GARNET GROWTH IN A KYANITE-BEARING PERALUMINOUS PEGMATOID FROM THE MOLDANUBIAN ZONE (BOHEMIAN MASSIF, AT)

An 8 mm sized almandine-spessartine garnet from the Moldanubian Gföhl Unit (Bohemian Massif, AT) displays asymmetric morphology and several zoning types, reflected by varying inclusion microstructures and garnet colour. Different characteristics of oscillatory, sector, concentric and non-concentric zoning serve to decipher several phases of garnet crystallization from pegmatoid melt.

Directed garnet growth is corroborated by (i) asymmetric crystal morphology with irregular shape on one side and idiomorphic facets on the other, (ii) asymmetric compositional zoning, (iii) absence of two rim zones on the irregular side which are clearly discernible on the idiomorphic side and (iv) presence of a garnet reaction rim on the irregular side exclusively.

The garnet composition and microstructural zoning are not necessarily spatially correlated. Major element composition can show continuous trends, despite striking microstructural changes. Contrastingly, domains with the same microstructural characteristics can display differences in compositional details. Therefore, compositional and microstructural zoning seem to be governed by different parameters that change during garnet crystallization.

Sector and oscillatory zoning are reflected by inclusion microstructure and colouring of the garnet host. Grt{110} and Grt{112} growth sectors contain various inclusion types in different ratios, where dodecahedral sectors show prevalence of phosphates over rutile, vice versa for trapezohedral sectors. In addition to non-concentric zoning, these observations indicate a local and crystallographically assisted inclusion origin. Instead of external forcings (Jamtveit 1991, Am Min 76), oscillatory zoning is interpreted to reflect a self-organized process at the garnet-melt interface (Allègre et al. 1981, Nature 294).

The Austrian Science Fund (FWF): I4285-N37 is acknowledged.