A newly discovered high-pressure terrane in eastern Hokkaido, Japan

Abstract The upper Jurassic Nikoro greenstone complex of eastern Hokkaido suffered high-pressure intermediate type metamorphism. Characteristic minerals include lawsonite, aragonite, sodic pyroxene of the aegirinejadeite series, winchite. sodic amphibole of the glaucophane-riebeckite series, pumpellyite, epidote and actinolite.
High-pressure metamorphism of the Nikoro greenstone complex is related to subduction of the Kula plate toward the Palaeo-Okhotsk Land during Cretaceous time.     

五大连池、天池和腾冲火山岩Sr、Nd同位素地球化学特征与岩浆演化

在的年来对长白山天池火山、五大连池火山和腾冲火山三个火山区火山岩岩石学的主、微量元素研究的基础上，新做出18个火山岩Sr-Nd同位素数据，进一步讨论地幔源区特征与岩浆成因演化。五大连池富钾火山岩浆源区由原始地幔与EMI两个地幔端员混合而成，岩浆直接来自地幔，未受地壳物质明显混染和分离结晶作用影响；天池火山各阶段火山岩一致的似原始地幔特征，可能指示存在巨大的壳内岩浆房和持续的幔源岩浆的补给；腾冲火山岩的高钾钙碱性岩浆源区为由陆内壳－幔相互作用导致的原始地幔与EMⅡ两个地幔端员不同程度混合而成。     

Petrology of peridotite xenolith-bearing basaltic to andesitic lavas from the Shiribeshi Seamount,off northwestern Hokkaido,the Sea of Japan

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 spinel 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.     

Geochemistry of the Hawi lavas,Kohala Volcano,Hawaii

Hawi lavas form the late stage alkalic cap on Kohala Volcano and range in composition from hawaiite to trachyte. New, detailed field mapping of Kohala and reinterpretation of previously published age data suggest that there was no significant eruption hiatus between the Hawi and underlying Pololu shield lavas as was previously suggested. Mineral and whole-rock chemical data are consistent with a crystal fractionation origin for the hawaiite to trachyte compositional variation observed within the Hawi lavas. Plagioclase, clinopyroxene, Ti-magnetite, olivine and apatite fractionation are needed to explain this variation. The clinopyroxene fractionation may have occurred at moderate pressure because it is virtually absent in these lavas and is not a near liquidus phase at pressures of less than 8 Kb. Plagioclase would be buoyant in the Hawi hawaiite magmas so a mechanism like dynamic flow crystallization is needed for its fractionation and to account for the virtual absence of phenocrysts in the lavas. Hawi lavas are distinct in Sr and Nd isotopic ratios and/or incompatible element ratios from the Pololu lavas. Thus they were derived from compositionally distinct sources. Compared to other suites of Hawaiian alkalic cap lavas, Hawi lavas have anomalously high concentrations of phosphorus and rare earth elements. These differences could be due to greater apatite content in the source for the Hawi lavas.     

Mineral Composition of Manganese Crusts from Belyaevskii Volcano (Sea of Japan)

Doklady Earth Sciences - The polygenic mineral composition of Mn crusts from Belyaevskii Volcano of the Central Basin of the Sea of Japan is determined. The crusts are found for the first time to...     

Geochemistry of surface sediments in the southwestern East/Japan Sea

We analyzed 77 surface sediment samples collected in the southwestern East/Japan Sea from the Korea Strait through the Ulleung Basin and the Korea Plateau for grain size, calcium carbonate, organic carbon, and major (Na, Mg, Al, Fe, K, Ca, and Ti) and trace elements (P, Mn, Sr, Li, Sc, V, Cr, Co, Ni, Zn, Cu, and Pb).The chemical composition of the surface sediments was found to be highly variable spatially. Cluster analysis of surface sediment chemical compositions indicated five major geochemical sedimentary environments: basin, lower slope, coast and upper slope, inner shelf, and outer shelf. Continental-shelf sediments were rich in shell fragments and had relict and coarse-grained characteristics. Recent fine-grained sediments were only distributed in coastal, slope, and basin areas. Concentrations of Al, K, Ca, Ti, Cr, and Sc were highest in the coastal and upper slope areas and decreased with water depth. Elemental ratios using major and trace elements indicated that coastal and upper slope detrital sediments were mixtures of sediments derived from the Changjiang (Yangtze) and Nakdong Rivers. Although the concentrations of organic carbon, P, Mn, V, Co, Ni, Cu, and Pb increased with water depth, their distribution patterns indicated authigenic (V, Cu, and Pb) and diagenetic (Fe, P, Mn, Co, and Ni) origins. The distribution pattern with water depth suggested that the chemical composition of surface sediment was determined by sedimentologic and geochemical processes, such as the supply of detrital and biogenic materials, and authigenic and post-depositional diagenetic processes in sediments.     

Geochemistry of recent hydrothermal systems of Mendeleev Volcano, Kuril Islands, Russia

Among the thermal waters of Mendeleev Volcano three main types can be distinguished: sodium-chloride, acid-sulfate and chloride–sulfate–bicarbonate waters. Contents and behaviors of siderophile, chalcophile, lithophile, and rare-earth elements are discussed. These data, together with the result of isotopic studies (oxygen and hydrogen), are used to discuss the origin of these waters. From the three types of thermal waters, only acid-sulfate waters have a significant input on the chemical composition of the surrounding surface waters.     

Geochemistry of sediments from Lake Grand,Northeast Russia

Major and trace element distribution in the bottom sediments from Hole 13 drilled in Lake Grand, Magadan district, was studied using the method of principal components. It was established that geochemical characteristics are correlated with environmental changes. The sediments of cold MIS2 and MIS4 are characterized by the enriched TiO₂, MgO, Al₂O₃, Fe₂O₃, and Cr and low Na₂O, K₂O contents, which is related to the grain-size composition of sediments. Sediments of warm stages show an opposite tendency. High concentration peaks of iron, phosphorus, and manganese correspond to the accumulation levels of vivianite and ferromanganese rocks. Silica is represented by biogenic and abiogenic varieties. Maximum SiO₂ contents were found in the Late Holocene sediments and mark the high biological productivity of the basin. Revealed variations of some elements are correlated with the Heinrich events.     

A 90,000-year tephrostratigraphic framework of Aso Volcano, Japan

A detailed 90,000-year tephrostratigraphic framework of Aso Volcano, southwestern Japan, has been constructed to understand the post-caldera eruptive history of the volcano. Post-caldera central cones were initiated soon after the last caldera-forming pyroclastic-flow eruption (90 ka), and have produced voluminous tephra and lava flows. The tephrostratigraphic sequence preserved above the caldera-forming stage deposits reaches a total thickness of 100 m near the eastern caldera rim. The sequence is composed mainly of mafic scoria-fall and ash-fall deposits but 36 silicic pumice-fall deposits are very useful key beds for correlation of the stratigraphic sequence. Explosive, silicic pumice-fall deposits that fell far beyond the caldera have occurred at intervals of about 2500 years in the post-caldera activity. Three pumice-fall deposits could be correlated with lava flows or an edifice in the western part of the central cones, although the other silicic tephra beds were erupted at unknown vents, which are probably buried by the younger products from the present central cones. Most of silicic eruptions produced deposits smaller than 0.1 km³, but bulk volumes of two silicic eruptions producing the Nojiri pumice (84 ka) and Kusasenrigahama pumice (Kpfa; 30 ka) were on the order of 1 km³ (VEI 5). The largest pyroclastic eruption occurred at the Kusasenrigahama crater about 30 ka. This catastrophic eruption began with a dacitic lava flow and thereafter produced Kpfa (2.2 km³). Total tephra volume in the past 90,000 years is estimated at about 18.1 km³ (dense rock equivalent: DRE), whereas total volume for edifices of the post-caldera central cones is calculated at about 112 km³, which is six times greater than the former. Therefore, the average magma discharge rate during the post-caldera stage of Aso Volcano is estimated at about 1.5 km³/ky, which is similar to the rates of other Quaternary volcanoes in Japan.     

Carbon and helium isotopic ratios at Kusatsu-Shirane Volcano,Japan

We have measured the chemical compositions, He/Ne ratios, He and C isotopes of 14 gas samples collected from the crater lake, fumaroles and hot springs associated with Kusatsu-Shirane Volcano, Japan. The³He/⁴He ratio decreases with increasing distance from the central crater of the volcano to the sampling site whereas the δ¹³C value of CO₂ increases with the distance. This tendency suggests that high³He/⁴He-low δ¹³C magmatic gas is mixed with and/or diluted by low³He/⁴He-high δ¹³C crustal gas with increasing distance from the crater. Alternatively, the variation of δ¹³C values may be the result of isotope fractionation during migration since the isotope shift is relatively small. Based on the apparent mixing trend in a³He/⁴He-δ¹³C diagram, the magmatic He and CO₂ in the volcano may have a³He/⁴He ratio of 8 R_atm and a δ¹³C value of −3.2%, respectively. The CO₂/³He ratios show a positive correlation with temperature of fumaroles or hot springs, which may be caused by a difference of temperature dependencies of the solubilities between CO₂ and He.     

Petroloy of the Taga Volcano Group,Izu Peninsula,Central Japan

The Taga Volcano Group of the Izu Peninsula is divided into four volcanoes: Older Taga, Atami, Shimo-Taga and Himenosawa. Each volcano consists of basalt-andesite lavas and volcaniclastic sediments, mainly of low alkali tholeiite composition.The activity of the Shimo-Taga Volcano is divided into three stages (early, middle and later). Rocks of the first two stages consist of lavas and volcaniclastic sediments of olivine basalt, phyric and aphyric basaltic andesite. The later stage rocks consist of volcaniclastic sediments which were erupted as mudflow deposits.Basalts and andesites of the first two stages are considered to have been derived from a zoned magma chamber. They were produced by fractional crystallization, which involved the gravitational separation of plagioclase, olivine and/or pyroxene crystals. Eruption of aphyric and phyric basaltic andesite from the upper part of the chamber occurred first, followed by olivine basalt from lower parts of the chamber.

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Zusammenfassung Die Taga-Vulkan Gruppe besteht aus dem älteren Taga, dem Atami, dem Shimo-Taga und dem Himenosawa Vulkan. Jeder dieser Vulkane wird aus basaltisch-andesitischen Laven und vulkano-klastischen Sedimenten, hauptsächlich subalkalisch tholeiitischer Zusammensetzung, aufgebaut.Die Eruptionstätigkeit des Shimo-Taga Vulkans läßt sich in drei Phasen, eine Früh-, Mittel- und Spätphase untergliedern. Laven und vulkanoklastische Sedimente der ersten beiden Phasen bestehen aus Olivinbasalten, sowie aus basaltischen Andesiten mit porphyrischem und nichtporphyrischem Gefüge. Während des letzten Stadiums wurden vulkanoklastische Sedimente in Form von vulkanoklastischen Schlammströmen eruptiert.Die Basalte und Andesite der ersten beiden Stadien werden als Produkte einer zonierten Magmenkammer gedeutet. Sie entstanden durch fraktionierte Kristallisation und durch die gravitative Abtrennung von Plagioklas, Olivin und/oder Pyroxenkristallen. Der Eruption der porphyrischen und nichtporphyrischen basaltischen Andesite aus dem obersten Teil der Magmakammer folgte die Eruption von Olivinbasalten aus dem tieferen Teil der Magmakammer.

     

Geochemistry of Rock-Forming Minerals in Mantle Xenoliths from Basalts of Sverre Volcano,Spitsbergen Archipelago

The paper reports the results of SIMS and SEM-EDS study of rock-forming minerals from melt pockets in the central part of a spinel peridotite xenolith taken from Quaternary alkaline basalts of Sverre Volcano in the northwestern part of West Spitsbergen Island. Olivine and clinopyroxene are analyzed to trace changes related to the metasomatic interaction between spinel lherzolite and a carbonate melt with formation of corresponding secondary minerals and silicate glass. It is established that the metasomatic interaction of the carbonate melt with minerals of host spinel lherzolite is accompanied by partial recrystallization of olivine and clinopyroxene, or crystallization of the second generation of these minerals. Percolating carbonate melt caused significant changes in the major, trace, and rare-earth element composition of the considered minerals, thus placing constraints on the use of the composition of these minerals for calculation of P–T parameters, estimating equilibrium, and modeling petrological processes in mantle.     

Petrogenetic Evolution of a Transitional Tholeiitic--Calc-alkaline Series: Towada Volcano, Japan

Detailed petrological and geochemical data of stratigraphicallywell-defined tholeiitic and calc-alkaline rocks from Towada,Japan, are used to evaluate the system's evolution over 0•2myr. Available data suggest that Towada progressed from severalsmall independent magma chambers to one large chamber, illustratedby systematic changes in mineral composition, isotopic and traceelement ratios. This is attributed to variations in intracrustalprocesses in the shallow chambers, with the following evolutionarysequence divisible into four main units:

1. the Precaldera Stage:consisting of several basaltic to daciticlava cones, independentlyfed by tholeiitic and calc-alkalinemagma chambers. These arecharacterized by periodic replenishment,fractionation and magmamixing, with some undergoing crustalassimilation during latestages, increasing the ⁸⁷Sr/⁸⁶Sr ratioat constant SiO₂.
2. TheCaldera Stage: characterized by large volume tholeiiticandcalc-alkaline andesitic to dacitic pyroclastic eruptions,associatedwith caldera collapse. Geochemical data indicatethat this stageresulted from some of the precaldera chambersamalgamating.Magma in this enlarged chamber underwent mixing,continued fractionalcrystallization and assimilation.
3. The Ninokura Stage: characterizedby primitive tholeiitic basalticlavas and scoria deposits,which underwent fractional crystallizationplus minor crustalassimilation.
4. The Goshikiiwa Pyroclastic Deposits: consistingof evolved calc-alkaline±tholeiiticmagmas, which underwentassimilation and fractional crystallization.

KEY WORDS: AFC; calc-alkaline; Japan; magma evolution; tholeiitic     

中国东北地区4个活火山群的火山灾害

东北地区有4个活火山群,它们是长白山火山群 、五大连池火山群、龙岗火山群和镜泊湖火山群。这些活火山群中的活火山有潜在的喷发危险,一旦再次爆发,由火山喷发空中降落堆积物、火山泥流、火山碎屑流、火山碎屑崩塌和熔岩流等造成的灾害和环境污染将是巨大的。当务之急是重塑火山喷发历史和模式,编制火山灾害预测图。东北地区火山灾害类型如此之多,影响范围如此之广,进行专项火山灾害调查研究是必须的。     

Isotope and Trace Element Geochemistry of Augustine Volcano, Alaska: Implications for Magmatic Evolution

Augustine Volcano, a Quaternary volcanic centre of the easternAleutian Arc, produces predominantly andesites and dacites oflow- to medium-K calc-alkaline composition. Mineralogical andmajor element characteristics of representative lavas suggestthat magmatic evolution has been influenced by both crystalfractionation and magma-mixing processes. However, incompatibletrace element variations (e.g. K/Rb) indicate that these evolvedlavas have been contaminated by the mafic arc crust of the underlyingTalkeetna accreted terrane. The limited range of isotope compositionsalso supports the assimilation of non-radiogenic mafic crust(e.g. ⁸⁷Sr/⁸⁶Sr = 0.7032–0.7034; ¹⁴³Nd/¹⁴⁴ Nd = 0.51301–0.5130).In addition, Pb-isotope compositions parallel the North Pacificmean oceanic trend (²⁰⁶Pb/²⁰⁴ Pb = 18.3–18.8; ²⁰⁷Pb/²⁰⁴Pb= 15.5–15.6; ²⁰⁸Pb/²⁰⁴Pb = 38.2–38.3) and do notrequire a subducted sediment component in the source. Relativelyhigh (Ba/La) N (0.79–18.10) and B/Be (14.5) ratios do,however, suggest a metasomatic fluid component derived fromthe dehydration of the subducting plate. The thickened continental crust (35 km) of the eastern AleutianArc prevents the ascent of basaltic melts, which fractionateand assimilate at various depths to produce andesitic magmas.These andesites evolve towards more silicic compositions byfractional crystallization. The absence of evidence for a largehigh-level crustal magma chamber implies that the magmatic systembeneath the volcano is young and at an immature stage of evolution. KEY WORDS: Augustine Volcano; Aleutians; assimilation; melasomatism; geochemistry ^*Corresponding author. Present address: Department of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148, USA     

Organic Geochemistry of Black Sea cores

Six Black Sea bottom sediments samples were analyzed for their organic content (chloroform extract and humic compounds) by elemental analysis, liquid and gas chromatography, various chemical methods and especially infra-red spectroscopy. Four of these samples came from an euxinic facies (“unit 2”) and two from a normally oxygenated facies below (“unit 3”).Even though important differences exist between the two sets of samples, the origin of the organic matter was found to be the same for both facies; this origin is mixed, with the marine contribution slightly predominant over the terrestrial one.Consequently, the compositional differences originate in the different environmental conditions in the sedimentary basin. More precisely, the euxinic facies appears to “preserve” the organic matter, on both quantitative and qualitative grounds. This effect is due more to the anoxic and toxic nature of the environment than to the reducing conditions.The sulfur incorporation in relatively small molecules does not proceed by a rapid biochemical pathway, but by slow and abiotic reactions.     

Petrogenesis of Mafic Inclusions in Rhyolitic Lavas from Narugo Volcano, Northeastern Japan

Mafic inclusions present in the rhyolitic lavas of Narugo volcano,Japan, are vesiculated andesites with diktytaxitic texturesmainly composed of quenched acicular plagioclase, pyroxenes,and interstitial glass. When the mafic magma was incorporatedinto the silica-rich host magma, the cores of pyroxenes andplagioclase began to crystallize (>1000°C) in a boundarylayer between the mafic and felsic magmas. Phenocryst rim compositionsand interstitial glass compositions (average 78 wt % SiO₂) inthe mafic inclusions are the same as those of the phenocrystsand groundmass glass in the host rhyolite. This suggests thatthe host felsic melt infiltrated into the incompletely solidifiedmafic inclusion, and that the interstitial melt compositionin the inclusions became close to that of the host melt (c.850°C). Infiltration was enhanced by the vesiculation ofthe mafic magma. Finally, hybridized and density-reduced portionsof the mafic magma floated up from the boundary layer into thehost rhyolite. We conclude that the ascent of mafic magma triggeredthe eruption of the host rhyolitic magma. KEY WORDS: mafic inclusion; stratified magma chamber; magma mixing; mingling; Narugo volcano; Japan     

中天山前寒武纪基底中新发现铁铜矿床

池西铁铜矿床是近年来新发现的赋存在中天山前寒武纪基底卡瓦布拉克群内的块状硫化物型矿床,具上铁下铜的分带性,在成因上可能与海相火山作用有关。已发现的2个矿体呈层状、透镜状产出,产状与围岩地层一致。该矿床可能是中天山地区卡瓦布拉克群内首次发现的块状硫化物型矿床,对今后在中天山乃至东天山地区寻找与海相火山作用有关的块状硫化物型矿床(尤其是铁铜共生矿床)具有重要的参考意义。