Precambrian of Central India

Renewed intt'rest in the study of the Indian Precambrian which started with the publication of Memoir 4 of the Geological Society of India on the' Precambrian of South India' in 1983, resulted in two other volumes which were issued by the Geological Society in quick succession on the 'Precambrian of the Aravalli Mountain, Rajasthan, India' (Memoir No.7) and the' Precambrian of the Eastern Indian Shield' (Memoir No.8). A separate volume on the Precambrian of Central India was long over due. This lacuna has now been filled up with the pUblication by the Geological Survey of India, Central Region, of a handsomely bound volume containing papers on the geology of the differ­ ent sections of the Central part of India. This region lying at the heart of India is of great interest because of the Son-Narmada rift trend­ ing in a E. W. direction across the subcontinent and dividing it into two segments of contrasting geology. An account of the geological and structural evolution of this important region is, therefore, to be welcomed.

In a compilation of this kind, we would have liked to have an intro­ ductory summary of the geology of the region as a whole. Instead. we find a number of individual papers describing segments of the terrain with a bewildering array of new names making it difficult to comprehend the geological evolution of this terrain as a whole. The absence of a comprehensive geological map also stands in the way of our understand­ ing of the complex geology of this region.

There is no set pattern in the arrangement of papers. The main emphasis appears to be on stratigraphy and tectonics. The volume starts appropriately with a paper by Yedekar et al. emphasizing the impor­ tance of the Son-Narmada lineament as a suture joining the northern Bundelkhand and the southern Deccan continent. The region is marked by a thick zone of ductile deformation represented by mylonites and phyllonites. The authors visualize a southward subduction of the Bundelkhand block beneath the overriding Deccan block. They place the colHsion event in early Proterozoic. The Sausars are conceived as sediments deposited in a basin marginal to the northern Bundelkhand continent. It is interesting to note' that this collision zone has deve­ loped two-pyroxene granulites. Illustrations accompanying the paper unfortunately are too crowded with many irrelevant details, making the main thrust of the paper-the establishment of a suture marking the line of junction of two continental blocks of contrasting geology­ somewhat obscure. 208 PRECAMBRIAN OF CENTRAL INDIA Ramakrishnan has presented a brief and concise account of the geology of south Bastar including the Sukma, Bengpal and the Bailadilla Groups. He finds no similarity with the successions identified in the Dharwar craton, underlining the fact that the two cratons belong to two separate tectonic provinces.

A granite-greenstone belt-the Sonakhan belt-having some simila­ rities with the Dharwar schi!;ts is described by Nitish Das and others. The sulphide-bearing auriferous cherts of this belt are deserving of fur­ ther study as sources of precious and base metals.

Sedimentation· pattern in the Bija.war Basin forms the subject matter of a paper by N. C. I·ant and D. M. Banerjee. Balrniki Prasad has attempted a broad-brush summary of the Precambrian of the Central region. He includes the Bengpal-Sukma in the Archaean, while the Bailadil1a, the Mahakoshal, the Sausar-Sakoli and the Chilpi Groups are brought under Early Proterozoic (2500-2000 m.y.). The Khaira­ garh and Abhujmar (2000-1600 m.y.), the Chattisgarh and the Vindhyan (1600-570) are the other Precambrian Groups. A geological map of the Central region showing the extension of rocks belonging to different age groups would have helped in following the argument more easily, but such a map is badly missing. Instead, we have the familiar geo­ logical map of the Peninsular shield. Even here, the contrasting charac­ ter of the geology of the Northern Bundelkband and the Southern Deccan blocks have not been clearly brought out. The preparation of such a map should, in our opinion. be taken up on a priority basis.

In no o'ther terrain of India is the need for more geochronological data vital than in this Central block. Available geochronologiical data on rocks of the Central region are presented by Sarkar et al. Chrono­ logy, however, remains confusing. The sequence of events which have shaped the geology of the region have yet to be clearly established. Information on the ages of the southern and northern blocks, the time of collision of the two blocks and of the intrusives lying within the collision belt is badly needed.

Roy and Bandyopadhyay present an interesting account of the Bija­ war of Central India (now termed' the Mahakoshal Group '). \Vithout precise age data and detailed geologiCal map, structural data alone does riot help in understanding the geological complexity. It is obvious this' mobile belt at the margin of the northern continent needs more intensive study. A detailed geological map of the belt is essential.

The study of lineament fabric by Rajurkar and others, brings out clearly the distinctive and contrasting pattern of lineaments in the northern Bundelkhand segment and in the southern Deccan segment, strengthening the inference that we are dealing here with two different continental blocks which have come together along the Son-Narmada suture. PRECAMBRIAN OF CENTRAL INDIA 209 Several papers on petrography follow, describing the Dongargarh volcanics, Tirodi gneiss, anorthosites of Sidhi district, and the manga~ nese belt of Balghat district.

Sedimentological studies on the Vindhyan, Penganga are then presented. Stromatolites, microbiota and trace fossils of Chattisgarh basin and the Bijawar Group of Joga area are described. Shukla and Sharma describe the fossils of Suket Shale and point to the possibility of Metazoan development in India, earlier than in other parts of the world. Obviously, this is an important field of research worth pursuing.

Papers dealing with metallogenetic aspects are unfortunately too few and not quite representative. The col1ectionof papers ought to have included descriptions of the world famous manganese belt of Central India and the fabulous iron ores of Bailadilla. The significant features of the largest deposit of base meta] in India - the Malanjkhand pros~ pect - has not been well brought out. Description of the only produc­ ing diamond mine is missing. Central India, especiaUy the Central Indian'Tectonic Zone with its polymetamorphic history holds promise of being an important zone of metal\concentration and is deserving of closer attention.

In our opinion, a special mineral exploration project for the Central Tectonic Zone. ought to be formulated. . Precambrian basement map of the zone falling within parts of the States of Gujarat, Rajasthan, Maha­ rashtra, Madhya Pradesh, Uttar Pradesh, Bihar and West Bengal should be attempted. All drill holes so far dri11ed in this zone should be care­ fully logged and cores sampled and geochemical1y tested for their precious and base meta] content. Quality gravity and aeromagnetic data should be gathered and made available to all workers. Structural trends and boundaries between rock type should be identified. Geology of the region has to be reinterpreted in terms of the tectono-stratigraphic concept.

The middle Proterozoic anorogenic granite-rhyolite terrain should be the main target of exploration for the location of Olympic Dam type deposits. The Olympic Dam at Roxby Downs, South Australia, is one of the world's premier meta] deposit. It is reported to contain at least 2000 M tonnes of ore that has an average grade of] .6% Cu, 0.06 per cent of uranium and 0.6 gm of gold and 3.5 gm of silver. The deposit occurs within an area demarcated by Mid~Proterozoic anorogenic magmatism and the ores are hosted by coarse clastic sedimentary rocks which are interpreted mainly as breccias. Marwar craton in W. Rajasthan and the Central tectonic zone in Madhya Pradesh present similar environmental set up. A new generation of field mapping; of these terranes is caned for.

We have nothing but praise for the excellent job that has been done by the team of workers of the Centra] region of the Geological Survey of 210 PRECAMBRIAN OF CENTRAL INDIA India under the leadership of Dr. B. C. Poddar in bringing out thi.s volume which forms an important addition to our knowledge. It is not an easy task to bring together such a wealth of information within the short time at their disposal. Certain deficiencies have been pointed out here not with a view to finding fault, but to emphasize the need for further work on certain critical areas. There is no doubt that the memoir forms an important Gontributioll to our understanding of the Pre~ambrian of India and will stimulate interest in all those engaged in understanding the different stages of crustal development of the Indian subcontinent. We commend the publication to the attention of our readers. Copies of the memoir can be had from the Director-General, Geological Survey of India, No. 29, lawabarlal Nehru Road, Calcutta 700016. The volume is reasonably priced at Rs. 165/- and all libraries would do well to possess a copy.

B. P. RADHAKRISHNA

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