PB ISOTOPIC COMPOSITION OF LATE ARCHEAN GRANITES AND THE EXTENT OF RECYCLING EARLY ARCHEAN CRUST IN THE SLAVE PROVINCE, NORTHWEST CANADA

Initial Pb isotopic compositions have been determined for potassium fe ldspar from ca. 2.58 to 2.62 Ga plutonic rocks in the southern and cen tral Slave Province of northwestern Canada to evaluate the extent of r ecycling of ancient crust within the province. Large differences in in itial Pb compositions were measured which correlate with geographical areas of the province. Plutons in the east-central part of the provinc e have initial compositions only slightly more radiogenic than estimat ed mantle values (Pb-207/Pb-204 14.8-14.9), and were dominantly derive d from juvenile crustal sources. In contrast, plutons in the Point Lak e and western Contwoyto Lake areas of the western Slave Province have radiogenic compositions (Pb-207/Pb-204 15.1-15.2), and indicate signif icant recycling of pre-3.5 Ga crust. The Pb data support previous inte rpretations, based on Nd isotopes, for a major isotopic boundary in th e central part of the province. Granites from the southern part of the province, near Yellowknife, have intermediate compositions which indi cate: (1) the age of the protolith to the granitoids in the Yellowknif e area is younger than at Point Lake, but older than in the eastern Sl ave; or (2) the granitoids in the Yellowknife area contain a mixture o f an older Point Lake-type component and younger crust. The absence of pre-3.2 Ga crust in the Yellowknife area and lack of evidence for pre -2.8 Ga inherited zircons in the Yellowknife granitoids favour the for mer possibility. Evidence for recycling of ancient crustal sources, su ch as the Acasta Gneiss, is limited to a relatively small area of the west-central part of the province, suggesting that Acasta aged, or der ived, crust is not widespread in the province. The marked regionality of isotopic composition may reflect a basement in the western part of the province which is itself a collage of crust of different age, bein g younger (ca 3.2-2.9) in the south, relative to the Point Lake region (3.9-3.2 Ga).