GROUP DISCUSSION ON 'THE PENINSULAR GNEISSIC COMPLEX OF THE SOUTHERN INDIAN SHIELD' , 8th to 11th March 1975

SUMMARY OF PROCEEDINGS

Introduction A group discussion on ' The Peninsular Gneissic Complex of the Southern Indian Shield '-the second in a continuing series of group discussions on important problems of Indian geology, under the auspices of the Geological Society of -was held at the Society's Headquarters in Bangalore from March 8-11, 1975 (see Jour. Geol. Soc. Ind., Vol. 15, No.4, 1974, for the proceedings of the first such group discussion on 'The Geochemistry of the Precambrian Rocks of India ').

Choice of the Topic The Council of the Society selected the 'Peninsular Gneissic Complex' as the topic for the second group discussion for the following reasons: The bimodal associa­ tion of sodic of tonalitic or trondhjemitic composition and meta-basalt, and the world-wide occurrence of this association (age, 2700-3600 m.y.) as basements underlying Archaean greenstone belts have recently tempted some geologists to invoke plate-tectonic concepts to trace the paths of evolution of Early Precambrian lithologies. Concepts such as these that are so vital for the progress of geology caIl for renewed thinking and vigorous debate on comparable rock associations in India. Interesting in this context is the possibility that the' Peninsular Gneissic Complex' (age, 2600-3300m.y.) of the Dharwar is quite similar to the' Ancient Tonalitic Gneisses' of other continents. This possibility has far-reaching implications for the chronostratigraphic, litho-tectonic, and metallogenic evolution of the Indian Shield. Unfortunately, however, the' Peninsular Gneissic Complex' is one of the most vaguely defined, poorly characterised, and cursorily studied rock components of the Indian Precambrian despite the fact that it has been a subject for discussion over the last hundred years. Objectives The main objective of the group discussion was to generate a high-level intel­ lectual interaction between the participating scientists (Annexure I) in the hope that it would lead to a more meaningful, and fuller understanding of this important rock complex. This, it was thought, would, help in unravelling the problematical aspects of comparable gneissic complexes in the Singhbhum, and Rajasthan .

Themes The group discussion was conducted in six thematic sessions (the problem and historical perspective; geochronology, structure, and stratigraphy; geochemistry; regional geologic studies; evolutionary models; some new approaches to the problem) in which twenty papers were presented as the basis for discussion (Annexure 2). The session on regional geologic studies was preceded by an entire day's field work in critical exposures of the' Peninsular Gneissic Complex' (' PGC ') of the Dharwar 240 PROCEEDINGS Craton in, and around, Bangalore, Thippasandra, , Nademavinpura, Kabbal­ durga, and Kanakapura, so as to emphasise and reiterate the obvious, subtle, and concealed problems involved in a clear understanding of the' PGC' in other parts of the Southern Indian Shield. A significant feature was an exhibition of' problematic maps', field photographs, hand specimens, and thin sections of critical rock sui tes of the' PGC' from different areas. In his introductory address, Professor C. S. Pichamuthu highlighted some of the problems posed by the Peninsular Gneisses, and the gaps in our knowledge regarding the relationship ofthese rocks to the Dharwar and charnockites. The following are some of the points referred to by him as requiring further detailed examination: (1) define the term' Peninsular ' as its exact connotation is still not quite clear; (2) what is the base of the Dharwar sequence? are there any true basal conglomerates? These questions should be answered before any meaningful assess­ ment can be made regarding the relative ages of the schists and gneisses; (3) some of the localities where intrusion of into Dharwar schists has been recorded should be re-examined, and definitely decided one way cr the other; (4) petrological, geochemical, and geochronological evidences should be obtained for the existence of pre-Dharwar rocks; (5) are there' mantled gneiss domes'? One typical example, like Saulanga, should be examined for any metamorphic effects, and whether the surrounding metasedimentary rocks dip away from domes; (6) are the felsic and mafic xenoliths in the Peninsular gneisses different from the rocks in the adjoining -greenstone belts? If so, in what respects? (7) is there a front, and can it be demarcated? (8) Can we work out the early Precambrian crustal evolution in ? Are there evidences of a primordial crust? (9) Are the charnockites part of the primordial crust? Have they retrograded into Peninsular gneisses, 01' have the Peninsular gneisses transformed into charnockites under granu­ lite facies metamorphism? (10) Gravity anomaly patterns of South India.

Results of Recent Studies The more significant of the conclusions reported in. some of the papers are: (I) The Peninsular Gneissic Complex of the Archaean-Proterozoic Shield of India represents the primordial sialic basement crust which repeatedly constituted cratonic zones for the succeeding geosynclinal-orogenic pro­ vinces, and had thus undergone repeated periods of folding and deformation, repeated emplacements of and other igneous rocks, and repeated episodes of metamorphism and remobilisation. The complexity of multi­ deformational, multi-intrusional, and poly-metamorphic history of the Peninsular Gneissic Complex is faithfully reflected by the varying isotopic ages reported for these rocks ranging from over 3OCOm.y. to 500-600m.y. (S. Narayanaswamy). (2) The Peninsular gneisses are essentially migmatitic gneisses within which occur several poorly foliated and unfoliated pink and grey 'granites'. Since gross mineralogy is unlikely to be helpful in distinguishing the various types of gneisses, morphological and statistical studies ofzircons are strongly recommended (M. V. N. Murthy). (3) In Tamil Nadu there is extensive development of granulite facies rocks of the khondalite-charnockite group set in a wide stretch of quartzo-felspathic gneisses (). The gneisses comprise a varied assemblage ofquartzo- PENINSULAR GNEISSIC COMPLEX 241 felspathic units with and as the important mafic minerals, and interbanded with magnetite quartzite, crystalline limestone, calc-granu­ lite and quartzite (N. G. K. Murthy). (4) Detailed structural mapping in the North Arcot district of Tamil Nadu shows five periods of deformation interspersed with three periods of mig­ matitic and granitic activities, and four periods of dyke emplacement. These gneisses are considered to have been derived from migmatisation of charnockites and other associated rock types, such as magnetite-quartzite, pyroxene granulite, norite and ultrabasics, constituting the 'basement group' (E. B. Sugavanam). (5) Recent work in parts of the Bangalore, Tumkur, Mandya, Bellary, and North Kanara districts of shows that the Peninsular Gneissic Complex represents a wide zone of migmatisation in different stages of resorption and homogenisation culminating in the development of anatectic granite, , or diorite, depending upon the earlier palaeosome (recognised to be pelitic to semi-pelitic sediments). The migmatisation process culminated in the formation of homophanous granite and grano­ diorite in some places (M. Ziauddin). (6) Recent stratigraphic work in Southern Karnataka has shown that the Peninsular gneisses constituted the basement for greenstone belts, which may be Jithostratigraphically grouped into a Dharwar Supergroup. Some high­ grade schists ( Supergroup) occurring as enclaves and linear belts or bands in the Gneissic Complex are older than the greenstone belts not only because of their sharp metamorphic break, but also because of the abrupt truncation oftheir structural trends by the greenstone belts. The Peninsular Gneiss is quite possibly a polymigmatite spanning several episodes (M. Ramakrishnan, M. N. Viswanatha and J. Swami Nath). (7) The Peninsular Gneisses in the northern parts of Andhra Pradesh represents a group of composite gneisses, mostly banded or layered, formed as a result of migmatisation of older meta-pelites (predominantly biotite-schists), meta­ psammite and meta-basics (predominantly and hornblende­ schists) by a grey quartzo-feldspathic injection (P. Perraju and V. Natarajan). (8) in Peninsular Gneisses from Bangalore include a high percentage of euhedral to subhedral crystals with simple and complex terminations, and a small percentage showing overgrowths. The elongation frequency curves show a prominent monomodal peak at 2.15, and the RMAs indicate that the zircons are self-nucleated. These features suggest that the Peninsular Gneisses are magmatic in origin (T. V. Viswanathan}, (9) Some of the tonalitic pebbles in the Kaldurga conglomerates of Karnataka have yielded a Rb-Sr isochron age of 3300 m.y. (V. S. Venkatasubra­ manian). (10) Most of the pre-Dharwar gneissic, granitic, granodioritic, and tonalitic pebbles of the conglomerates from the Shimoga, Chitaldrug, and Bababudan belts of Karnataka have low K~O, and high Na.OjK.O ratios (0.71 to 38.75; mode, 4-5) in which more than 95% have Na.OjK.O ratios higher than 1, whereas the post-Dharwar gneisses have high K.O, and low Na.O/K.O ratios (0.4 to 12.1; mode, about 1) in which more than 70% have Na~O/K10 ratios less than 1. The enrichment pattern of soda and 242 PROCEEDINGS potash in the pre-, and post-Dharwar acid rocks, respectively, indicates: differences in poT conditions in the upper mantle in the respective periods. (S. M. Naqvi, V. Divakara Rao and K. Satyanarayana). (11) The Peninsular gneissic terrains of the Southern Indian Shield (age, more than 2600 m.y.) contain sodic granitic gneisses, similar geochernically, to the 3000-3400 m.y. old tonalitic gneisses of Barberton, Johannesburg­ Pretoria Dome, and Swaziland of the Southern African Shield. The similarities are striking with respect to their high Na,O/K,O ratios (2.8-3.2), and their low Rb/Sr ratios (0.06-0.07). A significant geochemical feature of the Southern Indian gneissic terrains is the interlayering of low-silica mafic metamorphites (SiO" ca 50 per cent), with high-silica felsic metamorphites (SiO" ca 75 per cent). These features, when viewed against the plausible similarities in protoglobal geologic processes, suggest a common mode of origin for the' Peninsular Gneissic Complex' of the Southern Indian Shield and the' Ancient Tonalitic Gneisses' of the Southern African Shield-both representing rocks that consolidated from a distinctive tonalitic that was widely generated and developed from a major differentiation episode within the earth's upper mantle (the development of this magma being largely a complementary response to the immediately preceding vast out­ pouring of the largely pristine komatiites - recognized in the Southern Indian Shield also). Several of the circular, or oval-shaped 'granitic'. plutons within the' Peninsular Gneissic Complex' of Karnataka might be tonalitic-granodioritic diapiric domes formed by the reactivation, rheo­ morphism, and remobilization of the 'Peninsular Gneisses' (S. Viswa­ nathan).

Recommendations The major conclusions and recommendations of the participating scientists are: (l) Terms such as 'Peninsular Gneisses', and 'Dharwars' that have got entrenched in the Indian geological literature shot!ld be defined rigorously, precisely, and unambiguously. It is important to evolve appropriate chronolithostratigraphic terminology that would facilitate easy comparisons of the various rock components of the Indian Shield with those in other Shields. (2) Critical areas, each of about 1000 sq. km iii different regions of the Southern Indian Shield should be identified, and subjected to detailed studies involving structural-geologic, stratigraphic, petrographic, geo­ chronometric, isotopegeologic, geochemical, ore-geologic, and geophysical techniques. The institutions capable of undertaking these studies are: Geological Survey of India (structural-geologic, stratigraphic, and petro­ graphic studies); Atomic Minerals Division, and the Indian Institute of Science (geochronometric, isotope-geologic, and geochemical studies); Chitradurga Copper Company, and Manasa Gangotri (ore-geologic studies); National Geophysical Research Institute (geophysical and geochemical studies). (3) The geophysical studies should be aimed at determining the depths of the various greenstone (schist) belts. PENINSULAR GNEISSIC COMPLEX 243 (4) The margins of the greenstone belts, for example, the contact relations of the auriferous Kolar greenstone belt with the Peninsular gneiss, require closer scrutiny than hitherto accorded. Likewise, the contacts between the Peninsular gneisses, and the suspected mobile belts should also be investi­ gated. (5) The Sargur schists, whose constituent rock units occur as enclaves, and as linear belts or bands within the 'Peninsular Gneissic Complex' of the Dharwar Craton, is reported to be older than the other greenstone belts of the Craton because of its higher metamorphic grade (upper to lower granulite facies), and the truncation of its structural trends by the greenstone belts. In view of this possibility, critical areas of the Sargur schist belt should be examined. (6) The structural styles of the suspected mantled domes within the' Peninsular Gneissic Complex', for example, the Saulanga and Honnali gneisses, should be worked out by the modern techniques ofstructural analysis. (7) The limits, if any, of' migmatisation fronts' in the Dharwar Craton should be mapped. (8) In disciplines where facilities are not available within the country, for example, application of oxygen, hydrogen, and sulphur isotope geo­ chemistry, fluid inclusion techniques, and electron probe microanalysis to problems of the' Peninsular Gneissic Complex', national ego should not stand in the way of establishing contacts with competent earth scientists of the western world, and in collaborating with them in joint research projects. (9) The Geological Society of India will give quick publicity to research papers that contain new information and meaningful data based on innovative approaches to any enigmatic aspect of the 'Peninsular Gneissic Complex' by promptly publishing them in its quarterly Journal. The proceedings of the group discussion will be published soon by the Society.

S. VISWANATHAN B. P. RADHAKRISHNA

ANNEXURE 1 List of Participants 1. BALASUBRAMANYAN, M. N., Director, Geological Survey of India, 27, J. N. Road, Calcutta-700013. 2. DIVAKARA RAo, V., Scientist, National Geophysical Research Institute, Uppal Road ,- Hyderabad-500007. 3. MURTHY, M. V. N., Deputy Director-General, Geological Survey of India, Hyderabad-500009. 4. MURTHY, N. G. K., Director, Geological Survey of India, 35, Haddcws Street, Madras-600006. 5. NARAYANASWAMY, S., Deputy Director-General, Geological Survey of India, Airborne Mineral Surveys and Exploration Wing, Bangalore. 244 PROCEEDINGS 6. PERRAJU, P., Geological Survey of India, Hyderabad. 7. PICHAMUTHU, C. S., President, Geological Society of India, 1, Hayes Road Cross, Bangalore-560025. 8. QURESHY, M. N., Director, Department of Science and Technology, Techno­ logy Bhavan, New Delhi-l10029. 9. RADHAKRISHNA, B. P., Secretary, Geological Society of India, 16, Ali Asker Road, Bangalore-560052. 10. RAMAKRISHNAN, M., Senior Geologist, Geological Survey of India, Circle, Jayanagar, Bangalore-560011. 11. RAo, P. S., Geological Survey of India, 35, Haddows Street, Madras-600006. 12. SANKARAN, A. V., Scientific Officer, Atomic Minerals Division, 1-11-252/1, Begumpet, Hyderabad-500016. 13. SUGAVANAM, E. B., Senior Geologist, Geological Survey ofIndia, 35, Haddows Street, Madras.600006. 14. SWAMI NATH, J., Director, Geological Survey of India, Mysore Circle, Jaya­ nagar, Bangalore-560011. 15. UDAS, G. R., Director, Atomic Minerals Division, 1-11-200, Begumpet, Hyderabad-500016. 16. VENKATASUBRAMANIAN, V. S., Indian Institute of Science, Bangalore-560012. 17. VISWANATHA, M. N., Geological Survey ofIndia, Bangalore. 18. VISWANATHAN, S., Scientific Officer, Atomic Minerals Division, 1-11-252/1, Begurnpet, Hyderabad-500016. 19. VISWANATHAN, T. V., Geological Survey ofIndia, Hyderabad. 20. ZIAUDDIN, M., Director, Geological Survey of India, Mysore Circle, Jayanagar, Bangalore-560011.

ANNEXURE 2 Themes and Papers Presented

THEME 1. The Problem and Historical Perspective 1. Problems of the Peninsular Gneissic Complex of the Southern Indian Shield. By PROF. C. S. PICHAMUTHU. 2. Evolution of thoughts on the Peninsular Gneissic Complex of the Dharwar Craton. By DR. B. P. RADHAKRISHNA.

THEME 2. Geochronology, Structure, and Stratigraphy 1. Review of isotopic age data on the Precambrian gneisses and granites of the Southern Indian Shield. By PROF. V. S. VENKATASUBRAMANIAN. 2. Major Problems in correlation of the Precambrian gneisses and granites of the Southern Indian Shield. By DR. M. N. BALASUBRAHMANYAN. 3. Structural and tectonic features of the Peninsular gneissic terrain. By S. NARAYANASWAMY. 4. Structural and tectonic features of the Peninsular gneissic terrain of Tamil Nadu and Kerala. By E. B. SUGAVANAM. PENINSULAR GNElSSIC COMPLEX 245

THEME 3. Geochemistry 1. Major, and trace-element geochemistry of the Precambrian gneisses and granites of the Southern Indian Shield. By DR. S. VISWANATHAN. 2. Isotope geochemistry of the Precambrian gneisses and granites of the Southern Indian Shield. By PROF. V. S. VENKATASUBRAMANIAN. 3. Geochemistry of the pre-, and post-Dharwar gneissic rocks from Karnataka and its significance. By DR. S. M. NAQVI, DR. V. DIVAKARA RAO* and K. SATYANARAYANA.

THEME 4. Regional Geologic Studies 1. The Peninsular gneisses of the Southern Indian Shield: a strategy for studies. By DR. M. V. N. MURTHY. 2. Peninsular Gneissic Complex of Karnataka State. By M. ZIAUDDIN. 3. Basement-cover relationships among Peninsular gneisses, greenstone belts and high-grade schists of Southern Karnataka. By M. RAMAKRISHNAN *, M. N. VISWANATHA, and J. SWAMI NATH. 4. The Peninsular Gneissic terrain of Kerala. By P. S. RAO. 5. Peninsular Gneisses of Tamil Nadu. By N. G. K. MURTHY. 6. 'Peninsular Gneiss' in northern parts of Andhra Pradesh. By P. PERRAJU * and V. NATARAJAN.

THEME 5. Evolutionary Models 1. Peninsular Gneissic Complex and related rocks m place and time. By M. ZIAUDDIN.

THEME 6. Some New Approaches to the Problem 1. Application of rare earth geochemistry to problems of the Peninsular gneissic complex of the Southern Indian Shield. By DR. A. V. SANKARAN. 2. studies in interpreting the origin of granitic rocks-an approach to the problem of Peninsular gneisses. By T. V. VISWANATHAN. 3. Application of oxygen isotope geochemistry to problems of the Precambrian gneisses and granites of the Southern Indian Shield. By DR. S. VISWA­ NATHAN. 4. Application of remote sensing to problems of locating new mineral deposits in the Precambrian gneissic-granitic terrains of the Southern Indian Shield. By DR. M. N. QURESHY.

• Speaker.