The generation and evolution of the continental crust

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					Journal of the Geological Society, London, Vol. 167, 2010, pp. 229–248. doi: 10.1144/0016-76492009-072.




Review

                           The generation and evolution of the continental crust

     C . J. H AW K E S WO RT H 1,2 * , B. D H U I M E 1 , A . B. P I E T R A N I K 3 , P. A . C AWO O D 4, A . I . S . K E M P 5
                                                        & C . D. S TO R E Y 1,6
     1
       Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK
                 2
                   Present address: School of Geography and Geosciences, University of St. Andrews, North Street,
                                                      St. Andrews KY16 9AL, UK
                         3
                           Institute of Geological Sciences, University of Wrocław, 50-205 Wrocław, Poland
   4
     School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
            5
              School of Earth and Environmental Sciences, James Cook University, Townsville, QLD 4811, Australia
               6
                 School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth PO1 3QL, UK
                                        *Corresponding author (e-mail: cjh21@st-andrews.ac.uk)

                     Abstract: The continental crust is the archive of the geological history of the Earth. Only 7% of the crust is
                     older than 2.5 Ga, and yet significantly more crust was generated before 2.5 Ga than subsequently. Zircons
                     offer robust records of the magmatic and crust-forming events preserved in the continental crust. They yield
                     marked peaks of ages of crystallization and of crust formation. The latter might reflect periods of high rates of
                     crust generation, and as such be due to magmatism associated with deep-seated mantle plumes. Alternatively
                     the peaks are artefacts of preservation, they mark the times of supercontinent formation, and magmas
                     generated in some tectonic settings may be preferentially preserved. There is increasing evidence that
                     depletion of the upper mantle was in response to early planetary differentiation events. Arguments in favour of
                     large volumes of continental crust before the end of the Archaean, and the thickness of felsic and mafic crust,
                     therefore rely on thermal models for the progressively cooling Earth. They are consistent with recent estimates
                     that the rates of crust generation and destruction along modern subduction zones are strikingly similar. The
                     implication is that the present volume of continental crust was established 2–3 Ga ago.




The continental crust constitutes some 40% of the surface area                  Plank 2005; Hawkesworth & Kemp 2006a,b) and return of
of the Earth, and yet it constitutes almost 70% of the total                    residue or cumulate to the mantle. Differentiation of the
volume of the Earth’s crust. It is andesitic in composition, 25–                continental crust primarily involves igneous processes, and an
70 km thick, and less dense than the thinner (,10 km) oceanic                   idealized crustal section consists of a lower part dominated by
crust of largely mafic composition. The differentiation of the                   residue or cumulate and/or new mafic crust and an upper part
crust of the Earth into these contrasting chemical–mechanical                   composed mainly of rocks of granitic to granodioritic composi-
components in part reflects the horizontal movement of the                       tion. The residence times of elements in the upper crust appear
lithosphere (crust and upper mantle) through plate tectonics. The               to be much longer than those in the lower crust (Hawkesworth &
contrasting density structure of continental and oceanic crust                  Kemp 2006b).
results in a pronounced bimodal elevation, a buoyant continental                   The longstanding questions are when the continental crust was
crust and an oceanic crust that is, except for very young rocks,                generated, and how the processes involved in the generation of
gravitationally unstable (e.g. Cloos 1993) and sinks back into the              the continental crust have changed with time. This review is
asthenospheric mantle at subduction zones, resulting in no                      written at a time of considerable conceptual upheaval when old
oceanic crust being older than 200 Ma.                                          approaches are being questioned and new analytical techniques
   The continental crust is the geological archive of Earth history,            have recently come into play. It has long been argued that the
it is different from analogues on nearby planets, and it influences              upper mantle was depleted by the extraction of the continental
global climate by being a sink for CO2 (e.g. Garrels & Perry                    crust, and so the two reservoirs are complementary (Jacobsen &
1974; Zhang & Zindler 1993; Lowe & Tice 2004). It is andesitic                                                             `
                                                                                Wasserburg 1979; O’Nions et al. 1980; Allegre et al. 1983). The
in composition, and such magmas are not commonly in equili-                     implication was that the record of continental crust generation
brium with the upper mantle (e.g. Rudnick 1995; Walter 2003).                   could then be broadly investigated from the evolution of the
Most models for the generation of new continental crust there-                  depleted upper mantle. If that depleted mantle reservoir is well
fore involve the generation of basalt and subsequent differentia-               mixed, its radiogenic isotope ratios should offer a more robust
tion, by fractional crystallization and/or remelting, to higher                 indication of the volumes of crust extracted than the remaining
silica compositions (Kuno 1968; Ellam & Hawkesworth 1988;                       vestiges of old continental crust. Yet recent evidence suggests
Arndt & Goldstein 1989; Kay & Kay 1991; Rudnick 1995;                           that the upper mantle was initially depleted by processes much
Arculus 1999; Kemp & Hawkesworth 2003; Zandt et al. 2004;                       older than the preserved continental crust (Carlson & Boyet

                                                                          229
230                                                        C . J. H AW K E S WO RT H E T A L .

2008; Tolstikhin & Kramers 2008). Similarly, it has been widely                Rousseau 1984). Strictly speaking, they therefore describe the
assumed that the geological record provides a representative                   increase in the volume of continental crust that survived for long
record of the evolution of the continental crust, and yet now                  enough for the isotope ratios to change in response to radioactive
there is increasing evidence that it may be biased by the tectonic             decay, which is of the order of several hundred million years. As
settings 
				
DOCUMENT INFO
Description: The continental crust is the archive of the geological history of the Earth. Only 7% of the crust is older than 2.5 Ga, and yet significantly more crust was generated before 2.5 Ga than subsequently. Zircons offer robust records of the magmatic and crust-forming events preserved in the continental crust. They yield marked peaks of ages of crystallization and of crust formation. The latter might reflect periods of high rates of crust generation, and as such be due to magmatism associated with deep-seated mantle plumes. Alternatively the peaks are artefacts of preservation, they mark the times of supercontinent formation, and magmas generated in some tectonic settings may be preferentially preserved. There is increasing evidence that depletion of the upper mantle was in response to early planetary differentiation events. Arguments in favour of large volumes of continental crust before the end of the Archaean, and the thickness of felsic and mafic crust, therefore rely on thermal models for the progressively cooling Earth. They are consistent with recent estimates that the rates of crust generation and destruction along modern subduction zones are strikingly similar. The implication is that the present volume of continental crust was established 2-3 Ga ago. [PUBLICATION ABSTRACT]
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