Geochemical characteristics of back-arc basin lower crust and upper mantle at final spreading stage of Shikoku Basin: an example of Mado Megamullion

Ken-ichi Hirauchi,Yumiko Harigane,Shiki Machida,Alessio Sanfilippo,Valentin Basch,Hisashi Asanuma,Hiroyuki Yamashita,Norikatsu Akizawa,Atlanta Sen,Yasuhiko Ohara,Kyoko Okino,Jonathan E. Snow,Osamu Ishizuka,Takafumi Hirata,Katsuyoshi Michibayashi,Masakazu Fujii

doi:10.1186/s40645-021-00454-3

Abstract

This paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling.

Highlights

Back-arc basins (BAB) are extensional basins formed behind subduction zones by rifting and in some cases, seafloor spreading developing on the overriding plate
The island-arc basalts (IAB)-type Back-arc basin basalt (BABB) are characterized by increased concentrations of large ion lithophile elements (LILE) and reduced concentrations of high field strength elements (HFSE): the former is linked to elemental transport by the hydrous fluid, and the latter is linked to increased mantle melting as the result of the hydrous fluid influx (e.g., Stolper and Newman 1994; Pearce and Stern 2006)
The plagioclases in the peridotite overlap in chemical composition to those in the oxide gabbro (Fig. 6b), being consistent with the fact that the plagioclases in the peridotite are a constituent mineral of the oxide gabbroic vein

Summary

Introduction

Back-arc basins (BAB) are extensional basins formed behind subduction zones by rifting and in some cases, seafloor spreading developing on the overriding plate. The subduction-related elemental input and mantle depletion associated with BABB genesis have been documented by perspectives from isotopes, halogens, and noble gasses (e.g., Hickey-Vargas 1991; Ikeda et al 1998; Pearce et al 1999; Kendrick et al 2020). These data are in good agreement with geophysical investigations that depicted a thicker and more differentiated BAB crust located close to the arc, as a consequence of enhanced mantle melting caused by hydrous fluid supply from the down-going slab (Arai and Dunn 2014; Eason and Dunn 2015). The gradual decrease in IAB signature with increasing distance from the arc reflects the maturing of the BAB (Taylor and Martinez 2003; Langmuir et al 2006)

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