Continental crust Archaean

Green M. G., Sylvester P. I., and Buick R. (2000) Growth and recycling of early Archaean continental crust geochemical evidence from the Coonterunah and Warrawoona Groups, Pilbara Craton, Australia. Tectonophysics 322, 69-88. 

The two layers in continental lithosphere represent extreme end-members in the compositional spectrum of magmatic rocks and each forms through independent processes. Archaean continental crust has an allochthonous relationship to underlying lithospheric mantle. 

Luais, B. Hawkesworth, C. 1994. The generation of continental crust an integrated study of crustforming processes in the Archaean of Zimbabwe. Journal of Petrology, 35, 43-93. 

Taylor, P. N., Moorbath, S., Goodwin, R. Petrykowski, a. C. 1980. Crustal contamination as an indicator of the extent of early Archaean continental crust Pb isotopic evidence from the late Archaean gneisses of West Greenland. Geochimica et Cosmochimica Acta, 44, 1437-1453. 

Bickle, M. J. Nisbet, E. G. 1993. The Geology of the Belingwe Greenstone Belt, Zimbabwe - a Study of the Evolution of Archaean Continental Crust. Geological Society of Zimbabwe Special Publication, 2, 239. 

Dougherty-Page, j. S. 1994. The evolution of the Archaean continental crust of northern Zimbabwe. PhD thesis, Open University, Milton Keynes. 

Of these, late Archaean sedimentary basins are the most "modern" in their form and can be clearly recognized as upper crust. Similarly, the relatively low metamorphic grade of many Archaean greenstone belts indicates that they too represent upper Archaean crust. In contrast, Archaean granite-gneiss terrains more commonly belong to the deeper continental crust and variously represent middle to lower continental crust. There are only a few places... 

Kapuskasing Canada 2.5-2.8 An exposed Archaean crustal cross section from upper to lower continental crust... 

The growing consensus that ancient crust is always underlain by ancient lithospheric mantle has been challenged recently by Wu et al. (2003). These authors report an unusual absence of xenoliths with Archaean ages beneath the Archaean North China Craton. They propose that in some cases, therefore, ancient subcontinental mantle can be removed from beneath ancient continental crust by delamination - a process which has previously been postulated but never demonstrated. The subject of mantle delamination is discussed more fully in Chapter 5, Section 5.5.2. 

If therefore, the modern subarc mantle is the site where Phanerozoic subcontinental lithosphere is created, we are still left with a large number of questions about the earlier history of the subcontinental lithosphere. Why for example is the Archaean subcontinental lithosphere so different in composition, heat production and thickness from more recent subcontinental mantle What different processes were operating early in Earth history which are recorded in this mantle domain Is there a link with komatiite extraction, as suggested by Boyd (1989), or with the extraction of basaltic melts Or, is there a close link between the formation of this type of mantle and the over-lying continental crust We will return to these issues when we discuss the origin of the continental crust in Chapter 4 (Section 4.5.1). 

Fragments of the Archaean mantle -xenoliths from the subcontinental lithosphere As geochronological determinations on xenoliths from the subcontinental lithosphere become more robust, a consistent observation is emerging - that the subcontinental lithosphere beneath Archaean continental crust is very ancient (see Section 3.1.3.2). Recent studies... 

Alternative views of early Archaean mantle evolution require that mantle depletion started as early as ca. 4.5 Ga (see compilation in Rollinson, 1993). These models imply significant mantle Sm-Nd fractionation in the very early Archaean and have major implications for the differentiation of the early Earth. One such study is that of Bennett et al. (1993) who measured very high eNd values (+3.5 to +4.5) in 3.81 Ga Amitsoq gneiss samples. Collerson et al. (1991) also calculated an isochron eNd value of +3.0 for 3.8 Ga-old peridotites from northern Labrador. The extreme deviation from CHUR early in Earth history (Fig. 3.27) was interpreted by Bennett et al. (1993) as evidence for an extreme and very early fractionation of the Earth s mantle relative to CHUR. Such an event implies the formation of extensive continental crust prior to 3.8 Ga, for which there is no independent geological evidence. This apparent paradox and the claim for very early extensive mantle differentiation led to a detailed reexamination of the Bennett... 

Current models of Archaean crust generation imply that subduction is a relatively ancient process operating since at least about 3.5 Ga. An outstanding question is the relative contribution that plume magmatism has made to the generation of felsic continental crust. 

The purpose of this chapter therefore, is to explore in detail how the Earth came to be so different from its planetary neighbors in possessing an extensive, evolved continental crust. In particular it will be argued that a major portion of the Earth s continental crust had formed by the end of the Archaean and that... 

Archaean crust-forming processes play a pivotal role in shaping the structure of the Earth s continental crust,. .. so that... we need to increasingly focus our attention on the distant past, as the key to understanding the present (Kemp Si Hawkesworth, 2003)... 

Granitoid magmas in the compositional range Tonalite-Trondhjemite-Granodiorite. These are the magmas which have built the Archaean continental crust... 

FIGURE 4.7 A plot of Nb/U ratio over time, showing the range of values for primitive mantle, the continental crust and the depleted mantle (represented by MORB and OIB and various Archaean and proterozoic basalts). Also shown is the curve for Nb/U versus age from Collerson and Kamber (1999). 

Their best-fit models are shown in Fig. 4.8 and support a progressive growth model for the continental crust, with about 50% crust formed by the end of the Archaean. Their calculations show that there was little crustal recycling before the mid-Proterozoic. 

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