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Remelting of an Andesitic Crust as a Possible Origin for Rhyolitic Magma in Oceanic Arcs: an Example from the Izu-Bonin Arc 总被引:10,自引:0,他引:10
The IzuBonin volcanic arc is an excellent example ofan intra-oceanic convergent margin. A total of 1011 chemicalanalyses of 17 Quaternary volcanoes of the arc are reviewedto estimate relative proportions of magmas erupted. Basalt andbasic andesite (SiO2 < 57 wt %) are the predominant eruptiveproducts of the IzuBonin arc, and rhyolite (SiO2 >70 wt %) forms another peak in volume. Such rhyolites possesscompositions identical to those of partial melts produced bydehydration-melting of calc-alkaline andesites at low pressure(<7 kbar). Meanwhile, the major element variation of theShirahama Group Mio-Pliocene volcanic arc suite, Izu Peninsula,completely overlaps that of the Quaternary IzuBonin arcvolcanoes, and groundmasses of Shirahama Group calc-alkalineandesites have compositions similar to those of IzuBoninrhyolites. Moreover, phenocryst assemblages of calc-alkalineandesites of the Shirahama Group resemble restite phase assemblagesof dehydration-melting of calc-alkaline andesite. These linesof evidence suggest that the rhyolite magmas may have been producedby dehydration-melting of calc-alkaline andesite in the upperto middle crust. If so, then the presence of large amounts ofcalc-alkaline andesite (35 times more abundant than therhyolites) within the oceanic arc crust would be expected, whichis consistent with a recently proposed structural model acrossthe IzuBonin arc. The calc-alkaline andesite magmas maybe water saturated, and would crystallize extensively and solidifywithin the crust. The model proposed here suggests that rhyoliteeruptions could be triggered by an influx of hot basalt magmafrom depth, reheating and partially melting the calc-alkalineandesite component of the crust. KEY WORDS: bimodal magmatism; calc-alkaline andesite; oceanic arcs; rhyolite 相似文献
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NORIYUKI NASU SHOJI FUJII KANTARO FUJIOKA CHIAKI IGARASHI HOTAKA KAWAHATA HARUNA MAPPA KENJI KONISHI TAKEO TANAKA TAKASHI SHIMODA 《Boreas: An International Journal of Quaternary Research》1983,12(1):13-16
Tree stumps in a standing position have been discovered in 20 to 40 meters of water in Toyama Bay, Japan. Radiocarbon dated samples from two of these stumps yielded ages of 8.480±70 and 9.060±80 years B.P., indicating that the stumps are remnants of a forest that existed during thc early Holocene. 相似文献
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Boron isotope variations in the atmosphere 总被引:2,自引:0,他引:2
YOSHIKI MIYATA TAKAYUKI TOKIEDA HIROSHI AMAKAWA MITSUO UEMATSU YOSHIYUKI NOZAKI 《Tellus. Series B, Chemical and physical meteorology》2000,52(4):1057-1065
We report here the first measurements of boron isotope ratios in the maritime atmosphere together with those of precipitation. The δ11 B values of atmospheric condensates in the western North Pacific and Japanese coast and snow in Tokyo range from −12.8 to +5.1‰ and from −0.4 to +0.4‰, respectively, which are significantly lower than those of rainwater (+18.9 to +34.7‰) collected mostly over the North Pacific. Since the 11 B/10 B ratios of the atmosphere are lower than those of volcanic emissions (δ11 B=+2.3 to +21.4‰), we must seek sources for atmospheric boron other than volcanism. We postulate that the sea may be an important supplier for atmospheric boron under some dynamic conditions and that boron isotope fractionation during evaporation from seawater and removal from the atmosphere may account for the large variations of 11 B/10 B ratios observed in the atmosphere and precipitation. 相似文献
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SHOJI FUJII NORIYUKI NASU ALEC J. SMITH NORIO FUJI YOSHIHIKO MIZUTANI MISABURO SHIMAKURA KENJI KONISHI CHIAKI IGARASHI JUNRYO MURAMOTO TOSHIO TAKEMURA TAKASHI SHIMODA SAM BOGGS JR. KANTARO FUJIOKA HARUNA MAPPA HOTAKA KAWAHATA YONG SAE KONG TAKEO TANAKA 《Boreas: An International Journal of Quaternary Research》1986,15(4):265-277
Remnants of a submerged ancient forest have been found on the continental shelf of NW Japan off the Kurobe alluvial fan in Toyama Bay. The remains, standing tree stumps and roots, are located at depths between 20 and 40 m and ages of 8,000 years B.P. to 10,000 years B.P. have been determined by the 14 C method. Alnus and Salix are the most commonly preserved genera and the trees grew on the now sub-merged seaward margin of the alluvial fan. Pollen analysis permits an estimate of palaeo-temperatures 2°C to 4°C lower than at present. Diatoms give some indication of salinity variations at the time of forest development and suggest brackish water, possible lagoonal conditions. The distribution of the stumps and roots suggests that there were probably two separate forests which were overwhelmed by rising sea-level and flash flood deposits. The present day submarine exposures may be due to recent submarine erosion related to the seaward movement of flood waters which passed from the fan into the upper reaches of the Toyama submarine canyon system. 相似文献
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The Petrology and Geochemistry of Volcanic Rocks on Jeju Island: Plume Magmatism along the Asian Continental Margin 总被引:2,自引:0,他引:2
TATSUMI YOSHIYUKI; SHUKUNO HIROSHI; YOSHIKAWA MASAKO; CHANG QING; SATO KEIKO; LEE MOON WON 《Journal of Petrology》2005,46(3):523-553
The incompatible element signatures of volcanic rocks formingJeju Island, located at the eastern margin of the Asian continent,are identical to those of typical intraplate magmas. The sourceof these volcanic rocks may be a mantle plume, located immediatelybehind the SW Japan arc. Jeju plume magmas can be divided intothree series, based on major and trace element abundances: high-aluminaalkalic, low-alumina alkalic, and sub-alkalic. Mass-balancecalculations indicate that the compositional variations withineach magma series are largely governed by fractional crystallizationof three chemically distinct parental magmas. The compositionsof primary magmas for these series, using inferred residualmantle olivine compositions, suggest that the low-alumina alkalicand sub-alkalic magmas are generated at the deepest and shallowestdepths by lowest and highest degrees of melting, respectively.These estimates, together with systematic differences in traceelement and isotopic compositions, indicate that the upper mantlebeneath Jeju Island is characterized by an increased degreeof metasomatism and a change in major metasomatic hydrous mineralsfrom amphibole to phlogopite with decreasing depth. The originalplume material, having rather depleted geochemical characteristics,entrained shallower metasomatized uppermost mantle material,and segregated least-enriched low-alumina alkalic, moderatelyenriched high-alumina alkalic, and highly enriched sub-alkalicmagmas, with decreasing depth. KEY WORDS: Jeju Island; magma genesis; mantle plume; subcontinental mantle 相似文献
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