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Metamorphic processes cannot be either observed directly or demonstrated practically. They are very gradual, large-scale processes involving geological times measurable in terms of millions of years. It is only through the study of their effects on the rocks that conclusions can be drawn regarding the nature of the process through which those rocks have passed.
Different rocks behave differently under given conditions of metamorphism. Metamorphic effects on many different types of rocks exposed in different areas of the world have been studied in sufficient details by many petrologists.
The following examples indicate only trends of changes observed in some common rock types as illustrations:
1. Igneous Rocks:
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(i) Acid Igneous Rocks:
These rocks are rich in free silica in the form of quartz (SiO2). Granite is a typical example of this group of igneous rocks. Granites are generally very hard and resistant rocks. They are effected only slightly under normal metamorphic conditions but when the degree of metamorphism is very high, quite a few new types of rocks are formed from granite.
Under conditions of dynamic metamorphism, granites are simply broken and crushed forming a newly structured rock called crush breccia. In other cases, the hard minerals of granite like quartz and felspars may be flattened and elongated under the imposed loads so that ultimately they remain embedded in the finely crushed ground mass produced due to crushing of other minerals of granite. Such metamorphosed granites are called flaser granites. Similar changes may be produced in gabbros and other coarse grained igneous rocks.
In many cases, the crushing effect due to imposed loads may be practically complete involving all the constituents. Such extremely crushed metamorphic rocks are described as mylonites.
Contact Metamorphism of granites involves, most commonly, simple recrystallisation of the constituent minerals. In regional metamorphism, however, when the heat factor is actively associated with pressure and chemically active fluids, granites and other related igneous rocks show very considerable and conspicuous changes.
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They may pass through a flaser-rock type stage under the influence of directed stress and suffer simultaneous and substantial recrystallisation due to intense heating and interplay of chemically active fluids. The net result may be a new foliated coarse-grained rock called gneiss.
(ii) Basic Igneous Rocks:
Formation of new minerals and imposition of new textures and structures are major changes shown by basic igneous rocks that have been subjected to metamorphism.
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Under contact metamorphism, only mineralogical reconstitution is observed while the original textures and structures are retained. Rocks like gabbros and dolerites having suffered such changes are described as meta-gabbros and meta-dolerites.
Under dynamothermal (regional) metamorphism, however, complex mineralogical changes are associated with development of foliation, crushing and granulation of mineralogical constituents. Rocks like gneisses and schists of great variety are produced. Hornblende schists and amphibolites are a few important examples.
2. Sedimentary Rocks:
(i) Argillaceous Rocks:
These are extremely fine-grained sedimentary rocks made up of fine particles of clays, felspars, quartz and micas. Clays and shales are typical examples of this group.
Contact metamorphism of shales leads to the formation of new minerals in them like corundum, rutile, andalucite and cordierite etc. through the process of recrystallisation. The resulting rocks are classed under the group name of hornfels. Under the conditions of optalic metamorphism, the shales simply get indurated (baked without much recrystallisation) and change into an exceedingly hard rock called hornstone.
When the shales are highly siliceous in nature, these are completely recrystallised into a very hard metamorphic rock called novaculite.
Under dynamic metamorphism, shales behave differently. As stress factor is dominating, not much new mineral formation takes place. The original constituents, however, rearrange themselves in such a way that a perfect orientation is introduced into the rock in which the maximum, medial and minimum diameters of the constituent grains all lie in parallel positions. This type of perfect parallelism at the grain-axis level results in a typical slaty cleavage. When shales develop such a cleavage due to intense pressure and good amount of heating, they result into a new rock type called slate.
When subjected to dynamothermal (regional), metamorphism, the changes in shale precede a step or two ahead of the slate stage; there is new mineral formation as well. The resulting rock becomes coarsely crystalline, the effect of recrystallisation becomes pronounced with the formation of micas, in addition to growth of crystals of garnet, staurolite, andalucite and other minerals.
This rock with pronounced rough parallelism of flaky and platy minerals is called schist, the structure being known as Schistose and the new property is called Schistosity. At an intermediate stage between schist and slate, the metamorphosed shale is known as phyllite. This rock is characterised by a good amount of micaceous minerals, which may not be as coarsely crystallized as in schists.
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(ii) Arenaceous Rocks:
These are sedimentary rocks composed chiefly of siliceous grains ranging between 2 mm and 1/16 mm in diameter. Sandstone is a common example of an arenaceous rock. When subjected to conditions of contact metamorphism, sandstones generally recrystallize into a new rock called Quartzite (para-quartzite, to distinguish it from sedimentary rock of similar composition, which is called ortho-quartzite).
Metamorphism of impure sandstone, like those containing argillaceous or calcareous cement results in Schistose-grits and other schistose rocks.
Under Dynamic metamorphism, sandstones are either deformed or crushed depending upon the quality of the rock and the magnitude of the stress. The resulting rock may be a crush-breccia or a micro-breccia or a mylonite.
(iii) Carbonate Rocks:
Metamorphism of limestones and other carbonate rocks is of considerable importance and interest.
Three broad conclusions may be drawn from the cases:
First:
When pure, limestone simply recrystallizes under conditions of contact metamorphism without any new mineral formation but with a definite change in grain size to a granular texture. The perfectly recrystallised granular metamorphic equivalent of limestone is the so well-known rock-MARBLE.
Second:
When the limestone is impure, recrystallization may involve formation of many new minerals in the resulting rock depending upon the nature of the impurities and also the temperature and pressure conditions. The underlying principle in all such chemical changes is that calcium oxide (CaO) is formed at the first step from dissociation of calcium carbonate due to heat factor. The calcium oxide so formed reacts with the impurities in the second step whereby many new minerals may be formed.
Thus, when limestone contains silica as the sole impurity, a new mineral called wollastonite (calcium silicate) may result as a metamorphic product. However, when alumina (Al2O3) and magnesia (MgO) are present as impurities, minerals like anorthite and an amphibole respectively may be the end products due to recrystallisation.
Following chemical reactions may show these changes:
Third:
When limestone is originally dolomitic in composition, metamorphic changes of different varieties may take place depending upon the type of the impurities present in the dolomite. This process of breaking down of dolomite accompanied by reconstitution of calcite is known as dedolomitization (compare dolomitization).
The changes involved in the process are expressed by the following equation:
