limits of metamorphism

1. Metamorphisam
Sajith sadanandan
Assistant professor
Gems arts and science college

2. What is Metamorphisam
 process that changes the mineralogical and chemical composition, as well as
the structure of rocks
 Associated with elevated temperature and pressure,
 Affects rocks within the earth’s crust and mantle
 Metamorphism does not include, by definition, similar processes that
occur near the earth’s surface such as weathering, cementation and
diagenesis

4. example
 Coal (s) -> Anthracite
 Sand stone(s) -> Quartzite
 Lime stone(s) -> Marble
 Mud stone(s) -> Slate
 Basalt(i) -> Granulite
 Granite(i) -> Gneiss
 Shale(s) -> Schist

5. Conditions of Metamorphisam
 Temperatures at which metamorphism sets in are strongly dependent on the material under
investigation.
 Transformation of evaporites, of vitreous material and of organic material, for example, begins
to take place at considerably lower temperatures than chemical reactions in most silicate and
carbonate rocks. Eg: coalification (maturation) its not metamorphisam

6.  Transformations begin shortly after sedimentation and proceed continuously
with increasing burial. Whether such reactions are called “diagenetic” or
“metamorphic” ?????...
 Further examples of processes that are intimately related to metamorphism
include low temperature alteration of volcanic rocks, precipitation of mineral
coatings in fractures and low-temperature rock–water reactions that produce
mineral-filled veins and fissures
 Not metamorphic…..

7. Low temperature limit
 the low-temperature limit of metamorphism has been arbitrarily
 set to 150C ( +/- 50C)
 lawsonite, paragonite,
 prehnite, pumpellyite,

8. High temp limit
 Melting temperatures are strongly dependent on pressure, rock composition
and the amount of water present.
 granitic rocks begin to melt 660C will not melt below about 1,000C if no
water
 while basaltic rocks need 800C. If H2O is absent, basalt require > 1,120C to
melt
 The highest temperature reported from crustal metamorphic rocks are 1,000–
1,150C
 Such rocks are termed ultra high temperature (UHT) metamorphic rocks and
are typically magnesian- and aluminous-rich gneisses, e.g. Napier Complex
(Antarctica),

9.  Temperatures in the lower continental crust of geologically active areas are
inferred to be about 750–850C and the rocks produced under these conditions
are termed granulites.
 convecting mantle continuously undergoes metamorphic processes such as
recrystallization and various phase transformations in the solid state at
temperatures in excess of 1,500C.

10. Low pressure limit
 Ascending hot silicate magmas are a typical and globally important
occurrence in geologically active areas.
 The heat released by the cooling magma causes metamorphism in the
surrounding country rocks to produce so-called contact aureoles
 at shallow depth and pressures of a few megapascal.

11. High-Pressure Limit of Metamorphism
 Mantle rocks such as garnet peridotites (or garnet–olivine– pyroxene–
granofelses) from ophiolite complexes or from xenoliths in kimberlite record
pressures of>3–4 GPa (Gneisses with pure pyrope garnet containing coesite
inclusions)
 Consequently, it is possible to find and study rocks that formed 100–200 km
below the earth surface corresponding to pressures >3–6 GPa.(eclogites with
coesite inclusions in garnet and even diamond-bearing eclogites have
been reported)

13. Agents of metamorphisam
 Heat
 Uniform pressure
 Directed pressure
 Chemically active fluids and gases

14. temprature
The degree or intensity of heat present in a substance or object, especially as
expressed according to a comparative scale
Typically the most important factor in metamorphism

15. Increasing temperature has several effects
1) Promotes recrystallization Increased grain size
2) Drive reactions (Endothermic)
3) Overcomes kinetic barriers Equilibrium

17. Pressure
Continuous physical force exerted on or against an object by something in
contact with it.
Lithostatic Pressure
Overburden pressure, also called lithostatic pressure, confining pressure or
vertical stress, is the pressure or stress imposed on a layer of soil or by the
weight of overlying material

18. Normal pressure gradient is in two types:
1. High T/P geotherms in areas of plutonic activity of rifting.
2. Low T/P geotherms in subduction zones.

20. a stress component in a system which consist of unequal principal-stresses.
(Deviatoric stress = pressure unequal in different directions )
Resolved into three mutually perpendicular stress (sigma) components:
Sigma1 is the maximum principal stress
Sigma2 is an intermediate principal stress
Sigma3 is the minimum principal stress

22. A small volume of water and other fluid is present in
virtually in all rocks, and it is known as fluids and volatiles.
The presence of fluids makes metamorphism more easy.
Conversely, a dry rock is very hard to get to change. Without
the fluid chemical changes are just harder to take place.

23. Process of metamorphisam
Granulation
 The pressure shatters rock and the friction melts rocks partially
 This crushing of rocks without loosing coherence………..
Plastic deformation
Shifting of shape and regaining when a pressure is applied and removed
Recrystallisation
Formation of new mineral or crystal causing mineralogical and textural change

24. Metasomatisam
 Original composition of rocks have changed by addition or removal of material
 Change is caused by movement of hydrothermal fluid in high temperature and
pressure
 All above process are mutually dependent

25. Geo thermal gradient
•Geo thermal gradient is the rate of increasing temperature with
respect of increasing depth, in the earths interior.
•Amount the earths temperature increases with depth, indicating
heat flowing from the earth’s warm interior to it’s surface
•Away from tectonic boundaries.
•It is about 25-30°C/Km

27. Types of metamorphisam
 classification based on the principal agents (thermal, dynamic, dynamo-
thermal, hydrothermal);
 based on geological setting – contact, shock, high-strain, regional (burial,
ocean-ridge, orogenic);
 based on plate tectonic setting – metamorphism at convergent, divergent,
transform plate margins and plate interiors.

28. Orogenic
metamorphisam(dynamothermal)
 (Miyashiro 1973
 most significant type of metamorphism affecting the rocks of the continental
crust on a large scale
 Rocks subjected to orogenic metamorphism usually extend over large belts,
hundreds or thousands of kilometers long and tens or hundreds of kilometers
wide.

29.  Orogenic metamorphism and granitic plutons are often intimately associated.
 In the middle and upper crust, rising granitic magma carries heat and thereby
contributes to the increase of temperature on a regional scale leading to
typical high-temperature low-pressure terranes.
 In the lower to middle crust, the granitic magmas were generated by partial
melting as a consequence of high-grade metamorphism.

30. Types of orogenic metamorphism
 An early high-pressure low-temperature type metamorphism is related to a
subduction zone process
 a later younger regional metamorphism following a moderate P–T gradient is
related to continental collision
 metamorphic recrystallization in orogenic belts is accompanied by
deformation.

31.  Such metamorphic rocks exhibit a penetrative fabric with preferred
orientation of mineral grains.
 Examples are phyllites, schists and gneisses

32. Ocean floor metamorphisam(ridge)
 The ocean-floor metamorphic rocks are mostly of basic and ultrabasic
composition (basalt, peridotite).
 extensive veining and metasomatism, produced by the convective circulation
of large amounts of heated sea water

33. Burial metamorphism
 low temperature regional metamorphism affecting sediments and
interlayered volcanic rocks in a geosyncline without any influence of
orogenesis or magmatic intrusion.
 Lack of schistosity is an essential characteristic of resultant rocks
 cannot be sharply distinguished from deep-seated diagenesis.
 Original rock fabrics are largely preserved.

34. Contact metamorphism(thermal)
 takes place in rocks in the vicinity of plutonic or extrusive igneous bodies.
 Metamorphic changes are caused by the heat given off by the cooling igneous
body and also by gases and fluids released by the crystallizing magma.
 The zone of contact metamorphism is termed a contact aureole

35.  Rocks adjacent to small dikes, sills or lava flows are barely metamorphosed,
whereas larger igneous bodies give rise to a well-defined contact aureole of
metamorphic rocks.
 Contact metamorphic rocks are generally fine-grained and lack schistosity.
 most typical example is called hornfels

37. Cataclastic
metamorphisam(dynamic/high strain)
 confined to the vicinity of faults and overthrusts,
involves purely mechanical forces causing crushing and granulation of the rock
fabric
 is favored by high strain rates under high shear stress at relatively low
temperatures.
 resulting cataclastic rocks are non-foliated and are known as fault breccia,
fault gauge, or pseudotachylite.

38. Hydro thermal metamorphisam
 In hydrothermal metamorphism, hot aqueous solutions or gases flow through
fractured rocks causing mineralogical and chemical changes in the rock
matrix.
 Water–rock interaction and hydrothermal processes are
 particularly relevant for the making of ore deposits, rock leaching and
alteration, formation of vein systems and fissure deposits

39. Impact metamorphism
 the shock waves and the observed changes in rocks and minerals result from
the hypervelocity impact of a meteorite.
 The duration is very short, i.e. a few microseconds resulting in melting and
vaporization of the impacted rocks
 presence of shocked quartz
 high pressure coesite and stishovite as well as minute diamonds

40. Lightning metamorphism
 localised impact
 metamorphism caused by lightning strikes at very high temperatures that can
exceed 2,000C,
 the products of lightning metamorphism are typically glassy tubes called
fulgurites

41. Combustion metamorphism
 type of pyrometamorphism caused by spontaneous combustion of organic
matter, coal, oil and gas at or near the earth’s surface.
 Temperatures can be extreme, i.e., 1,000–1,500C,
 only a few meters thick, and more rarely may be up to several tens of meters,