Apr 28 2014
Migmatite is a rock composed of both igneous and metamorphic rocks. The rock derived its name from a Greek word which means “mixed rock”. It was first discovered by Jakob Johannes Sederholm in 1907. The rock contains alternating layers of light-colored bands, known as leucosomes, and dark-colored bands, known as melanosomes. Leucosomes are made up of light-colored minerals, such as muscovite, feldspar and quartz, and melanosomes are made of dark-colored minerals, such as biotite and amphibole. Migmatites may sometimes be classified into metatexites, having low degrees of partial melting, and diatexites, having high degrees of partial melting.
The heterogeneous nature of migmatite is due to the partial melting of pre-existing rock when exposed to high pressure and temperature. The light-colored layers have igneous characteristics, which denote that they have been crystallized from a melt. Migmatites are formed under extreme temperature conditions during prograde metamorphism. They are not produced as a result of solid-state reactions, and are crystallized from a totally molten material.
Migmatites generally occur in extremely deformed rocks covering the base of eroded mountain ranges. They are commonly related to gneisses and granulites, and are spatially associated to granitic intrusions. The partial melting of crustal rocks, resulting in migmatites, is also responsible for the formation of granite magmas and a banded rock, due to segregation of rigid restite, and partial melts upon differential stress.
Migmatites consist of clast-like bodies made of restite, which are the solid blocks of partial melt residue. Leucosomes in migmatites often appear to have mafic minerals, such as selvages, formed by the reaction with a coexisting fluid or the melt.
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