Hygrochronometry of punctuated metasomatic events during exhumation of the Cycladic blueschist unit (Syros, Greece)

The spatiotemporal scales at which fluid flow events occur along the subduction interface remain poorly constrained. This study illustrates the benefit of using in‐situ Rb/Sr dating on metasomatic white mica to constrain the timing of metasomatic events, taking advantage of the well‐studied reaction zones of the Kampos‐Lia unit of Syros, Greece. This unit is a subducted block‐in‐matrix structure corresponding to a preserved fragment of a discontinuous, slow‐spreading oceanic domain. Results reveal that the main metasomatic event took place at ~36 Ma, coincident with the transition from blueschist to greenschist facies and syn‐ to post‐orogenic exhumation in the Cyclades marking the end of a first exhumation stage along the subduction interface. Results highlight that fluid–rock interaction along the subduction interface cannot be treated as the result of continuous fluid influx, but rather reflect punctuated, heterogeneously distributed events (both in time and space) tied to specific tectonometamorphic episodes.