What controls partial melting in migmatites?

Abstract

Abstract. The layers of six stromatic migmatites from Northern, Western, and Central Europe display small but systematic chemical and mineralogical differences. At least five of these migmatites do not show any signs of largescale metamorphic differentiation, metasomatism, or segregation of melts. It is concluded, therefore, that the compositional layering observed in most of the investigated migmatites is due to compositional differences inherited from the parent rocks. Almost isochemical partial melting seems to be the most probable process transforming layered paragneisses, metavolcanics, or schists into migmatites.The formation of neosomes is believed to be caused by higher amounts of partial melts formed due to higher amounts of water moving into these layers. The neosomes have less biotite and more K-feldspar, if K-feldspar is present at all, than the adjacent mesosomes. These differences are small but systematic and seem to control the access of different amounts of water to the various rock portions. Petrographical observations, chemical data, and theoretical considerations indicate a close relationship between rock composition, rock deformation, transport of water, partial melting, and formation of layered migmatites.

Melting and diffusion processes in closed‐system migmatization

“…This petrogenetic model differs from those of Mehnert & Biisch (1982) and Johannes (1988) by not involving water, and from that of Olsen (1982Olsen ( , 1985 by not involving the extraneous addition of fluids or magmas.…”

“…It is evident from the migmatites studied here and from those described by Olsen (1982) and Johannes (1988) that leucosomes are frequently enriched in microcline. In addition, the microcline crystals tend to be larger than the plagioclase and quartz crystals.…”

“…that leucosomes are formed where the original gneiss had a leucocratic composition. Johannes (1988) considered that the formation of leucosome melt is due to a concentration of water in the leucosome layers, an origin also considered by Mehnert & Biisch (1982). A metasomatic process was invoked by Misch (1968) and Olsen (1985), who also suggested the intrusion of magma from an external source.…”

“…The nomenclature used here is identical to that of Johannes (1988). The leucosome is the leucocratic layer formed during migmatization, and the melanosome is the selvage between the leucosome and mesosome.…”

“…Anatexis and segregation of melt into the leucosome was a model proposed by Winkler (1961) and Mehnert (1968). Johannes & Gupta (1982) and Johannes (1988) suggested an isochemical mode of origin, i.e. that leucosomes are formed where the original gneiss had a leucocratic composition.…”