Back-arc Paleo-Tethys related blueschist from Central Iran, south of Chupanan, Isfahan Province     

Ghodrat Torabi  Shoji Arai 《Petrology》2013,21(4):393-407

Late Paleozoic blueschists present good exposures in the Pateyar metamorphic complex (south of Chupanan, Isfahan Province). They are formed by metamorphism of primitive basaltic lavas. Petrography and microprobe analyses show that the studied rocks consist of glaucophane, actinolite, actinolitic hornblende, plagioclase (albite), sphene, magnetite, quartz and apatite. Secondary minerals are epidote, chlorite, pyrite, hematite and calcite. Mineralogical assemblages are consistent with blueschist facies metamorphism, which is followed by a retrograde metamorphism in greenschist facies. Estimation of the metamorphic conditions suggests 300–400°C and 7–11 kbar. Chemical signatures of the studied metamorphic rocks conclude that they retain main geochemical characteristics of the protoliths, which allow the petrochemical interpretations. Geochemical analyses of these blueschists show that they were originally tholeiitic basalts. Evident negative anomalies of Nb, Ta and Ti relative to Th, La and Ce, in the primitive mantle normalized spider-gram, reveal subduction role in their petrogenesis. The studied metavolcanics exhibit an intermediate chemistry between the N-MORB (normal mid-ocean ridge basalt) and IATB (island arc tholeitic basalt). Enrichment in LREE (light rare earth elements) and LILE (large ion lithophile elements) and relative depletion in HFSE (high field strength elements) suggest a back-arc basin paleotectonic setting for the Chupanan samples. The primitive magma of the analyzed samples possibly have been produced by 8–13% melting of a spinel lherzolite. The field and petrological data propose that the studied Paleozoic metavolcanics were formed in a back-arc basin above the northward subduction of Paleo-Tethys oceanic lithosphere in Central Iran. The chemical criteria of the LILE/HFSE ratio suggests that the subduction zone was young and immature during the volcanism.  相似文献   

Geochemistry and mineralogy of a feldspathic lunar meteorite (regolith breccia), Northwest Africa 2200     

Hiroshi Nagaoka  Yuzuru Karouji  Tomoko Arai  Mitsuru Ebihara  Nobuyuki Hasebe 《Polar Science》2013,7(3-4):241-259

The lunar meteorite Northwest Africa (NWA) 2200 is a regolith breccia with a ferroan feldspathic bulk composition (Al₂O₃ = 30.1 wt.%; Mg^# = molar 100 × Mg/(Mg + Fe) = 59.2) and low Th content (0.42 μg/g). Lithologically, NWA 2200 is a diverse mixture of lithic and glassy clasts, mineral fragments, and impact glass spherules, all embedded in a dark glassy matrix. NWA 2200 contains some feldspathic brecciated rock components (ferroan anorthositic granulitic breccia, poikiloblastic granulitic breccia, and glassy melt breccia with an intersertal texture). The bulk compositions of these brecciated components indicate they are derived from ferroan troctolitic or noritic anorthosite lithologies (bulk Al₂O₃ = 26–30 wt.%; bulk FeO/MgO > 1.0). The bulk composition of NWA 2200 is more ferroan and feldspathic than the Apollo feldspathic regolith samples and feldspathic lunar regolith meteorites, and is also more depleted in incompatible elements (e.g., rare earth elements) than Apollo 16 feldspathic regolith samples. We conclude that NWA 2200 originated from a location different to the Apollo landing sites, and may have been sourced from the ferroan KREEP-poor highlands, “KREEP” materials are enriched in such elements as potassium (K), rare earth elements (REE), phosphorus (P).  相似文献   

The revised form of characteristic actions ℏ (s) in the structure of the Universe     

Keisuke Kaminisi  Kenzo Arai 《Astrophysics and Space Science》1987,134(1):197-198

It is shown that the recently obtained Jordan-Brans-Dicke solutions by Chauvet and Guzmán (1986) are either inconsistent, or only special power-law solutions derived previously by Lorenz-Petzold in various papers.  相似文献   

Podiform chromitites in the lherzolite-dominant mantle section of the Isabela ophiolite, the Philippines     

Tomoaki  Morishita  Eric S.  Andal  Shoji  Arai Yoshito  Ishida 《Island Arc》2006,15(1):84-101

Abstract The Isabela ophiolite, the Philippines, is characterized by a lherzolite‐dominant mantle section, which was probably formed beneath a slow‐spreading mid‐ocean ridge. Several podiform chromitites occur in the mantle section and grade into harzburgite to lherzolite. The chromitites show massive, nodular, layered and disseminated textures. Clinopyroxene (±orthopyroxene/amphibole) inclusions within chromian spinel (chromite hereafter) are commonly found in the massive‐type chromitites. Large chromitites are found in relatively depleted harzburgite hosts having high‐Cr? (Cr/(Cr + Al) atomic ratio = ～0.5) chromite. Light rare earth element (LREE) contents of clinopyroxenes in harzburgites near the chromitites are higher than those in lherzolite with low‐Cr? chromite, whereas heavy REE (HREE) contents of clinopyroxenes are lower in harzburgite than in lherzolite. The harzburgite near the chromitites is not a residual peridotite after simple melt extraction from lherzolite but is formed by open‐system melting (partial melting associated with influx of primitive basaltic melt of deeper origin). Clinopyroxene inclusions within chromite in chromitites exhibit convex‐shaped REE patterns with low HREE and high LREE (+Sr) abundances compared to the host peridotites. The chromitites were formed from a hybridized melt enriched with Cr, Si and incompatible elements (Na, LREE, Sr and H₂O). The melt was produced by mixing of secondary melts after melt–rock interaction and the primitive basaltic melts in large melt conduits, probably coupled with a zone‐refining effect. The Cr? of chromites in the chromitites ranges from 0.65 to 0.75 and is similar to those of arc‐related magmas. The upper mantle section of the Isabela ophiolite was initially formed beneath a slow‐spreading mid‐ocean ridge, later introduced by arc‐related magmatisms in response to a switch in tectonic setting during its obduction at a convergent margin.  相似文献   

Corundum-bearing mafic granulites in the Horoman (Japan) and Ronda (Spain) Peridotite Massifs: Possible remnants of recycled crustal materials in the mantle     

Tomoaki  Morishita  Eiichi  Takazawa  Shoji  Arai  Masaaki  Obata  Tadahiro  Kodera  Fernando  Gervilla 《Island Arc》2006,15(1):2-3

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Petrological characteristics of podiform chromitites and associated peridotites of the Pan African Proterozoic ophiolite complexes of Egypt   总被引：10，自引：0，他引：10  

Ahmed Hassan Ahmed  Shoji Arai  Abdel Kadar Attia 《Mineralium Deposita》2001,36(1):72-84

Mafic-ultramafic fragments of a dismembered ophiolite complex are abundant in the late Precambrian Pan African belt of the Eastern Desert of Egypt and north-east Sudan. The ultramafic bodies in the Eastern Desert of Egypt are mostly characterised by the harzburgite–dunite–chromitite association. Because of their severe metamorphism, almost all primary silicates were converted to secondary minerals and we use the chrome spinel as a reliable petrogenetic indicator. The podiform chromitite deposits are common as small and irregularly shaped masses in the central and southern parts of the Eastern Desert. They strongly vary in texture, degree of alteration and chemical composition of chrome spinel. The podiform chromitites exhibit a wide range of composition from high Cr to high Al varieties. The Cr of chrome spinel ranges from 0.65 to 0.85 in dunite, quite similar in the high-Cr chromitite, whereas it is around 0.5 in harzburgite. Primary hydrous mineral inclusions, amphibole and phlogopite, in chrome spinel are reported for the first time from the Pan African Proterozoic podiform chromitites. The petrological characteristics of Pan African podiform chromitites and associated peridotites of Egypt are similar to those of Phanerozoic ophiolites. The Proterozoic podiform chromitites may have formed in the same way as the Phanerozoic ones, namely by melt-harzburgite reaction and subsequent melt mixing. The similarity of the mantle section of the late Proterozoic and the Phanerozoic ophiolites suggests that the thermal conditions controlling genesis of the crust–mantle system basically have not changed since the late Proterozoic era. The Pan African harzburgite is very similar to abyssal peridotite at fast-spreading ridges, and the high-Cr, low-Ti character of spinel in chromitite and dunite indicates a genetic link with a supra-subduction zone setting. The late Proterozoic ophiolites of Egypt are possibly a fragment of oceanic lithosphere modified by arc-related magmatic rocks, or a fragment of back-arc basin lithosphere. Received: 26 October 1999 / Accepted: 28 June 2000  相似文献   

⁴⁰Ar–³⁹Ar analysis of phlogopite in the Horoman Peridotite Complex, Hokkaido, Japan and implications for its origin     

Ichiro Kaneoka  Natsuko Takahashi and  Shoji Arai 《Island Arc》2001,10(1):22-32

Abstract ⁴⁰Ar–³⁹Ar analysis of phlogopite separated from a plagioclase lherzolite of the Horoman Peridotite Complex, Hokkaido, Japan, has yielded a plateau age of 20.6 ± 0.5 Ma in an environment where the metamorphic fluid was characterized by an almost atmospheric Ar isotopic ratio. The age spectrum is slightly saddle-shaped, implying some incorporation of excess ⁴⁰Ar during the formation of the phlogopite at a depth. As the phlogopite has been inferred to have formed in veins and/or interstitials during exhumation of the peridotite body, metasomatic fluids, to which ground- and sea water might have contributed, were probably involved in the formation of phlogopite in the crustal environment. A total ⁴⁰Ar–³⁹Ar age of 129 Ma of a whole rock sample of the plagioclase lherzolite, from which the phlogopite was separated and is representative of the main lithology of the Horoman Peridotite Complex, indicates the occurrence of excess ⁴⁰Ar. Hence, the age has no geological meaning.  相似文献   

Behavior of gases in the Nojima Fault Zone revealed from the chemical composition and carbon isotope ratio of gases extracted from DPRI 1800 m drill core   总被引：2，自引：0，他引：2  

Takashi Arai  Tamotsu Okusawa  Hiroaki Tsukahara 《Island Arc》2001,10(3-4):430-438

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Evolution of spinel–pyroxene symplectite in spinel–lherzolites from the Horoman Complex,Japan     

Tomoaki Morishita  Shoji Arai 《Contributions to Mineralogy and Petrology》2003,144(5):509-522

We examined the textural and geochemical characteristics of spinel-pyroxene symplectites in spinel-lherzolites collected from the lowest, middle, and upper parts (LZ1, MZ, and UZ1, respectively) of the Horoman Peridotite Complex, Japan. The modal proportion of the minerals within symplectite is almost the same, i.e., orthopyroxene:clinopyroxene:spinel = 2:1:1. The size of the symplectite minerals increases from the lowest through to the middle to the upper parts in the complex. The reconstructed major element composition of the bulk symplectites is intermediate between pyrope-rich garnet and olivine. The model garnet compositions of the LZ1satisfies garnet stoichiometry and those of the MZ and UZ1 are not consistent with garnet stoichiometry. The primitive mantle-normalized pattern in trace elements for the LZ1 symplectite is similar to that of pyrope-rich garnet from fertile peridotites, particularly in its enrichment of HREE and a positive Zr anomaly. Thus, the LZ1 symplectite has inherited both major and trace element signatures from pre-existing garnet whereas the compositions of the MZ and UZ1 symplectites were modified during and/or after breakdown of pre-existing garnet. Geochemical and textural variations of symplectites might basically correspond to temperature differences within the complex during upwelling of the Horoman Complex. The basal part of the complex (LZ1) experienced the lowest temperature decompression path in the complex, which resulted in less textural and chemical modification. On the other hand, the higher part of the complex (UZ1) experienced a relatively higher temperature decompression path than other parts of the complex, resulting in chemical equilibration among the constituent minerals and coarsening of the symplectite minerals. Selective enrichment of Sr and LREE in the symplectite may indicate that the metasomatism by a Sr- and LREE-rich melt/fluid occurred during and/or after the formation of symplectite.  相似文献   

Insights into Petrological Characteristics of the Lithosphere of Mantle Wedge beneath Arcs through Peridotite Xenoliths: a Review   总被引：4，自引：0，他引：4  

Arai  Shoji; Ishimaru  Satoko 《Journal of Petrology》2008,49(4):665-695

The petrological characteristics of peridotite xenoliths exhumedfrom the lithospheric mantle below the Western Pacific arcs(Kamchatka, NE Japan, SW Japan, Luzon–Taiwan, New Irelandand Vanuatu) are reviewed to obtain an overview of the supra-subductionzone mantle in mature subduction systems. These data are thencompared with those for peridotite xenoliths from recent orolder arcs described in the literature (e.g. New Britain, WesternCanada to USA, Central Mexico, Patagonia, Lesser Antilles andPannonian Basin) to establish a petrological model of the lithosphericmantle beneath the arc. In currently active volcanic arcs, thedegree of partial melting recorded in the peridotites appearsto decrease away from the fore-arc towards the back-arc region.Highly depleted harzburgites, more depleted than abyssal harzburgites,occur only in the frontal arc to fore-arc region. The degreeof depletion increases again to a degree similar to that ofthe most depleted abyssal harzburgites within the back-arc extensionalregion, whether or not a back-arc basin is developed. Metasomatismis most prominent beneath the volcanic front, where the magmaproduction rate is highest; silica enrichment, involving themetasomatic formation of secondary orthopyroxene at the expenseof olivine, is important in this region because of the additionof slab-derived siliceous fluids. Some apparently primary orthopyroxenes,such as those in harzburgites from the Lesser Antilles arc,could possibly be of this secondary paragenesis but have beenrecrystallized such that the replacement texture is lost. TheTi content of hydrous minerals is relatively low in the sub-arclithospheric mantle peridotites. The K/Na ratio of the metasomatichydrous minerals decreases rearward from the fore-arc mantleas well as downward within the lithospheric mantle. The lithosphericmantle wedge peridotites, especially metasomatized ones frombelow the volcanic front, are highly oxidized. Shearing of themantle wedge is expected beneath the volcanic front, and isrepresented by fine-grained peridotite xenoliths. KEY WORDS: mantle wedge; lithospheric mantle; peridotite xenoliths; melting; metasomatism  相似文献