Abstract

A comparison of average values of U–Pb, Rb–Sr, and K–Ar isotopic dates from Precambrian shields in North America, Europe, and northern Asia suggests three major orogenies common to all areas. U–Pb and Rb–Sr age determinations, qualified by the more numerous K–Ar data, suggest mean values of 2600 ± 100 m.y., 1800 ± 100 m.y. and 1000 ± 100 m.y. An earlier event at 3300 to 3500 m.y. is locally recognizable in the Baltic Shield, the Ukrainian Shield, and the north-central United States. Anorogenic igneous events intermediate between the major orogenies are indicated in the Canadian, Ukrainian, and Aldan Shields, but these are as yet unmatched outside their own restricted settings.Isotopic dates are divided into two categores: anorogenic and orogenic. Within a specified orogeny, dates are further divided into precrystalline, syncrystalline, and postcrystalline. Regional correlations ideally should be based only on syncrystalline dates, recording times of final crystallization, unmodified by subsequent processes except initial cooling. Depth of crystallization and rate of uplift determine the difference between true and apparent time of crystallization. Precrystalline dates (survival values) and postcrystalline (rejuvenated or overprinted) dates should be excluded from the designated span of major orogenies.Rigorous statistical treatment of isotopic dates seems unwarranted on the basis of uncertainties in the geologic history of most rock bodies. Histograms may aid in the identification of anorogenic and precrystalline dates.

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