part iii petrography, mineralogy and

PART III

PETROGRAPHY, MINERALOGY AND GEOCHEMISTRY

6 2

OF THE METAMORPHIC ROCKS.

PETROGRAPHY, MINERALOGY AND GEOCHEMISTRY

OF THE METAMORPHIC ROCKS.

Introduction

The general geology and structure of the mapped

area have been discussed in Parts I and II, and many of

the major features of the geology of this region have

now emerged. Attention will now be devoted to petro-

logic aspects of the metamorphic rocks. The majority of

this discussion is concerned with the lithologies of the

Tia Complex, as most of the petrologic work is concen-

trated on the rocks of this major structural subdivision.

The integration of the structural, petrographic,

mineralogical and geochemical data will be attempted in

Part VI after discussion of the Tia Granodiorite and411.1.01.1.101.

the Nowendoc Ultrabasic Belt.

PETROGRAPHY OF THE METAMORPHIC ROCKS

Chapter (9 )THE OXLEY AND WYBEENA METAMORPHICS

Introduction

The metamorphosed metabasic horizons in these

two subdivisions are abundant throughout a broad belt

southeast and east of Nowendoc, which narrows north-

wards through Tia into the Wybeena Metamorphics, This

is an ideal arrangement in which to study the progressive

metamorphism of an essentially isochemical succession.

The rocks of this belt show a progressive regional in-

crease in metamorphic grade from south to north, with

the amphibolites of highest metamorphic grade outcropp-

ing adjacent to the Tia Granodiorite„ Four metamorphic

zones labelled alphabetically Zones A, B, C, D have been

delineated, basically according to the mineral assemblages

of the metabasalts. The scarcity of this lithology else-

where prevented the establishment of these metamorphic

zones throughout the remainder of the Tia complex.

The accompanying phyllites and schists of these

two subdivisions show much more limited mineralogical

indications of the progressive metamorphism to which they

60

were presumably subjected. Three zones labelled

numerically 1, Transition Zone, 2 have been delineated

in these lithologies, however these are not progressive

metamorphic zones like A, B, C, D of the metabasic rocks

but reflect in part a superposition of metamorphic

episodes.

The distribution of the various mineral species

throughout each zone is illustrated in Figs 4 and 5,

with the correlation between the zonation of the meta-

basic and siliceous metasedimentary rocks shown on Fig

5. The geographic relationship between these meta-

morphic zones is illustrated in Map (2).

Character of the Metamorphic Zones

Metamorphosed Basic Rocks

Zone A

The metamorphosed basalts of this zone are

incompletely reconstituted and contain complex mineral

assemblages, (see Table 1). These assemblages have

replaced the original basalt mineralogy and have also

crystallized within post-tectonic veins cutting the meta-

basalts throughout this zone. The gradual decrease and

eventual disappearance of the original igneous clinopyroxene

defined a simple, easily mappable metamorphic boundary,

FIG. 4.

6c;

ZONE A ZONE B ZONE C ZONE 0

--.....a.

MIND ,

OM OM WWI 1.10 OM

•11•0 NOM

MO

■••• MI. OM moll NM OM WM Mb

a.. MN .0

....

r ow., www•

. ...

ALBITE

CALCIC PLAGIOCLASE

ACTINOLITE

CROSSITE

NORNBL &WOE

GRUNERITE

DIOPSIDE

EPIDOTE

PUMPELLY/TE

CHLORITE

GREEN BIOTITE

RED - BROWN BIOTITE

STILPNOMELANE

WHITE MICA

CALCITE

RELIC IGNEOUSCLINOPY'ROXENE

QUARTZ

The distribution of the minerals of the

metabasic rocks with respect to the metamorphic

zones.

FIG. 5.

6 -1

ZONE A ZONE B ZONE C ZONE D

ZONE 1 TRANSITIONZONE ZONE 2

- .

......., ...... ....,

....... in

...., ......

...... .....

.......... in... ....

ALBae

CALCIC PLAGIOCLASE

ORTHOCLASE

SCHISTOSE MUSCOVITE

PORPHYROBLASTICMUSCOVITE

BIOTITE

CHLORITE

GARNET

ST1LPNOMELANE

TOURMALINE

OUARTZ

The distribution of the minerals of the

siliceous metasedimentary rocks with respect

to the metamorphic zones. The correlation with

the zones of the metabasic rocks is shown for

comparison.

0

ri E ;

gttU

O

k3

L43tAJCt

r...'NJI... 44ct $....

.-40 Z.6

ki)'q 65J ...10 TI

.i.■.0yxO

C.)

- 6 8TABLE. I.

949694979498949995579558

9559956295699570

957/957295739574

9575.9576

9577

958095829583958495859586.9587

95889589

9590959/959295939595959695989604

9608

960996/0

96/1/0004

• ? • • • • • a

• • • • • • •• • • • • • •• • • • • • • •• • • • • • •• • • a • • • • •• ? • • • • • • • •• ? • • • • • • •• • • • • • •• • • • • a • • • •• • • • • • • •• • • • ' • * • • • • •• • • • • • • •• 0 III • • • • •• • • • • • • •• • • • • ► • • •• • • • • •• • • • • • • •• • • • • • •• • • • • • 0 • •• • • • • • •• • • • • • • •• • • • • • • •• • • • • • • • • 1

• • • • • • • •• • • • • • • • •• • a • • • • • • •• • • • • • • •• • • • a • • •• • • • • • • a? • •• • • • • • •• • • • • • • • •• • • • • • • •• • • • • • • •• • • • • •• ? • • a • • • •• • • • • • • •• • • • a • • • • •• • • • • • • • • •

Mineral assemblages recorded in the Zone A

metabasalts.

and this was therefore used as the upper limit of Zone A.

Division of Zone A into pumpellyite-bearing and pumpell-

yite-free sub-zones may be possible after more exhaustive

sampling, but it appears that the variety and complexity

of the mineral assemblages found in these rocks is due

to the superposition of successive metamorphic episodes.

Zone B

The following mineral assemblages have been found

in this zone:

(1) Actinolite epidote - chlorite - albite -

quartz

(2) Actinolite epidote - chlorite - crossite -

albite - quartz

(3) Actinolite epidote - green biotite -

chlorite - stilpnomelane crossite

albite - quartz

(4) Hornblende - epidote - albite - quartz

Sphene, opaque oxides and sulphides are additional minor

mineral phases.

The high grade boundary of this zone is defined by

an abrupt decrease, almost to disappearance, of epidote.

Within the upper part of this zone, chlorite also decreases

and disappears, and it is inferred that this coincides

with the transition from an actinolitic to an aluminous

hornblende. The latter, however, is not a useful index

of metamorphic grade because of the difficulty distin-

guishing actinolite from pale coloured hornblende.

Apart from the absence of pumpellyite, the

mineral assemblages of the lower grade part of this zone

are similar to those of Zone A, with assemblage (1) most

common. Within the higher grade part of this zone, the

metabasalts have recrystallized to assemblage (4).

Zone C

The only assemblages recorded in the metabasalts

of this zone are:

(1) Hornblende - Plagioclase - Quartz


red brown biotite.

Opaque oxide mainly ilmenite and sphene are additional

minor mineral phases.

A lower boundary of this zone, defined by the

virtual disappearance of epidote from the amphibolites of

Zone B, coincides with the crystallization of a more calcic

plagioclase. Throughout this zone there is substantial

variation in the absorption tints of the hornblendes in

rocks of similar mineralogic composition. This

variation was not systematically related to the inferred

metamorphic gradient, and is probably the result of

variations in host rock chemistry, (see Chapter 1q-).

This zone includes the majority of the meta-

basalts of Domain II plus the metabasalts of Domain

VIIIA within the Wybeena Metamorphics, The amphibolites

within the Karinya Metamorphics (Domain IX) also contain

the above assemblages, but have undergone a much simpler

structural history (see Part II, Chapter 6).Zone D

Adjacent to the granodiorite the amphibolites

increase in grainsize, the hornblende deepens in colour

and undergoes a change from a fibrous and ragged platy

habit to a more prismatic form. This takes the form of

superposition of prismatic hornblende on the more fibrous

hornblendes of the Zone C amphibolites, and implies that

the crystallization of these prismatic hornblendes took

place during a superimposed metamorphic episode. The

incoming of prismatic hornblende has been taken as the

lower boundary of Zone D. Within the very highest grade

amphibolite adjacent to the granodiorite, the hornblende

is much more deeply coloured than that of Zone C and may

be accompanied by diopsidic clinopyroxene.

71

The following assemblages have been recorded

from this zone:


(2) Diopside - Hornblende - Plagioclase - Quartz

Sphene, magnetite and pyrite are additional minor mineral

phases.

Siliceous Metasedimentary Rocks

Zone 1

Quartz-rich muscovite and muscovite-chlorite

schists and phyllites are characteristic of this zone.

Epidote, calcite and tourmaline are accessory constituents,

and finely divided opaque material is abundant in certain

horizons as well as being a common accessory constituent.

The following assemblages have been recorded:

(1) Quartz - Albite - Muscovite

(2) Quartz - Muscovite

(3) Quartz - Muscovite - Chlorite

(4) Quartz - Albite - Muscovite - Chlorite

(5) Quartz - Chlorite - Calcite - Albite

(6) Quartz - Muscovite - Chlorite - Tourmaline

(7) Quartz - Muscovite - Albite - Epidote

(8) Quartz - Muscovite - Haematite - Chlorite

(9) Quartz - .epidote - Calcite - Muscovite

Chlorite - Haematite

Within Domain I, this zone corresponds to all of

Zone A of the metabasalts plus the lowest grade portion

of Zone B. Within Domains the rocks of this low

grade zone occupy an approximate north-south belt between

the Nowendoc Fault and the Transition Zone and higher

grade biotite rich schists (see Map 2).

Transition Zone

This zone contains highly veined and laminated

coarse micarich schists, in which the segregation laminae

are thicker, better developed and more pod-like than in

Zone 1. Towards its high grade boundary, remnants of

original metamorphic biotite are visible, having under-

gone partial replacement by white mica and chlorite. The

following assemblages have been recorded:

(1) Quartz - albite - white mica - epidote

chlorite

Quartz •-albite --white mica .-stilpnomelane )

- chlorite )

Quartz - albite - white mica - chlorite

Quartz - albite • microcline white mica• epidote

- Relict

) Biotite

Finely divided opaque mineral phases and Ephene are

common accessories.

This transition zone is inferred to be a zone

of retrogressive metamorphism in which higher grade

biotite-bearing assemblages were replaced by a lower

grade, white-mica rich assemblage, in which the absorp-

tion colours of the majority of white micas suggest

they are of phengitic composition. The boundary between

Zone 1 and this zone is based on a coarsening of the

grainsize of the mica rich layers and a concurrent in-LA, in 44-

crease in the thickness @ grainsize of the quartz richsegregation laminae.

The schists of the Transition Zone are best ex-

posed along the Cooplacurripa River and Uriamukki Creek

and Back Creek, north-west of Nowendoc. Further north,

within Domains III and IV near Oorundumby, the schists

show similar evidence of retrogression, however the

segregation laminae are not as strongly developed as in

Domain

Zone 2

The characteristic mineral throughout this zone

is brown and red-brown metamorphic biotite. The follow-

ing mineral assemblages have been recorded:

74

(1) Quartz - plagioclase - biotite - muscovite

(2) Quartz - plagioclase . biotite - muscovite

garnet

(3) Quartz - plagioclase - biotite - muscovite

orthoclase

(4) Quartz - plagioclase - biotite - muscovite -

orthoclase - garnet

(5) Quartz - plagioclase - biotite - tourmaline

Opaque oxides, sulphides, sphene and tourmaline were minor

accessory mineral phases.

The boundary between this zone and the Transition

Zone is defined by the absence of partially retrogressed

brown biotite. The metamorphic rocks of this zone occupy

an enormous area, as minerals such as almandine, staurolite,

cordierite, andalusite, sillimanite, or kyanite that might

permit further zonation are absent.

76Chapter (10)

PETROGRAPHY OF THE OXLEY AND WYBEENA METAMORPHICS

Basic Metamorphic Rocks

Zone A

The original igneous clinopyroxenes of the basalts

range from colourless to pink or pinkish-brown, resembling

the titaniferous clinopyroxenes of typical alkali-olivine

basalts. Occasional clear pools of chlorite with a

squashed lenticular outline may represent the site of

original phenocrystic olivine in less deformed meta-

basalts, however in general there is little textural

evidence that olivine was ever a major mineral phase of

the original basalts. Considering the ease with which

olivine is altered in comparatively recent basic lavas,

it is possible that the olivine of these basalts has

disappeared and textural evidence of its original presence

obliterated by deformation. The other important original

mineral phase, opaque oxide, is preserved as anhedral

cores within aggregates of finely divided sphene.

The analysed metabasalt MB8 is illustrated in

Plate 7A and is typical of the equigranular metamorphosed

basalts of this zone. The aggregates of rounded clino-

pyroxene grains of this rock have undergone marginal re-

placement by actinolite,chlorite and crossite, and the

7 'iremainder, inferred to have been predominantly calcic

plagioclase, has been replaced by pumpellyite, epidote,

albite and quartz along with actinolite and chlorite

from replacement of finer grained interstitial clinopyro-

xene. Finely divided sphene is distributed evenly through-

out the rock. The original basaltic fabric has been

almost completely destroyed.

Plate 7B illustrates MB7 a less deformed meta-

morphosed porphyritic basalt containing large (5mm)

clinopyroxene phenocrysts replaced marginally and along

fractures by chlorite, actinolite, and linear aggregates

of finely divided opaque oxide. The remainder of this

rock is again made up of epidote, pumpellyite, chlorite,

actinolite, albite, quartz and sphene which has replaced

the original interlocking laths of calcic plagioclase

and finer grained interstitial clinopyroxene. In some thin

sections from this zone e.g. S9591, 59589, 59588, the

shadowy outlines of original phenocrystic plagioclase is

preserved, deformed and flattened parallel to foliation.

These plagioclase phenocrysts have been replaced by a

finely divided aggregate of albite, quartz, epidote,

pumpellyite and tiny flakes of white mica (7).

Fibrous pale coloured actinolite surrounds and

replaces the original igneous clinopyroxene as well as

occurring as independent plates and fibrous aggregate

throughout the bulk of the rock. This fringing growth

surrounding the clinopyroxene is commonly zoned, with

an inner zone in contact with the clinopyroxene con-

sisting of crossite, and an outer zone of pale coloured

actinolite, with a sharp, well defined boundary separat-

ing the two amphiboles. Several examples of this

textural feature are illustrated in Fig. 6, Some largerplates of actinolite contain cores of crossite, and it

is inferred that these arose from complete replacement

of original clinopyroxene, thereby producing zoned

amphibole plates. Plate 7C and Figs. 6c, 6d illustrate

cases in which the original clinopyroxene has disappeared

from the core, but left a fine grained aggregate of

sphene and opaque oxide. Plate 8A illustrates another

example from the lower grade part of Zone B. Less common-

ly the crossite may occupy the fractures within clino-

pyroxene grains, e.g. Fig 6f. This textural feature is

interpreted to mean that the crystallization of cross-

ite is a late metamorphic event in these rocks, succeeding

a period when actinolite or chlorite replaced the

original igneous clinopyroxene. Chlorite may also replace

the original clinopyroxene, in which case no crossite is

observed. The replacement of clinopyroxene also releases

finely divided opaque and semi-opaque sphene.-like material

PUMPELLYITE

0.3mm

Replacement of original igneous clinopyroxene. C.

A. B.

I---10.02mm

CROSSITE

ACTINOLITE

CLINOPYROXENE

EPIOOTE

0.Imm0.Imm

Replacement of epidote by_pumpellytteL

G.

0.3mm

I.

Mode of crystallization of stilpnomelane.

CHLORITE

STILPNOMELANE

F.

FIG. 6.

TEXTURAL FEATURES OF THE METABASALTS OF ZONE A.

which aggregates within and marginal to the amphibole

and chlorite (e.g. Fig 6 and Plate 70).

The remainder of the rock consists of a fine

grained disordered aggregate of quartz, albite, pumpelly-

ite, epidote, chlorite, actinolite and calcite in vary-

ing proportions, accompanied by anhedral aggregates and

streaks of dirty-brown material. A weak to moderate

foliation may be discerned, parallel to these streaks

and to lensoidal aggregates of chlorite and epidote.

Typical veins are irregularly shaped bodies

ranging in size from a fraction of a millimetre to

several centimetres in width. These have similar assem-

blages to the remainder of the rock, but with different

proportions of the various minerals. The common vein

assemblages are listed below:

(1) Albite

(2) Epidote-Quartz (t Albite)

(3) Epidote-Pumpellyite-Quartz

(4) Epidote-Pumpellyite-Actinolite-Quartz

(5) Epidote-Chlorite

(6) Epidote-Chlorite-Stilpnomelane

(7) Quartz-Chlorite-Stilpnomelane

(8) Quartz-Chlorite-Stilpnomelane-Actinolite

0

(9) Quartz-Calcite-Green Biotite-Chlorite

(10) Epidote-Pumpellyite

A broad sequence of veining may be outlined. The

youngest veins, cutting all older veins, contain albite.

The main part of the sequence, cutting the foliation of

the metabasalt, contain all the above assemblages except

(1). The youngest veinlike bodies are semi-penetrative

parallel to the foliation of the metabasalt, and contain

assemblages (5), (6), (10). The majority of the veins are

non-penetrative, and cut across the foliation of the host

metabasalt. It is clear that they developed subsequent

to the penetrative deformation of the host basalt.

Throughout these veins it is apparent that

pumpellyite has crystallized by partial and complete re-

placement of epidote. Textural relationships believed to

illustrate this are shown in Figs, 6h, 6g and Plate 8B.

Pumpellyite pseudomorphs the aggregates of vein epidote,

and in many veins, cores of cloudy, rounded epidote

granules are surrounded and replaced by pumpellyite which,

in contrast with epidote, has a much more prismatic,

idioblastic habit. The textural relationships indicate

that replacement was a static phenomenon, unaccompanied by

penetrative deformation, and took place subsequent to

the development and infilling of the veins by an epidote

rich assemblage. The pumpellyite of the remainder of

the metabasalt is texturally continuous with that of

the vein, and it seems likely that this pumpellyite

crystallized at the same time as that within the vein.

Stilpnomelane is an important constituent of

the veins, in which it occurs as blade-like aggregates

radiating from the surface of chlorite patches. A

sketch illustrating this textural feature is given in

Fig 6i, and stilpnomelane with a similar habit from the

lower part of Zone B is illustrated in Plate 8c. This

mode of crystallization suggests that the stilpnomelane

nucleated and grew on chlorite grain boundaries under

static conditions, subsequent to crystallization within

the vein of earlier minerals. Fibrous actinolite has

also crystallized in these veins, as a stable associate

of pumpellyite, stilpnomelane and chlorite. Anhedral

calcite is present in some veins but is not a major con-

stituent. Green biotite in veins was observed in S9562

and S9593, in both cases partially replaced by chlorite.

The evidence above suggests that in this zone the

metabasalts have undergone two distinct periods of meta-

morphism, the first of which coincided with the penetrative

deformation of the surrounding schists and was of essent-

ially dynamic character. A subsequent metamorphic episode

S

was of a static nature, with the crystallization of

distinctive minerals throughout the basalts and within

veins cutting the metabasalts.

Zone B

In Zone B the original igneous clinopyroxene

has been completely replaced by metamorphic minerals and

all evidence of an original basaltic fabric is obliter-

ated. A typical metabasalt consists of ragged plates of

actinolitic amphibole or hornblende, accompanied by

abundant granules of epidote, finely divided albite and

quartz, and aggregates and streaks of opaque oxide and

finely divided sphene.

Plate 9A illustrates the analysed metabasalt MB6,

collected from the Cooplacurripa River just above the

lower boundary of this zone. This sample contains cross-

ite cores within amphibole plates and radiating sheafs

of stilpnomelane, (see Plate (8A) and (8c) ), which is

not typical of the metabasalts of this zone. The habit

of the stilpnomelane again suggests late, post-tectonic

crystallization, similar to that of Zone A. Plate 9B

illustrates a more typical Zone B metabasalt from a slight-

ly higher grade horizon. The fibrous and platy nature of

the actinolitic amphibole is apparent, interspersed with

patches of tiny epidote granules and chlorite. Towards

the upper part of this zone, the amphibole plates coarsen,

8 4

but are otherwise optically indistinguishable from those

of lower grade. Epidote grains also show a slight in-

crease in size, and chlorite disappears. The transition

from an actinolitic to an aluminous hornblende is inferi-

ed to take place at this point, and Plate 9C illustrates

such a hornblende-epidote-amphibolite from the upper part

of this zone.

Zone C

The hornblendes of this zone maintain the platy,

ragged and fibrous habit, but show a further increase in

grainsize. Plate 10A illustrates such an amphibolite

(MB5) from south of Tia, (see Map 2). Epidote has com-

pletely disappeared, and the hornblende is accompanied

by granoblastic quartz and calcic plagioclase and an

anhedral opaque mineral phase. The calcic plagioclase

of this zone is usually untwinned, and possesses a com-

position within the range An29 - An46'

A minor mineral

of this zone is bright red-brown biotite, which tends to

crystallize within patches of quartz and feldspar s only

rarely in contact with the hornblende. The hornblende

shows considerable variation in colour throughout this

zone, but as mentioned earlier, this could not be correl-

ated with metamorphic grade.

Plate 10B illustrates a similar metabasalt (MB2)

from this zone, collected from near Tia Post Office just

85

below the Zone D boundary. Their similarity illustrates

the uniformity, both textural and mineralogical, of the

amphibolites of this zone.

Zone D

Plate 10C illustrates the textural character of

an amphibolite (MB1) from this zone, collected from the

horizon adjacent to the Tia Granodiorite. The hornblendes

of this zone are characterised by a more prismatic habit,

and the amphibolite possesses a microscopic and megascopic

layering, consisting of alternating quartz-feldspar and

hornblende laminae, giving outcrops a characteristic fluted

appearance (Plate 11A). Diopsidic clinopyroxene co-exists

with hornblende in parts of this horizon. Granoblastic

quartz and feldspar accompany the above minerals, and the

slightly more calcic plagioclase of this zone (An50 ) shows

rudimentary Albite twin lamellae.

Some later alteration is evident in this highest

grade horizon within zoned, irregular, pod-like sheets up

to 6 inches thick cutting across the penetrative layering.

The outermost zone consists of normal amphibolite, followed

by a zone of diopsidic clinopyroxene, a zone of epidote,

with the core of these bodies containing grossular-andradite

garnet. The structural relationship of this body suggests

that it must have originated after the metamorphism and

imposition of the fabric and layering on the host

amphibolite, In addition a grunerite bearing amphibolite

was collected from about the middle of this zone,(see

Map 2. This outcropped beside an impure quartzite

horizon containing the assemblage garnet-grunerite-quartz-

biotite-stilpnomelane, and the texture of the amphibolite

suggests that grunerite is replacing the earlier crystall-

ized hornblende, see Plate 11B,

Siliceous Metasedimentary Rocks

Zone 1

A fine grained white-mica rich schist typical of

this zone is illustrated in Plate 11C, This shows the

intense microfolding characteristic of these schists, with

the axial plane of the microcrenulations corresponding

to the S3 surface described in Part II, Chapter (4). This

crenulation cleavage cuts an earlier mica schistosity,

which has undergone varying degrees of transposition into

S3 in some parts of this zone. Parallel to this earlier

schistosity there is a metamorphic lamination consisting

of granoblastic quartz or quartz plus subordinate albite

alternating with layers rich in white mica and chlorite.

The major constituent of the mica rich layers is

colourless, fine grained white mica. Haematite-rich schist

layers and some additional less siliceous horizons contain

a slightly pleochroic white mica, which exhibits yellow

or pale green maximum absorption colours and low 2V

suggesting a phengitic composition, (Ernst, 1963).

Sufficient haematite is concentrated in some horizons

to produce a scarlet or maroon coloured schist contain-

ing haematite, white mica and chlorite, otherwise it is

sporadically distributed as a minor constituent.

The quantity of chlorite in these schists varies

widely. It is completely absent from some horizons, but

is a major constituent of the mica-rich layers in others.

It is typically a pale green pleochroic variety, with

anomalous birefringent colours of blue, violet or brown.

Rounded epidote granules are a common additional minor

component of the mica-rich layers, and in one horizon

sampled (S9806), tiny prisms (0.05mm) of tourmaline are

an abundant minor constituent.

The quartzites of this zone contain a grano-

blastic aggregate of fine grained quartz and subordinate

albite, with widely varying quantities of opaque oxides,

sulphides, and dark, murky semi-opaque material. With

increasing quantities of white mica and chlorite, these

lithologies show a gradation into massive and layered

siliceous schists. Many of the darker quartzitic horizons

contain bands rich in various sulphides. A polished sec-

tion of a specimen from a massive sulphide horizon within

a black, fissile quartzite near Nuggetty Gully showed

it is predominantly pyrite with subordinate chalcopyrite

and sphalerite.

Throughout this zone there is a gradual increase

in the average grainsize of the constituents of the mica-

rich layers, from 0.02 mm within the lower grade schists

increasing to 0.1 mm toward the Transition Zone. Quartz

grainsize shows wide variation, apparently dependent on

factors other than grade of metamorphism.

Transition Zone

The hypothesis that retrogressive metamorphism

has taken place is based on the occurrence and distribution

of partially altered remnants of brown metamorphic biotite

in this zone. These remnants are absent along the bound-

ary with Zone 1, but become more common toward the higher

grade boundary, where the retrogressed schists merge with

those of Zone 2,

Plates 12A and 12B show the substantial increase

in grainsize within this zone compared with Zone 1, (Plate

11C). Plate 12A illustrates a specimen from the lower

part of this zone in which all the biotite has undergone

reaction and been replaced by white mica and chlorite.

The specimen depicted in Plate 12B comes from close to

the Zone 2 boundary and contains biotite that has been

only partially retrogressed.

8 8

The predominant white mica of this zone is a

pleochroic variety, with pale green or yellow maximum

absorption colours and low 2V suggesting a phengitic

composition. Colourless white mica is also present,

but is in each case found to co-exist with substantial

chlorite. The retrogressed white-mica rich layers

contain numerous rounded granules of sphene and smeared

out patches and lenticles of opaque oxide parallel to the

earlier mica schistosity as well as in microscopic domains

in which S3 is dominant. It is inferred that this

material is, in part, a product of the retrogressive

reaction of earlier formed biotite. Fine granules of

epidote are an additional minor constituent. The schists

of this zone along Uriamukki and Back Creek northwest of

Nowendoc contain stilpnomelane as an additional component.

This has crystallized as radiating blade-like aggregates

showing no preferred orientation, and is of very similar

textural character to that found in the metabasalts of

Zone A.

In specimens from about the middle of this zone,

the textural relationships of the biotite remnants to the

subsequent white mica and chlorite is important evidence

that retrogressive crystallization of the biotite to white

mica and chlorite took place during the F3 regional deform-

9 ,r)

ation, (Part II, Chapter4.). It is found that unretro-

gressed biotite is preserved in the flexures of micro-

crenulations, corresponding to microscopic domains in

which the earlier S1 , S2 schistosity is preserved. In

adjoining microscopic domains where transposition of

the earlier schistosity into S3 has taken place, biotite

is absent, and white mica-chlorite assemblages containing

scattered sphene granules and opaque oxide has crystalliz-

ed. An example that illustrates this correlation is

given in Fig 7. This is considered good evidence thatcrystallization of biotite in this zone was concurrent

with the early fold periods (Fl, F2) and that metamorphic

conditions altered so that during the formation of

F3 folds, the biotite grade assemblage partially or complet-

ely recrystallized to a lower metamorphic grade assemblage.

Zone 2

The remainder of the pelitic and semi-pelitic

schists of the Oxley Metamorphics and all these lithologies

within the Wybeena Metamorphics belong to this zone, (see

Map 2). The single diagnostic mineral of this zone is

brown metamorphic biotite. A colourless white mica is

almost invariably an accompanying mineral. The mineralogy

of the interlaminated metamorphosed basic rocks show there

is an overall increase in metamorphic grade in this zone,

however this is not obvious mineralogically in the pelitic

FIG. 7.

1I s2/ 1

10.2mm.

i

Textural relationship of the successive S-

surfaces and the mineral assemblages, suggestingret rogression of biotite during development ofS3 (F3).

and semi-pelitic lithologies. Monoclinic orthoclase is

abundant in the higher grade schists of this zone

adjacent to the eastern contact of the granodiorite, but

it can be shown that metasomatic transfer of potassium

from the granodiorite has taken place during its emplace-

ment. Garnet is a relatively uncommon minor constituent

of the schists of this zone, and it is shown in Chapter

(6) that this is a spessartine-rich variety showing onlyslight variation in composition with grade of metamorphism.

There is also little systematic variation in the composition

of the sodic plagioclase. Other minerals useful as

indicators or diagnostic of metamorphic grade are absent

throughout this zone.

The progression from the Transition Zone to Zone

2 was drawn where biotite had not retrogressed during F3,

and was stable within narrow microscopic domains parallel

to S3 as well as being preserved in the flexures of micro-

folds and microcrenulations as an earlier schistosity

parallel to Sl, S2. Closer to the granodiorite at a

slightly higher metamorphic grade, the earlier Sl, S2

schistosity becomes progressively replaced by an S3

schistosity. The earlier surface is still clearly marked,

however, by the folded penetrative quartzose layering.

The micas also undergo a progressive increase in grainsize,

with some of the muscovite crystallizing as coarser equant

flakes athwart the schistose biotite fabric. Adjacent

to the granodiorite the muscovite assumes a strongly

porphyroblastic habit, usually without any preferred

orientation, but occasionally also semi-parallel to

the biotite fabric, e.g. Plate 13B. The accompanying

biotite shows variable behaviour. It is sometimes strong-

ly schistose, and it may also have a poorly oriented

fabric like that of the porphyroblastic muscovite but

of finer grainsize. This behaviour of the biotite is

inferred to result from small scale local differences

throughout the higher grade parts of this zone in the

relationship between crystallization and deformation

during the F3 folding.

Plate 12C illustrates a garnet bearing schist of

the Wybeena Metamorphics. This comes from the lower

grade parts of this zone and the garnet shows its typical

habit in crystallizing as a large number of small rounded

grains. Plate 13A illustrates a slightly higher grade

schist from the Oxley Metamorphics. The biotite has

coarsened in grainsize and is not strongly schistose.

Abundant pleochroic yellow to blue tourmaline prisms

accompany the biotite and these are inferred to result

from recrystallization of original sedimentary boron, as

this sample comes from over 5 miles south of the Tia

91Granodiorite. A sample of low grade schist described

earlier from Zone 1 also contains an unusually high

quantity of fine grained tourmaline.

A high grade schist of this zone, collected about

30 yards from the granodiorite contact, is illustrated in

Plate 13B. Both the bulk chemistry and chemical composition

of the biotite, muscovite, orthoclase and garnet of this

rock have been determined, and are discussed later. This

sample was collected beside the outcrop of highest grade

amphibolite, of which the bulk chemistry (MB1) and

hornblende chemistry (HB1) is also known. The biotite of

this sample shows a high degree of preferred orientation,

to which the much coarser grained muscovite is semi-

parallel. The remainder of the rock consists of grano..

blastic quartz, potassium feldspar and sodic plagioclase.

Most of the quartzite horizons of this zone are

relatively pure, with any deepening in colour clearly

correlated with the presence of a small percentage of

anhedral opaque material. Plate 13C illustrates an

assemblage consisting of quartz-garnet-grunerite-biotite-

stilpnomelane found in a single quartzite horizon of

this zone. This rock is without any clear penetrative

structure, and grunerite has crystallized as rosettes up

to 1 cw. accompanied by aggregates of rounded garnets.

The illustrated specimen was collected beside the

grunerite-bearing amphibolite described earlier.

Summary.

It has been shown that these rocks have under-

gone a complex metamorphic history. The earliest recog-

nizable metamorphic event in the siliceous metasediments

is the development of segregation laminae and crystalliza-

tion of a mica schistosity parallel to the surfaces S1

and S2. During this early metamorphic event, biotite

crystallized as a stable mineral throughout the Transition

Zone and Zone 2. During the F3 regional deformation, the

schists of the Transition Zone were retrogressed to a

lower metamorphic grade, however growth of prograde meta-

morphic minerals continued throughout and subsequent to

the F3 deformation at higher metamorphic grades.

This contraction of metamorphic activity is

clearly the result of the changing distribution of temper-

ature and pressure during evolution of the Tia complex.

The close relationship of the highest grade metamorphics

and the Tia Granodiorite also seems clear. This contrac-

tion is thus seen as a consequence of the slow ascent of

the Tia Granodiorite magma which, as it rose from its site

of generation provided a progressively more localized heat

source.

The apparent superposition textures of the

metamorphosed basic rocks of Zone D is believed to

result from late crystallization associated with the

emplacement of the granodiorite. The lack of evidence

of retrogressive metamorphism within the metabasalts

corresponding to the Transition Zone is believed to be

due primarily to the widely differing stability fields

of the metamorphic minerals contained in these two

lithologies, but there were no basic horizons exposed

where a direct comparison with the Transition Zone schists

could be made.

The mineral assemblages of the non-penetrative

veins and segregations of the metamorphosed basic rocks

of Zone A identifies a further late metamorphic event.

The structural relationship of these veins and the

textures within the veins suggest that these minerals

crystallized within an essentially hydrostatic environ-

ment after the cessation of penetrative regional deforma-

tion.

9fi

Chapter (ii)

THE BRACKENDALE METAMORPHICS

Introduction

The monotony and uniformity of the lithologies

of this subdivision are reflected in the metamorphic

mineral assemblages that have crystallized. Throughout

this subdivision, the following have been the only ones

recorded:

(1) Quartz - Albite - Epidote - Biotite

Muscovite

(2) Quartz - Sodic Plagioclase - Calcite

Biotite - Muscovite (1 Specimen)

(3) Quartz - Sodic Plagioclase - Biotite -

Muscovite (-7 minor Epidote)

(4) Quartz - Sodic Plagioclase - Orthoclase -

Biotite - Muscovite

Sphene, opaque oxides and tourmaline are additional minor

constituents, Garnet is absent. Graphite is an import-

ant additional mineral phase in some layers.

Stages of Metamorphism

Biotite is a stable mineral throughout all this

subdivision, but additional minerals useful in the erec-

tion of zones of metamorphism are absent, The transition

91

from albite to sodic plagioclase indicates the direction

of overall increase in metamorphic grade, but this was

not a useful metamorphic reaction on which to subdivide

the schists because of the difficulty in determining

the composition of the fine grained plagioclase. The

probability of metasomatism limits the usefulness of

orthoclase as an indicator of metamorphic grade, as it

can be shown that potassium from the granodiorite has

been introduced into the surrounding envelope schists.

Although not based on mineralogic criteria,

three stages of progressive metamorphism can be roughly

delineated. These stages are based on the progressive

reconstitution of the coarse greywackes, siltstones and

slates towards the granodiorite, and the concurrent

increase in grainsize of metamorphic minerals in this

direction. In addition there is a progressive change in

the direction of preferred orientation of the micas,

from an S1 schistosity at low grade to an S2 orientation

closer to the granodiorite. The boundary between each

stage is gradational, as their basis is essentially the

varying relationship between deformation and crystalliza-

tion. The distribution of the rocks belonging to each

stage is shown on Map 2.

Character and Petrography of the Metamorphic Stages

Stage 1

This stage is found in the northern part of

this subdivision where the original greywackes, silt-

stones and shales have undergone only partial recon-

stitution. Original detrital material has survived but

has been flattened and smeared out parallel to the S1

foliation. Recognizable detrital material consists of

mudstone chips, clastic quartz and feldspars and quartzo-

feldspathic rock fragments. Detrital quartz grains invar-

iably show strong undulatory extinction and frequently

also marginal cataclasis. The detrital feldspars are

cloudy, and may be replaced by patches of fine grained

albite and epidote. Quartzofeldspathic rock fragments,

some still recognizably porphyritic, probably represent

partially digested acid volcanic debris.

The typical metamorphic assemblage of this stage

is (1), which forms a finer grained matrix surrounding

the surviving detrital material. The biotite and muscovite

have crystallized as a strong Si schistosity. These micas

are accompanied by streaks of semi-opaque murky material

parallel to S1 and fine grained granoblastic quartz and

albitic feldspar. Within Stage 1 the younger S2 surface

is only weakly developed as penetrative minor offsets

of S1 and the S1 schistosity shows little tendency to

9 9

align itself parallel to S2. A typical specimen

illustrating these textural features is shown in Plate

14A.

Stage 2

The transition to Stage 2 is marked by the

reconstitution of all detrital material and an overall

increasing grainsize of the metamorphic minerals. This

is accompanied by the development parallel to S1 of

narrow, quartz-rich segregation laminae which gradually

increase in thickness toward the granodiorite. Within

this stage the numerous F2 mesoscopic folds develop and

the earlier schistosity parallel to S1 undergoes progress-

ive re-orientation during grain growth throughout this

zone to become a mica schistosity parallel to S2, the

axial plane of the F2 mesoscopic folds. This is illus-

trated in Plate 14B, which shows the hinge of an F2 fold

with the boundary of the quartz segregation lamina

parallel to S1 and the biotite schistosity parallel to

S2.

The biotite may also show a less highly oriented

fabric such as that illustrated in Plate 14C, in which

the biotite has crystallized with a semi-porphyroblastic

habit and only a relatively weak schistosity. The S1

compositional layering of this sample has been transposed

during F2 folding. This variation in the crystallization

7

behaviour of biotite is very similar to that described

from the Oxley and Wybeena Metamorphics in Chapter (0).

Apart from one specimen, (3) is the only mineral

assemblage recorded throughout Stage 2. The variation in

bulk composition of the schists is clearly accommodated

by the varying proportions of minerals of this assemblage.

Graphite is present throughout the schists as trails of

extremely fine, flaky, opaque granules parellel to Sl.

This is inferred to have crystallized from organic

material more strongly concentrated in the pelitic or

shalb horizons of the original sediments.

Within the layers rich in graphite, biotite tends

to have a porphyroblastic habit and flakes of graphite

form trails of helicitic inclusions that run unobstructed

through the transverse flakes of biotite. Plate 15A

illustrates such a specimen, which in addition shows

several other textural features of interest. The micas

of this sample show three different modes of crystalliz-

ation. The earliest is as a fine grained biotite and

muscovite schistosity parallel to Sl. This is accompanied

by large elliptical, or roughly diamond-shaped transverse

porphyroblasts of biotite cutting across S1 but pre-S2.

This biotite has clearly crystallized by the replacement

of an earlier formed mineral. An adjacent specimen of

graphite-free, quartzose schist contains assemblage (2),

with the calcite occurring as a replacement of transverse

porphyroblasts of identical habit to those described

above. The identity of the replaced mineral is unknown.

The third mode of crystallization is as smaller por-

phyroblasts that have grown after F2 and have engulfed

the fine graphite-rich laminae as helicitic inclusions.

In the graphite rich layers throughout Stage 2,

muscovite is found, in general, only as fine schistose

flakes parallel to Sl l whereas the accompanying biotite

crystallizes as post-F2 porphyroblasts. In the majority

of the less graphite-rich layers, muscovite has crystall-

ized as post-F2 porphyroblasts rather than biotite. This

relationship suggests that the presence of graphite in

these layers has in some way suppressed muscovite grain

growth while promoting that of biotite, whereas the

relative absence of graphite in other layers is correl»

ated with substantial syn- and post-F2 grain growth of

muscovite as well as biotite.

Stage 3

The gradual transiton to Stage 3 takes place in

the vicinity of the granodiorite. These rocks are

characterised by well developed and highly contorted

veining which is in places pe4matitic. The small-scale

doming and basining of S1 described in Part II, Chapter

(A-) is also typical. At some localities the granodiorite

contact rocks are of migmatitic aspect, possessing irreg-

ularly shaped patches of coarser grained, biotite-bearing

P

J

granitic material surrounded by highly folded, finer

grained siliceous schist,

Assemblages (3) and (4) are found within thisStage. Clusters of blades or needles resembling silli-

manite were found in one sample of migmatitic schist,

however on closer examination these were found to be

muscovite. The schists of this Stage also contain ortho-

clase, which is most abundant within the immediate contact

rocks, and which gradually decreases away from the grano-

diorite. An example of the migmatitic schist is illustrated

in Plate 15B. It is shown in Part IV, Chapter (19) that

the potassium feldspar of the migmatites is an inter-

mediate microcline, while that of the unmigmatized en-

velope schist is monoclinic orthoclase, and that there

has been metasomatic introduction of potassium into these

envelope rocks.

This stage is also characterised by the presence

of large, randomly oriented porphyroblasts of muscovite,

such as those illustrated in Plate 15C, These porphyro-

blasts are most abundant within the lighter coloured,

originally arenaceous bands. In rare instances the basal

muscovite cleavage shows a weak preferred orientation.

Biotite shows a similar tendency for porphyroblastic

growth within this Stage, especially in the graphitic

layers within which muscovite porphyroblasts are absent,

1 0 4

but it is always of smaller grainsize than the muscovite.

These muscovites (and some biotites) are interpreted to

have mainly crystallized after the F2 folding, with some

syn.-F2 grain growth suggested by instances of weak

preferred orientation. A connection between the late F3

folding and this development of preferred orientation

could not be established.

Summary

The metamorphic evolution of the Brackendale

Metamorphics has been shown to have many features in

common with that of the Oxley and Wybeena Metamorphics.

There is evidence of a progressive contraction of meta-

morphic activity toward the granodiorite with time. In

addition, the extent of the succeeding deformations through-

out the Brackendale Metamorphics shows similar contraction

and a declining intensity with time.

The earliest recognizable metamorphic event is

widespread regional metamorphism concurrent with the Fl

deformation, with subsequent grain growth and recrystalliz►

ation during the following F2 within a region progressively

contracting toward the granodiorite. The progressive

development of S1 veining in this direction also indicates

that early crystallization during Fl was also zoned with

respect to the present site of the granodiorite. The

presence of the muscovite porphyroblasts and potassium

I 0 5

feldspar within the contact rocks of the Oxley Meta-

morphics and Brackendale Metamorphics shows their

crystallization is independent of the rock types of

each of these subdivisions.

Chapter (12)

THE KARINYA METAMORPHICS AND LOCHABER GREYWACKES

Introduction

The lithologic properties and simple structural

history of these two subdivisions have already been

described. There is also evidence of a simple metamor-

phic history consisting of one major period of crystall-

ization. The relationship between S1 and the fabric of

the newly crystallized metamorphic minerals suggests that

this period of metamorphism reached its peak after the

Fl regional deformation.

The Karinya Metamorphics contain several amphib-

olite horizons and the interbedded metamorphosed greywackes

all contain metamorphic biotite. The Lochaber Greywackes

may be subdivided according to the metamorphic mineral

assemblages, with the crystallization of biotite defin-

ing an isograd, (see Map 2). South of this isograd the

metamorphic grade progressively increases through the

Karinya Metamorphics toward the Tiara Fault.

Metamorphic Mineral Assemblages

Zone A The greywackes, siltstones and slates north of

the biotite isograd contain the following assemblage:

(1) Quartz - Albite --White Mica - Chlorite (+- Epidote

- Calcite)

This is accompanied by patches and streaks of opaque

and semi-opaque material, No metamorphosed basic rocks

were found in this zone,

Zone B South of the biotite isograd the following assem-

blages were recorded:

Quartz - Albite - Epidote - Biotite - White Mica

Chlorite

Quartz - Sodic Plagioclase - Biotite - White Mica

(t Epidote I Calcite)Quartz - Sodic Plagioclase - Biotite - Hornblende

(t Epidote)

Metamorphosed Basic Rocks


(6) Hornblende - Biotite - Plagioclase - Quartz

Opaque oxides and finely divided sphene are additional

minor constituents,

Petrography

Zone 1

Within the northern part of this zone the grey-

wackes show little microscopic evidence of deformation or

recrystallization, The onset of metamorphism is observed

in the silty and clay-rich greywacke matrix which shows

progressive recrystallization towards Zone 2, This

Metamorphosed Greywackes

(2)

(3)

(4)

matrix has crystallized to a finely divided (0.005

0.01 mm) mixture of quartz, albite, epidote, white mica

and chlorite, and finely divided opaque and semi.-opaque

material. These new mineral phases show no preferred

orientation parallel to S1 within the greywackes however

a stronger microscopic alignment of platy minerals is

apparent in the interbedded silty and slaty rocks.

The coarser (0.5 - 5.0 mm) clastic material is

also affected. The detrital plagioclase becomes clouded

and its outlines blurred, and white mica and chlorite has

crystallized within acid volcanic rock fragments and

angular mudstone chips. Detrital quartz shows undulatory

extinction and some marginal recrystallization. Some of

this quartz is strongly embayed and has a recognizable

- quartz bipyramidal outline.

The identification of chlorite in the recrystall-

ized matrix was impossible optically, however X-ray

diffractometer scans of ground rock chips and crushed samples

showed it was an important constituent, and also confirmed

the identity of the other mineral phases.

Zone 2

Biotite first appears as tiny flakes (0.02 -

0.03 mm) scattered throughout the recrystallized matrix

of the greywackes. At its first appearance as three or

four flakes per thin section, the preferred site of

crystallization appears to be beside opaque and semi-

opaque material in the matrix. Co-existing chlorite is

also found at this stage.

South of the isograd biotite rapidly becomes

more abundant as it crystallizes at other sites through-

out the greywacke. It occurs as a large number of tiny

flakes (0.05 - 0,1 mm) with no preferred orientation,

accompanied by minor white mica and granoblastic quartz

and albite or sodic plagioclase. There is an overall

blurring of the outlines of the original coarse clastic

material within this zone. The detrital plagioclase

feldspars have been replaced by a highly poikiloblastic

albite or sodic plagioclase. The composition of these

plagioclases was impossible to determine accurately how-

ever their optic sign and relief compared with quartz

was used to distinguish albite and sodic plagioclase.

The larger grains of original clastic quartz also remain

and the murky, semi-opaque material clearly marks the site

of the original mudstone and siltstone clasts.

Assemblages (4) was found in a greywacke adjacent

to an amphibolite horizon in which about 5 modal percent

of pale green fibrous hornblende has crystallized along

with biotite during metamorphism. This could indicate a

slight mixing of basic rock material with greywacke during

sedimentation.

The amphibolites are massive with typical

hornfelsic textures showing no preferred orientation

of any constituent. They are also relatively fine

grained and no surviving original basaltic textures

could be discerned. Fibres and ragged plates of horn-

blende show the following pleochroic scheme.

Pale yellow green to pale green

P

Pale green

Pale green to pale blue green

Calcic plagioclase (np = 1.556, An1f7 ) and quartz are

the other major constituents. In addition some layers

contain up to about 10 modal percent of red-brown biotite.

This could also indicate a mixing of these two litho-

logies during sedimentation.

THE WOOMBI GREENSTONES

Introduction

This poorly exposed succession of basic lavas,

laminated quartzites and siliceous siltstones and slates

are of uniformly low metamorphic grade throughout. Evidence

of only one period of low grade metamorphism and one re-

gional deformation are contained in these rocks.

3.1 3

The metamorphosed basic rocks contain the

following assemblage:

Quartz - Albite - Epidote Actinolite - Chlorite

(I Calcite)

Sphene is an important additional minor constituent.

Adjacent to the Tiara Fault the metamorphosed lavas con-

tain a trace of fine grained biotite (7) as an additional

mineral phase.

Petrography

The basic lavas are homogeneous with the original

basaltic textures well preserved. Clear examples of

ophitic and porphyritic textures are very common however

no original igneous clinopyroxene similar to that found

within Zone A of the Oxley Metamorphics is found in these

rocks. This pyroxene has undergone replacement by

actinolite, which forms large ragged plates at the site of

the original pyroxene phenocrysts as well as occurring as

smaller ragged plates and fibres at the site of smaller

interstitial clinopyroxene. The actinolite is, in general,

a strongly coloured variety with pale blue-green maximum

absorption tints.

The sites of the original plagioclase are now

occupied by albite (nil= 1.532), epidote and chlorite.

Epidote and chlorite also tend to form larger patches in

112

which idioblastic epidotes are completely surrounded

by fine grained matted chlorite. The chlorite is a

pale green pleochroic variety with anomalous blue or

purple birefringence. Xenoblastic calcite shows irregular

distribution but is always a minor constituent. Finely

divided sphene is inferred to have crystallized at the

sites of interstitial opaque oxides in the original lava.

There is no evidence of any penetrative deform-

ation in these lavas except adjacent to the Tiara Fault

where the metabasalts have a strong schistosity. This

is inferred to have developed during localized penetrative

deformation associated with fault movement.

Except for the quartzites, the interbedded litho-

logies consisting of siliceous slates and siltstones are

very poorly exposed. These are made up of finely divided

opaque and semi-opaque material and fine grained clastic

quartz grains, and show very little evidence of any

metamorphic crystallization. The quartzites are found to

consist of fine grained quartz (0.05 mm) with scattered

white mica and chlorite. This is criss-crossed by irregular

fractures infilled with coarser quartz and twinned albite

(nfl= 1.530) containing abundant submicroscopic opaque

inclusions,

Summary

The subdivisions discussed in this chapter show

evidence of a metamorphic history of quite different

character to that of the subdivisions discussed earlier.

The textures of the metamorphic minerals show that re-

crystallization took place after the single recognizable

regional deformation Fl under relatively static conditions.

This resulted in the crystallization of typical hornfelsic,

non-directional textures within the greywackes and

amphibolites of the Karinya Metamorphics and Lochaber

Greywackes. The mineral assemblages of the Woombi Green-

stones suggest they are of about the same metamorphic

grade as the Zone A metabasalts of the Oxley Metamorphics,

however they contain no relict igneous clinopyroxene

and show much better preservation of original basaltic

textures than the Zone A metabasalts.

The relationship of the metamorphism of these

rocks to that of the subdivisions described earlier is

not clear, as the direction and amount of movement along

the Tiara Fault is unknown. The character of the meta-

morphism of these subdivisions is however very similar

to that of the Moona Plains area, described in the next

chapter.

part iii petrography, mineralogy and

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