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Journal of Petrology Volume 43 Number 1 Pages 3-16 2002
© Oxford University Press 2002
The Petrology and Geochemistry of Calc-Alkaline Andesites on Shodo-Shima Island, SW Japan
1INSTITUTE FOR FRONTIER RESEARCH ON EARTH EVOLUTION, JAPAN MARINE SCIENCE AND TECHNOLOGY CENTER, YOKOSUKA 237-0061, JAPAN
2INSTITUTE FOR GEOTHERMAL SCIENCES, KYOTO UNIVERSITY, BEPPU 874-0903, JAPAN
Received October 15, 2000; Revised typescript accepted June 18, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGYEXPERIMENTAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Petrographical and geochemical characteristics of calc-alkaline andesites on Shodo-Shima Island, SW Japan, having bulk compositions largely identical to the continental crust, are presented. The following petrographic observations suggest a role for magma mixing in producing such andesite magmas: (1) two types of olivine phenocrysts and spinel inclusions, one with compositions identical to those in high-Mg andesites and the other identical to those in basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe3+ relations, respectively; (2) the presence of orthopyroxene phenocrysts with mg-number >90 suggests the contribution of an orthopyroxene-bearing high-Mg andesite magma to production of calc-alkaline andesites; (3) reversely zoned pyroxene phenocrysts may not be in equilibrium with Mg-rich olivine, suggesting the involvement of a differentiated andesite magma as an endmember component; (4) the presence of very Fe-rich orthopyroxene phenocrysts indicates the association of an orthopyroxene-bearing rhyolitic magma. Contributions from the above at least five endmember magmas to the calc-alkaline andesite genesis can also provide a reasonable explanation of the Pb–Sr–Nd isotope compositions of such andesites.
KEY WORDS: calc-alkaline andesites; high-Mg andesites; magma mixing; continental crust; SW Japan
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGYEXPERIMENTAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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The continental crust possesses an andesitic composition broadly identical to calc-alkaline andesites (Christensen & Mooney, 1995
; Rudnick & Fountain, 1995; Taylor & McLennan, 1995) that are produced at convergent plate boundaries and are characterized by lower FeO*/MgO ratios than tholeiitic andesites (Miyashiro, 1974). An understanding of the origin of such orogenic, calc-alkaline andesites is thus essential for comprehending not only the magma flux in the modern Earth but also the role of subduction-related magmatism in the evolution of the solid Earth. Since the pioneering work of Eichelberger (1975, 1978), Anderson (1976), Sakuyama (1979, 1981, 1984), and Luhr & Carmichael (1980), the majority of petrologists have considered mixing of mafic and felsic magmas to be one of the major mechanisms of generation of calc-alkaline andesites. Continental crust formation, however, may not be elucidated solely by such magma mixing processes. The reasons for this are twofold. First, calc-alkaline andesite magmatism typifies continental arcs at least on the modern Earth, suggesting that the continental crust itself plays an important role in such andesite formation, possibly as the source of the felsic endmember component. Second, there is a slight but significant difference in MgO contents between the bulk continental crust and the typical calc-alkaline andesite (e.g. Kelemen, 1995), i.e. 4·4 and 3·2 wt % MgO at 
60 wt % SiO2, respectively (Gill, 1981); Taylor & McLennan, 1995. Such Mg-rich andesites may not be formed by simple differentiation of basaltic magmas (Kelemen, 1995), so that some additional mechanisms must have been operational.
One of the processes responsible for the production of Mg-rich continental crusts may be differentiation of mantle-derived andesitic, not basaltic, magmas (Shirey & Hanson, 1984; Stern et al., 1989; Stern & Hanson, 1991). Observations supporting this mechanism include the following: (1) andesitic melts can be generated by partial melting of peridotites at mantle pressures under hydrous conditions (e.g. Kushiro, 1969; Mysen & Boettcher, 1975; Hirose, 1997); (2) particular natural andesites with high MgO contents and high Mg/(Mg + Fe) ratios, which have been referred to as high-mg-number andesites (HMAs), may represent such unfractionated, mantle-derived melts (e.g. Tatsumi & Ishizaka, 1981, 1982; Umino & Kushiro, 1989); (3) the compositional variation of Archean monzonite–granodiorite suites can be explained by crystallization differentiation of mantle-derived andesitic magmas (e.g. Stern & Hanson, 1991). It is thus interesting to examine the genetic linkage between HMAs and calc-alkaline andesites.
This paper presents petrographical and geochemical characteristics of porphyritic and plagioclase-phyric calc-alkaline andesites from Shodo-Shima Island, SW Japan, where mantle-derived HMAs and basalts are also distributed, and provides constraints on the genesis of such calc-alkaline andesites.
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GEOLOGY |
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The Philippine Sea plate is actively subducting beneath the Eurasian plate to form the arc–trench system of SW Japan (Fig. 1). Although the clearly defined seismicity and active volcanism in Kyushu does not continue to Honshu and Shikoku, a study of the ScSp phase (Nakanishi, 1980
) suggests that the leading edge of the subducting slab may have reached the upper mantle beneath the Quaternary volcanic front in SW Honshu (Fig. 1).
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) are distributed in the present forearc region, to the trench side of the Quaternary volcanic front (dashed lines). Setouchi volcanic rocks on Shodo-Shima Island, which cover mainly granitic basement (cross), are divided into two groups, the Kankakei (1–11) and Uchinomi (12) Formations in descending order. EUR, Eurasian plate; PHS, Philippine Sea plate; PAC, Pacific plate; 1, Seiho lavas; 2, Toho lavas; 3, Utsukishi-nohara lavas; 4, Utsukushi-nohara volcaniclastic rocks; 5, Dan-yama lavas; 6, Kaerugo-ike volcaniclastic rocks; 7, Kiyotaki stock; 8, Choshikei lavas; 9, Kamikake-yama volcaniclastic rocks; 10, basaltic sanukitoid lavas; 11, andesitic sanukitoid lavas; 12, Uchinomi group.
Miocene igneous rocks in SW Japan are located in the present forearc region, more than 80 km trenchward of the Quaternary volcanic front (Fig. 1). In the near-trench region of the SW Japan arc, felsic volcano-plutonic complexes, which intruded into an accretionary prism built in the Cretaceous to Miocene period, are distributed (Fig. 1). The K–Ar ages of these complexes concentrate remarkably at 14 ± 1 Ma (Shibata, 1978).
The Setouchi volcanic belt extends for 600 km along the SW Japan arc, with five major volcanic regions (Fig. 1). Formation of the Setouchi volcanic belt parallel to the arc–trench system may indicate the involvement of the subducting lithosphere of the Philippine Sea plate in producing Setouchi magmas. New K–Ar age data for Setouchi lavas (Tatsumi et al., 2001) confirmed an earlier suggestion (Tatsumi, 1983a) that the Setouchi magmatism took place within the short period of 13 ± 1 Ma. This is largely synchronous with the timing of a 40–50° clockwise rotation of the arc sliver of SW Japan or the major mode of rifting of the Japan Sea backarc basin at 14–16 Ma (Otofuji et al., 1991). It is thus suggested that southward drift of the SW Japan arc caused by the clockwise rotation of the arc sliver resulted in the subduction of very young (17 Ma; Okino et al., 1994) oceanic crust from the Shikoku Basin beneath the arc lithosphere (Tatsumi & Maruyama, 1989). On the basis of numerical simulations, Furukawa & Tatsumi (1999) demonstrated that the temperature at the surface of such a young subducting slab is high enough for partial melting of both sediments and oceanic crust, thereby providing support for the proposal by Shimoda et al. (1998) and Shimoda & Tatsumi (1999) that the Setouchi magmatism was triggered by partial melting of subducting sediments.
Shodo-Shima Island, which is located to the NE of Shikoku (Fig. 1), is distinctive for the collective occurrence of major types of Setouchi volcanic rocks ranging from phenocryst-poor andesites (including HMAs) to basalts referred to as sanukitoids (Tatsumi & Ishizaka, 1981), porphyritic and plagioclase-phyric calc-alkaline andesites, garnet-bearing dacites or rhyolites, and pitchstones (Tatsumi, 1983b). Because HMAs in the Setouchi volcanic belt are distinct from boninites, well-known HMAs originally described from the Bonin Island, in the absence of clinoenstatite phenocrysts, the presence of plagioclase in a groundmass, and the enriched trace elements and isotopic signatures, Setouchi HMAs are referred to as HMA sanukitoids hereafter.
The older Setouchi volcanic rocks (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 compositions (Fig. 1). K–Ar dates for Setouchi volcanic rocks on Shodo-Shima Island (Tatsumi et al., 2001) yielded the mean ages of 12·82 ± 0·12 Ma and 13·78 ± 0·17 Ma for Kankakei and Uchinomi Formations, respectively, which is consistent with the stratigraphic relations. The present volume of Setouchi volcanic rocks on Shodo-Shima Island is estimated to be 30 km3 (Tatsumi, 1983b). Tatsumi (1983b) suggested that most lavas and volcaniclastic rocks on Shodo-Shima Island were formed under subaqueous conditions. Some samples show typical water-chilled structure with cracks perpendicular to the rock surface.
The andesite samples used in the present study are lavas and volcanic breccias from the Kankakei Formation (Fig. 1). To evaluate the role of granitic basements in the formation of calc-alkaline andesite magmas, we analyzed four representative basement rocks on Shodo-Shima Island.
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EXPERIMENTAL METHODS |
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Major and trace element compositions were measured by RIGAKU Symaltics 3550 and 3070 X-ray fluorescence (XRF) spectrometers, on fused glass beads and pressed powder pellets, respectively. Detailed analytical procedures were described by Goto & Tatsumi (1994
, 1996).
Sr, Nd, and Pb isotope compositions were analyzed with a Thermoquest MAT 262 mass spectrometer. The measured 143Nd/144Nd ratios were normalized to a 146Nd/144Nd value of 0·7219. The average value for the JNdi-1 standard supplied from the Geological Survey of Japan during this study was 143Nd/144Nd = 0·512100 ± 12 (n = 23; 2
), which corresponds to a value for La Jolla of 0·511842 (Tanaka et al., 1996). For the NBS SRM 987 standard, we obtained a value of 87Sr/86Sr = 0·710259 ± 10 (n = 32). The values 206Pb/204Pb = 16·902 ± 0·004, 207Pb/204Pb = 15·445 ± 0·005, and 208Pb/204Pb = 36·553 ± 0·017 were obtained for NBS SRM 981 lead. The correction factors per a.m.u. for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb were 0·10%, 0·10%, and 0·11%, respectively. Total analytical blanks were below 100 ng for all elements.
Mineral compositions have been analyzed using a JEOL XMA-8800 electron-probe micro-analyzer. Analyses were made using an excitation potential of 15 kV (25 kV for olivine analyses), specimen current of 1·2 nA (2·0 nA for olivine analyses) and analytical time of 20 s (200 s for olivine analyses). The correction procedures followed those of ZAF.
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGYEXPERIMENTAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Major and trace element, and isotopic compositions of Shodo-Shima lavas are listed in Table 1 together with modal compositions of phenocrysts. Porphyritic and plagioclase-phyric andesites on Shodo-Shima Island possess a wide range of compositions with SiO2 contents from 52 to 65 wt % and show a broad calc-alkaline trend (Fig. 2). In connection with Fig. 3, the bulk continental crust compositions are found to lie largely within the geochemical trend formed by Shodo-Shima andesites, except for samples of rather high concentration of Al2O3 in Shodo-Shima andesites. Further, calc-alkaline andesites on Shodo-Shima Island show incompatible element patterns which typify both subduction zone magmas and bulk continental crust compositions; i.e. relative depletion and enrichment in Nb and Pb, respectively, when compared with N-MORB compositions (Fig. 4).
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Figure 3 demonstrates that HMA sanukitoids and porphyritic andesites, except for ‘evolved’ andesites described below, form broadly a single compositional trend. Such trends were also reported from Archean monzodiorite and trachyandesite suites in Superior Province and were explained by crystallization differentiation of mantle-derived HMA magmas (Shirey & Hanson, 1984; Stern & Hanson, 1991).
Porphyritic calc-alkaline andesites on Shodo-Shima Island show more enriched isotopic characteristics than both basalts and HMA sanukitoids (Fig. 5), suggesting the involvement of enriched endmember components such as rhyolitic magmas and granitic basements (Fig. 5) in their formation.
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Most porphyritic andesites exhibit the following disequilibrium petrographic characteristics: (1) the presence of plagioclase phenocrysts with a dusty zone containing fine melt inclusions and with wide range of compositions (Fig. 6); (2) the presence of reversely zoned pyroxene phenocrysts with rounded cores mantled by rims of higher mg-number (Fig. 6); (3) the presence of subhedral to rounded olivine phenocrysts rimmed by pyroxenes. These petrographic observations suggest the role of magma mixing processes in formation of such magmas (Eichelberger, 1975; Sakuyama, 1979; Bloomfield & Arculus, 1989; Kawamoto, 1992; Yang et al., 1999). It is thus confirmed that porphyritic calc-alkaline andesite magmas are not produced by simple crystallization differentiation processes from HMA sanukitoid magmas, although they form a single compositional trend (Fig. 3).
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On the other hand, particular porphyritic andesites to basaltic andesites show little evidence for the above disequilibrium features. A basaltic andesite SD931, as shown in Fig. 6, contains olivine phenocrysts, which are not rimmed by pyroxenes and are in equilibrium with the bulk composition in terms of Fe–Mg exchange partitioning, and is characterized by the absence both of reversely zoned clinopyroxene phenocrysts and of dusty plagioclase phenocrysts. An andesite SD936 also does not include disequilibrium phenocrysts, although only minor amounts of reversely zoned orthopyroxene are found (Fig. 6). To emphasize those distinctive petrographic features, such andesites are called ‘evolved andesites’ hereafter. The origin of evolved andesites will be examined geochemically in the next section.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGYEXPERIMENTAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Assimilation and fractional crystallization
Evolved andesites possess geochemical characteristics distinct from other mixed andesites. First, they occupy the most silica-deficient part of the andesite trend and show a gentle slope close to the boundary between calc-alkaline and tholeiitic trends in an FeO*/MgO–SiO2 diagram (Fig. 2). Second, basalts and evolved andesites form major and trace element trends distinct from those of HMAs and mixed andesites (Fig. 3). It may be thus suggested, based on such distinctive chemistry together with equilibrium mineralogy in those evolved andesites, that evolved andesite magmas are derived originally from basalt magmas not from HMA sanukitoid magmas.
To clarify the origin of evolved andesites and to examine the role of crystallization differentiation of basaltic magmas in their formation, the fractionation trend of a Mg-rich, primitive basalt magma was formulated by MELTS calculations (Ghiorso & Sack, 1995). Figure 7 indicates the fractional crystallization trends obtained for the basalt SDSYB (Table 1) at 0·3 GPa in the presence of 1·5 wt % H2O under the QFM buffer. The crystallization sequence of silicate phases under those conditions is olivine 
clinopyroxene plagioclase orthopyroxene, which is consistent with the petrographic observations. Although the compositions of the evolved andesites can be largely explained by fractional crystallization of a basalt magma, the significant difference between inferred and observed andesites, especially in CaO and Na2O, should be stressed (Fig. 7). One of the possible processes responsible for such compositional changes would be assimilation of granitic upper crust. If we assume the basement granites as such an assimilant, then evolved andesite magmas can be produced by a process of assimilation–fractional crystallization (AFC) as intuitively shown in Fig. 7. Further examination of the role of AFC processes in evolved andesite genesis is not easy, because of the presence of a variety of basement rocks (e.g. gabbro, gneiss, granite, sedimentary rocks, etc.). However, the isotopic compositions of basalts and evolved andesites support such AFC processes; that is, more radiogenic isotopic ratios are observed for those rock series with increasing degrees of crystallization (i.e. from basalt, via basaltic andesite, to andesite), as shown in Fig. 5.
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) with 1·5 wt % H2O at 0·3 GPa under the QFM buffer. This process can largely explain the compositions of evolved andesites (
). To fully explain the evolved andesite compositions, assimilation of granite basements (oval fields) by crystallizing magmas may be needed.
Mafic to intermediate components
Porphyritic calc-alkaline andesites on Shodo-Shima Island, except for evolved andesites, may be produced by mixing of magmas with different compositions, as suggested by petrographical observations. We will further examine the endmember components for such mixed magmas based mainly on mineral compositions.
Possible mafic endmember components for Shodo-Shima porphyritic andesites are basalts and HMA sanukitoids, which were formed in close spatial and temporal relation to the andesites on this island (Fig. 1). Thus, we first examine the contribution of such mafic magmas to the production of calc-alkaline andesite magmas. Olivine phenocrysts in Shodo-Shima andesites have mg-numbers greater than those obtained by assuming equilibrium Fe–Mg partitioning between olivine and the bulk magma (Fig. 6). One possible explanation for this observation may be the derivation of disequilibrium olivine from a mafic magma. Tatsumi & Ishizaka (1981) and Nakashima et al. (2000) demonstrated the compositional difference of olivine phenocrysts and spinel inclusions in olivine between basalts and HMA sanukitoids on Shodo-Shima Island. Olivines and spinels in basalts are more enriched in NiO at a given mg-number and Fe3+, respectively, than those in HMAs (Fig. 8). It is shown in Fig. 8 that olivine and spinel in andesites SD512 and SD513 may be derived from HMA sanukitoid, rather than from basalt magmas. On the other hand, the compositions of these minerals in the andesite SD516 require the contribution of basalt, in addition to HMA sanukitoid magmas, in the formation process. It is thus likely that both basalt and HMA sanukitoid magmas are mafic endmember components for the mixed calc-alkaline andesites on Shodo-Shima Island. If so, then rather Mg-rich, normally zoned clinopyroxene in porphyritic andesites may also be derived from HMA sanukitoid and/or basalt magmas. It is hard to identify the original magma for such clinopyroxenes because of little compositional difference in major element compositions of pyroxenes between HMA sanukitoids and basalts (Tatsumi & Ishizaka, 1981; Nakashima et al., 1999). Further examination of the trace element compositions of clinopyroxene phenocrysts (e.g. Yogodzinski & Kelemen, 1998) is needed, to clarify the origin of such pyroxenes.
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It has been documented that two types of HMA sanukitoids, cpx-HMA and opx-HMA sanukitoids, are present in the Setouchi volcanic belt (Tatsumi, 1982). Cpx-HMA and opx-HMA sanukitoids crystallize clinopyroxene and orthopyroxene after olivine, respectively. The melting phase relations at high pressures for these two types of HMA sanukitoids suggest that opx-HMA magmas are produced at higher degrees of partial melting than cpx-HMA, because they are multiply saturated with harzburgitic phases (olivine + orthopyroxene), and lherzolitic phases (olivine + orthopyroxene + clinopyroxene), respectively, at upper-mantle pressures and temperatures (Tatsumi, 1981, 1982). Differences in the degree of partial melting may be supported by the compositions of the constituent minerals in these HMA sanukitoids; olivine is more enriched in Mg and Ni, and spinel is more chromian in opx-HMA than in cpx-HMA sanukitoids (Fig. 8).
The andesite SD516 contains olivine phenocrysts and spinel inclusions with compositional characteristics identical to those in opx-HMA sanukitoids (Fig. 8). Furthermore, the presence of very Mg-rich orthopyroxenes in SD516 (mg-number 90; Fig. 6) may also imply that the opx-HMA sanukitoid magma is one of the mafic endmembers. Although opx-HMA sanukitoids are not found on Shodo-Shima Island, the generation and involvement of such HMAs in the production of calc-alkaline andesites may reasonably explain the petrographic characteristics of porphyritic andesites. An opx-HMA sanukitoid is, on the other hand, distributed in the Takamatsu region, NE Shikoku (Fig. 1), 30 km to the SW of Shodo-Shima Island (Kushiro & Sato, 1978).
One of the distinctive petrographic features of calc-alkaline porphyritic andesites on Shodo-Shima Island is the presence of reversely zoned pyroxenes with mg-number 70 (Fig. 6). Such Mg-poor pyroxenes are observed neither in HMA sanukitoids nor in basalts. Furthermore, HMA sanukitoids and basalts do not contain plagioclase phenocrysts, whereas plagioclase is the major phenocryst phase in porphyritic andesites. These observations may suggest the involvement of differentiated magmas. Evolved andesites with little evidence of magma mixing may be produced by fractional crystallization of basalt magmas with a certain amount of crustal assimilation as shown in the previous section. Although phenocryst compositions, especially those of plagioclase, in a particular evolved andesite SD936 do not completely match compositions of inferred phenocrysts in differentiated magmas, the presence of evolved andesites on Shodo-Shima Island reinforces the involvement of variably differentiated and assimilated, basalt-derived magmas in the magma mixing processes.
Felsic component
The porphyritic andesite SD513 contains reversely zoned orthopyroxenes with Fe-rich compositions (mg-number 50; Fig. 6). This andesite does not contain clinopyroxene, which should be in equilibrium with such Fe-rich orthopyroxene (Fig. 6). This may suggest that a magma containing only orthopyroxene as a mafic phase contributed to the formation of this particular andesite. In the Setouchi volcanic belt including Shodo-Shima Island, dacites or rhyolites containing such Fe-rich orthopyroxene phenocrysts are present (e.g. Shimoda & Tatsumi, 1999). Compositions of this type of rhyolites are shown in Fig. 3, suggesting that such rhyolite magmas may be a possible felsic endmember component. Also, Sr–Nd–Pb isotope compositions of rhyolites in the Setouchi volcanic belt can reasonably explain the isotopic trends of calc-alkaline andesites (Fig. 5).
Origin of endmember magmas
Major and trace element, and isotopic compositions of five endmember magmas and porphyritic calc-alkaline andesites (Figs 3 and 5) support the present discussion that mixing of five such magmas can reasonably account for the production of calc-alkaline andesites. The petrographical characteristics of the five potential endmember magmas, which are required for the formation of calc-alkaline andesites on Shodo-Shima Island, are summarized in Table 2. Although quantitative estimation of the contributions of five endmember components to the formation of a particular calc-alkaline andesite is interesting, it is difficult because of the variable compositions of basalt-derived, differentiated magmas and rhyolite magmas.
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One of the characteristic endmember magmas for the Shodo-Shima calc-alkaline andesites is an HMA magma, which possesses high mg-number and hence can be in equilibrium with upper-mantle peridotite (e.g. Sato, 1977; Kuroda et al., 1978; Tatsumi & Ishizaka, 1981; Crawford et al., 1989). High-pressure experiments on hydrous peridotites confirmed that such HMA melts can be generated by partial melting of peridotites at mantle pressures under hydrous conditions (e.g. Kushiro, 1969; Hirose, 1997). On the other hand, most researchers have favored a mechanism including partial melting of the subducting lithosphere and interaction of these slab-derived, hydrous silicic melts with overlying mantle peridotites (Kay, 1978; Pearce et al., 1992; Yogodzinski et al., 1994; Kelemen, 1995). Shimoda et al. (1998) demonstrated that Pb, Sr, and Nd isotope compositions of HMAs in the Setouchi volcanic belt are explained by a contribution of partial melts derived from subducting sediments, rather than aqueous fluids from the dehydrating slab. This mechanism was further supported by geochemical formulation of partial melting of subducting sediments and subsequent melt–mantle interaction (Tatsumi, 2001).
It has been widely accepted that generation of basalt magmas in subduction zones is triggered by addition of slab-derived fluids (e.g. Gill, 1981; Tatsumi & Eggins, 1995). A possible mechanism for production of basalt magmas in the Setouchi volcanic belt is, on the other hand, interaction of slab-derived sediment-melts with overlying mantle peridotites (Shimoda & Tatsumi, 1999). Although the origin of basalt magmas is not clearly understood, fractional crystallization of such basalt magmas together with assimilation of basement granitic rocks contributes to the formation of evolved and least-mixed andesite magmas on Shodo-Shima Island.
Setouchi rhyolites are characterized by their highly radiogenic isotope signatures identical to those of trench-fill, terrigenous sediments, i.e. higher Pb and Sr, and lower Nd isotope ratios relative to basalts and HMAs (Fig. 5), which led Shimoda & Tatsumi (1999) to speculate that such rhyolite magmas are produced by partial melting of subducting sediments and reach the surface with little interaction with mantle peridotites. However, the process that formed such an extremely enriched rhyolite magma should be further examined.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONGEOLOGYEXPERIMENTAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Differentiation of mantle-derived HMA magmas has been considered as a possible mechanism for continental crust formation, because of the continuous compositional trends which include HMAs, the bulk continental crust, and Archean calc-alkaline rocks. Such compositional trends are also observed for volcanic rocks on Shodo-Shima Island in the Setouchi volcanic belt, SW Japan. However, the petrographical and geochemical characteristics of these andesites do not support the proposed crystallization differentiation processes, and instead can be explained by the mixing of at least five endmember magmas, i.e. basalt, two types of HMA sanukitoid, differentiated andesite, and rhyolite. It may thus be suggested that differentiation of mantle-derived HMA magmas may not have played the major role in the production of andesitic magmas with compositions similar to those of the bulk continental crust, at least in the case of magmatic suites on Shodo-Shima Island.
Petrographical and geochemical examination of lavas on Shodo-Shima Island demonstrated that evolved andesites, which contribute to formation of mixed calc-alkaline andesites as one of the endmember components, are produced by an AFC process from basalt magmas. In other words, no andesite shows evidence for its derivation from HMA magmas, suggesting that HMA magmas could not differentiate to produce evolved magmas. Melting experiments of HMA sanukitoids in the Setouchi volcanic belt indicated that at least 7 wt % H2O is required for generation of those magmas in the upper mantle (Tatsumi, 1981, 1982). Such H2O-rich magmas have experienced extensive crystallization with falling temperature and may solidify more rapidly within the crust than H2O-poor basalt magmas.
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ACKNOWLEDGEMENTS |
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We thank Takashi Sano and Hiroshi Shukuno for their help in electron probe microanalyses and MELTS calculation, respectively. Positive reviews by Peter Kelemen, Mark Defant, and Richard Arculus are greatly appreciated. This work was partially supported by Grants-in-Aid from the Ministry of Education, Science, and Culture of Japan (08404029 and 09440177) and a Special Coordination Fund for Promoting Science and Technology (Superplume) from the Science and Technology Agency of Japan.
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FOOTNOTES |
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*Corresponding author. Telephone: 81-468-67-9760. Fax: 81-467-66-9625. E-mail: tatsumi{at}jamstec.go.jp
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REFERENCES |
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TOPABSTRACTINTRODUCTIONGEOLOGYEXPERIMENTAL METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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