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THE PETROLOGY OF A PORTION OF THE MELLEN
GABBRO COMPIEX AND ITS RELATIONSHIP TO‘

THE M‘J'EENAWAN BASALTS AND THE. TYLER FORMATION

BY

JGIN H. PUFFER

A THESIS

SUBMITTED TO THE SCHOOL OF SCIENCE AND ARTS OF

MICHIGAN STATE UNIVERSITY IN PARTIAL FUIFILIMENT

(IF THE REQUIRE-ENTS FOR THE DEGREE O‘F

MASTER OF SC IENCE

DEPARTMENT OF GEOLOGY
196LL

ABSTRACT

Immediately northeast of the town of Mallen,'Wisconsin there
occurs a basic intrusive mass that separates the Tyler Formation from
the Keweenawan Basalts. However, toward the east, the intrusion cuts
into the Keweenawan Basalts. Field investigation combined with a
petrographic analysis indicates that the intrusion has greatly
affected the basalts through contact metamorphism whereas the effects
on the Tyler Formation are less obvious. The intrusive itself is

differentiated and becomes more acidic toward the north.

AC KNO NLE DGI‘J'LE NT 8

I wish to express my sincere thanks to Dr. Justin Zinn, Dr. Harold
Stonehouse, and Dr. James Trow for their assistance and encouragement

in the development of this thesis.

My thanks are also offered to Mr. James Olmstead whose advice and

counsel has proven to be very helpful.

TABLE OF CONTENTS

Introduction
Purpose of Investigation
Method of Investigation
Review of the Literature
General Geology
Archeozoic
Huronian
Keweenawan
The Tyler Formation
Megascopic Description

Microscopic Ibscription

Environment of Deposition

Metamorphism
Structure

Dip

Faulting
The Lower Keweenawan
The Keweenawan Basalts

Unaltered Basalts

Fbta-Basalts

Southern Meta-Basalt Mass
Albite—Epidote-Hornfels Facies

Hornblende hornfels-Pyroxene hornsfels facies

Northern Basalt

Diabase Dikes

Page Number

1

13' Lo

O\O\O\

l2
l2
13
2O
21
22

22

TABLE OF CONTENTS (continued)

The Mellen Gabbro Intrusive
The Zones of Differentiation
Chill Zone
Gabbro Zone
Diorite—Cranodiorite Zone
Granite Differentiate
The Kellen Gabbro-Mellen Granite Contact
Conclusion
Suggestions for Further Study

Bibliography

Page Numjer
39
LL].
bl
b8
58
60
63
6h
66

1.

2.

10.

11.

12.

13.

1h.

16.
17.

18.

19.

LIST OF FIGURES

The town of Mellon, Wisconsin

A glacial drift bank along the Bad River

Tyler Formation outcrop displaying wavy bedding
Photomicrograph of 26-9h showing quartzitic texture

Photomicrograph of 26-58 illustrating rounding of quartz
grains

Photomicrograph of 32-2 showing chlorite-biotite inter-
growth in quartzite

Photcmicrograph of 26-58 illustrating a typical plagio-
clase grain in quartzite

Photomicrograph of 32~22 illustrating limonite cement

Photomicrograph of 32-21 illustrating yellow unknown in
plain light

Photomicrograph of 32-21 under crossed niccols

Photomicrograph of 26-82 illustrating magnetite rim around
a pegmatoid in the metaébasalt

Photomicrograph of 26-83 illustrating hornfels texture

Photomicrograph of 26—92 illustrating penninite in the
meta—basalt

Photcmicrograph of 26-7L illustrating a poikolitic horn-
blende metacryst in the meta—basalt

Photomicrograph of 22-15 illustrating a calcite—chalcedony
amygdu le

Gabbro-diabase contact
Xenolith of gabbro within diabase

Photcmicrograph of 32-60 showing a poikilitic augite
crystal containing olivine and plagioclase

Photomicrograph of reaction rim around olivine grain in

32-5

Page NUmber
2

2

15

17

l?

18

18

19

19

3b
3b

LIST CF FIGURES (continued)

)age Number

20. Photomicrograph of 32-66 illustrating magnetite skeleton h?
21. "Black Granite" quarry hoist at Loon Lake 50
22. Drill operator at work at Loon Lake quarry SO
23. Quarry wall illustrating granodiorite intruded into gabbro 51
2h. Mellen Granite outcrop containing gabbro and diabase

xenoliths 51
2S. Photomicrograph of 27-15 illustrating a crushed plagioclese

crystal in the gabbro Sh
26. Photomicrograph of 32-61 illustrating pigeonite-augite

exsolution in the gabbro 5h
27. Equilibrium diagram of pyroxenes after Hess (l9hl) 55

28. Photomicrograph of 32-17 illustrating hypersthene exsolving
augite 55

l.

2.

3.

LIST OF TABLES

Pre-Cambrian Stratigraphy of the Gogebic District

Mineral Percentages

Mineral Percentages
Meta-Basalts

Mineral Percentages

Mineral Percentages

Mineral Percentages
Granite Zone

in the

in the

in the

in the

in the

Tyler Formation

Keweenawan Basalts and

Chill Zone

Gabbro Zone

Diorite-Granodiorite-

Page Number

11

21a

to

56

INTRODUCTION

It was stated as early as 1889 by R. V. Irving, that "For its
variety of geological problems, structural, mineralogical, petrogenic,
and metamorphic, there is probably no area comparable to the environs
of Mellen. It would be an excellent location for a school of field
geology". After studying the "environs of Mellen", I must whole-
heartedly agree with Irving.

I have found the town of Mellen, Wisconsin to be very picturesque
(see figure 1) and friendly. The greater part of Mellen is located
in the upper half of section 6 (T. hlLN. - R. 2w.) The town itself
marks the extreme southwestern portion of the investigated area
which includes sections 20-23, 26-30, 31 and 32 and the northern
portion of section 33 and 3h of T. hSN. - R. 2W. in Ashland County,
Wisconsin.

The area was chosen immediately after consulting the map published
by Aldrich in 1929. Further limitations were made on the basis of
a more recent map composed by the Bear Creek Company. Despite the
fact that the area had been previously mapped, I felt that I would
require a first hand observation of the field relationships, an accurate
sampling of the rock types, and a relatively high degree of map detail

in order to fulfill the purposes of my investigation.

PURPOSE OF INVESTIGATION

The chief purposes of my investigation consist of gathering factual

information relative to three questions not adequately answered by

 

 

 

1300118311

The town of Mellen W

 

iillll ll.

 

A glacial drift bank along the Bad

Figure 2

previous investigations:
(1) What are the effects of a basic intrusive on a (l) subgray-
wacke and (2) basalt?
(2) What lithologic variations or differentiations occur within
the basic intrusive?
(3) What is the structural history of the area and the origin

of the rock types that make up the areafi
METHOD OF INVESTIGATION

Pace and compass mapping, using aerial photographs and 7.5
minute topographic maps, was conducted during the summer of l96h.
A sample of each outcrop was taken and its location plotted on the
field map. Structural information was observed and also plotted
wherever possible. Thin sections of 82 representative samples
were then made and analyzed. "Point counts" to determine mineral
percentages were made on the majority of sections. The pyroxenes
and olivine were then determined with the 5 axis universal stage
on the basis of 2V measurements. The plagioclases were identified
by using the Michel Levy method and extinction angles of combined earls-
bad-albite twins. Samples which appeared to contain potash feldspar
were stained so as to readily distinguish it from untwinned plagioclase,
quartz and others. The stain that was quite successfully used was
sodium cobaltinitrite using techniques developed by S. Rosenblum
(1956), and F. Chayes (1952). Oil immersion techniques were

employed to identify questionable mineral grains.

REVIEW OF THE LITERATURE

In 1892, Irving and Van Hise made one of the first studies of
what was then known as the Penokie iron bearing series. A detailed
description of the Huronian formations supported by thin section
observation was carried out with emphasis on the iron formation.

A large part of the Keweenawan was also mapped, but no attempt was
made to define the contacts of the Mellen Gabbro. However, in 1911
Van Hise and Leith recognized the gabbro and correlated it with the
Duluth gabbro which they observed was similar in every respect.

Grout in 1918 suggested that the gabbro in Wisconsin may be an
extension of the Duluth gabbro. Hotchkiss in 1919 described the
“Ironwood Iron Formation in detail and recognized the Pabst member

at the base of the Tyler Formation suggesting that it represented

a Middle - Upper Huronian unconformity. In 1923 he developed the theory
that the Lake Superior Syncline was formed by subsidence during and
immediately following the emptying of the basalt magma chamber which
supplied the flows and intrusions. He also suggested that the

gabbro intruded along a western extension of the Keweenawan thrust
fault. In 1929, Aldrich redescribed in detail the Huronian sediments
of the Gogebic district. He mapped the Huronian and Keweenawan
formations and included a magnetic dip needle analysis and structural
analysis of the area. In 1935, Leith, Lund, and Leith discussed the

stratigraphic relationships of the Gogebic area with the entire

Lake Superior region. In l95h, M. W. Leighton described the
petrogenesis of the westernmost exposure of the Mellen gabbro ~
granophyre complex using petrographic and chemical data. In

1956, the Bear Creek Mining Company remapped the area including the
Keweenawan intrusives in the Gogebic Area. Aeromagnetic data was
also obtained. At the present time (l96h) extensive petrologic
work is being conducted by James Olmsted on the portion of the
Mellen gabbro complex located between the portion studied by
Leighton and the granite mass that separates his area from the

map area of this report. In addition, this same granite mass is

being presently studied by M. Katsman.

GENERAL GEOLOGY

mammals

 

The oldest formation found in the western Gogebic area is
greenstone schist. The schist has been described by Irving and
Van Hise (1892) as being quite finely laminated, dark colored and
fine grained. The schist is composed chiefly of feldspar, horn-
blende, chlorite, and quartz, but the composition is quite variable.
According to Irving and Van Hise, the greenstone is easily dis-
tinguished from the coarse grained granites and gneisses that
occur west of the Bad River Valley. The granites and granite
gneisses, also unconformably underlying the Huronian, are
generally thought to be later than the greenschists. However,
Within the eastern Gogebic area, granite has been found intrusive
into the Huronian. It may therefore be concluded that at least
some of the granite in the Gogebic area may be post-Huronian. In
addition, a large mass of granite north west of Mellen has been
found to cut Keweenawan rock and has been classified by Leith -

Lund and Leith (1936) as of Killarney age.

HURONIAN

After the orogeny that separated Archeozoic from Proterozoic
time Lower Huronian deposition began with the deposition of Bad
River dolomite in the western Gogebic area. The formation is very
thin and is found only intermittently at the base of the Huronian
series in Wisconsin. Aldrich (1929) has reported a thickness of

100 feet at the Penokee Gap where it grades from a lower cherty

dolomitic phase to an upper dolomitic cherty phase. The individual
beds range from 1 to 2 inches in thickness.

Unconformably above the Bad River dolomite lies the Palms
formation. The formation grades from a conglomerate at the base to
quartz - feldspathic slate to a quartzite at the top. The conglomerate
facies is 3-10 feet thick; the slate facies is about 390 feet thick;
and the quartzite facies is about 50-60 feet thick.

Conformably overlying the Palms formation lies the Ironwood
iron formation. Aldrich has reported that the formation is
composed of roughly 65 percent quartz and 35 percent iron materials.
The formation has been divided into zones of alternating ferruginous
cherts, and cherty carbonates. The ferruginous chert beds are
thick and wavy and have an oolitic texture. The cherty carbonate
beds are thin and straight averaging an eighth of an inch in
thickness. The thickness of the Ironwood in Wisconsin is about
650 feet.

Although there is considerable uncertainty, the Tyler Formation
probably overlies the Ironwood unconformably. Although there is no
divergence in dip or strike at the contact, a thin conglomerate
which was named the Pabst member of the Tyler Formation by Hotchkiss
in 1919 separates the two formations. According to Leith, Lund, and
Leith (1936), the Pabst conglomerate is similar in every respect to
the basal conglomerate of the Copps slate of the eastern Gogebic

range. Since the Tyler formation is known to be equivalent to the

Copps formation which has a basal conglomerate that unconformably
bevels the underlying iron formation, it may be concluded that the
Pabst and the Copps conglomerate have the same unconformable re-
lationship.

In the same article Leith, Lund, and Leith correlated the
Tyler Formation with the Michigamme slate, the Copps slate, the

Virginia slate, and the Rove slate of the Lake Superior area.

The Tyler Formation grades from a thin bedded, almost black
graywacke at the base through a more typical graywacke phase to
a subgraywacke and finally to a quartzite at the top of the forma-
tion. Its thickness is about 10,000 feet. The majority of the
formation is poorly bedded. It is also a relatively soft formation

and is poorly exposed.

KEWEENAWAN

A large time gap separates the Huronian Tyler Formation from
the Keweenawan during which time a large amount of Tyler Formation
was eroded away. The erosion however, was not uniform. Toward the
east at Wakefield, Michigan, the overlying Keweenawan basalts bevel
the Tyler Formation and are in contact with the Ironwood Formation.
At the west end of the Tyler Formation, the entire Huronian is
missing and is overlain by the Keweenawan igneous complex. One
reason that the Tyler Formation was not uniformly eroded may be the
presence of a resistent quartzite facies at the top of the portion

of the formation which has escaped great erosion. In the map area

of this report, where the formation is thickest, such a quartzite
facies has been found to exist.

The first Keweenawan event to occur was the deposition of a thin
quartz conglomerate and sandstone formation now existing as a pink
quartzite. This lower Keweenawan deposition may correlate with the
Puckwunge Formation of the Grand Portage area.

Following the deposition of the lower Keweenawan a thick series
of flows were extruded. The great majority of the flows are basalt
however, felsite flow layers are also included in the Keweenawan.
The individual flows vary from a few feet to over 100 feet thick.
According to Aldrich (1929), the lava pile probably exceeded 20,000
feet in total thickness. Following the extrusion of the flows, a
series of Upper Keweenawan sediments starting with the Great Con-
glomerate was deposited. This conglomerate is composed largely of
felsite pebbles. The deposition of the conglomerate was then
followed by a continuation of extrusive activity known as the Lake
Shore Traps. This final extrusion was followed by the deposition
of about 800-1200 feet or more of conglomerate known as the Outer
Conglomerate. Upper Keweenawan deposition continued with the
formation of the Nonesuch Shale which is about 120-350 feet thick.
The Nonesuch Shale is overlain by 12,000 feet of Freda Sandstone
which is in turn overlain by 200 feet of Eileen Sandstone. Deposition
finally ended.with 5,000 feet of Amnicon shale and arkose and an
upper group of about 7,000 feet of sandstone.

Before the close of the Keweenawan a large amount of intrusive

10

activity had also taken place. The result of the intrusive activity
was the implacement of the Mellen gabbro - granophyre complex, a

portion of which occurs in the map area of this report.

ll

PRE CAMBRIAN STRATIGRAPHY
OF

THE GOGEB IC DISTRICT

 

 

TABLE 1
Series Gogebic
(Unconformity)
Killarney Granite Intrusives

 

(Killarney Urogeny)

Basic Intrusives

. Sandstone, Shale. and
Keweenawan Conglomerate

' Basic flows

Quartzite and Conglomerate

 

 

(Unconformity)

(upper) Tyler Formation

 

 

(Unconformity) ?

Huronian (middle) Ironwood Iron-Formation
Palms Quartzite

 

(Unconformity)

 

(lower) Bad River Dolomite
Sunday Lake Quartzitew

 

 

 

t (Unconformity)

Laurentian Granites and Gneisses

 

 

Kewatin Greenstone

 

* occurs in eastern Gogebic District at Wakefield, Michigan but
was not deposited in Wisconsin

12

THE TYLER FORMATION

The upper Tyler Formation ranges from a recrystalized subgray-
wacke to a quartzite. The central part consists of thick bedded
recrystalized graywacke and subgraywacke with the lower portion a
darker, thin bedded graywacke. From east to west there is also a
variation. westwardly, according to earlier literature, grain size,
magnetite, hematite, biotite, and feldspar content increases with
a corresponding decrease in chlorite and carbonate content.

The choice of the term "slate" in reference to the Tyler
Formation is an unfortunate one and could cause confusion. Van Hise
(1892), stated that "It is not meant to imply by the term 'slate'
that any of the rocks have a slate cleavage". The cleavage when
visible, rather than being at an angle to the bedding, is parallel
to the bedding, unlike a slate. However, the term continues in
the literature with the qualification that it is a graywacke slate.

The major part of the Tyler Formation is located in a valley
bordered to the south by the range of hills formed by the Ironwood
Iron Formation and to the north by the Keweenawan igneous mass.

The upper Tyler formation is eXposed quite well along the northern

slopes of this valley just north of Mentreal Creek.

PEGASCOPIC DESCRIPTIQE

The quartzite of the Tyler formation is usually completely
massive in hand specimens and breaks with conchoidal fracture. In
outcrops, however, a definite but sometimes poorly defined cleavage

along the bedding planes occurs. All the samples examined were fine

.Il If

13

grained and gray to light gray. The gray color in hand specimens is
misleading to the extent that it tends to indicate a "dirtier" rock
than is actually the case when examined in thin section. The majority
of the samples are fresh, impermeable and completely structureless
except for a faint bedding in some cases. The poorly defined bedding
is quite variable in thickness. Definite crossébedding was observed
on three samples. Figure 3 illustrates an exposure with a wavy type

of bedding often found in the upper Tyler Formation.

MICROSCOEIC DESCRIPTION

 

The mineralogy of the upper Tyler Formation is surprisingly
simple in the sense that only a relatively few minerals occur in

it. (See table 2).

QUARTZ: The average grain size of the quartz is .l99mm. This places
the size on the wentworth scale as fine sand.

The extreme development of authigenic growth and recrystallization
(figure D) has in most cases obliterated original grain boundaries.
However, in section 26-51 and 32-21, authigenic growth is less well
developed allowing observation of original grain shape. In both cases
it was observed that the grains were well rounded to spherical.

(See figure 5).

In each quartzite thin section only fresh, inclusinn free quartz

was observed, with minor exceptions. Strain shadows are also almost

completely absent. The grains are almost all recrystallized with

1h

interlocking sutured boundaries. In the sub-graywacke, most of the
quartz is also fresh and free from inclusion. However, inclusions
were quite abundant in some of the grains and many of the grains are

fractured and strained.

BIOTITE: In most cases, the biotite occurs as unaltered flakes
which are poorly or entirely unoriented. The flakes are large,
fresh, and extremely pleochroic crystals with a few inclusions of
zircon. In some cases there has been retrograde metamorphic
alteration to chlorite. (See figure 6). In a few of the samples

examined the biotite occurs as small faintly oriented grains.

PLAGIOCLASE: Albite was found in every Tyler Formation thin section
examined. 'With few exceptions the crystals are fresh, unaltered,
anhedral and twinned. (See figure 7). The grain size of the plagio-
clase roughly corresponds with the average quartz grain size . In
some cases, the albite contains inner cores of a more calcic plagio-
class, which may be explained as authigenic growth around a calcic
nucleus. The occurence of unaltered plagioclase has been reported
by Van Hise to exist throughout the entire Tyler Formation. Staining

has indicated the absence of orthoclase feldspar.

OPAQUES: Hematite, titaniferous magnetite, limonite and pyrite are
the chief opaque minerals in the Tyler Formation. The relative
amount of each, however, was not determined, as all were grouped
together in the point count analyses. The red rim around some of

the thinner crystals was used as an easy identification for hematite.

\I“ (ll. l.

 

15

 

. - ——_———

 

Figure 3 Tyler Formation displaying wavy
bedding taken from sectign 6,0utcrop #1.

 

NOV ' 64

 

Figure h Photomicrograph of 26—9h showing
quartzitic texture. 80 power, crossed niccols.

16

In several thin sections, limonite occurs as crack fillings and
interstitial cement around quartz grains. The cement is limited

to grains near the cracks which have served as a source. (See figure 8).

MUSCOVITE: Muscovite most typically occurs as fresh, colorless
crystals corresponding in size in each section to the biotite and
in some cases, the muscovite flakes are even a little larger than

the biotite.

CHLORITE: Chlorite occurs as a retrograde product of biotite. (See
figure 6). MCst commonly it occurs as pale olive greenish gray
flakes that occur in zones separated from similar biotite zones.

The flakes contain opaque dust and zircon. The chlorite strongly
resembles chloritoid except that the birefringence and index is a

little too low.

YELLOW UNKNOWN: A systematic investigation of all mineral possibili-
ties with reference to optical properties has failed to identify a
certain yellow mineral found in almost half of the Tyler thin
sections studied. (See figure 9,10). Its properties include:

(1) pale to bright yellow color

(2) lack of any noticable pleochroism

(3) extremely low birefringence with a maximum

(h) an index of refraction of about 1,653

(5) a length fast orientation

(6) parallel extinction

(7) very poor cleavage

l7

\

 

. .1.-

NO! 7°__64-

Figure 5 Photomicrograph of 26-58 illustrating
rounding of quartz grains, crossed niccols

 

_Figure 6 Photomicrograph of 32-2 showing chlorite-
biotite intergrowth, plain light

 

o; uov -. 64

-—l _, __ ._, _— _

Figure 7 Photomicrograph of 26-58, illustrating
a typical plagioclase grain, crossed niccols
X 80 diameters

 

I
7—i— 7* —7 — ._.__4

 

Figure 8 Photomicrpgraph of 32-22 illustrating limonite
cement in plain light X 60 diameters

_._._.

19

 

uov ‘. 64

— _._ - 17"— i , fl,-— ‘____

 

Figure 9 Photomicrograph of 32-21 illustrating
yellow unknown (gray) plain light X 80 diameters

 

NOV 0 64

 

Figure 10 Same as fig. 9 under crossed niccols
illustrating low birefringence of yellow
unknown

20

Its index of refraction is not exact because of the difficulty
of distinguishing it from grains of biotite normal to the c axis.
Under the universal stage, the birefringence proved to be too low to
permit an accurate determination of 2V or optic sign.

It is quite possibly a surface weathering effect due to the
introduction of chemicals other than those involved in original
deposition and lithification. The mineral may therefore have
formed as a rare clay mineral or alteration product of biotite
from solutions that have invaded the intersticies of the original
grains. Its occurence is largely confined to samples that were
taken at the surface whereas samples that were taken more than

6 inches below the surface are free from the unknown mineral.

ENVIRONMENT OF DEPOSITION

 

The well developed rounding and sphericity and the large
percentage of quartz content of the upper Tyler quartzite facies
indicates a high degree of maturity. The presence of cross-
bedding is an indication of shallow water deposition. These, plus
other indications, such as the fact that the upper Tyler is well
sorted, indicate that the upper Tyler was deposited in a.paralic
environment.

The transition from a graywacke at the base of the Tyler to
quartzite at the top indicates that Tyler deposition occured in

water that became progressively shallower near the last stages.

21

mTAMORPH ISM

 

Despite the fact that many of the samples of Tyler formation
were taken within 100 feet of the Mellen Gabbro intrusive contact,
I have observed nothing in any of the hand specimens or thin sections
that could definitely be called a contact metamorphic effect. The
usual contact metamorphic minerals such as andalusite, cordierite,
etc. are conspicuously absent. The probable reason for this is
the resistance of quartzite to metamorphism. This resistance may
be due to the low mafic content of the quartzites (see table 2).
A larger amount of mafic material is needed to supply the elements
for many contact metamorphic minerals. Another reason is that
quartzite recrystallization causes impermeability and rigidity, thus
reducing later solution or stress effects. This second reason would
however, necessitate recrystallization of the Tyler Formation before
intrusion of the gabbro sill. Whether the quartzite actually was
recrystallized before the intrusion is uncertain. However, during
the Huronian-Keweenawan time gap, this recrystallization could
have occured.

Another factor to be considered is the relatively dry nature
of the Mellen gabbro. Gabbro in general which does not contain
as much water and other volatiles as granite has usually been
found to have less extensive metamorphic effects than granite.
Grout (1933) has observed a much thinner contact metamorphic zone

for gabbro than for granite in the Duluth area.

Outcrop number

Section

A)
ox
I
\o
4:—

6-1
26-58
27-13
26-072
27-81
32-21
33-9h
32-65
32-22
31-2
33-83
33-103

Average

21a

MINERAL PERCENTAGES IN THE

TYLER FORMATION

Tflfle2 E

x (D

“N a» E a a

-H +9 m :> r! ea
N ”’- 8 '9 :3 a g a 8 a
'fi 5 E ea c :3 h o o .H «a E

' -P 0 0* o 0 r1 +3 km
2 as a a :2 : ... w :3 a
5’ w m r: o o N a a o. a

79 .250 12 7 2 - - - - T -

(2) an .125 - T 2 2 - - - 2 -

89 .100 10 - T - T - - l T

78 .075 10 l 2 - T l - 8 T

(1) Lower Keweenawan quartzite

(2) Xenolith of Tyler Formation found within the gabbro

Hornblende

22

Therefore, although a quartzite should not be expected to
show abundant contact metamorphic effects, a subgraywacke such
as the subgraywacke facies of the upper Tyler Formation, should
theoretically yield the same contact metamorphic minerals that
form in argillaceous rocks. The reason that the subgraywacke facies
has also evidently escaped contact metamorphism is that it has
probably been sufficiently shielded from the intrusion by the over-
lying quartzite facies.

Below the subgraywacke in the graywacke portion of the Tyler,
Van Hise and Irving have found garnet which indicates some kind of
metamorphism. However, near the gabbro contact garnet has not been

found.

STRUCTURE
‘DIE: The average of dip measurements taken along the upper Tyler is
76 degrees north. This figure agrees with what might be expected in
this part of the Lake Superior Syncline. The trough of the syncline
is located about 25 miles north of Mellen. The steepness of the dip
is an indication of the extent to which the development of the
syncline has effected the area. Hotchkiss has suggested that this
syncline was formed as the result of the gradual collapsing of an
underlying magma chamber. The collapsing took place during the
extrusion of the Keweenawan flows and continued during the deposition
of the overlying Keweenawan sediments. Evidence for this theory

includes a gradual fanning of northward dips from 80 degrees at the

23

base of Keweenawan flows to 30 degrees at the base of the overlying
Keweenawan sediments. However, although this explanation has been
widely accepted, it remains highly theoretical.
FAULTING: Aldrich has mapped several cross-faults passing through
the Huronian and Keweenawan. Two such cross-faults were thought
by him to exist in the map area of this report. The effect of
both faults according to Aldrich, has been the southward displace-
ment of a 1% mile wide fault block. However, on the basis of the
outcrop data that I have accumulated in order to draw the northern
Tyler Formation contact, such displacement would be impossible.
The western fault was drawn by Aldrich on the basis of several
hundred feet of alleged displacement, located in the south west
corner of section 32. The eastern fault was drawn through the
northwest corner of section 33 where offsetting displacement is
said by Aldrich to occur, although he has also stated in l92h,
that (p.227) "the actual faulting has nowhere been seen". The
existence of the eastern displacement would place the northern
Tyler Formation contact within section 28, however, the existence
of several Keweenawan exposures south of section 28 indicates that
the contact is actually located south of the northern edge of
section 33.

The northward extension of both faults has supposedly isolated
a rhomboid area of anorthosite. But, on the basis of my own field
work, it has become apparent that the block of anorthosite actually

grades gradually into a more typical type of gabbro.

THE LOWER H‘hEEIxIAWAN

After an interval of unmeasured duration that separates the
Huronian from the Keweenawan, a thin sequence of Lower Keweenawan
sediment was deposited. The formation is very distinctive, but
in the map area of this report it is exposed only in a small area
in the central part of section 26. The base contains a bed of
conglomerate about 10 feet thick. The matrix of the conglomerate
consists of dark gray, medium grained, silica cemented uartzite
which contains % to 1 cm well rounded milky quartz pebbles. Above
this, the formation changes to a medium grained, pinkish gray
quartzite that is well bedded. At the top just beneath the basalt
flows the formation changes to a light pink, well bedded quartzite.
In thin section, a sample of the pink quartzite proved to be 9h
percent quartz, h percent muscovite, and 2 percent chlorite plus
traces of zircon and magnetite. Sodium cobaltnitrite staining
indicated that the pink color is not due to orthoclase content as
was previously suggested by Aldrich (1929 p. 217). Instead the
pink color may be caused by an impurity in the quartz or the quartz
cement.

Locally the lower Keweenawan possesses the same strike and
dip as the upper Tyler Formation. In Michigan,however, the
Huronian-Keweenawan unconformity is much more obvious and clearly
indicates that upper Huronian-lower Keweenawan deposition was not

continuous.

25

THE KEWEENAWAN BASALTS

The basalt occurs as flow layers which vary greatly in thick-
ness from a few feet to over 100 feet. The basalts are generally
thought of as being the product of large fissure eruptions somewhere
in the interior of the Lake Superior basin. Evidence to show that
the lava came from the north has not been found in the map area
of this thesis, but elsewhere I have observed pipe amygdules bent
toward the south, and flow layers which thicken down dip.

In general, the basalts may be classified as of a typical
tholeiitic type. The petrographic characteristics of a tholeiitic
basalt such as little or no olivine and a prevalence of pigeonite
pyroxene are both shown by the basalt in the map area. (See table 3).

The field occurence characteristics of tholeiitic flood basalts
are also met by the Keweenawan lavas of Lake Superior in general.
According to Turner and Verhoogen (1960), tholeiite basalts are
usually:

(1) disected with contemporaneously injected swarms
of diabase dikes and sills

(2) of enormous volume and extent

(3) are characteristically found in a nonorogenic
continental environment

(h) take the form of thin flows

(5) are rarely associated with tuff or explosive products.

An important implication of a tholeiitic classification is that
chemically there is a tendency for tholeiitic basalts to contain more

SiOQ and less Na20, K20, and MgO than typical alkaline olivine basalts.

26

UNALTERED BASALTS

 

As a norm for comparing the metamorphism of the basalt near

the gabbro contact, a relatively fresh and only slightly altered
xposure of basalt was found in section 1h approximately 3000 feet
from the gabbro contact. The only alteration that has occurred

is a relatively slight alteration of the plagioclase into paragonite
and the pyroxene into chlorite. The pyroxene is apparently pigeonite
on the basis of 2V measurements of 10 degrees and 12 degrees. The
pyroxene occurs as interstitial material between laths of plagioclase
in a well developed diabasic texture. Occasional phenocrysts of
plagioclase occur in the basalt that are of the same composition

as the rest of the plagioclase and this appears to be labradorite

(An 63-65). Euhedral crystals of magnetite occur as the chief
accessory. A single small crystal of orthoclase was found in thin
section lh-2. The rock is fine grained with the average length of
the plagioclase laths averaging .3mm however, grain size is variable

and ranges from fine grained to very fine grained in some samples.

META-BASALTS

 

Unlike the Tyler Formation which shows relatively little contact
metamorphic effects, the basalts near the Mellen Gabbro intrusion
have been strongly contact metamorphosed. The area that best
illustrates these effects occurs south of the gabbro intrusion in
section 23 and 26. The series of basalt flows in section 23 and 26
is no thicker than 1100 feet at its thickest portion and pinches out

completely somewhere in section 27. The zone of high ranked

2?

metamorphism is of course much thinner, being not over 500 feet
thick on the basis of the outcrop data.
Southern Meta-Basalt Mass: Three contact metamorphic facies are
represented in the metaébasalt near the southern contact of the
gabbro intrusion. These are from the remote edge of the metamorphic
aurola to the contact:

1. The Albite-epidote-hornfels facies.

2. The Hornblende-hornfels facies.

3. The Pyroxene hornfels facies.
In many ways the sequence resembles the metavolcanics of the Peavy
Pond area as described by R. W. Bailey (1959), and the metabasalts
in contact with the Duluth gabbro as described by G. S. Schwartz (192h).
Albite-Epidotengrnfels facies: The basalts farthest away from the
intrusion are typically dark gray,medium fine grained basalts some
of which show amygdules. Also observed are small pegmatoids composed
of coarse grained white plagioclase, pink orthoclase and quartz, and
rimmed with hornblende. Often these blebs are stretched out in a
linear fashion, or flattened. Their origin is uncertain, however,
they may be a variety of segregation structure, such as described
by W. Q. Kennedy. (1933). Local concentrations of water may have
been responsible for their formation. They are easily noticed,
quite common and very distinctive. Under the microscope (figure 11),
it can be seen that during their growth magnetite grains have been
"pushed" away and segregated along the edge of blebs.

Thin sections indicate that all the Keweenawan flows located

28

south of or beneath the gabbro intrusion in the map area have under-
gone at least some degree of alteration. In the outermost contact
metamorphic aureole, alteration consisted of recrystallization of
an otherwise diabasic texture. Most of the pyroxene in the rocks
of this outer aureole was converted to chlorite and uralite. The
relic pyroxene is a colorless pigeonite. As the hornblende-hornfels
facies is approached, the original pyroxene becomes completely
uralized and chloritized, and albite appears as small rounded crystals
that are often untwinned and are always free from inclusions. The
remaining plagioclase is badly altered and occurs as long corroded
laths which are clouded with an opaque dust. The most obvious change,
'however, is the development of the characteristic hornfels texture
(See figure 12). Two varieties of amphiboles are present. The most
common variety is the typical pleochroic brown variety. However in
addition, a variety is also present that is pleochroic in pale green
to yellow and possesses a smaller extinction angle. This second
variety is evidently a member of the trenolite-actinolite group and
because it is probably secondary after pigeonite, it may be called
uralite. Usually the two varieties of amphibole do not occur in the
same thin section. Another difference between the two varieties of
amphibole is that the pale green variety has a more feathery or
fibrous habit whereas the brown hornblende has a typical prismatic
cleavage.

Two varieties of chlorite are also present. Penninite (see
figure 13) occurs in thin sections 26~82, 26-83, and 26-92 (see

table 3) as pale green clusters of flakes. A more typical variety

29

of chlorite also occured in a few of the samples as very small
bright grass green pleochroic flakes. This second variety of
chlorite however, occured in only very small amounts.

The basis upon which the samples were classified as albite-
epidote hornfels was the presence of relic diabasic texture, relic
pigeonite, altered plagioclase, and the presence of epidote,
clorite and recrystallized albite.

Hornblende hornfels-Pyroxene hornsfels facies: In hand specimens
taken near the intrusive sill, the more strongly metamorphosed

samples lack pegmatoids and amygdules. Another difference is that

the hornblende hornfels rocks possess distinctive euhedral meta-
crysts of hornblende, about 3/h cm across. These metacrysts typically
weather out in relief and often occur in great abundance.

In thin section, the hornblende hornfels and pyroxene hornfels
differ from the less strongly metamorphosed basalts in many respects.
The texture is completely hornfelsic except for where the large
extremely poikolitic metacrysts of hornblende occur. (See figure 1h).
In some of the thin sections such as 23-h, the majority of the
hornblende present occurs as poikolitic metacrysts. The plagioclase
is unclouded and rather than occuring as albite, it occurs largely
as andesine and labradorite. The plagioclase occurs as small short
stubby laths that are evenly distributed among hornblende crystals
that have the same shape and size. The average size of the crystals
varies greatly from sample to sample (see table 3) however, in each

individual thin section, the grain size of the various component

3O

minerals is quite uniform.

Hornblende makes up a large part of the composition of the
rocks of the hornblende hornfels facies. It occurs largely as
small rounded individual crystals but also frequently occurs as
large poikolitic metacrysts. The hornblende metacrysts are rarely
free from inclusions and often inclusions are so numerous that the
same crystal may be separated into several parts. However, when
the hornblende metacrysts are relatively free from inclusions,
they possess an euhedral shape. The hornblende is pleochroic from
brown to olive green to grass green and is easily recognized by
its perfect amphibole cleavage, small extinction angle, and length~
slow orientation. In each case the hornblende is evenly distributed
throughout the rock.

Diopside which makes its first appearance in the rocks of the
hornblende hornfels facies has a similar occurance to that of the
hornblende which also occurs both as individual rounded crystals
and large anhedral to euhedral poikioblastic metacrysts. In the
majority of the thin sections, pyroxene occurs in smaller amounts
than the accompanying hornblende. However, in 2 thin sections,
27-75 and 26-90, it greatly exceeds the hornblende content and in
thin section 27-12, it occurs without any accompanying hornblende.
The samples in which pyroxene is the dominant mafic mineral, plagioclase
increases in An content to the labradorite stage which is the same
plagioclase composition found in the chill zone of the gabbro

intrusive.

31

Several crystals have been found in the hornfels which appear
to be one half hornblende and one half diopside. This observation
infers either one of two explanations. Either the diopside is
replacing the hornblende which has in turn altered from the original
pigeonite, or the hornblende is replacing the diopside. The first
interpretation would imply an intermediate hornblende stage for the
diopside that is present. Although we know that it is possible
through metamorphism to convert pyroxene into amphibole and amphibole
into pyroxene, an alternative possibility may be a direct metamorphic
conversion of one pyroxene into another which in this case would
involve pigeonite and diopside. If this second explanation is
accepted, then the occurence of hornblende in the combined horn-
blende-diopside crystals may be explained as a retrograde meta-
morphic product of diopside.

Magnetite together with a small amount of ilmenite are the
only opaque minerals that were observed. 'With few exceptions,
the magnetite occurs as a small anhedral grains varying in size
from .050mm to .125mm. In a few cases however, large poikiolitic
magnetite crystals were observed in which case biotite was found
to be associated with the magnetite. The magnetite content
averages three percent. However, in the case of two xenoliths
of metabasalt that were found within the basic intrusive (samples
27-79 and 28-h table 3) the magnetite content was extremely high

amounting to over 12 percent. In each case, the magnetite grains

32

occur as small evenly distributed crystals. The source for
most of the magnetite was evidently the basic magma however,
the reason that so much magnetite was attracted to the basalt
is uncertain.

Ilmenite and bright red rutile were often present in trace
amounts in several of the hornfels indicating a relatively high
titanium content. Apatite was also found to be present in trace
amounts in several of the samples (see table 3). In one sample,
quartz was also observed in small amounts and in another sample,
orthoclase was observed. The presence of these two minerals is
probably the result of small relic pegmatoids or amygdules in

the hornfels.

NORTHERN BASALT

 

North of the intrusive sill in Section 22, there occurs a
group of basalt outcrops. A characteristic of the samples taken
from these outcrops is that they are full of calcite amygdules.
Most of the calcite has been dissolved by ground water, leaving
cavities. The calcite is pink or white and is easily confused in
hand specimen with orthoclase phenocrists that are also found in
the basalt. The calcite amygdules are surrounded with chalcedony
which in many cases make up the entire armgdule. (See figure 15)
The chalcedony shows well developed radiated or cockade structure.

Chlorite occurs as very fine flakes throughout the ground-
mass in large amounts as a secondary mineral after pyroxene which

has been largely replaced. The plagioclase laths are clouded with

33

a fine opaque powder and shows some alteration to paragonite.
The plagioclase appears to be andesine (An 25 - An hh), although
clouding and alteration of the feldspar makes the determination
uncertain. However, the original diabasic texture of the basalt
has been preserved quite well.

The metamorphism of the northern basalts should have been even
more extensive than the basalts to the south of the intrusion because
of the added presence of volatiles associated with the granite
differentiate. However, the lack of appreciable observed metamorphism
above the sill may be explained by the probability that the rock
examined is a little too far from the intrusion to have been strongly
recrystallized. If in the future the northern contact is studied
it may be of considerable interest to compare the metamorphic effects
at the northern contact with the southern contact. A zenolith was
found in the diorite zone of the basic intrusive, however in thin
section it was comparable in every respect to the hornblende

hornfels of the southern metabasalts.

 

3h

 

NOV ° 64

 

Figure 11 Photomicrograph of 26-82 illustra-
ting magnetite rim around a coarse grained
bleb indicating post magmatic growth, plain
light x 60 diameters.

 

| nov- 64

7—- “a“--wfire —~— —-——— -— -

Figure 12 Photomicrograph of 26-83 illustra-
ting hornfels texture, crossed niccols x 60
diameters.

 

35

 

NOV 0 64

Figure 13 Photomicrograph of 26-92 illustra-
ting penninite in plain light x 80 diameters.

 

NOV 0 64

Figure 1h Photomicrograph of 26-7h illustra-
ting a single poikolitic hornblende crystal
(dark gray) altering into pyroxene (light gray).
Plagioclase is white. Plain light x 80 diameters.

36

 

Figure 15 Photomicrograph of 22-15 illustra-
ting calcite-chalcedony amygdule, crossed
niccols X 60 diameters.

 

Figure 16 Gabbro-diabase contact taken
in section 31 outcrop 6.

Section
Outcrop

22-15
22-18

30-h0

26-Sh
26-82
26-8 3
26-91

26-92

23-10
23—5

26—76
26-78
26-1

26-7h
26-79
26—90
27-12

27-79

QR-h

Plagioclase

62

SO
67

5h

53
hl
6h
81
36

76
50
57

58
S7
69
67
75

72

Maximum.% An

65

36
35

38
hO
b0
38
32

37
50

68
5h
51
57
51
h3

LR

Average grain
size in mm.

.325

.275

.200

.750

.500

.hOO
.225
.500
.100

.100

37

MINERAL.PERCENT GES OF THE

Pyroxene

l\)
U1

30

20
20
22
12
17

10

'18

Table 3
<1)
'8
a :2 a 8 :2
,o °H «H 5 ea
c :4 > 0* a
a w -H m o
o h r4 94 :4
:2 :> C) C) .c
:2
Fresh Basalt
- - - u 3
- - - s 3
Northern Nets-Basalt Mass
- ~ - 12 13
- - - 7 u
Low Rank Meta-Basalt
3S - - 1 10
- 3h - h 6
2h - - h 6
- - - T -
53 - - 3 7
High Rank Meta-Basalt
h - - 9 -
37- - u -
3h - - 3 -
9O - T T 5
2O - — 2 -
2O - - 3 T
6 - - 3 -
8 12 - l -
_ 5 - 3 -
5 - - l2 -
16 - 11 -

erENAwAN BASALTS AND META-BASALTS

Apatite

Paragonite

Biotite

Quartz

Calcite

Rutile

Orthoclase

I-3

Epidote

38

DIABASE DIKES

Near the contact of the Tyler Formation with the overlying
intrusive Keweenawan igneous series, several diabase dikes Cut the
Keweenawan flows and underlying sediments. The dikes that out
the Tyler must be examined carefully to distinguish them from the
graywacke which is of an identical color and grain size. Similar
dikes have also been reported to cut the Huronian as deep as the
Ironwood where they vary from a few inches to 90 feet thick
according to Hotchkiss.

Thin sections from two such dikes, (22—1h and 26-60), reveal
a diabasic texture and composition. They have grain size of .hmm
and .25mm respectively and are composed of labradorite plus uralite,
which seems to be secondary after pigeonite. The most distinctive
feature of both samples is that their composition and texture is
very similar to other exposures in the area that have a different
but possibly closely related origin. Many other thick diabase
dikes have been found within the igneous mass to the north of the
Tyler Formation. Many of these dikes are often thick and have
sharp gabbro-diabase contacts (see figures 17 and 18). However,
where the exact field relationships and contacts cannot be seen,
the dikes are often confused with other surrounding rock types

of an identical lithologic appearance.

39

THE MEIIEN'GABBRO INTRUSIVE

The Keweenawan Mellen gabbro-granophyre intrusive complex
taken as a whole, extends from a point 3% miles west of the
Michigan border in section 7 of T. h6N., R. 2E. to section 3
in T. hh N., R. 6W. Its length is approximately hO miles with
an average strike of approximately N.65 degrees E. The width of
the intrusion varies appreciably. At one point in T. h5N., R. 2W.
the width decreases to less than a mile whereas elsewhere in T. th.,
R. MW. a thickness of about 6 miles is reached. At the town of
Mellen, the intrusive complex is separated by a large mass of
granite which appears to have been emplaced following the intrusion
of the larger basic intrusive. The eastern contact of this Mellen
granite with the Mellen gabbro is located in the map area of this
report.

Since at the eastern end of the Hellen gabbro complex the
Middle Keweenawan basalt is cut by the intrusion well up into
the series, it appears that the intrusion was not emplaced until
after most or all of the Keweenawan basalt had been extruded. The
magma would not have crystallized with differentiation unless it
had a thick cover of rock.

The gabbro is thought by Aldrich to have been intruded along
a proposed western extension of the Keweenawan Thrust which he
thought extended into Wisconsin from the Lake Gogebic area of
Michigan.

The original attitude of the intrusion was probably approximately

horizontal which would account for the well developed "upward" or

LO

northern differentiation, which has resulted in a basic base and
an acidic top. The degree of northward tilting that has followed
its emplacement is uncertain. However, dip measurements of a
planar orientation of plagioclase laths taken in the map area,
average about north h5 degrees. Therefore if it is accepted that
the planar orientation of the plagioclase laths at the time of
crystallization was approximately horizontal, then the northward
dip is therefore established. However, dip of the plagioclase
orientation was quite variable and may have been strongly affected
by internal flowing or movement during crystallization. A h5
degree dip, being less than that of the Tyler Formation, indicates
intrusion at a point in time after the regional northward inclina-
tion had begun to develop but before it had been completed.

The occurrence of banding or layering so often found in
differentiated intrusives, such as the rhythmic layering of the
Stillwater and Skaergaard Igneous Complex, is absent in the map
area of this report. Hess (1960) has stated that its presence can
be attributed to motion in the magma possibly caused by convection
currents. Since the presence is attributed to motion within the
magma chamber, its absence therefore implies quiescence and a
lack of any convection currents. The reason for this may be the
sheetlike shape of the intrusive sill. According to Hess, the
ideal shape for a convection cell is one in which the horizontal
diameter is only slightly larger than.the vertical dimension.

Therefore a sheetlike intrusion would not be expected to have had

L1

a convection cell.
Although rhythmic banding is not found in the intrusion,
cryptic layering is well developed which has made possible the

division of the intrusion into several zones of differentiation.
THE ZONES OF DIFFERENTIATION

An important characteristic of the basic intrusion, is the
nature of its differentiation, which has developed at least four
recognizable zones. The thicknesses of these zones in the map

area of this report are as follows:

granite lOO-hOO feet thick
granodiorite-

diorite h00-1,200 feet thick
gabbro-

anorthosite l,OOO-8,000 feet thick
chill zone O-hOO feet thick

CHILL ZONE

 

Hand specimen study shows the lower contact of the gabbro
to be a thin, fine grained chill zone varying in thickness, but
probably no greater than LOO feet thick. Although similar to the
metabasalts to the east, certain differences do exist, which make
recognition of this chill zone possible. The most obvious difference
in hand specimens is lack of amygdules and metacrysts that are
found in the basalts. Other differences are slightly coarser grain
size and a fresher, less altered appearance. In some of the hand
specimens large phenocrysts of a glassy plagioclase are found in

the chill zone.

h2

Oiabase dikes that in no known way can be distinguished
from the chill zone have been found intruding the gabbro in
the map area. Since the lithology of the dikes is identical to
the greater part of the chill zone, they apparently intruded
along fractures while the first gabbro was crystallizing or
immediately thereafter. If they had been intruded much later,
their composition would be less basic due to differentiation
in the magma chamber.

An important characteristic of the chill zone is its
olivine content. Olivine is not present throughout the chill
zone, being noticeably lacking in the very fine grained zone in
contact with the Tyler Formation. However, farther from the
contact where the grain size has slightly increased, but not
enough to be called gabbro, olivine is present in significant
amounts. Still further north, at the base of the coarse grained
gabbro, olivine is also present, but is lacking in the interior
of the gabbro zone. It therefore seems likely that the very
first rock to crystallize did so before differentiation or
gravitational crystal settling had a chance to operate and
therefore reflects the true composition of the intrusive as a
whole. (Thin sections 28-92, 31-6, 32-62, 32-63 represent this
zone). Then before conditions became favorable for the crystalli-
zation of large plagioclase crystals, typical of the gabbro,
gravitational crystal settling had already begun to take place.
This may be why the interior of the chill zone contains so much

olivine.

h3

The texture of the interior of the chill zone is ophitic and
resembles in every detail the base of the gabbro except for the
difference in grain size.

On the basis of 2V measurement, the olivine appears to have
a composition of F0 70-Fa 30. The olivine is often very fresh
(see figure 18) but in some cases magnetite and serpentine have
partly replaced some of the crystals. Olivine crystals in a few
samples are surrounded by what appears to be a magmatic stage
reaction rim. (See figure 19). The reaction rim is composed of
hornblende flakes and pyroxene in a confused fine grain pattern.
The olivine is completely unzoned.

The plagioclase is labradorite in nearly all cases and is
zoned. The zoning, however, is not generally continuous. The
irregular shaped interior zone is more calcic than the surrounding
plagioclase. Plagioclase cores of An 60-65 often have outer zones
of An 35-h0.

The pyroxene is all colorless in thin section and often shows
well developed pyroxene cleavage. It occurs both as individual
crystals and interstitially between plagioclase laths in an
ophitic nature. The samples lacking olivine contain pigeonite
which was probably the first pyroxene to crystallize. Pigeonite
was recognized by its 2V which varied from 10 to 22. In the
samples containing olivine, measurements ranged from 19 to 51

suggesting that two varieties of pyroxene are present. The lower

uh

2V measurements indicating pigeonite and the higher measurements
suggesting a variety of augite. However, in the coarse grained
samples of the chill zone, pigeonite was not found.

Titaniferous magnetite and a few rare crystals of pyrite
are the only Opaques that were found to exist in the chill
zone. Quite often the ilmenite has been completely eaten away
and replaced by plagioclase leaving only a skeleton of magnetite.
(See figure 20).

Small euhedral apatite crystals and small, rounded, bright
red rutile crystals also occur in the chill zone.

Hernblende is a frequent and sometimes plentiful constituent
and occurs as fresh pleochroic crystals in browns and greens,
With well developed amphibole cleavage. Its presence in large
quantities suggests that water was available during the crystal-
lization of the chill zone. Some of the water may have been
supplied by the Tyler Formation in contact with the chill zone.
Biotite is also found within the chill zone and is closely

associated with the magnetite.

 

Figure 17 Xenolith of gabbro within
diabase taken from outcrop 6 section 31.

 

NOV °

o 64 V, -7 V ,__.1_,
Figure 18 Photomicrograph of 32-60 showing
fresh olivine and plagioclase within.poikilitic
augite crystal. X 100 diameters.

D6

NTAGES IN THE

.l
£3

MINERAL PERC

CHILL ZONE

h

Table

opwoamo
maflpsm
eefieowm
coopHN
opwpmmm
opwcommhmm

mpasoaeo

mpHCoEHH
Impwpocmmz

mcH>HHo
mpaamsp
mpcmfincnom

mflmKOE

.EE CH msflm
aflmmw omwmo><

G¢ R_Edeflxmz

emeOOflmmHm

monopso
nowpoom

36 .50
52

70

6-6

25
36

0 L10
.85

.30

66

6-10

58 SS

6-h2

27

58 S7

6—h3

.hO
.25
.25

h?

62

26-60%

59

27-lh* Sh

28

63

66

27-80

52 .80 20

79

6-hO

27 u9 .10 so 20 17
15

28-90

.30

62

28-105 73

6h
57

66

30-3

55
72

31-3

.33
.hO

55

32-13

10 20

T

h0

3O 56

32-62

71

32-63

h6 51 .30

33-100 85

33-92

12 -

.35

56

.30 12

S9

33-10h 73

* Dikes cutting Tyler Formation

h?

o‘._

 

l .Nov...

, ._____*..- -_

Figure 19 Photomicrograph of reaction rim
around olivine grain, 32-5, crossed niccols.

 

NOV 0 64

Figure 20 Photomicrograph of 32-66 illustra-
ting magnetite skeleton left after ilmenite
has been eaten away in plain light X 80 diameters.

h8

THE GABBRO‘ZONE

 

The coarse grained gabbro intrusive differs from the chill
zone in lacking olivine, containing smaller amounts of mafic
minerals and a proportionately higher amount of plagioclase.
(See table 5 and 6). An obvious difference is the great increase
in grain size. In hand specimens, typical gabbro samples are
very coarse grained and are composed largely of plagioclase.

The plagioclase is a gray labradorite variety. Green silicates
and occasional magnetite crystals fill the space in between the
plagioclase laths producing a very attractive rock.

The gabbro is being quarried as "black granite" in section
29 and has been quarried previously at two other locations, also
in section 29. The quarrying operation (see figure 21 and 22)
has eXposed large areas of gabbro, making possible the observation
of large granodiorite and pegmatite veins that have intruded
the gabbro. (See figure 23). The granodiorite - gabbro contact
is extremely sharp. In an interview with Mr. Louis Jager, the
quarry foreman, I was informed that most of the granodiorite
intrusions dip toward the north at about h5 degrees. Test
drilling has shown that the intrusions become thicker at depth.

The source of the granodiorite sills or veins is evidently
the large granite intrusion west of the Bad River known as the
Mellen granite. It may be that during the emplacement of the
Mellen granite just west of the map area, cracking and jointing

occurred in the gabbro mass. These cracks may have then been

h9

filled by apophyses from the granite magma. Granodiorite has cut

the gabbro at all three quarries and at outcrops within the gabbro

at several other locations. As the Mellen granite is approached,
these granodiorite veins become more and more numerous. At the

quarry south of Loon Lake (outcrop l2—section 29), the granodiorite
contains broken off xenoliths of gabbro. Upon observing the xenolith-
granodiorite relationship of the Loon Lake exposure, it is clear

that there is a strong resemblance in every respect to outcrop

12h in section 30 where the Nellen granite intrusion contains

numerous xenoliths of basic rock. (See figure 23 and 2h).

Outcrop 33 in section 29 south of Loon Lake is located at
the summit of a well exposed gabbro dome with 160 feet of relief.
At this location, the gabbro possesses very unusual magnetic
properties. When the Brunton compass is held four feet away from
the rock surface, the needle points north as verified from a
bearing taken on a church tower that could be seen in the town of
Mellen, less than two miles away. However, when the compass was
placed directly on the rock surface, the needle pointed due west.
This is probably caused by polarity in a local concentration of
magnetite occuring within the gabbro.

In thin sections, the gabbros are composed chiefly of large
laths of labradorite in an ophitic texture. The grain size is
quite variable. There seems to be some correlation between grain
size and plagioclase content. The plagioclase is often well zoned
with irregular shaped inner zones. The individual laths are often

cracked and broken in a way that would seem to indicate crushing

5O

 

Figure 21 "Black Granite" quarry hoist
with Loon Lake in the background.

' 'E- . ,
M‘ cup“ '(FP'* ' ‘
.. 4 'a' ' ‘
...' " 1“.st~ I 'f

‘u

-.— —~‘~

 

 

 

L. _ __ ,-, - -_.,_ - - .lu__ ._l_ }

Figure 22 Drill operator at work at the
Loon Lake quarry.

51

a... as;

.

a
a,

as;

 

in

Figure 23 Quarry wall illustrating

grano

truded into gabbro.

diorite

...u -

“-5--

 

 

‘i'lll I 1""!

 

igure 2h White granite outcrop (30-l2h)
containing gabbro and diabase xenoliths.

F

52

before the interstitial liquid had crystallized. (See figure 25).
In an experiment performed by Wagner, Brown and Wadsworth
(1960) it was shown that simple packing of aluminum tablets,
(of a shape comparable with plagioclase of the type found in
anorthosite) if loosely accumulated in water, the resulting void
space amounted to about h5 percent. Shaking to improve the
packing only decreases the voids to about 35 percent. Since
the amount of plagioclase in the anorthositic variety of gabbro
found in section 28, 29 and 30 reaches 95 percent, it seems
probable that about 35 percent of the plagioclase grew from
material supplied by the interstitial liquid after crystal
settling. This reasoning assumes an origin by crystal accumula-
tion, but in view of the gradual upward differentiation, this type
of origin seems to apply. An explanation to account for the
enlargement of the plagioclase has been offered by Hess (1939).
He has suggested that diffusion from the overlying magma through
the interstitial liquid between the crystals supplied the material
necessary to continue growth. While this diffusion was occuring,
a simultaneous diffusion of the unused material was occuring in
the opposite direction. As growth continued, the interstitial
liquid would gradually be pushed out until a small amount would be
trapped. This would necessitate very slow cooling and require a
thin layer of "crystal mush" in order for the diffusion to take
place. As a result of this type of crystal growth, large extremely

poikilitic pyroxene crystals have grown from the pore-space liquid.

53

Each pyroxene crystal includes within it several large plagioclase
laths. It would seem that in this igneous body, plagioclase has
a greater tendency to form crystal nuclei at the beginning of
crystallization than pyroxene thus accounting for the ophitic
texture.

The pyroxene is chiefly a colorless augite. However, hyper-
sthene also occurs in smaller amounts. The hypersthene reveals
well developed exsolution structure where augite has exsolved as
oriented lamellae along the (001) cleavage direction of the hyper-
sthene (see figure 28).

The order of crystallization making use of information supplied
by the chill zone, can now be roughly stated. It appears that the
first pyroxene to crystallize was pigeonite which makes up the
majority of the fine grained chill zone pyroxene. As the tempera-
ture decreased slightly, augite also crystallized and exsolved
from the pigeonite (see figure 26). As cooling continued while
the gabbro was crystallizing, the pigeonite inverted to hypersthene
while continuing to exsolve augite (see figure 27).

The following pyroxene contents were developed as a result
of differentiation with approximate percentages:

(1) pigeonite 90% plus augite 10% - chill zone

(2) pigeonite 10% plus augite 85% plus 5% pigeonite
with exsolved augite parallel to (001) lamellae -
interior of chill zone and base of gabbro

(3) hypersthene 5; plus augite 90% and 5% hypersthene
with exsolved augite parallel to (001) lamellae -
gabbro

The presence of augite plates exsolved from pigeonite before

 

EV" ° 64

Figure 25 Photomicrograph of 27-15 illustra-
ting a crushed plagioclase crystal under
crossed niccols.

 

NOV ° 64

Figure 26 Photemicrograph of 32-61 illustra-
ting pigeonite-augite exsolution. Crossed
niccols.

pigeonite
+-
quwd

Pj'QEOhiT€[p)5€on}Te + Augi‘f’e: AugiTC

 

 

 

Hypergfhehe+ AungC

 

     

 

 

Figure 27 Equilibrium diagram of pyroxenes,
after Hess (l9hl)

-\'-»' ‘. ", /
fit? v 13.5%; 3°- - '
.nl; 31--53"..." , '3 '
. {"1” ~ " I

Figure 28 Photomicrograph of 32-17 illustra-
ting hypersthene exsolving augite lamalle
under crossed niccols X 100 diameters.

 

Section
Outcrop

27-15
29-8a
31-131
32-17
32-8
23-120
32-6
32-12
32-101
32-66
32-60
32-61
28-106
31-5

Plagioclase

CI)

3

85
83
79
7‘4
61
90
9S
85

75
35

Maximum % An

65
6h
62
57
S6
36
6h
67
6h
58
55
S 9
5h

Average length
of plagioclase laths

H
0
U1

Lo
7.5
h.o

2.5
1.0

11.8 .

3.5
6.0
2.5
1.5
2.5
1.2

3.0

56

hINERAL PERCENTAGES IN THE

Pyroxe ne

26
13
65

Hornblende

GABBRG ZONE

Table 5
(D

m -p

+3 -H
°H F—I
H O
(U H

5-: .53

D O
10 T
25 T
- T
- T

- T

35 T

5 ..

2 _

- 2

Paragonite

w

I-3

Opaques

F: pr

+4 x» tr :‘

U)

Olivine

10

10

35

Apatite

Zeolite

F3

Muscovite

Calcite

I-E!

Biotite

57

inversion has also been described by Hess and Henderson (l9h9).
According to Hess (1960), orthopyroxenes are not usually found

in dolerites (such as the interior of the chill zone). Instead
its place is taken either by uninverted pigeonite or a variety of
orthopyroxene which he calls the Palisades Type. The "Palisades
Type" is caused when cooling is fast and inversion takes place
before exsolution. The fast cooling therefore does not permit
excess CaO from the pigeonite to become absorbed by augite lamellae
and therefore causes the formation of diopside specks in the ortho-
pyroxene. Since diopside specks were not found, it can be con-
cluded that cooling was slow enough for the excess CaO to become
absorbed by the augite lamellae or by the plagioclase.

The average 2V for the augite is only h? degrees which in-
dicates that it is probably diopsidic variety bordering on sub-
calcic variety. The augite is completely colorless in thin
section. The pigeonite of the chill zone, which is also colorless,
has an average 2V of 21 degrees.

Maximum extinction angles for the pigeonite averaged 3h
degrees whereas maximum extinction angles for augite averaged
hl degrees.

Hypersthene was readily distinguished by its parallel
extinction and faint red to colorless pleochrism. Schiller
structure is well developed in most of the hypersthene and was

observed in some of the augite.

58

In samples which contain much uralite, the pyroxene content
is proportionately low. Uralitization has almost completely
replaced pyroxene in several cases.

Titaniferous magnetite is the only opaque that I have
observed in thin section. It occurs in amounts ranging from one
to four percent. Traces of apatite have occasionally been
observed in very small amounts. Biotite is seldom present, but
has been observed associated with crystals of magnetite. Olivine
(Fa 30), as has already been mentioned, occurs only near the
base of the intrusion. Pleochroic brown to green hornblende has
also been observed in two gabbro samples located near the base.

In each of the samples listed in table 6, the texture is

ophitic.

DIORITE - GRANODIORITE ZONE

In traversing northwardly up dip from the dark gray gabbro,
a diorite zone is encountered.

Samples taken from the diorite zone are easily distinguishable
from the coarse grained gray gabbro below. In hand specimen,
the diorite occurs as a medium grained rock consisting of black
amphibole crystals equal in size to white feldspar crystals
which make up the majority of the rock. In this section the
rock is hypautomorphic granular. The composition of the diorite

is represented by thin section 23—31 (see table 6). The plagioclase

appears to be andesine and the chief mafic mineral is amphibole
thus indicating the rock is definitely not gabbro although the
sample was taken from an area that has previously been mapped
as a gabbro. The amphibole is a pale green variety that is
occasionally feathery but occurs chiefly as prismatic moderately
pleochroic crystals.

A small amount of pyroxene which has been largely altered
to amphibole also occurs in the diorite. A small amount of ortho-
clase was found in thin section 23-31 which occurs as small round-
ed blebs within the amphibole and feldspar crystals and never as
distinct crystals. In the overlying granodiorite zone the ortho-
clase content increases.

The granodiorite is typified by sample 25 in section 23
(see table 6) where quartz makes its first appearance and total
mafic content greatly decreases. The pyroxene appears to be a
colorless augite. The hornblende is fresh and euhedral and is
pleochroic in brown and olive green. Staining for the positive
identification of orthoclase has indicated a close quartz-
orthoclase association with intergrowths of quartz within the
orthoclase. Magnetite is often found as euhedral crystals in
contrast to its occurence in the gabbro. As the granite zone is
approached it is interesting to note the high apatite content in
several samples (see table 6). In some cases, the apatite occurs

in fairly large grains and remains quite euhedral.

CRANITE DIFFERENT IATE

The granite zone is especially well exposed along two east-
west trending ridges located along the south edge of section 21
and in the center of section 23. The granite differentiate differs
from the major part of the large granite body immediately to the
west of the gabbro in lacking a porphyritic texture. Certain
differences have also been observed with the granite in section
30 which is not a porphyritic variety. The chief differences consist
of a higher plagioclase, quartz, hornblende, apatite, and opaque
content in the differentiate. (See table 6). The granite differ-
entiate apparently differs from.the granophyre described by
Leighton (l95h) in having a lesser development of micrographic
texture and a higher percentage of mafic material in the granite
differentiate. The granite differentiate is medium grained to coarse
grained. The orthoclase in it is bright reddish orange. It does
not contain the great number of xenoliths which may be observed in
the Mellen granite to the west. However, at least two large basalt
xenoliths and one medium grained xenolith of a gabbroic nature
were found within it. The medium grained xenolith may represent
a remnant of the upper chill zone, if such a chill zone originally
developed. The upper edge of the differentiated sill was not
found exposed at any place. Pegmatite (outcrop 23-22) found in
contact with the granite, contains crystals ranging from l-2cm.

It is composed of roughly 50 percent red orthoclase, 30 percent

61

very euhedral black hornblende, and 20 percent milky quartz. The
quartz weathers out strongly in relief. The pegmatite probably
originated along a crack in the granite as material "sweated out"
of the surrounding rock shortly following magmatic crystallization.

In thin section, the granite differentiate is best represented
by sample 28-32. The texture is equigranular. The amount of
orthoclase is less than one would expect after observing the rock
in hand specimen. Actually, the majority of the brick red feldspar
is microcline and plagioclase. All three types of feldspar are
colored by'a red dust that is also present in the feldspar of the
granophyres described by Leighton. Leighton indicates that the
coloring agent is probably hematite.

The orthoclase that is present grades into microcline which
is present in large amounts. Since in hand specimen all of the
feldspar is brick red, this evidently includes the plagioclase that
is present. The plagioclase is albite (An 6%) and is unzoned.
The red dust in the plagioclase is quite blotchy, whereas in the
orthoclase and microcline it is evenly distributed. Quartz is
present only in rounded, regular blebs associated with the potash
feldspar, but avoiding the hornblende. The hornblende is pleochroic
in green and.yellows and occurs as discrete individual crystals.

Although the amount of granite probably amounts to no more
than 3 or h percent of the total intrusive, this figure according

to (Hess 1960), is quite high. Hess has stated that "In the most

Section
Outcrop

3o -29»
21-32a
21-32b
23-21
28-98
21-6
23-25
22-3
22-7
23-31

29-8b*

2":

Plagioclase

29
28

71
69
76
6O
78
66
25

Maximum % An

MINERAL PERCENTAGES IN THE

62

DIGRITE - GRM‘JO‘DIOHITE - GRANITE ZONE

Average grain

size

m
0
U1

1.5
1.7

.7
2.0
1.5
1.5
1.0
1.5

2.0

1.5

Orthoclase

030)
N

145

30

Table 6
Q)
a
£11 8
o N m
o -P M
safe
- 7 -
23 19 T
2h 17 T
_ 35 -
-85
-ST
- 2 5
- 9 -
- 1 3
--s
10 23 -

Hornblende

l—’ 12"
V1

15

25

Uralite

Biotite

Zircon

I-S

Apatite

M

M m m 5' *3 OD aque

«3 tr +4

sample taken from granodiorite vein at gabbro quarry

sample taken from Mellen Granite intrusive

Rutile

L

Serpentine

63

extreme cases of fractional crystallization, in which the chance
of obtaining a granite residue would be most favorable, it seems
very unlikely that large bodies of granite are ever derived from
fractional crystallization of basalt". Despite this observation
I feel that the near perfect gradation to the granite can best

be explained as an acid residual of a fractional crystallization

process.

THE MELLEN GABBRO~MELLEN GRANITE CONTACT

The western contact of the Mellen Gabbro instrusion with a
large granite mass to the west (tentatively called the Mellen
Granite), is very irregular. At no place does gabbro litho-
logically grade into the Mellen Granite. Instead, the contact
is very sharp, with no intermediate rock type present. The
most characteristic feature of the granite is the large quantity
of xenoliths it contains. The larger xenoliths are usually
gabbro. However, medium to fine grained basalt or diabase
fragments are also found in the granite mixed in with those

of gabbro.

61:

CONCLUSION

The conclusion drawn from the above research can best be
summarized by a brief discussion of the sequence of events that
occurred.

The deposition of the Tyler Formation probably ended in an
increasingly shallow water environment as indicated by the nature
of the quartzite that resulted. The Huronian-Keweenawan time
gap that followed was ended with the deposition of a thin basal
Keweenawan conglomerate and quartzite layer before the extrusion
of the thick Keweenawan flows that followed. While the magma
chamber was emptying, a regional tilting as described by
Hotchkiss (1923), may have occurred, causing the area to dip
toward the northern basin thus created. Before the slumping had
ceased, a large gabbro laccolith was intruded, possibily along
the plane of a thrust fault that split the basaltic mass in the
eastern end of the map area and followed the northern contact
of the Tyler Formation in the western portion of the area.

The contact metamorphic effects of the intrusion on the
Tyler Formation were slight, as indicated by the absence of
contact metamorphic minerals. The reason for this fact appears
to be partly because of the resistence of the recrystallized
silica cemented facies of the Tyler Formation to temperature
effects and its impermeability to solution effects. Also

quartzites evidently lack sufficient mafic content to supply

65

the material for the crystallization of contact metamorphic
minerals. The quartzite facies has also shielded the underlying
subgraywacke and graywacke facies of the Tyler Formation from
any contact metamorphic effects. However, the effect of the
gabbro intrusion on the Keweenawan basalt was appreciable and
reaches the pyroxene hornfels facies of contact metamorphism.

As the intrusion began to crystallize, a thin chill zone
was formed. Diabase dikes of an identical lithologic nature
were also being intruded into the Tyler Formation as offshoots
of the main gabbro mass. As the crystallization continued,
olivine separated out by gravity, sinking to the bottom of the
intrusive. A typical to anorthositic gabbro was then formed
which makes up the bulk of the intrusive body. The gabbro,
however, grades into a diorite near the top, which in turn
grades into a granodiorite and finally a granite.

The last igneous event to occur in the area was the

intrusion of granodiorite dikes, probably originating from

the Mellen Granite, which cut the Mellen Gabbro.

66
SUGGESTIONS FOR FURTHER STUDY

Although it is unlikely that contact metamorphic effects
will be found in any future study of the Tyler Formation, it may
be very useful to examine the possibility of regional metamorphic
effects on the Tyler. It is quite possible that during the
Huronian ~ Keweenawan unconformity, some degree of metamorphism
was impressed upon the Tyler Formation.

A contact metamorphic problem involving the basalt that I
have left unanswered concerns the question as to whether the
pigeonite was directly converted to diopside or whether it passed
through an intermediate hornblende stage. This question would
necessitate an explanation of the chemical changes that went on
during the metamorphism. For additional field evidence, the area
directly to the east of the map area of this report may prove
useful. A comparison of the contact metamorphic potential of the,
base of the gabbro sill with the acid differentiate may also be
of interest. Such a comparison would require good basalt exposure
at both contacts. However, adequate exposures may exist in the
area to the east of section 23.

NOW that it has been demonstrated that the intrusive sill has
undergone differentiation it may be useful, through a series of
normative analysis, to describe the sequence of chemical changes
that occurred as crystallization progressed. Such an investiga-
tion may aid in any further speculation concerning the relative
importance of crystal zoning, gravity settling, and possibly

partial fusion and filter pressing as differentiation mechanisms.

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1-145.
’1

ll.“‘.ll|1'l«,
’4; )J‘

 

 

.1

he!

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