It is widely accepted that there is only one parental magma of basaltic composition and all the different varieties of igneous rocks were supposed to have originated from this magma of uniform composition. The origin of diverse igneous rocks with regards to mineralogical composition and texture can be attributed to two causes:
It may be defined as “the process whereby, magma originally homogeneous splits up into contrasted parts, which may form separate bodies of rocks or may remain within the boundaries of single unitary mass”. The process of differentiation is usually favoured by two factors:
(a) Rate of cooling.
(b) Settling of early crystallized heavy minerals.
Stages of differentiation:
According to Tyrrel there are two stages, in the first stage, there is preparation of units such as crystals, liquid submagma etc. In the second stage the prepared units are separated and accumulate separately to form distinct masses.
Differentiation in an igneous magma involves processes like:
1. Fractional crystallisation.
2. Gravity separation.
3. Filter pressing.
4. Liquid immiscibility.
5. Gaseous transfer.
1. Fractional crystallisation:
With the cooling of the magma, crystallisation begins and earliest minerals start crystallising. Differentiation may be brought about by at least two distinct processes:
(a) The localisation of crystallisation aided by diffusion and convection.
(b) The localised accumulation of crystals in several different ways, with the concommitant segregation of the liquid magmatic residuum.
Crystallisation may be localised at a cooling margin, where the temperature is lower than the central parts of the magma. Thus two phases-a solid and a liquid are formed.
The concentration of the molecules of the growing crystals at the site of crystallisation is supposed to be due to (a) free ionic diffusion of that substance from all parts of the magma, (b) by convection current with a concommitant movement of other substances in the opposite direction. But these suppositions were later on found untenable.
During crystallisation, there is a tendency for equilibrium to be maintained between the solid and liquid phases. To maintain equilibrium, early formed crystals react with the liquid and changes in composition take place.
In case of plagioclase, for instance, the first formed crystals are those richest in lime; as reaction proceeds with falling temperature, the crystals become progressively sodic. Thus a continuous series of homogeneous solid solution is produced, which constitute the ‘continuous-reaction series’.
Certain ferromagnesian minerals on the other hand react with the melt to give rise to a new mineral with a new crystal- structure and a definite composition. Olivine, for example, may be transferred to pyroxene, and pyroxene to amphibole. Such abrupt changes constitute the discontinuous reaction series.
Certain minerals in igneous rocks are associated because they crystallize over the same range of temperature. Early high-temperature minerals of both series generally crystallize together. As a result while some minerals are characteristically associated with some specific minerals, others are incompatible with them.
‘Bowen’s Reaction principle’ illustrates how a primary basaltic magma may solidify as a gabbro or it may give rise to rocks varying from dunite through gabbro, diorite, tonalite, granodiorite to granite depending upon the degree of fractionation and the extent to which early formed minerals are removed from further reaction with the melt.
Thus two magmas of identical initial composition but cooling at different rate produces different rock types, in the absence of volatiles the normal minerals of the discontinuous reaction series cannot form.
The products of early crystallisation are concentrated at one end of a differentiation series and the products of later crystallisation, at the other end.
2. Gravitational settling:
It is the tendency of the heavy minerals to sink to the bottom and those having lower specific gravity than the melt rise up and float at the top of the magma chamber. The perfection of this process depends on the size, shape and specific gravity of individual crystals and also on the viscosity of the magma. Olivine seems to be the most important mineral affected by this process and its gravitational settling forms stratification in igneous rocks.
3. Filter pressing:
As crystallisation continues a loose mesh -or frame-work of crystals with residual liquid in the interstices will ultimately be formed. If, at this stage, deformation of the mass occurs, either by the lateral earth pressure or downward pressure of the lifted strata, the interstitial liquid will be squeezed out. The liquid will tend to move towards the region of least pressure. Thus, this process of separation of solid crystals from the fluid magma is known as filter-pressing and is found to be very helpful in bringing about effective and appreciable differentiation in magma.
4. Liquid immiscibility:
A mix of two different components may be homogeneous at a particular temperature, but with falling of temperature both of them become immiscible fractions and separate from each other by the difference in specific gravity. In a similar manner, components of an igneous magma may be perfectly miscible at higher temperature but with gradual cooling the magma mass may separate out into distinctly different and mutually immiscible components.
5. Gaseous transfer:
Being excellent solvents, volatile constituents continually go on collecting the otherwise sparsely disseminated metallic and non-metallic constituents as they rise upwards through the magma chamber. Again the escaping gas bubbles may attach themselves to growing crystals and float them upwards. The -volatile constituents are capable of making selective transfer of material from lower to higher levels. In this way, pronounced heterogeneity may develop in magma.
Thus, differentiation is a major process that is responsible for bringing about diversity in igneous rock masses.