Isotopic dating of very early Precambrian amphibolite facies gneisses from the Godthaab district, West Greenland

Abstract

Amphibolite-facies feldspathic gneisses (the Amitsoq gneisses) from the Godthaab area of West Greenland give hitherto unique age and isotope data, including (i) a Rb-Sr whole-rock isochron age of 3980 ± 170 my. (t 1/2 = 5.0 × 10 10y) with an initial 87Sr/ 86Sr ratio of 0.6992 ± 0.0010, (ii) a Pb-Pb whole-rock isochron age of 3620 ± 100 my, and (iii) extremely unradiogenic whole-rock leads, with 206Pb/ 204Pb and 207Pb/ 204Pb values extending down to 11.51 and 13.14 respectively. Taken together, these data provide the first direct evidence for the existence of a granitic crust so early in the Earth's history. The isotopic evidence also demonstrates that these rocks could not have had a lengthy (i.e. more than a few tens of millions of years) history prior to the Rb-Sr isochron age. Possible interpretations of the isochron ages are discussed in detail. Several other petrologically distinctive rocks from the same area are shown to have been affected by thermal events somewhere in the range 2000–2800 my ago.