Malaita, Solomon Islands - Leicester Research Archive - University

338 Pages·2013·17.66 MB·English

Geochemical and petrological investigations
of the deeper portions of the Ontong Java
Plateau: Malaita, Solomon Islands
Thesis submitted for the degree of
Doctor of Philosophy
at the University of Leicester
by
Tanya Louise Babbs BSc. (Leics.)
Department of Geology
University of Leicester
June 1997
UMI Number: U096028
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Abstract
Geochemical and petrological investigations of the
deeper portion of the Ontong Java Plateau: Malaita,
Solomon Islands
Tanya Louise Babbs
The island of Malaita in the Solomons Islands represents a unique opportunity to study
the deeper portions of the largest oceanic plateau discovered to date - the Ontong Java Plateau
(OJP). Malaita exposes several kilometres of Cretaceous OJP lavas, stratigraphically named the
Malaita Volcanic Group (MVG), within en-echelon inliers. Most of the lavas have suffered low-
grade (brownstone facies) metamorphism. The MVG basalts are low-K tholeiites, displaying a
limited range of major and trace element concentrations, consistent with the lavas being well mixed
prior to eruption, probably in shallow level magma chambers. However, two distinct groups,
Type A and Type C-G, can be identified from their rare earth elements (REEs), platinum group
elements, Sr and Nd radiogenic isotopes, indicating that mixing has not effectively wiped out all
source-related and melting signatures.
The MVG basalts were formed by extensive melting, mostly within the spinel Iherzolite
field as indicted by their chondritic REE patterns, suggesting a ridge-centred eruption. However,
melting in the MVG’s source started within the garnet Iherzolite field, consistent with a thermal
anomaly such as a decompressing mantle plume. Paradoxically for a ridge centred plume, the
MVG erupted in a deep submarine environment, when the elevation caused by a surfacing
plume would surely imply a subaerial eruption environment, such as seen in Iceland. In addition,
the MVG’s overlying sedimentary sequence implies a history of uplift, rather than that of
subsidence which would be expected as the plume wanes or crust is transported away.
The OJP plume was probably heterogeneous, consisting of enriched (HIMU) streaks in a
depleted matrix. This depleted component is thought not to be entrained upper MORB material,
but a depleted component intrinsic to the plume itself, possibly entrained as the plume ascended
through the lower mantle.
Acknowledgements
Acknowledgements
First and foremost I would like to thank all the staff at the Ministry of Natural Resources,
Honiara, Solomon Islands. This project was only made possible by their collaboration, hard work,
use of resources and invaluable field experience. I would also like to thank all the chiefs,
landowners and local labourers from Malaita for their co-operation and help; their handiwork with
machetes and axes was most impressive, even if a little worrying at times. Funding for the project
was provided by the Natural Environment Research Council. Thanks also to SOPAC and the
Solomon Island government for allowing me to use their sidescan imaging and interpretations of
the Malaita-Melanesian Arc Gap.
Many thanks to Mike Petterson (BGS), who's help, enthusiasm and encouragement in
difficult field conditions and with several chapters in this thesis was invaluable. Mike always
seemed to be able to laugh when I wanted to scream.
I am grateful to John Mahoney, Marissa Tejada (SOEST, Hawaii) and Clive Neal (Notre
Dame, Indiana) for letting me use unpublished data from the Singgalo River in chapter 4, and
especially to Clive for providing ICPMS data on the upper Singgalo River. I also thank John and
Marissa for unselfishly hosting a Malaita workshop in Hawaii, although it 'unfortunately' meant the
rest of us had to travel long distances. My trip to Hawaii was made all the more memorable by
Andy Harris ("H"), Martin Wooster and Dave Rothery from the Open University. They
generously allowed me to tag along for a week on their trip to the Big Island and their hikes out to
the active flows. Without their help I would never have seen 'the red stuff', let alone melt the
bottom of my boots and singe my eyebrows.
Many thanks to Paul Sylvester and Ian Campbell (ANU, Canberra) for generously
providing PGE and trace element data for the thesis. This project also greatly benefited from
radiogenic isotope work which was carried out at NIGL, Keyworth. Thanks to Pamela Kempton
and Geoff Nowell for their support and assistance, not only during my time spent in the NIGL
laboratories but their continuing help with the development of ideas within this thesis.
A big thank to my supervisors at Leicester, Andy Saunders and Mike Norry, especially to
Andy for his continuous guidance, support and encouragement with the project and, on a more
personnel note, I would particularly like to thank Andy for rescuing me from various oversized
spiders in the Solomon Islands.
Many thanks to all academic and technical staff at the Department. Special thanks to Andy
Smith and Ansel Dunham for their assistance in understanding the strange world of clay minerals.
Andy Kerr who's witticisms often 'took me to the fair' and always seemed to be planning his next
'jolly'. Ray Kent for listening to my geochemical ramblings and Dickson Cunningham for patiently
Acknowledgements
listening to my attempts to grasp the finer points of tectonics. Also thanks to Chris Beckett for his
help with the computers, being a good drinking partner and his rather bad 'anal' jokes.
I would like to thank all my fellow PhD students, both past and present. Matt for his
friendship and company on numerous Friday afternoon 'pound and pint' drinking sessions, also
for his fast and entertaining lifts back to Ipswich on many a weekend. Chris for showing us all
how it is done by finishing before us all and Vivi for continuously being madder than myself.
Lindsey for patiently putting up with my rants and bad moods around the office, and thanks to
Marion, Kate, Tiff, Ross, Sarah, Phil, Karine, Sam, Neil and anyone else I have forgotten, for their
friendship.
I would especially like to acknowledge those I had the pleasure of not only working with,
but also cohabiting. Cayley for her company drinking far too many pints in the 'Dead' Horse and
travelling around Turkey. Ceri for his weird Welsh wisdom, I also forgive you Ceri for keeping me
awake at night 'tottering' around in your flip flops. Loz for sharing many bottles of vin and garlic
laden pizzas. Jo and Rob who put up with both myself and Duncan writing up together, and
Magnus whose emails in the wee small hours of the morning always put a smile on my face. Also
an extra big thank you to Dunes, for your company and friendship during the long nights spent in
the department writing up, all those times I tried to strangle you I really didn't mean it...honest!?
Finally, my biggest thanks go to my parents and to Sean. Mum and dad never blinked
when I told them I wanted to give up a perfectly well paid job to go back to university and their
encouragement, support and love were, as always, unwavering. And Sean, who was always
there for me and without whose strength and confidence I would never have finished this thesis.
Contents
Contents
Chapter 1 - The Ontong Java Plateau - rationale for study
1.1 Introduction to LIPs, plumes and plateaus
Large Igneous Provinces
Plumes
Oceanic plateaus versus continental flood basalts
1.2 The Ontong Java Plateau
1.3 Aims of this study
Chapter 2 - The Solomon Islands and their tectonic evolution
2.1 Introduction
2.2 A troublesome history of geological expeditions in the Solomon Islands
2.3 Coleman's Provinces within the Solomon Islands
2.4 Age constraints of tectonic events effecting the Solomon Islands
2.5 The tectonic evolution of the Solomon Islands
2.5.1 Introduction to the Ontong Java Plateau and the tectonic history of the Solomon
Islands
2.5.2 Extension of the Ontong Java Plateau and younger volcanism
2.5.3 Vitiaz (Stage 1) Arc development
2.5.4 The Ontong Java Plateaus docking with the North SolomonA/itiaz Trench
2.5.5 Stage 2 Arc development
2.5.6 Obduction of the Malaita Anticlinorium
2.6 Seismic studies in the Solomon Islands
2.7 Present day anomalous volcanic activity in the Solomon Islands
2.8 Development of a new geological framework for the Solomon Islands
2.9 Proof that Malaita is an obducted fragment of the Ontong Java Plateau
Chapter 3 - The Geology of Malaita
3.1 Introduction
3.2 Topography, vegetation and climate
3.3 Field Logistics
3.4 Nomenclature changes to the stratigraphy of Malaita
3.5 The Malaitan Volcanic Group (MVG)
3.5.1 Stratigraphic relationships, exposure and age
Contents
3.5.2 Lithological description
3.5.3 Fateleka Gabbro Intrusions
3.5.4 Spotty Rock
3.5.5 Eruption Environment of the Malaita Volcanic Group
3.6 Malaita's cover sequence
3.6.1 Kwara'ae Mudstone Formation
3.6.2 Alite Limestone Formation
3.6.3 Maramasike Volcanics Formation
The Southern Younger Volcanics (from Hughes and Turner, 1976 and 1977)
The Northern Malaita Alkali Suite (from Barron, 1993)
Eruption environment of the Maramasike Volcanic Formation
3.6.4 Haruta Limestone Formation
3.6.5 Suafa Limestone Formation plus local stratigraphic units
3.6.6 Silica Budget of the Pacific Ocean during the Eocene
3.7 The Malaita alnoites
3.8 Structural deformation of Malaita - a brief outline
3.8.1 Fold axial trends
3.8.2 Thrust faults - Hughes and Turner (1970 -1974)/ Barron (1993)
3.8.3 Thrust faults and structural areas - Petterson et al (1992 -1995)
Chapter 4 - Growth and obduction models of oceanic plateaus: the formation of the
Malaita Anticlinorium
4.1 Introduction
4.2 The formation of large igneous provinces
4.2.1 Lateral growth models
4.2.2 Vertical growth models
4.3 Criteria for preservation of oceanic plateaus
4.4 Formation of decollements in oceanic plateaus
4.5 Structure of the Malaita Anticlinorium - clues to obduction
4.5.1 Shallow level thrusting in the MVG - shallow level detachment layers?
4.5.2 Geochemical evidence for shallow-level thrusting in the MVG
4.5.3 Structural deformation on Malaita - clues to the obduction mechanism
4.6 Obduction models for the Malaita Anticlinorium
4.6.1 The tectonic flake model
4.6.2 NE thrusting and imbrication of the OJP
4.6.3 Discussion
4.7 The future of the OJP - potential plume or greenstone belt?
4.8 Conclusions
Contents
Chapter 5 - Petrography and mineral chemistry of the Malaita Volcanic Group
5.1 Introduction
5.2 Petrography
5.2.1 Petrography of the phenocryst-bearing basalts
5.2.2 Petrography of the dolerites
5.2.3 Plagioclase megacrysts
5.3 Mineral chemistry
5.3.1 Chemical mineralogy of the MVG olivines
5.3.2 Chemical mineralogy of the MVG plagioclases
5.3.3 Chemical mineralogy of the MVG pyroxenes
5.3.4 Chemical mineralogy of the MVG opaques
5.4 Discussion of the petrography and chemical mineralogy
Crystallisation order of the MVG tholeiites
Pressure of formation of clinopyroxenes in the MVG
Magma chamber processes and plagioclase megacrysts
Chapter 6 - The effect of alteration and element mobility on the Malaita Volcanic Group
6.1 Introduction
6.2 X-ray diffraction studies of the MVG
6.3 Chemistry of the MVG secondary minerals
6.4 Secondary element mobilisation affecting the MVG
6.4.1 Introduction
6.4.2 High field strength elements (i.e. Nb, Zr, Y, P, Ti, Ta, Hf)
6.4.3 Major elements (Si, Fe, Al, Ca, Mg, Mn)
6.4.4 Transition metal elements (V, Zn, Ni, Cr, Cu)
6.4.5 Large ion lithophile elements (Sr, Rb, Ba, K, Na)
6.4.6 Rare-earth elements
6.4.7 Radiogenic isotopes
6.4.8 Platinum group elements (Pd, Pt, Rh, Ru, Ir)
6.5 Conclusions of alteration studies of the MVG basalts
Chapter 7 - Petrogenesis of the Malaita Volcanic Group
7.1 Introduction
7.2 Classification of the MVG
7.3 Major and trace element chemistry of the MVG
7.4 Mineral phases involved in the fractionation of the MVG
7.5 Rare-earth elements of the MVG
7.6 Sr, Nd and Pb isotope ratios of the MVG
7.7 New Hf isotopes for the MVG
Contents
7.8 New PGEs for the MVG
7.9 Concluding points
Chapter 8 - Characteristics of the mantle source of the Malaita Volcanic Group
8.1 Introduction
8.2 Effects of partial melting
8.2.1 Depth of partial melting
8.2.2 Degree of partial melting
8.3 Eruption Environment of the MVG - intra-plate or ridge-centred?
8.4 Post-emplacement uplift of the MVG
8.5 Rapid emplacement of the Ontong Java Plateau
8.6 Mantle sources
8.6.1 Plume-Upper mantle mixing
8.6.2 Depleted isotopes but chondritic implications from trace element ratios
8.6.3 Plume Origins-D" or 670 km?
8.7 The OJP plume
8.8 The Fate of the OJP plume - Louisville or R.I.P.?
8.9 Concluding Remarks
Appendix A - River Traverses (enclosed in back pocket)
Ai Singgalo River
Aii Kwaleunga River
Aiii Ala'olo River
Appendix B - Analytical techniques
Bi Sample preparation
Bii X-ray fluorescence techniniques
Biii Instrumental neutron activation techniques
Biv Isotope dilution techniques
Bv XRD techniques
Bvi Electron microprobe techniques
Bvii ICPMS techniques
Appendix C - Data Accuracy and precision
Ci XRF standard tables
Cii XRF operating conditions
Ciii INAA standard tables
Civ XRF and INAA standard graphs
Contents
Appendix D - XRF Data
Di Malaita Volcanics Group
Dii Maramasike Volcanics (NMAS)
Diii Maramasike Volcanics (SYV)
Div Makira Volcanics (MORB and OJP-type)
Dv Ulawa Volcanics
Appendix E - INAA and ICPMS data (including PGEs)
Ei Leicester INAA
Eii ANU ICPMS data (including PGEs)
Appendix F - Radiogenic isotopes
Appendix G - Electron microprobe data
Gi Olivines
Gii Pyroxenes
Giii Feldspars
Giv Oxide minerals
Gv Alteration minerals
References