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Naxos in the Greek Cyclades preserves a type example of polymetamorphism. The southern and northern parts of the island record different Tertiary P–T histories between Eocene and Miocene times, including a blueschist facies event, one or more amphibolite/greenschist facies overprint(s) and contact metamorphism. Age attributions for these events are inconsistent in the literature. Here, we propose a new approach that combines electron probe microanalyzer (EPMA) characterization of the white mica (WM) with $^{39}$Ar-$^{40}$Ar–Rb-Sr multichronometry. Textural–petrographic–compositional observations reveal that the polygenetic WM consists of five different generations: pre-Eocene relicts, paragonite, high-Si phengite, low-Si phengite and muscovite. EPMA mapping of four WM samples, previously analysed by Rb-Sr, reveals major element compositions heterogeneous down to the μm scale. Each WM consists of chemically distinct generations, documenting submicron-scale retrogression of high-pressure (HP) phengite grains to muscovite. Four WM samples from a N-S traverse across the island were analysed by $^{39}$Ar-$^{40}$Ar stepheating, comparing coarse and fine sieve size fractions to obtain overdetermined K-Ar systematics. Fine sieve fractions are richer in Cl than coarse ones. Linear arrays in Cl/K-age isotope correlation diagrams show two predominant WM generations (one Cl-poor at ca. 38 Ma and one Cl-rich at

Dating deformation is difficult, as textures and petrogenesis of deformed rocks are complex. Moreover, geochronometer categories are pursued by communities that often do not communicate. Hygrochronology dates the retrograde metasomatic/metamorphic reactions caused by aqueous fluid circulation events. Thermochronology models time-temperature histories by assuming that mineral ages can be uniquely assigned to a ‘closure temperature T c ’, the only process occurring in rocks being Fick's Law diffusion. Diffusion by definition produces a bell-shaped concentration profile. In contrast, patchy intra-grain isotope concentration profiles denounce aqueous retrogression, whose rate is orders of magnitude faster than diffusion. Petrochronology is based on opposite assumptions, as the mobility of structure-forming major cations is higher than that of radiogenic Pb, Ar, and Sr. Whenever the formation of a mineral occurs at T T c , its apparent age dates its formation. Nanochronology analyses samples at the nanometre-scale. These analyses illuminate atomic-scale processes, e.g. open-system transport of soluble ions along self-sealing networks of nanopores. The key to dating deformation and producing correct, regional-sized (up to hundreds of kilometres) tectonic models is the realization that minerals consist of atoms, whose behaviour is only firmly constrained by nanometre-scale analyses. Thematic collection: This article is part of the Isotopic Dating of Deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation

Radiogenic strontium isotopes (87Sr/86Sr) of vein carbonates play a central role in the tectonometamorphic study of fold-and-thrust belts and accretionary wedges and have been used to document fluid sources and fluxes, for example, along major fault zones. In addition, the 87Sr/86Sr ratios of vein carbonates can trace the diagenetic to metamorphic evolution of pore fluids in accreted sediments. Here we present 87Sr/86Sr ratios of vein carbonates from the Infrahelvetic flysch units of the central European Alps (Glarus Alps, Switzerland), which were accreted to the North Alpine fold-and-thrust belt during the early stages of continental collision. We show that the vein carbonates trace the Sr isotopic evolution of pore fluids from an initial seawater-like signature towards the Sr isotopic composition of the host rock with increasing metamorphic grade. This relationship reflects the progressive equilibration of the pore fluid with the host rock and allows us to constrain the diagenetic to low-grade metamorphic conditions of deformation events, including bedding-parallel shearing, imbricate thrusting, folding, cleavage development, tectonic mélange formation and extension. The strontium isotope systematics of vein carbonates provides new insights into the prograde to early retrograde tectonic evolution of the Alpine fold-and-thrust belt and helps to understand the relative timing of deformation events.

Radiogenic strontium isotopes (87Sr/86Sr) of vein carbonates play a crucial role in the tectono-metamorphic study of fold-and-thrust belts and accretionary wedges and have been used to document fluid sources and fluxes, for example, along major fault zones. Moreover, the 87Sr/86Sr ratios of vein carbonates can trace the diagenetic to metamorphic evolution of pore fluids entrapped in accreted sediments. Here we present 87Sr/86Sr ratios of vein carbonates from the paleo-accretionary complex of the central European Alps (Glarus Alps, Switzerland) that formed during early stages of continental collision. We show that the vein carbonates trace the Sr isotopic evolution of pore fluids from an initial seawater-like signature towards the isotopic composition of the host rock. This relation allows us to constrain the diagenetic to low-grade metamorphic conditions of deformation events, including imbricate thrusting, folding, cleavage development, stratal disruption and tectonic transport of thrust slices, bedding-parallel shearing, and extensional vein-formation. Taken together, the strontium isotope systematics of vein carbonate provides new insights into the prograde to early retrograde tectonic evolution of Alpine accretionary complex and helps to understand aspects that are not sufficiently clear from traditional cross-cutting relationships.