A comparative study of compositional zonation in greenschist and granulite facies almandine garnets

Almandine garnets in greenschist (Loch Lomond, Scotland) and granulite (Sri Lanka) facies pelitic rocks are studied with the electron microprobe analyser. Greenschist facies garnets show strong compositional zonation. MnO has its highest value at the centre of the garnet and it decreases towards the margin. FeO shows antipathetic variation to MnO. CaO and MgO exhibit less marked variation but their trend is similar to that of MnO and FeO respectively. In contrast zonation in granulite facies garnets is displayed only by CaO and MgO. Furthermore, this zonation is weak and it is confined to the margin of the garnets. The contrasting pattern of zonation is the consequence of difference in temperature of formation of the two rock groups. Models explaining the formation of compositional zonation in garnets of low to medium grade rocks are discussed. The weak zonation in the granulite facies garnets cannot be interpreted as due to cation exchange between garnets and the matrix minerals, in response to a retrograde cooling phase since biotite and plagioclase are absent in the studied rocks. Consistent with the petrographic evidence, it is suggested that the garnet rim grew with falling temperature.