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ABSTRACT

The intergranular surface area of a deformed rock can be used to
describe its strain state. The strain state will be a function of
strain-rate and temperature. The Little Eau Claire River shear zone
is shown to develop polygonized and microfractured grains leading to
an overall increase in intergranular surface area per unit volume with
increasing strain4rate. The strain state is considered a function of
strain-rate only, since plagioclase compositional data indicates that
the entire shear zone was raised to temperatures very near the crest
of the peristerite solvus during the deformational episode.

The total rock intergranular surface area per unit volume increases
from 12.37 mmZ/mm3 to 82.84 mm2/mm3 in a sheared quartz monzonite.

This is a quantitative measure of change in strain state with increased
shearing. The phase-phase surface areas of quartz, plagioclase, and
K-feldspar, plotted as a function of changing strain state, show dis-
tinct and different granulation profiles. All three indicate that the
deformational mechanisms dominant in the shearing of the quartz monzonite
led to increases in the phase—phase surface areas. It is suggested that
“the mechanisms responsible for this phase-phase surface area increase

are grain microfracture and polygonization and subsequent grain-boundary
sliding. Grain growth by recrystallization occurs but is not the
dominant deformational mechanism.

The peristerite solvus has been intersected during shearing. Only

the fine-grained plagioclase fraction is able to equilibrate with the

highest temperatures reached near the crest of the peristerite solvus.

A TEXTURAL ANALYSIS OF THE STRAIN STATE OF A SHEAR ZONE

By

Kevin John Freeman

A THESIS

Submitted to

Michigan State University
in partial fulfillment of the requirements

for the degree of

MASTER OF SCIENCE

Department of Geology

1974

U" ACKNO‘.'.IEDG?2TFII~ITS

I am indebted to Dr. Hugh Bennett and Dr, Bill Cambray for
their assistance and review of this manuscript, A very special
thank you goes to Tom Vogel, without whose patient assistance very
little science would have been generated.

I am grateful to my family for their guidance and assistance
throughout my studies, both graduate and undergraduate.

But those for whom I feel the most gratitude at this time are
the people who have made my educational experience the memorable
event it has been. I will not forget the many adventures and
tribulations I have shared with Stoney, Gary, Bruce and Graham in
the pursuit of knowledge, wine, women and song (not necessariLy in
that order), And to Sue, Kathy, Jan and Geri, I am indebted for
helping me understand the importance of my feelings and the
necessity for honest communication between people.

To all of you my thanks and my hopes for freedom of purpose
and fulfillment of ideals, "...,yes, I get by with a little help

from me friends,"

ii

TABLE OF CONTENTS

Page
List of Figures ......... ............................. .... . . ..... 1V
Introduction ..... . ........... ................. . .. . .. . . .. 1
Model: Surface Area Response to Dynamic Metamorphism ...... ,,,,,,,, 2

Model: Plagioclase Compositional Response to Dynamic MEtamorphism.. 5

The Little Eau Claire River Shear Zone ,,

........................... 6

Interpretation of Surface Area Results ....... ,..,,,,.,,,,, ,,,,, .... ll
Plagioclase Compositions .......... .............. .......... ......... 14
Conclusions ........... ................ . ........... ................. 18

Bibliography cocooOooooooooooooooooooooooooooooooooooooooooooooooooo 2O

- - iii

Figure

Figure

Figure

Figure.

Figure
Figure
Figure
Figure

Figure

Figure

Figure

10

10.

11.

,LIST OF FIGURES

Page
Photomicrographs of relativeLy unsheared quartz
monzonite, Little Eau Claire River shear zone ....... .... 6A
Plot of quartz—quartz surface area versus Sv ..... ....... 7A
Plot of K-feldspar-K-feldspar surface area
versus Sv ..... 00.00.00.000... ...... c on o. co. .00 7B
Plot of plagioclase-plagioclase surface area
versus Sv.......OOOOOOOOOOOOOOOOO ...... O OOOOOOOOOOOOO 0 8A
Polygonization textures in quartz and plagioclase ....... 8B
Post—sliding strain—free quartz mosaics..... ...... ...... 9A
Plagioclase subgrain mosaiCS....... ..... ... ... ....... 9B
Textural disequilibrium within the shear zone ,,,,,,,, ... 10A
Development of curved grain boundaries within
plagioclase subgrain mosaiCS....................... ..... 12A
Plagioclase compositions as determined by ARL
EMX micrOprobe.............................. ............ 14A

Illustration of apparent peristeritic trend in

plagioclase compositions with increased shearing..,,,,,, 16A

iv

INTRODUCTION

 

The purpose of this paper is to develop a model characterizing
the response of intergranular surface area and plagioclase compositions
to dynamic metamorphism. These parameters are chosen since they are
sensitive to the energy environment associated with dynamic metamorphism
and are easily measurable.

The surface energy of grains in a crystalline aggregate has long
. been considered a significant variable in petrogenesis (DeVore, 1959;
Spry, 1969). The intergranular surface area of a polycrystalline
aggregate is intimately related to the surface energy and can be
easily quantified using stereologic techniques developed by metall-
urgists and recently applied to geologic problems (Ehrlich g£_§l,
1972).

During low temperature metamorphism plagioclase compositions are
sensitive indicators of the metamorphic thermal history since sodic
plagioclase compositions are controlled by the peristerite solvus

(Crawford, 1966).

MODEL: Surface Area Response to Dynamic Metamorphism

The strain state of a deformed rock is a function of the temp-
erature and strain-rate of deformation. Variations in the final strain
state will indicate the relative magnitude of the strain-rate, provided
that the temperatures across the deformed interval are relatively con-
stant. One quantitative measure of the strain state of a deformed rock
is its total-rock intergranular surface area per unit volume (Sv).

During dynamic metamorphism, the effect of high strain-rates is
to impede the onset of grain growth by recrystallization at a given
temperature (Carter, 1971). Therefore, such high strain-rates will
favor a grain polygonization process over a recrystallization grain
growth process during the reduction in the strain energy of the system.
Polygonization, wherein dislocations rearrange themselves into sub-
grain boundaries enclosing relatively strain-free cyrstals (Carter,
1971), will lead to a dramatic increase in the intergranular surface
area per unit volume of a crystalline mosaic providing subsequent sub-
grain boundary sliding and rotation occur.

Prior to and contemporaneous with the onset of polygonization,
brittle failure induced by transgranular microcrack propagation will
lead to the development of cracked grains (Kingery, 1960). Micro-
crack nuclei will be present initially in the large primary grains and
Inore will be produced as dislocations pile up at natural barriers such
£13 grain boundaries, impurities, etc., (Kingery, 1960). If subsequent

Irotation and sliding occurs along these transgranular fractures,

J

microcrack propagation will also lead to an increase in intergranular
surface area per unit volume.

Within a shear zone, recrystallization grain growth will be fav-
ored over recovery processes when the temperatures produced by fric-
tional heating are sufficiently high to overcome the impeding effects
of high strain-rates, and favorably oriented grains will grow at the
expense of their neighbors (Carter, 1971) resulting in a decrease in
intergranular surface area per unit volume. The surface area decrease
would result from the destruction of phase-phase boundaries during nor-
_ mal grain growth.

Microfracture and polygonization will dominate as the deformational
mechanisms of shearing when the high strain-rates are capable of imped-
ing the onset of recrystallization grain growth and the intergranular
surface area per unit volume should increase as a direct function of
strain-rate. Steady-state flow will not be achieved and grain growth
by recrystallization will not dominate unless temperatures during post-
tectonic relaxation remain sufficiently high to promote it.

If polygonization and microfracture are the most significant defor-
mational modes during shearing, then the fraction of the intergranular
surface area attributable to like phases in contact with one another
should increase with increasing strain-rate, since this type of bound-
ary will be generated by both processes. Host grain polygonization
phenomena should be identifiable in the rock mosaics (Hobbs, 1968).
21f grain growth by recrystallization predominates, such like-like
lihase boundaries will be destroyed. Curved grain boundaries indi-

crltive of grain growth should be observed (Coble and Burke, 1963).

The model proposed is that mylonites are the result of extremely
high strain-rates, impeding recrystallization grain growth and pro-

moting microcrack induced brittle fracture and grain polygonization.

MODEL: Plagioclase Compositional Response to Dynamic Metamorphism

As frictional heating increases during dynamic metamorphism,
plagioclase compositions will be increasingly controlled by the
peristerite solvus. Assuming equilibrium conditions and an initially
homogeneous plagioclase compositional population across the shear
zone, the plagioclase compositions should imprint the thermal regime
affecting a given sheared rock. Temperatures associated with the
crest of the peristerite solvus at equilibrium range between 600°C
and 650°C (Crawford, 1966). If temperatures across the shear zone
cover the range of the peristerite solvus, then it is expected that
with increasing temperature albite will develop, then the two-feldspar
regime will be encountered, and finally the crest of the peristerite
solvus will be exceeded. There should be a gradation from an albitic
phase to a two-plagioclase compositional population to a single
Ca-bearing plagioclase compositional population, assuming equilibrium
has been achieved. An apparent peristeritic plagioclase compositional

population has been previously reported in the Scourie shear zone,

Scotland (Beach, 1973).

THE LITTLE EAU CIAIRE RIVER SHEAR ZONE

A series of parallel to subparallel shear zones trending approx-
imately N3OOE are exposed in the PreCambrian rocks of Marathon County
in central Wisconsin (LaBerge, 1972; LaBerge and Myers, 1973). The
Little Eau Claire River shear zone occurs within this belt and inter-
sects a quartz monzonite on the western margin of the Wolf River Bath-
olith. Hand sample textural variation in the sheared quartz monzonite
ranges from primary igneous to an ultramylonite over a lateral interval
of less than four miles (LaBerge, 1972). Extreme textural changes occur
within a severely sheared one-mile section on the northern side of this
larger interval.

The Little Eau Claire River shear zone was selected for this
study because of its obvious textural variability and also because
it is compositionally similar to a regionally metamorphosed grandiorite
previously studied for surface area variation (Ehrlich g£_§l, 1972).
The quartz monzonite (Fig. 1) has slightly less plagioclase and slightly
more quartz and potassium feldSpar than the granodiorite, but compo-

sitions are close enough for textural comparisons to be made.

Shear Zone Mineralogy
Modal per cent of quartz and plagioclase across the Little Eau
Claire River shear zone show slight variations not unlike those ex-
‘pected in a normal igneous intrusive. Plagioclase and quartz exhibit

zaverage modal values of 35% and 34% respectively. Potassium feldspar

Figure l,

Photomicrographs of relatively unsheared quartz
monzonite, Little Eau Claire River shear zone,

V A and B. Note the large grain size of the pri-

mary quartz, K-feldspar, and plagioclase,

C and D. Compositional zoning is common in the
primary plagioclase grains. Transgranular micro-
fracture occurs in many of the plagioclase pheno-
crysts. E and F, Microcracks in these quartz
phenocrysts have been filled by opaques, When
post-cracking grain boundary sliding occurs during
shearing, the quartz grains will become opticalky
discontinuous across the crack boundary, leading
to an increase in measured quartz—quartz surface
area per unit volume,

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is more variable across the shear zone, averaging 25% in the less
sheared samples and dropping to less than 5% in the extremely sheared
samples. Hornblende, biotite and magnetite are minor phases in the
less sheared samples. Biotite, muscovite, epidote, and hematite are

accessory minerals in the more severely sheared samples.

Surface Area Per Unit Volume (Sv)

The three-dimensional surface area per.unit volume (Sv) of a
mosaic of contiguous grains is equal to twice the number of inter-
sections of intergranular boundaries with randomly placed test secants
of known length imposed on a two-dimensional thin section through
that aggregate (DeHoff and Rhines, 1968; Underwood, 1970). Units of
surface area are mmZ/mmd. The total rock intergranular surface area
can be partitioned into its constituent components, like-like boun-
daries (arc boundaries), and unlike boundaries (a-B boundaries). In
this study, the phase-phase surface area will refer to that fraction
of the total rock intergranular surface area per unit volume attrib-
utable to like mineral phases in contact with one another and sharing
mutual grain boundaries (a-a boundaries).

Within the Little Eau Claire River shear zone, quartz-quartz,
K-feldspar-K-feldSpar, and plagioclase-plagioclase surface areas have
been documented. These variables are presented graphically as a
function of total-rock surface area. The variation of interest in
this study is the change in phase-phase surface area as a function of
increasing strain-rate. In a given outcrop of a sheared rock, there
can be extreme variation in strain state over relatively small inter-

vals (Ramsay and Graham, 1970). This variation will be reflected in

Figure 2.

Plot of quartz-quartz surface area per unit volume
versus the total rock surface area per unit volume
(Sv) -- a quantitative measure of the strain state

of the deformed rocks, The dashed reference line
indicates the ratio of quartz-quartz surface area per
unit volume to SV to be expected if a one-to—one
correspondence exists between changes in these two
variables. In the low—shearing zone, such a
correspondence seems to exist, but as shearing in-
tensifies, a greater percent of SV is contributed

by quartz-quartz boundaries, since microfracture

and polygonization followed by grain—boundary sliding
dominate —- thus selectively producing such like—
like phase boundaries,

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Plot of K-feldspar-K—feldspar surface area per unit
volume versus Sv, The rapid departure from the
dashed reference line indicates the generation of
cracked and polygonized K-feldspar grains during
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the degree of granulation achieved within a particular Specimen.
Generally, development of a fine-grained fraction is the obser-
vable variation used to determine degree of granulation. However,
changes occur not only in the size of grains but in the shape of
grains as well. Surface area is a fundamental textural parameter
sensitive to both size and shape of grains within a rock. Textural
inhomogeneities in size and shape of grains are obvious across the
shear zone, and these inhomogeneities reflect the variations in
strain-rate. Therefore, within a shear zone, the total rock inter-
granular surface area is a much more accurate indicator of shear
intensity than is the spatial distribution of samples within the
outcrop. For this reason, the phase-phase surface area is plotted
as a function of total-rock surface area to indicate the response
of phase-phase surface area to increasing strain-rate.

A reference line is included in all three graphs to facilitate
comparisons between the "granulation profiles" of the individual
mineral phases. The line represents that amount of phase-phase
surface area eXpected if a direct one-to-one functional relation-
ship exists between phase-phase and total-rock intergranular surface
area. The initial ratio between the phase-phase and total-rock
surface area in the relatively unsheared quartz monzonite is used
to construct this line for each phase. It is emphasized that this
is a reference line and no particular petrogenetic significance is
attached to it. Measured total-rock intergranular surface area
increases from 12.37 mmz/mm3 in the relatively unsheared quartz

monzonite to 82.84 mmZ/mm3 in the most severely sheared sample.

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Figure 4,

Plot of plagioclase-plagioclase surface area per
unit volume versus Sv- The per cent of SV con-
tributed by plagioclase-plagioclase boundaries in—
creases dramatically across the shear zone; An
initial increase in plagioclase-plagioclase boun-
daries through post-cracking and post—polygonization
sliding is followed by the development of even finer
grained plagioclase subgrain mosaics, presumably

by continued subgrain polygonization.

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Figure 5, A and B. Quartz polygonization leads to minor
optical offsets in strained primary crystals,
Subsequent grain boundary sliding will lead to
unstrained optically heterogeneous quartz mosaics,
C and D, PoLygonization and subsequent grain
boundary sliding produce well—defined plagioclase
subgrain mosaics,

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Quartz-quartz surface area (Fig. 2) increased from .70 mmz/mm3 in the

relatively unsheared quartz monzonite to a high of 12.48 mm2/mm3 in the

most severely sheared sample. The quartz-quartz surface area does not

deviate appreciably from the one-to-one reference line in the low-

shearing interval. The intermediate zone shows a slight increase in

the rate of quartz-quartz surface area development, and a rapid increase

in surface area development is evident in the highly-sheared zone. The

K-feldspar-K-feldspar surface area (Fig. 3) ranges from an initial low

of .63 mmZ/mm3 to a high in the intermediate shearing interval of 13.55

mmz/mma. In the highly-sheared zone, K-feldspar is lost from the sys-

.tem. K-feldSpar-K-feldspar surface area deviates much more rapidly

from the one-to-one reference line than does quartz-quartz surface

area. Measurements are somewhat erratic in the intermediate zone,

but several very high phase-phase surface area samples are recorded.
Plagioclase-plagioclase surface area (Fig. 4) ranges from a low

of .77 mm2/mm3 in the relatively unsheared quartz monzonite to a high

of 32.02 mmz/mm3 in the highly-sheared zone. The plagioclase-plagioclase

surface area in the highly-sheared rock is 41 times that of the unsheared

rock. The plagioclase-plagioclase surface area shows a marked deviation

from the one-to-one reference line in the low-shearing zone and sys-

tematically increases through the intermediate and highly-sheared zones.

Microtextural Features
All three of the mineral phases of interest show evidence of host-
grain microcracking (Fig. 1E, F, G) and polygonization (Fig. 5) in

thin section. Those samples from the low-sheared zone show fractures

 

 

Figure 6. . A and B. Strain-free quartz mosaics resulting

from post-polygonization grain boundary sliding.
The mosaics are generalty ovate in outline,

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Figure 7.

A and B, The development of finely crystalline
plagioclase subgrain mosaics is the result of con-
tinuing subgrain polygonization with increasing
strain rates, Grain boundary sliding under shear
stress produces an augen-like outline around the
potygonized "ghost" plagioclase porphyroclasts,
C, D, and E. Polygonal shapes characterize the
fine—grained plagioclase, F, Some larger sub-
grains are preserved within the finer—grained
material.

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in many of the larger porphyroclasts. In the intermediate and highly-
sheared zones, quartz grains develop a stretched vein-like texture
with rectangular subgrains (Fig. 6). Plagioclase and K-feldspar
polygonize rapidly thus developing subgrain mosaics (Fig. 7). Many
polygonized plagioclase porphyroclasts are stretched into ovate augen
structures (Fig. 7A, B).

Textural disequilibrium (Fig. 7F, 8A, B) is evident throughout
the shear zone. A polygonized stretched plagioclase grain may well
be spatially close to a fractured remnant of a larger primary plagio-
clase'porphyroclast. Such broken fragments exist in all samples,
many with igneous zoning still intact. The intensely sheared sample
that exhibits the highest plagioclase-plagioclase surface area con-

tains many such fragments embedded in a finely crystalline matrix.

Figure 8. A and B, Textural disequilibrium is commonplace
across the shear zone, Large primary porphyro-
elastic plagioclase grain fragments are often

spatially close to totalLy polygonized plagioclase
host grains,

scale-“100)!

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INTERPRETATION OF SURFACE AREA RESULTS

The dramatic increases in phase-phase surface area documented in
this study support the contention that brittle grain microfracture and
host-grain polygonization were the dominant deformational mechanisms in
the Little Eau Claire River shear zone. An initial reduction in grain
size is caused by microcrack propogation (Fig. 1D, E, F). Crack
nuclei are abundant in large primary crystals and will significantly
alter the deformational behavior of these crystals at lower strain-
rates. With increasing strain-rate, work-hardening intensifies and
increased frictional heating activates polygonization as a strain
recovery process (Carter, 1971). Both microfracture and polygonization
increased the phase-phase surface area of the rock mosaic.

Granulation profiles for quartz, K-feldspar, and plagioclase show
distinct differences. Quartz-quartz surface area is much less than
K-feldspar and plagioclase in the lower and middle zones and much less
than plagioclase in the highest-sheared zone. In experimental syntec-
tonic quartz deformation, no significant recovery occurs until temper-
atures of 800°C - 900°C (Hobbs, 1968). Grain size of quartz at low
strain-rates is 10 times that at high strain-rates (Hobbs, 1968).
However, the presence of water greatly affects the deformational habit
of quartz. The presence of epidote veining within the shear zone suggests
the presence of a fluid phase, which would enhance the ability of all
silicates to recover. The difference in surface area reSponse of the

three phases may reflect differences in physical properties of these

11

12

phases as well as differences in primary grain size and modal percents.
The relatively lower values of quartz-quartz and K-feldspar-K-feldspar
surface area as compared to plagioclase-plagioclase surface area may
indicate that recrystallization grain growth more readily offsets the
effects of microfracture and polygonization in these phases than in
the plagioclase within the shear zone. Detailed microsc0py (Fig. 9A,
B) reveals that it is not until the extremely sheared samples that
curved grain boundaries indicative of recrystallization grain growth
occur in the fine-grained plagioclase. This indicates that the major
deformational mechanism in plagioclase was grain microfracture and
polygonization since these processes will generate high plagioclase-
plagioclase surface areas. Recrystallization grain growth did occur
but was impeded by the high strain-rates. In the most severely sheared
samples, the strain-rates were high enough to severely impede quartz
grain growth by recrystallization, and a very fine-grained quartz
fraction developed. It is likely that the development of stretched
augen and vein-like grains of quartz, K-feldspar, and plagioclase
was caused by a high-temperature deformational mechanism such as
grain-boundary sliding (Elliott, 1973; Kingery, 1960; Kirby and
Raleigh, 1971) occurring after the development of subgrain mosaics.
The disappearance of K-feldspar from the system is difficult to
evaluate but perhaps can be explained with the aid of the surface area
data. Several very high R-feldspar-K-feldspar surface area measure-
uents occur in the intermediate sheared interval immediately before
the loss of K-feldspar. A possible interpretation of the data is

that the polygonization of the K-feldspar, which is indicated by the

Figure 9, A. Plagioclase subgrain boundaries are angular
within the zone of intermediate shearing.
B. Plagioclase subgrain boundaries become curved
within the zone of intense shearing, indicating
the activation of recrystallization grain growth,

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13

high surface area measurements, aided in the atomic diffusion of
potassium from the feldspar lattice. Each new subgrain boundary
serves as a potential perfect source or sink for atomic vacancies,

thus enhancing diffusion of potassium out of the system.

PLAGIOCLASE COMPOSITIONS

All Grains

The dramatic variation in plagioclase-plagioclase surface area
within the Little Eau Claire River shear zone documents the intense
textural readjustment of plagioclase associated with the shearing
episode. Plagioclase composition, a sensitive petrogenetic indicator,
also responded to the energy conditions of the shear zone.

Plagioclase compositions were recorded using an ARL EMX micro-
probe. In the initial sampling, compositions were recorded for each
plagioclase grain encountered on parallel line traverses across thin
sections from ten selected samples within the shear zone. When large
porphyroclastic plagioclase grains or fragments were encountered,
compositions were recorded at standard 5 micron intervals across the
grains. This compositional data reflects not only variation between
the finely crystalline plagioclase matrix grains, but also the varia-
tion within and between the larger porphyroclasts. The data are
presented in frequency histograms with class intervals of 2% Ab (Fig.
10, all grains). Because of textural and compositional disequilibrium
encountered in samples from the shear zone, it is reasonable to assume
that the position of the compositional modes of these frequency diagrams
most closely indicates those compositions that for any particular sam-
ple represent an attempt by the system to reach equilibrium. In other
words, if the compositions are indeed responding to the temperatures

and strain-rates associated with the deformation, then a mode should

14

Figure 10.

Plagioclase compositions determined by an ARL
EMX microprobe are presented in histograms with
class interval 2% Ab, Compositions were re-
corded for samples within the entire population
of plagioclase grains in each thin section
(labelled "all grains") and also for samples
within the fine-grained fraction exclusively
(labelled "fine grains"), Thin sections are num—
bered in order of increasing Sv- Mode
positioning is used to most closely approximate
the stable compositions for each sample. Com—
positional modes within "all grains" indicate a
possible peristeritic reaction within the
plagioclase. The apparently unimodal calcic
populations developed within the "fine grains"
suggest that the entire shear zone eventually
reached temperatures near the crest of the
peristerite solvus (6000—65000) and only the
fine-grained plagioclases equilibrated with
these temperatures,

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develop at or near the most stable compositions for a given temper-
.ature and strain-rate. An examination of the histograms using this
criterion reveals an interesting compositional variation across the
shear zone.

The initial relatively unsheared sample 1 shows an obviously
hetergeneous plagioclase compositional population. This is reflected
in the zoning evident in the primary igneous grains. Using total-rock
surface area as an indication of the intensity of shearing, it is appar-
ent from samples 2 and 3 that at low to moderate frictional heating
and relatively minor granulation a marked decalcification of the plag-
ioclase occurs. Compositions are pushed towards higher albite percents
and the distributions tighten considerably. As total-rock surface area
increases (sample 4), a distinct compositional subpopulation with a
mode centered between 78-80%.Ab is indicated. Sample 5 exhibits a
calcic mode between 84-88% Ab and also the first appearance of a high-
albite subpopulation between 94-98% Ab. Samples 6 and 7 show varia-
tion in calcic and albitic mode development, another indication of
compositional disequilibrium within the shear zone. In sample 8, the
two modes have converged and subpopulations are centered between
82-84%.Ab and 88-90% Ab. Sample 9 exhibits a somewhat skewed normal

distribution with an intense mode between 84-86% Ab.

Interpretation Plagioclase Compositions: All Grains
The less sheared samples show marked decalcification of the
plagioclase feldspars, perhaps aided by the effects of microfracture

and beginning polygonization. The calcic arm of the peristerite solvus

16

is indicated by the compositional population of sample 4 and the increas-
ingly sheared samples by our measurements have strong indications of bi-
modal populations until the highly-sheared samples 8 and 9 are encountered.
Here the distributions tighten and 9 appears to be nearly normally dis-
tributed. Beach (1973) has interpreted plagioclase compositional vari-
ation in the Scourie shear zone as evidence for a peristeritic reaction
within the sheared interval. A similar interpretation can be made from
this data, and a hypothetical peristerite solvus (Fig. 11) is presented
based on mode positioning (in % Ab) versus the shearing intensity based
on total-rock surface area. Due to the compositional and textural dis-
equilibrium apparent across the shear zone, no attempt is made to accu-
rately describe a peristerite solvus for these rocks. Rather, a trend

is indicated which suggests that a peristerite-type reaction has occurred
in these rocks. The data suggest that the two highly-sheared samples
have attempted to equilibrate with temperatures very near or slightly

above the crest of the peristerite solvus.

Plagioclase Compositions Within the Fine-Grained Fraction

Microfracture and polygonization have developed a fine-grained
subpopulation within the intermediate and highly-sheared samples
from the shear zone. In order to establish what effect the development
of this fine-grained fraction has had on plagioclase composition, a
second compositional sample was recorded from selected fine-grained
lareas within the six thin sections whose plagioclase compositions were
interpreted as peristeritic. The data are again presented in frequency
histograms with a class interval of 2% Ab (Fig. 10,-fine grains).

In the less sheared samples 4 and 5, very little difference can

be seen in comparison with the whole grain assemblage histograms.

Plot of "all grains" compositional mode positions
(in % Ab) versus SV to illustrate the development
of a peristeritic trend in plagioclase compositions
with increased shear intensity.

Figure 11,

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However, in the more intensely sheared samples, obvious differences can
be noted. The compositional distributions within the fine-grain mat-
erial tighten, and modes indicate a calcic compositional population
in each of the highly-sheared samples. Apparently, the albitic and
calcic limbs of the peristerite solvus are approximately equal within
the fine-grained material. This indicates that the frictional heat-
ing associated with shearing eventually raised the entire shear zone
to a temperature near the crest of the peristerite solvus. The fine-
grained material formed by the shearing was able to equilibrate with
this temperature, the coarser material was not. The albitic side of
the peristerite solvus therefore occurs within the coarser grain frag-
ments, and represents compositional disequilibrium within the coarse-

grained plagioclases.

CONCLUSIONS

 

Quantitative surface area data and qualitative thin section obser-
vations support the hypothesis that strain-rates within the Little Eau
Claire River shear zone were sufficiently high to impede recrystalliza-
tion grain growth as the major deformational mechanism. A comparison
with surface area measurements of the regionally metamorphosed Cross
Lake granodiorite (Ehrlich g£_§l, 1972) exemplifies the differences
between a predominantly recovery phenomenon such as is seen in the ‘
shear zone and the recrystallization phenomena within the regionally
metamorphosed rocks. In the Cross Lake granodiorite, normalized plag-
ioclase surface area shows a very high initial surface area in the
middle greenschist facies. Then, as the metamorphic grade increases,
the surface area decreases systematically until an apparent equili-
brium value is reached. The plagioclase-plagioclase surface area
(unpublished data) shows the exact same trends. The initial surface
rarea high is attributable to early granulation effects in the green-
schist facies. It is possible that within these rocks the strain-
rates were high enough to foster recovery over recrystallization
grain growth in the greenschist facies, much as in the shear zone.
However, as soon as the temperatures became sufficiently high, recry-
stallization grain growth became dominant and the surface area decreased.
In the middle amphibolite facies, an apparent steady-state flow regime
is attained, as recrystallization grain growth and nucleation of new
grains balance one another to achieve an equilibrium surface area

- 18

l9

configuration.

Within the Little Eau Claire River shear zone, temperatures were
not high enough to promote recrystallization grain growth as the dom-
inant mechanism due to the strain-rates associated with the deformation.
No dramatic decrease in plagioclase surface area occurs across the shear
zone. Whole rock surface area increases as more and more granulated
phases occur. Therefore, it is concluded that within shear zones de-
veloping classical mylonitic textures, strain-rates are extremely high,
leading to grain microfracture and polygonization and subsequent grain
boundary sliding as the dominant deformational mechanisms.

Plagioclase compositional data indicate temperatures associated
with the dynamic metamorphism of the Little Eau Claire River shear
zone exceeded the crest of the peristerite solvus. Temperatures in the
range of 600°C - 650°C are indicated (Crawford, 1966). Strain-rates
necessary to impede recrystallization grain growth in plagioclase at
these temperatures may be reproducible in laboratory experiments.

The data also suggest that the ability of plagioclase to equili-
brate with the temperatures generated during shearing was kinetically
(enhanced by the intergranular surface area generating processes of mi-
crofracture and polygonization.

The N30°E trending shear zones of central Wisconsin must result
from a tectonic episode capable of producing strain-rates high enough
to explain the observed textures and inferred temperatures documented

in this study.

BIBLIOGRAPHY

 

Beach, A., 1973. .The mineralogy of high temperature shear zones at
Scourie, N.W. Scotland. J. Petrol., 14: 231-248.

Carter, N. L., 1971. Static deformation of silica and silicates. g;
GeOphy_. Res., 76: 5514-5538.

 

Coble, R. L. and Burke, J.E., 1963. Sintering in ceramics. In: J. E.
Burke (Editor), Progress in Ceramic Science, Vol. 3. McMillan, New
York, pp. 197-2490

 

Crawford, M. L., 1966. Composition of plagioclase and associated
minerals in some schists from Vermont, U.S.A., and South Wéstland,
New Zealand, with inferences about the peristerite solvus. Contr.
Mineral and Petrol. 13: 269-294.

 

DeHoff, R. T. and Rhines, F. N., 1968. Quantitative Microscopy. McGraw-
Hill, New York, 422 pp.

 

DeVore, G. W., 1959. Role of minimum interfacial free energy in deter-
mining the maCroscopic features of mineral assemblages: I. The
Model. J. Geol., 67: 211-226.

Ehrlich, R., Vogel, T. A., Weinberg, B., Kamilli, D. C., Byerly, G.
and Richter, H., 1972. Textural variation in petrogenetic analyses.
Bull. Geol. Soc. Am., 83: 665-676.

 

Elliott, D., 1973. Diffusion flow laws in metamorphic rocks. Bull.
Geol. Soc. Am., 84: 2645-2664.

 

Hobbs, B. E., 1968. Recrystallization of single crystals of quartz.
Tectonophysics, 6: 353-401.

 

Kingery, W. D.. 1960. Introduction to Ceramics. John Wiley and Sons,
New York, 781 pp.

 

Kirby, S. H. and Raleigh, C. B., 1973. Mechanisms of high-temperature,
solid-state flow in minerals and ceramics and their bearing on the
creep behavior of the mantle. In: E. Irving (Editor), Mechanisms
of Plate Tectonics, Tectonophysics, 19: 165-194.

 

 

La Berge, G., 1972. Lineaments and mylonite zones in the PreCambrian
of northern Wisconsin (abstract). Annual Institute on Lake Superior

Geology, 18: p. 27.

20

21

La Berge, G. and Myers, P., 1973. PreCambrian geology of Marathon
county. In: Guidebook to the PreCambrian Geology of Northeastern
and Northcentral Wisconsin, Annual Institute on Lake Superior Geology,
19: 31-36.

 

 

Ramsay, J. G. and Graham, R. H., 1970. Strain variation in shear belts.
Can. J. Earth Sci., 7: 786-813.

 

Spry, A., 1969. Metamorphic Textures. Pergamon Press, Oxford, 350 pp.

 

Underwood, E., 1970. Quantitative Stereology. Addison-Wesley Publish-
ing Co., Reading, Mass., 274 pp.

 

 

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