Development of different light and dark coloured minerals.

1. METAMORPHIC DIFFERENTIATION
MID SEM ASSIGNMENT
SESSION – 2019-20
SUBMITTED TO - SUBMITTED BY –
PROF. M.K. YADAV GOPALJI GUPTA
M.SC. 2ND
SEM
ROLL NO. 190014215018
DEPARTMENT OF GEOLOGY
UNIVERSITY OF LUCKNOW, LUCKNOW

2. CONTENTS
1.INTRODUCTION
2.KINDS OF METAMORPHIC DIFFERENTIATION
• SEGREGATION
• COMPOSITIONAL LAYRING
3.FACTORS CONTROLLING CHEMICAL POTENTIAL
GRADIENT
4.MECHANISM OF METAMORPHIC
DIFFERENTIATION
• PRESERVATION OF ORIGINAL COMPOSITIONAL
LAYERING
• TRANSPOSITION OF ORIGINAL BEDDING
• SOLUTION AND REPRECIPITATION
• PREFERENTIAL NUCLEATION
• MIGMATIZATION

3. The most prominent macroscopic structure in regional
metamorphic is a foliation. This is commonly parallel to a
compositional layering and even in coarse grained gneisses,
granulites and charnokites is tacitly assumed to be bedding by
many workers although Not all bendings which are generally
found in the metamorphic rocks like layering, striping or
foliation in schist, gneisses, amphibolites, quartzites and
marble is necessarily relict bedding. It may be an entirely
metamorphic structure due to metamorphic differentiation or
it may be relict bedding which has been folded, transposed and
rotated around to coincide with a metamorphic foliation.
Metamorphic differentiation is a collective term for the
varius processes by which minerals or mineral assemblages are
localy segregated from an initially uniform parent rock during
metamorphism.
As we know that minerals differ in their ability to glide,
thus gneissic or layered ores may develop by metamorphic
differentiation. Thus there is development of pseudo-
stratification by the metamorphic differentiation process.
Metamorphic differentiation is redistribution of mineral
grains and/or chemical components in a rock as a result of
metamorphic processes. Metamorphic process by which
mineral grains or chemical components are redistributed in

4. such a way to increase the modal or chemical anisotropy of a
rock (or portion of a rock ) without changing the overall
composition of the rock.
Metamorphic differentiation appears to act contrary to most
metamorphic processes which tends to flatten out chemical
gradients. It involves an increase of order from more
disordered, homogeneous material and thus a decrease in
antropy.
There are two kinds of metamorphic differentiation which are
most important are –
1.Segregation under low to high pressure, but no direct
pressure, to produce segregates rich in one or more
minerals. The chemical concentration results from
gradients in chemical potential due to pressure
differences, to some original chemical discontinuity, to
difference in grain size and shape, to differences in ionic
migration and to other lesser known factors.

5. 2.The formation of a compositional layring parallel to a
metamorphic foliation. The layring is due to segregation
of light coloured minerals such as quartz and feldspar on
one hand, and of dark coloured mineral such as biotite,
hornblend, pyroxene, epidote, garnet etc. on the other.
This layring occurs in mylonite and is very comman schists.
Massive igneous rocks pass into layered greenschists and
amphibolotes
The gradients in chemical potential are created by 5 main
factors which are as follows –
1. Differences in Temperature
2. Differences in Pressure or non-hydroststic stress
3. Differences in the chemical composition of
minerals
4. Differences in the size of minerals
5. Differences in the surrounding media

6. There are several mechanisms to explain the
metamorphic differentiation.
1.Preservation of Original Compositional Layering. In
some rocks the compositional layering may not represent
metamorphic differentiation at all, but instead could
simply be the result of original bedding. For example,
during the early stages of metamorphism and
deformation of interbedded sandstones and shales the
compositional layering could be preserved even if the
maximum compressional stress direction were at an
angle to the original bedding.
In such a case, a foliation might develop in the shale
layers due to the recrystallization of clay minerals or the
crystallization of other sheet silicates with a preferred
orientation controlled by the maximum stress direction.

7. Here, it would be easy to determine that the
compositional layers represented original bedding
because the foliation would cut across the
compositional layering.
2.Transposition of Original Bedding. Original
compositional layering a rock could also become
transposed to a new orientation during
metamorphism. The diagram below shows how this
could occur. In the initial stages a new foliation begins to
develop in the rock as a result of compressional stress at
some angle to the original bedding. As the minerals that
form this foliation grow, they begin to break up the
original beds into small pods. As the pods are
compressed and extended, partly by recrystallization,
they could eventually intersect again to form new
compositional bands parallel to the new foliation.

8. 3.Solution and Re-precipitation. In fine grained
metamorphic rocks small scale folds, called kink bands,
often develop in the rock as the result of application of
compressional stress. A new foliation begins to develop
along the axial planes of the folds. Quartz and feldspar
may dissolve as a result of pressure solution and be
reprecipitated at the hinges of the folds where the
pressure is lower. As the new foliation begins to align
itself perpendicular to 1, the end result would be
alternating bands of micas or sheet silicates and quartz or
feldspar, with layering parallel to the new foliation.

9. 4.Preferential Nucleation. Fluids present during
metamorphism have the ability to dissolve minerals and
transport ions from one place in the rock to another.
Thus felsic minerals could be dissolved from one part of
the rock and preferentially nucleate and grow in another
part of the rock to produce discontinuous layers of
alternating mafic and felsic compositions
5.Migmatization. As discussed previously, migmatites are
small pods and lenses that occur in high grade
metamorphic terranes that may represent melts of the
surrounding metamorphic rocks. Injection of the these
melts into pods and layers in the rock could also produce
the discontinuous banding often seen in high grade

10. metamorphic rocks. The process would be similar to that
described in 4, above, except that it would involve partially
melting the original rock to produce a felsic melt, which
would then migrate and crystallize in pods and layers in the
metamorphic rock. Further deformation of the rock could
then stretch and fold such layers so that they may no longer
by recognizable as migmatites.

11. REFERENCES :-
• SPRY ALAN, METAMORPHIC TEXTURES (1974),
PERGAMON PRESS. (P. 9, 236, 237, 260, 287,
334)
• BWHER KART & GRAPES RODNEY,
PETROGENESIS OF METAMORPHIC ROCKS (8TH
ED.), SPRINGER. (P. 29)
• METAMORPHIC PETROLOGY CONCEPTS AND
METHODS , GSI PUBLICATIONS
• https://www.tulane.edu/~sanelson/eens212/
metatexture.htm