Faculty Profiles - MICHIBAYASHI Katsuyoshi

Spatial variations in antigorite fabric across a serpentinite subduction channel: Insights from the Ohmachi Seamount, Izu-Bonin frontal arc Reviewed

Ken-ichi Hirauchi, Katsuyoshi Michibayashi, Hayato Ueda, Ikuo Katayama

EARTH AND PLANETARY SCIENCE LETTERS Vol. 299 ( 1-2 ) page： 196 - 206 2010.10

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We conducted a microstructural study of samples from a natural serpentinite shear zone in the Ohmachi Seamount, Izu-Bonin frontal arc. The serpentinite samples consist mainly of columnar antigorite grains that show marked variations in texture from two approximately orthogonal sets of grains (interpenetrating) to aligned (schistose) forms. Because the two types of grains have similar compositions, these textural differences are interpreted to reflect the existence of a strain gradient toward a plate interface in a subduction zone. The crystal-preferred orientation (CPO) of antigorite with interpenetrating texture is almost randomly oriented, whereas in the case of schistose texture the CPO shows a typical [010](001) pattern. We also found that with increasing intensity of schistosity, the polarization plane of Vs(1) for antigorite grains becomes aligned parallel to the flow plane, consistent with a plane oriented normal to the maximum concentration of slow antigorite c-axes. This configuration results in seismic anisotropy that is approximately five times higher than that for olivine grains. These findings indicate that if a serpentinite layer on the plate interface attains large bulk shear strains (gamma>similar to 2), the resultant alignment of antigorite grains within the layer strongly influences the orientation and magnitude of seismic anisotropy in the mantle wedge, depending on the dip angle of the subducting slab. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2010.08.035

Uppermost mantle anisotropy beneath the southern Laurentian margin: Evidence from Knippa peridotite xenoliths, Texas Reviewed International coauthorship

Takako Satsukawa, Katsuyoshi Michibayashi, Urmidola Raye, Elizabeth Y. Anthony, Jay Pulliam, Robert Stern

GEOPHYSICAL RESEARCH LETTERS Vol. 37 ( 20 ) 2010.10

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

Peridotite xenoliths from southern Texas consist of spinel lherzolite, harzburgite and minor dunite. Based on phase relations and temperature of equilibration, Knippa xenoliths come from the uppermost mantle, 40-70 km deep. Knippa xenoliths provide rare snapshots of upper mantle processes and compositions beneath south-central Laurentia. They preserve olivine a-axis fiber fabrics with a strong concentration of [100] and girdles of [010] and [001]. Assuming a lithospheric mantle having a horizontal flow direction parallel to fast directions, the mantle lithospheric fabric revealed by the xenoliths mostly explains the magnitude of observed shear-wave splitting observed along the southern margin of the Laurentian craton. Citation: Satsukawa, T., K. Michibayashi, U. Raye, E. Y. Anthony, J. Pulliam, and R. Stern (2010), Uppermost mantle anisotropy beneath the southern Laurentian margin: Evidence from Knippa peridotite xenoliths, Texas, Geophys. Res. Lett., 37, L20312, doi:10.1029/2010GL044538.

DOI： 10.1029/2010GL044538

Grain-size-sensitive deformation of upper greenschist- to lower amphibolite-facies metacherts from a low-P/high-T metamorphic belt Reviewed

Takamoto Okudaira, Daisuke Ogawa, Katsuyoshi Michibayashi

TECTONOPHYSICS Vol. 492 ( 1-4 ) page： 141 - 149 2010.9

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

To identify the dominant deformation mechanism in continental middle crust at an arc-trench system, we used an SEM-EBSD system to measure the lattice-preferred orientations of quartz grains in fine-grained metachert from the low-grade (chlorite and chlorite-biotite zones) part of the low-P/high-T Ryoke metamorphic belt, SW Japan. Quartz c-axis fabrics show no distinct patterns related to dislocation creep, although the strain magnitudes estimated based on deformed radiolarian fossils are high enough that a distinct fabric might be expected to have formed during deformation. Fabric intensities are very low, indicating a random distribution of quartz c-axes. Quartz grains are equant in shape and polygonal, and free of intracrystalline plasticity. These observations suggest that the dominant deformation mechanism in the metacherts was grain-size-sensitive flow (diffusion creep accompanied by grain-boundary sliding) rather than dislocation creep, possibly reflecting the relatively low strain rate or low flow stress compared with that in high-strain zones. The development of grain-size-sensitive flow in metamorphic tectonites at mid-crustal conditions would result in a significant decrease of the rocks strength of the continental middle crust. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2010.06.002

Effect of grain growth on cation exchange between dunite and fluid: implications for chemical homogenization in the upper mantle Reviewed

Tomohiro Ohuchi, Michihiko Nakamura, Katsuyoshi Michibayashi

CONTRIBUTIONS TO MINERALOGY AND PETROLOGY Vol. 160 ( 3 ) page： 339 - 357 2010.9

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER

The effect of grain growth on the cation exchange between synthesized forsterite aggregates (i.e., dunite) and nickel-rich aqueous fluid was evaluated experimentally at 1.2 GPa and 1,200A degrees C. The grain boundary (GB) migration caused nickel enrichment in the area swept by the GBs in a fashion similar to that reported for stable isotope exchange in the quartz aggregates. The progress of the grain growth resulted in an increase in the average nickel concentration in the dunites of up to similar to 80 times that was calculated for a system having stationary GBs. The overall diffusivity of the nickel along the wet GBs and interconnected fluid networks was found to be 6.5 x 10(-19)-6.7 x 10(-18) m(3)/s, which is 4-5 orders of magnitude higher than the grain boundary diffusivity in the dry dunite. These results show that the grain growth rate is a fundamental factor in the evaluation of the time scale of chemical homogenization in the upper mantle.

DOI： 10.1007/s00410-009-0481-7

Preliminary results of elastic wave velocity measurements on a serpentinite obtained from the deep-sea floor Reviewed

Grain growth experiments of quartz aggregates in agate under 1 GPa and 800°C Reviewed

Imoto Hisashi, Michibayashi Katsuyoshi

Geoscience reports of Shizuoka University Vol. 37 ( 0 ) page： 13 - 19 2010.7

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Institute of Geosciences, Faculty of Science, Shizuoka University

We investigated grain growth process of quartz aggregates in agate. The agate samples were annealed in a piston-cylinder solid medium apparatus (MK65S) of Shizuoka University. The experiments were performed at temperature of 800°C, confining pressure of 0.5 GPa for 0.5 hr, 24 hr and 66 hr, respectively. The agates were well crystallized during experiments. Mean grain sizes increased from a few to ten microns with increasing time. Aspect ratios were nearly constant at around 0.7 (b/a; b < a). Shape preferred orientation (SPO) of quartz grains were developed with increasing time. Crystal-preferred orientation of c-axes, which were primarily girdled, appears to control the development of SPO with time.

DOI： 10.51053/shizuoka.37.0_13

CiNii Research

Shinkai Yuri, Watanabe Tohru, Michibayashi Katsuyoshi, Harigane Yumiko, Ohara Yasuhiko

Geoscience reports of Shizuoka University Vol. 37 ( 0 ) page： 27 - 34 2010.7

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Institute of Geosciences, Faculty of Science, Shizuoka University

Elastic wave velocities were measured on a deep seafloor serpentinite sample at room temperature and confining pressures of up to 180 MPa, using a high-pressure apparatus at University of Toyama. The sample was collected from a dredge site D17 at the Godzilla Megamullion, Parece Vela Rift, Phillipine Sea during the cruise KR07-02 (R/V Kairei). The serpentinite sample has the density of 2.37 g/cm3 and consists mainly of lizardite, chrysotile and magnetite, showing a banded structure with a moderately developed lineation and foliation. Mutually orthogonal axes were set for velocity measurements; the x-axis is parallel to the lineation, and the z-axis normal to the foliation. Lizardite mainly has a mesh texture, whereas fibrous chrysotile fills veins, most of which are subperpendicular to the lineation. The compressional wave velocity at 180 MPa is the fastest (4.56 km/s) in the y-axis direction, the slowest (3.97 km) in the direction parallel to the lineation (x-axis direction), and intermediate (4.25 km/s) normal to the foliation (z-axis direction). The azimuthal anisotropy is 13.85% at 180 MPa. The shear wave velocity at 180 MPa is 2.07~2.35 km/s. The shear waves oscillating parallel to the yz-plane have the fastest velocity, and that normal to the yz-plane the slowest. The polarization anisotropy is thus strong along the y- (9.35%) and z- (12.24%) axes, whereas weak along the x-axis (1.39%). Based on microstructural examinations, observed velocity anisotropy may be mainly controlled by the orientation of chrysotile veins rather than the banded structure.

DOI： 10.51053/shizuoka.37.0_27

CiNii Research

Grain growth experiments of quartz aggregates in flint under 1GPa and 800℃ Reviewed

Ueda Naohiko, Michibayashi Katsuyoshi

Geoscience reports of Shizuoka University Vol. 37 ( 37 ) page： 21-26 - 26 2010.7

Exsolution of dolomite and application of calcite-dolomite solvus geothermometry in high-grade marbles: an example from Skallevikshalsen, East Antarctica Reviewed

H. Mizuochi, M. Satish-Kumar, Y. Motoyoshi, K. Michibayashi

JOURNAL OF METAMORPHIC GEOLOGY Vol. 28 ( 5 ) page： 509 - 526 2010.6

Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY-BLACKWELL

Calcite-dolomite solvus geothermometry is a versatile method for the estimation of metamorphic temperature because of its simplicity. However, in medium- to high-grade metamorphic rocks the accuracy of estimating temperature by the integration of unmixed dolomite and calcite is hampered by the heterogeneous distribution of unmixed dolomite, difficulties in distinguishing between preexisting and exsolved dolomite and demarcating grain boundaries. In this study, it is shown that calcite-dolomite solvus thermometry can be applied to calcite inclusions in forsterite and spinel for the estimation of peak metamorphic temperature in granulite facies marbles from Skallevikshalsen, East Antarctica. The marbles are comprised of a granoblastic mineral assemblage of calcite + dolomite + forsterite + diopside + spinel + phlogopite +/- apatite, characteristic of granulite facies metamorphic conditions. Forsterite, spinel and apatite frequently contain 'negative crystal' inclusions of carbonates that display homogeneously distributed dolomite lamellae. On the basis of narrow ranges of temperature (850-870 degrees C) recorded from carbonate inclusions compared with the range from matrix carbonate it is regarded that the inclusion carbonates represent a closed system. Furthermore, this estimate is consistent with dolomite-graphite carbon isotope geothermometry, and is considered to be the best estimate of peak metamorphic temperature for this region. Matrix calcite records different stages of retrograde metamorphism and re-equilibration of calcite that continued until Mg diffusion ceased at similar to 460 degrees C. Electron backscattered diffraction (EBSD) results together with morphological features of unmixed coarse tabular dolomite suggest anisotropic diffusion and mineral growth are influenced by crystallographic orientation. Identification of sub-grain boundaries and formation of fine-grained unmixing in calcite rims suggest the presence of grain boundary fluids in the late retrograde stages of metamorphic evolution. These results, thus, demonstrate the usefulness of carbonate inclusion geothermometry in estimating the peak metamorphic temperatures of high-grade terranes and the application of EBSD in understanding the unmixing behaviour of minerals with solid solutions.

DOI： 10.1111/j.1525-1314.2010.00877.x

Two Contrasting Fabric Patterns of Olivine Observed in Garnet and Spinel Peridotite from a Mantle-derived Ultramafic Mass Enclosed in Felsic Granulite, the Moldanubian Zone, Czech Republic Reviewed

Akira Kamei, Masaaki Obata, Katsuyoshi Michibayashi, Takao Hirajima, Martin Svojtka

JOURNAL OF PETROLOGY Vol. 51 ( 1-2 ) page： 101 - 123 2010.1

Language：English Publishing type：Research paper (scientific journal) Publisher：OXFORD UNIV PRESS

The Mohelno peridotite is a medium-sized ultramafic body (2 km x 4 km in size) enclosed in the Gfohl granulites in the eastern part of the Bohemian Massif. It consists mainly of coarse spinel peridotite (harzburgite and dunite); garnet peridotite occurs only in the sheared and deformed margins of the body. To decipher the origin and history of this mantle-derived peridotite, we determined the mineral chemistry by electron microprobe analysis and olivine fabric patterns by the electron backscattered diffraction method for each rock type. We found two distinct types of olivine fabric (crystal-preferred orientation; CPO) in the peridotite, which can be correlated with the mineralogy and thermal history of each. The olivine CPO in coarse-grained spinel peridotite shows a strong concentration of [100] slightly oblique to the lineation and [010] and [001] girdles normal to the lineation (which is the so-called {0kl}[100] pattern typical of medium-temperature deformation). Olivine in coarse-grained garnet peridotite, on the other hand, shows a strong concentration of [010] normal to the foliation and a concentration of [100] parallel to the lineation (which is the so-called (010)[100] pattern typical of high-temperature deformation). These fabric patterns become diffuse as the grain size is reduced for each mineralogical type. We interpret the development of these contrasting fabric patterns and mineralogical types based on the pressure-temperature history of each rock type determined by applying published geothermometers and geobarometers to the constituent minerals. Starting from a high-temperature (> 1200 degrees C) spinel peridotite, during exhumation and cooling in contact with surrounding granulites, the marginal part of the body was transformed to garnet peridotite, whereas the interior remained in the spinel-peridotite facies because cooling was slower inside the body. Because of the slow cooling and continuous deformation in the interior of the body, the original high-temperature fabric pattern in the spinel peridotite was converted to a lower-temperature type. The high-temperature fabric was preserved only at the margin of the body where cooling was more rapid. Reduction of grain size that occurred during later, low-temperature, deformation partly obliterated the high-temperature fabric patterns for both garnet and spinel peridotites. The initial rapid cooling at high temperatures associated with deformation probably occurred after the mantle peridotite was emplaced within the crustal granulites, which implies that the spinel- to garnet-peridotite transformation took place in the continental crust.

DOI： 10.1093/petrology/egp092

Trench-parallel anisotropy produced by serpentine deformation in the hydrated mantle wedge Reviewed

Preliminary results of elastic wave velocity measurements on a serpentinite obtained from the deep-sea floor Reviewed

新海優里, 渡辺了, 道林克禎, 針金由美子, 小原泰彦, 小原泰彦

静岡大学地球科学研究報告 ( 37 ) 2010

Katayama Ikuo, Hirauchi Ken-ichi, Michibayashi Katsuyoshi, Ando Jun-ichi

NATURE Vol. 461 ( 7267 ) page： 1114-U209 2009.10

Rock seismic anisotropy of the low-velocity zone beneath the volcanic front in the mantle wedge Reviewed

Katsuyoshi Michibayashi, Tatsuya Oohara, Takako Satsukawa, Satoko Ishimaru, Shoji Arai, Victor M. Okrugin

GEOPHYSICAL RESEARCH LETTERS Vol. 36 ( 12 ) 2009.6

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

Peridotite xenoliths derived from the low velocity zone beneath the Avacha frontal volcano, Kamchatka, preserve a-axis slip fabrics, comparable with those in xenoliths from the back-arc region of the NE Japan. Although low-velocity zones are commonly attributed to zones of partially melted mantle, migration of the melt does not erase the existing olivine fabrics and related seismic anisotropies. These anisotropies may counteract the anisotropies associated with c-axis slip fabrics, if they exist, along the slab or in the high-pressure zone. Citation: Michibayashi, K., T. Oohara, T. Satsukawa, S. Ishimaru, S. Arai, and V. M. Okrugin (2009), Rock seismic anisotropy of the low-velocity zone beneath the volcanic front in the mantle wedge, Geophys. Res. Lett., 36, L12305, doi:10.1029/2009GL038527.

DOI： 10.1029/2009GL038527

Peridotites from a ductile shear zone within back-arc lithospheric mantle, southern Mariana Trench: Results of a Shinkai 6500 dive Reviewed International coauthorship

K. Michibayashi, Y. Ohara, R. J. Stern, P. Fryer, J. -I. Kimura, M. Tasaka, T. Ishii

GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS Vol. 10 ( 5 ) 2009.5

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：AMER GEOPHYSICAL UNION

Two N-S fault zones in the southern Mariana fore arc record at least 20 km of left-lateral displacement. We examined the eastward facing slope of one of the fault zones (the West Santa Rosa Bank fault) from 6469 to 5957 m water depth using the submersible Shinkai 6500 (YK06-12 Dive 973) as part of a cruise by the R/V Yokosuka in 2006. The dive recovered residual but still partly fertile lherzolite, residual lherzolite, and dunite; the samples show mylonitic, porphyroclastic, and coarse, moderately deformed secondary textures. Crystal-preferred orientations of olivine within the peridotites show a typical [100](010) pattern, with the fabric intensity decreasing from rocks with coarse secondary texture to mylonites. The sampled peridotites therefore represent a ductile shear zone within the lithospheric mantle of the overriding plate. Peridotites were probably exposed in association with a tear in the subducting slab, previously inferred from bathymetry and seismicity. Furthermore, although the dive site is located in the fore arc close to the Mariana Trench, spinel compositions within the sampled peridotites are comparable to those from the Mariana Trough back arc, suggesting that back-arc basin mantle is exposed along the West Santa Rosa Bank fault.

DOI： 10.1029/2008GC002197

P- and S-wave velocities of the lowermost crustal rocks from the Kohistan arc: Implications for seismic Moho discontinuity attributed to abundant garnet Reviewed

Yoshio Kono, Masahiro Ishikawa, Yumiko Harigane, Katsuyoshi Michibayashi, Makoto Arima

TECTONOPHYSICS Vol. 467 ( 1-4 ) page： 44 - 54 2009.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

P- (Vp) and S-wave (Vs) velocities of garnet-free (two-pyroxene granulite) and garnet-bearing (garnet granulite and garnet pyroxenite) lowermost crustal rocks collected from the Kohistan arc, northern Pakistan, were measured at 0.1-1.0 GPa and 25-400 degrees C. Garnet granulite had higher Vp (+0.31 km/s) and Vs (+0.27 km/s) than two-pyroxene granulite. Although Vp and Vs increased with increasing volume percent of garnet, plagioclase-free garnet pyroxenite showed significantly higher Vp and Vs than plagioclase-rich garnet granulite mainly due to the low Vp and Vs of plagioclase. In contrast, we observed two quasi-linear relationships between Vp (Vs) and SiO(2) content for the garnet-bearing and garnet-free rocks. The garnet-bearing rocks had relatively higher Vp and VS and stronger SiO(2) dependences than the garnet-free rocks. The stronger SiO(2) dependences of Vp and Vs in the garnet-bearing rocks suggest that the garnet formation in mafic to ultramafic rocks (e.g., pyroxenite and hornblendite), having relatively lower SiO(2), leads to more pronounced increases in Vp and Vs than that of relatively felsic rocks (e.g., felsic-to-mafic granulite). Indeed, the Vp and Vs of the garnet pyroxenite were significantly higher than those of garnet granulite but comparable to those of dunite. The significantly high Vp and Vs of the garnet pyroxenite yielded high reflection coefficients between the garnet granulite and garnet pyroxenite of up to 0.13 for P-waves and 0.14 for S-waves, comparable to values expected for Moho reflection. Thus the lithological boundary between plagioclase-rich garnet granulite and plagioclase-free garnet pyroxenite in the lowermost crust of the Kohistan arc corresponds to the seismic Moho discontinuity. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2008.12.010

Determination of slip system in olivine based on crystallographic preferred orientation and subgrain-rotation axis: examples from Ichinomegata peridotite xenoliths, Oga peninsula, Akita prefecture

Satsukawa Takako, Michibayashi Katsuyoshi

The Journal of the Geological Society of Japan Vol. 115 ( 6 ) page： 288 - 291 2009

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Geological Society of Japan

In this study of peridotite xenoliths from Ichinomegata volcano, Oga peninsula, NE Japan, we used electron backscattered diffraction to assess the validity of two methods of identifying the olivine slip system that operated during deformation. The xenoliths are harzburgite in composition and contain a distinct foliation and lineation defined by aligned spinel and pyroxene grains. Using the first method, the dominant slip system in olivine was estimated based on the crystallographic preferred orientation (CPO) of olivine with respect to the foliation and lineation. The second method was based on an analysis of subgrain rotation: given that intracrystalline subgrain boundaries were observed within several olivine grains, we measured the misorientation angles across these boundaries to define sets of slip planes, slip directions, and rotation axes. The olivine slip systems estimated using the two methods are essentially identical. The CPO pattern provides an important constraint on the dominant slip system that operated during deformation, whereas the analysis of subgrain rotation yields the slip system responsible for the development of individual subgrain boundaries composed of edge dislocations. The two methods are therefore complementary.

DOI： 10.5575/geosoc.115.288

CiNii Research

Hydration due to high-T brittle failure within in situ oceanic crust, 30 degrees N Mid-Atlantic Ridge Reviewed

Katsuyoshi Michibayashi, Takehiro Hirose, Toshio Nozaka, Yumiko Harigane, Javier Escartin, Heike Delius, Margaret Linek, Yasuhiko Ohara

EARTH AND PLANETARY SCIENCE LETTERS Vol. 275 ( 3-4 ) page： 348 - 354 2008.11

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Analysis of an in situ fault zone within the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge) provides clues to the relevant deformation mechanisms and their temporal evolution within oceanic crust. IODP EXP304/305 drilled a succession of gabbroic lithologies to a final depth of 1415 m below the sea floor (mbsf), with very high recovery rates of up to 100% (generally similar to 80%). We identified an intra-crustal fault zone between 720 and 780 mbsf in a section of massive gabbro, olivine gabbro, oxide gabbro units, and minor diabase intrusions of particular interest is the section between 744 and 750 mbsf, which unfortunately was marked by low recovery rates (17%). Electrical borehole-wall images show a I-m-thick zone of east-dipping fractures within this interval, which is otherwise dominated by N-S dipping structures. Despite the high fracture density in this section, the hole walls are smooth, with rare breakouts, suggesting that the low recovery rate was due to a change in lithology rather than well conditions. The recovered rocks include ultracataclasite and possibly incohesive fault gouge that formed in the upper amphibolite regime, with mostly amphibole infill. Logging data suggest that the gabbroic rocks in this interval are rich in hydrous phases, consistent with increased amounts of amphibole found in the core. Equilibration temperature conditions of about 640 degrees C were obtained for plagioclase clasts and aluminous actinolite, assuming a pressure of 200 MPa. The permeability of the fault zone is in the range of 10(-19) to 10(-17) m(2). Although the permeability appears to be high within the fault zone relative to other parts of the section, it is no higher than that in typical lower crustal material. As a consequence, because brittle failure occurred at high temperatures, the fault zone was subsequently completely sealed by hydrous minerals, thereby preventing further fluid circulation and preserving water in the crust. (c) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2008.08.033

Shearing within lower crust during progressive retrogression: Structural analysis of gabbroic rocks from the Godzilla Mullion, an oceanic core complex in the Parece Vela backarc basin Reviewed

Yumiko Harigane, Katsuyoshi Michibayashi, Yasuhiko Ohara

TECTONOPHYSICS Vol. 457 ( 3-4 ) page： 183 - 196 2008.10

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Microstructural and petrological analyses of gabbroic rocks sampled from the Godzilla Mullion, located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea, reveal the development of a ductile shear zone in the lower crust. The shear zone is interpreted to represent a detachment fault within an oceanic core complex. Microstructures indicative of intense deformation. characterized by porphyroclastic textures consisting dominantly of coarse plagioclase porphyroclasts and lesser clinopyroxene porphyroclasts in a fine-grained matrix, are observed within samples of gabbroic rocks dredged near the breakaway area of the Godzilla Mullion (dredge site D6). Samples are classified into three types based upon the grain-size of fine-grained plagioclase in the matrix: coarse (80-130 mu m), medium (25 mu m), and fine (similar to 10 mu m). Although the chemical composition of plagioclase porphyroclasts is consistently An 40-50 among all sample types, the compositions of fine grains in the matrix vary with decreasing grain-size, being An 40-50 for the coarse-type, An 30-40 for the medium-type, and An 5-30 for the fine-type. This finding implies that the composition of fine-grained plagioclase in the matrix is related to the following retrograde reaction that occurred during deformation: clinopyroxene+plagioclase+Fe-Ti oxide+fluid -> hornblende+plagioclase. Plagioclase crystal-preferred orientations also show a gradual change with grain-size, varying from a (010)[100] pattern for the coarse-type, (010)[100] and (001)[100] patterns for the medium-type, and a weak (001)[100] pattern or random orientations for the fine-type. These patterns are interpreted to result from a change in the deformation mechanism of plagioclase from dislocation creep to grain-size-sensitive creep with decreasing temperature, thereby leading to strain softening and localization during cooling. Although secondary amphibole occurs ubiquitously within all samples, the chemical composition of amphibole varies from pargasitic hornblende (i.e. brown hornblende) to actinolite (i.e. green hornblende) within each of the sample types. However, amphibole in the coarse-type shows no evidence of deformation, whereas brown hornblende in the medium- and fine-types is plastically deformed. As a consequence, we argue that the microstructural development of the gabbroic rocks occurred during uplift-related cooling of the gabbro body and that a primary shear zone developed near the breakaway area at depth under anhydrous conditions at high temperatures above 850 degrees C; the shear zone subsequently evolved during progressive retrogression in association with hydration of the shear zone, possibly resulting in the development of the detachment fault that gave rise to the Godzilla Mullion. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2008.06.009