Description of MIGMATITEs, Common structures associated with MIGMATITEs, and its occurrence.

1. MIGMATITE
GEY 305

2. CONTENT:-
 INTRODUCTION
 PARTS IN IN-SITU MIGMATITE
 APPEARANCE OF MIGMATITE
 STRUCTURE OF MIGMATITE
 ORIGIN OF MIGMATITE
 CONTROVERSIES IN ORIGIN OF MIGMATITE
 EXPERIMENTAL WORK
 GRANITIC SYSTEM
 CRUSTAL MELTING ( ANATEXIS)
 PETROGRAPHIC SIGNIFICANCE OF MIGMATITE
 OCCURRENCE OF MIGMATITE

3. INTRODUCTION
• The term “migmatite”
was first introduced by a
Finnish petrologist
Jakob Sederholm (1907)
for rocks within the
Scandinavian craton in
Southern Finland.
• The term derived from
Greek word “migma”
means mixed rock.
• Migmatite is a rock that
is mixture of
metamorphic and
igneous rock

4. PARTS IN IN-SITU MIGMATITES
Mehnert (1968)
1. PALEOSOME- Unaltered or slightly modified
parent rock or country rock.
2. NEOSOME - Newly formed rock portion.
It is generally of two types :
 Leucosome : containing more light
minerals (quartz and or felspar ) with
respect to the paleosome.
 Melanosome : containing mainly dark
minerals, such as biotite, hornblende,

5. APPEARANCES OF MIGMATITES
• Migmatites appear to represent the culmination
of high grade metamorphism under anhydrous
condition that characterize granulite facies.
• Migmatites are best developed in metapelites
but also occur in metamorphosed sandy and
arkosic sediments, mafic rocks, and granitoids.

6. • Minerals of the dark colored have
preferred orientation .
• Fabric of the leucosomes is
characteristic of a rock formed by
magmatic crystallization.
• The foliation characteristic of
gneisses is modified so that the
individual light and dark colored
layers may have a thickness of a
few centimeters or a tens of
centimeters or even meters.

7. STRUCTURES OF MIGMATITES
Typical migmatite structures
1. Agmatic (breccia) structure 11. Schlieren structure
2. Diktyonitic (net like)structure 12. Nebulitic structure
3. Schollen (raft) structure
4. Phlebitis (vein) structure
5. Stromatic structure
6. Surreitic (dilatation) structure
7. Folded structure
8. Ptygmatic structure
9. Ophthalmitic (augen) structure
10. Stictolithic (fleck) structure

8. Agmatic (breccia)structure
• Fragments of the
paleosome are surrounded
by relatively veins of the
neosome.
• Name derived from Greek
“agma” means fragment.
• Mostly their sharp edges
corresponds exactly in
outline i.e., their origin due
to simple fracture of the
paleosome.

9. Diktyonitic
(net like)structure
• The paleosome is interlaced by
net-like arrow veins of the
neosome.
• In contrast to the agmatic
fabric this structure exhibits
shear movement within the
country rock.
• For instance , structure of the
paleosome is bent parallel to
the adjacent veins in manner
of flexures.
• The mineral content of the
neosomes is generally granitic,
aplitic or rarely Pegmatitic.

10. Schollen (raft) structure
 Fragments of the
paleosome are generally
smaller than in the
preceding and somewhat
rounded, floats in the
neosome like rafts.

11. Phlebitic (vein) structure
• Paleosome is irregularly
traversed by the vein –like
neosomes
• Resulting structure has the
rough appearance of the
vein system of a human
body.

12. Stromatic(layered) structure
• Neosome form light
and dark layers in the
paleosome generally
parallel to the plane of
schistosity.
• As a rule the
neosomatic layers are
not entirely even, but
thicken and thin out
irregularly

13. Ptygmatic fold
• Ptygmatic structures
exhibit highly disharmonic
and extremely tortuous
folds.
• Normal granitic minerals
present in ptygmatic fold.
• The microfabric is always
plutonitic granular and
generally coarser than
host rock(gneiss)
• The folds were formed
from an original flat layer
or straight vein by

14. Ophthalmitic (augen)
structure
• Here the neosome is
distributed or rather
dispersed, within the
paleosome in the shape of
eyes
• The neosome consists of
felspar phenocryst which
are often surrounded by
mafic streaks (like eyelids)
consistent with the main
planes of schistosity

15. Nebulitic structure
• It carries ghost- like
relicts of pre –existing
rocks.

16. Schleiren Structure
• Dilation structure in
which the leucosome
fills openings in
stretched component
layers and schleiren,
stretched or sheared
irregular streaks of
melanosome that taper
at ends.

17. ORIGIN OF MIGMATITES
Two theories
Sedimentary source
pelites+quartzo-felspathic
Metamorphic gneiss
Partial melting
Migmatites
Igneous source
Granites
Partial melting
Migmatites

18. CONTROVERSIES IN ORIGIN OF MIGMATITES
• There are three principle theories
• Migmatites are formed by injection of granitic leucosome into dark high
grade schistose rocks.
• Migmatites form by localized partial melting (anatexis). The first melts
are granitoids which compose the leucosome. The melanosome is
generally considered to be the restitute or the somewhat refractory
residuum from which the melt were extracted.
• Migmatites are created by metamorphic differentiation or metasomatic
growth of the leucosome and melts are not involved.

19. EXPERIMENTAL WORK
 Well established observation of
geologists.
 Read formulates as follows: “ When
we follow rocks into higher
metamorphic grades, we finally end
in granitic core. This cannot be
accidential; the association of
metamorphites, migmatites and
granites must mean something.”
 The origin of granites and migmatites
in deep-seated parts of the orogenic
belts must be considered as directly
connected with the high-grade
metamorphism

20. • Granitic system is
essential to understand
the migmatites
formation.
• Granitic system-Qtz-
Albite-Orthoclase-H2O
system .

21. Crustal melting (anatexis)
(a) Simplified P-T phase diagram for
melting of dry and wet granite like
composition.
(b) quantity of melt generated during
the melting of muscovite-biotite-
bearing crustal source rocks
Shaded areas in (a) indicate melt
generation.

22. Petrographic significance of
migmatites
• In Mineralization
• Academic purpose- migmatites
presence in the field gives idea
about extreme conditions of
pressure and temperature in
metamorphism.

23. OCCURRENCES OF MIGMATITES
• The high grade metamorphic belts of the northern
Appalachians in New England comprise one of the classic
migmatite terrains of the world.
• Migmatites occur in the highest grade metamorphic zones in
roughly linear north – south belts running from Long Island
Sound to near Canadian Border at a distance about 500 km.
• In the High Himalayan Crystallines of Zanskar (NW India)
migmatites and leuco granites are found.

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