The behaviour of the phengite 40Ar/39Ar system has been explored at different scales on Sifnos and Syros, two key islands of the Upper Cycladic Blueschist system (UCB). These islands expose well-preserved eclogites and blueschists that were exhumed via extensional collapse of the Aegean subduction/accretionary wedge during the Eocene and Miocene. Both islands record the switch from convergence to extension via a nappe-scale thermal-kinematic reworking marked by a sharp (~2 km thick) metamorphic transition from preserved high-pressure low-temperature (HP/LT) eclogites and blueschists at the top of the stack (peak P-T conditions of 22 ± 2 kbar and 550 ± 25 °C), to retrogressed greenschists at the base (overprinted at P-T conditions of 12–10 kbar and 500-450 °C). Extensive mapping of regional- to grain-scale 40Ar/39Ar age variations along this gradient reveals that phengite ages consistently decrease from 51 Ma down to 18 Ma towards the base of the stack, in parallel with progressive strain localization during exhumation and reworking by the regional greenschist ductile overprint. Homogeneous grain-scale Ar distributions are recorded in the poorly deformed topmost HP samples as Eocene (51–37 Ma) plateau ages and homogeneous in situ UV ablation ages, and by Miocene (20–18 Ma) plateaus in the intensely deformed basal greenschist samples. The homogeneous eclogite and blueschist ages are interpreted to record protracted crystallization in the subduction channel between 51 and 37 Ma, while the homogeneous basal ages likely record complete resetting and/or neocrystallization at 20–18 Ma. All intermediate samples across the metamorphic transition display variably complex age spectra and discordant in situ ages. Sub-grain scale Ar disequilibrium is ubiquitous in the form of within-sample and within-grain variations, with in situ (≪1 mm) variations locally encompassing almost the entire age span recorded regionally (up to 26 Myr). These variations occur mostly independent of microstructures and overlapping textural relationships. We interpret the bulk downward younging and associated internal discordance as the result of competing mechanisms of 40Ar retention and partial resetting operating during the regional greenschist thermo-kinematic overprint at the sub-crystal level. Grain-scale variations in solid-state Ar retention properties, fluid-phase dissolution/precipitation creep, and shear localization resulted in a mosaic of subdomains composed of partially reset to fully preserved relics of early HP clasts coexisting with neo-crystallized domains formed during metamorphic re-equilibration. This domain-like Ar behaviour operated largely below the spatial resolution of the UV probe (<50 μm), irrespective of phengite compositional variations, indicating that structural and thermal-kinetic crystallographic processes driving 40Ar and stoichiometric recombinations in phengite are coupled at a more elementary level, possibly the K-interlayer proper. A major outcome of this study is that it is clearly not possible to properly interpret 40Ar/39Ar phengite ages from HP/LT settings without integrating the information at all scales, from the crystal proper to the regional relationships. On Syros and Sifnos, the interplay between thermal-induced volume diffusion and strain localization is temporally and spatially related to the exhumation dynamics at the scale of the UCB, but these appear locally decoupled at the microscale. Such a complex behaviour is likely to be prevalent in other HP/LT successions exhumed elsewhere.