They are all azoic, i.e., there was no existence of life at the time of their formation. The close of the Archaean era was marked by intense earth movements, as a consequence mountains and plateaus were formed and were acted upon by denuding agencies. Their subsequent subsidence and deposition of sediments from neighbouring land gave rise to Eparchaean Unconformity.
The important rock types are gneisses, schists, charnockites, khondalites, gondite, kodurite, pyroxenite, hyperite, quartzite, phyllite, marble, anorthosite etc.
In structure and composition, the Archaean rocks are very complex and are therefore known as basement complex, gneissic complex etc.
1. Archaeans of South India:
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It was first studied by R.B. Foote. The Archaean rocks of South India, are known as Dharwar system, and are best developed in Mysore and Southern Bombay. They are constituted by schists, gneisses and granites. The schistose rocks are isoclinally folded and the dip is towards east. The Regional Strike is N.N.W.-S.S E.
(i) Classification:
According to W.F. Smith:
(Younger) Chloritic DivisionàChlorite and Mica schist, Quartzite, Marble, conglomerates etc.
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(Older) Hornblendic Divisionà Hornblende schist, Calc-granu-lite, Hematite, Quartzite etc.
(ii) Rama Rao:
Felsite and Porphyry dykes closepet granite (Bellary gneiss).
Charnockites
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Norite dykes
Hornblendic dykes
Peninsular gneiss
Champion gneiss
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Unconformity
Upper Dharwar :
(Sedimentary): Conglomerates, calcareous silts, cherty and ferrugious silts and clay.
Middle Dharwar:
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Metamorphosed igneous and sedimentary rocks. The upper part of the sub-division consists of metamorphosed granitic rocks with gneissose structure. The lower part consists of micaceous genisses, schistose conglomerates, banded iron stones etc.
Lower Dharwar :
Upper part is made up of highly crushed micaceous quartz-schists and gneisses, formed due to metamorphism of pre-existing acid volcanic rocks. Lower part is made up of Hornblende schist; green stones etc. the products of metamorphism of ancient dykes and sills.
Base is not known
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Rama Rao has classified the exposures of Dharwarian rocks into five distinct zones as
(a) Eastern most zone:
Known as Kolar Schist Belt.
(b) East central zone:
Composed of granulites, gneisses, schists of varying composition.
(c) Central zones:
Composed of limestones, ferruginous rocks and metamorphosed igneous rocks.
(d) West-central zone:
Composed of banded manganiferous and ferruginous rocks.
(e) Western most zone:
Composed of hornblende schist, within which bands of hematite, quartzite occurs.
The Dharwarian rocks of south Bombay exhibit the lowest grade of metamorphism while those of the Southern Mysore are of the highest grade.
2. Archaeans of Madhya Pradesh:
The areas where archaean rocks are developed characteristically in Madhya Pradesh and their characteristic features are as follows:
(i) In Bastar and adjacent areas the rocks are made up of schists and gneisses of both igneous and sedimentary origin, which have been subsequently intruded by igneous masses.
(ii) In Raipur, the rocks are mica-schist, phyllite, quartzites and Banded-hematite-quartzite etc.
(iii) In Bilaspur-Balaghat area, the oldest rocks occur towards the northern part and are known as Sonawani series, which is made up manganese ores, calc-gneisses, crystalline limestone, quartzites and schists.
The Sonawani series is overlain by the chilpighat series which is made up of trap rocks, grits, conglomerates, green stones etc.
It may noted that both Sonawani and Chilpighat series contaia a manganese ore horizon and in part represent the Sausar series of Nagpur-Chindwara.
(iv) Nagpur-Chindwara:
The chilpi rocks continue westwards and bifurcate, the southern strip occupying parts of Nagpur and Bhandara districts are known ar Sakoli series. The northern strip goes into Nagpur-Chhindwara, known as Sausar series.
Rocks of this series are granulites, marbles, schists and manganiferous rocks.
The Sausar series is divided into nine stages of which five are well developed in Nagpur area and four in Chhindwara area. The rocks of the Sausar series generally dip to the South and S.S E. me stratigraphic succession is as follows:
Sapghota stage
Sitapar stage
Bichua stage
Junawani stage
Chorbaoli stage
Mansar stage-Gondites, Manganese ore etc.
Lohnangi stage-Marbles and Manganese ores
Utekata stage
Kadbikhera stage.
(v) Nagpur-Bhandara area:
This is the Sakoli series .The rocks belonging to this series are less metamorphosed than the Sausar series. The rocks generally dip to the NNW and are made up of quartzites, dolomites, amphibolites, schists, phyllites, conglomerates etc.
The lower part of the sequence of metamorphic rocks is to some extent similar to Chorbaoli, Bichua and Sitapar stages of the Sauser series.
3. Archaeans of Rajasthan:
Archaean rocks of Rajasthan occur
(a) in the Aravalli range region in the form of a part of the very large synclinorium;
(b) in the plains lying to the east of Aravalli ranges and .separated from the Vindhyan country further east by a great fault, known as ‘Great Boundary Fault’, which strikes approximately N.E.-S.W. ;
(c) in the desert regions of jodhpur and Marwar to the west.
Important feature:
The archaean and other pre-cambrian rocks have together formed the conspicuous Aravalli ranges, which traverse the country along a N.E -S.W. direction.
Special attention is attached to the Aravalli Mountains where the Dharwars were laid down in an ancient basin upheaved by orgen forces towards the close of the Archaean era.
The archaean stratigraphy of Rajasthan has been studied worked out by C.A. Hacket, C.S. Middlemiss, A.M. Heron and many others. The stratigraphic succession is as follows;
Bundelkhand gneiss:
It is also called ‘Berach granite’ and may possibly represent the granitic portion of Banded Gneissic Complex. The exact relation between these two could not be established directly since their junction is concealed by the overlying Aravalli system.
Aravalli system:
They are made up of mostly argillaceous materials. They show increasing metamorphism from east to west. The basal beds are arenaceous which are overlain by shales, phyllites and the associated volcanic rocks of basic composition.
(i) Reddish quartzites forming uppermost part of the Aravalli system are known as Ranthambhor quartzites.
(ii) Binota shales On the eastern side of the Great Boundary Fault, the unmetamorphosed Aravalli rocks are known as Binota shales which are succeeded by ‘Jiran sandstone’.
(iii) Gwalior series:
Arenaceous rocks around Gwalior which are of the same age and are similar to the unmetamorphosed Aravalli rocks are called Gwalior series. Here the grits are called khardeola Grit.
Raialo series:
It overlies the Aravalli system and is made up limestones. In Chitor area it is represented by unmetamorphosed dolomites which are known as Bhagwanpura limestone. Stromatolites are found in it. It is believed that Raialo series belong to the Period represented by Eparchaean Unconformity.
An outcrop of archaean rocks near Baroda is called Champaner series which contains quartzites, conglomerates, phyllites, marbles.
4. Archaeans of Eastern ghats:
The Eastern-ghats region between Bezwada and Cuttack, which attains the greatest width in the Ganjam, cuttack, tract, is composed of ridges trending in a N.E.-S. W. direction. This is also the regional strike of the rocks.
The hills are made up of gneisses, charnockites and khondalites. Thus the Eastern ghats is a belt of high grade metamorphism as evidenced by the abundanee of garnet and sillimanite. The khondalites as well as the charnockites are rocks of granulite facies.
The khondalites are also named as Bezwada gneiss, Kailash gneiss etc.
A hybrid rock called ‘Kodurite’ is also found in the Eastern Ghats. The Kodurite rocks consist of felspar, spessartite-andradite garnets, rhodonite, quartz and apatite in varying amounts. They are believed to have been produced by assimilation of manganese ore and manganese silicate rocks by an acid igneous magma.
Fermor was of the opinion that the khondalites and charnockites were formed at greater depths and that the whole Eastern Ghats belt was uplifted at a later period bringing those high grade metamorphic rocks to the surface.
Near Koraput, an igneous complex of alkali-gabbro, calc-alkali syenite and nepheline-syenites occurs concordant to the foliation of the gneissic rocks of the region.
5. Arghaeans of Singhbhum:
The archaean rocks of Singhbhum have been studied by V. Ball, H.C. Jones, J.A. Dunn and A.K. Dey. The archaeans of Southern Singhbhum are separated from those of the Northern Singhbhum by a distinct thrust zone, running approximately East-West for about 100 miles.
The stratigraphic succession in Singhbhum as a whole is as follows:
The rocks within the thrust zone are metamorphosed shales and sandy shales in the main with subordinate metavolcanics, viz., chlorite schist amphibole-schist, mica-schist talc-schist etc. The pelitic materials have been extensively migmatised resulting in felspathic mica-schist, quartz-inuscovite-biotite-schist and highly felspathic granophyric rocks known as Soda granite or Arkasani granophyre.
The iron-ore series is made up of conglomerates, sandstones, limestones, shales, banded-hematite-quartzites and basic lavas.
The thrust zone of Singhbhum has been mineralised to a great extent. Important deposits of apatite, magnetite and of copper ore are found to occur in this belt.