Fluid inclusions trapped in quartz veins hosted by a leucogneiss from the southern part of the Naxos Metamorphic Core Complex (Attic‐Cycladic‐Massif, Greece) were studied to determine the evolution of the fluid record of metamorphic rocks during their exhumation across the ductile/brittle transition. Three sets of quartz veins (V‐M2, V‐BD & V‐B) are distinguished. The V‐M2 and V‐BD are totally or, respectively, partially transposed into the foliation of the leucogneiss. They formed by hydrofracturing alternating with ductile deformation accommodated by crystal‐plastic deformation. The V‐B is discordant to the foliation and formed by fracturing during exhumation without subsequent ductile transposition. Fluids trapped during crystal–plastic deformation comprise two very distinct fluid types, namely a CO2‐rich fluid and a high‐salinity brine, that are interpreted to represent immiscible fluids generated from metamorphic reactions and the crystallization of magmas respectively. They were initially trapped at ∼625 °C and 400 MPa and then remobilized during subsequent ductile deformation resulting in various degrees of mixing of the two end‐members with later trapping conditions of ∼350 °C and 140 MPa. In contrast, brittle microcracks contain aqueous fluids trapped at 250 °C and 80 MPa. All veins display a similar δ13C pointing to carbon that was trapped at depth and then preserved in the fluid inclusions throughout the exhumation history. In contrast, the δD signature is marked by a drastic difference between (i) V‐M2 and V‐BD veins that are dominated by carbonic, aqueous‐carbonic and high‐salinity fluids of metamorphic and magmatic origin characterized by δD between −56‰ and −66‰, and (ii) V‐B veins that are dominated by aqueous fluids of meteoric origin characterized by δD between −40‰ and −46‰. The retrograde P–T pathway implies that the brittle/ductile transition separates two structurally, chemically and thermally distinct fluid reservoirs, namely (i) the ductile crust into which fluids originating from crystallizing magmas and fluids in equilibrium with metamorphic rocks circulate through a geothermal gradient of 30 °C km−1 at lithostatic pressure, and (ii) the brittle upper crust through which meteoric fluids percolate through a high geothermal gradient of 55 °C km−1 at hydrostatic pressure.

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International audienceThis contribution presents an original study combining detailed mapping, petrography, whole-rock geochemistry and geochronological constraints on the recently identified LREE (Light Rare Earth Elements) occurrences associated with pegmatitic granite dykes (PGD) from the central Grenville (Lac Okaopéo region). These PGD intrude paragneisses or meta-igneous complexes with a REE mineralization hosted either in monazite-(Ce) or in allanite-(Ce) respectively. The investigated samples display peraluminous signatures and are dominated by a quartz+K-feldspar+plagioclase+biotite+monazite/allanite assemblage. Field relationships and the magmatic textures of the dykes combined with U-Pb dating of magmatic monazite grains at 1005.4±4.4 Ma and 996.7±5.3 Ma (concordant igneous ages) imply that the LREE-rich PGD were emplaced in a post-tectonic setting. Allanite-(Ce) and monazite-(Ce)-bearing PGD have ΣREE contents up to 9242 ppm and 7048 ppm, respectively. The allanite-rich assemblage is consistent with the petrographic assemblage of LREE-enriched PGD identified in the southwestern Grenville Province and elsewhere in the world, but this study constitutes the first evidence for a sole presence of monazite as LREE-bearing phase in strongly peraluminous PGD from the Grenville Province

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