Microstructural controls on sites of metamorphic reaction: a case study of the inter-relationship between deformation and metamorphism
Spiess, Richard; Bell, Tim H.
European Journal of Mineralogy Volume 8 Number 1 (1996), p. 165 - 186
published: Feb 22, 1996
manuscript accepted: Oct 3, 1995
manuscript received: Feb 24, 1995
ArtNo. ESP147050801017, Price: 29.00 €
Abstract Compositional zonation in garnet porphyroblasts in two samples from the same outcrop within the Meran-Mauls Basement of the Austro-Alpine of South Tyrol show at least four growth stages with three of them characterized by repetitive grossular zoning. This occurred due to repeated cycles of growth commencing early during a deformation and then ceasing and resulted from reaction overstepping and termination of micrometasomatic access of material to the porphyroblast margins as the matrix foliation intensified. The composition of these garnets varies from smooth to abrupt reductions in grossular and spessartine content (and a rise in XMg) in different microstructural sites. The transitions are smooth where the deformation event accompanying each phase of garnet growth was partitioned such that it was only weakly developed against the porphyroblast. However, they are abrupt where the deformation was more intensely developed against the earlierformed garnet core due to dissolution of the rim of this portion. Both features indicate a strong microstructural control on the sites of porphyroblast growth and associated chemical reaction within syntectonically metamorphosed rocks. This control has resulted from the effects of partitioning of the deformation, and consequent variation in the intensity of foliation development, on both the access of material to the growth site in zones of progressive shortening plus dissolution of portions of the porphyroblast rims along zones of considerable progressive shearing. In one of the samples this microstructural control on sites of growth and associated chemical reaction has resulted in cessation of growth throughout one whole period of deformation.