Material transfer through the interface between peraluminous metapelite and gedrite-bearing gneiss at high temperatures and moderate pressures

Abstract

Relations between gedrite-bearing gneisses and aluminous metapelites were studied in amphibolite- and granulite-facies rocks in the Sharyzhalgai Complex, Eastern Sayan. The rocks show reaction relations and provide evidence of simultaneous diffusion of components through the interface between these chemically unequilibrated rocks, with the displacement of the boundary between them. In the vicinity of the gedrite-bearing gneiss, garnet and sillimanite of the aluminous metapelite are replaced (up to complete disappearance) by cordierite. The XMg of the rock and its minerals simultaneously increase. The diffusion coefficients of major oxides were evaluated as follows (cm2/s): TiO2 = 6.98 × 10−15, Al2O3 = 1.45 × 10−14, FeO = 9.70 × 10−15, MgO = 3.27 × 10−14. The thicknesses 2h (cm) of the zones of diffusion-controlled transfer of various components were estimated as follows: TiO2 = 6.6, FeO = 7.8, K2O = 8.5, Al2O3 = 9.5, Fe2O3 = 14.1, MgO = 14.3. The average calculated velocity of the interface is V = 4.2 × 10−6 cm/year. The relative mobility of major oxides was determined as follows: MgO > Fe2O3 > Al2O3 > K2O > FeO > TiO2.