HR: 0800h
AN: MR11A-0930    [Abstracts]
TI: "Fossil" bright layer recorded in the low-P/T metamorphic rocks
AU: * Terabayashi, M
EM: tera@eng.kagawa-u.ac.jp
AF: Department of Safety Systems Construction Engineering, Kagawa University, Kagawa, 761-0396 Japan
AU: Yamamoto, H
EM: hyam@sci.kagoshima-u.ac.jp
AF: Department of Earth and Environmental Sciences, Kagoshima University, Kagoshima, 890-0065 Japan
AU: Kitajima, K
EM: saburo@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, 152-8551 Japan
AB: The "fossil" (geological time) bright layer was recognized in the Cretaceous low-P/T Ryoke metamorphic rocks in the Iwakuni-Yanai area, southwest Japan. Silicified pelitic schists distribute as layers or lenticular bodies several to fifteen meters in thickness, and they are restricted in the greenschist facies conditions within structurally vertical thickness about one kilometer. Silicified pelitic schist is mainly composed of fine-grained quartz and minor muscovite and biotite, and some of colored minerals are decolored by alteration more or less. The boundary between silicified layer and underlying pelitic schist is fairly distinct but that between the overlying pelitic schist is rather gradual. Quartz veins crossing high angles with schistosity were preferentially developed in the silicified rocks, while schistosity-parallel quartz veins, which underwent ductile flow, were observed in the pelitic schist. En echelon quartz vein and fishnet-like quartz veins are characteristic of silicified rocks. The mode of occurrences of quartz veins indicates that silicified rocks are competent relative to underlying pelitic schist. Fluid inclusion studies were conducted from two kinds of quartz-filled veins: crosscutting foliation in silicified pelitic schist and foliation-parallel in pelitic schist. Fluid inclusions in quartz from a vein crosscutting foliation in silicidied pelitic schist occur as isolated individual inclusions or clusters with preferred spatial arrangement. The isolated inclusions display negative crystal geometries, and are generally range in size from 5 to 10 $\mu$m, with some inclusions up to 20 $\mu$m across. Fluid inclusions in quartz from a foliation-parallel vein are rounded and usually small about a few $\mu$m. Homogenization of the vapour and liquid phases to a single liquid phase occurred at temperatures (Th) between 275 and 330 $\deg$C except in rare instances. The value is considered to be close to the condition of vein formation. Importantly, the homogenization-temperature distributions for vein quartz in silicifeid rocks and pelitic schists are similar. The Raman spectra for the vapour phase within water-rich inclusions indicate that these inclusions contain CO$_{2}$ alone or CO$_{2}$ and CH$_{4}$, in varying molar proportions. The volatile phase is dominated by CO$_{2}$ with small amount of N$_{2}$. All these observations suggest that the silicification of the Ryoke pelitic rocks was a result of silica precipitation from fluids and occurred at the fore-arc region before or coincident with the low-P/T metamorphism. The subducting slab releases fluids into wedge mantle. Deeper fluids may be channelized updip. Silicification occurred in the middle crust and caused sealing process to form impermeable-competent layers. Fluids may likely concentrate just below the bright layer.
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 8159 Rheology--crust and lithosphere
DE: 8164 Stresses--crust and lithosphere
DE: 3660 Metamorphic petrology
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting