Abstract
Granitic rocks of the Iwakuni-Yanai district in the southwestern Japanese mainland (Honshu) have been classified into three distinct series: Sanyo, Ryoke (younger and older) and Sannin types. The present study involeves the former two.
Examinations of the mode of occurrences and petrographical studies including the variation of chemical compositions of Sanyo and Ryoke granites revealed that they are in reality cogenetic. They can be called as the "Ryoke-Sanyo" series. This study points out also the existence of migmatitic concordant granites occurring in small dimensions in the Ryoke metamorphic rocks of high grade part.
No compositional differences were observed between slightly metamorphosed sediments of the Kuga formation (Jurassic) and Ryoke metamaophic rocks constituting the country rocks of these granites. The latter can be considered as metamorphic (green schist facies to high grade part of the amphibolite facies) equivalent of the former.
Granite masses of the "Sanyo type" granites are intruded discordantly into their country rocks (the Kuga formation and low grade part of the Ryoke metamorphics). They are homogeneous in each masses with great dimensions of batholithic order. They manifest on the variation diagram, compositions of advanced differenciation characterized by extremely poor MgO content and Q: Or: Ab ratio close to that of the "ternary minimum (or eutectic depending on vapour pressure) composition of the granite system".
The "Younger Ryoke" granites are intruded discordantly into the country rocks. Dimensions of each individual masses of this types are much smaller than those of the "Sanyo" granites. The varaiation range of their chemical compositions are wider than that of the "Sanyo" granites, but the trend is same. It is considered from the mode of occurrences and petrographical characters that the granites of the "Younger Ryoke" granites represent rock facies formed in levels deeper than olaces where the "Sanvo" granites were consolidated.
Besides these discordant granites, there are gneissose granites of small dimensions in several places (typical example can be seen at Oshima) in high grade part of the Ryoke metamorphics. They are concordant to the country rock. Their chemical composition is different from above two types and show the same trend in MgO-AL2O3, Total Fe-AL2O3, TiO2-AL2O3 and K2O-MgO relations as those of the Kuga formation and Ryoke metamorphic. These granites (migmatites) are considered to be formed in situ in the Ryoke metamorphics through partial fusion of metamorphic rocks whose compositions were modified by the metasomatic replacement caused by percolation of fluids of deep origin.
From these obserations, it is concluded that granites of the Sanyo and Ryoke type of the district belong to the same series named as the "Ryoke-Sanyo" granite series.
