The compositional range of groundmass plagioclase
in the basalts is AnTe.aAbzo.rOroto.r An63
Ab31Or1w hereast he cores and narrow marginso f
plagioclase phenocrysts in the andesites are An65
Ab33Or2to An32Ab63Ors It. is clear that the most
sodic margins of the former are still less sodic than
the cores of the latter. The compositions of the
cores of phenocrysts tend to be more sodic with
increase in SiO2 content of the host rocks, but the
sodic margins are not strikingly different from one
another. The Or content increases from 0.3 to 6.9
with increase in the Ab content (Fig. 5).