Changes in the composition of anatectic melt and its complementary residue by forward-modelling using THERMOCALC

Abstract

This paper presents results from forward modelling of graywacke and shale average bulk compositions assuming closed-system and open-system partial melting processes. For each process, several compositions of anatectic melt and corresponding solid residue are calculated, and the results compared in binary, ternary and normative diagrams to evaluate how their compositions evolve during an isobaric heating path. Melt compositions are peraluminous, with A/CNK increasing from 1.05 to 1.45, as P and T decrease. The mg# ranges from 0.22 to 0.39, increasing with consumption of biotite and other FeMg phases by melting reactions as T rises. On FeO + MgO, CaO + Na2O, K2O ternary, K2O vs. Na2O binary and chemical normative diagrams, compositions display trends with an increase in the K2O content of melt as T rises, and an increase in FeO + MgO content after the complete consumption of biotite. An increase in orthoclase component along the heating path, followed by an increase in the proportion of quartz component after K-feldspar/plagioclase breakdown is illustrated on mesonorm diagrams. Total melt volume produced by graywacke bulk compositions is in the range 33–53 vol% for closed-system and 21–33 vol% for open-system melting. For the shale bulk composition volumes are in the range 50–70 vol% during closed-system and 30–45 vol% during open-system melting. The rate of melt production varies with the type of melting reaction and is between 0.19 and 4.73 vol% °C−1 for water-present melting and 0.01–6.82 vol% °C−1 for hydrate-breakdown melting. Retrograde metamorphic assemblages can be formed even with the chemical depletion caused by melt extraction, but an external source of water and other components would make the consumption of the high-T assemblages more efficient. The correlation between data from this study and that from melt inclusions and natural leucosomes indicates that the melt compositions calculated via forward modelling are a useful guide in understanding how leucosome compositions and their mineral assemblages evolve.