Archaean lithological associations

Archaean Cratons contain three different types of lithological association, each of which provides important information about the early stages of the Earth System. These associations are Archaean greenstone belts, late Archaean sedimentary basins, and Archaean granite-gneiss terrains. We will consider each in turn. 

TEXT BOX 1.1 Radioactive dating methods applicable to the early Earth 

1 FIGURE 1 Isochron diagram for the Sm-Nd system showing a suite of 13 samples which formed at the same time, but which have slightly different Sm/Nd ratios. The calculated age for the 13 samples is 3,540 30 Ma. 

Isochron age calculations are commonly made for the Rb-Sr (rubidium-strontium], Sm-Nd (samarium-neodymium], and U-Pb (uranium-lead] radioactive systems. They are most commonly applied to whole-rock systems, that is, a suite of samples thought to have formed at the same time, such as an igneous plu-ton or a suite of lavas. Isochron age calculations may also be made for a suite of minerals in a rock, in which case they date the time at which the minerals lost isotopic contact with each other, that is, became closed systems. This approach can be useful in dating metamorphism. 

Isochron age calculations are as good as the assumptions behind them - namely that the samples all formed at the same time and that they have been geochemically undisturbed since that time. The best isochron ages will have errors of a few tens of millions of years on an age of 2000-3000 million years. 

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Later in this chapter, each of the major Archaean lithological associations - greenstone... 

TABLE 1.4 Key lithological associations recognized in Archaean greenstone belts (modified from Eriksson et ah, 1994). ... 

Eriksson et al. (1994) defined seven dominant lithological associations, which they used as a basis for defining the different tectonic environments of greenstone belt formation in the Archaean (Table 1.4). What is clear from their analysis is that tectonic environments between 3.8 and 2.5 Ga were remarkably similar to those in the modern Earth. This study also emphasizes the fact that greenstone belts... 

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