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. Trachyte 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 explanied by fractional crystallization of Ni, Co and Cr contents in residual magma. Clinopyroxene then plagiocalse and iron-titanium oxides fractionated in the next stage on produce the change in Sc, Sr, and V, Ti contents, respectively. In the middle stage of differentiation, Rb and Y contents were modified by biotie and amphibole fractionation, respectively. Crystallization of alkali feldspar showed strong decrease of Ba in trachytic magma. Minor constituents such as zircon and allanite also controlled the change in Zr and Ce contents, respectively, in alkali rhyolite.
Mechanical mixing seemed to have played a role in producing compositional variation in picritic basalt and central cone trachyte. Microscopic observation and incompatible element ratios suggest that basaltic magma, originate from different source from alkali olivine basalt, might have mixed with cumulatvie phase including olivine, chromian diopside and opaque phases. Trachytic magma might have changed its composition by mixing with altered basement quartz syenite.
Comparing with contemporaneous volcanism occured in the Oki-Islands revealed differnet genetic and evolutional relation between Dozen and Dogo.
The genetic and evolutional model Oki-Dozen volcanic rocks, which was revealed by the present study is different from that for Oki-Dogo volcanic rocks.