Assimilation

Assimilation - petrogenesis

1. Seminar on “ASSIMILATION”

2. CONTENTS 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. INTRODUCTION 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

4. Crustal Assimilation The composition of the crust is generally different from the composition of magmas, there is always the possibility that reactions between the crust and the magma could take place. If crustal rocks are picked up, incorporated into the magma, and dissolved to become part of the magma, we say that the crustal rocks have been assimilated by the magma .

5. Evidence for Assimilation 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 of the wallrock -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. How foreign rock matter are incorporated by magma? 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. Factors affecting assimilation 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. Effect of assimilation 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. Importance of assimilation as a petrogenic process 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. Conclusion 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. References 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 Website - chestofbooks.com ›  Science  ›  Introduction To Geology www.britannica.com/EBchecked/topic/282318/... rock /.../ Assimilation www.preservearticles.com/.../ assimilation