Archaean-Proterozoic boundary

Archaean cratons have long been prime targets for a broad array of scientific studies, partly because they form the oldest cores of the continents, but also because they have economic significance as a major source of the world s mineral wealth. (The term craton is confined here to Archaean crust, although the word is commonly applied to Proterozoic shields as well. The Archaean-Proterozoic boundary is, as always, imprecisely defined, but in southern Africa is marked by a sufficient hiatus in time that there is no difficulty... 

Fig. 1. Relative probability histograms of Slave craton detrital zircons (continuous curve with black infill below based on data from Sircombe et al. 2001), Ar/ Ar ages of impact spherules in lunar soil samples (dash-dot curve after Culler et al. 2000), and Ar/ Ar ages of impact glasses in lunar meteorites (dashed curve after Cohen et al. 2000). Time interval spans from 4500 Ma, the approximate age of formation of the Moon, to 2500 Ma, the defined Archaean-Proterozoic boundary. Vertical scales of the three curves are independent. Shaded age bars with roman numerals represent main events in basement of the Slave craton that were initially defined on the basis of individual rock age and their inheritance (see Bleeker Davis 1999). The detrital zircon data represent c. 300 zircon grains from five widely distributed samples of a c. 2800 Ma quartzite unit overlying the Mesoarchaean to Hadean-age basement complex of the Slave craton. These data represent a least-biased record of pre-2.8 Ga components of the Slave craton. The broad complementarity in the datasets should be noted. With the first major peak in Slave crustal ages (event V 3100-3200 Ma) immediately following the last major peak in the lunar spherule data. Both lunar soil and meteorite data sets support a lunar cataclysm or late heavy bombardment that appears to have erased or swamped out the pre-4.0Ga lunar record.

Richter, F.M., 1988. A major change in the thermal state of the earth at the Archaean-Proterozoic boundary consequences for the nature and preservation of continental lithosphere. J. Petrol. (Special Lithosphere Issue), 39-52. 

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