The Dharwar craton, southern India, and its Late Archaean plate tectonic setting: current interpretations and controversies BRIAN CHADWICK*, V N VASUDEV @and G V HEGDEt * Earth Resources Centre, University, Exeter EX~ 4QE, UK 120/~5(A) III Block, Thyagarajanagar, Bangalore 560 028, India t Department of Mines and Geology, Government of Karnataka, Bangalore 560 027, India In spite of detailed geological investigations of the Dharwar craton since the 1890s, its principal lithological units, structure and chronology remain contentious. Important new work on lithostrati- graphy, basin development, structure, geochemistry and geochronology has led to wide-ranging speculation on the Late Archaean plate tectonic setting. Much of the speculation is based on uniformitarian models which contrast with a recent proposal that the evolution of the craton was controlled by gravity-driven processes with no crustal shortening. 1. The Dharwar craton The craton can be subdivided into two principal parts (figure 1) which are separated by a steep belt of The Late Archaean Dharwar craton (figure 1), in the mylonites, cl-l.5km wide, trending approximately sense of Ramakrishnan (1993), is an important part of N-S or NW-SE (Chadwick et al 1989). The belt of the collage of Archaean and Proterozoic terrains in mylonites has been interpreted as a thrust by many Peninsular India. Areas east and south of the craton workers on the grounds of a relatively shallow, are characterized by structures, metamorphism and easterly dipping reflector identified by Kaila et al igneous bodies related to the Pan-African assembly of (1979), but there is no obvious curvature along the Gondwana, but the interior of the craton largely length of its outcrop to suggest that the steep mylo- escaped significant Pan-African overprinting. The nites are part of a listric structure. Shallow and northern margin of the craton is concealed by steeply plunging linear fabrics in the mylonites Proterozoic sedimentary rocks and the Deccan lavas, (Chadwick et al 1989) are ambiguous in terms of the whereas the east is overlain by the Meso-Neoproter- principal displacement direction within the belt. ozoic Cuddapah basin. Late Archaean metamorphism in much of the western part of the craton varies from LT greenschist to amphibolite facies in contrast with 2. Western part of the craton HT greenschist to amphibolite facies in the eastern part which is related to the emplacement of volumi- 2.1 Peninsular Gneiss and the Sargur Group nous granite. Effects and possible causes of Late Archaean granulite facies metamorphism in the West of the mylonite belt the craton is characterized extreme south of the craton have been reviewed by by Late Archaean volcanic and sedimentary rocks Hansen et al (1995), among others. This contribution (Dharwar Supergroup; Swami Nath et al 1976) that comprises a brief review of recent findings in the were deposited in the period c2900-2600 Ma (Taylor Dharwar craton north of the terrain affected by et a11984; Nutman et a11996; Kumar et a11996) on a Archaean granulite facies metamorphism. sialic basement of orthogneisses and granodiorites Keywords. Dharwar craton; Late Archaean; plate tectonics; Dharwar batholith. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 106, No. 4, December 1997, pp. 249-258 Printed in India 249 250 Brian Chadwick et al Taylor et al 1984; Bhaskar Rao et al 1991; Naha et al 1993; Peucat et a11993). Whereas much of the Penin- sular Gneiss is part of a tonalite-trondhjemite- granodiorite suite whose precursors had short periods of crustal residence (Bhaskar Rao ct al 1983; Monrad 1983; Stroh et al 1983; Jayaram et al 1983; among others), its tectonic evolution prior to c2900Ma is poorly understood. The gneisses include tracts of older volcanic and sedimentary rocks and stratiform gabbro-anorthosite complexes (Sargur Group; Swami Nath et al 1976). SHRIMP and single grain evaporation ages of zircon (Nutman et a11992; Ramakrishnan et a11994; Peucat et al 1995) show that the Sargur Group has an age of c2960-3300 Ma, and it includes detrital zircon grains as old as 3580 Ma. It is likely that the Sargur Group includes a number of different tracts of supracrustal rocks of different ages. The Sargur Group includes banded iron formations, fuchsite quartzites of detrital origin (Chadwick et al 1986; Argast 1995), metape- lites, basaltic amphibolites, metaperidotites and stra- tiform gabbro-anorthosites: the depositional, volcanic and tectonic setting remains unclear. Deformation of the Sargur Group in the Hole- narsipur belt (figure 1) took place in three distinct Figure 1. Simplified geological map of the Late Archaean periods (Chadwick et a11978). Bouhallier et al (1993) Dharwar craton (ornamented) in southern Peninsular India: argued that the structure of this belt was controlled DS: Dharwar Supergroup; PG: Peninsular Gneiss (basement by diapiric emplacement of the adjacent components to the Dharwar Supergroup); H: Holenarsipur schist belt, of the Peninsular Gneiss in amphibolite facies condi- > c.3000Ma; m: steep belt of mylonites (see text); black: Late Archaean volcano-sedimentary schist belts in the eastern part tions and subsequent sinistral transcurrent shearing in of the craton (Hu: Hutti; K: Kolar; R: Ramagiri; S: Sandur); greenschist facies conditions. The diapiric emplace- DB: Dharwar batholith; g: Late Archaean granulite facies ment appears to have taken place prior to c2900 Ma, metamorphism; KA: Kibbanahalli Arm (see text). Blank areas whereas the sinistral shearing appears to be Late are mostly Proterozoic and younger rocks, including Archaean. Chardon et al (1996) extended the diapiric CB: Cuddapah basin; PSZ: Proterozoic shear zones; PAT: model to structures in the Peninsular Gneiss and terrain dominated by Pan-African phenomena. B: Bangalore; C: Chitradurga; Ma: Madras; My: Mysore; T: Trivandrum. Sargur rocks which underlie the Dharwar Supergroup in the Kibbanahalli Arm (figure 1) west of the Chitra- durga schist belt. They noted that orthogneisses are which are known collectively as the Peninsular Gneiss. generally located in domes, whereas Sargur supra- Many workers have included the younger Archaean crustal rocks are mainly found in intervening basins. plutonic rocks in the eastern part of the craton as part This distribution contrasts with the common inter- of the Peninsular Gneiss following W F Smeeth who sheeting of the gneisses and Sargur rocks elsewhere in coined the term nearly a century ago. Because of the the western part of the craton, the intersheeting confusions that have arisen with its application to having been due principally to intrusion of the rocks with a wide range of ages (see Naha et al 1993; supracrustal rocks by precursors of the gneisses. The Srinivasan and Naha 1996, for recent examples), intersheeted relationship suggests that a significant Radhakrishna and Vaidyanadhan (1994) proposed sub-horizontal regime of plutonic intrusion and that the term be abandoned. They suggested that the deformation characterized much of the Peninsular term Older Gneiss Complex be used for gneisses older Gneiss before the generation of upright structures than 3000 Ma, and Younger Gneiss Complex for those like those described by Bouhallier et al (1993) and c2600 Ma. Because the younger gneisses are a high- Chardon et al (1996). strain component of the Dharwar batholith which is discussed later, we recommend that the term Penin- 2.2 Dha~war Supergroup sular Gneiss be retained, but restricted to gneisses older than c2900 Ma. Although the unconformable relationship between the The basement orthogneisses and low-strain plutonic Dharwar Supergroup and its basement of Peninsular rocks in the western part of the craton have ages in Gneiss is disputed by some workers (Naha et al 1993; the range c2900-3300Ma (Beckinsale et al 1980; Srinivasan and Naha 1996), the field evidence south of The Dharwar craton in southern India 251 Bababudan, north of Sigegudda, west of the Chitra- The second stage of basin development saw the durga belt and on the west and east of the accumulation of alluvial and shallow marine fans, Kibbanahalli Arm points overwhelmingly to an uncon- debris flows, quartz arenites, greywackes and local formable relationship which shows that deposition of stromatolitic limestones, with sporadic basic and the Dharwar Supergroup did not begin until cooling, acid volcanism. The coarse polymict conglomerates uplift and peneplanation of the basement had taken in the alluvial fans and debris flows include not place after c3000Ma (Swami Nath et al 1976). The only crowded clasts of Dharwar metabasalt, gabbro, age of the Dharwar Supergroup is constrained to the banded iron formation and quartzite, but also period c2900-2600 Ma on the grounds of the age of its orthogneisses which show that areas of basement basement, intrusion of the Chitradurga Granite were elevated and eroded during basin development. (2605 • 18 Ma; Taylor et a11984) and a Pb/Pb meta- The final stage was marked by deposition of a morphic recrystallization age of 2639 • 32 Ma of lime- widespread, but relatively thin, banded iron formation stones from various parts of the Dharwar stratigraphy which was followed by thick fine-grained greywackes (Russell et al 1996). SHRIMP zircon age data from with intercalations of chert and volcanic rocks. acid volcanic rocks at a relatively high stratigraphic The mixed-mode basin development and volcanic level indicate melt crystallization at 2614• facies (Bhaskar Rao and Drury 1982; Drury 1983) (Nutman et al 1996), whereas well constrained arrays suggest an incipient back-arc or an active continental of Sm-Nd data interpreted as isochrons by Kumar margin, with similarities in the Mesozoic-Cenozoic et al (1996) show that some of the oldest basic setting of New Zealand and the western United States volcanic rocks in the supergroup are 2911 • (Chadwick et al 1992). old and other volcanic suites low in the stratigraphic Interpretation of the Late Archaean structure of the sequence are 2848 i 70Ma and 2747 • 15Ma old. Dharwar Supergroup and its sialic base~nent is Isotopic whole rock ages of 2565 + 28Ma (Pb/Pb; controversial (Mukhopadhyay 1986).