A Garnet tale: chemical and mechanical responses in Dora Maira Whiteschists

Dora Maira is one of the internal crystalline massifs of the Western European Alps, formed by basement nappes of the Penninic Domain. The massif is characterized by high-pressure (HP) and ultra-high-pressure (UHP) rocks and is famous for the presence of coesite-bearing whiteschists. These rocks can be employed as source of information for Earth’s subduction-exhumation cycle, as well as window into (U)HP mechanical processes. For these reasons, Dora Maira whiteschists have attracted the attention of petrologists for the last four decades. However, most of the previous studies focused on petrological aspects and little attention has been given to the peculiar microstructures of these intriguing rocks. This contribution shows the preliminary results of a microstructural and compositional study performed on the whiteschists, with a focus on garnet crystals.These rocks are characterized by a spatially variable foliation, defined by the alignment of phengite and garnet crystals. Where the foliation is spaced, palisade quartz develops between phengite crystals with an orientation mostly at high angle to the main schistosity. Palisade quartz also formed within garnet crystals, surrounding or completely substituting pre-existing coesite inclusions.Garnet grains are both elongated parallel to the rock’s foliation or rounded in shape. They show two sets of fractures: a parallel set developed at high angle to the rock schistosity, and radial fractures around coesite/palisade quartz inclusions. The formation of this second set of radial fractures is due to the large volumetric change involved in the coesite-quartz transition.We adopted SEM-EDS, (HR-)EBSD techniques and performed microprobe analyses to study both microstructures and composition of garnet crystals. Their composition is ca. 88 up to 98% pyrope. However, they also show a distinctive chemical zoning around inclusions, which results in a higher grossular content.These observations raise questions on the mechanical behaviour of garnets at UHP. In particular, the coesite-quartz transition provokes large volumetric changes which likely result in a mechanical modification of the host garnet. The question is whether the volumetric change of the phase transition and related fractures can trigger also a chemical redistribution. More investigations are still needed, however a strong influence of mechanics on garnet crystals’ behaviour in these rocks is undeniable. Only a meticulous microstructural and compositional analysis can shed light on the history written in the pyrope crystals.