Ceol. Sac. Malaysia, Bulletin 6, July 1973; pp. 213-226. Geochronology of the Indian Precambrian * M.S. BALASUNDARAMI AND M.N. BALASUBRAHMANYAN1 Abstract: Precambrian geochronology of India is now known with greater precision because of the dating of samples carefully se lected, in respect of geological consideration and set up, from several areas in the pas t decade. Vestiges of older nuclei dating more than 3000 m.y. have been identified in southern, eastern and western India. The Charnockite-Khonda­ lite supergroup (c.2900-2600 m.y.) is now considered to be older than the Dharwar super­ group; the Eastern Ghats (sensu stricto) rocks are now considered belonging to the Charnoc­ kite-Khondalite supergroup proper but with impress of events at 1650 m.y. and 1400 m.y. at about the same time as events in Cuddapah Basin, Nellore Schist Belt and Amgaon group. The Aravalli, Bailadila, Bengpal and Iron Ore groups are correlated wi th the Dharwar and range in age from c.2500-<:.2000 m.y. The Delhi supergroup appears to be coeval with the lavas of the Gwalior 'series'. Vindhyan sedimentation which started at c.1400 m.y. continued into Cambrian (?) un­ like the Cuddapah supergroup where sedimentation ended by about 1300 m.y. The success­ ively younger dates for the mobile belts from south to north, with respect to the southern nucleus, indicates a migration of geosynclines in the same direction. The Satpura 'group' despite valid e.vidences for its abolition as a supergroup appears to indicate the reactivation of the Narmada-Son lineament at 950/ 1000 m.y. in part or eastern India. The manifestation of carbonatite and anorogenic granites dating 725 m.y. in southern India, high-level granites in western India, dating 730 m.y., Mylliem granite at 765 m.y. in eastern India and carbona­ tite in Rajasthan at 960 m.y. appear to be rel ated to reactivation of lineaments in these areas at these dates. The overprint of minera I ages at c.500 m.y. along the east coast is also suggest­ ive of activity of lineaments but further studies are necessary to confirm this view. INTRODUCTION Dates on I~ss than a score of minerals were available when Arthur Holmes (1955, p. 81) presented the first account of the geochronology of the Precambrian tectonic cycles in the Indian Shield. At present, nearly six hundred dates are available re­ presenting a much wider spectrum of the Indian Shield: (Aswathanarayana, 1956; 1959 ; 1962 and 1964; Sarkar et al., 1964 ; 1967 and 1969; Venkatasubramanian er al. , 1968 and 1971 ; Crawford I 969a, 1969b and 1970 ; Crawford and Compston, 1970). A review of the stratigraphy and geochronology by Sarkar (1968) has been supplemented by a review by Chaterji et al., (/971). Even fission track methods have been applied for pegmatite minerals (Mehta el al. , 1968 and 1970). At present many institutions in India, including the Geological Survey of India, have commissioned mass spectro­ meters and a wealth of data will be flowing from these institutions in the years to come. PRECAMBRIAN DATES General Fig. I depicts the histograms of isotopic ages of the Indian Precambrian Shield. Even though it will be hazardous to interpret the average of dates or the histograms (Cahen and Snelling, 1966, p. 16), it is now fairly well established that peaks of histo­ grams do reflect orogenies (Clifford, 1968 , p. 302). An attempt is made here to focus attention on the dates of major tectonic events and also elucidate the concordance or discordance between different methods of dating in the same group of rocks. * Published by kind permission of the Director General, Geological Survey of India. 1 Geological Survey ofTndia, Calcutta. 214 M. S. BALASUNDARAM AND M. N. BALASUBRAHMANYAN 30 Ii d. K - Ardatu 20 10 " ~ IT1 20 C . Rb- Sr dates 0 C IT1 b. U -Th/Pb, Common Pb. Pb'£' dofes Z () -< o. All Dotes 500 1000 1500 2000 2500 3000 3500 ISGTOPIC AGE IN MILLION YEARS M .N .8olclubrohmonya n G. S . I . Fig. 1. Indian Precambrian Dates (Histograms) Histograms a, b, c and d of Fig. 1 represent respectively the dates available i) by all the methods ii) by U-Th/Pb method, lead alpha method and common lead method and iii) by Rb/Sr methods pertaining to single whole rock analysis with initial ratio at 0.7 and mineral dates and iv) K/Ar method on whole rock basic igneous rocks and mineral dates. The histogram 'a' represents major events at 500 m.y., 950-1000 m.y., 1500- 1600 m.y., 2100 m.y., 2600 m.y. , 3100-3200 m.y. and 3500 m.y. matched by U­ Th/Pb date peaks at 1000 m.y., 1600 m.y., 2600 m.y. and 3500 m.y. The lack of other dates is possibly due to lack of datable material only. The peaks of Rb/Sr dates are at 500 m.y., 800 m.y.-900 m.y., 1500 m.y., 2100 m.y., and 2600 m.y.-2700 m.y. However Rb-Sr whole rock isochrons (Fig. 2) are concordant to the peaks of U-Th/Pb at 1000 m.y. , 1700 m.y., 2000 m.y. and 2600 m.y. , indicating that these pertain to major pluto­ nic activity. The KjAr dates with peaks at 500 m.y., 900 m.y., 1500-1600 m.y., 2100 m.y., 2300 m.y. , 2500 m:y., and 3200 m.y;, based as they are mainly on mineral dates, lend to reflect the metamorphic and uplift dates in general. GEOCHRONOLOGY OF THE INDIAN PRECAMBRIAN 215 - '- - ,-; --"~-l fl,. 2 THE INDIAN ~CA"BAIAH (S~~~) 't . .. ' ~. j' ... , .. _ ... ... u::::iJJiJ .... ~ ....... ............... I)M ••• " ... 0: ..,,. ••• ~ ' ,"" =G8 .:;::..;.!. ';':....~.: ..:: ;:-; '."., ..,., • • gQ · .......... .... ·1·. .. .... ·_ ... .. ··00 .. , CJI!IJ t ... " . .... , " ... .... .. ... .... ... \e . .. , ,"110 .. , ~ s ....,.'I •••~ . s " ••.•. G4 .... ' . ' •• I . U . .. t ' ••• ,."oo .. , . ,/ ~ oo,~ . ..... " ... U IClO .. , o:J ..... ~ .. •. _ .... _ ... ,." ..... 1lO . .. '_' I" " . 2 ~ ClO .. , ~ :::.:::~::2:::~ '::: ., .'.: ~" , .I., ~ " ... ,... ~ ......., . Fig. 2. The Tndi an Preca mbrian (Shield) Relating these peaks to the regions where they are pertinent (Fi g. 2), it is obse rved that: (i) a 500 m.y. event is seen only in southern, east coast and Shillong plateau areas and it indicates a biotite rejuvenation date due to ( ?) reactivation of basement fractures. (ii) a 950-1000 m.y. event in eastern India, in what has been described as areas showing a 'Satpura trend' pari passu with magmatism in northwest Tndi a. This event is perhaps similar to the 500 ill.y. event. (iii) a 1600- 1700 m.y. event related to the development ofCuddapah Basin with effusion of lavas, Nellore Schist Belt with emplacement of pegmatites, east coast with a metamorphic/uplift event and in northeast India with emplace­ ment of granitic bodies. (iv) a 2100 m.y. event related to the retrogression of granulite facies rocks to amphibolite facies in southern India and migmatisatioll of the associated paragneisses. 216 M. S. BALASUNDARAM AND M. N. BALASUBRAHMANYAN (v) a 2600 m.y. plutonic event of crystallisation of charnockites under granulite facies conditions in the region pari passu with regional migmatisation result­ ing in the evolution of Peninsular Gneisses of Mysore-Hyderabad region. and (vi) a 3000 m.y.-3100 m.y. plateau and a peak at 3500 m.y. revealing nuclei in southern India, eastern India and western India. The periodicity of the events is similar to those in other continents, particularly similar to the geochronology of Africa (Clifford, 1968, p. 303) except for the event at 1400 m.y. which is recorded insignificantly in India. Classification into supergroujJs Fig. 3 details the stratigraphy and geochronology of the different supergroups in the Indian Precambrian. The approximate time range in the tectonic and post tectonic evolution of these are also detailed together with histograms of dates pertinent to the supergroups shown. The Dharwar supergroup is shown separately from that of the Charnockite-Khondalite supergroup; the Eastern Ghats formations belong to the Charnockite-Khondalite group, but certain dates have attributes of the adjoining Cuddapah supergroup also and hence shown alongside for comparison of dates. Earlier views that the Charnockites represent metamorphosed Dharwar and that they belong to two periods (Pichamuthu, 1962) and they intrude the Dharwar (Rama Rao, 1936) are now questionable hypotheses. Evidences have been presented by Narayanas­ wami (1963) , Iyengar and Banerji (1971) and Balasundaram and Balasubrahmanyan (1971) that the Charnockite-Khondalite supergroup forms a craton to the Dharwar. On the basis of isotopic age data, the east coast Eastern Ghats rocks have been thought to be of a younger age (Crawford, 1969b), in relation to their southern equivalents. The status of the Nellore Schist Belt, which is now included in the Dharwar super­ group, will be subjected to review perhaps on more modern geological data and isotopic age, as suggested by Crawford (1968). The data for central India groups ofKhairagarh, Sausar, Sakoli, Amgaon and as those of the Singhbhum region are tentative and will have to await Rb/Sr studies as the present data rely on K /Ar dates only. The events in the Aravalli and Delhi supergroups together appear to span the time range from 3000 m.y. to 700 m.y. and future work will throw much light on the detailed correlation of the region. Thus Fig. 3 is partly based on existing data as such and partly on reinter­ pretation of such data by the authors, well supported by other workers, as already referred to. Charnockite-Khondalite Supergroup In lithology, regional structure and metamorphism, the rock formations in the Charnockite Province of Fermor (1936) are similar. The province comprises the eastern coastal tract of Eastern Ghats group of Andhra Pradesh and Orissa and the southern extensions in Tamil Nadu and Kerala proper.