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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.

<p>The nature and processes occurring at the subduction plate interface remain poorly constrained. In particular, the behavior of fluids and its impact on the rheology and the chemistry of the plate interface are mostly unknown. Based on detailed fieldwork, petrographic, geochemical analyses and thermodynamic modelling, the present study documents an example of a reacted “mélange” and metasomatism along the subduction interface: the Lia mélange zone on Syros island. Syros island is located in the Cycladic Archipelago in the centre of the Aegean domain which corresponds to the deepest exhumed parts of the Hellenides–Taurides belt. We show that this particular mélange zone is a disrupted yet still relatively coherent fragment of transitional lithosphere (i.e., OCT type from the Pindos Ocean), which has undergone dominant exhumation-related deformation with top to the east shearing. A large part of the “mélange” structure is inherited from the initial lithostratigraphic setting. Through detailed mapping and a statistical study of the nature of blocks and matrix we show that, as a first approximation, metasomatism occurs in contact between metavolcanite layers and serpentinite, with diffusion of Ca from the metavolcanites to the matrix and diffusion of Mg from matrix to metavolcanite. Most of the metavolcanite layers and blocks (mafic and carbonate) are mostly only partly digested but the ultramafic matrix has been largely metasomatised forming a tremolite-chlorite-talc schist, a “hybrid” rock, with an intermediate chemical composition. Geochemical data suggest that exhumation-related metasomatism is probably triggered and/or enhanced by the arrival of fluids from the dehydrating slab underneath. The Lia mélange zone shows that hybrid rocks can be formed by metasomatism along the subduction interface. Due to the absence of major tectonic mixing and of evidence of prograde reactions, this metasomatism may not be representative of deeper hybridization (as a potential source of arc volcanism). However, by changing the mineralogy of the matrix, the metasomatism changes the rheological properties of the mélange and thus could impact that of the subduction interface and the exhumation processes. This study highlights the significance of rock hybridization through metasomatism, largely in the context of a syn-convergent exhumation, along the slab interface and emphasizes its potential chemical and rheological impacts.</p>

Abstract. The high-pressure metagranite of La Picherais belongs to the Cellier Unit (part of the lower allochthon of the Champtoceaux Complex; Armorican Massif, western France), where it crops out as an undeformed body embedded within the orthogneisses of the Cellier Unit and is closely associated with numerous mafic eclogite lenses and seldom metahornfels. The petrographic observations of this metagranite reveal the presence of well-developed reaction textures: (1) pseudomorph after plagioclase, (2) garnet and phengite coronae at biotite–plagioclase interfaces, (3) garnet and phengite coronae at biotite–K-feldspar interfaces, and (4) garnet and rutile coronae at ilmenite–plagioclase interfaces, attesting that it underwent high-pressure and low-temperature conditions after the granite intrusion and its cooling. The analysis of the coronae and of a xenolith inclusion found in this granite points to pressure (P) and temperature (T) estimates of P>1.7 GPa and T=600–650 ∘C for the peak of metamorphism. P–T estimates performed on the mafic eclogite collected in the vicinity of the metagranite give values of 2.0–2.2 GPa and 640–680 ∘C, in good agreement with previous estimates made in other places within the Cellier Unit. The La Picherais metagranite is a key example of undeformed high-pressure metagranite allowing the study of the reactivity and degree of transformation of quartzofeldspathic rocks during subduction and constitutes a Variscan equivalent of the Alpine Monte Mucrone or Brossasco–Isasca metagranitoids.