Rubidium-Strontium Whole-Rock Ages of Banded and Incipient Charnockites from Southern Karnataka

Abstract

The Late Archaean regional amphibolite facies to granulite facies progression in southern Karnataka contains two textural varieties of charnockite (orthopyroxene-bearing quartzofeldspathic rocks). Banded chamockites typified by the banded pyroxene gneisses in the vicinity of Halaguru are thought to be older than the "incipient" coarse-grained charnockitic alteration of am.phibolite facies gneisses, as at Kabbaldurga, and similar alteration of the banded charnockites in many places south of there.

Whole-rock Rb-Sr isochrons were obtained for banded chamockites from the Chillapura quarry near Halaguru, and for incipient charnockite in amphibolite facies gneiss from a quarry near Honganuru, just east of Chamarajnagar. Large multi-layered samples and individual layers (light versus dark layers) were analysed separately in an attempt to discriminate older and younger metamorphic ages. For both quarries, all data yielded good isochrons with ages near 2.5 Ga, or terminal Archaean (2.50 ± 0.05 Ga with initial ⁸⁷Sr/⁸⁶Sr = 0.70500 ± 0.00009 for Chillapura and 2.54 ± 0.17 Ga, 0.7088 ± 0.0016 for Honganuru). More than one age of charnockitic metamorphism based on isotope systematics of samples collected from single quarries is not evident.

The high initial Sr ratios for both localities suggest, however, that the country rocks in southern Kamataka had a protracted crustal history prior to 2.5 Ga ago. Moreover, when the mean isotopic data of both quarries are plotted on the same diagram, "a regional isochron" close to 3.0 Ga with low initial Sr ratio results. This plausibly corresponds to a crustal accretion age, in accord with the interpretaiton of previous whole-rock isotope studies of this region based on combining data from widely-separated quarries. A widespread high-grade metamorphic event coinciding with, or following shortly after, crustal accretion remains an open possibility.