Shigeru Takizawa

DILATANT CLAYEY MICROSTRUCTURE IN THE BARBADOS DECOLLEMENT ZONE

Abstract Dilatant clayey microstructures in cores from ODP Leg 156, which drilled the basal decollement zone of the northern Barbados accretionary prism, are obtained from freeze-dried samples. The microstructure in the scaly fabric portion shows heterogeneous strain characterized by domains with strong preferred orientation of clay minerals and with sigmoidal, spindle, or tube-shaped coarse pores in fabric domains with random particle orientation. These dilatant microstructures have not been observed in clayey sediments from the Barbados decollement zone or other active shear zones. The dilatancy of the microstructure might be formed under the condition of high pore-fluid pressure in the course of shearing clays and due to tectonic stress.

The chemistry of allanite from the Daibosatsu Pass, Yamanashi, Japan

Abstract The crystal structure of allanite from granitic pegmatite, the Daibosatsu Pass, Yamanashi, Japan, has been refined under the constraint of chemical composition determined by electron microprobe analysis of rare earth elements. Back-scattered-electron images and X-ray element maps of the allanites show that each of their crystal grains has chemically homogeneous distribution of major elements. A typical formula for the chemistry is: (Ca0.920 ⃞0.080)∑1.000(La0.238Ce0.443Pr0.048Nd0.100Sm0.019Th0.042Mn0.008 ⃞0.102)∑1.000(Al0.607Fe3+0.317Ti0.076)∑1.000(Al1.000)(Fe2+0.543Fe3+0.365Mn0.055Mg0.037)∑1.000(SiO4)(Si2O7)O(OH). The crystal structure of allanite, monoclinic, a 8.905 (1), b 5.7606 (5), c 10.123 (1) Å, β 114.78°(1), space group P21/m, Z = 2, has been refined to an unweighted R factor of 3.46% for 1459 observed reflections. Although the H atom position was not determined on the Difference-Fourier map, inspection of the bond valence sums demonstrates that the H atom is uniquely located at the O10 atom and involved in a hydrogen bond to O4. A systematic examination as to crystal chemistry of allanites suggests that the isolated SiO4 tetrahedron has the largest distortion of three kinds of the tetrahedron containing Si2O7 groups in the allanite structure. This observation is common to the epidote group minerals, while the larger distortion of A2 sites caused by occupancy by REE in allanites contrasts with the smaller one of A sites in other epidote group minerals. In the allanite groups the bond angles between the O10−H bond and hydrogen bond H…O4 are found to range from 170 to 180°. Compilation of the chemical compositions of the title allanite and the others from granitic rocks, Japan, which reveals Th-incorporation as the coupled substitution of 3Th4+ + ⃞ (vacancy) ⇌ 4REE3+, provides an explanation for the observation that higher Th concentrations characterize allanites from the island arcs. The ternary Al2O3-Fe2O3-∑REE diagram illustrates that allanites are grouped, according to their origins, into three classes suggestive of tectonic backgrounds for the crystallization localities; (1) intracontinental, (2) island arc and (3) continental margin.

Delineation of deformation grades of low-strain granitoids using assemblages of elementary deformation textures

Abstract Based on microscopic observations of the Miocene Tokuwa granodiorite, central Japan, a method of delineating deformation grades of low-strain granitoids has been derived which utilizes assemblages of elementary deformation textures (EDTs) such as subdomains in quartz, bent twins in plagioclase and kink-bands in biotite. Although the granodiorite is comprised of equant grains and shows no evidence of deformation to the naked eye except for brittle fractures, it shows a variety of EDTs at the microscale. In thin section, assemblages of EDTs belonging to quartz, plagioclase or biotite are used for deformation grading of the corresponding rock. The deformation grading by EDTs based on quartz correlates with deformation grading based on plagioclase or biotite. Eigenvalues derived from a principal component analysis indicate that the first principal component explains approximately three-quarters of the variance of deformation grading by EDTs based on quartz, plagioclase, and biotite. The deformation grading by EDTs using quartz correlates well with the first principal component score. These results indicate that the deformation grading by assemblages of EDTs based on quartz, as well as on the first principal component score, is useful for deformation grading of low-strain granitoids.