Analysis of uplifting processes of Sambagwa Belt based on quartz microtextures Ikuo HARA*, Tsugio SHIOTA**, Tatsuhiko NAKAMURA* and Teijiro SAITO*** Summary On the basis of analysis of rock-structures and relations between Si-fabric and Se-fabric for metamorphic minerals such as plagioclase, garnet and amphiboles, tectonic history of Sambagawa belt of Central Shikoku may tentatively be explained as shown in Fig. 1 (cf. HARA et al., 1977, 1983, 1984, 1985a, b, 1986, present data ; MAEDA & HARA, 1984). During the Se-deformation (= deformation of Nagahama phase) began the retrogressive metamorphism of Sambagawa schists and their uplifting as nappes such as Saruta nappe I, Saruta nappe II, etc, giving rise to distinct schistosity of single set (bedding schistosity) (HARA et al., 1983, 1984, 1985a, b). The folding of the bedding schistosity occurred in two phases, Ozu phase and Hizikawa phase (HARA et al., 1977 ; FAuRE,-1981). The folding of Ozu phase was of southward vergence and associated with the formation of nappes of Sambagawa schists emplaced on the Mikabu nappe, while the folding of Hizikawa phase occurred in left-hand en echelon fashion forming upright buckle folds with wavelength of less than 20km. In order to understand movement picture of the Saruta nappes I and II during the Se-deformation, in this paper have been analyzed quartz microtextures of Sambagawa siliceous schists from the River Asemi, Central Shikoku (Fig. 2). The quartz microtextures of siliceous schists from the River Asemi, which show neither folds of Ozu phase nor those of Hizikawa phase, appear to be divided into three types, Type I, Type II and Type HI. Type I (Fig. 3a) consists mainly of quartz grains (Se-grains) strongly elongated parallel to the bedding schitosity, weakly associating subgrains and recrystallized grains in polygonal shape. In the siliceous schists with the quartz microtextures of Type I are found two types of muscovite, old muscovite and new muscovite, both flakes being preferrably oriented parallel to the bedding schistosity. New muscovite is tiny thin plate and old muscovite is referred to as "mica fish" (LISTER & SNOKE, 1984) in shape. From mica fabric it has been inferred that the quartz microtexture of Type I (= bedding schistosity) was produced under shear deformation and the shear sense was left-lateral along the WNW-ESE mineral lineation. Type II (Fig. 3b) is an equant mosaic microtexture of recrystallized grains (Se-B-grains), rarely associating remnant Se- grains. Type III (Fig. 3c) is an elongate mosaic microtexture of recrystallized grains (B-grains). The long axes of B-grains are preferrably oriented forming a distinct schistosity of single set (B-schistosty). The B-schistosity is oblique to the
bedding schistosity at low angles, which is now defined by the preferred orientation of muscovite, biotite and chlorite. Thus it may be said that the siliceous schists with the quartz microtextures of Type III are comparable with the Type II S-C mylonite after LISTER & SNOKE (1984). But, the intermediate phase between the Se-deformation related to the formation of Se-grains and that related to the formation of B-grains (B-schistosity) appears to have been static under decreasing temperature. The microtextural evolution of quartz from Type I to Type II can be ascribed to increasing strain. The deformation related to the formation of the B-schistosity appears to have also been left-lateral shear along WNW-ESE mineral lineation and to have been concentrated in the basal parts of the Saruta nappes I and II.
Fig. 1. A tentative diagram showing the relationship between tectonic history and temperature-pressure path of the biotite zone of the Sambagawa belt of Central Shikoku. Sic : Sic-deformation, Bic : Bicdeformation, Si : Si-deformation, Bi : Bi-deformation, R : R-deformation, Se : deformation of Se-stage B : B- deformation, Oz : Ozu deformation, Hz : Hizikawa deformation. P : Pressure, T : Temperature. Gc : garnet core, Gim : garnet inner mantle, Gom : garnet outer mantle. Fig. 2. Diagram showing the localities of specimens in which quartz microtextures have been analyzed. I : Type I of microtexture, II : Type II of microtexture, III : Type III of microtexture, S I : Saruta nappe I, S II : Saruta nappe II, a, b, p. specimen numbers (a : Al2, b : All, c : A10, d: A9, e: A8, f : A7, g: A6, h: A5, i : A4, j : 1905, k : 2904,1: 1904, m : 1903, n : A2, o : 1908, p : 1902), 1 : siliceous schist, 2: 5th Shirataki amphibole-schist, F : Fuyunose, S : Sakase.
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