A Note on "Concentric" Folding of Multilayered Rocks

The structural features of a "concentric" fold of a multilayered system consisting of alternating quartz-rich layers and mica-rich layers have been analysed in detail, with regard to the strain distribution and the nature of behaviour of rock material in all the layers involved in the system. On the basis of obtained data, the strain distribution and geometry of the "concentric" folds of multilayered rocks have been discussed, and a new kinematic and geometric model of "concentric" folding of multilayered rocks has been proposed. In order to understand physical conditions in which the "concentric" folding can occur in multilayered rocks, some schematic patterns of flexural folding of multilayered rocks odserved in nature have been also examined with reference to mutual relationships between them

Quartz Fabrics in a Kink Band

Quartz fabrics in a kink band in the Sambagawa crystalline schist of the Kune district, Shizuoka Pref., Central Japan, has been described and discussed with reference to the stress and strain pictures developed in the system during the phase of the deformation related to the formation of the kink band

Petrofabric Study of the Lamellar Structures in Quartz

The lamellar structures of quartz so far described in the literature have been classified in-to four types, that is, type L1, type L2, type L3 and type L4 (see p. 56). The lamellae of type L1 and of type L2 have been concluded to be relict lamellae which have been evolved from the lamellae of type L3 by annealing. As to the formation of the lamellae of type L3, the basic lamellae, deformation features of quartz grains have been analyzed in detail. The lamellae of type L3 have been regarded as a deformation band inclined at high angles to the glide line parallel or subparallel to the crystallographic c. axis Undulatory extinction bands developed in the grains containing lamellae have been classified into three types: a) undulatory extinction bands with wide width displayed as banding of parts with lamellae and those without lamellae in a grain, b) those with narrow width developed only in parts showing lamellae in a grain, and c) marginal undulatory extinction bands developed parallel or subparallel to the grain bound-aries. On the basis of detailed examination of the undulatory extinction bands of the second type, it is pointed out that the boundaries of them in the host crystal coincide with the glide plane, which is responsible for the formation of lamellae, and that the formation of the undulatory extinction bands of the second type may be attributed to inhomogeneous rotation of the secondary glide plane parallel or subparallel to the lattice plane (0001) in each of intervening spaces bounded by the glide plane responsible for the formation of lamellae

Dynamic Interpretation of the Simple Type of Calcite and Quartz Fabrics in the Naturally Deformed Calcite-Quartz Vein

The calcite and quartz fabrics in a deformed calcite-quartz vein found in the Sangun metamorphic formation at Hirose, Yamaguchi Pref., Western Japan, have been analysed, and the syn-chronization of both fabrics has been examined. The stress systems in the deformations, which produced the observed calcite fabrics, have been established in the light of what has been determined in the fabrics of experimentally deformed single crystals of calcite and marbles, and the quartz fabrics have been inter-preted with reference to those stress systems. Small circle girdle pattern with angular radius of ca. 30° in the quartz lattice fabric in the vein has been clearly correlated with compressive stress directed parallel to the center of the small circle girdle, accompanied with almost equal amount of extension in all of the directions within the plane normal to the compression axis. It is pointed out that the c-axes of quartz may be stable at ca. 30° to the compression axis or at ca. 60° to the tension axis under such stress condition. The lamellae in quartz grains have been interpreted as the structure of late stage unrelated to the deformation which induced the lattice and dimensional fabrics of the quartz grains. The regular positional relationships between the c-axis of the host crystal and the corresponding lamellar pole and between the c-axis of the lamellar crystal and that of the corresponding host crystal have been recognized in the composite diagrams of those axial data for the quartz grains containing lamellae. It is pointed out that those relationships will be used to synthesize the stress system in the deformation related to the formation of lamellae

The Quartz Lamella Fabrics in a Concentric Fold

Lamellar structures in quartz grains occurring in a concentric fold (multiple-layered system made of the alternation of mica-rich layers and quartz-rich layers) found in the Sangun metamorphic formation at Atetsu, Okayama Pref., Southwest Japan, have been described. Two types of lamellar structure have been found in quartz grains of the quartz-rich layers, that is, the Böhm lamellae and the deformation bands. The crystallographic location of the deformation bands has been tentatively determined on the basis of assumption that the rotational axis of the change in the lattice orientation from the band to the host crystal coincides with either the a-axis or the a*-axis. At this time the deformation bands show a wide range of the crystallographic location, but many of them are inclined at high angles to the c-axis. Quartz grains containing the deformation bands are found in very sharply restricted parts in the fold, and these parts all correspond approximately to the "knee of fold". The distribution of quartz grains containing the Böhm lamellae is also sharply restricted in two narrow zones through the fold, which are approximately placed on the knee of fold, though the distribution area is more enlarged to the limbs than that for the deformation bands. The Böhm lamellae occurring in the central part of the knee are different in geometric properties from those occurring in the marginal parts of the knee and in the limbs. Namely, most of the former is inclined at high angles to the c-axis, and, in the diagram for the pole of lamellae and the c-axis for the former, great circles containing these axial data for individual grains have a constant direction and sense, while the latter shows a wide range of the crystallographic location and the diagram in question does not show any regularity. In the specimens described so far in the literature, the Böhm lamellae referred to the type of the former and these to the type of the latter appear to be commonly coexisting. The rule for establishing the directions of the principal normal stresses developed in the system concerned during the deformation related to the formation of the lamellae, previously introduced by the author (1961a and 1963), has been successfully used also for the present specimen. The cause of the fact that quartz grains containing the deformation bands or the Böhm lamellae are found in some very sharply restricted parts in the fold (especially the "knee of fold") has been also discussed with reference to the mechanics of rock folding in the final phase of meta-morphic deformation

Studies on the Structure of the Ryoke Metamorphic Rocks of the Kasagi District, Southwest Japan

Structural features on various scales of the Ryoke metamorphic rocks of the Kasagi district, Southwest Japan, were described and discussed, and the genetic history of geologic structure of the district was examined in connection with the plutonic and metamorphic history

A Note on Deformation Bands in Quartz

Variation in width of deformation bands in quartz in four specimens collected from the Sangun crystalline schist of the Nakase district, Hyôgo Pref., and the Sambagawa crystalline schist of the Kôtsu district, Shikoku, has been examined. The band width is considerably variable. Within a restricted domain (e. g. each specimen), however, it is fairly constant, showing a single marked maximum (the most frequent width) in the variation diagram of the band width for each specimen. Both the most frequent width and the maximun width are different between four specimens, and the former is proportional to the latter, that is, the former increases with increase in the latter. The band width does not appear to be always dependent upon the grain size. The band width may be dependent upon the rate and temperature of deformation, that is, it may increase with increase in temperature and decrease in strain rate, as is the case of creep deformation of metalic crystals. The present knowledges of some other properties of the deformation bands in quartz are also briefly described and discussed. Three types of syntectonic recrystallization of quartz which are also referred to three types of paracrysta-lline deformation of quartz have been distinguished as follows: 1) Type I － it is shown by poly-gonization of bending lattice (e.g. band-boundaries)and formation of new grains which do not show preferred lattice and dimensional orientation. 2) Type II － the shapes of recrystallized quartz grains are strongly elongated parallel to the direction of mass elongation of the system concerned but the reorientation of the c-axes (lattice) of those grains are scarcely induced. And 3) Type III － the formation of preferred lattice and dimensional orientation. Deformation bands do not appear to be formed in quartz grains giving rise to the syntectonic recrystallization of Types II and III

Deformation Bands in Calcite and Quartz Crystals

Deformation bands in some of calcite and quartz grains in a naturally deformed calcite-quartz vein found in the Sangun metamorphic formation near the Kawayama mine, Yamaguchi Pref., Western Japan, have been described. The deformation bands in calcite are inclined at moderate angles to the active glide plane, a {0112} plane (e1), and the axis of the lattice rotation in the band is parallel to the glide plane and normal to the glide direction, the edge [e1: r3]. The angle between the c-axis of the host crystal and the surface of the band boundary is between 14° and 44°, and the angle between the glide direction and the surface of the band boundary is between 40° and 70°. For many of the deformation bands the lattice in the band approaches the completely twinned condition for e1, while in the host crystals the lattice is about 10 per cent twinned on e1. The sense of lattice rotation in the band is opposed to that for twin gliding on e1. The maximum value of the angle of lattice rotation in the band is 87°. Generally, the band boundaries are distinctly displayed by the sharp change in the trend of e1 and in the degree of e1 twinning, but not by such a manner as that e1 twinning progressively increases toward the margins of the band. For the deformation bands, in which the angle of lattice rotation exceeds 44°, the band boundaries are commonly displayed as a sharp discontinuous plane whose crystallographic location can be directly measured by the U-stage. The crystallographic location of the deformation bands in quartz has been tentatively determined on the basis of assumption that the rotational axis of shift in the lattice orientation from the host to the band coincides with either the a-axis or the a*-axis. At this time many of the quartz deformation bands are inclined at high angles to the c-axis. They are approximately tautozonally oriented and are grouped in two sets of planes in the system concerned. The rule for establishing the directions of the principal stresses developed in the system concerned during the deformation related to the formation of the deformation bands, previously introduced by the senior author (1961a and 1963), has been successfully used also for the present specimen. It has been clarified that the sense of lattice rotation in the band for quartz deformation bands is opposed to that for the calcite deformation bands of the present type in the same stress system.今村外治教授退官記念特集

Garnets in a Biotite Schist from Southwestern Part of Mikawa Plateau, Central Japan

The garnets in the analysed biotite schist consist of two types, euhedral garnets showing equiaxial euhedral shape and anhedral garnets showing ellipsoidal shape whose longest axes are oriented parallel to the schistosity. The euhedral garnets have inclusion-rich radial zones separated by inclusion-poor sectors (or sectors of clear garnet) which are oriented along the rational crystallographic directions, while the ellipsoidal garnets have homogeneously distributed inclusions which are oriented parallel to the schistosity in their matrix. The euhedral garnets are rarely included in the ellipsoidal garnets, suggesting that the latter postdates the former. Antipathetic relation in zonal variation between MnO and FeO are found in the garnets of both types. In the euhedral garnets normal zoning and reverse zoning develop in the cores and outermost margins respectively, while in the ellipsoidal garnets zonal variation (reverse zoning) developes only in the outermost margins. It has been assumed that, during the phase of appearance of the ellipsoidal garnets, their nucleation and growth occurred independently upon the pre-existing euhedral garnets and the overgrowth of the latter hardly occurred and that the appearance of the ellipsoidal garnets began during the phase of growth of the parts of euhedral garnets at which (= near the rim) MnO is minimum.The present study is financially partly supported by the Grant in Aid for Scientific Re­searches of the Ministry of Education

Epidote Porphyroblasts in Sambagawa Schists of Central Shikoku, Japan

Epidote grains are commonly found as inclusions in plagioclase porphyroblasts of the schists in the biotite zone of the Sambagawa belt of Central Shikoku, Japan. Two types of fabrics of inclusion epidotes are observed: Type I) though the growth of plagioclase porphyroblasts occurred under non-deformational condition (cf. TAKAGI and HARA, 1979; MAEDA and HARA, 1983a and b), the epidote grains show preferred lattice and dimenssional orientation, in individual plagioclase porphyroblasts forming a single set of schistosity and of lineation. And Type II) the epidote grains occur with random lattice and dimensional orientation in individual plagioclase porphyroblasts, developing as porphyroblasts which contain other metamorphic minerals as inclusions. Only in the schists in which inclusion epidote has the fabric of Type II, matrix epidote appears to develop as porphyroblasts. The appearance of the epidote grains as por-phyroblasts under non-deformational condition appears to have begun earlier to some extent than that of plagioclase porphyroblasts