Metamorphism with respect to the migmatites

1. Seminar on
MIGMATISATION
AND
METAMORPHISM

2. CONTENTS
Migmatisitation
Migmatite
Structures of Migmatites
Metamorphism
Association of metamorphism of pelitic rocks with
Migmatites
Occurences
Conclusion
Reference

3. MIGMATISATION
 The process of formation of a migmatite is called migmatisation.
The term “migmatite” was introduced by Sederholm(1908) to
designate certain gneissic rocks which looked like mixed rock .
 It is created when a metamorphic rock such as gneiss partially
melts, and then that melt recrystallises into an igneous rock,
creating a mixture of the unmelted metamorphic part with the
recrystallised igneous part.
 Migmatites form under extreme temperature conditions during
prograde metamorphism, where partial melting occurs in pre-
existing rocks. Many migmatites probably represent the partial
fusion of the metamorphic host during extreme metamorphism.
 Migmatite also can form near large intrusions of granite when
some of the magma is injected into the neighbouring
metamorphic rocks.

4. Mehnert classified the following parts that can generally be
distinguished in insitu migmatites:
1) Paleosome-unaltered or slightly modified parent rock or country
rock i.e., a gneiss.
2) Neosome-newly formed rock portion.
a) Leucosome-Containing more light minerals with respect to the
paleosome.
b) Melanosome-containing mainly dark minerals such as biotite,
hornblende, cordierite, garnet, sillimanite and others .
3) Mesosome- Rock portion of a migmatite,that is intermediate in
colour between leucosome and melanosome. If present, the
mesosome is mostly a more or less unmodified remnant of the
parent rock (protolith) of the migmatite.

5. PALEOSOME
MELANOSOME LEUCOSOME

6. MIGMATITE
 Migmatite is a rock that is a mixture of both metamorphic and
igneous rock. 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.
 Migmatite is a rock composed of a metamorphic (altered ) host
material that is streaked or veined with granite rock. The name
means mixed rock.
 Such rocks are usually gneissic (banded) and felsic rather than
mafic in composition. They may occur in regional scale in areas of
high grade metamorphism.

7. STRUCTURES OF MIGMATITES
DIETZONITIC(NET) STRUCTURE
 Paleosome is
interlocked by
net-like narrow
veins of
neosome.
Two sets of Granite veins combine to form
a net-like pattern.
Broken Hill area, New South Wales,
Australia.

8. SCHOLLEN(RAFT) STRUCTURE
 Paleosome occur
as raft-like
fragments
partially dissolved
in the neosome
and showing
distinct borders.
The rafts are paleosome which occur in a
coarse-grained, light coloured host.
Ashuanipi subprovince, Northern Quebec,
Canada.

9. PHLEBITIC(VEIN) STRUCTURE
 The paleosome is
irregularly traversed
by veins of
neosomes hence
resulting in a veined
structure.
Veins of light coloured neosome traversing
dark coloured paleosome.
Nemiscau, North Quebec, Canada.

10. STROMATIC (LAYERED) STRUCTURE
 The light coloured
layers of leucosomes
and dark layers of
neosomes generally
occur parallel.
The neosomes are
not entirely even,but
thicken and thin out
irregularly.
The thickest leucosome is at the bottom and it
has the largest grain size.
Glenelg River Complex, Victoria, Australia.

11. SURREITIC (DILATION) STRUCTURE
 Dilatent sites are
produced as crystals
in the paleosome
were pulled in a
direction parallel to
the foliation in the
host.
Tabular segments of leucosome which are
oriented parallel in mafic paleosome.
Georgian Bay, Grenville Province, Canada.

12. AGMATIC (BRECCIA) STRUCTURE:
 Agma means
fragment. Here
fragments of
paleosome are
surrounded by
narrow veins of
neosome.
 Brecciation is due to
simple fracturing of
paleosome.
Dark coloured paleosome surrounded
by veins of neosome
Quetico Subprovince, Canada.

13. FOLDED STRUCTURE
 It is formed by
compressional
pressure acting on a
rock.
 Here the
incompetent layers
are bent in contrast
to competent layers
thus resulting in
buckling and
bending of layers.
The light coloured neosome are bent in
contrast to grey coloured paleosome.
Colorado Front Range, U.S.A.

14. PTYGMATIC STRUCTURE
 Here The folds
are highly
disharmonic
which thicken and
thin out
irregularly.
The fold at the bottom is thickened and at the
top it is thinned out.
Saint Fulgence, Grenville provine, Canada

15. OPTHALMIC(AUGEN)STRUCTURE
 Here neosome
material is
distributed within
the paleosome in
the form of augen
(eye-like)
structures.
chitradurga schist belt, karnataka.

16. STICTOLITHIC (FLECK) STRUCTURE
 This is a rare
structure.
Here the mafic
minerals are
concentrated in
flecks leaving around
them a mantle poor
in mafic minerals.
Orthopyroxene flecks.
Southern closepet region , karnataka.

17. METAMORPHISM
 Metamorphism is the process by which pre-existing rocks are
transformed or altered in a solid state under relatively high
pressure, temperature, and/or hot circulating fluids.
 The effects of metamorphism are the formation of new minerals,
changes in shape and size of mineral grains, and the development
of new structures in the rock.

18. ASSOCIATION OF THE METAMORPHISM OF PELITIC ROCKS
WITH MIGMATITES
 Pelitic rocks are derived from clay-rich sediment and are of
particular importance in metamorphism because they develop a
wide range of distinctive minerals. The term ‘pelitic rocks’ is used
to signify all slaty or schistose rocks. They are usually alumina rich
rocks such as shales, mudstones.
 Migmatites are form in siliceous metasediments , metabasic
rocks etc but are best developed in pelitic rocks .
 The chemical composition of pelites can be represented by
the system K2O-FeO-MgO-Al2O3-SiO2-H2
 The chemistry of pelitic metamorphic rocks change with
increasing metamorphic grade. Fe, Mg and Ca contents increase,
and K content decreases with increasing metamorphic grade.

19. • Most pelitic rocks are foliated type in composition
with quartz and feldspars. These are formed due to
the regional metamorphism
Eg: Slate, Phyllite, schist , gneiss.

20. OCCURENCES
 The bulk of the earth’s migmatites occur in extensive terranes,
often of Precambrian age. Eg: Eastern Ghats, Orissa.
 The higher Himalayan crystalline in Sikkim consist of pelitic
migmatites interlayered with calc- silicate rock.
 Migmatised amphibolite, a few metres are traceable in the
supracrustal rocks and in major part of peninsular gneiss of
Archean dharwar craton of southern india.
 Cu can be found in migmatitic terrain at Kalyadi and Aladhalli in
Karnataka.
 Kolar district consist of immense expanse of migmatitic gneiss.
 Migmatites and gneisses are found in Thirthahalli taluk, Shimoga
district .

21. CONCLUSION
 The process of formation of a migmatite is called migmatisation.
 Paleosome, Neosome, Mesosome are some important terms
used in migmatites.
 Metamorphism is the process by which pre-existing rocks are
transformed or altered in a solid state under relatively high
pressure, temperature, and hot circulating fluids.
 The term ‘pelitic rocks’ is used to signify all slaty or schistose
rocks. They are usually alumina rich rocks such as shales,
mudstones.
 The chemistry of pelitic metamorphic rocks change with
increasing metamorphic grade. Most pelitic rocks are foliated
type in composition with quartz and feldspars. These are
formed due to the regional metamorphism.

22. REFERENCE
 Helmut G. Winkler, 1976, Petrogenesis of metamorphic rocks,
springer-verlag New York Inc. Pp 278-282.
 Myron G. Best, 1986, Igneous and Metamorphic petrology, W.H.
Freeman and company, New York, Pp 120,393, 425, 533.
 ASHWORTH, J.R. (1985): Migmatites. Ashworth, J. R. (editor).
Blackie, Glasgow, 302 pp.
 MEHNERT, K.R. (1968): Migmatites and theorigin of granitic
rocks. Developments inPetrology 1. Elsevier, Amsterdam, 393 pp.
 www.angelfire.com/sc3/farooqs/askot/migmatites.html.
 en.wikipedia.org/wiki/peninsular-gneiss.
 en.wikipedia.org/wiki/pelite