Rb-Sr and Pb-Pb Whole Rock Isochron Ages of Basement Gneisses in Karnataka Craton

Abstract

Rb-Sr and Pb-Pb whole rock isotopic systematics and petrochemical data for two suites of Peninsular Gneiss immediately underlying the first order regional unconformity at the base of the main greenstone successions of Dharwar Supergroup in Bababudan and Chitradurga belts are presented.

The Chikmagalur granite (s.l.) and associated migmatite gneisses (CMG suite) underlying the Bababudan greenstones define an eleven point Rb-Sr isochron of 3080 ± 80 m.y. (MSWD4.5) with an initial ⁸⁷Sr/⁸⁶86 isotope ratio (I.R.) of 0.7016 ± 0004. Pb isotope data for the same samples yield a 3185 ± 60 m.y. isochron (MSWO 7.0) with single stage 238_U/204_Pb (μ₁) ratio of 7.99. The crust-forming event in this area spans a range of 3000-3200 m.y. within the analytical error of the isochrons. The mantle-type I.R. and μ₁ values preclude interaction with or reworking of earlier continental crust in their genesis. Petrological and chemical studies of the CMG suite show that the Chikmagalur 'granite' is a biotite granodiorite and the intruded gneisses are low alumina trondjhemites having a calc-alkaline trend. It is significant that the CMG suite is located within a low strain zone in Central Karnataka.

The gneisses and granitoids close to the western margin of the Chitradurga belt (CDG Suite) give a Rb-Sr age of 2970 ± 100 m.y. (MSWD 11) with I.R. of 0.7035 ± 0.0013. The corresponding Pb-Pb isochron defines a date of 3044 ± 150 m.y. with μ₁ = 7.69. The μ₁ and I.R. values suggest the possible involvement of previous sialic crust in their generation during the gneiss-forming event around 3000 m.y. ago. The CDG suite is heterogeneous, but all its components are regarded as a part of the Peninsular Gneiss. The composition of the CDG suite has a wide range from tonalite to granite, with a typical calcalkaline granitic trend.

Recent intensive geochronological studies in central Karnataka suggest a widespread crust-forming event around 3000 m.y. together with older gneissic components of Ca 3300 m.y. The present data bridge the time gap between 3000 and 3300 m.y. suggesting that continental crust was being generated in Karnataka craton from 3300 to 3000 m.y. semi-continuously. The unambiguous field relations in central Karnataka suggest that this event occurred prior to the deposition of main greenstone piles of Dharwar Supergroup.