Abstract
SummaryRb-Sr, Sm-Nd and Pb/Pb whole-rock isochron methods and the U-Pb zircon method have been widely applied to dating Precambrian igneous and meta-igneous rocks. Application of these methods in regional geochronological studies provides reliable constraints on the age and temporal evolution of rock units, even when some parent-daughter systems have suffered disturbance in metamorphic and/or metasomatic events which significantly post-date primary rock formation. Initial isotopic compositions of Sr, Nd and Pb aid geological interpretation of isochron age data, constrain crustal residence time of the protoliths and can be used to assess the relative contributions of mantle and crust in magma genesis.Precambrian sediments can be indirectly dated by reference to age data on interbedded volcanics, or by age-bracketing between dated basement rocks and dated cross-cutting intrusives. Significant progress has also been made in the direct dating of sedimentary rocks. Published isotopic age determinations on early Precambrian sediments containing biogenic remains are briefly reviewed. Provided that stratigraphical and geochronological correlations are correct, it appears that stromatohtic limestones were being deposited by c.3400 - 3300 Ma ago, and that true microfossils occur in rocks dated at c.3200 - 3000 Ma. Evidence relating to possible biogenic markers in sediments reliably dated at c.3800 - 3700 Ma is not yet definitive.
Publisher
Geological Society of London