By H. Wänke, G. Dreibus, E. Jagoutz (auth.), Professor A. Kröner, Professor G. N. Hanson, Professor A. M. Goodwin (eds.)

Archaean Geochemistry 1972 - 1984 The realisation that the continental crust includes well-preserved relics which date way back to 4/5 of the Earth's age has given an outstanding impetus to the research of early Precambrian terrains. As overdue because the mid-sixties the Archaean nonetheless constituted the 'terra-in­ cognita' of earth technology. excessive metamorphic grades, bad out­ crop, and never least a generally assumed obliteration of early crustal documents by way of convective recycling and thermal transforming had com­ bined to deter learn during this box. many fantastic neighborhood reports existed, particularly round gold mining centres, yet remained unrelated to a broader neighborhood and theoretical figuring out. this example has replaced because the final result of 2 inter-related components: (1) advances in isotopic equipment and their software to Precambrian rocks, and (2) the popularity that a few of the oldest terrains have retained a wealth of fundamental igneous and sedi­ mentary textures or even geochemical characteristics.

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Extra resources for Archaean Geochemistry: The Origin and Evolution of the Archaean Continental Crust

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Recently, Cattell et al. (to be published) and Chauvel et al. 6) and Kambalda, W. 4). These data strongly suggest mantle source isotopic heterogeneities and/or contamination by sialic crust within a single greenstone belt. 5 Ga ago. g. Tilton 1983; Chauvel et al. 1983, 1984). 7 Ga) is indicated by available data. ll and eNd values has yet been established. It is important to point out that, as in the case of Sr and Nd isotope studies, crustal contamination may also be responsible for some of the Pb isotope variation especially those observed in layered intrusive mafic-ultramafic complexes.

Sun and Nesbitt 1978b). Enrichment of Rb, Ba, and K are commonly observed but not well defined due to secondary mobility problems. The occurrences in several localities in Yilgarn and Pilbara, Western Australia (Sun and Nesbitt 1978b) and in Barberton, South Africa (Jahn et al. 1982), would suggest that this type of volcanic rock is common in the Archaean. Other examples of major/trace elemental decoupling are tholeiites from Theo's flow in Munro Township, Canada (Arndt and Nesbitt 1982). One picrite sample from this flow shows continuous enrichment from heavy REE to light REE.

1982; Menzies et al. 1982) also suggest that reactivation of ancient sub-continental lithosphere, triggered by lithosphere subduction or continental rifting, could be a major mechanism for generation of much of the modern continental basalts. 6 Effect of Archaean Tectonism and Magmatism on Mantle Evolution On the basis of the discussions presented above, it is expected that Archaean and pre-Archaean magmatic and mantle-crust interaction processes have extensively modified the chemical composition of the primitive mantle and have caused longlasting effects on the chemical and isotopic composition of the Earth's upper mantle.

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