NEOARCHEAN TRONDHJEMITIC AND TONALITIC ORTHOGNEISSES IDENTIFIED WITHIN THE NORTHERN GRENVILLE PROVINCE IN ONTARIO BY PRECISE U-PB DATING AND PETROLOGIC STUDIES

Integrated field, petrologic and geochronological studies reveal the p resence of heretofore unrecognized Neoarchean trondhjemitic and tonali tic orthogneisses north and northwest of Lake Nipissing in the norther n Grenville Province of Ontario. The high-Al type trondhjemitic orthog neiss, termed Warren Trondhjemite, forms several intrusions with a tot al exposed area of similar to 300 km(2). The tonalitic orthogneiss, in cluding the Tilden Tonalite, forms a discontinuous zone of intrusions, about 20 km long and up to 1.6 km wide. Both trondhjemitic and tonali tic orthogneisses are geochemically distinct from paragneiss that form s the country rocks. Precise U-Pb zircon and titanite dating of the Wa rren Trondhjemite at six locations, and of the Tilden Tonalite at one location, indicates that the former was emplaced at 2679 +/- 2 Ma wher eas the latter was intruded at 2669 +/- 4 Ma. A 996 to 975 Ma metamorp hism, shown by the lower intercept ages of concordia plots of all samp les, caused partial Pb loss from primary Neoarchean zircons and titani tes and also induced new growth of zircons in a local pegmatitic patch and of titanite in orthogneisses. Intrusive relationships between the orthogneiss and the country rock paragneiss indicate that the paragne iss is also Archean in age. The orthogneiss and the paragneiss togethe r constitute an Archean crustal sequence southeast of the Grenville Fr ont Boundary Fault within the Ontario segment of the Grenville Provinc e. This Archean sequence resembles the Pontiac Subprovince of the Supe rior Province in western Quebec in several important ways, including t he dominance of compositionally similar metasediments, comparable depo sitional ages of the metasediments and contemporaneous granitoid pluto nism at about 2680 to 2670 Ma. Differences also exist between the two; for example, the major granitoids in the Pontiac Subprovince are gene rally more K-rich.