Sideritic rocks that occur in the freshwater sediments of the Ishikari coalfield contain authigenic calcite with or without dolomite in addition to siderite. The cause and stages of carbonate mineral precipitation were studied on the basis of the modes of occurrence, Mn/Fe atomic ratios, and carbon and oxygen isotope compositions of the carbonate minerals.

The precursor of the sideritic rocks deposited at the bottoms of rivers or lakes in the Ishikari coalfield is ferric hydroxide. With increasing sediment burial, ferric hydroxide was reduced to ferrous ions by reaction with organic matter contained in the sediment. Calcite precipitated at this stage due to an increase in the alkalinity of interstitial waters. The source of Ca to form calcite was Ca²⁺ ions dissolved in the river water and that of CO₃^2- was both atmospheric CO₂ dissolved in river water and organic matter contained in the sediments. The precipitation of calcite increased the Mg/Ca ratio of the interstitial waters, which resulted in the precipitation of dolomite, as in the case of the Ikushunbetsu Formation. When ferric hydroxide in the sediments had been completely reduced, bacterial methanogenesis began to occur. Siderite with a high positive δ¹³C value was then precipitated at this stage. In the Ishikari coalfield, early diagenesis of the fresh water sediments produced authigenic calcite and siderite, as in the case of the Horokabetsu and Yubari Formations, while calcite, dolomite, and siderite were produced in the Ikushunbetsu Formation with increased sediment burial.

There are two high-P/T type metamorphic belts distinguished as c. 300 Ma Renge belt and c.

200 Ma Suo belt in the Inner zone of SW Japan. In the Gotsu area of the Suo metamorphic belt, blueschists occur as lenses or layers intercalated in pelitic schists. The blueschists consist mainly of epidote, amphibole

(glaucophane, magnesioriebeckite, winchite, ferrowinchite, actinolite and magnesiohornblende) and chlorite, with minor amounts of phengite, albite, titanite, K-feldspar, biotite, calcite, apatite, hematite and quartz. A schistosity is defined by preferred orientation of amphibole, chlorite and epidote. Coarse-grained glaucophane- and epidote-rich layers are also developed along the schistosity. In the blueschists, three different metamorphic stages, i.e. prograde, peak and retrograde metamorphic stages, are distinguished on the basis of texture and compositional zoning of constituent minerals. The prograde stage is defined by inclusions in porphyroblastic epidote and amphibole, and core to mantle of matrix minerals, such as amphibole

(winchite and glaucophane), phengite (Si 6.45–6.9 pfu), chlorite, albite, epidote, hematite and titanite. These minerals suggest high-pressure greenschist facies to glaucophane schist facies. The peak metamorphic stage is well-defined by schistosity-forming minerals of epidote, glaucophanic amphibole (glaucophane, magnesioriebeckite), phengite (Si 6.9–7.1 pfu), chlorite and titanite. The peak mineral assemblage suggests the metamorphic conditions of epidote-blueschist facies at 380–540 °C and 1.1–1.4 GPa, which are likely to be the highest grade metamorphic conditions of the Suo belt. Magnesiohornblende/actinolite, chlorite and albite replacing the peak metamorphic minerals suggest the conditions of the greenschist facies at a retrograde metamorphic stage.