U–Pb Ages for Sphene from Early Precambrian Igneous Rocks in Northeastern Minnesota – Northwestern Ontario

Abstract

Five sphene concentrates from early Precambrian igneous granitic rocks in northeastern Minnesota and northwestern Ontario give 207Pb–206Pb ages of 2680–2750 m.y., which are in good agreement with discordant U–Pb ages for zircon and whole-rock Rb–Sr ages (λβ = 1.39 × 10−11y−1) from these rocks. Three of the sphene concentrates give concordant ages, two discordant.Although the rocks have undergone at least one low-grade metamorphism, which has affected some of the K–Ar and Rb–Sr mineral ages from the region, the 207pb–206pb ages for sphene do not appear to have been affected.An electroplating technique was employed for recovering the lead from the sphenes after decomposition in perchloric and hydrofluoric acid.