Migmatite


Migmatite
Ptygmatic folding in migmatite
Migmatite on the coast of Saaremaa.

Migmatite is a rock at the frontier between igneous and metamorphic rocks. 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. Migmatites are composed of a leucosome, new material crystallized from incipient melting, and a mesosome, old material that resisted melting. 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. The light colored material has the appearance of having been mobilized or molten. Once enough leucosomes join up to form a network and granite is produced, the residual material is known as restite.

Contents

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 to 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.

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 sensu stricto 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 sensu stricto 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

  • Blatt, Harvey and Tracy, Robert J.; 1996, Petrology: Igneous, Sedimentary, and Metamorphic, 2nd ed., p. 463-466, W. H. Freeman, ISBN 0-7167-2438-3
  • Sawyer, Edward W. 2008. Atlas of Migmatites. The Canadian Mineralogist Special Publication 9." Mineralogical Association of Canada, Quebec; NRC Research Press, Ottawa. ISBN 978-0-660-19787-6

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Look at other dictionaries:

  • migmatite — [ migmatit ] n. f. • 1931; t. créé en suédois 1907, du gr. migma « mélange » ♦ Minér. Roche métamorphique, gneissique, d aspect mélangé, formée d une « trame » sombre (amphibole, par ex.) et d un apport magmatique clair (feldspaths, quartz). ●… …   Encyclopédie Universelle

  • Migmatite — Une migmatite correspondant à une métatexite. On observe deux rubanements. L un de couleur claire, l autre plus sombre Les migmatites (du grec : migma , mélange) sont des roches métamorphiques issues d anatexie crustale partielle. On les… …   Wikipédia en Français

  • migmatite — Rock composed of a metamorphic host material that is streaked or veined with granite; the name means mixed rock. Many migmatites probably represent partial fusion of the metamorphic host; some components of the rock are fused and gather to… …   Universalium

  • migmatite — migmatitas statusas T sritis chemija apibrėžtis Nevienalytė uoliena, susidedanti iš metamorfizuotos uolienos ir artimos granitams magminės medžiagos. atitikmenys: angl. migmatite rus. мигматит …   Chemijos terminų aiškinamasis žodynas

  • migmatite — noun any rock of mixed appearance, being an intimate mixture of granite and older rock, specifically from intense metamorphism which partially melts the rock, causing it to recrystallize in a state intermediate between igneous and metamorphic …   Wiktionary

  • Migmatite — Migmatịte   [zu griechisch mĩgma »Mischung«], Singular Migmatịt der, s, Mischgesteine, in ihrer chemischen Zusammensetzung und ihrem Gefüge zwischen metamorphen Gesteinen und Magmatiten stehende Gesteine mit einem unveränderten, hochmetamorphen …   Universal-Lexikon

  • migmatite — mig·ma·tì·te s.f. TS petr. roccia cristallina mista, prodotto di solidificazione del migma {{line}} {{/line}} DATA: 1957. ETIMO: der. di migma con 2 ite …   Dizionario italiano

  • migmatite — pl.f. migmatiti …   Dizionario dei sinonimi e contrari

  • migmatite — [ mɪgmətʌɪt] noun Geology a rock composed of two intermingled but distinguishable components, typically a granitic rock within a metamorphic host rock. Origin early 20th cent.: from Gk migma, migmat mixture + ite1 …   English new terms dictionary

  • migmatite — mig·ma·tite …   English syllables


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