PARTIAL MELTING, PARTIAL MELT EXTRACTION AND PARTIAL BACK REACTION IN ANATECTIC MIGMATITES

Abstract

Anatectic migmatites commonly show both prograde (entropy producing) and retrograde reactions between minerals and melt. The final textures, mineral modes and mineral chemistries are affected by four successive processes: (i) prograde partial melting and small-scale segregation into melt-rich domains and restitic domains; (ii) partial melt extraction; (iii) partial retrograde reactions (back reaction) between in situ crystallizing melt and the restite; (iv) crystallization of remaining melt at the solidus, releasing volatiles.A new model is presented which combines the four successive processes. Partial melting is assumed to affect all textural elements of a migmatite unit in a closed system. Hence, the protolith (palaeosome) is separated into restite (now mesosome)+ melt. A batch melting model is assumed with segregation of all batches except the last. The segregated, but not extracted, melt back reacts only with the adjacent portions of the mesosome, resulting in a melanosome-leucosome pair. The last unsegregated melt batch, with a local volume fraction below the melt segregation threshold, back reacts with the surrounding mesosome. Any non-reacting melt crystallizes at or near the solidus, releasing volatiles. An important consequence of this model is that melanosome-leucosome-mesosome compositions do not necessarily show linear compositional trends in a closed system. This affects liquid compositions deduced from leucosomes, mineral modes and compositions as well as mass balance in migmatites and the possible granite-migmatite connection may therefore be blurred. Allowing water fluxes into and out of the system does not seriously affect the conclusions.A simple graphical analysis suggests that texturally observable back reaction between melt and restite may occur if certain conditions are fulfilled, most importantly: (i) fluid-absent, incongruent melting; (ii) crystallization far above the solidus; (iii) incomplete melt escape. Melanosome biotite is relictic in water-saturated partial melting as well as in reactions not consuming biotite. In other cases, melanosome biotite is partly relictic and partly produced during back reaction. The ratio of retrograde versus prograde biotite will thus depend on the protolith, on the melting reaction, and on kinetic factors.