Sequential porphyroblast growth and crenulation cleavage development during progressive deformation

Publishing

SEQUENTIAL CLEAVAGE

171

I 94

Tectonophysics, 92 ( 1983) 17 IElsevier Scientific

Company,

Amsterdam

PORPHYRORLAST

DEVELOPMENT

- Printed

in The Netherlands

GROWTH AND CRENULATION

DURING PROGRESSIVE

DEFORMATION

T.H. BELL and M.J. RUBENACH James

Cook University,

(Received

September

Qld. 4811 (Australia)

Townsville,

1, 1982)

ABSTRACT

Bell, T.H. and Rubenach, ment during

Six stages

deformation.

of crenulation

in both matrix

Australia,

development

homogeneous

ment

in the schistose

cleavage The

matrix.

if a ~~hyrobl~t development,

porphyroblasts

growth

cleavage

to the stages

sequence Dissolution staurolite

developProcesses

including

event

can

of garnet

deformation

and differentiated

(S,)

(D2)

River Formation,

and differentiated

crenulation schistosity

be

N.E. of S,, (S,),

to

garnet,

relative

to the stage of crenulation

history

involved

on all scales, partly development

during

metamorphism

that were unstable

in schistosity occurred

Dz and chloritoid,

staurolite,

formed

progressive,

at various

it could overgrow

type to the next, cleavage

bulk,

as a consequence locally

andaiu-

which commonly develop-

inhomogeneous

of this. Hence each times

during

D,.

all stages of crenutation

the very early ones.

others

of porphyroblasts, accompanied

a single

of these trails varies from one mineral

grew late in the deformation,

increased

overgrew

during

The deformation

tion. Hence the precise timing of mineralogical relative

cleavage

Deformation

Most rocks are rich in po~hyroblasts,

trails. The geometry

of porphyroblast

of crenulation

temperature

cleavage

metamorphism

and the strain is very heterogeneous

Therefore,

and crenulation

trails in the Robertson

to crenulation

crenulation

have bee.n mapped.

inclusion

on the timing

shortening

during

inclusion

from undeformed

prograde

isograds

well-defined

of the six stages

growth

and S. Cox (Editors),

(S,).

These rocks underwent

depending

development

of differentiated

foliation

site, and sillimanite contain

cleavage

and porphyrobtast

These stages progress

through

porphyroblast

In: M. Etheridge

Tecronophysics, 92: 17 I- 194.

in Tectonophysics.

recogmsed

M.J., 1983. Sequential

progressive

in decreasing during

andalusite

reactions

development order

growth

in any

one

locality

in the new conditions, (deduced

and

preserving

from isograds

can also be determined.

consequently

some

them from destruc-

and mineral

In the andalusite

chemistry) zone the

of age, is biotite-g~et-biotite-staurolite-~daiusite,

of staurolite

and especially

andalusite,

and dissolution

of

growth.

INTRODUCTION

Robertson River Formation outcrops extensively over the Robin Hood cattle station in north Queensland and forms part of the Proterozoic Georgetown Inlier, the regional geology of which is summarised in Withnall et al. (1980). The Robertson oo40- 195 l/83/~-~/$03.~

0 1983 Elsevier Scientific

Publishing

Company

172

River Formation quartzites

consists

of a suite of multiply

and amphibolites,

first two deformation

which underwent

events

deformed prograde

at 1570 m.y. (D, ) and

phyllites,

pelitic

metamorphism

schists,

during

the

1468 m.y. (D2 ) respectively

(Black et al., 1979), and local retrogressive metamorphism during the last four deformation events. In the area covered by Fig. 1, the first two deformations developed

penetrative

with no associated Numerous shown

foliations,

crenulation

spectacular andalusite,

retrograde

alteration

are present

local crenulations

in most outcrops,

(see later). They consist

cordierite, related

the last four produced

cleavage.

porphyroblasts

to have grown syn-D,

staurolite,

whereas

plagioclase

to later crenulation

and

of chloritoid,

sillimanite.

deformations

and can be

garnet,

Locally,

biotite,

they

or weathering,

show but in

general they are well preserved and contain abundant inclusion trails forming an internal foliation (S,). They therefore provide an excellent opportunity for studying all aspects of porphyroblast nucleation, growth, dissolution and reactions across a grade transition in a variety of pelitic rock types. Timing of porphyroblast microstructures with respect to deformation has been described

and discussed

at some length in recent years (Ferguson

Dixon, 1976; Vernon, 1978; Olesen, problems associated with alternative

and Harte,

1975;

1978; Vernon and Flood, 1979). A number of origins of porphyroblast/matrix relationships

and internal versus external inclusion trails have been revealed. In the Robertson River Formation, multi-stages of sequential porphyroblast growth, reaction and dissolution resulting from a temperature increase during D, can be seen in thin section. These stages can be demonstrated to occur during D,. We also show that

Fig. 1. Isograd map of part of the Robertson intersection

of the chloritoid

River Formation.

The heavy dashed lines are faults. The

and andalusite isograds with the staurolite isograd in the northwest

map coincides with a change in pelitic lithologies.

of the

173

certain porphyroblast microstructures that previously were accepted as indicators of pre- or post-defo~ation growth are of syn-deformation origin in these rocks and delineate criteria for distin~is~ng this. We have worked with thin sections, oriented perpendicular to the mineral-elongation lineation (N) and parallel to the mineral elongation but perpendicular to S, (P), for all examples of porphyroblasts described in this paper. The photographs are labelled P or N depending on the section used. We have each made an equal contribution to this paper, which presents the initial part of a major structural-microstructural and chemical-metamorphic study of the Robertson River Formation, which we began in 1974. PROBLEMS

WITH

WITH DIFFERENT

GEOMETRIC DEFORMATION

RELATIONSHIPS

BETWEEN

STRUCTURES

ASSOCIATED

EVENTS

In multiply deformed terrains there is ample scope for confusing foliations and lineations of different generations. This problem is intensified if younger crenulation producing deformations also macroscopically fold earlier developed foliations as orientation is no longer a guide. Inhomogeneous strain is essential to the structural geologist/metamorphic petrologist for ascertaining the structural/metamorphic history of a multi-deformed area. However, it also causes confusion because foliations can change in character from well developed to absent from one outcrop to the next for any particular deformation event. If S,, S, and S,, can all develop to similar intensities, ~stinguis~ng between them in individual outcrops may become quite difficult. Even if they were distinguished correctly, specimens collected could later be thin-sectioned incorrectly, because the thin sections might be cut relative to the wrong foliation. Labelling of foliations in the field, plus sketches of specimens in field notes, is the only way to overcome this very real problem. In the Robertson River Formation, S, generally forms a penetrative schistosity. However, locally Dt is present only as open crenulations of S,. In such localities S, could have been mistaken for S, and the crenulation as D, or younger, or if the foliations were not marked on the specimens in the field, the thin sections could have been cut oriented relative to S, and mineral elongation L\ under the misapprehension that they were S, and Lz (terminology after Bell and Duncan, 1978). All po~hyroblasts would be identified as post-D*, syn-D,, rather than syn-D,. This problem can even arise where S, is developed as a spaced crenulation cleavage. It occurs where S, is oriented at a high angle to mineral elongation (L’,) and just one N-section is cut perpendicular to L’,, for it is quite likely that no sign of S, will occur in a section with this orientation. However, a P-section immediately rectifies the problem. Most information regarding porphyroblast timing relative to schistosity and fold development in the Robertson River Formation has come from sections cut normal to L:. However, in this section problems can arise with vergence changes in

174

(a) Fig. 2. a. Sketch of a refolded

Fit fold. The dotted rectangle

parallel

a D, fold. However,

S, inclusion

trails in the porphyroblasts

across it and hence the porphyroblasts laboratory changes

for S,, then thin sections in inclusion

interpreted

could be interpreted cut parallel

trails in the porphyroblasts

this area could be mistaken

would not change vergence

Fp fold. If S, is only weakly developed

b. Sketch of a refolded

of the Fy fold hinge in which

covers a portion

to the axial surface of the FF fold. In outcrop

S, is approximately

for

with respect to S,

as post Dr. and S, in this diagram

was mistaken

in the

to the axial planes of D, folds would show no vergence across

the F,’ fold hinge, and therefore

they would be

as post D,.

porphyroblasts across folds, as Similar problems arise where S, S,, steeply inclined to Ff (Fig. 2 ) not only (mistaking it for F”

it was easy to be distracted by FF folds (Fig. 2a). is not strongly developed and lies at a high angle to 2b), as cutting sections normal to the Fp fold axis reveals no vergence change in porphyroblast inclu-

sions, but also can result in misidentification DEFORMATION

HISTORY,

STRAIN

of porphyroblasts

HETEROGENEITY,

AND

as post-D,.

CRENULATION

CLEAVAGE

DEVELOPMENT

Rocks commonly strain very inhomogeneously. This is particularly characteristic of rocks which have undergone deformation histories involving bulk, inhomogeneous shortening

(Bell, 1981), such as those of the Robertson

River Formation.

These rocks

contain numerous criteria supporting this including millipede microstructure in crenulations and in plagioclase (Bell and Rubenach, 1980), chloritoid and staurolite porphyroblasts (see below). Another criterion occurs in locations adjacent to boudinaged pegmatites and quartz veins. Here D2 crenulations are inferred by us to have overprinted earlier formed S,. The crenulations change asymmetry along their axial planes in a section containing the mineral elongation (Fig. 3). Within the Robertson River Formation, the degree of S, development is relatively independent of metamorphic grade (except in the upper sillimanite zone-see below). From the chloritoid to the andalusite zone, all stages of development of S,, from 1 to 6 (see below) can be seen from locality to locality, though early stages

175

Fig. 3. Sketch of micaceous schist surrounding a pegmatite pod. The foliation shown is Sz. Mineral elongation lineation Z$ is parallel to the plane of the page. Crenulations with opposite asymmetry have formed in the micaceous schist around the pegmatite. Their axial planes are parallel to the bulk scale S, in this outcrop and they are interpreted as having formed during Da as deformation continued.

occur less commonly in the andalusite zone. Figure 4 shows the progressive development of S, from open crenulations of S, through crenulation cleavage to a penetrative S, schistosity with no F: microfolds or quartz-rich microlithons remaining. Stage 3 shows the mica-rich and quartz-rich microlithons inferred to have been produced by rotation of S, micas and solution transfer of quartz, whereas in stage 4, new mica grains in the mica-rich layers have grown parallel to the axial plane S,. Stage 5 shows a layer-differentiated schistosity, in which the rare mica grains of the quartz-rich layers have grown parallel to the S, axial plane. In stage 6, quartz from the quartz-rich layers in stage 5 is inferred to have dissolved and nucleated in the mica-rich layers (and vice versa), producing a uniform, penetrative S, schistosity, The heterogeneity of strain during D, in the Robertson River Formation was such that all the “stages” of Fig. 4 are preserved and their distribution appears to be random over the area mapped, with the exception of the upper sillimanite zone. In this zone mineral growth in the matrix dominated over deformation of individual mineral grains and the stage 6 foliation was uniformly developed. The presence of numerous po~h~oblasts in these rocks enabled us to examine earlier stages in the deformation history than that preserved in the local schistose matrix. Inclusion trails in the porphyroblasts commonly reveal that the porphyroblasts grew at an earlier stage of S, development than that represented in the matrix.

1

2

3

4

5

6 Fig. 4. Six stages of development of a new sc~st~ty via a crenelation cleavage. f; or incipient S, is oriented N-S. Stage 1 shows the original foliation S,. Stage 2 shows crenulation of S,. Stage 3 shows crenulation accompanied by solution transfer and consequent metamorphic differentiation. Stage 4 shows growth of new micas parallel to S,. Stage 5 shows destruction of relic crenulations in Q-domains. Stage 6 shows homogenized foliation Sz,.

Fig. 5. Stauraiite

(in extinction)

in contact E-W)

with quartz-muscovite-biotitr

developed

to stage 6 (oriented

and is much coarser

staurolite,

which ciearly show a stage 3 crenulation

Fig. 6. The matrix of the schist (specimen A garnet grain has preserved grains (sporadically have preserved

2100.1)

cleavage development.

distributed

across photograph I.5 cm.

S, in the matrix

(top R.H. corner),

e.g. top and bottom

inclusrons

Length 5.2 mm. Specimen

shows a stage 5 quartz-muscovite

stage 4 of the cleavage development

stage 5. Length

matrix.

in grain size than the quartz

L.H.-corners

has

in the BC9.

layer differentiation. whereas the staurolite and below the garnet)

Fig. 7. Sequentiat growth of porphyroblasts during schistosity development, and beterogeneity of a strain on the scale of a single thin section. a. Photograph of thin section Z163A taken normal (N) to the mineraf elangation. b, Sketch of this section. The porphyroblasts are: S = staurolite; G = garnet; A = andalusite; B = biotite; A/S Q staurohte replaced by andalusite and biotite. The heavy black laths are ilmenite grains. Most biotite grams grew eady (on information seen in other slides) and are now bigMy strained. Staurolite grams, which have replaced biotite porphyroblasts as well as matrix muscovite and quartz, grew when the S, schistosity was inhomogeneously foided to open Fi crenulations. One staurolite porphyroblast (on LHS of figure) shows a good “millipede” microstructure. Most of the andah&te oikoeryst (at ~ttom-~~i~ grew after staurolite, preserving tighter F: cremdations than the staurolite, but not as tight as those in most of the matrix. The matrix crenufations are generally very tight stage 2, but in more highly strained domains the matrix grades rapidly towards stage 6, represented by the continuous thin lines in the matrix. Length of photograph is 3.8 mm.

17x

For example,

Fig. 5 shows a staurolite

trails, whereas

the matrix

stage 4, whereas

porphyroblast

the matrix

has developed

Formation,

stage 3 inclusion that has overgrown

to stage 5. Even the one thin section can

display extreme heterogeneity of strain in the matrix inclusion trails in porphyroblasts (e.g., Fig. 7). In the Robertson

containing

is at stage 6, and Fig. 6 shows garnet

we can demonstrate

and consequently

also within

from the composition

zoning

and chemical changes of minerals with increasing grade, that the garnet porphyroblasts of the garnet, staurolite, andalusite and lower sillimanite zones all grew under the temperature conditions of the garnet zone (Rubenach and Bell, in prep. a; Bell and Rubenach, in prep.). Garnet grains in the staurolite, andalusite and lower sillimanite zones were partly dissolved as the temperature rose, the cores remaining stable only because they were richer in MnO. The garnet porphyroblasts contain stages 1-5 of crenulation cleavage development preserved within them. Thus, Fig. 6 shows stage 4, and Fig. 8, stage 1. Even more commonly

preserved

in garnets

are

stages 1, 2 and 3 (Figs. 8. 9 and 14), where they have overgrown F7] microfolds during deformation and the external schistosity has developed to further stages. Other porphyroblasts, such as staurolite and andalusite, have also overgrown the S, foliation at various stages in its development. However, they more commonly show a later stage of development of S, than the average garnet porphyroblast. i.e., stages 3, 4. 5 and 6, although growth of andalusite and staurolite synchronous with stage 2 does occur where garnet has overgrown

Fig. 8. Specimen ellipsoidal

V56.1, N section.

shape of the garnet

Si in the garnet

is interpreted

stage 1.

is at stage 1, whereas

to be a result of dissolution

the matrix against

is stage 5-6.

mica. Length:

The

2.1 mm.

Fig. 9. Specimen

16.1, P section.

garnet

preserving

relics of stage 2 also occur. The truncation during

the cleavage

development.

Length:

stage 2 microfolds.

of Si in the garnet

The matrix

is inferred

IS mamly

stage 6, but

to be the result of dissolution

5.2 mm.

DISSOLUTION

Chloritoid, garnet and staurolite all show microstructural evidence of dissolution during prograde reaction processes. At the top of the chloritoid zone and in the garnet zone, chloritoid porphyroblasts are partly to totally replaced by quartz and chlorite. Below the staurolite isograd, garnet porphyroblasts are invariably dodecahedral. However, above this isograd, they have commonly lost their dodecahedral shape where mica occurs on their boundaries. We have made detailed microprobe and microstructural studies of these garnet porphyroblasts and the adjacent biotites which we are currently preparing for publication. Some of the results of this work are briefly indicated below. The garnet porphyroblasts show MnO zoning which we have contoured. In some cases contouring suggests that tabular garnets grew that way; such garnets never have biotite rims. However, in most cases contouring of tabular garnet grains in contact with biotites, has demonstrated sharp truncation of the zoning on those boundaries parallel to S,. We can demonstrate that these garnets have not been sheared on S, and consequently this geometry is consistent with dissolution against the mica (cf. Fig. 11). Cores of garnet porphyroblasts are preserved through to the sillimanite isograd apparently because their higher manganese content extends their thermal stability relative to the rims. However,

1x0

some of the resulting microstructures interpreted wrongly on classic criteria. We interpret dodecahedral

are described

here, because

that the garnet shapes in thin sections have commonly sections

boundaries

in contact

especially

in the upper

to elongate

rectangles,

with muscovite staurolite

or biotite

and andalusite

garnet boundaries due to chemical relationships between the external

due

to solution

they could changed

transfer

be

from

on those

(Figs, 8, 9 and 10). (In some rocks, zones,

biotite

reaction (3) discussed schistosity and the

has grown

on the

below.) The resultant preserved within the

porphyroblasts (S,) are such that the garnet could be interpreted as pre-S,, because of the sharp truncation of Si by S, micas. However, we can demonstrate that these rectangular garnets originally grew syn-D,. Additional evidence for dissolution comes from rocks which show staurolite porphyroblasts containing garnet inclusions (e.g., Fig. 11). Garnet porphyroblasts with dodecahedral sections are preserved within the staurolite whereas those within the matrix are rectangular parallel to S,. Figure 11 also shows two garnet grains which were not wholly included within the staurolite porphyroblast. Those edges not protected by the staurolite have gone. We interpret that this was due to dissofution rather than shearing as S,, in this rock is not displaced in a section parallel to the stretching lineation in S,. Criteria which indicate that dissolution may have occurred are rectangular

Fig. 10. Specimen has been dissolved.

or ellipsoidal

shaped

TR7. Relic, elongate Length:

2.1 mm.

garnet

with mica on those boundaries

garnet grains in stage 6 matrix.

Most of each original

parallel

garnet grain

Fig.

11. Specimen

idioblastic,

having

have be-en dissolved

V56-1,

N section.

been protected against

The garnet

inclusions

from dissolution.

the matrix mica. Length:

in this large

The two garnet

staurolite

porphyroblast

are

grains on the RH edge of staurolite

2.1 mm.

to S,. It is important to examine the P-section as well as the N-section in such cases to determine whether the rectangular garnet shapes could have arisen by shear of the garnet parallel to L:. If this had occurred en-echelon arrays of rectangular garnets across S, should be visible in the P-section. Staurolite grains in rocks containing andalusite commonly show evidence of dissolution and reaction. In most cases they are partly replaced by coarse-grained muscovite, and in some rocks only small isolated staurolite cores showing optical continuity remain within a patch of muscovite which crudely conforms to the shape of the original staurolite porphyroblast. In rare circumstances, staurolite grains are enclosed by large andalusite porphyroblasts which have partly pseudomorphed them. The outline of the original staurolite grain is obvious (see Fig. 13) and when the andalusite has pseudomorphed the staurolite it also contains small biotite inclusions which are quite different in shape and size to the normal matrix biotite grains.

182

SEQUENTIAL

GROWTH

OF DIFFERENT

PQRPHYROBLASTS

DURING

Dr

In the southern part of the area mapped, the chloritoid zone is followed up grade by the garnet zone, which contains both chloritoid and garnet-bearing layers (resulting from s~~~t~y effect bulk compositions) but never both po~hyrob~asts in the same layer. However, in the upper part of the garnet zone, chloritoid grains show partial to complete replacement by quartz and chlorite, and some garnet-bearing rocks contain such pseudomorphs after chloritoid. Many of the microstructures associated with chloritoid in most rocks are ambiguous, as the porphyroblasts could be interpreted as post-D,/pre-D, or early syn-D,. However, where inclusion trails are preserved in chloritoid, or where relict F2’ microfolds are preserved in the pressure shadows at chloritoid ends. the chloritoid po~hyroblasts can be shown to be early D,, stages 1-2 (Fig. 12). In a few rocks containing chloritoid pseudomorphs, which occur just above the staurohte isograd, garnet grew syn-D,, stages 2-4. Where staurolite also occurs in these rocks, it always shows inclusion patterns of stages 5 and 6. Hence it appears

Fig. 12. Chioritoid centre

whereas

chloritoid opening,

porphyroblast

the schistose

porphyroblast concave

showing

matrix

S, inclusion

outwards

fine scale, stage

is at stage

trails with a millipede

S, trails. Specimen

1 inclusion

6 with S, oriented

trails (S, ) oriented E-W.

like geometry

50.1, N section.

Length:

Towards

are preserved. 5.2 mm.

each

N-S

in its

end of the

That is, opposite

183

that staurolite grew some time after garnet in these particular rocks when they had reached a higher temperature. Dissolution of chloritoid could have proceeded by the following two reactions; however, we believe that only reaction (1) occurred (see Section B): chloritoid + chlorite + quartz = garnet + water (continuous)

(1)

chloritoid + chlorite + muscovite = biotite + staurolite + quartz + water (discontinuous)

(2)

However, we have no unequivocal textural evidence that garnet grew after ehloritoid, and have inferred that reaction (1) has taken place where garnet grains occur in the same rock as pseudomo~hed chloritoids. All chloritoids have been pseudomorphed some 100 m before the staurolite isograd is reached. We interpret the quartz and chlorite pseudomorphing of chloritoid as a Carmichael-type replacement (Carmichael, 1969; Rubenach and Bell, in prep. b), i.e., an intermediate step in net reaction (1). (b) Sequential growth of garnet-staurolite-biotite-andalusite Rocks in the staurolite and andalusite zones commonly contain the porphyroblast assemblages garnet-staurolite-biotite and garnet-staurolite-biotite-andalusite, respectively. These rocks are excellent for studying the microstructural relationships between porphyroblast and matrix. Above the staurolite isograd, garnet was refractory and its Mn poor rims were unstable. Garnets preserved in the matrix schist are commonly tabular. In a few cases garnet grains are preserved in staurolite as dodecahedra whereas those in the matrix are tabular (compare Figs. 10 and 11). Staurolite growth appears to have involved dissolution of garnet (reaction 3) but was rapid relative to strain rate (see discussion). Consequently staurolite overgrew some garnet grains unaffected by dissolution and thus preserved them as dodecahedra. We interpret the tabular garnet shapes in the matrix schist of rocks above the staurolite isograd as due to dissolution by solution transfer. Garnet inclusions preserved inside the andalusite po~hyroblasts are commonly frayed, with no signs of their original dodecahedral outlines. Therefore dissolution appears to have occurred pre- or syn-growth of the andalusite (Fig. 13). Vernon (1977) discussed the possible problems with respect to the relative nucieution and growth times of porphyroblasts in contact with one another. He showed for example, that a frayed garnet grain enclosed in a large andalusite porphyroblast could have nucleated before, during or even after the andalusite. The andalusite porphyroblast in any of these cases would have overgrown the garnet grain after it had been partly dissolved. Timing of andalusite and garnet nucleation is difficult in specimen Z163A (Fig. 7) for example, because the andalusite porphyroblast has overgrown a number of stages of foliation development, including that preserved within the garnet grain. However, in the majority of cases, garnet contains inclusion trails which show an

Fig. 13. Specimen andalusite

2163A.

P section.

(light), which is optically

The prismatic continuous

are not shown). This pseudomorphous from biotite grains elsewhere pseudomo~hed replaced have

stauroiite,

by andalusite)

been largely

oikocrysts. cleavage

earlier

it has been dissolved

The schistosity development

stage

andalusite

contains

in the rock. A garnet

but where in contact

dissolved,

staurolite

regardless

porphyroblast

with a large oikocryst

small biotite inclusions

grains shows good crystal

with the original and partly

of whether

surrounded

than

that

replaced

boundaries

by

of which

which differ texturally

faces where enclosed

in the

of which has been

Garnet

grains elsewhere

in the matrix

or in andalusite

by biotite.

they are enclosed

of this P section is S,, and timing-of-growth

development

ts partly

matrix (the muscovite

of the porphyroblasts

is obvious only in the N section of this specimen,

of schistosity

(dark)

(the external

relative

shown in Fig. 7. Length:

preserved

in the

to

5.2 mm.

andalusite

porphyroblasts. Therefore it can be clearly demonstrated that garnet nucleated and grew before the growth of andalusite and that it was dissolved pre- or syn-andalusite porphyroblast growth. The nature of the inclusion trails preserved inside the garnet, staurolite, biotite and andalusite is such that the sequence of events just described is reflected in a change in degree of development of S,. For example, Figs. 14 and 6 show inclusion trails preserved in garnet at stages 2 and 3-4, whereas those in the staurolite are at stages 3 and 5 respectively. Other rocks show that stages 1 and 2 are preserved in the garnet, whereas stages 3, 4 and 5 are preserved in the andalusite. Spectacular examples of staurolite containing stages l-2 and andalusite surrounding the staurolite containing stages 2 and 3 are shown in Fig. 7. The main inferred reaction involved during dissolution of garnet at the staurolite

Fig. 14. Specimen crenulations),

2235.1,

whereas

N section.

the staurolite

Garnet

(left side) preserves

(in extinction,

an early stage 2 pattern

right side) and the matrix

(i.e. open Fi

shows stage 3. Length:

5.2

mm.

isograd

was the “discontinuous”

muscovite

reaction:

+ garnet + chlorite = staurolite

+ biotite + quartz + water

(3)

Chlorite was the first reactant used up, so garnet persisted in most rocks above the staurolite isograd. Equation (4) is another obvious reaction for the dissolution of garnet

above the staurolite

are found in andalusite garnet + muscovite

isograd,

and indeed,

(or sillimanite)

= biotite + andalusite

bearing

the best dissolution

microstructures

rocks.

+ quartz (continuous)

(4)

However, the solution of garnet also occurs in some staurolite-bearing rocks (all of which are chlorite free) where andalusite is absent. Reaction (5) would therefore appear

to be a possibility:

garnet + muscovite

+ water = staurolite

+ biotite + quartz (continuous)

(5)

However, reaction (5) appears to be retrograde rather than pro-grade with respect to temperature (Thompson, 1976, p. 415). Therefore the only explanation for garnet dissolution, is reaction (3) which proceeded until all the chlorite was used up, and possibly reaction (4). The fact that garnet is zoned with higher MnO in the core, and that increasing MnO will progressively stabilize garnet dissolution to higher temper-

186

atures

(Thompson,

1976) would

staurolite

isograd

additional

component,

Staurolite, overgrown

(in

other

account

words,

MnO, makes reaction

a product

of reaction

(3) continuing

of significant

above

amounts

the

of an

(3) continuous).

(3), has not

the garnet and thus preserved

must have proceeded

for reaction

the presence

replaced

garnet,

it from dissolution.

it has locally

So reactions

by the types of coupled ionic exchanges

discussed

such as (3)

by Carmichael

(1969). Staurolite did not grow by reaction (2) for the following reasons. Garnet and chloritoid never occur in the same layers, and although staurolite occurs in rocks containing quartz-chlorite-biotite pseudomorphs contain garnet, which on microstructural criteria was consumed Dissolution staurolite

after chloritoid, these same rocks preceeded staurolite. So chloritoid

by reaction (1) before the staurolite grew. of staurolite proceeded by the continuous reaction:

+ muscovite

+ quartz = biotite + andalusite

+ water

(6)

This is inferred by us to have usually occurred via Carmichael-type interchanges, with muscovite commonly replacing staurolite, and andalusite obviously replacing muscovite (Fig. 16). However, where large andalusite porphyroblasts envelope staurolite,

the straurolite

has been partly

pseudomorphed

by andalusite

small texturally unique biotite grains (Figs. 7 and 13). Thus in rocks containing garnet, staurolite and andalusite, ural evidence crystallisation

from the stages of crenulation was garnet-staurolite-andalusite.

containing

we have microstruct-

cleavage development and that reaction

that the order of (3) was followed

1

2

.___-__

1

1

2 A

___-

2 ._.__-._ __-. __.. ~.__._3 -____-.-.__-.

3 A

GT

ST,

44

RISING

Fig.

15. With the andalusite

porphyroblasts porphyroblast

FINAL

PND,bB*

3)

(REACTION

6)

MATRIX

>

this diagram illustrates

with respect to the D, crenulation cleavage development.

ity, the final stage of development

6 4

TEMPERATURE

zone as the example,

4

5

BI

(REACTION

.~.~ ~~___

the sequential

growth of

As a result of strain heterogene-

of S, varies from locality to locality. Regardless of this, the sequence of

reaction and growth was maintained.

regardless of whether it has overgrown stage 6 or 2.

For example, andalusite always followed

staurolite,

187

Fig.

16. Specimen

overgrown original

stage

mica-rich

V27.1, N section. 3/4

Part of a large andalusite

of the crenulation

layers, before replacement

left) which overgrew

stage 2. Length:

cleavage

development,

by andalusite,

oikocryst, essentially

partly wrap around

almost

in extinction.

replacing

muscovite.

It has The

biotite grains (e.g., bottom

5.2 mm.

by reaction (6) as the temperature increased during the D, event. The heterogeneity of strain and the sequential porphyroblast growth and reactions for the andalusite zone are summarised in Fig. 15. DISCUSSION

P~rp~~ro~~ast timing

This study would have given extremely complex results if the structural geometry/history had not been firmly established and carefully related to the metamorphism. It is possible that in some other multideformed terrains, where multistage growth of porphyroblasts pre, syn and post several deformation events has been proposed, the metamorphic story is much simpler and that confusion of foliations with one another and direct application of the criteria of Zwart (1962) have resulted in anomalous and/or erroneous timings of mineral growth. As long as care is taken to identify foliations, crenulations, and lineations and to look at both N and P sections of initial specimens, the criteria of Zwart (1962) in general hoId. Ambiguities such as those discussed in Vernon (1978) and Olesen

IX8

(1978, p. 280) and problems caused by lack of inclusion trails in porphyroblasts in some specimens, were always resolved where we cut other sections from the same or an adjacent

outcrop,

which

stages of S, development, tosity (S/S,)

of the strain internal

heterogeneity inclusion

showed

different

trails/external

schis-

relationships.

One potential garnet.

because

and unambiguous

problem

For example,

S, (Fig. 9) would porphyroblasts

in timing-of-growth

a superficial

easily

allow

examination

criteria

the dissolution

of

of garnet with curved Si truncated

by

it to be interpreted

in the Robertson

River

Formation

concerns

as pre-kinematic,

but garnet

grew early syn-D,.

Where

the

matrix foliation has developed to stage 6, the garnet grains commonly grown stage 2 microfolds or quartz-rich domains of stage 3. Subsequent

have overdissolution

against mica grams still during

of Si. Such

S, development,

produced

the truncations

syn-kinematic dissolution and truncation is easily recognised where other porphyroblasts have overgrown some garnet grains and protected them (Fig. 11). Truncations of the chemical zoning in garnets, as determined by contouring microprobe analysis spots, are also indicative of dissolution. Biotite reaction rims around irregular garnets also occur and are an indication of dissolution or replacement, but these are not always present. Other chemical effects and repercussions, such as changes in Mn distribution between garnet and biotite with increasing grade, will be described and discussed in another paper. Since garnet is so common in pelitic rocks, it is essential that possible dissolution used for timing of growth. Minerals plagioclase tend to be pseudomorphed during

reactions

effects are born in mind if this mineral is to be such as chloritoid, andalusite, staurolite and rather than truncated against the matrix, and

do not cause as many

problems

in their use as timing

of growth

criteria. Porphyroblast

versus matrix growth

Porphyroblast

growth appears

to have been extremely

in these rocks as the porphyroblasts

commonly

rapid relative

show no change

to strain rate

in inclusion

trail

geometry from core to rim. Exceptions occur such as the millipede porphyroblasts (Bell and Rubenach, 1980) but even they show evidence of rapid growth of at least the cores. However, there does not appear to be much of an increase in the schistose matrix grain size until the sillimanite isograd is reached. Thus it appears that the matrix rather than the porphyroblasts took up the strain during D,. This implies that the porphyroblasts grew in areas removed from zones of higher strain at the time of their growth, such as the ellipsoidal pod like zones of low shortening strain depicted in Fig. 17. These growth sites do not have to be zones of no strain. Earlier in the deformation they may have been sites of higher strain which have ceased straining as the strain was taken up elsewhere. The sources of material for porphyroblasts were very likely the more highly strained regions associated with crenulation cleavage development, and also those

189

Fig. 17. This figure is reproduced deformation

history involving

shortening

only. The anastomosing

component

of shear and shortening.

zones

where

mica

from Bell (1981). it shows the strain

bulk, inhomogeneous

shortening.

zones of high strain

anastomose

around

quartz

field resulting

The ellipsoidal

around

these ellipsoidal

or feldspar

from a non coaxial

pods of low strain form by

grains

pods contain

a large

in the fashion

discussed in Bell (198 1). These zones were possible sites for generation of steep local chemical potential gradients in which dissolution occurred more readily. No sinks for unwanted ions from porphyroblast sites appear to be available in the schistose matrix at stages 2 and 3 of crenulation cleavage development as there is no new mineral growth parallel to S,. We consider it unlikely that new material would nucleate as overgrowths on minerals within the deformed foliation S, (the minerals of which show undulose extinction) and no fibrous overgrowths of any type were observed. However, sinks are readily available at stages 4, 5 and 6 of crenulation cleavage development where strong new mineral growth parallel to S, has occurred (Fig. 4). The only sites available for precipitation of new matrix grains at stages 2 and 3 were: (1) pressure shadow regions around (2) syntectonic veins;

porphyroblasts;

(3) sites where dissolution of porphyroblasts and matrix schist and/or coarse micas (e.g., after staurolite) (e.g., after chloritoid).

concurrent substitution by and/or chlorite and quartz

If such sites were not present, it appears that unwanted ions or molecules from porphyroblast sites must have diffused out of the system or to adjacent more highly strained regions where crenulation cleavage development was at stages 4, 5 or 6. The biggest porphyroblasts overgrow matrix at stages 5 and 6. This is to be expected because their growth involves a large turnover of matrix material such as that which must occur during the formation of stages 5 and 6.

190

Homogenization During nucleating ated

the development of stage 6 in the matrix, material is diffusing into and as new grains within the mica and quartz-rich domains of the differenti-

layering,

nucleate domains

of the schist matrix

such

as to homogenize

in quartz-rich initially

arise

(Q) domains because

it. That

is, mica

begins

to preferentially

and quartz in mica (M) domains. of deformation

of a previously

The Q and M well-developed

foliation (anisotropy), and they result from inhomogeneous deformation of the rock with some portions taking up a shear plus shortening component and others taking up only a shortening component. This generates large strain gradients, and therefore chemical potential gradients. with consequent solution transfer and metamorphic differentiation (Marlow and Etheridge, 1977: Bell, 198 1: Boswort h, 198 1). However. when stage 5 is reached and all the minerals are elongate and aligned parallel to Sz, the anisotropy inhomogeneity has also been inhomogeneity

due to S, has been removed. Consequently the regular strain that produced the crenulations and subsequently the solution transfer removed and dissolution of old grains becomes dependant on the of strain on a grain scale (cf. Bell, 1981. p. 287). Nucleation of new

grains will depend on grain boundary energy alone instead of being cc>ntrolled by an imposed crenulation geometry. A similar phenomenon is observed in mylonites that have reached the ultimate phyllonitic stage. They also have undergone a homogenization of the fabric due to the preferential nucleation of new mineral phases on the boundaries of other mineral phases. This can possibly be explained in terms of the effect of interfacial energy on nucleation of new grains. Devore (1956, 1959) concluded that the interfacial or grain boundary free energy largely controls the sites of nucleation and crystal growth, and that it could play a dominant role during nucleation. crystal growth, exsolution, replacement and diffusion transfer. Smith (1952) measured interfacial energy in metals. His data shows that the energy of the interface between crystals of different metallic phases in contact with each other is generally less than that of the interface between two differently oriented crystals of the same phase. Lower interfacial energies for unlike phases may favour nucleation of new grains against other mineral

phases once all other constraints

on recrystalliza-

tion have been removed. This is supported by the work of White (1968) on phase distribution in ceramics and Vernon (1968). Flinn ( 1969) and Kretz ( 1966) on grain contacts in metamorphic rocks. Their results showed that unlike phase contacts are statistically more frequent than like phase contacts. Byerly and Vogel (1973, p. 187) attempted to refute this, but their reasoning does not accord with Smith’s (1952) and Vernon’s (1968) measurements. Hence, it appears energetically favourable for a new mineral to nucleate on an unlike rather than a like phase grain boundary. It is possible that similar phenomena are involved in development of stage 6 with quartz nucleating on mica grain boundaries and mica nucleating on quartz grain boundaries. However, it is also likely that other factors such as those discussed by Etheridge et al. (1974) came into play. That is, mica nucleated and grew in the micaceous layers

191

at stages 4 and 5 because plane

in these

domains

zones.

also became

available

However, elongate

grain boundaries with

parallel

further

and their reprecipitation

parallel

to the S,

grains

in the Q

the quartz

to S, and consequently

grow parallel to S, on their boundaries. Another point worth discussing here is cyclical zones or sites of high strain

were aligned

strain,

exchange

mica could of certain

as new grains

readily

ions from

elsewhere.

For

example, Si and 0 ions from highly strained quartz boundaries might have preferentially nucleated as quartz in M domains and thus lowered the overall strain energy of these latter zones by replacing

the highly strained

mica grain boundaries.

This would

only occur after the differentiated layering developed and newly grown mica grains in these sites became highly strained themselves. The components of mica from these sites could then precipitate on the highly strained quartz grain boundaries establishing a local source-sink Temperature

cycle that lowers the strain energy.

rise

Previous authors (e.g., Hollister, 1969) have deduced overprinting reactions resulting from a temperature rise on the basis of mineral assemblages and chemical compositions. However, we have been able to demonstrate on microstructural criteria

alone (i.e., relationships

between

Si and S,) that the different

porphyroblasts,

and therefore the reactions which produced them, did not grow or occur simultaneously in any given rock, but that there are consistent sequences of porphyroblast growth for each zone of the Robertson River Formation. The exact metamorphic reactions, of course, must be determined from mineral assemblages and careful chemical studies (Rubenach and Bell, in prep. b) for although they are directly observable in some cases (e.g., Figs. 7 and 13), the reactions generally proceed by coupled ionic diffusion between sites of dissolution and growth, and may involve other phases in a “catalytic” capacity as suggested by Carmichael (1969). The most likely explanation for the sequence of porphyroblast growth and associated reactions in many rocks is a temperature rise during the crenulation cleavage development; shift of isograds theoretical alternative

this is consistent

towards

with the spacing of isograds,

the lower grade

sides during

the progressive

the D, development,

and

and experimental data on the reactions (e.g., Thompson, 1976). The explanation of a constant temperature for a long period of time and all

porphyroblast growth restricted to a single period of intense deformation considered likely. The temperature rise during D, is believed to be related

is not to the

emplacement of the Forsayth Batholith, the nearest outcrop of which is about 15 km to the northeast of the area studied. Some phases of the batholith contain weak to moderate S, schistosity, whereas others are undeformed (R. Holmes, pers. commun., 1981). The metamorphism and granite development are both associated with the same regional thermal event. However, the emplacement of the batholith at the level of the exposed Robertson River Formation is believed to have had a more dramatic

192

effect on the thermal event.

history

of the rocks studied

than the overall regional

thermal

Rotation of porphyroblasts Rotation

of Si in porphyroblasts

processes, between

all of which

relative

probably

Si and S, it is important

do occur to ascertain

generated

by the same deformation

However,

assuming

(1) Rotation of Schoneveld, 1978).

to pre-existing the

(3) Differential rotation 197 I; Olesen, 1978).

rocks.

When

the foliation

deformations

the timing

to a number

(Ramsay,

(Cox,

1962; Wilson,

of the porphyroblast

Spry,

distinguishing

(cf. Olesen,

of S, relative

1969;

of

in both cases was 1978).

to S,, rotation

S, or newly formed Si relative

porphyroblasts

(2) Rotation of the matrix 1970; Olesen, 1978).

in some whether

or separate

that one can identify

pre-existing Si relative S, can be a result of:

to S, has been attributed

of

to newly formed

1969;

Dixon,

1976;

1971; Powell and Treagus.

and matrix (Ramsay,

1962; Wilson,

Ramsay (1962) recognised that porphyroblasts would not rotate if the deformation involved bulk coaxial flattening, and he showed (figs. 17, 18 in Ramsay, 1962) that

if shear

porphyroblast inclusion

was homogeneously contained

distributed

in 2-D across

a square

within it would rotate. Very few porphyroblasts

trails in the Robertson

River Formation

show rotation

axial planes or schistosity relative to the externally developed it is only very slightly and in such cases the porphyroblasts

of rock,

a

containing

of D, crenulation

S,. If they are rotated, (except garnet) show

some degree of undulose extinction. S, inclusion trails, however, show a wide variety of orientations relative to the external S, or S,. Most of the porphyroblasts (with the exception of biotite) in the Robertson River Formation

have undergone

little or no deformation

after they nucleated

and grew.

Consequently, they appear to have acted as the relatively unstrained ellipsoidal cores which are surrounded by anastomosing zones of high strain, as described by Bell ( 198 I, figs. 6, 7). If they behaved

in this fashion

during

progressive,

bulk, inhomoge-

neous shortening, and the schist matrix took up the shear component of the strain in the zones of high strain that anastomose around them, they would not have rotated even during non-coaxial deformation. This can be readily understood by carefully examining Fig. 17. The non-coaxial component of the deformation is totally localized in the anastomosing zones of high shear and shortening strain surrounding the ellipsoidal pods of lower strain within this diagram. Porphyroblasts in the Robertson River Formation have overgrown deformed S, and newly formed S, at a variety of stages as described herein. Consequently they have overgrown differently oriented S, depending on when they nucleated and grew and also the local strain. Hence Si derived from S, shows a variety of relationships

193

to immediately adjacent S, and indeed locally to S, in adjacent porphyroblasts (e.g. is cf. Fig. 7). However, S,, whether internal or external to the po~h~oblasts, constant in orientation. Garnets porphyroblasts that have been rotated rather than overgrown a crenulated foliation that has continued to develop around them, presumably form in zones of deformation undergoing progressive, simple or inhomogeneous simple shear. ACKNOWLEDGEMENTS

We would both like to acknowledge the Australian Research Grants Committee for their several years of support. We greatly appreciated critical reviews by Ron Vernon and Vie Wall which much improved the final manuscript. Trevor Steele made the superb thin sections. REFERENCES

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