high-Mg Andesite Magma Research Articles

Parent magma of the Cretaceous Northern Kyushu batholith and heat source of large-scale igneous activity: Petrological constraints on magmatism of the Northeastern Asian continental margin in the Cretaceous period, example for the Kita-Taku mafic complex in northern Kyushu, SW Japan

The Japanese island in the Northeastern Asian continental margin is located in a subduction zone, and granitoid covers about 30% of the island arc. The Cretaceous Northern Kyushu batholith (CNKB) is classified as 19 bodies based on previous studies and consists of a granitic body and a small amount of associated mafic bodies. Of these, the Kita-Taku mafic complex body (KMC) mainly consists of gabbroic rocks and can be classified into cumulus textures (Cumulus group: Types 1–4) and those without (Non-cumulus group: Types 5–7), which can provide information on the heat source of the large-scale igneous activity in northern Kyushu from the Northeastern Asian continental margin at Cretaceous period.Melt compositions inferred from the high Mg# pyroxene compositions of the olivine-bearing pyroxene hornblendite (Type 1) member of KMC were estimated using melt-pyroxene partition coefficients. The clinopyroxene-bearing equilibrium melts are similar to the Sanukitic high-Mg andesite (HMA) in multi-element and rare-earth element patterns. Besides, the trace element and rare earth element patterns of the Cretaceous high-Mg diorites (HMDs) in northern Kyushu have similar patterns to the clinopyroxene-bearing equilibrium melts of the KMC and Sanukitic HMA magma. This implies that Sanukitic HMA magmas were substantially activated in northern Kyushu during the Cretaceous period.Analysis of the parent magma (heat source) and the small-scale plutonic bodies that consist of CNKB indicates that the essential granitic magma is formed by partial melting of the lower crust and mixing with mantle-derived Sanukitic HMA magma. The Sanukitic HMA magma could be the parent magma for the heat source and crustal growth. The lithofacies of CNKB can be reconstructed from the examination of small-scale plutonic bodies due to the mixing and assimilation of various end-member magmas and crustal materials, and their associated differentiation and accumulation processes. As the Northeastern Asian continental margin during the Cretaceous is assumed to be situated within the extensional setting due to slab rollback, the results of this study provide new insights into the complicated batholith formation processes and heat source.

マントルウェッジ流体の化学組成

Subduction-zone magmatism is triggered by the addition of H2O-rich slab-derived flux: aque- ous fluids, hydrous partial melts or supercritical fluids from the subducting slab through reac- tions. Whether the slab-derived flux is an aqueous fluid, a partial melt, or a supercritical fluid remains an open question. In general, with increasing pressure, aqueous fluids dissolve more silicate components and silicate melts dissolve more H2O. Under low-pressure conditions, those aqueous fluids and hydrous silicate melts remain isolated phases due to the miscibility gap. As pressure increases, the miscibility gap disappears and the two liquid phases becomes one phase. This vanishing point is regarded as critical end point or second critical end point. X-ray radiog- raphy experiments locate the pressure of the second critical end point at 2.5 GPa (83 km depth) and 700℃ for sediment-H2O, and at 2.8 GPa (92 km depth) and 750℃ for high-Mg andesite (HMA) -H2O. These depths correspond to the depth range of a subducted oceanic plate beneath volcanic arcs. Sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, may react with the mantle peridotite to form HMA supercritical fluids due to peritectic reaction between silica-rich fluids and olivine-rich mantle peridotite. Such HMA su- percritical fluids may separate into aqueous fluids and HMA melts at 92 km depth during ascent. HMA magmas can be erupted as they are, if the HMA melts segregate without reacting to the overriding peridotite. Partitioning behaviors between aqueous fluids and melts are determined with and without (Na, K) Cl using synchrotron X-ray fluorescence. The data indicate that highly saline fluids effectively transfer large-ion lithophile elements. If the slab-derived supercritical fluids contain Cl and subsequently separate into aqueous fluids and melts in the mantle wedge, then such aqueous fluids inherit much more Cl and also more or less amounts of large ion litho- phile elements than the coexisting melts. In contrast, Cl-free aqueous fluids can not effectively transfer Pb and alkali earth elements to the magma source. Enrichment of some large-ion litho- phile elements in arc basalts relative to mid-oceanic ridge basalts has been attributed to mantle source fertilization by such aqueous fluids from a dehydrating oceanic plate. Such aqueous fluids are likely to contain Cl, although the amount remains to be quantified. If such silica-rich mag- mas survive as andesitic melts under a limited reaction with mantle minerals, they may erupt as HMA magmas having slab-derived signatures.

瀬戸内火山帯に分布するHMAのK-Ar, Ar-Ar, FT 年代測定

瀬戸内火山帯に分布するHMAのK-Ar, Ar-Ar, FT 年代測定

HIGH-MG ANDESITES IN THE SETOUCHI VOLCANIC BELT, SOUTHWESTERN JAPAN: Analogy to Archean Magmatism and Continental Crust Formation?

The occurrence of unusual high-Mg andesite (HMA) characterizes the Setouchi volcanic belt in SW Japan, which was activated at 13.7±1.0 Ma by subduction of the young and hot Shikoku Basin lithosphere into the high-temperature upper mantle. This tectonic setting may be analogous to the thermal regime during Archean times, which suggests more ubiquitous production of HMA. A plausible process that can comprehensively account for the petrological and geochemical characteristics of Setouchi HMAs involves partial melting of subducting lithosphere, subsequent melt-mantle interactions, and final equilibration with the upper-most mantle. HMAs and more differentiated andesites, which are coined sanukitoids, are distinct in that they are phenocryst-poor (<10%), compact, and nearly anhydrous, despite HMA magmas originally containing ∼7 wt% H2O, and commonly form composite lava flows. One mechanism for explaining these features is formation of a mostly solidified HMA pluton, remelting of the HMA pluton by intrusion of a high-temperature basaltic magma, consequent production of a nearly dry HMA magma, and mixing of this HMA magma with overlying residual felsic melts during ascent to form a zoned magma reservoir.

The Petrology and Geochemistry of Oto-Zan Composite Lava Flow on Shodo-Shima Island, SW Japan: Remelting of a Solidified High-Mg Andesite Magma

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 (� 7w t %H 2O) HMA magma. However, Oto-Zan sanukitoids contain little H2O and are phenocryst-poor. The liquid lines of descent obtained for an Oto-Zan HMA at 0� 3GPa in the presence of 0� 7–2� 1w t %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 H2O 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 H2O-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.

紀伊半島大峯花こう岩質岩から見いだされたＭｇに富む火成包有物

Magnesian igneous enclaves showing sub-angular shape and up to 2 m in the maximum dimension were found from granodiorite in the Dorogawa pluton, the northernmost pluton of the Ohmine granitic rocks in Kii peninsula, southwest Japan. Igneous enclaves are composed mainly of plagioclase and hornblende, as phenocrysts, and of plagioclase, hornblende, quartz, and biotite as groundmass minerals. They also contain hydrous mafic mineral aggregates consisting of chlorite, actinolite, and biotite with rare chromian spinel microcrystals. Their whole rock compositions were characterized by high MgO content (5.8-7.0 wt%), low FeO*/MgO ratio (0.80-0.92), high Cr (256-346 ppm), and Ni (81-138 ppm) contents. Patterns of trace element abundances are close to those of the high-Mg andesite (HMA) of the Setouchi volcanic rocks. The compositional range of microcrystals of chromian spinel within hydrous mafic mineral aggregates is similar to those of included in olivine or orthopyroxene phenocrysts of the Setouchi HMA. These geochemical and mineralogical characteristics of the igneous enclaves suggest the activity of HMA magma closely in space-time with the intrusion of the Dorogawa pluton.

A Late Neoproterozoic (∼630 Ma) high-magnesium andesite suite from southern Israel: implications for the consolidation of Gondwanaland

The East African Orogen formed as a result of collision between portions of East and West Gondwanaland as the Mozambique Ocean closed in Late Neoproterozoic time, but it is not known exactly when. We use distinctive chemical and isotopic composition of deformed ‘schistose’ dykes in southern Israel to argue that this collision occurred after about 630 Ma, when the dykes were emplaced. These magmas had compositions of basaltic andesites and andesites but had high Mg# (100 Mg/Mg+Fe; 55–70 ppm), Ni (70–240 ppm), and Cr (100–400 ppm) indicating that the most primitive samples were in equilibrium with mantle peridotite; evolved samples suffered modest fractionation. The schistose dykes are a medium-K, calc-alkaline suite, strongly enriched in light rare earth elements and depleted in heavy rare earth elements. They are high-magnesium andesites and are similar to low-Ca type 2 boninites; similar magmas today only form over active subduction zones. The schistose dykes have non-radiogenic initial 87Sr/ 86Sr (0.7026–0.7033) and radiogenic 143Nd/ 144Nd, with ϵ Nd(630 Ma) of +2.7 to +4.9 and T DM=0.77–0.94 Ga. Hf isotopic compositions ( ϵ Hf(630 Ma)=+6.8 to +8.8) confirm the juvenile nature of these magmas. The dykes are closely related to a nearby quartz diorite, although it is not clear whether the dykes represent magma that fed into a magma body now filled with the quartz diorite, or issued from it. The generation of high-Mg andesite magma at 630 Ma involved reactive porous flow of a slab-derived melt through the mantle, requiring an active subduction zone, and strongly suggesting that young, hot seafloor – perhaps the spreading ridge of the Mozambique Ocean – was subducted. This indicates that collision between components of E. and W. Gondwana to form the East African Orogen must have occurred more recently than 630 Ma.

The Petrology and Geochemistry of Calc-Alkaline Andesites on Shodo-Shima Island, SW Japan

Petrographical and geochemical characteristics of calc-alkaline McLennan, 1995) that are produced at convergent plate boundaries and are characterized by lower FeO∗/MgO andesites on Shodo-Shima Island, SW Japan, having bulk comratios than tholeiitic andesites (Miyashiro, 1974). An positions largely identical to the continental crust, are presented. understanding of the origin of such orogenic, calc-alkaline The following petrographic observations suggest a role for magma andesites is thus essential for comprehending not only mixing in producing such andesite magmas: (1) two types of olivine the magma flux in the modern Earth but also the role phenocrysts and spinel inclusions, one with compositions identical of subduction-related magmatism in the evolution of the to those in high-Mg andesites and the other identical to those in solid Earth. Since the pioneering work of Eichelberger basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe (1975, 1978), Anderson (1976), Sakuyama (1979, 1981, relations, respectively; (2) the presence of orthopyroxene phenocrysts 1984), and Luhr & Carmichael (1980), the majority of with mg-number >90 suggests the contribution of an orthopyroxenepetrologists have considered mixing of mafic and felsic bearing high-Mg andesite magma to production of calc-alkaline magmas to be one of the major mechanisms of generation andesites; (3) reversely zoned pyroxene phenocrysts may not be in of calc-alkaline andesites. Continental crust formation, equilibrium with Mg-rich olivine, suggesting the involvement of a however, may not be elucidated solely by such magma differentiated andesite magma as an endmember component; (4) the mixing processes. The reasons for this are twofold. First, presence of very Fe-rich orthopyroxene phenocrysts indicates the calc-alkaline andesite magmatism typifies continental arcs association of an orthopyroxene-bearing rhyolitic magma. Conat least on the modern Earth, suggesting that the contributions from the above at least five endmember magmas to the calctinental crust itself plays an important role in such andesite alkaline andesite genesis can also provide a reasonable explanation of formation, possibly as the source of the felsic endmember the Pb–Sr–Nd isotope compositions of such andesites. 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,

Generation of Setouchi high-Mg andesite magmas by partial melting of subducting sediments and subsequent melt-mantle interaction : evidence from trace element and Os isotopic compositions

Generation of Setouchi high-Mg andesite magmas by partial melting of subducting sediments and subsequent melt-mantle interaction : evidence from trace element and Os isotopic compositions

Melting of a subducting slab and production of high‐mg andesite magmas: Unusual magmatism in SW Japan at 13∼15 Ma

Characteristic high‐Mg andesite magmas were produced in the SW Japan arc at 13∼15 Ma that was synchronous with the commencement of subduction of a very young (<11 m.y.) lithosphere of the Shikoku Basin. Numerical simulation suggests that temperature at the surface of such a young subducting plate is high enough for partial melting both of the subducting sediments and oceanic crust at the beginning of the subduction. High‐Mg andesite magmas were likely to be produced by interaction between silicic slab melts and the overlying mantle wedge. HMA magmas may be commonly produced in the Archean subduction zones under relatively high mantle temperature conditions, contributing to making continental crusts.

ORIGIN OF HIGHLY REFRACTORY PERIDOTITES: IMPLICATIONS FOR DIVERSITY OF HIGH-MG ANDESITE MAGMA GENESIS

Highly refractory peridotites are defined here as peridotites more depleted in melt components than abyssal peridotite, i.e. peridotites with spinel showing Cr# (Cr/(Cr+Al) atomic ratio) higher than 0.6. These correspond to the type III peridotite defined by Dick and Bullen (1984) and are reported from forearc regions, such as Izu-Ogasawara, Mariana and Tonga trenches (e.g. Bloomer and Hawkins, 1983; Bloomer and Fisher, 1987; Ishii et al., 1992) and from the Kamuikotan belt, the central axial zone of Hokkaido, northern Japan (e.g. Katoh and Nakagawa, 1986; Makita and Arai, 1997). These peridotites are, however, not common in the upper mantle and their genesis has not been explained thoroughly. We discuss the genesis of highly refractory peridotite from three complexes, the Takadomari, Iwanaidake and Nukabira, in the Kamuikotan belt, Japan, and its bearing on the process of high-Mg andesite magma genesis. The Kamuikotan belt is a tectonic melange zone composed of metamorphic rocks, ultramafic rocks, greenstones and sedimentary rocks. The metamorphic rocks are of typical high-pressure/low-temperature type and ophiolitic rocks which recorded low-pressure ocean-floor metamorphism are distributed in the Horokanai area, northern part of the belt (Ishizuka et al., 1983). Ultramafic rocks form the basal member of the ophiolite in the northern part and are exposed as peridotite complexes of various size in the southern part of the Kamuikotan belt. They are generally in fault contact with surrounding Kamuikotan metamorphic rocks, sediments and mafic volcanic rocks (e.g. Niida and Katoh, 1978). The peridotite complexes mainly consist of harzburgite and dunite which suffered serpentinization to various extent. The Takadomari complex from the northern part, and the Iwanai-dake complex from the southern part of the belt consist of harzburgite, dunite and small amounts of orthopyroxenite. The Nukabira complex (southern part of the belt) consists of lherzolite and harzburgite with dunite and pyroxenites. Podiform chromitite deposits sometimes accompany dunite in the latter two complexes. Primary hydrous minerals are sometimes included in chromian spinel of the former two complexes. The Cr# of chromian spinel and Fo (forsterite) content of olivine in peridotites from the Takadomari complex are high (Cr#=0.64-0.92, Fo=91.9- 94.0), and are higher in dunite than in harzburgite on average. The Cr# and Fo are also high (Cr#=0.43-0.87, Fo=90.8-93.5) in the Iwanai-dake complex and have wider ranges (Cr#=0.18-0.86, Fo=89.0-93.2) in the Nukabira complex. The Cr# of spinel and Fo of olivine in dunite complexes are sometimes similar to or even lower than the values in harzburgite from Iwanai-dake and Nukabira complexes. The mineral chemistry of dunites in the Iwanai-dake and Nukabira complexes depends on their thickness within harzburgite or lherzolite bodies. Thicker dunite layers tend to show higher Cr# of chromian spinel and Fo content of olivine. In addition to this, there are clear systematic variations in lithology across dunite layers and surrounding peridotites. In the case of thick dunite layers, the Cr# of chromian spinel and the Fo content of olivine gradually increase from the surrounding peridotite to the central part of the dunite layer (Cr#>0.6 and Fo>91.5 in dunite). In contrast to this fact, these values in thin dunite layers are similar to the surrounding peridotite, lherzolite or harzburgite (Cr#=0.4- 0.6 and Fo=89-91). These values are generally lower in thinner dunite layers than in the thick ones compared at the center. The harzburgites and dunites in the Takadomari complex are interpreted to be a series of refractory residue after extraction of high-Mg andesite magma; partial melting of the fertile peridotite possibly occurred under hydrous conditions. Harzburgite and lherzolite are mostly simple residue in the Iwanai-dake and Nukabira complexes but their dunites are not simple residue because the Cr# and Fo content are not systematically higher in the dunite than in the harzburgite (e.g. Arai, 1987; 1994). The dunite may be produced by a reaction between wall peridotite and a melt formed at higher pressures (e.g. Quick, 1981; Fisk, 1986; Kelemen, 1990). The differences in thickness and mineral chemistry of dunite layers are due to the difference of the melt/wall peridotite volume ratio. Supplying a large amount of melt caused formation of thick dunite layers and generated high-Mg andesitic magma by the reaction process.

Setouchi high-Mg andesites revisited: geochemical evidence for melting of subducting sediments

In order to evaluate the mechanism of production of unusual high-Mg andesite (HMA) magmas, Pb–Nd–Sr isotopic compositions were determined for HMAs and basalts from the Miocene Setouchi volcanic belt in the SW Japan arc. The isotopic compositions of Setouchi rocks form mixing lines between local oceanic sediments and Japan Sea backarc basin basalts, suggesting a significant contribution of the subducting sediment component to the HMA magma generation. Mixing calculations using compositions of an inferred original mantle and local oceanic sediments suggest that a sediment-derived melt, neither an H 2O-rich fluid nor an amphibolite/eclogite-derived melt, could have been produced first and served as a plausible metasomatic agent for the HMA magma source. The unusual tectonic setting, including subduction of a newly-borne hence hot plate, may be responsible for melting of subducting sediments.