[S-CG42] Hard-Rock Drilling Science: Continental and Deep-Sea Drilling, and Ophiolite

[SCG42-05] Deformation associated with compositional change of plagioclase in gabbroic rocks in Mado Megamullion, the Shikoku Basin, Philippine Sea

★Invited Papers

*Kohei Nimura¹, Katsuyoshi Michibayashi^1,2, Yumiko Harigane³, Yasuhiko Ohara^1,2,4 (1.Department of Earth and Planetary Sciences, GSES, Nagoya University, 2.Volcanoes and Earth’s Interior Research Center, IMG, JAMSTEC, 3.Research Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, 4.Hydrographic and Oceanographic Department of Japan)

Keywords:oceanic core complex, lower crust, gabbroic rocks, plagioclase

The understanding of lower crustal deformation within oceanic core complexes (OCCs) has progressed dramatically over the last two decades with improvements in microstructural analyses. OCCs are domal bathymetric highs characterized by surface corrugations oriented parallel to the spreading direction and are dominant lower crustal and upper mantle rock such as serpentinized peridotite and gabbroic rock.This study evaluates the lower crustal deformation within OCCs, as revealed by microstructural analyses. The gabbroic rocks from the OCCs experienced the following high-temperature to low temperature deformation events: (i) deformation by crystal plasticity and diffusion creep, (ii) semi-brittle to brittle deformation associated with pervasive fluid-rock interaction, and (iii) dissolution-precipitation creep, reaction softening, and cataclastic flow. Since the microstructures of gabbroic rocks are possibly overprinted by these events, recent improvements in microstructural analyses have facilitated an understanding of the deformation mechanisms in gabbroic rocks with the help of experimental studies. However, the relationship between the deformation and compositional change processes of minerals such as plagioclase remains ambiguous because of the difficulty in combining deformation textures and their compositional variations.
Here, we introduce new results obtained using scanning electron microscopy (SEM) combined with both electron backscatter diffraction (EBSD) and electron dispersive spectroscopy (EDS) for plagioclase in deformed gabbroic rocks sampled from the transform fault zone of the Mado Megamullion (dive site YK20-18S-6K-1569) located in the Shikoku Basin, Philippine Sea. The SEM-EBSD-EDS analysis of plagioclase from the Mado Megamullion can be used to understand the deformation associated with compositional changes. Plagioclase was classified into five types: porphyroclasts with high anorthite content [An = Ca/(Ca+Na)], porphyroclasts with low An, matrices with high An, matrices with low An, and veinlets with low An. These types suggest that plagioclase has experienced either recrystallization without compositional changes or recrystallization with compositional changes possibly due to the presence of fluids. This study highlights the deformation associated compositional changes occurred heterogeneously.