The influence of the low-high quartz transformation on recrystallization and grain growth during contact metamorphism (Traversella intrusion, North Italy)

Tectonophysics,

120 (1985) 107-117

Elsevier Science Publishers

THE INFLUENCE

107

B.V., Amsterdam

- Printed

in The Netherlands

OF THE LOW-HIGH

RECRYSTALLIZATION METAMORPHISM

QUARTZ

TRANSFORMATION

ON

AND GRAIN GROWTH DURING CONTACT

(TRAVERSELLA

INTRUSION,

NORTH ITALY)

R. WIRTH Mmeralogisches (Received

Institut der Lkersitiit

August

zu Kijln, Ziilprcherstr.

30, 1984; revised version

accepted

- I (West-Germany)

49, 5000 Kijln

February

25, 1985)

ABSTRACT Wirth,

R., 1985. The influence

growth

during

contact

of the low-high

metamorphism

quartz

transformation

(Traversella

intrusion,

on recrystallization

North

Italy).

and grain

Tectonophysics,

120:

107-117. Recrystallization recrystallization at about

and gram

transformation low-high

recrystallization

by

disordered

of atoms

transformation. the atomic

structure during

which

The remarkable and grain

displacement

the low-high

are transferred

migration

This limit of

of 620 f 20°C. which coincidence

growth

becomes

from

quartz

with the

mechanism

of the grain boundary

to the other grain. A lowering

results such that grain boundary

temperature

of recrystallization

of the SiO, tetrahedra

of the number

35 m of the contact.

maximum

quartz

is explained

The already

and from the grain boundary migration

only within

for the beginning

of quartz

by the fluctuations

occur

with an inferred

for the low-high

620°C

transformation.

in an increase

boundary, boundary

temperature

quartz

more distorted

of about

of quartz

coincides

6-7 km is the temperature

of the temperature

results

growth

and grain growth

of the

becomes

transformation,

one grain

of the activation

even This

into the grain energy

the most important

for grain

process

for

and grain growth.

INTRODUCTION

The Sesia-Lanzo Zone is one of the main structural units of the internal Western Alps. It comprises polymetamorphic and some monometamorphic rocks and is part of the Austro-alpine continental crust. During an Early Alpine event the Sesia unit was overprinted by high-pressure low-temperature metamorphism owing to a subduction process. The Sesia-Lanzo zone is characterized mainly by widespread jadeitic pyroxene-garnet-quartz assemblages in paraschists and metagranitoids (Compagnoni et al., 1977). These rocks were affected by the Late Alpine and post-tectonic Traversella Intrusion. Most of the country rocks are glaucophane-mica schists (Reinsch, 1979). The quartz crystals in these mica schists have been deformed during subduction .and uplift of the Sesia unit. Contact heating of the intrusion initiated processes like recovery, recrystallization and grain growth of formerly

0040-1951/85/$03.30

0 1985 Elsevier Science Publishers

B.V.

108

strained

crystals.

Generally,

recovery

and point defect agglomerates, annihilation loops

of dislocations

of opposed

(Haessner,

1978). The

more favourable

configurations migration is continued

migrate

the average

grain

from the magnitude

specific smaller

further diameter.

of dislocations of point

are the basic processes after the completion

through

the recrystallized

The driving

of point

of dislocation

forming

energetically

defects and dislocations

of recrystallization

(Haessner,

of recrystallization, structure,

force for grain

defects

of these defects, and the

sign as well as the shrinking

and the absorption

1978). If annealing

by reactions

the annihilation

rearrangement

by grain-boundary boundaries

is characterized

in particular

growth

the grain

thereby

increasing

(coarsening)

stems

boundary energy of the grains; it is about three orders of than that of recrystallization. Consequently, grain growth phe-

nomena proceed much more slowly (Haessner, 1978). Voll(l960) has pointed out the relevance of these processes for the grain-boundary distribution of naturally deformed rocks. Recovery, recrystallization, and grain growth are supposed to be thermally activated processes (Haasen, 1972; Haessner, 1978). For contact metamorphic heating of quartzites at the Ballachulish granitic intrusion-Scottish Highland Voll (1968) derived a threshold temperature of 59O’C for the beginning of grain growth of quartz.

He pointed

out a possible

relationship

between

the threshold

temperature

for grain growth of quartz in a contact aureole and the low-high quartz transformation temperature (Voll, 1968). This idea is supported by a recent paper of Joesten (1983) which reports a temperature quartz in a contact aureole.

of 600°C

for the beginning

of grain growth

of

In this paper a model is proposed, which shows, how recrystallization and grain growth are affected by the atomic transformation mechanism of the low-high quartz transformation. This model is based on investigations of recrystallization and grain growth of quartz in the contact aureole of the Traversella Intrusion. METHODS AND MATERIAL STUDIED

Microscopic investigations concerning recrystallization and grain growth of quartz in the contact aureole of the Traversella Intrusion have been carried out by polarized light analysis. The investigated rocks in the contact containing up to 47% quartz by volume.

aureole are glaucophane-mica The quartz grains in these

schists rocks are

arranged in clusters of several hundred grains. These clusters contain only occasionally micas or feldspars. The grain size of the quartz was determined by measuring the smallest and the largest diameter of each quartz grain, and calculating the mean value. About 450 grains from different clusters were measured in every thin section. Then the mean value of 450 grains and the standard deviation were calculated. Grains not completely surrounded by other quartz grains (quartz-mica, quartz-feldspar phase boundaries) were not measured. Also omitted were deformed

109

i.e., those containing

grains,

these measurements microscope. The total amount Early

Alpine

Dauphin6 electron studies,

of quartz,

deformation,

were determined ture (Comer,

undulant

were carried

and

the amount

low-angle

of newly

of Dauphin6

thin sections

schists. Suitable

and mounted

mm and the thickness

areas of quartz

on copper

about

All

using a light

grains

deformed

recrystallized

through

the inversion

twins was confirmed

microscopy (TEM), and scanning electron samples of quartz clusters were prepared

glaucophaneemica

grown

boundaries.

ocular

by

quartz

by means of a Swift pointcounter.

in quartz by cooling

1972). The existence

grain

of a calibrated

of the host quartz

the amount

from thin sections

twins are produced

extinction,

out by means

microscopy by cutting clusters

grids. The diameter

0.03 mm. The samples

tempera-

by transmission

(SEM). For TEM thin sections from

were extracted of the samples

were thinned

from the is about

3

for TEM by ion

beam milling (argon 99.999/5 kV-80 PA), and coated with carbon. Electron microscopy was performed in a Jeol JEM 200 CX microscope with side entry goniometer. The microscope was operated at 200 kV. For SEM studies thin sections with a polished surface were etched by hydrofluoric acid vapor (HF 40%) for 35 min. The samples were coated with carbon, and additionally with chromium tilting the samples at about 45”. Scanning electron microscopy was performed in a Cambridge SEM, operated

at 30 kV.

RESULTS

The Traversella km has a vertical estimated intrusions

Intrusion, outcrop

a quartz diorite with horizontal of about

dimensions

1200 m. The temperature

of 4.5 x 1.5

of the intrusion

is

in the range of 800”-900°C referring to the data of Winkler for granitic (Winkler, 1962, 1967). The depth of the instrusion was derived from the

breakdown reaction of phengite, one of the most common minerals of the glaucophane-mica schists of the contact aureole (Wirth, 1984). At a distance of about

36 m to the intrusion

phengite

+ quartz + biotite

the following + sanidine

f cordierite

reaction

+ oligoclase

was observed: + andalusite/sillimanite

+ spine11 k corundum

+ H,O

At an interval of 36-16 m from the intrusion andalusite contact, sillimanite appears, and the amount of andalusite a distance

of about

10 m to the intrusion

only sillimanite

occurs. Approaching is strongly diminished. occurs.

the At

The breakdown

reaction of phengite with sanidine and cordierite indicates that at a distance of 36 m to the intrusion contact metamorphism has reached the alkalifeldspar-cordierite hornfels facies. That means temperatures of 580 + 20°C at a pressure of 0.5 kbar, or 630 + 20°C at a pressure of 2 kbar (Winkler, 1967). From the appearance of andalusite followed by sillimanite, a depth of the intrusion of less than 7 km ( -c 2 kbar) is derived, using the stability curves of Al,SiO, (Holdaway, 1971). and

110

muscovite pressure

+ quartz

The maximum

(622 + lO”C/2

temperature

ture of the intrusion for the intrusion,

(Winkler,

of the contact

kbar, Althaus

The breakdown

temperature

reaction

that a temperature

a temperature

at the contact

of 660”-690°C

range of 800°-900°C

rocks of about

150°C

(7

would be in the range of 630”-690°C.

at a distance at the contact

the maximum

and no andalusite.

to be 60% of the tempera-

of the surrounding

of phengite

80 m to the intrusion

et al., 1970). A confining

sillimanite

is estimated

1967). Assuming

and a temperature

km), the maximum

of about

reaction

of more than 2 kbar would have created

of 36 m to the intrusion is more reasonable.

temperature

indicates

At a distance

in the contact

aureole

has

reached 500”-52O’C. This value is derived from the sanidine-microcline transformation temperature (Laves, 1960). At an interval of 80 m to the contact the alkali-feldspar is sanidine. The distribution of the maximum temperatures in the contact aureole can be assumed to be 660”-690°C at the contact, about 630°C at 36 m, and 500-520°C at about 80 m. It is possible to report exact distances to the intrusion, contact

because

the contact

plane has an exposed

The temperature

Fig. 1. Quartz boundary, caused

crystal

plane

is vertical

vertical

dimension

of the low-high

with Dauphin6 damage.

At the outcrop

the

of 250 m.

transformation

twin (12 m from the intrusion).

visible with a (1011) reflection.

by radiation

quartz

at the outcrop.

is pressure

Fringe contrasts

dependent

distinguish

the twin

The small black dots on both sides of the twin boundary

are

117

(Buntebarth,

1980). At a pressure

625°C. Thus, the low-high of about

aggregates

the literature

P-quartz

transformation

36 m to the intrusion.

that the quartz From

quartz

of 2 kbar (7 km) the transformation

through

that Dauphine

the inversion

was reached

the stability

twinning

temperature

evidence

field of P-quartz.

can be produced

(Comer,

is

at a distance

there is some microstructural

from this area have reached

it is well known

by cooling

temperature

In addition,

temperature

from

1972). The twinned

parts are related by a rotation of 180” about the c-axis and are of the same hand. Dauphin6 twins cannot be detected by optical methods but are visible on suitably etched specimens. Dauphins microtwins can be identified by fringe patterns bonding the

twin,

microscope

visible

with

(Comer,

strong

and SEM at an interval which

did not

(1Oil)

reflections,

1972). Four different

reach

the inversion

temperature

Fig. 2. SEM

by hydrofluoric

and additionally

of quartz

line marks the Dauphin6

around

the c-axis.

two samples m). The TEM

acid (Fig. 2). Etch pits are to be seen on the surface

by the dashed

dashed

(54-80

electron by TEM

Dauphin5 twins on different electron by SEM studies from samples (36-O

micrograph showing a quartz crystal etched with hydrofluoric

The etch pits, separated

a transmission

have been investigated

of 36-O m to the intrusion,

studies from these four samples reveal many micrographs (Fig. 1). This result is supported m) etched

using

samples

line, show different

twin boundary.

orientations

The etch pits (triangles)

of

acid (25 m from the intrusion). (v left side; A right side). The are related by a rotation

of 180”

112

those quartz

grains which have their c-axis normal

etch pits are shaped arrangements to have

occasionally

the c-axis of the quartz reached

the P-quartz

Dauphine

The investigated

to the surface of the sample.

It is very common

of etch pits with different

of 180” about not

like triangles.

orientations. grain.

that in one grain They are related

In contrast,

temperature

by a rotation

the two samples

conditions

(54-80

The

there are inferred

m) show

only

twinning.

rocks are glaucophane-mica

schists containing

up to 47% quartz

by volume. 51 samples were collected at an interval of 317 m from the intrusion. At a distance of 317 m quartz appears as polycrystalline aggregates inside the glaucophane-mica schists. There are two different groups of quartz grains. Quartz. which have suffered following deviation tion,

Early Alpine

deformation,

are called host quartz

grains

in the

sections. The mean grain size of these host quartz is 328 pm (standard = 145 pm). The host quartz exhibit deformation lamellae, undulose extinc-

sutured

grain

boundaries,

incipient

polygonization

recrystallization

(formation

at grain

of

boundaries

low-angle (Fig.

grain

boundaries),

and

recrystallized

quartz grains have a mean grain size of 50 pm, and they are the second

3). These

group of quartz grains. The recrystallized quartz very often show undulose extinction, indicating that they have been deformed again or that they were formed by dynamic recrystallization. The undulose extinction makes them different from the statically recrystallized quartz, formed by the heating of the intrusion. At a distance

Fig. 3. Host undulose

quartz

extinction,

grains

which

sutured

grain boundaries.

have suffered

Early

Alpine

polygonization.

deformation. and incipient

The quartz recrystallization.

grains

exhibit

113

%

DEF WART2 RECRYST WART2

100

x loo

I% I ,

.*a.

.“.:

.

*. *.*..* .

.

10

1.. .

.. ‘.

..a..

.

.

.

.

.

.

.

.

i

l0.5

0.J

1

5

10

50 DISTANCE

Fig. 4. Percentage as a function

of deformed

of the distance

host quartz

TO

? 1000

1w THE

INTRUSION

grains and statically

[ml recrystallized

quartz

(by contact

heating)

to the intrusion.

of 40-317 m from the contact the volume percentage of host quartz grain to recrystallized quartz grains is high: 95575% and 5-25%, respectfully. In this range there is no significant change of the percentage. At a distance of 40-20 m to the contact the amount of host grains strongly decreases, and the amount of recrystallized quartz increases (Fig. 4). Quartz grains which have recrystallized by the heating of the intrusion

show no undulose

quartz grains increases At a distance grains decreases

towards

extinction.

the contact

The grain

size of the recrystallized

by grain growth (Fig. 5).

of 20-O m from the intrusion the percentage of the host quartz to 10% by volume, while the percentage of statically recrystallized

GRAIN SIZE bml T

L. 5

1

Fig. 5. Grain function

10

size of the quartz

of the distance

grains

to the intrusion.

50

100

DISTANCE

TO

which

th

40 THE

INTRUSION

have undergone

(error bar: standard

Iml

grain

growth

deviation).

(recrystallized

grains)

as a

Fig. 6. Coarsened

quartz

angles at triple points.

grains

near the intrusion

and without

undulose

(1.6 m) with straight

grain

boundaries.

equilibrium

extinction.

grains increases. From a distance of 40 m to the contact of the intrusion the grain size of the recrystallized grains increases from 50 pm to 260 pm by grain growth (Fig.

5). The coarsened

quartz

grains

are the result

of three

different

processes:

polygonization of the deformed host quartz, recrystallization and grain growth. The result is an arrangement of coarsened quartz grains with straight grain boundaries, equilibrium

angles at triple points and the absence of undulose

extinction

(Fig. 6).

The most important result is that recrystallization and grain growth are initiated by the heating bf the intrusion at a distance of about 35 m to the contact. As mentioned

above,

this distance

can be correlated

620 k 20°C. Referring to an intrusion depth ture for the low-high quartz transformation.

with a maximum

of about

temperature

of

5-7 km, this is the tempera-

DISCUSSION

Voll (1968) supposed that the beginning of grain growth during contact metamorphism may be initiated by the low-high quartz transformation. This assumption may be supported by the data of Joesten (1983) and the data presented in this paper. Recrystallization and grain growth are both explained by migrating high-angle grain boundaries. The migration of grain boundaries is suggested to occur the following way (Gleiter and Chalmers, 1972): Suppose, two grains are separated by a

115

grain

boundary

emission absorption model

of definite

of atoms

from

thickness.

the steps

of the same number

refers to metals.

SiO, tetrahedra

of atoms

The grain

area in the crystal lattice.

Grainboundary

on the surface

occurs

by

the

grain

and

by

at the steps of the growing

boundary

is assumed

It has not been proven

is involved

migration

of the shrinking

in the migration

grain.

to be a strongly

that the transfer

of grain

This

distorted

of single atoms or

boundaries

between

quartz

crystals. Assuming a transfer of single atoms, the strong Si-0 bond has to be broken. Grain boundary migration would then be correlated with a high activation energy.

This

migration

situation

of grain

hydrolysis,

is more

boundaries

and a transfer

favorable (Hobbs,

of single

if water

molecules

1968). The Si-0

atoms

are involved

bond

will be favored

in the

will be broken

(hydrolytic

by

weakening,

Griggs 1967). However, investigations to understand the nature and the behaviour of grain boundaries in silicates still have to be done. Experiments with metal alloys have shown that the mobility of grain boundaries depends on the energy of the grain boundary. That is, it depends on the disordered state of the crystal lattice in the grain boundary (Gleiter and Chalmers, 1972). An increase of the disordered state of the atoms in the grain boundary would result in a reduction of the activation energy for grain-boundary grain growth

migration.

should

In quartz

The threshold

temperature

for recrystallization

and

then decrease.

bearing

materials,

an

additional

distortion

of the grain-boundary

structure is caused by the low-high quartz transformation. The low quartz structure consists of a tetrahedral network in which tetrahedra are joined in a double spiral with two nonintersecting

threads

around

the trigonal

axis. The two states (Y, and (Ye

of low quartz of the same enatiomorph (left or right handed) are transformed into each other by a diad axis coinciding with the trigonal axis. These two states correspond to Dauphine twin domains which differ by tilt angles of opposite direction. Low-quartz, thus, consists of two equilibrium states (Y, and a*. An individual described Megaw

chain parallel to directions [loo], [OlO] or [llO] of the tetrahedra may be as a particle in a double minimum potential (Liebau and Bohm, 1982). (1973) suggested

temperature,

that in a temperature

the low-quartz

single crystal

These Dauphine

twin domains

of the domain

walls

transition states

temperature

were

having

observed

the chains

(Y, and CX*.These fluctuations

start

region

breaks

slightly

strongly

temperature

by Van

Tendeloo

to fluctuate

increase

below

the transition

up into small (r, and a2 domains. dependent et al. (1976).

between

fluctuations Near

the

the two permissible

when the temperature

is raised above

the transformation temperature T,. Below T, such fluctuations are correlated, yielding microdomains (Liebau and Bohm, 1982). Above T, the long-range correlation ceases. Each chain fluctuates independently between the two equivalent minimum states in a double minimum potential. The P-quartz structure can be concieved as both a geometrical and a time average of the two equilibrium positions. The most important point of the low-high quartz transformation mechanism is the fluctuation of the SiO, tetrahedra at the transformation temperature T,. These

116

fluctuations

are supposed

An increase

of the disordered

activation

to increase

energy needed

the disordered

state of a quartz grain-boundary.

state of the grain boundary

for grain-boundary

migration;

from the surface of one grain into the grain boundary, surface

of the growing

The

low-high

temperature

of the

of atoms

and from the boundary

to the

grain.

quartz

range

results in a lowering that is, the transfer

transformation

between

530”

and

shows 572°C

a volume (Taylor,

change

1972).

of 1.6% in a

This

results

in an

additional drfving force for grain-boundary migration. The quartz aggregates in the glaucophane-mica schists are enclosed by other minerals like phengite, feldspar. chlorite and biotite. An increase of the volume of quartz grains in the quartz aggregates causes a stress in the quartz, and of course in the surrounding minerals. However, stress on the quartz grains results in an additional driving force for grain boundary migration, thus, accelerating grain-boundary migration. Voll (1976) recrystallization

reported a threshold temperature of 290°C for the beginning of quartz in the Swiss Alps. These quartz were heated during

of the

Alpine orogeny. Voll has pointed out that the reason for the lower threshold temperature for recrystallization of quartz in regional metamorphic rocks is the great difference heating

in time during

time is supposed

the intrusion,

which the rock series were heated. to be lOOO-100,000

the depth

of the intrusion,

years, depending

and the dimension

In contact

aureoles

the

on the temperature

of

of the intrusive

body.

However, the heating time during regional metamorphism is up to four orders of magnitude higher. At the threshold temperature of 290°C for the recrystallization of quartz. only a few atoms will be transferred from one grain to the other grain in a short time, like the heating time of an intrusion. The only small transfer of atoms in a short time (1000 years) results in a nearly invisible displacement of the grain boundary. However, the threshold temperature for recrystallization is not only a function of the heating time, but it is also affected by the stored strain energy. The threshold

temperature

rocks from outside

decreases

quartz

as the rocks inside

stored

strain

recrystallization

energy

with increasing

the Traversella the contact

of the quartz

of quartz

contact aureole. grains

is really initiated

deformation

aureole

Therefore, does

(Haasen,

1974). The

show the same highly not

it can be assumed differ

by the low-high

very quartz

Knowing the depth of an intrusion, a low-high quartz transformation be derived by measuring the beginning of the quartz recrystallization aureole.

much,

strained that the and

the

transformation. isograde may in a contact

ACKNOWLEDGEMENTS

The author would like to thank Prof. Dr. G. Voll for the problem reported here, for critical reading the manuscript, and sions. Thanks are also due to Prof. Dr. H. Gleiter for use of the facilities in his institute. My thanks also go to Dr. S. Vogler and

introduction to the stimulating discuselectron microscopy H. Hofer for critical

117

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