The metamorphic structures are determined by definite mechanical conditions and also by recrystallisation. Relict (remnant of the original structure) and crystalloblastic (metamoiphic crystalline structure) structures may exist side by side. The relict structure may be use d to trace back the nature of the original rock and the magnitude of alteration it has undergone. Five major types of metamorphic structures have been recognised as follows:
(i) Cataclastic texture.
(ii) Maculose structure.
(iii) Schistose structure.
(iv) Granulose structure.
(v) Gneissose structure.
Metamorphic rocks derived from the sedimentary rocks are known as parametamoiphic rocks. Those which are derived from gravel rocks are known as ‘psephitic rock’, from aranaceous rocks are called psammitic rocks and those from argillaceous ones are said to be ‘pelitic rocks.’
(i) Cataclastic texture:
It is produced under stress and in absence of high temperature, whereby rocks are subjected to shearing and fragmentation. Only the durable mineral partly survive the crushing force and the less durable ones are powdered. Thus, when resistant minerals and rock fragments stand out in a pseudo porphyritic manner in the finer materials, it is known as ‘porphyroclastic structure.’ Phenocrysts are called ‘porphyroclasts’. Argillaceous rocks develop salty cleavage, harder rocks may be shattered and crushed forming crush breccia and crush conglomerate. When the rocks are highly crushed into fine grained rocks, they are known as mylonites Since these structures are formed due to cataclasis, they are, as a whole, known as cataclastic structure.
(ii) Maculose structure:
It is produced by thermal metamorphism of argillaceous rocks like shales. Here, larger crystals of andalusite, cordierite and biotite are sometimes well developed giving a spotted appearance to the rocks. The well developed crystals are known as ‘porphyroblasts’ with increasing degree of metamorphism, the spotted slates pass into extremely fine grained granular rock known as Hornfels.
(iii) Schistose structure:
Here the platy or flaky minerals like the micas and other inequidimensional minerals show a preferred orientation along parallel planes, under the effect of the stress dominating during metamorphism. The longer directions are parallel to the direction of maximum stress. Schistosity is the property or tendency of a foliated rock, whereby it can be readily split along foliation plane.
(iv) Granulose structure:
This is found in the rocks composed of equidimensional minerals like quartz, feldspar and pyroxenes. They are formed by the recrystallisation of pre-existing rocks, under uniform pressure and great heat. The typical texture is coarsely granoblastic. These structures are also known as ‘sacchroidal. Quartzites and marbles are typical examples of this structure.
(v) Gneissose structure:
It is a banded structure due to alternation of schistose (dark coloured) and granulose (light coloured) bands and is produced by highest grade of metamorphism, typically by regional metamorphism. The bands differ from ore another in colour, texture and mineral composition. Gneisses typically show this type of structure, hence the name.