TEXTURAL, CHEMICAL, AND MICROSTRUCTURAL INVESTIGATIONS OF SNOWBALL GARNET PORPHYROBLASTS

Usually, the formation of snowball garnet is explained by either the rotational or non-rotational models. Chemical mapping, electron backscattered diffraction (EBSD) analysis and computed tomography imaging on snowball garnets from the Lukmanier area (Swiss Alps) indicate that snowball garnets can be explained by the combination of the two models operating consecutively during garnet growth. The configuration of the internal foliation, the crystallization sequence revealed by Mn contouring and the distribution of the crystallographic orientations within the spiral indicate that the final stage of garnet growth occurred within a non-rotational regime, whereas microstructural arguments imply a rotational behavior during the first stage of growth. In this view, the overall spiral geometry is thought to overestimate the true amount of rotation experienced by the garnet. In addition, EBSD data on different snowball garnets reveal that garnets displaying rotation angle below 300˚ are formed by one single crystallographic domain for the whole spiral. In contrast, garnets with an apparent rotation angle exceeding 300˚ are characterized by a central sector of about 300˚ of rotation and a terminal portion of the spirals consisting of several domains with distinct crystallographic orientations. EBSD data also indicate that the central domain displays a crystallographic orientation characterized by a [001] pole showing a systematic angle of about 18˚ with respect to the rotation axis of the garnet whereas in most crystallographic sectors, one of the two other [100] poles is (sub)parallel to the orientation of the internal foliation. This feature suggests that the crystallographic orientation across the garnet spiral is not random and that a relation between rotation axis, internal foliation and crystallographic orientation does exist. Lastly, the crystallization sequence of garnet, the distribution of crystallographic orientations within the spiral, and the arrangement of internal foliation strongly suggest that as long as the growth of the garnet is accompanied by rotation, the primary core orientation is preserved, but once the rotation stopped the crystallographic orientation may change. In this view, EBSD data can potentially be used to distinguish between the rotational and non-rotational models.