DATING THE OLDEST TERRESTRIAL ROCKS - FACT AND FICTION

Abstract

This paper is a review of well-documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks, mainly using Rb/1bSr, Sm/1bNd and U/1bPb decay schemes. The oldest known stable sialic crust of true continental character, predominantly composed of calc-alkaline orthogneisses, was produced during major mantle-differentiation some 3.7−3.5 Ga ago and is represented on several continents. Enclaves of older supracrustal rocks, as well as a major remnant of a volcano-sedimentary sequence of greenstone belt affinity dated at ∼3.8 Ga, are preserved within the surrounding gneisses in southern West Greenland. Recent ionmicroprobe evidence for ∼4.1-Ga-old detrital zircon grains in a younger early Archaean metasedimentary sequence in Western Australia is also discussed.Several cases are reviewed where claims for early Archaean (> 3.4 Ga) dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data. Some of the geological and geochemical mechanisms which have given rise to claims for spuriously old Sm/1bNd, U/1bPb and Pb/Pb dates are critically discussed.To supplement published data, we present several new sets of whole-rock Sm/1bNd and Pb/Pb isotopic data from West Greenland, Labrador, Zimbabwe and eastern India relevant to the overall topic of this review.