Basement and Sedimentary Recycling and Continental Evolution

Abstract

The present day cumulative distribution of areas of continental basement age provinces (as defined by radiometric estimates) follows an exponential function. A similar distribution is displayed by the reserves of their associated economic deposits and by the thickness and areal distribution data for the overlying sediments. This exponential pattern, similar to isotope decay systematics, is a result of recycling (s.l.). Each theoretically possible growth model must be complemented by an appropriate average recycling constant in order to generate the observed present day distribution. Computer simulation of the system suggests that: (a) the theoretical recycling (radiometric rejuvenation) constant b for the continental basement is $$\leq 6.5 \times 10^{-4}/10^{6}a$$. Strontium isotopic constraints favor $$2 - 4 \times 10^{-4}/10^{6}a$$ as the most likely b. Correcting for the effect of recycling, the emerging pattern of continental accretion (based on the available age data) is of a slow growth during the early Archean, fast during the ~3.0 - 2.0 Ga period, and slow during the subsequent interval: (b) the theoretically possible b for deeply buried sediments on continents is $$\leq 16 \times 10^{-4}/10^{6}a$$, but geometrical and isostatic considerations favor a b similar to that of their underlying basement; (c) the probable b for surficial sediments on present day continents is $$~45 - 67 \times 10^{-4}/10^{6}a$$. Thus it is mostly this surficial balnket, which has been involved in a rapid cannibalistic recycling during the entire geological history; (d) sediments display mafic $$\rightarrow$$ felsic secular trends in chemical composition, with a clear transition during the 3.0 - 2.0 Ga period; consistent with a major development of the continental crust during this interval. However, due to ~65 % cannibalistic nature of the sedimentary cycle, the initial mafic composition of the primordial sedimentary mass was partly perpetuated, resulting in the present day sedimentary mass, which is more mafic than its presumed source (upper continental crust); (e) consideration of the basement recycling rates suggests that the high frequency of Fe, Cr, Ni, (Au) deposits in the Early Proterozoic and the Archean is a primary phenomenon, not yet obliterated by subsequent recycling.