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 exceptional impetus to the examine of early Precambrian terrains. As past due because the mid-sixties the Archaean nonetheless constituted the 'terra-in cognita' of earth technology. excessive metamorphic grades, terrible out crop, and never least a largely assumed obliteration of early crustal documents by way of convective recycling and thermal remodeling had com bined to deter examine during this box. many fantastic neighborhood stories existed, particularly round gold mining centres, yet remained unrelated to a broader nearby and theoretical figuring out. this case has replaced because the outcome of 2 inter-related components: (1) advances in isotopic tools and their program to Precambrian rocks, and (2) the popularity that the various oldest terrains have retained a wealth of fundamental igneous and sedi mentary textures or even geochemical characteristics.
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Extra info for Archaean Geochemistry: The Origin and Evolution of the Archaean Continental Crust
In this respect they resemble subduction zone arc basalts. However, in similarity with Archaean mafic volcanics such as spinifex-textured basalts, they do not have an obvious Sr overabundance. These chemical characteristics suggest that they were derived from mantle sources which have previously been affected by Archaean mantle enrichment processes related to eclogite melting. It is necessary to point out, however, that the possibility of a metasomatic enrichment process operating under the subcontinental lithosphere (without subduction) and different from that operating under the oceanic lithosphere has not yet been critically evaluated.
There are two basic patterns observed among Archaean mafic volcanic rocks (Fig. 7). Samples of the first type show coherence of major and trace elements. On a chondri tic normalized plot they have flat or light REE depleted, heavy REE flat patterns and Ti0 2/P 20 s ratios close to 10. Chondritic ratios are commonly observed among refractory lithophile elements such as Ti/Zr, Ti/Y, Ti/heavy REE, AlITi, and Ca/Ti. This regularity is basically the same as that observed in modern 'typical' MORB (Fig.
These samples have AI 20 3/Ti0 2 ratios considerably lower than chondrites. In this regard (and on account of heavy REE depletion) they are similar to the Barberton type komatiites. A third type of pattern, which is rarely observed among Archaean samples, was proposed by Jahn et al. (1982). ). It also shares geochemical characteristics of modern boninites. A literature survey indicates that the degree of elemental decoupling in Archaean basalts depends on the extent of light REE enrichment. The extreme cases are found in some light REE highly enriched basalts from Finland (Jahn et al.
Archaean Geochemistry: The Origin and Evolution of the Archaean Continental Crust by H. Wänke, G. Dreibus, E. Jagoutz (auth.), Professor A. Kröner, Professor G. N. Hanson, Professor A. M. Goodwin (eds.)