Syntectonic fluids redistribution and circulation coupled to quartz recrystallization in the ductile crust (Naxos Island, Cyclades, Greece)

Abstract The presence of external fluids in metamorphic rocks has been shown to have a profound impact on rock rheology as high fluid pressure processes promote embrittlement and favor ductile deformation by recrystallization. Moreover, it has been proposed that brittle deformation guides fluid circulation and that intracrystalline deformation is responsible for fluid redistribution at the grain scale. Nevertheless, the amount of fluid present in the metamorphic ductile crust is debated and the nature of the interaction between fluids and recrystallization processes are not clearly identified. The aim of this study is to document the spatial distribution of fluid inclusions relative to microstructures in quartz grains and aggregates from veins sampled in amphibolite facies metamorphic rocks, exposed in the island of Naxos in the center of the Attic-Cycladic Metamorphic Complex in Greece. The veins, ranging from discordant structures with sharp contacts to totally transposed structures into the metamorphic foliation, display a large variety of microstructures and fluid evidences interpreted as recording exhumation processes through the ductile/brittle transition: (i) remnants of primary quartz grains contain abundant CO 2 -H 2 O fluid inclusions, decrepitated for the most part, distributed in clusters and in fluid inclusion trails, (ii) fluid inclusions with a similar composition are less abundant in recrystallized zones and in subgrains but are concentrated along grain boundaries indicating that grain boundary migration is responsible for redistribution of CO 2 -H 2 O fluids, (iii) subgrains of the last generation are almost devoid of fluid inclusions and are characterized by thick grain boundaries with abundant metamorphic fluids locally forming a continuous film. CO 2 -H 2 O fluid inclusions aligned in parallel, regularly spaced intragranular trails, locally rooted into grain boundaries, are interpreted as reflecting the spatial redistribution of these fluids in quartz slip planes owing to the increase of fluid pressure in grain boundaries. This proposition is corroborated by the parallelism between slip planes and fluid inclusion trails. Perpendicular sets of fluid inclusion trails formed at the junctions of subgrains suggest that redistribution of fluids along quartz slip planes contributes to the initiation of subgrain rotation recrystallization. Transgranular fluid inclusion planes oriented perpendicular to the regional mineral and stretching lineations of the host metamorphic rocks mark a transition from grain to rock scale brittle behavior associated with the infiltration of H 2 O fluid, mixing to various degrees with the initial CO 2 -H 2 O fluid already present in the metamorphic rocks. These features indicate that, in the ductile-metamorphic crust, fluid redistribution is intimately linked to recrystallization mechanisms allowing fluids circulation under lithostatic pressure.