The Japanese granitic rocks cover 12 percent of the surface and consist mainly of granite and granodiorite. The plutonism that began in Silurian time and closed in the Miocene is subdivided into four episodes: Paleozoic, Triassic-Jurassic, Cretaceous-Paleogene, and Miocene. The location of the plutonism is inferred to have migrated oceanward in zones nearly parallel with the Honshu arc. This migration is in harmony with the migration of associated geosynclines.
Some of the granitic rocks are closely related to regional metamorphic rocks and others to volcanic rocks commonly represented by a vast volume of welded tuffs. The Cretaceous-Paleogene and Miocene granites are especially notable for their associated volcanic activity.
The major element chemistry of the granitic rocks is characterized by high SiO2 content and relatively low Fe2O3, Na20, and K2O. The initial 87Sr/86 Sr ratio varies between 0.704 and 0.710 but averages about 0.705 or slightly higher. Biotites of Cretaceous-Paleogene granites yield Mg/Mg+Fe2++Fe3+Mn values of between 0.2 and 0.6 and Si of around 5.5 on the basis of O,OH=24. Amphiboles of the same granites are mostly magnesio- or ferro-hornblende.
The Japanese granitic rocks can be grouped into magnetite series and ilmenite series. The two series seem to correspond roughly to areal variations in chemistry and mineralogy and also to the distribution of associated ore deposits.