内部・環太平洋地域の新生代ソレアイト質～カルクアルカリ質火山岩類のSr同位体比にみられる広域的変異

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タイトル別名
  • Regional Variations in the Sr Isotopic Composition in the Cenozoic Tholeiitic-Cal-calkaline Volcanic rocks from the Intra- and Circum- Pacific Regions
  • ナイブ カンタイヘイヨウ チイキ ノ シンセイダイ ソレアイトシツ カルク ア

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説明

In the present paper, the writers reexamined the regional variations of Sr isotopic composition in the Cenozoic tholeiitic and calc-alkaline volcanic rocks from the Intra- and Circum- Pacific regions, based on many published and new data. The L zone is denned as area of the volcanic rocks with 87Sr/86Sr ratio lower than 0.7055 which is the upper limit of 87Sr/86Sr in the tholeiitic basalts of oceanic area. The L zone comprises the Pacific Ocean areas such as Type II island arc, oceanic island, oceanic rise, deep ocean bottom, and the Pacific coastal area of the Cenozoic Circum-Pacific erogenic belt. On the contrary, the area of the volcanic rocks with higher 87Sr/86Sr ratio than the above figure is defined as H zone, which includes the inner continental side of the above erogenic belt and Type I island arc, with the exception of the northern and central Japan belonging to the L zone. It is noteworthy that the volcanics with low Sr isotopic composion also occur in the H zone. Moreover, the Mesozoic and Cenozoic granitic rocks with low initial Sr isotopic ratio occur mainly in the L zone, whereas in the H zone there occur frequently grantic rocks with higher initial Sr isotopic composition. Considering from the facts mentioned above, the Cenozoic acid volcanic rocks and the Mesozoic to Cenozoic granitic rocks of each zone seem to have been derived from certain different source material, respectively. It may be the most reasonable interpretation that both the acid volcanic rocks in the L zone and those with low 87Sr/86Sr ratio in the H zone were formed by fractional crystallization of certain basaltic magma, or by direct partial fusion of certain hydrous peridotitic upper mantle source material. On the other hand, the probable source material of the acid volcanic rocks with higher 87Sr/86Sr ratio in the H zone is certain basic material, which is slightly different in its chemistry from the above basaltic magma and is similar to the source material of granitic magma proposed by GORAI et al. (1972).

収録刊行物

  • 地球科学

    地球科学 29 (2), 75-86, 1975

    地学団体研究会

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