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_Journal of Structural Geology 32 (2010) 2–14_ _Comparison of petrofabrics with composite magnetic fabrics of S–C mylonite in paramagnetic granite_ _Takaaki Ono a,1, Yukinobu Hosomi a,2, Hiroyoshi Arai b, Hideo Takagi b,*_ _a Department of Earth Sciences, Graduate School of Science and Engineering, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo 169-8050, Japan_ _b Department of Earth Sciences, Faculty of Education and Integrated Arts and Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo 169-8050, Japan_ _Article history: Received 30 January 2008; Received in revised form 17 April 2009; Accepted 18 April 2009; Available online 3 May 2009_ _Keywords: AMS Shear zone Composite planar fabric Paramagnetic granite S–C mylonite_ _Abstract_ _An anisotropy of magnetic susceptibility (AMS) analysis was conducted for a typical S–C mylonite in a small-scale ductile shear zone derived from Late Cretaceous magnetite-free granite in the Ryoke Belt, Teshima Island, southwest Japan. In such mylonites, paramagnetic minerals such as biotite and hornblende, which define foliations, are assumed to control the AMS. Accordingly, we attempted to measure the orientations of both minerals to correlate the S–C–C0 fabric to the AMS using techniques of microscopic and back-scattered electron (BSE) image analyses. A measured magnetic fabric of the S–C mylonite that is expressed by a Kmin normal plane (Kmax–Kint plane) approximated S foliation, Kmax orientation coincided well with the orientations of mean resultant vectors for long axes of biotite and minor hornblende, whereas Kmax did not coincide well with those of the mean resultant vectors for long axes of the brighter domain (aggregates of biotite and minor hornblende) from the binary BSE image. These results show that the magnetic fabric fairly reflects the shape preferred orientation of individual grains of paramagnetic monoclinic minerals, especially biotite, which forms the S–C–C0 fabric._ _? 2009 Elsevier Ltd. All rights reserved._ _1. Introduction_ _The anisotropy of magnetic susceptibility (AMS) is affected by the sum of the contributions of each magnetic (diamagnetic, paramagnetic, ferromagnetic) mineral type in proportion to its volume fraction, mean susceptibilities and mineral intrinsic AMS (e.g., Borradaile and Jackson, 2004). AMS is a symmetrical second-order tensor (Kij), which relates the intensity of the applied field (Hj) to the acquired magnetization (Mi) of a mineral through the equation: Mi ? KijHj (Hrouda, 1982). The tensor is geometrically expressed by an ellipsoid specified by its principal eigenvalues (susceptibility magnitudes) and eigenvectors (their orientations), Kmax ! Kint ! Kmin (e.g., Tarling and Hrouda, 1993). In the case of deformed magnetite-free granitic rocks, paramagnetic Fe-bearing silicates, which define foliations, are responsible for AMS (e.g., Rochette, 1987; Archanjo et al., 1994). For instance, when single preferred orientation fabrics exist in a rock, AMS is a powerful tool to investigate petrofabric, because its principal axis (Kmax, Kint, Kmin) often corresponds with the principal axis (X, Y, Z) of finite strain (e.g., Borradaile and Henry, 1997). In contrast, when multiple mineral orientations are present in a rock, a measured magnetic fabric will reflect a composite of these orientation fabrics (Borradaile and Tarling, 1981). Such composite magnetic fabrics do not correspond to finite strain. A situation of importance to structural geology where this issue might arise is in the use of AMS to infer deformation in an S–C mylonite. For example, Tomezzoli et al. (2003) reported that the magnetic foliation, defined as the plane normal to Kmin, of S–C mylonite departs from the orientations of the S-plane. The magnetic foliation of deformed rocks with S–C structure can show an intermediate orientation between the S- and the C-planes reflecting the additive effect of two planar structures (Aranguren et al., 1996)._ _The main aim of this study is to use a single sample across an S–C mylonitic shear zone to investigate the relationships of the AMS geometry with rock fabric geometry. The advantage of this approach is that a detailed spatial comparison from the perimeter to a shear zone center is achieved. For this analysis, the AMS is characterized by the content of paramagnetic Fe-bearing minerals, the mineral shape-fabric by optical inspection, and the mineral-aggregate shape fabric by back-scatter electron (BSE) image analysis._ _* Corresponding author. E-mail address: hideo@waseda.jp (H. Takagi)._ _1 Present address: Daiwa Institute of Research, Ltd., Tokyo 135-8460, Japan._ _2 Present address: Mainichi Broadcasting System, Inc., Osaka 530-8304, Japan._ _0191-8141 $ – see front matter ? 2009 Elsevier Ltd. All rights reserved. doi:10.1016 j.jsg.2009.04.009_ _2. Geological setting_ _The S–C mylonite sample is from magnetite-free, biotite-hornblende granite in the Teshima Island, Seto Inland Sea, southwest Japan (34 x142206000 N, 133 x143905500 E). This island is located in the Ryoke Belt, which is composed of Late Cretaceous foliated older granite and non-foliated younger granite, and metamorphic rocks, which occur as a roof pendant on the granitoid (Fig. 1). The granitic rocks in the Ryoke belt are low-susceptibility, ilmenite-series granitoid (Ishihara, 1981). Weakly mylonitized coarse-grained hornblende-biotite older granite, which strikes approximately ENE–WSW and dips subvertically, is widely exposed in the south of the island (Arita, 1988). Within the older granite, about 25 small-scale dextral ductile shear zones with a thickness ranging from several centimeters to a few meters are observed around the southernmost coastal area of the island. The shear zones have S–C– C0 fabrics and shear zone orientation strike WNW with subvertical dip, that crosscuts an older mylonitic foliation striking ENE._ _The analyzed S–C mylonite sample (Fig. 2) was collected as a half side of the cross section of an entire shear zone that strikes N59 x14W with vertical dip and is about 50 x14 oblique to the older foliation in the surrounding host older granite (N62–74 x14E, 90 x14). The S-foliation (Fig. 2) is defined by a shape fabric of biotite, quartz aggregate and elongated porphyroclasts. A sigmoidal deflection of the S foliation varying in angles with respect to the shear zone center (C-plane) from 60 x14 to 10 x14 gives a dextral shear sense (Fig. 2). This shear zone can be divided into three by grade of mylonitization; strongly, moderately and weakly mylonitized zones (SMZ, MMZ, WMZ). The distance from the shear zone center for each zone is 0–6 cm for SMZ, 6–14 cm for MMZ and over 14 cm for WMZ. Shear bands (C0), which are inclined at 5–25 x14 clockwise to the C-plane dominate at the MMZ to SMZ near the shear zone center (Fig. 3). This oblique angle range between C0 and C is the same as that described by Michibayashi and Murakami (2007)._ _3. Sample preparation and analytical methods_ _The sample analyzed covers about 35 cm from the shear zone center where the C plane and stretching lineation can be observed (Fig. 2). The angle between the C plane and the cut plane is about 75 x14, and thus, the polished surface (Fig. 2a) is not exactly the XZ plane. When we estimated shear strain g using g ? 2 tan2qo after Ramsay and Graham (1970), we corrected the measured apparent angle qo into the real angle qr as a formula, tanqr ? cos15 x14tanqo where 15 x14 is the angle between the XZ plane and the sample plane. However, the difference between qo and qr is within the measuring error (for example, qr ? 45 x14 in the case where qo ? 46 x14 for sample 8a). Therefore, no correction for the 15 x14 obliquity above is included in the analysis. Cylindrical core samples were drilled into the direction subparallel (about 15 x14) to the Y-axis normal to the polished XZ plane (Fig. 2b). Twenty-four cylindrical oriented specimens (diameter ? 2.54 cm) were extracted by drilling with a non-magnetic diamond drill bit. AMS measurements were performed on 2.54 cm (diameter) ? 2.2 cm (length) core samples, cut from the oriented specimens using a diamond tipped nonmagnetic saw blade. Thin sections subparallel to the XZ plane were also prepared from the oriented core samples. Little asymmetric fabric was detected in the YZ thin sections, therefore the mylonite was characterized as having monoclinic fabric symmetry. Accordingly, we measured the fabrics and content of macrominerals for each cylindrical sample in only the XZ section._ _3.1. Microscopic analyses_ _To compare petrofabrics with the AMS data, foliations defined by biotite and hornblende were analyzed. All individual grains of biotite and hornblende within the same 16 ? 16 mm sample areas as the BSE images (samples 3a to 8a) were analyzed, if grains of these minerals could be resolved microscopically. The minimum grain size measured is about 20 mm using 10 (eyepiece) ? 20 (objective) lenses. The length of long axis a and short axes b, the angle q between the long axis of biotite (hornblende) and the C plane, and the modal percentage of each mineral were measured for samples 3a to 8a microscopically by point-counting (Fig. 4). The grain orientation (q) data are_ Ключевые слова: correspond, increase, hemisphere projection, grain size, grain area, geophysical, magnetic anisotropy, nite strain, biotite ?akes, dene foliation, recrystallized quartz, island, granite, measuring error, image, ono journal, anisotropy, recrystallized, rochette, bright domain, takagi, wa, minor hornblende, mmz, ishihara, journal structural, earth, aggregate, university, axis, shear zone, structure, fabric, foliation, polished, smz, sample, decrease, ryoke belt, magnetite, parallel, volume fraction, angle, ?ne-grained, magnetic susceptibility, borradaile, size, stephens, paramagnetic, jackson, bse, frequency, dominantly developed, deformed rock, magnetic, form, planar, kmaxkint plane, fm, orientation data, waseda, arita, plane, sample plane, grain, preferred orientation, tectonophysics, distribution, circular, ferromagnetic magnetite, individual, calculated, wmz, arm, observed, journal structural geology, journal, center, magnetic fabric, orientation frequency, de?ned, doe, modal, size progressively, mm, long, contribution, table, addition, housen, plane normal, volume, ams, data, mylonite, application, kmax, tarling, difference, characteristic, biotite, zone, kint, structural geology, nye, area, resultant vector, result, mylonites, strain, zapletal, mineral, wmz sample, biotite akes, composite, long axis, granitic rock, magnetic foliation, structural, mac mineral, series, mmz sample, rock, qm, bright, physical, circular uniformity, shape, jelinek, japan, analyzed, individual grain, picture, binary image, geology, s–c, watsons test, tomezzoli, feldspar, susceptibility, cm, quartz, zone center, hornblende, microscopic analysis, measured, signicance level, biotite hornblende, anticlockwise, composite magnetic, rose diagram, shear strain, ?akes, subparallel, platt, kmin, angle interval, qo, hrouda, aranguren, bse image, normal, vector, domain, ono, microscope, smz sample, respect, analysis, orientation, ?o °, ferromagnetic, shear