<研究報告>大分県杵築地域に分布するNbに富む安山岩

概要

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Nb-rich andesites (NRAs ; Nb 27-15ppm) are found in the Miocene Usa Formation which distributes in Kitsuki area, Oita Prefecture. Phenocrysts of the NRAs are euhedral olivines, orthopyroxenes, clinopyroxenes, hornblendes and plagioclases. The NRAs posses SiO_2 contents of 58.1 62.6 and FeO^*/MgO ratios of 1.39 - 2.19,and are rich in TiO_2,K_2O, Ba, Nb, Rb, Pb, Th, Zr and light rare earth elements as well as the alkali basalts. N-MORB-normalized incompatible element patterns of the NRAs are similar to those of the SW Kyushu alkali basalt (JB-1a) and the Oninomi alkali basalt (within-plate type alkali basalt) rather than the Kuju-type basalts (island arc type basalt) and Yabakei-type basalt (intermediate between island arc and within-plate type basalts). The above chemical characteristics suggests that alkali basalt magmas or magma sources might play an important role of the origin of the NRAs.
山口大学の論文