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Per migma ad magma downunder
Author(s) -
White Allan J. R.,
Chappell Bruce W.
Publication year - 1990
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3350250305
Subject(s) - migmatite , geology , felsic , igneous rock , magma , geochemistry , partial melting , melt inclusions , volcanic rock , petrology , gneiss , mafic , basalt , metamorphic rock , volcano
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 2 O that is not lost from the migmatitic system. Because H 2 O 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.