広島-岩国地域の広島花崗商岩の化学組成

抄録

The Cretaceous Hiroshima granite of the Hiroshima-Iwakuni area is divided petrographically into two types, e. g. biotite granite and hornblende-bearing biotite granite. The dark inclusions in the latter are characterized by containing hornblende commonly, while those associated with the former granite are hornblende-free, tiny in size, and also small in amount.
Plotting the chemical compositions on the normative An-Ab-Or diagram, these two types of granite are quite different in trend, although there is no distinct difference on the Thornton-Tuttle's variation diagrams. This cannot be explained by the difference of fractionation and also by the change in partial P_H2O of magma under cooling condition. Irregular shape and uneven distribution of hornblende in hornblende-bearing biotite granite indicate that this hornblende was not the crystallization product of granite magma, but was accidental xenocryst transported into the magma. This fact and the difference of trend in the An-Ab-Or diagram suggest the contamination of granite magma by the hornblende-bearing dark inclusions.