UNUSUAL PYRALSPITE-UGRANDITE GAGNETS FROM THE SANBAGAWA METAMORPHIC ROCKS IN CENTRAL SHIKOKU

Unusual garnets, exceedingly close to the mid-point between the two end members almandine and grossular, have been found in metagabbros of the epidote amphibolite facies in the Sanbagawa metamorphic terrain. The major minerals associated with them are subcalcic hornblende and zoisite. The chemical compositions of these garnets range from Alm38Spe12Pyr1Gro49And0 to Alm54Spe3Pyr3Gro39And1, thus filling the previously empty gap in the pyralspite-ugrandite series of glaucophanitic metamorphic rocks. The reactions concerned in the formation of these minerals, assuming different environments, are given. Most appropriate equations can not, however, be definitely determined because of no exact mineralogical information on the intrusive rocks before metamorphism. Each of the solid solution spessartine-grossular and almandine-grossular covers, at least, the range from pyralspite end member to about 50 molecular per cent grossular under the conditions of glaucophanitic metamorphism. Ca-rich pyralspites are widespread in, but not restricted to, glaucophanitic metamorphic rocks.