JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 2
PAGES 369–389
2004
DOI: 10.1093/petrology/egg100
Petrology of Peridotite Xenoliths from Iraya
Volcano, Philippines, and its Implication for
Dynamic Mantle-Wedge Processes
SHOJI ARAI1*, SHUICHI TAKADA1, KATSUYOSHI MICHIBAYASHI2
AND MEGUMI KIDA1
1
DEPARTMENT OF EARTH SCIENCES, FACULTY OF SCIENCE, KANAZAWA UNIVERSITY, KANAZAWA 920-1192, JAPAN
2
INSTITUTE 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 constrain the nature of the mantle-wedge materials and processes. They
can be classified into two types: C-type (coarse-grained type) and Ftype (fine-grained type) peridotites. C-type peridotites are mostly
coarse-grained (olivine, 1 mm across) harzburgites with porphyroclastic to protogranular textures but include subordinate dunites. Ftype 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 peridotites 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 Fe3 þ /(Cr þ Al þ Fe3 þ ) atomic
ratio of chromian spinel are 02---03 and 501, respectively, in
C-type peridotites, and 04---07 and around 01, 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 crystallization from a primitive arc magma, assisted the recrystallization of the
*Corresponding author. Telephone: 81-(0)76-264-5724. Fax: 81-(0)76264-5746. E-mail: [email protected]
C-type peridotites to the F-type peridotites with metasomatic chemical 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 peridotite
xenoliths entrained in arc magmas, are rare relative to
mantle samples from non-arc settings, i.e. from oceanic
hotspots and continental rift zones (e.g. Nixon, 1987).
This means that there is a paucity of xenolith-based
direct petrological information about the mantle wedge
relative to other tectonic settings. Hence the rare examples
of arc-derived peridotite xenoliths need to be investigated
systematically and in detail to explore the nature of
mantle-wedge materials and processes.
Peridotite xenoliths of possible mantle-wedge origin
have 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 Cascades, USA (Brandon & Draper, 1996; Ertan & Leeman,
1996), Mexico (Luhr & Aranda-Gomez, 1997), Papua
New Guinea (Gregoire et al., 2001; McInnes et al.,
2001; Franz et al., 2002) and Kamchatka (Kepenzhiskas
Journal of Petrology 45(2) # Oxford University Press 2004; all rights
reserved
JOURNAL OF PETROLOGY
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et al., 1995; Arai et al., 2003). In this study we focus on the
peridotite xenoliths hosted in arc-type andesite of the
Iraya volcano, in the Luzon arc (Richard, 1986; Maury
et al., 1992). Among the Iraya peridotite xenoliths
extremely fine-grained peridotites [F-type of Arai &
Kida (2000)] predominate over coarse-grained types
(C-type). Peridotite xenoliths with similar characteristics
are also known from the Avacha volcano, Kamchatka,
and it has been proposed that the fine-grained peridotites
are characteristic of the mantle wedge beneath island
arcs (Arai et al., 2003). Their distinctive characteristics
have not been observed in other tectonic settings (e.g.
oceanic hotspots and continental rift zones), but are
probably common to mantle-wedge peridotites. In a
previous paper (Arai & Kida, 2000) we presented
basic petrographical and mineral chemical data and
referred to a possible deserpentinization (¼ dehydration
recrystallization from serpentinite) origin for the F-type
peridotites. Here we present a new interpretation, based
on a more detailed petrological study of the peridotite
xenoliths from the Iraya volcano, and discuss the petrological characteristics of the mantle wedge. We focus
especially on the origin of the F-type peridotites, based
on petrological and fabric analyses in the context of the
tectonic situation of the mantle wedge.
GEOLOGICAL AND TECTONIC
BACKGROUND
Batan is the main island of the Batanes Province, bounding 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 junction of the western and eastern chains of the Taiwan--Luzon arc (Yang et al., 1996). The Babuyan Segment has
evolved on the western part of the Philippine Sea plate,
which is subducted by the South China Sea plate and the
Eurasian plate along the Manila Trench (e.g. Lallemand
et al., 2001) (Fig. 1). The underthrust plate has a very high
angle of subduction or is even overturned beneath Batan
(Yang et al., 1996; Lallemand et al., 2001). The Philippine
Sea plate is moving northwestward with a velocity of
about 7 cm/year relative to the Eurasian plate (Seno,
1977).
Batan comprises three volcanoes, Mahatao, Matarem
and Iraya (Fig. 1) with different ages (Richard et al.,
1986a, 1986b). Mahatao volcano is the oldest; its
eruption started during the Late Miocene in the central
part of Batan. Matarem volcano in the southern part of
the island has been strongly dissected and its volcanic
products are covered by volcanics from the two younger
volcanoes, sediments and coral reefs. Matarem volcano is
Pliocene to Early Pleistocene in age. Iraya volcano has
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been active since the Late Pleistocene in the northern
part of the island (Fig. 1). The volcanic rocks from Batan
are andesitic to basaltic (Richard et al., 1986a). Gabbroic
cumulate xenoliths and hornblende megacrysts have
been reported from pyroclastics of the Mahatao volcano
(Richard et al., 1986a).
Xenoliths are especially abundant in recent pyroclastics of calc-alkaline series lavas (1480 years BP) erupted
from Iraya volcano (Richard et al., 1986a, 1986b), collected mainly from cliffs at Song-Song Bay and Balugan
Bay. They are ultramafic (peridotitic) to mafic (gabbroic)
in composition, rounded to subangular in shape and are
up to 25 cm across. Subordinate peridotite xenoliths have
also been found in volcanics from Matarem volcano.
Xenoliths of basement crystalline schists are common in
the lavas from Matarem volcano but are very rarely
found in the lavas from Iraya volcano. Fine-grained
(F-type) peridotite xenoliths are predominant over coarsegrained (C-type) types (Arai et al., 1996; Arai & Kida,
2000). Typical C-type peridotite xenoliths, with porphyroclastic to protogranular texture, are very rare, consisting
of about 4% of all the xenolith samples examined.
The volcanics hosting the peridotite xenoliths were
analyzed by XRF at Kanazawa University. They contain
49---60 wt % SiO2 and are mostly andesites with relatively high K2O contents, belonging to the high-K series.
They plot in the calc-alkaline field and around the
boundary between calc-alkaline and tholeiitic series on
a SiO2---FeO (total FeO)/MgO diagram.
PETROGRAPHY OF THE
XENOLITHS
Arai et al. (1996) classified the peridotite xenoliths
from Iraya volcano into two types in terms of grain
size, C-type (coarse-grained type) and F-type (finegrained type). The two types of peridotite are very
different in appearance, petrography and mineral chemistry (Arai et al., 1996). C-type and F-type peridotites
have a light olive green color and very pale yellowish
green color, respectively, in hand specimen. Some
xenoliths are intermediate between the two types:
coarse-grained peridotite is either cut or enclosed by a
fine-grained part. This suggests a transformation from
C-type to F-type peridotites as described in detail below.
Modal proportions of minerals were determined
by point-counting, involving 2000---3000 points covering
the whole area of a thin section (Fig. 2). Some uncertainty is expected for the C-type peridotite xenoliths
because of their small sample size. F-type peridotites
are often too fine-grained for point-counting analysis;
consequently, only F-type peridotites with relatively
coarse-grained textures were analyzed by the pointcounting method (Arai & Kida, 2000) (Fig. 2).
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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). The
arrow indicates motion of the Philippine Sea plate relative to the Eurasian plate (Seno, 1977).
The peridotite xenoliths of both types from Iraya have
a hornblendite selvage of which the thickness is highly
variable from sample to sample (Arai et al., 1996). They
are occasionally entirely enclosed in hornblende gabbro,
although the hornblende gabbro is never in direct contact with the peridotite. In extreme cases, hornblendite
encloses small angular peridotite clasts up to 1 cm across,
which are partly disintegrated and darker-colored (dull
yellowish green), indicating partial digestion and chemical modification.
C-type (coarse-grained) peridotites
C-type peridotites are mostly harzburgites (Figs 2 and 3),
and usually exhibit protogranular to weakly porphyroclastic textures (Fig. 3a and c). The rare dunites have a
tabular equigranular texture (Fig. 3e and f ). The volume
ratio of clinopyroxene/pyroxenes is mostly less than 01 in
C-type harzburgites. C-type harzburgites are occasionally
higher in orthopyroxene content than abyssal peridotites
(Fig. 2) partly because of the presence of orthopyroxenerich pockets that are olivine-orthopyroxenite in mode
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Fig. 2. Modal amounts of olivine (ol), orthopyroxene (opx) and clinopyroxene (cpx) in peridotite xenoliths from Iraya volcano. Determined by
point counting (see text). [Note that orthopyroxene addition (dotted line with an arrowhead) is distinct for particular samples (60-12, E19-12 and 721) of metasomatized C-type peridotites that preserve primary textures.] Square drawn with a dotted line indicates the protolith (dunite for 60-12
and E19-12; harzburgite for 72-1) before metasomatism. Fields for harzburgite xenoliths from Noyamadake and Kurose, in the SW Japan arc, are
shown 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 (Arai
et al., 2003).
and are 505 cm across (e.g. sample 124-1 of Fig. 2).
The C-type harzburgites rarely contain clinopyroxenerich bands. Olivine in harzburgites is up to 5 mm across,
and is clear but partly turbid as a result of glass inclusion
trails (Fig. 3b). Olivine and orthopyroxene frequently
show wavy extinction or kink bands (Fig. 3a), and orthopyroxene porphyroclasts, up to 1 cm across, commonly
contain thin exsolution lamellae of clinopyroxene
especially in their central part (Fig. 3d). Clinopyroxene
is anhedral, fine-grained and small in amount; it is commonly associated with orthopyroxene porphyroclasts. It
is subhedral and is selectively turbid in many samples.
Chromian spinel is anhedral and brown-colored in thin
section (Fig. 3c). Plagioclase and hydrous minerals are
totally absent.
F-type (fine-grained) peridotites
F-type peridotite xenoliths contain green-colored speckles
up to 1 cm across, which can be identified as concentrations of minute grains of chromian spinel in thin section.
F-type peridotites are similar in their modal mineralogy
to C-type peridotites (Fig. 2); foliation occurs in some
samples. Olivine is around 60---70 mm across and contains minute spherical inclusions of orbicular glass with
chromian spinel and bubbles (Schiano et al., 1995)
(Fig. 4a---d). Chromian spinel typically occurs in finegrained aggregates of various shapes and is dark brown
to black (Fig. 4e and f ), often accompanied by glass. This
glass is interstitial to the spinel aggregate and is larger in
size than other types of glass. The chromian spinel often
exhibits pull-apart textures, suggesting that the original
coarse spinel grains were flattened and split into pieces
perpendicular to the foliation plane (Fig. 4e and f ).
Globules of Fe---Ni sulfide are characteristically found in
F-type peridotites. Plagioclase is very rarely found as an
anhedral grain interstitial to olivine (Arai et al., 1996).
Very small amounts of amphibole with a pale greenish
color occur in the F-type peridotites.
The F-type peridotites frequently contain two types
of coarser olivine. One is clear and euhedral to
anhedral in shape, and is medium in size up to 1 mm
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Fig. 3. Photomicrographs of C-type peridotite xenoliths from Iraya volcano. Scale bar represents 05 mm. (a) Harzburgite with porphyroclastic
texture. [Note the deformation of olivine and orthopyroxene (upper left).] Cross-polarized light. (b) Coarse olivine with secondary inclusion trails in
C-type harzburgite. Plane-polarized light. (c) Brown anhedral chromian spinel in C-type harzburgite. Plane-polarized light. (d) Partly metasomatized C-type harzburgite (72-1; Fig. 2). Opx-1 is a primary orthopyroxene with clinopyroxene lamellae inside. Opx-2 is secondary orthopyroxene
partly 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 and
crystallographic 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).
across (Fig. 4g and h). It is characteristically free from
strain and is equivalent to the ‘tablet olivine’ described in
peridotite xenoliths from kimberlites (Boullier & Nicolas,
1975). The other is as coarse as olivine in the C-type
peridotites, and is turbid owing to minute glass inclusions
(Fig. 4g). This type of olivine occasionally includes the
tablet olivine above (Fig. 4g). This coarse olivine is considered to be a relic of olivine from a C-type peridotite
protolith (Fig. 3). Fine olivine grains were annealed to
form medium strain-free olivine, or more favorably, the
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Fig. 4. Photomicrographs of F-type peridotite xenoliths from Iraya volcano (a) Fine turbid olivine with abundant inclusions. Plane-polarized
light. (b) Cross-polarized light. (Note the small grain sizes of olivine.) (c) Numerous inclusions of chromian spinel and glass (gl) in fine
olivine. 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 coarse
olivine enclosing clear strain-free medium-sized olivine. Cross-polarized light. (h) Clear strain-free olivine within fine-grained olivine matrix. Crosspolarized light.
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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-grained
olivine aggregates are generated probably by subgrain rotation from transitional peridotite (b). Left and right panels in plane-polarized light and
cross-polarized light, respectively. (See text for detailed description.)
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 peridotite
xenoliths is shown in Fig. 5. In C-type peridotites,
relatively coarse grains of olivine show triple junctions
with straight grain boundaries. The grain-size distribution of olivine is nearly log-normal and its mean grain
size is 800 mm (Fig. 6). In some intermediate-type
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Fig. 6. Olivine grain-size distributions in Iraya peridotite xenoliths. (Note that the previous coarse grains comprise subgrains that are one-twentieth
smaller in size in transitional peridotites between C-type and F-type peridotites.) (See Fig. 5b and text.)
peridotites, these coarse grains of olivine contain uniform
subgrains (Figs 5b and 6). It should be noted that subgrains occur in old coarse grains and they have remarkably uniform sizes from one grain to another (Figs 5b and
6). The size distribution of these subgrains is log-normal
and the mean subgrain size is 50 mm (Fig. 6). In F-type
peridotites, extremely fine olivine grains occur and their
grain sizes are extremely uniform in each xenolith
(Figs 5c and 6). The grain-size distribution is log-normal
and the mean grain size is 70 mm (Fig. 6).
Olivine CPO patterns
Olivine CPO (crystallographic preferred orientation) of
both C-type and F-type peridotites was measured by
scanning electron microscopy (SEM) on highly polished
thin sections using a JEOL 5600 system equipped with
electron back-scattered diffraction (EBSD). A total of 290
and 299 olivine crystal orientations were determined
respectively and the computerized indexation of the diffraction pattern was visually checked for each orientation. Although the structural reference frame is unknown
in these samples, the measured olivine CPO is presented
on equal area, lower hemisphere projections, where the
maximum 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 characterized by strong concentrations of [100] and [010]
axes (Fig. 7a). The CPO occurs as a single crystal-like
point 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 also
characterized by a single crystal-like point maximum
similar to that in the C-type peridotite (Fig. 7b). However, the concentrations of axes are significantly weaker
that those in the C-type peridotite.
Secondary orthopyroxene
Secondary orthopyroxenes are commonly found in
C-type and subordinately in F-type peridotite xenoliths
from Iraya. The secondary orthopyroxene has ragged
boundaries with olivine and contains irregular-shaped
fine-grained olivine grains (Figs 3e, f and 8a, b, d). The
fine-grained olivine inclusions have the same crystallographic orientation as the surrounding coarse olivine,
suggesting a replacement origin for the orthopyroxene
(Fig. 8b and d). Small secondary orthopyroxene grains
also form within coarse primary olivine (Fig. 8c). Enrichment in orthopyroxene can be demonstrated in some of
the samples. A coarse-grained harzburgite (72-1) contains secondary orthopyroxene replacing olivine but has
still preserved the primary minerals and texture (Fig. 3d).
It contains about 21% of primary orthopyroxene and
about 20% of secondary orthopyroxene, the total orthopyroxene being over 40% by volume (Fig. 2). In this
harzburgite, the primary orthopyroxene has also begun
to be converted to finer secondary orthopyroxene laths
around the rim (opx-2 of Fig. 3d), as described in subarc peridotite xenoliths from the Avacha volcano,
Kamchatka (Arai et al., 2003). Two relatively coarsegrained samples (60-12 and E19-12) were initially dunites
with equant chromian spinel, and have only secondary
orthopyroxene replacing olivine (Fig. 3e and f ). The
primary texture is well preserved, mainly composed of
coarse to medium grains of olivine that are only partly
replaced by relatively fine orthopyroxene (Fig. 3e and f ).
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Fig. 7. Olivine CPO (crystallographic preferred orientation) of C-type (a) and F-type (b) peridotites from Iraya, measured by SEM using a JEOL
5600 system equipped with electron back-scattered diffraction (EBSD). Equal area projection, lower hemisphere. Contours in 1% area. The
maximum density of the [100] axis was aligned east---west, whereas the maximum density of the [010] axis was aligned north---south. pf J, pole figure
J-index. Foliation cannot be shown because of arbitrary cutting in making thin sections.
The amount of secondary orthopyroxene reaches 16 and
25 vol. %, respectively, for 60-12 and E19-12 (Fig. 2).
The secondary orthopyroxene is sometimes unusual in
texture, showing spherulitic or radial aggregates, no sign
of deformation, and no exsolution of clinopyroxene
(Fig. 8e). It is clearly distinguished from ordinary mantle
orthopyroxene similar to that in the coarse-grained harzburgites (Fig. 3a---d). McInnes et al. (2001) and Arai et al.
(2003) described metasomatic orthopyroxenes with
exactly the same texture in sub-arc harzburgite xenoliths
from the Lihir volcano, Papua New Guinea, and from
the Avacha volcano, Kamchatka, Russia. As noted by
Arai et al. (1996) and Arai & Kida (2000), spherulitic
orthopyroxene has been found as porphyroblasts in
deserpentinized peridotites from thermal aureoles
around granitic intrusions (see also Arai, 1974, 1975;
Matsumoto et al., 1995). Phlogopite with pale brownish
colors is sometimes associated with these secondary
orthopyroxenes, especially in F-type peridotites. Clinopyroxene is occasionally associated with the secondary
orthopyroxene, especially with that recrystallized from
primary orthopyroxene.
Host andesites
The xenoliths were entrained mainly by calc-alkaline
andesites with phenocrysts of plagioclase, hornblende,
augite, biotite, olivine and occasionally hypersthene.
Plagioclase is optically zoned but its form is varied from
euhedral to subhedral. Some of the plagioclase is clear
and some is turbid with numerous glass inclusions. Hornblende is euhedral to subhedral, and is brown to dull
green in thin section. Opacite rims are sometimes
observed around hornblende phenocrysts. Augite is
euhedral to subhedral and pale greenish in color.
Magnetite inclusions are common. Olivine is euhedral
to round in shape, and coarse euhedral grains enclose
brownish euhedral chromian spinel. Orthopyroxene is
relatively fine, if present, and frequently has a reaction
rim of clinopyroxene. The groundmass is intersertal, with
plagioclase, clinopyroxene, magnetite and glass.
MINERAL CHEMISTRY
Minerals and glasses were analyzed with a JEOL
electron microprobe ( JXA8800) at the Center for
Co-operative Research of Kanazawa University (accelerating voltage 15 kV and beam current 12 nA) and with
a JEOL 8800 superprobe at the Tokyo Institute of
Technology (accelerating voltage 15 kV and beam
current 12 nA). Special caution was taken in the NiO
analysis of olivine, using 25 kV accelerating voltage,
20 nA beam current and a longer counting time (100 s
instead of 20 s for other elements). Ferrous and ferric iron
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Fig. 8. Photomicrographs of secondary orthopyroxenes in Iraya peridotite xenoliths. Cross-polarized light. (a) Orthopyroxene (opx-2) in the finegrained olivine matrix of F-type peridotite. (Note the minute inclusions of olivine.) (b) Secondary orthopyroxene pool (opx-2) replacing medium
strain-free olivine in F-type peridotite. (Note the fine olivine grains included in orthopyroxene along the boundaries with olivine, indicating
replacement.) (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 between
olivine 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 peridotite
complex (Arai, 1975), SW Japan. [Note the textural similarity to the orthopyroxene in (e).] (See text for explanation.)
contents of chromian spinel were calculated assuming
spinel stoichiometry. Cr number is Cr/(Cr þ Al) atomic
ratio of chromian spinel. Mg number is Mg/(Mg þ total
Fe) atomic ratio for silicates and is Mg/(Mg þ Fe2 þ )
atomic ratio for chromian spinel. The minerals of the
C-type peridotites are almost homogeneous in chemistry,
except in samples strongly affected along grain
boundaries by the host magma. The minerals in F-type
peridotites sometimes show grain-by-grain chemical
heterogeneity, and the minerals are too small and too
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turbid to examine intra-grain chemical heterogeneity in
some samples (e.g. Fig. 4a and e). Only the core compositions 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 ranges
from 03 to 06 in C-type peridotites (Table 1; Fig. 9).
Dunites have slightly more magnesian olivine (Fo91---93)
than harzburgites (Fo91---92). The Cr number of spinel
increases steeply with an increase in the Fo content of
olivine within the olivine---spinel mantle array, a spinel
peridotite restite trend (Arai, 1994), in C-type peridotites
(Fig. 9). Olivine is Fo89---91 and the Cr number of spinel
ranges from 04 to 08 in the F-type peridotites (Fig. 9). It
is noteworthy that F-type peridotites have lower Fo content of olivine on average than C-type peridotites despite
the higher Cr number of spinel (Fig. 9). Olivine composition is not appreciably different between the intact part
and the metasomatized part in which a large amount of
secondary orthopyroxene has formed at the expense of
olivine (60-12 and E19-12 of Table 1; Fig. 3e and f ). The
NiO content of olivine ranges from 04 to 05 wt % in
C-type harzburgites (Table 1). It is, however, variable in
F-type peridotites: it is sometimes high, 05---06 wt %
(e.g. sample 52 of Table 1), and is sometimes low, around
03 wt % (e.g. sample 1 of Table 1). The Fe3 þ /(Cr þ
Al þ Fe3 þ ) atomic ratio of spinel is low, around 005 in
C-type peridotites (Fig. 10), and is slightly higher, around
01, in F-type peridotites than in C-type peridotites
(around 005) (Fig. 10). The Cr---Al---Fe3 þ ratio of chromian spinel gradually changes from the C-type to F-type
peridotites: the Fe3 þ /(Cr þ Al þ Fe3 þ ) ratio of the
former spinel tends to increase sharply with increase in
Cr number (Fig. 10). This transitional spinel is found in
peridotites intermediate between the C-type and F-type
peridotites mentioned above. Spinel with the chemical
signature of F-type peridotite is frequently found in
C-type harzburgites with secondary orthopyroxene (e.g.
sample A of Table 1). This spinel compositional trend is
very different from that related to chemical modification
by the hornblendite selvage (Fig. 10). It is noteworthy
that the Mg/(Mg þ Fe2 þ ) atomic ratio of spinel is slightly
higher at a given Cr number in F-type than in C-type
peridotites (Arai & Kida, 2000).
Clinopyroxene is chromian diopside with more than
05 wt % of Cr2O3 in C-type peridotites (Fig. 11). Clinopyroxene is slightly poorer in Al2O3, Cr2O3 and Na2O
on average in F-type peridotites than in C-type peridotites (Fig. 11). The Na2O content of clinopyroxene is
variable from 0 to 1 wt % in individual grains but is
low, 506 wt % on average, for each sample of C-type
peridotite (Fig. 11). Clinopyroxenes in gabbros and as
phenocrysts in the host andesite are clearly distinguished
from those in peridotites in having higher TiO2 and
lower Cr2O3 contents (Fig. 11). Clinopyroxenes in hornblendite selvages, as well as interstitial cpx in peridotites
adjacent to the selvages, are intermediate in composition
between the peridotite and gabbro/andesite clinopyroxenes (Fig. 11).
Secondary orthopyroxenes (opx-2 in Table 1), sometimes exhibiting radial aggregation, are characterized by
low contents of CaO, Al2O3 and Cr2O3 relative to primary opx in C-type peridotites (Arai & Kida, 2000)
(Table 1). The secondary orthopyroxene replacing olivine (e.g. opx-2 of Fig. 3f ) tends to be lower in Ca, Al and
Cr than that recrystallized from primary orthopyroxene
(e.g. opx-2 of Fig. 3d). The secondary orthopyroxenes
are very similar in chemistry to those in metasomatized peridotite xenoliths from the Avacha volcano,
Kamchatka (Arai et al., 2003) and from the Colorado
Plateau (Smith & Riter, 1997; Smith et al., 1999). The
textural and chemical features (especially the radiating
form and low Ca content) of the secondary orthopyroxene are very similar to those of metasomatic orthopyroxene in deserpentinized peridotites (Arai & Kida, 2000).
Hornblende and phlogopite are generally low in TiO2,
512 wt % and528 wt %, respectively. Rare plagioclase
is very calcic and is An94---98.
Glass compositions
Schiano et al. (1995) reported the compositions of glasses
mainly included in olivine. Complementary to their data,
the glasses interstitial to chromian spinel in F-type peridotite and metasomatized C-type harzburgite were analyzed by electron microprobe (Tokyo Institute of
Technology) for major elements (Table 2). They are
highly silicic and contain 60---66 wt % of SiO2,
24---40 wt % of Na2O and 538 wt % of K2O, and
are similar in major-element chemistry to the glasses that
occur mostly as inclusions in olivine (Schiano et al., 1995)
except for the high analytical totals of our data. This may
be due to a difference in volatile contents depending on
the mode of occurrence: glasses completely included by
minerals [primary inclusions of (Roedder, 1984)] may
have higher volatile contents than those from the secondary inclusions analyzed by Schiano et al. (1995). The
presence of H2O and almost complete absence of CO2
and other volatiles in the glasses was preliminarily determined by IR microspectroscopy. Normative quartz content varies from 14 to 26 wt %, and the normative
quartz/(normative quartz þ hypersthene) weight ratio
is high and remarkably constant, ranging from 071 to
075 (Table 2).
Thermobarometry
We calculated equilibrium temperature for the peridotites using the two-pyroxene geothermometers of
Wells (1977) and Wood & Banno (1973) for pyroxene
pairs adjacent to each other. They yield no systematic
379
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 (metasomatized)
C-type harzburgite
C-type dunite (metasomatized)
71-1-2
72-1
A
60-12
Sample no.:
Mineral:
SiO2
ol
opx
41.06
TiO2
0
Al2O3
0
Cr2O3
FeO
0
8.92
MnO
380
55.25
0.03
52.47
0.05
3.92
0.82
3.65
1.28
5.88
0 .1
1.98
0.06
33.67
0.53
16.4
23.78
sp
0.04
0
41.99
26
14.05
0.09
17.5
40.77
0
0
0.03
7.74
0.13
50.69
0.04
opx
56.5
cpx
53.51
0
2.39
0
3.44
0.71
4.56
1.65
1.52
0.17
34.46
0.11
16.55
1.17
22.23
sp
ol
0.03
0.02
40.66
30.37
39.12
0
0.02
13.95
0.08
8.85
0.03
15.57
0.03
48.94
0.08
0
opx-1
56.09
0
2.47
0.88
5.83
0.1
34.35
n.d.
0.12
0
0.41
0
0.07
0.42
0.01
0.35
99.96
0.599
99.56
0.913
99.89
0.597
98.57
99.85
100.01
100.04
99.21
0.686
99.04
0.908
100.31
0.916
0.13
98.99
0.951
0.464
0.026
0.17
99.96
0.913
0.527
0.898
0.017
0.913
0.004
0.52
0.453
0.085
0.084
0.027
0
8.2
0.2
48.63
0.06
0.914
0
0
0.37
99.12
0.914
0.922
0.596
0.905
0.01
0.647
0.690
0.909
0.015
0.086
0.077
Al
0.286
0.688
0.457
0.528
0.501
0.45
0.53
0.36
0.631
0.284
Fe3 þ
0.026
0.015
0.049
0.11
0.085
Cr
FEBRUARY 2004
0.068
0
0
12.77
0.24
0
sp
NUMBER 2
0
0.03
0.07
0.032
0.13
99.02
0
0
0.07
0.088
0.07
99.42
0
0
0.46
Fe
0
0.01
0.01
0
n.d.
0.04
Ca
0.01
0.03
n.d.
0
0.03
0.496
0.479
0.01
0.24
0.01
0
0.05
0.91
0.022
13.63
34.23
0.52
0
0.42
0.474
0.494
33.87
0.77
49.45
0.02
n.d.
0.3
0.901
0.01
49.19
0.07
13.22
0.03
Mg
20.25
0.37
18.39
22.28
0
Cr no.
5.1
0.24
35.16
0.19
0 .1
0.913
8.3
0.17
23.73
0
0.951
14.92
49.49
5.78
0 .2
0
0.37
0.931
1.08
0.28
8.32
0.13
K2O
0.921
0
0.01
19.64
0.07
0
0.294
0.03
0.18
1.69
0.03
0
0.937
57.92
0.05
5.77
0.15
0
0.911
41
19.42
42.64
n.d.
0.909
41.08
0.04
2.48
0.65
1
Total
opx-2
0
0
0.44
0.16
ol-2
0.4
0.04
0.03
Mg no.
55.93
ol-1
0
n.d.
100.69
40.79
sp
0
25.16
41.71
0.32
99.99
0.725
opx
2.49
0.72
0
0.04
ol
1.09
0.09
0
100.02
sp
52.97
0.04
CaO
100.29
cpx
57.72
0.04
Na2O
100.4
0.9
opx-2
0.04
NiO
0.02
ol
VOLUME 45
MgO
0.13
49.88
cpx
JOURNAL OF PETROLOGY
C-type harzburgite
72-2
C-type dunite (metasomatized)
F-type
F-type
F-type
F-type
E19-12
52
21
C
1
ol-1
ol-2
opx-2
SiO2
40.86
0.03
40.95
0.01
58.38
0.01
TiO2
Al2O3
0
0
0.5
Cr2O3
0
0
0
FeO
6.87
0.19
6.53
0.19
4.33
0.19
50.31
0.05
50.07
0.03
34.9
0.23
MnO
MgO
CaO
381
Na2O
0
0
0
K2O
0.01
0.01
0
sp
0.01
0.05
21.29
43.19
18.51
ol
opx-2
cpx
sp
ol
41.15
0.05
56.69
54.3
0.08
0.48
0.19
24.59
41.29
0.04
56.84
0.11
0.09
0.02
1.83
0.42
7.81
0.19
5.4
0.2
49.81
0
35.09
0.4
0
0
0
0
1.55
0.05
8.87
0.25
5.91
0.39
0.15
0.09
2.71
0.14
15.77
0.02
48.7
0.13
34.7
0.44
17.46
22.86
0.01
0.03
0
0.01
0.03
0.02
NiO
0.39
0.42
0.08
0.14
0.28
0.28
Total
98.71
0.929
98.21
0.932
98.62
0.935
99.41
0.717
99.39
0.907
99.96
0.913
Mg no.
Cr no.
1.67
0.5
0
0.02
0
99.94
0.92
0.576
Mg
Ca
Fe
37.98
22.62
0
13.34
0.18
0
0.22
0.3
99.95
0.608
0.43
99.68
0.919
opx-2
ol
opx-2
40.47
0.11
57.08
0.07
53.1
0.11
0.32
0.11
1.32
0.13
2.34
0.7
8.53
0.23
5.87
0.27
2.78
0.17
14.04
0.34
49.18
0.1
34.36
0.61
17.6
23.16
0.36
n.d.
0
0
0.25
0.19
0
n.d.
0
0.01
0.02
0
0
0
0.04
0
0.04
0
0.493
0.464
0.087
0.043
0.13
0.04
23.96
38.84
22.02
0
cpx
sp
0.09
0.13
27.62
40.6
0.09
56.51
0.04
0.56
0.17
25.1
0
0.19
48.23
0.09
0.52
0.43
0.23
0.34
0.47
99.49
0.911
99.96
0.913
100.3
0.919
98.82
0.665
0.902
0.012
0.491
0.465
0.086
0.043
0.079
sp
0.22
14.8
0.09
99.89
0.639
0.521
opx-2
0
9.39
0.18
0.914
0.008
ol
34.23
21.13
100.83
0.921
0.509
0.905
0.008
sp
0.63
99.47
0.902
1.81
0.35
6.47
0.13
33.42
0.67
0.21
99.84
0.902
0.454
35.27
21.04
0
14.28
0.15
0.04
96.61
0.662
0.485
0.89
0.013
0.097
Al
0.527
0.387
0.461
0.445
0.469
0.431
0.406
0.492
0.435
0.461
Fe3 þ
0.086
0.094
0.1
0.099
0.104
Cr
ol, olivine; opx, orthopyroxene; cpx, clinopyroxene; sp, chromian spinel. Primary phase and secondary phase (or remnant of primary one) are indicated by suffixes 1 and
2, 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 Fe2 þ ) atomic ratio for chromian
spinel. 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 ) of
pyroxenes. Cr, Al and Fe3 þ, atomic fraction of Cr, Al and Fe3 þ respectively to (Cr þ Al þ Fe3 þ ) of chromian spinel.
SUB-ARC PERIDOTITE XENOLITHS FROM PHILIPPINES
Mineral:
ARAI et al.
Sample no.:
JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 2
FEBRUARY 2004
Fig. 9. Relationships between the Fo content of olivine and Cr/(Cr þ
Al) 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). Arrow
indicates the variation from C-type peridotites to F-type peridotites
as a result of metasomatism.
Fig. 11. Compositional variations in clinopyroxenes in the C- and
F-type peridotite xenoliths and associated rocks Phenocrysts are from
the host rocks; gabbro-----gabbroic crust around the peridotite xenoliths;
selvage-----hornblendite selvage around the peridotite xenoliths;
interstitial-----interstitial clinopyroxene in peridotites adjacent to the
hornblendite selvage; vein-----hornblende-rich veins in peridotites.
Forty-five samples were analyzed to produce this plot.
Fig. 10. Trivalent cation ratios of chromian spinels in the Iraya peridotite xenoliths. Two compositional trends of chromian spinel are
clearly distinguished: one from C-type to F-type peridotites, and the
other for metasomatic modification by the hornblende selvage.
difference between the C-type and F-type peridotites,
giving the same temperature range, 950---990 C for the
Wells (1977) geothermometer. The thermometer of
Wood & Banno (1973) gives basically the same range
but the nominal temperatures are higher by about 100 C
than the Wells’ temperatures. Arai et al. (2003) also
reported a similar equilibrium temperature, about
900 C based on Wells (1977), for both the C-type and
F-type peridotite xenoliths from the Avacha volcano,
Kamchatka. This result is apparently contradictory to
the generally low contents of Ca and Al in the secondary
orthopyroxene (Arai & Kida, 2000; Arai et al., 2003). The
secondary orthopyroxene is, however, relatively high in
Ca and Al if accompanied by clinopyroxene, thus
382
ARAI et al.
SUB-ARC PERIDOTITE XENOLITHS FROM PHILIPPINES
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
Analysis no.:
B3sp2
Bsp1re
C1sp1
C7sp5
64.34
0.36
61.81
0.19
60.35
0.13
63.62
0.23
19.55
0.82
19.79
2.59
20.79
3.17
2.44
0.08
2.94
2.28
0.07
2.20
8.39
2.46
IV D
IV 12
D1sp1
D7sp5
64.07
0.26
65.69
0.13
65.05
0.17
58.61
0.16
61.51
0.02
61.89
0.84
21.25
0.82
19.05
0.50
19.62
0.63
19.42
0.30
17.20
16.97
17.24
2.93
0.07
2.96
2.37
0.10
2.62
2.62
0.13
2.14
2.15
0.01
2.12
2.06
0.05
1.61
2.37
0.06
0.52
2.54
0.11
0.63
1.35
0.02
1.70
5.75
3.55
7.49
2.36
8.47
2.82
6.40
2.73
4.71
3.57
5.01
4.03
3.78
4.21
3.01
5.03
2.47
3.60
0.29
0.00
0.00
3.15
0.00
0.00
0.83
0.00
0.01
0.08
0.00
0.00
2.06
0.00
0.03
3.75
0.00
0.02
3.47
0.05
0.00
2.92
0.51
2.46
0.38
4.04
0.02
MgO þ FeO
101.66
5.38
101.39
4.48
101.08
5.89
102.40
4.99
99.98
4.76
102.39
4.27
101.21
3.63
90.25
2.89
92.66
3.17
93.17
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
0.68
0.00
0.36
0.00
0.25
0.00
0.44
0.00
0.49
0.00
0.25
0.00
0.32
0.36
0.13
1.16
0.04
0.87
1.60
0.05
1.71
20.81
18.61
30.02
4.90
19.96
0.47
23.85
12.17
23.09
22.15
30.19
20.50
34.08
17.26
35.62
14.54
42.56
23.87
30.46
1.18
1.21
1.10
3.81
1.42
4.67
1.15
1.21
1.27
0.74
1.04
0.93
1.00
0.44
0.76
0.85
0.44
41.45
0.00
28.52
0.09
37.15
2.40
42.01
1.13
31.74
0.70
23.36
1.13
24.52
0.05
15.42
1.42
12.45
1.47
12.12
2.50
0.13
8.56
0.00
5.35
0.00
7.51
0.00
8.01
0.00
7.40
0.00
6.74
0.00
5.64
0.00
4.30
0.00
4.94
0.00
4.61
0.00
26.02
0.00
13.59
0.00
22.94
0.00
24.20
0.00
22.48
0.00
16.68
0.00
14.62
0.00
14.14
0.00
14.98
0.00
17.55
3.04
0.18
2.54
0.15
3.05
0.18
3.02
0.20
3.04
0.15
2.45
0.16
2.60
0.12
3.29
0.09
3.03
0.08
3.81
0.25
0.20
0.23
0.16
0.18
0.20
0.23
0.22
0.25
0.16
0.18
0.17
0.20
0.14
0.16
0.10
0.11
0.09
0.11
0.28
0.31
SiO2
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
NiO
Total
C10sp6
IV 4
Norm
il
ap
or
ab
mt
cm
an
C
di
hy
ol
Q
Q/hy
KD-1
KD-2
KD-3
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---Fe2 þ between glass and olivine [¼ (Mg/Fe)glass (Fe/Mg)olivine ]
assuming that the Fe3 þ /total Fe atomic ratio is 0, 0.1 and 0.2, respectively.
indicating relatively high equilibrium temperatures. Arai
& Kida (2000) reported a slightly higher Mg number
MgÿFe
of spinel cores and lower ln KD
between olivine
cores and spinel cores at a given Cr number of spinel,
MgÿFe
where KD
is the apparent partition coefficient normalized to a constant Fe3 þ ratio (005) after Evans &
Frost (1975). Although not examined in detail, this
possibly resulted from heating after entrainment by a
383
JOURNAL OF PETROLOGY
VOLUME 45
magma that formed gabbroic selvages at depth before
entrainment of the xenolith by the present host andesite.
Because of the smaller grain sizes of both olivine and
chromian spinel in the F-type peridotites, Mg and Fe2 þ
may have diffused sufficiently, changing the core compositions, even after heating for only a short period (see
Ozawa, 1983). The coexistence of calcic plagioclase
(An94---98) with magnesian olivine in some of the F-type
peridotites indicates that some peridotites were derived
from shallow mantle depths (51 GPa) (e.g. Kushiro &
Yoder, 1966).
DISCUSSION
Characterization of C-type peridotites
The C-type peridotites from Iraya are different from
abyssal harzburgites (Hebert et al., 1983; Dick, 1989;
Cannat et al., 1990; Arai & Matsukage, 1996; Dick &
Natland, 1996) in having characteristic orthopyroxene
enrichment (Fig. 2). They are also different from the
most common ophiolitic harzburgites, for example
mantle harzburgites from the Oman ophiolite, in which
orthopyroxene is around 20% in volume (e.g. Lippard
et al., 1986; Kadoshima, 2002). Instead they are similar to
some sub-arc harzburgites; e.g. harzburgite xenoliths
from 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 discrete
clinopyroxenes is also one of the characteristics of some
sub-arc peridotites and abyssal peridotites (Arai, 1994).
In summary, we consider that the C-type peridotites
from Iraya reflect the petrographical and mineral chemical signatures of the mantle wedge.
Origin of F-type peridotites
Arai & Kida (2000) concluded that the fine-grained
peridotites were formed by fluid metasomatism, or alternatively, but less possibly, by deserpentinization of
serpentinite in the mantle wedge. The similarity of the
characteristic radial aggregate of orthopyroxenes supports a deserpentinization origin for the F-type peridotites. However, we also re-examined the F-type
peridotites to see if they could have been transformed
from 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 contain
uniform subgrains, indicating that the coarse grains
have been dynamically recrystallized. The size of the
subgrains is remarkably uniform from one grain to
another, suggesting that the recrystallization mechanism
is subgrain rotation (e.g. Passchier & Trouw, 1996).
Furthermore, the grain-size distributions in Fig. 6 show
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FEBRUARY 2004
that the mean size of subgrains is slightly smaller than the
mean grain size of olivine in the F-type peridotites. This
size difference between the subgrains and fine olivine
grains in the F-type peridotites has also been documented in experimental studies (e.g. Jung & Karato, 2001).
Therefore, this suggests that F-type peridotites could
have resulted from deformation of C-type peridotites.
The transitional process can be illustrated from observations of appropriate samples, where fine-grained olivine
aggregates defined by similar crystallographic orientations preserve previous coarse olivine microstructures
(Fig. 5b). This shows that the coarse olivine grains in Ctype peridotites in Fig. 5a were dynamically recrystallized into aggregates of far smaller subgrains as a result of
a subgrain rotation mechanism as shown in Fig. 5b.
Further rotation of subgrains because of increasing strain
resulted in their weak crystallographic orientations and
finally produced F-type peridotites (Fig. 5c).
The grain-size distributions of both C- and F-type
peridotites are log-normal and their microstructures are
rather uniform. Therefore, assuming that their grain
sizes represent steady-state grain sizes, we can estimate
the flow stress by a grain-size paleopiezometer. We use
the stress versus recrystallized grain-size relationship of
Jung & Karato (2001). The estimated flow stress yields
40 MPa for F-type peridotites, but the mean grain size
of olivine in C-type peridotites is too coarse to estimate
the flow stress by this paleopiezometer.
Although the structural reference frame is unknown, it
is noted that the CPO patterns of both C-type and F-type
peridotites show a similar pattern to the {0kl}[100] system, which is the most commonly activated slip system in
naturally deformed peridotite (e.g. Nicolas & Poirier,
1976). The overall CPO strengths in the F-type peridotites are remarkably weak compared with those in the
C-type peridotites (Fig. 7). This may be predominantly
due to the subgrain rotation recrystallization, which
tends to weaken strong maxima by rotating the crystals
away from the ‘ideal’ positions (e.g. Heidelbach et al.,
2003).
Minute inclusions of glass and chromian spinel are
very common in olivine, especially within its central
part, in the F-type peridotites (Fig. 4a and c). This type
of inclusion is categorized as a ‘primary inclusion’ (e.g.
Roedder, 1984), suggesting entrapment of melt/fluid
during the growth of the host fine olivine. This is in
strong contrast to the trail of inclusions cutting the coarse
olivine in the C-type peridotites (Fig. 3b). This type of
inclusion is ‘secondary’ (Roedder, 1984), and was formed
along cracks after the formation of the host olivine. The
melt/fluid invaded after the formation of the C-type
peridotites and during the formation of the F-type peridotites from C-type protoliths, suggesting a dynamic
recrystallization of the C-type peridotites assisted by
this melt/fluid. Downes (1990) recognized a preference
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ARAI et al.
SUB-ARC PERIDOTITE XENOLITHS FROM PHILIPPINES
for mantle metasomatism in deformed or sheared parts
of peridotite mantle xenoliths.
Fluid or melt for the metasomatic agent?
The orthopyroxene, which is especially characteristic of
the fine-grained peridotites, was most probably formed
by reactive replacement of olivine by a Si-rich melt/fluid.
The relatively low contents of CaO, Al2O3 and Cr2O3 in
the secondary orthopyroxene from the Iraya xenoliths
are also characteristic of secondary orthopyroxenes from
Avacha (Arai et al., 2003), Lihir (McInnes et al., 2001) and
the Colorado Plateau (Smith & Riter, 1997; Smith et al.,
1999) that have been interpreted to have formed by
metasomatism by aqueous fluids of slab origin. Aqueous
fluids in equilibrium with peridotite under high-pressure
and high-temperature conditions can be reactive with
olivine to form orthopyroxene at lower-pressure conditions (e.g. Nakamura & Kushiro, 1974; Stalder et al.,
2001; Mibe et al., 2002). Direct information on the nature
of the metasomatic agent involved in the formation of the
secondary orthopyroxene is not available. The secondary
orthopyroxene itself is not accompanied by glass,
although glasses are more frequently found in F-type
peridotites where the secondary orthopyroxene is most
common.
Relatively low partition coefficients (KD values) of
Mg---Fe, from 01 to 03 (mostly 01 to 02), between
glass and olivine are obtained from pairs of host olivine
and glass inclusions (Schiano et al., 1995). Our data also
yield low KD values, from 01 to 02, for the pairs of glass
associated with spinel and olivine in F-type peridotites
(Table 2). The KD values depend on the Fe2 þ /Fe3 þ ratio
of the glass: we assumed that the Fe3 þ /(total Fe) atomic
ratio is 0, 01 or 02 in our calculations because the redox
state is unknown (Table 2). The KD values of Mg---Fe
between glass and olivine demonstrate positive and negative correlations with (MgO þ total FeO) and (Na2O þ
K2O) of the glass, respectively (Table 2). Combined with
the KD values of Schiano et al. (1995), which are generally
low, this is totally consistent with the tendency of KD to
change depending on the alkali content (Falloon et al.,
1997; Draper & Green, 1999) and the (MgO þ total
FeO) content (Kushiro & Walter, 1998) of the melt,
although our KD values, assuming the Fe3 þ /(total Fe)
ratio of the glass as 0, 01 or 02, are slightly lower than
the experimental data. The melts of Falloon et al. (1997)
and Draper & Green (1997) are anhydrous to slightly
hydrous low-degree partial melts of peridotite, and are
nepheline-normative even when they are silicic (with
around 60 wt % of SiO2). Taking all the characteristics
of the glasses into account, the metasomatic agent could
have been a silicate melt with a high H2O content.
The relatively low KD values for olivine and glass in the
F-type peridotites from Iraya (01---02) may be due to
the relatively high contents of normative quartz and
H2O in the melt.
The H2O content of the glasses seems to be systematically variable in the Iraya peridotite xenoliths. As a
result of the almost exclusive presence of H2O as a
volatile, the analytical total of the microprobe analyses
of the glass is expected to be lower than 100% depending
on the H2O content. The possible H2O content
decreases from the primary glass inclusions in the olivine
(around 5---10 wt %; Schiano et al., 1995) to the glass
associated with the chromian spinel (almost anhydrous in
this study, Table 2) through the secondary glass inclusions in olivine (around 5---7 wt %) (Schiano et al., 1995).
This difference of H2O content is due to the loss of H2O
on quenching of the melt to various degrees depending
on the degree of interconnectivity of the trapped melt.
The melt that initially invaded the peridotite was high in
H2O (?410 wt %), considering the presence of bubbles
in the glass inclusions in olivine (Schiano et al., 1995) as
well as the possible complete miscibility between SiO2rich silicate melts and hydrous fluids at upper-mantle
conditions (Bureau & Keppler, 1999). The melt that
formed the glasses was possibly saturated with olivine,
after formation of the secondary orthopyroxene, and was
not reactive with olivine. Overgrowth of olivine was
possible, but may not have occurred because any compositional halo has not been detected around the glass
inclusions in olivine.
Orthopyroxene spherulites (Fig. 8g and h) can be
formed by supersaturation of the (Mg,Fe)SiO3 component
in the fluid/melt and/or by its supercooling (e.g. Inoue
et al., 2000). Supersaturation in (Mg,Fe)SiO3 component was probably achieved by the contact of silicaoversaturated melt with olivine within the mantle
peridotite. Similar conditions may have operated in the
deserpentinized peridotites within the contact aureoles of
the granitic intrusions, mentioned above. Arai (1975)
reported a higher silica content of the deserpentinized
peridotites in the orthopyroxene zone than in the other
zones, suggesting silica enrichment from the granitic
magma. The silica-rich fluid that emanated from the
granitic magma invaded the highest-temperature zone
(orthopyroxene zone) of the dehydrating serpentinite
and produced spherulitic orthopyroxene in contact with
olivine.
There is no systematic increase in the amount of
orthopyroxene from the C-type to the F-type peridotites
(Fig. 2), although the replacement of olivine with orthopyroxene can be, at least locally, observed (Figs 3e, f
and 8b, d). In particular samples, however, orthopyroxene enrichment is discernible (Fig. 3d---f ). We cannot
conclude that there has been silica enrichment of the
mantle wedge based on the Iyara xenolith suite. The
migrating melts appreciably modified the peridotites in
chemistry (Fig. 9), suggesting that the melt was relatively
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Fe-rich. They were possibly residual melts fractionated
from partially solidified primitive arc magmas at deeper
levels.
Implications for tectonic setting
In this part of the Luzon---Taiwan arc the tip of the
mantle wedge and the overlying crust is being displaced
by shearing parallel to the trench, as a result of the strain
partitioning of oblique subduction (Fitch, 1972; Pinet &
Cobbold, 1992; Aurelio, 2000). This will lead to strain
within the mantle-wedge peridotites, and any melts or
fluids present may facilitate deformation/recrystallization (Figs 12 and 13). The South China Sea Basin started
to subduct along the Manila Trench beneath the Philippine Sea plate around Middle Miocene times (Stephan
et al., 1986). The obliquity of subduction at this time
was very high between Taiwan and Luzon; Seno &
Maruyama (1984) proposed a north-northwestward
movement of the Philippine Sea plate at this time. After
the change to the present northwestward movement of
the Philippine Sea plate the obliquity of subduction
lessened. Consequently, the continuous shearing caused
by oblique subduction that deformed the lithospheric
mantle to form the F-type peridotites from the C-type
peridotites concurrent with invasion of melt (Fig. 12) may
have ceased before the onset of the recent activity of the
Iyara volcano. The migrating melt dispersed into the
surrounding C-type peridotites through cracks and
formed trails of secondary glass inclusions (Fig. 3b). The
strain-free tablet olivine (Fig. 4g) formed by local recrystallization of strained olivine during annealing as a result
of a decrease in the obliquity of subduction (see Drury &
Van Roermund, 1989).
The processes of deformation and recrystallization
deduced from the peridotite xenoliths from Iraya,
Philippines, may be common to all supra-subduction
zone mantle wedges. Subduction not orthogonal to the
trench (i.e. oblique subduction) is common, especially
around the Western Pacific, and transcurrent faults possibly related to the oblique subduction are also common
(Fitch, 1972). F-type peridotite xenoliths were first
described from the Avacha volcano, which is located on
the volcanic front of the Kamchatka arc (Arai et al.,
2003). This is also consistent with the expected location
of 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-alkaline
andesites from the Iraya volcano, Philippines, can be
classified into two types, C-type (coarse-grained) and Ftype (fine-grained) peridotites. Harzburgites with porphyroclastic to protogranular textures are predominant
over dunites in the C-type peridotites. Secondary
Fig. 12. A schematic representation of the origin of the F-type peridotites from a C-type protolith. Strained peridotite (C-type peridotite) is
dynamically recrystallized to F-type peridotite with the assistance of
metasomatic melts rich in SiO2, H2O and Fe. (See also Fig. 13 and
text.)
orthopyroxene replacing olivine and sometimes exhibiting radial (spherulitic) aggregation is very common in the
F-type peridotites and, subordinately, in the C-type peridotites. Glasses included within olivine or interstitial to
fine-grained spinel aggregates are common in the F-type
peridotites.
(2) Mineral chemistry is distinctly different between the
two types of peridotite: olivine is around Fo91---92 and
Fo89---91 in the C-type and F-type peridotites, respectively. The Cr number and Fe3 þ /(Cr þ Al þ Fe3 þ )
atomic ratio of chromian spinel is 02---03 and 501,
respectively, in the C-type peridotites, and 04---07 and
around 01, respectively, in the F-type peridotites. The
secondary orthopyroxenes are appreciably lower in Al2O3,
Cr2O3 and CaO than the primary orthopyroxene.
(3) C-type peridotites are similar in mineral chemistry
to arc-type harzburgites, e.g. the harzburgite xenoliths
from the Japan arcs. The textural transition from C-type
to F-type peridotites can be observed under the microscope: coarse olivine (C-type peridotite) is recrystallized
to fine grains (F-type peridotite) through subgrains that
386
ARAI et al.
SUB-ARC PERIDOTITE XENOLITHS FROM PHILIPPINES
Fig. 13. Schematic profile (left) to demonstrate mantle wedge processes around the volcanic front (see Mibe et al., 1999). Dynamic recrystallization
of peridotite (Figs 5 and 12) is common in the mantle wedge, possibly as a result of the oblique convergence of the Philippine Sea plate with the
trench (right). [Note the transcurrent movement of the tip of overlying lithosphere behind the trench as a result of shear partitioning (e.g. Fitch,
1972; Aurelio, 2000).]
preserve the previous coarse size of the original grains.
Glasses, mainly trapped in F-type peridotites, are silicate
melts rich in SiO2, H2O and Fe. The melt may have
assisted the transformation of the C-type peridotites to
the F-type peridotites.
(4) The formation of F-type peridotites from C-type
peridotites was due to shearing of the mantle wedge
by oblique subduction. This may be common within
supra-subduction zone mantle wedges because oblique
subduction is common.
ACKNOWLEDGEMENTS
We are grateful to G. P. Yumul, Jr, the University of the
Philippines, for his arrangement of and assistance in our
field research. Crystal orientation measurements were
made by K. Kanagawa with an SEM---EBSD system at
the Department of Earth Sciences, Chiba University. We
thank E. Hellebrand and two anonymous reviewers for
their critical comments that improved an earlier version
of the manuscript. The editorial handling and comments
of 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 the
field. E. S. Andal kindly provided some literature on
Philippine geology. Y. Shimizu and S. Ishimaru helped
us in preparing the manuscript.
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