The Ryoke metamorphic belt, one of the most important regional metamorphic in later Cretaceous, ends in northern Kiso district, southern Nagano prefecture.
There occur in this district both non-metamorphosed Paleozoic rocks and their metamorphosed derivatives, from biotite slate of the lowest metamorphic grade to sillimanite gneiss of the highest-grade.
The metamorphic belt is divided into six zones, Ia, Ib, Ic, Ila, IIb, and III, based upon the stability range of minerals in clayslate, basalt and calcareous rocks. The appearance of the most characteristic minerals is shown in the following table.
Besides, the optical property of chlorite, and an assemblage, cordierite+ K-feldspar, are to be noticed.
This paper is concerned mainly with the petrographic description of such minerals.

View full abstract

The colour of rhodochrosite is pale light pink when pure, but commonly it is pale pink, dark pink, purple, reddish brown and brown in natural crystals. In the Yakumo mine, colour of rhodochrosite crystallizing in the early stage is pale or dark pink, or rarely purple, and the saturation is pale, while those crystallizing in the later stage are variable from reddish pink to reddish dark orange(brown) in colour, and the saturation is deep. It is remarkable that the former is almost free from iron component, whereas the latter always contains about 10 percent of FeO.

View full abstract