The Dharwar , southern , 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, 560 028, India t Department of Mines and Geology, Government of , 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 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 and the Group nous . 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

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- suite whose precursors had short periods of crustal residence (Bhaskar Rao ct al 1983; Monrad 1983; Stroh et al 1983; Jayara