Flow kinematics in the deeper part of a subduction channel, as inferred from inclusion trails in plagioclase porphyroblasts from the Sambagawa metamorphic rocks, south-west Japan |
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| Authors: | T TAKESHITA K ISHII K KANAGAWA |
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| Institution: | Department of Earth and Planetary Sciences, Hokkaido University, Sapporo 060-0810, Japan (); Department of Earth and Life Sciences, Osaka Prefecture University, Sakai 599-8531, Japan; Department of Earth Sciences, Chiba University, Chiba 263-8522, Japan |
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| Abstract: | The subduction and exhumation of accretionary prism metasedimentary rocks are accompanied by large‐strain ductile deformations which may be recorded in microstructures. Porphyroblast microstructures have been a key to unravel the kinematics in such deformed belts. Shape‐preferred orientation (SPO) of epidote and amphibole inclusions that define S‐shaped trails in prograde cores of plagioclase porphyroblasts were analysed from the high‐P/T Sambagawa metamorphic rocks. Inclusions are found to be elongate parallel to the 010] and 001] directions, respectively, and their long‐axis orientations define an internal foliation Si (best‐fit great circle) and lineation Li (maximum on the Si). S‐shaped inclusion trails in the orthogonal sections do not exhibit the same geometries, but rather are grouped into two types, where the foliation intersection axes (FIAs) are nearly perpendicular and parallel to Li, respectively. These two types of S‐shaped inclusion trails are seen in the sections inclined at low and high angles to the Li, respectively. However, the latter type commonly consists of composite trails, where the Si is first rotated about an FIA perpendicular to the Li (i.e. unique axis), and then about an FIA parallel to the Li. The S‐shaped inclusion trails are interpreted to have formed by the successive overgrowth of matrix minerals and rotation of the plagioclase porphyroblast cores about a unique axis in non‐coaxial deformation. The rotation of Si about an FIA nearly parallel to the Li is perhaps an apparent rotation, caused by the deflection of foliation around the growing prismatic plagioclase grain prior to inclusion into the porphyroblast. This study has for the first time documented the 3‐D geometry of S‐shaped inclusion trails in porphyroblasts from accretionary prism metasedimentary rocks and identified their origin, which helps to understand the flow kinematics in the deeper part of a subduction channel. |
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| Keywords: | accretionary prism 3-D geometry of S-shaped inclusion trails plagioclase porphyroblast |
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