Growth rate of the preserved continental crust: II. Constraints from Hf and O isotopes in detrital zircons from Greater Russian Rivers

Abstract

Detrital zircons from the Ob, Yenisey, Lena, Amur, Volga, Dnieper, Don and Pechora rivers have been analyzed for U–Th–Pb, O and Lu–Hf isotopes to constrain the growth rate of the preserved continental crust in Greater Russia. Four major periods of zircon crystallization, 0.1–0.55, 0.95–1.3, 1.45–2.0 and 2.5–2.9 Ga, were resolved from a compilation of 1366 zircon U/Pb ages. The Archean zircons have δ 18O values lying between 4.53‰ and 7.33‰, whereas Proterozoic and Phanerozoic zircons have a larger range of δ 18O values in each of the recognized U/Pb time intervals with maximum δ 18O values up to 12‰. We interpret the zircons with δ 18O between 4.5‰ and 6.5‰ to have been derived from a magmatic precursor that contains little or no sedimentary component. The variable δ 18O values of the zircons were used to constrain the 176Lu/ 177Hf ratios of the crustal source region of the zircons, which, in turn, were used to calculate Hf model ages (T DM V). The crustal incubation time, the time difference between primitive crust formation (dated by T DM V) and crustal melting (dated by zircon U/Pb age), varies between 300 to 1000 Myr for the majority of analyzed zircon grains, but can be up to 2500 Myr. The average T DM V Hf model age weighted by the fraction of zircons in the river load is 2.12 Ga, which is in reasonable agreement with the area-weighted average of 2.25 Ga. The T DM V Hf model age crustal growth curve for zircons with mantle-like δ 18O values (4.5–6.5‰), weighted by area, shows that growth of the Great Russian continental crust started at 4.2 Ga, and that there are two principal periods of crustal growth, 3.6–3.3 Ga and 0.8–0.6 Ga, which are separated by an interval of low but more or less continuous growth. An alternative interpretation, in which the average 176Lu/ 177Hf ratio (0.0115) of the continental crust is used for the Paleoproterozoic zircons from the Lena River, lowers the average T DM V age of these grains by about 500 Myr and delays the onset of significant crustal growth to 3.5 Ga. The two principal growth periods recognized in Greater Russia differ from those identified from the Gondwana and the Mississippi river basin, which show peaks at 1.7–1.9 and 2.9–3.1 Ga ( Hawkesworth and Kemp, 2006a) and 1.6–2.2 and 2.9–3.4 Ga ( Wang et al., 2009), respectively. The older 3.6–3.3 Ga or 3.5–3.3 Ga peak for Greater Russia is slightly older than the older Gondwana-Mississippi peaks, whereas the younger 0.8–0.6 Ga peak is distinctly younger than the youngest peak in either Gondwana or the Mississippi river basin. This suggests that the two major peaks of crustal growth identified in Gondwana and the Mississippi river basin may not be global periods of enhanced continental growth and that the major periods of crustal growth may differ from continent to continent.