Transformation-induced strain localization in a lherzolite

ELSEVIER Tectonophysics 313 (1999) 411–432www.elsevier.com/locate/tecto

Transformation-induced strain localization in a lherzolite mylonitefrom the Hidaka metamorphic belt of central Hokkaido, Japan

Masaya Furusho a, Kyuichi Kanagawa b,*

a Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japanb Department of Earth Sciences, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan

Received 18 January 1999; accepted 12 July 1999

Abstract

The Uenzaru peridotite complex in the northern part of the Hidaka metamorphic belt of central Hokkaido, Japan,contains a mylonitized, plagioclase lherzolite. The plagioclase-lherzolite mylonite consists of porphyroclasts of olivine,orthopyroxene, clinopyroxene and spinel, and fine-grained matrix of olivine, orthopyroxene, clinopyroxene, plagioclase andspinel. Symplectite composed of plagioclase, olivine and chromian spinel occurs around porphyroclasts of orthopyroxene,clinopyroxene and spinel. A fine-grained aggregate of plagioclase, olivine and chromian spinel also occupies the pressureshadows around porphyroclasts of orthopyroxene, clinopyroxene and spinel. The fine-grained aggregates in the pressureshadows laterally connect with each other to form layers, which characterize the mylonitic foliation. Symplectite with thesame mineral assemblage as the fine-grained aggregate, a bulk rock chemistry of the mylonite close to that of pyrolite, anda reverse zoning of plagioclase such that the anorthite component increases toward its rim, indicate that the fine-grainedaggregate is derived from the subsolidus phase-transformation reaction from spinel lherzolite to plagioclase lherzolite, butnot from melt. The mylonitic foliation defined by layers of fine-grained aggregate implies that strain is localized intothe reaction products. Therefore the phase-transformation reaction may have enhanced mylonitization of the lherzolite.Compositional zoning of pyroxenes and plagioclase in the mylonite suggests an adiabatic decompression at temperaturesabove 960ºC within the spinel-lherzolite stability field, followed by a rapid isobaric cooling down below 760ºC at about700–800 MPa. The lherzolite which initially ascended as a mantle diapir was mylonitized through the phase-transformationreaction and thrusting onto crustal rocks, the contact with which resulted in a rapid cooling of the lherzolite. 1999Elsevier Science B.V. All rights reserved.

Keywords: strain localization; phase transformation; lherzolite; mylonite; Hokkaido

1. Introduction

Deformation in the lithosphere is largely accom-modated within inter- and intra-plate shear zones.Crustal shear zones extending down to the upper

Ł Corresponding author. Fax: C81-43-290-2859; E-mail:[email protected]

mantle inferred from seismic reflection profiles (e.g.Flack et al., 1990; Reston, 1990; Baird et al., 1995;McBride et al., 1995), peridotite mylonites in mantlexenoliths (Nielson and Schwarzman, 1977; Harte,1983) and peridotite massifs (Drury et al., 1990;Vissers et al., 1991, 1995; Hoogerduijn Strating etal., 1993; Suhr, 1993; Newman et al., 1999), and thedeformation conditions of these mylonites indicate

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412 M. Furusho, K. Kanagawa / Tectonophysics 313 (1999) 411–432

that strain localization occurs in lithospheric mantleperidotites. Because the basal part of a collisionalor obducted ophiolite is often occupied by stronglydeformed peridotites (Ishiwatari, 1985; Boudier etal., 1988; Drury et al., 1990; Vissers et al., 1991,1995; Hoogerduijn Strating et al., 1993; Suhr, 1993;Boudier and Nicolas, 1995), strain localization inperidotites may also play an important role duringthe emplacement of mantle peridotites into the crust.

It has been suggested that strain localization inperidotites occurs due to grain-size-sensitive creepof dynamically recrystallized fine-grained olivine orto the formation of polyphase fine-grained layersformed by local hydration (Drury et al., 1990, 1991;Jin et al., 1998). We report in this paper anotherstrain localization process in lherzolite induced bya phase-transformation reaction (see also Newmanet al., 1999). We show that in a lherzolite mylonitestrain is localized into a fine-grained aggregate ofplagioclase, olivine and chromian spinel formed bythe phase-transformation reaction from spinel lher-zolite to plagioclase lherzolite. Since any type ofexhumation of a lherzolite body is necessarily ac-companied by this phase-transformation reaction, the

Fig. 1. Geologic map of the Uenzaru area. Rectangle indicates the route map area shown in Fig. 2. Inset shows the locations of theHidaka metamorphic belt and the study area.

strain localization process reported in this paper maycommonly occur during ascent of a mantle diapir orlithospheric extension.

2. Geologic setting

The NNW-trending, 10–20-km-wide and 140-km-long Hidaka metamorphic belt in south-central Hok-kaido, Japan (Fig. 1) consists of high-temperaturetype metamorphic rocks with metamorphic ages of16–41 Ma (Shibata et al., 1984; Saheki et al., 1991;Arita et al., 1993). The Hidaka metamorphic belt isdivided into the Main Zone and the Western Zone(Fig. 1; Komatsu et al., 1983). The Main Zone occu-pies the eastern main part of the belt, and consists ofpelitic and psammitic metamorphic rocks, and plu-tonic rocks such as gabbro, diorite and granitic rocks(Komatsu et al., 1983; Osanai et al., 1986, 1991).The metamorphic rocks of the Main Zone gradu-ally change eastward from granulite-facies rocks tounmetamorphosed sedimentary rocks. Accordingly,intrusive plutonic rocks change in lithology fromgabbro through diorite to granitic rocks. Hence the

M. Furusho, K. Kanagawa / Tectonophysics 313 (1999) 411–432 413

Main Zone is considered to represent a tilted is-land-arc crustal section with its exposed thickness ofabout 23 km (Komatsu et al., 1983). In contrast, theWestern Zone consists of pelitic and greenschists,amphibolite, metagabbro and mafic to ultramafic cu-mulate, and is inferred to be a metaophiolite assem-bly (Miyashita, 1983). The boundary between theMain Zone and the Western Zone is the Hidaka MainThrust (HMT; Fig. 1) which has a dextral strike-slipcomponent (Jolivet and Miyashita, 1985; Arita et al.,1986). Some Alpine-type mantle peridotite bodiesare discontinuously distributed along the HMT. TheWestern Zone is in thrust contact with the Creta-ceous accretionary complex (Kiyokawa, 1992) alongthe Western Boundary Thrust (WBT; Fig. 1).

The Uenzaru peridotite complex is located in thenorthern part of the metamorphic belt (Fig. 1). It isin thrust contact with biotite gneiss of the Main Zone

Fig. 2. Route map along the Uenzaru trail showing outcrops, attitudes of foliation and lineation, and the localities of the plagioclase-lher-zolite protomylonite and mylonite described in this study. Dots indicate the localities where foliation and lineation are measured. Alsoshown are lower-hemisphere, equal-area projections of pole to foliation (square) and lineation (circle) in three lithologic units of theUenzaru peridotites.

in the east, and with amphibolite of the WesternZone in the west (Figs. 1 and 2). Based on lithol-ogy, microstructures, and attitudes of foliation andlineation, the complex is divided into three units:dunite=harzburgite unit, plagioclase-lherzolite unitand harzburgite=spinel-lherzolite unit from west toeast (Fig. 2). The dunite=harzburgite unit consistsof dunite and harzburgite with amphibolite dikes.Dunite and harzburgite show banding characterizedby alternating coarse-grained layers of olivine andorthopyroxene, and fine-grained layers of olivine,tremolite, chlorite, talc, spinel and magnetite. Thisbanding defines NNW-striking vertical foliation withsteeply plunging mineral lineation (Fig. 2). Theplagioclase-lherzolite unit consists of plagioclaselherzolite with clinopyroxenite layers. Plagioclaselherzolite is preferentially mylonitized. Alignmentof fine-grained layers, and elongate porphyroclasts


and pressure shadows define foliation and lineationwhich differ in orientation between the western andeastern parts of the unit. In the western part whereplagioclase lherzolite is weakly mylonitized, foli-ation strikes northwest to west-northwest, dippingmoderately to the northeast, and lineation gentlyplunges toward the east (Fig. 2). In contrast, in theeastern part where plagioclase lherzolite is stronglymylonitized, foliation strikes northeast to east-north-east, and dips moderately to steeply to the southeast,while lineation gently plunges toward the south orwest (Fig. 2). The harzburgite=spinel-lherzolite unitconsists of porphyroclastic harzburgite and spinellherzolite (Mercier and Nicolas, 1975) with clinopy-roxenite layers. Alignment of elongate porphyro-clasts of olivine and orthopyroxene defines subhori-zontal foliation and lineation gently plunging towardthe north (Fig. 2).

Although foliation and lineation considerablyvary in orientation among the above three unitsand even in the plagioclase-lherzolite unit (Fig. 2),no overprinting relationships are observed betweenfoliations of different orientations.

3. Microstructures of the plagioclase-lherzolitemylonites

Microstructures of the two plagioclase-lherzolitemylonite samples, protomylonite and mylonite (seeFig. 2 for their localities), are described below. Theyhave been observed in X Z sections perpendicularto foliation and parallel to lineation. The X Z sec-tion of the protomylonite dipping gently toward thesouth-southeast has been viewed upward, while theNNW-striking subvertical X Z section of the my-lonite has been viewed from the east.

3.1. Protomylonite

The protomylonite consists of coarse-grainedolivine (grain size d D 350 µm to 2.5 cm), orthopy-roxene, clinopyroxene and spinel, medium-grainedpolygonal olivine (d D 30–230 µm) surroundingolivine porphyroclasts, and matrix of fine-grainedolivine (d D 10–50 µm), plagioclase, orthopy-roxene, clinopyroxene, spinel and opaque minerals(Fig. 3a). Undulatory extinction, deformation bands

and subgrains are common in olivine porphyroclasts.Orthopyroxene porphyroclasts are elongate with ax-ial ratios up to 11. Clinopyroxene porphyroclasts areless elongate with axial ratios up to 7, and containplagioclase lamellae in their cores. Spinel porphyro-clasts are rimmed by plagioclase grains. Symplectitecomposed of fine-grained olivine, plagioclase andspinel occurs along boundaries between spinel andorthopyroxene porphyroclasts (Fig. 3b). Fine-grainedaggregate with the same mineral assemblage as thesymplectite occupies pressure shadows adjacent toporphyroclasts of orthopyroxene, clinopyroxene andspinel (Fig. 3c). The fine-grained aggregates in thepressure shadows laterally connect with each otherto form thin, fine-grained layers (Fig. 3a,d).

3.2. Mylonite

The mylonite consists of porphyroclasts ofcoarse-grained olivine (d D 300 µm to 1 cm),orthopyroxene, clinopyroxene and spinel, medium-grained polygonal olivine (d D 30–330 µm) sur-rounding olivine porphyroclasts and, and fine-grained matrix of olivine (d D 10–100 µm), plagio-clase (d D 10–100 µm), orthopyroxene, clinopyrox-ene, chromian spinel (d D 30 µm) and opaque min-erals (Fig. 4a). Fine-grained aggregates of olivine,plagioclase and spinel occupy pressure shadowsaround porphyroclasts of orthopyroxene, clinopy-roxene and spinel. The aggregates laterally connectwith each other to form fine-grained layers (Fig. 4a),which characterize the mylonitic foliation. The longaxes of elongate porphyroclasts are oriented slightlyclockwise with respect to the foliation (Fig. 4a,b,f).

Undulatory extinction, deformation bands andsubgrains are common in olivine porphyroclasts.Olivine porphyroclasts are mantled by dynami-cally recrystallized medium-grained olivine, exhibit-ing core-and-mantle structure (Fig. 4b). A gradualtransition is observed between subgrains and re-crystallized grains. The olivine porphyroclast-mantlesystem is isolated as a lenticular domain in fine-grained layers (Fig. 4b).

Elongate orthopyroxene porphyroclasts with axialratios up to 17 are aligned subparallel to foliation(Fig. 4a). Symplectite composed of olivine, vermic-ular plagioclase and very fine-grained spinel occursaround orthopyroxene porphyroclasts, occasionally


Fig. 3. Optical micrographs of the X Z section of the plagioclase-lherzolite protomylonite. OL D olivine, OPX D orthopyroxene, CPX Dclinopyroxene, SP D spinel, and PL D plagioclase. (a) Porphyroclast-in-matrix microstructure. Crossed polarized light. (b) Symplectiteof vermicular plagioclase, olivine and very fine-grained spinel, between orthopyroxene and spinel porphyroclasts. Crossed polarized light.(c) Pressure shadow composed of fine-grained plagioclase, olivine, spinel and orthopyroxene adjacent to an orthopyroxene porphyroclast.Plane polarized light. (d) Fine-grained layer composed of plagioclase, olivine and spinel. Plane polarized light.

being deflected and sheared (Fig. 4c). Pressure shad-ows adjacent to orthopyroxene porphyroclasts con-sist of fine-grained plagioclase, olivine, spinel andorthopyroxene.

Clinopyroxene porphyroclasts are elongate withaxial ratios up to 5, and contain fine exsolutionlamella of plagioclase in their cores (Fig. 4d). Sym-plectite around clinopyroxene porphyroclasts is com-posed of fine-grained olivine, plagioclase, clinopy-roxene and very fine-grained spinel (Fig. 4e). Pres-sure shadows adjacent to clinopyroxene porphyro-clasts consist of fine-grained plagioclase, olivine,spinel and clinopyroxene (Fig. 4d).

Spinel porphyroclasts show a heterogeneous dis-tribution of Al and Cr contents such that they arealuminous in long-axis rims while chromian in short-axis rims (Fig. 4f,g). Symplectite lobe around spinelporphyroclasts is composed of olivine, plagioclaseand vermicular chromian spinel (Fig. 4h). The pres-sure shadows adjacent to the porphyroclasts havethe same mineral assemblage as the symplectite(Fig. 4f). The back scattered electron (BSE) imageof a pressure shadow adjacent to a spinel porphyro-clast (Fig. 4i) indicates that plagioclase grains in thepressure shadow are richer in anorthite component(brighter) at their rims than at their cores, so that they


Fig. 4 (for description see p. 418).



have a reverse compositional zoning. The distribu-tion of Ca content in plagioclase grains surroundingan isolated spinel porphyroclast (Fig. 4j) reveals thatplagioclase grains adjacent to the long-axis rim ofthe porphyroclast are relatively coarse-grained andricher in anorthite component, while those adjacentto the short-axis rim are relatively finer-grained andpoorer in anorthite component.

The fine-grained layer defining the mylonitic fo-liation is mainly composed of plagioclase (d D 100µm), olivine (d D 100 µm) and chromian spinel(d D 30 µm) (Fig. 4k,l). Fine-grained orthopyroxeneoccurs in the fine-grained layer adjacent to orthopy-roxene porphyroclasts, while fine-grained clinopy-roxene occurs adjacent to clinopyroxene porphyro-clasts. Fine-grained olivine is elongate with axialratios up to 5 and oriented parallel to foliation, butit does not contain subgrain boundaries nor exhibitundulatory extinction (Fig. 4l). Plagioclase in thefine-grained layer is equigranular (Fig. 4k,l). Opti-cally visible zoning in plagioclase (Fig. 4l) as wellas the increase in anorthite component from coreto rim (Fig. 4k), indicate that the compositional re-verse zoning in individual grains is preserved, andhence that plagioclase is not dynamically recrystal-lized after nucleation, although tapered deformationlamellae and undulatory extinction (Fig. 4l) imply

Fig. 4 (see p. 416, 417). Microstructures of the X Z section of the plagioclase-lherzolite mylonite. The NNW-striking subvertical X Zsection is viewed from the east. Mineral abbreviations are the same as in Fig. 3. (a) Optical micrograph of porphyroclast-in-matrixmicrostructure. Plane polarized light. (b) Optical micrograph of core-and-mantle structure around an olivine porphyroclast. Crossedpolarized light. (c) Optical micrograph of symplectite composed of vermicular plagioclase, olivine and spinel, between orthopyroxeneand spinel porphyroclasts. Plagioclase is finer-grained adjacent to the orthopyroxene porphyroclast. Note a deflection of alignment ofvermicular plagioclase and elongate olivine grains, which indicates shearing along the orthopyroxene porphyroclast boundary. Crossedpolarized light. (d) Optical micrograph of a clinopyroxene porphyroclast with plagioclase lamellae at the core. Pressure shadows onboth sides are composed of fine-grained plagioclase, olivine, spinel and clinopyroxene. Plane polarized light. (e) Optical micrographof symplectitic plagioclase, olivine and spinel around a clinopyroxene porphyroclast. Plagioclase is finer-grained adjacent to theporphyroclast. Plane polarized light. (f) Optical micrograph of a spinel porphyroclast which is aluminous (light-colored) in the long-axisrim and chromian (dark-colored) in the short-axis rim. Its pressure shadows are composed of plagioclase, olivine and chromian spinel.Plane polarized light. (g) Distribution of Al (upper) and Cr (lower) contents (higher shown brighter) in the spinel porphyroclast shownin (f). Al content is higher at the long-axis rim than at the short-axis rim, while Cr is concentrated at the short-axis rim. (h) Opticalmicrograph of a symplectite lobe composed of plagioclase, olivine and vermicular spinel along a spinel porphyroclast boundary. Planepolarized light. (i) BSE micrograph of a pressure shadow adjacent to a spinel porphyroclast. Plagioclase grains have dark cores andbright rims, indicating a reverse zoning such that anorthite component increases toward their rims. (j) Distribution of Ca content (highershown brighter) in plagioclase around a spinel porphyroclast. Note that plagioclase grains around the long-axis rim are coarser-grainedand richer in Ca content than those around the short-axis rim. (k) BSE micrograph of a fine-grained layer composed of plagioclase,olivine and chromian spinel. Note that some plagioclase grains have very dark rims, in addition to a reverse zoning as in (i). (l)Optical micrograph of a fine-grained layer composed of fine-grained mixed aggregate of plagioclase, olivine and spinel. Plagioclase isequigranular, while olivine is slightly elongated parallel to the foliation (horizontal). Note optically visible zoning in some plagioclasegrains (white arrows). Roundish tiny spinel grains are commonly included in plagioclase and olivine grains. Plane polarized light.

intracrystalline deformation. Plagioclase grains inthe fine-grained layer also show a steep decrease inanorthite component at their rims especially in thedirection parallel to foliation (Fig. 4k). They havestraight grain boundaries with dihedral angles of ap-proximately 120º at triple junctions (Fig. 4i,k,l). Thegrain boundaries between plagioclase and olivine areconcave toward olivine (Fig. 4i,k,l).

In symplectite, pressure shadow or fine-grainedlayer, relatively large spinel grains (d D 10–30 µm)occur along olivine and=or plagioclase grain bound-aries (Fig. 4i,k), while tiny, rounded or vermicularspinel inclusions (d ³ few µm) are usually includedin plagioclase and olivine grains (Fig. 4e,h,i,k,l).

4. Modal compositions of theplagioclase-lherzolite mylonites

The modal compositions of the plagioclase-lher-zolite protomylonite and mylonite are measured inX Z sections (Table 1). The modal content of por-phyroclast in the protomylonite is 61.7%, while thatin the mylonite is 30.0% (Table 1). We follow theclassification of these mylonites after Sibson (1977).

The modal content of porphyroclast olivine andthat of medium-grained mantle olivine are 45.0% and


Table 1Modal compositions (in %) of the plagioclase-lherzolite my-lonites determined from 4000 point measurements with pointspacing of 200 µm

Protomylonite Mylonite

OL 45.0 12.6OPX 11.0 9.1CPX 4.5 6.8SP 1.3 1.6m.OL 13.4 18.7f.OL 22.0 42.5f.OPX 1.0 0.8f.CPX 0.5 1.0f.SP 0.3 2.1PL 1.0 4.6AMP 0.0 0.1OP 0.2 0.2

Porphyroclast 61.7 30.0Matrix 38.3 70.0PL C f.OL C f.SP 23.3 49.2

OL D porphyroclast olivine, OPX D porphyroclast orthopyrox-ene, CPX D porphyroclast clinopyroxene, SP D porphyroclastspinel, m.OL D medium-grained olivine, f.OL D fine-grainedolivine, f.OPX D fine-grained orthopyroxene, f.CPX D fine-grained clinopyroxene, f.SP D fine-grained spinel, PL D plagio-clase, AMP D amphibole, and OP D opaque minerals.

13.4%, respectively, in the protomylonite, while theyare 12.6% and 18.7%, respectively, in the mylonite(Table 1). This indicates that grain-size reduction ofolivine proceeded with mylonitization of the lherzo-lite. The modal content of plagioclase and that offine-grained spinel is 1.0% and 0.3%, respectively,in the protomylonite, while they are 4.6% and 2.1%,respectively, in the mylonite (Table 1), indicating theincrease in modal content of these two minerals inthe mylonite.

5. Lattice preferred orientation (LPO) of olivinein the plagioclase-lherzolite mylonite

Orientations of olivine crystallographic axes havebeen measured for medium-grained olivine in theporphyroclast mantle and olivine in the fine-grainedlayer, respectively, using an optical microscopeequipped with a universal stage (Fig. 5).

The LPO patterns of medium-grained olivine arecharacterized by a point maximum of [100] sub-

parallel to lineation, a diffuse maximum of [001]subperpendicular to foliation, and a point maximumof [010] subparallel to foliation and subperpendicu-lar to lineation (Fig. 5a). The positions of [100] and[001] maximum are slightly anticlockwise oblique tolineation and foliation-normal, respectively (Fig. 5a).

[100] of olivine in the fine-grained layer exhibitsa point maximum subparallel to lineation, while its[010] and [001] are rather scattered on poorly definedgirdles around the [100] maximum (Fig. 5b). Theposition of [100] maximum of fine-grained olivineis also slightly anticlockwise oblique to lineation(Fig. 5b).

6. Bulk chemical compositions of the Uenzaruperidotites

Bulk chemical compositions of the 13 Uenzaruperidotite samples (see Fig. 2 for their localities)were determined by a Philips X-ray fluorescence(XRF) spectroscopy (PW1480) at the Department ofGeology, University of Tokyo (Table 2). We followedthe sample preparation and analytical proceduresdescribed in Yoshida and Takahashi (1997).

Incompatible elements such as Al, Ca, Ti, Na andY are rather poor in dunite, harzburgite and spinellherzolite, whereas these elements are relatively richin plagioclase lherzolite (Table 2). Plagioclase lher-zolite is close to pyrolite in bulk chemical com-position (Fig. 6). Ca content of the protomyloniteis slightly lower than that of the mylonite (Fig. 6;Table 2).

7. Mineral compositions of theplagioclase-lherzolite mylonites

Mineral compositions of two pyroxenes and pla-gioclase in the plagioclase-lherzolite mylonite, andthose of spinel in the protomylonite and mylonitewere determined by electron microprobe analysis(EPMA) using a JEOL JXA-8900 at the Chemi-cal Analysis Center, Chiba University, and a JEOLJXA-8900L at the Department of Geology, Univer-sity of Tokyo. All analyses except for orthopyroxenehave been performed using focused beam, while aprobe diameter of 20 µm was used in the analysis

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Table 2Bulk chemical compositions of the Uenzaru peridotite samples whose localities are shown in Fig. 2

Element W Protomylonite Mylonite ! E

1 D=H 9 D=H 2 D=H 10 D=H 11 D=H 3 D=H 12 P-L 4 P-L 5 P-L 6 H=S-L 7 H=S-L 16 H=S-L 8 H=S-L

(wt%)SiO2 41.26 41.60 41.01 40.93 41.12 42.12 43.78 44.26 43.70 41.73 41.11 42.21 41.87Al2O3 0.71 0.45 0.39 0.48 0.52 0.81 3.18 3.10 3.40 1.31 0.54 0.60 0.68CaO 0.79 0.51 0.79 0.73 0.83 0.98 2.95 2.45 3.32 1.69 0.79 0.77 0.92Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.13 0.08 0.23 0.00 0.00 0.00 0.00K2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.006Fe2O3 8.62 9.04 9.69 9.61 9.71 9.39 9.82 9.50 9.32 9.26 9.36 9.65 9.55MgO 45.61 46.96 47.02 47.42 47.42 46.84 41.57 40.37 40.28 44.08 46.30 48.38 47.01MnO 0.13 0.13 0.15 0.14 0.14 0.14 0.15 0.15 0.14 0.14 0.14 0.14 0.14TiO2 0.03 0.03 0.02 0.03 0.03 0.03 0.12 0.11 0.15 0.03 0.02 0.03 0.03P2O5 0.03 0.03 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0.03 0.03Total 97.18 98.75 99.09 99.36 99.80 100.34 101.73 100.05 100.57 98.26 98.29 101.81 100.23

(ppm)Cr 2640.0 2741.3 2306.8 2897.2 2861.5 2846.3 2732.6 2926.3 2498.3 2450.5 2728.5 2758.8 2665.7Ni 1929.6 1978.5 1935.3 1929.5 1909.3 1916.0 1660.3 1695.9 1644.9 1838.6 1915.2 1936.2 1912.3Sr 0.0 2.0 0.0 1.7 0.9 0.0 3.2 0.0 8.0 0.0 0.0 1.9 0.0Y 0.4 0.1 0.1 0.2 0.1 0.4 2.6 2.3 3.5 0.7 0.3 0.4 0.3Zr 0.0 0.4 0.0 0.5 0.2 0.0 2.1 0.9 5.3 0.0 0.0 0.2 0.0

D=H D dunite=harzburgite unit, P-L D plagioclase-lherzolite unit, and H=S-L D harzburgite=spinel-lherzolite unit.


Fig. 5. Stereographic projections of crystallographic axes of medium- and fine-grained olivine in the plagioclase-lherzolite mylonite.Lower-hemisphere, equal-area projections using Neil Mancktelow’s Stereoplot. Contour interval at multiples of uniform distribution.Horizontal line represents foliation trace with E–W direction being lineation. n D number of grains measured, and max D maximumdensity.

of orthopyroxene. ZAF correction procedures wereapplied. Line analyses across grains have been donethrough the center of chemical zoning and perpen-dicular to exsolution lamellae, if any.

The Al content in one of the largest orthopyroxeneporphyroclasts in the mylonite gradually decreasesfrom its core (0.22–0.24=6 oxygens) toward the rim(¾0.13=6 oxygens) (Fig. 7a; Table 3), while theCa content is roughly constant at the core (0.016–0.018=6 oxygens) but decreases at the rim (¾0.011=6oxygens) (Fig. 7b; Table 3).

The Al content in one of the largest clinopyroxeneporphyroclasts in the mylonite is nearly constant atthe core (¾0.30=6 oxygens), but abruptly decreasesat the rim (¾0.12=6 oxygens) (Fig. 8a). The Ca con-tent is roughly constant at the core (0.80=6 oxygens),and gradually increases toward the rim (¾0.91=6

oxygens) (Fig. 8b). The Na content is nearly con-stant at the core (0.15=6 oxygens), and gradually de-creases toward the rim (¾0.05=6 oxygens) (Fig. 8c).

The anorthite component in plagioclase grainsadjacent to a spinel porphyroclast continuously in-creases from their cores (An44–55) toward their rims(An75–80) (Fig. 9b,c). Plagioclase in the fine-grainedlayer exhibits essentially the same compositionalprofile, but additionally an abrupt decrease in anor-thite component at the margin to An45–50 (Fig. 9a)as seen on a BSE micrograph (Fig. 4k). The anor-thite component in plagioclase grains in symplectiteadjacent to orthopyroxene porphyroclasts is An40–48

(Fig. 9d,e). Some plagioclase grains in symplectitehave a normal compositional zoning such that theanorthite component decreases from their cores totheir rims (Fig. 9e).


Fig. 6. Bulk Al2O3 vs CaO diagram of the Uenzaru peridotites.Pyrolite composition is after Ringwood (1966).

Fig. 7. Al (a) and Ca (b) concentration profiles across one of thelarge orthopyroxene porphyroclasts in the plagioclase-lherzolitemylonite.

Table 3Chemical compositions of one of the largest orthopyroxene por-phyroclasts in the plagioclase-lherzolite mylonite

Element Rim Core

(wt%)Na2O 0.000 0.035SiO2 55.502 54.130Al2O3 3.119 5.625MgO 33.935 32.452FeO 6.433 6.214CaO 0.335 0.453TiO2 0.163 0.146MnO 0.193 0.137Cr2O3 0.223 0.410Total 99.903 99.601

(cations=6O)Na 0.0000 0.0024Si 1.9192 1.8762Al 0.1271 0.2304Mg 1.7492 1.6752Fe 0.1860 0.1803Ca 0.0124 0.0167Ti 0.0042 0.0039Mn 0.0057 0.0040Cr 0.0061 0.0112Total 4.0100 4.0004

Mg=(Mg C Fe) 0.904 0.903Wo 0.006 0.009En 0.898 0.895Fs 0.096 0.096Al (M1) 0.074 0.116

T (ºC) 758 (650 MPa) 958 (750 MPa)765 (1350 MPa) 965 (1400 MPa)

The core composition is averaged from measurements of 6 pointsnear the center of the compositional zoning. Al (M1) representsAl content of orthopyroxene in M1 site. Temperatures calculatedby using the Gasparik (1987) thermometry are also indicated.Pressures used for the temperature calculation are the lower andupper limits within the spinel-lherzolite stability field, given theAl content (see Fig. 12).

Chemical compositions of spinel porphyroclastcores in the protomylonite are roughly constant(Fig. 10a), while those in the mylonite show a vari-ation in Al content (Fig. 10b). Vermicular spinelgrains in symplectite around porphyroclasts in themylonite have lower Al contents than the adjacentporphyroclast cores (Fig. 10b). The Al content ofspinel in the fine-grained layer is comparable withthat of symplectite spinel, but is relatively lower thanthat of the porphyroclast core (Fig. 10b).


Fig. 8. Al (a), Ca (b) and Na (c) concentration profiles acrossone of the largest clinopyroxene porphyroclasts in the plagio-clase-lherzolite mylonite.

8. Discussion

8.1. Phase-transformation reaction in the Uenzarulherzolite

There are two possibilities for the origin of plagio-clase in plagioclase lherzolite: crystallization frommelt or nucleation by a subsolidus reaction. The

bulk chemical composition close to that of pyrolite(Fig. 6) as well as a smooth reverse zoning in pla-gioclase (Fig. 9a–c) indicate that plagioclase in theUenzaru plagioclase lherzolite is formed by a sub-solidus decompression reaction, but not from melt.

Symplectite composed of plagioclase, olivine andchromian spinel around porphyroclasts of ortho-pyroxene, clinopyroxene and spinel in the pla-gioclase-lherzolite mylonites (Fig. 3b, Fig. 4c,e,h)indicates a subsolidus reaction of orthopyroxene,clinopyroxene and spinel to produce the symplectite.The mineral assemblage of pressure shadows adja-cent to porphyroclasts and that of fine-grained layersare the same as that of symplectite. This, togetherwith vermicular spinel included in plagioclase andolivine grains in fine-grained layers, indicates thatplagioclase and most olivine grains in both pressureshadows and fine-grained layers are derived fromsymplectite, although some olivine grains may bederived from porphyroclast mantles.

In a closed system, clinopyroxene is the onlysource for Na in plagioclase nucleated in lherzo-lite. The Na content in porphyroclast clinopyroxeneactually decreases at the rim (Fig. 8c), suggestinga consumption of Na to produce plagioclase. Alower Al content of spinel in symplectite than inthe porphyroclast core (Fig. 10b) suggests a supplyof Al from porphyroclast spinel to plagioclase. Pro-vided that porphyroclast orthopyroxene, clinopyrox-ene and spinel are represented by enstatite, diopsideand chromspinel, respectively, and that plagioclase,olivine and chromian spinel in symplectite are rep-resented by anorthite, forsterite and picrochromite,respectively, the following reaction is suggested tohave occurred in the Uenzaru plagioclase lherzolite:

2.1� x/MgSiO3 C .1� x/CaMgSi2O6 Censtatite diopside

MgAl2.1�x/Cr2x O4! .1� x/CaAl2Si2O8 Cchromspinel anorthite

2.1� x/Mg2SiO4 C xMgCr2O4

forsterite picrochromite

where x is a variable dependent on the compositionof spinel porphyroclast. x ranges from 0.09 to 0.18in the Uenzaru plagioclase lherzolite. The abovereaction is the phase-transformation reaction from


Fig. 9. Anorthite component profiles in plagioclase grains. (a) Plagioclase in a fine-grained layer. (b) and (c) Plagioclase grains adjacentto a spinel porphyroclast. (d) and (e) Plagioclase in symplectite around orthopyroxene porphyroclasts.

spinel lherzolite to plagioclase lherzolite (Kushiroand Yoder, 1966).

8.2. Temporal relation betweenphase-transformation reaction and deformation

Ozawa (1989) suggested that a heterogeneousdistribution of Al and Cr contents in spinel such that


Fig. 10. Al–Cr–Fe3C atomic ratio of spinel in the plagioclase-lherzolite protomylonite (a) and mylonite (b). Plus D porphyroclast core,circle D symplectite, triangle D fine-grained layer, bold circle D symplectite adjacent to a specific porphyroclast core (bold plus).

it is aluminous in the long-axis rim while chromianin the short-axis rim, is formed due to unequaldiffusivities of Al and Cr ions during deformationof spinel grains by solid-state diffusion creep. Alin spinel porphyroclast is thus transported towardthe long-axis rim, while it is consumed during theplagioclase-producing phase-transformation reactionas discussed above.

The relatively coarse grain size and high anor-thite component of plagioclase grains adjacent tothe long-axis rim of an isolated spinel porphyroclastwith respect to those adjacent to the short-axis rim(Fig. 4j) indicate that grain growth of plagioclaseis promoted adjacent to the long-axis rim. This im-plies that the Al content necessary for the growthof plagioclase had been enriched at the long-axisrim of the spinel porphyroclast, as a result of de-

formation of the spinel porphyroclast by diffusioncreep. Growth of plagioclase has therefore occurredwhen an Al–Cr concentration gradient existed in thespinel porphyroclast. This indicates that the plagio-clase-producing reaction has occurred either duringor after deformation of the spinel porphyroclast. Thereaction after the deformation would modify or de-stroy the Al–Cr concentration gradient in the spinelporphyroclast, because Al is consumed to produceplagioclase. However, the Al–Cr concentration gra-dient is still well preserved in the porphyroclast(Fig. 4g), and therefore deformation of the spinelporphyroclast and the plagioclase-producing reac-tion are considered to have occurred simultaneously.

Elongate spinel porphyroclasts are aligned paral-lel to the other porphyroclasts whose long axes areoriented slightly clockwise to the foliation defined


Fig. 11. Schematic diagram showing a possible strain localization process in the Uenzaru lherzolite. Mineral abbreviations are the sameas in Fig. 3.

by fine-grained layers (Fig. 4a,b,f). Spinel porphyro-clasts have therefore likely been deformed togetherwith olivine, orthopyroxene and clinopyroxene por-phyroclasts during mylonitization of the lherzolite. Itis thus concluded that mylonitization of the lherzolitehas occurred simultaneously with the phase-transfor-mation reaction.

8.3. Transformation-induced strain localization

The fine-grained aggregate which defines the fine-grained layers and pressure shadows in the pla-gioclase-lherzolite mylonites is mostly formed fromsymplectite produced by the phase-transformationreaction, although relatively large olivine and spinel(e.g. Fig. 4i,k) grains are likely derived from porphy-roclasts. Deflection of pressure shadows into fine-grained layers (Fig. 3a,c, Fig. 4a,d,f) implies thatstrain is localized into the reaction products. Shear-ing of symplectite is also observed (Fig. 4c).

The modal content of reaction-derived plagioclasein fine-grained aggregate is 1.0% in the protomy-lonite, while it is 4.6% in the mylonite (Table 1).Provided that fine-grained olivine and spinel aretotally derived from the phase-transformation re-action, the modal content of reaction products offine-grained plagioclase, olivine and spinel is 23.3%in the protomylonite, while it is 49.2% in the my-lonite. Therefore, the reaction is more extensive inthe mylonite than in the protomylonite. Existence ofsmall strain-free grains produced by reactions en-hances rock deformation (White and Knipe, 1978;Rubie, 1990). If the modal content of weak minerals

reaches 20–30% in a rock, then they interconnect toform layers which control bulk rheology of the rock(Handy, 1990). The fine-grained aggregates formedby the phase-transformation reaction actually later-ally connect with each other to form fine-grainedlayers (Fig. 4a), which may have controlled bulkrock rheology (Fig. 11).

In the Uenzaru peridotites, mylonites are devel-oped only in plagioclase lherzolite where the phase-transformation reaction occurred, and not developedin dunite, harzburgite or spinel lherzolite. This alsoimplies that the mylonites have developed due totransformation-induced strain localization.

8.4. Pressure–temperature (P–T) path of theplagioclase-lherzolite mylonite

We inferred a possible P–T path of the plagio-clase-lherzolite mylonite based on the compositionalprofiles of two pyroxenes and plagioclase accordingto Ozawa and Takahashi (1995) (Fig. 12). Becausea primary bulk rock chemistry of mantle lherzoliteis preserved in the mylonite (Fig. 6), compositionalprofiles of these minerals should reflect the P–Tconditions. The Al content in orthopyroxene withinthe spinel-lherzolite stability field depends largely ontemperature (Gasparik, 1987), while that within thegarnet-lherzolite and plagioclase-lherzolite stabilityfields depends on both pressure and temperature(Gasparik, 1984, 1987). Pressure–temperature de-pendence of the Al content in clinopyroxene is simi-lar to that of orthopyroxene (Gasparik, 1984). The Cacontent in orthopyroxene and clinopyroxene largely


Fig. 12. Inferred P–T path of the Uenzaru plagioclase-lherzolite mylonite. Stability-field boundaries, isopleths of Al content inorthopyroxene, and those of An mol% in plagioclase are after Gasparik (1987). Dry lherzolite solidus is after Takahashi and Kushiro(1986), and wet solidus is after Kushiro et al. (1968). Temperature ranges indicated by Al contents at the core and the rim ofan orthopyroxene porphyroclast are shown by horizontal shadings with gradation. P–T ranges indicated by anorthite components inplagioclase are shown by dark gray areas parallel to An mol% isopleths. A possible P–T path is shown by a black arrowed curve. Seetext for details.

depends on temperature in all three fields (Lindsley,1983). The anorthite component in plagioclase inthe mineral assemblage of orthopyroxene, clinopy-roxene, forsterite, spinel and plagioclase increaseswith decreasing pressure or increasing temperaturewithin the spinel-lherzolite stability field (Gasparik,1987).

The absence of pyroxenes at the olivine–plagioclase grain boundaries indicates that the re-verse-phase transformation from plagioclase lherzo-lite to spinel lherzolite did not occur. The ratherconstant Al and Ca contents in the cores of twopyroxenes (Figs. 7 and 8) as well as the continuousincrease in anorthite component from the plagioclasecore toward its rim (Fig. 9a–c) suggest an isothermaldecompression after achieving an equilibrium withinthe spinel-lherzolite stability field (Fig. 12). The

temperature estimated from the average Al content(0.23=6 oxygens) at the core of a large orthopyroxeneporphyroclast is approximately 960ºC (Table 3). Be-cause Al content in the pyroxene porphyroclast coremay be decreased due to later diffusion, and alsobecause the obtained line profile may not exactlysample through the center of compositional zon-ing, this temperature is likely a minimum estimate(Fig. 12). Anorthite component An44 at the core ofthe largest plagioclase grain (Fig. 9c) indicates that,at temperatures above 960ºC, the formation of pla-gioclase has started at an approximate pressure of1000 MPa (Fig. 12). An extremely slow CaAl–NaSiinterdiffusion rate in plagioclase (Grove et al., 1984)as well as a steep peak of anorthite component pre-served (Fig. 9a–c) suggest that the compositionalprofile in plagioclase is not modified by later dif-


fusion. Since the anorthite component at the rimdoes not exceed An80, the isothermal decompressionhas not continued below 800 MPa at temperaturesabove 960ºC, as indicated by the An80 isopleth afterGasparik (1987) (Fig. 12).

The constant Al and Ca contents in the cores oftwo pyroxenes (Figs. 7 and 8) may totally be a relictof an equilibrium, rather than partly a result of theisothermal decompression as discussed above. In thatcase, the reverse zoning in plagioclase (Fig. 9a–c)is possibly due to a decompression accompanied bycooling. However, the temperature at the end of thedecompression should be higher than 950ºC, as in-dicated by the junction of the An80 isopleth and thespinel-lherzolite stability-field boundary (Fig. 12).Because the minimum temperature during the equi-librium is estimated to be 960ºC from the Al contentin orthopyroxene, cooling during the decompressionis likely not important.

The decreasing Al content at the two-pyrox-ene rims (Fig. 7a, Fig. 8a), decreasing Ca contentat the orthopyroxene rim (Fig. 7b), increasing Cacontent at the clinopyroxene rim (Fig. 8b), andthe steep decrease in anorthite component downto An45 at the plagioclase rim in the fine-grainedlayer (Fig. 9a), all indicate that the temperature hasdropped abruptly (Lindsley, 1983; Gasparik, 1984,1987). A sharp peak in anorthite component pre-served at the marginal region of plagioclase (Fig. 9a)suggests rapid cooling following the isothermal de-compression. The lowest Al content in the orthopy-roxene porphyroclast rim (0.13=6 oxygens) givesan approximate temperature of 760ºC (Table 3). Itshould be noted here that we estimated the tem-perature for a plagioclase-bearing assemblage usingthe Gasparik (1987) thermometry calibrated for pla-gioclase-free assemblages. Due to the possibility ofdynamic recrystallization of orthopyroxene at therim, this must be a maximum estimate (Fig. 12).Symplectite plagioclase grains with a normal zon-ing (Fig. 9e) and those with anorthite componentsof An40–42 (Fig. 9d) are likely formed during thiscooling. The An44 and An40 isopleths, the formerof which represents the anorthite component at therim of normally zoned plagioclase (Fig. 9e), givea pressure window of 700–800 MPa at tempera-tures below 760ºC (Fig. 12). Thus the lherzolite hascooled down below 760ºC rather isobarically along

the stability-field boundary between spinel lherzoliteand plagioclase lherzolite (Fig. 12).

The existence of aluminous spinel porphyroclastindicates that the phase-transformation reaction hasnot been completed. The absence of minerals sta-ble at lower temperatures such as amphibole, chlo-rite and serpentine also indicates that the lherzoliterapidly cooled down below the nucleation cut-offtemperature (Putnis and McConnell, 1980). The Al–Cr concentration gradient preserved in spinel por-phyroclasts suggests that the closure temperature forAl–Cr diffusion in spinel has been attained before re-distribution of these ions by chemical diffusion. Allthese mineral assemblages and microchemical fea-tures are consistent with the rapid isobaric coolinginferred from mineral compositional profiles.

In summary, the Uenzaru lherzolite is inferredto have been adiabatically decompressed from pres-sures higher than 1000 MPa to 800 MPa at temper-atures above 960ºC, and subsequently cooled downbelow 760ºC rather isobarically at pressures of 700–800 MPa. Since mylonitization of the lherzolite is as-sociated with the phase-transformation reaction, themylonitization condition should be within the rangeof 1000–700 MPa and 960–760ºC on the P–T path(Fig. 12).

The Horoman peridotite complex (Niida, 1984)in the southernmost part of the Hidaka metamorphicbelt contains two-pyroxene spinel symplectite whichis formed by a breakdown of garnet (Takahashiand Arai, 1989). Based on compositional profilesof two pyroxenes and plagioclase in the lherzolite,Ozawa and Takahashi (1995) inferred that the Horo-man peridotite complex adiabatically ascended asa mantle diapir from the garnet-lherzolite stabilityfield, then cooled down below 800ºC at 500 MPawithin the plagioclase-lherzolite stability field. Theplagioclase-lherzolite mylonite in the Uenzaru peri-dotite complex has a similar P–T path to that of theHoroman peridotite complex, but cooled at a slightlyhigher pressure.

8.5. Deformation mechanisms active in theplagioclase-lherzolite mylonite

Because olivine porphyroclasts exhibit a core-and-mantle structure in which subgrains laterallypass into recrystallized mantle grains, medium-


grained mantle olivine grains are formed by dynamicrecrystallization due to subgrain rotation. LPO pat-terns of medium-grained olivine such that [100] issubparallel to lineation, and that [001] is subper-pendicular to foliation (Fig. 5a), are attributable toactivation of the (001)[100] slip system (Nicolasand Poirier, 1976; Mercier, 1985). Porphyroclast andmantle olivine is therefore deformed by dislocationcreep with (001)[100] slip being active.

The fine-grained layers controlling rheology ofthe mylonite are mainly composed of a polyphasemixture of plagioclase, olivine and chromian spinel(Fig. 3d, Fig. 4k,l). Since chromian spinel mostlyoccurs as small inclusions in plagioclase and olivinegrains, its effect on bulk deformation of the fine-grained layer is negligible. Elongate olivine grainsin the fine-grained layer and their LPO patterns suchthat [010] and [100] are oriented roughly on a girdleperpendicular to [100] maximum subparallel to lin-eation, may suggest intracrystalline deformation dueto (0kl)[100] slip (Carter and Ave’Lallemant, 1970;Nicolas and Poirier, 1976; Mercier, 1985). However,in contrast to medium-grained olivine, fine-grainedolivine does not develop subgrains, and exhibits arather weak LPO. Plagioclase grains in the fine-grained layer with the center of the compositionalzoning at their cores (Fig. 4k,l) indicate that theywere originally fine-grained and have undergonegrain growth. Although deformation lamellae andundulatory extinction indicate the presence of dis-locations, the equigranular grain shape, absence ofsubgrains, and straight grain boundaries with ¾120ºdihedral angles at triple junctions (Fig. 4k,l) rathersuggest that plagioclase in the fine-grained layer ispossibly deformed by grain boundary sliding accom-modated by the activity of dislocations. The initialfine grain size of the reaction products as well asinhibited grain growth due to a polyphase aggre-gate (White and Knipe, 1978; Brodie and Rutter,1985) also favor superplastic deformation of thefine-grained aggregate by grain boundary sliding.

In order to give a further constraint on deforma-tion mechanisms in the porphyroclast-mantle olivineand in the fine-grained layer, we have measuredgrain sizes of dynamically recrystallized, medium-grained olivine in the porphyroclast mantles, and offine-grained olivine in the fine-grained layers. 884dynamically recrystallized olivine grains and 755

fine-grained olivine grains were traced from pho-tomicrographs, and their grain sizes were obtainedby image analysis as diameters of circles with thesame areas of individual grains. The obtained aver-age š standard-deviation grain size of dynamicallyrecrystallized olivine is 83 š 34 µm, while that offine-grained olivine is 34š 13 µm.

Using the dynamically recrystallized grain-sizepiezometer calibrated by van der Wal et al. (1993),the stress associated with dynamic recrystalliza-tion of porphyroclast olivine is estimated to havebeen 39–74 MPa. This dynamic recrystallizationmay have been occurred either during the isother-mal decompression at temperatures above 960ºC orduring the subsequent cooling down below 760ºC.We accordingly constructed deformation mechanismmaps for dry olivine at two different temperatures of1000ºC and 800ºC (Fig. 13) following Drury and FitzGerald (1998). It seems reasonable to assume dry de-formation conditions, because of a negligible amountof hydrous minerals in the plagioclase-lherzolite my-lonite (Table 1). On both deformation mechanismmaps of 1000ºC and 800ºC (Fig. 13), the range ofdynamically recrystallized grain size (83 š 34 µm)is plotted within the field of [a] dislocation creepwhose rate is grain-size sensitive because of ac-commodation mechanisms such as grain boundarysliding or diffusion creep (Drury and Fitz Gerald,1998). Strain localization into the fine-grained layermay have resulted in a change of stress-support-ing framework from the dynamically recrystallizedolivine to the fine-grained layer. Provided that fine-grained olivine has subsequently supported the stresslevels which had been supported by dynamically re-crystallized olivine (39–74 MPa), the former’s grainsize (34 š 13 µm) indicates a rheology close to theboundary between [a] dislocation creep and diffu-sion creep (Fig. 13). Thus it is likely that olivineas well as plagioclase in the fine-grained layer aredeformed by grain boundary sliding accommodatedby activity of dislocations.

8.6. Kinematics and tectonic implications

Diapiric ascent of a peridotite body results in anearly adiabatic decompression of the body. In con-trast, its uplift by thrust faulting leads to an isobariccooling. The adiabatic decompression followed by


isobaric cooling inferred for the plagioclase lherzo-lite of the Uenzaru peridotite complex therefore im-plies its diapiric ascent and subsequent uplift along athrust. Diapiric ascent of a peridotite body is closelyassociated with partial melting of the peridotite (e.g.Nicolas, 1986). In the Uenzaru peridotite complex,dunite, harzburgite and spinel lherzolite surround-ing the plagioclase lherzolite are residual peridotitesin which partial melting occurred, because they arepoor in incompatible elements such as Al, Ca, Ti, Na

and Y (Table 2). The plagioclase-lherzolite unit mayrepresent a block in which partial melting did notoccur in a mantle diapir.

The highest metamorphic P–T condition of thebasal granulite in the Main Zone of the Hidakametamorphic belt is estimated to be 720 MPa and870ºC (Osanai et al., 1991). The P–T conditionat the initiation of cooling of the lherzolite (¾800MPa and ½960ºC; Fig. 12) is thus well correlatableto the highest metamorphic P–T condition of theMain Zone, suggesting that the rapid cooling of thelherzolite is due to the contact with crustal rocks, bywhich the latter may have been metamorphosed.

All shear sense indicators in the plagioclase-lher-zolite mylonite such as asymmetric LPO patterns ofmedium- and fine-grained olivine (Fig. 5), alignmentof porphyroclast long axes oblique to the myloniticfoliation (Fig. 4a,b,f), and the heterogeneous Al–Crdistribution in spinel porphyroclasts (Fig. 4g), indi-cate a top-to-the-south sense of shear. The top-to-the-south sense of shear is also reported from the UpperZone of the Horoman peridotite complex (Sawaguchiand Takagi, 1997). In contrast, a dextral sense ofshear is inferred for the Main Zone mylonites alongthe HMT (Toyoshima et al., 1994). Recent seismicreflection profiling across the Hidaka metamorphicbelt revealed that the HMT dipping steeply eastwardat the surface is a listric fault dipping gently atdepth (Arita et al., 1998). The HMT with a dextralslip component has therefore a top-to-the-south slip

Fig. 13. Deformation mechanism maps for dry olivine at apressure of 1000 MPa, oxygen fugacity at the quartz–fayalite–magnetite buffer, and temperatures of 1000ºC (a) and 800ºC(b). Flow laws used, strain-rate calibrations for different oxygenbuffers, and the boundary between [a] dislocation-creep and [c]dislocation-creep fields are after Drury and Fitz Gerald (1998).The majority of strain in two dislocation-creep fields results from[a] slip, but accommodation strain occurs by [c] slip in the [c]dislocation-creep field, while by grain boundary sliding and dif-fusion creep in the [a] dislocation-creep field (Drury and FitzGerald, 1998). Thick broken line indicates the relationship be-tween dynamically recrystallized grain size and stress calibratedby van der Wal et al. (1993). Dark shaded area (DR) indicates theranges of grain size of dynamically recrystallized olivine (hor-izontal line) and associated stress (vertical line). Light shadedarea (FG) shows the grain size range of fine-grained olivine(horizontal line) and the same stress range with dynamicallyrecrystallized olivine (vertical line). See text for details.


component at depth, which may be preserved in theUenzaru and Horoman peridotite complexes. Thusthe Uenzaru lherzolite has probably been thrust upwith the Main Zone rocks along the HMT.

Acknowledgements

We thank S. Arai, Y. Hiroi, T. Ito, K. Ozawaand N. Takahashi for discussions and suggestions,and H. Yoshida for assisting with EPMA and XRFanalyses. We also thank M.R. Drury and an anony-mous reviewer for helpful comments and improvingthe manuscript. This study was supported in part bya grant from the Fukada Geological Institute to M.Furusho and the Grant 07454120 from the Ministryof Education, Science and Culture of Japan to K.Kanagawa.

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Transformation-induced strain localization in a lherzolite