FRONTISPIECE imagery of the Tweed Shield Volcano …
Transcript of FRONTISPIECE imagery of the Tweed Shield Volcano …
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