Abstract The oldest known, intact sedimentary record of a nascent intraoceanic arc was recovered in a ∼100-m-thick unit (IV) above ca. 49 Ma basaltic basement at International Ocean Discovery Program Site U1438 in the Amami Sankaku Basin. During deposition of Unit IV the site was located ∼250 km from the plate edge, where Izu-Bonin-Mariana subduction initiated at 52 Ma. Basement basalts are overlain by a mudstone-dominated subunit (IVC) with a thin basal layer of dark brown metalliferous mudstone followed by mudstone with sparse, graded laminae of amphibole- and biotite-bearing tuffaceous sandstone and siltstone. Amphibole and zircon ages from these laminae suggest that the intermediate subduction-related magmatism that sourced them initiated at ca. 47 Ma soon after basement formation. Overlying volcaniclastic, sandy, gravity-flow deposits (subunit IVB) have a different provenance; shallow water fauna and tachylitic glass fragments indicate a source volcanic edifice that rose above the carbonate compensation depth and may have been emergent. Basaltic andesite intervals in upper subunit IVB have textures suggesting emplacement as intrusions into unconsolidated sediment on a volcanic center with geochemical and petrological characteristics of mafic, differentiated island arc magmatism. Distinctive Hf-Nd isotope characteristics similar to the least-radiogenic Izu-Bonin-Mariana boninites support a relatively old age for the basaltic andesites similar to detrital amphibole dated at 47 Ma. The absence of boninites at that time may have resulted from the position of Site U1438 at a greater distance from the plate edge. The upper interval of mudstone with tuffaceous beds (subunit IVA) progresses upsection into Unit III, part of a wedge of sediment fed by growing arc-axis volcanoes to the east. At Site U1438, in what was to become a reararc position, the succession of early extensional basaltic magmatism associated with spontaneous subduction initiation is followed by a rapid transition into potentially widespread subduction-related magmatism and sedimentation prior to the onset of focused magmatism and major arc building.
The Izu-Bonin Mariana (IBM) island arc in the western Pacific basin is a volcanic chain formed by subduction of the Pacific lithospheric plate beneath the Philippine Sea plate. Worldwide, subduction causes high-magnitude earthquakes and explosive volcanic activity in addition to forming the Earth’s continental crust and major ore deposits, thus its understanding is important to human concerns. In the IBM, volcanic activity spans 50 million years to the present and the arc has undergone extension and periodic rifting, preserving remnants of its early history. The Kyushu Palau ridge (KPR), an inactive Oligocene remnant of the rifted IBM system, is a unique window to understanding the early subduction processes. In this work, contemporaneous volcaniclastic sediments drilled at Deep Sea Drilling Project (DSDP) Site 296, located in a basin at the crest of the northern KPR, and volcaniclastic sediments drilled at IODP (International Ocean Discovery Program) Site 1438, in the nearby Amami-Sankaku basin, were studied and compared to understand the early volcanic history of the IBM, from the early Oligocene to arc rifting and opening of the Shikoku basin in the early Miocene. Grains of feldspar, pyroxene and amphibole, together with enclosed melt inclusions, glass grains and lithic fragments were separated from the sediment at intervals along the drilled cores and analyzed for major and trace elements and radiogenic isotopes. In-situ analysis on minerals, glass grains and melt-inclusions was performed using electron probe microanalysis (EPMA) and laser-ablation inductively coupled plasma mass-spectrometry (LA-ICPMS). Lithic fragments were dissolved and analyzed for elemental abundances by solution introduction ICP-MS and for isotopes by multi-collector-ICP-MS. Mineral compositions were used to calculate equilibrium magma compositions using major element matched partition coefficients, to compare with glass grains and lithic fragments. Findings show that magma compositions of the arc became progressively more water-rich with time, with periods of explosive eruption evidenced by pumice layers, stabilization of amphibole and Ca-rich plagioclase feldspar. Interspersed incompatible element-depleted mafic magmas at intermediate depths in the Site 296 core probably represent the initial intrusions associated with arc rifting which may have begun contemporaneously with arc volcanism.
Abstract Understanding the petrologic and geochemical evolution of island arcs is important for interpreting the timing and impacts of subduction and processes leading to the formation of a continental crust. The Izu–Bonin–Mariana (IBM) Arc, western Pacific, is an outstanding location to study arc evolution. The IBM first arc (45–25 Ma) followed a period of forearc basalt and boninite formation associated with subduction initiation (52–45 Ma). In this study, we present new major and trace element data for the IBM first arc from detrital glass shards and clasts from DSDP Site 296, located on the northernmost Kyushu Palau Ridge (KPR). We synthesize these data with published literature for contemporaneous airfall ash and tephra from the Izu–Bonin forearc, dredge and piston core samples from the KPR, and plutonic rocks from the rifted eastern KPR escarpment, locations which lie within or correlate with KPR Segment 1 of Ishizuka, Taylor, Yuasa, and Ohara (2011). Our objective is to test ways in which petrologic and chemical data for diverse igneous materials can be used to construct a complete picture of this section of the Oligocene first arc and to draw conclusions about its evolution. Important findings reveal that widely varying primary magmas formed and differentiated at various depths at this location during this period. Changes in key trace element ratios such as La/Sm, Nb/Yb, and Ba/Th show that mantle sources varied in fertility and in the inputs of subducted sediment and fluids over time and space. Plutonic rocks appear to be related to early K‐poor dacitic liquids represented by glasses sampled both in the forearc and volcanic fronts. An interesting observation is that the variation in magma compositions in this relatively small segment encompasses that inferred for the IBM Arc as a whole, suggesting that sampling is a key factor in inferring temporal, across‐arc, and along‐strike geochemical trends.
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