Geochemistry and origin of piemontite-quartz schists in the Sanbagawa Metamorphic Belt, central Shikoku, Japan

Abstract

In the Sanbagawa Metamorphic Belt of central Shikoku, geochemical characteristics are used to demonstrate that piemontite-quartz schists had a manganiferous sedimentary protolith. The Sanbagawa Metamorphic Belt was formed during a Cretaceous regional intermediate high-pressure type of metamorphism which is present throughout Southwest Japan. Based on a detailed mineralogical study of the Asemi-gawa and Besshi areas in central Shikoku, the Sanbagawa piemontite-quartz schists can be divided into talc-bearing and talc-free types. Phlogopite occurs only in talc-bearing assemblages and the modal percentage of piemontite in talc-bearing types is higher than in talc-free types. Plotting analyses of piemontites and garnets from talc-bearing and talc-free types in the Al 2O 3–Fe 2O 3–Mn 2O 3–MnO(Al–Fe 3+–Mn 3+–Mn 2+) diagram, for the Asemi-gawa and Besshi areas separately, shows that the piemontite which accompanies talc is richer in Mn 3+ and poorer in Fe 3+. Also, garnets from talc-bearing assemblage contain a slightly higher amount of Mn 2+ and a lower amount of Al than talc-free types. The Al 2O 3–SiO 2–Fe 2O 3+FeO plot shows that Sanbagawa piemontite-quartz schists have a composition between deep-sea pelagic clay and pure silica. By the use of Pearson's product moment correlation coefficient method, and plotting elements against each other after removing silica and volatiles, it is possible to distinguish four components which are able to explain the variance of the samples. These components are: (first component: Al 2O 3, TiO 2, MgO, Na 2O, K 2O, Zr, Rb, Y, Nb and to lesser degree Fe 2O 3, FeO, Ni, Th, Ba); (second component: SiO 2); (third component: MnO) and (fourth component: CaO). Al 2O 3, TiO 2, Zr and other elements with positive coefficient relations with them have a detrital origin. Biogenic origins are considered for CaO and SiO 2 and it was concluded that MnO was derived from an hydrothermal source. Because the Sanbagawa Metamorphic Belt has been subjected to much detailed research and also contains talc-bearing and talc-free piemontite-quartz schists, it provides the best opportunity to find an explanation for the occurrence of talc in some types of piemontite schist and its absence from others. Mineralogical and geochemical evidences show that the oxygen fugacity for talc-bearing and talc-free types is similar. Oxygen fugacity is so high that almost all of the iron is present as Fe 3+ in hematite. Pressure and temperature is considered to have been constant for talc-bearing and talc-free types in each locality because the samples were collected from a known metamorphic context. The talc absent/present problem is discussed using phase relations of the samples with the greatest number of phases in each locality. Consideration of talc, clinochlore, piemontite and garnet in the system Al 2O 3–Mn 2O 3/MnO–MgO–CaO(ACMMn), with excess phases albite, quartz, hematite, phengite and water, shows that talc coexists with piemontite and garnet which are richer in the piemontite and spessartine end-members, respectively.