Assimilation is an important factor in bringing about diversity in igneous rocks. This is the process whereby rockmasses are incorporated by magmas; there is also commingling of two liquid magmas. Since these processes involve the re-mixing of rocks, they represent the reverse of the differentiation processes and heterogeneity results when the mixing is incomplete and non-uniform.
The laws of assimilation are governed by the same general laws of fractional crystallisation. Reaction between magma and wall-rock is a normal accompaniment to igneous intrusion. In the course of this reaction the magma becomes contaminated by incorporating materials originally present in the wall-rock. This broad process of modification is described as assimilation.
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.
In general it is a complex process of reciprocal reaction between magma and invaded rock. During the process of reaction, due to ionic exchange between liquid and crystals, minerals are changed into those crystalline phases with which the liquid was already saturated.
The end product is a contaminated igneous rock which was at no time entirely liquid and which is made up of materials contributed partly by the original rock and partly by the wall-rock. The rock formed in this way is naturally of hybrid origins, which are particularly common along the borders between intrusive and invaded rocks.
Factors affecting assimilation:
(i) Temperature of the magma at the time of intrusion.
(ii) Presence or absence of notable degree of superheat, i.e., the stage at which the inclusions are tapped in.
(iii) Composition of the inclusions.
(iv) Concentration of volatiles in the magma.
(v) Conditions which facilitate or retard the escape of volatiles into the surrounding-rocks.
Since the melt reacts with the minerals which are formed earlier at a higher temperature, and gives rise to the minerals which at the moment are in equilibrium; as in Bowen’s Reaction Series. These reactions are exothermic, that is, they proceed with the production of heat and not the absorption of it.
Only those inclusions made up of minerals belonging to the lower series can be directly dissolved; the heat required for dissolving the inclusions is supplied by the crystallisation of a thermally equivalent quantity of those phases with which the magma at that moment is saturated.
According to the above observations, a general rule has been enunciated (by Zirkel) “in acid magma acid inclusions are not assimilated but basic ones are ; likewise in basic magma basic inclusions are not digested but acid ones are”.
There are some petrochemical considerations underlie the reaction between magma and wall rock:
(a) Suppose, a magma of granitic composition has started reacting with the wall-rock of gabbroic composition; in such a case labradorite and augite of gabbro are earlier members than oligoclase and hornblende of granitic magma. A complex reaction takes place whereby the minerals from the walls of gabbro are changed into hornblende and oligoclase, minerals which are in equilibrium with the melt at that particular temperature. This is the assimilation of basic igneous rocks by acid magma.
(b) Assimilation of acid inclusions by basic-magma:
Basaltic magma is capable of melting acid igneous rocks, as its temperature is much above the melting point of acid igneous rocks. In such cases the members of the late-crystallization go into solution by the magma. To supply the necessary heat for fusion and reaction, an equivalent amount of the members of early crystallization get precipitated from the liquid.
(c) Assimilation of sedimentary-rocks by basic magma:
Since sedimentary rocks are mostly composed of quartz, alkali-feldspar, clay minerals and calcite which are low-temperature minerals, they are completely incorporated by basic magmas.
Assimilation of calcareous inclusions desilicate the magma by crystallizing out various lime silicates as melillite, garnet etc giving rise to a silica poor alkaline-residue and cause felspathoids to appear.
Incorporation of argillaceous matter may give rise to cordierite, sillimanite, spinel, garnet, anorthite etc.
Partial digestions of siliceous inclusions show reaction rims of augite, hypersthene, cordierite, plagioclase etc.
(d) Assimilation of sedimentary-rocks by acid-magma:
It leads to a parallel development of minerals in the xenolith and in the magma.
(e) Assimilation of metamorphic-rocks by basic magma:
Since metamorphic rocks are either altered igneous or sedimentary rocks, the effects of assimilation of such inclusions are guided by their temperature of formation as well as mineralogical composition.