Petrography, chemistry and isotopic ages of Peninsular Gneiss, Dharwar acid volcanic rocks and the Chitradurga granite with special reference to the late Archean evolution of the Karnataka craton, Southern India

Abstract

New petrographic, major and trace element, and isotopic age data are presented for four suites of rocks from the low- to medium-grade part of the South Indian Archaean craton in central Karnataka. Two suites are from the Peninsular Gneiss basement to the late Archaean Dharwar Supergroup, the third is a suite of Dharwar potassic rhyolites and the fourth is from the Chitradurga Granite which has intrusive contacts with the Dharwar Supergroup. 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 ± 110 Ma and 3175 ± 45 Ma, respectively, an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7013 ± 0.0009 and a μ₁ value (²³⁸U/²⁰⁴Pb ) of 7.98. Host tonalitic gneisses to the Chikmagalur Granite give an Rb---Sr whole-rock isochron date of 3060 ± 160 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7015 ± 0.0004 and a Pb/Pb whole-rock isochron date of 3190 ± 100 Ma with a model μ₁ value of 8.00. 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 ± 100 Ma and 3028 ± 28 Ma, respectively, with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7035 ± 0.0013 and a model μ₁ value of 7.62. The suite of Dharwar acid volcanic rocks from north of Honnali defines a poorly fitted ²⁰⁷Pb/²⁰⁶Pb linear array (MSWD 15.8) which gives an apparent age of 2565 ± 28 Ma and a model μ₁ value of 7.46. 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 ± 18 Ma with a model μ₁ value of 7.68. The U---Pb relations in these suites from Karnataka are in marked contrast with the commonly severe U depletion and, consequently, unradiogenic Pb isotopic compositions observed in deeply eroded, high-grade Archaean gneiss terranes such as those bordering the North Atlantic. This contrast indicates that the cratonic rocks in central Karnataka represent a relatively high level in the original Archaean continental crust.