Migmatite

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.They can also
be known as diatexite.
Migmatite: A composite silicate metamorphic rock, pervasively heterogeneous on a meso- to
megascopic scale. It typically consists of darker and lighter parts. The darker parts usually exhibit
features of metamorphic rocks where as the lighter parts are of igneous-looking appearance. Wherever
minerals other than silicates and quartz are substantially involved.
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. 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 . The light-colored material has the
appearance of having been mobilized or molten.
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.
A leucosome is the lightest-colored part of migmatite.The melanosome is the darker part, and
occurs between two leucosomes or, if remnants of the more or less unmodified parent rock (mesosome)

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are still present, it is arranged in rims around these remnants. When present, the mesosome is
intermediate in color between leucosome and melanosome.
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.

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 Structures of migmatite
(1) Agmatic (Breccia) structure
Agmatic structure consists of angular fragments of paleosome (gneiss and schist) and they are
intruded by net-like quartzofeldspathic vein. It has the appearance of a breccias. It may be assumed that
the vein like neosome originated by metasomatism along fissures. There are two possibilities, they are
simple brecciation and simultaneous filling of the veins, the so called “forceful intrusion” or injection of
magma or magmatic differentiates such as pegmatites, aplites and partial or relective resolution or
replacement of the wall rocks, starting from thin joints or cracks which there by grew wide .

(2) Schollen (raft structure)

The fragments of the paleosome are generally smaller than in the preceding types and are often
somewhat rounded, thus floating like “rafts” (German : “ Schollen”) in the homogeneous or slightly
heterogeneous neosome. They often exhibit deformation structures due to shearing and rotational
movement. The fragments are partially dissolved in the neosome showing indistinct borders. Such
irregular, rotated and deformed inclusions are typically developed within the border zones of
inhomogeneous granite massifs. Neosome is interspersed by numerous small, dark, generally flattened
lens- to podlike bodies was called “forellen” migmatite (German “Forelle” = trout, referring to the shape
of the dark patches) by ANGEL and STABER (1952). By some it is regarded as formed from more or
less resorbed fragments of a formerly continuous paleosome with local concentrations of the
melanosome by metamorphic or anatectic differentiation.

(3) Diktyonitic structure

The paleosome is cut by net-like narrow veins of the neosome. This type of structure is called
“Diktyonitic structure”. The palesome is gneiss or schist and the neosomes is granitic materials. The
explanation for this structure is that the formation of shear-zones appears to be contemporaneous with
the vein-filling process. The vein-filling process is either local re-melting or replacement partially or
predominately in the solid state.

(4) Phlebitic (vein) structure

In this structure, the paleosome is irregularly transversed by the vein-like neosome. This type of
structure originated by shearing stress principally as a system of shear fractures. The paleosome is
biotite schist and neosome is aplite. This can be seen by the dragged ends of the neosomes and their
parallel arrangement with respect to a system of shear zone.
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(5) Stromatic (layered) structure
In this structure the neosomes form light and dark layers of paleosome generally parallel to the
plane of schistosity, The paleosome is either banded biotite gneiss or schist and neosome ranges from
pegmatitic to aplitic composition, and either foliation or non-foliated. The neosomatic layers are
thickened and thinned out irregularly. The layering may be “ lit-per-lit” in nature, or in someplaces
metamorphic differentiation may give rise to stromatic migmatite. The leucosome are not closely
spaced, which indicates that the migmatisation is not intense. These layered or banded structures are
known as “Stromatic (layered) structure” by Mehnert (1968).
(6) Surreitic (dilation) structure
This structure comprises a number of phenomena of joint original and partial remobilization.
This structure has been described by Ramberg (1955, 1956) as follows : “Since differences in
mechanical strength are very commom, especially in layered metamorphic rocks, dilation parallel to the
layering is not rare, a type of structure can often be observed in a more or less characteristic pattern,
called “pinch and swell structure.” “Pinch and swell” structure is one of the common surreitic structure
of layered migmatite. The palesome is biotite gneiss and neosome is leucogranite. Perpetual alteration
of thickened and thinned neosome and paleosome characterized the structure. Another structure, in
which paleosome is hornblende biotite schist and neosome is leucogranitic.

(7) Folded structure

Folded structure is also common in the migmatite unit. This type of structure may be formed by
the shear folding accompanied by ductile deformation, which is characterized by thickened crest and
thinned limbs (Mehnert, 1968). The paleosome is gneiss or schist and leucosome is granitic material
which shows folding .The style of folding includes both simple and complex folding. Wynne-Edwards
(1963), described this structure as of the high grade migmatites consisting of very mobile materials. The
resulting folds are highly contored because the shear planes that from the folding pattern are folded in
themselves, and finally became more and more blurred by complex folding and refolding.

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(8) Ptygmatite structure
Ptygmatite structures exhibits highly disharmonic, extremenly tortuous flolds, the limbs of which
are thickenrd and the crests are thinned. The ptygmatic vein are mainly composed of quartzo-feldspathic
materials. The folds are formed by injection of a fluid phase into an anisotropic schistose rock.
According to Read (1927) and Buddington (1939), “Ptymatic folds” is of primary origin, i.e. it occurs
simultaneous with the formation of vein.
The injection material could be deflected by the planes of schistosity or by layers of differing
mechanical strength (Mehnert, 1968). The granitic leucosome injects into paleosome of hornblende
biotlte schist forming ptygmatic fold Ptgmatic structure is remarkably sinuous coarse and is thought to
be produced by injection into very plastic country rocks.

(9) Ophthalmitic (augen) structure

The feldspar porphyroblasts which are surrounded by biotite flakes are consistent with the
foliation plane of gneiss and schist of stromatic migmatite. This structure is called “ophthalmitic or
augen structure”. This can be observed “ ophthalmitic” neosome are composed of an aggregate of
feldspar quartz. Many occurrence the augen have developed within an ealier fabric and the formation
has been attended without any visible signs of general deformation by pure crystal growth by pushing
aside the adjoining minerals.The formation of these feldspar porphyroblasts is generally explained by a
process of metasomatic replacement called “Feldsparthization”.

(10) Stictolithic (fleck ) structure

The mafic minerals are concentrated in flecks leaving around them a halo or mantle poor in
mafites which thus appear as a light zone around a dark fleck. Such rocks are referred to as “stictolite”
(stictolith, from Greek “stictos” = flecky, spotted, dotted). The paleosome is generally fine-grained,
massive or gneissose, and obviously is not affected by the process of fleck formation. In the immediate
vicinity of the light halos the petrographic habit of the paleosome is exactly the same as at some distance
from the flecks. The fleck formation was described in detail by LOBERG (1963) from rocks in the
Vastervik area, southeastern Sweden. The dark core of the flecks mainly consists of biotite, andalusite,
or cordierite, associated with some quartz and plagioclase. The light halo or “mantle” is rich in light
minerals, i.e., quartz, potash feldspar, and plagioclase. Other types essentially contain hornblende in the
dark core, or even pyroxene, or zoisite, if the paleosome is richer in calcium than the normal paleosome
of generally leptitic composition. Quantitative analysis of the flecks shows that they were simply formed
by segregation of the dark components from the intermediate parent rock, leaving the light halo behind
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them impoverished in these dark components. Further considerations about the PT conditions during this
process of metamorphic differentiation. Another type of stictolithic structure of apparently more
advanced stage than the former consists of patches or clusters of dark minerals within pegmatoid veins
or schlieren. They generally consist of cordierite forming more or less idiomorphic crystals. Since the
bulk composition of the schlieren including the patches and clusters of cordierite as a rule corresponds to
that of the palesome, it can be assumed that cordierite was formed at the expense of biotite, the
prevailing dark mineral of the paleosome.

(11) Schliern structure

This type of migmatitic structure can be observed at Kanabaw Chaung. According to Mehnert
(1968), they are formed by the laminar flow, so that the original shape of heterogeneities within the
following medium may be completely deformed to more or less parallel streaks. It also proved a rather
hight degree of mechanical mobility, as is generally developed in migmatites of hig P-T condition in an
advanced stage of formation ( Mehert, 1968).

(12) Nebulithic structure

This structure occurs in the migmatite unit. It is observed at the closely contact with of the biotite
microgranite intrusion. In this structure, paleosome and neosome can here no longer be identified
separately. There are diffused portion of the rock to be distinguished by their slightly different mineral
content. This type of migmatite was named by Cole (1902) as “granitic gneiss” – the granite like gneiss
rock which is derived from previously foliated meta-sedimentary rock. Other synonymous names are
granite gneiss by Goldschmidt (1920), and “granitized gneiss” by Engle and Engle (1958).

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 a b

c d

e f

Figures show structures of migmatite in the area; (a) Stromatic structure, (b) Ptygmatic structure,
(c) Ophthalmitic structure, (d) Nebulitic structure, (e) Phlebitic structure, (f) Folded structure