1. A SEMINAR PRESENTATION ON
MIGMATITE
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2. CONTENT:- PARTS IN IN-SITU MIGMATITE APPEARANCE OF MIGMATITE
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)
 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, cordierite, garnet, sillimanite and others.

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
Agmatic (breccia) structure Schlieren structure
Diktyonitic (net like)structure Nebulitic structure
Schollen (raft) structure
Phlebitis (vein) structure
Stromatic structure
Surreitic (dilatation) structure
Folded structure
Ptygmatic structure
Ophthalmitic (augen) structure
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 deformation

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. GRANITIC SYSTEM SiO2 - NaAlSi3O8 - KAlSi3O8 - H2O Tuttle and Bowen,1958
H2O is present “in excess”, so that H2O need not be represented graphically as a component.
The temperature axis is parallel to the edges of the prism.
PH2O = 2000 bars

24. Crustal melting (anatexis)
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.

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

26. 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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