抄録
Detailed petrologic study revealed the rocks of Shirataka volcano were formed by two end-members mixing. The mafic end-members were mainly medium-K basalts, but in the beginning of the activity they were low-K ones. The low-K basalt is inconsistent with the systematic across-arc variation, because Shirataka volcano is situated in back arc side.
The degrees of partial melting of mantle in producing the low- and medium-K basalts had been fixed, because the HFSE abundances of these two are similar. In contrast, Rb as well as K abundance is obviously higher in the medium-K than low-K, which suggests different effects of subduction-derived fluids to their compositions. If the medium-K mafic magmas were derived from the up-welling hydrous mantle, which was newly metasomatized, then the low-K ones would be generated from yet metasomatized mantle surrounding the up-welling.
