Age and isotopic composition of late Archean leucogranites: implications for continental collision in the western Superior Province

U-Pb and Sm-Nd isotopic data are presented for leucogranites and pegmatites from the boundary zones of the English River, Winnipeg River, and Wabigoon subprovinces in the late Archean Superior Province of northwestern Ontario . The Ghost Lake batholith and the Separation Rapids pluton are posttectoni c, rare-metal-bearing, S-type leucogranites that were generated during the final stages of the amalgamation of the Superior Province. U-Pb dating of m onazites yields ages of 2650 +/- 3 Ma for a pegmatite from the Dryden area and 2646 +/- 2 Ma for the Separation Rapids pluton. Sm-Nd data from these g ranitoids are compared with similar data from late Archean intrusions and a djacent rocks from the same regions. Values of epsilon(Nd) range from 0 to +2 for pretectonic tonalites, from -2 to +2 for both the Ghost Lake batholi th and the Separation Rapids pluton, from +1 to +3.5 for metavolcanic rocks , and from -0.5 to -1.5 for metasedimentary rocks. There is an overall tren d of decreasing epsilon(Nd) values from pretectonic tonalites to the latest leucocratic pegmatites. This reflects the origin of more and more granitoi ds as a result of anatexis as the crust grew and thickened through accretio n. The ranges of epsilon(Nd) values found among leucogranites from the two regions overlap with the isotopic values of the basalts and sediments. This suggests that the leucogranites were generated by similar processes involv ing both a crustal and a juvenile mantle component. Furthermore, the ages f rom this study and from pegmatites (2652-2643 Ma) in the Bird River greenst one belt provide the best estimate of the age of rare metal mineralization in the western Superior Province. Similar ages (2651-2639 Ma) for rare-meta l-bearing leucogranites in southern Abitibi subprovince suggest a specific period of emplacement over a wide area.