Alteration processes recorded by back-arc mantle peridotites from oceanic core complexes,Shikoku Basin,Philippine Sea |
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| Authors: | Ken-ichi Hirauchi Izumi Segawa Yui Kouketsu Yumiko Harigane Yasuhiko Ohara Jonathan Snow Atlanta Sen Masakazu Fujii Kyoko Okino |
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| Institution: | 1. Department of Geosciences, Faculty of Science, Shizuoka University, Shizuoka, Japan;2. Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan;3. Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan;4. Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana, USA;5. Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas, USA;6. National Institute of Polar Research/SOKENDAI, Geoscience Group, Tokyo, Japan;7. Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba, Japan |
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| Abstract: | 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. |
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| Keywords: | back-arc basin detachment fault Mado Megamullion oceanic core complex peridotite serpentinite Shikoku Basin |
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