This study illustrates the field relationships of jadeitite-bearing block-in-matrix sequences on Syros and Tinos, Cycladic Blueschist Unit, and adds additional U–Pb zircon ages for jadeitites to the geochronological database. The results confirm the importance of Cretaceous (c. 80 Ma) and Eocene (c. 50 Ma) processes in their geological evolution. Interpretations suggesting that the jadeitites were formed by complete metasomatic replacement of a pre-existing rock are not fully supported by field observations. In at least some cases, the formation of jadeitite is likely due to precipitation from Na-Al-Si-rich aqueous fluids, which also caused variable metasomatic alteration of the host rock. Unambiguous age constraints for formation of the Syros and Tinos jadeitites are not available. A relationship to Eocene blueschist facies metamorphism recorded in the associated metamafic rocks seems plausible. However, since high-pressure overprinting of pre-Eocene jadeitite is also conceivable, there is a much larger time window for jadeitite formation, framed by Cretaceous (c. 80–76 Ma) protolith ages of various mélange blocks and the waning stages of blueschist facies metamorphism (c. 40 Ma). Field observations are consistent with the interpretation that the mélange-like occurrences on Syros and Tinos record, to varying extent, multi-stage processes that include detachment of mafic rocks from the subducting plate, local infiltration of Na-Al-Si-rich aqueous fluids, exhumation via a serpentinitic matrix in a subduction channel and reworking of the primary block-in-matrix fabric by sedimentary or tectonic processes during accretionary wedge formation.