Tracking high-grade metamorphism: from partial melting to the detrital record

High to ultra-high temperature (HT-UHT) metamorphism (>800°C) drives changes in rocks that range from the production of chemical heterogeneities to the disturbing of isotopic systems. To disentangle the complex history of these rocks, a multi-prong approach is needed, due to the different sets of mineral reactions that produce melts and fluids of varying compositions at different times. Nanorock inclusions preserved in zircon are critical for retrieving primary melt compositions through re-homogenization experiments, especially in garnet-free rocks, that can be further compared to melt compositions from different scales of observation. These observed melt compositions may then be contrasted with those predicted by thermodynamic models to evaluate the pressure-temperature evolution of partial melting. Nanorock inclusions can further assist in the deconvolution of HT-UHT rocks intricate geochronological record since these rocks often reveal a protracted zircon record with dates smeared along the concordia curve. This smearing may result from the partial resetting of existing grains or prolonged metamorphism and introduces ambiguity into U-Pb dating from igneous, sedimentary, and metamorphic rocks. The HT-UHT metamorphism has implications for maximum depositional ages and tectonic setting determinations, and its extent depends on whether metamorphism took place in the source that will be eroded and then deposited or when the rock was already deposited. A detailed investigation enables identifying protoliths and reconstructing metamorphic histories. Hf isotope analysis allows zircon sources to be distinguished, minimum crystallization ages to be retrieved, and provides additional constraints on the conditions of anatexis (e.g., open versus closed system melting; role of coexisting minerals).