ATOLL FORMATION FULL

Thermodynamic modelling indicated the formation of garnet by the breakdown of chlorite and lawsonite/zoisite, and peak-pressure phases were represented by garnet, omphacite, quartz, amphibole, rutile, and talc. Therefore, minor elements that were less vulnerable to diffusion than major elements and strongly sensitive to the broad spectrum of geochemical processes, such as YвЂ‰+вЂ‰REE, were used to track possible mineral reactions during the whole garnet growth path. The ring garnet II indicated an elevated concentration of Mn in comparison with the marginal parts of the interior garnet I.

ATOLL FORMATION FULL

Preliminary observations showed that the full (non-atoll) garnet grains had compositionally different cores (interior, or garnet I) and rims (ring, or garnet II) that were separated by a YвЂ‰+вЂ‰HREEвЂ‰+вЂ‰medium REE (MREE) concentration peak. Based on textural relations, we show that the atoll garnet grains in the studied rocks were formed during the prograde stage from blueschist- to eclogite-facies metamorphism. In this study we present an example of the resorption of garnet cores and formation of atoll garnet textures in eclogite from the KruЕЎnГ© hory (in the Saxothuringian tectonic zone of the Bohemian Massif). Yet rapid changes in the reaction progress of a rock during garnet growth, namely the resorptionвЂ“dissolution of minerals with high concentrations of YвЂ‰+вЂ‰REE, may create an anomaly or peak in the mantle or rim of garnet grains. Because of this, as the garnet porphyroblasts form mostly in equilibrium with the surrounding matrix, the concentration of these elements continuously decreases towards the porphyroblast rims.

Garnet commonly accommodates high contents of MnвЂ‰+вЂ‰YвЂ‰+вЂ‰heavy rare earth elements (HREE) that follow Rayleigh fractionation during garnet early growth, with the exception of overstepping nucleation (late crystallization owing to reaction overstepping).

Atoll formation