GROWTH OF CONTINENTAL CRUST BETWEEN 2.5 AND 1.2 GA AS CONSTRAINED BY ND AND HF ISOTOPES

Whole rock Nd isotopic data and Hf isotopes in detrital zircons indicate that in addition to the well known peak in juvenile crust production at 1.9-1.7 Ga, smaller peaks occur at 2.5-2.45, 2.1, 1.65, 1.45 and 1.27 Ga. Correlations with probable mantle plume events occur at 2.1, 1.27 and 1.45 Ga and perhaps at 2.5-2.45 Ga. The largest preserved remnants of 1.6-1.4 Ga juvenile crust are in eastern Laurentia, SW Baltica, SE China, western Amazonia and NE Australia. In addition, detrital zircon populations with positive EHf(T) from modern river sands in eastern Australia, NW India and the Ukraine record juvenile sources with ages of 1.6-1.5 Ga. Juvenile crust with 1.4-1.2 Ga ages occurs in SW Laurentia and Mexico and small exposures are found in South Africa and Colombia.

The Birimian-Transamazonian (2.1 Ga) crust in Amazonia and West Africa is the only significant volume of juvenile crust preserved in the 2.5-2.0 Ga time window. Although 2.5-Ga zircon ages are widespread in North China and southern India, a significant number of negative ENd(T) values from these regions indicates involvement of older crustal components. However, numerous positive EHf(T) from detrital zircon populations from modern river sands in India, Ukraine, and Australia indicate 2.5-Ga juvenile sources, the extent of which is unknown. The largest population of detrital zircons in the 2.4-2.2 Ga window are from Paleozoic and Proterozoic sediments in Canada, most of which have negative EHf(T) recording an older crustal component in their sources. Detrital zircons from modern river sands in Brazil, eastern Australia, Ukraine and SE Asia contain some 2.4-2.2-Ga zircons with positive EHf(T) recording at least partly juvenile sources. A scarcity of evidence for juvenile crust in the 2.4-2.2 Ga window may reflect 1) recycling and overprinting of crust of this age, or more likely 2) lack of sampling of juvenile detrital zircon populations of this age.