Variation curves for SiO₂, FeO+Fe₂O₃, and TiO₂ for the differentiation of Skaergaard, Palisade and Dillsburg, and Elephant's Head and New Amalfi intrusions, and the pigeonitic rock series of Izu-Hakone region are compared. Abrupt change of slope of the three curves occurs at approximately the same stage of fractionation for each differentiation series. It is also noticed that the later the stage of the change of slope in the series, the higher the concentration of FeO+Fe₂O₃ during fractionation. The change of slope is probably caused by appearance of titaniferous magnetite (or magnetite+ilmenite) as a solid phase in the crystallizing magmas, or possibly by sudden increase in the amount of the same mineral separating from the magmas. As the stage of the appearance of magnetite is probably determined by oxygen partial pressure of the magmas, which is in turn related to the H₂O content, it is inferred that the Skaergaard differentiation series, where the FeO+Fe₂O₃ concentration is the highest, was produced from magma with lowest H₂O, the pigeonitic rock series, where the FeO+Fe₂O₃ concentration is the lowest, was produced from magma with highest H₂O, and the other two from magmas of in ermediate H₂O. In the hypersthenic rock series of Izu-Hakone, the maximum concentration of FeO+Fe₂O₃ is seen at the earliest stage of fractionation and the maximum FeO+Fe₂O₃ thereof is even lower than that in the pigeonitic rock series, indicating higher H₂O content in the magma of the hypersthenic rock series. The high H₂O content in this series is also indicated by the occurrence of orthopyroxene in the groundmass and the appearance of hornblende and biotite as phenocrysts which are normally absent in the other series. The variation trend in chemistry of the hypersthenic rock series agrees with that generally accepted as characteristic of the calc-alkali rock series. It is proposed to confine the term calc-alkali rock series to basalt-andesite-dacite-rhyolite of the hypersthenic rock series and to its plutonic equivalent, if any. The essential factor for producing the calc-alkali rock series is therefore the high concentration of H₂O in the magma. It should be remembered that the distribution of the calc-alkali rock series is confined to the areas floored by the sialic crust and also that granitic xenoliths and xenocrysts are common in this series. It is not yet certain however whether contamination of the magma by granitic material is a necessary factor for concentrating H₂O in the magma and therefore for producing the calc-alkali rock series.

View full abstract

In the Kitakami-massif of north-eastern Japan, there develope many granitic masses, and they have been grouped into two types from their geological relation, i.e. older type of pre-Cretaceous and younger type of Cretaceous age. From these 23 rock masses, 52 samples were collected for K-A dating experiment. As a result, it is ascertained that the granitic rocks of older type would have emplaced more than 222×10⁶ years ago at least, though the value does not show the true age, because of the thermal effect of younger granitic intrusion. On the other hand, it is clarified that most of the younger granitic rocks intruded 120×10⁶ years ago, though some of them show the ages of 128×10⁶ years and 106×10⁶ years.
In conclusion, the underground structure of the Kitakami-massif is considered to be composed of one granitic batholith.

View full abstract

Jadeitic pyroxene has recently been found as pebbles of sandstone beds in lower part of lower Cretaceous formation of the Kanto Mts. This fact as well as previous discovery of lawsonite and pumpellyite-bearing rocks as clastic materials in the same horizon clearly show that the Sanbagawa metamorphism of the Kanto Mts. must have occurred before the deposition of lower Cretaceous formation of the Kanto Mts.

View full abstract