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Miocene and Pliocene Silicic Coromandel Volcanic Zone Tephras from ODP Site 1124-C: Petrogenetic Applications and Temporal Evolution

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Miocene and Pliocene Silicic Coromandel Volcanic Zone Tephras from ODP Site 1124-C: Petrogenetic Applications and Temporal Evolution Miocene and Pliocene silicic Coromandel Volcanic Zone tephras from ODP Site 1124-C: Petrogenetic applications and temporal evolution Matthew Thomas Stevens A thesis submitted for the partial fulfilment for the degree of Master of Science with Honours in Geology School of Geography, Environment and Earth Sciences Victoria University of Wellington November 2010 Abstract The Coromandel Volcanic Zone (CVZ) was the longest-lived area of volcanism in New Zealand hosting the commencement of large explosive rhyolitic and ignimbrite forming eruptions. The NW trending Coromandel Peninsula is the subaerial remnant of the Miocene-Pliocene CVZ, which is regarded as a tectonic precursor to the Taupo Volcanic Zone (TVZ), currently the most dynamic and voluminous rhyolitic volcanic centre on Earth. This study presents new single glass shard major and trace element geochemical analyses for 72 high-silica volcanic tephra layers recovered from well-dated deep-sea sediments of the SW Pacific Ocean by the Ocean Drilling Program (ODP) Leg 181. ODP Site 1124, ~720 km south and east from the CVZ, penetrated sediments of the Rekohu Drift yielding an unprecedented record of major explosive volcanic eruptions owing to the favourable location and preservation characteristics at this site. This record extends onshore eruptive sequences of CVZ explosive volcanism that are obscured by poor exposure, alteration, and erosion and burial by younger volcanic deposits. Tephra layers recovered from Site 1124 are well-dated through a combination of biostratigraphic and palaeomagnetic methods allowing the temporal geochemical evolution of the CVZ to be reconstructed in relation to changes in the petrogenesis of CVZ arc magmas from ~ 10 to 2 Ma. This thesis establishes major and trace element geochemical “fingerprints” for all Site 1124-C tephras using well-established (wavelength dispersive electron probe microanalysis) and new (laser ablation inductively coupled plasma mass spectrometry) in situ single glass shard microanalytical techniques. Trace element analysis of Site 1124-C glass shards (as small as 20 m) demonstrate that trace element signatures offer a more specific, unequivocal characterisation for distinguishing (and potentially correlating) between tephras with nearly identical major element compositions. 2 The Site 1124-C core contains 72 unaltered Miocene-Pliocene volcanic glass-shard- bearing laminae > 1 cm thick that correspond to 83 or 84 geochemical eruptive units. Revised eruptive frequencies based on the number of geochemical eruptive units identified represent at least one eruption every 99 kyr for the late Miocene and one per 74 kyr for the Pliocene. The frequency of tephra deposition throughout the history of the CVZ has not been constant, rather reflecting pulses of major explosive eruptions resulting in closely clustered groups of tephra separated by periods of reduced activity, relative volcanic quiescence or non-tephra deposition. As more regular activity became prevalent in the Pliocene, it was accompanied by more silicic magma compositions. Rhyolitic volcanic glass shards are characterised by predominantly calc-alkaline and minor high-K enriched major element compositions. Major element compositional variability of the tephras deposited between 10 Ma and 2 Ma reveals magma batches with pre-eruptive compositional gradients implying a broad control by fractional crystallisation. Trace element characterisation of glass shards reveals the role of magmatic processes that are not readily apparent in the relatively homogeneous major element compositions. Multi- element diagrams show prominent negative Sr and Ti anomalies against primitive mantle likely caused by various degrees of plagioclase and titanomagnetite fractional crystallisation in shallow magma chambers. Relative Nb depletion, characteristic of arc volcanism, is moderate in CVZ tephras. HFSEs (e.g. Nb, Zr, Ti) and HREEs (e.g. Yb, Lu) remain immobile during slab fluid flux suggesting they are derived from the mantle wedge. LILE (e.g. Rb, Cs, Ba, Sr) and LREE (e.g. La, Ce) enrichments are consistent with slab fluid contribution. B/La and Li/Y ratios can be used as a proxy for the flux of subducting material to the mantle wedge, they suggest there is a strong influence from this component in the generation of CVZ arc magmas, potentially inducing melting. CVZ tephra show long-term coherent variability in trace element geochemistry. Post ~ 4 Ma tephras display a more consistent, less variable, chemical fingerprint that persists up to and across the CVZ/TVZ transition at ~ 2 Ma. Initiation of TVZ volcanism may have occurred earlier than is presently considered, or CVZ to TVZ volcanism may have occurred without significant changes in magma generation processes. 3 Acknowledgements Professor Joel Baker and Professor Lionel Carter have generously donated time in supervising this thesis, providing me with much enthusiam and encouragement for this research, and a lot of understanding throughout this difficult year. I also wish to gratefully acknowledge the professional support provided by Joel and Lionel when I accepted a full- time position at GNS Science, thank you for all your patience as I settled in. Dr Richard Wysoczanski provided supervision prior to accepting a position at NIWA, your guidance was thoroughly appreciated. Aidan Allan has been my honorary supervisor throughout this research. I have been very fortunate in sharing a similar research topic with Aidan; our close collaboration has saved each other much effort at times. Aidan has provided me with invaluable support, from assistance with sample sieving and mount preparation (oh joy!) to discussion and constructive feedback. One day I hope to achieve your amazing knowledge and understanding. I would like to thank a number of fellow research students and staff at Victoria University whom have provided their time and support in various ways, I am sorry to anyone whom I have inadvertently missed. To all my fellow research students, you have been a fantastic source of dynamic discussion (many times off topic) and I thank you all for unselfishly sharing your mastery. John Patterson for your friendship and advice during work on the electron microprobe, you were a very patient instructor. Stewart Bush for your watchful eye during sample preparation. My office buddies, Aidan, Alex and Evie – I owe you all a special thank you for (mostly) keeping me on the straight and narrow except during those Friday afternoons when we shared the platform at the procrastination station. Thanks to staff at Victoria University including Euan Smith, Tim Stern and John Collen for their interest, discussion and advice. The text of this thesis has benefited from editorial critisim by my supervisors, but I would like to thank Michael Gazley, Katie Collins and Sarah Grain for their „speedy‟ editing during final stages of this thesis, especially during final proofreading. Michael and Sarah, thank you for the much-needed coffee breaks, I think we now own several of the cafes on campus. The companionship of my friends has carried me through the highs and lows of this project. I would like to acknowledge the staff at Geosphere Ltd, thank you for taking an interest in this research. To the GeoNet team, I extend my thanks to those whom have endured my endless discussions about my research (it will now stop), and seen me on the days where I forgot to sleep the night before (no more grumpy Matt). Kevin, I thank you for your quiet understanding and fantastic support, thank you for taking me on even though I carried some baggage. To my family, for their continued encouragement and support during my studies. Lesley, thank you for the last minute formatting assistance and cheering me to the finish line. Michaela, your constant love, encourgement and inspiration have carried me through this research. You provided me with the strength to reach this goal, and have stood by me completely during the highs and many lows. I dedicate this thesis to you. 4 Table of Contents ABSTRACT 2 ACKNOWLEDGEMENTS 4 TABLE OF CONTENTS 5 LIST OF FIGURES 8 LIST OF TABLES 11 1.0 INTRODUCTION 12 1.1 REGIONAL TECTONIC SETTING 12 1.2 COROMANDEL VOLCANIC ZONE 13 1.2.1 ONSHORE MIOCENE-PLIOCENE VOLCANIC RECORD OF SILICIC VOLCANISM 14 1.3 OFFSHORE VOLCANIC RECORD - OCEAN DRILLING PROGRAM SITE 1124 15 1.3.1 RECOGNITION OF TEPHRA LAYERS AT OCEAN DRILLING PROGRAM SITE 1124-C 16 1.3.2 AGE MODELS 17 1.3.3 FREQUENCY OF ERUPTIONS 19 1.4 TEPHROSTRATIGRAPHY/TEPHROCHRONOLOGY 22 1.4.1 NOMENCLATURE 22 1.4.2 REVIEW OF VOLCANIC ASH GENERATION, TRANSPORTATION AND DEPOSITION 23 1.4.3 TEPHRA FALL DEPOSITS 25 1.5 GEOCHEMICAL EVALUATION OF ODP SITE 1124-C TEPHRA LAYERS 26 1.5.1 ELECTRON PROBE MICROANALYSIS (EPMA) 27 1.5.2 LASER ABLATION INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY (LA-ICP-MS) 27 1.6 OBJECTIVES OF THIS THESIS 29 1.7 THESIS STRUCTURE 30 5 2.0 SAMPLE PREPARATION AND ANALYTICAL TECHNIQUES 32 2.1 SAMPLE ACQUISITION 32 2.2 SAMPLE PREPARATION 32 2.3 ELECTRON PROBE MICROANALYSIS (EPMA) 34 2.3.1 PRECISION AND ACCURACY OF MAJOR ELEMENT DATA 36 2.3.2 EXISTING MAJOR ELEMENT DATA FOR MIOCENE-PLIOCENE COROMANDEL VOLCANIC ZONE TEPHRAS 37 2.4 LASER ABLATION INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY (LA-ICP- MS) 39 2.4.1 LA-ICP-MS DATA ACQUISITION AND CALCULATION 41 2.4.2 PRECISION AND ACCURACY OF TRACE ELEMENT DATA 42 2.4.2.1 The Use of Different Internal Standards 45 2.4.3 EXISTING TRACE ELEMENT DATA FOR MIOCENE-PLIOCENE COROMANDEL VOLCANIC ZONE TEPHRAS 49 3.0 RESULTS – COMPOSITION OF THE SILICIC TEPHRAS 50 3.1
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