The evolution of early Precambrian crustal rocks of Isua, West Greenland-geochemical and isotopic evidence

Petrographic and chemical evidence suggests that boulders from a conglomeratic unit in the Isau supracrustal succession were derived by the erosion of an acid volcanogenic sediment. Six samples of the boulders and surrounding matrix yield a Rb-Sr whole rock isochron with a slope corresponding to an age of 3860+-240m.y.(2 sigma error), but consideration of the initial 87Sr/86Sr ratio constrains the possible age of formation of 3710+0-90)m.y. This is in general agreement with a published Pb/Pb age of 3760+- 70m.y. on Isua banded ironstones. (Auth.)