FLUID CONTROL OF CHARNOCKITIZATION

The reactions Bt+Qtz=Opx+Kfs+H2O, Bt1+Qtz+(K2O)=Or+Bt2+(n H2O), Mg-Ts(Opx)+Qtz+(K2O) = Or+En, Pl1+(K2O)=Pl2+Or+(Na2O) and Prg+En+Qtz+(K2O)=Or+Ed recorded in unique rock textures and mineral compositions from gneissic complexes of some granulite facies terrains have been studied. It is shown that chemical potentials of the perfectly mobile CO2, H2O and K2O in a metamorphic fluid govern the charnockitization process along the retrograde P-T paths typical of granulite facies rocks. The sensors indicating the contribution of thermodynamic parameters to the process are discussed. In many complexes charnockitization took place al a a(H2O)fl similar to or even higher than that for initial gneisses. The origin of arrested charnockites in the Precambrian gneissic complexes is considered to be CO2-H2O fluid/gneiss interaction, which causes migration of the majority of the basic components (MgO, FeO, CaO, TiO2 etc.) from initial metamorphic rocks toward a transition zone; the scale of the migration depends strongly on the intensity of the interaction. The migration of the elements from a given rock volume propagates the shift of its bulk composition toward the charnockite (Kfs + Qtz + Fs) eutectic and subsequent melting. This model may explain the existence of zoned metabasite-enderbite-charnockite complexes of Precambrian granulite facies terrains. The arrested charnockites are a small-scale model for the formation of these complexes.