[S-CG46] New Insights of Fluid-Rock Interactions: From Surface to Deep Subduction Zone

[SCG46-P02] Metasomatic alteration and deformation recorded in a tectonic mélange from the Nagasaki metamorphic rocks, Nomo Peninsula, Japan

*Matsutake Koumi¹, Ken-ichi Hirauchi¹, Asuka Yamaguchi² (1.Department of Geosciences, Faculty of Science, Shizuoka University, 2.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:metasomatism, mantle wedge, Nagasaki metamorphic rocks, tectonic mélange

The boundary zone between the slab and the mantle wedge is known to be a structurally and lithologically complex zone (Bebout and Penniston-Dorland, 2016). In this zone, various types of fluid-rock interactions including metasomatism occur, and rocks derived from the slab and the wedge are tectonically mixed to form a mélange (Bebout and Barton, 2002; Tarling et al, 2019). To understand how such a tectonic mélange zone forms in association with metasomatic alteration and deformation at the slab-mantle interface zone, we conducted a structural and geochemical analyses of a tectonic mélange exposed in the Nagasaki metamorphic rocks, Nomo Peninsula, Japan. The tectonic mélange located between basic schist and serpentinite exhibits a block-in-matrix structure, characterized by lenticular blocks of mafic rocks (e.g., basic schist and albite-rich rock) enclosed in a basic matrix. The formation of blocks seems to be related to simultaneous extensional (mode I) and extensional-shear (mode I-II) fracturing (i.e., fault-fracture meshes; Sibson, 2017) at supra-lithostatic pore fluid pressures. The matrix consists mainly of actinolite, chlorite, epidote, and albite, with minor kaersutite, edenite and Ca-carbonate. Mass-balance calculations using the isocon method indicate that for the matrix-rich mélange, CaO is lost and Cr, MgO, and Na₂O are gained relative to the basic schist. The source of Cr and MgO is likely serpentinization of the mantle peridotites, while that of CaO is unclear. Some parts of the matrix form an anastomosing network of aligned aggregates of actinolite and chlorite with minor muscovite. These results suggest that the fault-fracture mesh development in the slab-derived oceanic crust materials was followed by infiltration of mantle-wedge-derived fluids into the fractures, and led to precipitation of mechanically weak hydrous minerals, and then strain localization into the metasomatic matrix.


References: Bebout and Barton (2002), Chemical Geology, 187, 79-106. Bebout and Penniston-Dorland (2016), Lithos, 240-243, 228-258. Sibson (2017), Earth, Planets and Space, 69, 113. Tarling et al (2019), Nature Geoscience, 12, 1034-1042.