Volcaniclastic sandstones record the influence of subducted Pacific MORB on magmatism at the early Izu-Bonin arc

Anders Mccarthy,Gene M Yogodzinski,Michael Bizimis,Ivan P Savov,Rosemary Hickey-Vargas,Richard Arculus,Osamu Ishizuka

doi:10.1016/j.gca.2021.01.006

Abstract

The remnant rear-arc segment of the early Izu-Bonin arc, known as the Kyushu-Palau Ridge (KPR), is a key location where magmatic outputs can be constrained during the lifetime of an island arc. We present new geochemical data for coarse-grained basaltic to andesitic volcaniclastic sandstones derived from the KPR and deposited in the Amami Sankaku Basin (IODP Site U1438, Unit III rocks) in the time period 40–30 Ma. Bulk disaggregated and cleaned volcaniclastic sandstones of Unit III at Site U1438 retain primary magmatic signatures and can be used to infer the evolution of magmatic sources of the juvenile Izu-Bonin island arc through time. A sharp increase of slab-derived components to the source of KPR magmatism developed at about 35 Ma, indicated by increasing Th/La and decreasing Sm/La, Yb/Hf and Nb/Nd. Systematic variations in trace element ratios and increasing trace element abundances in younger samples through the 40–30 Ma time window are decoupled from Hf-Nd isotope ratios, which are measurably more depleted (εHf = 16.5–15, εNd = 9.6–8.2) than boninites produced during the preceding magmatic phase and sampled in the modern Izu-Bonin forearc. Hafnium isotopic compositions in the Unit III sandstones remain little-changed and similar to the subducting Pacific Plate after 40 Ma and do not revert to highly radiogenic compositions of the Indian-type MORB mantle wedge which is reflected in highly-depleted basalts produced at Site U1438 and in the forearc (commonly εHf ≥ 18.0). The overall pattern recorded in Unit III sandstones indicates that the Pacific-type MORB slab-melt component, which was present in the preceding boninite phase of magmatism, persisted after 40 Ma, while the subducted sediment component in the boninite source was lost or significantly reduced. Variations in trace element ratios (at constant εNd and near-constant and radiogenic εHf) and in high field strength element abundances of the early Izu Bonin arc are controlled by the addition of a subducted Pacific MORB melt or supercritical fluid to the mantle wedge. A subducted MORB (slab melt) component is thus sampled throughout the early life of the Izu-Bonin arc and in the currently active Izu-Bonin arc-backarc system.

Highlights

Convergent plate boundaries produce arc magmas with distinctive chemical and petrological signatures compared to plume and ocean-ridge magmatism
We present chemical data from coarse-grained portions of volcaniclastic sandstones deposited from 40 to 30 Ma to constrain the temporal evolution of the juvenile Izu-Bonin arc following subduction initiation
Hafnium isotopic compositions of these sandstones overlap the more radiogenic boninites formed during the preceding phase of volcanism upon subduction initiation along the forearc and which trend towards PacificMORB isotopic compositions

Summary

Introduction

Convergent plate boundaries produce arc magmas with distinctive chemical and petrological signatures compared to plume and ocean-ridge magmatism. In island-arc systems, these characteristics are attributed to source compositions and related processes that involve mixing of fluids and melts derived from subducted sediment and oceanic crust into the overlying mantle wedge. The earliest products of volcanism are buried beneath younger volcanic rocks and volcaniclastic sediment that accumulated during subsequent phases of arc growth. The implication of this common geologic succession is that the early history and evolution of arcs is recorded in the initial lavas and related intrusive rocks, and in volcaniclastic sedimentary sequences deposited in basins proximal to the site of nascent arc growth The implication of this common geologic succession is that the early history and evolution of arcs is recorded in the initial lavas and related intrusive rocks, and in volcaniclastic sedimentary sequences deposited in basins proximal to the site of nascent arc growth (e.g. Egeberg et al, 1992; Bryant et al, 2003; Straub, 2003; Gill et al, 1994; Robertson et al, 2018)

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