Abstract

A previously unrecognized pattern of garnet zoning has been preserved in garnets from the upper amphibolite facies Britt domain, Ontario Grenville province. Some large garnets contain discrete regions of high grossular content as well as variations in the Mg/(Mg+Fe) and Mn nearly parallel to grain boundaries. The latter are superimposed on and cut across regions of high grossular content. Such crosscutting zoning patterns have not been previously recognized in garnets. The Ca zoning involves changes of 4 to 10 mol% grossular and is interpreted to represent growth zoning, whereas the decreasing Mg/(Mg+Fe) and increasing Mn from core to rim are believed to be a diffusional retrograde effect. The Ca zoning indicates a pressure drop of ca. 5 kbar at 700 ±50°C (<5°C/km). Preservation of grossular-rich areas also suggests that the diffusion rate of Ca is slower than those of Fe, Mn, and Mg in garnet.

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