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:
I. Differentiation
II. Assimilation
I. Differentiation. It may be defined as “the process whereby, a 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 Tyrrell there are two stages, in the
first stage, there is preparation of units such as crystals, liquid sub magma
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.
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.
(a)
free ionic diffusion of that substance from all parts of the magma,
(b)
by convection current with a concomitant 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 product of early crystallisation is
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 rises 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.
ASSIMILATION
Assimilation is
an important factor in bringing about diversity in igneous rocks. This is the
process whereby rock masses are in corporate 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 a 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 rocks formed in
this way is naturally, of hybrid origin, which are particularly common along the
borders between intrusive and invaded rocks.
Factors
affecting assimilation:
1. Temperature of the magma at the time of intrusion.
2. Presence or absence of notable degree
of superheat, i.e., the stage at which the inclusions are tapped in.
3. Composition of the inclusions.
4. Concentration of volatiles in the magma.
5. 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 disilicate the magma by crystallizing out various lime
silicates as melilite, 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.
Posts
0 Comments