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Deposits and Basalts from Ori Massif, Shatsky Rise Oceanic Plateau

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Deposits and Basalts from Ori Massif, Shatsky Rise Oceanic Plateau VOLCANOLOGY AND GEOCHEMISTRY OF THE CRETACEOUS VOLCANICLASTIC DEPOSITS AND BASALTS FROM ORI MASSIF, SHATSKY RISE OCEANIC PLATEAU Irina Romanova BSc (2007), MSc (2009) Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy School of Earth, Environmental and Biological Sciences Science and Engineering Faculty Queensland University of Technology 2017 Keywords Shatsky Rise IODP Expedition 324 Large Igneous Province Oceanic plateau Cretaceous magmatism Basalt Volcaniclastic deposits Phreatomagmatic volcanism Surtseyan-style shallow-marine volcanism Paleoelevation Nd-Hf isotope geochemistry Magmatic sources Seawater alteration Magnesium Element mobility Mantle plume and spreading ridge interaction North-western Pacific i Abstract Oceanic plateaus are an important class of Large Igneous Provinces (LIPs) and represent the most voluminous examples of oceanic intraplate magmatism, with crustal thicknesses reaching 10-40 km and areal extents exceeding 0.1 Mkm2. Formation of oceanic plateaus remains highly debated, with arguments that they form during impact of a mantle plume head with oceanic lithosphere or that they represent anomalously high degrees of melting associated with shallow plate tectonic processes. Shatsky Rise in the North-Western Pacific is one of the largest (~500,000 km2) and oldest (145-135 Ma) non-accreted oceanic plateaus and provides evidence that is consistent with both models for oceanic plateau formation. Studies of magnetic lineations and bathymetry showed that Shatsky Rise was formed at a triple junction of oceanic spreading ridges and has three edifices (Tamu, Ori and Shirshov Massifs) progressively decreasing in age and in volume. Research from this thesis has been focused on Ori Massif, the second largest and oldest edifice from the central part of the Shatsky Rise, to provide constraints on its formation and evolution. The Ori Massif has been investigated in terms of paleoelevations that this edifice reached upon formation, source contributions to this edifice, and seawater alteration effect on the primary magmatic signatures of basalts from this edifice. This was achieved with the petrographic, lithostratigraphic, major and trace element and Hf-Nd isotope geochemical, mineralogical and multivariate analyses of samples retrieved from Ori Massif during the Integrated Ocean Drilling Program Expedition 324. Volcaniclastic deposits from the summit of Ori Massif, are ~30 m thick and predominantly composed of juvenile fragments (from 70 to 100 modal % on a cement- free basis) with minor amounts of accidental lithics and singular shallow-marine fossils. The abundance of poorly to moderately vesicular quenched glass fragments, mainly angular lithics along with cored juvenile clasts within the volcaniclastic succession suggests explosive fragmentation of basaltic magma and pre-existing rocks during phreatomagmatic eruptions. The volcaniclastic succession from Ori Massif is characterised by poor sorting, thick-bedded to massive structure and contains no sharp erosional boundaries or sedimentary interbeds, and therefore is interpreted to represent ii one eruptive unit. Preservation of gentle textures and forms of the glass fragments within the deposits imply relatively limited reworking processes during transportation from the vent. The volcaniclastic material was produced during shallow-marine to near sea-level Surtseyan style phreatomagmatic eruptions at the summit of Ori Massif, transported by mass flow processes and deposited below wave base. This indicates that Ori Massif reached very shallow water depths during its formation, but that there is no direct evidence for plateau emergence. Basaltic fragments from the volcaniclastic deposits, Ori Massif, demonstrate N- MORB-like and transitional to depleted MORB trace element and Hf-Nd isotope compositions, that are different to the underlying picrite-like lavas. This indicates change in chemical composition of magma during final stages of Ori Massif formation. The Hf-Nd isotope data (εHft = 13.3-18; εNdt = 8.9-10.8) suggest presence of a relatively more depleted Pacific MORB component in the source of Shatsky Rise. The Hf-Nd isotope compositions from Shatsky Rise, Hess Rise, Ojin Rise and Cooperation seamount follow a common compositional trend, suggesting mixing of a common most depleted Pacific MORB source mantle with an enriched component, such as PREMA or HIMU. Overall, basalts from Shatsky Rise demonstrate more depleted Hf-Nd isotope compositions and more variety within the MORB field, compared to those of Ontong Java, Manihiki and Hikurangi oceanic plateaus. Such compositional differences are interpreted as a result of Shatsky Rise magma interaction with an active MOR-system upon emplacement. Whole-rock samples from the flanks of Ori and Tamu Massifs show similar, predominantly smectitic and minor calcite alteration. The onset of seawater alteration of oceanic basalts is largely dependent on the initial crystallinity and presence and abundance of olivine. Two compositionally-contrasting alteration processes are defined: 1) olivine-normative Higher-Mg tholeiitic basalts undergo olivine decomposition to saponite clays and calcite, resulting in Mg-loss by 2-3 wt.%, which releases significant amounts of Mg2+ into seawater providing a source of Mg2+ for redistribution throughout the oceanic crust and to the ocean; and 2) Higher-Fe tholeiitic basalts contain traces of olivine phenocrysts and act as sinks for Mg2+ during low temperature alteration. High crystallinity rocks from both groups are associated with Ca-gain, whereas increasing contents of altered glass lead to Mg-uptake and Ca-loss. iii The HFSE and REE predominantly behave conservatively and correlate with MgO enrichment or depletion. Ori Massif is associated with the greatest range in vertical dynamics, compositional variability and alteration styles across the Shatsky Rise. Formation of Ori Massif can be explained by less intense interaction with an active spreading ridge system, relative to Tamu and Shirshov Massifs, and a more pronounced involvement of a mantle plume during construction of this Shatsky Rise edifice. iv Table of Contents Keywords ...................................................................................................................... i Abstract ........................................................................................................................ ii Table of Contents ......................................................................................................... v List of Figures ............................................................................................................. ix List of Tables............................................................................................................... xi List of Abbreviations.................................................................................................. xii Statement of Original Authorship ............................................................................. xiv Acknowledgements .................................................................................................... xv Chapter 1: Introduction ...................................................................................... 1 1.1 Research background and problem ..................................................................... 1 1.2 Research focus and questions ............................................................................. 6 1.3 Significance of this PhD project ......................................................................... 8 1.4 Research plan ...................................................................................................... 8 1.4.1 Research project 1 (Chapter 3) .................................................................. 9 1.4.2 Contribution to collaborative research and research project 2 .................. 9 1.4.2.1 Participation in IODP Expedition 324 collaborative research .... 9 1.4.2.2 Research project 2 (Chapter 4) ................................................... 9 1.4.3 Research project 3 (Chapter 5) ................................................................ 10 1.5 Sample database and materials ......................................................................... 10 1.6 Thesis structure ................................................................................................. 11 1.7 List of publications ........................................................................................... 11 Chapter 2: Previous investigations on LIPs and Shatsky Rise ...................... 13 2.1 Large Igneous Province formation hypotheses................................................. 13 2.2 Previous studies on Shatsky Rise ..................................................................... 20 2.2.1 Geological location and structure ............................................................ 20 2.2.2 Tectonic setting ....................................................................................... 21 2.2.3 Age of magmatism .................................................................................. 24 2.2.4 Basement stratigraphy ............................................................................. 25 2.2.5 Paleoelevations and subsidence .............................................................. 28 2.2.6 Igneous and alteration petrology ............................................................. 29 2.2.7 Geochemistry .........................................................................................
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