Alteration processes recorded by back‐arc mantle peridotites from oceanic core complexes, Shikoku Basin, Philippine Sea

• Published in: The island arc Vol. 30; no. 1
• Main Authors: Hirauchi, Ken‐ichi, Segawa, Izumi, Kouketsu, Yui, Harigane, Yumiko, Ohara, Yasuhiko, Snow, Jonathan, Sen, Atlanta, Fujii, Masakazu, Okino, Kyoko
• Format: Journal Article
• Language: English
• Published: Melbourne John Wiley & Sons Australia, Ltd 01.01.2021; Wiley Subscription Services, Inc
• Subjects: back‐arc basin; Chrysotile; Crystallization; Crystals; detachment fault; Dredging; Ductile-brittle transition; Dunite; Fault zones; Faults; Fluids; Localization; Mado Megamullion; Massifs; Minerals; oceanic core complex; Peridotite; Rocks; serpentinite; Serpentinization; Shikoku Basin; Silica; Silicon dioxide; Strain localization; Ultramafic materials; Ultramafic rocks; Upper mantle; Philippine Sea
• ISSN: 1038-4871; 1440-1738
• DOI: 10.1111/iar.12419

We determined the mineralogical and petrological characteristics of ultramafic rocks dredged from two oceanic core complexes: the Mado Megamullion and 23°30′N non‐transform offset massif, which are located within the Shikoku back‐arc basin in the Philippine Sea. The ultramafic rocks are strongly serpentinized, but can be classified as harzburgite/lherzolite or dunite, based on relict primary minerals and their pseudomorphs. Strongly elongated pyroxene porphyroclasts with undulatory extinction indicate high‐temperature (≥700 °C) strain localization on a detachment fault within the upper mantle at depths below the brittle–viscous transition. During exhumation, the peridotites underwent impregnation by magmatic or hydrothermal fluids, lizardite/chrysotile serpentinization at ≤300 °C, antigorite crystallization, and silica metasomatism that formed talc. These features indicate that the detachment fault zones formed a fluid pathway and facilitated a range of fluid–peridotite interactions.