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The Archaean Peninsular Gneiss of southern India is considered by a number of workers to be the basement upon which the Dharwar supracrnstai rocks were deposited. However, the Peninsular Gneiss in its present state is a composite gneiss formed by synkinematic migmatization during successive episodes of folding (DhFI, DhFI, and DhF2) that affected the Dharwar supracrustal rocks. An even earlier phase of migniatization and deformation (DhF,) is evident from relict fabrics in small enclaves of gneissic tonalites and amphibolites within the Peninsular Gneiss. We consider these enclaves to represent the original basement for the Dharwar supracrustal rocks. Tonalitic pebbles in conglomerates of the Dharwar Supergroup confirm the inferenoe that the supracrustal rocks were deposited on a gneissic basement. Whole rock Rb-Sr ages of gneisses showing only the DhF1 structures fall in the range of 310(O3200 Ma. Where the later deformation (DhF 2) has been associated with considerable recrystallization, the Rb-Sr ages are between 2500 Ma and 2700 Ma. Significantly, a new Rb-Sr analysis of tonafitic gneiss pebbles in the Kaldurga conglomerate of the Dharwar sequence is consistent with an age of ,,-2500 Ma and not that of 3300 Ma reported earlier by Venkatasubramanian and Narayanaswamy (1974). Pb-Pb ages based on direct evaporation of detrital zircon grains from the metasedimentary rocks of the Dharwar sequence fall into two groups, 3300-3100Ma, and 2800-3000 Ma. Stratigraphic, structural, textural and geochronologic data, therefore, indicate that the Peninsular Gneiss of the Dharwar craton evolved over a protracted period of time ranging from > 3300 Ma to 2500 Ma.

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Stromatolites associated with cherty dolomites of the Vanivilaspura Formation of the Archaean Dharwar Supergroup show a morphology indicative of the deposition of the latter in a intertidal to subtidal environment. The cherts are moderately high in their A1/A1 + Fe ratios but depleted in Fe203 and also most trace elements. Unlike most other Archaean cherts, the Vanivilaspur cherts exhibit significant negative Ce anomaly, which is interpreted to have resulted from contemporary manganese deposition. The Rb/Sr ratios in the cherts show a sufficient spread to define a linear correlation line in the Rb-Sr evolution diagram corresponding to an age of 2512 • 159 Ma and initial Sr ratio of 0.7128 • 0.0012 (2a). While this age is strikingly close to that of regional metamorphism in the Dharwar craton, the initial ratio is distinctly higher than that of the associated volcanics. Acid leaching experiments on the cherts suggest that they may have been isotopically equilibrated on a mm to cm scale about 500 Ma later than the time of regional metamorphism.

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