Published Papers - SHINJO Ryuichi

Gabbroic-dioritic dykes from the Sanadaj-Sirjan Zone: windows on Jurassic and Eocene geodynamic processes in the Zagros Orogen, western Iran

Deevsalar, R, Shinjo, R, Wang, K.-L, Yeganehfar, H, Neill, I

Journal of the Geological Society   175 ( 6 ) 915 - 933   2018 [ Peer Review Accepted ]

Type of publication: Research paper (other science council materials etc.)

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Spatial variations in dissolved rare earth element concentrations in the East China Sea water column.

Luong, L.D, Shinjo, R, Hoang, N, Shakirov, R.B, Syrbu, N

Marine Chemistry   205   1 - 15   2018 [ Peer Review Accepted ]

Type of publication: Research paper (other science council materials etc.)

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Sr-Nd-Hf isotopic composition of volcanic rocks from Yoneyama Formation, northern Fossa Magna region

Aizawa Masataka, Shinjo Ryuichi, Okamura Satoshi, Takahashi Toshiro, Research Group Yoneyama

Abstracts of Annual Meeting of the Geochemical Society of Japan ( GEOCHEMICAL SOCIETY OF JAPAN )  65 ( 0 ) 118 - 118   2018

Type of publication: Research paper (research society, symposium materials, etc.)

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Petrology of volcanic rocks from Yoneyama Formation, central Japan: REE constraints on the magma process of the Northern Fossa Magna

Aizawa Masataka, Shinjo Ryuichi, Okamura Satoshi, Takahashi Toshiro, Fujibayashi Norie, Yoneyama Research Group

PROGRAMME AND ABSTRACTS THE VOLCANOLOGICAL SOCIETY OF JAPAN ( The Volcanological Society of Japan )  2018 ( 0 ) 37 - 37   2018

Type of publication: Research paper (other science council materials etc.)

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Petrology of volcanic rocks from Tomari Formation, Shimokita Peninsula, Northern Honshu arc

Aizawa Masataka, Inaba Mitsuru, Okamura Satoshi, Shinjo Ryuichi

Annual Meeting of the Geological Society of Japan ( The Geological Society of Japan )  2018 ( 0 ) 368 - 368   2018

Type of publication: Research paper (other science council materials etc.)

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[Program canceled for a disaster] Program canceled for the 2018 Hokkaido Eastern Iburi earthquake. However, This abstract is quotable and viewable on PDF.

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Tracing subducted sediment inputs to the Ryukyu arc-Okinawa Trough system: Evidence from thallium isotopes

Shu Y.

Geochimica et Cosmochimica Acta ( Geochimica et Cosmochimica Acta )  217   462 - 491   2017.11 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Sediments are actively subducted in virtually every arc worldwide. However, quantifying their contributions to arc lavas and thereby establishing budgets of how sediments participate in slab-mantle interaction is challenging. In this contribution we use thallium (Tl) abundances and isotopic compositions of lavas from the Ryukyu arc (including south Kyushu) and its back-arc basin, Okinawa Trough, to investigate the influence of sediments from arc to back-arc. We also present extensive geochemical data for sediments and altered oceanic crust (AOC) outboard of the northern (DSDP Sites 296, 442B, 443 and 444) and central (DSDP Sites 294 and 295) part of the Ryukyu arc. The Tl isotopic compositions of sediments change systematically from lighter outboard of northern Ryukyu arc to heavier outboard of central Ryukyu arc. The feature reflects the dominance of terrigenous material and pelagic sedimentation outboard of the northern and central Ryukyu arc, respectively. Central and northern sections of Ryukyu arc and Okinawa Trough display larger range of Tl isotopic variation than southern section, which is consistent with more pelagic provenance for sediments outboard of central and northern Ryukyu arcs than that of expected sediments outboard of southern Ryukyu arc. Identical Tl, Sr, Nd and Pb isotope variations are found when comparing arc and back arc lavas, which indicates that sediments fluxes also account for the Tl isotopic variations in the Okinawa Trough lavas. Two-end-member mixing models of Tl with Pb, Sr and Nd isotopes require sediment inputs of&lt; 1%, 0.1-1% and 0.3-2% by weight to the depleted mantle source to account for all these isotopic compositions of lavas from northern, central and southern portion of the Ryukyu arc and Okinawa Trough. Bulk mixing between mantle and sediment end members predict very similar sediment fluxes when using Tl, Sr, Nd and Pb isotopes, which indicates that fractionation of these elements must have happened after mixing between mantle and sediments. This conclusion is corroborated by model calculations of mixing between sediment melts with fractionated Sr/Nd ratios and mantle wedge, which show that no arc lava plot on such mixing lines. Thus bulk sediment mixing, rather than sediment melt, is required for the generation of the lavas from the Ryukyu arc and Okinawa Trough. The requirement of bulk sediment mixing occurring before trace element fractionation in the sub-arc mantle is consistent with models where melange layers form at the top of the slab and are the principle source material for arc lavas. In addition, the fact that sediment components observed in the Ryukyu arc and OkinawaTrough lavas are similar, suggests that transport of melange material to the source regions of the arc and back arc is equally efficient. This feature is most readily explained if melange material is transported from the slab as diapirs. (C) 2017 Elsevier Ltd. All rights reserved.

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Mesozoic–Cenozoic mafic magmatism in Sanandaj–Sirjan Zone, Zagros Orogen (Western Iran): Geochemical and isotopic inferences from Middle Jurassic and Late Eocene gabbros

Deevsalar R.

Lithos ( Lithos )  284   588 - 607   2017.07 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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One of the consequences of Neo-Tethys ocean subduction beneath the Central Iranian Micro-continent (CIMC) is the development of rare gabbroic intrusions in the Malayer-Boroujerd Plutonic Complex (MBPC) located in the Sanandaj-Sirjan Zone (SaSZ) of the Zagros Orogenic belt. The MBPC is a suite of extensive felsic and lesser mafic magmatic products in the northern SaSZ with geochemical signatures of arc-like magmatism during the Middle Jurassic (Ghorveh-Aligudarz arc) and intraplate type in the Late Eocene. Middle Jurassic gabbros (non-cumulate and cumulate) have low-Ti concentrations (&lt;1 wt.%) and quite uniform isotopic compositions (initial Sr-87/Sr-86: 0.7035-0.70593 and epsilon Nd(t): -6.18 to -0.7), enriched LILE relative to HFSE, variable fractionation between the LREE and HREE ((La/Yb)(cn) : 227-7.45) and both negative to positive Eu anomalies. These distinctive features of arc-type magmatism are consistent with a subduction-modified mantle source for these rocks. Trace element and REE models indicate similar to 15% melting of a metasomatized amphibole-bearing garnet-spinel lherzolite (gamet:spinel similar to 7:3) in the sub-arc mantle wedge. The cumulate gabbros and non-cumulates belong to common liquid line of descent, with complementary trace element patterns. Much of the variation between samples can be modeled by fractional crystallization (FC) of a common parent; only one cumulate gabbro from this suite exhibits isotopic evidence of contamination, probably by Rb-depleted crustal materials. The Late Eocene gabbros have relatively high-Ti (&gt;1 wt.%) and display isotopically depleted Sr-Nd values (initial Sr-87/Sr-86: 0.7044-0.7087, epsilon Nd(t): 1.9 -+3.2, barring one crustally-contaminated sample). OIB-like trace element characteristics such as enriched HFSE, and only minor enrichment of LILE and LREE, reflect a within-plate character and asthenospheric source. Trace element modeling indicates small degree melting (f(melting): 0.05) of upper mantle lherzolite (garnet:spinel similar to 3:1) followed by higher degree melting (f(melting): 0.15) at shallower depths (garnet:spinel similar to 4.5:2). The Eocene parental magma underwent FC of olivine and clinopyroxene. We propose that Eocene asthenospheric upwelling was triggered by slab tearing in response to slab-rollback, which is elsewhere reported to have triggered a 'flare-up' of extension-related magmatism across Iran. Three stages of tectono-magmatic evolution in the Ghorveh-Aligudarz arc segment of the N-SaSZ are represented by: 1) arc-like magmatism during active subduction of the Neo-Tethys seaway at Middle Jurassic, 2) magmatic quiescence during an interval of shallow-angle or highly oblique subduction during the Cretaceous-Paleocene, and 3) asthenosphere melting during slab tearing shortly before the onset of the Arabia-Eurasia collision. (C) 2017 Elsevier B.V. All rights reserved.

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Petrologic and geochemical characterization of rift-related magmatism at the northernmost Main Ethiopian Rift: Implications for plume-lithosphere interaction and the evolution of rift mantle sources

Feyissa D.

Lithos ( Lithos )  282   240 - 261   2017.06 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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In this paper, we present petrography, K-Ar ages, whole-rock major- and trace-element concentrations, and Sr-Nd-Hf-Pb isotopic ratios of volcanic rocks from Debre Birhan area in the northernmost Main Ethiopian Rift (MER). The K-Ar ages of the mafic series range from 27 to 0.25 Ma, and two felsic rocks yield ages of 0.93 and 0.23 Ma. The mafic volcanics are classified into older and younger series based on their K-Ar ages. The Mg-numbers (Mg# &lt; 60) of both mafic series, along with low Ni and Cr contents, clearly indicate that these lavas have undergone fractionation en route to the surface. Geochemical and isotopic compositions of the older and younger mafic lavas indicate that crustal contamination did not play a significant role in the evolution of their magmas, hence reflect the geochemical characteristics of the sources. The older and younger mafic volcanic rocks display contrasting trace element and isotopic signatures. The older lavas have higher La/Nb, Zr/Nb, Ba/Nb and Sr-87/Sr-86(i) (0.70445-0.70681) and lower epsilon(Hf) (-2.58 to + 6.01) and epsilon(Nd) ( -1.25 to +3.43) and less radiogenic Pb isotopic ratios (Pb-206/(204)Pbi = 17.82-18.64, except TG-54 = 19.1) relative to the younger mafic lavas. Correlations among isotopic ratios and trace element concentrations provide evidence for the involvement of at least three major end-member components (C-1, C-2, and C-3) in the petrogenesis of these mafic lavas. The C-1 end-member component contributes dominantly to younger mafic lavas, and has isotopic composition similar to the common component 'C' deduced from oceanic basalts. The second end-member component (C-2) is represented by lavas near the transition between older and younger, and has isotopic composition similar to that of the seafloor basalts of the Red Sea and the Gulf of Aden. The third end-member component (C-3) is prominent in the older mafic series, and similar isotopically to Pan-African lithosphere. The younger felsic volcanic rocks generally have higher Sr-87/Sr-86(i) (0.70395-0.70689) than younger mafic series (0.70366-0.70454) but Nd, Hf and Pb isotopic ratios similar to those of the younger mafic lavas. The element concentrations and Sr, Nd, Hf and Pb isotopic compositions of the younger felsic rocks are explained by fractional crystallization of mafic melts in shallow-level magma chambers accompanied by limited assimilation of crustal material. Low degrees of partial melting of existing mafic underplate can account for the genesis of the least differentiated (intermediate) lavas, but failed to explain highly evolved younger felsic rocks. (C) 2017 Elsevier B.V. All rights reserved.

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Contribution of slab-derived fluid and sedimentary melt in the incipient arc magmas with development of the paleo-arc in the Oman Ophiolite

Kusano Y.

Chemical Geology ( Chemical Geology )  449   206 - 225   2017.01 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Major-element and trace-element geochemistry of fresh volcanic glass, and the whole-rock Hf and Nd isotopic compositions of volcanic rocks from the Oman Ophiolite were used to understand the extent to which slab-derived fluids and melts were involved in magma generation during incipient arc development. The spreading stage (V1) samples have trace-element characteristics similar to those of mid-ocean-ridge basalts, with Hf and Nd isotopic compositions of the Indian mantle domain. The incipient arc stage (V2) volcanic glasses have lower abundances of high field strength elements (HFSEs) and middle and heavy rare earth elements (MREE5 and HREES), and higher abundances of large ion lithophile elements (LILEs) than the V1 glasses. Progressive increases in B, Pb, and LILEs in the V2 stage glasses with age indicate an increasing contribution of slab-derived fluids from earlier arc tholeiite (LV2) to later boninite (UV2). The LV2 was generated by the contribution of amphibolite-derived fluid. The UV2 is subdivided into low-Si UV2 and high-Si UV2 magmas, with the former having lower SiO2, and LILE, and higher Na2O, HFSE and REE concentrations than the latter. The low-Si UV2 magma was generated with the involvement of high-temperature slab-derived fluid. In contrast, the high-Si UV2 shows a spoon-shaped trace-element pattern and Hf and Nd isotope chemistry, indicating the involvement of sediment melt in its magma genesis. Decreasing HFSEs and HREEs in the magmas with age indicate progressive depletion of the source mantle during the V2 arc magmatism, suggesting an absence of convection in the mantle wedge that resulted from subduction of a hot, buoyant slab beneath young, hot lithosphere. (C) 2016 Elsevier B.V. All rights reserved.

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Pollution assessment of various heavy metals in the surface sediment of Kendari Bay, Indonesia

Armid, A, Shinjo, R, Sabarwati, S.H, Ruslan, R

Nature Environment and Pollution Technology   16 ( 4 ) 1067 - 1074   2017 [ Peer Review Accepted ]

Type of publication: Research paper (other science council materials etc.)

地質調査で採取された堆積物試料中の間隙水の地球化学的解析

石橋純一郎, 正木裕香, 岡村慶, 野口拓郎, 土岐知弘, 新城竜一

SIP 『次世代海洋資源調査技術』(海のジパング計画) 海底熱水鉱床の調査技術プロトコル     2017

Type of publication: Research paper (other science council materials etc.)

Neogene volcanism associated with back-arc basin tectonics at the northern Fossa Magna, NE Japan

OKAMURA Satoshi, INABA Mitsuru, ADACHI Yoshiko, SHINJO Ryuichi

Earth Science (Chikyu Kagaku) ( The Association for the Geological Collaboration in Japan )  71 ( 2 ) 65 - 73   2017 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Isotopic and trace element data imply a temporal change in magma sources and thermal conditions beneath the northern Fossa Magna from the Oligocene to the Pliocene. Less radiogenic ¹⁷⁶Hf/¹⁷⁷Hf and Nd/¹⁴⁴Nd, and high Zr/Hf characterize the Early Miocene volcanism in the northern Fossa Magna region. Based on out isotope proxies, we propose that during the onset of subduction, influx of hot asthenospheric mantle provided sufficient heat to partially melt newly subducting sediment. Geochemical modeling demonstrates that slab-derived melt mixed with mantle wedge produces the observed isotopic and trace elemental characteristics. The Early Miocene Kiriake Basalt has similar geochemical composition to the basalts of the Japan Sea basement, suggesting the melting of asthenospheric mantle beneath the extensional rift system within the northern Fossa Magna region during the opening of the Japan Sea. In the Middle Miocene, the injection of hot and depleted asthenospheric material replaced the mantle beneath the northern Fossa Magna region. The mantle wedge was gradually cooled during the Middle Miocene to the Pliocene with back-arc opening ending in the Late Miocene. Slab surface temperatures were still high enough for sediments to melt but not too high (<~780 °C) to lose zircon as a residual phase.

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北部フォッサマグナ周辺の新生代火成活動と背弧海盆テクトニクス

岡村聡, 稲葉充, 足立佳子, 新城竜一

地球科学   71 ( 2 ) 65 - 73   2017 [ Peer Review Accepted ]

Type of publication: Research paper (other science council materials etc.)

Distribution of hydrothermal-derived chemical components around the ore body in Izena Hole

Toki Tomohiro, Ohtake Tsubasa, Ishibashi Jun―ichiro, Matsui Yohei, Kawagucci Shinsuke, Kato Yamato, Fuchida Shigeshi, Miyahara Rena, Tsutsumi Akihi, Nakamura Shunsuke, Kawakita Ryuhei, Uza Hirotaka, Uehara Riki, Shinjo Ryuichi, Nozaki Tatsuo, Kumagai Hidenori, Maeda Lena

Abstracts of Annual Meeting of the Geochemical Society of Japan ( GEOCHEMICAL SOCIETY OF JAPAN )  64 ( 0 ) 147 - 147   2017

Type of publication: Research paper (scientific journal)

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<p>From 16th November to 15th December 2016, hydrothermal field was drilled at Izena Hole in Okinawa Trough using D/V Chikyu. Five sites were drilled along the transect line from the top of the northern mound of the HAKUREI site to the east side, and 1 site about 500 m northwest of the mound as the reference site. From the collected cores, samples for headspace gas measurement and interstitial water extraction were taken on the ship. Regarding the interstitial water extracted from the sample, pH, alkalinity, nutrients and hydrogen sulfide concentration on ship, and main elements concentration and boron isotopic ratio were measured on land. For samples for gas measurement, hydrocarbon and hydrogen concentrations were measured on ship and carbon isotopic ratios of methane on land.</p>

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Fluid flow and water–rock interaction across the active Nankai Trough subduction zone forearc revealed by boron isotope geochemistry

Hüpers A.

Geochimica et Cosmochimica Acta ( Geochimica et Cosmochimica Acta )  193   100 - 118   2016.11 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Compositional changes, dehydration reactions and fluid flow in subducted sediments influence seismogenesis and arc magmatism in subduction zones. To identify fluid flow and water-rock interaction processes in the western Nankai Trough subduction zone (SW Japan) we analyzed boron concentration and boron isotope composition (delta B-11) of pore fluids sampled across the subduction zone forearc from depths of up to similar to 922 m below seafloor during four Integrated Ocean Drilling Program (IODP) Expeditions. The major structural regimes that were sampled by coring include: (1) sedimentary inputs, (2) the frontal thrust zone, (3) the megasplay fault zone, and (4) the forearc basin. From mass balance consideration we find that consumption of boron (B) by ash alteration and desorption of B from the solid phase, mediated by organic matter degradation, produces a net decrease in B concentrations with depth down to similar to 120 mu M and variable (delta B-11) values in the range of similar to+20 parts per thousand and +49 parts per thousand. Interstitial water in sediments on the incoming oceanic plate are influenced by more efficient mobilization of exchangeable B from the solid phase due to higher temperatures and alteration of the oceanic crust that acts as a sink for B-10. At the tip of the megasplay fault zone, elevated B concentration and B isotopic composition suggest that underthrust coarse-grained slope sediments provide a pathway for fluids out of the upper(&lt;2 km) accretionary prism. Silt and sand layers in the underthrust section of the downgoing plate favor fluid escape in seaward direction from depths equivalent to the temperature range of 60-150 degrees C. At both locations the (delta B-11) signature evolves during updip migration through re-adsorption. Mass balance considerations suggest a shallower fluid source depth compared to pore fluids sampled previously near the decollement zone along the central portion of the Nankai margin. (C) 2016 Elsevier Ltd. All rights reserved.

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Miocene-Pliocene mantle depletion event in the northern Fossa Magna, western NE Japan

Okamura S.

Journal of Geodynamics ( Journal of Geodynamics )  97   42 - 61   2016.07 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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New isotopic and trace element data presented here imply a temporal change in magma sources and thermal conditions beneath the northern Fossa Magna of NE Japan from the Miocene to the Pliocene. Less radiogenic Hf-176/Hf-177 and Nd-143/Nd-144, high Zr/Hf, and little or no Hf anomaly characterize the Early Miocene volcanism in the northern Fossa Magna region. The mantle wedge consisted of chemically heterogeneous mantle source. Based on out isotope proxies, we propose that during the onset of subduction, influx of hot asthenospheric mantle provided sufficient heat to partially melt newly subducting sediment. Geochemical modeling demonstrates that slab-derived melt mixed with mantle wedge produces the observed isotopic and trace elemental characteristics. In the Middle Miocene, the injection of hot and depleted asthenospheric material replaced the mantle beneath the northern Fossa Magna region of NE Japan. This caused the isotopic signature of the rocks to change from enriched to depleted. Then, the mantle wedge was gradually cooled during the Middle Miocene to the Pliocene with back-arc opening ending in the Late Miocene. Slab surface temperatures were still high enough for sediments to melt but not too high (&lt;similar to 780 degrees C) to lose zircon as a residual phase. The Late Miocene and Pliocene volcanism at the post stage of the back-arc opening is best explained by a partial melting of subducted metasediment saturated with trace quantities of zircon and rutile. (C) 2016 Elsevier Ltd. All rights reserved.

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Development of a New Method of Extraction of Interstitial Water from Low-Porosity Consolidated Sediments Recovered During Super-Deep Drilling Projects

Toki T.

Geostandards and Geoanalytical Research ( Geostandards and Geoanalytical Research )  40 ( 2 ) 291 - 300   2016.06 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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During the Integrated Ocean Drilling Program (IODP) Expedition 338, several methods were tested for the extraction of interstitial water in consolidated, low-porosity deep-sea sediments from Site C0002 in the Kumano Basin. On the basis of those tests, we propose a modified ground rock interstitial normative determination (GRIND) method of extraction of interstitial water. In separate runs of the new method, sediment samples were ground in a ball mill with either ultrapure water or a solution of HNO3. The interstitial water was then extracted with a conventional squeezer. Sufficient solution was extracted by this method to analyse most major and a few minor components of interstitial water that were comparable to those previously reported for samples extracted by the conventional squeezing method. The new method requires much smaller amounts of sediment than that of the conventional method and will be useful for analysis of samples recovered during superdeep drilling programmes.

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Finite element method (FEM) groundwater inflow modeling associated with an unconfined aquifer into the open-pit coalmine of the Phulbari area, NW Bangladesh

Islam M.

Arabian Journal of Geosciences ( Arabian Journal of Geosciences )  9 ( 5 ) 394   2016.05 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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This paper uses a two-dimensional finite element method (FEM) numerical modeling to predict groundwater inflow from an unconfined aquifer into the open-pit coalmine in the Phulbari area of northwest Bangladesh. The rock mass of the unconfined aquifer in the Phulbari coal deposit is highly porous and permeable. Interconnected pores within rock mass are the main paths for groundwater flow. Model assumes groundwater inflow rate through the rock mass associated with interconnected pores. Eleven stages of computation associated with variable overall pit slope angle (OPSA) 25 degrees, 26 degrees, 27 degrees, 28 degrees, 29 degrees, 30 degrees, 31 degrees, 32 degrees, 33 degrees, 34 degrees, and 35 degrees are presented here. Model reveals that the probable inflow rate from unconfined aquifer in the open-pit coalmine would be about 377.68, 364.93, 354.38, 340.20, 331.29, 326.36, 316.86, 315.36, 308.03, 302.86, and 298.00 m(3)/h with relevant slope angles of 25 degrees, 26 degrees, 27 degrees, 28 degrees, 29 degrees, 30 degrees, 31 degrees, 32 degrees, 33 degrees, 34 degrees, and 35 degrees, respectively. It is reasonable to emphasize that the computed inflow rate (m(3)/h) in the present numerical model would be effective through unit width of rock strata up to the pit depth of about 320 m in the Phulbari coal deposit. From the simulation results, it is thought that the open-pit coalmine in the Phulbari area would be an environmentally challenging issue because of regional unconfined mega-aquifer.

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The epilog of the western paleo-Pacific subduction: Inferred from spatial and temporal variations and geochemistry of the Late Cretaceous to Early Cenozoic silicic magmatism in coastal South China

Chen C.

Journal of Asian Earth Sciences ( Journal of Asian Earth Sciences )  115   520 - 546   2016.01 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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The Late Cretaceous to Early Cenozoic magmatism in the South China coastal area produced some amounts of rhyolitic rocks in two phases, which may be used to unravel the geohistory of the epilog of the paleo-Pacific plate subduction system. Essence of the Phase I rocks is the high temperature rhyolite (A-type)-trachydacite association in north Fujian (95-91 Ma) that was coeval with regional A-type granites. They succeeded the vast rhyolite-dacite-andesite (RDA) associations and I-type granitoids (113.5-96 Ma) and preceded the silicic-dominating rhyolite/basalt bimodal suites or monolithologic rhyolite in Zhejiang (89-86 Ma). Phase II rocks include (a) the RDA association or rhyolite alone in some drifted continental fragments nearby (83-56 Ma) and (b) the following rift-basin related rhyolite-tra chyte/basalt bimodal suites in Guangdong and west Taiwan (56-38 Ma). The silicic volcanism, spatially changed from a NE-SW to the nearly E-W direction after 83 Ma, may reflect tectonic-driven eruptions occurred in the post-orogenic extensional (Phase I), resumed plate subducting (Phase Ha) and continental margin rifting (Phase IIb) stages. Rhyolitic rocks basically are shoshonitic to high-K calc-alkaline affinities while the Phase Ha RDA associations are mostly concentrated in the high-K to medium-K calc-alkaline series. All these rocks generally possess a continental arc character in tectonic discrimination diagrams, except shoshonitic rocks that have within-plate signatures. Based on the trace element and Nd-Pb isotope data, A-type rocks are suggested to have derived from mixing between trachydacitic (or syenitic) magmas and crustal melts of various sources under the high temperature condition (+/- metasomatism), and the succeeding silicic rocks are derivatives of the contaminated lithospheric mantle melts through crystal fractionation. On the other hand, Phase II silicic rocks are mainly the fractionation products of mafic magmas originated either from the lithospheric or the asthenospheric mantle. In reflection to such magmatotectonics with time, break-off of the subducted slab shortly before 113.5 Ma, change of the plate-moving direction between 86 and 83 Ma, and termination of plate movement probably at similar to 56 Ma signify the epilog of the paleo-Pacific plate. (c) 2015 Elsevier Ltd. All rights reserved.

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Petrologic characteristics of eruption products around Jougan eruption of Chokai volcano

Takahashi Takuya, Ban Masao, Ohba Tsukasa, Hayashi Shintaro, shinjo Ryuichi

Abstracts for Annual Meeting of Japan Association of Mineralogical Sciences ( Japan Association of Mineralogical Sciences )  2016 ( 0 ) 186   2016

Type of publication: Research paper (research society, symposium materials, etc.)

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Chokai volcano is a Quaternary stratovolcano in NE Japan. The newest collapse was at BC466 and a caldera was formed. Effusion of lavas in this caldera has continued intermittently to present. This volcano erupted after the Jougan big earthquake. Thus, after 2011 Tohoku earthquake, it is important to examine the geologic and petrologic features around Jougan eruption to predict future eruption. We found the eruption rate decreased and compositions of mafic magma changed after the Jougan eruption.

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