Assimilation is the process by which magma incorporates country rock. As magma passes through the crust, it can break off and melt pieces of the surrounding rock. There are three processes of assimilation: melting, diffusion and dissolution, and reaction. Assimilation requires large amounts of heat from the magma in order to melt the country rock. Evidence of assimilation can be seen in xenoliths and isotopic signatures within the igneous rock. Assimilation affects the composition and evolution of magmas.

2.  Introduction
 Crustal Assimilation
 Evidence for Assimilation
 Assimilation processess
1.melting, 2.diffusion and dissolution, 3.reaction
 How foreign rock matter are incorporated by magma
 Factors affecting assimilation
 Effect of assimilation
 Importance of assimilation as a petrogenetic process
 Conclusion
 References

3. Assimilation is the process of reacting of the magma with
the wall rock,whereby the country rock are incorporated in
the magma and eventually melt.
Magma rising from their site of origin react with or melt
and mix with the wall rocks in the magma chamber

5. As magma passes upward through the crust, pieces of the country
rock through which it passes may be broken off and assimilated
by the magma.
Contamination of magmas by country rocks may contribute xenoliths
and xenocrysts to the magma
The presence of xenoliths does not always indicate that assimilation
has taken place, but if the xenoliths show evidence of having been
disaggregated with their minerals distributed throught the rest of
the rock it is likely that some contamination of the magma has
taken place.
Xenoliths and inclusions within a plutonic or volcanic rock provide
evidence for the assimilation process.
The best evidence of assimilation comes from studies of radiogenic
isotopes such as the Rb - Sr system.

6. -Assimilation is a thermodynamic process involving the following principles;
Heat of solution = heat of melting + heat of mixing
where:
Heat of solution is the ability of a magma to dissolve an inclusion.
Heat of melting is the heat capacity of the solid phases plus the heat of
crystallization of the minerals involved.
Heat of mixing is the heat required to mix the phases.

7. Assimilation consist of 3 processess
1.Melting of wallrock
2.Dissolution and Diffusion of wallrock
3.Reaction of fluid and the wallrock

8. -Melting required considerable amounts of heat
-Heat required to melt the wallrock has 2 essential
component
1.Heat energy necessary to raise the temperature of the
wall rock to melting temperature.
2.The latent heat of melting must be added to the system
before the rock will melt.
If assimilation is to occur by melting then
1.The magma must be superheated to provide
substantial amount of heat without freezing.
2.Large quantity of magma must freeze to provide
enough heat to melt the wallrock.
3.Only hotter magma will assimilate cooler wall rock.

9. Diffusion - solid-state diffusion is very slow, therefore
the effect may be limited. However, the break-up of wall-
rocks (stoping) can significantly increase the surface-area
and diffusion can become effective.
Dissolution requires that the magma be undersaturated in
the component to be dissolved

10. Reaction of fluid and the wallrock
Reaction requires ionic exchange between the magma
and the wall rock
Reaction is favoured by chemical potential gradient in
contact zone
Reaction is governed by
1.The type of alteration that actually proceed
2.wall-rock composition.
3.Pressure and temperature of the environment.

11. Examples of Assimilation
•For a basaltic liquid assimilating a granitic solid consisting of quartz, feldspar
(plagioclase and alkali feldspar and biotite.
Q, F and B will be melted,
the heat for melting comes from the heat generated by the crystallization of olivine
and pyroxene from the liquid,
not from the temperature of the basaltic liquid.
The granite inclusions will not be completely melted ====> partial melting.
The end result is a basaltic andesite with inclusions of chewed up, partially digested
granitic material.
•For the reverse process where granitic liquid incorporates basalt, the anhydrous
minerals in the basalt (olivine, pyroxene and plagioclase) become altered to micas,
amphiboles and epidote by the addition of H2O.
The heats of crystallization of quartz, feldspar and biotite from a granitic liquid are not
large enough to melt the basalt inclusions, resulting in very minor changes in the
original liquid composition.
The result is a granite with amphibolite inclusions.
Any changes caused by assimilation and hybridization are dependant on the nature of
the inclusions and the nature of the magmatic liquid into which the inclusions are
emplaced.

12.  The incorporation of foreign rock matter by magma
occurs in three ways as
 (a) Mechanical incorporation without chemical
reaction.
 (b) Reactions involving partial solution of the
incorporated matter and the precipitation involving the
replacement of one solid phase by another.
 (c) Total dissolution involving total disappearance of
the solid phase.

13.  Temperature of the magma at the time of
intrusion
 Presence or absence of notable degree of heat.
 Composition of inclusion.
 Concentration of volatile in magma.
 Condition which facilitate or retard the escape
of volatile into the surrounding rock.

14.  It effect the crystallisation.
 assimilation effect, constantly changing the
composition itself.
 It effect Process of paste formation, migration,
evolution and consolidation.
 Assimilation effect not only can change the
composition of magma, and the magma
cooling, crystallization, promote the
differentiation
 Assimilation effect mainly occurs in granite
intrusive rocks.

15.  As an evidence for interaction between mantle derived
magma and the crustal rocks.
 As the phase relation that tends to produce the same
assemblages of crystal from spontaneous fractionation.
 In attributing the two important group of rock i.e. the
alkaline rock and the calc-alkaline series.
 In understanding the origin of igneous rocks and in
bringing about diversity in igneous rocks.
 Progressive change in the composition of lava.

16.  Assimilation is the process of reacting of the
magma with the wall rock, whereby the country
rock are incorporated in the magma and
eventually melt.
 As magma passes upward through the crust pieces
of the country rock through which it passes may be
broken off and assimilated by the magma.
 Assimilation required large amount of heat to take
place.
 Contamination of magmas by country rocks may
contribute xenoliths and xenocrysts to the magma
 Melting,diffusion and reaction are the process of
assimilation.

17.  1. Donald W.Hyndman(1972):Petrology of igneous and
metamorphic rocks,Pp
 Loren.A.Raymond:the study of petrology of
igneous,sedimentary and metamorphic rocks,Pp-92
 G.B.Mahapatra:Textbook of geology,Pp202-204
 H.S.Yoder:evolution of the igneous rocks,Pp307-335
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