GENESIS OF THE SOUTHERN ABITIBI GREENSTONE-BELT, SUPERIOR-PROVINCE, CANADA - EVIDENCE FROM ZIRCON HF ISOTOPE ANALYSES USING A SINGLE FILAMENT TECHNIQUE

Hafnium isotopic analyses, together with U-Pb geochronology, have been performed on zircons from the main units of the Late Archean southern Abitibi greenstone belt in the Superior Province. A new analytical method in which Hf is loaded and run with a mixture of Ir-Mo and C on single filaments was used. The results indicate a relatively homogeneous isotopic composition of the greenstone belt, with epsilon(Hf) values of 6.3-4.2 for the 2730-2700 Ma preorogenic assemblages and 4.7-3.6 for the 2690-2675 Ma synorogenic intrusions. Hafnium isotopes and U-Pb ages indicate that the greenstone belt formed in a relatively short time interval within an ensimatic setting, with minimal interaction with older sialic crust. Primary magmas were derived from mantle sources with a depleted isotopic signature of epsilon(Hf) almost-equal-to 5.5 +/- 0.5, corresponding to published epsilon(Nd) almost-equal-to 2.5, Sr-87/Sr-86 almost-equal-to .7011, Pb-207/Pb-204 almost-equal-to 14.4, and Pb-206/Pb-204 almost-equal-to 13.2. Slightly lower epsilon(Hf) values in some rocks may record moderate degrees of source heterogeneity; in the case of the alkalic plutons, the lower epsilon-H f can be interpreted in terms of mantle wedge contamination by subducted components isotopically similar to sedimentary rocks in the adjacent Pontiac Subprovince. The consistent, positive epsilon(Hf) signature confirms that the Archean mantle was already depleted prior to formation of 3.0-2.7 Ga Superior Province crust.