The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists

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The contact-metamorphosed Mesozoic rocks of the Shimanto Belt at the northern part of the Akaishi Mountains are studied in detail. Two zones, i. e. Biotite and Cordierite Zones, are distinguished for pelitic rocks. The latter is further separated into two zones, Cordierite I and II Zones.
Recognition fo the Cordierite I and II Zones is based on whether or not chlorite occurs in contact with white mica in the presence of cordierite. The area where chlorite not with white mica is Cordierite II Zone.
The following reactions are proposed for each isograd on the basis of chemical, modal and paragenetic study of metamorphic minerals.
(1) Formation of biotite at biotite isograd.
celadonite molecule of white mica+chlorite→biotite+muscovite+quartz+H₂O
(2) Formation of cordierite at the first cordierite isograd.
chlorite+Al₂O₃+quartz→cordierite+H₂O
(3) Formation of cordierite at the second cordierite isograd.
celadonite molecule of white mica+chlorite→cordierite+biotite+quartz+H₂O
In the high grade part of Cordierite II Zone, garnet commonly occurs. This part is called “Garnet Zone” in this paper. The appearance of garnet is controlled by the difference of bulk chemical composition of host rocks, i. e. garnet occurs in metapelite rich in MnO, high in (Mn+Mg+Fe⁺²)/(Mn+Mg+Fe⁺²+Al) ratio and low in Mg/Fe⁺² ratio.
The following reactions are considered for the formation of garnet, although it is not clear which is more realistic.
chlorite+quartz→garnet+H₂O
celadonite molecule of white mica+cholriet→garnet+biotite+quartz+H₂O In either case chlorite rich in MnO must have participated in the reaction.
The zone of garnet+potassium feldspar which has been reported by previous studies is not recognized.

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The hornfelsic, gneissic and granodioritic rocks exposed below the Miocene the Miocene deposits in the southern part of Yamagata Prefecture have been studied by the author in field and laboratory. Eleven rock specimens, purposely selected by him were placed in K-Ar dating by the courtesy of Prof. Y. Ueda. The result will be briefly given below.
There are two groups in respect to K-Ar ages. Namely, four rock specimens of them (Nos. 1, 2, 3, and 11 in Table 1) show the Middle to the Late Cretaceous in age, and one specimen shows Early Palaeogene. Other six specimens including granodioritic and hornfelsic rocks are dated as young as the Miocene, llthough they are unconformalbly covered by the lower Miocene deposits in field. The younger ages of the same samples may suggest that the originally older rock complexes may have been rejuvenated by igneous activith during Miocene time, although presence of corresponding younger igneous rocks has been not yet evidently proved.

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