Petrology of Oki-Dozen volcano. Part 1. Petrography and, major and trace element compositions.

Petrographical and geochemical study of alkaline volcanic rocks from Oki-Dozen, southwest Japan, revealed that principal mechanisms of differentiation were fractional crystallization and mechanical mixing.Alkaline volcanic rocks from Oki-Dozen belong to potassic series and can be classified into picritic basalt, alkali olivine basalt, trachybasalt, tristanite, trachyte and alkali rhyolite.Tra chyte is geologically and petrographically subclassified into central cone trachyte and parasitic volcano trachyte.Major and trace element variation among alkali olivine basslt, trachybasalt, tristanite, parasitic volcano trachyte, and alkali rhyolite are explained by fractional crystallization of observed phenocrysts; i.e. early fractionation of olivine and the inclusions of ore decreased Mg, Ni, Co and Cr contents in residual magma.Clinopyroxene then plagioclase and iron-titanium oxides fractionatedin the next stage to produce the change in Sc, Sr, and V, Ti contents, respectively.In the middle stage of differentiation, Rb and Y contents were modified by biotite