Petrology of Chokai volcano. Part I

Petrography and major element compositions

Abstract

The rocks of Chokai volcano, northeastern Japan, are described, and 6 basaltic rocks, 48 andesitic rocks, and 2 cognate inclusions were analyzed.
Hayashi (1984) divided the evolution history of the volcano into three stages. The volcanic products of the first stage exceed more than 60% of the entire erupted materials, while those of the third stage are less than 6%.
The rock suite of the first stage consists mainly of calc-alkaline andesite with a minor amount of high alumina basalt. Hydrous minerals were rarely found in felsic andesite.
The volcanic products of the second stage are composed largely of calc-alkaline andesite with a small amount of calc-alkaline basalt. Most of them contain hornblende as phenocryst and chromian spinel as inclusion in olivine phenocryst.
The third stage volcanic products are made up of only mafic to felsic calc-alkaline andesite. No basalts has been found so far in the lavas and pyroclastics of this stage. Hydrous phenocryst minerals are scarcely recognized.
The major element chemical characteristics of the rocks from the Chokai volcano are as follows:
1) They generally plot in the upper region of the high-alumina basalt field on total alkalies-SiO₂ diagram (Kuno, 1966). The rocks of the first stage are slightly higher in total alkalies than those of the second and third stage samples.
2) The rocks of the first stage represent a single differentiation trend in the AFM diagram, whereas those of the second and third stages are scattered.
3) MgO content of the rocks of the first stage decreases rapidly in the earlier stage of differentiation, but slowly in the later stages. On the other hand, MgO content of the rocks of the second and third stage decreases monotonously from basalt to andesite, and show a linear differentiation trend.