A COMBINED EMP/EBSD/µCT STUDY OF SNOWBALL GARNETS IN PELITES

Snowball garnets are recurrently found in regional metamorphic rocks and are commonly interpreted as the result of simultaneous growth and rotation of the crystals during syn-metamorphic deformation. Investigations of the chemical composition, crystallographic orientation, and three-dimensional shape of snowball garnets from the Lukmanier Pass, in the Swiss central Alps revealed several characteristics which enable us to better understand their formation. Chemical major elements maps show an overall concentric zoning pattern which likely reflects a single metamorphic event and a temperature increase during garnet growth. However, a detailed examination of the Mn pattern reveals a more complex behavior illustrated by anomalous high-Mn regions systematically located at the outermost edge of the bridges that connect two shells of snowball garnets. Considering Mn concentrations as time lines, the anomalous high-Mn regions appear to have crystallized earlier than the adjacent part of the external shell of the garnet. This feature strongly suggests that static garnet crystallization along micas-rich layers subsequently connected the high-Mn regions of the same shell. On the other hand, electron back-scattered diffraction (EBSD) analyses show evidence of misorientation domains whose boundaries do not necessarily correspond to compositional variation. The crystallographic orientation maps are characterized by the presence of one single grain at the core of the snowball garnets and by the occurrence of several garnet grains with multiple crystallographic orientations at the outermost shells. In addition, most of those latter grains share a common crystallographic axis with the central sector. This feature strongly suggests that the different crystallographic sectors are related and did not randomly nucleate. The opportunity of nucleation adjacent to the central growing garnet grain supports a garnet growth controlled by interface-reaction process. Application of high-resolution X-ray computed tomography(µCT) imaging shows the complex three dimensional spiral geometry such as predicted by rotational model of snowball garnet formation, but highlights evidences of final static garnet growth as well.