Abstract
The magmatic character of early subduction zone and arc development is unlike mature systems. Low-Ti-K tholeiitic basalts and boninites dominate the early Izu-Bonin-Mariana (IBM) system. Basalts recovered from the Amami Sankaku Basin (ASB), underlying and located west of the IBM’s oldest remnant arc, erupted at ~49 Ma. This was 3 million years after subduction inception (51-52 Ma) represented by forearc basalt (FAB), at the tipping point between FAB-boninite and typical arc magmatism. We show ASB basalts are low-Ti-K, aluminous spinel-bearing tholeiites, distinct compared to mid-ocean ridge (MOR), backarc basin, island arc or ocean island basalts. Their upper mantle source was hot, reduced, refractory peridotite, indicating prior melt extraction. ASB basalts transferred rapidly from pressures (~0.7-2 GPa) at the plagioclase-spinel peridotite facies boundary to the surface. Vestiges of a polybaric-polythermal mineralogy are preserved in this basalt, and were not obliterated during persistent recharge-mix-tap-fractionate regimes typical of MOR or mature arcs.
Highlights
The magmatic character of early subduction zone and arc development is unlike mature systems
We examine the most important petrological characteristics of the basalt comprising Unit 1 of Amami Sankaku Basin (ASB) basalt, and explore the distinctive conditions under which it was generated
On the basis of morphology, chemical composition, and isotopic characteristics, the basalt sheet lavas and pillows comprising Unit 1 have been divided into several subunits (a–f; Fig. 1c)[26,27]
Summary
The magmatic character of early subduction zone and arc development is unlike mature systems. Prior to the establishment of a (quasi-) steady state in a given subduction zone system, the initial magmatic outputs in nascent arcs are of prime interest for their potential in revealing fundamental characteristics such as the nature of the earliest mantle wedge, temporal changes in the inputs from the subducted slab, depths from which the magmas were derived, and petrological characteristics that serve to identify examples in the geological record. Neither of these magma types is derived by partial melting of the slab. The stratigraphy exposed both along the IBM trench wall and the western flank of the Ogasawara Ridge collectively reveals peridotite succeeded by gabbro, FAB and boninite[11,15,17,18]
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