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Neoarchean trondhjemitic and tonalitic orthogneisses identified within the northern Grenville Province in Ontario by precise UPb dating and petrologic studies

1995; Elsevier BV; Volume: 72; Issue: 3-4 Linguagem: Inglês

10.1016/0301-9268(94)00094-8

1872-7433

Y.D. Chen, T. E. Krogh, S.B. Lumbers,

Geochemistry and Elemental Analysis

Integrated field, petrologic and geochronological studies reveal the presence of heretofore unrecognized Neoarchean trondhjemitic and tonalitic orthogneisses north and northwest of Lake Nipissing in the northern Grenville Province of Ontario. The high-Al type trondhjemitic orthogneiss, termed Warren Trondhjemite, forms several intrusions with a total exposed area of ∼ 300 km2. The tonalitic orthogneiss, including the Tilden Tonalite, forms a discontinuous zone of intrusions, about 20 km long and up to 1.6 km wide. Both trondhjemitic and tonalitic orthogneisses are geochemically distinct from paragneiss that forms the country rocks. Precise UPb zircon and titanite dating of the Warren Trondhjemite at six locations, and of the Tilden Tonalite at one location, indicates that the former was emplaced at 2679 ± 2 Ma whereas the latter was intruded at 2669 ± 4 Ma. A 996 to 975 Ma metamorphism, shown by the lower intercept ages of concordia plots of all samples, caused partial Pb loss from primary Neoarchean zircons and titanites 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 paragneiss is also Archean in age. The orthogneiss and the paragneiss together constitute an Archean crustal sequence southeast of the Grenville Front Boundary Fault within the Ontario segment of the Grenville Province. This Archean sequence resembles the Pontiac Subprovince of the Superior Province in western Quebec in several important ways, including the dominance of compositionally similar metasediments, comparable depositional ages of the metasediments and contemporaneous granitoid plutonism at about 2680 to 2670 Ma. Differences also exist between the two; for example, the major granitoids in the Pontiac Subprovince are generally more K-rich.