Magmatic Response to Subduction Initiation: Part 1. Fore‐arc Basalts of the Izu‐Bonin Arc From IODP Expedition 352

Abstract

The Izu‐Bonin‐Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore‐arc basalt (FAB). FABs have mid‐ocean ridge basalt (MORB)‐like compositions, however, FAB are consistently lower in the high‐field strength elements (TiO2, P2O5, Zr) and Ni compared to MORB, with Na2O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off‐axis magmas return to more primitive compositions. Melt models require a two‐stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400 °C–1,480 °C) at relatively low pressures (1–2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle.

Subduction Initiation During Collision‐Induced Subduction Transference: Numerical Modeling and Implications for the Tethyan Evolution

“…On the other hand, the geological records for SI are even more complex (Guilmette et al, ; Hall, ; Pandey et al, ; Parlak et al, ; Patriat et al, ; Searle, ; Stern, ; Stern et al, ). The emplacement of magmatic rock is generally later, by several million years, than the exact time of SI (Arculus et al, ; Hall, ; Shervais et al, ). Thus, the timing of SI may be delayed according to the magmatic records.…”

Geochemistry and Microtextures of Vein Calcites Pervading the Izu‐Bonin Forearc and Rear Arc Crust: New Insights From IODP Expeditions 352 and 351

“…These modified seawater signatures may be related to the lack of selvedges possibly representing a protection from fluid‐rock interaction, higher extent of fluid‐rock interaction as indicated by pervasively altered boninite compared to localized alteration patterns of FAB, and/or higher proportion of antitaxial and transitional veins, whose diffusion‐fed growth facilitated host rock leaching. As a consequence of fluid‐rock interaction, the alkalinity in the fluid was increased, leading to calcite supersaturation (Sample et al, ); the Y/Ho ratios of the fluid were reduced toward the host rock value (~26) (Shervais et al, ); and the fluid δ 18 O was slightly lowered relative to seawater (Gregory & Taylor, ; Muehlenbachs & Clayton, ). The latter is indicated by clumped isotopes, which yield slightly lower parental fluid δ 18 O compositions than FAB‐hosted vein calcites.…”

“…Magmatic activity in the Izu-Bonin protoforearc initiated~52 Ma when the dense Pacific Plate subsided into the mantle along a transform fault or fracture zone and induced asthenospheric upwelling (Ishizuka, Tani, et al, 2011;Ishizuka et al, 2018;Reagan et al, 2013;Reagan et al, 2019). This early subduction initiation stage caused decompression melting that generated mainly aphyric basaltic pillow lavas, sheet flows, and hyaloclastites with FAB composition (Reagan et al, 2017;Reagan et al, 2019;Shervais et al, 2019). FAB show a similar major element geochemistry (MgO, FeO*, and CaO) as mid-ocean ridge basalts but have lower Zr/Y and Ti/V ratios (Reagan et al, 2017;Shervais et al, 2019).…”

Postmagmatic Tectonic Evolution of the Outer Izu‐Bonin Forearc Revealed by Sediment Basin Structure and Vein Microstructure Analysis: Implications for a 15 Ma Hiatus Between Pacific Plate Subduction Initiation and Forearc Extension

“…The ASB basement commonly encompasses basaltic sheet flows and dikes of high‐Mg, low‐Ti, tholeiitic basalts showing variable alteration and veining (Arculus, Ishizuka, Bogus, Gurnis, et al, ; Arculus, Ishizuka, Bogus, & the Expedition 351 Scientists, ). The geochemistry of the basement lavas (Hickey‐Vargas et al, ; Yogodzinski et al, ) indicates derivation from mantle source rocks that were more melt depleted than those of typical mid‐ocean ridges, similar to the IBM FABs (Shervais et al, ). The basaltic basement of the ASB (IODP Site U1438) is currently interpreted as part of the initial basement produced during subduction initiation (Arculus, Ishizuka, Bogus, & the Expedition 351 Scientists, ), although ages of the Amami‐Sankaku basement are concurrent with high‐Si boninite volcanism on the Bonin Ridge (Ishizuka et al, ), allowing an alternative explanation that this was the first IBM back arc basin (Reagan et al, ).…”

“…It resulted from ~52 Ma of subduction of the Pacific Plate beneath the eastern margin of the Philippine Sea plate (Reagan et al, ). IODP expedition results were published recently, notably concerning the magmatic evolution (Brandl et al, ; Yogodzinski et al, ; Hickey‐Vargas et al, ; Shervais et al, ), and the age of IBM volcanic rocks (Barth et al, ; Ishizuka et al, ; Reagan et al, ). In addition, the drilled cores provided (hemi)pelagic sedimentary‐volcaniclastic successions and tectonic structures that bear information on the tectonic evolution of the outer IBM forearc.…”