In this article, we will discuss The Archaean Formations (Pre-Cambrian). So, let’s get started.
The Archaean Formations (Pre-Cambrian)
The Archaean Era is also known as the Precambrian Period. This is the division of geologic time scale from the formation of the Earth (about 4.6 billion years ago) to the beginning of the Cambrian Period of the Paleozoic Era (about 570 million years ago).
The Precambrian time constitutes about 86.7% of the Earth’s history. The term “Archaean”
introduced by J.D. Dana in 1782. refers to the oldest plutonic rocks of the Earth’s crust. The
oldest known rocks of the Earth, the evolutionary atmosphere, the first chemosynthesis the
first photosynthesis, the life-supporting atmosphere and the Earth’s modern atmosphere, were developed during the Precambrian Era (Archaean and Protozoic). Rocks of the Archaean system are devoid of any form of life. In other words, the Archaean rocks are all azoic or unfossiliferous.
They are thoroughly crystalline, extremely contorted, faulted, and practically devoid of any
sediment. They are largely intruded by plutonic intrusions and generally have a well-defined
foliated structure. These rocks are known as the basement complex or fundamental gneisses. Thus, all over the world, the Archaean rocks are the foundation of all the great ancient plateaus, and they form the core of all the great folded mountain ranges of the world.
In the Indian Geological Time Scale, advocated by T.S. Holland, the Precambrian Era is known as the Purana. The Archean system includes the Aravalli, Dharwar, Cuddapah, Vindhyan, Meghalaya Plateau, and Mikir Hills. These are also called the Archean gneiss. The mineral composition of Archaen gneiss varies from granite to gabbro. The constituent minerals are: orthoclase, oligoclase, quartz, muscovite, biotite, and hornblends. The Archaen rocks cover two-thirds of Peninsular India. They also occur in the roots of the mountain peaks all along the Greater Himalayas from the western most part of Kashmir to the eastern-most part of
Arunachal Pradesh as well as in the Trans-Himalayan ranges of Zaskar (Zanskar), Ladakh, and the Karakoram. The Archaean rocks cover two-thirds of Peninsular India. In the Peninsular region, the Archaean rocks are known to be of three well-defined types:
(i) The Bengal Gneiss
• The Bengal gneiss is highly foliated, which is distributed in the Eastern Ghats, Odisha (known as Khodolites after Khond tribes in Koraput and Bolangir districts), stretching over Manbhum and Hazaribagh districts of Jharkhand, Nellore district of Andhra Pradesh and Salem district of Tamil Nadu.
• They also occur in the Son Valley, Meghalaya Plateau and Mikir Hills. These formations are very thinly foliated. These rocks were identified for the first time in the Midnapur district of West Bengal.
(ii) The Bundelkhand Gneiss
• The Bundelkhand gneiss are massive granitoid which form the second group of fundamental
gneiss of the Archaean age.
• It is geographically distributed in Bundelkhand (UP), Baghelkhand (MP), Maharashtra, Rajasthan, Andhra Pradesh, and Tamil Nadu.
• It is a coarse grained gneiss which looks like granite.
• The Bundelkhand gneiss is conspicuously criss-crossed and characterised by quartz veins.
(iii) The Nilgiri Gneiss
• These are massive, eruptive dark-coloured gneiss. The name being given in honour of Job
Charnock whose tombstone in Kolkata was made of this rock.
• The Nilgiri gneiss is bluish-grey to dark coloured rock, medium to coarse grained in texture.
• This is plutonic gneiss intrudes into the other Archaean rock masses.
• Nilgiri gneiss is popularly recognised as belonging to the Charnockite series.
• It is widely found in South Arcot, Palni Hills, Shevaroy Hills and Nilgiri in Tamil Nadu, Nellore in Andhra Pradesh, Balasore in Odisha, Karnataka, Kerala, Malabar, Jharkhand, Chhattisgarh and Aravallis (Rajasthan).
• The Archaean rocks are the repositories of the mineral wealth of India. These rocks are rich in ferrous and non-ferrous minerals like iron ore, copper, manganese, mica, dolomite, lead, zinc, silver and gold.