Garnet in pelitic schists from a quartz-eclogite unit of the southern Dora-Maira massif, Western Alps

On the basis of grain size distribution, chemical zoning patterns and the nature of inclusions, two types of garnet are recognized in pelitic schists from a quartz-eclogite unit of the southern Dora-Maira massif, 1) an Alpine-stage gar net, and 2) a multi-stage (Alpine and pre Alpine) garnet. Garnet, associated with chloritoid, paragonite, albite, phengite and quartz, occurs either as porphyroblasts with unimodal grain size distribution or as large and small grains with bimodal distribution. Porphyroblastic garnet commonly contains chloritoid inclusions throughout the grain, and has a concentric chemical zoning pattern with Mn decreasing and Ca increasing from core to rim. This suggests that the garnet grew during a single high-pressure metamorphic event, i.e., the high-pressure Alpine stage. The cores of the larger garnets show various zoning patterns. The rims of such garnets are Ca-rich and Mn-poor. Some grains have a clear chemical discontinuity between the core and rim. Chloritoid inclusions in such garnets are only found in the chemically distinct rims. The smaller garnets contain chloritoid throughout the grain and the composition is similar to that of the rim of the larger garnets. This suggests that the chemically distinct rim overgrew a pre-Alpine garnet relict during the high-pressure Alpine stage: the larger garnet records a multi-stage growth history. This is direct evidence for polymetamorphism of the quartz-eclogite unit in the Dora-Maira massif.