Grades of metamorphism

Grades of metamorphism

1. GRADe OF METAMORPHISM SEMINAR ON

2. CONTENTS 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. GRADE OF METAMORPHISM 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 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 of metamorphism 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 .

10. Medium – grade metamorphism These grade having the temperature range up to from400oc to 550 ⁰ c 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.

11. Con…………. 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.

12. Blueschists Amphibolite ,

13. High grade metamorphism 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 H 2 O , 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.

14. Cont ……. 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.

15. INDEX MINERALS 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.

16. Isograde 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 .

17. 21 Reactions and appearance/disappearance of critical minerals can be used to determine P-T conditions.

18. 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.

19. 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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