四万十帯北帯に分布する現地性玄武岩の希土類元素組成とその起源に関する示唆

概要

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Major, trace and rare earth elements (REE) abundances are reported for in-situ basalts from the Mugi Formation, southeastern Shikoku, and the Ryujin Formation, western Kii Peninsula in the Upper Cretaceous Shimanto Belt. High field strength element (HFSE), REE, Th and Hf concentrations of the in-situ basalts are approximately the same as normal type mid-ocean ridge basalt (N-MORB). However, their large ion lithophile element (LILE) concentrations are much greater than those of N-MORB. The basalts are characterized by flat or light REE-depleted chondrite-normalized REE patterns, which are similar to those of typical tholeiitic basalt at MOR or at some island arc. A relationship between La/Yb ratios and LILE concentrations indicates that a LILE-enrichment of these basalts was caused by weathering and/or alteration processes. Based on the La/Yb ratios and HFSE concentrations, it is concluded that most of in-situ basalts are N-MORB in origin, although a depletion of Nb in the basalts from the Itogo area of the Ryujin Formation suggests that these basalts may be of partly island-arc tholeiite origin.
日本鉱物科学会の論文
2000-11-30