Journal of Petrology Advance Access originally published online on December 8, 2005
Journal of Petrology 2006 47(3):595-629; doi:10.1093/petrology/egi087
© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
The Petrology and Geochemistry of Oto-Zan Composite Lava Flow on Shodo-Shima Island, SW Japan: Remelting of a Solidified High-Mg Andesite Magma
Y. TATSUMI1,*,
T. SUZUKI1,
H. KAWABATA1,
K. SATO1,
T. MIYAZAKI1,
Q. CHANG1,
T. TAKAHASHI1,
K. TANI1,
T. SHIBATA2 and
M. YOSHIKAWA2
1 INSTITUTE FOR RESEARCH ON EARTH EVOLUTION (IFREE), JAPAN AGENCY FOR MARINE-EARTH SCIENCE AND TECHNOLOGY (JAMSTEC), YOKOSUKA 237-0061, JAPAN
2 INSTITUTE FOR GEOTHERMAL SCIENCES, KYOTO UNIVERSITY, BEPPU 974-0907, JAPAN
RECEIVED
DECEMBER 16, 2004;
ACCEPTED
OCTOBER 20, 2005
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ABSTRACT
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The Oto-Zan lava in the Setouchi volcanic belt is composed of
phenocryst-poor, sparsely plagioclase-phyric andesites (sanukitoids)
and forms a composite lava flow. The phenocryst assemblages
and element abundances change but Sr–Nd–Pb isotopic
compositions are constant throughout the lava flow. The sanukitoid
at the base is a high-Mg andesite (HMA) and contains Mg- and
Ni-rich olivine and Cr-rich chromite, suggesting the emplacement
of a mantle-derived hydrous (
7 wt % H
2O) HMA magma. However,
Oto-Zan sanukitoids contain little H
2O and are phenocryst-poor.
The liquid lines of descent obtained for an Oto-Zan HMA at 0·3
GPa in the presence of 0·7–2·1 wt % H
2O
suggest that mixing of an HMA magma with a differentiated felsic
melt can reasonably explain the petrographical and chemical
characteristics of Oto-Zan sanukitoids. We propose a model whereby
a hydrous HMA magma crystallizes extensively within the crust,
resulting in the formation of an HMA pluton and causing liberation
of H
2O from the magma system. The HMA pluton, in which interstitial
rhyolitic melts still remain, is then heated from the base by
intrusion of a high-
T basalt magma, forming an H
2O-deficient
HMA magma at the base of the pluton. During ascent, this secondary
HMA magma entrains the overlying interstitial rhyolitic melt,
resulting in variable self-mixing and formation of a zoned magma
reservoir, comprising more felsic magmas upwards. More effective
upwelling of more mafic, and hence less viscous, magmas through
a propagated vent finally results in the emplacement of the
composite lava flow.
KEY WORDS: high-Mg andesite; sanukitoid; composite lava; solidification; remelting
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INTRODUCTION
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The majority of andesites that typify subduction zone magmatism
are derived from parental basaltic magmas through variable differentiation
processes such as fractional crystallization, mixing with felsic
magmas, crustal contamination and anatexis of pre-existing basaltic
crustal materials (e.g. Gill, 1981
; Hildreth, 1981
; Sakuyama,
1981
; Hunter, 1998
; Temel
et al., 1998
; Couch
et al., 2001
;
Tatsumi
et al., 2002
; Grove
et al., 2003
). On the other hand,
mantle-derived, primary andesitic magmas also exist in arcs.
Such unusual andesites are referred to as high-Mg andesites
(HMAs), as they are characterized by high MgO contents and/or
Mg-number (= 100
x Mg/(Mg + Fe)) and are in equilibrium with
Mg-rich mantle minerals. The occurrence of HMAs has been reported,
for example, from the Bonin Islands (Kuroda
et al., 1978
), Baja
California (Saunders
et al., 1987
), Piip volcano in W Aleutians
(Yogodzinski
et al., 1994
), and the Setouchi volcanic belt in
SW Japan (Tatsumi & Ishizaka, 1981
).
High-pressure melting experiments on both simple and natural peridotite systems (e.g. Kushiro, 1969; Hirose, 1997) have established that partial melting of upper mantle peridotite under hydrous conditions is a possible mechanism for HMA magma production. It has been further demonstrated that some HMAs are multiply saturated with peridotitic phases under hydrous conditions at mantle pressures (e.g. Kushiro & Sato, 1978; Tatsumi, 1981, 1982; Umino & Kushiro, 1989; van der Laan et al., 1989). These experimental results led Crawford et al. (1989) and Tatsumi & Maruyama (1989) to the conclusion that the direct overprinting of aqueous fluids released from the subducting lithosphere onto the mantle wedge is a likely mechanism for HMA magma generation. However, this rather simple process is not the only way to attain equilibration between hydrous HMA magmas and mantle peridotite. A process involving partial melting of subducting lithosphere and subsequent reaction of such hydrous felsic slab melts with mantle wedge peridotite has been widely accepted as a likely mechanism for the generation of hydrous HMA magmas (Kay, 1978; Pearce et al., 1992; Yogodzinski et al., 1994; Kelemen, 1995; Shimoda et al., 1998; Tatsumi, 2001; Hanyu et al., 2002). Geochemical modelling suggests that the observed Sr–Nd–Pb–Hf isotopic signatures of HMAs can be reasonably and quantitatively understood by this process (Tatsumi & Hanyu, 2003).
HMA magmas contain larger amounts of H2O than basalt magmas when the magma left the mantle (e.g. Tatsumi, 1982), and will be oversaturated with H2O during ascent, causing extensive crystallization. It is thus likely that HMAs are rich in phenocrysts and contain a larger amount of H2O than other arc basaltic magmas, as observed for boninites (e.g. Dobson & O'Neil, 1987; Umino & Kushiro, 1989; Sobolev & Danyushevsky, 1994). In contrast, HMAs in the Setouchi volcanic belt are rather aphyric and poor in H2O (e.g. Tatsumi & Ishizaka, 1981, 1982a, 1982b). This apparent paradox needs to be assessed.
In this paper, the petrography and geochemistry of a single composite lava flow that contains HMA at its base are presented. The origin of the composite lava flow and a likely mechanism that may overcome the above paradox will be discussed.
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GEOLOGICAL SETTINGS
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The Oto-Zan lava flow is located on Shodo-Shima Island, SW Japan
(
Fig. 1). The current SW Japan arc is being built by subduction
of the Philippine Sea plate from the Nankai Trough beneath the
Eurasian plate (
Fig. 1). The Quaternary volcanic front is located
100 km above the top of the subducting slab, as is the case
for most arc–trench systems (Tatsumi & Eggins, 1995
).
Miocene igneous rocks are distributed in the present fore-arc
region of the SW Japan arc. In the near-trench region of this
arc (the Outer Zone), felsic volcano-plutonic complexes were
emplaced at 14 ± 1 Ma (Shibata, 1978
; Sumii, 2000
) into
a Cretaceous to Miocene accretionary prism or subduction complex
(
Fig. 1). Synchronous with this near-trench magmatism, i.e.
at 13·7 ± 1·0 Ma (Tatsumi
et al., 2001
,
2003
), volcanism took place in the Setouchi volcanic belt (
Fig. 1).
The zonal arrangement of Miocene magmatism parallel to the
arc–trench system suggests a contribution from the plate
subduction to the formation of these magmatic belts.
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Fig. 1. Tectonic setting of the Setouchi volcanic belt (inset) and a simplified geological map of Shodo-Shima Island after Tatsumi (1983). The Setouchi volcanic belt (open circles) and coeval Outer Zone felsic complexes (hatched circles) are distributed to the trench side of the Quaternary volcanic front (QVF) of the SW Japan arc. Setouchi volcanic rocks on Shodo-Shima Island, which cover granitic basement (4), are divided into two groups, the Kankakei (1, porphyritic andesite; 2, sanukitoid) and the Uchinomi (3) formations in descending order. The Oto-Zan lava flow forms the lowermost part of the Kankakei Formation. Bathymetric contours are given in 2000 m intervals.
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Miocene magmatism, both in the Setouchi region and the Outer
Zone of the SW Japan arc, was largely synchronous with the timing
of a 40–50° clockwise rotation of the arc sliver at
14–16 Ma that was caused by the opening of the Sea of
Japan back-arc basin (Otofuji
et al., 1991
). The Shikoku Basin,
situated to the south of the SW Japan arc (
Fig. 1), is also
a back-arc basin that was created behind the Izu–Bonin–Mariana
arc by rifting at 30–15 Ma (Okino
et al., 1994
, 1998
,
1999
). It is thus inferred that the southward drift of the SW
Japan arc in association with the clockwise rotation of the
SW Japan arc sliver resulted in subduction of the young (<16
my) oceanic lithosphere of the Shikoku Basin. Such an unusual
tectonic setting and accompanying thermal conditions could have
caused slab melting to produce magmatism unusually close to
the trench in the Miocene SW Japan arc (Furukawa & Tatsumi,
1999
; Tatsumi & Hanyu, 2003
).
Rocks distributed in the Setouchi volcanic belt include phenocryst-poor, sparsely plagioclase-phyric andesites (including HMAs) and basalts referred to as sanukitoids (Tatsumi & Ishizaka, 1981), porphyritic and plagioclase-phyric ‘normal’ calc-alkalic andesites, garnet-bearing dacites/rhyolites, and pitchstones. These volcanic rocks are collectively distributed on Shodo-Shima Island, which is located to the NE of Shikoku (Fig. 1). The older Setouchi volcanic rocks on the island (Uchinomi Formation), which cover basement granites and gneisses, are composed of felsic lava flows, lava domes, sheets, dykes and volcaniclastic rocks, whereas the younger group (Kankakei Formation) consists of volcanic rocks of intermediate to mafic composition (Fig. 1). K–Ar dates for Setouchi volcanics on Shodo-Shima Island (Tatsumi et al., 2001) yield mean ages of 12·82 ± 0·12 and 13·78 ± 0·17 Ma for the Kankakei and Uchinomi Formations, respectively, which is consistent with the stratigraphic relations. Tatsumi (1983) suggested that most lavas and volcaniclastics on Shodo-Shima Island were emplaced under subaqueous conditions, as they show typical water-chilled structures with cracks perpendicular to the block surface.
The Oto-Zan lava flow, which is located in the western part of the island, forms the lowermost part of the Kankakei Formation and directly covers basement rocks (Fig. 1). The maximum thickness of this lava flow is 100 m. Although the lowermost part of the lava flow forms a lava clinker consisting of water-chilled volcanic breccias and volcaniclastics, the main part of the lava flow is massive. Importantly, no flow unit boundary is observed within the lava flow, suggesting that the Oto-Zan lava is a single lava flow.
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ANALYTICAL AND EXPERIMENTAL METHODS
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Chemical analyses
Major and trace element (Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba,
Pb and Th) compositions were measured using RIGAKU® Simaltics
3512 and Rix 3000 X-ray fluorescence (XRF) spectrometers on
fused glass beads and pressed powder pellets, respectively.
Detailed analytical procedures have been described by Goto &
Tatsumi (1994
, 1996
) and Tani
et al. (2005)
.
Rock samples for trace elements (ICP-MS) and Sr–Nd–Pb isotope analysis were crushed to coarse chips (<0·5 mm3) and fresh pieces were hand-picked. To avoid surface contamination, the rock chips were washed with ethanol and then leached with 0·5M HCl at room temperature for 1 h. Finally, the chips were rinsed three times with Milli-Q water. The chips were ground to less than 200 mesh size using a vibration mill made of alumina ceramic.
Concentrations of rare earth and 14 other trace elements (Sc, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Tl, Pb, Th and U) were determined using a VG Elemental® PQ3 inductively coupled plasma mass spectrometer (ICP-MS) enhanced with a chicane lens system, following the procedures described by Chang et al. (2003). Trace element data, except for high-field-strength elements (HFSE: Zr, Nb, Hf and Ta), were obtained after HF–HClO4–HNO3 digestions. For HFSE analysis, alkali fusion (LiBO2/Li2B4O7, Spectroflux® 100B of Johnson Matthey) was applied to ensure a complete decomposition of refractory minor phases. Analytical accuracy and precision for ICP-MS analyses, estimated from repeated measurements of international reference rocks, were better than 10 and 2–5%, respectively.
The analytical procedure used for chemical separation and mass spectrometry for Sr, Nd and Pb isotope determinations was outlined by Yoshikawa et al. (2001), Shibata et al. (2003) and Miyazaki et al. (2003). Total procedural blanks for Sr, Nd and Pb were less than 10, 10 and 5 pg, respectively. Mass spectrometry was performed on a Thermo-Finnigan® Triton TI equipped with nine Faraday cups, using a static multi-collection mode. Normalizing factors used to correct for isotopic fractionation in the Sr, Nd and Pb isotope analyses were 86Sr/88Sr = 0·1194, 146Nd/144Nd = 0·7219, and 0·145 % per atomic mass unit, respectively. Measured isotopic ratios for standard materials were 87Sr/86Sr = 0·710268 ± 19 (2
) for NIST 987 (n = 10), 143Nd/144Nd = 0·511844 ± 11 (2) for La Jolla (n = 10), and 208Pb/204Pb = 36·712 ± 11 (2), 207Pb/204Pb = 15·495 ± 4 (2), 206Pb/204Pb = 16·939 ± 3 (2) for NIST 981 (n = 35).
Mineral compositions were analysed using JEOL JXA-8800 and -8900 electron-probe micro-analysers following the method described by Shukuno (2003). The excitation potential, specimen current, and analytical time were: 15 kV, 15 nA and 20 s (25 kV, 20 nA and 100 s for Mn, Ca and Ni analyses) for olivine; 15 kV, 12 nA and 20 s for spinel; 15 kV, 15 nA and 20 s for pyroxene and plagioclase. ZAF correction procedures were employed.
Melting experiments
Starting material
An HMA (SD-249), which forms a part of the Oto-Zan lava flow, was used as the starting material for the experiments (Table 1). This HMA sample possesses a composition almost identical to the sample from the lowermost part of the Oto-Zan lava flow (OTO-1 in Table 1), but with lower Na2O and higher K2O contents. Sample SD-249 was selected because it is completely fresh, whereas OTO-1 contains altered glass in the groundmass. In order to adjust the above compositional differences and to apply the experimental results to the Oto-Zan composite lava compositions, we used ‘corrected’ melt compositions, as described later.
Experimental
Experiments were performed in a Kobelco 500 MPa type internally
heated pressure vessel in which pure Ar gas was used as the
pressure medium. Three glasses with different water contents
were used as starting materials, and were prepared as follows.
The powdered sample was first sealed in a Pt capsule (4·7
mm inner diameter, 0·15 mm wall) with a small amount
of distilled water. The sample was then heated to 1300–1350°C
at 0·2 GPa for 20–30 min, and quenched isobarically.
The amounts of H
2O in the recovered glass are 0·7, 1·5
and 2·1 wt %, measured using an FT-IR micro-spectrometer.
Although slight Fe-loss occurred (from 5·6 to 5·2–4·9
wt % FeO
* from the original sample to the recovered glasses,
respectively), no change was observed for other elements.
The glass sample was set in an Au25Pd capsule (2·0 mm inner diameter, 0·15 mm wall), and was hung by Mo wire in the hotspot of a Mo furnace within the pressure vessel and held for 6–75 h at 0·3 GPa and a set temperature. The experimental pressure corresponds to middle-crustal depths beneath the Setouchi volcanic belt. Pressures were measured with a strain-gauge pressure transducer. Temperatures were monitored with two W5Re–W26Re thermocouples spaced vertically 5 mm apart, and the observed temperature gradient across the sample was less than 10°C. At the end of the run, the hanging wire was cut with a surging current, thereby letting the capsule fall to the cold (<250°C) bottom of the vessel and quench isobarically.
Oxygen fugacity during melting experiments was estimated based on a solution model for coexisting magnetite and ilmenite (Spencer & Lindsley, 1981). Although the observed compositions of magnetite and ilmenite are beyond the upper limit of their oxygen barometer, we tentatively show these estimates in Fig. 2. The experimental device generated redox conditions within a well-defined range at NNO+3, which is 2 log units higher than the oxygen fugacity estimated for the Oto-Zan HMA magma based on olivine–spinel compositions (see below). If this is the case, then the difference in oxygen fugacity will have significant impacts on the stability of oxide minerals and, thereby, could influence the variation of FeO*/MgO and TiO2 during magmatic differentiation.
Mineral and glass compositions
Mineral and glass compositions were analysed using JEOL JXA-8800
electron-probe micro-analyser. The excitation potential, specimen
current, and analytical time were 15 kv, 12 nA and 20 s, respectively.
A defocused electron beam of 10 µm in diameter was used
for glass, whereas a focused beam was employed for the measurement
of the crystals. ZAF correction procedures were employed.
Modal compositions were calculated by mass balance based on compositions of the starting material, minerals and glasses.
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RESULTS
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Petrography
Modal proportions of phenocrysts and representative mineral
compositions are shown in
Tables 2–
6. Rocks forming the
Oto-Zan lava flow are distinct from typical orogenic andesites
in two important ways. First, they are poor in phenocrysts (<15
vol. %). Second, they do not contain plagioclase as a phenocryst
phase, which is the major phenocryst phase in typical andesites.
These petrographic signatures characterize sanukitoids occurring
in the Setouchi volcanic belt. An additional petrographic feature
of sanukitoid is its compact nature, i.e. the absence of vesicles.
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Table 2: Modal mineralogy of the Oto-Za lava flow as a function of stratigraphic height above the base of the flow
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One petrographic signature observed for the Oto-Zan lava is
that both the amount and the assemblage of phenocryst phases
change gradually within the single lava flow (
Fig. 3), indicating
that the lava is a composite flow. The basal part of the lava
flow is composed of augite–olivine andesite, with the
amount of olivine and augite decreasing upwards. Orthopyroxene,
instead of olivine, appears as a phenocryst phase
20 m from
the base. The upper part of the lava flow comprises augite–orthopyroxene
(bronzite to hypersthene) andesite.
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Fig. 3. Variation of modal minerology throughout the Oto-Zan lava flow as a function of height above the base. The phenocryst assemblage changes from olivine (ol) + clinopyroxene (cpx) in the basal part, via olivine + clinopyroxene + orthopyroxene (opx), to orthopyroxene + clinopyroxene in the upper part of the lava flow, indicating that the Oto-Zan lava forms a composite lava flow. The absence of plagioclase phenocrysts, which characterizes sanukitoids in the Setouchi volcanic belt, is significant.
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Olivine phenocrysts at the base of the lava flow (OTO-1) are
more magnesian and characterized by a narrower compositional
range than those in the sample at 19 m above the base (OTO-7;
Fig. 4). Olivine in OTO-1 has a composition in equilibrium with
the bulk rock in terms of Fe–Mg exchange partitioning,
whereas that in OTO-7 has Mg-numbers lower than that of inferred
equilibrium olivine (
Fig. 4). The NiO content at a given Mg-number
of olivine phenocryst in OTO-7 is higher than that in OTO-1.
The compositional trend for olivine in OTO-1 can be explained
by simple olivine fractionation inferred on the basis of Fe–Mg–Ni
exchange partitioning between olivine and silicate melts (Roeder
& Emslie, 1970
; Kinzler
et al., 1990
) and an assumption
of Fe
2+/(Fe
2+ + Fe
3+) = 0·9 in the magma (
Fig. 4), whereas
some additional processes may be needed for understanding the
unusually nickeliferous trend for OTO-7 olivine. Such unusually
nickeliferous olivine phenocrysts have been found in some basalts
and andesites (e.g. Sato & Banno, 1983
; Nabelek & Langmuir,
1986
; Nakamura, 1995
; Tatsumi
et al., 2003
, 2004
). Nakamura
(1995)
examined the compositional zoning of olivine phenocrysts
in calc-alkalic andesites from the Yatsugatake volcano, Central
Japan, by using a growth and diffusion model in the Mg–Fe–Ni
system. He showed that the composition of nickeliferous olivine
phenocrysts could be explained by diffusion processes within
normally zoned olivine, causing Fe-enrichment without a marked
depression in the Ni content. This is due to a greater Fe–Mg
inter-diffusion coefficient than the Ni tracer-diffusion coefficient
in olivine. Tatsumi
et al. (2003)
favored this explanation for
the occurrence of unusually nickeliferous olivine in rather
olivine-rich (
20 vol. %) sanukitoids (
Fig. 4). Although OTO-7
contains only 7 vol. % of olivine phenocrysts, it may be inferred
that a long residence time of olivine phenocrysts in the magma
may cause such unusual compositions.
Many olivine phenocrysts contain chromite inclusions. In a sample
from the base of the lava flow (OTO-1), these spinels show a
limited compositional range, whereas those in OTO-7 range from
chromite to chromian titanomagnetite (
Fig. 5a). The following
compositional differences in chromite inclusions should be stressed:
(1) chromites in OTO-7 are enriched in Fe
2+, Fe
3+, Ti and depleted
in Mg, Al, Cr compared with those in OTO-1 (
Fig. 5a–c);
(2) such compositional differences are also observed for chromite
included in individual olivine phenocrysts in OTO-7 as a function
of the distance from the rim of the olivine crystal. These compositional
changes are documented for spinel inclusions in rather phenocryst-rich
HMAs from the Setouchi volcanic belt (
Fig. 5a–c). Scowen
et al. (1991)
documented similar chemical variability of chromite
inclusions in olivine phenocrysts from the 1959 Kilauea Iki
lava lake and proposed that the compositional changes may be
caused by re-equilibration with the residual melt by cationic
diffusion (Mg, Al, Cr outwards and Fe
2+, Fe
3+, Ti inwards) through
the olivine crystals. Re-equilibration of chromite inclusions,
but not of the Ni content in the host olivine as described before,
implies a significantly larger flux of Al
3+,Cr
3+, Fe
3+ and Ti
4+ through the olivine crystal than Ni
2+. Although the diffusivities
of such elements having high charge have not been measured experimentally,
they are likely to be significantly slower than the divalent
cations such as Ni. If so, then the explanation for the elevated
Ni content in olivine, involving a slow cooling rate, could
not be valid for the variability in the composition of chromite.
Further work is needed to understand the compositional relationship
between chromite inclusions and the host olivine.
Ballhaus
et al. (1990
, 1991)
and Ballhaus (1993)
examined Cr–Al-rich
spinel compositions and demonstrated that Fe
3+ in spinel may
provide a reasonable estimate of the
fO
2 relative to the FMQ
buffer for a magma.
Figure 2 indicates
fO
2 estimates based on
spinel crystals in the Oto-Zan lava flow and suggests that the
fO
2 of the Oto-Zan magma may be close to, or one log-unit higher
than, the NNO buffer, which is a typical value for subduction
zone magmas (Ballhaus, 1993
).
Clinopyroxene phenocrysts in the lower part of the Oto-Zan lava flow are oscillatory zoned and show a wide range in composition (e.g. OTO-1 in Fig. 6). On the other hand, those in olivine-free pyroxene andesites in the upper part of the lava flow (Fig. 3) include both normally zoned and reversely zoned crystals (Table 5). Orthoyroxene-phyric rocks also contain both normally zoned and reversely zoned orthopyroxene phenocrysts (Fig. 6 and Table 6), suggesting some disequilibrium processes operated during magmatic differentiation, such as mixing of magmas with different compositions.
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Fig. 6. Compositions of pyroxene phenocrysts in three samples from the Oto-Zan composite lava flow. Both reversely zoned (RZ) and normally zoned (NR) orthopyroxene phenocrysts are observed in Oto-Zan sanukitoids, suggesting mixing of compositionally different magmas during magmatic differentiation processes.
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In the middle to upper part of the lava flow, i.e. typically
at >20 m above the base and higher, pegmatitic streaks, which
consist of cristobalite and minor alkali feldspar and biotite,
are developed along platy joints.
Bulk chemical compositions
Major and trace element compositions for all samples, obtained by XRF analysis, are listed in Table 7. Trace element concentrations (ICP-MS) and Sr–Nd–Pb isotopic compositions for representative samples are shown in Tables 8 and 9.
Sanukitoids from the Oto-Zan lava flow exhibit more of a calc-alkalic
trend than andesites from Quaternary volcanoes in the NE Japan
arc and other sanukitoids in the Setouchi volcanic belt (
Fig. 7a).
Oto-Zan sanukitoids have N-MORB-normalized incompatible
element patterns identical to those in the other regions of
the Setouchi volcanic belt (
Fig. 7b), which typify both subduction
zone magmas and bulk continental crust compositions, i.e. higher
concentrations of elements with higher incompatibility during
mantle melting and relative depletions and enrichment in Nb–Ta
and Pb, respectively. In contrast, boninite, another type of
HMA, is more depleted in incompatible trace elements and REEs
than sanukitoids (
Fig. 7b and c).
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Fig. 7. Geochemical characteristics of sanukitoids from the Oto-Zan lava flow and other regions of the Setouchi volcanic belt (Shimoda et al., 1998; Tatsumi & Ishizaka, 1982a, 1982b; Tatsumi et al., 2003), boninites (Pearce et al., 1992; Taylor et al., 1994) and Quaternary calc-alkalic volcanic rocks from the NE Japan arc (NEJ CA; Tatsumi & Kogiso, 2003). The liquid line of descent obtained for an HMA from the Shasta region, California (Grove et al., 2003) is shown in (a). Sanukitoids comprising the Oto-Zan lava flow show an extreme calc-alkalic trend and have trace element characteristics identical to those from other regions of the Setouchi volcanic belt and are distinct from boninites. TH, tholeiitic series; CA, calc-alkalic series (Miyashiro, 1974). Normalization values from Sun & McDonough (1989). (a) FeO*/MgO vs wt% SiO2; (b) N-MORB normalized incompatible element patterns; (c) chondrite-normalized REE patterns. Normalization values from Sun & McDonough (1989).
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The Sr–Nd–Pb isotopic compositions of Oto-Zan sanukitoids
are shown in
Fig. 8, together with previously reported HMAs
and basement rocks from Shodo-Shima Island (Shimoda
et al.,
1998
; Tatsumi
et al., 2002
). Oto-Zan sanukitoids exhibit more
enriched isotopic signatures than other HMAs from the Mito Peninsula
(
Fig. 1), only
5 km away from the Oto-Zan.
The compositional variation in the Oto-Zan lava flow as a function
of height from the base of the lava flow is shown in
Figs 9 and
10. Concentrations of most elements change with increasing
height, synchronously with the changes in modal composition
(cf.
Fig. 3), although the abundance of some elements such as
Ba, Pb, Sr, Y and Th, and Nd and Pb isotopic ratios remain essentially
constant throughout the lava flow (
Figs 9 and
10). Two samples
(SD303 at the top and OTO-7 at 19 m from the base) possess higher
87Sr/
86Sr (>0·7057) than the other samples (
0·7055).
This is unlikely to have resulted from selective crustal contamination,
because of rather constant
143Nd/
144Nd compositions for all
the samples. An unusual ‘spike’ in those samples
is also documented for Ba (
Fig. 9). One possible explanation
for such spikes in Sr isotopic composition and Ba abundance
may be selective transport of these elements in gas-rich phases
that could extensively react with crustal components; there
is a higher concentration of Ba and Sr in the pegmatitic streaks
that are ubiquitous in the middle to upper part of the lava
flow.
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Fig. 9. Variations in element concentrations in the Oto-Zan lava flow as a function of height from the base. Concentrations of most elements change with increasing height synchronously with the changes in modal minerology (cf. Fig. 2), although the abundance of some elements such as Ba, Pb, Sr, Y and Th remains constant throughout the lava flow.
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Fig. 10. Variations in Sr–Nd–Pb isotopic ratios in the Oto-Zan lava flow as a function of height from the base. Although some ‘spikes’ are documented, these isotopic ratios are broadly constant throughout the lava flow (shaded), suggesting a rather closed-system differentiation from a common parental magma for the Oto-Zan sanukitoid samples.
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Melting phase relations and liquid compositions
The phase assemblages and the average compositions of minerals
and glasses are listed in
Table 10, and the modal compositions
of experimental products in the presence of 0·7, 1·5
and 2·1 wt % H
2O are shown in
Fig. 11. In order to apply
the melt compositions obtained in the experiments to the Oto-Zan
lava compositions, ‘corrected’ melt compositions
were calculated by adjusting the Na
2O and K
2O contents in the
starting material (SD-249) to those in the OTO-1 (
Tables 1 and
10) and recalculating to 100% total. This should provide reasonable
estimates for K
2O and at least for Na
2O in rather high-temperature
melts, as these elements are not strongly partitioned into the
mineral phases present (e.g. pyroxene;
Table 10) and behave
largely as incompatible elements.
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Fig. 11. Modal minerology of experimental run products estimated from mass balance calculations and phase compositions. Although amphibole and biotite are present in the low-T run products, modal proportions could not be estimated because of the absence of phases of analysable size. Plag, plagioclase; opx, orthopyroxene; cpx, clinopyroxene; Fe–Ti, magnetite and/or ilmenite; ol, olivine; amph, amphibole; bt, biotite.
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An important criterion for equilibrium is the achievement of
regular and consistent partitioning of major elements between
crystalline phases and melt. The calculated orthopyroxene–melt
and clinopyroxene–melt Fe–Mg exchange distribution
coefficients (
KDFe–Mg) are 0·24 ± 0·02
(1
) and 0·31 ± 0·03 (1
), respectively.
The plagioclase–melt Ca–Na exchange distribution
coefficient is 2·1 ± 0·2 (1
). Although
the clinopyroxene–melt
KDFe–Mg is slightly higher,
the other exchange distribution coefficients are close to those
previously reported (e.g. Gaetani
et al., 1993
; Sisson &
Grove, 1993
; Grove
et al., 2003
). This, together with constant
melt and mineral compositions within each run product (
Table 10),
suggests a sufficiently close approach to equilibrium.
The ‘corrected’ melt compositions are plotted in Fig. 12, together with the compositional range of the Oto-Zan composite lava flow. In the FeO*/MgO vs SiO2 diagram, the liquid line of descent obtained for the Oto-Zan HMA is close to that for an HMA from the Mt Shasta region (Grove et al., 2002). In a strict sense, this crystallization path, especially in its early stage, does not follow a calc-alkalic differentiation trend because the degree of increasing FeO*/MgO against SiO2 content is greater than that defined by Miyashiro (1974). It should be stressed that differences in the H2O content of the starting materials (0·7, 1·5 and 2·1 wt %) do not yield significant changes in the liquid line of descent, although small but systematic differences are recognized for TiO2 and Al2O3 (Fig. 12).
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Fig. 12. Comparison of the compositions of the quenched experimental melts and the Oto-Zan lava. The liquid line of descent for the Oto-Zan HMA (continuous lines for 0·7 wt % H2O experiments), which is similar to that of a Mt Shasta HMA (grey arrow in the FeO*/MgO diagram) from Grove et al. (2002), cannot explain the differentiation trend of the Oto-Zan magmas. Instead, mixing between an HMA magma and a highly differentiated rhyolitic magma (star) can reproduce the Oto-Zan trend. However, a felsic magma with a much lower concentration of K2O, which can be produced in the presence of biotite, is required for the end-member component (see text). TH, tholeiitic series; CA, calc-alkalic series (Miyashiro, 1974); FeO*, total iron as FeO.
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The experimental product after a run at 900°C in the presence
of 0·7 wt % H
2O contains only a small amount of glass,
suggesting that this temperature is close to the solidus (
Fig. 11).
At this temperature, amphibole and biotite are also observed
in the run products, suggesting that the subsolidus mineral
assemblage includes amphibole and biotite in addition to plagioclase,
Fe–Ti oxide and pyroxene. Although the modal abundance
of these phases could not be determined from the present experiments,
the total amount of H
2O held in the subsolidus phases can be
constrained to less than 1 wt %. It is worth pointing out that
even if the subsolidus modal assemblage were 100% hornblende,
it would still only contain
2 wt % H
2O. As the experimental
starting material has a composition consistent with a mantle-derived
HMA that originally contains
7 wt % H
2O (Tatsumi, 1982
), this
result indicates that H
2O can be effectively extracted from
a mantle-derived hydrous HMA magma through solidification.
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DISCUSSION
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The reasons for interest in the origin of the Oto-Zan composite
lava flow are twofold. First, there is a paradox concerning
the origin of Setouchi HMA magmas; andesites forming the composite
lava flows are poor in both phenocrysts and H
2O, although the
HMA compositions found at the base of the lava flows may have
originally contained a large amount of H
2O during their final
equilibration in the uppermost mantle. Second, HMAs in the Setouchi
volcanic belt often form composite lava flows and dykes, suggesting
that some particular processes may have contributed to the formation
of Setouchi HMAs.
Mantle-derived hydrous HMA magma
HMA at the base of the Oto-Zan lava flow
Andesites comprising the Oto-Zan lava flow possess FeO*/MgO ratios smaller than unity (Fig. 7a and Table 7) and are referred to as HMA magmas. In particular, those forming the basal part of the lava flow (e.g. OTO-1) contain Mg-rich olivine (up to Mg-number of 90) and Cr-rich chromite (Cr/(Cr + Al + Fe3+) > 0·7; Figs 4 and 5, and Tables 3 and 4), which typify HMAs in the Setouchi volcanic belt and other subduction zones (e.g. Tatsumi & Ishizaka, 1981, 1982a, 1982b). However, the relationship between Mg-number and NiO content in olivine from OTO-1 (Fig. 4) suggests that this HMA is differentiated, as the olivine that is in equilibrium with the bulk OTO-1 is less nickeliferous and less magnesian than the most Ni- and Mg-rich olivine phenocrysts in the sample.
Two HMAs (TGI from the Osaka region of the Setouchi volcanic belt and SD-261 on Shodo-Shima Island; Table 7) have been proposed as least-differentiated, near primary andesite magmas that were produced in the upper mantle leaving harzburgitic and lherzolitic residues, respectively, based on both petrographic examination and melting phase relations at high pressures (Tatsumi, 1981, 1982; Tatsumi & Ishizaka, 1981, 1982a, 1982b). Although the composition of the HMA OTO-1 may be modified a little by olivine fractionation, as mentioned above, the compositional similarity between those HMAs and OTO-1 suggests that the parental magma for the Oto-Zan lava flow was andesitic and possessed a composition close to the OTO-1 HMA. Furthermore, the Oto-Zan HMAs have incompatible trace element and REE characteristics identical to those of HMAs in other regions of the Setouchi volcanic belt (Fig. 7b and c), providing compelling evidence that the Oto-Zan HMA magma was produced in the upper mantle.
Although experimental results indicate that Setouchi HMA magmas can equilibrate with mantle peridotites solely under hydrous conditions, this does not necessarily require the direct addition of slab-derived aqueous fluids to the mantle wedge. Many researchers (e.g. Pearce et al., 1992; Yogodzinski et al., 1994; Kelemen, 1995) have favoured a mechanism for HMA magma production that involves partial melting of the subducting lithosphere and interaction of these slab-derived, hydrous, silicic melts with the overlying mantle wedge peridotite, as originally proposed by Kay (1978). The geochemical characteristics of Setouchi HMAs can be quantitatively explained by this mechanism (Tatsumi, 2001; Hanyu et al., 2002; Tatsumi & Hanyu, 2003).
If such a mechanism was the case for generation of Oto-Zan and other Setouchi HMA magmas, then the Oto-Zan HMA magma must have contained a large amount of H2O when it was in final equilibrium with mantle peridotite. Experimental results for Setouchi HMAs under H2O-undersaturated conditions (Tatsumi, 1981, 1982) suggest that 7 wt % H2O in the magma is needed for equilibration at 1·0 GPa—a pressure equivalent to the depth immediately below the Moho beneath the Setouchi volcanic belt. It should be stressed, however, that the Oto-Zan HMA, as well as other Setouchi HMAs, contain little H2O as supported by the present XRF data (Table 7); a HMA from the Oto-Zan lava flow (SD-249) contains 0·89 wt % H2O(+) (Tatsumi & Ishizaka, 1982a), much smaller than boninites (2–5 wt %; Dobson & O'Neil, 1987; Sobolev & Danyushevsky, 1994). Boninites are also distinct from Setouchi HMAs in containing much larger amounts of phenocrysts, up to 40% (e.g. Umino & Kushiro, 1989), which is consistent with extensive crystallization caused by H2O-oversaturation during magmatic ascent. It may, thus, be suggested that some particular processes may operate for producing phenocryst-poor and H2O-poor Setouchi HMAs that are originally rich in H2O and must become saturated with H2O within the crust.
Source heterogeneity
The Sr–Nd–Pb isotopic compositions of the Oto-Zan lava samples are clearly different from those of other HMAs from Mito Peninsula on Shodo-Shima island (Figs 1 and 8). This is also the case for Hf isotopes (Hanyu et al., 2002); an Oto-Zan HMA sample (SD-249) possesses an 
Hf of 6·5, significantly lower than other Shodo-Shima HMAs (7·6–8·9). One possible explanation for the difference in isotopic composition, especially in terms of Sr–Nd isotopes, would be contamination by crustal materials, as basement rocks, including granite, gneiss and gabbro, have much higher 87Sr/86Sr and lower 143Nd/144Nd than the HMAs (Fig. 8). The presence of xenocrystic minerals such as quartz and Na-rich plagioclase in the Oto-Zan lava flow (Table 2) may be consistent with this mechanism. However, Pb isotopic compositions for the Shodo-Shima basement rocks do not support the involvement of crustal materials in the magmatic differentiation of the Oto-Zan sanukitoid magma (Fig. 8). Therefore, source heterogeneity beneath Shodo-Shima Island in terms of Sr–Nd–Pb isotopic composition, is suggested to be responsible for the observed isotopic spatial variations.
There is additional evidence suggesting that the HMA magma source beneath Shodo-Shima Island is heterogeneous. It is well established that olivine in the upper mantle possesses rather constant NiO contents of 0·35–0·5 wt %, whereas its Mg-number can be variable (Fig. 4; Sato, 1977; Takahashi, 1990). Some olivines that crystallized from primitive HMAs in the Setouchi volcanic belt, including those on Shodo-Shima Island, have Mg-number and NiO compositions consistent with a mantle signature (Tatsumi & Ishizaka, 1981, 1982a, 1982b; Tatsumi et al., 2003). Olivine phenocrysts embedded in the Oto-Zan HMA (OTO-1) are characterized by significantly higher NiO contents, up to 0·6 wt % at Mg-number of 90 (Fig. 4), suggesting the presence of unusually nickeliferous olivine in its source.
The depleted chemical signature, in terms of high Ni contents in olivine, for the Oto-Zan HMA source is reinforced by the composition of spinel inclusions in the olivine. Two types of HMAs are recognized in the Setouchi volcanic belt (Tatsumi, 1982): one with a crystallization sequence of olivine 
orthopyroxene (opx-HMA) and the other with olivine clinopyroxene (cpx-HMA). The results of high-pressure melting experiments (Tatsumi, 1981, 1982) indicate that harzburgitic and lherzolitic peridotites are the melting residues for opx- and cpx-HMA magmas, respectively, leading Tatsumi (1982) to the conclusion that the opx-HMA magmas are produced either by higher degrees of partial melting or from a relatively depleted mantle peridotite source compared with the cpx-HMA magmas. This is consistent with the difference observed in the spinel compositions, with higher Cr/(Cr + Al) for the opx-HMA magmas (Fig. 5a). Although the Oto-Zan HMA is classified, in terms of its phenocryst assemblage and crystallization sequence, as a cpx-HMA, its spinel inclusions have compositions identical to those in opx-HMAs (Fig. 5a and f).
Magmatic differentiation
Fractional crystallization and assimilation
Continuous sampling and analysis of the Oto-Zan lava flow clearly document a variation in mineralogical and chemical compositions within the single lava flow as a function of height from the base (Figs 3 and 9). It may thus be suggested that magmas having different chemistry and mineralogy have contributed to the lava formation.
Andesites forming the lower part of the lava flow are more depleted in Si, Na, K, Rb and Zr, and more enriched in Fe, Mg, Ca and Ni than those from the upper part (Fig. 9). This, together with constant Nd–Pb isotopic compositions throughout the lava flow (Fig. 10) would suggest the derivation of more felsic magmas through fractional crystallization of a hydrous HMA magma such as OTO-1, which occurs at the base of the lava flow. Grove et al. (2003) determined the phase relations of an HMA from the Mt Shasta region over a range of pressure and temperature conditions under H2O-saturated conditions and demonstrated the liquid line of descent through fractional crystallization of such a hydrous HMA magma (Fig. 7a). Their results may be applicable for examining possible magmatic differentiation paths for the Oto-Zan HMA magma. The reasons for believing so are twofold. First, the experimental starting composition was similar to the Oto-Zan HMA composition. Second, the original H2O content of the Setouchi HMAs (7 wt %) would cause H2O-oversaturation in a shallow crustal magma reservoir, which is the condition reproduced by the Grove et al. (2003) experiments. A marked increase in FeO*/MgO with increasing SiO2 content in the differentiated melt was observed in their crystallization experiments (Fig. 7a), which is a trend quite different from that of the Oto-Zan magma differentiation.
In order to further examine the crystallization differentiation process, a simple mass-balance calculation was made assuming an initial composition equal to the HMA OTO-1 and a final composition equal to one of the most differentiated samples within the lava flow (OTO-9). The results indicate that fractionation of plagioclase and magnetite is needed to produce the differentiated magma from the parental HMA (Table 11). However, neither plagioclase nor magnetite is present as a phenocryst phase in the Oto-Zan samples. Petrographic evidence also does not support this simple process. The presence of both normally zoned and reversely zoned pyroxene phenocrysts in a single sample, especially in differentiated rocks, indicates an open-system differentiation process such as mixing of two compositionally different magmas. It may thus be concluded that a fractional crystallization process was not responsible for differentiation of the Oto-Zan magma.
Simultaneous assimilation of crustal materials and fractional
crystallization (e.g. DePaolo, 1981
; Hildreth & Moorbath,
1988
; Dias & Leterrier, 1994
) is another possibility for
producing differentiated magmas from an HMA magma in the Oto-Zan
lava flow, because plagioclase and quartz, which are major constituent
minerals in basement granitic rocks, occur ubiquitously, although
not voluminously, as xenocrysts in the lava flow (
Table 2).
A simple mass-balance calculation based on compositions of Oto-Zan
lava samples, phenocryst phases and a granite from Shodo-Shima
Island (Tatsumi
et al., 2002
) yields results consistent with
assimilation coupled with fractional crystallization (
Table 11).
However, the isotopic compositions of the Oto-Zan samples
and the basement rocks, especially for Pb, do not support this
(
Fig. 8); a contaminant having a much higher
207Pb/
204Pb and
208Pb/
204Pb composition than the Shodo-Shima basement rocks
is required for the assimilation process, and there is no evidence
for such materials.
Magma mixing
Many calc-alkalic andesites are characterized by the following petrographic and chemical signatures: (1) the presence of reversely zoned pyroxene phenocrysts with rounded cores mantled by rims with higher Mg-number; (2) the presence of plagioclase phenocrysts with a wide range of compositions and with a dusty zone containing fine melt inclusions; (3) the presence of disequilibrium mineral assemblages such as Mg-rich olivine and quartz; (4) linear chemical trends typically demonstrated for MgO and FeO against SiO2. These observations led several authors (e.g. Eichelberger, 1975; Sakuyama, 1979, 1981; Cribb & Barton, 1997; Hunter, 1998; Rorolo & Castorina, 1998; Feeley et al., 2002; Tatsumi et al., 2002) to the conclusion that mixing of mafic and felsic magmas plays a significant role in the production of calc-alkalic andesite magmas. Orthopyroxene-bearing andesites from the Oto-Zan lava flow, i.e. samples from 19 m above the flow base, contain ortho- and clino-pyroxenes that exhibit both normal and reverse zoning (Fig. 6, Tables 5 and 6). Furthermore, the differentiation trends observed for this lava flow are generally linear (Fig. 13). It may thus be suggested that mixing of compositionally different magmas may have played a role in the evolution of the Oto-Zan magma.
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Fig. 13. Chemical differentiation trends for the Oto-Zan sanukitoid samples. The highly linear trends observed can be explained by mixing of two magmas: one having a composition close to the least differentiated Oto-Zan HMA and the other with a rhyolitic composition (open circle). Rhyolites on Shodo-Shima Island, also belonging to the Setouchi volcanic rocks, are not a suitable candidate for the felsic end-member, as the rhyolite compositions do not match those of the inferred rhyolite (see text).
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A possible, and the most plausible, candidate for the mafic
end-member magma would be the HMA OTO-1 sample that forms the
basal part of the lava flow. Rhyolites do exist on Shodo-Shima
Island and could represent a possible felsic end-member. The
compositions of such rhyolites are shown in
Fig. 13, together
with those of the Oto-Zan lava flow. Linear chemical trends
observed for Oto-Zan sanukitoids, such as those for SiO
2 vs
MgO, FeO and Th, can be reasonably explained by the contribution
of a Shodo-Shima rhyolite magma as the felsic end-member in
a magma mixing process. However, chemical trends for other elements
do not support this (
Fig. 13).
Alternatively, the relatively constant Sr–Nd–Pb isotopic compositions throughout the lava flow would be consistent with derivation of the felsic end-member magma from the parental Oto-Zan HMA magma, which can be referred as to ‘internal mixing’ or ‘back mixing’. Formation processes of such a differentiated felsic magma are hereafter discussed based on the present experimental results.
Although the compositional trends of melts produced at high temperatures, especially those for TiO2 and Al2O3, are governed by the amount of H2O in the system, the composition of melts produced at lower temperature (1000°C) are broadly similar (Fig. 12 and Table 10). This may be attributed to H2O-saturation in such small-volume, low-temperature, rhyolitic melts, even with different H2O contents in the overall system, because 4–7 wt % H2O in a rhyolitic melt (0·7, 1·5 and 2·1 wt % H2O in the system with 90, 60 and 50% solids and 10, 40 and 50% melt, respectively) may be close to or above the solubility limit of H2O (Burnham & Nekcasil, 1986; Moore et al., 1998; Yamashita, 1999). It thus seems reasonable to assume a melt with a composition that plots on an extended portion of the liquid line of descent as a possible felsic melt derived from the Oto-Zan HMA magma (Fig. 12). If we accept this as the end-member felsic magma, then the differentiation trends of the Oto-Zan andesites, including the unusually linear trend for MgO and the decrease in TiO2 with increasing SiO2, can be successfully explained by mixing of the basal HMA magma with this felsic magma (Fig. 12). However, the behaviour of alkali elements, especially K2O, is less clear; a felsic magma with a lower concentration
TOP
ABSTRACT
INTRODUCTION
