Migmatite

A migmatite, or "mixed rock" in Greek, is a banded, heterogenous rock composed of intermingled metamorphic and igneous components. Veins, contorted layers, and irregular pods of silica-rich granite occur within the structure of foliated iron and magnesium-rich metamorphic rocks like gneiss , schist and amphibolite. Because metamorphic rocks form by recrystallizaton of minerals without melting , and igneous rocks like granite form by crystallization of minerals from molten magma , it is difficult to explain their coexistence in a single rock. It is clear, however, that migmatites form at the threshold between high-grade metamorphic recrystallization, and complete igneous melting. Migmatites were partially melted during formation.

Some migmatites appear to have formed by intrusion of liquid granitic melt into a preexisting banded metamorphic rock . In these examples, the granite inclusions have sharp contacts with the metamorphic bands, and cut across the metamorphic fabric in places. In other cases, the boundaries between metamorphic and igneous components are gradational, or indistinct, suggesting that at least some migmatites form during a single phase of partial melting and fractional recrystallization. Metamorphic and igneous petrologists have rigorously debated these two hypotheses regarding the formation of migmatites. As is sometimes the case, both hypotheses are probably correct, and some migmatites form in several phases of metamorphism and melting, while others from during a single phase.

Migmatites generally occur in plate tectonic settings where regional belts of continental crust have been subjected to very high temperatures and pressures. The metamorphic portion of most migmatites includes the minerals horneblende, plagioclase feldspar , and garnet. This mineral assemblage indicates so-called amphibolite-grade metamorphism typical of convergent plate tectonic boundaries where rocks are subjected to very high pressures, strong directional stresses, and high temperatures.

See also Plate tectonics

migmatite A coarse-grained, heterogeneous mixed rock consisting of: (a) a high-grade metamorphic component with a gneissose texture (see METAMORPHIC GRADE); and (b) an igneous component with a granite mineralogy and a foliated or unfoliated texture (see FOLIATION). Migmatites are found in high-grade metamorphic terrains where a sequence from high-grade metamorphic rocks through migmatites to granite bodies is often seen in the field. The granite component is thought to form by partial melting of the rock during extreme metamorphism. Migmatites may thus be a record of the initial stages in the generation of large bodies of granite magma and, as such, they would represent the high-temperature boundary between metamorphic and igneous rocks. Migmatites have an attractive appearance, often being marked with irregular small stripes or patches of contrasting shades ranging from almost white to dark grey, and are widely used as building stone, sometimes being polished for ornament.

migmatite ‘Migmatite’ is a textural term that is used to describe a coarse-textured metamorphic rock, usually of high grade, that contains pockets of crystallized melt, either in the form of small lenses or large sheets. The term was introduced by J. J. Sederholm in 1907. Migmatites have long been thought of as having been formed near the boundary between metamorphic and igneous conditions. For example, a migmatite could represent a gneiss in which some pockets of melt had begun to form. Some migmatites have now been shown to be the source regions of granite, from which granite melts have been extracted, leaving an unmelted residue containing a few patches of trapped melt.

Migmatites find uses as ornamental building stones, where their striped or patchy appearance is displayed to good effect, especially on polished surfaces.

David A. Rothery