Petrology of Peridotite Xenoliths from Iraya Volcano, Philippines, and its Implication for Dynamic Mantle-Wedge Processes

Petrology of Peridotite Xenoliths from IrayaVolcano, Philippines, and its Implication forDynamic Mantle-Wedge Processes

SHOJI ARAI1*, SHUICHI TAKADA1, KATSUYOSHI MICHIBAYASHI2

AND MEGUMI KIDA1

1DEPARTMENT OF EARTH SCIENCES, FACULTY OF SCIENCE, KANAZAWA UNIVERSITY, KANAZAWA 920-1192, JAPAN

2INSTITUTE OF GEOSCIENCES, FACULTY OF SCIENCE, SHIZUOKA UNIVERSITY, SHIZUOKA 422-8529, JAPAN

RECEIVED NOVEMBER 12, 2002; ACCEPTED AUGUST 14, 2003

Peridotite xenoliths entrained in calc-alkaline andesites from the

Iraya volcano, Philippines, were petrologically examined to con-

strain the nature of the mantle-wedge materials and processes. They

can be classified into two types: C-type (coarse-grained type) and F-

type (fine-grained type) peridotites. C-type peridotites are mostly

coarse-grained (olivine, �1 mm across) harzburgites with porphyro-

clastic to protogranular textures but include subordinate dunites. F-

type peridotites are fine-grained (olivine, �60---70 mm across).

Secondary orthopyroxenes that replace olivine and sometimes show

radial (spherulitic) aggregation are very common in F-type perido-

tites and, subordinately, in C-type peridotites, in which the total

amount of orthopyroxene increased in volume. Fine-grained olivine

in F-type peridotites characteristically has minute glass and chromian

spinel inclusions. Mineral chemistry is clearly different between the

two types of peridotite: olivine is around Fo91---92 and Fo89---91 in

C-type and F-type peridotites, respectively. The Cr/(Cr þ Al)

atomic ratio (Cr number) and Fe3þ/(Cr þ Al þ Fe3þ) atomic

ratio of chromian spinel are 0�2---0�3 and 50�1, respectively, in

C-type peridotites, and 0�4---0�7 and around 0�1, respectively, in

F-type peridotites. The secondary orthopyroxenes are appreciably

lower in Al2O3, Cr2O3 and CaO than the primary ones. A textural

transition from C-type to F-type peridotites can be observed; coarse

olivine becomes recrystallized into fine grains through subgrains that

preserve the previous coarse texture. The C-type harzburgites are

similar in mineral chemistry to arc-type harzburgites, e.g. mantle

xenoliths from the Japanese island arcs, and may represent samples

of the sub-arc lithospheric mantle. The C-type harzburgites beneath

the Iraya volcano may have been strained and deformed during

oblique subduction of the South China Basin. A silicate melt rich

in SiO2, H2O and Fe, possibly derived by fractional crystalliza-

tion from a primitive arc magma, assisted the recrystallization of the

C-type peridotites to the F-type peridotites with metasomatic chem-

ical modification. Oblique subduction is common in arc---trench

systems, suggesting that F-type peridotite formation may be common

within the mantle wedge.

KEY WORDS: mantle wedge; peridotite; metasomatism; Iraya volcano;

Philippines

INTRODUCTION

Samples of the sub-arc mantle, represented by peridotitexenoliths entrained in arc magmas, are rare relative tomantle samples from non-arc settings, i.e. from oceanichotspots and continental rift zones (e.g. Nixon, 1987).This means that there is a paucity of xenolith-baseddirect petrological information about the mantle wedgerelative to other tectonic settings. Hence the rare examplesof arc-derived peridotite xenoliths need to be investigatedsystematically and in detail to explore the nature ofmantle-wedge materials and processes.

Peridotite xenoliths of possible mantle-wedge originhave been described from the Japanese island arcs (e.g.Takahashi, 1978; Aoki, 1987; Abe, 1997; Abe et al.,1998; Arai et al., 1998, 2000), the Colorado Plateau(e.g. Smith & Riter, 1997; Smith et al., 1999), the Cas-cades, USA (Brandon & Draper, 1996; Ertan & Leeman,1996), Mexico (Luhr & Aranda-G�oomez, 1997), PapuaNew Guinea (Gr�eegoire et al., 2001; McInnes et al.,2001; Franz et al., 2002) and Kamchatka (Kepenzhiskas

JOURNAL OF PETROLOGY VOLUME 45 NUMBER 2 PAGES 369–389 2004 DOI: 10.1093/petrology/egg100

*Corresponding author. Telephone: 81-(0)76-264-5724. Fax: 81-(0)76-

264-5746. E-mail: [email protected]

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et al., 1995; Arai et al., 2003). In this study we focus on theperidotite xenoliths hosted in arc-type andesite of theIraya volcano, in the Luzon arc (Richard, 1986; Mauryet al., 1992). Among the Iraya peridotite xenolithsextremely fine-grained peridotites [F-type of Arai &Kida (2000)] predominate over coarse-grained types(C-type). Peridotite xenoliths with similar characteristicsare also known from the Avacha volcano, Kamchatka,and it has been proposed that the fine-grained peridotitesare characteristic of the mantle wedge beneath islandarcs (Arai et al., 2003). Their distinctive characteristicshave not been observed in other tectonic settings (e.g.oceanic hotspots and continental rift zones), but areprobably common to mantle-wedge peridotites. In aprevious paper (Arai & Kida, 2000) we presentedbasic petrographical and mineral chemical data andreferred to a possible deserpentinization (¼ dehydrationrecrystallization from serpentinite) origin for the F-typeperidotites. Here we present a new interpretation, basedon a more detailed petrological study of the peridotitexenoliths from the Iraya volcano, and discuss the petro-logical characteristics of the mantle wedge. We focusespecially on the origin of the F-type peridotites, basedon petrological and fabric analyses in the context of thetectonic situation of the mantle wedge.

GEOLOGICAL AND TECTONIC

BACKGROUND

Batan is the main island of the Batanes Province, bound-ing the northernmost territory of the Philippines (Fig. 1).The volcanoes of Batan belong to the Babuyan Segment,the least evolved of four segments of the Luzon arc(Defant et al., 1989, 1990). Batan is located at the junc-tion of the western and eastern chains of the Taiwan---Luzon arc (Yang et al., 1996). The Babuyan Segment hasevolved on the western part of the Philippine Sea plate,which is subducted by the South China Sea plate and theEurasian plate along the Manila Trench (e.g. Lallemandet al., 2001) (Fig. 1). The underthrust plate has a very highangle of subduction or is even overturned beneath Batan(Yang et al., 1996; Lallemand et al., 2001). The PhilippineSea plate is moving northwestward with a velocity ofabout 7 cm/year relative to the Eurasian plate (Seno,1977).

Batan comprises three volcanoes, Mahatao, Mataremand Iraya (Fig. 1) with different ages (Richard et al.,1986a, 1986b). Mahatao volcano is the oldest; itseruption started during the Late Miocene in the centralpart of Batan. Matarem volcano in the southern part ofthe island has been strongly dissected and its volcanicproducts are covered by volcanics from the two youngervolcanoes, sediments and coral reefs. Matarem volcano isPliocene to Early Pleistocene in age. Iraya volcano has

been active since the Late Pleistocene in the northernpart of the island (Fig. 1). The volcanic rocks from Batanare andesitic to basaltic (Richard et al., 1986a). Gabbroiccumulate xenoliths and hornblende megacrysts havebeen reported from pyroclastics of the Mahatao volcano(Richard et al., 1986a).

Xenoliths are especially abundant in recent pyroclast-ics of calc-alkaline series lavas (1480 years BP) eruptedfrom Iraya volcano (Richard et al., 1986a, 1986b), col-lected mainly from cliffs at Song-Song Bay and BaluganBay. They are ultramafic (peridotitic) to mafic (gabbroic)in composition, rounded to subangular in shape and areup to 25 cm across. Subordinate peridotite xenoliths havealso been found in volcanics from Matarem volcano.Xenoliths of basement crystalline schists are common inthe lavas from Matarem volcano but are very rarelyfound in the lavas from Iraya volcano. Fine-grained(F-type) peridotite xenoliths are predominant over coarse-grained (C-type) types (Arai et al., 1996; Arai & Kida,2000). Typical C-type peridotite xenoliths, with porphyro-clastic to protogranular texture, are very rare, consistingof about 4% of all the xenolith samples examined.

The volcanics hosting the peridotite xenoliths wereanalyzed by XRF at Kanazawa University. They contain49---60 wt % SiO2 and are mostly andesites with relat-ively high K2O contents, belonging to the high-K series.They plot in the calc-alkaline field and around theboundary between calc-alkaline and tholeiitic series ona SiO2---FeO�(total FeO)/MgO diagram.

PETROGRAPHY OF THE

XENOLITHS

Arai et al. (1996) classified the peridotite xenolithsfrom Iraya volcano into two types in terms of grainsize, C-type (coarse-grained type) and F-type (fine-grained type). The two types of peridotite are verydifferent in appearance, petrography and mineral chem-istry (Arai et al., 1996). C-type and F-type peridotiteshave a light olive green color and very pale yellowishgreen color, respectively, in hand specimen. Somexenoliths are intermediate between the two types:coarse-grained peridotite is either cut or enclosed by afine-grained part. This suggests a transformation fromC-type to F-type peridotites as described in detail below.

Modal proportions of minerals were determinedby point-counting, involving 2000---3000 points coveringthe whole area of a thin section (Fig. 2). Some uncer-tainty is expected for the C-type peridotite xenolithsbecause of their small sample size. F-type peridotitesare often too fine-grained for point-counting analysis;consequently, only F-type peridotites with relativelycoarse-grained textures were analyzed by the point-counting method (Arai & Kida, 2000) (Fig. 2).

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The peridotite xenoliths of both types from Iraya havea hornblendite selvage of which the thickness is highlyvariable from sample to sample (Arai et al., 1996). Theyare occasionally entirely enclosed in hornblende gabbro,although the hornblende gabbro is never in direct con-tact with the peridotite. In extreme cases, hornblenditeencloses small angular peridotite clasts up to 1 cm across,which are partly disintegrated and darker-colored (dullyellowish green), indicating partial digestion and chem-ical modification.

C-type (coarse-grained) peridotites

C-type peridotites are mostly harzburgites (Figs 2 and 3),and usually exhibit protogranular to weakly porphyro-clastic textures (Fig. 3a and c). The rare dunites have atabular equigranular texture (Fig. 3e and f ). The volumeratio of clinopyroxene/pyroxenes is mostly less than 0�1 inC-type harzburgites. C-type harzburgites are occasionallyhigher in orthopyroxene content than abyssal peridotites(Fig. 2) partly because of the presence of orthopyroxene-rich pockets that are olivine-orthopyroxenite in mode

Fig. 1. Location of Batan in the Luzon---Taiwan arc and the Iraya volcano on Batan. After Defant et al. (1989) and Lallemand et al. (2001). Thearrow indicates motion of the Philippine Sea plate relative to the Eurasian plate (Seno, 1977).

ARAI et al. SUB-ARC PERIDOTITE XENOLITHS FROM PHILIPPINES

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and are 50�5 cm across (e.g. sample 124-1 of Fig. 2).The C-type harzburgites rarely contain clinopyroxene-rich bands. Olivine in harzburgites is up to 5 mm across,and is clear but partly turbid as a result of glass inclusiontrails (Fig. 3b). Olivine and orthopyroxene frequentlyshow wavy extinction or kink bands (Fig. 3a), and ortho-pyroxene porphyroclasts, up to 1 cm across, commonlycontain thin exsolution lamellae of clinopyroxeneespecially in their central part (Fig. 3d). Clinopyroxeneis anhedral, fine-grained and small in amount; it is com-monly associated with orthopyroxene porphyroclasts. Itis subhedral and is selectively turbid in many samples.Chromian spinel is anhedral and brown-colored in thinsection (Fig. 3c). Plagioclase and hydrous minerals aretotally absent.

F-type (fine-grained) peridotites

F-type peridotite xenoliths contain green-colored specklesup to 1 cm across, which can be identified as concentra-tions of minute grains of chromian spinel in thin section.

F-type peridotites are similar in their modal mineralogyto C-type peridotites (Fig. 2); foliation occurs in somesamples. Olivine is around 60---70 mm across and con-tains minute spherical inclusions of orbicular glass withchromian spinel and bubbles (Schiano et al., 1995)(Fig. 4a---d). Chromian spinel typically occurs in fine-grained aggregates of various shapes and is dark brownto black (Fig. 4e and f ), often accompanied by glass. Thisglass is interstitial to the spinel aggregate and is larger insize than other types of glass. The chromian spinel oftenexhibits pull-apart textures, suggesting that the originalcoarse spinel grains were flattened and split into piecesperpendicular to the foliation plane (Fig. 4e and f ).Globules of Fe---Ni sulfide are characteristically found inF-type peridotites. Plagioclase is very rarely found as ananhedral grain interstitial to olivine (Arai et al., 1996).Very small amounts of amphibole with a pale greenishcolor occur in the F-type peridotites.

The F-type peridotites frequently contain two typesof coarser olivine. One is clear and euhedral toanhedral in shape, and is medium in size up to 1 mm

Fig. 2. Modal amounts of olivine (ol), orthopyroxene (opx) and clinopyroxene (cpx) in peridotite xenoliths from Iraya volcano. Determined bypoint counting (see text). [Note that orthopyroxene addition (dotted line with an arrowhead) is distinct for particular samples (60-12, E19-12 and 72-1) of metasomatized C-type peridotites that preserve primary textures.] Square drawn with a dotted line indicates the protolith (dunite for 60-12and E19-12; harzburgite for 72-1) before metasomatism. Fields for harzburgite xenoliths from Noyamadake and Kurose, in the SW Japan arc, areshown for comparison (Arai et al., 1998, 2000). The sub-arc harzburgites are enriched in orthopyroxene relative to abyssal harzburgites (Dick,1989). The arrow indicates a silica-enrichment trend observed in the metasomatized peridotite xenoliths from Avacha in the Kamchatka arc (Araiet al., 2003).


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across (Fig. 4g and h). It is characteristically free fromstrain and is equivalent to the ‘tablet olivine’ described inperidotite xenoliths from kimberlites (Boullier & Nicolas,1975). The other is as coarse as olivine in the C-typeperidotites, and is turbid owing to minute glass inclusions

(Fig. 4g). This type of olivine occasionally includes thetablet olivine above (Fig. 4g). This coarse olivine is con-sidered to be a relic of olivine from a C-type peridotiteprotolith (Fig. 3). Fine olivine grains were annealed toform medium strain-free olivine, or more favorably, the

Fig. 3. Photomicrographs of C-type peridotite xenoliths from Iraya volcano. Scale bar represents 0�5 mm. (a) Harzburgite with porphyroclastictexture. [Note the deformation of olivine and orthopyroxene (upper left).] Cross-polarized light. (b) Coarse olivine with secondary inclusion trails inC-type harzburgite. Plane-polarized light. (c) Brown anhedral chromian spinel in C-type harzburgite. Plane-polarized light. (d) Partly metasoma-tized C-type harzburgite (72-1; Fig. 2). Opx-1 is a primary orthopyroxene with clinopyroxene lamellae inside. Opx-2 is secondary orthopyroxenepartly recrystallized from the rim of opx-1. Olivine (ol) is partly replaced by opx-2 with ragged grain boundaries. (e) Long acicular orthopyroxene(opx-2) replacing olivine in C-type dunite (60-12). Opx-2 encloses small irregular-shaped olivine grains (ol-2), which have the same composition andcrystallographic orientation as the primary olivine (ol-1). (See Table 1 for mineral chemistry.) (f ) Laths of secondary orthopyroxene (opx-2)replacing olivine in C-type dunite (E19-12).


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Fig. 4. Photomicrographs of F-type peridotite xenoliths from Iraya volcano (a) Fine turbid olivine with abundant inclusions. Plane-polarizedlight. (b) Cross-polarized light. (Note the small grain sizes of olivine.) (c) Numerous inclusions of chromian spinel and glass (gl) in fineolivine. Plane-polarized light. (d) Reflected light. [Note the inclusions of spinel (bright spots) and glass (gl).] (e) Coarse chromian spinel (dark,center) in fine-grained olivine matrix. Cross-polarized light. (f ) Reflected light. (Note the pull-apart texture of chromian spinel.) (g) Turbid coarseolivine enclosing clear strain-free medium-sized olivine. Cross-polarized light. (h) Clear strain-free olivine within fine-grained olivine matrix. Cross-polarized light.


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coarse strained olivine grains, with tablet olivine (Fig. 4g),were selectively recystallized into fine grains (Fig. 4h).

Textural variation from C-type to F-type peridotitexenoliths is shown in Fig. 5. In C-type peridotites,

relatively coarse grains of olivine show triple junctionswith straight grain boundaries. The grain-size distribu-tion of olivine is nearly log-normal and its mean grainsize is �800 mm (Fig. 6). In some intermediate-type

Fig. 5. Transition from C-type peridotite to F-type peridotite through dynamic recrystallization. (a) C-type peridotite. (Note the coarse olivine.)(b) Transitional peridotite. (Note the original coarse olivine grains transformed into aggregates of subgrains.) (c) F-type peridotite. Fine-grainedolivine aggregates are generated probably by subgrain rotation from transitional peridotite (b). Left and right panels in plane-polarized light andcross-polarized light, respectively. (See text for detailed description.)


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peridotites, these coarse grains of olivine contain uniformsubgrains (Figs 5b and 6). It should be noted that sub-grains occur in old coarse grains and they have remark-ably uniform sizes from one grain to another (Figs 5b and6). The size distribution of these subgrains is log-normaland the mean subgrain size is �50 mm (Fig. 6). In F-typeperidotites, extremely fine olivine grains occur and theirgrain sizes are extremely uniform in each xenolith(Figs 5c and 6). The grain-size distribution is log-normaland the mean grain size is �70 mm (Fig. 6).

Olivine CPO patterns

Olivine CPO (crystallographic preferred orientation) ofboth C-type and F-type peridotites was measured byscanning electron microscopy (SEM) on highly polishedthin sections using a JEOL 5600 system equipped withelectron back-scattered diffraction (EBSD). A total of 290and 299 olivine crystal orientations were determinedrespectively and the computerized indexation of the dif-fraction pattern was visually checked for each orienta-tion. Although the structural reference frame is unknownin these samples, the measured olivine CPO is presentedon equal area, lower hemisphere projections, where themaximum density of the [100] axis was aligned east---west and the maximum density of the [010] axis north---south (Fig. 7).

Olivine CPO in the C-type peridotite sample is char-acterized by strong concentrations of [100] and [010]axes (Fig. 7a). The CPO occurs as a single crystal-likepoint maximum, which is similar to a typical (010)[100]pattern (e.g. Michibayashi & Mainprice, 2004, fig. 5).The olivine CPO in the F-type peridotite sample is alsocharacterized by a single crystal-like point maximum

similar to that in the C-type peridotite (Fig. 7b). How-ever, the concentrations of axes are significantly weakerthat those in the C-type peridotite.

Secondary orthopyroxene

Secondary orthopyroxenes are commonly found inC-type and subordinately in F-type peridotite xenolithsfrom Iraya. The secondary orthopyroxene has raggedboundaries with olivine and contains irregular-shapedfine-grained olivine grains (Figs 3e, f and 8a, b, d). Thefine-grained olivine inclusions have the same crystallo-graphic orientation as the surrounding coarse olivine,suggesting a replacement origin for the orthopyroxene(Fig. 8b and d). Small secondary orthopyroxene grainsalso form within coarse primary olivine (Fig. 8c). Enrich-ment in orthopyroxene can be demonstrated in some ofthe samples. A coarse-grained harzburgite (72-1) con-tains secondary orthopyroxene replacing olivine but hasstill preserved the primary minerals and texture (Fig. 3d).It contains about 21% of primary orthopyroxene andabout 20% of secondary orthopyroxene, the total ortho-pyroxene being over 40% by volume (Fig. 2). In thisharzburgite, the primary orthopyroxene has also begunto be converted to finer secondary orthopyroxene lathsaround the rim (opx-2 of Fig. 3d), as described in sub-arc peridotite xenoliths from the Avacha volcano,Kamchatka (Arai et al., 2003). Two relatively coarse-grained samples (60-12 and E19-12) were initially duniteswith equant chromian spinel, and have only secondaryorthopyroxene replacing olivine (Fig. 3e and f ). Theprimary texture is well preserved, mainly composed ofcoarse to medium grains of olivine that are only partlyreplaced by relatively fine orthopyroxene (Fig. 3e and f ).

Fig. 6. Olivine grain-size distributions in Iraya peridotite xenoliths. (Note that the previous coarse grains comprise subgrains that are one-twentiethsmaller in size in transitional peridotites between C-type and F-type peridotites.) (See Fig. 5b and text.)


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The amount of secondary orthopyroxene reaches 16 and25 vol. %, respectively, for 60-12 and E19-12 (Fig. 2).

The secondary orthopyroxene is sometimes unusual intexture, showing spherulitic or radial aggregates, no signof deformation, and no exsolution of clinopyroxene(Fig. 8e). It is clearly distinguished from ordinary mantleorthopyroxene similar to that in the coarse-grained harz-burgites (Fig. 3a---d). McInnes et al. (2001) and Arai et al.(2003) described metasomatic orthopyroxenes withexactly the same texture in sub-arc harzburgite xenolithsfrom the Lihir volcano, Papua New Guinea, and fromthe Avacha volcano, Kamchatka, Russia. As noted byArai et al. (1996) and Arai & Kida (2000), spheruliticorthopyroxene has been found as porphyroblasts indeserpentinized peridotites from thermal aureolesaround granitic intrusions (see also Arai, 1974, 1975;Matsumoto et al., 1995). Phlogopite with pale brownishcolors is sometimes associated with these secondaryorthopyroxenes, especially in F-type peridotites. Clino-pyroxene is occasionally associated with the secondaryorthopyroxene, especially with that recrystallized fromprimary orthopyroxene.

Host andesites

The xenoliths were entrained mainly by calc-alkalineandesites with phenocrysts of plagioclase, hornblende,

augite, biotite, olivine and occasionally hypersthene.Plagioclase is optically zoned but its form is varied fromeuhedral to subhedral. Some of the plagioclase is clearand some is turbid with numerous glass inclusions. Horn-blende is euhedral to subhedral, and is brown to dullgreen in thin section. Opacite rims are sometimesobserved around hornblende phenocrysts. Augite iseuhedral to subhedral and pale greenish in color.Magnetite inclusions are common. Olivine is euhedralto round in shape, and coarse euhedral grains enclosebrownish euhedral chromian spinel. Orthopyroxene isrelatively fine, if present, and frequently has a reactionrim of clinopyroxene. The groundmass is intersertal, withplagioclase, clinopyroxene, magnetite and glass.

MINERAL CHEMISTRY

Minerals and glasses were analyzed with a JEOLelectron microprobe ( JXA8800) at the Center forCo-operative Research of Kanazawa University (accel-erating voltage 15 kV and beam current 12 nA) and witha JEOL 8800 superprobe at the Tokyo Institute ofTechnology (accelerating voltage 15 kV and beamcurrent 12 nA). Special caution was taken in the NiOanalysis of olivine, using 25 kV accelerating voltage,20 nA beam current and a longer counting time (100 sinstead of 20 s for other elements). Ferrous and ferric iron

Fig. 7. Olivine CPO (crystallographic preferred orientation) of C-type (a) and F-type (b) peridotites from Iraya, measured by SEM using a JEOL5600 system equipped with electron back-scattered diffraction (EBSD). Equal area projection, lower hemisphere. Contours in 1% area. Themaximum density of the [100] axis was aligned east---west, whereas the maximum density of the [010] axis was aligned north---south. pf J, pole figureJ-index. Foliation cannot be shown because of arbitrary cutting in making thin sections.


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contents of chromian spinel were calculated assumingspinel stoichiometry. Cr number is Cr/(Cr þ Al) atomicratio of chromian spinel. Mg number is Mg/(Mg þ totalFe) atomic ratio for silicates and is Mg/(Mg þ Fe2þ)atomic ratio for chromian spinel. The minerals of the

C-type peridotites are almost homogeneous in chemistry,except in samples strongly affected along grainboundaries by the host magma. The minerals in F-typeperidotites sometimes show grain-by-grain chemicalheterogeneity, and the minerals are too small and too

Fig. 8. Photomicrographs of secondary orthopyroxenes in Iraya peridotite xenoliths. Cross-polarized light. (a) Orthopyroxene (opx-2) in the fine-grained olivine matrix of F-type peridotite. (Note the minute inclusions of olivine.) (b) Secondary orthopyroxene pool (opx-2) replacing mediumstrain-free olivine in F-type peridotite. (Note the fine olivine grains included in orthopyroxene along the boundaries with olivine, indicatingreplacement.) (c) Orthopyroxene (opx-2) replacing olivine in C-type harzburgite. [Note the fine orthopyroxene grains (white dots) within olivine(lower right).] (d) Aggregate of secondary acicular orthopyroxene (opx-2) replacing olivine in F-type peridotite. (Note the ragged boundary betweenolivine and orthopyroxene.) (e) Radial (or spherulitic) aggregate of secondary orthopyroxene in F-type peridotite. (f ) Radial (or spherulitic)aggregate of orthopyroxene in deserpentinized peridotite (orthopyroxene zone) in the contact aureole of a granitic intrusion. Tari-Misaka peridotitecomplex (Arai, 1975), SW Japan. [Note the textural similarity to the orthopyroxene in (e).] (See text for explanation.)


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turbid to examine intra-grain chemical heterogeneity insome samples (e.g. Fig. 4a and e). Only the core compo-sitions of the minerals were considered in this study.Representative analyses are listed in Table 1.

Olivine is Fo91---93 and the Cr number of spinel rangesfrom 0�3 to 0�6 in C-type peridotites (Table 1; Fig. 9).Dunites have slightly more magnesian olivine (Fo91---93)than harzburgites (Fo91---92). The Cr number of spinelincreases steeply with an increase in the Fo content ofolivine within the olivine---spinel mantle array, a spinelperidotite restite trend (Arai, 1994), in C-type peridotites(Fig. 9). Olivine is Fo89---91 and the Cr number of spinelranges from 0�4 to 0�8 in the F-type peridotites (Fig. 9). Itis noteworthy that F-type peridotites have lower Fo con-tent of olivine on average than C-type peridotites despitethe higher Cr number of spinel (Fig. 9). Olivine composi-tion is not appreciably different between the intact partand the metasomatized part in which a large amount ofsecondary orthopyroxene has formed at the expense ofolivine (60-12 and E19-12 of Table 1; Fig. 3e and f ). TheNiO content of olivine ranges from 0�4 to 0�5 wt % inC-type harzburgites (Table 1). It is, however, variable inF-type peridotites: it is sometimes high, 0�5---0�6 wt %(e.g. sample 52 of Table 1), and is sometimes low, around0�3 wt % (e.g. sample 1 of Table 1). The Fe3þ/(Cr þAl þ Fe3þ) atomic ratio of spinel is low, around 0�05 inC-type peridotites (Fig. 10), and is slightly higher, around0�1, in F-type peridotites than in C-type peridotites(around 0�05) (Fig. 10). The Cr---Al---Fe3þ ratio of chro-mian spinel gradually changes from the C-type to F-typeperidotites: the Fe3þ/(Cr þ Al þ Fe3þ) ratio of theformer spinel tends to increase sharply with increase inCr number (Fig. 10). This transitional spinel is found inperidotites intermediate between the C-type and F-typeperidotites mentioned above. Spinel with the chemicalsignature of F-type peridotite is frequently found inC-type harzburgites with secondary orthopyroxene (e.g.sample A of Table 1). This spinel compositional trend isvery different from that related to chemical modificationby the hornblendite selvage (Fig. 10). It is noteworthythat the Mg/(Mgþ Fe2þ) atomic ratio of spinel is slightlyhigher at a given Cr number in F-type than in C-typeperidotites (Arai & Kida, 2000).

Clinopyroxene is chromian diopside with more than0�5 wt % of Cr2O3 in C-type peridotites (Fig. 11). Clino-pyroxene is slightly poorer in Al2O3, Cr2O3 and Na2Oon average in F-type peridotites than in C-type perido-tites (Fig. 11). The Na2O content of clinopyroxene isvariable from 0 to 1 wt % in individual grains but islow, 50�6 wt % on average, for each sample of C-typeperidotite (Fig. 11). Clinopyroxenes in gabbros and asphenocrysts in the host andesite are clearly distinguishedfrom those in peridotites in having higher TiO2 andlower Cr2O3 contents (Fig. 11). Clinopyroxenes in horn-blendite selvages, as well as interstitial cpx in peridotites

adjacent to the selvages, are intermediate in compositionbetween the peridotite and gabbro/andesite clinopyrox-enes (Fig. 11).

Secondary orthopyroxenes (opx-2 in Table 1), some-times exhibiting radial aggregation, are characterized bylow contents of CaO, Al2O3 and Cr2O3 relative to pri-mary opx in C-type peridotites (Arai & Kida, 2000)(Table 1). The secondary orthopyroxene replacing oliv-ine (e.g. opx-2 of Fig. 3f ) tends to be lower in Ca, Al andCr than that recrystallized from primary orthopyroxene(e.g. opx-2 of Fig. 3d). The secondary orthopyroxenesare very similar in chemistry to those in metaso-matized peridotite xenoliths from the Avacha volcano,Kamchatka (Arai et al., 2003) and from the ColoradoPlateau (Smith & Riter, 1997; Smith et al., 1999). Thetextural and chemical features (especially the radiatingform and low Ca content) of the secondary orthopyr-oxene are very similar to those of metasomatic orthopyr-oxene in deserpentinized peridotites (Arai & Kida, 2000).Hornblende and phlogopite are generally low in TiO2,51�2 wt % and52�8 wt %, respectively. Rare plagioclaseis very calcic and is An94---98.

Glass compositions

Schiano et al. (1995) reported the compositions of glassesmainly included in olivine. Complementary to their data,the glasses interstitial to chromian spinel in F-type peri-dotite and metasomatized C-type harzburgite were ana-lyzed by electron microprobe (Tokyo Institute ofTechnology) for major elements (Table 2). They arehighly silicic and contain 60---66 wt % of SiO2,2�4---4�0 wt % of Na2O and 53�8 wt % of K2O, andare similar in major-element chemistry to the glasses thatoccur mostly as inclusions in olivine (Schiano et al., 1995)except for the high analytical totals of our data. This maybe due to a difference in volatile contents depending onthe mode of occurrence: glasses completely included byminerals [primary inclusions of (Roedder, 1984)] mayhave higher volatile contents than those from the sec-ondary inclusions analyzed by Schiano et al. (1995). Thepresence of H2O and almost complete absence of CO2

and other volatiles in the glasses was preliminarily deter-mined by IR microspectroscopy. Normative quartz con-tent varies from 14 to 26 wt %, and the normativequartz/(normative quartz þ hypersthene) weight ratiois high and remarkably constant, ranging from 0�71 to0�75 (Table 2).

Thermobarometry

We calculated equilibrium temperature for the perido-tites using the two-pyroxene geothermometers ofWells (1977) and Wood & Banno (1973) for pyroxenepairs adjacent to each other. They yield no systematic


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Table 1: Selected electron microprobe analyses of minerals in C-type and F-type peridotite xenoliths from Iraya volcano, Batan, the Philippines

C-type harzburgite C-type harzburgite C-type harzburgite (metasomatized) C-type harzburgite C-type dunite (metasomatized)

Sample no.:

72-2 71-1-2 72-1 A 60-12

Mineral: ol opx cpx sp ol opx cpx sp ol opx-1 opx-2 cpx sp ol opx sp ol-1 ol-2 opx-2 sp

SiO2 41.06 55.25 52.47 0.04 40.77 56.5 53.51 0.03 40.66 56.09 57.72 52.97 0.04 40.79 55.93 0.44 41.08 41 57.92 0.03

TiO2 0 0.03 0.05 0 0 0 0 0.02 0 0 0.04 0.04 0 0 0 0.16 0.04 0 0.05 0.18

Al2O3 0 3.92 3.65 41.99 0 2.39 3.44 30.37 0 2.47 1.09 2.49 25.16 0.4 2.48 19.42 0 0 1.08 14.92

Cr2O3 0 0.82 1.28 26 0.03 0.71 1.65 39.12 0.02 0.88 0.09 0.72 41.71 0.04 0.65 42.64 0 0.01 0.28 49.49

FeO� 8.92 5.88 1.98 14.05 7.74 4.56 1.52 13.95 8.85 5.83 5.77 1.69 19.64 8.32 5.78 23.73 8.2 8.3 5.1 20.25

MnO 0.13 0.1 0.06 0.09 0.13 0.17 0.11 0.08 0.03 0.1 0.15 0.03 0.07 0.13 0.2 0 0.2 0.17 0.24 0.37

MgO 49.88 33.67 16.4 17.5 50.69 34.46 16.55 15.57 48.94 34.35 35.16 18.39 13.22 49.45 34.23 12.77 48.63 49.19 33.87 13.63

CaO 0.04 0.53 23.78 0.02 0.04 1.17 22.23 0.03 0.08 0.9 0.19 22.28 0 0.02 0.52 0.24 0.06 0.07 0.77 0

Na2O 0 0 0.32 n.d. 0.03 0 1 n.d. 0 0 0 0.24 n.d. 0 0 0 0 0.01 0.01 0.01

K2O 0 0 0 n.d. 0 0 0 n.d. 0 0 0.03 0.01 n.d. 0 0 0.07 0.01 0 0.03 0.01

NiO 0.37 0.1 0.03 0.3 0.42 0.05 0.03 0.04 0.46 0.07 0.07 0.13 0.12 0.41 0.17 0.42 0.35 0.37 0.07 0.13

Total 100.4 100.29 100.02 99.99 99.85 100.01 100.04 99.21 99.04 100.69 100.31 98.99 99.96 99.56 99.96 99.89 98.57 99.12 99.42 99.02

Mg no. 0.909 0.911 0.937 0.725 0.921 0.931 0.951 0.686 0.908 0.913 0.916 0.951 0.599 0.913 0.913 0.597 0.914 0.914 0.922 0.647

Cr no. 0.294 0.464 0.527 0.596 0.690

Mg 0.901 0.474 0.91 0.496 0.898 0.913 0.52 0.905 0.909

Ca 0.01 0.494 0.022 0.479 0.017 0.004 0.453 0.01 0.015

Fe� 0.088 0.032 0.068 0.026 0.085 0.084 0.027 0.086 0.077

Cr 0.286 0.457 0.501 0.53 0.631

Al 0.688 0.528 0.45 0.36 0.284

Fe3þ 0.026 0.015 0.049 0.11 0.085

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C-type dunite (metasomatized) F-type F-type F-type F-type

Sample no.:

E19-12 52 21 C 1

Mineral: ol-1 ol-2 opx-2 sp ol opx-2 cpx sp ol opx-2 sp ol opx-2 cpx sp ol opx-2 sp

SiO2 40.86 40.95 58.38 0.01 41.15 56.69 54.3 0.48 41.29 56.84 0.13 40.47 57.08 53.1 0.09 40.6 56.51 0.56

TiO2 0.03 0.01 0.01 0.05 0.05 0 0.08 0.19 0.04 0.11 0.04 0.11 0.07 0.11 0.13 0.09 0.04 0.17

Al2O3 0 0 0.5 21.29 0 1.55 1.67 24.59 0.09 1.83 23.96 0.32 1.32 2.34 27.62 0 1.81 25.1

Cr2O3 0 0 0 43.19 0.05 0.25 0.5 37.98 0.02 0.42 38.84 0.11 0.13 0.7 34.23 0 0.35 35.27

FeO� 6.87 6.53 4.33 18.51 8.87 5.91 2.71 22.62 7.81 5.4 22.02 8.53 5.87 2.78 21.13 9.39 6.47 21.04

MnO 0.19 0.19 0.19 0.39 0.15 0.09 0.14 0 0.19 0.2 0 0.23 0.27 0.17 0.22 0.19 0.13 0

MgO 50.31 50.07 34.9 15.77 48.7 34.7 17.46 13.34 49.81 35.09 14.04 49.18 34.36 17.6 14.8 48.23 33.42 14.28

CaO 0.05 0.03 0.23 0.02 0.13 0.44 22.86 0.18 0 0.4 0.34 0.1 0.61 23.16 0.09 0.09 0.67 0.15

Na2O 0 0 0 0.01 0 0.03 0 0 0 0.36 n.d. 0 0.02 0 0 0.25 0.19 0

K2O 0.01 0.01 0 0.03 0.01 0.02 0.02 0.22 0 0 n.d. 0.01 0 0 0.04 0 0.04 0

NiO 0.39 0.42 0.08 0.14 0.28 0.28 0 0.3 0.43 0.18 0.52 0.43 0.23 0.34 0.47 0.63 0.21 0.04

Total 98.71 98.21 98.62 99.41 99.39 99.96 99.94 99.95 99.68 100.83 99.89 99.49 99.96 100.3 98.82 99.47 99.84 96.61

Mg no. 0.929 0.932 0.935 0.717 0.907 0.913 0.92 0.608 0.919 0.921 0.639 0.911 0.913 0.919 0.665 0.902 0.902 0.662

Cr no. 0.576 0.509 0.521 0.454 0.485

Mg 0.905 0.493 0.914 0.902 0.491 0.89

Ca 0.008 0.464 0.008 0.012 0.465 0.013

Fe� 0.087 0.043 0.079 0.086 0.043 0.097

Cr 0.527 0.461 0.469 0.406 0.435

Al 0.387 0.445 0.431 0.492 0.461

Fe3þ 0.086 0.094 0.1 0.099 0.104

ol, olivine; opx, orthopyroxene; cpx, clinopyroxene; sp, chromian spinel. Primary phase and secondary phase (or remnant of primary one) are indicated by suffixes 1 and2, respectively. FeO�, total iron as FeO. n.d., not determined. Mg no., Mg/(Mg þ total Fe) atomic ratio for silicates and Mg/(Mg þ Fe2þ) atomic ratio for chromianspinel. Cr no., Cr/(Cr þ Al) atomic ratio of chromian spinel. Mg, Ca and Fe�, atomic fractions of Mg, Ca and Fe� (total iron) respectively to (Mg þ Ca þ Fe�) ofpyroxenes. Cr, Al and Fe3þ, atomic fraction of Cr, Al and Fe3þ respectively to (Cr þ Al þ Fe3þ) of chromian spinel.

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difference between the C-type and F-type peridotites,giving the same temperature range, 950---990�C for theWells (1977) geothermometer. The thermometer ofWood & Banno (1973) gives basically the same rangebut the nominal temperatures are higher by about 100�Cthan the Wells’ temperatures. Arai et al. (2003) alsoreported a similar equilibrium temperature, about900�C based on Wells (1977), for both the C-type andF-type peridotite xenoliths from the Avacha volcano,Kamchatka. This result is apparently contradictory tothe generally low contents of Ca and Al in the secondaryorthopyroxene (Arai & Kida, 2000; Arai et al., 2003). Thesecondary orthopyroxene is, however, relatively high inCa and Al if accompanied by clinopyroxene, thus

Fig. 9. Relationships between the Fo content of olivine and Cr/(Cr þAl) atomic ratio of chromian spinel in Iraya peridotite xenoliths.OSMA, olivine---spinel mantle array, a spinel peridotite restite trend(Arai, 1994). Shaded area, abyssal peridotites (Arai, 1994). Arrowindicates the variation from C-type peridotites to F-type peridotitesas a result of metasomatism.

Fig. 10. Trivalent cation ratios of chromian spinels in the Iraya peri-dotite xenoliths. Two compositional trends of chromian spinel areclearly distinguished: one from C-type to F-type peridotites, and theother for metasomatic modification by the hornblende selvage.

Fig. 11. Compositional variations in clinopyroxenes in the C- andF-type peridotite xenoliths and associated rocks Phenocrysts are fromthe host rocks; gabbro-----gabbroic crust around the peridotite xenoliths;selvage-----hornblendite selvage around the peridotite xenoliths;interstitial-----interstitial clinopyroxene in peridotites adjacent to thehornblendite selvage; vein-----hornblende-rich veins in peridotites.Forty-five samples were analyzed to produce this plot.


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indicating relatively high equilibrium temperatures. Arai& Kida (2000) reported a slightly higher Mg numberof spinel cores and lower ln K

�MgÿFeD between olivine

cores and spinel cores at a given Cr number of spinel,

where K�MgÿFeD is the apparent partition coefficient nor-

malized to a constant Fe3þ ratio (0�05) after Evans &Frost (1975). Although not examined in detail, thispossibly resulted from heating after entrainment by a

Table 2: Selected electron microprobe analyses of glass associated with chromian spinel in metasomatized

C-type or F-type peridotite xenoliths from Iraya volcano, Batan, the Philippines

Glass associated with spinel in metasomatized C-type or F-type peridotite Glass included by olivine (Schiano

et al., 1995)

Sample no.: 71-1-2 72-2 124-6 IV 4 IV D IV 12

Analysis no.: B3sp2 Bsp1re C1sp1 C7sp5 C10sp6 D1sp1 D7sp5

SiO2 64.34 61.81 60.35 63.62 64.07 65.69 65.05 58.61 61.51 61.89

TiO2 0.36 0.19 0.13 0.23 0.26 0.13 0.17 0.16 0.02 0.84

Al2O3 19.55 19.79 20.79 21.25 19.05 19.62 19.42 17.20 16.97 17.24

Cr2O3 0.82 2.59 3.17 0.82 0.50 0.63 0.30

FeO� 2.44 2.28 2.93 2.37 2.62 2.15 2.06 2.37 2.54 1.35

MnO 0.08 0.07 0.07 0.10 0.13 0.01 0.05 0.06 0.11 0.02

MgO 2.94 2.20 2.96 2.62 2.14 2.12 1.61 0.52 0.63 1.70

CaO 8.39 5.75 7.49 8.47 6.40 4.71 5.01 3.78 3.01 2.47

Na2O 2.46 3.55 2.36 2.82 2.73 3.57 4.03 4.21 5.03 3.60

K2O 0.29 3.15 0.83 0.08 2.06 3.75 3.47 2.92 2.46 4.04

P2O5 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.51 0.38 0.02

NiO 0.00 0.00 0.01 0.00 0.03 0.02 0.00

Total 101.66 101.39 101.08 102.40 99.98 102.39 101.21 90.25 92.66 93.17

MgO þ FeO� 5.38 4.48 5.89 4.99 4.76 4.27 3.63 2.89 3.17 3.05

Na2O þ K2O 2.75 6.70 3.19 2.90 4.79 7.32 7.50 7.13 7.49 7.64

Norm

il 0.68 0.36 0.25 0.44 0.49 0.25 0.32 0.13 0.04 1.60

ap 0.00 0.00 0.00 0.00 0.00 0.00 0.36 1.16 0.87 0.05

or 1.71 18.61 4.90 0.47 12.17 22.15 20.50 17.26 14.54 23.87

ab 20.81 30.02 19.96 23.85 23.09 30.19 34.08 35.62 42.56 30.46

mt 1.18 1.10 1.42 1.15 1.27 1.04 1.00 0.76 0.85 0.44

cm 1.21 3.81 4.67 1.21 0.74 0.93 0.44

an 41.45 28.52 37.15 42.01 31.74 23.36 24.52 15.42 12.45 12.12

C 0.00 0.09 2.40 1.13 0.70 1.13 0.05 1.42 1.47 2.50

di 0.13 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

hy 8.56 5.35 7.51 8.01 7.40 6.74 5.64 4.30 4.94 4.61

ol 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Q 26.02 13.59 22.94 24.20 22.48 16.68 14.62 14.14 14.98 17.55

Q/hy 3.04 2.54 3.05 3.02 3.04 2.45 2.60 3.29 3.03 3.81

KD-1 0.18 0.15 0.18 0.20 0.15 0.16 0.12 0.09 0.08 0.25

KD-2 0.20 0.16 0.20 0.22 0.16 0.17 0.14 0.10 0.09 0.28

KD-3 0.23 0.18 0.23 0.25 0.18 0.20 0.16 0.11 0.11 0.31

Selected analyses of glass inclusions in olivine (Schiano et al., 1995) are listed for comparison. FeO�, total iron as FeO. KD-1,KD-2 and KD-3; apparent partition coefficient of Mg---Fe2þ between glass and olivine [¼ (Mg/Fe)glass � (Fe/Mg)olivine]assuming that the Fe3þ/total Fe atomic ratio is 0, 0.1 and 0.2, respectively.


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magma that formed gabbroic selvages at depth beforeentrainment of the xenolith by the present host andesite.Because of the smaller grain sizes of both olivine andchromian spinel in the F-type peridotites, Mg and Fe2þ

may have diffused sufficiently, changing the core compo-sitions, even after heating for only a short period (seeOzawa, 1983). The coexistence of calcic plagioclase(An94---98) with magnesian olivine in some of the F-typeperidotites indicates that some peridotites were derivedfrom shallow mantle depths (51 GPa) (e.g. Kushiro &Yoder, 1966).

DISCUSSION

Characterization of C-type peridotites

The C-type peridotites from Iraya are different fromabyssal harzburgites (H�eebert et al., 1983; Dick, 1989;Cannat et al., 1990; Arai & Matsukage, 1996; Dick &Natland, 1996) in having characteristic orthopyroxeneenrichment (Fig. 2). They are also different from themost common ophiolitic harzburgites, for examplemantle harzburgites from the Oman ophiolite, in whichorthopyroxene is around 20% in volume (e.g. Lippardet al., 1986; Kadoshima, 2002). Instead they are similar tosome sub-arc harzburgites; e.g. harzburgite xenolithsfrom Kurose and Noyamadake, the SW Japan arc,both in mineral chemistry and mode (Fig. 2; Arai et al.,1998, 2000). The relatively low Na2O content of discreteclinopyroxenes is also one of the characteristics of somesub-arc peridotites and abyssal peridotites (Arai, 1994).In summary, we consider that the C-type peridotitesfrom Iraya reflect the petrographical and mineral chem-ical signatures of the mantle wedge.

Origin of F-type peridotites

Arai & Kida (2000) concluded that the fine-grainedperidotites were formed by fluid metasomatism, or alter-natively, but less possibly, by deserpentinization ofserpentinite in the mantle wedge. The similarity of thecharacteristic radial aggregate of orthopyroxenes sup-ports a deserpentinization origin for the F-type peri-dotites. However, we also re-examined the F-typeperidotites to see if they could have been transformedfrom C-type peridotites, assisted by melt migration,through dynamic recrystallization processes.

Transition from C-type to F-type peridotites

In C-type peridotites, coarse grains of olivine containuniform subgrains, indicating that the coarse grainshave been dynamically recrystallized. The size of thesubgrains is remarkably uniform from one grain toanother, suggesting that the recrystallization mechanismis subgrain rotation (e.g. Passchier & Trouw, 1996).Furthermore, the grain-size distributions in Fig. 6 show

that the mean size of subgrains is slightly smaller than themean grain size of olivine in the F-type peridotites. Thissize difference between the subgrains and fine olivinegrains in the F-type peridotites has also been documen-ted in experimental studies (e.g. Jung & Karato, 2001).Therefore, this suggests that F-type peridotites couldhave resulted from deformation of C-type peridotites.The transitional process can be illustrated from observa-tions of appropriate samples, where fine-grained olivineaggregates defined by similar crystallographic orienta-tions preserve previous coarse olivine microstructures(Fig. 5b). This shows that the coarse olivine grains in C-type peridotites in Fig. 5a were dynamically recrystal-lized into aggregates of far smaller subgrains as a result ofa subgrain rotation mechanism as shown in Fig. 5b.Further rotation of subgrains because of increasing strainresulted in their weak crystallographic orientations andfinally produced F-type peridotites (Fig. 5c).

The grain-size distributions of both C- and F-typeperidotites are log-normal and their microstructures arerather uniform. Therefore, assuming that their grainsizes represent steady-state grain sizes, we can estimatethe flow stress by a grain-size paleopiezometer. We usethe stress versus recrystallized grain-size relationship ofJung & Karato (2001). The estimated flow stress yields�40 MPa for F-type peridotites, but the mean grain sizeof olivine in C-type peridotites is too coarse to estimatethe flow stress by this paleopiezometer.

Although the structural reference frame is unknown, itis noted that the CPO patterns of both C-type and F-typeperidotites show a similar pattern to the {0kl}[100] sys-tem, which is the most commonly activated slip system innaturally deformed peridotite (e.g. Nicolas & Poirier,1976). The overall CPO strengths in the F-type perido-tites are remarkably weak compared with those in theC-type peridotites (Fig. 7). This may be predominantlydue to the subgrain rotation recrystallization, whichtends to weaken strong maxima by rotating the crystalsaway from the ‘ideal’ positions (e.g. Heidelbach et al.,2003).

Minute inclusions of glass and chromian spinel arevery common in olivine, especially within its centralpart, in the F-type peridotites (Fig. 4a and c). This typeof inclusion is categorized as a ‘primary inclusion’ (e.g.Roedder, 1984), suggesting entrapment of melt/fluidduring the growth of the host fine olivine. This is instrong contrast to the trail of inclusions cutting the coarseolivine in the C-type peridotites (Fig. 3b). This type ofinclusion is ‘secondary’ (Roedder, 1984), and was formedalong cracks after the formation of the host olivine. Themelt/fluid invaded after the formation of the C-typeperidotites and during the formation of the F-type peri-dotites from C-type protoliths, suggesting a dynamicrecrystallization of the C-type peridotites assisted bythis melt/fluid. Downes (1990) recognized a preference


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for mantle metasomatism in deformed or sheared partsof peridotite mantle xenoliths.

Fluid or melt for the metasomatic agent?

The orthopyroxene, which is especially characteristic ofthe fine-grained peridotites, was most probably formedby reactive replacement of olivine by a Si-rich melt/fluid.The relatively low contents of CaO, Al2O3 and Cr2O3 inthe secondary orthopyroxene from the Iraya xenolithsare also characteristic of secondary orthopyroxenes fromAvacha (Arai et al., 2003), Lihir (McInnes et al., 2001) andthe Colorado Plateau (Smith & Riter, 1997; Smith et al.,1999) that have been interpreted to have formed bymetasomatism by aqueous fluids of slab origin. Aqueousfluids in equilibrium with peridotite under high-pressureand high-temperature conditions can be reactive witholivine to form orthopyroxene at lower-pressure condi-tions (e.g. Nakamura & Kushiro, 1974; Stalder et al.,2001; Mibe et al., 2002). Direct information on the natureof the metasomatic agent involved in the formation of thesecondary orthopyroxene is not available. The secondaryorthopyroxene itself is not accompanied by glass,although glasses are more frequently found in F-typeperidotites where the secondary orthopyroxene is mostcommon.

Relatively low partition coefficients (KD values) ofMg---Fe, from 0�1 to 0�3 (mostly 0�1 to 0�2), betweenglass and olivine are obtained from pairs of host olivineand glass inclusions (Schiano et al., 1995). Our data alsoyield low KD values, from 0�1 to 0�2, for the pairs of glassassociated with spinel and olivine in F-type peridotites(Table 2). The KD values depend on the Fe2þ/Fe3þ ratioof the glass: we assumed that the Fe3þ/(total Fe) atomicratio is 0, 0�1 or 0�2 in our calculations because the redoxstate is unknown (Table 2). The KD values of Mg---Febetween glass and olivine demonstrate positive and nega-tive correlations with (MgO þ total FeO) and (Na2O þK2O) of the glass, respectively (Table 2). Combined withthe KD values of Schiano et al. (1995), which are generallylow, this is totally consistent with the tendency of KD tochange depending on the alkali content (Falloon et al.,1997; Draper & Green, 1999) and the (MgO þ totalFeO) content (Kushiro & Walter, 1998) of the melt,although our KD values, assuming the Fe3þ/(total Fe)ratio of the glass as 0, 0�1 or 0�2, are slightly lower thanthe experimental data. The melts of Falloon et al. (1997)and Draper & Green (1997) are anhydrous to slightlyhydrous low-degree partial melts of peridotite, and arenepheline-normative even when they are silicic (witharound 60 wt % of SiO2). Taking all the characteristicsof the glasses into account, the metasomatic agent couldhave been a silicate melt with a high H2O content.The relatively low KD values for olivine and glass in theF-type peridotites from Iraya (0�1---0�2) may be due to

the relatively high contents of normative quartz andH2O in the melt.

The H2O content of the glasses seems to be system-atically variable in the Iraya peridotite xenoliths. As aresult of the almost exclusive presence of H2O as avolatile, the analytical total of the microprobe analysesof the glass is expected to be lower than 100% dependingon the H2O content. The possible H2O contentdecreases from the primary glass inclusions in the olivine(around 5---10 wt %; Schiano et al., 1995) to the glassassociated with the chromian spinel (almost anhydrous inthis study, Table 2) through the secondary glass inclu-sions in olivine (around 5---7 wt %) (Schiano et al., 1995).This difference of H2O content is due to the loss of H2Oon quenching of the melt to various degrees dependingon the degree of interconnectivity of the trapped melt.The melt that initially invaded the peridotite was high inH2O (?410 wt %), considering the presence of bubblesin the glass inclusions in olivine (Schiano et al., 1995) aswell as the possible complete miscibility between SiO2-rich silicate melts and hydrous fluids at upper-mantleconditions (Bureau & Keppler, 1999). The melt thatformed the glasses was possibly saturated with olivine,after formation of the secondary orthopyroxene, and wasnot reactive with olivine. Overgrowth of olivine waspossible, but may not have occurred because any com-positional halo has not been detected around the glassinclusions in olivine.

Orthopyroxene spherulites (Fig. 8g and h) can beformed by supersaturation of the (Mg,Fe)SiO3 componentin the fluid/melt and/or by its supercooling (e.g. Inoueet al., 2000). Supersaturation in (Mg,Fe)SiO3 compo-nent was probably achieved by the contact of silica-oversaturated melt with olivine within the mantleperidotite. Similar conditions may have operated in thedeserpentinized peridotites within the contact aureoles ofthe granitic intrusions, mentioned above. Arai (1975)reported a higher silica content of the deserpentinizedperidotites in the orthopyroxene zone than in the otherzones, suggesting silica enrichment from the graniticmagma. The silica-rich fluid that emanated from thegranitic magma invaded the highest-temperature zone(orthopyroxene zone) of the dehydrating serpentiniteand produced spherulitic orthopyroxene in contact witholivine.

There is no systematic increase in the amount oforthopyroxene from the C-type to the F-type peridotites(Fig. 2), although the replacement of olivine with ortho-pyroxene can be, at least locally, observed (Figs 3e, fand 8b, d). In particular samples, however, orthopyrox-ene enrichment is discernible (Fig. 3d---f ). We cannotconclude that there has been silica enrichment of themantle wedge based on the Iyara xenolith suite. Themigrating melts appreciably modified the peridotites inchemistry (Fig. 9), suggesting that the melt was relatively


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Fe-rich. They were possibly residual melts fractionatedfrom partially solidified primitive arc magmas at deeperlevels.

Implications for tectonic setting

In this part of the Luzon---Taiwan arc the tip of themantle wedge and the overlying crust is being displacedby shearing parallel to the trench, as a result of the strainpartitioning of oblique subduction (Fitch, 1972; Pinet &Cobbold, 1992; Aurelio, 2000). This will lead to strainwithin the mantle-wedge peridotites, and any melts orfluids present may facilitate deformation/recrystalliza-tion (Figs 12 and 13). The South China Sea Basin startedto subduct along the Manila Trench beneath the Philip-pine Sea plate around Middle Miocene times (Stephanet al., 1986). The obliquity of subduction at this timewas very high between Taiwan and Luzon; Seno &Maruyama (1984) proposed a north-northwestwardmovement of the Philippine Sea plate at this time. Afterthe change to the present northwestward movement ofthe Philippine Sea plate the obliquity of subductionlessened. Consequently, the continuous shearing causedby oblique subduction that deformed the lithosphericmantle to form the F-type peridotites from the C-typeperidotites concurrent with invasion of melt (Fig. 12) mayhave ceased before the onset of the recent activity of theIyara volcano. The migrating melt dispersed into thesurrounding C-type peridotites through cracks andformed trails of secondary glass inclusions (Fig. 3b). Thestrain-free tablet olivine (Fig. 4g) formed by local recrys-tallization of strained olivine during annealing as a resultof a decrease in the obliquity of subduction (see Drury &Van Roermund, 1989).

The processes of deformation and recrystallizationdeduced from the peridotite xenoliths from Iraya,Philippines, may be common to all supra-subductionzone mantle wedges. Subduction not orthogonal to thetrench (i.e. oblique subduction) is common, especiallyaround the Western Pacific, and transcurrent faults pos-sibly related to the oblique subduction are also common(Fitch, 1972). F-type peridotite xenoliths were firstdescribed from the Avacha volcano, which is located onthe volcanic front of the Kamchatka arc (Arai et al.,2003). This is also consistent with the expected locationof the strike-slip faults caused by oblique subduction, i.e.around the volcanic front (e.g. Fitch, 1972).

SUMMARY AND CONCLUSIONS

(1) Peridotite xenoliths entrained within calc-alkalineandesites from the Iraya volcano, Philippines, can beclassified into two types, C-type (coarse-grained) and F-type (fine-grained) peridotites. Harzburgites with por-phyroclastic to protogranular textures are predominantover dunites in the C-type peridotites. Secondary

orthopyroxene replacing olivine and sometimes exhibit-ing radial (spherulitic) aggregation is very common in theF-type peridotites and, subordinately, in the C-type peri-dotites. Glasses included within olivine or interstitial tofine-grained spinel aggregates are common in the F-typeperidotites.

(2) Mineral chemistry is distinctly different between thetwo types of peridotite: olivine is around Fo91---92 andFo89---91 in the C-type and F-type peridotites, respect-ively. The Cr number and Fe3þ/(Cr þ Al þ Fe3þ)atomic ratio of chromian spinel is 0�2---0�3 and 50�1,respectively, in the C-type peridotites, and 0�4---0�7 andaround 0�1, respectively, in the F-type peridotites. Thesecondary orthopyroxenes are appreciably lower in Al2O3,Cr2O3 and CaO than the primary orthopyroxene.

(3) C-type peridotites are similar in mineral chemistryto arc-type harzburgites, e.g. the harzburgite xenolithsfrom the Japan arcs. The textural transition from C-typeto F-type peridotites can be observed under the micro-scope: coarse olivine (C-type peridotite) is recrystallizedto fine grains (F-type peridotite) through subgrains that

Fig. 12. A schematic representation of the origin of the F-type perido-tites from a C-type protolith. Strained peridotite (C-type peridotite) isdynamically recrystallized to F-type peridotite with the assistance ofmetasomatic melts rich in SiO2, H2O and Fe. (See also Fig. 13 andtext.)


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preserve the previous coarse size of the original grains.Glasses, mainly trapped in F-type peridotites, are silicatemelts rich in SiO2, H2O and Fe. The melt may haveassisted the transformation of the C-type peridotites tothe F-type peridotites.

(4) The formation of F-type peridotites from C-typeperidotites was due to shearing of the mantle wedgeby oblique subduction. This may be common withinsupra-subduction zone mantle wedges because obliquesubduction is common.

ACKNOWLEDGEMENTS

We are grateful to G. P. Yumul, Jr, the University of thePhilippines, for his arrangement of and assistance in ourfield research. Crystal orientation measurements weremade by K. Kanagawa with an SEM---EBSD system atthe Department of Earth Sciences, Chiba University. Wethank E. Hellebrand and two anonymous reviewers fortheir critical comments that improved an earlier versionof the manuscript. The editorial handling and commentsof K. Ozawa are gratefully acknowledged. A. Ninomiya,K. Kadoshima, N. Abe, M.V. Manjoorsa and C. P.David collaborated with us to collect the samples in thefield. E. S. Andal kindly provided some literature onPhilippine geology. Y. Shimizu and S. Ishimaru helpedus in preparing the manuscript.

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Documents

Petrology of Peridotite Xenoliths from Iraya Volcano, Philippines, and its Implication for Dynamic Mantle-Wedge Processes