Peninsular Gneiss - Geological Age

Geological Age

Initial studies on the Gneiss samples of the Lalbagh hill and other locations in Bangalore, carried out in the early 1970s, have attributed two major events of 2.9 –3.0 Ga and 2.5 Ga –2.6 Ga for development of the Peninsular Gneisses of Bangalore. Recent studies carried out with precision techniques indicate that the gneisses have accreted in the following major episodes.

Geological/Radiometric ages	Study Region/Belt
3.4 Ga.	The oldest Peninsular Gneiss found in areas in the Hosur – Gorur – Holenarsipur – Hunsur belt
3.3–3.2 Ga.	The second generation Peninsular Gneiss found in the Bangalore– Chickmagalur –Holenarsipur region
3.0–2.9 Ga	a) Grandiorite facies in the Bangalore Gneisses established by the Single Zircon Kober evaporation 207Pb/206Pb data. b) In the Dharwar Craton studies done on the trondhjemites of Holenarsipur, major crust forming event are attributed through the Rb–Sr whole rock isochron ages. c) In Karighatta – Kunigal areas also, a similar age is indicated. In this case, data of U–Pb SHRIMP of Zircons from amphibolite facies gneisses from Kobaladurga and Kober single zircon evaporation ages of grey gneisses have been considered

Geological/Radiometric ages

Study Region/Belt

3.4 Ga.

The oldest Peninsular Gneiss found in areas in the Hosur – Gorur – Holenarsipur – Hunsur belt

3.3–3.2 Ga.

The second generation Peninsular Gneiss found in the Bangalore– Chickmagalur –Holenarsipur region

3.0–2.9 Ga

a) Grandiorite facies in the Bangalore Gneisses established by the Single Zircon Kober evaporation 207Pb/206Pb data.

b) In the Dharwar Craton studies done on the trondhjemites of Holenarsipur, major crust forming event are attributed through the Rb–Sr whole rock isochron ages.

c) In Karighatta – Kunigal areas also, a similar age is indicated. In this case, data of U–Pb SHRIMP of Zircons from amphibolite facies gneisses from Kobaladurga and Kober single zircon evaporation ages of grey gneisses have been considered

Thus, three major episodes, namely, 3.4 Ga., 3.3–3.2 Ga., and 3.0–2.9 Ga., are accreted to form the Peninsular Gneiss of the region, dated 2500 to 3400 million years.

The Sargur schist belts within the Peninsular Gneiss could be the oldest suture zones in the Indian subcontinent.

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