Contacts between rocks recording large differences in metamorphic grade are indicative of major tectonic displacements. Low‐P upon high‐P contacts are commonly interpreted as extensional (i.e. material points on either side of the contact moved apart relative to the palaeo‐horizontal), but dating of deformation and metamorphism is essential in testing such models. In the Western Alps, the Piemonte Ophiolite consists of eclogites (T ≈550–600 °C and P≈18–20 kbar) structurally beneath greenschist facies rocks (T ≈400 °C and P≈9 kbar). Mapping shows that the latter form a kilometre‐wide shear zone (the Gressoney Shear Zone, GSZ) dominated by top‐SE movement related to crustal extension. Rb–Sr data from micas within different GSZ fabrics, which dynamically recrystallized below their blocking temperature, are interpreted as deformation ages. Ages from different samples within the same fabric are reproducible and are consistent with the relative chronology derived from mapping. They show that the GSZ had an extensional deformation history over a period of c. 9 Myr between c. 45–36 Ma. This overlaps in time with the eclogite facies metamorphism. The GSZ operated over the entire period during which the footwall evolved from eclogite to greenschist facies and was therefore responsible for eclogite exhumation. The discrete contact zone between eclogite and greenschist facies rocks is the last active part of the GSZ and truncates greenschist facies folds in the footwall. These final movements were therefore not a major component of eclogite exhumation. Pressure estimates associated with old and young fabrics within the GSZ are comparable, indicating that during extensional deformation there was no significant unroofing of the hangingwall. Since there are no known extensional structures younger than 36 Ma at higher levels in this part of the Alps, exhumation since the final juxtaposition of the two units (at 36 Ma) seems to have been dominated by erosion. Key words: deformation age, eclogite, exhumation, Rb–Sr dating, tectonic.

The deformation-related microstructure of an Indian Ocean zircon hosted in a gabbro deformed at amphibolitegrade has been quantified by electron backscatter diffraction. Orientation mapping reveals progressive variations in intragrain crystallographic orientations that accommodate 20° of misorientation in the zircon crystal. These variations are manifest by discrete low-angle (<4º) boundaries that separate domains recording no resolvable orientation variation. The progressive nature of orientation change is documented by crystallographic pole Zircon (ZrSiO 4) is an extremely significant accessory phase due to its ability to incorporate and retain geologically important trace and rare earth elements (REE), including elements produced by radioactive decay. These characteristics enable the geochemistry of zircon to be applied to a wide range of geoscience disciplines, for example rock petrogenesis (Belousova

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Hypervelocity impact processes are uniquely capable of generating shock metamorphism, which causes mineralogical transformations and deformation that register pressure (P) and temperature (T) conditions far beyond even the most extreme conditions created by terrestrial tectonics. The mineral zircon (ZrSiO 4) responds to 26 shock deformation is various ways, including crystal-plasticity, twinning, 27 polymorphism (e.g., transformation to the isochemical mineral reidite), formation of 28 granular texture, and dissociation to ZrO 2 + SiO 2 , which provide robust 29 thermobarometers that record different extreme conditions. The importance of 30 understanding these material processes is twofold. First, these processes can mobilize 31 and redistribute trace elements, and thus be accompanied by variable degrees of 32 resetting of the U-Pb system, which is significant for the use of zircon as a 33 geochronometer. Second, some features described herein form exclusively during 34 shock events and are diagnostic criteria that can be used to confirm the hypervelocity 35 origin of suspected impact structures. We present new P-T diagrams showing the 36 phase relations of ZrSiO 4 polymorphs and associated dissociation products under extreme conditions using available empirical and theoretical constraints. We present case studies to illustrate zircon microstructures formed in extreme environments, and present electron backscatter diffraction data for grains from three impact structures (Mistastin Lake of Canada, Ries of Germany, and Acraman of Australia) that preserve different minerals and microstructures associated with different shock conditions. For each locality, we demonstrate how systematic crystallographic orientation relationships within and between minerals can be used in conjunction with the new phase diagrams to constrain the P-T history. We outline a conceptual framework for a zircon-based approach to 'extreme thermobarometry' that incorporates both direct observation of high-P and high-T phases, as well as inferences for the former existence of phases from orientation relationships in recrystallised products, a concept we refer to here as 'phase heritage'. This new approach can be used to unravel the pressure-temperature history of zircon-bearing samples that have experienced extreme conditions, such as rocks that originated in the Earth's mantle, and those shocked during impact events on Earth and other planetary bodies.