Grade of metamorphism

1. SEMINAR ON

2.  INTRODUCTION
 GRADE OF METAMORPHISM
 VERY LOW-GRADE METAMORPHISM
 LOW-GRADE METAMORPHISM
 MEDIUM GRADE METAMORPHISM
 HIGH GRADE METAMORPHISM
 ISO GRADE
 CONCLUSIONS
 REFERENCE

3. INTRODUCTION
 The word "Metamorphism" comes from the Greek: meta = change,
morph = form, so metamorphism means to change form.
 In geology this refers to the changes in mineral assemblage and
texture that result from subjecting a rock to conditions such
pressures, temperatures, and chemical environments different from
those under which the rock originally formed.
 Metamorphism, may be defined as the mineralogical and textural
changes which occur in a rock in response to change in temperature
and pressure imposed on it.
 The term metamorphic grade is widely used to indicate relative
conditions of metamorphism.
 Metamorphic Grade is a scale of metamorphic intensity which
uses indicator minerals as geo-thermometers and geo-barometers.

4.  Metamorphic grades is a general term for
describing the relative P-T condition under which
the metamorphic rocks from.
 As the temperature and/or pressure increases on
a body of rock we say the rock undergoes prograde
metamorphism or that the grade of metamorphism
increases.
 Metamorphic grade refers to degree or state of
metamorphism.

5. The temperature , pressure increases with increase of
depth due to the GEOTHERMAL GRADIENT.

6.  As the grade of metamorphism increases, new minerals
becomes stable and crystallize.
 Tailley (1924) had identified four metamorphic grade,
such as:
Very low grade
Low grade
Medium grade
High grade

7.  Very low grade metamorphism defined to comprise the
temperature pressure field ,which is bounded on the low
temperature side by the beginning of metamorphism.
 Very low grade metamorphism occurs below 200⁰c
 It indicated by the first appearance of a non-sedimentary
truly metamorphic mineral and on the higher temperature
side by a number of reactions all of which have in common
the formation of ziosite
 Lawsonite is one of the diagonostic mineral of very low grade
metamorphism.
 Mafic rocks are the diagnostically significant rocks in very low
grade metamorphsm.

8. CHANGES FROM VERY LOW GRADE TO LOW GRADE
METAMORPHISM DEMONSTRATION
1 disappearance of lawsonite and formation of ziosite
(or) clinoziosite
2 disappearance of pumpellyite by reaction with
chlorite and quartz, producing clinoziosite beside
actinolite
3 At low pressure metamorphism where lawsonite
cannot from and reaction
4 Good crystallinity of illite equal to kubler’s valve
of 4.0

9.  Low grade metamorphism takes place at temperatures
between about 200 to 320oC, and relatively low
pressure.
 Low grade metamorphic rocks are generally
characterized by an abundance of hydrous minerals.
With increasing grade of metamorphism, the hydrous
minerals begin to react with other minerals and/or
break down to less hydrous minerals.
 Low grade metamorphic rock indicated by the begging
of green schist facies.

12.  These grade having the temperature range up to
from400oc to 550⁰c and is indicated by the index
minerals like staurolite and cordierite.
 Pressure variations play an important role in
determining the stability of various minerals formed
in this grade.
 Medium grade face the problem is mark the higher
temperature boundary of the low grade metamorphic
division against the medium grade division.

13.  The mineralogical changes that the characterize the
beginning of the amphibolite facies these changes
should preferably occur with in large pressure range.
 The mineral parageneses of low grade metamorphism
(greenschist facies ) which disapper at medium grade
(amphibolite facies ).
 under the medium grade temperature ranges the
formation of schists, blueschists, amphibolite,
greenschists generally occur.

14. Blueschists Amphibolite,

15.  High grade metamorphism takes place at temperatures
greater than 600oc -800oc and relatively high pressure.
 As grade of metamorphism increases, hydrous
minerals become less hydrous, by losing H2O, and non
hydrous minerals become more common.
 High grade metamorphism generally occurs at greater
depths their high pressure and high temperature can
act, it can also occur other than the high depth’s where
the high amount of temperature supplied by intruded
magma.

16.  A typical examples indicative of high grade
metamorphism is provided by the breakdown of
muscovite mica in the presence of quartz and
plagioclase.
 They depending on (a)pressure and (b)absence of or
very low partial pressure of H₂O this is significant in
granulite terrains.

18.  These grades are indicated by the presence of a set of
minerals that are called index minerals.
 The formation of different set of minerals restricted to
different grades, by study of these minerals we can
find out the metamorphic grade of the particular rock.
 These minerals are stable only with the temperature
pressure range considered characteristic of that
particular grade.

20.  Isograd = ‘’line’’ to indicate same grade of metamorphism.(First appearance
of mineral of increasing metamorphic grade.)
 This concept has been found very convenient in tracing the progress of
metamorphism in the given region.
 In practive an assemblage of index minerals rather an individual mineral is
used for drawing isograde .
 Another term isoreaction grade is some time used when similar reaction as
indicated by mineral assemblage at different place in a metamorphosed area
are clearly understood .

21. 21
 Reactions and appearance/disappearance of critical minerals can be used to
determine P-T conditions.

22. CONCLUSION :
Metamorphic rocks are formed from the older rocks when they are subject to
increasing T,P, & chemically active fluid.
The degree or intensity of metamorphism that has affected a rock, is called
grade of metamorphism
Tilley has described as four types of grades
1.Very low grade
2.Low grade
3.Medium grade
4.High grade
Increasing the grade of metamorphism is also accompanied by increasing the
grain size of the rock.
Metamorphic Grade is a scale of metamorphic intensity which uses
indicator minerals as geothermometers and geobarometers.
A group of metamorphic rocks that have formed under the same set of physico-
chemical condition.

23. REFERENCE :
 Helmut G. H. Winkler 1915. Petrogenesis of metamorphic rocks , third
edition, pp:-66-81
 Turner F.J., 1981. Metamorphic Petrology, 2nd ed. McGraw-Hill, New York,
pp 524-528 .
 A text book of Engineering & General Geology-PRABIN SINGH-
2011(pp:320-322).
 Metamorphic petrology-B.BHASKAR RAO-1986, (pp:85-126).
 Principles of Engineering Geology-K.M.BANGAR-2010 (pp:202-205).
 Geology.com

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