NdとSrの同位体組成からみた帝釈緑色岩類の起源

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Several late Paleozoic limestone plateaus sporadically occur in Southwest Japan. The Taishaku limestone plateau located at the northeastern part of Hiroshima Prefecture is one of them. The Taishaku greenstones are classified geologically and petrologically into two different types; one is of alkaline basalt forming the basement of limestone and the other is of tholeiitic basalt cropping out in clastic rocks in and around the limestone plateau. Sr isotopic ratios of the alkaline basalts range from 0.70546 to 0.70622. Some relict clinopyroxenes separated from the alkaline basalts have isotope ratio slower than their respective host basalts and the ratios range from 0.70522 to 0.7560. Sr isotopic ratios of the tholeiitic basalts are lower than those of the alkaline basalts, ranging from 0.70476 to 0.70553. Two clinopyroxenes from the tholeiitic basalts also have lower isotopic ratios from 0.70432 to 0.70485. In a ^<87>Rb/^<86>Sr versus ^<87>Sr/^<86>Sr isochron diagram, the plots of the alkaline basalts indicate a broad clustering which does not express a single isochron, however some points suggest a coincidental fit with the reference isochron of 370 Ma. A single clinopyroxene-bulk rock join also shows nearly the same inclination as the 370 Ma reference isochron. However, the steep inclination of other clinopyroxene-bulk rock pairs suggests the possibility of Sr enrichment in rocks during metamorphism. The plots of tholeiitic basalts are scattered widely and do not indicate isochronous trend. Nd isotopic ratios of the alkaline basalts can be divided into two groups; normal alkaline (N-type) and enriched alkaline (E-type) basalts. The N-type alkaline basalts have Nd isotopic ratios ranging from 0.51262 to 0.51269, which are similar to those of the tholeiitic basalts. In a ^<147>Sm/^<144>Nd versus ^<143>Nd/^<144>Nd diagram, a clinopyroxene-bulk rock pair of tholeiitic basalt shows 370 Ma isochron, and those of other two tholeiitic basalts and two N- type alkaline basalts express 366 Ma. Five analyses out of seven enriched alkaline basalts also give a linear fit for the 370 Ma reference isochron. In a Nd-Sr epsilon diagram, the ε_^<Nd> values of the enriched alkaline basalts are from -0.7 to -3.5, whereas those of the normal alkaline basalts and tholeiitic basalts form a cluster around +4 to +5. The enriched basalts of the Kerguelen Islands volcanics in the south of the Indian Ocean and the similar basalts from Tristan da Cunha in the south of Atlantic Ocean have nearly the same ε_^<Nd> values as the Taishaku enriched alkaline basalts. These enriched basalts are characterized by their ε_^<Nd> <0 andε_^<Sr>=+10 to +20. In other words, these alkaline basalts are plotted on the "mantle array" which extends to ε_^<Nd> < 0. The present study supports the existence of an anomalous EMI type or TGK type alkaline basalt seamount of latest Devonian age. These anomolous basalts also suggest that there was once "EM I or TGK" like LIL elements enriched source mantle under the old Pacific Ocean of latest Devonian age.
日本地質学会の論文
1989-04-26

NdとSrの同位体組成からみた帝釈緑色岩類の起源

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