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Journal of Petrology | Volume 43 | Number 6 | Pages 1029-1047 | 2002
© Oxford University Press 2002
Remelting of an Andesitic Crust as a Possible Origin for Rhyolitic Magma in Oceanic Arcs: an Example from the Izu–Bonin Arc
INSTITUTE FOR FRONTIER RESEARCH ON EARTH EVOLUTION (IFREE), JAPAN MARINE SCIENCE AND TECHNOLOGY CENTRE (JAMSTEC), YOKOSUKA 237-0061, JAPAN
Received February 20, 2001; Revised typescript accepted January 8, 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONTHE IZU-BONIN ARCTERTIARY SHIRAHAMA GROUP, IZU...DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The Izu–Bonin volcanic arc is an excellent example of an intra-oceanic convergent margin. A total of 1011 chemical analyses of 17 Quaternary volcanoes of the arc are reviewed to estimate relative proportions of magmas erupted. Basalt and basic andesite (SiO2 < 57 wt %) are the predominant eruptive products of the Izu–Bonin arc, and rhyolite (SiO2 > 70 wt %) forms another peak in volume. Such rhyolites possess compositions identical to those of partial melts produced by dehydration-melting of calc-alkaline andesites at low pressure (<7 kbar). Meanwhile, the major element variation of the Shirahama Group Mio-Pliocene volcanic arc suite, Izu Peninsula, completely overlaps that of the Quaternary Izu–Bonin arc volcanoes, and groundmasses of Shirahama Group calc-alkaline andesites have compositions similar to those of Izu–Bonin rhyolites. Moreover, phenocryst assemblages of calc-alkaline andesites of the Shirahama Group resemble restite phase assemblages of dehydration-melting of calc-alkaline andesite. These lines of evidence suggest that the rhyolite magmas may have been produced by dehydration-melting of calc-alkaline andesite in the upper to middle crust. If so, then the presence of large amounts of calc-alkaline andesite (3–5 times more abundant than the rhyolites) within the oceanic arc crust would be expected, which is consistent with a recently proposed structural model across the Izu–Bonin arc. The calc-alkaline andesite magmas may be water saturated, and would crystallize extensively and solidify within the crust. The model proposed here suggests that rhyolite eruptions could be triggered by an influx of hot basalt magma from depth, reheating and partially melting the calc-alkaline andesite component of the crust.
KEY WORDS: bimodal magmatism; calc-alkaline andesite; oceanic arcs; rhyolite
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONTHE IZU-BONIN ARCTERTIARY SHIRAHAMA GROUP, IZU...DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The northern Izu–Bonin volcanic arc extends for 550 km from Izu Peninsula, Japan, to near the Nishinoshima Trough or Sofugan Tectonic line (Yuasa, 1985
) (Fig. 1). This is the northernmost segment of the Izu–Bonin–Mariana arc system, which extends 2500 km from the Izu Peninsula to beyond Guam (e.g. Taylor, 1992). The Izu–Bonin–Mariana volcanic arc formed by subduction of the Pacific plate beneath the Philippine Sea plate, and is an excellent example of an intra-oceanic convergent margin. Here, we show that modern magmatism at the northern Izu–Bonin arc is bimodal with basalt and rhyolite predominating. The origin of rhyolite in oceanic arcs is a matter of considerable interest. We suggest that this rhyolite is a partial melt of calc-alkaline andesite occurring at depth within the oceanic island-arc crust. Our study thus casts doubt on the importance of the partial melting or ultimate fractionation of basalt in producing such rhyolites and suggests the fundamental role of calc-alkaline andesite in the formation of island-arc crust. An original calc-alkaline andesite magma is likely to be water saturated and will therefore solidify in the crust, forming an andesite source region at depth, which could be reheated and remobilized by influxes of basalt.
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THE IZU–BONIN ARC |
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TOPABSTRACTINTRODUCTIONTHE IZU-BONIN ARCTERTIARY SHIRAHAMA GROUP, IZU...DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Geological setting and submarine eruptions
The Izu–Bonin arc forms more than half of the Izu–Bonin–Mariana arc system, and is divided into north and south segments, based on submarine topography, chemical composition of volcanic rocks, and distributions of hypocentres and back-arc rifts (Yuasa, 1985
). It appears that subduction along the northern Izu–Bonin segment began several million years earlier than along the southern segment, and the difference in alkalinity of modern volcanic rocks between the two segments reflects differences in crustal thickness and thermal structure of the mantle wedge below the crust (Yuasa, 1992). We will here focus on the northern Izu–Bonin volcanic arc (
550 km) (Fig. 1) to avoid complexities related to the along-arc geological variations of the overall Izu–Bonin–Mariana arc system.
Six Quaternary island volcanoes and nine Quaternary submarine silicic calderas lie along the volcanic front of the Izu–Bonin arc between latitudes 35°N and 30°N (Nagaoka et al., 1991; Yuasa et al., 1991; Iizasa et al., 1999; Fig. 1). Before these calderas were recognized, basalts were believed to be the predominant eruptive products in the Izu–Mariana arc (Aramaki & Ui, 1978). However, samples recently dredged from two large submarine caldera volcanoes (Omurodashi and Kurose Hole) are mostly rhyolite (Uto, 1983) as is the thick syn-caldera pumice deposit at Myojin Knoll Caldera (Fiske et al., 2001). Modern volcanism at the Izu–Bonin arc thus contrasts sharply with that of the NE Japan arc, which is dominated by andesites (Aramaki & Ui, 1978).
Suga & Fujioka (1990) used topography to determine the distribution and volumes of volcanoes along the northern Izu–Bonin arc. Back-arc seamount chains, made up of individual Mio-Pliocene volcanoes having volumes >100 km3, extend obliquely to the volcanic front (Ishizuka et al., 1998). However, a greater volume of modern volcanic material lies along the volcanic front, and this is contained in basalt-dominant island volcanoes and rhyolite-dominant submarine calderas. Myojin Knoll Caldera, which was the first of these silicic structures to receive detailed submersible-based study, was a site of a submarine eruption that produced 35–40 km3 of rhyolite tephra (Naka et al., 1995; Yuasa, 1995; Fiske et al., 2001). A post-caldera hydrothermal system is now producing a modern Kuroko-type polymetallic sulphide deposit (Iizasa et al., 1999).
Fiske et al. (1998) documented the unique characteristics of the shallow submarine 1952–1953 eruption of Myojinsho. The flushing action of water convecting through the hot rubble at the volcano’s summit removes fine-grained matrix vigorously and persistently, resulting in the fines-depleted characteristics of proximal deposits and the dispersal of fine ash over wide areas by ocean currents (Fiske et al., 1998). Tephras at Sites 787, 792, and 793 of Ocean Drilling Program (ODP) Leg 126, which might actually result from such tephra dispersal from arc submarine volcanoes, clearly show the bimodal nature of magmatism in the Izu–Bonin arc (Fujioka et al., 1992).
Turbidites, which have been delivered to the forearc largely through submarine canyons, provide a more complete record of arc volcanism than arc lavas, and are more voluminous and proximal than ashes (Gill et al., 1994). Hiscott & Gill (1992) and Gill et al. (1994) characterized the turbidite geochemistry of the Izu–Bonin arc by using 271 samples of volcaniclastic sand and sandstone collected from cores at the six ODP Leg 126 sites (787, 788, 790, 791, 792 and 793). These turbidites are andesitic on average (60 wt % SiO2) but are bimodal in detail (Gill et al., 1994).
Bimodal volcanism
Relative proportions of erupted magmas
A total of 1011 chemical analyses of samples from 17 Quaternary volcanoes of the Izu–Bonin arc (30°N–35°N) were reviewed to estimate the relative proportions of erupted magmas (Table 1). All discussions in this paper refer to analyses that have been normalized to 100% on a volatile-free basis with total iron calculated as FeO. Figure 2a shows the frequency distribution of the SiO2 content of samples. The histogram showing the number of analyses is converted into volume-weighted histograms (Fig. 2b and c) by the method of Aramaki & Ui (1978). If, for example, there are 10 analyses available for a volcano with a volume of 20 km3, a volume of 2 km3 is allotted to each analysis. Volumes of volcanoes are from data provided by Suga & Fujioka (1990) and the Committee for the Catalog of Quaternary Volcanoes in Japan (1999). Myojinsho caldera (326 km3) was not included in this review, because safety restrictions have prevented dredging and submersible study of this area after the tragedy of the 1952 Myojinsho eruption (Fiske et al., 1998). Erupted materials from this eruption, however, were mostly dacitic in composition.
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The silica frequency relationship based on the number of analyses (Fig. 2a) is similar to that of the volume-weighted histogram (Fig. 2b). Basalt and basic andesite (<57 wt % SiO2) are clearly the predominant eruptive products of the Izu–Bonin arc, but rhyolite (>70 wt % SiO2) also forms a major mode. The shift of the rhyolite peak from Fig. 2a relative to Fig. 2b reflects the relatively low silica contents of rhyolites forming the large submarine calderas. Volumetrically, the Izu–Bonin magmas do not simply decrease linearly from basalt to rhyolite, but pass through the minimum in acid andesite and increase again in dacite and rhyolite (Fig. 2c). The latter peak would be emphasized even more strongly if (1) the Myojinsho caldera (326 km3) actually consists of dacite and rhyolite and (2) pumices from submarine caldera-forming eruptions, dispersed far from their source volcanoes, were taken into consideration. For example, more than half of the sediment layers drilled in the Sumisu Rift during ODP Leg 126, at Sites 790 and 791 (Fig. 1), were made up of thick layers of two-pyroxene rhyolite pumice derived from nearby arc volcanoes (Nishimura et al., 1992).
Average magma composition
Plots of major element abundances vs SiO2 content for the Izu–Bonin arc Quaternary volcanoes are shown in Fig. 3. Although the eruptive products are volumetrically bimodal, magmas range from basalt, through andesite and dacite, to rhyolite. It should be stressed that the average turbidite of Gill et al. (1994) plots in the centre of the eruptive products of the Quaternary Izu–Bonin arc. A large amount of silicic magma production, comparable with the volumes of basalt and basic andesite produced in the arc, is required to yield an average SiO2 content of 60 wt %.
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) and average Izu–Bonin arc turbidite (large
Relatively dry basalt
Basalt magmas on the volcanic front of the Izu–Bonin arc are either anhydrous or contain very little water (Aramaki & Fujii, 1988; Fujii et al., 1988; Nakano et al., 1988a, 1991; Nakano & Yamamoto, 1991; Takada et al., 1992). Fujii et al. (1988) showed that aphyric basalt of the 1986 Izu–Oshima eruption has a composition close to an anhydrous 1 atm cotectic line projected on the pseudoternary normative diagram Pl–Opx–(Qz + Or). Lower H2O contents in basaltic melts will result in higher melt density so that plagioclase crystals will be likely to float rather than sink (Aramaki & Fujii, 1988; Nakano et al., 1988a, 1991; Nakano & Yamamoto, 1991). There is, for example, petrological evidence that plagioclase phenocrysts have accumulated in the upper parts of magma chambers beneath Izu–Oshima volcano where plagioclase phenocryst contents in lavas vary between 0 and 20 vol. % and mafic phenocrysts are rare, yet all the lavas show similar groundmass (melt) compositions. For plagioclase to float in a basaltic liquid, the H2O content must be <0·7% (Aramaki & Fujii, 1988; Fujii et al., 1988; Nakano et al., 1988a; Nakano & Yamamoto, 1991). Hachijyojima and Aogashima volcanoes are also characterized by magmas in which the composition is controlled by the abundance of plagioclase phenocrysts, which ranges from 1 to 43 vol. %, with groundmass (liquid) compositions being constant across the range of rock compositions (Nakano et al., 1991; Takada et al., 1992). Aramaki & Fujii (1988) demonstrated that an average Izu–Oshima basalt can be generated by fractionation of a primary olivine basalt by removing 50 wt % crystals. Consequently, <0·4 % H2O would have been present in such primary basalts.
Wet calc-alkaline andesite
Arculus & Bloomfield (1992) studied ashes recovered during ODP Leg 125 (Sites 782, 784 and 786) from the Izu–Bonin arc. These ashes are made up of volcanic glass, strictly representing liquid compositions. Groundmass compositions of volcanic rocks can also be used to suggest liquid lines of descent for arc magmas (Tamura, 1995). Figure 4 shows Miyashiro diagrams (FeO*/MgO vs SiO2) of the Izu–Bonin arc ashes (Arculus & Bloomfield, 1992) and groundmasses of the Shirahama Group (Tamura, 1995). Interestingly, there is a striking similarity between these two liquid trends. First, in both cases, liquid compositions range from basalt to rhyolite. Second, tholeiitic andesites (FeO*/MgO > 0·156 x SiO2 – 6·68) exist in a liquid state, but there are no analyses to indicate that the calc-alkaline andesites (FeO*/MgO < 0·156 x SiO2 – 6·68) were erupted in a liquid state. Similarly, glasses from the Mariana Trough fallout tephra contrast with the contemporaneous basaltic to dacitic lavas of the Mariana arc volcanoes (Straub, 1995); all compositions of glasses with SiO2 < 57% plot in the tholeiitic field, whereas FeO*/MgO vs SiO2 variation in the Mariana arc volcanoes shows a complete chemical gradation from the tholeiitic to calc-alkaline fields (Straub, 1995).
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) of the Shirahama Group plotted on FeO*/MgO vs SiO2 (Tamura, 1995
One possible explanation for the curious lack of calc-alkaline andesitic melts in the Izu–Bonin and Mariana arcs is that such liquids were originally water saturated. Water-saturated liquidi have negative slopes in P–T space and this results in the crystallization of these magmas before they can be erupted at the surface. For example, a water-saturated andesitic composition has a liquidus temperature of 970°C at 5 kbar, but the liquidus temperature rises to 1200°C at 1 atm (Green, 1982).
Rhyolites in the Izu–Bonin arc
Crustal partial melts
Beard & Lofgren (1991) showed that dehydration-melting of basaltic and andesitic rocks at 1, 3 and 6·9 kbar yields partial melts with compositions similar to island-arc tonalites and dacites. Their starting materials had major element compositions close to Izu–Bonin arc basalts and andesites, and it is therefore informative to compare the compositions of these dehydration melts with silicic rocks (60–79 wt % SiO2) from the Izu–Bonin arc Quaternary volcanoes (Fig. 5). Comparison can also be made with melts from experiments at higher pressure (10 kbar) carried out by Wolf & Wyllie (1994) and Nakajima & Arima (1998). The starting materials in these experiments were similar Izu–Bonin basalts and the experiments were carried out under water-deficient conditions.
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The melts produced by dehydration-melting at lower pressures (<7 kbar) by Beard & Lofgren (1991) have major element compositions identical to those of Izu–Bonin arc dacites and rhyolites except that Na2O contents are lower. Sodium is thought to have been lost during microprobe analysis. Interestingly, the melts from the higher-pressure (>10 kbar) experiments of Wolf & Wyllie (1994) and Nakajima & Arima (1998) have calc-alkaline dacite or rhyolite compositions, but these differ from those of most Izu–Bonin arc dacites and rhyolites in having lower TiO2 and FeO and higher Al2O3. Moreover, the phase assemblages produced during the 10 kbar experiments contain hornblende and garnet (Wolf & Wyllie, 1994), but these differ from the phenocryst assemblages of Izu–Bonin rhyolite pumices (Nishimura et al., 1992).
Partial melting of calc-alkaline andesite
Figure 6 and Table 2 show modal variations and melt composition as a function of temperature for the dehydration-melting experiments carried out by Beard & Lofgren (1991) at 3 kbar for their composition 557 (low-K calc-alkaline andesite). The data show that rhyolitic melts can be produced by <35% partial melting of calc-alkaline andesite at depths equivalent to the middle crust. The dehydration-melting experiments yielded rhyolitic melts coexisting with the anhydrous restite assemblage plagioclase + orthopyroxene + clinopyroxene + magnetite (Fig. 6, Table 2). This assemblage, without amphibole, is consistent with the phenocryst assemblage of rhyolite pumices cored from the Sumisu Rift (Nishimura et al., 1992). Rhyolite pumices from Myojin Knoll, Sumisu and South Sumisu Calderas also have a two-pyroxene assemblage (± hornblende ± quartz), but quartz phenocrysts are commonly resorbed (Yuasa & Nohara, 1992; Yuasa, 1995), probably caused by reheating of the magma.
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TERTIARY SHIRAHAMA GROUP, IZU PENINSULA |
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TOPABSTRACTINTRODUCTIONTHE IZU-BONIN ARCTERTIARY SHIRAHAMA GROUP, IZU...DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The Mio-Pliocene Shirahama Group medium-K volcanic arc suite of the Izu Peninsula is characterized by the occurrence of both a tholeiitic series (basalt–dacite) and a calc-alkaline series (andesite–dacite) (Tamura, 1994,
1995; Tamura & Nakamura, 1996). Magmatic temperatures inferred by the two-pyroxene thermometer show unambiguous differences between the two. Generally, temperatures of 950–1100°C are obtained from tholeiitic samples, whereas 800–900°C is indicated for calc-alkaline samples (Tamura, 1994). Chemical variations in the tholeiitic series and the calc-alkaline series are consistent with crystal fractionation from basalt and assumed magnesian andesite, respectively (Tamura, 1994). Both the tholeiitic series and the calc-alkaline series are isotopically identical (Tamura & Nakamura, 1996). Although rocks of the Shirahama Group were classified into the tholeiitic series and the calc-alkaline series on the basis of the fractionation models (Tamura, 1994), most of the tholeiitic series rocks and all of the calc-alkaline series rocks satisfy Miyashiro’s (1974) requirements of FeO*/MgO > 0·156 x SiO2 – 6·68 and FeO*/MgO < 0·156 x SiO2 – 6·68, respectively (Tamura, 1994).
Figure 7 shows that the major element variation of the Shirahama Group lies within that of the Quaternary Izu–Bonin arc volcanoes; the latter extends to higher-silica rhyolite (78 wt % SiO2) and more magnesian basalt (9 wt % MgO).
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Groundmasses of the Shirahama Group
The SiO2 contents of the tholeiitic and calc-alkaline rocks of the Shirahama Group, classified on the basis of the fractionation models of Tamura (1994), range from 47 to 67 wt % SiO2 and from 61 to 69 wt % SiO2, respectively. Mixed magmas have SiO2 abundances ranging from 54 to 60 wt % SiO2 (Tamura, 1994). The Shirahama Group is a shallow submarine sequence of volcaniclastic deposits, lava flows and intrusive bodies (e.g. Cashman & Fiske, 1991; Tamura et al., 1991); all of these rocks have rapidly cooled groundmasses. Thus late-stage microlite growth and devitrification of glass, observed in equivalent subaerial eruptions (e.g. Sparks et al., 2000) were minimal. Tamura (1995) determined the groundmass compositions by electron probe microanalysis (EPMA) using the following procedure: (1) squares with sides ranging from 200 to 500 µm, which included microlites (<50 µm), were taken to represent groundmasses; (2) 49 points were arranged equally in the square of each sample and measured by EPMA (grid analyses) by using a beam of 20 µm diameter; (3) average values were calculated as the groundmass compositions.
Representative whole-rock major element analyses, modal composition (wt %) and groundmass analyses of calc-alkaline andesite and dacite in the Shirahama Group are given in Table 3. Most calc-alkaline magmas in the Shirahama Group (61–69 wt % SiO2) consist of rhyolitic groundmasses (70–78 wt % SiO2) containing phenocrysts of augite, hypersthene, plagioclase and titanomagnetite; minor quartz and/or hornblende can be found in about half of these (Tamura, 1995). Figure 8 shows plots of major element abundances against wt % SiO2 for tholeiitic and calc-alkaline groundmasses (liquids) of the Shirahama Group. Rhyolitic groundmass compositions of the calc-alkaline series contrast sharply with the wide range of compositions shown by tholeiitic liquids (49–71 wt % SiO2). These calc-alkaline liquids resemble rhyolites from the Quaternary Izu–Bonin arc volcanoes (Fig. 8, Table 3) except that Na2O contents are lower; this feature suggests loss of this component during microprobe analysis. Moreover, the phenocryst assemblages of calc-alkaline andesites of the Shirahama Group are similar to restite phase assemblages of dehydration-melting of a calc-alkaline andesite (Fig. 6).
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Figure 9 shows modes of phenocrysts (wt %) of calc-alkaline andesites and dacites of the Shirahama Group in descending order of host liquid SiO2 content. Although the Shirahama calc-alkaline bulk-rock samples have SiO2 contents between 61 and 69 wt % in bulk composition, Fig. 9 corresponds approximately to the dehydration-melting experiments of a single calc-alkaline andesite shown in Fig. 6. In the Shirahama Group calc-alkaline andesites and dacites, quartz and/or hornblende exist at lower temperature (<850°C) and the wt % SiO2 of the liquid decreases and the fraction of liquid increases as the magmatic temperature rises (Fig. 9). Interestingly, liquid compositions, phenocryst assemblages and magmatic temperatures are related to each other, but they are not related in a simple way to bulk-rock compositions (Fig. 9). Hindered crystal fractionation of a cooling magma body or remobilization of a reheated and partially melted pluton may result in variable liquid compositions irrelevant to bulk-rock compositions.
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Phases to be subtracted from the assumed parental magma to produce calc-alkaline series rocks in the Shirahama Group are olivine + orthopyroxene + clinopyroxene + plagioclase (Tamura, 1994), which are different from phenocryst phases actually found in the calc-alkaline rocks (Fig. 9, Table 3). This discrepancy, not explained by Tamura (1994), can be reconciled by a partial melting model.
Figure 10a shows compositional relationships between bulk rocks and their groundmasses for the Shirahama Group. There is no correlation between these in the calc-alkaline series magma. On the other hand, tholeiitic andesites and dacites do not develop high-silica rhyolites (75–79 wt % SiO2) (Fig. 10a), but they show a positive correlation between bulk-rock and liquid compositions in the range of 50–70 wt % SiO2 (Tamura, 1995). Figure 10b shows wt % SiO2 of liquids vs magmatic temperatures from Tamura (1995). In the calc-alkaline liquids of the Shirahama Group, there is a negative correlation with temperature, similar to the results obtained in the dehydration experiments of Beard & Lofgren (1991) (Fig. 10b). The significantly higher temperature of the experimental results is probably due to the lower SiO2 content of their starting material (57 wt % SiO2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONTHE IZU-BONIN ARCTERTIARY SHIRAHAMA GROUP, IZU...DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Genesis of rhyolite
Does melting of basalt or andesite produce rhyolite?
Hydrous basalt and/or andesite are likely source rocks from which rhyolites could be produced by partial melting. It is commonly accepted that rhyolites form by the melting of hydrous basaltic rocks in the crust (Beard, 1995
), but basaltic magmas along the volcanic front of the Izu–Bonin arc are inferred to be almost anhydrous, and fractional crystallization would be inevitable within their crustal magma chambers. In other words, these basalt magmas cannot solidify within the crust without undergoing significant differentiation. On the other hand, the absence of calc-alkaline andesite glasses along the Izu–Bonin arc (Fig. 4) and the Mariana volcanic arc (Straub, 1995) suggests that liquids with these compositions could be water saturated, causing them to solidify at depth and never erupt to the surface. Thus, the partial melting of solidified calc-alkaline andesites rather than basalts might play an important role in producing rhyolite. The fundamental role of basalt magma in the genesis of rhyolite would therefore be to provide heat to masses of solidified calc-alkaline andesite at depth.
Partial melting of calc-alkaline andesite
Rhyolites of the Izu–Bonin arc possess major element compositions similar to those of melts produced by dehydration-melting of calc-alkaline andesite at low pressure (<7 kbar) (Figs 5 and 6, Beard & Lofgren, 1991). Moreover, compositions of these rhyolites are close to groundmass compositions of calc-alkaline andesites of the Shirahama Group, Izu Peninsula (Figs 8 and 9, Tamura, 1995). These lines of evidence support the concept that rhyolites are produced by 20–30% dehydration-melting of calc-alkaline andesites in the upper to middle crust.
The foregoing argument does not necessarily rule out a fractionation origin for rhyolites, but we prefer a melting origin because (1) fractional crystallization from basalt magma (tholeiitic series) does not develop high-silica (>75 % SiO2) rhyolite in the Shirahama Group (Fig. 8, Tamura, 1995); (2) hydrous calc-alkaline andesite magma of the Izu–Bonin arc would solidify within the crust rather than develop rhyolite through fractional crystallization.
Concealed calc-alkaline andesite
Calc-alkaline andesite is abundant in evolved island arcs having crustal thicknesses >30 km, and in continental marginal regions (Green, 1982). The volcanic front of the Izu–Bonin arc, with a crustal thickness of 22 km (Suyehiro et al., 1996), is characterized by bimodal volcanism, which produces large amounts of rhyolite magma as well as basalt and basic andesite magmas (Fig. 2c). The genesis of rhyolite in the Izu–Bonin arc leads us to an interesting conclusion that a much larger amount of calc-alkaline andesites (3–5 times greater than the rhyolites) is concealed at depth within the oceanic arc crust.
A detailed structural model across the Izu–Bonin arc along 32°15'N (Suyehiro et al., 1996) indicates that the oceanic arc contains a middle crust with a P-wave velocity of 6 km/s and occupying 25% of the crustal volume. Moreover, this middle-crustal component is confined beneath the arc and is absent beneath the Shikoku back-arc basin. The relatively low velocity (6 km/s) and its small increase with depth in the middle crust beneath the arc might be attributable to granitic rocks (Suyehiro et al., 1996). On the basis of the geological and petrological studies, Kawate & Arima (1998) suggested that the middle crust of the Izu–Bonin arc would be similar to the Miocene Tanzawa plutonic complex, central Japan, which is a tonalitic suite exposed at the northern end of the Izu–Bonin arc system (Fig. 1). The most voluminous intrusions in this suite comprise rocks with 60 wt % SiO2 (Kawate & Arima, 1998). It is thus possible that voluminous calc-alkaline andesites may have accumulated in the oceanic arc middle crust even though it is generally accepted that andesitic volcanism typifies continental arcs.
Remobilization of calc-alkaline andesite
If calc-alkaline andesite magma is water saturated, it will solidify in the crust. Additional heat is necessary to reheat and remobilize this highly crystalline calc-alkaline material. Reheating and remobilization of calc-alkaline magmas has been envisaged to have occurred in the Adamello massif, Italy (Blundy & Sparks, 1992), the Lascar volcano, Chile (Matthews et al., 1999) and the Soufriere Hills volcano, Montserrat (Murphy et al., 2000). Murphy et al. (2000) concluded that crystal-rich calc-alkaline andesite of the 1995–1999 eruption represents highly crystalline magma with physical properties that are more probably related to a partially molten but rigid and relatively immobile pluton rather than a mobile body of magma. The current eruption was triggered by influx of hot mafic magma, which reheated and softened the andesite magma body (Murphy et al., 2000), permitting it to erupt (Sparks et al., 2000).
Were the calc-alkaline andesites and dacites of the Shirahama Group also the result of heating and mobilization caused by mafic magma? The silicic liquids (groundmasses) contained in calc-alkaline andesites and dacites do not overlap the whole-rock calc-alkaline compositional trends (Fig. 8). The tholeiitic series of the Shirahama Group formed via fractionation of crystallizing phases (phenocrysts) from basalt through andesite to dacite (Tamura, 1994), and tholeiitic liquids show a wide range of composition, which overlaps the whole-rock trends (Tamura, 1995). Calc-alkaline series rocks lack both of these characteristics. Further, the calc-alkaline series includes a wide range of rhyolitic liquids (Fig. 10a). For example, calc-alkaline rocks with 64 wt % SiO2 are associated with rhyolitic liquids with SiO2 contents ranging from 69 to 79 wt % (Fig. 10). These lines of evidence suggest that calc-alkaline magmas in the Shirahama Group represent magmas formed by partial melting and consist of low- to high-percentage partial melts with restite crystals.
Segregation of partial melt from restite crystals would produce a magma of rhyolitic composition, and complete mobilization would produce a magma of the same composition as the source rock. This interpretation of some orogenic andesite-to-rhyolite sequences as the products of partial melting is increasingly supported by accumulating evidence, such as reversed zoning of orthopyroxene phenocrysts in terms of temperature (Matthews et al., 1999; Murphy et al., 2000) and hydrogen isotopic zoning of some amphiboles (Harford & Sparks, 2001).
Melt, solidify and melt again
Our contention is that mantle-derived hydrous magnesian andesite, not basalt magmas, may be parental to the calc-alkaline series rocks in the Shirahama Group (Tamura, 1994). Tamura (1994) developed this hypothesis based on the following interpretations: (1) a mantle diapir consisting of hydrous peridotite is formed in the lower part of the mantle wedge above the slab and ascends buoyantly through the mantle wedge; (2) this diapir is heated during ascent through the hot and dry mantle wedge, which is consistent with the model of Tatsumi et al. (1983) for the thermal structure; (3) finally, the heated diapir, which still has a wet and cool interior and heated dry and hot rind, produces both magnesian andesite and basalt, respectively (Tamura, 1994).
Hirose’s (1997) experiments are consistent with this hypothesis. He showed that wet magnesian andesite magma (54·4 wt % SiO2 and 6 wt % MgO on anhydrous basis) containing 6·3 wt % H2O is produced by melting of lherzolite KLB-1 with 1 wt % H2O at 1 GPa and 1050°C. The same peridotite produces basalt (50·5 wt % SiO2 and 10·1 wt % MgO) at 1 GPa and 1300°C under dry conditions (Hirose & Kushiro, 1993). The degrees of melting are 16 wt % and 12 wt %, respectively.
Low H2O contents in pre-eruptive calc-alkaline andesite magmas, which are not saturated with water, are commonly highlighted as one of the major problems inherent in fractionation models from hydrous magnesian andesite, because H2O contents are too low to have been produced by crystal fractionation of H2O-rich mantle-derived magmas (e.g. Tamura, 1995). Given solidification and remobilization of calc-alkaline magma, H2O contents can no longer be a constraint for genesis of calc-alkaline andesite. Water-saturated magmas solidify in the crust. Melting of such a solidified body produces water-deficient magmas; partial melting of largely solidified pluton cannot produce volatile-rich silicic magmas, because the volatile content of the constituent minerals is too low (Matthews et al., 1999).
Figure 11 shows the two-step model for genesis of calc-alkaline magmas. The primary step is anhydrous or hydrous melting of mantle peridotite, which produces basalt or magnesian andesite, respectively. Tholeiitic series rocks are produced by fractionation of basalt (Fig. 11b), but water-saturated magnesian andesite and/or calc-alkaline series rocks, which are produced by fractionation of magnesian andesite, will solidify in the crust (Fig. 11b). Thus, calc-alkaline magmas would not appear on the surface without reheating and the second-stage melting by hot and dry basalt magmas, which are produced in the same mantle diapir (Tamura, 1994, Fig. 11c). Tamura et al. (2000) presented evidence for supercooling of arc basalt at Daisen volcano, Japan; such a process would be complementary to remelting.
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Implications for continental genesis
The andesitic major element composition of the continental crust can be formed only via a two-stage process from the upper mantle: (1) generation of basalt (or magnesian andesite); (2) differentiation of this protolith (Arculus, 1999).
If continental crust formed mainly as a result of arc processes, the results of this study could have a bearing on the genesis of continental crust. In this study, the second stage of differentiation, described by Arculus (1999), involves both fractional crystallization of basalt magma and partial melting and remobilization of solidified andesite (protolith) by basalt magma (Fig. 11). Importantly, water-saturated andesite magma will solidify in the crust not as a result of the sudden drop in temperature, but as a result of decompression (Fig. 11b). In that case, the solidified andesite magma bodies could still have a high temperature (800°C) before subjacent basalt magmas are emplaced (Fig. 11c). Thus basalt magma might ‘strike while the protolith is hot’, which could make the resulting differentiation and continental genesis both rapid and efficient.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONTHE IZU-BONIN ARCTERTIARY SHIRAHAMA GROUP, IZU...DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The Izu–Bonin arc is characterized by bimodal, basalt–rhyolite, magmatism and it is commonly accepted that the rhyolite forms by the partial melting of basaltic rocks in the crust. However, basalt magmas of the Izu–Bonin arc are anhydrous and if a reservoir of basalt magma exists, it will not freeze within the crust, but will gradually crystallize to fractionate into a layered intrusion. In contrast, original calc-alkaline andesite could be saturated in water and is more likely to freeze within the crust, because the cooling path crosses the water-saturated solidus (negatively sloping on a P–T plot). Rhyolites of the Izu–Bonin arc could be produced by dehydration-melting of the solidified hydrous calc-alkaline andesite, which is also consistent with melting experiments in terms of both composition and mineralogy. Thus the fundamental role of basalt magmas in the generation of calc-alkaline silicic rocks is to provide heat to the solidified calc-alkaline magma bodies, thereby causing remelting and remobilization. The middle crust of the Izu–Bonin arc, a typical oceanic island arc, might consist of solidified calc-alkaline andesite, which is being partially melted by hot and dry basalt to produce rhyolite.
A two-stage process, involving mid-crustal solidification of water-saturated calc-alkaline magmas followed by partial melting related to reheating by subjacent, relatively anhydrous basaltic magmas, would generate melts that would have equilibrated with a phase assemblage differing significantly from that expected from direct fractional crystallization from a parent. This new interpretation complements Tamura’s (1994) fractionation models of calc-alkaline series rocks from mantle-derived magnesian andesite and provides a more complete explanation of how calc-alkaline series rocks low in H2O are produced from hydrous magnesian andesite.
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ACKNOWLEDGEMENTS |
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We thank R. S. J. Sparks for a review of the draft version of the manuscript. R. C. Price, T. H. Green and R. Arculus are thanked for thorough and critical reviews. R. S. Fiske helped with manuscript revisions. Many thanks go to K. Aoike, K. Fujioka, T. Ishii, S. Kawate, S. Morita, I. Moriya, K. Suga, A. Takada, T. Yamamoto, M. Yuasa for their help in compiling Izu–Bonin data.
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FOOTNOTES |
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*Corresponding author. Telephone: +81-468-67-9761. Fax: +81-468-67-9625. E-mail: tamuray{at}jamstec.go.jp
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REFERENCES |
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Y. Tamura, K. Tani, Q. Chang, H. Shukuno, H. Kawabata, O. Ishizuka, and R. S. Fiske Wet and Dry Basalt Magma Evolution at Torishima Volcano, Izu Bonin Arc, Japan: the Possible Role of Phengite in the Downgoing Slab J. Petrology, October 1, 2007; 48(10): 1999 - 2031. [Abstract] [Full Text] [PDF]
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S. Saito, M. Arima, T. Nakajima, K. Misawa, and J.-I. Kimura Formation of Distinct Granitic Magma Batches by Partial Melting of Hybrid Lower Crust in the Izu Arc Collision Zone, Central Japan J. Petrology, September 1, 2007; 48(9): 1761 - 1791. [Abstract] [Full Text] [PDF]
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N. Takahashi, S. Kodaira, S. L. Klemperer, Y. Tatsumi, Y. Kaneda, and K. Suyehiro Crustal structure and evolution of the Mariana intra-oceanic island arc Geology, March 1, 2007; 35(3): 203 - 206. [Abstract] [Full Text] [PDF]
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 P. T. LEAT, R. D. LARTER, and I. L. MILLAR Silicic magmas of Protector Shoal, South Sandwich arc: indicators of generation of primitive continental crust in an island arc Geological Magazine, January 1, 2007; 144(1): 179 - 190. [Abstract] [Full Text] [PDF]
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M. ELBURG, V. S. KAMENETSKY, I. NIKOGOSIAN, J. FODEN, and A. V. SOBOLEV Coexisting High- and Low-Calcium Melts Identified by Mineral and Melt Inclusion Studies of a Subduction-Influenced Syn-collisional Magma from South Sulawesi, Indonesia J. Petrology, December 1, 2006; 47(12): 2433 - 2462. [Abstract] [Full Text] [PDF]
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C. J. GARRIDO, J.-L. BODINIER, J.-P. BURG, G. ZEILINGER, S. S. HUSSAIN, H. DAWOOD, M. N. CHAUDHRY, and F. GERVILLA Petrogenesis of Mafic Garnet Granulite in the Lower Crust of the Kohistan Paleo-arc Complex (Northern Pakistan): Implications for Intra-crustal Differentiation of Island Arcs and Generation of Continental Crust J. Petrology, October 1, 2006; 47(10): 1873 - 1914. [Abstract] [Full Text] [PDF]
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Y. TAMURA, K. TANI, O. ISHIZUKA, Q. CHANG, H. SHUKUNO, and R. S. FISKE Are Arc Basalts Dry, Wet, or Both? Evidence from the Sumisu Caldera Volcano, Izu-Bonin Arc, Japan J. Petrology, September 1, 2005; 46(9): 1769 - 1803. [Abstract] [Full Text] [PDF]
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 S. E. Kesler, I. H. Campbell, C. N. Smith, C. M. Hall, and C. M. Allen Age of the Pueblo Viejo Gold-Silver Deposit and Its Significance to Models for High-Sulfidation Epithermal Mineralization Economic Geology, March 1, 2005; 100(2): 253 - 272. [Abstract] [Full Text] [PDF]
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Y. TAMURA, M. YUHARA, T. ISHII, N. IRINO, and H. SHUKUNO Andesites and Dacites from Daisen Volcano, Japan: Partial-to-Total Remelting of an Andesite Magma Body J. Petrology, December 1, 2003; 44(12): 2243 - 2260. [Abstract] [Full Text] [PDF]
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P. D. CLIFT, G. D. LAYNE, Y. M. R. NAJMAN, A. KOPF, N. SHIMIZU, and J. HUNT Temporal Evolution of Boron Flux in the NE Japan and Izu Arcs Measured by Ion Microprobe from the Forearc Tephra Record J. Petrology, July 1, 2003; 44(7): 1211 - 1236. [Abstract] [Full Text] [PDF]
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P. T. Leat and R. D. Larter Intra-oceanic subduction systems: introduction Geological Society, London, Special Publications, January 1, 2003; 219(1): 1 - 17. [Abstract] [PDF]
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I. E.M. Smith, T. J. Worthington, R. B. Stewart, R. C. Price, and J. A. Gamble Felsic volcanism in the Kermadec arc, SW Pacific: crustal recycling in an oceanic setting Geological Society, London, Special Publications, January 1, 2003; 219(1): 99 - 118. [Abstract] [PDF]
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Y. Tamura Some geochemical constraints on hot fingers in the mantle wedge: evidence from NE Japan Geological Society, London, Special Publications, January 1, 2003; 219(1): 221 - 237. [Abstract] [PDF]
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P. T. Leat, J. L. Smellie, I. L. Millar, and R. D. Larter Magmatism in the South Sandwich arc Geological Society, London, Special Publications, January 1, 2003; 219(1): 285 - 313. [Abstract] [PDF]
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