Geological Survey of Western Australia

RECORD 2011/18

P–T–t EVOLUTION OF THE FRASER ZONE, ALBANY–FRASER OROGEN, WESTERN AUSTRALIA

by CW Oorschot

Government of Western Australia Department of Mines and Petroleum

Government of Western AustraliaDepartment of Mines and Petroleum

Record 2011/18

P–T–t EVOLUTIONOF THE FRASER ZONE, ALBANY–FRASER OROGEN, WESTERN AUSTRALIA

byCW Oorschot

Perth 2011

MINISTER FOR MINES AND PETROLEUMHon. Norman Moore MLC

DIRECTOR GENERAL, DEPARTMENT OF MINES AND PETROLEUMRichard Sellers

EXECUTIVE DIRECTOR, GEOLOGICAL SURVEY OF WESTERN AUSTRALIARick Rogerson

About this publicationThis Record is a BSc Hons thesis researched, written and compiled through an ongoing collaborative project between the Geological Survey of Western Australia (GSWA) and Curtin University. Although GSWA has provided fi eld and sample support for this project, the scientifi c content of the Record, and the drafting of fi gures, has been the responsibility of the author. No editing has been undertaken by GSWA.

REFERENCEThe recommended reference for this publication is:Oorschot, CW 2011, P–T–t evolution of the Fraser Zone, Albany–Fraser Orogen, Western Australia:

Geological Survey of Western Australia, Record 2011/18, 101p.

National Library of Australia Card Number and ISBN 978-1-74168-377-6

Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Locations mentioned in the text are referenced using Map Grid Australia (MGA) coordinates, Zone 50. All locations are quoted to at least the nearest 100 m.

Published 2011 by Geological Survey of Western AustraliaThis Record is published in digital format (PDF)and is available online at <http://www.dmp.wa.gov.au/GSWApublications>.

Further details of geological publications and maps produced by the Geological Survey of Western Australia are available from:Information CentreDepartment of Mines and Petroleum100 Plain StreetEAST PERTH, WESTERN AUSTRALIA 6004Telephone: +61 8 9222 3459 Facsimile: +61 8 9222 3444<http://www.dmp.wa.gov.au/GSWApublications>

Curtin University Department of Applied Geology

P-T-t evolution of the Fraser Zone, Albany-Fraser Orogen,Western Australia.

Christopher W. Oorschot

November 2010

Dissertation submitted in partial fulfillment of the requirements for the degree of BSc. Applied Geology Honours

Abstract

The Fraser Zone resides within the Mesoproterozoic Albany-Fraser orogenic belt

believed to have formed during the formation of the supercontinent Rodinia. Field

investigations, metamorphic petrology, phase equilibria modeling and in situ

SHRIMP U-Pb monazite geochronology are utilized in order to a refine the

understanding of the pressure, temperature and timing of metamorphism within the

Fraser Zone. Metamorphic petrology and phase equilibria modeling confines the peak

metamorphic conditions to 900 ºC and 8 kbar based on quartz rich metapelite

samples obtained from the Gnamma Hill locality. Similar conditions of 800-900 ºC at

6.5 kbar were also recorded from quartz rich metapelites samples from the Mount

Malcolm locality. Phase equilibria modeling suggests a narrow ‘pin head’ P-T path

where temperature increased steadily with pressure suggesting a transient heat source.

Monazites from the same lithologies record metamorphic ages between 1285 and

1266 Ma (207Pb/235U ages). The interpretation of data obtained from this study

combined with previously published and unpublished data suggests the Fraser Zone

developed in a shortened back arc basin environment. The advection of hot mafic

material into the back arc during extension provided a heat source when the basin was

shortened during the first stage of the Albany-Fraser orogeny.

Acknowledgements

Foremost I would like to thank Dr. Chris Clark for supervising this project. His

knowledge and understanding in the fields of metamorphic geology and

geochronology were invaluable. Catherine Spaggiari from GSWA should also be

thanked for her supervision and assistance with fieldwork logistics. Thanks also go to

Richard Taylor for his advice regarding all things related to the SHRIMP II and the

preparation of mounts. Chris Kirkland from GSWA is also thanked for his advice and

discussion towards the end of the project. Thankyou for all your

assistance throughout the year.

The Geological Survey of Western Australia should also be acknowledged for loan of

a vehicle for field work and the use of unpublished age data.

Table of contents

1. Introduction 1 1.1 Objectives 2 1.2 Affirmation of research 2

2. Regional geology 3 2.1Regional Geology of the Albany-Fraser orogenic belt 3

2.1.1 Northern Foreland 3 2.1.2 Mount Barren Group 5 2.1.3 Stirling Range Formation 6 2.1.4 Kepa Kurl Boya Province: Biranup Zone 6 2.1.5 Kepa Kurl Boya Province: Nornalup Zone 8 2.1.6 Recherche and Esperance Supersuite 9

2.2 Previous investigation: Fraser Zone 10 2.3 Metamorphic history of the Albany-Fraser Orogen 15 2.4 Tectonic context 17

3. Methods and analytical techniques 21 3.1 Fieldwork 21 3.2 Metamorphic petrology 21 3.3 XRF major element analysis 22 3.4 Pseudosections 23 3.5 Geochronolgy 24

4. Mapping and field observations 25 4.1 Metapelitic rocks 25 4.2 Quartzite 30 4.3 Mafic Granulite 30 4.4 Granitic intrusions 30 4.5 ‘Pavement’ formation 31 4.6 Felsic gneiss 31 4.7 Geophysical interpretations 32 4.8 Regional structure 32 4.9 Sampling 34

5. Metamorphic petrology 37 5.1 Quartz rich metapelites 37 5.2 Mafic granulites 41 5.3 Quartzite (metasandstone) 41 5.4 Granites 42 5.5 Felsic Gneiss 43 5.6 Samples used for detailed analysis

43 5.6.1 Sample: 182446 43 5.6.2 Sample: FR-10-005 44 5.6.3 Sample: FR-10-007 46 5.6.4 Sample: FR-10-009 47 5.6.5 Sample: FR-10-011 48 5.6.6 Sample: FR-10-017 50 5.6.7 Sample: FR-10-019 50

6. Phase equilibria modeling 52 6.1 FR-10-005 (Gnamma Hill) 52 6.2 FR-10-009 (Gnamma Hill) 52

6.3 FR-10-017 (Mount Malcolm) 55 6.4 FR-10-019 (Mount Malcolm) 57

7. Geochronology 62 7.1 FR-10-011 (Gnamma Hill) 62 7.2 FR-10-007 (Gnamma Hill) 66 7.3 182447 (Mount Malcolm) 66

8. Discussion 75 8.1 Previous work on metamorphic age 75 8.2 The interpretation of monazite ages 76 8.3 The use of 207Pb/235U ages 78 8.4 Age interpretations 78 8.5 Comparison with zircon ages 80 8.6 Link between igneous crystallization age of mafic granulites and

metamorphic age 81 8.7 Summary of Fraser Zone age data 81 8.8 P-T evolution 82 8.9 Tectonic model 85 8.10 What does the Biranup Zone represent? 89 8.11 Recommended future investigations 89

9. Conclusions 91 10. References 93

Appendix 1: Sample tracking 97 Appendix 2: XRF results 98 Appendix 3: SHRIMP data 99

Outcrop map 1: Gnamma Hill accompanies this document Outcrop map 2: Mount Malcolm accompanies this document

List of figures

Figure 2.1. Lithotectoic components of the Albany-Fraser Orogen (adapted from Spaggiari et al. 2009)………………………………………………………………… 4

Figure 2.2. Geological map depicting the sourthern Fraser Zone with a schematic cross section (After Spaggiari et al. 2009)…………………………………...………12

Figure 4.1. Location of field areas Gnamma Hill and Mount Malcolm……………...26

Figure 4.2. Images from the field. a. garnet rich mesosome within metapelitic rock. b. Leucosomes preserves evidence of shearing, evident in the garnet-k-feldspar porphyroblasts with strain shadows and recrystallizes A-symetric tails. c. A-symmetric folds with meta-pelitic rocks. folds are defined by garnet rich bands. d. Late shear bands obliquely cutting the regional foliation.150 mm ruler for scale………………27

Figure 4.3. Images from the field. a. symmetrical ‘M’ shaped folds from Mount Malcolm, these folds are defined by quartz rich bands.b. Quartzite with folded oxide band. c. Deformed microgranite with narrow leucocratic veins that have been folded. d. ‘Pavement’ formation alternating bands of mafic granulite and garnet bearing felsic gneiss. 150mm ruler and hammer for scale………………………………………… 29

Figure 4.4. Magnetic response of the two field areas a. Gnamma Hill b. Mount Malcolm……………………………………………………………………………...33

Figure 4.5. Sample sites for Gnamma Hill locality…………………………………..35

Figure 4.6. Sample sites for Mount Malcolm locality………………………………..36

Figure 5.1. a. FR-10-005, aligned sillimanite within garnet porphyroblasts, also contains inclusions of quartz, opaque minerals and spinel.b. FR-10-005, post-peak biotite replacing garnet.c. FR-10-009, strain envelope around garnet defined by biotite, evidence of post-peak shearing. d. 182445, random sillimanite growth suggesting peak mineral growth after first deformation. (Bar represent 1mm for scale)………………………………………………………. ………………………...45

Figure 5.2. a. FR-10-011, Relict cordierite grain. b. FR-10-011, single shear zone (thick dashed line) deflecting main fabric evidence of a possible third deformation. c. FR-10-004, Granite with multiple oblique shear bands cutting fabric. d. FR-10-16, mafic granulite, single garnet being replaced by orthopyroxene…………………….49

Figure 6.1. Pseudosections constructed for sample FR-10-005. a. With garnet modal isopleths. b. With sillimanite modal isopleths……………………………………….53

Figure 6.2. a. Pseudosections constructed for sample FR-10-005 with biotite modal isopleths, arrows show predicted P-T path. b. Pseudosections constructed for sample FR-10-009 with Sillimanite modal isopleths………………………………………..54

Figure 6.3. Pseudosections constructed for sample FR-10-009. a. With garnet modal isopleths. b. With biotite modal isopleths……………………………………………56

Figure 6.4. Pseudosections constructed for sample FR-10-017. a. With sillimanite modal isopleths. b. With garnet modal isopleths…………………………………….58

Figure 6.5. a. Pseudosections constructed for sample FR-10-017 with biotite modal isopleths, arrows show predicted P-T path. b. Pseudosections constructed for sample FR-10-019 with sillimanite modal isopleths………………………………………...59

Figure 6.6. Pseudosections constructed for sample FR-10-019. a. with garnet modal isopleths. b. with sillimanite modal isopleths………………………………………..60

Figure 7.1. Back scatter electron images of monazite grains from sample FR-10-011. ………………………………………………………………………………………..63

Figure 7.2. Inverse concordia plot of all analyses from sample FR-10-011. Analyses produce a 207Pb/235U age of 1266±a8 Ma…………………………………………….64

Figure 7.3. Plot of 207Pb/235U ages for sample FR-10-011. 10 analyses not included because of high % of Figure 4: Plot of 207Pb/235U ages for sample FR-10-011. Major outliers removed……………………………………………………………………...64

Figure 7.4. Inverse concordia plot of analyses from sample FR-10-011. Red ellipse = garnet bound monazite, yellow represents analysis of monazite growth rim, clear ellipse represents monazite from leucosome matrix. How garnet bound monazite ages show little difference to the whole population. Three rim analyses display a slightly younger age………………………………………………………………….65

Figure 7.5. Plot of ages from garnet bound monazites producing a slightly older age. ………………………………………………………………………………………..65

Figure 7.6. Plot of 207Pb/235U ages for the three rim analyses with a younger age. …………………………………………………………………………………….….67

Figure 6.7. Lack of Th trends in monazites from sample FR-10-011………………..67

Figure 7.8. Back scatter electron images of monazite grains from sample FR-10-07……………………………………………………………………………………..68

Figure 7.9. Inverse concordia plot of all analyses from sample FR-10-007. Analyses produce a 207Pb/235U age of 1285±16 Ma…………………………………………….69

Figure 7.10. Plot of 207Pb/235U ages for sample FR-10-007………………………….69

Figure 7.11. Lack of Th trends in monazite from sample FR-10-007……………….70

Figure 7.12. Back scatter electron images of monazite grains from sample 182446...72

Figure 7.13. Inverse concordia plot of all analyses from sample 182447. Analyses produce a 207Pb/235U age of 1268 ±12 Ma……………………………………………73

Figure 7.14. Plot of 207Pb/235U ages for sample 182447……………………………..73

Figure 7.15. Inverse concordia plot of all analyses from sample 182447. Red ellipse represents garnet bound monazite analyses, white represent monazite from the matrix. ……………………………………………………………………………………….74

Figure7.16. Lack of Th trends in sample 182447……………………………………74

Figure 8.1. Interpreted P-T path for granulite facies rocks. Solid line represents Gnamma Hill rocks. The dashed line represents Mount Malcolm rocks. The diagram illustrates a tight P-T loop. The peak field for each sample is also drawn on for comparison…………………………………………………………………………...84

Figure 8.2. Cartoon depicting the formation of the Fraser Zone within the Albany-Fraser Orogen………………………………………………………………………...86

List of Tables

Table 5.1. Deformation history preserved in metapelitic rocks………….…………..38

Table 5.2: Summary of mineral growth history for metapelitic rocks…………….....40

List of Maps

Outcrop map 1: Gnamma Hill…………………………… Accompanies this document

Outcrop map 2: Mount Malcolm…………………...…… Accompanies this document

1. Introduction

The Albany-Fraser belt is an elongate orogen that developed along the south and

southeast margin of the Archaean Yilgarn Craton in Western Australia. Clark et al.

(2000) suggest the tectonic and metamorphic features developed during a

Mesoproterozoic orogenic event can be subdivided into two stages; Stage I is the

result of continental collision occurring during 1345 to 1290 Ma. This is followed by

Stage II which is interpreted as the product of reactivation within an intracratonic

setting occurring between 1215-1140 Ma (Clark et al., 2000).

The Fraser Zone forms a major component of the northeastern Albany-Fraser belt.

Originally proposed as a complex of metamorphosed and tectonically dismembered

layered basic intrusions, Myers (1985) identified five zones bounded by faults

interleaved with orthogneiss and paragneiss. Condie and Myers (1999) argued

against the concept of a single stratiform intrusion and suggested it was rather

remnants of multiple magmatic arcs. This led to the current name convention of the

Fraser Zone rather than the Fraser Complex (Spaggiari et al. 2009). The Fraser Zone

has undergone a number of older isotopic studies (Bunting et al., 1976; Condie &

Myers, 1999; Fletcher et al., 1991). Geochronological and metamorphic

investigations have also been undertaken by a number of authors (Clark, 1999; Clark

et al., 2000; Clark et al., 1999; Clark, 1995; De Waele & Pisarevsky, 2008; Spaggiari

et al., 2009).

Currently there has been no monazite geochronology undertaken on Fraser Zone

rocks. There are also no recent pressure-temperature (P-T) investigations using

current techniques and little in the way of an up to date model that can sufficiently

account for the occurrence of such a large mafic body within the Albany-Fraser belt.

Recent interest from the mineral exploration industry is also fueling the need to

understand the processes that affected the Albany-Fraser region.

1.1 Project Aims

The aim of this project is to combine metamorphic petrology, phase equilibria

modeling and monazite geochronology performed on metapelitic samples from the

Fraser Zone. The combined results will allow a valid interpretation of the P-T-t

(pressure-temperature-time) evolution of the Fraser Zone within the Albany-Fraser

Belt. The P-T-t conditions will allow an interpretation of the heat source and drivers

of metamorphism. This will result in the construction of a tectonic model accounting

for the Fraser Zones current position, age and preserved metamorphic record.

1.2 Affirmation of research

Sample 182447 a thin section was provided by the Geological Survey of Western

Australia (GSWA). All geophysical images and aerial photographs were provided by

the GSWA and then altered for this dissertation. Unpublished age data presented in

the discussion was provided by the GSWA and referenced as such. XRF results were

conducted at Franklin and Marshal College, Department of Earth and Environment,

PA in the U.S.A run by Dr. Stanley A. Mertzman. All other data, observations and

results were attained by the author.

3

2. Regional Geology

2.1 Regional Geology of the Albany-Fraser orogenic belt

The Albany-Fraser Orogen is a Proterozoic orogenic belt that outcrops along the

south coast of Western Australia, representing the suture between the West

Australian, South Australian and Mawson Cratons (Bodorkos & Clark 2004a). The

orogen can be subdivided into a number of lithotectonic units based on geophysical,

structural and age characteristics (Spaggiari et al. 2009). These subdivisions include

the Northern Foreland, the Kepa Kurl Booya Province and the Recherche and

Esperance Supersuites (figure 2.1 illustrates the regional components of the Albany-

Fraser orogenic belt). The Kepa Kurl Booya Province can the be further subdivided

into the Biranup Zone, Fraser Zone and Nornalup Zone (Spaggiari et al. 2009).

2.1.1 Northern Foreland

As seen in figure 2.1 illustrating the geology of the Albany-Fraser region the

Northern Foreland is the northernmost component of the orogenic belt and represents

the southern portion of the Yilgarn Craton that preserves evidence of the Albany-

Fraser Orogeny (Myers 1990). The Munglinup Gneiss is the dominant lithology of

the Northern Foreland. An amphibolite to granulite facies orthogneiss interlayered

with lenses of metamafics, minor metachert, amphibolite schist, serpentinite and

metamorphosed ultramafics (Beeson et al. 1988, Myers 1995b, Spaggiari et al.

2009). Previous geochronological investigation of the gneissic units within the

Munglinup Gneiss produced three igneous crystalisation ages of 2680, 2660 and

2630 Ma (Spaggiari et al. 2009 and references cited therein). These ages are

comparable to typical Yilgarn Craton granite ages and add weight to the

4

5

suggestion that the Munglinup Gneiss was originally part of the Yilgarn Craton

(Spaggiari et al. 2009). In the northeast of the Northern Foreland the Munglinup

Gneiss is indistinguishable from the Biranup Zone (Spaggiari et al. 2009). The Red

Island Fault Zone represents the southern margin of the Northern Foreland and is

interpreted as the suture between the southern Yilgarn Craton and the Kepa Kurl

Booya Province. The northern margin is defined by the Jerducuttup and Cundeelee

Faults (Spaggiari et al. 2009). Orogen sub-parallel dykes (Gnowangerup-Fraser Dyke

Suite) and thrust sheets of metasediments transported from the south (including the

Stirling Range Formation and Mount Barren Group) provide evidence of collision

and reworking of the Yilgarn Cratons southern margin (Myers 1990). There are well

defined northsouth trends within the Northern Foreland representing increased

influence of orogenesis on the margin of the Yilgarn Craton (Beeson et al. 1988).

This includes the progressive overprinting of northwest Archaean structures by west

to southwest dextral shear zones and gneissic foliations. There is an increased

influence of metamorphism north to south and large mafic dykes are increasingly

more deformed from north to south (Beeson et al. 1988).

2.1.2 Mount Barren Group

The Mount Barren Group occurs on top of and runs parallel the Southern margin of

the Yilgarn Craton for 120km from Bremer Bay to the east of Ravensthorpe (Figure

2.1; Spaggiari et al. 2009). The Mount Barren Group consists of Proterozoic

metasediments that can be subdivided into the lower Steer Formation, the Kundip

Quartzite and the Kybulup Schist (Thom et al. 1984). Geochronology suggests a

maximum depositional age of c. 1700 Ma based on detrital zircons (Dawson et al.

2003). The detrital zircon age populations are variable suggesting a heterogeneous

source (Hall et al. 2008) (Spaggiari et al. 2009).

6

2.1.3 Stirling Range formation

The Stirling Range Formation lies 100km west of the Mount Barren Group and is

composed of quartzite, shale, slate and phyllite. The metasediments have a minimum

depositional age of c. 1800 ±14 Ma based on authogenic xenotime of diagenetic

origin (Rasmussen & Fletcher 2004) and a maximum depositional age of c. 2016 ±6

Ma based on detrital zircon and xenotime ages (Rasmussen & Fletcher 2004).

Comparisons between Stirling Range Formation and Mount Barren Group detrital

zircon populations show both similarities and differences leading to a lack of

certainty surrounding the relationship of these two formations (Spaggiari et al. 2009).

2.1.4 Kepa Kurl Boya Province: Biranup Zone

The Biranup zone is an 800 km long, 25km wide belt of mid crustal rocks that wraps

around the southern and southeastern boundary of the Yilgarn Craton (figure 1;

Spaggiari et al. 2009). This comprised of two components the Dalyup Gneiss and the

Coramup Gneiss, both of which are of granitic composition with prototlith ages

between 1690 to 1660 Ma (Bodorkos & Clark 2004a). Both the Dalyup and Coramup

Gneiss display similar age and litological characteristics. The difference between the

two gneissic units is the presence of paragneiss in the Coramup Zone (Spaggiari et

al. 2009). The Biranup rocks have all been intruded by the Recherche Supersuite at c.

1300 Ma (Nelson et al. 1995). Structurally the Northern Foreland is separated from

the Biranup Zone (Kepa Kurl Boya Province) by the Miller Point Thrust and Bremer

Fault in the west. The Southern Ocean Shear Zone and the Red Island Fault Zone

define the eastern boundary. The Biranup Zone is separated from the Nornalup Zone

by the Heywood-Cheyne fault (Spaggiari et al. 2009). Within the Biranup the

7

Coramup Fault Zone separates the Dalyup Gneiss from the Coramup Gneiss

(Spaggiari et al. 2009).

The Dalyup Gneiss dominates the Biranup Zone and is composed of heterogeneous

granitic gneisses of granodiorite, monzogranitic and syenogranitic composition with

minor mafic litholologies (Spaggiari et al. 2009). SHRIMP U-Pb ages of zircons

provide an age for the granitic protoliths of c. 1680 Ma (Nelson et al. 1995,

Spaggiari et al. 2009). The Dalyup Gneiss was also intruded by the Recherche

Supersuite. The Dalyup Gneiss has been intensively deformed and metamorphosed to

granulite facies conditions (Spaggiari et al. 2009). High strain shear structures

dominate the Dalyup Gneiss though there is evidence for extension in the form of

large boudins in the Bremer Bay region, see Beeson et al. (1988) for detail. The

Dalyup Gneiss preserves a detailed structural history which is summarised well by

Spaggiari et al. (2009).

The Coramup Gneiss differs from the Dalyup in that it contains paragneisses as well

as orthogneiss and is separated from the Dalyup Gneiss to the south-east by the

Heywood-Cheyne Fault Zone. Typical rock types include layered grantitic to

granodiorite gneisses, tonalite gneisses and lenses/boudins of mafic lithologies,

psammitic migmatitic metapelite and minor calc-silicates (Spaggiari et al. 2009).

Igneous crystallisation ages for the Coramup Gneiss are similar to those observed in

the Dalyup Gneiss suggesting it is part of the same tectonic unit (Bodorkos & Clark

2004a, 2004b, Spaggiari et al. 2009). The Coramup Gneiss was also intruded by the

Recherche Supersuite. It is believed the Coramup represents a zone of high strain

(Bodorkos & Clark 2004a, 2004b). Bodorkos & Clark (2004b) described an early

sub-horizontal gneissic foliation that developed in response to extension. This was

subsequently transposed into southeast dipping shear zones and was accompanied by

8

tight recumbent asymmetric folding and northwest directed thrusting. These

structures were then overprinted by southeast dipping dextral shear zones, a shallow

southwest plunging mineral lineation and northwest verging folds whose age is

constrained by syn-kinematic pegmatites (1168 ±12 Ma; Bodorkos & Clark 2004b).

Last to develop were late sinistral mylonite zones and pseudotachylites which post

date the intrusion of the c. 1140 Ma Esperance Supersuite (Nelson et al. 1995,

Bodorkos & Clark 2004a).

2.1.5 Kepa Kurl Boya Province: Nornalup zone

The most southern and eastern lithotectonic component of the Albany-Fraser Orogen,

the Nornalup Zone, is separated from the Biranup Zone by the Heywood-Cheyne

Fault and extends from the Darling Fault in the south and continues north as far as

the Fraser Zone. Further north the Noranlup Zone becomes obscured by the

overlying Eucla Basin (figure 2.1; Spaggiari et al. 2009). The zone continues further

offshore however due to the extent of Recherche and Esperance Supersuites how far

is difficult to interpret.

The Nornalup Zone is composed of the Malcolm Gneiss and the Western Paragneiss.

The Malcolm Gneiss being older consists mostly of siliclastic meta-sedimentary

rocks, but also includes orthogneiss, mafic amphiboite schist, quartzofeldspathic

gneiss and minor calcsilicate rocks. 1330-1280 Ma intrusions of the Recherche

Supersuite are also present (Nelson et al. 1995, Clark et al. 1999, Spaggiari et al.

2009). The dominant metasediments include muscovite-biotite psammites, quartzites,

and minor garnet-biotite-sillimantite metapelites that are often migmatitic (Clark

1999). The protolith of the Malcolm Gneiss is approximately 1560 Ma in age (Clark

et al. 2000) and underwent low-pressure, high-temperature metamorphism at the

9

same time as the intrusion of the Recherche Supersuite (c. 1330 Ma and c. 1314 Ma;

Clark et al. 2000). The Western Paragneiss is less well exposed and is dominated by

garnet-sillimanite migmatites and quartzites (Love 1999). Detrital zircons have been

dated from 1750-1720 Ma by Love (1999) who identified a maximum depositional

age of c. 1360 Ma. Early deformation is evident in the Western Paragneiss with

recumbent folding and related flat lying compositional layering. This is overprinted

by a northeast trending sub-vertical foliation which is believed to have developed in

response to northwest vergent thrusting and dextral transpression (Duebendorfer

2002, Fitzsimons & Buchan 2005). The Salisbury Gneiss and Mount Ragged

Formation are also included in the Nornalup Zone and represent Mesoproterozoic

cover rocks (Spaggiari et al. 2009). The relatively undeformed Esperance Supersuite

intruded the Nornalup Zone rocks at c. 1140 Ma (Nelson et al. 1995, Clark et al.

2000).

2.1.6 Recherche and Esperance Supersuite

Clark (1999) identified two types of rocks comprising the Recherche Supersuite in

the east of Nornalup Zone. A 1330 ±14 Ma biotite hornblende monzogranite and a

1324 ±21 Ma peraluminous garnet-bearing granodiorite gneiss (Spaggiari et al.

2009). The Recherche Supersuite also contains syn-plutonic mafic dykes and

intrusive aplite dykes (c. 1313 ±16 Ma; Clark et al. 2000). The Recherche Supersuite

is less abundant in the west of the Nornalup Zone. A U-Pb zircon age of 1289 ±10

Ma (Fitzsimons & Buchan 2005) was obtained at Whale Rock near Albany and

SHRIMP U-Pb zircon age of 1302 ±7 Ma was also obtained at the same locality

(Love 1999). The Recherche Supersuite also occurs in the Biranup zone and

Northern Foreland. In the Biranup Zone ages for the Esperance Supersuite include

SHRIMP U-Pb zircon age of 1299 ±18 Ma within the Dalyup Gneiss (Spaggiari et

10

al. 2009), 1283 ±13 Ma in the Coramup Hill Orthogneiss (Spaggiari et al. 2009),

1288 ±12 Ma in the foliated leucogranite at Observatory Point and 1314 ±21 Ma in

the granodiorite gneiss at Israelite Bay (Spaggiari et al. 2009).

The Esperance Supersuite is relatively undeformed and intruded during the later

stages of the Albany-Fraser Orogeny (Clark et al. 2000). Nelson et al. (1995), and

Clark et al. (2000) place the age of its emplacement around c. 1140 Ma. The

Esperance Supersuite is confined to the Nornalup Zone.

2.2 Previous investigations: The Fraser Zone

The Fraser Zone or Fraser Range Metamorphics are dominated by granulite facies

metamafic assemblages with interlayered lenses of metagranitic and

metasedimentary lithologies (Myers 1985). In the southeast the Fraser Zone is bound

by the Fraser Fault and Coramup Fault Zone. The southwest margin is overlain by

the Eucla Basin however previous investigations (Clark 1999) suggest a fault contact

exists with the Nornalup Zone. Geophysical data suggests the Fraser Zone is a 425

km long northeast trending zone up to 50 km thick (Spaggiari et al. 2009). Myers

(1985) divided the Fraser Zone into five structurally interleaved units (figure 2.2).

Unit one consists mainly of garnet amphibolites (metagabbro) with thin layers of

metaultramafic, metamelanogabbro, metaleucogabbro and metamorphosed

anorthosites. Units two and four are similar as they are composed of pyroxene

granulites. Unit three is made up of metamorphosed leucogabbro, anorthosites,

gabbro and melanogabbro. Unit five is less deformed and comprises metamorphosed

gabbros and olivine gabbronorite (Myers 1985).

Early work on the Fraser Zone described the region as being composed of mafic

intrusive rocks (Doepel & Lowry 1970). Doepel (1973) interpreted the Fraser Zone

11

as an upfaulted wedge of lower crust. However, Bunting et al. (1976) later

interpreted the rocks to be mafic extrusive given the amygdales, relict pillows basalts

that were observed interlayered with quartzite. Bunting et al. (1976) attempted the

first subdivision of the Fraser Zone. Myers (1985) interpreted the rocks of the Fraser

Zone to represent a large layered intrusion. However Condie & Myers (1999)

suggested that the rocks were not part of a large intrusive body but rather remnants of

multiple oceanic magmatic arcs with trace element chemistry suggesting a

subduction related source.

The first significant subdivisions within the Fraser Zone (then known as the Fraser

Complex, due to the interpretation of it being a large intrusive body) was provided by

Myers (1985). Myers (1985) identified five structurally interlayered mafic units

separated by faults which included slivers of quartzofeldspathic gneiss,

metasedimentary gneiss, quartzite, metagranite and pegmatites. Structurally the

Fraser Zone displays a northeast trending steep northwest to southeast dipping

foliation with a northeast to southwest moderately plunging lineation and fold axis

(Clark et al. 1999). The boundaries between units are believed to represent major

thrust faults with the Fraser Fault as the leading northwest verging fault. Sinistral and

thrust kinematics were identified in the Fraser Fault by Wilson (1969), Bunting et al.

(1976) interpreted this as evidence suggesting the Fraser Zone was thrust north over

the Western Gneisses. A zone of shearing containing schistose and mylonitic rocks,

the Fraser Fault is a 2 km wide zone retrogressed from granulite to amphibolite

facies (Myers 1985).

Clark et al. (1999) recognized two deformational events D1-D2, two metamorphic

events M1-M2 and four episodes of recrystalisation M1a-M1b. It was found that

12

13

isoclinally folded leucosomes and the destruction of M1b disequilibrium textures in

high strain zones point to M2a occurring at lower temperatures than M1a/b. Fletcher et

al. (1991) suggested that the pyroxene granulite unit was retrogressed to garnet

amphibolite during D2 (M2a) and showing increased pressure relative to M1,

suggesting burial of the Fraser Zone, crustal thickening and tectonic interleaving

(Clark 1999). Exhumation is believed to have led to D3 during which localised

mylonitic fabrics were developed accompanied by retrogression of granulite facies

assemblages to greenschist facies (Clark et al. 1999)

Rb-Sr and Sm-Nd model ages were obtained by Bunting et al. (1976) and Fletcher et

al. (1991) which yielded a poorly constrained mixed age of c. 1330-1300 Ma

(Spaggiari et al. 2009). An igneous crystallisation age for the metagabbro of unit five

based on Sm-Nd isochron ages was c. 1291±21 Ma. The same sample provided Rb-

Sr whole rock biotite age of c. 1268±20 Ma, potentially representing time the rock

cooled below the biotite closure temperature (Fletcher et al. 1991). Pyroxene-bearing

mafic granulites from unit two were dated by SHRIMP U-Pb zircon by De Waele &

Pisarevsky (2008) who obtained an age of 1291±8 Ma, which is interpreted as the

igneous crystallization age of the mafic protolith. Charnockite lenses near Mt

Malcolm were dated using SHRIMP U-Pb on zircons producing an age of 1301±6

Ma as the igneous crystallization age (Clark et al. 1999), which is believed to

represent the minimum age of metagabbros that the charnockite intruded during

M1a/D1. Orthopyroxene bearing orthogneisses north of Mt Malcolm yielded a U-Pb

SHRIMP zircon age of 1293±9 Ma which Clark et al. (1999) interpreted as the

igneous crystallisation age of intrusion post-D1 and pre-D2. A monzogranite gneiss

from the Fantasia Dimension Stone quarry produced SHRIMP U-Pb zircon ages of

1287±14Ma which represents the igneous crystallization age of the protolith

14

(Spaggiari et al. 2009 and references cited therein). These granite protolith ages are

all within error and it has been proposed they are all of the same suite (Spaggiari et

al. 2009). De Waele and Pisarevsky (2008) did find some younger ages for granitic

gneisses 1280±10 Ma and 1256±23 Ma. These ages were not well constrained

(Spaggiari et al. 2009). The younger granitic gneisses could be related to an aplite

dyke dated in unit one by Clark et al. (1999), with an igneous crystallisation age of

1288±12Ma which is believed to have intruded after M2. Paragneiss occurs at

Gnamma Hill between units one and two and include quartzofeldspathic gneisses

which are garnet rich and also contain minor sillimanite biotite and rare cordierite

(Clark et al. 1999). Composite S1/S2 fabrics related to M1/M2 were described by

Clark et al. (1999) at this locality. A sample from this site produced a reversely

discordant age of 1388 ± 12 Ma interpreted as a detrital grain from a homogenous

proximal igneous source which Clark et al. (1999) interpreted as a volcanogenic

sequence that formed prior to stage one of the Albany-Fraser Orogeny. Quartz

metasandstone interlayered with mafic granulites between units one and two

produced a depositional age of 1466 ± 17 Ma (Spaggiari et al. 2009 and references

cited therein). Detrital ages of c. 1582, 1640, 1680 Ma and a small number at c. 2651

Ma suggest different sources possibly derived from the Biranup Zone (Spaggiari et

al. 2009). A metamorphic age from zircon rims in the sandstone of 1304 ± 7 Ma that

is within error of igneous protolith ages for the granitic gneisses was also observed

(Spaggiari et al. 2009 and references cited therein). De Waele & Pisarevsky (2008)

identified xenocrystic zircons from granitic gneisses which produced poorly

constrained ages of 1876 ± 44, 1814 ± 134, 1738 ± 28, 1674 ± 22 and 1665 ± 96 Ma,

the youngest ages are similar to those found in the Dalyup and Coramup Gneiss of

15

the Biranup Zone. Spaggiari et. al (2009) thus suggested that the likely source of

these rocks is the Biranup Zone or recycled components of it.

Previous investigations indicate a short period for igneous crystallisation, granulite

facies metamorphism, retorgression and cooling (Clark et al. 1999). Spaggiari et al.

(2009) in a review of the Albany-Fraser region point out that supposed igneous

crystallisation ages proposed by De Waele & Pisarevsky (2008) are more likely

metamorphic ages rather igneous crystallisation ages of the protolith, or a

combination of both. Spaggiari et al. (2009) also point out that there is plenty of

evidence of tectonothermal acivity during Stage-I of the Albany-Fraser Orogeny but

no traces of Stage-II. This is odd given the Fraser Zone is wedged between two zones

that were subject to Stage-II deformation and metamorphism.

2.3 Metamorphic history of the Albany-Fraser Orogen

Stage-I occurred from c. 1345-1260 Ma and is best preserved in the Nornalup Zone

(Clark et al. 2000). Deformation and high-temperature low-pressure metamorphism

(700˚C, 4-6 kbar; Clark et al. 2000) in the Malcolm Gneiss predates intrusion of c.

1330 ± 14 Ma felsic plutons representing the earliest Recherche Granite Supersuite

(c. 1330-1280 Ma; Myers 1995b). The development of upright northwest verging

folds was contemporaneous with the intrusion of the Recherche Granite Supersuite

that were subsequently cut by a suite of 1313 ± 16 Ma aplite dykes (Clark et al.

2000). Further to the northwest, in the Coramup Gneiss, Stage-I commenced ~30-40

Ma later (Bodorkos & Clark 2004a). The Fraser Zone was subject to granulite facies

metamorphism (800˚C, 6-7 kbar) at c. 1301 ±6 Ma, followed by a high-pressure

amphibolite facies event (650˚C, 8-10 kbar) bracketed by the intrusion of 1293 ± 9

Ma granites and 1288 ± 12 Ma aplite dykes (Clark et al. 1999). Metamorphism of the

16

Coramup Gneiss coincided with the emplacement of the Recherche Granite

Supersuite at c. 1288 ±12 Ma and 1283±13 Ma (Nelson et al. 1995, Myers 1995b).

Bodorkos & Clark (2004) investigated the metamorphic evolution of the Coramup

Gneiss and determined that Stage-I of the Albany-Fraser Orogeny can be further

subdivided. M1a peak conditions of 800-850 ˚C, 5-7 kbar took place at c. 1300 Ma.

This was followed by burial and recrystallisation at higher pressure conditions of M1b

(10 kbar; Bodorkos & Clark 2004a). Stage-I was terminated by high-temperature M1c

decompression (700-800 ˚C, 7-8 kbar) coincident with the emplacement of the

Recherce Granite (c. 1290-1280 Ma; Bodorkos & Clark 2004a).

Stage-I was followed by 45 Ma of quiescence throughout the Albany-Fraser Orogen

(Clark et al. 2000). The Fraser Complex cooled through the biotite Rb-Sr closure

temperature (300-350˚C) between c. 1270-1260 Ma (Fletcher et al. 1991), the

preservation of which suggests the Fraser Zone was not noticeably affected by Stage

II activity. At this time the Nornalup Zone was undergoing significant erosion of the

Stage-I crust, with sediments deposited in intracratonic basins. These represent the

protolith for Mount Ragged Formation and Stirling Range metasediments (Clark et

al. 2000, Fitzsimons 2003).

Stage-II of the Albany-Fraser Orogeny began with the intrusion of the Fraser dolerite

dyke swarm at 1212 ±10 Ma in the Northern Foreland (Dawson et al. 2003), and the

deformation of the southern margin of the Yilgarn Craton (Beeson et al. 1988). The

Salisbury Gneiss in the southeast of the Nornalup Zone underwent granulite facies

metamorphism (800˚C, 5 kbar) at approximately 1214 ± 8 Ma. This was followed by

high-temperature exhumation of the Nornalup Zone at c. 1182 ± 13 Ma (Clark et al.

2000). The Nornalup zone contains c. 1165 ± 5 Ma pegmatities in the Malcolm

Gneiss (Clark et al. 1999). The deformation of the Mount Ragged Formation

17

sediments followed amphibolite facies metamorphism (500-600˚C, 4 kbar) at around

c. 1154 ± 15 Ma (Clark 1999, Clark et al. 2000). Bodorkos & Clark (2004a)

recognised evidence of Stage-II metamorphism in the Coramup Gneiss from c. 1215-

1140 Ma. They further subdivided this event; M2a being high-temperature low-

pressure metamorphism (750-800˚C, 5-6 kbar) within dextral D2 shear zones

(Bodorkos & Clark 2004a). M2b is present in the form of fluid present amphibolite

facies retrogression. D3 mylonites and pseudotachylites post-date the intrusion of c.

1140 Ma Esperance Supersuite (similar to what is seen in the nearby Nornalup

zone)(Bodorkos & Clark 2004a). The end of Stage-II is marked by the intrusion of

the relatively underformed c. 1140 Ma Esperance Supersuite in the Nornalup

Complex (Nelson et al. 1995).

3.4 Tectonic context

The North Australian, West Australian and Mawson Cratons all underwent some

tectonic activity leading to the construction of Proterozoic Australia, a component of

the supercontinent Rodinia. Sutures between orogens are of Grenville age (1300-950

Ma) containing high-temperature and medium- to low-pressure metamorphic rocks

(Clark et al. 2000). The study of these sutures can lead to an understanding of the

processes and drivers that led to the formation of Proterozoic Australia.

Amalgamation of the Yilgarn Craton and the Albany-Fraser Orogen occurred

sometime between c. 1345 Ma and c. 1260 Ma (Clark et al. 2000). Stage I represents

the period in time when all lithotectonic units underwent some degree of

tectonothermal or magmatic activity at the same time (Spaggiari et al. 2009). Such

activity is believed to have occurred in response to the suturing of the Keba Kurl

Booya Province to the Southern Margin of the Yilgarn Craton via northwest vergent

18

thrusting (Myers et al. 1996, Clark et al. 2000, Bodorkos & Clark 2004b, Spaggiari

et al. 2009). There are, however, a number of unknowns surrounding the

lithotectonic evolution of the Albany-Fraser Orogen. The spatial relationships of the

lithotectonic units is uncertain particularly prior to collision(Spaggiari et al. 2009).

There is also very little paleogeographic data. One interpretation by Clark et al.

(2000) and Spaggiari et al. (2009) proposes that the Biranup Zone represents an

exotic terrane whose protolith formed independently from the Albany-Fraser Orogen.

However this poses the questions; how did it become part of the Kepa Kurl Booya

Province and was Biranup Zone part of the Kepa Kurl Booya Province prior to its

collision with the Yilgarn Craton (Spaggiari et al. 2009)? The c. 1690-1660 Ma

detrital and xenocrystic zircons in the Fraser Zone metasediments and granites

suggest the Biranup Zone was linked to the Fraser Zone at c. 1300 Ma (Spaggiari et

al. 2009). If the Fraser Zone represents a remnant arc, as Condie & Myers (1999)

suggested, such an arc could have possibly developed next to the Biranup Zone prior

to, synchronously or post collision with the Yilgarn Craton (Spaggiari et al. 2009).

Nelson et al. (1995) suggest the Biranup Zone was caught in a convergent zone

between the Western Australian, North Australian and Mawson Cratons during Stage

I. It has been suggested that subduction during convergence dipped to the southeast

due to the presence pre 1313 Ma magmatic and tectonothermal activity in the

Nornalup Zone (Bodorkos & Clark 2004b). As convergence continued the Biranup

Zone collided with the Nornalup Zone and Spaggiari et al. (2009) suggest this might

have led to a flip in subduction polarity causing it to begin to step backwards towards

the southern margin of the Yilgarn Craton. Spaggiari et al. (2009) proposed that

inferred transform faults along which the Biranup Zone may have migrated could

provide weaker zones to aid migration of the subduction zone. Such a phenomenon

19

would have coincided with early Stage-I deformation in the Nornalup Zone

(Spaggiari et al. 2009); such as northwest verging folds cut by an aplite dyke at c.

1313 ±16 Ma (Clark et al. 2000). Spaggiari et al. (2009) also suggested that

continued retreat of the subduction zone could lead to the extension of the Biranup

Zone which in turn promoted the development of magmatic arcs, thus producing the

Fraser Zone. Continued convergence resulted in the final collision of the Western

Australian, North Australian and Mawson Cratons between c. 1300-1280 Ma

(intrusive age of biotite-granodiorite gneiss in the Northern Foreland c. 1299 ±14

Ma; Spaggiari et al. 2009). The stitching of the Biranup and Nornalup Zones had

occurred by these late stages supported by the presence of the c. 1300-1280 Ma

Recherche Supersuite granitic plutons (Clark et al. 1999, Clark et al. 2000, Bodorkos

& Clark 2004a, 2004b). Finally thrusting of the Nornalup over the Biranup Zone

along the Heywood-Cheyne Fault represents the timing of thickening and high-

pressure high-temperature metamorphism of the Fraser Zone (Clark 1999, Clark et

al. 1999).

Between Stage I and Stage II (c. 1260-1215 Ma) the sedimentary protoliths for the

younger metasedimentary groups such as the Mount Ragged Formation were

deposited into shallow intracratonic basins (Clark et al. 2000). These basins probably

developed in response extensional collapse promoted by the thickened crust (Clark et

al. 2000, Bodorkos & Clark 2004a, 2004b).

Stage II reflects a number of episodes of intracontinental reworking of the orogen

over a 75 Ma period at high-temperatures (Spaggiari et al. 2009). Stage II began with

the high-temperature metamorphism of the Salisbury Gneiss in the east of the

Nornalup Zone and in the Coramup Gneiss in the Biranup Zone (c. 1225-1215 Ma;

Clark et al. 2000, Spaggiari et al. 2009). This was followed by the widespread

20

emplacement of the c. 1210 Ma Gnowangerup-Fraser Dyke Suite into the southern

margin of the Yilgarn Craton, Northern Foreland and the Kepa Kurl Booya Province

(Clark et al. 2000, Spaggiari et al. 2009). High-temperature metamorphism was most

widespread at c. 1180 Ma and this is preserved within the Northern Foreland,

Biranup Zone and Nornalup Zone (Clark et al. 1999, Clark et al. 2000, Bodorkos &

Clark 2004a, 2004b, Spaggiari et al. 2009). There were large differences in structural

styles and kinematics along strike of the orogen and within different units at this time

(Bodorkos & Clark 2004b, Spaggiari et al. 2009). Another phase of high-temperature

metamorphism began at c. 1170-1150 Ma in the Biranup Zone and possibly the

Munglinup Gneiss (Spaggiari et al. 2009). At a similar time thrusting along the

Rodona Fault and deformation of the Mount Ragged Group occurred (Clark et al.

2000). Stage II is also recognised to have occurred within the Mount Barren Group

(Dawson et al. 2003). Major lithotectonic and unit bounding faults cross-cut

dominant structures and fabrics and are believed to have been active during the later

period of Stage-II (Spaggiari et al. 2009). Such major structures (such as the Red

Island Fault Zone) are proposed as the main structures along which the high grade

rocks were exhumed (Spaggiari et al. 2009).

3. Methods and analytical techniques

3.1 Fieldwork

Fieldwork was conducted from late April to early May 2010. The field locations of

Gnamma Hill and Mount Malcolm were chosen because: 1. There is known outcrop

of metapelites at both localities that could be sampled and used for geochronology,

metamorphic petrology and provide P-T constraints. 2. Both sites were subject to or

close to areas of previous study. Previous investigations such as Clark (1999), Clark

et al. (1999) and Clark et al. (2000) as well as published and unpublished

investigations by the Geological Survey of Western Australia (GSWA) provide

additional data and a broader geological context for the study.

The fieldwork was conducted to create detailed outcrop maps of both Gnamma Hill

and Mount Malcolm localities, to sample lithologies within both regions and record

any structural/petrological relationships observed. Aerial photographs of the

mapping region on a 1: 5000 scale and maps were provided by the GSWA. Mapping

was undertaken at a 1:5000 scale, structural data was collected and diagrams and

photos were taken of key field relationships. The location of sample sites was

recorded using a grid coordinates from the Map Grid of Australia using Datum:

WGS 84. Samples were stored in clearly labeled green plastic sample bags.

3.2 Metamorphic petrology

Thin sections were made of all samples (sample tracking Appendix 1). Petrological

analysis was undertaken using a transmitted light microscope with a Leica DC 2000

digital imaging system at Curtin University. Peak assemblages were noted, as well as

any prograde and retrograde assemblages based on textural relationships observed in

thin section. The occurrence of accessory phases such as monazite and zircon and

their textural relationship within the sample were noted.

3.3 XRF major element analysis

The bulk elemental compositions of a number of the metapelites sampled were

obtained for the construction of P-T pseudosections. The four samples analyzed were

chosen because their assemblages contained the lowest variance mineral assemblage

(i.e. contained the most number of phases and would most usefully constrain the

prograde, peak and retrograde mineral evolution of the sample).

Samples of FR-10-005, FR-10-009, FR-10-017, and FR-10-019 collected from the

field were first crushed. The crusher was cleaned using acetone between samples to

prevent contamination between samples. The samples were then crushed to a fine

powder in a shatterbox so that the whole sample could pass through a size 8 sieve

mesh screen. The shatterbox was cleaned using acetone between samples to prevent

contamination between samples. The crushed sample was then sent to Franklin and

Marshal College, Department of Earth and Environment, Pennsylvania in the U.S.A.

for XRF analysis.

Once sent to the lab the crushed rock powder (0.4 grams) is mixed with

lithiumtetraborate (3.6 grams), placed in a platinum crucible and heated until molten.

The molten material is transferred to a platinum casting dish and quenched. This

produces a glass disk that is used for XRF analysis of SiO2, Al2O3, CaO, K2O, 5,

TiO2, Fe2O3, MnO, Na2O and MgO. The lab used a PAN analytical 2404 XRF

vacuum spectrometer equipped with a PW2540 X-Y sample handler and a 4kW Rh

super sharp X-ray tube. Working curves for each element are determined by

analyzing geochemical rock standards (Abbey 1983, Govindaraju 1994). Between 30

and 50 data points are gathered for each working curve; various elemental

interferences are also taken into account. Results are calculated and presented as

weight percent oxide.

Ferrous iron titration and Loss on Ignition was also conducted so the percentage of

Fe2+ can be calculated. The amount of ferrous Fe is determined by titration using a

modified Reichen and Fahey (1962) method. XRF determines total iron as Fe2O3.

Loss on ignition is determined by heating an exact aliquot of the sample at 950 C

for one hour. Results of the XRF analysis can be found in the appendix 2

3.4 Thermobarometry

Isochemical phase dagrams (pseudosections) based on bulk rock compositions are

used to investigate the metamorphic evolution of the metapelitic rocks from the

Gnamma Hill and Mount Malcolm localities within the Fraser Zone. The calculations

were performed using THERMOCALC 3.33i (Powell & Holland 1988) and the

internally consistent thermodynamic dataset of Holland & Powell (1988; dataset

tcds55, created in November 2003). Calculations were undertaken in the chemical

system Na2O – CaO - K2O – FeO – MgO - Al2O3 - SiO2 -H2O - TiO2 - Fe2O3

(NCKFMASHTO). Mineral abbreviations are as follows (Kretz 1983): Bt - biotite;

Crd - cordierite; Grt - garnet; Ilm - ilmenite; Kfs – K-feldspar; Ky – kyanite; Liq –

liquid/melt; Opx – orthopyroxene; Pl – plagioclase; Qtz – quartz; Sil – sillimanite.

Samples containing both mesosomes of restitic material and leucocosomes

interpreted as the crystallized remains of melt were analyzed for their chemical

composition by X-ray fluorescence spectroscopy (details outlined above). Isopleths

representing modal proportions of garnet, sillimanite and biotite were also calculated

to aid in the interpretation of the pseudosections.

3.5 Geochronology

Monazite, once identified in thin section, was cut out and mounted in 25mm epoxy

disks. Monazite standard Ind-1 (509 Ma, 206Pb/238U = 0.082133) was also mounted

with a small chip of Broken Hill K-feldspar used to identify the 204Pb peak. The

surface of the disk was then polished using a 1-micron polish. The mounts were then

cleaned using deionised water and petroleum ether repeatedly. The epoxy disk was

then coated with a thin membrane of gold that produced 10-20 of resistivity across

the surface. The coated disc was then imaged using a backscatter electron detector

(BSE) to identify any growth zoning in preparation for analysis. U-Th-Pb isotopic

measurements were collected using the SHRIMP-II at Curtin University Western

Australia and analyzed with a ~0.5 nA O 2 primary beam focused onto ~10μm spots,

a 5-scan duty cycle and a mass resolution of ~5000. A detailed account of the Curtin

SHRIMP monazite analysis procedure is found in Foster et al. (2000) . Data was

processed using the program SQUID and the application of off line corrections. A

summary of the SHRIMP data is contained in appendix 3.

4. Mapping and field observation

The Gnamma Hill field area is located between the 470800mE to 472800mE and

64414000mN and 6439100mN (Figure 4.1). The Mount Malcolm field area is

located between the 482400mE to 484400mE and 6435200 mN to 6433100 mN

(Figure 4.1). The grid used from mapping is the Map Grid of Australia and the

Datum: WGS 84. Mapping was undertaken at a scale of 1: 5000, the maps

accompany this dissertation (Outcrop map 1: Gnamma Hill and outcrop map 2:

Mount Malcolm). Access to the field area is via the Eyre Highway 90 km east of

Norseman then south on unsealed roads through the Fraser Range and Southern Hills

Stations (see Figure 4.1). A brief description of the lithologies and structure of both

mapping localities can be found below. Geological maps of the Gnamma Hill and

Mount Malcolm areas are provided with this document.

4.1 Metapelitic rocks

Quartz-rich metapelitic rocks were mapped through the centre of the Gnamma Hill

field area. The rocks were dominated by a medium to coarse-grained gneissic

foliation and the typical assemblage contained quartz - K-feldspar – garnet - Fe/Ti

oxides – sillimanite – biotite – cordierite ± plagioclase. The oxides displayed a weak

to moderate magnetic response suggesting the presence of magnetite or ilmenite.

Quartz-K-feldspar leucosomes are a prominent feature of this lithology and are

interpreted to represent the former presence of a melt phase. The strong gneissic

foliation is defined by compositional variations between garnet rich restite and quartz

rich leucosomes. Sillimanite rich bands are less common and, define the foliation and

preserve a lineation. There are also mesosomes comprised of garnet and Fe-Ti oxides

(figure 4.2a). The mesosomes are interpreted as a product of a heterogeneous

protolith that might have contained more iron rich zones. It might also reflect an

early differentiated fabric (Clark, 1999).

The majority of leucosomes were sub parallel to the regional foliation. A smaller

number of leucosomes were observed to cut across the dominant gneissic foliation.

The more leucocratic zones of the metapelitic gneiss appear to have been more

susceptible to deformation preserving large porphyroblasts that exhibit recrystallised

tails and strain shadows as evidence of deformation (Figure 4.2b). The kinematics

recorded by these structures suggests that dextral shearing. In the northern outcrops

of paragneiss are zones where leucocratic bands dominate the lithology with minor

garnet rich mesosomes representing restitic material. In the same area outcrops

preserve small leucosomes that have garnet rich selvages. Lecosomes have also been

tight to isoclinally folded. Garnet-rich mesosomes are also tight to isoclinally folded

(figure 4.2c). Fold axial planes are similar in orientation to the regional foliation.

Asymmetric folds (figure 4.2c) suggest north to northwest vergence. Narrow shear

bands obliquely cut the dominant fabric (figure 4.2d). These shear bands are

interpreted to have formed at a late stage in the deformational history as they

overprint all the previously described structures and fabrics. Thin aplite veins were

also observed cross cutting the foliation.

Mount Malcolm metapeltic rocks contain similar structural and lithological

relationships. They are also quartz-rich with similar assemblages and a similar

gneissic foliation. Leucosomes are abundant and there is evidence of folding (figure

4.3a).

4.2 Quartzite

Quartzites at both Gnamma Hill and Mount Malcolm are strongly foliated. They

contain K-feldspar – biotite rich bands as well as garnet and Fe-oxide rich bands that

had a good magnetic response. These bands are interpreted to be a product of

heterogeneities in the quartz rich protolith. Garnet and oxide rich bands could

represent iron rich zones in the protolith that Clark (1999) suggested were

metamorphosed banded iron formations. Quartzite typically occurs as discontinuous

slivers within and around metapelitic units. They are well exposed due to their

resistant nature. They also preserve evidence of folding. Figure 4.3b illustrates the

tight to isoclinal folds preserved in the quartzite. These are similar in geometry and

orientation to the folds observed within the quartz rich metapelitic rocks.

4.3 Mafic granulite

Outcrops of mafic granulite are more prominent within the Mount Malcolm field

area but were also identified within the Gnamma Hill mapping area. They are

characterized by the assemblage orthopyroxene - plagioclase - oxides - bioite ±

clinopyroxene - garnet. In the Mount Malcolm area this lithology exhibits a massive

texture showing little evidence of the regional foliation. In contrast, the Gnamma Hill

examples of mafic granulite preserve a weak foliation. At the Mount Malcolm

locality mafic granulite outcrops as elongate ellipsoid boulder strewn hillocks that

are enveloped by the regional structure.

4.4 Granitic intrusions

There are a number of intrusive granites seen in the field areas, the most voluminous

being the Mount Malcolm Granite that occurs on the eastern and western margins of

the Mount Malcolm mapping region. At Gnamma Hill three different granite types

were observed. An undeformed microgranite, has no observed boundary

relationships and was poorly represented in the outcrop. A K-feldspar rich, ‘pink’

granite is believed to have intruded the metapelites. This granite was observed over

short distances to cross cut the regional foliation and there was a K-feldspar rich vein

separating the granite from the metapelite. A strongly deformed microgranite was

also observed it Gnamma Hill. It displays a strong S-C fabric and contains narrow

veins of coarse quartz and K-feldspar (Figure 4.3c).

4.5 ‘Pavement’ formation

This lithology was identified only at the Gnamma Hill locality, defined as mafic

layers (similar in mineralogy to the mafic granulite) with parallel layers of garnet

bearing felsic gneiss (figure 4.3d). The layers are sub parallel to the regional foliation

and vary in thickness from narrow 20-30cm wide to 3-5 meter thick layers. This

outcrop is cross cut by numerous leucocratic veins. This association outcropped in

the north-west corner of the field area. The mafic-felsic layering may have been a

product of the partial melting of a mafic protolith, the melt separating into parallel

layers. Alternatively the compositional variation between the layers could be a

product of compositional variation in the protolith. Perhaps periods of mafic

volcanism followed by the deposition of felsic volcanics or sediments which were

then metamorphosed. It could also represent a metamorphosed layered mafic

intrusion similar to what was described by Myers (1985).

4.6 Felsic Gneiss

This unit was only observed in the Mount Malcolm area. Rich in quartz and K-

feldspar with minor oxides and biotite, the unit appears to be granitic in composition.

It contains a strong foliation. This unit is differentiated from the granites by its

geometry within the field. This unit is conformable with the major regional fold

structure. It is parallel with metasedimentary units, fairly homogenous and does not

demonstrate any intrusive relationship with neighboring lithologies. This unit is

interpreted to be of volcanic origin.

4.7 Geophysical Datasets

Magnetic data provided by the GSWA enables a more detailed interpretation of the

geology under cover. A higher magnetic response occurs through the centre of

Gnamma Hill locality (figure 4.4a). In the south this magnetic anomaly coincides

with meta-sedimentary rock types. Quartzites and metapelitic rocks have moderate to

weak magnetic response suggesting the presence of magnetite and/or ilmenite. It is

reasonable to assume that the magnetic anomaly is defined by the presence of the

metasediments.

Mount Malcolm displays a similar magnetic anomaly (figure 4.4b) which combined

with structural data highlights the presence of a kilometer scale fold. Through

mapping this anomaly corresponds to meta-sedimentary lithologies and the felsic

gneiss.

4.8 Regional Structure

Gnamma Hill has a dominant average foliation of 025 / 71 E (Map 1). Lineations

either plunge steeply NNE or moderately to the south within the foliation plane. The

majority of leucosomes are concordant to this regional foliation. There is a second

generation of leucosomes that displays a more northerly strike but a similar

dip (345 / 71 E). The many folds observed in the field displayed a fold axial plane

that was again similar to the regional foliation. Some asymmetrical folds were

observed suggesting vergence to the west or northwest. Late narrow shear bands

observed in metapelitic rocks displays an average strike and dip of 055 / 78 N.

Mount Malcolm was lacking in structural data due to the prevalence of mafic

granulites with a massive texture. However the structural data collected suggests the

presence of a large scale tight fold with one limb orientated at approximately 341/71

E and the second limb 022 / 79 E. This feature is evident within magnetic and gravity

data for the region.

The gneissic foliation and small scale folds are attributed to D1. Large scale folding

observed in the Mount Malcolm field area is most likely a product of D1 also. The

leucosomes are mostly concordant to the D1 foliation, though post D1 leucosomes

were observed suggesting melt generated after D1. Asymmetric folds suggests north

to northwest vergence. The deformation of the leucosomes (Figure 4.2b) is attributed

to a second deformation (D2) that is believed to be similar to D1 in regards to

orientation of differential stress. While the oblique narrow shears which cross cut all

fabrics are the product of a third deformational event (D3). The undeformed

microgranite and ‘pink’ granite provide evidence for the post-deformational

emplacement of small volumes of granitic material.

4.9 Sampling

19 samples were collected in the field. The location of the samples is marked in

figures 4.5 and 4.6. Sample 438447 was provided by GSWA. Appendix 1, sample

tracking contains exact coordinates and details regarding the samples. Samples are

also marked on outcrop maps 1 and 2.

5. Petrology

The high temperature metamorphic assemblages of metapelitic rocks observed and

sampled from both Gnamma Hill and Mount Malcolm preserve micro-structural and

mineral reaction histories. Monazite is also abundant within most of the metapelitic

rocks making them useful for monazite geochronology. Other lithologies such as

meta-basites, granitic dykes and quartzite also contain useful micro-structural

histories and have also been described.

Samples of all lithologies were taken from two localities, both of which contained

metapelitic rocks. The Gnamma Hill and Mount Malcolm localitites represent faulted

slivers between large mafic units as identified by Myers (1985) in the Fraser Zone.

For these two localities Gnamma Hill sits between unit 1 and unit 2, and Mount

Malcolm sits between unit 4 and unit 5. Thus the lithologies particularly the

metapelitic rocks should provide a record of the structural, metamorphic and

geochronologic history of the surrounding units and the Fraser Zone itself

5.1 Quartz rich meta-pelites

Metapelites from both Gnamma Hill and Mount Malcolm display similar

characteristics, both having a strong gneissic foliation defined by garnet rich

mesosomes and quartz-feldspar rich leucosomes. The leucosomes are thought to

represent re-crystallised melt that has separated from the restitic mesosome during

peak conditions. The peak assemblage of quartz, K-feldspar and garnet with minor

sillimanite and opaque minerals ± spinel, plagioclase and biotite suggests the rock

reached granulite facies metamorphic conditions (table 5.1). Leucosomes contain

coarse quartz and K-feldspar with minor opaques with rare garnet. Prograde minerals

within garnet porphyroblasts include quartz, sillimanite

Table 5.1: Deformation history preserved in metapelitic rocks

Deformation history preserved in quartz rich meta-pelitic rocks

Evidence Pre D1 D1 Post D1 D2 Post D2 D3 Post D3

Unorientated sillimanite within garnet with no strain envelope x

Gneissic fabric S1 x

Leucosomes parallel to Gneissic foliation x

Foliation parallel sillimanite within garnet x

Randomly orientated sillimanite cutting gneissic foliation and leucosomes not oriented to S1

x

Grain size reduction in leucosomes S2 x

Strain shadows around garnet grains S2 x

Pinched ends of Blocky sillimanite grains S2 x

Oblique narrow shear zones lightly deflecting gneissic foliation S3

x

and opaque minerals ± biotite and K-feldspar. Spinel is also identified as a pro-grade

mineral, it has been partially consumed by opaque minerals (magnetite or hematite).

Post peak minerals include boitite (that is associated with and partially rims garnet)

and opaque minerals. There is evidence of possibly three stages of deformation (see

table 5.2). Un-orientated sillimanite within garnet suggests M1 commenced prior to

D1. The pervasive strong gneissic fabric, orientated sillimanite within garnet and

leucosomes parallel to the foliation (S1) suggest D1 occurred during M1. Some

samples preserve sillimanite growth cutting the dominant fabric which is evidence of

M1 continuing after D1, which is also supported by a second type of leucososome

cutting the gneissic fabric in field observations. D2 is recognized by the grain size

reduction that occurred in many of the leucosomes and might have been responsible

for the strain shadows around garnet porphyroblasts (S2). Blocky sillimanite also

displays pinched ends suggesting they had been affected by D2. Grain size reduction

is most likely a product of shearing and all the leucosomes appear to show evidence

of this. Given their felsic composition it is likely leucosomes could accommodate

shear rather than the more rigid garnet bearing host. Narrow shear bands (S3) that

only lightly deflect the dominant gneissic fabric are evidence of a third late D3 event.

Isoclinally folded garnet rich mesosomes were observed within the meta-pelitic

rocks. These folds suggest vergence to the west. The relative timing of the folding is

unknown but they could be related to D1 and the mesosomes could represent iron

rich bands in the protolith. There is very little evidence of a second metamorphic

event with no obvious disequilibrium textures observed. The D2 shearing parallel to

S1 is associated with grain size reduction and strain envelopes around garnet that

could represent a recrystallization event.

Table 5.2: Summary of mineral growth history for metapelitic rocks

Mineral growth history for meta-pelitic rocks

Evidence Prograde Peak Post-peak Retrograde

Quartz Present in garnet In contact with all phases

K-feldspar Present in garnet In contact with all phases

Plagioclase In contact with all phases

Garnet In contact with all phases

Sillimanite Present in garnet In contact with all phases

Opaques minerals Present in garnet In contact with all phases

Biotite Minor inclusions within garnet

In contact with all phases

Partially rimming garnet grains or opaque minerals

Spinel Present in garnet? Always in contact with opaque minerals and possibly replacing

Perthite Exsolutionof perthite in K-feldspar

Sericite Alteration of K-feldspars

Chlorite Chlorite alteration present around feldspars

Zircon Present in garnet In contact with all phases

Monazite Present in garnet In contact with all phases

Small rims on large monazite grains suggesting a second episode of growth is the

only other evidence supporting a second M2 event.

Accessory zircon and monazite is abundant in all samples of the quartz rich meta-

pelites. Zircon is typically small and round, 10-20 μm in diameter and occurrs

amongst the stable assemblage and preserved within garnet with a high birefringence

and relief. Monazite displays similar properties to zircon except for more irregular

grain shapes and it is difficult to distinguish from zircon amongst the stable

assemblage and within garnet. However large, elongate, coarse monazite grains (up

to 1 mm in length) are present within leucosomes and are easily identified due to

their high relief and birefringence.

5.2 Mafic granulites

The dominant lithology at both Gnamma Hill and Mount Malcolm, these mafic units

preserve abundant medium-grained plagioclase, medium-grained orthopyroxene, less

abundant clinopyroxene and minor biotite and opaque minerals. Xenoblastic garnet is

present in a number of samples and partially rimmed and being consumed by

orthopyroxene and biotite (figure 5.2d). Garnet is thought to represent a prograde

phase. Minor chlorite is present as a retrograde mineral. Samples display a D1

gneissic foliation which is evident in hand samples.

5.3 Quartzite (metasandstone)

One polished thin section of quartzite from Gnamma Hill was made. Composed of

coarse grained quartz with minor fine-grained opaque minerals (ilmenite and

magnetite) it contains thin bands of garnet and small zones containing minor

sillimanite. White mica is also present in small proportions. Accessory zircon and

monazite is also evident. A strong foliation is defined by compositional variations in

the sample (S1). The presence of garnet and minor sillimanite suggests minor

heterogeneities in the composition of the protolith. The quartzite contained dark

isoclinally folded bands of ferro-magnesium minerals parallel to the S1. These are

interpreted as either primary sedimentary layering or as Clark (1999) proposed an

early differentiated fabric.

5.4 Granites

Samples FR-01-001, FR-01-004 and FR-10-015 are granitic lithologies observed and

sampled in the field. Sample FR-10-001 contains equal proportions of fine to

medium-grained plagioclase and K-feldspar (showing minor sericite alteration) with

fine-medium-grained quartz and minor fine-grained biotite and opaque minerals

(either ilmenite or magnetite). The sample has a massive texture except for a small

zone of grain size reduction. Relict xenoblastic garnet grains are present being

replaced by biotite. In the field the sample is cut by a plagioclase rich vein, the

composition enables the name of quartz-bearing monzanite.

Sample FR-10-004 contains fine-grained quartz, fine-grained K-feldspar and fine-

grained biotite with minor garnet and white mica. Accessory monazite and zircon is

also present. This garnet bearing microgranite displays strong S-C fabrics were S1 is

overprinted by an oblique S2 (Figure 4.2c). The fabric provides strong evidence for

D2 being overprinted by D3. This would suggest the microgranite intruded after D1.

Sample FR-10-015 has equal proportions of medium-grained quartz, medium-

grained K-feldspar and minor fine-grained biotite, muscovite and opaque minerals.

This K-feldspar rich micro-granite shows a weak foliation in the alignment of biotite

grains. Muscovite grains are randomly orientated. In the field this pink microgranite

is seen intruding into and cutting across meta-pelitic rocks separated by a quartz-K-

feldspar vein rich band. The lack of deformation in the granite suggests possible post

D2 intrusion.

5.5 Felsic Gneiss (metamorphosed felsic volcanic)

Samples FR-10-014 from Gnamma Hill and FR-10-018 from Mount Malcolm

present coarse granitic compositions that preserve a strong foliation similar to

surrounding lithologies. at Mount Malcolm mapping suggests this lithology is strata-

bound rather than intrusive. This lithology is interpreted as a felsic volcanic.

5.6 Samples used for detailed analysis

The following detailed petrographic descriptions are of samples that were used for P-

T analysis and/or geochronology.

5.6.1 Sample: 182445

This ample has similar proportions of equant medium-grained (0.2-2mm in diameter)

quartz and porphyroblasts of sub-idioblastic medium-grained garnet, needle like

medium-grained sillimanite, opaque minerals, minor plagioclase and biotite. These

phases represent the peak assemblage and are all in direct contact with each other. A

strong gneissic foliation is evident defined by compositional variations. Garnet

contains prograde inclusions of quartz, opaques and spinel. There is a very minor

component of post-peak biotite that occurs within close proximity to garnet and

opaque minerals. Sillimanite shows a preferred orientation. However a 10-15 percent

of grains show a more randomly oriented growth (Figure 5.1d). This sample does not

show evidence of shearing. Monazite occurs in both amongst the stable assemblage

and within garnet grains.

5.6.2 Sample: FR_10_05

Sample FR_1-_05 contains mesosomes composed of medium-grained (up to 2mm in

diameter) quartz , medium-grained K-feldspar and coarse-grained (up to 5mm in

diameter) garnet with minor opaque minerals (ilmenite and magnetite), sillimanite

and plagioclase. These phases represent the peak assemblage and define a strong

gneissic fabric and are typically all in direct contact. Recrystalised residual melt is

also present in the form of leucosome bands/rafts which are composed of coarse

quartz and K-feldspar with medium to fine-grained opaque minerals (less than 0.2

mm in diameter). The leucosome also preserves a strong fabric. Sub-idioblastic

garnet contains inclusions of prograde opaque minerals, spinel, sillimanite quartz and

rare K-feldspar and/or biotite (Figure 5.1a-b). The boundaries of garnet

porphyroblasts have intergrowths of quartz and sillimanite (it is undetermined

whether this is a retrograde or stable relationship). Garnet also contains irregular

fractures. Biotite is present as a post-peak phase, in contact with and partially

rimming garnet grains (Figure 5.1b). Spinel is always present with opaque minerals

and a majority occurs in garnet (figure 5.1a). Spinel is interpreted to be a prograde

phase that has been partially replaced by opaque minerals (most likely illmenite).

There is minor chlorite alteration around K-feldspar grains. K-feldspar also contains

perthite exsolution and sericite alteration. Zircon and monazite are also present.

Zircon occurs as fine round equant grains both among the peak assemblage and

isolated within garnets. Monazite grains are fine to medium grained (50-200 m) and

can be distinguished by their irregular grain shape and occur in the matrix and as

inclusions in garnet. Monazite is larger and more prominent within the leucosome.

Garnet preserves sillimanite inclusions that are both aligned (Figure 5.1a) and

randomly orientated suggesting garnet growth pre and syn-deformation. Within the

main assemblage and leucosomes there are zones of grain size reduction affecting

quartz and K-feldspar. The zones of grain size reduction suggest a post peak

deformation/shearing. This assemblage represents a quartz rich pelitic protolth.

5.6.3 Sample: FR_10_007

Coarse to medium-grained quartz, and K-feldspar, and coarse, sub-idioblastic garnet

with minor medium-grained blocky sillimanite, fine-grained opaque minerals, and a

small component of fine biotite constitute the peak mineral assemblage and define a

strong gneissic foliation where all phases are in direct contact. Garnet contains

inclusions of quartz, randomly oriented sillimanite and opaque minerals. Garnet also

contains minor fractures. Post-peak medium-grained biotite partially rims garnet and

opaque minerals. Garnet also suffers intergrowths of quartz and sillimanite (possibly

post peak?). Fine-grained spinel (similar to the previous sample) is adjacent to

opaques and looks to have been partially consumed by opaque minerals. Spinel is

also more abundant within garnet. Minor chlorite is present close to K-feldspar

grains which also demonstrate small exsolution lamellae and sericite alteration. This

sample displays thin bands of coarse-grained quartz and K-feldspar that could

represent compositional layering or represent melt (the latter is favored). The quartz-

rich bands also show grain size reduction (evidence of post-peak shearing). Strain

shadows containing quartz and K-feldspar and high strain zones defined by biotite,

fine-grained quartz and K-feldspar surround garnet porphyroblasts. This would

suggest garnet growth prior to a second shearing event which followed similar

kinematics to the first deformation that created the gneissic foliation. Monazite was a

prominent accessory phase within this sample. Monazite grains is irregular in shape

the largest up to 1 mm in diameter. Monazites are more common within leucosomes

and grains were also identified preserved within garnet. The peak assemblage

suggests the sample reached granulite facies metamorphism. The assemblage

suggests a pelitic protolith.

5.6.4 Sample: FR_10_009

Medium-grained quartz, and K-feldspar, and coarse sub-idioblastic garnet

porphyroblasts dominate peak phases in this sample with minor medium-grained

blocky sillimanite, medium-grained plagioclase, and fine-grained opaque minerals.

These peak phases define a strong gneissic foliation. Garnet porphyroblasts preserve

prograde quartz, randomly oriented sillimanite, opaque minerals and minor biotite

inclusions. Garnet also displays quartz and biotite boundary intergrowths and is

fractured. Fine-grained biotite is a post-peak phase that occurs in the high-strain

envelopes around garnet porhyroblasts along side fine-grained quartz and K-feldspar

(Figure 5.1c). The strain envelope again suggests garnet growth prior to shearing.

Sillimanite grains are aligned with the foliation and pinched at the ends, this suggests

they have been subject to shearing and their growth predates the shearing event.

Spinel is always adjacent to and seems to have been replaced by opaque minerals,

both within and outside of garnet and is a prograde phase. K-feldspar displays

perthite exsolution and sericite alteration is evident. The assemblage suggests a

pelitic protolith and that the rock was subjected to granulite facies metamorphic

conditions.

5.6.5 Sample: FR_10_011

Medium-grained quartz, medium-grained K-feldspar, and coarse sub-idioblastic

garnet porphyroblasts dominate with minor medium-grained blocky sillimanite,

medium-grained plagioclase, and fine grained opaque minerals. These minerals

constitute the peak metamorphic assemblage. These peak phases define a strong

gneissic foliation and all phases are in contact. Garnet porphyroblasts preserve

prograde quartz, sillimanite, and opaque minerals. Fractures are prominent within

garnet. Garnet also displays quartz and biotite boundary intergrowths. As a post-peak

phase biotite occurs in high strain envelopes around garnet porphyroblasts alongside

fine-grained quartz and K-feldspar. Sillimanite grains exist as fibrous needles. Spinel

is absent from this sample. K-feldspar displays perthite exsolution and sericite

alteration is evident. This sample contains a coarse quartz K-feldspar leucosome

which again is believed to represent former melt. Grain size reduction is also present

in the leucosome (concerning quartz and K-feldspar) indicating it was also subject to

shearing. A single coarse xenoblastic elongate grain of cordierite was also present

which appears to be an unstable phase (Figure 5.2a). There is a late shear band

cutting the foliation obliquely through the polished thin section that has deflected the

main foliation. This provides evidence of a definite late stage-shearing event (Figure

5.2b). This sample also contains accessory monazite. Monazite grains are irregular in

shape and reach up to 0.5mm in diameter. The assemblage suggests a less aluminous

quartz-rich pelitic protolith and that the rock was subjected to granulite facies

metamorphic conditions.

5.6.6 Sampple: FR_100_017

Medium-grained quartz, and k-feldspar, coarse sub-idioblastic garnet porphyroblasts,

minor fine-grained blocky sillimanite, opaque minerals (hematite/ilmenite), and

medium-grained blocky biotite are the peak phases that are all in equilibrium and

define a strong gneissic foliation. K-feldspar exhibits minor exsolution of perthite

and sericite alteration. Garnet contains prograde inclusions of quartz and sillimanite.

Sillimanite weakly follows the dominant foliation however there are zones where

sillimanite grains have grown in random orientations. Randomly oriented sillimanite

suggests the minerals growth after the development of the dominant gneissic

foliation. Spinel grains have been partially overgrown by opaque minerals. Post-peak

biotite is evident forming radial coronas around opaque grains. Bands of grainsize

reduction rich in quartz and K-feldspar leucosomes provide evidence of later

deformation. The assemblage suggests a pelitic protolith that reached granulite facies

metamorphic conditions.

5.6.7 Sample: FR_10_019

The peak assemblage is dominated by medium-grained, sub-idioblastic garnet,

medium-grained K-feldspar, and quartz, blocky medium-grained sillimanite, with

minor opaque minerals, and biotite. These phases define a strong gneissic foliation.

Garnet appears stable with prograde inclusions of quartz, sillimanite and opaque

minerals. Zones of grainsize reduction are evident in quartz/K-feldspar rich bands,

which are possibly leucosomes though could represent compositional layering.

Blocky biotite partially rims and may be replacing opaque phases. Accessory

monazite and zircon are also present amongst the stable assemblage. The minerals

present suggest a pelitic protolith that reached its metamorphic peak in granulite

facies metamorphic conditions.

6. Mineral equilibria modeling

The conditions of metamorphism of the quartz rich granulite facies metapelitic rocks

from the two localities Gnamma Hill and Mount Malcolm are considered using

isochemical phase diagrams (pseudosections). Bulk chemical compositions of four

samples were used. These samples represent the approximate composition of the

pelites and both garnet rich restite material with leucosomes were sampled. These

compositions (see appendix 2) reflect the combined effects of processes such as melt

loss from the original protolith. Processes such as melt loss should be considered

when interpreting prograde assemblages represented in these pseudosections.

6.1 FR-10-005 (Gnamma Hill)

Peak assemblage included quartz + K-feldspar + garnet + opaque minerals (ilmenite

or magnetite) + sillimanite + liquid/melt. Based on the constructed pseudosection,

this peak assemblage is predicted to be stable at temperatures greater than 900 °C

and pressures exceeding 8 kbar (figures 6.1a, 6.2b, 6.2a). Prograde minerals quartz +

opaques + sillimanite ± K-feldspar + biotite preserved in garnet suggest a prograde

path through the field Kfs-Pl-Sil-Grt-Bt-Qtz-Ilm, through the solidus, past the Pl out

field and into the Liq-Kfs-Sil-Grt field. Figure 6.1b shows the calculated modal

isopleths for sillimanite plotted onto the pseudosection. A majority of sillimanite

preserved in garnet occurs in the middle of the grains as finer needle like grains and

occurs less closely to the boundary. Sillimanite isopleths support the proposed

prograde path with the proportion of sillimanite decreasing with increasing pressure

and temperature (figure 6.1b). Garnet isopleths display increased mode proportions

along the proposed prograde path (figure 6.1a). According to the prograde path

FWFigure 6.1:With garnet

Pseudosectimode isopl

ions construleths. b. Wit

ucted for sath sillimanit

ample FR-10te mode iso

0-005. a. opleths.

Fmfo

Figure 6.2:mode isopletfor sample F

a. Pseudoseths, arrows

FR-10-009 w

ections consshow predi

with Sillima

structed for icted P-T paanite mode

sample FR-ath. b. Pseud

-10-005 witdosections c

th biotite constructedd

biotite would decrease to zero mode proportion in the stable field (figure 6.2a). This

may reflect the lack of peak biotite amongst the stable assemblage. Btotite exists as a

post peak phase suggesting the retrograde path moved into the biotite stability field.

Of note is the lack of cordierite identified through petrology. This would suggest that

the rocks did not move into the cordierite stability field. A retrograde path that

passed through similar fields as the prograde path is interpreted from this (figure

6.2a).

6.2 FR-10-009 (Gnamma Hill)

Peak assemblage includes quartz + K-feldspar + garnet + sillimanite + plagioclase +

opaque minerals + melt ± cordierite. Given the constructed pseudosections (figures

6.2b, 6.3a, b) this peak assemblage is stable at temperatures exceeding 900 °C and

pressures greater than 8 kbar. Prograde minerals preserved in garnet are quartz +

sillimanite + opaques and minor biotite. This prograde assemblage suggests a

prograde path passing through the large Kfs-Pl-Sil-Grt-Bt field, through the solidus

and into the peak field. The sillimanite and biotite mode isopleths support this (figure

6.2b and 6.3b) with a low proportion of sillimanite through peak and post peak

conditions. Post-peak biotite suggests cooling back into the biotite stability field.

Garnet isopleths support continued growth during the prograde path. And a decrease

in the proportion of garnet during post peak conditions that is interpreted from

observations in thin section may have enabled the growth of biotite. It should be

noted that there is evidence of cordierite in the Fraser Zone metasediments (FR-10-

011 for example) though none was observed in any of the samples used to construct

pseudosections for this sample. Previous investigations such as Clark (1999)

identified cordierite in meta-pelitic rocks that had been pseudomorphed by biotite,

sillimanite

constrain t

e and quar

the retrogra

rtz. Howev

ade path to a

ver the lack

above the co

k of cordie

ordierite sta

erite in thi

ability field.

is sample h

.

helps to

That it is possibly present in other samples as an unstable phase and that it has been

reported as being pseudomorphed suggests cordierite may be more stable in other

samples due to differences in bulk compositions. Its rare occurrence suggests the P-T

path passed through or clipped the cordierite stability field at some stage during the

rocks P-T evolution.

6.3 FR-10-017 (Mount Malcolm)

Peak assemblage is made up of quartz + K-feldspar + garnet + plagioclase +

sillimanite + opaques ± biotite. The presence of coarse biotite amongst the peak

assemblage places the peak conditions in a lower temperature field than was

observed in the two Gnamma Hill samples. The pseudosection constructed for this

samples bulk composition places this assemblage in a field greater than 8-8.5 kbar

and at temperatures higher than 825-900 °C (figure 6.4a). Quartz + sillimanite +

opaque inclusions in garnet grains imply a prograde path similar to the previous two

samples. Btotite also exists as a post-peak phase, partially rimming or replacing

opaque phases. The pseodosecction calculated for this sample bulk composition

displays similar trends in the mode isopleths for sillimanite, garnet and biotite

(figures 6.4a, b and 6.5a respectively). The absence of prograde or post-peak

cordierite supports the interpretation of a tight ‘hair pin’ P-T loop that peaked in the

field Kfs-Pl-Sil-Grt-Bt-Qtz-Ilm-Liq and does not cross into the cordierite stability

field (figure 6.5b).

6.4 FR-10-019 (Mount Malcolm)

Figure 6.3: Pseudosections constructed for sample FR-10-009. a. With garnet mode isopleths. b. With biotite mode isopleths.

Quartz + K

peak asse

assemblag

K-feldspar +

mblage. Th

ge at lower t

+ garnet + s

he construc

temperature

sillimanite +

cted pseudo

es and press

+ opaque m

osection for

sure than the

minerals + b

r this samp

e previous G

biotite const

ple places t

Gnamma Hi

titute the

the peak

ill

FsFigure 6.4:illimanite m

Pseudosectimode isoplet

ions construths. b. With

ucted for sah garnet mod

ample FR-10de isopleths

0-017. a. Ws.

With

Figumodeconst

ure 6.5: a. Pe isopleths ,tructed for s

seudosectio, arrows shosample FR-

ons construcow predicted-10-019 with

cted for samd P-T path. h Sillimanit

mple FR-10-b. Pseudos

te mode iso

-017 with biections pleths.

iotite

samples due to observed peak biotite. The peak assemblage exists at temperatures

between 800 °C and 850 °C and pressures greater than 6.5 kbar given the constructed

pseudosection (figure 6.5b). This lower range is due to the inclusion of biotite in the

peak assemblage. Petrology suggests a similar P-T path as previous samples. The

post-peak growth of biotite and lack of cordierite support a retrograde path that

passes through similar fields as the prograde path. Sillimanite, garnet and biotite

isopleths display similar trends to those observed in all other samples (figure 6.5b,

6.6a, and b respectively).

The pseudosections provide a good tool for the interpretation of P-T history. Mode

isopleths for the phases sillimanite, garnet and biotite are consistent over all four

samples. Prograde garnet with quartz, sillimanite, opaque minerals and minor biotite

inclusions suggests temperature increased with pressure. Peak assemblages indicate

conditions greater than 900 °C and pressures greater than 8 kbar for the Gnamma

Hill samples. Pseudosections for Mount Malcolm samples suggest a lower peak

temperature between 800-900 °C and pressures greater than 6.5 kbar. These lower

ranges are due to the observed coarse biotite in the peak assemblage. It should be

noted that in Mount Malcolm samples biotite partially replaced opaque phases that

were well distributed through the matrix. Therefore the coarse blocky biotite could

represent opaque phase that had been totally replaced with biotite and therefore

interpreted as post peak biotite growth. This would place the samples in a similar

Figure 6.6: Pseudosections constructed for sample FR-10-019. a. With sillimanite mode isopleths. b. With garnet mode isopleths.

temperature and pressure range as the Gnamma Hill samples. Petrology combined

with pseudosections suggests a tight P-T path with temperature steadily increasing

with pressure.

7. Geochronology

7.1 FR-10-011 (Gnamma Hill)

Monazites from this sample are anhedral and spherical to acicular in shape and up to

1000 μm in length. Monazites appeared clear with a dusty appearance under plain

polarized light. Monazites from sample FR-10-011 (figure 7.1) occurr in three

textural settings: included within garnet, in the matrix of the garnet-rich assemblage

and within the matrix of leucosomes (believed to represent crystallised melt).

Monazite in the leucosome were the largest (up to 1000 μm) in length and the most

abundant, often elongate with irregular boundaries (figure 7.1a to k). Monazites

within the matrix of the garnet rich zones and within garnets themselves were much

smaller and more spherical (figure 7.1 l to p). Grains display little zoning under

electron backscatter imaging except for a small faint rim on seen in figure 7.1h. 56

analyses were conducted on 13 monazite grains. Figure 7.1 shows electron

backscatter images of each analysed grain and the location of each SHRIMP analysis

spot.

An inverse condordia plot of the 238U/206Pb ratios against the 207Pb/206Pb for all the

analyses (figure 7.2) displays relatively concordant data. Of the 56 analyses 10

displayed a variance greater than 10% and were excluded from further interpretation.

Monazite U-Pb in situ analysis for sample FR-10-011 produced a concordant

weighted mean 207Pb/235U age of 1266.1 ± 7.6 Ma (2 , MSWD = 1.3) (figure 7.3)

with 3 of 46 analyses rejected. A comparison between analyses from garnet bound

grains (red), monazite rim ages (yellow) and matrix grains from the leucosome

(white) in figure 7.4. The garnet bound monazites do not deviate far from the general

Figure 7.2

Figure 7

207 Pb/206 Pb

2: Inverse c

7.3: Plot of

15

0.06

0.07

0.08

0.09

0.10

3.6

207 Pb/206 Pb

950

1050

1150

1250

1350

1450

M

concordia plproduce a

207Pb/235U abecause of

1450

550

6 4.0

Mean = 1266.1±7.7Wtd by data-pt e

MSWD = 1.3, p(error b

lot of all ana207Pb/235U a

ages for samhigh percen

13500

0 4.23

7 [0.61%] 95% rrs only, 3 of 46 r

probability = 0.06bars are 2s)

alyses fromage of 1266

mple FR-10-ntage of disc

1250

.4 438U/206Pb

data-point e

conf.rej.69

m sample FR6±a8 Ma.

-011. 10 ancordance.

1150

4.8 5

error ellipses

box heights a

R-10-011. A

alyses not i

0

5.2

are 2

are 2

Analyses

nclude

Figure 7represenrim, cleamonazit

Figure samp

207 Pb/206 Pb

.4: Inverse nts garnet boar ellipse rete ages show

7.5:Plot of ple FR-10-0

15

0.06

0.07

0.08

0.09

0.10

3.6

207 Pb/206 Pb

1140

1180

1220

1260

1300

1340

concordia pound monazepresents mow little diffe

displ

207Pb/235U a011 produci

1450550

6 4.0

plot of analyzite, yellowonazite fromerence to thay a slightly

ages (Ma) fng a slightly

popula

1350

0 4.23

yses from saw represents m leucosom

he whole popy younger a

from monazy older age

ation.

1250

.4 438U/206Pb

data-point e

MWM

ample FR-1analysis of

me matrix. Hpulation. Th

age.

zite inclusiowhen comp

1150

4.8 5

error ellipses

box heig

Mean = 1275±15 Wtd by data-pt errsMSWD = 0.78, pro

(error bars

box heig

10-011. Redf monazite gHow garnet bhree rim ana

ns in garnetpared to ent

5.2

are 2

hts are 2

[1.2 95% conf.s only, 0 of 9 rej.obability = 0.62s are 2s)

hts are 2

d ellipse growth boundalyses

t from tire

age population, and their combined age was within error of the total population

(figure 7.5). Three monazite rim analyses in yellow are all of a slightly younger age.

Based on a small number of analyses they produce an average 207Pb/235U age of

1236 ± 22 Ma (2 , MSWD = 0.79)(figure 6.6). Data displayed no trends in Thorium

content with age (figure 7.7).

7.2 FR-10-007 (Gnamma Hill)

Sample FR-10-007 contained anhedral, small, rounded monazites present amongst

the stable assemblage and within garnet (figure 7.8 a to e). The largest grain (figure

7.8a) was 150-200 μm in diameter and displayed no zoning. The remaining grains

were much smaller, as small as 20 μm in diameter. Monazites are clear appearing

slightly dirtier compared to zircon grains. These grains did not exhibit any major

zoning. Zircons are also abundant and display similar characteristics to monazite.

Thus it was difficult to distinguish the two accessory phases. Five grains were

analysed with thirteen SHRIMP spots.

Figure 7.9 displays an inverse Concordia plot of the 238U/206Pb ratios against the

207Pb/206Pb for all the analyses from sample FR-10-007. Monazite U-Pb analyses for

sample FR-10-007 produced a concordant 207Pb/235U age of 1285 ± 16 Ma (2 ,

MSWD = 0.79) based on 13 analyses (figure 7.10). No trend in Th was observed

(figure 7.11).

7.3 182447 (Mount Malcolm)

Grains from 182447 are anhedral and spherical to ellipsoid in shape. Electron

backscatter imaging revealed no major zoning. The largest grain was approximately

200 μm in length the smallest only 50 μm in length. Monazites are relatively clear

Figur

Figure 6

207 Pb/206 Pb

re 7.6: Plot o

6.7

1180

1200

1220

1240

1260

1280

1300

MWM

1

0.06

0.07

0.08

0.09

0.10

3.

207 Pb/206 Pb

of 207Pb/235U

Mean = 1236±22 [Wtd by data-pt errMSWD = 0.24, pr

(error bar

145

550

.6 4

U ages (Mayounge

[1.7%] 95% confrs only, 0 of 3 rej.robability = 0.79rs are 2s)

1350

50

4.0

a) for the threr age.

f..

1250

0

4.4238U/206P

data-p

ree rim anal

box heights a

110

4.8

Pb

point error elli

lyses. Displ

are 2

150

5.2

pses are 2

lay a

Th ppm

7000

19000

Figure 7.9

207 Pb/206 Pb

9: Inverse c

Figure 7

0.075

0.077

0.079

0.081

0.083

0.085

0.087

0.089

3.

1180

1220

1260

1300

1340

1380

concordia plproduce a

7.10

1

1400

9 4.1

Mean = 12Wtd by da

MSWD (

lot of all ana207Pb/235U a

207Pb/235U

1320

1360

4.3 423

data-poi

285±16 [1.2%] ata-pt errs only, = 1.3, probabilierror bars are 2

alyses fromage of 1285

U ages for sa

1240

1280

4.5 4.738U/206Pb

int error ellip

95% conf.0 of 13 rej.

ity = 0.212 )

m sample FR5±16 Ma.

ample FR-1

11

1200

0

4.9b

ses are 68.3

box heig

R-10-007. A

10-007.

160

5.1

3% conf

ghts are 2

Analyses

Figure 7

207 Pb/206 Pb

.11

0.078

0.080

0.082

0.084

0.086

0.088

4.

134

1380

1 4.3

1300

40

3 4.523

1

1260

4.738U/206Pb

data-point

118

220

4.9b

error ellipses

80

5.1

s are 2

T

5

4

Th

000

0000

and difficult to distinguish from zircon. 9 analyses from 5 monazites were performed

on sample 182447 (figure 7.12). Three monazite grains were inclusions bound within

garnet (Figure 7.12, a, b, and c). The remaining two monazites were located amongst

the stable assemblage (figure 7.12 d.and e) Three with no results had excessive

thorium counts overloading the SHRIMP II collector and no results were collected

(figure 7.12 f, g and h).

Figure 6.13 displays a inverse Concordia plot of the 238U/206Pb ratios against the

207Pb/206Pb ratios for all the analyses from sample 182447. Monazite U-Pb analysis

for sample 182447 produced a concordant 207Pb/235U age of 1268 ±12 Ma (2 ,

MSWD = 1.3) based on 8 analyses (figure 7.14). One analysis was rejected and

might represent a small amount of Pb loss. There was little difference observed

between garnet bound monazites and those from amongst the matrix (figure 7.15).

There were no trends in Th content with age observed (figure 7.16).

Figure 7

207 Pb/206 Pb

7.13: Inverse

Figure

0.076

0.078

0.080

0.082

0.084

0.086

4.

1180

1200

1220

1240

1260

1280

1300

1320

1340

e concordiaproduce a 2

e 7.14: Plot

134

1 4.3

Mean = 126Wtd by datMSWD =

(er

a plot of all a207Pb/235U a

of 207Pb/235

1300

40

3 4.523

data-poi

8±12 [0.99%] ta-pt errs only, 00.79, probabilityrror bars are 2

analyses froage of 1268

5U ages for

1

1260

4.738U/206Pb

int error ellip

95% conf.0 of 8 rej.y = 0.60)

om sample ±12 Ma.

sample 182

118

220

4.9b

ses are 68.3

box heig

182447. An

2447.

80

5.1

3% conf

ghts are 2

nalyses

Figure 7.1represent

Figure

207 Pb/206 Pb

207 Pb/206 Pb

15: Inverse garnet boun

7.16:

0.078

0.080

0.082

0.084

4.

0.078

0.080

0.082

0.084

4.

concordia pnd monazite

2 4

2 4

plot of all ane analyses, w

1

1290

1310

4.423

1

1290

1310

4.423

nalyses fromwhite repres

12

1250

1270

4.638U/206Pb

data-point

12

1250

1270

4.638U/206Pb

data-point

m sample 18sent monaz

1

1210

230

4.8b

error ellipses

1

1210

230

4.8

b

error ellipses

82447. Redite from the

190

5.0

s are 2

190

5.0

s are 2

T

6

1

d ellipses e matrix.

Th

000

8000

8. Discussion

8.1 Previous work on metamorphic age

The Fraser Zone has been subject to a number of geochronological investigations.

Most of these investigations use U-Pb isotope zircon ages. A majority of ages

obtained from the Fraser Zone are interpreted a igneous crystallisation ages or ages

for detrital zircons (Clark 1999, Clark et al. 1999, De Waele & Pisarevsky 2008,

Spaggiari et al. 2009), and there is a lack of work focused on the age of

metamorphism. The only metamorphic ages obtained include Fletcher et al. (1991)

who obtained biotie-whole rock two point Rb-Sr age of 1268 ±20 Ma. A similar age

for muscovite bearing rocks on the western boundary of the Fraser Zone of 1268 ±

21 Ma was obtained by Bunting et al. (1976). The Rb-Sr age reflects timing of

cooling below the biotite closure temperature after peak metamorphic conditions.

Clark (1999) obtained SHRIMP U-Pb zircon ages for metasedimentary gneiss at

Gnamma Hill. Most of the ages for this sample were interpreted as detrital however

six analyses of low U zircon rims produced a 207Pb/206Pb age of 1305 ± 80 Ma

attributed to zircon growth during high grade metamorphism. Evidently this age is

poorly constrained. Clark (1999) also dated a syn-M1a/D1 charnockite at 1301 ± 6

Ma. De Waele & Pisarevsky (2008) provided SHRIMP U-Pb zircon ages for a mafic

granulite at 1291 ± 8 Ma, interpreted as the age of igneous crystallisation. Spaggiari

et al. (2009) suggested this might indicate a metamorphic age rather than an igneous

crystallisation or a combined mixed age. An age of 1304 ± 7 Ma was reported by

Spaggiari et al. (2009) from metamorphic rims on zircons obtained from quartz

meta-sandstone.

8.2 The interpretation of monazite ages

The growth of monazite in granulite facies terranes is typically attributed to

metamorphism. However there is uncertainty in what the monazite age represents

and how it should be interpreted. For instance there is uncertainty surrounding the

timing of monazite growth within metamorphic rock, does it represent peak

metamorphism? How does the composition of the rock effect the timing of monazite

growth? How does a monazite age compare to an equivalent zircon age and how

significant is Pb diffusion in monazite.

Rubatto et al. (2001) investigated monazite response to prograde metamorphism in

the Reynolds Range, Central Australia. The authors found evidence of early

monazite growth in amphibolite facies rocks recording regional metamorphic ages.

In the same region they also identified abundant monazite all yielding an older age in

granulite faies rocks (Rubatto et al. 2001). This suggests monazite growth occurs

during both amphibolite and granulite facies conditions. Foster et al. (2000)

recognized episodic growth of monazite grains from two chronologically well

constrained areas of the Himalayan Orogen. Monazite inclusions within garnet grew

prior to garnet growth during greenschist facies conditions while monazite grains in

the matrix were younger and grew during amphibolite facies conditions (Foster et al.

2000). Both studies demonstrate that metamorphic rocks can preserve multiple

episodes of monazite growth.

Rubatto et al. (2001) compared the response of both monazite and zircon growth in

the Reynolds Range. While monazite growth is recorded at both amphibolite and

granulite conditions, zircon growth was only observed at granulite facies conditions.

The authors found that new zircon growth occurred at the expense of detrital and

inherited zircon cores. Rubatto et al. (2001) found evidence that the amount of

metamorphic growth of both accessory minerals increased with temperature. It was

also established that new zircon growth was influenced by rock composition and

driven by partial melting. Monazite growth was not affected by either of these factors

(Rubatto et al. 2001). Fitzsimons et al. (2005) compared monazite ages from rocks

with differing bulk compositions from the Mount Barren Group. The authors

suggested that varying bulk rock compositions produced different monazite ages for

rocks that had all undergone identical metamorphic conditions. This indicates that

bulk rock composition does play a role in the timing of monazite growth.

Furthermore, zircon displays different growth characteristics to monazite. These

differences should be taken into account when comparing and using ages from both

zircon and monazite.

Pb diffusion is also a concern when interpreting monazite isotopic data. Pb closure

temperatures for artificial monazite has been observed in excess of 900 C (Cherniak

et al. 2004) and in large natural monazites the closure of U-Pb system has been

observed at 750-800 C (Rubatto et al. 2001). Kelsey et al. (2008) conducted

thermobarometric modeling of zircon and monazite growth in melt bearing systems.

There results confirmed previous studies suggesting zircon ages are generally older

than monazite ages (Machado et al. 1989, Parrish 1990). However Kelsey et al.

(2008) suggested that the difference in age is not due to monazite having a lower

closure temperature than zircon, rather the variation in age is attributed to differences

in growth behavior. Because of this Kelsey et al. (2008) suggested that zircon and

monazite ages that do not overlap in error suggests: 1. A slow metamorphic cooling

rate. 2. The growth of zircon began at significantly different temperatures compared

to monazite. There could also be a combination of the previously mentioned

scenarios. The authors also suggested that the growth rate of accessory minerals in

rocks is higher at or near melt saturation temperatures. Thus differences in monazite

and zircon ages may not depend on variations in closure temperature. Further more

monazite and zircon growth rate increases with temperature suggesting that

accessory phases amongst the peak assemblage are more likely to represent the age

of peak metamorphism (Rubatto et al. 2001, Kelsey et al. 2008).

8.3 The use of 207Pb/235U ages

Reverse discordance has been observed in monazite and is believed to be a product

of excess 206Pb produced by the decay of 230Th (Kirkland et al. 2009). The excess

206Pb is incorporated into the monazite structure during crystallization and the

measured 206Pb/238U daughter parent ratio is larger creating the reverse discordance.

Kirkland et al. (2009) point out that this process does not affect the 207Pb/235U ratio

and thus the 207Pb/235U age is considered to be more reliable. Therefore all ages are

calculated using 207Pb/235U ratios.

8.4 Age interpretations

Three samples were used for in-situ monazite analysis; FR-10-007 and FR-10-011

from Gnamma Hill and sample 182447 from Mount Malcolm. All three samples

represent quartz-rich semi-pelitic rocks that have been subject to granulite facies

metamorphic conditions.

Sample FR-10-007 yielded a 207Pb/235U age of 1285 ± 16 Ma. Monazites from this

sample were bound within garnet or amongst garnet restite matrix. This is interpreted

as a metamorphic age but not necessarily the timing of peak metamorphism. The

smaller garnet-bound or restitic monazites were much less common than the larger

more abundant monazites grains observed in leucosomes. This suggests that a

majority of monazite growth occurred during the crystallisation of the rock. Sample

FR-10-011 yielded a significant quantity of large monazites that occurred amongst

the matrix of leucosomes. These monazites yielded a more refined 207Pb/235U age of

1266.1 ± 7.7 Ma. This is interpreted as the age of the leucosomes crystallisation.

Three rim analyses from the same sample provided a younger age of 1236 ± 22 Ma.

This is interpreted a either evidence of a second metamorphic event that was

recognized by Clark (Clark 1999) or the possible influence of hydrothermal fluid

alteration of the monazite grains. Garnet bound monazites from sample FR-10-011

provided a slightly older 207Pb/235U age of 1275 ± 15 Ma however it was within error

of the general age population for this sample. Kelsey et al. (2008) suggested that

accessory mineral growth increased with temperature. This accounts for the

significant amounts of monazite growth present within leucosomes which represent

the crystallised remains of melt. This age could be interpreted as being close to the

timing of peak metamorphic conditions or time spent above the solidus.

Sample 182447 from Mount Malcolm yielded a constrained 207Pb/235U monazite age

of 1268.2 ± 12 Ma. This age was obtained from monazites both bound by garnet and

from the matrix. There was no difference between matrix and garnet-bound analyses.

This is interpreted as a metamorphic age for the sample. The similarity of matrix and

garnet-bound monazite ages from both sample 182447 and FR-10-011 suggests rapid

metamorphism. This is based on the concept that garnet bound monazite grew during

prograde metamorphism while matrix grains represents either the timing of peak

metamorphism or the onset of cooling below monazites closure temperature (Foster

et al. 2000).

Multiple episodes of monazite growth preserving different ages have been reported

by authors such as Foster et al. (2000) and Rubatto et al. (2001) and have been used

to constrain the duration of peak metamorphism. Monazite ages from this study are

within error or close to within error of each other. This suggests relatively rapid

metamorphic history given the lack of variation between garnet and matrix bound

monazite ages.

The nature of the protolith must also be considered in interpreting the monazite ages.

Field observations suggest the protolith for the quartz rich meta-pelitic rocks was

heterogeneous with lenses of quartzite and areas abundant in garnet and oxides.

Fitzsimons et al. (2005) suggested that compositional variation in protoliths could

affect the timing of monazite growth for example iron rich samples were found to

yield older monazite ages. This might account for the minor variation in monazite

ages observed.

The results of monazite geochronology suggest metamorphic monazite growth

occurred between 1285 Ma and 1266 Ma. Monazites from the matrix of leucosomes

produced the youngest ages while garnet bound and matrix monazite produced

slightly older ages. These monazite grains are all typically homogenous indicating

they grew during metamorphism and were not inherited. The small variation in ages

suggests a relatively short lived metamorphic event.

8.5 Comparison with zircon ages

Previous age data obtained through SHRIMP U-Pb analysis of zircon for meta-

sedimentary rocks of the Fraser Zone appear older than the calculated monazite ages.

The 207Pb/206Pb age of 1305 ± 80 Ma calculated by Clark (1999) for low uranium

rims is poorly constrained and within error of the presented monazite ages. Clark

(1999) also dated a syn-metamorphic charnockite at 1301±6 Ma. Spaggiari et al.

(2009) reported 1304±7 Ma zircon rims from quartz metasandstone. These ages are

all older than the monazite ages reported here.

Unpublished data from GSWA provides additional zircon ages. Leucosomes from a

psammitic gneiss yielded a concordia age of 1285 ±7 Ma, which was interpreted as

the age of the leucosome crystallisation (unpublished data; sample 194715 GSWA).

A psammitic gneiss provided a concordia age of 1291±5 Ma for zircon rims

representing age of migmatization (unpublished data; sample 194714 GSWA). These

ages are older than the equivalent monazite ages identified in sample FR-10-011. In

general all zircon ages appear to be older than monazite ages. This illustrates that

there was a difference in the timing of both accessory minerals during peak

metamorphism.

8.6 Link between igneous crystallization age of mafic granulites and

metamorphic age

The igneous crystallisation age for the mafic granulites that make up the majority of

the Fraser Zone has been reported by authors such as De Waele & Pisarevsky (2008).

Unplished data provides a similar zircon age of 1292 ± 6 Ma for the mafic granulites

This was interpreted as the age emplacement or possibly when the mafic protolith

was metamorphosed (unpublished data; sample 194718 GSWA). These ages are

close to or within error of zircon metamorphic ages for the Fraser Zone. Such an

occurrence suggests there is a temporal link between the crystallization and/or

emplacement of the mafic granulites and the timing of metamorphism in the region.

8.7 Summary of Fraser Zone age data

Unpublished SHRIMP U-Pb zircon age data from GSWA of 1313±19 Ma provides

the oldest detrital age for the Fraser Zones metasediments (sample 194714 GSWA).

This is interpreted as the youngest depositional age of the sedimentary protolith.

SHRIMP U-Pb zircon data places the crystallization and/or emplacement of the

mafic granulites at c. 1291 Ma (De Waele & Pisarevsky 2008) this is supported by

unpublished SHRIMP U-Pb zircon data from the GSWA . The oldest record of

metamorphism is recorded at c. 1304 Ma by metamorphic rims on zircons (Spaggiari

et al. 2009). The youngest zircon age has been reported at c. 1285 Ma (unpublished

data; sample 194715). Monazites from this study preserve ages between 1285 to

1266 Ma. Rb-Sr whole rock biotite ages suggest the Fraser zone cooled below the

biotite closure temperature at c. 1268 Ma (Fletcher et al. 1991).

The relationship between zircon and monazite ages obtained from metasediments

and mafic granulites suggests the intrusion or emplacement of the mafic granulites

played a role in the metamorphism of the Fraser Zone. Peak metamorphism took

place relatively quickly evident in the minor difference in the age of garnet bound

monazite and matrix monazite. The youngest ages for both zircon and monazite are

preserved in leucosomes representing the crystallised remains of melt. The

preservation of Rb-Sr biotite ages suggest no major metamorphic events significantly

affected the Fraser Zone after 1268 Ma.

8.8 P-T evolution

Petrological and phase equilibria modeling constrains peak metamorphic conditions

to >900 °C and pressures as low as 8 kbar in Gnamma Hill and between 800-900°C

and pressures greater than 6.5 kbar are recorded in rocks from Mount Malcolm.

Prograde quartz + sillimanite + opaques and minor K-feldspar + biotite prerved in

garnet suggest a prograde path where temperature increased with pressure. This is

supported by the lack of kyanite preserved in the prograde assemblage. Post-peak

biotie growth suggest during retrogression the rocks moved back into the biotite

stability field. Lack of post-peak cordierite confirms a retrograde path where pressure

decreased steadily with temperature. This suggests a sharp ‘hair pin’ P-T path (figure

8.1). This P-T path indicates heating occurred alongside thickening. Cooling and

decompression followed. This leads to the interpretation of a transient heat source.

The disparity between peak metamorphic conditions of Gnamma Hill rocks

compared to Mount Malcolm rocks occurs because biotite appears to exist as a peak

phase in the Mount Malcolm samples. The peak assemblage consisting of Qtz-Grt-

Sil-Ilm-Kfs--Liq-Bt, whereas Gnamma Hill displayed a peak assemblage consisting

of Qtz-Grt-Sil-Ilm-Kfs--Liq-Bt. Including biotite in the peak assemblages places the

rocks in a lower temperature field. The peak biotite may represent the replacement of

opaque phases by post-peak biotite. If this were so the peak conditions of Mount

Malcolm rock would be similar to Gnamma Hill peak metamorphic conditions.

The only other P-T investigation for Fraser Zone rocks was conducted by Clark

(1999) and Clark et al. (1999). Clark (1999) identified two episodes of

metamorphism evident in disequilibrium textures preserved in the rock. The author

suggested the first event preserved higher temperature assemblages but was partially

overprinted by a lower temperature higher pressure second event. No significant

evidence of a second episode of metamorphism was identified in samples for this

study. No disequilibrium textures were identified in thin section. Clark (1999) and

Clark et al. (1999) suggest M1 occurred at similar temperatures and pressure as those

put forward in this study (T 800ºC and P = 6-7 kbar). M1 was then followed by

near isobaric cooling followed by M2 characterized by increased pressure. Clark

Figure 8.1possible PrepresentsMalcolm reach samp

1: P-T pathP-T conditios representsrocks. The dple is also dr

h interpretedons interpres the possdiagram Illurawn for co

d for granuleted from tsible P-T custrates a tig

omparison.

lite facies rthe Gnammconditions ght ‘hair pin

rocks. Solidma Hill rock

interpretedn’ P-T path

d line represks. The dasd from theh. The peak

sents the hed line

e Mount field for

(1999) suggested M1 was evident throughout the interior of the Fraser Zone. The

strongest evidence of M2 occurred at the boundary of mafic units and was most likely

localized along major bounding structures. This study’s lack of evidence is probably

due to the focus on metasedimentary lithologies rather than the mafic lithologies that

preserve evidence of a second metamorphic event and make up a majority of the

Fraser Zone (Clark 1999).

8.9 Tectonic model

Collins (2002a) discussed the formation of extensional accretionary orogens. Such

extensional orogens are generated when multiple episodes of large arc/back arc basin

systems are repeatedly thickened. Collins (2002a) suggested that such extensional

accretionary orogens could be recognised by a number of characteristics. Some of

these characteristics described by Collins (2000a) can be recognised within the

Fraser Zone such as the dominance of mafic lithologies, the presence of volcanic and

volcaniclastic sediments, Low P variable T metamorphism and evidence of

shortening. Collins (2002b) suggests that many granulite facies terrains formed in

accretionary orogens during tectonic switching. Heat is generated during slab retreat

when basaltic magmas and advected into the extending back arc basin (Collins

2002b). When slab retreat ends extension stops and thickening occurs. This creates a

transient heat source which ends once thickening begins (Collins 2002b). Such a

model has been used to explain the production of ultra high temperature

metamorphism by Brown (2006) and a similar model has been used by Clark et al.

(2009).

The model proposed by Collins (2002a, b) is used to account for the formation of the

Fraser Zone within the Albany-Fraser orogenic belt. Figure 8.2 illustrates the

simple model. P-T work indicates that the Fraser Zone underwent rapid heating and

thickening followed by relatively rapid cooling. It was suggested that this required a

transient heat source and Collins (2002b) argued that tectonic switching produces

short-lived heat sources. The Fraser Zone is dominated by mafic lithologies with thin

slivers of metasediments. The abundance of mafic lithologies is a major

characteristic of extensional orogenesis driven by the formation and destruction of

back arc basins (Collins 2002a). Collins (2002a) also suggested that that volcanic

sediments would be evident and field mapping within the Fraser Zone identified

possibly volcanic related rock types. Collins (2002a) also argued that low pressures

were characteristic in the recognition of extensional orogensis. P-T works suggests

high temperatures with comparatively low pressures.

It is suggested that the Fraser Zone mafic lithologies represent hot mafic material

advected into a back arc basin that possibly formed between what is now recognized

as the Biranup Zone. The older detrital zircons in the Fraser Zones metasedimentary

rocks are of similar age to the Biranup Zone (Spaggiari et al. 2009). This suggests

the Biranup Zone as a source of sedimentary material. Bodorkos & Clark (2004b)

suggested subduction was occurring towards the southeast due to evidence of pre-

1313 Ma magmatic and tectonothermal activity in the Nornalup Zone. The back arc

may have developed in response to slab retreat. The youngest detrital zircon age

found in psammitic gneiss suggests deposition of sediments into the basin was as late

as 1313 Ma (figure 8.2). Once extension ceased the back arc basin was thickened

through shortening between the Biranup Zone and the protolith for the Nornalup

Zone (the Mawson Craton) (figure 8.2). Whether this occurred prior to, during or

after the Biranup Zone’s collision with the Yilgarn Craton is unknown. Thickening

was accommodated by the repeated thrusting of the mafic back arc material onto and

over the Biranup zone along major thrusts. Back arc basin sediments were included

as the basin was progressively thrust over itself reaching pressures greater than 9

kbar and temperatures greater than 900 °C. Heat from the advected mafic material

was rapidly transferred into the included sediments as it began to cool. If the Fraser

Zone represents multiple episodes of tectonic switching you would expect more

evidence of felsic volcanics and granitic plutons between mafic units representing

remnants of small magmatic arcs (Collins 2002a). Thus it is proposed that the Fraser

Zone represents one major closure of a back arc system. Zircon U-Pb data from

mafic units suggests they crystallized and cooled below the zircon closure

temperature c. 1292 Ma. The time at which the sediments reached peak metamorphic

conditions is difficult to establish. The oldest record of metamorphism is recorded by

zircon growth rim with an age of c. 1304 Ma. Homogenous zircons and zircon rims

extracted from leucosomes dated between 1291 and 1285 Ma interpreted as the age

of leucosome crystallisation. Monazites preserved in garnet yield a minimum garnet

age of c. 1285 Ma. The monazite age from leucosomes of 1266 Ma provides the

youngest record of metamorphism. Thus peak metamorphism is believed to have

occurred between 1291 and 1267 Ma (figure 5.2). This coincides well with the

emplacement of granitic material dated between 1293-1288 Ma reported by Clark et

al. (1999), suggestive of crustal thickening. The mafic units cooled below the biotite

closure temperature around 1268 Ma (Fletcher et al. 1991), the similarity of this age

to the crystallisation age of the leucosomes is evidence of rapid cooling . The

preservation of this Rb-Sr ages suggests the region was not affected by any

subsequent metamorphic events. Authors such as Clark et al. (2000) and Bodorkos &

Clark (2004a, b) suggest the Albany-Fraser belt was subject to two episodes of

metamorphism. The Fraser Zone records Stage-I however there is little evidence of

Stage-II metamorphism. It is suggested that the Fraser Zone and particularly the

metasediments were not affected by the second event due to good insulation and

rigid properties reflecting its dominantly mafic composition. Also it must be taken

into account that the Fraser Zone had been subject to granulite facies conditions.

Fluid loss from the first event may have inhibited the development of any Stage-II

reactions. Due to the dry nature of the rocks stage-II reactions could not take place.

8.10 What does the Biranup Zone represent?

The origin of the Biranup zone was not within the scope of this study however it

plays a significant role in the formation of the Fraser zone so its origin should be

briefly discussed. The Biranup represent a large piece of exotic middle crust

(Spaggiari et al. 2009) and there is uncertainty regarding the origin of the Biranup

Zone due to the lack of tectonothermal activity in the adjacent blocks. The lack of

tectonothermal activity indicates it did not form in its present position but represents

an exotic terrane (Spaggiari et al. 2009). It has been suggested that there a link

between the Biranup Zone and the Gawler Craton or the southern Arunta orogen due

to similarities in granite ages (Clark et al. 1999, Spaggiari et al. 2009). Recent

evidence suggests the Biranup Zone was initially part of the Yilgarn Craton due to

the presence Yigran Craton material on the southern margin of the Biranup Zone

(Pers. Comm. C. Kirkland; GSWA). It is suggested that the Biranup Zone may have

formed on the boundary of the Yilgarn Craton at c. 1700 Ma (figure 8.2). The

protolith to the Biranup Zone was then rifted away from the Yilgarn Craton during a

period of extension. At some stage after its separation from the Yilgarn Craton the

small terrain boundary became the locus for subduction.

8.11 Recommended future investigations

The model proposed for the formation of the Fraser Zone could be significantly

strengthened or disproved through a geochemical investigation of the zone’s

geology. If the mafic units displayed a geochemical signature similar to the back arc

basin related mafic lithologies reported by Collins (2002a) it would be strong

eveidence supporting the back arc origin of the Fraser Zone. The Timing of

metamorphism could also be more refined with trace element studies of accessory

phases and mineral thermobarometry.

9. Conclusions

P-T analysis using THERMOCALC indicates that the quartz rich metapelitic rocks

from Gnamma Hill reached conditions greater than 900°C and pressures greater than

8 kbar. Similar rocks from Mount Malcolm record temperatures between 800-900 °C

and pressures greater than 6.5 kbar. Prograde assemblages suggest rapid heating with

increasing pressure. Post peak phases suggest rapid cooling and decompression

following a retrograde path that passed through similar fields to the prograde path.

This produced a tight pin head P-T loop. This suggests a transient heat source .

Monazite geochronology produced three ages. Sample FR-10-007 produced a

207Pb/235U age of 1285 ± 16 Ma. Sample FR-10-011 produced a 207Pb/235U age of

1266.1 ± 7.7 Ma and sample 182447 produced a 207Pb/235U age of 1273 ± 6.7Ma.

These ages help to define the timing of metamorphism within the Fraser Zone.

However there is a difference in equivalent zircons that record older ages. The

difference in age between the two accessory phases is attributed to differences in the

timing of mineral growth.

The similarity in the timing of metamorphism and the crystallisation age of mafic

lithologies within the Fraser Zone suggests the mafic units played a significant role

during metamorphism. The Fraser Zone represents a back arc basin system that has

been shortened as a result of the Albany-Fraser orogenic event. Extension possibly

due to slab retreat led to extension thinning the crust enabling the emplacement of

hot mantle material into the back arc basin. Once extension stopped and shortening

began the mafic material began to cool. Shortening was accommodated by thrust

faults. Supracrustal sediments were caught between thrusts and subject to granulite

facies conditions. The mafic units rapidly transferred heat into the sediments that

preserved a P-T path suggesting temperature increased with pressure. The mafic

rocks cooled rapidly and evidence for this is preserved by post peak assemblage

suggesting cooling with decompression. The back arc basin is believed to have

formed adjacent adjacent to the Biranup Zone through the retreat of a southeast

subducting slab.

9. Conclusions

P-T analysis using THERMOCALC indicates that the quartz rich metapelitic rocks

from Gnamma Hill reached conditions greater than 900°C and pressures greater than

8 kbar. Similar rocks from Mount Malcolm record temperatures between 800-900 °C

and pressures greater than 6.5 kbar. Prograde assemblages suggest rapid heating with

increasing pressure. Post peak phases suggest rapid cooling and decompression

following a retrograde path that passed through similar fields to the prograde path.

This produced a tight pin head P-T loop. This suggests a transient heat source .

Monazite geochronology produced three ages. Sample FR-10-007 produced a

207Pb/235U age of 1285 ± 16 Ma. Sample FR-10-011 produced a 207Pb/235U age of

1266.1 ± 7.7 Ma and sample 182447 produced a 207Pb/235U age of 1273 ± 6.7Ma.

These ages help to define the timing of metamorphism within the Fraser Zone.

However there is a difference in equivalent zircons that record older ages. The

difference in age between the two accessory phases is attributed to differences in the

timing of mineral growth.

The similarity in the timing of metamorphism and the crystallisation age of mafic

lithologies within the Fraser Zone suggests the mafic units played a significant role

during metamorphism. The Fraser Zone represents a back arc basin system that has

been shortened as a result of the Albany-Fraser orogenic event. Extension possibly

due to slab retreat led to extension thinning the crust enabling the emplacement of

hot mantle material into the back arc basin. Once extension stopped and shortening

began the mafic material began to cool. Shortening was accommodated by thrust

faults. Supracrustal sediments were caught between thrusts and subject to granulite

facies conditions. The mafic units rapidly transferred heat into the sediments that

preserved a P-T path suggesting temperature increased with pressure. The mafic

rocks cooled rapidly and evidence for this is preserved by post peak assemblage

suggesting cooling with decompression. The back arc basin is believed to have

formed adjacent adjacent to the Biranup Zone through the retreat of a southeast

subducting slab.

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Appendix 1 Sample tracking sheet

Sample ID Locality Easting * Northing * Sampler Type of sample rock type Petrology P-T SHRIMP Remnants 182447 Mt Malcolm 483447 6433869 GSWA thin section Metasediment x x NoFR-10-001 Gnama 471369 6439685 C. Oorschot Whole rock Micro Granite x YesFR-10-002 Gnama 471622 6440182 C. Oorschot Whole rock Mafic granulite x YesFR-10-003 Gnama 471678 6440366 C. Oorschot Whole rock Meta Pelite x YesFR-10-004 Gnama 471416 6441314 C. Oorschot Whole rock Micro Granite x YesFR-10-005 Gnama 471579 6439508 C. Oorschot Whole rock Meta Pelite x x YesFR-10-006 Gnama 471600 6439386 C. Oorschot Whole rock Mafic granulite x YesFR-10-007 Gnama 471515 6439369 C. Oorschot Whole rock meta pelite x x YesFR-10-008 Gnama 471457 6439520 C. Oorschot Whole rock quartzite x YesFR-10-009 Gnama 471528 6439409 C. Oorschot Whole rock Meta Pelite x x YesFR-10-010 Gnama 471561 6439396 C. Oorschot Whole rock mafic granulite x YesFR-10-011 Gnama 471529 6439392 C. Oorschot Whole rock Sheared Leucosomes x x YesFR-10-012 Gnama 471529 6439392 C. Oorschot Whole rock shear zone x NoFR-10-013 Gnama 471140 6440890 C. Oorschot Whole rock Pavement' formation x YesFR-10-014 Gnama 471518 6440626 C. Oorschot Whole rock sheared granite x YesFR-10-015 Gnama 471508 6439953 C. Oorschot Whole rock pink micro granite x YesFR-10-016 Mt Malcolm 483782 6434511 C. Oorschot Whole rock Mafic granulite x YesFR-10-017 Mt Malcolm 483456 6433874 C. Oorschot Whole rock Meta Pelite x x YesFR-10-018 Mt Malcolm 483220 6434905 C. Oorschot Whole rock meta-igneous volcanic x YesFR-10-019 Mt Malcolm 483149 6433995 C. Oorschot Whole rock Meta Pelite x x Yes

* Using MGA datum: WGS 84

All remaining specimens used in this investigation and cited in this dissertation are stored at the Department of Applied Geology, Curtin University. The locations of hand samples specified in the above table are arcurate to the nearest 5 meter.

Appendix 2

Oxide FR-10-005 FR-10-009 FR-10-017 FR-10-019 Element FR-10-005 FR-10-009 FR-10-017 FR-10-019SiO2 63.18 70.71 57.95 63.02 Rb 105.1 57.1 73.2 101.5TiO2 0.72 0.81 0.97 0.77 Sr 121 243 196 99

Al2O3 16.11 13.53 18.55 15.95 Y 9.8 21.4 25.1 18.1Fe2O3 6.81 4.74 7.64 7.03 Zr 189 163 137 122

FeO 4.24 3.21 4.03 5.05 V 140 124 152 112MnO 0.41 0.20 0.36 0.36 Ni 46 49 50 30MgO 3.04 2.37 3.46 2.53 Cr 110 98 85 105CaO 0.24 1.24 1.02 0.17 Nb 3.9 5.5 4.7 6.7

Na2O 0.73 1.03 1.77 1.11 Ga 25.5 17.3 25.2 24.6K2O 3.79 2.13 3.42 3.47 Cu 11 8 12 14P2O5 0.04 0.05 0.09 0.06 Zn 149 107 130 100LOI 0.68 0.58 0.68 0.78 Co 56 48 51 59Total 99.99 100.60 99.94 100.30 Ba 808 1087 899 810

Fe2O3T 11.52 8.31 12.12 12.64 La 27 31 36 33Ce 39 59 74 48U 1.3 1.3 0.9 0.6Th 5 7.7 7.6 11.7Sc 9 10 18 11Pb 39 19 35 32

Tables 1 . Major oxide composition (weight %). Table 2 . Minor elements (ppm)

Age (Ma)Spot Name Figure U (ppm) Th (ppm) Th/U 207Pb/235U ±% 206Pb/238U ±% 238U/206Pb ±% 206Pb/206Pb ±% 207Pb/235U ± Discord %

1.1 a 195 12749 65 2.48 2.3 0.2188 1.9 4.57 1.9 0.0823 1.4 1267 ±17 -21.2 217 15545 71 2.47 2.5 0.2190 1.9 4.57 1.9 0.0818 1.6 1263 ±18 -31.3 141 15478 110 2.46 2.9 0.2188 2.0 4.57 2.0 0.0817 2.1 1261 ±21 -31.4 142 14600 103 2.54 2.6 0.2162 2.0 4.63 2.0 0.0853 1.6 1285 ±19 +51.5 90 13584 152 2.48 3.6 0.2224 2.1 4.50 2.1 0.0807 2.9 1265 ±26 -71.6 164 13055 79 2.44 3.0 0.2159 1.9 4.63 1.9 0.0819 2.3 1254 ±22 -21.7 144 12804 89 2.41 2.9 0.2130 1.9 4.69 1.9 0.0821 2.1 1246 ±21 +01.8 317 12772 40 2.45 2.0 0.2153 1.8 4.65 1.8 0.0827 0.8 1259 ±14 +04.4 d 280 12589 45 2.55 2.1 0.2218 1.8 4.51 1.8 0.0833 1.1 1286 ±16 -14.5 65 13625 210 2.18 5.2 0.2030 2.2 4.93 2.2 0.0778 4.7 1174 ±36 -54.6 66 12877 196 2.29 4.8 0.2104 2.2 4.75 2.2 0.0791 4.3 1210 ±34 -54.7 141 13373 95 2.45 2.5 0.2149 1.9 4.65 1.9 0.0826 1.7 1256 ±18 +05.2 f 45 11531 257 2.76 5.0 0.2259 2.4 4.43 2.4 0.0888 4.4 1346 ±37 +75.3 95 11367 119 2.38 4.9 0.2236 2.3 4.47 2.3 0.0771 4.3 1236 ±35 -175.4 91 11293 124 2.59 4.0 0.2244 2.3 4.46 2.3 0.0836 3.3 1297 ±30 -25.5 66 11638 176 2.11 6.8 0.2148 2.4 4.66 2.4 0.0711 6.3 1151 ±47 -345.6 75 12007 160 2.52 4.4 0.2195 2.1 4.56 2.1 0.0832 3.9 1277 ±32 -15.7 85 9526 112 2.44 4.1 0.2253 3.1 4.44 3.1 0.0786 2.6 1255 ±30 -145.8 93 11479 124 2.48 4.4 0.2234 2.3 4.48 2.3 0.0806 3.7 1267 ±32 -86.1 e 131 11977 92 2.42 2.9 0.2151 2.0 4.65 2.0 0.0816 2.2 1248 ±21 -27.2 c 299 15384 51 2.65 2.2 0.2297 1.9 4.35 1.9 0.0836 1.2 1314 ±16 -47.3 158 15787 100 2.54 2.9 0.2255 2.3 4.43 2.3 0.0815 1.8 1282 ±21 -77.4 150 15000 100 2.50 2.8 0.2229 2.2 4.49 2.2 0.0814 1.8 1273 ±21 -67.5 136 15104 111 2.60 3.2 0.2257 2.0 4.43 2.0 0.0836 2.5 1301 ±24 -27.6 148 14757 100 2.80 5.7 0.2423 5.4 4.13 5.4 0.0838 1.9 1356 ±43 -97.7 298 15109 51 2.60 2.7 0.2274 2.2 4.40 2.2 0.0829 1.6 1301 ±20 -57.8 278 15797 57 2.46 2.9 0.2226 1.9 4.49 1.9 0.0802 2.2 1261 ±21 -99.1 h 160 16394 103 2.41 2.6 0.2108 2.0 4.74 2.0 0.0831 1.8 1247 ±19 +39.2 149 13339 89 2.36 2.8 0.2133 1.9 4.69 1.9 0.0803 2.0 1231 ±20 -49.3 147 14754 100 2.36 2.6 0.2097 2.0 4.77 2.0 0.0816 1.7 1230 ±19 +19.4 125 13001 104 2.41 3.6 0.2039 2.1 4.90 2.1 0.0857 3.0 1245 ±26 +119.5 168 16974 101 2.29 2.7 0.2054 2.0 4.87 2.0 0.0810 1.8 1211 ±19 +29.6 168 15215 90 2.36 3.2 0.2104 2.6 4.75 2.6 0.0813 1.7 1230 ±22 -0

Radiogenic ratios

Appendix 3 Table 1. Summary of SHRIMP U-PB monazite results for sample

Common Pb corrected using measured 204Pb.

Age (Ma)Spot Name Figure U (ppm) Th (ppm) Th/U 207Pb/235U ±% 206Pb/238U ±% 238U/206Pb ±% 206Pb/206Pb ±% 207Pb/235U ± Discord %

1.1 a 102 26833 263 2.57 3.9 .2265 2.9 4.42 2.9 0.0824 2.6 1293 ±29 -51.2 86 25778 300 2.56 3.6 .2255 2.7 4.44 2.7 0.0822 2.3 1293 ±26 -51.3 86 25299 295 2.62 3.6 .2292 2.8 4.36 2.8 0.0829 2.3 1306 ±26 -61.4 88 31187 356 2.59 3.5 .2293 2.7 4.36 2.7 0.0819 2.2 1298 ±26 -81.5 80 29595 372 2.50 3.8 .2215 2.9 4.51 2.9 0.0819 2.5 1272 ±28 -41.6 87 30675 354 2.53 3.5 .2225 2.7 4.49 2.7 0.0826 2.3 1282 ±26 -31.7 104 36422 349 2.49 3.8 .2175 2.9 4.60 2.9 0.0832 2.5 1271 ±28 +02.1 b 736 17013 23 2.63 2.2 .2270 1.9 4.41 1.9 0.0842 1.0 1310 ±16 -23.1 c 421 12696 30 2.68 2.4 .2308 2.0 4.33 2.0 0.0842 1.4 1323 ±18 -43.2 181 5693 31 2.46 3.1 .2162 2.2 4.63 2.2 0.0826 2.2 1261 ±22 -04.1 d 99 18815 189 2.47 3.3 .2145 2.6 4.66 2.6 0.0834 2.0 1262 ±24 +25.1 e 166 6780 41 2.43 2.9 .2139 2.2 4.68 2.2 0.0823 1.9 1250 ±21 +05.2 146 5377 37 2.45 2.6 .2083 2.2 4.80 2.2 0.0852 1.3 1257 ±19 +8

Age (Ma)Spot Name Figure U (ppm) Th (ppm) Th/U 207Pb/235U ±% 206Pb/238U ±% 238U/206Pb ±% 206Pb/206Pb ±% 207Pb/235U ± Discord %

1.1 a 567 15347 27 2.52 2.4 .2233 2.0 4.48 2.0 0.0819 1.4 1279 ±18 -53.1 c 675 17175 25 2.48 2.5 .2190 2.0 4.57 2.0 0.0820 1.5 1265 ±18 -33.2 611 16698 27 2.54 2.4 .2257 2.0 4.43 2.0 0.0816 1.5 1284 ±18 -73.3 615 14724 24 2.53 2.4 .2233 2.0 4.48 2.0 0.0823 1.3 1282 ±17 -46.1 d 504 6846 14 2.38 2.4 .2113 2.0 4.73 2.0 0.0816 1.4 1236 ±17 +06.2 476 12448 26 2.47 2.4 .2178 2.0 4.59 2.0 0.0822 1.5 1263 ±18 -26.3 195 17785 91 2.51 2.9 .2210 2.2 4.53 2.2 0.0824 1.8 1275 ±21 -38.1 e 536 8457 16 2.47 2.5 .2214 1.9 4.52 1.9 0.0810 1.7 1264 ±18 -6

Table 2. Summary of SHRIMP U-PB monazite results for sample FR-10-007

Radiogenic ratios

Radiogenic ratios

Table 3. Summary of SHRIMP U-PB monazite results for sample 182447

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This Record is published in digital format (PDF) and is available as a free download from the DMP website at <http://www.dmp.wa.gov.au/GSWApublications>.

Information CentreDepartment of Mines and Petroleum100 Plain StreetEAST PERTH WESTERN AUSTRALIA 6004Phone: (08) 9222 3459 Fax: (08) 9222 3444<http://www.dmp.wa.gov.au/GSWApublications>

Further details of geological products produced by theGeological Survey of Western Australia can be obtained by contacting:

67 58

61

54

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75

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471000E 472000E

6440

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6440

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Meters

1:5 000250 5000N

orth

Symbols

Geological boundaryInferred geological boundaryStrike and dip of foliationStrike and dip of leucosomePlunge and plunge direction of lineationUnsealed roadSample location; number represents last two digits of sample number ie: 1 refers to FR-10-001

Map Grid of Australia Datum: WGS 84

61

60

60

610

9

12

7

5

8

1

4

13

14

3

2

15

1

lithologies

Alluvium and colluvium - clay to boulder deposits; derived by sheet wash

Pavement formation - layers mafic and felsic gneiss

Mafic Granulite

Granulite Facies quartz rich metapelite

Quartzite - metasandstone

Granitic intrusions

Prot

eroz

oic

C

aino

zoic

Outcrop Map 1: Geology of Gnamma Hill, Western AustraliaCreated November 2010 - Christopher W. Oorschot - Curtin University.

75

80

82

7579

8069

69

57

32

79

6434

000

6435

000

483000 484000

Outcrop Map 2: Geology of Mount Malcolm, Western AustraliaCreated November 2010 - Christopher W. Oorschot - Curtin University.

53

47

17

17

18

16

Meters

1:5 000250 5000N

orth

Symbols

Geological boundaryInferred geological boundaryStrike and dip of foliationStrike and dip of leucosomePlunge and plunge direction of lineationUnsealed roadSample location; number represents last two digits of sample number ie: 1 refers to FR-10-001

Map Grid of Australia Datum: WGS 84

61

60

60

1

lithologies

Alluvium and colluvium - clay to boulder deposits; derived by sheet wash

Mafic Granulite

Granulite Facies quartz rich metapelite

Quartzite - metasandstone

Felsic Gneiss - metavolcanics

Granitic intrusions

Prot

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C

aino

zoic