Metasomatism and deformation of block-in-matrix structures in Syros: The role of inheritance and fluid-rock interactions along the subduction interface

Abstract

The behavior of fluids at the subduction plate interface and their chemical and rheological impacts remain poorly constrained. Based on detailed fieldwork, petrographic and geochemical analyses and thermobarometry, the present study documents an example of a block-in-matrix ‘mélange’ metasomatized along the subduction interface (Kampos-Lia mélange zone, Cycladic Blueschists, Syros island). We show that this particular mélange zone is a preserved fragment of discontinuous oceanic domain associated with the Pindos basin, which underwent dominant exhumation-related deformation with top to the east shearing. A large part of the ‘mélange’ structure is inherited from the initial intraoceanic setting and should not be considered as a tectonic ‘mélange’. Through exhaustive, meter-scale mapping of the nature of blocks and matrix we show that metasomatism dominantly occurs at the contact between metavolcanic layers and serpentinite, with diffusion of Ca from the metavolcanics to the matrix and diffusion of Mg in the opposite direction. Most of the metavolcanic layers and blocks (mafic and carbonate) are only partly digested contrary to the ultramafic matrix which has been largely metasomatized and forms a tremolite-chlorite-talc schist, i.e. a ‘hybrid’ rock with intermediate chemical composition. Geochemical data demonstrate that metasomatic element mobilization (e.g. Li, B, U, LILE and REE) varies at the scale of the unit, suggesting a complex evolution of fluid composition at the kilometer scale. We suggest that the fluid which intruded the Kampos subunit was derived from the subducting slab and caused an enrichment in Li, B, U, LILE (Rb, K, Na, Ba, Eu), MREE and HREE. Mineralogical and P-T constraints indicate that the dominant part of metasomatism and rock hybridization occurred during exhumation, at ~ 1 GPa, well after peak burial and detachment from the slab (~ 2 GPa). Due to the absence of major tectonic mixing and the scarce preservation of prograde and peak metasomatic reactions, this metasomatism may be representative of fore-arc hybridization processes at depths of ~ 35 km. Deeper hybridization processes as hypothesized to produce a potential source for arc magmas are not well preserved in the studied examples. However, by changing the mineralogy of the matrix, metasomatism changes the rheological properties of the mélange and could thus manipulate the rheology of the subduction interface and influence exhumation processes.