Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

C. S. Pichamuthu ¹

Affiliations
1 Department of Geology, University of Malaya, SG

Abstract

A preliminary account is given in this paper of the results of a statistical study made in Mysore State on the distribution of about 500 basic dykes, with special reference to the occurrence in them of clouded plagioclase feldspars.

The basic dykes are of different geological ages and of various petrographic types. They are distributed all over the State in a fairly uniform manner.

Attention is drawn to the important fact that the clouded dykes are concentrated in the southern half of Mysore State, and that they occur not only in the charnockite region but in a zone adjoining it. Two kinds of charnockites have been recognized in Mysore: an older gneissic type caused by an earlier regional metamorphism, and a younger granitic variety produced by subsequent palingenetic fusion. The two metamorphisms which were responsible for these differing types of charnockites are separated by the period during which the basic dykes were intruded.

Clouding of plagioclase feldspars can be caused by thermal metamorphism. In Mysore, the basic dykes have chilled against the first type of charnockites and transect the foliation of the country rocks. They are consequently younger in age. It is suggested that the later formed charnockites were responsible for the subsequent clouding of the plagioclase in those dykes.

Clouded feldspars in the basic dykes are, therefore, of great significance in recording the metamorphic history of some aspects of Precambrian geology in Mysore State.

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C. S. Pichamuthu ¹

Affiliations
1 Department of Geology, University of Malaya, Kuala Lumpur, MY

Abstract

A preliminary account is given in this paper of an example of rock transformation from near the village of Kabbal in southern Mysore State. In an area which is predominantly composed of Peninsular Gneiss, there is a development of charnockite in patches of varying shapes and dimensions. The gneiss is grey. banded and often contorted. It is generally biotitic but contains inclusions of hornblende schist. The charnockite is coarse-grained, greasy looking, and brownish-green in colour. Hypersthene occurs in big crystals. The gneissic foliations can often be traced right through the charnockitc patches,-evidence of the transformation of the banded gneisses into coarse grained charnockites.

Photographs taken in the field and photomicrographs of thin sections of some of the rock types illustrate stages in this interesting mode of formation of charnockites in Peninsular India.

The geological sketch map shows the position of Kabbal in relation to the occurrence of charnockites further south, in and towards the east of the Biligirirangan Hills, an area which forms part of the huge Nilgiri massif in Madras State. The locality described in this paper appears to be situated on the border of a zone of charnockitization where stages in the transformation of foliated gneiss into coarse charnockitc can be seen.

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C. S. Pichamuthu ¹

Affiliations
1 Bangalore, IN

Abstract

Peninsular India, like other shield areas of the world, displays an ingrained pattern of successive orogenic belts. The description of these belts is accompanied by a tectonic map in which are noted the main structural trends as well as many of the reliable ages so far determined.

The variations in the grades of metamorphism which can be recognised in South India are delineated in a sketch map. There is a general progressive increase in the intensity of metamorphism from north-west to south-east. The highest grades are met within the southern and south-eastern parts of the Peninsula where charnockites and khondalites are prevalent.

The successive stages in the geological evolution of Peninsular India are next described. The structural pattern as well as relative ages indicate that the oldest rocks are found in the type area for Dharwar Schists in Mysore State. This region is a continental nucleus, and the Eastern Ghats and Satpura provinces are later Precambrian accretions which have been folded by pressures directed towards this Archaean nucleus.

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C. S. Pichamuthu ¹

Affiliations
1 Department of Geology, Andhra University, Waltair, IN

Abstract

Bruce Foote gave the name 'Dharwar System' to the 'Lower Transition' rocks occurring near Dharwar in Mysore State, when he first separated these schistose rocks from the 'great granitoid gneiss system'.

It was Fermor, however, who generalized the term and commended its use for designating all the sedimentary schists lying below the Eparchaean unconformity. Lithological resemblances constituted an important factor in the correlation of these ancient schistose formations, and for the identification of 'Dharwar' rocks in many parts of India. The occurrences in widely separated areas, of crystalline limestones, manganese-bearing rocks, banded ferruginous quartzites and iron ores, and even calc-granulites and pyroxene gneisses, have been used as criteria not only for assigning certain formations to the Dharwars, but also as indicating the homotaxial relationship of these formations scattered in different parts of India. By doing so, the stratigraphical importance of certain types of rocks have been exaggerated, in the unfounded belief that some types of sediments were formed only at certain specific periods in the geological history of the earth.

Recent work has shown that Peninsular India like other shield areas of the world, exhibits an ingrained pattern of successive orogenic belts. Age determinations and the dispositions of the tectonic units of the Precambrian formations, indicate that the oldest rocks are found In Mysore State in southern India, which is the type area for the Dharwars (c. 2300 m.y.). This region possesses several important characteristics of a continental nucleus, and is bounded on the east by the Eastern Ghats province (c. 1600 m.y.), and on the north by the Satpura province (c. 1000 m.y.). These provinces must be considered as later Precambrian accretions composed of younger sediments with their own orogenic cycles, and so the real Dharwars are older than both the Eastern Ghats and Satpura belts.

It is necessary, therefore, to discontinue the use of the term 'Dharwar' to designate the schistose rocks outside the type area, because all these un fossiliferous deposits cannot be considered as synchronous or even homotaxial, since some of the lithologically similar rocks are known now to be associated with younger orogenic cycles.

The term 'Dharwar System' should be strictly confined to the schistose rocks of the type area in the continental nucleus situated in South India.

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C. S. Pichamuthu ¹

Affiliations
1 Department of Mines and Geology, Bangalore, IN

Abstract

Geochronological data for the Precambrian formations of such a large region as Peninsular India are still far too few. However, with the available data, certain broad generalisations can be made regarding their relative ages, correlation, and stratigraphy.

Mysore, Kerala, Singhbhum, and Bundelkhand contain rocks nearly 3000 m.y. old. which probably represent the nuclei around which grew the continental mass of the Peninsula. The Dharwar craton appears to be the most ancient part of the Peninsula. The existing age determinations do not help, however, in determining which is the oldest rock formation in this region.

The Eastern Ghats belt was formerly considered to be about 1600 m.y. old, but it is now known to contain rocks ranging in age from 2100 to 2600 m.y. The base of the Cuddapahs is about 1400 m.y. old, and the Kurnools about 1150 m.y. Southernmost India contains rocks of a wide variety of ages. Some are very ancient being over 3000 m.y. old. There are also evidences of 500 m.y. and 700 m.y. events which have been recognised in Ceylon.

In Rajasthan many formations are now known to be much older than previously thought. The base of the Aravallis appears to be about 2500 m.y. old and the Delhis 1800 m.y. The Bundelkhand and Berach granites are of the same age (2555 ± 55 m.y.). The Banded Gneiss Complex has components varying in age from about 2100 to 950 m.y. The base of the Vindhyans is probably 1400 m.y. old, and the Upper Kaimur Series about 910 m.y. No dates are available for fixing the upper age limit of the Vindhyans. They probably span the Precambrian-Cambrian boundary.

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C. S. Pichamuthu ¹

Affiliations
1 Department of Mines and Geology, Bangalore 1, IN

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C. S. Pichamuthu ¹

Affiliations
1 Department of Mines and Geology, Bangalore, IN

Abstract

Job Charnock was an officer of the East India Company in Bengal due to whose initiative the British were able to acquire Calcutta. He died in 1692 and his tombstone is found in a mausoleum in the compound of St. John's Church. Holland discovered that this tombstone was made of the hypersthene-bearing rock occurring rear Madras and whose petrology he was investigating at that time. As this was a new type of rock, Holland suggested for it the name Charnockite in honour of Charnock, the Founder of Calcutta.

In this paper, a description is given of the mausoleum, the tombstone and the epitaphs on it. The life, career and character of Job Charnock are briefly outlined.

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C. S. Pichamuthu ¹

Affiliations
1 Geological Society of India, Bangalore, IN

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B. P. Radhakrishna , C. S. Pichamuthu

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C. S. Pichamuthu ¹

Affiliations
1 Bangalore, IN

Abstract

Banded iron formations, as elsewhere in the world, are characteristic of the Precambrian schistose rocks in India. In this paper an attempt has been made to bring together most of the information available on these interesting rocks which have contributed largely to the iron ore resources of India. They occur chiefly in the States of Bihar, Orissa, Madhya Pradesh, and Karnataka (former Mysore State).

Though the iron formations are fairly constant in lithology and chemical composition, there are variations in their structure, texture, and mineralogy, the metamorphism undergone by these rocks being an important factor in bringing about the changes.

It is now known that there are banded iron formations of different ages in the Precambrian of India and while there are no reliable radiometric age data available, stratigraphically they can be assigned to different horizons.

Though mineralogically the rock is fairly simple, there is great difference of opinion regarding the mineral paragenesis. In particular, the origin of the iron ore minerals has given rise to considerable speculation especially as to whether hematite or magnetite was primary. The silicate minerals appear to have generally formed by metamorphism.

The source of the iron and silica, the environmental conditions (both depositional and atmospheric) which produced the banded iron formations, and the mode of origin of the banding, are all matters on which there has been wide difference of opinion. A brief account of the various views on these questions has been given which points to the conclusion that many of these important problems have not yet been solved.

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C. S. Pichamuthu

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C. S. Pichamuthu ¹

Affiliations
1 Geological Society of India, IN

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C. S. Pichamuthu ¹

Affiliations
1 Geological Society of India, IN

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C. S. Pichamuthu

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C. S. Pichamuthu

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C. S. Pichamuthu

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C. S. Pichamuthu

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C. S. Pichamuthu

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C. S. Pichamuthu ¹

Affiliations
1 Geological Society of India, IN

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C. S. Pichamuthu ¹

Affiliations
1 Geological Society of India, Bangalore, IN

Abstract

Since the terms Peninsular gneiss and Dharwar schist have recently been used by some writers without reference to the original meaning ascribed to them by the geologists who first coined those terms, attention is drawn here to their exact scope and significance. In Karnataka, 'Peninsular gneiss' comprised all granites and gneisses except those of Closepet, Chitaldrug, Hosdurga, Arsikere, Banavar, and Saulanga; and 'Dharwar schist' included all the crystalline schists (metavolcanics and metasediments) as opposed to the felsic gneisses. peninsular gneiss was considered to be intrusive into the Dharwars. No early geologist in India was entirely of the opinion that the Dharwars were younger than the gneisses.

The Bababudan Syncline contains the oldest Dharwar sequence composed of mafic and ultramafic volcanics and intrusives (some of komatiitic affinity) intercalated with iron formations. Limestones are entirely absent.

The actual base of the Dharwars has nowhere been definitely proved. Vertical shafts more than 3km deep in the Kolar Gold Fields have not touched the bottom. The fact that in some places the schists are seen 'resting on gneiss' does not mean that they constitute the lowest Dharwars.

A general progressive metamorphism in the schist belts in southern Karnataka can be noticed from north to south, culminating in high grade granulite facies rocks. There is no field evidence which lends support to the presumption that the chamockites are overthrust on the Dharwars.

Numerous examples are available throughout Karnataka to prove the intrusive relationship between Peninsular gneiss and Dharwars. There are probably some schistose rocks which are older than the Dharwars, but, so far, they have not been definitely identified. The suggestion that the Sargur schists of southern Karnataka are pre-Dharwars is not based on precise stratigraphical, petrological, geochemical, or geochronological data. These highly metamorphosed isolated strips of schists could weIl be constituents of the lowest Dharwar sequence.

The problem of the nature of the primordial crust, as elsewhere in the world, has not been satisfactorily resolved in southern India. The occurrence of very old migmatitic gneisses suggest a probable primitive simatic crust.

As in many precambrian shield areas, the granites and gneisses of Karnataka can be generally grouped into three types which were formed at different times in varying tectonic environments.

While the granulite facies rocks in the most ancient rock complexes are probably related to a higher thermal regime of the earth in early Precambrian time, it must be realised that the existence of pyroxene granulite facies assemblages alone is insufficient evidence to infer that such rocks have had an exclusive origin in the lower crust. It is now known not only in south India but in many parts of the world that gneisses have been transformed into charnockites. There are charnockites of different geological ages and of different modes of formation. The prevalence of enderbites in southern India is probably due to the conversion of early tonalitic rocks in the high grade metamorphic terrains.

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C. S. Pichamuthu

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C. S. Pichamuthu

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C. S. Pichamuthu ¹

Affiliations
1 Geological Society of India, 1, De Souza Road, Bangalore 560025, IN

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Considerable amount of investigation has recently been done on the minerals commonly present in the charnockites of India. In this paper a very concise account is given of the optical properties and chemical composition of orthopyroxene, clino-Pyroxene, hornblende, biotite, garnet, plagioclase, and potassium feldspars. Mention is made of some of the minor minerals such as monazite, magnetite, ilmenite, pyrites, Pyrrhotite, apatite, zircon, spinel, and sapphirine.

The work done on the chemistry of coexisting pyroxenes, and the cation distribution in the coexisting silicate minerals is also briefly reviewed.

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C. S. Pichamuthu ¹

Affiliations
1 Geological Society of India, 1, De Souza Road, Bangalore 560025, IN

Abstract

A brief review is made of the investigations conducted on the Archaean rocks of southern India from the time of Newbold (1838) to the present. Problems connected with Dharwar stratigraphy and the relationship of Peninsular Gneiss to Dharwar schists and charnockites have given rise to considerable differences of opinion and the main features of these controversies are summarised. Much of the confusion is due to the fact that terms like 'Dharwar' and 'Peninsular Gneiss' have been used in very different senses by individual workers. Geochemical and geochronological investigations are of recent origin and have yielded some interesting results. An account is given at the end of some of the problems in Precambrian geology that are still awaiting solution in southern India.

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