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

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.