Earth and Space Sciences Department, SUNY Stony Brook, Stony Brook, NY 11794, U.S.A.

All metaigneous rocks lie on a ¹⁴³Nd/¹⁴⁴Nd vs. ¹⁴⁷Sm/¹⁴⁴Nd isochron with an age of 2737 ±42 Ma and an initial ¹⁴³Nd/¹⁴⁴Nd of 0.509178 ±33 (ε_Nd = +1.9). This age is consistent with U-Pb zircon ages, which suggests the Nd isotopic system has been unaffected since the crust-forming events. The positive initial ε_Nd's are further evidence for time-averaged depletion in Sm/Nd relative to CHUR for the Archean mantle. The similarity of the initial Nd isotopic composition for both mantle-derived and crustally-derived rocks suggests rapid recycling of crustal components, which were previously derived from depleted mantle sources.

Initial ¹⁴³Nd/¹⁴⁴Nd ratios on individual rocks range from ε_Nd = +3.3 to ε_Nd = ?0.4. Younger granitoids have lower ε_Nd values (+1.5 to ?0.1) relative to tholeiites and monzodiorites crystallized from mantle-derived melts (+3.3 to +1.0). Thus, incorporation of slightly older crust (ca. 100–200 Ma) in some of the granitoid source areas is possible. Mantle-derived rocks form an isochron of 2764 ±58 Ma that represents a minimum age for enrichment processes in the mantle sources for the Rainy Lake area. Consideration of data from the Abitibi belt suggests such enrichment processes in the mantle may have preceded crust-forming events in a wide area of the Superior Province, perhaps by as much as 50–70 Ma.