Garnet Compositions at Broken Hill, New South Wales, as Indicators of Metamorphic Processes

Electron microprobe analyses of garnets in the finely bedded‘banded iron formations’ (BIF) of the Willyama Complexat Broken Hill reveal marked compositional changes from onegarnet to the next on a scale of 1–2 mm. Further, systematicanalytical traverses across bedding and along bedding show thecompositions of the garnets to change markedly from one finebed to the next, but to remain extremely uniform within individualbeds. In view of the minuteness of the domains involved it appearsevident that compositional variation cannot be attributed tovariations in metamorphic pressures, temperatures or oxygenfugacities. Neither can they be attributed to variation in garnet-matrixpartition functions, as most of the garnets occur in one simplematrix—quartz. It is concluded that in spite of the high (sillimanite) gradeof the relevant metamorphism, any equilibration of garnet compositions,and hence any associated inter-grain metamorphic diffusion,has been restricted to a scale of less than 1 mm; that garnetcompositions here reflect original rock composition on an ultra-finescale, and have no connotations concerning metamorphic grade;that, hence, the garnets must derive from a single precursormaterial, earlier suggested to be a manganiferous chamositicseptechlorite; and that the between-bed variation: within-beduniformity of garnet composition reflects an original patternof chemical sedimentation—a pattern preserved with theutmost delicacy through a period of 1800 x 10⁶ years and a metamorphicepisode of sillimanite grade.