The arc arises: The links between volcanic output, arc evolution and melt composition

Philipp A Brandl,Morihisa Hamada,Richard J Arculus,Kyle Johnson,Kathleen M Marsaglia,Ivan P Savov,Osamu Ishizuka,He Li

doi:10.1016/j.epsl.2016.12.027

Abstract

Subduction initiation is a key process for global plate tectonics. Individual lithologies developed during subduction initiation and arc inception have been identified in the trench wall of the Izu–Bonin–Mariana (IBM) island arc but a continuous record of this process has not previously been described. Here, we present results from International Ocean Discovery Program Expedition 351 that drilled a single site west of the Kyushu–Palau Ridge (KPR), a chain of extinct stratovolcanoes that represents the proto-IBM island arc, active for ∼25 Ma following subduction initiation. Site U1438 recovered 150 m of oceanic igneous basement and ∼1450 m of overlying sediments. The lower 1300 m of these sediments comprise volcaniclastic gravity-flow deposits shed from the evolving KPR arc front. We separated fresh magmatic minerals from Site U1438 sediments, and analyzed 304 glass (formerly melt) inclusions, hosted by clinopyroxene and plagioclase.Compositions of glass inclusions preserve a temporal magmatic record of the juvenile island arc, complementary to the predominant mid-Miocene to recent activity determined from tephra layers recovered by drilling in the IBM forearc. The glass inclusions record the progressive transition of melt compositions dominated by an early ‘calc-alkalic’, high-Mg andesitic stage to a younger tholeiitic stage over a time period of 11 Ma. High-precision trace element analytical data record a simultaneously increasing influence of a deep subduction component (e.g., increase in Th vs. Nb, light rare earth element enrichment) and a more fertile mantle source (reflected in increased high field strength element abundances). This compositional change is accompanied by increased deposition rates of volcaniclastic sediments reflecting magmatic output and maturity of the arc. We conclude the ‘calc-alkalic’ stage of arc evolution may endure as long as mantle wedge sources are not mostly advected away from the zones of arc magma generation, or the rate of wedge replenishment by corner flow does not overwhelm the rate of magma extraction.

Highlights

Subduction initiation and accompanying onset of arc magmatism are critical processes in global plate tectonics
The first magmatic products related to subduction initiation in the Izu–Bonin–Mariana (IBM) subduction system (Fig. 1) are 52–48 Ma basalts exposed on the arc-ward trench wall and have been termed forearc basalts (FAB) (e.g., Ishizuka et al, 2011a; Reagan et al, 2010)
Most exposed volcanism is limited to the final phase of 25–28 Ma that is recorded by the sharp decrease in deposition rate at Site U1438 (Fig. 2)

Summary

Introduction

Subduction initiation and accompanying onset of arc magmatism are critical processes in global plate tectonics. The first magmatic products related to subduction initiation in the Izu–Bonin–Mariana (IBM) subduction system (Fig. 1) are 52–48 Ma basalts exposed on the arc-ward trench wall and have been termed forearc basalts (FAB) (e.g., Ishizuka et al, 2011a; Reagan et al, 2010). The bathymetry, crustal structure, and pre-IBM sediments in basins between the Mesozoic–Lower Cenozoic arc ridges (Amami Plateau, Daito and Oki-Daito) west of Site U1438 show no sign of compressional uplift and sediment shedding, consistent with subsidence and extensional rifting accompanying a spontaneous mode of subduction inception for the IBM system (Leng and Gurnis, 2015; Arculus et al, 2016)

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