PETROLOGY, GEOCHEMISTRY AND TECTONIC IMPLICATIONS
OF MAGMATISM ALONG THE
NORTHERN HUNTER RIDGE AND KADAVU ISLAND GROUP, FIJI
by
Alicia Verbeeten BSc. (Hans) LaTrobe University
Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy (Geology)
University of Tasmania, Hobart December, 1996
, .
STATEMENT
This thesis contains the result of research done in the Geology Department, University of Tasmania, between 1993 and 1996. This thesis contains no material which has been accepted for the award of any other degree or diploma in any tertiary institution and, to the best of the author's knowledge and belief, this thesis contains no material previously published or written by another person, except where due reference is made in the text of the thesis.
(i) I agree/ dG-fl<>t..agree that the thesis may be made available for loan (ii) I agree/do-nGt-agFee that the thesis may be made available for photocopying
1
c..ii./£fkP-~''-Alicia Verbeeten
University of Tasmania December, 1996
ABSTRACT
The submarine Hunter Ridge separates the inactive South Fiji
Basin from the actively spreading North Fiji Basin, and is a newly
recognised intra-oceanic arc in the SW Pacific. Magmatic activity along
the Hunter Ridge is linked to the northward subduction of the crust of
the Oligocene (26.0-32.5Ma) South Fiji back-arc basin. Subduction began in
response to establishment of an E-W orientated spreading ridge at -7Ma
across the North Fiji Basin, and the accompanying anticlockwise rotation
of the Fiji Platform.
Rocks suites dredged along the northern part of the Hunter Ridge
include basalts to dacites transitional between high-Ca boninites and
typical arc tholeiites, and calc-alkaline basalts to rhyolites. Mineral
compositions (e.g. Cr-spinel with Cr#>70), coupled with whole rock high
CaO/ Ah03 (0.91-1.13) and low abundances of HFSE (0.37-0.54% Ti02; 0.27-
0.S6ppm Nb; 0.02-0.05ppm Ta), Y (1l-15ppm) and HREE of the basalts
relative to N-MORB show that both the arc tholeiite and calc-alkaline
basalts are derived from sources more refractory than the N-MORB
source, probably peridotite residual after production of North Fiji Basin
oceanic crust. The enrichment in LILE and LREE of the Hunter Ridge
rocks reflects addition to this refractory source of a slab-derived fluid for
the arc tholeiitic suite and a slab melt component for the calc-alkaline
suite.
The Kadavu Island Group, in southwesternmost part of the Fiji
archipelago, is the northeastern exposed end of the Hunter Ridge and can
be divided into four geochemically distinct magmatic groups.
The Astrolabe Group shoshonites (-3.4Ma) have major and trace
element and isotopic compositions very similar to other shoshonites in
Fiji. The low HFSE abundances (-0.64%Ti02, O.llppm Ta, 2.0ppm Nb),
high CaO/ Al203 values (0.SS-1.1) and high Cr# of Cr-spinel (CrLS5) of
the mafic lavas (absarokites) of the shoshonite suite indicate a particularly
refractory peridotite source for the Astrolabe lavas. The strong LILE and
LREE enrichments of the Astrolabe shoshonites are extreme variants of
the same enrichment shown by primitive arc tholeiites on the Hunter
Ridge and in the Vanuatu arc, and are attributed to relatively low degrees
of partial melting of a mantle source similarly affected by ingress of slab
derived hydrous fluids. The mantle metasomatism responsible for
producing the source peridotite of the Astrolabe suite magmas may have
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,,(curred during Oligocene to Miocene subduction associated with the
Viti.l!' em: system. Emplacement of the Fijian shoshonites, including the
:\~lrLlbbe Croup, is considered to be related to lithospheric extension in
the early-mid Tertiary Fijian arc, in response to reorganisation of
spreading systems in the North Fiji Basin and initiation of spreading in
the adjacent Lau Basin.
Pleistocene to Recent (2.9-0.4SMa) volcanism on Kadavu is
represented by the Western Kadavu and Central/Eastern/Ono Croups,
.mel records the effects of initiation of subduction of the South Fiji Basin
Cl'ust beneath Fijian arc lithosphere . The dominant rock types are
medium to high-K adakitic andesites and dacites. Low abundances of Y
(15.6-19.6ppm), high Sr contents (553-1667ppm), high Sr/Y (79.2-SS.9), and
strongly fractionated REE patterns (La/YbN =12-25), are consistent with an
origin involving partial melting of subducted basaltic oceanic crust
consisting mainly of garnet and clinopyroxene (eclogite). Furthermore, Sr,
Nd and Pb isotopic analyses plot within the range of Pacific MORB,
consistent with derivation of these magmas by partial melting of
subducted MORB, with no pelagic sediment involvement.
Lavas of the Ngaloa Croup form a volumetrically small part of the
Kadavu Island Croup and consist of unusual basalts and basaltic andesites
with high Na20 (2.7-5.5%), Ti02 (1.4-1.7%), and Sr (2055-2957ppm) and
low FeO' (5.S-6.S%) and relatively high Nb contents (S-16ppm) for supra
subduction basalts. They are temporally and spatially associated with the
Western Kadavu adakitic andesites and dacites, but major and trace
element considerations rule out any direct genetic link between them
through differentiation. However, Ngaloa Croup lavas also have specific
geochemical features (Sr/Y=93-205, La/YbN=19-29 and MORB-like isotopic
compositions) similar to the Western Kadavu and Central/Eastern/Ono
Croup adakites, indicating involvement in their petrogenesis of a
component formed by partial melting of the subducted oceanic crust.
Their high MgO (5.1-7.9%), and Ni (up to 200ppm) contents and primitive
phenocryst compositions (e.g. F089-91) however, preclude their derivation
solely from the partial melting of the subducted crust. Trace element
considerations suggest that the Ngaloa Croup basalts were prod uced by
partial melting of mantle wedge peridotite that had interacted with and
been strongly modified by slab melts probably similar to the Western
Kadavu adakitic lavas. Lavas compositionally similar to the Ngaloa
Croup volcanics in some other arcs also occurring in close association
with adakitic andesites, have been termed Nb-enriched arc basalts (NEAB)
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. .
and are also thought to have formed by melting of a slab-melt
metasomatised mantle wedge.
Partial melting of subducted oceanic crust in eclogite facies (-50-
80km depth) is thus implicated in the petrogenesis of the Ngaloa Group
and the Central/Eastern/Ono and Western Kadavu adakitic lavas. As the
subducted slab is too cool to partially melt at the amphibolite-eclogite
transition beneath normal forearc regions of oceanic arcs, a mechanism to
elevate the isotherms beneath the northern end of the Hunter Ridge is
required. The subducting South Fiji Basin oceanic crust is too old (26.0-
32.5Ma) and cold to provide the necessary heat. An alternative heat
source is ascending asthenospheric mantle beneath the northwestern end
of the Lau backarc basin. This convecting MORB-source mantle is
hypothesised to move via the mantle window at the end of the
subducting plate to abut subducted South Fiji Basin oceanic crust.
Conductive heat transfer from this hot asthenospheric mantle enabled
partial melting of the subducted South Fiji Basin slab, and partial melting
of the same asthenosphere mantle is considered to be responsible for
generation of the unusual OlB-type basalts that occur directly above the
slab window between Viti Levu and Vanua Levu, Fiji.
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ACKNOWLEDGMENTS
Many people have provided advice, assistance, support and friendship over the last three and a half years, and although the following list attempts to cite and thank all these, I also extend a general acknowledgment to any others I may have overlooked.
- My supervisor, Dr Tony Crawford provided constant support and encouragement. He contributed immeasurably to the development of the ideas and the assessment of the data presented in this thesis.
- Dr. Leonid Danyushevsky, Dr. Vadim (Dima) Kamenetsky and Professor Rick Varne offered invaluable advice and assistance.
- Professor David Green for access to the laboratory facilities (clean lab) and the ICP-MS at the Research School of Earth Sciences (RSES) at the Australian National University, and Dr. Richard Price for access to laboratory facilities at LaTrobe University.
- Dr. Steve Eggins (RSES) for his time analysing my samples (ICP-MS) and for his enthusiasm and advice.
- Dr. Roland Mass (Latrobe University), supervised the radiogenic isotope work and spent long days running the Finnigan MAT 262 mass spectrometer with me. A further thanks to Roland for providing a detailed description of the analytical procedure outlined in Appendix 2.
- I am grateful to the Mineral Resources Department, Fiji and the District Chiefs of the Kadavu Islands for allowing me to do fieldwork and to the people of Kadavu (especially Pauliasi Gudru from Waisomo Village, Tavuki) whose friendly help and hospitality made working in their country extremely enjoyable.
- Crew and Captain of the R.V Alis are thanked for an enjoyable and productive cruise along the northern part of the Hunter Ridge.
- Numerous post doctoral fellows and fellow Ph.D students at the University of Tasmania (Geology /CODES department) provided friendship and support throughout my Ph.D years. In particular the people I have shared an office with over the last three and a half years: Ingvar Sigurdsson, Fernando Della Pasqua, Massimo Gasparon, Ruth Lanyon, Robina Sharpe, Rohan Wolfe (thanks Rohan for helping with many computer problems and for drafting the last diagram) and Cathryn Gifkins. Many thanks also to Karen Orth, Andrew McNeill, Ali Raos, Mark Doyle, Paul Kitto, Anthea Hill, Bill Wyman, Briony Sinclair, and many others. I would like to make a special thanks to Fernando and Robina who have been enormous support throughout my Ph.D.
- Invaluable technical assistance was provided by Peter Cornish, Marylin Feast, Jeanette Harris, June Pongratz, Christine Higgins, Simon Stevens, Phil Robinson (XRF), Wislaw
. Jablonski (electron microprobe), Nilar Hliang and Katie McGoldrick
- Dr. Peter Fleming (Bald Eagle) and Dr. Peter Jackson (Jacko) from LaTrobe University for their continuing support and friendship.
- A special thanks to my extremely supportive housemates Megan Humrich, Ross Edwards and Paul Scott.
Finally, I dedicate this thesis to my family. To my parents Harry and Denise for their ongoing support throughout my long years as a student and their emotional support in the final months and to Danny, Sonja, Peter, Andrew, Simone and Darren just for being there.
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TABLE OF CONTENTS
CHAPTER 1: INTRODUCTION
1.1 Setting of the Hunter Ridge and the Kadavu Island Group 1.2 Previous work on the Kadavu Island Group 1.3 Fieldwork 1.4 Aims and presentation of this thesis 1.5 Acronyms and abbreviations
CHAPTER 2: TECTONIC SETTING AND GEOCHEMICAL EVOLUTION OF FIJI
2.1 Introduction 2.2 Regional setting of Fiji 2.3 Plate tectonic history of Fiji 2.4 Geochemical evolution of Fiji
2.4.1 Introduction 2.4.2 Early arc stage (>lOMa) 2.4.3 Mature arc stage (-1O-5Ma) 2.4.4 Early rifting stage (-5.5-3.0Ma) 2.4.5 Late rifting stage (3Ma to present)
2.5 Summary
CHAPTER 3: THE HUNTER RIDGE: A newly recognised intra-oceanic arc in the SW Pacific.
3.1 Introduction 3.1.1 Tectonic setting and previous studies
3.2 Petrography and mineral chemistry 3.2.1 Petrography 3.2.2 Phenocryst mineral chemistry
3.3 Magma crystallisation conditions 3.3.1 Temperature 3.3.2 Oxygen fugacity
3.4 Major element geochemistry 3.4.1 Major elements 3.4.2 Primary magmas
3.5 Trace element geochemistry 3.6 Radiogenic isotope chemistry 3.7 Petrogenesis
3.7.1 Introduction 3.7.2 Depleted component 3.7.3 Enriched component
3.8 Conclusion
VI
1
1 3 4 4 6
7
7 7 8 10 10 10 13 14 18 18
20
20 20 25 25 33 41 41 43 44 44 49 50 53 55 55 55 58 63
CHAPTER 4: ASTROLABE GROUP: The Shoshonitic Lavas 64
4.1 Introduction 4.2 Geology 4.3 Petrography and mineral chemistry 4.4 Geochemistry of the astrolabe lavas
4.41 Major and trace element geochemistry 4.4.2 Radiogenic isotope chemistry
4.5 Discussion 4.5.1 Introduction 4.5.2 Nature of the mantle wedge 4.5.3 Incompatible element enrichment and the nature of the subduction component
4.6 Relationship to tectonics
CHAPTER 5: NGALOA GROUP VOLCANICS: An example of interaction between siliceous melts from the subducted slab
64 65 67 75 75 79 82 82 82
86 89
and mantle wedge peridotite? 91
5.1 Introduction 91 5.2 Setting of the Ngaloa Group volcanics 92 5.3 Petrography and mineral chemistry 94
5.3.1 Olivine 94 5.3.2 Clinopyroxene 102 5.3.3 Spinel 108 5.3.4 Plagioclase 110 5.3.5 Phlogopite 110 5.3.6 Melt inclusions in olivine phenocrysts 110
5.4 Geochemistry of the Ngaloa Group 113 5.4.1 Major element geochemistry 113 5.4.2 Trace element geochemistry 113 5.4.3 Radiogenic isotope chemistry 117
5.5 Summary 117 5.6 Origin of the Ngaloa Group volcanics 119
5.6.1 Mixing between OIB and Adakite magmas 119 5.6.2 Nature of the mantle wedge 120 5.6.3 The slab melt signature 123
5.7 Discussion 124 5.8 Conclusions 128
CHAPTER 6: WESTERN KAD A VU AND CENTRAL/EASTERN/ONO GROUPS: Melts from subducted oceanic crust 130
6.1 Introduction 131 6.2 Geology 131
6.2.1 Introduction 131 6.2.2 Lithology 131 6.2.3 Sample locations 134
6.3 Petrography and mineral chemistry 137
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6.4 Geochemistry of the Western Kadavu and Central/Eastern/Ono Groups
6.4.1 Major element geochemistry 6.4.2 Trace element geochemistry 6.4.3 Radiogenic isotope geochemistry 6.4.4 Summary
6.5 Geochemical constraints on the origin of the Western Kadavu and Central/Eastern/Ono andesites and dacites.
6.5.1 Crystal fractionation from a more basic parent. 6.5.2 Source variations and partial melting
6.6 Conclusion
CHAPTER 7: SYNTHESIS: Tectonic implications of the composition of the Hunter Ridge and Kadavu Island Group magmas
7.1 Introduction 7.2 Tectono-magmatic evolution of the Hunter Ridge 7.3 Tectono-magmatic evolution of the Kadavu Island Group
7.3.1 introduction 7.3.2 The Astrolabe Group 7.3.3 Western Kadavu, Central/Eastern/Ono and Ngaloa Group volcanics 7.3.4 Petrogenetic relationships between the N galoa Group volcanics & adakites of the WK and C/E/O Groups 7.3.5 Why did the South Fiji Basin subducted slab melt? 7.3.6 The significance of Fijian Alkali Basalts
7.4 Summary
REFERENCES
APPENDIX 1: Catalogue of rocks from the Kadavu Island Group and the northern part of the Hunter Ridge
APPENDIX 2: ANALYTICAL TECHNIQUES
A2.1 Whole rock major and trace element analyses A2.2 Whole rock radiogenic isotope analyses A2.3 Mineral chemistry analyses
APPENDIX 3: Petrographic descriptions of the Kadavu . Island Group northern Hunter Ridge rocks
APPENDIX 4: Electron microprobe analyses of mineral phases in the Kadavu Island Group and northern Hunter Ridge rocks
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148 148 148 151 151
154 154 155 162
163
163 164 165 165 165
168
169 170 172 173
177
A1
A6
A6 A8 A14
A16
A48
