Melanite from the endoskarn of a spessartite dyke intruding into the Kinshozan limestone in Ogaki City, Gifu Prefecture

摘要

A spessartite dyke that intruded into the Permian Kinshozan limestone formed endoskarn that is rich in melanite, along with clinopyroxene and apatite, in a matrix of secondary calcite, thomsonite, stilbite, heulandite and apophyllite. Garnet grains, ranging in size from 0.5 to 3 mm, have dark brown cores and pale to colorless mantles. Detailed analysis of a typical grain reveals TiO_2 contents from 2.70 to 11.90 wt.% in the core and from 0.42 to 2.25 wt.% in the mantle. Cation proportions can be attributed to a melanite chemistry controlled by two kinds of substitutions, Ti~(4+)(Z)=Si(Z) and Fe~(2+)(Y)Ti~(4+)(Y)=2M~(3+)(Y). Iron occurs in both ferric and ferrous states, with Ti substitutions negatively correlated with intermediate to high Fe~(3+)V(Fe~(3+)+Al). Decreasing ferrous iron from core to mantle is consistent with reverse Mg/(Mg+Fe~(2+)) zoning in co-genetic clinopyroxene. Low f_o_2 conditions probably enhanced Fe~(2+)(Y)Ti~(4+)(Y)=2M~(3+)(Y) substitution in garnet.