中部太平洋における白亜紀の火成活動

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Cretaceous volcanic rocks have been collected from seamounts, linear islands chains, oceanic plateaus such as Manihiki Plateau and Ontong Java Plateau, and various types of deep-seated intrusions which are distributed in the cent ral Pacific Ocean. They may be classified into two types; 1) ocean island tholeiites, alkalic basalts and their differenciated rocks such as hawaiite, mugearite and trachyte, and potassic nephelinites; 2) ocean plateau tholeiites. Volcanic rocks of the first named type are found at seamounts, linear islands chains, and small sized deep-seated intrusions emplaced during the late Cretaceous (Campanian to Coniacian). While rocks of the latter type are found at oceanic plateaus and large sized deep-seated intrusions empl aced during the early Cretaceous (Barremian to Hauterivian and/or Valanginian). The age span of the ocean plateau tholeiite might continue to some extent until middle Cretaceous. Each volcanic suite is characterized by approximately the same duration of magma gene ration and by a regional distribution of volcanic activity. Ocean island tholeiites are chemically intermediate between Hawaiian tholeiites and Hawaiian alkalic basalts (typical hot-spot type in the Pacific Ocean). Potassic nephelinites are characterized by containing amphiboles and biotites as xenocrysts and ph enocrysts but never containing tectonized xenolithic peridotites. Moreover, compared to Hawaiian post-erosional lavas the potassic nephelinites have higher Al2O3 content and K2O/Na2O ratio. Also there are no garnets and pyroxenes enriched in Ca -Tshermak or jadeite molecule which are stable under high pressure. Therefore the potassic nephelinites might be generated at shallower depth than Hawaiian posterosional lavas. Ocean plateau tholeiites are chemically distinguished from ocean ridge tholeiites (typical abyssal tholeiites) by their higher FeO*/MgO ratio and lower TiO2 conten and by being quartz-and strongly hyperthene-normative. From these evidences, the basaltic liquids of the ocean plateau tholeiites might be generated at shallower depth (<5 Kbar) than ocean ridge tholeiites. While, it is different from the Hawaiian tholeiites of typical hot-spot type with lower K2O and TiO2 content. Consequently these two volcanic suites erupted during the Cretaceous are different from ocean ridge volcanics and also from Hawaiian hot-spot volcanics.
社団法人東京地学協会の論文

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徳山英一
東京大学海洋研究所

中部太平洋における白亜紀の火成活動

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