435 resultados para 7137-127


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The western Pacific includes many volcanic island arc and backarc complexes, yet multi-isotopic studies of them are rare. Basement rocks of the Sea of Japan backarc basin were encountered at Sites 794,795, and 797, and consisted of basaltic sills and lava flows. These rocks exhibit a broad range in isotopic composition, broader than that seen in any other western Pacific arc or backarc system: 87Sr/86Sr = 0.70369 to 0.70499, 143Nd/144Nd = 0.51267 to 0.51317, 206Pb/204Pb = 17.64 to 18.36. The samples form highly correlated arrays between very depleted mid-ocean ridge basalt (MORB) and the Pacific pelagic sediment fields on Pb-Pb plots. Similarly, on plots of Sr-Pb and Nd-Pb, the Sea of Japan samples lie on mixing curves between depleted mantle and enriched mantle ("EM II"), which is interpreted to be of average crustal or pelagic sediment composition. The source of these backarc rocks appears to be a MORB-like mantle source, contaminated by pelagic sediments. Unlike the Mariana and Izu arc/backarc systems, Japanese arc and backarc rocks are indistinguishable from each other in a Sr-Nd isotope plot, and have similar trends in Pb-Pb plots. Thus, sediment contamination of the mantle wedge appears to control the isotopic compositions of both the arc and backarc magmas. Two-component mixing calculations suggest that the percentage of sediments in the magma source varies from 0.5% to 2.5%.

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Suites of basalts drilled during Legs 127 and 128 can be distinguished by their mineral assemblages and compositions of phenocrysts and groundmass phases. An upper suite of plagioclase phyric basaltic sills with a groundmass composed of plagioclase, augite, and magnetite was recovered from Site 794. The upper, evolved part of this suite is highly plagioclase phyric, including calcic plagioclases (~An90). The most primitive, lower part of this upper suite, in addition, contains olivine, but lacks calcic plagioclase. A lower suite at Site 794 is plagioclase and olivine phyric to aphyric basaltic sills and flows with a groundmass of plagioclase, augite, olivine (~Fo75-83), and magnetite. At Site 795, plagioclase and augite phyric basalts and andesites were recovered. The relatively low Ti and Cr contents of augite of these basalts suggest typical arc tholeiitic parental magmas. Two suites of basalt were recovered from Site 797, an upper suite of plagioclase and olivine phyric to aphyric olivine basalts, and a lower suite of evolved plagioclase phyric basaltic sills. The most evolved sills at both sites lack olivine as phenocryst and groundmass phases, while this phase is present in the relatively primitive sills. The olivine-bearing suites contain plagioclase with relatively low potassium content and augite with relatively high sodium content. An exception is the olivine-bearing sills of the upper suite at Site 794 that contains plagioclase with relatively high potassium content similar to the associated olivine-free sills. The olivine-free suites contain plagioclase with high potassium content and augite with low sodium content and have the most evolved compositions of any of the Japan Sea rocks.

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