 

present in all rhyolites of the Portage Lake Volcanics. Glass 

is extremely rare in silicic volcanic rocks older than 

Cretaceous age (Lofgren, 197 Ib). However, the presence of 

glassy material in Midcontinent rift volcanic rocks from the 

North Shore Volcanic Group and the Michipicoten Island 

Formation (Roedder and Weiblen, 1971; Palmer and others, 

1988) attests to remarkable preservation due to the lack of 

extensive alteration, posteruptive deformation, or deep 

burial of Midcontinent rift rocks. No pristine glass has been 

identified in the Portage Lake Volcanics, but the original 

presence of glass is demonstrated by delicate hydration and 

incomplete devitrification textures preserved in these rhyolites for more than a billion years.

5Macdonald and others (1987a). 

6Christiansen and others (1984, 1986). 

7This report. 

"Total iron as Fe2O3 .

Lofgren (1971a,b) successfully produced common 

devitrification textures by varying conditions of temperature, pressure, time, and fluid composition in subsolidus 

experimental runs with natural rhyolite obsidians. He 

defined two devitrification stages an initial glassy stage 

followed by a spherulitic stage. In addition to intergrown 

spherulites, the spherulitic stage is characterized by the 

presence of micropoikilitic quartz enclosing spherulites and 

feldspar grains, the so-called snowflake texture (Lofgren, 

1971b). In older volcanic rocks, the fibrous minerals in 

spherulites typically are recrystallized, resulting in a finegrained equidimensional or felsitic texture of quartz and 

feldspar (Cas and Wright, 1987).

Textures produced experimentally by Lofgren 

(1971a,b) duplicate textures in modern rhyolite bodies (for 

example, Christiansen and Lipman, 1966; Bonnichsen and 

Kauffman, 1987; Cas and Wright, 1987; Manley and Fink,

B16 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
I- z '"i^^syj&§w'^'r

?^^c^l|^f^lA^

i-fx/^^^m^ *<*¥ * :.: '7&':^&: J^^v

fe:^^%jt-fe«^

if.^.^*^jf^X.

t^^^^ffi^l 'T-..-'^'.^i/ i^i^ "*:." - * '"« '.""

\t^< '%»*H.

,^T^ ^3g***&

m

:1 !fefrffi#4

Figure 14. Photomicrographs of rhyolitic textures. A, Thin section (plane-polarized light) of perlitic texture and isolated 

spherulites preserved in rhyolite breccia fragment from Keystone Point. 6, Thin section (plane- polarized light) of bow-tie 

spherulites in perlitic matrix in rhyolite breccia fragment from Keystone Point. C, Thin section (plane-polarized light) of 

coalescing spherulites from north side of rhyolite of Mt. Houghton. D, Thin section (x-nicols) of snowflake texture 

preserved near top of south side of rhyolite of Mt. Houghton.

1987) and have been documented at least locally in some 

rhyolites of the Portage Lake Volcanics, despite their 

Proterozoic age. Some of the most delicate textures representing the earliest stages of devitrification are preserved in 

silicified breccia fragments from northern margins of the 

rhyolites of Mt. Houghton and Keystone Point. In a breccia 

sample from Keystone Point (fig. 14A), delicate perlitic 

texture and isolated spherulites are preserved, representing 

very low temperature hydration. Such marginal breccias are 

only locally preserved; more commonly, rhyolite at the 

present margins of the stratiform bodies contains small 

spherulites, indicative of devitrification at temperatures 

lower than 400 °C.

Larger and more widely spaced spherulites are typical 

of samples closer to the interior of the rhyolite bodies. 

Well-preserved bow-tie spherulites at Keystone Point (fig. 

145) and coalescing spherulites or axiolites (fig. 14C) near 

the margins of the rhyolites of Mt. Houghton and Montreal 

River suggest that these samples underwent devitrification 

at intermediate temperatures, resulting from slower cooling

rates than encountered at the margins of the bodies. The 

most common devitrification textures are "snowflake" and 

felsitic textures characteristic of large parts of the rhyolites 

of Bare Hill, West Pond, and Fish Cove as well as the stony 

cores of the rhyolites of Mt. Houghton (fig. 14D) and the 

Montreal River. Recrystallization to felsitic textures obliterates earlier textural features of silicic glass and is common 

in many older rhyolites (Green, 1970, 1977).

In summary, the stratiform rhyolite bodies show a 

wide range of devitrification textures, from perlitic and (or) 

spherulitic at their margins to felsitic in their interiors. The 

discordant rhyolite bodies are dominated by felsitic texture 

and only locally show spherulites near their margins.

Alteration around Vesicles and Fractures

Numerous small vesicles and fractures are present 

near the margins of some Type I rhyolite bodies. They are 

generally aligned subparallel to the dominant foliation in the 

rhyolite (fig. 15A). Strings of rounded to slightly flattened

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B17
Figure 15. Photograph and photomicrographs of en echelon fractures in rhyolite. A, Field shot of en echelon fractures 

(low dip, light color due to local alteration) at Mt. Houghton. Coin is 2.2 centimeters in diameter. B, Thin section 

(plane-polarized light) of vesicles and tension fractures. Interiors of vesicles filled with quartz, Fe-Ti oxides, and some 

feldspar. From rhyolite sample from Mt. Houghton. C, Thin section (plane-polarized light) of zoning around quartz-filled 

fracture. Phenocryst is a rare quartz grain; rhyolite of Montreal River.

vesicles commonly occur in the same area as discontinuous 

en echelon fractures (fig. 155). Some fractures may be 

collapsed vesicles that became elongated during flowage as 

the rhyolite cooled (Fink, 1983; Bonnichsen and Kauffman, 

1987; Hausback, 1987); most, however, probably resulted 

from shearing as the flow advanced, providing avenues for 

vapor migration through the flow (Manley and Fink, 1987). 

Equant vesicles commonly are a few millimeters to 1 cm 

(centimeter) in diameter, whereas the fractures typically 

range up to 5 cm or more in length and 1-2 mm in width and 

are spaced about 0.5 cm to several centimeters apart.

Both vesicles and fractures are typically filled by 

coarse euhedral quartz and lesser Fe-Ti oxides. Surrounding 

individual vesicles and fractures is a light-colored zone 

several millimeters wide that resulted from an abrupt 

decrease in Fe-Ti oxide concentration in the rhyolite 

groundmass (fig. 15C). This zone near the fracture is 

dominated by potassium feldspar, and by fewer and coarser 

disseminated oxides compared with unaffected rhyolite.

Locally within this zone, there is a slight increase in size of 

the potassium feldspar grains, suggesting that locally higher 

contents of volatile elements may have affected grain size.

FIELD RELATIONS OF RHYOLITES OF 

PORTAGE LAKE VOLCANICS

Early workers recognized that several stratiform rhyolite bodies occur along nearly the same horizon and exhibit 

features that suggest an extrusive origin, whereas several 

other rhyolite bodies occur stratigraphically below the 

stratiform rhyolite horizon and appear to be "intrusive," 

disrupting adjacent basalt flows (Hubbard, 1898). The 

conclusions of these early observers remain valid. Recent 

work in young rhyolite fields (for example, Christiansen 

and Lipman, 1966; Benson and Kittleman, 1968; Fink, 

1983; Bonnichsen and Kauffman, 1987; Fink and Manley, 

1987; Manley and Fink, 1987; Sampson, 1987) has docuB18 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
LAKE SUPERIOR

Copper 

Harbor

25

25

30

30

50

35

30

25

35

30 Mt. Houghton JT ». + , D -

-g _ _ _ _ J~^ Montreal River 25

5 KILO METERS

EXPLANATION

pT] Jacobsville Sandstone

Copper Harbor Conglomerate

Portage Lake Volcanics 

[ | Low-TiO2 basalts

I I High-TiO2 basalts and

andesitic flows 

Bohemia conglomerate ?v

35 Strike and dip

© Horizontal bed 

49 Individual rhyolite or clastic bodies

Figure 16. Location of Type I rhyolite bodies and some andesite and dacite bodies, and the regional dips of the Portage 

Lake Volcanics and surrounding units, on the eastern tip of Keweenaw Peninsula.

mented the characteristics typical of modern rhyolite extrusive flows and has led to a more complete understanding of 

the relation between rhyolitic pyroclastic and lava flows. It 

is clear, from comparison with these studies, that many 

features of modern flows are preserved in the rhyolites of 

the Portage Lake Volcanics, despite their Middle Proterozoic age.

This section summarizes the field relations of rhyolites of the Portage Lake Volcanics, compares modern 

rhyolite systems with these rhyolites, and discusses the 

relation of the rhyolite bodies to the associated "conglomerates."

Stratigraphy

The Portage Lake Volcanics dips to the north beneath 

Lake Superior, is 3-5 km thick, and is faulted, but the unit 

has not been penetratively deformed. Dips are steepest 

(about 60°-80° N.) near the Keweenaw fault and become 

more shallow near the lake, where the upper part of the

Portage Lake Volcanics and the overlying Copper Harbor 

Conglomerate dip about 20°-30° N. (fig. 16). The Portage 

Lake Volcanics is at the margin of a central graben segment 

of the rift adjacent to the Keweenaw fault, which acted as a 

major normal growth fault during extension. Subsidence 

was greatest within the central graben, and volcanism 

became focused there. When flows of the Portage Lake 

Volcanics were extruded, rapid subsidence resulted in early 

flows tilting into the graben and succeeding flows erupting 

on a surface that continued to subside. Thus, the fanning of 

dips in the Portage Lake section is in part a primary feature, 

steepened by later compression.

Because the rhyolite bodies are near the base of the 

section where dips are steep, the map view (Cornwall, 

1954b, 1955) is essentially a cross section through the 

bodies. Individual rhyolite bodies are resistant to erosion, 

and the larger bodies form hills above the erosional level of 

the basalts. For this reason, the rhyolite hills and associated 

conglomerate were called the "Bohemian Range" by Irving 

(1883) and earlier workers.

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B19
0 km

3km

.9>0

:> ro

> 0

COPPER HARBOR 

CONGLOMERATE

"1087.2 ±1.6 Ma 

("Lake Shore traps")

 1094.0 ±1.5 Ma 

(Greenstone flow)

Bohemia conglomerate 

Rhyolite of Mt. Houghton

"1096.2 ±1.8 Ma 

(Basalt flow at Copper City)

Rhyolite of Bare Hill 

Keweenaw fault 

JACOBSVILLE SANDSTONE

EXPLANATION

Ijjj^l Sandstone and conglomerate 

[ | Interflow conglomerate 

^^ Fault

Rhyolite

High-TiO2 basalt 

Low-TiO2 basalt

Figure 17. Generalized stratigraphic section of Portage 

Lake Volcanics in vicinity of rhyolite of Mt. Houghton, 

illustrating the location of high-TiO2 basalt intervals and 

the location of associated felsic extrusive and subvolcanic 

bodies. Ages for the Copper City basalt flow, the Greenstone flow, and the "Lake Shore traps" (within the Copper 

Harbor Conglomerate) from Davis and Paces (1990).

The general stratigraphy in the vicinity of the rhyolite 

bodies at the eastern end of the Keweenaw Peninsula is as 

follows (fig. 17). Low-TiO2 ophitic basalts are the lowest 

unit of the Portage Lake Volcanics, and their base is 

faulted. About 1 km above the Keweenaw fault, a 0.5-km 

interval of high-TiO2 basalts and more evolved rocks 

overlies the low-TiO2 basalts. Stratiform or extrusive rhyolite bodies occur near the top of this interval of high-TiO2 

basalt, basaltic andesite, and andesite. Andesite commonly 

underlies the pyroclastic units, which typically underlie the 

stratiform rhyolite bodies. Discordant or "intrusive" rhyolite bodies occur below the stratiform rhyolite horizon. The 

Bohemia conglomerate, a distinctive felsic pebble conglomerate associated with the stratiform rhyolite horizon, has 

been traced laterally for tens of kilometers (Stoiber and 

Davidson, 1959a,b). Overlying the stratiform rhyolite horizon and Bohemia conglomerate are about 3 km of basalt and 

minor interlayered sedimentary rocks. In the Mt. Houghton

area, the first basalt flows to extrude after deposition of the 

Bohemia conglomerate were high-TiO2 basalts. Subsequently, low-TiO2 basalts were extruded and thereafter 

dominated the volcanism. High-TiO2 basalts recur only 

near the top of the Portage Lake Volcanics (Paces, 1988). 

No rhyolite bodies are exposed above the Bohemia conglomerate.

Twenty-two interflow conglomerates have been 

mapped within the Portage Lake Volcanics and used extensively for correlation (Butler and Burbank, 1929). Two 

types of conglomerates are recognized: amygdaloidal basaltic conglomerates are more common in the lower part of the 

formation, and dominantly felsic conglomerates are common in the upper part (above the felsic Bohemia conglomerate). Where the two types occur together, amygdaloidal 

basalt conglomerate generally underlies felsic conglomerate; their positions suggest that local erosion of basalt flows 

was followed by stream deposition of debris from more 

distant sources (Butler and Burbank, 1929).

Despite the restricted exposure of rhyolite bodies in 

the Portage Lake Volcanics, felsite and granophyric pebbles 

account for nearly 70 percent of the clastic material that 

makes up the numerous interflow conglomerates above the 

Bohemia conglomerate (Merk and Jirsa, 1982). Such a 

large volume implies a larger felsic source than can be 

accounted for by the presently exposed rhyolite bodies. In 

addition, some of the rhyolite pebbles contain abundant 

quartz and feldspar phenocrysts, an assemblage that is 

absent from the outcrop at the east end of the Keweenaw 

Peninsula. The presence of this distinctive rhyolite type in 

conglomerates above the observed rhyolite exposures 

(which lack this rhyolite type) has been a puzzle. Paleocurrent studies suggest northwestward flow from sources, now 

eroded or buried, that were located to the south-southeast of 

the present exposure of the Portage Lake Volcanics (Merk 

and Jirsa, 1982) and, thus, outside the central rift graben.

The major rhyolite and intermediate bodies, from east 

to west, are described below.

Keystone Point

At Keystone Point, a southern point near the eastern 

tip of the Keweenaw Peninsula, felsic rocks crop out at the 

shore of Lake Superior along strike with the stratiform 

Montreal River bodies to the west (fig. 16). Because the 

felsic rocks extend into the water, the dimensions of the 

body are indeterminate, and ihe body is at most less than a 

meter above the lake level. Hubbard (1898) described two 

small promontories as banded and jointed felsite masses that 

locally contain spherulites. Contact relations are obscure, 

but to the north of these promontories, altered and brecciated felsic rock is overlain by a breccia containing fragments of felsic and mafic rocks. Similar breccia crops out at 

the lake's edge just to the east of Hoar Creek, less than 2 km 

west of Keystone Point.

B20 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
Montreal River Area

Rhyolite of Montreal River

Outcrop of three small stratiform rhyolite bodies in 

the vicinity of the Montreal River is nearly 3 km long (fig. 

16). This group of rhyolite bodies lies along strike about 3 

km west of Keystone Point and strikes N. 70° W. to 

east-west. In map view, the widest area of outcrop is 200 m 

(meters), but generally the width is less than about 20 m. 

Outcrop is scattered, but small ridges and glaciated northern 

slopes afford reasonable exposure. Present-day relief is at 

most 25 m. Hubbard (1898), Papadakis and Raza (1954), 

and Cornwall (1955) mapped and described the rhyolites of 

Montreal River and associated conglomerates.

Hubbard (1898) considered this series of felsite bodies to be extrusive. The main mass of each Montreal River 

body is maroon rhyolite containing sparse feldspar phenocrysts. The most striking field characteristic is a welldeveloped foliation that generally parallels the margins of 

the body. This foliation is defined by maroon to gray 

laminae several millimeters thick interlayered on a scale of 

a few millimeters to centimeters. Adjacent layers vary in 

degree of crystallinity and devitrification, grain size, and 

content of spherulites and vesicles. Trails of spherulites are 

noticeable throughout the outcrops. Folding of interbanded 

layers is visible. Closely spaced parallel or en echelon 

fractures that are aligned with the flow foliation typically 

are filled with secondary quartz and feldspar as well as 

some carbonate minerals and epidote.

A coarse felsic pebble conglomerate overlying the 

Montreal River bodies has been correlated with the laterally 

extensive Bohemia conglomerate. Blocks within the felsic 

conglomerate become increasingly coarse and angular adjacent to the rhyolite, indicating that the breccia along the 

northern margin of the bodies was a local source of detritus.

Pyroclastic Units and Associated Conglomerate of 

Montreal River

Previous workers mapped at least three felsic fragmental units that they termed "conglomerates" adjacent to 

the rhyolites of Montreal River (Hubbard, 1898; Papadakis 

and Raza, 1954). West of the Montreal River, the "conglomerates" coalesce (Hubbard, 1898) and are correlated 

laterally with the Bohemia conglomerate (Cornwall, 1955). 

However, these three "conglomerates" are not identical; 

two are pyroclastic units. The coarse felsic pebble conglomerate that directly overlies the rhyolites of Montreal River is 

the most similar to the Bohemia conglomerate to the west.

The "conglomerate" directly underlying the rhyolite 

bodies, and in some places underlying the Bohemia conglomerate, is a lavender, partly welded pyroclastic unit (fig. 

ISA). The unit contains small fragments of rhyolite and 

basalt as well as rounded and broken feldspar phenocrysts 

and collapsed pumice fragments visible in hand sample and 

thin section (fig. 18B). This distinctive unit can be traced

Figure 18. A, Hand specimen of ashflow tuff that underlies 

the rhyolite domes of Montreal River, showing partly 

flattened pumice and angular lithic fragments. B, Photomicrograph of thin section (plane-polarized light) of tuff 

showing fiamme.

beneath the rhyolite bodies near the Montreal River itself, 

but it is poorly exposed away from the river. Directly 

beneath this lavender pyroclastic unit is a coarse breccia 

whose angular blocks are dominantly mafic. Another lavender pyroclastic unit is exposed in limited outcrops 

between Mt. Houghton and Bare Hill (fig. 16). Although 

this unit is on strike with the lavender unit beneath the 

rhyolites of the Montreal River to the east, correlation of the 

two is equivocal.

Fish Cove Knob, Fish Cove Dike, and West Pond

The rhyolites at Fish Cove and West Pond, described 

by Hubbard (1898), Papadakis and Raza (1954), and 

Cornwall (1955), are considered to be intrusive. Two 

rhyolite exposures east of West Pond may be part of a single 

body, but their junction is covered. One exposure trends 

east-west, the other north-south, clearly cutting across the 

regional strike of the adjacent basalt units. Papadakis and

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B21
Raza (1954) described a few basalt inclusions at the margin 

of one West Pond body.

Fish Cove knob is an irregular body rising about 30 m 

above Lake Superior. The orientations of basalt flows 

adjacent to both the West Pond and Fish Cove knob bodies 

have been disrupted locally, suggesting displacement during intrusion of the bodies. Fish Cove dike is a small 

outcrop that cuts across the regional strike of the flows at a 

high angle and extends into the lake about 300 m west of 

Fish Cove knob. This brecciated outcrop is cemented by 

carbonate and contains small amounts of native copper.

The rhyolites of Fish Cove and West Pond have 

sparse feldspar and rare quartz phenocrysts and thus are 

similar to the rhyolite of Montreal River. Flow foliation is 

present only near contacts. The interiors typically are 

massive and are brecciated locally. Small quartz-filled 

vesicles are present in some areas. Spherulitic texture is 

preserved locally at the margin of Fish Cove knob. The two 

rhyolite bodies of West Pond and the two of Fish Cove are 

directly south of the rhyolite domes of Montreal River and 

may represent feeders to the overlying domes. However, no 

direct connection among the bodies is traceable in the field.

Bare Hill Area

Bare Hill is named for its steep, unvegetated, southfacing cliffs, which rise nearly 300 m above Lake Superior 

and more than 100 m above the erosional level of the 

surrounding basalts. Hubbard (1898) and more recent workers (Papadakis and Raza, 1954) recognized that several sills 

make up this body, although there is no obvious connection 

between the rhyolite visible on the lake shore and the main 

mass of several sills that underlie Bare Hill itself. The upper 

sill is the largest, nearly 1 km long and up to 0.5 km wide 

(fig- 16).

Emplacement of the Bare Hill sills resulted in a 

flexure in the orientations of the overlying basalt flows and 

conglomerates, both above and below the Bohemia conglomerate (Papadakis and Raza, 1954). The eastern contacts of the upper and lower rhyolite bodies of Bare Hill 

trend about N. 25° W., clearly crosscutting the regional 

east-west strike of the adjacent basalts and interflow conglomerates. Along the southern margin of the uppermost 

sill, small basalt fragments are included in the rhyolite and 

clearly have reacted with the rhyolite. Flow foliation is 

common throughout the lowermost sill but is present only at 

the margins of the upper sills. The rhyolite sills of Bare Hill 

contain sparse feldspar phenocrysts and rare quartz phenocrysts, similar to the West Pond and Fish Cove bodies.

Four small, closely spaced outcrops of strongly brecciated, carbonate-cemented rhyolite lie on the lake shoreline 

just west of the stream immediately west of Bare Hill 

(Cornwall, 1954b). The largest outcrop trends N. 80° E. 

(Cornwall, 1954b), but it may be truncated by a fault 

(Papadakis and Raza, 1954). These outcrops, called the

"western felsite" by Papadakis and Raza (1954), are mineralogically similar to the rhyolite of Bare Hill.

Mt. Houghton Area

Rhyolite of Mt. Houghton

Mt. Houghton is a prominent topographic feature 

rising about 150 m above the current erosional level of the 

surrounding basalts. Mapped by Hubbard (1898), Fritts 

(1952), and Cornwall (1954b), the lenticular rhyolite body 

of Mt. Houghton is more than 2 km long and strikes 

east-west. This stratiform extrusive body is 300 m wide at 

its widest point, but it pinches out to the east and west (fig. 

16). Steep slopes along the north and south sides of Mt. 

Houghton afford good rock exposures.

Along the northern flank of Mt. Houghton near the 

contact with the overlying basalts, a fine-grained breccia is 

locally preserved (see fig. 20). This breccia grades laterally 

into felsic conglomerates that adjoin the Mt. Houghton 

body at several places. Most of the conglomerates pinch out 

within a kilometer of the rhyolite body, but the Bohemia 

conglomerate, which adjoins the rhyolite body near its base, 

has been correlated laterally with extensive conglomerate 

exposures east and west of Mt. Houghton. Also on the 

northern flank, abundant lithophysae as large as 2-3 cm in 

diameter are well exposed (fig. 19A). The cavities generally 

are spherical, but locally vesicles and lithophysae coalesce.

The main mass of Mt. Houghton is composed of 

maroon aphyric rhyolite, which has a well-developed foliation that generally parallels the margins. Interbedded 

maroon to gray laminae several millimeters to centimeters 

thick define the foliation (fig. \9E). Adjacent layers vary in 

degree of crystallinity and devitrification, grain size, and 

content of spherulites and vesicles. Locally, some layers 

have been pulled apart, forming boudins (fig. 19C), and 

folding of interbanded layers is common. Near the summit 

of Mt. Houghton, foliation orientations define a roughly 

concentric pattern (fig. 20). Closely spaced parallel or en 

echelon fractures commonly are filled with secondaryquartz and feldspar and are aligned with the flow foliation. 

Along the northern contact, the fractures and flow foliation 

generally become less regular and more widely spaced.

A steep face on the south side of Mt. Houghton 

provides an oblique cross section of the stony foliated 

interior of the rhyolite body, its southern contact, and part 

of the underlying pyroclastic unit. The upper part of the 

pyroclastic unit has been welded, rendering the contact 

between the pyroclastic unit and the overlying rhyolite body 

indistinct. Numerous internal zones of fine-grained breccia 

can be traced throughout the basal section of the rhyolite 

body. Flow foliation generally parallels the southern margin 

of the body, but locally it defines small folds and boudins. 

Spherulites are also present near the base. No xenoliths 

were observed, and none has been reported from this body.

B22 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
p .-'- x - » ' « 

L v LK^ar '"

Ir PN^i^y^v Jr ' 

- , ¥ tfcj ^ "*° |r «'-4^ -<-% Figure 19. Features in the rhyolite of Mt. Houghton. A, 

| T^y*- - / v 7 , ^miR Lithophysae on the north flank of Mt. Houghton range up 

1 ~* r ^ f ^^|Hj to several centimeters in size. Coin is 2.2 centimeters in 

= ' # :fA ? ' ' s^x diameter. B, Foliation defined by alternating layers of 

'.:\ , '?'* '''- .'.*.' fi varying crystallinity, vesicularity, color, and texture. Ham- 

* '* /' * w / T^*l ' >»i mer for scale. C, Vesicular layers forming boudins, with 

!^/u ;/'^ . >? i . .. ^ foliation flowing around the boudins. Coin is 2.2 centime- 

^*"i: i-% , .yA-'- ^iv.rf --- . fr'- " ters in diameter.

^ 

Lithophysae

 ^^ -

Basalt ^^^ Ll^Aj 70\ Spherulites p|^-|

Bohemia conglomerate X""^ -4o ^73 1 19B,C | y

.^X ^btj 60^

j$$~S^ CC CC 67

Crumble ^ri&*/ ^ -^ -i* ^ Spherulites

\,-fQiyC' I 1 "00000000000 Breccia x^^^/ IUD 1 -" w ^?^^

Tuff ring /

Welded tuff Andesite

Basaltic andesite

N 

0 0.5 MILES A

0 0.5 KILOMETERS

^^ Breccia

^\^ / Basalt ^v^-j

^^uBo h e m i a conglomerate

"\. Basalt

80 ^x ^~\ Conglonnerate_

 OT^7"-' '-

EXPLANATION 

PISA] Figure number

Figure 20. Plan view of rhyolite of Mt. Houghton showing location of diagnostic features, orientations of foliations, and 

adjacent rock units. Numbers correspond to thin-section photographs in figure 14, and to field photographs in figures 

15 and 19. The Mt. Houghton body most likely represents a rhyolite lava dome or flow.

Along the western margin of the main rhyolite mass, 

a coarse silicified breccia overlies the pyroclastic unit. 

Perlitic texture is visible within the brecciated rhyolite 

fragments.

Pyroclastic Unit of Mt. Houghton

Immediately underlying the rhyolite body of Mt. 

Houghton is a pyroclastic unit that was mapped as a

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B23
"conglomerate" by previous workers (Hubbard, 1898; 

Fritts, 1952). This unit appears to be a local feature 

preserved directly beneath Mt. Houghton itself; beyond the 

margins of the rhyelite body, preservation and outcrop 

exposure of the pyroclastic unit are poor to nonexistent. The 

thickness of the unit ranges from 0 to 30 m. In one locality, 

beneath the eastern half of Mt. Houghton, a coarse maficclast-dominated breccia is present and the fine-grained 

pyroclastic unit is not exposed, being either absent or very 

thin.

Glass shards and basaltic and rhyolitic fragments are 

aligned parallel to the regional strike in this pyroclastic unit. 

The unit shows little welding at its base, but the upper 

several meters are welded where the unit is in direct contact 

with the overlying rhyolite body (fig. 20). Although exposures of the unit are poor, clastic fragments appear to 

become generally finer up-section. Beneath the west end of 

the rhyolite of Mt. Houghton, the coarsest fragments 

decrease in size from about 5-20 cm near the base to 2-4 

cm near the top of the unit. Beneath the eastern end of the 

body, the coarsest fragments, about 3-6 cm, are near the 

top. Foliated rhyolite fragments and glass shards dominate 

the clasts, but mafic fragments are present, though they 

commonly are smaller than the rhyolite fragments. 

Although the pyroclastic unit underlies Mt. Houghton, it is 

not possible to correlate it directly with a similar unit that 

crops out beneath the Montreal River bodies. No chemical 

analyses that would aid in correlation have been done on 

this unit, outcrop is discontinuous, and the pyroclastic unit 

of Mt. Houghton contains a larger proportion of mafic and 

intermediate fragments than the unit beneath the Montreal 

River domes.

Suffolk and Mt. Gratiot Areas

Other small, discordant bodies have been mapped to 

the west of Mt. Houghton, including the bodies at Mt. 

Gratiot (Cornwall, 1954a) and Suffolk (Wright and Cornwall, 1954) (fig. 5). These small, irregular bodies occur 

below the Bohemia conglomerate in a stratigraphic position 

similar to that of the rhyolites of Bare Hill and Fish Cove to 

the east (fig. 16). The dacite at Mt. Gratiot occurs as 

discontinuous exposures at the top of Mt. Gratiot (Cornwall, 1954a). Browne and Haynes (1956) described flow 

foliation in the lower part and abundant vesicles and 

brecciation near the upper contact. The interior is massive 

felsite with sparse feldspar phenocrysts.

The Suffolk intrusion, originally called the Praysville 

porphyry (Irving, 1883), is a dark-reddish-brown andesite 

containing feldspar phenocrysts (fig. 5). Contacts with 

surrounding basalts are covered, but its dimensions are 

estimated at 500 m long and 100 m wide (Wright and 

Cornwall, 1954).

Surface 

breccia

Obsidian ~ 

Stony rhyolite 

Basal breccia

Pumice fall 

deposit

B

Contorted 

flow banding

Ramp structure Massive 

flow banding

Talus

Autobrecciated lava 

Spherulitic lava

Figure 21. A, Schematic cross section of a typical rhyolite 

lava flow illustrating distribution of lithologies (after Cas 

and Wright, 1987). B, Schematic cross section of a typical 

small rhyolite dome and short flow illustrating distribution 

of internal and surficial features (after Self, 1982). See text 

for discussion.

Copper City Area

The rhyolite of Copper City occurs below the Bohemia conglomerate but significantly to the west of the other 

rhyolite bodies near the tip of the Keweenaw Peninsula (fig. 

5) (White and others, 1953; Davidson and others, 1955). It 

is the only rhyolite body on the Keweenaw Peninsula that is 

gray white in outcrop and has abundant quartz phenocrysts. 

One or two additional small occurrences of gray quartzbearing rhyolite in the vicinity of the rhyolite of Copper 

City were noted by Hubbard (1898), but they have not been 

located by more recent workers.

Although internal breccia zones are numerous, flow 

foliation is not common. The contacts are not exposed; 

thus, it is difficult to determine whether the body was 

extrusive or subvolcanic. Because of poor outcrop in the 

area, it is also not known whether a pyroclastic phase is 

present. However, the body occurs below the Bohemia 

conglomerate and appears to crosscut the strike of the basalt 

units. Until further evidence is found, it is considered to be 

subvolcanic.

Comparison of a Modern Rhyolite Lava Flow 

with Extrusive Rhyolites of Portage Lake 

Volcanics

A comparison of a typical modern rhyolite lava flow 

and a well-exposed example of an extrusive rhyolite body of

B24 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
the Portage Lake Volcanics, such as at Mt. Houghton, 

reveals that original lithologies and delicate textures are 

remarkably well preserved despite the ancient age of the 

rhyolites of Portage Lake. An idealized cross section (fig. 

21 A) illustrates the spatial distribution of lithologies within 

a typical recent rhyolite lava flow. The vertical stratigraphy 

upward in a rhyolite flow begins with pyroclastic material 

from a coeruptive pumice fall deposit that precedes the 

eruption of the lava flow itself (Christiansen and Lipman, 

1966; Fink, 1983). As the rhyolite lava extrudes, its surface 

quenches and fractures, and blocks slide off the flow front 

and are overridden by the advancing lava (Fink, 1983), 

producing a basal breccia and brecciated flow margins. If 

the rhyolite flow is sufficiently thick, then a quenched 

obsidian margin forms an envelope around the stony foliated interior of the flow. The glassy marginal envelope may 

be somewhat thicker at the top of the flow than beneath it 

and may show evidence of hydration, vesiculation, interbanding of lithologies, flow foliation, and upon cooling, 

incipient development of spherulites (Cas and Wright, 

1987). The stony interior retains heat, allowing devitrification, spherulite development, and crystallization. Thicker 

flows commonly have disproportionately thicker devitrified 

cores than thinner flows, owing to the exponential increase 

in heat retention with increasing flow thickness (Manley 

and Fink, 1987). Overlying the upper obsidian zone is an 

upper vesicular and brecciated glassy zone.

Flow foliation or flow banding is the primary internal 

feature within a rhyolite flow or dome (fig. 2IB). This 

foliation is typically defined by laminae a few millimeters to 

a few centimeters thick that show variations in color, 

texture, lithology, vesicularity, or phenocryst content 

(Christiansen and Lipman, 1966; Benson and Kittleman, 

1968; Fink, 1983; Cas and Wright, 1987). Phenocrysts and 

microlites commonly are aligned with their long axes 

approximately parallel to the direction of flow foliation 

(Benson and Kittleman, 1968). Studies of modern rhyolite 

flows and domes (Christiansen and Lipman, 1966; Benson 

and Kittleman, 1968; Fink, 1980, 1983) have used orientations of surficial ridges, folds, diapirs, and ramp structures to constrain the geometry and emplacement history of 

the flows.

In modern rhyolite flows, the location of the vent 

ideally can be determined from the orientation of flow 

foliations. Where the vent is a pipelike conduit, such as in 

the obsidian Inyo domes of California, foliations measured 

on the upper surface of the flow commonly dip shallowly 

away from the vent in a concentric pattern (Sampson, 

1987); at some distance from the vent, foliations locally 

may steepen and overturn. Within the conduit itself, steep 

flow foliations parallel the sides of the conduit. A radial 

fracture pattern may occur around a pipe vent in addition to 

the concentric foliation pattern (Sampson, 1987). Not all 

rhyolite flows emanate from pipelike vents; some are

extruded along fissures. Fink (1983) and Fink and Pollard 

(1983) documented rhyolitic fissure eruptions in the Medicine Lake Highland area of California. The strike of flow 

foliations is parallel to the strike of the fissure, and flow 

foliations dip steeply away from the fissure on either side. 

Fractures in the rhyolite tend to parallel the orientation of 

the fissure.

Comparison of the distribution of lithologies within 

the Mt. Houghton body with an ideal lava flow reveals 

remarkable similarities (figs. 20, 21). The rhyolite of 

Mt. Houghton most likely represents a small rhyolite dome, 

or a lava flow lobe associated with a dome. The glassy 

carapace is still intact locally along the northern contact. 

Well-developed spherulites and lithophysae are evident 

stratigraphically just below the carapace. The core is stony 

and generally exhibits noticeable flow foliation. At the west 

end of the dome, a fragmental unit representing a probable 

marginal flow-front breccia has been silicified at the toe of 

the dome. The dome itself probably has relatively steep 

sides, but the breccia apron surrounding it softens the 

marginal slope. The relatively complete internal stratigraphy of the rhyolite of Mt. Houghton suggests that the 

present exposure represents a cross section near the middle 

of the body, whereas the abundance of spherulitic rhyolite 

with a strong flow foliation in the rhyolites of Montreal 

River suggests that these outcrops represent a cross1 section 

through the thinning margins of several domes or flows.

The concentrically zoned flow foliation pattern near 

Mt. Houghton's summit (fig. 20) suggests that it could 

represent the site of a vent; however, there is no direct 

evidence for the location of a vent, such as a feeder dike or 

localization of coarse pyroclastic material beneath the 

rhyolite body. Although the origin of the concentric fracture 

pattern is unknown, it may be analogous to the crosssectional concentric fracture pattern illustrated in figure 2 IB 

above the vent area. Orientations of surficial ridges and 

folds could be helpful in determining the flow direction; 

however, rhyolites of the Portage Lake Volcanics are 

exposed only in cross section, and where the upper surface 

of a body is exposed, erosion typically has stripped away 

many surficial features.

Although mineral composition data are not available 

to constrain the original temperatures and volatile compositions of rhyolites of the Portage Lake Volcanics, the 

aphyric to sparsely porphyritic character of the rhyolites 

suggests that temperatures were high. No hydrous or 

anhydrous mafic phenocrysts have been recognized in 

rhyolites of the Portage Lake Volcanics. In addition, the 

preservation of glass (or the minor degree of devitrification 

of glass) and the small volume of pyroclastic material 

associated with the rhyolitic eruptions are characteristics 

that have been used as evidence of high temperature and 

low volatile contents of rhyolite lava flows in the Snake 

River Plain (Bonnichsen and Kauffman, 1987).

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B25
Comparison of Modern and Portage Lake 

Volcanics Pyroclastic Deposits

Many modern rhyolitic lava flows are preceded by 

phases of explosive activity. Three types of pyroclastic 

deposits, called fall, flow, and surge deposits, can be 

distinguished on the basis of mode of transport and deposition (Cas and Wright, 1987). Pyroclastic fall deposits 

result from particles settling out of an eruption column and 

being deposited downwind. Typically, the ash and lithic 

fragments mantle the topography, are generally better 

sorted than other pyroclastic deposits, and locally may be 

stratified. Walker (1973) described a classification of pyroclastic fall deposits that related the eruption style to the 

dispersal area and the degree of fragmentation. Pyroclastic 

flow deposits result from collapse of a lava dome, a flow, or 

an eruption column, causing the emplacement of multiple 

high-temperature dense mixtures of solids and gases that are 

gravity controlled and typically fill topographic depressions. Poor sorting is characteristic, although inverse grading of pumice fragments and normal grading of lithic 

fragments are observed locally. Pyroclastic surge deposits 

are well stratified and result from turbulent, low-density 

mixtures of solids, gases, and water associated with a 

variety of pyroclastic events.

All exposed extrusive rhyolites of the Portage Lake 

Volcanics clearly overlie pyroclastic units. However, studies of the areal distribution of the pyroclastic units are 

precluded by the present near-vertical orientation and limited lateral exposure. At Mt. Houghton, the progressive 

welding in the underlying pyroclastic unit toward the 

overlying lava suggests that the pyroclastic unit is a tuff ring 

produced by a small pyroclastic eruption that preceded the 

extrusion of the rhyolite flow itself. Pyroclastic activity 

probably was confined to the earliest stages of rhyolite 

eruption, as no pyroclastic units overlie the rhyolite of Mt. 

Houghton (or the rhyolite of Montreal River). The pyroclastic unit is poorly sorted and has an ash matrix. Locally, 

some internal stratification and inverse grading of glass 

shards may be present. The unit is matrix supported, and the 

abundance of glass shards relative to lithic fragments varies 

over several centimeters. Lithic fragments range from 

rhyolitic to basaltic but are dominated by rhyolite.

Underlying the Montreal River bodies are several 

pyroclastic units that may or may not be related to each 

other or to the overlying rhyolite domes. Outcrop exposure 

is poor, but these pyroclastic units do not appear to be 

laterally persistent; they may reflect irregular topography at 

the time of deposition. The basal unit, which is inferred to 

be pyroclastic, is exposed west of the Montreal River and 

overlies an andesite flow. The basal pyroclastic unit is 

characterized by coarse, angular mafic blocks in an ashy 

matrix and probably represents a block-and-ash flow. Overlying this basal unit is a lavender to maroon pyroclastic unit 

that contains abundant angular lithic fragments ranging

from rhyolite to basalt. Feldspar phenocrysts, commonly 

broken, and flattened fiamme are visible in outcrop and thin 

section. Sorting is poor, and the outcrop is generally 

massive. The distribution of this unit appears to reflect 

topographic valleys. In different localities, it is overlain by 

the Bohemia conglomerate, by the extrusive rhyolite flow, 

and by a coarse rhyolite breccia that may represent the flowfront breccia of an adjacent dome or flow. These characteristics suggest that the lavender pyroclastic unit is a 

welded ash-flow tuff, and that the basal mafic breccia unit 

may represent an early phase of the same eruptive event.

Correlation of pyroclastic units from area to area is 

difficult owing to poor exposure. Nonetheless, it is clear 

that several local pyroclastic events preceded rhyolite lava 

extrusion and the formation of the domes or flows. Because 

the outcrop exposure is limited to a cross section, there is no 

record of other rhyolite domes that may have been updip 

and are now completely eroded or that may be buried 

downdip. Thus, not all exposed ash flows are necessarily 

related to the few domes that are exposed.

Summary of Field Relations

An analogy with modern rhyolites indicates that the 

rhyolites of Mt. Houghton, the Montreal River, and Keystone Point were erupted subaerially as domes or flows on 

a basalt plain. Characteristics of the extrusive domes 

include lithophysae, spherulites, flow foliation, the absence 

of xenoliths, and the presence of underlying rhyolitic 

pyroclastic units and associated felsic conglomerates bearing locally derived rhyolite fragments. Adjacent basalt units 

are not disrupted by emplacement of these lens-shaped 

rhyolite bodies. In contrast, rhyolites in the Bare Hill, West 

Pond, Fish Cove, and Copper City areas have all been 

emplaced stratigraphically below the Bohemia conglomerate and the extrusive rhyolite horizon. Some rhyolite 

contacts are at high angles to the regional strike of the 

basalts, adjacent basalt units above the rhyolite bodies are 

bowed, no pyroclastic or conglomeratic units are consistently associated with these bodies, and basalt inclusions are 

found locally at the margins. Flow foliation is minor or 

absent, and lithophysae do not occur in these bodies, but 

vesicles may be present locally.

Thus, the rhyolites associated with the Bohemia 

conglomerate most likely were extrusive, whereas rhyolites 

occurring below the conglomerate were emplaced as 

"cryptodomes" or subvolcanic bodies. The two rhyolite 

bodies of West Pond and the two at Fish Cove are directly 

south of the rhyolite domes of Montreal River and may 

represent feeders to the overlying domes, although no direct 

connection can be traced in the field.

Interflow conglomerates become increasingly abundant in the upper part of the Portage Lake Volcanics. 

Amygdaloidal basaltic conglomerates are most common 

near the base of the section. Above the extrusive rhyolite

B26 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
South

EXPLANATION 

Sedimentary rocks 

Volcanic rocks 

Pre-Keweenawan basement 

Mantle

horizon, felsite clasts dominate the interflow conglomerates. However, the volume and, locally, the composition of 

the rhyolite clasts cannot be accounted for solely by the 

currently exposed rhyolite bodies of the Portage Lake 

Volcanics.

Figure 22. Interpretation of development of Midcontinent rift in Lake Superior region (from Cannon and 

others, 1989). A, A broad crustal sag develops in the 

early stage. Volcanic rocks are erupted into the basin, 

and minor normal faults develop. B, Extension continues, resulting in large normal growth faults and continued eruption of volcanic material. C, Extension 

continues as the central graben fills with an enormous 

amount of volcanic material. D, After the thermal 

anomaly wanes, the entire system subsides, creating a 

basin, which is filled by sediments. £, Postrift compression reactivates the normal growth faults as large 

reverse faults, resulting in uplift of the central rift 

graben and exposure of Portage Lake Volcanics (PLV).

DISCUSSION

Decompression melting above anomalously hot mantle plumes has been postulated as the cause of enormous 

volumes of basalt associated with major continental flood 

basalt provinces and volcanic continental margins (White 

and McKenzie, 1989). Such a plume may have caused 

volcanism in the Midcontinent rift system (MRS) (Hutchinson and others, 1990; Nicholson and Shirey, 1990). Active 

plume-related rifting that produces tholeiitic basaltic to 

rhyolitic rocks in Iceland provides a modern analog for the 

environment in which Keweenawan volcanic rocks were 

erupted. Green (1977, 1989) compared North Shore Volcanic Group basalts with Tertiary basalts from eastern 

Iceland, citing such similarities as subaerial eruption from 

fissures, compositional range, abundances of rock types, 

and volcanological characteristics. Although Icelandic rhyolitic centers have received much study (for example, 

Walker, 1963; Carmichael, 1964; Blake, 1970, 1984; 

Sigvaldason, 1974; Sigurdsson and Sparks, 1981; Macdonald and others, 1987b; Gustafsson and others, 1989), MRS 

rhyolites are only now being integrated into our understanding of the magmatic evolution of the Midcontinent rift.

The following discussion reviews the tectonic development of the MRS, examines the occurrence of rhyolitic 

volcanic rocks in the Portage Lake Volcanics within the rift, 

and compares rhyolites of the Portage Lake Volcanics with 

other MRS rhyolites. Finally, MRS rhyolitic volcanic rocks 

are compared with rhyolite from Icelandic central volcanoes 

to evaluate the hypothesis that plume-related rifting in 

Iceland can be viewed as a modern analog for Midcontinent 

rifting.

Development of the Midcontinent Rift

The following summary of the tectonic evolution of 

the Midcontinent rift (fig. 22) is based on an interpretation 

of GLIMPCE seismic-reflection data by Cannon and others 

(1989). The rift was initiated by a broad crustal sag in which 

thin quartz sandstones were deposited. Early widespread 

flood basalts erupted, thickening toward the middle of the

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B27
:High-TiO2 basalt;

ELow-TiO2 basalt =

-1096 MaB

Interflow conglomerate

D

1094 Ma

Mt. Houghton Bare Hill Montreal River

West Pond and

Fish Cove

Figure 23. Schematic representation of emplacement of Type 

I rhyolites of Portage Lake Volcanics. A, Low-TiO2 basalts 

erupt from fissures, followed by a localized group of highTiO2 basalts and intermediate rocks. Near the end of this 

interval, a pyroclastic rhyolitic eruption forms the initial tuff 

ring of the Mt. Houghton center. B, Rhyolite of Mt. Houghton is extruded to overlie the tuff ring. C, Local sedimentation of clasts, in part derived from erosion of the rhyolite 

dome, results in deposition of the Bohemia conglomerate. 

Pyroclastic eruptions at Montreal River center. D, The three 

rhyolite lobes or domes are erupted at Montreal River. Small 

subvolcanic bodies (West Pond and Fish Cove) are emplaced 

along the feeder system for the overlying domes. Erosion of 

the domes contributes to sedimentation. Basalts continue to 

erupt. High-TiO2 basalt erupts above the Bohemia conglomerate and then is covered by fissure-fed low-TiO2 basaltic 

flows, f, Fissure eruptions of low-TiO2 basalt continue, 

ultimately covering the rhyolite extrusions and conglomerate. Subvolcanic emplacement of the Bare Hill rhyolite sills 

disrupts the overlying basalts and Bohemia conglomerate. 

End of rhyolitic activity.

sag, the area that ultimately became the rift axis. The 

Archean basement was thinned; minor normal faulting near 

the rift axis probably occurred in addition to mild subsidence (fig. 22A). As crustal extension and subsidence continued, large normal growth faults developed, resulting in 

an asymmetric central rift valley within which most of the 

magmatism and sedimentation became focused (fig. 22B). 

Within the central graben, at least 15 km of basalt were 

deposited before extension ceased and volcanism diminished. The earliest volcanic rocks were preserved at the 

margins of shallow flanking basins, whereas voluminous 

later eruptions were focused within the central graben and 

locally may have spilled out into the flanking basins (fig. 

22C). Sedimentary rocks up to 7 km thick were deposited 

during subsidence of the entire area as the thermal anomaly 

waned (fig. 22D). After the rifting event, the area underwent mild compression, resulting in the transposition of 

large normal growth faults into reverse faults and the 

surficial exposure of rocks that were originally deposited in 

the central graben (fig. 22E).

Rhyolitic Volcanism of Portage Lake Volcanics

Volcanism that led to the formation of the Portage 

Lake Volcanics occurred near the margin of a deep central 

graben during the main phase of rifting; a large volume of 

magma was extruded rapidly under subaerial conditions 

(Cannon and others, 1989; Hutchinson and others, 1990). 

At the margin of a central graben segment, eruption kept 

pace with subsidence so that streams deposited sediments 

only intermittently. Escarpments must have been present 

locally, certainly while the Keweenaw fault was active, 

shedding volcanic debris into alluvial fans. Figure 23 

illustrates the emplacement of rhyolites of the Portage Lake 

Volcanics. Extrusion of low-TiO2 basaltic flows gave way 

to extrusion of high-TiO2 basalts and intermediate rocks. 

Near the end of this interval, pyroclastic eruptions preceded 

the extrusion of small-volume rhyolite domes (figs. 23A, 

23B). These domes were rapidly eroded; the erosion provided material for alluvial fan conglomerates and extensive 

stream deposits such as the Bohemia conglomerate (fig. 

23C)- Rhyolitic subvolcanic bodies were emplaced into 

basalts below the rhyolite domes, causing some disruption 

of the overlying units. Basaltic flows, most likely erupted 

from fissures, lapped up against the rhyolite domes and 

ultimately covered the domes with additional low-TiO2 

basalt more than 3 km thick. As basaltic volcanism waned, 

interflow conglomerates became more common, until conglomerates were deposited with only minor associated 

basalt flows (for example, the "Lake Shore traps" in the 

Copper Harbor Conglomerate).

Rhyolite bodies and intermediate rocks do not crop 

out above the Bohemia conglomerate. However, the large 

volume of rhyolitic clastic material that constitutes the 

interflow conglomerates above the Bohemia conglomerate

B28 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
suggests that other rhyolite complexes may occur stratigraphically higher elsewhere in the formation and are not 

currently exposed, or may have formed in flanking basins 

outside the central graben and shed debris into the graben. 

The Porcupine Mountains area (summarized below) may 

represent such a complex southwest of the Keweenaw 

Peninsula.

Midcontinent Rift Rhyolites

Rhyolites make up about 10 percent of the exposed 

MRS volcanic rocks, but as noted above, their distribution 

is not uniform. Rhyolites make up 10-25 percent of the 

volcanic rocks in the flanking basins, whereas they make up 

less than 1 percent of the exposed Portage Lake Volcanics 

at the margin of the central graben. Although the central 

graben setting of these rhyolites differs from the settings of 

other MRS rhyolites, a comparison of stratigraphy and 

emplacement styles among MRS rhyolites reveals several 

common characteristics.

Within the flanking basins, some rhyolites occur 

without associated intermediate rocks. For example, the 

early volcanic sequences of the Mamainse Point Formation 

and the Osier Group are characteristically bimodal 

(Annells, 1973; Mcllwaine and Wallace, 1976). Mafic 

rocks are restricted to picritic to tholeiitic basalt and basaltic 

andesites, and rhyolites are the only silicic rocks; no 

intermediate rocks occur in these two suites. Locally, some 

rhyolites were extrusive, but most rhyolites of the 

Mamainse Point Formation and the Osier Group probably 

were intruded as sills or plugs (Mcllwaine and Wallace, 

1976; Green, 1977).

Most MRS rhyolite bodies of somewhat younger age 

in the flanking basins (and in the central graben) are 

associated with accumulations of intermediate rocks, and 

the rhyolites themselves are more commonly extrusive. For 

example, in the northeast limb of the North Shore Volcanic 

Group, near the top of the basal Grand Portage sequence, a 

trachyandesite underlies a porphyritic rhyolite flow, and the 

overlying Ho viand Lava includes a range of basaltic, 

intermediate, and rhyolitic units (Green, 1972; Green and 

others, 1980). Younger rhyolite units of the North Shore 

Volcanic Group range from about 150 to 1,000 m in 

thickness and commonly overlie intermediate rocks (Green, 

1972). These rhyolites exhibit a variety of emplacement 

styles and include large lava flows, ash flows, and rheoignimbrites (Green, 1972, 1977, 1989; Fitz, 1988; Green and 

Fitz, 1989): a few rheoignimbrites can be traced 40 km 

along strike (Fitz, 1988). There is some evidence of local 

mixing of basaltic and rhyolitic magmas near Duluth, Minn.

In Michigan, the Kallander Creek Formation of the 

Powder Mill Group consists of at least 1,200 m of mostly 

intermediate to rhyolitic rocks that overlie the predominantly basaltic basal unit, the Siemens Creek Formation 

(Hubbard, 1975b). Emplacement style1 of these silicic rocks

is not well described, but Cooper (1973) suggested that the 

rhyolites are extrusive.

Thick sections of intermediate to rhyolitic rocks on 

Michipicoten Island (Annells, 1974) and in the Porcupine 

Volcanics of the Porcupine Mountains area in Michigan 

(Leonardson, 1966; Hubbard, 1975a,b; Kopydlowski, 

1983) constitute up to 25 percent of these volcanic sections. 

Emplacement styles of these rhyolites are poorly described 

in the literature, but the rhyolites occur near the top of the 

sections as both extrusive and sub volcanic bodies.

Recognition of the characteristic association of intermediate and rhyolitic rocks in the Porcupine Mountains 

area, on Michipicoten Island, and in the North Shore 

Volcanic Group led White (1972), Annells (1974), Green 

(1977, 1989), and Kopydlowski (1983) to postulate that the 

large accumulations of intermediate and rhyolitic rocks 

represent central volcanoes, similar to those found in 

eastern Iceland. Detailed stratigraphic studies have not been 

conducted for most MRS rhyolite occurrences, but it is the 

contention of this study that the analogy to Iceland's central 

volcanoes can be applied more broadly than previously 

postulated.

Iceland: A Modern Volcanological Analog

Active plume-related rifting that produced and is still 

producing tholeiitic basaltic to rhyolitic rocks in Iceland 

provides a likely modern analog for the environment in 

which Keweenawan volcanic rocks were erupted. In Iceland, basalts make up about 85 percent of the Tertiary and 

postglacial series; intermediate and acidic rocks make up 

another 3 percent and 12 percent, respectively (Sigurdsson 

and Sparks, 1981). These proportions are similar to proportions for MRS volcanic rocks.

Silicic rocks and intermediate rocks in Iceland are 

confined to major Tertiary and neovolcanic central volcanic 

complexes (Walker, 1966; Saemundsson, 1978). The composition and form of these volcanic centers differ between 

within-rift and off-rift settings. Within-rift volcanic centers 

typically erupt tholeiitic basalts, whereas off-rift centers 

erupt alkalic or transitional tholeiitic basalts (Oskarsson and 

others, 1982). Within-rift central volcanoes tend to be 

low-relief shield volcanoes only a few hundred meters high 

(Saemundsson, 1978) or clusters of low hills interspersed 

with basaltic shields (Walker, 1966). In some, a central 

caldera may develop. The low relief of within-rift central 

volcanoes may be due in part to high tectonic subsidence 

rates. In contrast, off-rift central volcanoes are commonly 

cone-shaped stratovolcanoes with summit craters. Less 

tectonic subsidence in the off-rift regions in part accounts 

for the higher relief of the stratovolcanoes. Off-rift central 

volcanoes have higher proportions of silicic rocks and 

pyroclastic rocks than their counterparts in the active rift 

zone (Saemundsson, 1978).

Although central volcanic complexes occur in both 

within-rift and off-rift settings in Iceland, within-rift volGeochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B29
Present cross section through Type I rhyolites of Portage Lake Volcanics may 

represent a cross section through the edge of silicic central volcanic complex T,

EXPLANATION

Thin basic and intermediate lava flows j

DU i-+- i ti j u j Breiddalur Rhyohtic lava flows and welded V vo| cano

tuffs, felsic tuffs, and agglomerates 

Intrusions, mainly felsic J

I 1 Flood basalts forming envelope enclosing 

I____I central volcanic complex

Length of section approximately 35 kilometers 

Vertical scale approximately twice the horizontal scale

Figure 24. Stratigraphy in a cross section of the Breiddalur central volcano in Iceland, modified from Walker (1963). A 

localized accumulation of basalts and intermediate rocks is surrounded by an envelope of fissure-erupted basalt flows. 

Rhyolites occur near the top of the volcano, emplaced as both extrusive and subvolcanic bodies. The small parasitic 

rhyolite bodies at Breiddalur provide a good analog for the emplacement of rhyolites of Portage Lake Volcanics.

canism provides the stronger analogy to MRS volcanism. 

Fracturing in Icelandic axial rift zones is concentrated into 

fissure swarms up to 20 km broad and more than 100 km 

long. Most fissure swarms are focused on a central volcano, 

together constituting a volcanic system between 50-100 km 

long and 10-30 km wide. Basalt flows dip gently on a 

regional scale toward the center of the active rifting zone 

(Saemundsson, 1979); this regional tilt is imparted to the 

pile by subsidence during growth, but eruption rates are 

rapid enough to allow eruption to occur subaerially. In the 

well-exposed eroded Tertiary central volcanoes of eastern 

Iceland, the ratio of basalts to intermediate and rhyolitic 

rocks is about 3:1 (Walker, 1966). Volumes of central 

volcanoes can range from 200 to 600 km3 (Walker, 1966). 

Icelandic within-rift shield-type central volcanoes 

(fig. 24) are characterized by accumulations of intermediate 

and silicic rocks, high eruption rates of tholeiitic basalt, and 

the presence of dikes, shallow intrusions, and localized 

hydrothermal alteration that is not characteristic of the 

surrounding flood basalts (Saemundsson, 1978). Central 

volcanoes are interpreted to overlie shallow magma chambers (Saemundsson, 1979; Oskarrson and others, 1982) that 

fill episodically. When lithostatic pressure is overcome by 

magmatic pressure, eruption occurs (Saemundsson, 1979). 

As these major volcanic centers are being built, fissure-fed 

basaltic lava flows may erupt simultaneously. Once a 

volcanic center wanes, it is partially or completely buried 

by later basalts (Walker, 1966). The lifespan of a volcanic 

center is on the order of 1 million years or less (Saemundsson, 1979).

Comparison of Icelandic and Midcontinent Rift 

Rhyolites

Green (1977, 1989), Green and others (1980), and 

White (1972) have argued that areas of abundant rhyolite 

such as parts of the North Shore Volcanic Group and those 

of the Porcupine Volcanics and the Michipicoten Island 

Formation are comparable to Icelandic central volcanoes. It 

is argued here that this analogy can be more broadly applied 

to other MRS rhyolite occurrences, including the occurrence of Type I rhyolite bodies of the Portage Lake 

Volcanics.

The stratigraphy within the vicinity of the extrusive 

rhyolite horizon of the Portage Lake Volcanics is relatively 

simple: extrusive rhyolites occur only near the top of a 

stratigraphically restricted group of high-TiO2 basalts, 

intermediate, and rhyolitic rocks near the faulted base of the 

Portage Lake Volcanics. This interval is completely 

enclosed by low-TiO2 basalts. Alteration appears to be 

localized around this area. Rhyolitic extrusive rocks typically overlie pyroclastic deposits and subvolcanic rhyolite 

bodies are present below the extrusive rhyolite horizon. 

Interflow conglomerates contain much felsitic material 

above the Bohemia conglomerate and very little below the 

extrusive rhyolite horizon.

Specific characteristics of Icelandic central volcanoes 

in active rift zones include (1) abundant quartz tholeiites 

and more evolved rocks, (2) thinner basalt flows within the 

volcanoes than outside the volcanoes, (3) angular discordance between basalts within and outside the volcanic center,

B30 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
(4) emplacement of abundant dikes near the core of the 

volcano, (5) gabbroic to granophyric intrusions near the 

volcanic core, and (6) localized alteration in the core of the 

volcanic center (fig. 24). Each of these features is comparable to features of the group of intermediate and rhyolitic 

rocks of the Portage Lake Volcanics.

The restricted association of intermediate rocks and 

rhyolites of the Portage Lake Volcanics surrounded by 

olivine tholeiites initially prompted the analogy to Icelandic 

central volcanoes. High-TiO2 basalts and intermediate 

rocks are interlayered over a 0.5-km-thick interval; similar 

rocks occur again only as a few high-TiO2 basalt flows at 

the top of the Portage Lake section. In general for the 

Portage Lake section, the thickness of basalt flows is about 

10-30 m for subophitic to ophitic basalts (which are 

commonly low- TiO2 basalts) and about 5 m for melaphyric 

basalts (Paces, 1988). A group of melaphyric basalts was 

recognized in outcrop just below the Bohemia conglomerate 

at the east end of the Keweenaw Peninsula (Cornwall, 

1954a,b; 1955; Davidson and others, 1955). This group of 

melaphyric, high-TiO2 basalts pinches out east of the 

rhyolite of Montreal River but thickens to an outcrop width 

of about 0.5 km west of Mt. Houghton. This field contact 

closely corresponds to the boundary between low-TiO2 

basalts and high-TiO2 basalts inferred on geochemical 

grounds. Thus, there is at least circumstantial evidence that 

flows of intermediate composition are thinner than underlying and overlying fissure-fed basalts.

There is little field evidence for a discordance in dips 

between low-TiO2 basalts and high-TiO2 basalts, that is, 

between flows outside and within the inferred volcanic 

center. Careful examination of drill core may resolve this 

point. If little discordance exists, it may indicate that the 

present-day exposures of the Portage Lake Volcanics cut 

through a distal edge of a volcanic center, where angular 

discordance should be minimal. The lack of a high density 

of dikes and intrusive rocks also suggests that the exposed 

section is not through the core of a volcanic center. 

Hydrothermal alteration, in the form of alkali metasomatism, has affected these rocks and may reflect localized 

hydrothermal cells near shallow intrusions that are not 

exposed.

If rhyolites of the Portage Lake Volcanics do represent the distal facies of a central volcano, it is fortuitous that 

the volcano is exposed at all, given the steep dip of the 

Portage Lake Volcanics in this area. Thus, there may have 

been numerous central volcanoes that were emplaced updip 

and then eroded away, or that were emplaced farther out in 

the central rift graben and are now buried, or that were 

emplaced in the flanking basins and were covered by 

sediment during the waning stages of rifting. If several 

volcanic centers existed at different times, they could have 

produced a steady supply of felsic material, including the 

exotic rhyolite and granophyric pebbles that are observed in

conglomerates in the upper parts of the Portage Lake 

Volcanics.

The association of intermediate and rhyolitic rocks in 

many localities of the MRS suggests that detailed stratigraphic analysis of these areas may allow recognition of 

volcanic centers that have been eroded to different levels. 

The abundance of intermediate and intrusive felsic rocks in 

the Porcupine Mountains and Michipicoten Island areas 

implies that the present outcrop may expose the cores of 

volcanic centers. Likewise, intrusions of rhyolites in the 

oldest sequences, the Osier Group and the Mamainse Point 

Formation, and intrusions of granophyres in the North 

Shore Volcanic Group may have been feeders to overlying 

rhyolites that have been eroded away. Thus, it is likely that 

juxtaposition of primitive and evolved volcanic rocks 

throughout the MRS occurred in a manner analogous to the 

development of silicic central volcanoes in Iceland. Further 

documentation of stratigraphic and structural relations is 

needed to fully evaluate this hypothesis, however.

IMPLICATIONS FOR MINERAL RESOURCES

Precious- and base-metal mineralization associated 

with rhyolites occurs in several tectonic environments 

(Anderson, 1969; Hutchinson, 1973; Sillitoe, 1977), 

including extensional environments related to hotspot activity beneath continents (Sawkins, 1976, 1984, 1986). 

Although the hotspot-related Midcontinent rift system hosts 

some exceptional mineral deposits, including the world's 

largest native copper deposits (see, for example, Butler and 

Burbank, 1929; White, 1968) and very large copper-nickel 

deposits (Nicholson and others, in press), most MRSrelated mineral deposits are associated with mafic and 

sedimentary rift rocks, not with intermediate to felsic rocks. 

The only exception is the copper-molybdenum deposit in 

rhyolite-hosted breccia pipes, believed to be Keweenawan 

in age, in the Tribag area of Ontario (Blecha, 1965; 

Armbrust, 1969; Norman, 1978; Norman and Sawkins, 

1985; Richards and Spooner, 1989).

No mineralization is known to be directly associated 

with rhyolites of the Portage Lake Volcanics on Keweenaw 

Peninsula. However, an understanding of modern volcanic 

facies and cyclicity has been a useful mineral exploration 

tool in ancient volcanic terranes (Easton and Johns, 1986; 

Thurston, 1986) and can be applied to this terrane. For 

example, based on alteration mineralogy, major- and tracemetal content, and associated igneous rock types, three 

major categories of modern rhyolite-hosted epithermal 

precious-metal deposits have been reviewed by Bonham 

(1988). The first is a low-sulfur epithermal deposit characterized by adularia in the alteration assemblage, a high 

Ag/Au ratio, and varying amounts of base-metal sulfides. A 

second type of epithermal deposit is dominated by argillic 

alteration, enargite mineralization, high total sulfur, and a

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B31
spatial and temporal relationship to calc-alkalic intermediate-composition rocks. The third type is also a lowsulfur type, with a low Ag/Au ratio and a genetic relationship to alkalic rocks. The tholeiitic igneous association of 

rhyolites of the Portage Lake Volcanics and the generally 

low sulfur content of MRS rocks suggest that within 

Bonham's broad classification of epithermal deposits, only 

the first type is a possibility for rhyolites of the Portage 

Lake Volcanics.

Type I Rhyolite

Type I rhyolite bodies occur as small subaerial flows 

or domes and subvolcanic bodies within the dominantly 

basaltic Portage Lake Volcanics. These rhyolites and underlying intermediate rocks most likely represent the distal 

facies of an Icelandic-style silicic volcanic complex. If this 

interpretation is correct, it is possible that a caldera existed 

in the central part of the silicic volcanic complex. However, 

there is no direct evidence of subsidence of a caldera or of 

ring fractures in the cross section through the Portage Lake 

Volcanics that is preserved. Thus, it is assumed that the 

Type I rhyolites formed as domes and flows away from or 

independent of other major volcanic edifices.

Sillitoe and Bonham (1984) correlated the occurrence 

of specific mineral deposits with the volcanic landform with 

which the deposits are associated. Flow-dome complexes 

similar to those formed by Type I rhyolite of the Portage 

Lake Volcanics can develop in three environments: (1) near 

the summit or as parasitic domes on a stratavolcano, (2) 

along ring fractures or within the core of resurgent calderas, 

or at the eruptive sites of ignimbrite shields, and (3) in 

domes that may have formed without benefit of preexisting 

volcanic edifices (Sillitoe and Bonham, 1984). Shallowlevel mineralization accompanying flow-dome complexes 

may produce precious- and base-metal stock works, veins, 

and breccia fillings (fig. 25) as well as replacement bodies; 

thus, it usually occurs late in the evolution of the dome. 

Additionally, hot-spring-type deposits of mercury and precious metals may occur locally in and near silicic domes 

(White, 1981).

The Midcontinent rift is dominated by tholeiitic 

rocks; less abundant rhyolites are compositionally similar to 

rhyolites in bimodal suites elsewhere (table 6). Type I 

rhyolite has been shown to be derived by partial melting of 

MRS-related mafic rocks (Nicholson, 1990), but this rhyolite type occurs stratigraphically within an interval of 

high-TiO2 basaltic to intermediate-composition subaerial 

flows that are related to each other by fractional crystallization. The magmatic system in general is deficient in 

sulfur, and so models for possible mineralization must be 

consistent with low-sulfur compositions. Features that 

might indicate the presence of mineralization related to flow 

domes are not observed in the Type I rhyolite bodies. 

Despite the fact that the feeders to some extrusive rhyolite

Figure 25. Schematic cross section of a hypothetical flowdome complex, independent of other volcanic edifices 

(after Sillitoe and Bonham, 1984; Christiansen and others, 

1986). Possible sites for mineralization include the crumble breccia forming a talus apron around the dome, 

sinters if hot springs are associated with the dome, and 

veins, breccia pipes, or stockwork fractures that may 

underlie the dome. Topaz rhyolite volcanism may represent the surficial expression of mineralized porphyry systems at depth. Based on a summary by Christiansen and 

others (1986), possible sites and types of mineralization 

are keyed by number and include (1) preexisting clastic 

sediments beneath tuffs (U); (2) mineralized pyroclastic 

deposits (Be, Li, F, U, Cs); (3) hot-spring deposits within 

rhyolite (Ag, Au, W, Mn); (4) fractured and flow-banded 

lavas (Sn); (5) base- and precious-metal veins (Ag, Pb, Zn, 

Au, Sn, W); (6) vent and contact breccias (F, U); (7) 

mineralized breccia pipes (Mo, Ag, Au, F); (8) stockwork 

porphyry deposits (Mo, Sn, W); and (9) greisen-bordered 

veins in subvolcanic environment (Sn, W, Cu, Zn, Be).

flows are exposed locally, there is no evidence of adularia 

in alteration zones, stockwork vein systems, breccia pipes, 

or sinters on the Keweenaw Peninsula. In addition, gold in 

Type I rhyolites is generally below the detection limit (^6 

parts per billion; table 6). For comparison, there is no 

reported metallic mineralization associated with the active 

central volcanoes in Iceland, despite numerous hightemperature geothermal fields, some of which occur with 

silicic rocks (Sigurdsson and Torfason, 1989).

Although mineralization directly related to rhyolite 

emplacement is lacking, stratiform copper sulfide mineralization (dominantly chalcocite) has been identified in a zone 

about 30 km long and 3 km wide (fig. 26) between the 

Bohemia conglomerate and the Keweenaw fault; the mineralization is associated with the interval of intermediate 

and basaltic flows (Robertson, 1975; Michigan Copper 

Company, 1989). The sulfides occur as open-space fillings 

and replacements in amygdaloidal flow tops. Low-angle 

dacite dikes may be related to the mineralization (Robertson, 1975). The copper sulfide mineralization does not

B32 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
Table 6. Chemical analyses of rhyolites of Portage Lake Volcanics and barren and productive Cenozoic 

topaz rhyolites in the Western United States

[In parts per million, except as noted, b.d., below detection limit; empty cells, no data. Data from Christiansen and others, 1984, 

1986]

Portage Lake Volcanics (1,095 Ma)

Element

Nb

Rb

Sr

Zr

Y

Cs

Ta

Th

U

Cl

F

Cu

Zn

As

Sb

Sn

Mo

Au1

Type 1 rhyolite

41

112

55

240

56

1.2

2.9

15.1

2.5

b.d.

.01

83

43

4.4

.27

2.6

.4

b.d.

Type II rhyolite

51

465

28

144

74

2.0

4.9

63.0

13

<.0045

<.03

22

30.3

61.5

.47

5.8

<1

<6

Cenozoic topaz

Thomas Range 

(barren)

53

423

28

129

58

11.3

5.6

55

22

~600

^2,000^,000

~2

rhyolites

Spor Mountain 

(productive)

109

1,010

6

110

116

56

56

67

37

1,370

10,000

~30

'in parts per billion.

appear to be directly related to observed rhyolite units. 

Crosscutting relations and the low-temperature gangue minerals associated with the sulfides (Robertson, 1975) indicate 

that the sulfide mineralization is slightly younger than the 

native copper deposits of the area. Grade and tonnage 

estimates are about 7 million tons of 2.3 percent copper ore 

(Michigan Copper Company, 1989).

Type II Rhyolite

The incompatible trace element content of Type II 

rhyolite is consistent with that of topaz rhyolite, a subclass 

of high-silica rhyolites. Cenozoic topaz or rare-metal rhyolites in the Western United States occur in extensional 

environments in areas underlain by Precambrian crust. 

Topaz rhyolites typically are enriched in Li, Rb, Cs, U, Th, 

Nb, Ta, and Sn. These rhyolites have been linked spatially 

and genetically to several types of ore deposits: (1) volcanogenic deposits of Be, Li, U, and F such as those found at 

Spor Mountain, Utah, (2) subvolcanic breccia, porphyry, 

and greisen deposits of Mo, W, and Sn such as those found 

at the Climax deposit in Colorado, and (3) fluorite- and 

manganese-rich San Juan-type base- and precious-metal 

veins in volcanic rocks, or fluorite-rich skarn replacements 

in carbonate rocks (Burt and others, 1982). Where other 

types of mineralization, such as Hg and Au-Ag, are 

associated with topaz rhyolites, the common denominators 

may not be compositional but tectonic (thermal regime and 

(or) style of emplacement) (Christiansen and others, 1986).

Most topaz rhyolites are not associated with volcanogenie mineralization, but those topaz rhyolites that do 

contain productive deposits have common characteristics. 

These include a massive or blocky texture, few miarolitic 

cavities, 10-40 percent phenocrysts, >1.0 wt percent 

fluorine, and extreme enrichment in lithophile elements. In 

contrast, nonproductive topaz rhyolites are commonly flow 

banded and have abundant lithophysal cavities, few phenocrysts, 0.1-0.3 wt percent fluorine, and more moderate 

enrichment in lithophile elements.

The Spor Mountain beryllium deposit is the most 

important ore deposit directly associated with topaz rhyolite 

(Christiansen and others, 1986). Beryllium, uranium, and 

fluorine mineralization is contained within a rhyolitic tuff 

that underlies rhyolite flows; the mineralization is believed 

to result from vapor-phase crystallization accompanied by 

redistribution by meteoric water (Burt and Sheridan, 1981; 

Burt and others, 1982). Topaz rhyolites may also represent 

the surface expression of fluorine-rich intrusions, which, if 

there is little venting, may host porphyry molybdenumtungsten mineralization. Mineralization in the form of base 

and precious metals also could occur in fractures and veins 

emanating from the top of an intrusion, in breccia pipes, 

and in stockworks, as illustrated in a schematic cross 

section of a small rhyolite complex and its possible mineralization (fig. 25).

The small-volume Type II rhyolite near Copper City 

is most similar in composition to barren topaz rhyolites such

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B33
0

EXPLANATION 

Jacobsville Sandstone 

Freda Sandstone 

Nonesuch Shale 

Copper Harbor Conglomerate 

Portage Lake Volcanics 

Area of sulfide mineralization 

Area of native copper mineralization

I 

10 KILOMETERS

10 MILES N

-i A

Figure 26. Location of disseminated massive sulfide deposits associated with intermediate rocks, and the major native 

copper deposits, on east end of Keweenaw Peninsula (Michigan Copper Company, 1989). Dashed line represents the 

stratigraphic horizon of the Bohemia conglomerate.

as those of the Thomas Range, Utah, having only moderate 

enrichment in lithophile elements, a low fluorine content, 

and no recognized topaz (although the sample suite is small) 

compared with the productive rhyolite of Spor Mountain 

(table 6). Compared with Type I rhyelites, Type II rhyolites 

are enriched in Sn, As, U, Th, and Sb and are depleted in 

Mo, Zn, and Cu (table 6); apparent depletions may result 

from inconclusive analyses.

Many barren topaz rhyolites may have simply lost 

much of their metal content during volatile loss accompanying eruption, and this could be the case for the Type II 

rhyolite of the Portage Lake Volcanics. It is not clear from 

the field relations what the margins of the body are like and 

whether there is an associated pyroclastic phase. If the Type 

II rhyolite was extrusive, tin (and topaz) may have been 

concentrated in the upper brecciated parts of the flow as a 

result of vapor-phase crystallization (Christiansen and others, 1986).

Type II rhyolite could have been emplaced as a 

shallow intrusion. There is no field evidence for vent 

breccias or breccia pipes, although such features could be 

obscure because exposure is poor and the dip of the units 

provides only a single cross section through the body. 

Topaz rhyolites have not been recognized elsewhere in the 

rift, but there is no reason the rhyolite of Copper City 

should be the only occurrence of rhyolite enriched in 

lithophile elements. Deeper intrusions of Type II rhyolites 

could have retained more of the elements for mineralization.

CONCLUSIONS

The exposed rhyolites extruded within the central rift 

graben in central Lake Superior during the main stage of 

rifting account for less than 1 percent of the Portage Lake

B34 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
Volcanics. The rhyelites are restricted to a stratigraphic 

interval also characterized by high-TiO2 basalts and by 

intermediate rocks. Small-volume rhyolite domes near the 

top of this interval typically overlie thin pyroclastic units. 

Subvolcanic bodies were emplaced within the pile of 

basaltic and intermediate lavas beneath the extrusive rhyolite horizon. Due to lack of alteration and extensive postrift 

deformation, delicate hydration and devitrification textures 

are still preserved after 1 billion years.

The setting and field relations of intermediate and 

felsic rocks in the Portage Lake Volcanics are similar to 

those of central volcanoes in the active rift zone of Iceland. 

Similarities include a well-defined, chemically evolved 

group of basalts and intermediate rocks capped by rhyolites 

and associated with localized alteration, and burial of this 

group of rocks by more primitive fissure-erupted basalts. 

By analogy with well-described Icelandic central volcanoes, these characteristics suggest that the rhyolites of the 

Portage Lake Volcanics may represent the distal facies of a 

central volcano complex. The large proportion of silicic 

clasts in conglomerates adjacent to and overlying the 

extrusive rhyolite bodies suggests that rhyolitic centers are 

present elsewhere in the Portage Lake Volcanics or in an 

adjacent flanking basin but are not currently exposed. 

Although other workers have postulated a similarity 

between some MRS rhyolites and those of Iceland, a broad 

application of this analogy awaits detailed stratigraphic 

studies of additional rhyolite occurrences in the Midcontinent rift.

Type I rhyolite is a subalkalic rhyolite derived by 

partial melting of MRS-related rocks. Chemical analyses 

and field relations indicate that metallic mineralization 

potential related to this rhyolite type is limited. The lack of 

observed adularia in alteration assemblages and the lack of 

structural ground preparation (stockwork, veins, breccia) 

beneath the rhyolite flow domes would decrease the number 

of possible suitable sites for mineralization directly related 

to rhyolite emplacement. However, it is intriguing that 

late-stage copper sulfide mineralization underlies the extrusive rhyolite horizon and is intimately associated with the 

intermediate rocks of the Portage Lake Volcanics. It is 

possible that the rhyolitic horizon acted as a cap for 

percolating hydrothermal fluids.

Type II rhyolite is compositionally similar to some 

unmineralized Cenozoic topaz rhyolites in the Western 

United States. The small volume and geochemical characteristics of Type II rhyolite suggest that it is unlikely to 

contain mineralization. However, this rhyolite could represent the surficial expression of an intrusive system that may 

contain mineralization at depth.

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Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B39

APPENDIXES 1-3
APPENDIX 1-Sample Locations and 

Descriptions___________________

Mt. Houghton area: Lake Medora, Mich., 7.5-minute

____________quadrangle________________

85KE74: Location: NEV4SWV4 sec. 24, T. 58 N., R. 29 W. 

Description: Basalt, mottled dark-gray, mediumgrained ophitic texture.

86KE215: Location: NWV4EV4 sec. 24, T. 58 N., R. 29 W.

Description: Basalt, olive-gray to olive-black, finegrained massive ophitic texture.

88KE308: Location: NEV4SEV4 sec. 24, T. 58 N., R. 29 W. 

Description: Basalt, dusky-yellowish-brown, 

medium-grained ophitic texture with stringers of 

chlorite.

86KE230: Location: NEV4SWV4 sec. 24, T. 58 N., R. 29 W. 

Description: Basalt, olive-black, massive finegrained ophitic texture.

88KE309: Location: NEV4SEV4 sec. 24, T. 58 N., R. 29 W. 

Description: Basalt, brownish-black, mediumgrained ophitic texture.

88KE305 Location: SEV4SEV4 sec. 23, T. 58 N., R. 29 W. 

Description: Basalt, dark-gray, fine-grained, massive.

86KE218: Location: NEV4SWV4 sec. 24, T. 58 N., R. 29 W. 

Description: Basalt, overall color is dusky yellowish 

brown, but close inspection reveals fine-grained 

feldspar and pyroxene grains. Fine-grained massive 

ophitic texture.

86KE217: Location: SWV4SEV4 sec. 24, T. 58 N., R. 29 W. 

Description: Basalt, dark-gray, fine-grained massive 

texture.

88KE303: Location: SEV4SEV4 sec. 23, T. 58 N., R. 29 W. 

Description: Basalt, grayish-red, fine-grained, massive, sparse amygdules filled with carbonate minerals 

and chlorite.

88KE306: Location: SEV4SEV4 sec. 23, T. 58 N., R. 29 W. 

Description: Basalt, brownish-black, fine-grained, 

massive.

88KE304: Location: SEV4SEV4 sec. 23, T. 58 N., R. 29 W. 

Description: Basaltic andesite, dusky-yellowishbrown, fine-grained, massive.

88KE311: Location: NEV4SWV4 sec. 19, T. 58 N., R. 28 W. 

Description: Andesite, strongly banded; black bands 

(0.5-3 cm wide) containing many minute feldspar 

microlites and sparse feldspar phenocrysts are interlayered with dark-red aphanitic bands (0.1-1.5 cm 

wide) containing very sparse feldspar phenocrysts; 

small angular to subangular xenoliths also present.

88KE310: Location: NEV4SWV4 sec. 19, T. 58 N., R. 28 W. 

Description: Andesite, grayish-red, aphanitic, massive, thinly jointed.

85KE167: Location: SWV4SEV4 sec. 23, T. 58 N., R. 29 W. 

Description: Andesite, very dusky red, massive, 

fine-grained.

88KE307: Location: SWV4SEV4 sec. 23, T. 58 N., R. 29 W.

Description: Dacite, grayish-brown, fine-grained, 

massive.

85KE83: Location: NEV4SEV4 sec. 23, T. 58 N., R. 29 W. 

Description: Rhyolite, dull grayish-red aphanitic 

matrix. Massive, spherulitic texture, locally contains 

small (1-5 mm wide) vugs, typically filled with 

feldspar and (or) quartz.

85KE64: Location: NEV4SEV4 sec. 23, T. 58 N., R. 29 W. 

Description: Rhyolite, massive, dark-reddish-brown 

aphanitic matrix. Breakage occurs along joint surfaces.

85KE65: Location: NEV4SEV4 sec. 23, T. 58 N., R. 29 W. 

Description: Rhyolite, dull grayish-red aphanitic 

matrix with a few discontinuous en echelon subparallel fractures typically 1-2 mm wide, filled by 

quartz and potassium feldspar. Fractures are typically 

spaced 0.5-1 cm apart and are symmetrically zoned: 

a narrow black selvage occurs at the margin of the 

fracture, a red feldspar-rich zone occurs within the 

fracture, and the center of the fracture is filled with 

white feldspar and quartz.

86KE224: Location: NWV4SWV4 sec. 24, T. 58 N., R. 29 W. 

Description: Rhyolite, dull grayish-red aphanitic 

matrix. Massive, with mottled texture due to elongated spherulites. Dark oxides appear to be preferentially located around the spherulites. One large 

fracture is 5 mm wide and is symmetrically zoned 

with red feldspar lining the walls of the fracture and 

a mixture of red and white feldspar and quartz filling 

the center of the fracture.

86KE219: Location: NEV4SWV4 sec. 24, T. 58 N., R. 29 W. 

Description: Rhyolite, massive, moderate-red aphanitic matrix, conchoidal fracture.

85KE63: Location: NW1/4SE1/4 sec. 23, T. 58 N., R. 29 W. 

Description: Rhyolite, dull grayish-red aphanitic 

matrix with discontinuous en echelon subparallel 

fractures typically 1-2 mm wide and several centimeters long, filled by quartz and potassium feldspar. 

Fractures are typically spaced 0.3-1 cm apart and are 

symmetrically zoned: a narrow black selvage occurs 

at the margin of the fracture, a red feldspar-rich zone 

occurs within the fracture, and the center of the 

fracture is filled with feldspar and quartz. 

85KE67A: Location: NEV4SEV4 sec. 23, T. 58 N., R. 29 W. 

Description: Rhyolite, dull grayish-red aphanitic 

matrix with relict spherulitic texture. Conchoidal

______fracture.________________________

Montreal River area: Fort Wilkins, Mich., 7.5-minute quad-

_____________rangle__________________

88KE302: Location: NEV4NEV4 sec. 27, T. 58 N., R. 28 W. 

Description: Basalt, mottled grayish-red, finegrained, plagioclase and chlorite grains visible, diabasic texture.

86KE255: Location: SEV4NEV4 sec. 27, T. 58 N., R. 28 W. 

Description: Basalt, dusky-yellowish-brown, finegrained massive basalt.

B42 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
86KE261: Location: SEV4NWV4 sec. 26, T. 58 N., R. 28 W. 

Description: Basaltic andesite, very dusky red, massive, very fine grained groundmass in which a faint 

foliation is defined by changes in grain size.

86KE256: Location: SEV4NEV4 sec. 27, T. 58 N., R. 28 W. 

Description: Basaltic andesite, very dusky red, massive texture.

85KE101: Location: NWV4NEV4 sec. 27, T. 58 N., R. 28 W. 

Description: Rhyolite breccia, coarse spherulitic 

brecciated rhyolite fragments in a fine-grained, red 

aphanitic volcanic matrix with incipient spherulitic 

texture. Breccia pieces are up to 6 cm in diameter; 

few fragments are smaller than 0.5 cm. Breccia 

fragments make up at least 50 percent of the rock; the 

aphanitic rhyolitic matrix makes up the remainder.

85KE107: Location: SWV4NWV4 sec. 26, T. 58 N., R. 28 W. 

Description: Rhyolite, grayish-red massive aphanitic 

groundmass containing small sparse feldspar phenocrysts.

85KE105: Location: SWV4NWV4 sec. 26, T. 58 N., R. 28 W. 

Description: Rhyolite, dusky-red aphanitic groundmass with small sparse feldspar phenocrysts, conchoidal fracture.

85KE104: Location: SEV4NEV4 sec. 27, T. 58 N., R. 28 W. 

Description: Rhyolite, lavender, partly welded ash 

flow containing small fragments of layered rhyolite, 

elongated and flattened fiamme, and very small 

fragments of probable mafic volcanic material; massive texture.

86KE259: Location: SWV4NWV4 sec. 26, T. 58 N., R. 28 W. 

Description: Rhyolite, grayish-red, massive aphanitic groundmass containing small sparse feldspar 

phenocrysts, conchoidal fracture.

85KE108: Location: SWV4NWV4 sec. 26, T. 58 N., R. 28 W. 

Description: Rhyolite, grayish-brown aphanitic 

groundmass containing small sparse feldspar phenocrysts. Widely spaced (3-4 cm apart, 0.5 cm wide) 

discontinuous fractures are common, filled with 

quartz and feldspar. Alteration has affected the surrounding rhyolite, producing a narrow black selvage 

which encloses a bright-red zone surrounding the 

fracture.

85KE98: Location: NEV4NEV4 sec. 27, T. 58 N., R. 28 W. 

Description: Rhyolite, very dark red massive aphanitic matrix with sparse small (1-2 mm) feldspar 

phenocrysts. Local internally brecciated zones are 

filled with feldspar and quartz.

85KE106: Location: SWV4NWV4 sec. 26, T. 58 N., R. 28 W. 

Description: Rhyolite, dark-reddish-brown aphanitic 

matrix containing small sparse feldspar phenocrysts. 

Locally, small joints have been filled with carbonate 

______minerals.________________________ 

Bare Hill area: Lake Medora, Mich., 7.5-minute quadrangle 

86KE241: Location: SEV4NEV4 sec. 30, T. 58 N., R. 28 W. 

Description: Basalt, dark-gray, coarsely ophitic texture.

86KE236: Location: NWV*SWV4 sec. 29, T. 58 N., R. 28 W. 

Description: Basalt, dark-gray, coarsely ophitic texture.

86KE242: Location: SEV4NEV4 sec. 30, T. 58 N., R. 28 W. 

Description: Rhyolite, dusky-red apahanitic groundmass containing small sparse feldspar phenocrysts, 

minor alteration along secondary fractures.

86KE240: Location: SWV4NEV4 sec. 30, T. 58 N., R. 28 W. 

Description: Rhyolite, dusky-red aphanitic groundmass containing small sparse feldspar phenocrysts, 

conchoidal fracture.

85KE154: Location: SEV4SEV4 sec. 25, T. 58 N., R. 29 W. 

("western felsite")

Description: Rhyolite, grayish-red aphanitic massive 

groundmass, internally brecciated zones with minor 

carbonate infilling.

85KE173: Location: NWV4NWV4 sec. 29, T. 58 N., R. 28 W. 

Description: Rhyolite, dusky-red aphanitic groundmass containing small sparse feldspar phenocrysts, 

massive with fractures filled by feldspar and quartz.

85KE172: Location: NEV4NEV4 sec. 30, T. 58 N., R. 28 W. 

Description: Rhyolite, dark-reddish-brown aphanitic 

massive groundmass containing small sparse feldspar 

phenocrysts.

86KE244: Location: NEV4NWV4 sec. 29, T. 58 N., R. 28 W. 

Description: Rhyolite, dusky-red apahanitic groundmass containing small sparse feldspar phenocrysts; 

sample strongly fractured, breaking into small 

pieces.

85KE176: Location: NWV4NWV4 sec. 29, T. 58 N., R. 28 W. 

Description: Rhyolite, dusky-red aphanitic groundmass containing small sparse feldspar phenocrysts, 

massive.

85KE175: Location: NWV4NWV4 sec. 29, T. 58 N., R. 28 W. 

Description: Rhyolite, dusky-red aphanitic groundmass containing small sparse feldspar phenocrysts, 

massive.

85KE156: Location: SEV4NWV4 sec. 29, T. 58 N., R. 28 W. 

Description: Rhyolite, dusky-red, massive aphanitic 

groundmass containing small sparse feldspar phenocrysts, locally brecciated with minor feldspar and

______quartz infilling.____________________

West Pond area; Fort Wilkins, Mich., 7.5-minute quadrangle

85KE132: Location: NWV4SEV4 sec. 27, T. 58.N., R. 28 W. 

Description: Basalt, dark-gray, medium-grained 

ophitic texture.

85KE129: Location: NEV4SWV4 sec. 26, T. 58 N., R. 28 W. 

Description: Rhyolite, grayish-red aphanitic groundmass containing small sparse feldspar phenocrysts; 

massive texture.

85KE128: Location: NEV4SWV4 sec. 26, T. 58 N., R. 28 W. 

Description: Rhyolite, grayish-red aphanitic groundmass containing small sparse feldspar phenocrysts; 

massive texture. Local alteration along secondary 

fractures, resulting in bleaching of color.

85KE130: Location: NEV4SWV4 sec. 26, T. 58 N., R. 28 W.

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B43
Description: Rhyolite, grayish-red aphanitic groundmass containing small sparse feldspar phenocrysts; 

______massive texture.___________________ 

Keystone Point area: Fort Wilkins, Mien., 7.5-minute quad- 

____________rangle__________________ 

85KE135: Location: SWV4SEV4 sec. 30, T. 58 N., R. 27 W. 

Description: Rhyolite, dusky-red, strongly spherulitic texture with small sparse feldspar phenocrysts. 

85KE136: Location: SWV4SEV4 sec. 30, T. 58 N., R. 27 W. 

Description: Rhyolite, dusky-red, strongly spherulitic texture with small sparse feldspar phenocrysts. 

85KE137: Location: SWV4SEV4 sec. 30, T. 58 N., R. 27 W.

Description: Rhyolite, dusky-red, strongly spheru- 

______litic texture with small sparse feldspar phenocrysts. 

Copper City area: Ahmeek, Midi., 7.5-minute 

___________quadrangle__________________ 

88KE301: Location: SWV4SWV4 sec. 4. T. 56 N., R. 32 W. 

Description: Rhyolite, light-gray aphanitic groundmass containing sparse feldspar and quartz phenocrysts, massive.

85KE150: Location: SWV4SWV4 sec. 4, T. 56 N., R. 32 W. 

Description: Rhyolite, light-gray aphanitic groundmass containing feldspar and quartz phenocrysts, 

massive.

85KE147: Location: SEV4SWV4 sec. 4, T. 56. N., R. 32 W. 

Description: Rhyolite, light-gray aphanitic groundmass containing feldspar and quartz phenocrysts, 

massive.

85KE151: Location: NEV4SWV4 sec. 4, T. 56 N., R. 32 W. 

Description: Rhyolite, light-gray aphanitic groundmass containing feldspar and quartz phenocrysts, 

massive but with secondary fractures, some of which 

______have been filled by carbonate minerals._______ 

Suffolk area; Bruneau Creek, Mich., 7.5-minute quadrangle 

85KE37: Location: SWV4SWV4 sec. 10, T. 57 N., R. 31 W. 

Description: Andesite, very dusky red aphanitic 

groundmass containing abundant elongated feldspar 

phenocrysts.

85KE39: Location: SWV4SWV4 sec. 10, T. 57 N., R. 31 W. 

Description: Andesite, very dusky red aphanitic 

groundmass containing abundant elongated feldspar 

______phenocrysts.______________________ 

Mt. Gratiot area: Delaware, Mich., 7.5-minute quadrangle 

85KE49: Location: SEV4SWV4 sec. 26, T. 58 N., R. 30 W. 

Description: Andesite, grayish-red, massive aphanitic groundmass, with small concentrations of oxide 

minerals (<2 percent of specimen).

85KE45: Location: NEV4NWV4 sec. 34, T. 58 N., R. 30 W. 

Description: Dacite, grayish-red aphanitic groundmass containing small sparse feldspar phenocrysts; 

massive texture.

85KE48: Location: SWV4SWV4 sec. 26, T. 58 N., R. 30 W. 

Description: Dacite, very dusky red, fine-grained 

massive texture.

85KE50: Location: SEV4SWV4 sec. 26, T. 58 N., R. 30 W. 

Description: Dacite, grayish-red aphanitic groundmass, with small elongated vugs. Vugs are filled,

______entirely or partially, by quartz and feldspar.____

Fish Cove area: Fort Wilkins, Mich., 7.5-minute quadrangle

85KE122: Location: NEV4NWV4 sec. 35, T. 58 N., R. 28 W. 

Description: Basalt, grayish-brown, fine-grained 

groundmass containing abundant feldspar phenocrysts (up to 1 cm long). Small vugs, many of which 

are filled with chlorite.

85KE113: Location: SW'ASW'A sec. 26, T. 58 N., R. 28 W. 

("Fish Cove dike")

Description: Rhyolite, grayish-red aphanitic groundmass containing small sparse feldspar phenocrysts. 

Sample is brecciated, with minor alteration and 

carbonate minerals occurring along secondary fractures; small vugs are filled with quartz.

85KE119: Location: NEV4NWV4 sec. 35, T. 58 N., R. 28 W. 

Description: Rhyolite, grayish-red aphanitic groundmass containing small sparse feldspar phenocrysts; 

sparse small vugs are filled with quartz.

85KE117: Location: NEV4NWV4 sec. 35, T. 58 N., R. 28 W. 

Description: Rhyolite, grayish-red aphanitic groundmass containing small sparse feldspar phenocrysts; 

sparse small vugs are filled with quartz.

85KE118: Location: NEV4NWV4 sec. 35, T. 58 N., R. 28 W. 

Description: Rhyolite, grayish-red aphanitic groundmass containing sparse small to medium feldspar 

phenocrysts, most of which are altered; sparse small

______vugs are filled with quartz.______________

Mt. Bohemia area: Delaware and Lake Medora, Mich., 7.5-

____________minute quadrangle_____________

88KE313: Location: SEV4NWV4 sec. 27, T. 58 N., R. 29 W. 

Description: Basalt, dark-gray, finely ophitic, massive.

88KE312: Location: NWV4SEV4 sec. 27, T. 58 N., R. 29 W. 

Description: Basalt, dark-gray, coarsely ophitic, 

massive.

85KE51: Location: NWV4SEV4 sec. 29, T. 58 N., R. 29 W. 

Description: Basalt, dark-gray, coarsely ophitic, 

massive.

B44 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
APPENDIX 2 Major-oxide and trace-element data for 70 samples of Portage Lake Volcanics

[All major oxides and volatile elements are in weight percent. All trace elements are in parts per million except Au, which is reported in parts per billion, 

and Na and Fe, which are reported in weight percent. XRF, X-ray fluorescence; INAA, instrumental neutron activation analyses; ppm, parts per million; 

ppb, parts per billion; n.d., not determined. Totals are rounded to the nearest 0.01. USGS chemical analyses conducted by A.J. Bartel, E. Brandt, J.H. 

Christie, W.B. Crandell, M.W. Doughten, J. Evans, J. Grossman, J.S. Kane, M.G. Kavulak, B.J. Libby, J.W. Marinenko, J.S. Mee, M. Picketing, N. 

Rait, E. Robb, J. Sharkey, D.F. Siems, C.J. Skeen, H. Smith, J. Taggert, and G.A. Wandless]

Mt. Houghton

SiO2

TiO2

A12O3

Fe203

FeO

MnO

MgO

CaO

N^O

K2O

P205

Subtotal

H2O+

H2O~

CO2

Cl

F

Total

XRF analyses (ppm):

Nb

Rb

Sr

Zr

Y

Ba

Other analyses (ppm): 1

Cu

Sn

Mo

INAA (ppm):

Na (%)

Sc

Cr

Fe (%)

Co

Ni

Zn

As

Se

Sb

Cs

La

Ce

Nd

Sm

Eu

Tb

Yb

Lu

Hf

Ta

Th

U

Au (ppb)

85KE74

43.90

1.60

16.43

10.20

3.50

.22

9.40

6.20

2.20

2.30

0.27

96.22

3.5

.63

.03

.018

0.022

100.42

10

33

477

101

24

176

28

<1

<1

1.62

28.0

152

9.7

53.0

162

118

<2

<12

<.7

1.54

10.40

23.0

13.6

3.73

1.25

.58

2.08

.305

2.3

.524

.80

.36

n.d.

86KE215

44.80

2.29

15.20

10.54

5.18

.18

6.44

6.83

2.65

1.86

.25

96.22

3.08

.80

.02

<.01

.03

100.15

13

39

235

152

33

549

79

2.7

<1

2.02

32.2

100.9

11.57

49.9

116

136

<5

<2

<.6

.63

16.5

36.0

24.2

5.66

1.70

.939

3.02

.457

3.78

.814

1.64

<1

13

88KE308

45.10

2.48

14.60

8.10

7.72

.35

6.76

5.81

1.48

4.35

.27

97.02

3.3

.32

.06

n.d.

n.d.

100.70

14

53

744

153

27

269

145

n.d.

n.d.

1.14

33.0

101.9

11.13

53.8

98

138

<.9

n.d.

<.09

2.20

18.2

38.7

23.2

6.05

1.90

1.02

3.24

.477

4.24

.91

1.79

.87

<4

86KE230

45.50

1.52

16.20

7.02

5.10

.20

8.89

5.36

2.03

3.51

.16

95.49

4.40

.43

.03

<.01

<.01

100.35

<10

132

781

97

20

1300

100

<1

<1

1.21

26.4

154

8.96

55.6

185

116

3.4

<3

<.4

1.01

10.7

22.9

14.0

3.67

1.25

.609

1.83

.27

2.45

.60

1.07

<.9

14

88KE309

46.30

1.97

17.70

8.98

3.80

.18

5.66

8.04

2.40

1.77

.21

97.01

2.7

.60

.08

n.d.

n.d.

100.39

11

58

239

125

29

580

58

n.d.

n.d.

1.86

28.2

117

9.67

45.5

88

111

<.9

n.d.

<.l

1.04

15.1

33.5

17.9

4.98

1.63

.84

2.65

.404

3.41

.72

1.60

.34

<4

88KE305

47.80

2.52

14.60

6.31

7.10

.23

6.07

9.63

2.71

1.07

.28

98.32

1.9

.26

.08

n.d.

n.d.

100.56

13

35

205

192

44

278

9

n.d.

n.d.

2.05

34.7

73.5

9.86

47.2

53

110

<.9

n.d.

<.l

.45

20.6

43.2

26.6

6.38

1.89

1.09

3.89

.559

4.82

.93

2.76

.66

<10

area

86KE218

48.40

2.63

14.80

7.68

5.14

.21

6.69

4.88

4.04

.85

.46

95.78

3.71

.44

.02

.01

.04

100.00

14

24

274

198

32

190

41

<1

<1

3.15

22.4

189

9.70

45.8

156

208

<4

<3

<.5

.52

15.2

35.4

23.5

6.16

1.88

.99

2.82

.46

4.77

.81

1.26

<-9

<25

86KE217

48.90

2.49

14.90

8.19

5.13

.21

5.66

9.17

2.84

1.15

.29

98.93

1.47

.25

.16

.01

.03

100.85

17

27

272

193

51

282

110

<1

<1

2.11

34.6

73

9.68

44.6

64

116

<4

<2

<.6

.84

20.7

43.1

21

6.41

1.87

1.04

3.70

.59

4.63

.843

2.58

.40

<22

88KE303

49.00

3.31

14.00

9.91

3.50

.22

5.05

5.44

4.14

1.55

.51

96.63

2.7

.78

.19

n.d.

n.d.

100.30

,,

21

24

163

253

52

660

26

n.d.

n.d.

3.19

30.5

5.4

9.7

38.5

<50

206

2.2

n.d.

.18

.44

29.5

63.5

34.1

9.02

2.52

1.50

4.84

.693

6.43

1.50

3.56

.82

<3

88KE306

49.30

2.51

14.60

8.85

5.80

.25

4.53

6.35

3.14

2.31

.40

98.04

1.7

.24

.03

n.d.

n.d.

100.01

17

72

226

246

60

536

20

n.d.

n.d.

2.34

31.2

5.4

10.60

43.5

51

152

1.9

n.d.

.19

1.76

25.5

53.9

28.2

8.12

2.29

1.45

5.23

.731

6.11

1.23

3.85

.99

<2

88KE304

53.50

2.38

14.80

5.39

4.60

.36

3.50

4.55

3.19

3.91

.93

97.11

1.8

.57

.21

n.d.

n.d.

99.69

26

107

290

355

84

3190

22

n.d.

n.d.

2.41

23.9

<.4

7.5

18.1

<60

173

2.1

n.d.

.229

.89

46.3

98.0

53.3

12.94

3.67

1.99

6.66

.912

8.74

1.87

5.50

1.20

<9

:Cu concentrations were determined by flame atomic absorption, Sn concentrations were determined by graphite furnace atomic absorption, and Mo 

concentrations were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES).

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B45
APPENDIX 2 Major-oxide and trace-element data for 70 samples of Portage Lake Volcanics 

Continued

Mt. Houghton area Continued

SiO2

TiO2

A1203

Fe203

FeO

MnO

MgO

CaO

Nap

K2O

P205

Subtotal

H20+

H20"

C02

Cl

F

Total

XRF analyses (ppm):

Nb

Rb

Sr

Zr

Y

Ba

Other analyses (ppm): 1

Cu

Sn

Mo

INAA (ppm):

Na (%)

Sc

Cr

Fe (%)

Co

Ni

Zn

As

Se

Sb

Cs

La

Ce

Nd

Sm

Eu

Tb

Yb

Lu

Hf

Ta

Th

U

Au (ppb)

88KE311

57.60

1.50

15.30

7.68

.76

.17

2.79

3.83

4.47

2.94

0.62

97.66

2.0

.04

.06

n.d.

n.d.

99.76

28

44

98

468

94

626

<2

n.d.

n.d.

3.35

18.1

9.4

5.88

8.76

<9

115

4.6

n.d.

<.l

.2

47.2

100.2

51.6

12.90

3.33

2.09

7.25

1.017

10.84

2.11

6.80

1.84

<1

88KE310

60.10

1.39

14.60

8.20

.36

.13

2.52

2.00

5.32

2.82

.59

98.03

1.3

.12

<.01

n.d.

n.d.

99.45

31

27

77

467

88

938

19

n.d.

n.d.

4.10

16.9

4.2

6.17

9.25

<29

119

3.3

n.d.

.28

.19

43.1

99.7

50.8

13.33

3.63

2.16

7.23

1.008

11.26

2.10

6.90

1.49

<2

85KE167

61.40

1.10

15.60

5.90

1.20

.15

2.30

2.00

4.80

3.60

.37

98.42

1.7

.49

.02

.015

.084

100.73

32

60

95

620

84

919

12

2.9

<1

3.35

15.1

7.8

4.80

5.38

<120

222

2.7

<4

<.6

<1

51.8

112

50.8

12.87

3.11

2.19

7.93

1.11

13.0

2.38

7.46

1.56

n.d.

88KE307

62.00

.82

15.70

3.72

1.90

.17

1.87

4.98

4.58

1.52

.29

97.55

2.0

.17

.01

n.d.

n.d.

99.73

35

24

137

813

101

446

19

n.d.

n.d.

3.43

13.57

3.4

3.97

3.59

<7

144

2.4

n.d.

.087

.3

60.0

124.0

57.0

13.80

3.14

2.19

8.70

1.25

17.6

2.34

8.90

2.24

<2

85KE83

73.80

.09

13.40

1.70

.20

.03

.08

.05

1.20

9.70

.02

100.27

.26

.13

.06

.0050

<.008

100.73

52

200

16

196

76

1260

27

1.6

<1

.797

2.53

4.3

1.30

.34

<60

13.2

29.3

<3

.42

.31

27.6

132

35.2

11.0

1.16

1.89

7.02

.980

7.34

3.36

16.0

2.76

n.d.

85KE64

74.30

.08

13.40

1.40

.16

.02

.20

.07

2.40

7.80

.02

99.85

.40

.16

.06

0.0037

<.008

100.47

47

183

53

207

68

2290

60

3.2

<1

1.79

2.43

3.9

1.145

1.06

<50

270

2.01

<4

.24

.47

39.9

55.6

35.6

8.0

.767

1.42

7.04

.954

7.50

3.55

16.3

2.97

n.d.

85KE65

74.30

.09

13.90

1.80

.12

.02

.09

.04

3.50

6.80

.02

100.68

.41

.15

.06

0.0042

<.008

101.30

50

145

21

216

80

1070

21

1.2

<1

2.50

2.39

<4

1.276

<.7

<50

18.6

2.11

<3

.24

.28

64.1

98

51.2

10.7

.98

1.50

6.77

.939

7.22

3.37

16.4

2.59

n.d.

86KE224

74.90

.05

12.80

1.72

.06

.01

.05

.05

3.72

5.63

.03

99.02

.26

.096

<.01

<.01

<.01

99.38

49

105

35

186

62

981

13

1.3

<1

2.78

2.48

1.5

1.333

.219

<30

24.4

2.4

<1

.33

.15

34.5

76.5

30.9

7.09

.72

1.46

6.53

.899

7.02

3.49

16.0

2.50

<20

86KE219

75.20

.05

12.40

1.62

.05

.01

.05

.03

2.01

7.97

.03

99.42

.32

.07

<.01

.01

<.01

99.82

51

117

18

170

72

814

31

<1

<1

1.50

2.20

1.8

1.20

.143

<30

8.7

19.6

<2

.84

.15

15.8

52.0

15.4

5.69

.688

1.51

5.60

.797

6.23

3.14

14.1

1.98

<16

85KE63

75.90

.08

12.90

1.50

.20

.04

.06

.06

3.70

5.60

.02

100.06

.43

.11

.06

.011

<.008

100.67

41

131

24

180

61

1110

9.6

3.0

<1

2.69

2.26

2.5

1.164

<.7

<50

21.1

2.51

<3

.33

.19

58.5

144

55.4

12.4

1.13

1.77

5.28

.732

6.27

3.07

14.0

2.86

n.d.

85KE67A

78.60

.07

9.70

1.50

.08

.04

.06

.03

.72

7.30

.02

98.12

.37

.1

.08

.0060

<.008

98.67

34

101

16

167

46

967

6.6

1.2

<1

.53

1.560

2.2

1.087

<.6

<40

19.1

5.00

<6

<.3

.208

12.0

27.2

11.7

3.28

.289

.79

4.70

.64

5.39

2.43

11.4

3.61

n.d.

!Cu concentrations were determined by flame atomic absorption, Sn concentrations were determined by graphite furnace atomic absorption, and Mo 

concentrations were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES).

B46 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
APPENDIX 2 Major-oxide and trace-element data for 70 samples of Portage Lake Volcanics 

Continued

Montreal River area

SiO2

TiO2

A1203

Fe203

FeO

MnO

MgO

CaO

Nap

K2O

P2C>5

Subtotal

H2O+

H2O~

C02

Cl

F

Total

XRF analyses (ppm):

Nb

Rb

Sr

Zr

Y

Ba

Other analyses (ppm): 1

Cu

Sn

Mo

INAA (ppm):

Na (%)

Sc

Cr

Fe (%)

Co

Ni

Zn

As

Se

Sb

Cs

La

Ce

Nd

Sm

Eu

Tb

Yb

Lu

Hf

Ta

Th

U

Au (ppb)

88KE302

45.90

1.56

15.90

10.36

2.20

.16

7.72

7.10

2.60

2.65

0.16

96.31

3.2

.79

.02

n.d.

n.d.

100.32

8.0

38

1340

92

15

173

46

n.d.

n.d.

2.07

27.3

158

9.17

56.1

181

99

<1

n.d.

<.2

1.72

11.3

24.5

14.0

3.98

1.34

.629

2.26

.311

2.71

.55

1.03

.25

<4

86KE255

47.90

2.45

14.70

10.19

3.47

.17

5.49

8.61

2.78

.82

.31

96.89

1.76

.70

.03

<.01

.02

99.40

20

33

207

215

52

266

210

1.5

<1

2.11

34.2

17.1

10.6

46.0

65

126

<5

<3

<.6

.83

21.3

44.4

21.7

7.23

1.95

1.23

4.28

.67

5.24

1.07

3.04

.73

<27

86KE261

52.00

2.50

14.90

7.93

3.39

.15

4.13

5.90

3.58

1.72

1.27

97.47

2.42

.46

.02

.02

.09

100.48

26

44

250

348

72

410

6

1.9

1.4

2.75

23.9

2.6

8.82

18.1

<80

133

<5

<.3

<.5

1.27

47.9

101.9

56

13.96

3.69

2.11

6.30

.940

8.09

1.82

5.09

1.36

<20

86KE256

52.80

2.40

14.90

8.23

2.76

.37

4.22

4.42

3.99

2.48

.98

97.55

2.49

.39

.04

.01

.09

100.57

22

79

288

359

65

576

100

1.9

1.2

3.03

23.3

2.9

7.81

17.9

<80

192

<5

<2

<.5

1.25

44.7

94.1

52.6

12.77

3.47

1.98

6.22

.88

8.24

1.76

5.00

1.27

<13

85KE101

71.40

.14

14.60

2.40

.08

.05

.08

.55

3.50

7.40

.02

100.22

.17

.18

.33

.0065

.008

100.91

38

109

62

266

47

975

12

2.3

<1

2.45

2.18

2.7

1.63

.74

<50

25.1

4.2

<2

.23

.38

22.0

52.8

17.6

4.37

.516

1.04

5.00

.688

7.55

2.81

14.1

2.00

n.d.

85KE107

71.40

.13

14.00

2.20

.16

.06

.08

.67

3.00

7.40

.04

99.14

.26

.12

.45

.0090

.010

99.99

38

98

51

244

45

931

14

2.3

<1

2.05

2.20

2.4

1.62

.72

<50

25.9

4.6

<3

.24

.33

20.9

56.9

18.4

4.67

.506

1.01

4.97

.673

7.32

2.65

13.8

1.97

n.d.

85KE105

72.80

.04

14.00

2.30

.12

.04

.17

.19

3.40

6.90

.04

100.00

.26

.18

.06

.011

.009

100.52

39

101

116

253

45

1470

26

1.6

<1

2.51

2.35

8.3

1.73

1.72

<50

25.2

1.83

<3

.21

.44

15.3

64

16.1

4.53

.578

.98

5.09

.734

7.71

2.67

14.32

2.46

n.d.

85KE104

72.90

.27

12.90

2.20

.04

.04

.52

.08

.11

10.30

.02

99.38

.68

.42

.04

.0085

.031

100.56

19

116

26

255

38

999

190

<1

<1

.085

3.25

5.1

1.51

3.82

24

22.4

3.02

<2

.33

.72

8.62

90

<12

3.84

.65

1.11

3.26

.457

6.8

1.55

16.1

2.30

n.d.

86KE259

73.40

.11

13.60

1.96

.04

.02

.05

.09

2.86

7.61

.03

99.77

.23

.08

<.01

<.01

<.01

100.08

40

99

66

247

59

895

14

1.9

1.3

2.21

2.31

1.7

1.469

1.14

<30

23.4

6.2

<1

1.01

.27

47.3

97.4

35.8

7.90

1.020

1.51

5.71

.799

8.16

3.00

15.25

2.89

<16

85KE108

73.70

.13

14.10

2.20

.08

.03

.06

.10

3.20

7.20

.02

100.82

.43

.12

.07

.0050

.010

101.46

36

81

45

263

51

585

52

3.2

<1

2.29

2.22

<3

1.56

.67

<50

16

4.2

<2

<.4

.24

27.7

64

21.3

4.72

.67

1.20

5.32

.734

7.72

2.80

14.4

2.06

n.d.

85KE98

75.20

.13

12.90

2.00

.20

.04

.35

.11

2.20

7.30

.03

100.46

.51

.15

.04

.0070

<.008

101.17

34

138

75

259

49

1310

180

3.8

<1

1.552

2.82

2.9

1.483

1.08

<50

37.7

3.1

<4

.263

.61

17.1

75.8

16.0

4.53

.570

1.09

5.00

.718

7.87

2.71

14.4

2.49

n.d.

85KE106

75.40

.16

12.30

2.10

.08

.04

.57

.46

3.60

4.70

.04

99.45

.55

.20

.06

.010

.021

100.29

31

91

103

234

46

1310

84

2.5

<1

2.69

2.86

4.4

1.580

3.40

<50

52.4

6.0

<3

.14

.64

24.8

72

24.0

5.98

.850

1.24

4.98

.693

6.97

2.44

12.9

2.12

n.d.

'Cu concentrations were determined by flame atomic absorption, Sn concentrations were determined by graphite furnace atomic absorption, and Mo 

concentrations were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES).

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B47
APPENDIX 2 Major-oxide and trace-element data for 70 samples of Portage Lake Volcanics 

Continued

Bare Hill area

SiO2

TiO2

A1263

Fe2O3

FeO

MnO

MgO

CaO

Na2O

K,O

P205

Subtotal

H2O+

H2O~

CO2

Cl

F

Total

XRF analyses (ppm):

Nb

Rb

Sr

Zr

Y

Ba

Other analyses (ppm): '

Cu

Sn

Mo

INAA (ppm):

Na (%)

Sc

Cr

Fe (%)

Co

Ni

Zn

As

Se

Sb

Cs

La

Ce

Nd

Sm

Eu

Tb

Yb

Lu

Hf

Ta

Th

U

Au (ppb)

86KE241

44.60

1.77

16.20

8.69

4.23

.32

9.07

4.98

1.65

3.49

0.19

95.19

4.25

.92

.02

.01

.01

100.40

<10

58

1060

119

18

143

150

1.5

<1

1.27

28.9

186

9.40

55.6

185

113

<4

<2

<.5

1.16

13.1

27.6

17.7

4.30

1.35

.70

2.03

.348

2.84

.626

1.19

<.9

<19

86KE236

45.40

1.80

16.10

9.97

3.44

.16

7.20

8.13

3.56

.75

.19

96.70

3.01

.95

.04

<.01

.01

100.71

13

21

399

115

28

130

29

<1

<1

2.69

29.2

224

9.53

55.8

216

98

<4

<3

<.5

1.26

11.8

25.0

15.3

4.10

1.35

.68

2.10

.34

2.65

.59

1.05

<.9

<26

86KE242

72.30

.08

13.50

2.11

.06

.01

.18

.05

1.55

9.39

.03

99.26

.42

.19

<.01

<.01

<.01

99.87

45

94

27

218

62

655

24

2.9

1.5

1.13

2.48

1.50

1.62

.72

<40

44.8

2.1

<3

.232

.250

22.9

47.8

20.1

4.87

.550

1.25

6.90

.957

7.58

3.25

15.59

3.83

<15

86KE240

72.40

.08

13.40

1.96

.11

.01

.30

.07

1.68

9.08

.03

99.12

.34

.06

<.01

<.01

<.01

99.52

49

104

30

233

77

725

58

2.9

1.2

1.29

2.58

2.8

1.50

.79

<30

37.1

<1

<2

.15

.20

28.0

100.1

23.9

6.87

.828

1.67

6.98

.905

7.63

3.15

15.59

2.29

<14

85KE154

72.60

.09

12.20

1.50

.16

.06

.23

2.20

2.10

7.20

.01

98.35

.24

.13

.08

<.003

.008

98.81

38

114

41

193

52

924

20

2.7

<1

1.56

2.03

6.3

1.16

1.09

<50

45

5.9

<3

.12

.27

20.8

54.5

25.8

6.4

.82

1.12

4.90

.672

6.1

2.65

12.9

2.08

n.d.

85KE173

73.60

.10

13.70

1.70

.36

.03

.20

.06

1.40

9.00

.02

100.17

.46

.23

.03

.0055

<.008

100.90

39

109

36

218

53

1010

29

2.5

<1

1.11

2.26

<4

1.45

.54

<50

64

3.2

<2

.24

.68

70.8

48

60

11.8

1.13

1.49

6.07

.839

7.70

3.18

15.7

2.99

n.d.

85KE172

73.80

.10

13.90

2.00

.12

.03

.29

.15

3.40

6.30

.02

100.11

.35

.18

.02

.0030

<.008

100.66

47

126

72

234

56

1030

13

3.3

<1

2.62

2.49

3.1

1.49

.74

<50

61

3.9

<3

<.3

1.21

38.3

106

33

8.3

.88

1.24

6.21

.866

7.7

3.43

16.3

2.88

n.d.

86KE244

74.00

.09

13.70

1.84

.04

.01

.16

.07

4.41

5.09

.03

99.44

.29

.05

<.01

<.01

<.01

99.78

48

129

41

230

66

939

11

2.6

<1

3.34

2.50

2.1

1.368

.67

<30

38.8

<3

<2

.108

.30

49.8

108.5

44.4

9.27

.982

1.60

6.20

.881

7.43

3.18

15.4

2.46

<19

85KE176

75.00

.11

14.20

1.60

.08

.02

.16

.10

4.90

4.40

.04

100.61

.50

.17

.04

.0030

<.008

101.32

51

83

49

258

81

542

77

2.5

<1

3.45

2.50

8.7

1.101

.56

<50

22.7

<3

<2

.136

.24

66.4

137

58.4

12.7

1.31

1.86

6.06

.844

7.44

3.25

15.3

2.59

n.d.

85KE175

75.20

.11

14.00

1.70

.12

.02

.14

.12

6.00

3.00

.03

100.44

.50

.10

.03

.0030

<.008

101.07

44

61

28

236

57

438

13

2.8

<1

4.28

2.16

4.2

1.177

.54

<50

22

<2

<4

<.2

.26

35.2

59.5

28.5

6.5

.73

1.16

5.51

.737

7.39

3.16

15.1

2.12

n.d.

85KE156

75.40

.10

13.20

2.00

.12

.02

.19

.39

4.50

4.50

.01

100.43

.17

.35

.22

.0045

<.008

101.17

40

90

32

230

59

568

54

2.2

<1

3.18

1.95

6.0

1.35

.91

<40

23.1

2.4

<4

.18

.17

23.7

39.5

20.1

5.13

.60

1.17

5.66

.774

6.88

2.93

14.1

2.90

n.d.

'Cu concentrations were determined by flame atomic absorption, Sn concentrations were determined by graphite furnace atomic absorption, and Mo 

concentrations were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES).

B48 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
APPENDIX 2-Major-oxide and 

Continued

trace-element

West Pond area

SiO2

TiO2

A1203

Fe203

FeO

MnO

MgO

CaO

Na2O

K2O

P205

Subtotal

H2O+

H2CT

C02

Cl

F

Total

XRF analyses (ppm):

Nb

Rb

Sr

Zr

Y

Ba

Other analyses (ppm): 1

Cu

Sn

Mo

INAA (ppm):

Na (%)

Sc

Cr

Fe (%)

Co

Ni

Zn

As

Se

Sb

Cs

La

Ce

Nd

Sm

Eu

Tb

Yb

Lu

Hf

Ta

Th

U

Au (ppb)

85KE132

47.00

1.50

14.60

13.60

2.50

.09

5.70

6.60

2.00

4.40

0.21

98.20

2.7

.21

.85

.0055

0.041

101.97

14

35

238

107

22

340

240

<1

<1

1.46

24.8

194

9.3

37.0

180

72

<3

<9

<.5

1.09

9.57

20.3

13.9

3.76

1.06

.60

1.90

.327

2.45

.53

1.01

<.6

n.d.

85KE129

71.90

.18

14.60

2.40

.24

.04

.60

.20

3.90

5.60

.03

99.69

.65

.35

.07

.0045

.014

100.76

32

130

105

386

41

859

150

9.4

<1

2.77

3.01

3.5

1.80

1.34

<50

32

4.4

n.d.

.60

19.9

14.5

34.6

9.1

2.93

.54

.98

4.94

.725

10.2

2.54

13.6

2.61

n.d.

85KE128

73.00

.14

13.80

2.20

.12

.04

.39

.20

4.70

4.40

.03

99.02

.57

.24

.06

.010

.010

99.91

39

53

69

268

47

510

150

3.5

<1

3.42

2.33

2.7

1.59

1.94

<50

52

2.21

<5

.55

.307

16.4

87

12.2

3.36

.489

1.00

5.20

.724

8.0

2.92

15.4

2.17

n.d.

85KE130

73.80

.13

14.10

1.90

.12

.03

.30

.15

4.10

5.50

.03

100.16

.38

.23

.08

<.003

.009

100.86

39

147

108

258

48

1200

51

3.2

<1

3.03

2.35

<3

1.374

.89

<50

36.6

1.59

<2

.26

1.90

22.6

75.0

18.5

4.49

.600

1.09

5.17

.761

7.90

2.79

14.6

2.31

n.d.

data for 70 samples of Portage Lake Volcanics 

Keystone Point area

85KE135

66.50

.18

15.50

3.00

.04

.05

.72

2.30

6.60

3.00

.02

97.91

.57

.21

1.6

<.003

.023

100.31

26

65

102

193

36

711

1200

2.0

<1

4.86

1.79

7.6

2.17

2.98

<50

60

4.3

<3

.15

.38

10.9

19.7

10.0

2.42

.541

.71

3.87

.595

5.56

1.84

16.2

2.93

n.d.

85KE136

74.70

.17

13.30

1.80

.08

.03

.07

.20

4.10

4.80

.03

99.28

.23

.23

.04

.0045

<.008

99.78

24

122

122

213

38

1630

99

2.5

<1

3.02

1.65

3.1

1.31

.66

<40

3.5

4.7

<3

.28

.68

16.9

62.8

15.2

4.64

.787

.98

3.41

.494

6.12

.93

17.4

3.07

n.d.

85KE137

82.00

.16

9.60

1.60

.08

.03

.34

.19

3.10

3.00

.02

100.12

.45

.19

.02

.013

.010

100.80

21

38

49

178

31

594

140

1.6

<1

2.24

1.37

3.6

1.15

1.01

<40

12.6

3.1

<2

.26

.52

26.4

83

19.2

5.08

.72

.79

3.05

.422

5.15

1.68

14.9

2.37

n.d.

88KE301

74.30

.03

13.00

1.31

.06

<.02

.15

1.23

2.43

5.52

<.05

98.03

.72

.05

.88

n.d.

n.d.

99.68

41

421

28

139

59

460

n.d.

n.d.

n.d.

1.8

.921

3.6

.958

.177

<9

30.1

70.7

n.d.

.485

1.49

18.7

43.8

18.6

7.43

.050

1.31

6.75

.906

6.37

4.77

67.4

5.05

<6

Copper City area

85KE150

74.30

.06

13.40

1.40

.04

.04

.11

.86

2.00

6.90

.01

99.12

.60

.19

.64

.0045

.015

100.57

48

483

30

142

52

622

12

4.6

<1

1.50

.947

3.0

1.000

<.6

<40

29.3

73.2

<4

.65

1.77

14.4

49.9

20.4

6.64

.037

1.92

6.47

.934

6.35

4.59

68.8

4.54

n.d.

85KE147

74.40

.04

13.60

1.20

.04

.05

.14

1.00

3.10

5.00

.01

98.58

.55

.17

.74

<.003

.017

100.06

58

486

26

146

92

248

50

6.9

<1

2.27

.492

3.9

.797

<.4

<40

30.9

82.9

3.8

.76

3.58

9.48

26.1

17.8

7.85

.038

2.25

9.39

1.270

7.03

6.12

58.1

8.72

n.d.

85KE151

74.80

.05

12.20

1.40

.04

.04

.14

2.30

2.20

4.60

.01

97.78

.85

.25

1.7

.0030

.032

100.62

56

471

28

148

94

276

26

6.0

<1

1.60

.770

2.2

.947

<.5

<40

30.9

19.3

<5

<1

1.22

5.32

53.4

12.3

5.68

.041

1.91

8.12

1.15

5.46

4.11

57.7

35.1

n.d.

1Cu concentrations were determined by flame atomic absorption, Sn concentrations were determined by graphite furnace atomic absorption, and Mo 

concentrations were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES).

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B49
APPENDIX 2 Major-oxide and trace-element data for 70 samples of Portage Lake Volcanics 

Continued

Suffolk area

SiO2

TiO2

A1203

Fe203

FeO

MnO

MgO

CaO

Na2O

K2O

P205

Subtotal

H2O+

H2O"

CO2

Cl

F

Total

XRF analyses (ppm):

Nb

Rb

Sr

Zr

Y

Ba

Other analyses (ppm): 1

Cu

Sn

Mo

INAA (ppm):

Na (%)

Sc

Cr

Fe (%)

Co

Ni

Zn

As

Se

Sb

Cs

La

Ce

Nd

Sm

Eu

Tb

Yb

Lu

Hf

Ta

Th

U

Au (ppb)

85KE37

58.60

1.58

13.30

8.86

2.10

.12

1.43

2.75

2.84

4.67

0.56

96.81

1.50

.12

1.37

.01

0.05

99.86

32

143

109

549

82

1720

110

2.9

1.6

2.17

20.0

2.5

7.91

11.5

<70

225

8.0

<3

1.48

1.58

90.7

179

82

17.9

4.00

2.33

7.59

1.07

12.9

2.31

11.7

2.04

<12

85KE39

59.00

1.40

12.70

7.80

2.40

.15

1.60

2.80

3.30

4.00

.42

95.57

1.7

.15

1.6

.018

.082

99.12

30

100

100

552

81

1390

130

3.7

1.2

2.52

19.7

<7

7.62

11.81

<100

278

4.1

<6

.83

1.13

83.6

166

73.3

17.9

4.03

2.39

7.55

1.07

12.83

2.36

11.53

2.19

n.d.

85KE49

58.80

1.40

15.70

5.80

1.60

.20

2.80

2.00

3.80

4.40

.57

97.07

1.6

.50

.09

.019

.090

99.37

28

91

206

537

77

1320

8

2.1

1.2

2.89

17.5

2.7

5.47

6.69

<90

220

2.8

<5

<.4

.47

55.9

116.2

55

13.9

3.38

2.11

7.51

1.07

12.1

2.35

7.09

1.82

n.d.

Mt. Gratiot area

85KE45

63.70

.13

15.50

5.00

.68

.13

1.70

1.90

5.10

3.20

.18

97.22

1.6

.18

.07

0.012

.070

99.15

35

57

103

756

91

762

12

2.8

1.2

3.84

12.33

4.0

3.93

2.57

<70

108

<3

<3

<.3

.63

51.5

128

53

13.6

3.34

2.27

8.7

1.26

15.8

2.56

8.78

2.04

n.d.

85KE48

65.40

.48

15.20

3.60

1.40

.12

1.40

.81

3.60

6.00

.14

98.15

.61

.20

.08

.011

.050

99.10

31

74

72

720

86

882

16

3.8

1.0

2.78

10.46

<4

3.67

2.09

<70

122

1.6

<2

.31

.4

66.0

135

59

14.0

2.96

2.17

8.90

1.26

15.65

2.72

10.2

2.62

n.d.

85KE50

65.50

.49

15.60

5.00

.36

.06

.54

.57

4.40

6.60

.16

99.28

.43

.50

.08

.005

.044

100.34

31

67

46

684

81

858

24

2.3

<1

3.28

10.37

<6

3.83

1.50

<70

58

2.4

<2

.20

<.6

58.5

127.1

56.5

14.3

3.11

2.19

9.08

1.30

15.5

2.71

10.2

2.66

n.d.

85KE122

47.90

1.40

16.70

10.20

2.60

.34

6.60

1.70

5.00

1.90

.19

94.53

3.9

.72

.28

.006

.13

99.57

11

21

161

133

32

134

160

4.1

<1

3.75

27.0

39.6

9.61

45.2

95

288

<7

<4

.59

1.4

15.7

29.1

15

4.16

1.22

.76

2.34

.38

2.84

.64

2.65

<2

n.d.

Fish Cove area

85KE113

69.20

.16

14.40

2.60

.16

.03

.47

.32

1.50

9.20

.01

98.05

.72

.10

.17

.005

.016

99.06

43

115

35

293

64

1090

490

2.7

1.5

1.12

2.75

5.5

1.99

1.46

<50

50.0

<3

<3

.26

1.28

24.7

48.3

20.1

4.52

.75

1.41

6.59

.93

9.29

3.41

17.2

2.45

n.d.

85KE119

72.00

.13

13.90

2.00

.16

.04

.30

.27

4.40

4.80

.01

98.01

.66

.23

.03

.005

.013

98.95

39

100

70

271

48

900

390

2.0

1.8

3.38

2.34

5.5

1.579

1.07

<40

82

2.4

<2

.16

.57

36.6

97.4

24.5

5.88

.847

1.17

5.26

.718

8.09

3.01

15.4

2.36

n.d.

85KE117

72.20

.14

14.00

2.50

.12

.03

.30

.24

4.50

4.80

.01

98.84

.52

.29

.08

.005

.010

99.75

35

91

65

267

51

886

67

2.1

2.5

3.31

2.19

2.7

1.89

1.09

<40

62.8

7.9

<2

.099

1.87

33.5

78

27.0

6.74

.95

1.33

5.06

.73

8.0

2.79

14.8

1.21

n.d.

85KE118

72.20

.14

13.90

2.30

.16

.04

.33

.23

4.10

5.40

.01

98.81

.64

.23

.04

.005

.014

99.74

40

99

65

274

44

990

130

2.3

1.5

3.02

2.48

2.9

1.80

1.011

<50

73

3.1

<2

.145

.87

24.4

51.6

16.3

3.95

.555

.914

4.51

.690

8.15

2.99

15.33

1.86

n.d.

'Cu concentrations were determined by flame atomic absorption, Sn concentrations were determined by graphite furnace atomic absorption, and Mo 

concentrations were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES).

B50 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
APPENDIX 2 Major-oxide and trace-element 

data for 70 samples of Portage Lake Volcanics 

Continued

SiO2

TiO2

A1203

Fe203

FeO

MnO

MgO

CaO

N^O

K2O

P205

Subtotal

H2O +

H2O"

CO2

Cl

F

Total

XRF analyses (ppm):

Nb

Rb

Sr

Zr

Y

Ba

Other analyses (ppm): 1

Cu

Sn

Mo

INAA (ppm):

Na (%)

Sc

Cr

Fe (%)

Co

Ni

Zn

As

Se

Sb

Cs

La

Ce

Nd

Sm

Eu

Tb

Yb

Lu

Hf

Ta

Th

U

Au (ppb)

88KE313

46.10

1.42

15.40

6.31

5.30

.22

9.49

5.35

3.31

1.69

0.15

94.74

4.3

.61

.50

n.d.

n.d.

100.15

6.4

41

259

92

27

369

3

n.d.

n.d.

2.47

22.1

122

8.28

54.6

177

111

1.5

n.d.

<.l

.75

8.8

19.3

13.0

3.28

1.06

.539

1.69

.239

2.09

.501

.68

.21

<3

Mt. Bohemia area

88KE312

46.10

1.67

16.20

8.55

4.00

.19

8.09

5.83

2.53

3.05

.18

96.39

3.3

.17

.16

n.d.

n.d.

100.02

7.7

213

127

282

43

2590

9

n.d.

n.d.

1.93

26.9

177

9.35

56.8

198

109

2.9

n.d.

<.01

6.56

12.6

27.3

16.7

4.19

1.34

.694

2.29

.329

2.74

.613

1.22

1.00

<6

85KE51

46.40

1.60

16.40

5.20

6.50

.18

7.60

8.90

2.60

.94

.17

96.49

2.2

.54

.34

.014

.036

99.62

15

26

310

118

27

218

71

<1

<1

1.97

27.7

226

9.05

53.1

220

101

<4

<3

<.6

1.04

11.9

24.7

15.6

4.14

1.30

.71

2.12

.306

2.63

.63

.89

<1

n.d.

!Cu concentrations were determined by flame atomic absorption, Sn 

concentrations were determined by graphite furnace atomic absorption, 

and Mo concentrations were determined by inductively coupled plasmaatomic emission spectrometry (ICP-AES).

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B51
APPENDIX 3 Major-oxide and CIPW normative compositions of Portage Lake Volcanics

[Major-oxide compositions from appendix 2 recalculated on an anhydrous basis after correcting Fe2O3/FeO ratios of all rocks except basalts, according to 

Middlemost (1989). Basalt analyses were corrected using Fe2O3/FeO=0.15 (after Brooks,1976). Mg #=Mg/(Fe+Mg+Mn) (mole percent)]

Mt. Houghton area

SiO2

TiO2

A1203

Fe2O3

FeO

MnO

MgO

CaO

Na2O

K2O

P205

Mg#

CIPW norms:

Q

Or

Ab

An

Ne

C

Di

Wo

En

Fs

Wo

Hy

En

Fs

01

Fo

Fa

Mt

11

Ap

85KE74

46.05

1.68

17.20

1.77

11.70

.23

9.86

6.50

2.31

2.41

.28

.60

14.26

17.29

29.41

1.21

.86

.44

.24

.18

30.60

17.09

13.51

2.57

3.19

.61

86KE215

46.98

2.40

15.94

2.05

13.53

.19

6.75

7.16

2.78

1.95

.26

.47

11.54

32.51

25.22

7.18

3.61

1.66

1.92

.75

.35

.40

23.70

10.41

13.29

2.97

4.56

.57

88KE308

47.04

2.59

15.23

2.02

13.30

.37

7.05

6.06

1.54

4.54

.28

.49

26.86

12.23

21.18

.42

5.87

2.95

1.40

1.52

24.97

11.36

13.61

2.93

4.92

.61

86KE230

47.92

1.60

17.06

1.61

10.59

.21

9.36

5.65

2.14

3.70

.17

.61

21.89

16.30

25.96

.97

.80

.41

.23

.16

28.34

16.22

12.12

2.33

3.04

.37

88KE309

48.10

2.05

18.39

1.65

10.86

.19

5.88

8.35

2.49

1.84

.22

.49

10.88

21.05

33.52

5.39

2.71

1.30

1.37

7.94

3.86

4.08

14.46

6.68

7.78

2.39

3.90

.48

88KE305

49.17

2.59

15.02

1.75

11.57

.24

6.24

9.80

2.23

1.10

.29

.49

6.51

18.85

27.69

15.78

7.96

3.87

3.95

21.21

10.49

10.72

1.86

.87

.99

2.54

4.92

.63

86KE218

50.85

2.76

15.55

1.69

11.15

.22

7.03

5.13

4.24

.89

.48

.53

5.27

35.86

20.75

1.32

.67

.35

.30

17.61

9.53

8.08

10.44

5.39

5.05

2.45

5.25

1.05

86KE217

49.75

2.53

15.16

1.70

11.20

.21

5.76

9.33

2.89

1.17

.30

.48

6.92

24.43

24.91

16.13

8.12

3.87

4.13

13.84

6.70

7.14

5.83

2.68

3.15

2.47

4.81

.66

88KE303

51.15

3.45

14.61

1.73

11.41

.23

5.27

5.68

4.32

1.62

.53

.45

9.59

36.53

15.67

7.53

3.79

1.80

1.94

11.36

5.46

5.90

9.09

4.15

4.94

2.51

6.56

1.16

88KE306

50.65

2.58

15.00

1.89

12.45

.26

4.65

6.52

3.23

2.37

.41

.40

14.02

27.31

19.40

8.64

4.30

1.73

2.61

14.25

5.69

8.56

7.84

2.95

4.89

2.74

4.90

.90

88KE304

55.26

2.46

15.29

2.60

7.42

.37

3.61

4.70

3.29

4.04

.96

.47

4.68

23.92

27.84

14.99

1.73

.88

.45

.40

16.28

8.59

7.69

3.77

4.68

2.10

88KE311

59.31

1.54

15.75

2.37

5.76

.18

2.87

3.94

4.60

3.03

.64

.47

7.59

17.94

38.91

13.35

1.69

.86

.44

.39

12.75

6.74

6.01

3.44

2.93

1.40

88KE310

61.66

1.43

14.98

2.74

5.48

.13

2.59

2.05

5.46

2.89

.61

.46

9.76

17.10

46.17

6.20

.59

12.15

6.48

5.67

3.98

2.72

1.33

B52 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
APPENDIX 3 Major-oxide and CIPW normative compositions of Portage Lake Volcanics 

Continued

Mt.Houghton area Continued

SiO2

TiO2

A12O3

Fe203

FeO

MnO

MgO

CaO

Na2O

K2O

P205

Mg#

CIPW norms:

Q

Or

Ab

An

Ne

C

Di

Wo

En

Fs

Wo

Hy

En

Fs

Ol

Fo

Fa

Mt

11

Ap

85KE167

62.62

1.12

15.91

2.29

4.58

.15

2.35

2.04

4.90

3.67

.38

.48

11.05

21.72

41.43

7.65

1.06

10.82

5.88

4.94

3.32

2.13

.83

88KE307

63.70

.84

16.13

1.62

3.93

.17

1.92

5.12

4.71

1.56

.30

.47

16.32

9.23

39.82

18.24

4.33

2.18

1.07

1.07

7.46

3.73

3.73

2.35

1.60

.66

85KE83

73.68

.09

13.38

.59

1.19

.03

.08

.05

1.20

9.68

.02

.11

28.74

57.26

10.14

.12

.86

1.80

.20

1.60

.86

.17

.04

85KE64

74.48

.08

13.43

.49

.98

.02

.20

.07

2.41

7.82

.02

.27

29.88

46.34

20.41

.22

.90

1.35

.05

1.30

.71

.15

.04

85KE65

73.89

.09

13.82

.60

1.19

.02

.09

.04

3.48

6.76

.02

.12

26.55

39.85

29.32

.07

.73

2.40

.22

2.17

.87

.17

.04

86KE224

75.67

.05

12.93

.56

1.12

.02

.10

.05

3.76

5.69

.05

.14

31.20

33.66

31.78

.57

1.80

.25

1.55

.81

.10

.08

86KE219

75.66

.05

12.48

.56

1.05

.02

.10

.03

2.02

8.02

.05

.15

32.42

47.45

17.08

.45

1.70

.25

1.45

.75

.10

.05

85KE63

75.93

.08

12.90

.57

1.07

.04

.06

.06

3.70

5.60

.02

.09

32.18

33.11

31.26

.17

.67

1.58

.15

1.43

.83

.15

.04

85KE67A

80.19

.07

9.90

.50

1.01

.04

.06

.03

.73

7.45

.02

.10

46.69

44.07

6.17

.02

.61

1.55

.15

1.40

.73

.13

.04

88KE302

48.10

1.63

16.66

1.61

10.62

.17

8.09

7.44

2.72

2.78

.17

.58

16.45

18.00

25.00

2.71

8.89

4.52

2.45

1.91

23.16

12.45

10.71

2.34

3.10

.37

Montreal River area

86KE255

49.34

2.52

15.14

1.88

12.40

.18

5.65

8.87

2.86

.84

.32

.45

4.97

24.18

25.97

13.32

6.68

2.98

3.66

19.06

8.55

10.51

4.29

1.82

2.47

2.73

4.79

.70

86KE262

53.63

2.58

15.37

2.90

8.25

.15

4.26

6.09

3.69

1.77

1.31

.48

6.04

10.49

31.24

20.14

1.30

.66

.35

.29

18.80

10.32

8.48

4.21

4.91

2.86

86KE256

54.43

2.47

15.36

2.80

7.97

.38

4.35

4.56

4.11

2.56

1.01

.49

3.29

15.16

34.78

15.88

.16

.08

.04

.03

19.76

10.85

8.91

4.07

4.70

2.21

85KE101

71.36

.14

14.59

.77

1.54

.05

.08

.55

3.50

7.40

.02

.08

20.61

43.77

29.58

2.19

.36

.17

.02

.17

2.06

.18

1.88

1.12

.27

.04

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B53
APPENDIX 3 Major-oxide and CIPW normative compositions of Portage Lake Volcanics 

Continued

Montreal River area Continued

Si02

TiO2

A1203

Fe203

FeO

MnO

MgO

CaO

Na2O

K2O

P205

Mg#

CIPW norms:

Q

Or

Ab

An

Ne

C

Di

Wo

En

Fs

Wo

Hy

En

Fs

Ol

Fo

Fa

Mt

11

Ap

85KE107

72.21

.13

14.10

.74

1.49

.06

.08

.67

3.02

7.45

.04

.09

23.83

44.07

25.53

2.86

.18

.09

.01

.09

2.12

.19

1.93

1.07

.25

.09

85KE105

72.85

.12

14.01

.75

1.51

.04

.17

.19

3.40

6.91

.04

.17

25.18

40.88

28.74

.68

.67

2.45

.43

2.03

1.09

.23

.09

85KE104

73.47

.27

13.00

.70

1.40

.04

.52

.08

.11

10.38

.02

.40

31.45

61.40

.93

.27

1.46

2.92

1.30

1.62

1.02

.51

.04

86KE259

73.62

.11

13.64

.62

1.25

.02

.10

.09

2.87

7.63

.05

.13

26.79

45.13

24.26

.12

.59

1.89

.25

1.64

.90

.21

.11

85KE108

73.21

.13

14.01

.70

1.41

.03

.06

.10

3.18

7.15

.02

.07

26.27

42.29

26.88

.37

.89

2.00

.15

1.85

1.02

.25

.04

85KE98

74.95

.13

12.86

.69

1.38

.04

.35

.11

2.19

7.28

.03

.31

32.85

43.06

18.51

.35

1.23

2.70

.87

1.82

1.00

.25

.07

85KE106

75.93

.16

12.39

.68

1.37

.04

.57

.46

3.63

4.73

.04

.43

34.21

27.98

30.68

2.02

.54

3.19

1.43

1.76

.99

.30

.09

86KE241

47.20

1.87

17.14

1.70

11.23

.34

9.60

5.27

1.75

3.69

.20

.60

21.83

14.79

24.87

1.14

4.35

2.56

1.79

26.57

15.01

11.55

2.47

3.55

.44

86KE236

47.35

1.88

16.79

1.73

11.40

.17

7.51

8.48

3.71

.78

.20

.54

4.61

25.46

26.83

3.20

11.51

5.83

3.00

2.67

21.88

11.04

10.83

2.51

3.57

.44

Bare Hill area

86KE242

72.92

.08

13.62

.68

1.37

.02

.18

.05

1.56

9.47

.05

.19

26.48

56.02

13.19

.78

2.31

.45

1.86

.99

.15

.08

86KE240

73.12

.08

13.53

.65

1.30

.02

.30

.07

1.70

9.17

.05

.29

26.88

54.25

14.37

.02

.78

2.50

.75

1.75

.94

.15

.11

85KE154

73.89

.09

12.42

.53

1.06

.06

.23

2.24

2.14

7.33

.01

.28

29.44

43.36

18.09

2.58

4.00

1.96

.58

1.47

1.58

.77

.17

.02

85KE173

73.55

.10

13.69

.65

1.30

.03

.20

.06

1.40

8.99

.02

.22

29.83

53.18

11.83

.17

1.57

2.24

.50

1.74

.94

.19

.04

B54 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
APPENDIX 3 Major-oxide and CIPW normative compositions of Portage Lake Volcanics 

Continued

Bare Hill area Continued

SiO2

TiO2

A1203

Fe203

FeO

MnO

MgO

CaO

Na2O

K2O

P205

Mg#

CIPW norms:

Q

Or

Ab

An

Ne

C

Di

Wo

En

Fs

Wo

Hy

En

Fsl.77

Ol

Fo

Fa

Mt

11

Ap

85KE172

73.82

.10

13.90

.66

1.32

.03

.29

.15

3.40

6.30

.02

.28

28.45

37.27

28.74

.61

1.25

2.49

.73

.96

.19

.04

86KE244

74.42

.09

13.78

.59

1.17

.02

.16

.17

4.43

5.12

.05

.20

27.92

30.29

37.44

.52

.75

1.95

.40

1.55

.86

.17

.11

85KE176

74.63

.11

14.13

.52

1.04

.02

.16

.10

4.88

4.38

.04

.21

28.57

25.91

41.24

.24

1.26

1.73

.40

1.33

.75

.21

.09

85KE175

74.97

.11

13.96

.56

1.12

.02

.14

.12

5.98

2.99

.03

.18

27.76

17.69

50.54

.40

.73

1.80

.35

1.45

.81

.21

.07

85KE156

75.18

.10

13.16

.66

1.32

.02

.19

.39

4.49

4.49

.01

.20

30.02

26.56

37.94

1.87

.21

2.22

.47

1.75

.96

.19

.02

85KE132

49.84

1.59

15.48

1.70

11.23

.10

6.04

7.00

2.12

4.67

.22

.49

27.63

14.62

18.88

1.79

11.98

6.03

2.82

3.13

19.12

8.60

10.53

2.47

3.02

.48

West Pond area

85KE129

72.24

.18

14.67

.83

1.66

.04

.60

.20

3.92

5.63

.03

.39

25.70

33.30

33.13

.80

1.82

3.64

1.50

2.14

1.20

.34

.07

85KE128

73.83

.14

13.96

.73

1.47

.04

.39

.20

4.75

4.45

.03

.32

27.40

26.32

40.15

.80

1.02

2.91

.98

1.94

1.06

.27

.07

85KE130

73.78

.13

14.10

.63

1.26

.03

.30

.15

4.10

5.50

.03

.30

27.48

32.53

34.65

.55

1.19

2.38

.75

1.63

.91

.25

.07

Keystone Point

85KE135

68.06

.18

15.86

.97

1.93

.05

.74

2.35

6.76

3.07

.02

.40

11.47

18.16

57.14

3.84

6.55

3.21

1.36

1.86

1.16

.49

.67

1.41

.34

.04

85KE136

75.33

.17

13.41

.59

1.18

.03

.07

.20

4.13

4.84

.03

.10

31.71

28.63

34.91

.80

1.07

1.63

.18

1.45

.86

.32

.07

area

85KE137

81.99

.16

9.60

.52

1.05

.03

.34

.19

3.10

3.00

.02

.37

51.06

17.75

26.20

.81

.95

2.14

.85

1.29

.75

.30

.04

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B55
APPENDIX 3 Major-oxide and CIPW normative compositions of Portage Lake Volcanics 

Continued

Copper City area

Si02

TiO2

A1203

Fe203

FeO

MnO

MgO

CaO

Na2O

K2O

P205

Mg#

CIPW norms:

Q

Or

Ab

An

Ne

C

Di

Wo

En

Fs

Wo

Hy

En

Fs

Ol

Fo

Fa

Mt

11

Ap

88KE301

75.83

.03

13.27

.44

.87

.01

.15

1.26

2.48

5.63

.03

.24

36.48

33.30

20.96

6.06

.86

1.58

.38

1.20

.64

.06

.07

85KE150

75.03

.06

13.53

.45

.91

.04

.11

.87

2.02

6.97

.01

.18

34.03

41.23

17.07

4.25

1.08

1.55

.28

1.27

.65

.11

.02

85KE147

75.53

.04

13.81

.39

.78

.05

.14

1.02

3.15

5.08

.01

.24

34.90

30.05

26.62

5.00

1.28

1.49

.35

1.14

.57

.08

.02

85KE151

76.57

.05

12.49

.46

.92

.04

.14

2.35

2.25

4.71

.01

.22

40.00

27.86

19.02

10.02

1.36

.66

.15

.55

.95

.20

.75

.67

.10

.02

Suffolk area

85KE37

60.90

1.64

13.82

3.14

7.64

.12

1.49

2.86

2.95

4.85

.58

.26

14.49

28.71

24.95

10.10

.27

.13

.04

.10

12.53

3.69

8.84

4.56

3.12

1.27

85KE39

62.30

1.48

13.41

3.28

6.57

.16

1.69

2.96

3.48

4.22

.44

.31

15.76

24.98

29.43

8.47

2.83

1.40

.53

.90

10.00

3.70

6.30

4.76

2.81

.96

85KE49

60.81

1.45

16.24

2.12

5.15

.21

2.90

2.07

3.93

4.55

.59

.50

9.86

26.93

33.23

6.43

2.48

12.95

7.25

5.69

3.08

2.76

1.29

Mt. Gratiot area

85KE45

65.32

.70

15.89

1.85

3.71

.13

1.74

1.95

5.23

3.28

.18

.46

14.13

19.41

44.22

8.51

.61

8.72

4.35

4.36

2.68

1.33

.39

85KE48

66.77

.49

15.52

1.63

3.27

.12

1.43

.83

3.68

6.13

.14

.44

16.55

36.26

31.10

3.21

1.64

7.64

3.58

4.07

2.36

.93

.31

85KE50

66.20

.50

15.77

1.69

3.39

.06

.55

.58

4.45

6.67

.16

.22

11.33

39.45

37.61

1.84

.54

5.48

1.37

4.11

2.45

.95

.35

Fish Cove area

85KE122

51.14

1.49

17.83

1.68

11.07

.36

7.05

1.81

5.34

2.03

.20

.53

12.01

44.85

7.68

.16

4.02

25.27

12.34

13.23

2.44

2.83

.44

85KE113

70.70

.16

14.71

.88

1.76

.03

.48

.33

1.53

9.40

.01

.33

23.38

55.61

12.93

1.57

1.42

3.49

1.20

2.29

1.28

.30

.02

85KE119

73.56

.13

14.20

.69

1.38

.04

.31

.28

4.50

4.90

.01

.28

26.76

29.04

38.03

1.32

.98

2.60

.77

1.82

1.00

.25

.02

B56 Contributions to the Geology and Mineral Resources of the Midcontinent Rift System
APPENDIX 3-Major-oxide and CIPW r^rmative compositions of Portage Lake Volcanics 

Continued

Fish Cove areaContinued

SiO2

TiO2

A1203

Fe203

FeO

MnO

MgO

CaO

Na2O

K2O

P205

Mg#

CIPW norms:

Q

Or

Ab

An

Ne

C

Di

Wo

En

Fs

Wo

Hy

En

Fs

01

Fo

Fa

Mt

11

Ap

85KE117

73.10

.14

14.17

.83

1.66

.03

.30

.24

4.56

4.96

.01

.25

25.70

29.34

38.54

1.13

.87

2.93

.75

2.18

1.20

.27

.02

85KE118

73.18

.14

14.09

.78

1.56

.04

.33

.23

4.16

5.47

.01

.28

26.18

32.36

35.16

1.08

.92

2.89

.83

2.06

1.13

.27

.02

Mt.

88KE313

48.91

1.51

16.34

1.55

10.25

.23

10.07

5.68

3.51

1.79

.16

.64

10.59

28.11

23.51

.85

2.97

1.52

.90

.55

28.51

17.00

11.51

2.25

2.87

.35

Bohemia area

88KE312

48.17

1.75

16.93

1.63

10.76

.20

8.45

6.09

2.64

3.19

.19

.58

18.87

19.28

24.88

1.65

3.38

1.72

.95

.71

25.84

14.13

11.71

2.36

3.33

.41

85KE51

48.38

1.66

17.03

1.55

10.21

.19

7.89

9.24

2.70

.98

.18

.58

5.80

22.82

31.41

10.88

5.54

3.03

2.31

6.52

3.70

2.82

16.77

9.10

7.67

2.25

3.15

.39

Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan B57



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