Secular evolution of the continental crust: Implications for crust evolution models

Abstract

[1] The present-day flux from mantle to crust is basaltic and yet the average composition of the continental crust is andesitic. This is the crust composition paradox. A new solution to this paradox is proposed whereby the secular evolution in the composition of the continental crust reflects a changing flux from mantle to crust over time. Thus it is proposed that the present-day composition of the continental crust is a time-integrated average. Crustal growth curves show that 48-54% of the continental crust was formed by the end of the Archaean. A mass balance model based upon a tonalite-trondhjemite-granodiorite compositional model for the Archaean continental crust shows that the post-Archaean mantle to crust flux was predominantly basaltic and likely a mix of arc-plume basalts. Trace element modeling, however, reveals that additional processes also contributed to the average crust composition. Balancing Y, Ho, and Yb concentrations requires a garnetiferous mafic granulite lower Archaean crust, which in turn drives the post-Archaean flux toward a high mg # andesite. This suggests that there was a slab melt contribution to the continents, in addition to basalt. An excess of fluid mobile elements in the continental crust can be explained either by the addition of a slab melt or small fraction melts. A deficiency in Sr requires that the post-Archaean crustal composition has been modified by erosion. Both Archaean and post-Archaean continental crust contain contributions from basalt and a slab melt. In the Archaean crust the slab melt contribution is dominant. In the post-Archaean crust the basaltic contribution is dominant.

On the conditions of magma mixing and its bearing on andesite production in the crust

“…Conversely, an increase in crust thickness, correlated to a lower mantle heat flux, are bound to produce less, but more mafic, hybrids, and thus drive the bulk composition of the continental crust toward a more mafic composition. This may have helped to produce an Archean continental crust more silicic than the Phanerozoic one 64 , as there are no reasons to believe that mixing was a less important petrogenetic mechanism in the Archean than nowadays.…”

Detrital zircon evidence for Hf isotopic evolution of granitoid crust and continental growth

“…doi:10.1016/j.gca.2010.01.023 chemical compositions -from lower portions dominated by mafic rocks to upper portions dominated by granitoids (Taylor and Mclennan, 1985;Christensen and Mooney, 1995;Rudnick and Fountain, 1995;Wedepohl, 1995;Gao et al, 1998). Yet there is considerable debate as to when and how it has evolved to its present form (e.g., Rudnick, 1995;Albarède, 1998;Tatsumi, 2000;Hawkesworth and Kemp, 2006;Rollinson, 2008). Of the two contrasting models of continental growth, the first is that the volume of continental crust has increased progressively with time (Hurley and Rand, 1969;Moorbath, 1978), as manifested by the present areal extent of continental crust of different ages (Veizer and Jansen, 1985).…”

Constraints on continental crustal mass loss via chemical weathering using lithium and its isotopes

“…However, the magmas that generate the present-day continental crust are dominantly basalt (2 and references therein). This discrepancy has been referred to as the "crust composition paradox" (3). Various hypotheses have been proposed to solve this paradox, including stripping of Mg through chemical weathering (4)(5)(6), removal of mafic/ultramafic lower crust through foundering/delamination (7)(8)(9), subduction of continental crust followed by "relamination" of buoyant, felsic crust (10), or direct addition of tonalites to the crust through slab melting in a hotter Archean Earth (e.g., 2,3,11).…”