LATE PRECAMBRIAN U-PB AGES FOR THE BROOKVILLE GNEISS, SOUTHERN NEW-BRUNSWICK

Tonalitic orthogneiss from the Brookville Gneiss in southern New Brunswick has an igneous protolith age of 605±3 Ma based on U-Pb dating of zircon. Metamorphic titanite from the same sample gives a minimum age of 564±6 Ma for upper amphibolite facies metamorphism of the unit. Based on U-Pb analyses of single detrital zircons, a maximum sedimentary protolith age of about 641 Ma is suggested for associated paragneiss. These ages indicate that the igneous protolith of the orthogneiss was at most 35 Ma younger than the sedimentary rocks it intruded, and that the Brookville Gneiss is younger than the Green Head Group to which it was previously considered basement. Both orthogneiss and paragneiss in the Brookville Gneiss contain Proterozoic and Archean detrital zircon grains, suggesting an old, heterogeneous continental source. We conclude that the Brookville Gneiss does not represent the basement to the Avalon Terrane. -Authors