Pb isotopic composition of late Archaean granites and the extent of recycling early Archaean crust in the Slave Province, northwest Canada

Abstract

Initial Pb isotopic compositions have been determined for potassium feldspar from ca. 2.58 to 2.62 Ga plutonic rocks in the southern and central Slave Province of northwestern Canada to evaluate the extent of recycling of ancient crust within the province. Large differences in initial Pb compositions were measured which correlate with geographical areas of the province. Plutons in the east-central part of the province have initial compositions only slightly more radiogenic than estimated mantle values ( 207Pb/ 204Pb 14.8–14.9), and were dominantly deruved from juvenile crustal sources. In contrast, plutons in the Point Lake and western Contwoyto Lake areas of the western Slave Province have radiogenic compositions ( 207Pb/ 204Pb 15.1–15.2), and indicate significant recycling of pre-3.5 Ga crust. The Pb data support previous interpretations, based on Nd isotopes, for a major isotopic boundary in the central part of the province. Granites from the southern part of the province, near Yellowknife, have intermediate compositions which indicate: (1) the age of the protolith to the granitoids in the Yellowknife area is younger than at Point Lake, but older than in the eastern Slave; or (2) the granitoids in the Yellowknife area contain a mixture of 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 former possibility. Evidence for recycling of ancient crustal sources, such as the Acasta Gneiss, is limited to a relatively small area of the west-central part of the province, suggesting that Acasta aged, or derived, 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, being younger (ca 3.2-2.9) in the south, relative to the Point Lake region (3.9-3.2 Ga).