IMPLICATIONS FOR THE EVOLUTION OF CONTINENTAL-CRUST FROM HF-ISOTOPE SYSTEMATICS OF ARCHEAN DETRITAL ZIRCONS

The fractionation of zircons by sedimentary processes into continental margin sandstone deposits results in a biased preservation of pre-existing continental crust in the form of zircon in those sequences. This provides a unique opportunity to distinguish between the contrasting theories of gradual growth versus constant volume of continental crust over geologic time through Hf isotope ratios of detrital zircons. sol176Hf 177Hf ratios were determined for detrital zircon fractions from 2.0-2.5, 2.6-3.0, and pre-3.0 Ga old sandstones from the Canadian Shield, North Atlantic, Wyoming, and Kaapvaal Cratons. Proterozoic quartzites have 2.39 to 2.84 Ga Hf T(CHUR) ages, indicative of a large expanse of late Archean crust exposed to erosion at the time of deposition. The late Archean (2.6-3.0 Ga) sequences appear to be dominated by zircon populations of late Archean age. Hf model ages are less than 3.0 Ga and ε{lunate}(Hf) values are positive or slightly negative at the time of deposition for most of the Malene, Canadian Shield, Wyoming, and upper portions of the Kaapvaal sequences. Exceptions include basal samples of the Pongola (3.32 Ga), Dominion (3.11 Ga), and Witwatersrand (3.13 Ga), an arkose from Michigan (3.20 Ga), and one Malene sample (2.96 Ga), all of which either unconformably overlie or are closely associated with pre-3.0 Ga crust. Nd data for shales from the same sequences in the Canadian Shield and Kaapvaal sequences mimic the Hf results. Hf model ages, from pre-3.0 Ga old strata (Upernavik of Labrador and quartzites from Montana), range from 3.0 to 3.5 Ga and are broadly consistent with ages of coexisting volcanics or intrusives, suggesting little inheritance of significantly older material. The data strongly suggest inheritance of pre-3.0 Ga zircons only in areas where pre-3.0 Ga old crust exists today, and imply that the quantity of continental crust prior to 3.0 Ga ago was not much greater in extent than the pre-3.0 Ga crust exposed today. Small amounts of continental crust prior to 3.0 Ga ago and rapid addition of continental crust between 2.5 and 3.0 Ga ago are consistent with the gradual growth of continental crust, and argue against no-growth histories. © 1990.