Implications for the evolution of continental crust from hafnium isotope systematics of detrital zircons in Archean sandstones.
AuthorStevenson, Ross Kelley.
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PublisherThe University of Arizona.
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AbstractThe 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 episodic growth versus constant volume of continental crust over geologic time through Hf isotope ratios of detrital zircons. ¹⁷⁶Hf/¹⁷⁷Hf ratios were determined for detrital zircon fractions from 2.6-3.0 Ga old sedimentary sequences from the Canadian Shield, North Atlantic, Wyoming, and Kaapvaal Cratons. Hf T(CHUR) ages are less than 3.0 Ga and ε(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 sediments. Notable exceptions are 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 (2.97 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. The late Archean sequences appear to be dominated by zircon populations of late Archean age. Hf model ages, from pre-3.0 Ga strata (Upernavik of Labrador and quartzites from Montana), range from 3.1 to 3.6 Ga and are broadly consistent with ages of coexisting volcanics or intrusives, suggesting little inheritance of significantly older material. 2.0-2.5 Ga old quartzites from the Canadian Shield, Wyoming and South Africa have 2.58 to 2.84 Ga model Hf ages indicative of a large expanse of late Archean crust exposed at the time of deposition. 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 episodic growth theory of crustal evolution.