Section 20
Chapter 19,705

Petrography, chemistry and isotopic ages of Peninsular Gneiss, Dharwar acid volcanic rocks and the Chitradurga Granite with special reference to the late Archaean evolution of the Karnataka Craton, southern India

Taylor, P.N.; Chadwick, B.; Moorbath, S.; Ramakrishnan, M.; Viswanatha, M.N.

Precambrian Research 23(3-4): 349-375


ISSN/ISBN: 0301-9268
DOI: 10.1016/0301-9268(84)90050-0
Accession: 019704661

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The Chikmagalur Granite (granodiorite), which with its host gneisses forms part of the basement (Peninsular Gneiss) to the Dharwar Supergroup in the Bababudan basin, yields Rb-Sr and Pb/Pb whole-rock isochron dates of 3080+ or -110 Ma and 3175+45 Ma, respectively. Host tonalitic gneisses to the Chikmagalur Granite give an Rb-Sr whole-rock isochron date of 3190+ or -100 Ma. A suite of granites and granodioritic-tonalitic gneisses (Chitradurga granitic rocks and gneisses) forming part of the Peninsular Gneiss basement to the Dharwar Supergroup west of the Chitradurga belt yields Rb-Sr and Pb/Pb whole-rock isochron dates of 2970+ or -100 Ma and 3028+28 Ma. The suite of Dharwar acid volcanic rocks from north of Honnali defines a poorly fitted (super 207) Pb/ (super 206) Pb linear array (MSWD 15.8) which gives an apparent age of 2565+ or -28 Ma. The poor fit of this "isochron" shows that a closed system was not maintained, a conclusion supported by high K abundances which are attributed to a syn- or post-magmatic ion exchange process. The Chitradurga Granite which intrudes the older rocks of the Dharwar Supergroup in the Chitradurga belt yields a well-fitted Pb/Pb whole-rock isochron date of 2605+ or -18 Ma.

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