HR: 0800h
AN: T51A-0443    [Abstracts]
TI: Structural Analysis of Serpentinite in the Jikkoku Pass Area, Northwestern Kanto Mountains, Central Japan
AU: * Hirauchi, K
EM: kenny_h@arsia.geo.tsukuba.ac.jp
AF: Doctoral program in earth evolution sciences, Graduate School of Life and Environmental Sciences, the University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8572 Japan
AU: Hisada, K
EM: hisadak@arsia.geo.tsukuba.ac.jp
AF: Doctoral program in earth evolution sciences, Graduate School of Life and Environmental Sciences, the University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8572 Japan
AB: Serpentinite is a product made by hydrothermal alteration of ultramafic rocks such as peridotite. It has been understood that serpentinite along a fault or a plate boundary plays a role as a lubricant for rheological properties. However, it is unknown how serpentinite actually acts through its deformation processes. The study area is located on the Jikkoku Pass area, northwestern Kanto Mountains, central Japan. Serpentinite bodies are intermittently distributed along fault boundaries. In this study, we divided serpentinite into four deformation stages (D1-D4) based on meso- and microstructures. 1) D1 stage: It is represented by block-in-matrix structure. Block sizes are various, and their longest axis is about 1 m. Shapes of blocks are almost oblate type and rounded to subangular. The long axes of blocks nearly show a same orientation, and planar structures are nearly parallel to them. Inner structures of blocks are entirely deformed, and mesh texture as a pseudomorph of olivine and euhedral chromian spinels are observed. Serpentines consist mainly of lizardite and chrysotile. Fine-gained serpentines of various sizes are randomly scattered in the matrix. 2) D2 stage: It is represented by alternation in portions of different colors and textures. Colors consist mainly of dark green and yellowish green. Intervals of this alternation vary from 1 to 3 mm. In portions of dark green, needle-shaped antigorites have a preferred orientation. Antigorites are remarkably fine-grained in some cases, and are similar to characteristics of ultramylonite. Portions of yellowish green include opaque minerals such as magnetite, forming planar structures. A chromian spinel occurs as a porphyroclast (rounded shape) together with fine-grained recrystallized antigorites. 3) D3 stage: It is represented by fracture foliation. It is formed by foliation as fracture planes that are penetratively developed with a few hundreds of ŸEm intervals. Besides, intervals of this foliation also vary on meso-scale. Fracture planes are often filled up with chrysotile. Lens-shaped grains from 100 ŸEm to 5 mm are frequently observed. D3 foliation is recognized in the matrix of D1 tectonite. In this case, blocks of D1 tectonite are extremely flattened and rounded. 4) D4 stage: It is represented by cataclastic texture. Cataclastic textures are locally (a few meters on scale) observed in serpentinite deformed in previous stage or in non-deformed serpentinite. In fault zone, foliated cataclasite (gouge) is observed, and is often characterized by S-C structure. Foliation or cracks between microbreccias are filled up with chrysotile and/or carbonate minerals. Thus, it is clear that deformation facies of serpentinite in the study area show ductile-brittle deformation. Serpentines are composed mainly of lizardite and chrysotile in non-deformed serpentinite (inclusive of a block of D1 tectonite). In deformed serpentinite, serpentines vary from antigorite (high temperature type) to chrysotile (low temperature type) in relation to a difference of deformation facies. Strikes of D1-D3 foliation coincide nearly with an elongate direction of serpentinite body. It is inferred that deformation styles of serpentinite were constrained by a mode of serpentinite body. D4 stage seems to be the deformation not related to serpentinite emplacement causing D1-D3 stages. Deformation degree of serpentinite is independent of a distance from fault boundaries, and frequently varies on meso-scale. Hence, we can conclude that serpentinite in the study area records a deformation caused by solid intrusion as well as faulting.
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8025 Mesoscopic fabrics
DE: 8030 Microstructures
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting