市山 祐司

To constrain the incipient Pacific-type orogeny and tectonic processes in the Early Paleozoic proto-East Asian continental margin, the Motai–Matsugadaira–Yamagami (MMY) metamorphic rocks in the South Kitakami belt, northeast Japan were investigated. They are divided into two different types: amphibolite-facies rocks associated with serpentinite and blueschist-facies rocks associated with pelitic and psammitic schists. Three geochemical groups are identified from the MMY metamorphic rocks. Groups 1 and 2 resemble geochemical characteristics of mid-ocean ridge basalt and continental arc rocks, respectively. Group 3 exhibits considerable depletion of highly incompatible elements, which is caused by the high degree of partial melting of a hot mantle plume. The zircon U–Pb ages of Group 1 indicate that the protoliths experienced amphibolite-facies metamorphism soon after their formation in the Early Ordovician. Group 2 exhibits a coeval zircon U–Pb age with Group 1. The age distribution of detrital zircons in the MMY psammitic schists shows a peak of 500–400 Ma, the presence of Archean to Neoproterozoic zircons, and the youngest Late Devonian zircon. The following model is proposed for the tectonic evolution of the proto-East Asian continental margin: (1) the formation of an arc in the eastern margin of the South China craton in the Cambrian to Ordovician; (2) the subduction of a spreading axis and an oceanic plateau at the same time as the continental arc formation; (3) the consumption and subduction of arc materials by tectonic erosion; and (4) the formation of the Carboniferous accretionary complex and high-pressure metamorphic rocks under steady oceanic plate subduction. The proposed tectonic evolution model may also be applicable to equivalent Early Paleozoic rocks in southwest Japan.

The magmatic and tectonic evolution of the Oman ophiolite was investigated using the core samples obtained in the Oman Drilling Project. The core samples were recovered from three drilling sites (BA1B, BA3A, and BA4A) in the mantle section composed mainly of harzburgite and discordant dunite and wehrlite. A large number of mafic veins crosscut the structures of both mantle lithologies, indicating that they intruded after the formation of discordant dunites. The wehrlites include Fe3+-rich and TiO2-poor chromian spinel, suggesting that the discordant dunites and wehrlites were involved in hydrous, Ti-poor arc magmatism. The composition of plagioclase and clinopyroxene and rare-earth element (REE) patterns of clinopyroxene for the mafic veins suggest that the mafic veins were produced from mid-ocean ridge basalt (MORB)-like parental melts. On the other hand, early crystallization of clinopyroxene and Fe3+-rich and TiO2-poor chromian spinel in the mafic veins imply that the parental melts were also hydrous. Zircon U[sbnd]Pb dating of the mafic veins yields the igneous age of about 91 Ma, which is younger than the V1 and V2 volcanic sequences. Depending on the distance from the mafic veins, the various REE patterns of clinopyroxene and Ca amphibole in the Oman harzburgites indicate local peridotite metasomatism during intrusion and percolation of late-stage melts that formed the mafic veins. Ca amphibole in the harzburgites and mafic veins lack significant enrichment in fluid-mobile elements, indicating that the hydrous phases were formed by fluids associated with hydrothermal activities in a spreading environment instead of slab-derived fluids. The mafic veins were likely produced in a spreading environment on a supra-subduction zone. As their formation took place after the arc magmatism that formed the Oman V2 lava sequence and discordant dunites, as a result of slab roll-back. This seafloor re-spreading on a supra-subduction zone after the V2 magmatism recorded in the mantle section might have been involved in the formation of the plume-related V3 lava sequence overlying the V2 lavas.

The Wadi Ranga sulfidic jasperoids in the Southern Eastern Desert (SED) of Egypt are hosted within the Neoproterozoic Shadli metavolcanics as an important juvenile crustal section of the Arabian Nubian Shield (ANS). This study deals with remote sensing and geochemical data to understand the mechanism and source of pyritization, silicification, and hematization in the host metavolcanics and to shed light on the genesis of their jasperoids. The host rocks are mainly dacitic to rhyolitic metatuffs, which are proximal to volcanic vents. They show peraluminous calc-alkaline affinity. These felsic metatuffs also exhibit a nearly flat REE pattern with slight LREE enrichment (La/Yb = 1.19–1.25) that has a nearly negative Eu anomaly (Eu/Eu* = 0.708–0.776), while their spider patterns display enrichment in Ba, K, and Pb and depletion in Nb, Ta, P, and Ti, reflecting the role of slab-derived fluid metasomatism during their formation in the island arc setting. The ratios of La/Yb (1.19–1.25) and La/Gd (1.0–1.17) of the studied felsic metatuffs are similar to those of the primitive mantle, suggesting their generation from fractionated melts that were derived from a depleted mantle source. Their Nb and Ti negative anomalies, along with the positive anomalies at Pb, K, Rb, and Ba, are attributed to the influence of fluids/melt derived from the subducted slab. The Wadi Ranga jasperoids are mainly composed of SiO2 (89.73–90.35 wt.%) and show wide ranges of Fe2O3t (2.73–6.63 wt.%) attributed to the significant amount of pyrite (up to 10 vol.%), hematite, goethite, and magnetite. They are also rich in some base metals (Cu + Pb + Zn = 58.32–240.68 ppm), leading to sulfidic jasperoids. Pyrite crystals with a minor concentration of Ag (up to 0.32 wt.%) are partially to completely converted to secondary hematite and goethite, giving the red ochre and forming hematization. Euhedral cubic pyrite is of magmatic origin and was formed in the early stages and accumulated in jasperoid by epigenetic Si-rich magmatic-derived hydrothermal fluids; pyritization is considered a magmatic–hydrothermal stage, followed by silicification and then hematization as post-magmatic stages. The euhedral apatite crystals in jasperoid are used to estimate the saturation temperature of their crystallization from the melt at about 850 °C. The chondrite (C1)-normalized REE pattern of the jasperoids shows slightly U–shaped patterns with a slightly negative Eu anomaly (Eu/Eu* = 0.43–0.98) due to slab-derived fluid metasomatism during their origin; these jasperoids are also rich in LILEs (e.g., K, Pb, and Sr) and depleted in HFSEs (e.g., Nb and Ta), reflecting their hydrothermal origin in the island arc tectonic setting. The source of silica in the studied jasperoids is likely derived from the felsic dyke and a nearby volcanic vent, where the resultant Si-rich fluids may circulate along the NW–SE, NE–SW, and E–W major faults and shear zones in the surrounding metavolcanics to leach Fe, S, and Si to form hydrothermal jasperoid lenses and veins.

The Neoproterozoic pyroxene gabbros and gabbronorites in the El-Baroud mafic intrusion in the Northern Eastern Desert (NED) of Egypt host Fe-Ti oxide ore deposits. This study discusses the major and trace elements of both titaniferous iron ores and their host rocks, along with the mineral chemistry (major and in situ trace elements) of interstitial clinopyroxene (Cpx), to gain a deeper understanding of the Fe-Ti oxide genesis. These ores occur as disseminated (55–60 vol.% of Fe-Ti oxides) and massive types (85–95 vol.%) in the form of the dyke, layer, and lens. They are composed of titanomagnetite (80–87 vol.%) with subordinate ilmenite (10–15 vol.%) and magnetite (3–5 vol.%), in accordance with their high Fe2O3 (75.66 wt.% on average) and TiO2 contents (16.30–17.60 wt.%). The Cpx in the investigated ores is diopside composition (Mg#; 0.72–0.83) and exhibits a nearly convex upward REE pattern, similar to Cpxs in the ferropicrite that originated from the primitive mantle. Melts in equilibrium with this Cpx resemble Greenstone ferropicrite melts; the parent melt of El-Baroud gabbros is possibly a ferropicritic melt that was derived from the lithospheric mantle during plume interaction. The El-Baroud gabbroic rocks were generated during the arc rifting and crystallized under a high oxygen fugacity at a temperature of 800–1000 °C and a pressure of 3 kbar with a depth of 12 km. The Fe-Ti oxide ores have been formed from ferropicritic parent melts by two processes, including in situ crystallization that leads to the formation of disseminated Fe-Ti oxides in the iron-rich gabbros at the bottom and liquid immiscibility that is responsible for the formation of thick Fe-Ti ore lenses and layers at the top of the gabbroic intrusion. Initially, titanomagnetite crystallized from the primary Ti-rich oxide melt. As cooling progressed, some of the excess titanium in this melt was exsolved in the form of the exsolution ilmenite lamellae within the titanomagnetite. The Fe-Ti oxide layers in the NED follow the trend of NW-SE (Najd trend), where their distribution is possibly controlled by the composition of parent melts (rich in Ti and Fe), high oxygen fugacity, and the structure related to the Najd fault system. The distribution of Fe-Ti oxide ores increases from the NED to the Southern Eastern Desert (SED), suggesting the dominant mantle plumes and/or shear zones in the SED relative to the NED.

The Igla Ahmr region in the Central Eastern Desert (CED) of Egypt comprises mainly syenogranites and alkali feldspar granites, with a few tonalite xenoliths. The mineral potential maps were presented in order to convert the concentrations of total rare earth elements (REEs) and associated elements such as Zr, Nb, Ga, Y, Sc, Ta, Mo, U, and Th into mappable exploration criteria based on the line density, five alteration indices, random forest (RF) machine learning, and the weighted sum model (WSM). According to petrography and geochemical analysis, random forest (RF) gives the best result and represents new locations for rare metal mineralization compared with the WSM. The studied tonalites resemble I-type granites and were crystallized from mantle-derived magmas that were contaminated by crustal materials via assimilation, while the alkali feldspar granites and syenogranites are peraluminous A-type granites. The tonalites are the old phase and are considered a transitional stage from I-type to A-type, whereas the A-type granites have evolved from the I-type ones. Their calculated zircon saturation temperature TZr ranges from 717 °C to 820 °C at pressure < 4 kbar and depth < 14 km in relatively oxidized conditions. The A-type granites have high SiO2 (71.46–77.22 wt.%), high total alkali (up to 9 wt.%), Zr (up to 482 ppm), FeOt/(FeOt + MgO) ratios > 0.86, A/CNK ratios > 1, Al2O3 + CaO < 15 wt.%, and high ΣREEs (230 ppm), but low CaO and MgO and negative Eu anomalies (Eu/Eu* = 0.24–0.43). These chemical features resemble those of post-collisional rare metal A-type granites in the Arabian-Nubian Shield (ANS). The parent magma of these A-type granites was possibly derived from the partial melting of the I-type tonalitic protolith during lithospheric delamination, followed by severe fractional crystallization in the upper crust in the post-collisional setting. Their rare metal-bearing minerals, including zircon, apatite, titanite, and rutile, are of magmatic origin, while allanite, xenotime, parisite, and betafite are hydrothermal in origin. The rare metal mineralization in the Igla Ahmr granites is possibly attributed to: (1) essential components of both parental peraluminous melts and magmatic-emanated fluids that have caused metasomatism, leading to rare metal enrichment in the Igla Ahmr granites during the interaction between rocks and fluids, and (2) structural control of rare metals by the major NW–SE structures (Najd trend) and conjugate N–S and NE–SW faults, which all are channels for hydrothermal fluids that in turn have led to hydrothermal alteration. This explains why rare metal mineralization in granites is affected by hydrothermal alteration, including silicification, phyllic alteration, sericitization, kaolinitization, and chloritization.

Mass transfer at shallow subduction levels and its ramifications for deeper processes remain incompletely constrained. New insights are provided by ocean island basalt (OIB) clasts from the Mariana forearc that experienced subduction to up to ∼25–30 km depth and up to blueschist-facies metamorphism; thereafter, the clasts were recycled to the forearc seafloor via serpentinite mud volcanism. We demonstrate that the rocks were, in addition, strongly metasomatized: they exhibit K₂O contents (median = 4.6 wt%) and loss on ignition (median = 5.3 wt%, as a proxy for H₂O) much higher than OIB situated on the Pacific Plate, implying that these were added during subduction. This interpretation is consistent with abundant phengite in the samples. Mass balance calculations further reveal variable gains in SiO₂ for all samples, and increased MgO and Na₂O at one but losses of MgO and Fe₂O₃* at the other study site. Elevated Cs and Rb concentrations suggest an uptake whereas low Ba and Sr contents indicate the removal of trace elements throughout all clasts. The metasomatism was likely induced by the OIBs’ interaction with K-rich fluids in the subduction channel. Our thermodynamic models imply that such fluids are released from subducted sediments and altered igneous crust at 5 kbar and even below 200°C. Equilibrium assemblage diagrams show that the stability field of phengite significantly increases with the metasomatism and that, relative to not-metasomatized OIB, up to four times as much phengite may form in the metasomatized rocks. Phengite in turn is considered as an important carrier for K₂O, H₂O, and fluid-mobile elements to sub-arc depths. These findings demonstrate that mass transfer from the subducting lithosphere starts at low P/T conditions. The liberation of solute-rich fluids can evoke far-reaching compositional and mineralogical changes in rocks that interact with these fluids. Processes at shallow depths (&amp;lt;30 km) thereby contribute to controlling which components as well as in which state (i.e., bound in which minerals) these components ultimately reach greater depths where they may or may not contribute to arc magmatism. For a holistic understanding of deep geochemical cycling, metasomatism and rock transformation need to be acknowledged from shallow depths on.

© 2020 Elsevier B.V. Mafic and ultramafic clasts (mostly ~1–5 cm in size) were recovered from three different serpentinite mud volcanos in the Mariana forearc during Integrated Ocean Drilling Program (IODP) Expedition 366. Mafic clasts from drill sites distant from the trench bear lawsonite, Al-rich riebeckite, jadeitic pyroxene (~80 mol% jadeite), and aragonite as metamorphic minerals. In contrast, mafic clasts from drill sites closer to the trench are characterized by prehnite–pumpellyite-facies mineral associations and/or the presence of analcime and natrolite. An occurrence of antigorite-bearing ultramafic clasts becomes progressively more frequent with distance from the trench. One amphibolite clast from a mud volcano near the trench also has prehnite filling veins, and it also occurs as pseudomorphs after plagioclase. Amphibolite clasts at other mud volcanoes distant from the trench are partially overprinted by blueschist-facies minerals. The apparent metamorphic grades increase with distance from the trench; these metamorphic conditions represent the increasing depth from zeolite- to lawsonite–blueschist-facies conditions in a subduction zone. Considering the consistency of the low-temperature metamorphic grade of mafic and ultramafic clast mineralogy in each mud volcano, they likely reflect the thermal structure of the slab-mantle interface before the ascent. As a result, these clasts were brought up to the seafloor en masse by the serpentinite mudflow. The polymetamorphosed amphibolite clasts suggest cooling of the hot forearc-mantle at the initiation of Mariana subduction in the Eocene. The ultramafic clasts in the mud volcanoes distant from the trench frequently contain Ca amphibole and talc, which indicates hot mantle hydration by metasomatic fluids released from the slab at subduction initiation.

© 2021 John Wiley & Sons Australia, Ltd. The present study examines the petrology and geochemistry of the Early Paleozoic Motai serpentinites, the South Kitakami Belt, northeast Japan, to reveal the subduction processes and tectonics in the convergent margin of the Early Paleozoic proto East Asian continent. Protoliths of the serpentinites are estimated to be harzburgite to dunite based on the observed amounts of bastite (orthopyroxene pseudomorph). Relic chromian spinel Cr# [=Cr/(Cr + Al)] increases with decreasing amount of bastite. The compositional range of chromian spinel is similar to that found in the Mariana forearc serpentinites. This fact suggests that the protoliths of the serpentinites are depleted mantle peridotites developed beneath the forearc regions of a subduction zone. The Motai serpentinites are divided into two types, namely, Types 1 and 2 serpentinites; the former are characterized by fine-grained antigorite and lack of olivine, and the latter have coarse-grained antigorite and inclusion-rich olivine. Caamphibole occurs as isolated crystals or vein-like aggregates in the Type 1 serpentinites and as needle-shaped minerals in the Type 2 serpentinites. Caamphibole of the Type 1 serpentinites is more enriched in LILEs and LREEs, suggesting the influence of hydrous fluids derived from slabs. By contrast, the mineral assemblage, mineral chemistry, and field distribution of the Type 2 serpentinites reflect the thermal effect of contact metamorphism by Cretaceous granite. The Caamphibole of the Type 1 serpentinites is different from that of the Hayachine– Miyamori Ophiolite in terms of origin; the latter was formed by the infiltration of melts produced in an Early Paleozoic arc–backarc system. Chemical characteristics of the Ca-amphibole in the ultramafic rocks in the South Kitakami Belt reflect the tectonics of an Early Paleozoic mantle wedge, and the formation of the Motai metamorphic rocks in the forearc region of the Hayachine–Miyamori subduction zone system, which occurred at the Early Paleozoic proto-East Asian continental margin.

Early Paleozoic serpentinite melanges in Japan preserve the oldest high-P metamorphic rocks in the circum-Pacific orogenic belt. To understand the tectonic regime at the subduction initiation of the proto-Japan convergent plate boundary, whole-rock geochemistry, and zircon U-Pb geochronology were investigated for amphibolite blocks in the Omi serpentinite melange, central Japan. The studied amphibolites from two different localities have the mineral assemblage of albite + clinozoisite + amphibole +/- rutile +/- titanite, which characterize epidote-amphibolite facies metamorphism. Whole-rock trace element concentrations of the amphibolites suggest that gabbroic protoliths formed possibly in an oceanic setting. The zircon U-Pb weighted mean ages obtained from two amphibolite samples indicate that the protolith was formed in the Cambrian. The protolith ages of the studied amphibolites are comparable with those of reported Early Paleozoic ophiolite and high-pressure rocks in Paleozoic serpentinite melanges in Japan. This fact implies that the young hot oceanic crust was subducting into the East Asian convergent plate margin during the Cambrian.

The subduction of seamounts and ridge features at convergent plate boundaries plays an important role in the deformation of the overriding plate and influences geochemical cycling and associated biological processes. Active serpentinization of forearc mantle and serpentinite mud volcanism on the Mariana forearc (between the trench and active volcanic arc) provides windows on subduction processes.  Here, we present (1) the first observation of an extensive exposure of an undeformed Cretaceous seamount currently being subducted at the Mariana Trench inner slope; (2) vertical deformation of the forearc region related to subduction of Pacific Plate seamounts and thickened crust; (3) recovered Ocean Drilling Program and International Ocean Discovery Program cores of serpentinite mudflows that confirm exhumation of various Pacific Plate lithologies, including subducted reef limestone; (4) petrologic, geochemical and paleontological data from the cores that show that Pacific Plate seamount exhumation covers greater spatial and temporal extents; (5) the inference that microbial communities associated with serpentinite mud volcanism may also be exhumed from the subducted plate seafloor and/or seamounts; and (6) the implications for effects of these processes with regard to evolution of life. This article is part of a discussion meeting issue 'Serpentine in the Earth system'.

Secondary orthopyroxenes occur as veinlets (&lt;0.1 mm thick) cutting an olivine grain in a two-pyroxene peridotite xenolith from the Shiribeshi Seamount in the Sea of Japan. These orthopyroxenes are characterized by low Al2O3 (0.4-1.7 wt%), Cr2O3 (&lt;0.1 wt%), and CaO (0.3-0.4 wt%) contents, which are the same signatures of the secondary orthopyroxenes in peridotite xenoliths from island arcs. The trace-element patterns of the melts in equilibrium with the secondary orthopyroxenes show enrichment in light rare earth elements and Sr and depletion in heavy rare earth elements, Nb and Ti. These trace-element characteristics are highly consistent with those of slab-derived adakites. The involvement of slab-derived melts in the mantle beneath the Sea of Japan has been inferred from the geochemical characteristics of the volcanic rocks formed during opening of the Sea of Japan. The source mantle of the enriched basalts in the Sea of Japan is likely to have been metasomatized by adakitic melts in the same manner as the peridotite-hosted veinlet. The secondary orthopyroxenes in the peridotite xenolith from the Shiribeshi Seamount provide direct evidence for the metasomatic influx of adakitic melts into the back-arc mantle beneath the Sea of Japan. Adakitic metasomatism, as documented in the Sea of Japan, potentially plays an important role in mantle evolution and magma generation beneath global back-arc basins.

Orthopyroxene-rich and orthopyroxene-poor peridotite xenoliths were sampled from quaternary basaltic to andesitic lava flows of the Shiribeshi seamount, Japan Sea. These xenoliths were affected by reactions with the host magma during transportation to the surface, which caused partial orthopyroxene dissolution and intergrowth with vermicular spinel. Chromian spinel and clinopyroxene major element compositions in the Shiribeshi peridotite are similar to those in abyssal peridotites. REE modeling indicates that the Opx-rich peridotite experienced decompression partial melting from the garnet to the spinel peridotite stability field. Rare earth element (REE) patterns of clinopyroxene in the Opx-rich peridotite show various degrees of enrichment in light REE, which resulted from melt percolation through the reaction with host magma. Comparison with peridotite xenoliths from two other localities (Seifu and Oshima-O shima) in the Japan Sea suggests that the Oshima-O shima peridotite record higher degree of partial melting than the Shiribeshi and Seifu peridotites. Oxygen fugacities calculated from chromian spinel in the Japan Sea peridotites are comparable to those of arc peridotites. The high degree of partial melting of the Oshima-O shima peridotite was possibly caused by the infiltration of a H2O-bearing flux released from the subducted slab. The Shiribeshi peridotite is interpreted as the residue formed after the extraction of depleted back-arc basalts during a later stage of the Japan Sea opening in the Middle Miocene, whereas the Oshima-O shima peridotite is residual after the extraction of enriched basalts during an earlier stage of the opening of the Japan Sea.

We report detailed lithological and chemical characteristics of deep-sea sediments, including rare-earth elements and yttrium-rich mud (REY-rich mud), in the Japanese Exclusive Economic Zone (EEZ) around Minamitorishima Island. Three research cruises obtained fourteen sediment cores collected by piston coring. Based on the visual descriptions and geochemical analysis of the sediment cores, we confirm the presence of REY-rich mud containing more than 400 ppm total REY (Sigma REY) in the southern and northwestern areas of the Minamitorishima EEZ. The REY-rich mud layers are characterized by abundant grains of phillipsite, biogenic calcium phosphate, and manganese oxides, and are widely distributed in relatively shallow depths beneath the seafloor. In contrast, relatively thick, non-REY-rich mud lies near the seafloor in the northern areas of the EEZ. In the three cores from the southern part of the EEZ, we also confirm the presence of highly/extremely REY-rich mud layers. Further accumulation of geochemical data from the sediments will be required to constrain the extent of the highly/extremely REY-rich mud layers.

We have discovered deep-sea mud that is extremely enriched in rare-earth elements and yttrium (together called REY) in the Japanese Exclusive Economic Zone around Minamitorishima Island, in the western North Pacific Ocean. The maximum total REY concentration reaches approximately 7000 ppm, which is much higher than that reported for conventional REY deposits on land and other known potential REY resources in the ocean. The extremely REY-rich mud is characterized by abundant phillipsite and biogenic calcium phosphate. In addition, the stratigraphic layer with the highest REY concentration occurs just similar to 3 m beneath the seafloor. The shallow burial of these strata together with the high REY content, especially those of heavy rare-earth elements, suggest that the newly discovered extremely REY-rich mud may be a promising REY resource.

The Mikame ultramafic body, located in westernmost central Shikoku, Japan, forms a nappe accompanied by the Maana Formation and low- to medium-pressure Oshima metamorphic rocks. This body is divided into the Shigiyama and Korotokibana masses. The Shigiyama mass is composed of very fresh dunite, wehrlite and pyroxenite; whereas the Korotokibana mass is composed of antigorite-bearing meta-serpentinite derived from dunite and wehrlite. Estimated equilibrium temperatures are 600-700 degrees C for the Shigiyama mass and 400-500 degrees C for the Kototokibana mass, respectively. The geological and petrological characteristics of the Mikame ultramafic rocks are similar to those of the Higo belt in central Kyushu, rather than those of the Mikabu and Kurosegawa belts, and the Mikame body is possibly equivalent to the Higo belt. The Shigiyama ultramafic rocks are subdivided into Mg- and Fe-rich suites based on their olivine and chromian spinel compositions. The Mg-rich suite is characterized by magnesian olivines (Fo(84-92)) and high-Cr# [= Cr/(Cr + Al) = 0.5-0.8] spinels. On the other hand, the Fe-rich suite contains less magnesian olivines (Fo(74-88)) and low-Cr# (= 0.3-0.4) spinels. The Mg- and Fe-rich suites of the Shigiyama ultramafic rocks are cumulates formed from depleted and less depleted magmas, respectively. The chemistry of the chromian spinels in the Mikame ultramafic rocks indicates that they formed by crystal accumulation from magmas generated in an arc setting. The Korotokibana meta-serpentinites resemble those from the Mariana forearc regarding their mineral assemblage, and olivine and chromian spinel compositions. The Korotokibana meta-serpentinites experienced dehydration at 400-500 degrees C, after serpentinization caused by addition of H2O released from a subducting slab, whereas the Shigiyama ultramafic rocks contain no evidence for dehydration. The Mikame ultramafic body may have been the lower forearc crust produced by magmas with various degrees of depletion, later subjected to diverse hydration and dehydration processes.

Recent reassessment of abyssal peridotites obtained during the dredging of the oblique supersegment and the easternmost subsection of the Southwest Indian Ridge by the R/V Knorr Cruise 162 and the R/V Yokosuka YK98-07 revealed the occurrence of dunites containing podiform chromitites and dunites with variable chromite concentration closely associated with lherzolite and harzburgite. The size of the chromitite pods varies from a few mm to 2 cm in width. Chromites in the podifom chromitites have very low Cr# (=0.22-0.23) and low TiO2 (&lt; 0.17 wt%). They are almost free of silicate inclusions except for a few euhedral sulfide grains which occur far from cracks and lamellae and are considered primary in origin. The lherzolite which possibly represents the wallrock hosting the dunites with podiform chromitites also show low spinel Cr#(=0.16) and low Cr# in the clinopyroxenes (=0.09-0.10) and orthopyroxenes (=0.07-0.09). The small size of the SWIR podiform chromitites is strongly controlled by the low Cr/Al available in the wallrock and the invading melt. The presence of sulfide inclusions and the absence of PGEs further attest to the low Cr/Al (i.e. low refractoriness) in the system involved in the genesis of the SWIR podiform chromitites. Lastly, the discovery of podiform chromitites in the SWIR implies that the formation of podiform chromitite at mid-oceanic ridges, regardless of its spreading rate, is highly possible.

The Mikabu and Sorachi-Yezo belts comprise Jurassic ophiolitic complexes in Japan, where abundant basaltic to picritic rocks occur as lavas and hyaloclastite blocks. In the studied northern Hamamatsu and Dodaira areas of the Mikabu belt, these rocks are divided into two geo-chemical types, namely depleted (D-) and enriched (E-) types. In addition, highly enriched (HE-) type has been reported from other areas in literature. The D-type picrites contain highly magnesian relic olivine phenocrysts up to Fo(93.5), and their Fo-NiO trend indicates fractional crystallization from a high-MgO primary magma. The MgO content is calculated as high as 25 wt%, indicating mantle melting at unusually high potential temperature (Tp) up to 1,650 degrees C. The E-type rocks represent the enrichment in Fe and LREE and the depletion in Mg, Al and HREE relative to the D-type rocks. These chemical characteristics are in good accordance with those of melts from garnet pyroxenite melting. Volcanics in the Sorachi-Yezo belts can be divided into the same types as the Mikabu belt, and the D-type picrites with magnesian olivines also show lines of evidence for production from high T-p mantle. Evidence for the high T-p mantle and geochemical similarities with high-Mg picrites and komatiites from oceanic and continental large igneous provinces (LIPs) indicate that theMikabu and Sorachi-Yezo belts are accreted oceanic LIPs that were formed from hot large mantle plumes in the Late Jurassic Pacific Ocean. The E-and D-type rocks were formed as magmas generated by garnet pyroxenite melting at an early stage of LIP magmatism and by depleted peridotite melting at the later stage, respectively. The Mikabu belt characteristically bears abundant ultramafic cumulates, which could have been formed by crystal accumulation from a primary-magma generated from Fe-rich peridotitemantle source, and the HE-typemagma were produced by low degrees partial melting of garnet pyroxenite source. They should have been formed later and in lower temperatures than the E- and D-type rocks. The Mikabu and Sorachi Plateaus were formed in a low-latitude region of the Late Jurassic Pacific Ocean possibly near a subduction zone, partially experienced high P/T metamorphism during subduction, and then uplifted in association with (or without, in case of Mikabu) the supra-subduction zone ophiolite. The Mikabu and Sorachi Plateaus may be the Late Jurassic oceanic LIPs that could have been formed in brotherhood with the Shatsky Rise.

The Shiribeshi Seamount off northwestern Hokkaido, the Sea of Japan, is a rear-arc volcano in the Northeast Japan arc. This seamount is composed of calc-alkaline and high-K basaltic to andesitic lavas containing magnesian olivine phenocrysts and mantle peridotite xenoliths. Petrographic and geochemical characteristics of the andesite lavas indicate evidence for the reaction with the mantle peridotite xenoliths and magma mixing between mafic and felsic magmas. Geochemical modelling shows that the felsic end-member was possibly derived from melting of an amphibolitic mafic crust. Chemical compositions of the olivine phenocrysts and their chromian spinet inclusions indicate that the Shiribeshi Seamount basalts in this study was derived from a primary magma in equilibrium with relatively fertile mantle peridotites, which possibly represents the mafic end-member of the magma mixing. Trace-element and REE data indicate that the basalts were produced by low degree of partial melting of garnet-bearing lherzolitic source. Preliminary results from the mantle peridotite xenoliths indicate that they were probably originated from the mantle beneath the Sea of Japan rather than beneath the Northeast Japan arc. (C) 2013 Elsevier Ltd. All rights reserved.

Many samples of various rock types recovered directly from the ocean floor are housed at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). The GANSEKI database, maintained by JAMSTEC, provides a record of submarine rock samples and is currently available on the Internet, enabling the user to search for specific types of submarine rock samples and their conditions, such as size, lithology and degree of alteration/weathering. We used the GANSEKI database to obtain submarine samples recovered from various tectonic settings, such as mid-ocean ridge, back arc, and forearc, and here we report on the major and trace element compositions of volcanic glasses from mid-ocean ridge settings.

The GANSEKI (Geochemistry and Archives of ocean floor rocks on Networks for Solid Earth Knowledge Integration) database, managed by JAMSTEC, houses information and geochemical data of rock samples collected during research cruises. The database can be searched by two methods: by dive and dredge sites identified on a map, and by metadata. Of the rocks in the database, archived samples housed at JAMSTEC are available for observation and analysis free of charge, if an application is made and accepted, and geochemical data in the JAMSTEC archives have been compiled in GANSEKI.

Permian greenstones in the Jurassic Mino-Tamba accretionary complex, southwest Japan, are divided into three distinct series on the basis of their geological occurrence, mineralogy, and geochemistry. A low-Ti series (LTS) is associated with Lower Permian chert and limestone, and is the most voluminous of the three series. The LTS shows slightly more enriched geochemical and isotopic characteristics than MORB. A transition series (TS) is mainly associated with Lower Permian chert, and has more enriched geochemical signatures than MORB. Its isotopic characteristics are divided into enriched and depleted types. A high-Ti series (HTS) occurs as sills and hyaloclastites within Middle Permian chert and as dikes intruding the TS. Some HTS rocks have high MgO contents. The HTS is characterized by enrichment in incompatible trace elements and an isotopic composition comparable to HIMU-type basalt. The geochemistry of the voluminous LTS is similar to that of the oceanic basalt series of the Kerguelen plateau, suggesting production by partial melting of a shallow mantle plume head below thick oceanic lithosphere in Early Permian time. We infer that the TS formed simultaneously at the margins of the mantle plume head. In contrast, the HTS may have resulted from partial melting of a deep mantle plume tail in Middle Permian time. Permian greenstones in the Mino-Tamba belt may have thus formed by superplume activity in an intra-oceanic setting. Given the presence of two known contemporary continental flood basalt provinces (Siberia and Emeishan) and some accreted oceanic plateau basalts, the vast magmatism of the Mino-Tamba oceanic plateau suggests a large-scale superplume pulse in Permian time. Accretion of oceanic plateaux may have played an important role in the growth of continental margins and island arcs in Japan and elsewhere in the circum-Pacific region. (c) 2007 Published by Elsevier B.V.

Permian basalt showing typical spinifex texture with &gt; 10 cm-long olivine pseudomorphs was discovered from the Jurassic Tamba accretionary complex in southwest Japan. The spinifex basalt occurs as a river boulder accompanied by many ferropicritic boulders in a Permian chert-greenstone unit. Groundmass of this rock is holocrystalline, suggesting a thick lava or sill for its provenance. Minor kaersutite in the groundmass indicates a hydrous magma. The spinifex basalt, in common with the associated ferropicritic rocks, is characterized by high high field strength element (HFSE) contents (e.g. Nb = 62 ppm and Zr = 254 ppm) and high-HFSE ratios (Al2O3/TiO2 = 3.9, Nb/Zr = 0.24 and Zr/Y = 6.4) unlike typical komatiites. The spinifex basalt and ferropicrite might represent the upper fractionated melt and the lower olivine-rich cumulate, respectively, of a single ultramafic sill (or lava) as reported from the early Proterozoic Pechenga Series in Kola Peninsula. Their parental magma might have been produced by hydrous melting of a mantle plume that was dosed with Fe- and HFSE-rich garnet pyroxenite. The spinifex basalt is an evidence for the Pechenga-type ferropicritic volcanism taken place in a Permian oceanic plateau, which accreted to the Asian continental margin as greenstone slices in Jurassic time.

The Permian ferropicrite and picritic ferrobasalt occur in the Jurassic accretionary complexes of the Mino-Tamba belt as dikes intruded into the basaltic volcanic rocks. They are characterized by high MgO (11-27 wt.%), FeO* (16-20 wt.%) and HFSE (Nb = 24-86 ppm and Zr = 103-399 ppm) contents. Mineralogical and petrolographical evidences indicate that their unusual iron-rich nature is apparently magmatic in origin. The incompatible element contents and ratios indicate that the picritic ferrobasalt has close genetic kinship with the previously reported HFSE-rich, but iron-poor picrites, and that they were produced by the low degrees of partial melting of HFSE-enriched source material at high pressures (4-5 GPa). On the other hand, the ferropicrite may have been produced by the same degree of partial melting at a lower pressure, and subsequent olivine accumulation. The Sr and Nd isotopic signatures (Sr-87/Sr-86((i)) = 0.70266 to 0.70329 and epsilon Nd-(i) = +5.7 to +7.7) of these picritic and ferropicritic rocks are nearly constant and are equivalent to those of HIMU rocks, which require involvement of subducted oceanic crust material into their source region. Nevertheless, the ferropicritic melt cannot have been generated from the iron-poor picrite melt by crystal fractionation. Compared to the compositions of the melts obtained by some melting experiments, production of the unusual ferropicritic melts requires addition of an unreasonable amount of recycled basaltic component into the source mantle peridotite or partial melting at extremely high pressures. A possible source material for the ferropicrite is the mixture of the recycled Fe- and Ti-rich basalt (and/or gabbro) and mantle peridotite. Such a ferrobasalt occurs in the present ocean floor and also in some peridotite massifs as Fe- and Ti-rich eclogite bodies. The ferropicritic magma may have been derived from the Permian, deep mantle plume in an oceanic setting. The occurrence of the ferropicritic rocks and the HFSE-rich, iron-poor picrite in the Mino-Tamba belt implies that the greenstone-limestone-chert complexes in the Mino-Tamba belt may be fragments of an oceanic plateau formed by the Permian superplume activities in paleo-Pacific ocean and subsequently accreted to a continental margin through subduction process in the Jurassic time. (c) 2005 Elsevier B.V. All rights reserved.

During the Hakuho-Maru KH03-3 cruise and the Tansei-Maru KT04-28 cruise, more than 1000 rock samples were dredged from several localities over the Hahajima Seamount, a northwest-southeast elongated, rectangular massif, 60 km x 30 km in size, with a flat top approximately 1100 m deep. The rocks included almost every lithology commonly observed among the on-land ophiolite outcrops. Volcanic rocks included mid-oceanic ridge basalt (MORB)-like tholeiitic basalt and dolerite, calc-alkaline basalt and andesite, boninite, high-Mg adakitic andesite, dacite, and minor rhyolite. Gabbroic rocks included troctolite, olivine gabbro, olivine gabbronorite (with inverted pigeonite), gabbro, gabbronorite, norite, and hornblende gabbro, and showed both MORB-type and island arc-type mineralogies. Ultramafic rocks were mainly depleted mantle harzburgite (spinel Crsharp 50-80) and its serpentinized varieties, with some cumulate dunite, wehrlite and pyroxenites. This rock assemblage suggests a supra-subduction zone origin for the Hahajima Seamount. Compilation of the available dredge data indicated that the ultramafic rocks occur in the two northeast-southwest-oriented belts on the seamount, where serpentinite breccia and gabbro breccia have also developed, but the other areas are free from ultramafic rocks. Although many conical serpentinite seamounts 10 km in size are aligned along the Izu-Ogasawara (Bonin)-Mariana forearc, the Hahajima Seamount may be better interpreted as a fault-bounded, uplifted massif composed of ophiolitic thrust sheets, resembling the Izki block of the Oman ophiolite in its shape and size. The ubiquitous roundness of the dredged rocks and their thin Mn coating (&lt; 2 mm) suggest that the Hahajima Seamount was uplifted above sealevel and wave-eroded, like the present Macquarie Is., a rare example of ophiolite exposure in an oceanic setting. The Ogasawara Plateau on the Pacific Plate is adjacent to the east of the Hahajima Seamount, and collision and subduction of the plateau may have caused uplift of the forearc ophiolite body.

Alaskan-type zoned ultramafic complexes and ultramafic lavas, sills and dikes of Jurassic age occur in the Jurassic accretionary complex (Samarka-Nadanhada zone) of Russian Primorye and adjacent northeast China (Heilongjiang Province). The Alaskan-type complexes include ilmenite-rich pyroxenite and gabbro, and are locally associated with nepheline-bearing rocks and carbonatites. These complexes were intruded into Jurassic chert-shale-sandstone sequences, to which they gave a distinct contact metamorphism. The ultramafic volcanic rocks include massive lava, pillow lava, agglomerate, and tuff. They are dominated by picrite, but also include mennechite, a TiO(2)-rich ultramafic volcanic rock. The meimeechite-picrite association is also known from Japan and Sakhalin, such as in the central Hokkaido-Sakhalin belt (Jurassic). Mikabu bell (Jurassic), Mino belt (Permian), and Mineoka belt (Paleogene), although Alaskan-type complexes are not known in these belts. All these ultramafic volcanic rocks in Far East Russia, northeast China, and Japan are distinctly lower in TiO(2)/Al(2)O(3), Nb/Y, and Nb/Zr ratios than the mennechite series in northern Siberia, but are clearly higher in these ratios than the Japanese in-situ island-arc picrites, and closely resemble picritic rocks in oceanic islands, especially those with HIMU signatures. Occurrence of the Jurassic ultramafic magmatism over the 2000 km wide area of the East Asian margin, the vast development of Jurassic accretionary complexes in the same area, and the very short time interval between ultramafic magmatism and accretion suggest superplume activity in or near the subduction zone. The Permian, Jurassic, and Paleogene "oceanic meimeechites" among Japanese accretionary complexes suggests repeated superplume events through the Phanerozoic.

The Permian ophiolite emplaced in the Yakuno area, Kyoto Prefecture, consists of metavolcanic sequences, metagabbro and a troctolitic intrusion. The metavolcanics are associated with thick mudstone through a contact that shows the flowage of lava over unconsolidated mud layers on the sea floor. The metavolcanics and metagabbro have rare earth element (REE) patterns that are similar to enriched (E)- and transitional (T)-types ([La/Yb](N) = 0.77-11.2) of mid-oceanic ridge basalts (MORB), whereas their Nb/La ratios (0.40-1.20) are as low as those of back-arc basin basalts (BABB). Cr-spinels in the metavolcanic rocks have Crsharp of 40-73 and an Fe(3+)sharp of 9-24, numbers which are comparable to the values of BABB. These lines of evidence suggest that the Yakuno ophiolite originated more likely from an early stage back-arc basin rather than from an oceanic plateau, as has been suggested by some researchers. The troctolitic body that intrudes as a 0.5-km long lens in the metagabbro is composed of troctolite, olivine gabbro and microgabbro. The troctolite is marked by an olivine-plagioclase crystallization sequence, different from the commonly observed olivine-clinopyroxene sequence in other mafic/ultramafic cumulates of the Yakuno ophiolite. The microgabbro, with a composition close to that of the parental magma of the troctolite, is depleted in light REE ([La/Yb](N) = 0.18-0.55) so that it has an REE pattern that mimics normal (N)-type MORB. The interstitial clinopyroxene of the troctolite has highly variable TiO(2) contents (0.2-1.4 wt%), which is interpreted to result from postcumulus crystallization of heterogeneous intercumulus melts. The troctolitic intrusion may represent a late stage intrusion that formed in an off-ridge environment during sea floor spreading of the back-arc basin. The geochemical variation observed in the Yakuno ophiolite, ranging from N- to E-MORB affinities, reflects the changes in both mantle source compositions and processes involved in magma generation during the evolution of the back-arc basin.