SUBCALCIC HORNBLENDES IN THE EPIDOTE AMPHIBOLITES OF THE YAMAGAMI METAMORPHIC COMPLEX IN THE NORTHERN ABUKUMA PLATEAU

Abstract

The chemical compositions of subcalcic hornblendes, epidotes, white mics and bulk rocks were reported from the Yamagami epiclote amphibolites. The compositional variation of subcalcic hornblende solid solution was discussed in the system of tschermakite-pargasite-eckermanite-glaucophane under the conditions of glaucophanitic metamorphism. The solvus between subcalcic hornblende and, alkali amphibole is wide and subcalcic hornblende is soluble to about 1 in the M₄ site alkali content. There are two variation trends of subcalcic hornblendes in glaucophanitic metamorphism. Trend A, that is, the alkali content of the M₄ site increases with rising temperature, is the same as the contributions by Black (1973) and Toriumi (1975). Trend B, in which the alkali content, of the M₄ site decreases and that of the vacant site slightly increases with rising temperature, is compatible with the general trend of subcalcic to calcic hornblende variation. Trend A is produced by the reactions among albite, chlorite, epidote and quartz, but Trend B must be created by the reactions among iron ores, carbonates, albite and epidote or by the breakdown of subcalcic hornblende_C to clinopyroxene-garnet-subcalcic hornblende_D or to quartz-subcalcic hornblende_D. The change of the trend from A to B is due to not only the metamorphic grade but also the bulk composition, specially the scarcity of the modal content of chlorite, quartz and albite.