Summary: A detailed petrographic and microprobe study was carried out on atoll-shaped garnets from the aureole of the Ardara pluton, in which regionally metamorphosed rocks have been affected by thermal metamorphism. Textural evidence showed that the atoll garnets are formed by replacement mainly by biotite, some plagioclase, quartz, and opaques, and may proceed from the garnet interior towards the margins, or from the margins into the core. Core replacement occurs mainly in those garnets enclosed by quartz or quartz-rich matrix.
Microprobe analyses are presented of garnets in different stages of atoll formation. Compositional profiles at the garnet margins show abnormal (reversed) zoning with increase in MnO and Fe2O32, marked decrease of CaO, and decrease of MgO towards the extreme edge. With decreasing width of the atoll rims, there is generally an increase in MnO and to a lesser extent Fe2O3, a decrease in CaO; MgO is variable. The microprobe data support the textural observations and a model for atoll formation involving prograde resorption (i.e. garnet replacement during temperature increase) is presented.