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ARCHAEAN greenstone belts have commonly been correlated on the basis of similarities in their stratigraphic successions1. Conclusive radiometric tests of these correlations have rarely been made, because of the difficulty of dating the rocks of the greenstone belts themselves, and because of uncertainties in their relationships to the more easily dated granitoid rocks with which they are normally associated. We report here radiometric evidence from eastern Africa, which clearly contradicts previous lithostratigraphic correlations between the Tanzanian and Kaapvaal Cratons by establishing a minimum age difference between the uppermost parts of their respective greenstone belt successions.

THE Archaean–Proterozoic boundary, it has been suggested, marks a fundamental change in the behaviour of the Earth, ending an early phase of cratonisation and the beginning of a period characterised by stable shelf sedimentation and linear orogens1,2,3. Almost all Archaean terrains, other than those dominated by granulites and migmatites, seem to be characterised by a variety of granitoid rocks intimately associated with ultramafic through mafic to calc-alkalic sequences, and their sedimentary associations. These greenstone belt assemblages are remarkably similar from one craton to another; they are usually folded into keel-shaped troughs, subjected to greenchist facies metamorphism and usually show similar structural styles which suggest that they were acted on by predominantly vertical forces. Their alleged antiquity (2,000 Myr or more) and widespread distribution have been attributed to either a change in the geothermal gradient as the Earth became colder3 or the existence of a relatively thin, Archaean, continental crust4,5.

A RECONSTRUCTION of the continental blocks for the central and North Atlantic has led to the widespread acceptance of the continuity of the Palaeozoic orogenic belts on both sides of the Atlantic. A reconstruction of the land masses during the Carboniferous and Permian shows the Variscan Belt of Europe, the Mauritanides of Morocco, and the Southern Appalachians as being continuous1. Although the Hercynian Front has now been recognised in New Brunswick2 the situation in Newfoundland is unclear. We suggest, on the basis of the geochemical data outlined in Table 1, that the continuation of these belts can be interpolated between New Brunswick and south-west England to include at least the Central Mobile Belt and the Avalon Zone of Newfoundland. We find the earlier correlation by Holmes3 particularly attractive in view of our new isotopic data, a correlation which extends the Hercynian Front westwars from south-west England and Wales into Canada, where it passes along the Cabot Fault into the New Brunswick seaboard and the Bay of Fundy (Fig. 1). Cherkis et al.4 link the Charlie Gibbs Fracture Zone to the Hercynian Front on both the European and North American continents, and have recently offered new evidence for the emergence of the front in Newfoundland5. The intersection of the Charlie Gibbs Fracture Zone and the continental shelf of Newfoundland lies about 100km east of the Cabot Fault, so that their delineation of the front lies further to the south than the one outlined here.

THE susceptibility of fine-grained, volcanic rocks to open-system behaviour during regional metamorphic activity has long been suspected in Rb/Sr geochronology1–5. The evidence that we present here shows that not only can open-system behaviour occur, but that it is partly a function of bulk chemical composition. We have studied the Burlington Peninsula area of northern Newfoundland (Fig. 1) which is underlain by the Cape St John Group, a volcanic pile of sub-aerial, calc-alkalic volcanic and sedimentary material, about 3,500 m thick. Coarse calcareous units and minor conglomeratic beds form its basal part; overlying units consist of basic volcanic flows and sills, commonly vesicular, associated with andesitic and silicic pyroclastic rocks and flows. The Cape St John Group is similar to calc-alkalic rocks associated with modern island arcs, although there is a predominance of high-silica rocks in the former; a marked compositional gap between 62 and 67% SiO2 for the Cape St John succession corresponds to a lack of high-silica andesite and low-silica dacite and illustrates its bimodal nature. Sub- to low-greenschist facies metamorphism has affected the entire volcanic pile, and the metamorphic grade gradually increases from the southern part of the peninsula to the north, where it reaches amphibolite facies. An east-west trending syncline forms the major structure in the area and at least four phases of folding have been documented6. The Cape St John Group rests unconformably on the Snooks Arm Group (Arenig)7,8 and should, therefore, be younger than about 500 Myr.

A recent Rb–Sr study of selected suites of samples, mainly from Precambrian alkaline rocks of northern Ontario, may have some bearing on mantle differentiation and evolution of the continental crust. We show here that initial 87Sr/86Sr ratios from most of the complexes, plotted as a function of age, define a straight line, and imply a source region for the Sr with a Rb–Sr ratio of 0.018 ± 0.002 that has behaved as a closed system for at least 2,600 Myr. Furthermore, the intersection of this line with the development line for ‘bulk Earth’ at about 2,800 Myr can be interpreted as evidence for a mean time of differentiation of bulk Earth material into an enriched crust and a residual, depleted mantle.

WE present here geochronological evidence that parts of the Central Mobile Belt and the Avalon Zone of Newfoundland have been contiguous since the Ordovician or earlier, and that some time after about 4 Myr, significant movement took place along part of the boundary.

WHOLE-ROCK Rb–Sr isochrons from granitoid rocks intruded into the easternmost part of the Central Mobile Belt and the Avalon Platform of Newfoundland indicate that the bulk of the plutonic activity took place during Devonian and Carboniferous times. Of the nine intrusions that were dated, only one, a Cambrian granite from the Avalon Platform, was significantly older.

PALAEOMAGNETIC evidence does not support the model of a supercontinent made up of Laurentia and West Gondwanaland existing between 2,200 and 1,800 Myr BP. The palaeomagnetic poles (except those based on fewer than 10 samples) from the Laurentian Shield for the interval 2,200 to 1,800 Myr BP are shown in Fig, 1. The poles form a reasonably coherent array stretching from what is nowadays the Mackenzie delta to Peru, This polar path is referred to as polar track 5 (ref. 1), and the sense of apparent polar motion is probably from north to south.

The Cambridge Encyclopedia of Earth Sciences. Edited By David G. Smith. Pp.496. US ISBN 0-517-54370-2; UK ISBN 0-521-23900-1. (Crown/Cambridge University Press: 1982.) $34.95, £19.95.

McGraw-Hill Encyclopedia of the Geological Sciences, 2nd edn. Editor-in-chief Sybil P. Parker. McGraw-Hill:1988. Pp.722. $85, £65.