Mineralogical evolution of contaminated granitic pegmatites hosted in marbles. The role of CO₂ rich fluids on phase relationships of crystallizing granitic melts. An example from the Intermediate Unit of the Central Rhodope Metamorphic Province, Greece

The Intermediate Unit of the Rhodope Metamorphic Province in Greece intervenes between the Lower (Pangaion) Unit with continental passive margin affinities, composed of orthogneisses of Permo-Carboniferous magmatic age overlain by amphibolite facies marbles and minor schists, and the Upper Unit dominated with 150 Ma metagranites. The Intermediate Unit is an assemblage of strongly deformed and variably migmatized lithologies of oceanic and trench affinities. An important component of the Intermediate Unit is a migmatized (diatexitic) biotite-plagioclase gneiss, intercalated with marbles, calc-silicate rocks and minor garnet amphibolites. The leucosome components of the migmatites, representing in situ melts, with granitic and quartz monzinitic compositions and of pegmatitic or aplitic textures, are hosted in the surrounding parental gneisses or in the neighbouring marbles. In the cases they are hosted in marbles they have the anhydrous assemblage: quartz + plagioclase + Kfs + diopside-hedenbergite cpx + titanite + scapolite + zircon. This paragenesis is interpreted to arise from the interaction of silicic magma with the carbonate host rock. In carbonate rich environments the fluid phase composition should be rich in CO₂. Fluid phase compositions X_CO2fl>0.5 have been reported in analogous cases where silicic melts are in contact or intrude carbonate rocks. Under these circumstances, imposing nearly anhydrous conditions of crystallization, silicic liquids of extremely high temperature are required in order to crystallize rocks with pegmatitic texture It has been experimentally established that for P=5 to 10 kb and X_CO2fl=0.7 the granitic Ts is 800 ºC, whereas it is above 950 ºC for X_CO2fl=0.9. This implies that melting of the gneisses took place at a temperature well above the wet granitic solidus. It is suggested therefore that (HP?) granulite facies conditions existed during the partial melting of the gneisses and the formation of the studied granitic and monzonitic leucosomes in this part of the Rhodope.