THE NATURE OF THE BASEMENT IN THE ARCHEAN DHARWAR CRATON OF SOUTHERN INDIA AND THE AGE OF THE PENINSULAR GNEISS

The Archaean Peninsular Gneiss of southern India is considered by a nu mber of workers to be the basement upon which the Dharwar supracrustal rocks were deposited. However, the Peninsular Gneiss in its present s tate is a composite gneiss formed by syn-kinematic migmatization durin g successive episodes of folding (DhF1, DhF1a and DhF2) that affected the Dharwar supracrustal rocks. An even earlier phase of migmatization and deformation (DhF) is evident from relict fabrics in small enclav es of gneissic tonalites and amphibolites within the Peninsular Gneiss . We consider these enclaves to represent the original basement for th e Dharwar supracrustal rocks. Tonalitic pebbles in conglomerates of th e Dharwar Supergroup confirm the inference that the supracrustal rocks were deposited on a gneissic basement. Whole rock Rb-Sr ages of gneis ses showing only the DhF1 structures fall in the range of 3100-3200 Ma . Where the later deformation (DhF2) has been associated with consider able recrystallization, the Rb-Sr ages are between 2500 Ma and 2700 Ma . Significantly, a new Rb-Sr analysis of tonalitic gneiss pebbles in t he Kaldurga conglomerate of the Dharwar sequence is consistent with an age of approximately 2500 Ma and not that of 3300 Ma reported earlier by Venkatasubramanian and Namyanaswamy (1974). Pb-Pb ages based on di rect evaporation of detrital zircon grains from the metasedimentary ro cks of the Dharwar sequence fall into two groups, 3300-3 100 Ma, and 2 800-3000 Ma. Stratigraphic, structural, textural and geochronologic da ta, therefore, indicate that the Peninsular Gneiss of the Dharwar crat on evolved over a protracted period of time ranging from > 3300 Ma to 2500 Ma.