FRONTISPIECE

ERTS-A imagery of the Tweed Shield Volcano showing the

radial drainage pattern, cir~ular erosion caldera and

central intrusive complex.

'It if., pOM.-Lble. tha.t:the. .6toJttj M not an .-Lnve.YLtion'.

- Plato

THE MINERALOGY AND PETROLOGY OF THE SOUTHERN PORTION OF

THE TWEED SHIELD VOLCANO, NCRTHEASTERN NEW SOUTH WALES

by

Morris B. Duggan

Thesis submitted for the degree ofDoctor of Philosophy, Universityof New England, Armidale, N.S.W.

December, 1974.

I cel'ti.~y tha~ this thesis has not beel1 pl'Bvioua ly

submitted in 2ubstan~ fel' arty degree Ql1d that it is not being

cul'reJ'ltZy sub'1titted for art;, ethel' cWgree.

I fUl'thel' C€l':ij"y that any aBsistance in the p1'6parattcn

of this thesis~ ~d al~ S~Ul'ee8 of ir.fol'matian~ have been fOrMally

ackl10lJ ledged.

M. B. Ou.ggan.

i.

PREFACE

In 1968 Professor J.F.G. Wilkinson drew attention to the occurrence

of tholeiitic rocks within the Tweed Shield Volcano and suggested that

more than one lineage may be present. It l;yas suggested to the author by

Professor Wilkinson that a detailed mineralogical and chemical study of

part of the Volcano would be of considerable interest.

An investigation was commenced in February, 1969 and this thesis

presents the results of this project. With the exception of 7 major

element analyses the author is responsible for all analytical results

and conclusions. All technical assistance received during the study has

been specifically cited in the Acknowledgements and Appendices. The

results of all other workers are clearly al:knowledged in the text.

A paper written in collaboration with Professor J.F.G. Wilkinson

entitled "Tholeiitic andesite of high-pressure origin from the Tweed

Shield Volcano, northeastern New South Wales" has been published in

Contributions to Mineralogy and Petrology, Volume 39, pages 267-276, 1973.

~ J ..

AB:3TRACT

In the southern portion of the Tweed Shield Volcano, north­

eastern New South Wales, a sequence of alkaline rocks (the Kyogle Basalt)

thins eastward where it is overlain by a subalkaline succession composed

predominantly of tholeiitic andesite and rhyolite, with minor icelandite

and rhyodacite (in stratigraphic sequence, the Lismore Basalt, Nirnbin

Rhyolite and Blue Knob Basalt).

On the basis of detailed mineralogical and chemical data, two

distinct: tholeiitic series have been recognised, respectively designated

the low-Si series and the high-Si series. Members of the low-Si series

(almost exclusively tholeiitic andesites) contain gro'.1ndmass olivines but

lack groundmass Ca-poor pyroxenes. In members of the high-Si series

(tholeiitic andesite ~ rhyodacite), groundmass olivines are absent but

Ca-poor pyroxenes occur as phenocryst and groundmass phases and commonly

in reaction coronas around olivine phenocrysts. These differences reflect

differing levels of silica saturation in tile two series,

Rhyolites, including vitric and microcrystalline variants (the

latter often resulting from devitrific.ation), commonly contain phenocrysts

of quartz, sanidine and minor oligoclase. Microphenocrysts of ferrohypersthene

and ilmeni te may also occur in these rocks.

Some rocks contain relatively high pressure pyroxenes and

plagiocl,ases, In the low-Si series the pyroxenes (bronzite and subcalcic.

augite), which are enriched in AlVI r.:!lative to groundmass pyroxenes,

crystallized at moderately high press.lres (about 9 kb). In the high-Si

j jj .

series t early-formed ferroaugites ar~ not enriched in Al and these probably

crystallized at substantially lower )ressures (about 5 kb). The moderately

high pressure plagioclasesof both volcanic series are enriched in Na

relative to groundmass plagioclases. They typically consist of a relatively

sadie core surrounded by a zone of sieving and resorption which is in turn

mantled by a thin rim of glassy, mOrE~ calc.ic plagioclase.

Low silica activities inhib:~ted the crystallization of Ca-poor

pyroxenE~s in members of the low-5i sHries. The groundmass augi tes of this

series exhibit increasing Ca contents with increasing Fe/Mg ratios of the

pyroxenes and of their respective ho~.t rocks. This trend is interpreted

as a response to a decrease in the activity of Fe Z+ caused by a progressive

increase in fa through the series. Phenocrysts of ferroaugite commonlyZ

preceded more magnesian microphenocryst and groundmass pyroxenes in rocks

of the high-5i series. This is attributed to a combined effect of Fe

depletion in the host liquids through precipitation of ilmenite phenocrysts

and increasing fa in the liquids during ascent to near-surface c.onditions.2

In the low-5i series t with increase in the degree of evolution of

the host rocks there is a progressive change in the nature of the Fe-Ti

oxide assemblage from ilmenite alone through ilmenite plus magnetite to

magnetite alone. An RZ0 3-poor ilmenite is the dominant Fe-Ti oxide throughout

the high-5i series and in the rhyolites. For mas t of the tholeii ti.c rocks,

relatively low oxygen fugacities t estimated to be intermediate between the

fayalite-magnetite-quartz and the wuscite-magnetite buffer assemblages, are

inferred from the Fe-Ti oxide data.

iv.

The Fe and Or contents of plagioclases are discussed in relation

to their environment of crystallization. Entry of Fe into plagioclase is

favoured by rapid rates of crystallization" The Or contents of plagioclases

are a function of host rock chemistrias insofar as these control the

location of the solidus-solvus inters·action in the system Ab-An-Or.

The absence of primary hydrous phases, the abundance and &odic

character of plagioclase phenocrysts and the predominance of ilmenite over

magnetitla in most of the rocks collectively indicate that the tholeiitic

magmas were in general relatively wattar-deficient during intratelluric

crystallization and subsequent eruptitJn.

As a group the tholeiitic ro.;ks comprise a relatively alkali rich

suite (especially with respect to K20) exhibiting only moderate iron

enrichmen.t. The relatively high K contents of the mafic rocks possib 1y

resul t from the breakdown of small amounts of phlogopi te during partial

melting in an inhomogeneous source region. With increasing degree of

evolution there is a significantly st::onger trend toward normative quartz

enrichment in the high-Si series than in the low-Si series.

It is proposed that the low-Si series resulted from fractl0natlon

of a mafic tholeiitic parent at moderately high pressures (-9 kb) near

the crust mantle boundary involving the removal of plagioclase and aluminj an

clinopyroxene together with, or foll~Ted by olivine. Fractionation of

aluminian augite at elevated pressures prevented clinopyroxene from be::.omlng

a near liquidus phase at low pressures, thereby inhibiting its appearance

as a low pressure phenocryst phase in most of the evolved members of the

series.

v.

The high-Si series probably developed through fractionation at

intermediate depths in the crust (-5 kb) where the fractionating phases

were oli'ITine, ferroaugite, ilmenite and plagioclase. The two tholeiitic

series may conceivably be related via a common parent magma, the contrasting

chemical trends possibly reflecting fractionation under significantly

different pressure regimes.

The origin of the rhyoliticnagrna is critically assessed. The

available field and chemical data are more consistent with an origin

involving extreme fractionation than 1"ith a crustal anatexis model. It

is like~{ that the acid magma evolved through extreme fractionation of

a more mafic parent at moderate crustal depths under relatively anhydrous

condi tions •

The origin of the Tweed Shield Volcano is briefly discussed in

terms of a tectonic model involving plate movement over an upper mantle

melting source.

•

vi.

ACKNOIoJLEDGEMENTS

The author is deeply indeb ted to Professor J.F.G. Wilkinson for

the interested supervision, perceptivE~ advice and pentrative criticism

he provided throughout the course of the research and especially during

preparation of this thesis. Many other members of the University of New

England Geology Department are thanked for their helpful comments. In

particular the author benefi ted from several discussions with Dr. N.C.N.

Stephenson. Mr. G,I.Z. Kalocsai is thanked for his generous assistance

and guidance in all aspects of the major element analytical work.

All members of the technical staff of the University of New England

Geology Department provided assistance in technical aspects of the research.

Special thanks are due to Mr. J.S. Cock for his patience, care and skill

in preparation of polished thin sections used for electron microprobe

studies.

Professor A.E. Ringwood generously prOVided access to electron

microprobe facilities in the Research School of Earth Sciences, Australian

National University. Invaluable instruction in the use of the microprobe

by Mr. N.G. Ware and Dr. S.J.B. Reed is gratefully acknowledged.

For access to X-ray fluorescence facilities in the Department of

Geology and Mineralogy, University of ~ueensland, the author thanks Professor

A.F. Wilson and Dr. A. Ewart. Mr. A.S. Bagley provided advice and assistance

in the running of the equipment. Dr. A. Ewart is also thanked for valuab le

discussion of data on the Tweed Shield Volcano.

Mrs. R. Cuskelly patiently typed the final copy of the thesis.

Finally, many thanks to my Wife, Fiona, for undying encouragement

and support and also for typing early drafts of the thesis.

vii.

TABLE OF CONTENTS

PREFACE i

ABSTRACT ii

ACKNOWLEDGEMENTS vi

TABLE OF CONTENTS vii

CHAPTER 1: INTRODUCTION 1

CHAPTER 2: PRE-CAINOZOIC STRATIGRAPHY 8

CHAPTER 3: VOLCANIC STRATIGRAPHY 12

CHAPTER 4: PETROGRAPHY OF THE THOLEIITIC ROCKS 24

4.1 Nomenclature 24

4.2 Petrography 30

4.2.1 Low-Si tholeiitic andesites 304.2.2 High-Si tholeiitic andesites 364.2.3 Icelandites 404.2.4 Tholeiitic rhyodacites 424.2.5 Rhyolites and rhyolitic pitchstones 444.2.6 Pyroclastic rocks 48

CHAPTER 5: MINERALOGY 52

Iron-Titanium Oxides

Olivines

Pyroxenes

Feldspars

5.4.1 Feldspars of the low-Si series5.4.2 Feldspars of the high-Si series5.4.3 Feldspars of the rhyolites5.4.4 Discussion

Accessory Minerals

Residual Glass

52

57

5774

96

109

109114118124

136

138

Pyroxenes of the low-Si seriesPyroxenes of the high-Si series

5.2.15.2.2

5.3

5.4

5.5

5.6

5.1

5.2

TABLE or CONTENTS

CHAPTER 6: CHEMISTRY

6.1 Major Elemen t Data

6.2 Major Element Variatj.on

6.3 Trace Element Data

CHAPTER 7: DISCUSSION

viii.

143

144

158

166

181

Generation of Parent Magmas and the Potassiumcontent of tholeiitic rocks

Evolution of the Low--Si Series

Evolution of the High-Si Series

Relationships between the Low-Si Series and theHigh-Si Series

Genesis of the RhyoH tic Rocks

Tectonic Setting of the Tweed Shield

7.1

7.2

7.3

7.4

7.5

7.6

7.2.17.2.27.2.3

Summary of pertinent dataLow pressure fractionationHigh pressure fractionation

181

188

188189196

209

219

221

239

CHAPTER 8: SUMMARY AND PRINCIPAL CONCLUSIONS

BIBLIOGRAPHY

APPENDIX I FIELD AND LABORATORY TECHNIQUES

APPENDIX II MICROPROBE ANALYSES OF OLIVINES, PYROXENES ANDPLAGIOCLASES

APPENDIX III PETROGRAPHIC AND CHEMICP~ DATA ON THE ALKALINE ROCKS

APPENDIX IV SPECIMEN NUMBERS, ROCK TYPES AND GRID REFERENCES

243

249

269

278

292

297