Marginal basin geology : volcanic and associated sedimentary and tectonic processes in modern and ancient marginal basins
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Summary The type, relative abundance and stratigraphical relationships of volcanic rocks that comprise island volcanoes are a function of (i) depth of extrusion beneath water, (ii) magma composition, and (iii) lava-water interactions. The water depth at which explosions can occur is called the pressure compensation level (PCL) and is variable. Explosive eruptions that occur above the PCL and below sealevel can give rise to abundant hydroclastic and pyroclastic debris. Below the PCL, clastic material cannot form explosively; it forms from lava by thermal shock. The volcaniclastic products are widely dispersed in basins adjacent to extrusion sources by three principal kinds of marine transport processes. These are slides, sediment gravity flows and suspension fallout. Volcaniclastic debris can be derived in subaqueous and subaerial-to-subaqueous environments (i) directly from eruptions, (ii) from remobilization of juvenile volcaniclastics, or (iii) from epiclastic material which initially develops above sealevel. Sediment gravity flows (fluids driven by sediment motion) exhibit the phenomenon of flow transformation . This term is used here for the process by which (i) sediment gravity flow behaviour changes from turbulent to laminar, or vice versa, within the body of a flow, (ii) flows separate into laminar and turbulent parts by gravity, and (iii) flows separate by turbulent mixing with ambient fluid into turbulent and laminar parts. Dominant kinds of subaqueous volcaniclastic sediment gravity flows are debris flows, hot or cold pyroclastic flows and turbidites. Fine grained material can be thrown into suspension locally during flow transformations or underwater eruptions, but thin, regionally distributed subaqueous fallout tephra is mostly derived from siliceous Plinian eruptions.
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Summary Supra-subduction zone (SSZ) ophiolites have the geochemical characteristics of island arcs but the structure of oceanic crust and are thought to have formed by sea-floor spreading directly above subducted oceanic lithosphere. They differ from ‘MORB’ ophiolites not only in their geochemistry but also in the more depleted nature of their mantle sequences, the more common presence of podiform chromite deposits, and the crystallization of clinopyroxene before plagioclase which is reflected in the high abundance of wehrlite relative to troctolite in their cumulate sequences. Most of the best-preserved ophiolite complexes in orogenic belts are of this type. Geological reconstructions suggest that most SSZ ophiolites formed during the initial stages of subduction prior to the development of any volcanic arc. Evidence from these ophiolites suggests that the first magma to form in response to intra-oceanic subduction is boninitic in composition, derived by partial melting of hydrated oceanic lithosphere in the ‘mantle wedge’. As subduction proceeds, the magma composition changes to island-arc tholeiite, probably because the hydrated asthenosphere of the ‘mantle wedge’ eventually becomes the dominant mantle source. Other SSZ ophiolites formed in the early stages of back-arc spreading following splitting of a pre-existing arc. Nonetheless the more common mechanism for formation of SSZ ophiolites appears to have been pre-arc rather than back-arc spreading.
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Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type ( eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.
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1 What is Typical Calcalkaline Andesite?.- 1.1 Introduction.- 1.2 Definition of Orogenic Andesite.- 1.3 Magma Series Containing Orogenic Andesites.- 1.4 Overview.- 2 The Plate Tectonic Connection.- 2.1 Spatial Distribution of Active Orogenic Andesite Volcanoes.- 2.2 Initiation of Subduction.- 2.3 Cessation of Subduction.- 2.4 Collisions.- 2.5 Reversal of Subduction Polarity.- 2.6 Forearc and Transform Fault Volcanism.- 2.7 Anomalously Wide Volcanic Arcs.- 2.8 Andesites Clearly Not at Convergent Plate Boundaries.- 2.9 Conclusions.- 3 Geophysical Setting of Volcanism at Convergent Plate Boundaries.- 3.1 Topography, Gravity, Heat Flow, and Conductivity.- 3.2 Crustal Thickness, Structure, and Age.- 3.3 Upper Mantle Beneath the Forearc, Volcanic Arc, and Backarc Regions.- 3.4 Dipping Seismic Zones (Benioff-Wadati Zones) and Underthrust Lithosphere.- 3.5 Partial Melting and Magma Ascent Beneath Volcanic Arcs.- 3.6 Magma Chambers Beneath Orogenic Andesite Volcanoes.- 3.7 Conclusions.- 4 Andesite Magmas, Ejecta, Eruptions, and Volcanoes.- 4.1 Characteristics of Andesite Magma.- 4.1.1 Temperature.- 4.1.2 Density.- 4.1.3 Rheology.- 4.1.4 Miscellaneous Properties and Applications.- 4.2 Andesite Rock, Eruption, and Edifice Types.- 4.3 Variations in Magma Composition During and etween Historic Andesite Eruptions.- 4.4 Variations in Rock Composition During Evolution of Stratovolcanoes.- 4.5 Conclusions About Andesite Magma Reservoirs.- 4.6 Stress Fields and Volcano Spacings Within Volcanic Arcs.- 4.7 Relationships Between the Timing of Arc Volcanism and Plate Movements.- 4.8 Magma Eruption Rates at Convergent Plate Boundaries.- 4.9 Relative Proportions of Andesite.- 5 Bulk Chemical Composition of Orogenic Andesites.- 5.1 Rock Analyses: Significance, Averages, and Representative Samples and Suites.- 5.2 Major Elements.- 5.2.1 Silica Contents and Harker Variation Diagrams.- 5.2.2 Alkalies.- 5.2.3 Iron and Magnesium.- 5.2.4 Titanium.- 5.2.5 Aluminum and Calcium.- 5.2.6 Phosphorous.- 5.2.7 CIPW Normative Mineralogy.- 5.3 Volatiles.- 5.3.1 Water.- 5.3.2 Carbon Dioxide.- 5.3.3 Sulfur.- 5.3.4 Halogens.- 5.3.5 Oxygen.- 5.4 Trace Elements.- 5.4.1 The K-Group: Rb, Cs, Ba, and Sr.- 5.4.2 REE Group: Rare Earth Elements Plus Y.- 5.4.3 The Th Group: Th,U, and Pb.- 5.4.4 The Ti Group: Zr, Hf, Nb, and Ta.- 5.4.5 The Compatible Group: Ni, Co, Cr, V, and Sc.- 5.4.6 The Chalcophile Group: Cu, Zn, and Mo.- 5.4.7 Trace Element Systematics.- 5.5 Isotopes.- 5.5.1 Strontium.- 5.5.2 Lead.- 5.5.3 Neodymium.- 5.5.4 Inert Gases.- 5.5.5 U-Disequilibrium.- 5.5.6 Oxygen.- 5.5.7 Synthesis of Isotope Data.- 5.6 Comparison with Andesites Not at Convergent Plate Boundaries.- 5.7 Geochemical Distinctiveness of Volcanism at Convergent Plate Boundaries.- 5.8 Conclusions: Chemical Diversity of Orogenic Andesites.- 6 Mineralogy and Mineral Stabilities.- 6.1 Plagioclase.- 6.2 Pyroxenes.- 6.3 Amphibole.- 6.4 Olivine.- 6.5 Oxides.- 6.6 Garnet.- 6.7 Other Minerals.- 6.8 Inclusions in Orogenic Andesites.- 6.9 Mineral Stabilities in Andesite Magma.- 6.10 Trace Element Equilibria Between Minerals and Melt.- 6.11 Conclusions.- 7 Spatial and Temporal Variations in the Composition of Orogenic Andesites.- 7.1 Variations in Magma Composition Across Volcanic Arcs.- 7.2 Variations in Magma Composition Along Volcanic Arcs.- 7.3 Effects of Plate Convergence Rate on Magma Composition.- 7.4 Relationships Between Compositions of Orogenic Andesites and Adjacent Oceanic Crust.- 7.5 Changes in the Composition of Orogenic Andesites During Earth History.- 8 The Role of Subducted Ocean Crust in the Genesis of Orogenic Andesites.- 8.1 Characteristics of Subducted Ocean Crust Beneath Volcanic Arcs.- 8.2 Circumstantial Evidence of Slab Recycling in Arc Volcanism.- 8.3 Are Orogenic Andesites Primary Melts of Subducted Ocean Floor Basalt? No.- 8.4 The Sediment Solution.- 8.5 IRS Fluids and Maxwell's Demons.- 8.6 Conclusions.- 9 The Role of the Mantle Wedge.- 9.1 Characteristics of the Mantle Wedge.- 9.2 Circumstantial Evidence that Arc Magmas Originate Within the Mantle Wedge.- 9.3 Are Orogenic Andesites Primary Melts of Only the Mantle Wedge? Rarely.- 9.4 Fluid Mixing, Metasomatism, and Demonology in the Mantle Wedge.- 10 The Role of the Crust.- 10.1 Circumstantial Evidence for Crustal Involvement in Orogenic Andesites.- 10.2 Crustal Anatexis.- 10.3 Crustal Assimilation.- 11 The Role of Basalt Differentiation.- 11.1 General Arguments for and Against Differentiation.- 11.2 Roles of Plagioclase, Pyroxenes, and Olivine.- 11.3 Role of Magnetite and the Plagioclase-Orthopyroxene/Olivine-Augite-Magnetite (POAM) Model.- 11.4 Role of Amphibole.- 11.5 Role of Garnet.- 11.6 Role of Accessory Minerals: Apatite, Chromite, Sulfides, Biotite.- 11.7 Role of Magma Mixing.- 11.8 Role of Other Differentiation Mechanisms.- 11.9 Differentiation Processes Leading to Andesites in Anorogenic Environments.- 12 Conclusions.- 12.1 Andesite Genesis by POAM-Fractionation: the Most Frequent Mechanism.- 12.2 Some Outstanding Problems Requiring Clarification.- 12.3 Origin of Tholeiitic Versus Calcalkaline Andesites.- 12.4 Origin of Across-Arc Geochemical Variations.- 12.5 Epilog.- References.
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Five sequences of pumiceous dacitic debris interbedded with fossiliferous marine mudstones are recognized inthe Tokiwa formation (Miocene). They represent the quenched and laterally transported products of submarine explosions. The mechanisms and environment of deposition are discussed.
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