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R. L. STANTON, K. L. WILLIAMS, Garnet Compositions at Broken Hill, New South Wales, as Indicators of Metamorphic Processes, Journal of Petrology, Volume 19, Issue 3, August 1978, Pages 514–529, https://doi.org/10.1093/petrology/19.3.514
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Abstract
Electron microprobe analyses of garnets in the finely bedded ‘banded iron formations’ (BIF) of the Willyama Complex at Broken Hill reveal marked compositional changes from one garnet to the next on a scale of 1–2 mm. Further, systematic analytical traverses across bedding and along bedding show the compositions of the garnets to change markedly from one fine bed to the next, but to remain extremely uniform within individual beds.
In view of the minuteness of the domains involved it appears evident that compositional variation cannot be attributed to variations in metamorphic pressures, temperatures or oxygen fugacities. Neither can they be attributed to variation in garnet-matrix partition functions, as most of the garnets occur in one simple matrix—quartz.
It is concluded that in spite of the high (sillimanite) grade of 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 garnet compositions here reflect original rock composition on an ultra-fine scale, and have no connotations concerning metamorphic grade; that, hence, the garnets must derive from a single precursor material, earlier suggested to be a manganiferous chamositic septechlorite; and that the between-bed variation: within-bed uniformity of garnet composition reflects an original pattern of chemical sedimentation—a pattern preserved with the utmost delicacy through a period of ≈1800 × 106 years and a metamorphic episode of sillimanite grade.
