著者

河内 洋佑 渡辺 暉夫

出版者

地学団体研究会

雑誌

地球科学 (ISSN:03666611)

巻号頁・発行日

vol.28, no.1, pp.1-10, 1974-01-20 (Released:2017-07-26)

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ACF and similar other triangular and tetrahedral diagrams have been used widely to analyze mineral parageneses of metamorphic rocks. Listed parageneses, however, are contradictory to the accompaying diagrams in many papers; that is, they result in crossing tie lines and/or more number of phases than each compositional triangle/tetrahedron of the diagram can show. This contradiction has been commonly ignored or explained as such that the rock consists of far more numbers of components than the diagram can show or that the rock is in disequilibrium, However, as long as the assumptions employed to reduce the number of components into three or four are valid, the diagram should show perfectly the parageneses under consideration. When we take minerals in mutual contact under the microscope as paragenetic, but exclude minerals not in contact however closely they occur, an ACF diagram constructed from phases occurring side by side can represent successfully all the parageneses observed in the same metamorphic grade. This leads to a result that a series of ACF diagrams obtained from a progressively metamorphosed terrane can be arranged so as to represent a series of univariant reactions (WATANABE, in press; WATANABE & KAWACHI, in preparation). Thus each metamorphic isograd will be denned by respective univariant reaction. In low grade rocks, as the equilibrium is reached only within a few to several tens of microns, a grain of mineral should often be considered to represent a domain having bulk composition of that mineral. A wide variety of metamorphic mineral assemblages occur in basaltic lava flows in contrast to much simpler variety in tuffs intercalating with the former in the Mesozoic rocks of the Akaishi Mts. in Shimanto belt, central Japan (SEKINE & MATSUURA, in preparation): lava consisting of many small domains (phenocrysts and groundmass etc.) of different bulk compositions while tuff a few domains (laminations with small chemical difference). HASHIMOTO (1972) stressed that an actinolite overgrowth on relict clinopyroxene does not represent actinolite isograd while actinolite in a matrix of mafic rock does. This seems to be a matter of definition rather than an equilibrium as HASHIMOTO has claimed: either an isograd is based on a rather unusual bulk composition represented by relict clinopyroxene or an ordinary bulk composition represented by mafic tuffaceous matrix.