Matrix‐dependent garnet growth in polymetamorphic rocks of the Sesia zone, Italian Alps

Abstract

AbstractMushroom‐ and atoll‐shaped garnet crystals were found in high‐pressure quartz‐rich pelitic layers from the Monte Mucrone area (Western Alps, Italy). These garnet crystals are characterized by a peninsula‐shaped core surrounded by a partially crystallized, dodecahedral external rim. Textural observations and thermodynamic modelling point towards growth of the atoll garnet from the Monte Mucrone area during two distinct orogenic cycles. The core region and the inner part of the ring forming the edge of the atoll grew under Barrovian metamorphic conditions of likely Hercynian age, while the outer rim of the atoll structure developed under Alpine high‐pressure conditions. Electron backscatter diffraction analyses indicate that the atoll‐shaped structure has one single crystallographic orientation, despite its complex compositional zoning. Thermodynamic modelling reveals marked changes in equilibrium assemblage leading to changes in stoichiometry of the garnet‐forming reaction, which in turn explain the characteristic garnet morphology. Small amounts of quartz are consumed during the earlier stage of garnet growth history, whereas the production of garnet requires a much larger amount of quartz during the final stage of its growth. This leads to a change from initial poikiloblastic to non‐poikiloblastic textures. This change is responsible for the formation of atoll‐shaped garnet. Finally, garnet in intercalated mica‐rich layers forms idiomorphic crystals, continuous from the centre to rim. This study highlights the importance of the difference between the local matrix composition and the aggregate composition of the reactants needed for the garnet‐forming reaction. Finally, we show that interaction between matrix and reaction stoichiometry can lead to porphyroblast precipitation inside the already grown porphyroblast.