Abstract
Textural (optical) zones and patterns of chemical zoning of Fe, Mn, Mg, and Ca in garnets from regionally-metamorphosed rocks in western Scotland correlate in a simple way where growth proceeded at a uniform rate into a homogeneous reservoir. Garnets with three zones predominate in garnetiferous mica-schists, whereas garnets in semi-pelite have two textural zones. In both types of garnet, each zone represents a separate growth stage. However, the outermost zone in many three-zone garnets is incomplete, which suggests that local changes in the effectiveness of intergranular diffusion in the matrix, combined with changes in distance to local sources of components such as Ca and Al, dominated the growth rate.
The evolution of textural zones closely reflects the amount of particulate matter and the local conditions in the reservoir into which the garnets grew. In contrast, chemical zoning resulted from distinct and universal stages in the metamorphic history of the rock. Where the incomplete development of textural zones in two- and three-zone garnets is due to partial growth, insufficient concentrations of particulate matter, or fragmentation, we believe the chemical data provide the firmest base for unraveling the history of garnet growth.
