Per migma ad magma downunder

Abstract

According to some early definitions, 'magma' had to be completely molten so that granitic rocks containing residual material from their source (restite) were not considered to be magmatic. Granites and volcanics with abundant recognizable restite were called migmatites or migmatitic rocks, and this terminology appears to be still in use by some. However, many felsic granites and volcanics commonly contain at least some restite, and for this reason we suggest that magma be defined either as melt or as rock material with enough melt for it to become mobile and intrusive or extrusive. The rocks crystallizing from these magmas are igneous. They are not migmatites. On the other hand, we suggest that migmatites be defined as rocks that did not have sufficient melt to become mobile; they retain a structural coherency even though the structures may be complex. Differences between the two rock types result from the amount of melt phase. When a critical melt fraction is reached during partial melting, there is a rapid change from migma to magma (migmatite to granite).The geochemistry and mineralogy of migmatites tell us little about reactions that produce more melt and hence about granite formation because of back reactions and other processes that occur during the cooling of migmatite. Back reactions mostly involve H,O that is not lost from the migmatitic system. Because H 2 0 is lost by processes of second boiling and pressure quenching during the crystallization of granite magmas at or near the surface, it is these rocks and their inclusions, not migmatites, that provide the most information about sources and processes of granite formation.

Geochronology in migmatites – a Sm–Nd, U–Pb and Rb–Sr study from the Proterozoic Damara belt (Namibia): implications for polyphase development of migmatites in high‐grade terranes

“…Furthermore, migmatites are of interest because of their inferred role as a link between high‐grade metamorphism and the generation of larger‐scale granitic bodies (e.g. Brown & D'Lemos, 1991; for a different view see White & Chappell, 1990).…”

Metamorphism today: new science, old problems

“…Discussions still continue on the mechanisms by which these partial melts are collected to form massive intrusions, able to rise and cause contact metamorphism in the upper crustal levels. It should be noted that this process is only valid for some granites (S-type granites; White & Chappel 1990), whereas others (I-type granites) have no relation to a metamorphic environment and are best explained by magmatic differentiation. Bowen, who always claimed that magmatic differentiation was the only process by which all magmatic rocks are created, was not wrong, but he had missed the metamorphic counterpart.…”

Formation and evolution of granite magmas from migmatites :

“…Ashworth 1985, Olsen 1985, 1991, White & Chappell 1990, Clemens & Mawer 1992 because, in their studies, there appeared to be a discrepancy between the composition of granite and that of the leucosomes in the associated migmatites (see Olsen 1991). However, an intimate genetic connection between migmatites and granites was found by Read (1956), Wickham (1987), Sawyer (1998), Brown & Solar (1999), Castro et al (2000), Vanderhaeghe (2000Vanderhaeghe ( , 2001, Solar & Brown (2001), Ledru (2001), Milord et al (2001) and Slagstad et al (2005) who demonstrate that the partial melts formed in migmatites were able to segregate and coalesce to form a granitic magma, from which a granite pluton crystallized.…”