Precambrian geochronology and the geological record

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.

Pre-Brasiliano Orogenic Evolution in the Seridó Belt, Ne Brazil: Conflicting Geochronological and Structural Data

“…The 1.0 Ga mineral dates are thus consistent with an older, pre-Brasiliano age for D 2 , coupled with syn to late-D 3 reheating, perthitization and other unmixing textures of the K-feldspars, leading to the isotope disequilibria observed in these systems. The mineralogy of the rock offers few sites for retention of radiogenic Sr liberated from their crystal lattices (Moorbath & Taylor 1985). This is also reflected in the rock's bulk composition; for instance, the two most discrepant whole rock samples in Fig.…”

Sequence stratigraphy in Proterozoic successions

“…Biostratigraphic and paleomagnetic techniques, for example, provide only limited age resolution (e.g., Elston and Bressler, 1980;Bertrand-Sarfati and Walter, 1981;Vidal and Knoll, 1983). Isotopic ages from Proterozoic rocks are commonly problematic because isotopic systems tend to be reset by younger thermal events, and detrital minerals may yield the age of source terranes rather than the time of deposition (Moorbath and Taylor, 1985). Prominent angular unconformities have been recognized, but the limitations of existing dating have led Proterozoic strata to be widely regarded as relatively conformable, even in successions as much as several kilometres thick (e.g., Crittenden et aI., 1971;Preiss and Forbes, 1981).…”