Fluid record of rock exhumation across the brittleductile transition during formation of a Metamorphic Core Complex (Naxos Island, Cyclades, Greece)
Résumé
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 crystalplastic 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 approximate to 625 degrees C and 400MPa and then remobilized during subsequent ductile deformation resulting in various degrees of mixing of the two end-members with later trapping conditions of approximate to 350 degrees C and 140MPa. In contrast, brittle microcracks contain aqueous fluids trapped at 250 degrees C and 80MPa. All veins display a similar 13C pointing to carbon that was trapped at depth and then preserved in the fluid inclusions throughout the exhumation history. In contrast, the D 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 D between 56 parts per thousand and 66 parts per thousand, and (ii) V-B veins that are dominated by aqueous fluids of meteoric origin characterized by D between 40 parts per thousand and 46 parts per thousand. The retrograde PT 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 degrees C km1 at lithostatic pressure, and (ii) the brittle upper crust through which meteoric fluids percolate through a high geothermal gradient of 55 degrees C km1 at hydrostatic pressure.