Migmatite

Ptygmatic folding in migmatite
Migmatite on the coast of Saaremaa
Intricately-folded migmatite from near Geirengerfjord, Norway

Migmatite is a rock that is a mixture of metamorphic rock and igneous rock. It is created when a metamorphic rock such as gneiss partially melts, and then that melt recrystallizes into an igneous rock, creating a mixture of the unmelted metamorphic part with the recrystallized igneous part.[1] They can also be known as diatexite.

Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally molten material, and are not generally the result of solid-state reactions. Commonly, migmatites occur within extremely deformed rocks that represent the base of eroded mountain chains, typically within Precambrian cratonic blocks.

Migmatites often appear as tightly, incoherently folded (ptygmatic folds) dikelets, veins and segregations of light-colored granitic composition called leucosome, within dark-colored amphibole and biotite rich material called the melanosome. If present, the mesosome, intermediate in color between a leucosome and melanosome, is mostly a more or less unmodified remnant of the original parent rock (protolith).[2] The light-colored material has the appearance of having been mobilized or molten.

Arrangement of banded colors

A leucosome is the lightest-colored part of migmatite.[2] The melanosome is the darker part, and occurs between two leucosomes or, if remnants of the more or less unmodified parent rock (mesosome) are still present, it is arranged in rims around these remnants.[2] When present, the mesosome is intermediate in color between leucosome and melanosome.[2]

Textures

Migmatite textures are the product of thermal softening of the metamorphic rocks. Schlieren textures are a particularly common example of granite formation in migmatites, and are often seen in restite xenoliths and around the margins of S-type granites.

Ptygmatic folds are formed by highly plastic ductile deformation of the gneissic banding, and thus have little or no relationship to a defined foliation, unlike most regular folds. Ptygmatic folds can occur restricted to compositional zones of the migmatite, for instance in fine-grained shale protoliths versus in coarse granoblastic sandy protolith.

When a rock undergoes partial melting some minerals will melt (neosome, i.e. newly formed), while others remain solid (paleosome, i.e. older formation). The neosome is composed of lightly-colored areas (leucosome) and dark areas (melanosome). The leucosome lies in the center of the layers and is mainly composed of quartz and feldspar. The melanosome is composed of cordierite, hornblende and biotite and forms the wall zones of the neosome.[3]

Migmatite and the origin of granites

For migmatised argillaceous rocks, the partial or fractional melting would first produce a volatile and incompatible-element enriched rich partial melt of granitic composition. Such granites derived from sedimentary rock protoliths would be termed S-type granite, are typically potassic, sometimes containing leucite, and would be termed adamellite, granite and syenite. Volcanic equivalents would be rhyolite and rhyodacite.

Migmatised igneous or lower-crustal rocks which melt do so to form a similar granitic I-type granite melt, but with distinct geochemical signatures and typically plagioclase dominant mineralogy forming monzonite, tonalite and granodiorite compositions. Volcanic equivalents would be dacite, trachyte and trachydacite.

It is difficult to melt mafic metamorphic rocks except in the lower mantle, so it is rare to see migmatitic textures in such rocks. However, eclogite and granulite are roughly equivalent mafic rocks.

Etymology

The Finnish petrologist Jakob Sederholm first used the term in 1907 for rocks within the Scandinavian craton in southern Finland. The term was derived from the Greek word μιγμα: migma meaning a mixture.

See also

References

  1. ↑ Marshak, Stephen, Essentials of Geology, W. W. Norton 3rd Ed, 2009 ISBN 978-0393196566
  2. 1 2 3 4 Recommendations by the IUGS Subcommission on the Systematics of Metamorphic Rocks, Part 6. Migmatites and related rocks, p2.
  3. ↑ Mehnert, Karl Richard (1971). Migmatites and the origin of granitic rocks, Developments in Petrology. Elsevier.

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