PB ISOTOPIC GEOCHEMISTRY OF GRANITOIDS AND GNEISSES FROM THE LATE ARCHEAN PONTIAC AND ABITIBI SUBPROVINCES OF CANADA

Initial Pb isotopic compositions were determined on K-feldspars for thirty granitoid plutons and gneiss complexes in the Abitibi and the Pontiac Subprovinces of Canada. The gneisses cover a wide spectrum of initial Pb isotopic compositions and undoubtedly demonstrate that very radiogenic reservoirs were present in the Pontiac Subprovince and along its southern boundary with the Grenville Province in the late Archean. The granitoid rocks also define a large spectrum, covering 0.3 unit of the Pb-207/Pb-204 ratio, ranging from compositions typical of mafic mantle melts to those of sedimentary banded iron formations. The older intrusives emplaced between 2.72 and 2.68 Ga were principally derived from juvenile mantle sources. They may have been formed in a volcanic arc environment, but they still frequently record the signature of crustal precursors which may have been represented by subducted sediments or by older assimilated crustal components. A sharp change in the provenance of the granitoid magmas occurred close to 2.68 Ga, an age that also corresponds to an episode of alkaline activity and fault-related sedimentation. For the following 40 Ma, magmatism chiefly resulted from melting of crustal precursors in a thickened crustal lithosphere. The Pb isotopic compositions of these crustal melts demonstrate that the basement of the Pontiac Subprovince contains a major component of rocks older than 3.0 Ga. In comparison, the Abitibi Subprovince may not contain basement components older than 2.9 Ga, or else they represent a negligible fraction of the crust. This supports the concept that the Abitibi and Pontiac Subprovinces formed as discrete crustal segments tectonically juxtaposed. Finally, the Grenville Front tectonic zone was metamorphosed and partially melted in the late Archean and is probably the deep crustal equivalent of the Pontiac Subprovince.