The texture of the original rock which has undergone metamorphism is sometimes found to exist in the metamorphic rocks. Such textures are called ‘relict or palimpset texture.’ In describing metamorphic textures the terms ‘blastic’ or ‘blast’ are used as a suffix to represent the metamorphic equivalents of igneous textures of similar look.
Recrystallisation of minerals produces a ‘crystalloblastic texture’ which is similar to the holocrystalline texture of igneous rock. If during metamorphism a texture similar to porphyritic comes into existence, the same is described as ‘porphyroblastic texture’
In case of palimpset textures, if for example a porphyritic igneous rock is metamorphosed and the original texture continues to occur in metamorphosed ones, the resulting texture will be said to be ‘blasto-porphyritic’. Thus, palimpset textures are prefixed by ‘blasto’.
Similar to the igneous texture where the minerals have perfect crystal outlines, such grains are called ‘idioblastic’, if not ‘xeno- blastic’. Where the recrystallised mineral grains are found to be equidimensional, the texture is said to be ‘granoblustic’. ‘Helicilic texture’ is a term commonly applied to ‘S’ shaped or ‘Z’ shaped trails of inclusion in poikiloblastic crystals, especially garnets and staurolites found in regionally metamorphosed rocks.
Mineralogical Composition of Metamorphic Rocks
(i) Stress and anti stress minerals. Stress minerals are produced as a result of stress and have a stable existence only under stressed conditions. Kyanite, garnet, cliloritoid, staurolite, epidote, zoisite, glaucophane, anthophyllite etc. are common stress minerals.
The anti-stress minerals are those which are formed conveniently under uniform pressure. These minerals are incapable of withstanding high shearing stresses. Such minerals therefore do not occur in highly deformed rocks. They may include-sillimanite, cordierite, anorthite, felspathoids, andalusites, alkali feldspars etc.
Antistress minerals are of low-density while stress minerals in general are dense.
(ii) The following are the typical metamorpic minerals:
Aluminosilicates like andalusite, kyanite, sillimanite, staurolite, cordierite, epidote, tourmaline, talc, chlorite, zeolites, graphite, pyrite and pyrrohtite etc.
Metamorphic rocks have been classified on the basis of several factors like:
1. The parent rocks from which they have been metamorphosed.
2. Structure, texture and predominance of agents.
3. Mineralogical assemblages etc.
However, the first and the last factors are quite significant in the classification of metamorphic rocks.
In case the parent rocks are of igneous origin which have subsequently undergone some or other metamorphic changes, the resulting rocks are known as
(a) Orthometamorphic rocks or Meta-igneous rocks.
When sedimentary rocks undergo metamorphic changes, the resulting rocks are said to be (b) Para-metamorphic rocks or Meta- sedimentary rocks.
On the basis of the mineralogical assemblages also attempts have been made to classify the metamorphic rocks. As we know, whenever metamorphism is ideally complete, the product is an assemblage of minerals in chemical equilibrium with one another and in most metamorphic rocks; this ideal condition appears to be at least closely approached. The actual mineral composition of a metamorphic rock is determined by two factors:
(i)The initial composition of the rock and the extent to which material have been added or subtracted during metamorphism.
(ii) The degree of metamorphism.
Accordingly the facies concept also came into being in metamorphism. It has also been observed that the number of mineral assemblages produced by metamorphism is between two and six. On the basis of type of metamorphism, i.e., whether contact metamorphism or regional metamorphism as well as the range temperature the following classifications have been made:
1. Contact metamorphism:
(i) Albite-epidote-hornfels facies.
(ii) Hornblende-hornfels facies.
(iii) Pyroxene-hornfels facies.
(iv) Sanidinite facies.
These four facies have been distinguished in the ascending order of temperature of formation and have characteristic mineral assemblages.
2. Regional metamorphism:
There are six facies included in this group in ascending order of temperature of formation, as
(i) Zeolite facies.
(ii) Green-schist facies.
(iii) Glaucophane-schist facies.
(iv) Amphibolite facies.
(v) Granulite facies.
(vi) Eclogite facies.
All the possible mineralogical assemblages have been represented by the above facies types. Accordingly metamorphic rocks are classified.
Besides the above, some geologists classify metamorphic rocks into two broad categories:
(a) Foliated, and
(a) Foliated rocks:
These are characterised by parallel arrangement of salty minerals, such as the micas. Foliation is produced during regional metamorphism. The degree of foliation is related to the intensity of metamorphism. The foliations are ultimately changed into bands.
The most common examples are slates, schist’s and gneisses.
(b) Non-foliated metamorphic rocks:
In these rocks the mineral grains are equidimensional, hence there is no specific orientation. Mostly they are the products of thermal metamorphism or contact metamorphism.
Marble, quartzite and hornfels are some of the common examples of this type.