Zircon Lu-Hf systematics and the evolution of the Archean crust in the southern Superior Province, Canada

Abstract

A combined Lu-Hf and U-Th-Pb isotopic study was made of 25 zircons and 2 whole rocks from the late Archean crust (2,888-2,668 Ma) in the southern Superior Province, Canada. The relative abundances of U, Th, Lu and Hf in zircons from the low grade Michipicoten and Gamitagama greenstone belts show variable patterns which in part reflect the bulk compositional differences of their parent rocks. Zircons from the high grade lower crustal regions adjacent to these belts (Kapuskasing Structural Zone) are distinguished from the low grade zircons by their strong depletions of Lu and Hf. The low Hf contents imply that the growth of metamorphic zircon involves a significant fractionation of the Zr/Hf ratio. Initial Hf isotope ratios for Hf in zircons from the low grade rocks are correlated with silica enrichment of their host rocks. eHf varies from +9.2 to -1.3 and data from similar rock types exhibit correlations of eHf with time. Whole rock basalt analyses yield eHf values of +8.7 and +11.3 suggesting their derivation from a depleted mantle. The basalt data fall on an evolution trend which implies that differentiation from a chondritic mantle occurred at 3,100-2,900 Ma. Low eHf values (-1.3 to +1.4) for rhyolites and granites are consistent with a derivation involving remelting of old crust similar to a 2,888 Ma granite with eHF of +0.5. Significantly higher values (+1.4 to +3.9) are found in zircons from 2,748-2,682 Ma dacites and tonalites suggesting that their parent rocks had higher Lu/Hf ratios. This may indicate that their parent rocks were mafic. However, there is some evidence that the possible lower crustal source reservoirs of these rocks may have undergone processes early in their histories which increased their Lu/ Hf ratios. This would give rise to the higher eHf values observed in their derivatives. ?? 1987 Springer-Verlag.