Everything about Migmatite totally explained
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.
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's 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.
Further Information
Get more info on 'Migmatite'.
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