Rate of growth of the preserved North American continental crust: Evidence from Hf and O isotopes in Mississippi detrital zircons

• Published in: Geochimica et cosmochimica acta 2009-02, Vol.73 (3), p.712-728
• Main Authors: Wang, Christina Yan, Campbell, Ian H., Allen, Charlotte M., Williams, Ian S., Eggins, Stephen M.
• Format: Article
• Language: English
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2008.10.037

Detrital zircons from the Mississippi River have been analyzed for U–Th–Pb, Lu–Hf and O isotopes to constrain the rate of growth of the preserved North American continental crust. One hundred and forty two concordant zircon U/Pb dates on grains mounted in epoxy, obtained by Excimer laser ablation ICP-MS method, resolved six major periods of zircon crystallization: 0–0.25, 0.3–0.6, 0.95–1.25, 1.3–1.5, 1.65–1.95 and 2.5–3.0 Ga. These age ranges match the ages of the recognized lithotectonic units of the North American continent in the hinterland of the Mississippi River. Ninety-six zircons mounted on tape, which show no age zonation and were within 7.5% of concordance, were selected to represent the six U/Pb age time intervals and analyzed for Lu–Hf and O isotope by laser ablation MC-ICP-MS and SHRIMP II, respectively. The δ 18O values of the zircons show a small step increase in the maximum δ 18O values at the Archean-Proterozoic boundary from 7.5‰ in the Archean to 9.5‰, and rarely 13‰, in the Proterozoic and Phanerozoic. However, the average value of δ 18O in zircons changes little with time, showing that the increase in the maximum δ 18O values between 2.5 and 2.0 Ga, which can be attributed to an increase in the sediment content of the source regions of younger granitoids, is largely balanced by an increase in zircons with anomalously low δ 18O, which can be attributed to hydrothermally altered crust in the granitoid source region. εHf i values for the zircons range from 13.1 to −26.9. Zircons derived from juvenile crust, which we define as having mantle δ 18O (4.5–6.5‰) and lying within error of the Hf depleted mantle growth curve, are rare or absent in the Mississippi basin. The overwhelming majority of zircons crystallized from melted pre-existing continental crust, or mantle-derived magmas that were contaminated by continental crust. The average time difference between primitive crust formation and remelting for each of the recognized lithotectonic time intervals, which is defined as crustal incubation time in this study, is 890 ± 460 Myr. There is also a suggestion that the crustal incubation time increases with decreasing age in the Mississippi basin, which is consistent with the declining role of radioactive heat production in the lower crust with time. The average Hf model age (1.94 Ga), weighted by fraction of zircons in the river load is in reasonable agreement with the Nd model age (1.7 Ga) for the Mississippi River. However, if the zircons a