Garnets and their inclusions as key to unravel P-T paths, deformation history, and fluid-rock interaction in the ultrahigh-pressure metamorphic Lago di Cignana unit, Western Alps, Italy

The dynamic upper zone of the Earth, particularly at subduction zones, is responsible for volcanism, earthquakes and other natural hazards, large-scale elemental cycles, and the formation of economically viable mineral deposits. However, our knowledge of processes during subduction is hindered by the challenge of in-situ studies and limited exhumation of (ultra)high-pressure metamorphic rocks. Topics like the stress state and fluid-rock interaction at depth are of particular interest, yet poorly understood. The aim of this thesis is to utilize and further develop the use of garnets and inclusions therein as tool of unravelling metamorphic conditions, fluid-rock interaction, deformation, stress, and strain, in systems where these are otherwise unobtainable. The ultrahigh-pressure metamorphic Lago di Cignana unit (LCU) in the Western Alps of Italy is the focal point of this thesis. This locality provides an example of fluid-rich metamorphic rocks within the subduction zone, an ideal setting to improve our combined understanding of the extent and interaction of stress state, deformation, and fluid-rock interaction. This thesis combines petrological, microstructural, geochemical, and mineralogical aspects of garnet, rutile, quartz, and zircon, to gain a better understanding of the processes at work during (ultra)high-pressure metamorphism. Five studies are presented in this work, each on a different aspect of the intersection between fluid-rock interaction, deformation, metamorphism, and stress. The first study highlights an extreme case of fluid-rock interaction in a long-lived fluid pathway. Fluids derived from dehydrating serpentinites led to high amounts of dissolution of matrix minerals, resulting in the accumulation of garnet. Quartz inclusions in garnet that grew coeval with garnet pressure solution indicate that the system was at low differential stress. The fluids that circulated through these garnetites resulted in unique microstructures that reveal a history of pressure solution, grain-boundary migration, radial fracturing, and partial replacement accommodated by infiltrating fluids. The second study focuses on the microstructural aspects of the garnet in this system, showing that the preferred shape orientation has locally recorded a change in relative stress orientation, but also that local microstructures can be correlated to this shape preferred orientation, possibly indicating complex internal stresses within the garnetite. Evidence for grain-boundary migration is observed in all studied garnet-rich lithologies in the LCU while it is almost never observed elsewhere, suggesting a unique case of interaction between garnets and a fluid. The third study focuses on zircon and coesite inclusions trapped within garnet that was fractured and sealed, aiming to understand the resetting of elastic strains in the zircon and how coesite was preserved despite this fracturing occurring in the quartz stability field. The low-pressure conditions of fracturing are corroborated by elastic geothermometry on the zircon inclusions. The fourth study approaches the same system as in the previous chapters from a perspective of trace elements. The growth of garnet is studied by their rare earth element composition, and the trace element compositions of sub-micron zircon and rutile inclusions within garnet cores is calculated based on contaminated garnet measurements. The latter allowed for Zr-in-rutile thermometry to be applied, supporting low-pressure retrograde conditions of garnet fracturing and related garnet alteration. The fifth study targets rutile rather than garnet, combining a microstructural study with inclusions and trace elements to reveal that rutile in an UHP omphacite vein formed around the HP-UHP boundary and subsequently deformed. Low-angle boundaries that formed during this deformation then acted as fast diffusion pathways for trace elements, as is revealed by atom probe tomography.