THE EFFECT OF MN ON MINERAL STABILITY IN METAPELITES

Calculations based on a KMnFMASH petrogenetic grid derived using an in ternally consistent thermodynamic dataset indicate that the principal effect of the presence of Mn in average subaluminous pelite dynamic da taset indicate that the principal effect of the presence of Mn in aver age subaluminous pelite compositions is to stabilize garnet to higher and lower pressures and temperatures over a wide range of bulk composi tions. Garnet-bearing fields expand to lower temperatures and pressure s with the addition of Mn, and garnet appears as an extra phase at low pressures. The addition of Mn also increases the number and extent of four AMnFM phase assemblages and stabilizes five AMnFM phases along u nivariant reactions. The KMnFMASH system predictions for typical subal uminous pelite bulk compositions match the sequence of isograds and as semblages observed in the Barrovian zones. The sequence:of assemblages observed in the Stonehaven section can also be predicted if there is variation in bulk composition within the stratigraphic section. Mn app ears to be less important in producing the sequence of isograds and ga rnet-absent assemblages in the low-pressure Buchan zones. The addition of Mn to the calculations does not change the sequence of isograds th at are predicted to be stable in a regional metamorphic terrane, but t he P-T position of these isograds does change. In particular, the pred icted temperature of the garnet-in isograd is lowered by as much as 10 0 degrees C by the addition of Mn to KFMASH. Mn also increases the ran ge of metapelite bulk compositions that develop the assemblages tradit ionally identified as metapelite isograds.