Nd isotopic evidence for Early to Late Archean (3.4–2.7 Ga) crustal growth in the Western Superior Province (Ontario, Canada)

Abstract

New Sm–Nd analyses of Archean metavolcanic, plutonic, gneissic and metasedimentary rocks from Sachigo and Berens River subprovinces (North Caribou terrane) and Winnipeg River and Wabigoon subprovinces in conjunction with previously published data are used to trace the crustal evolution of the Western Superior Province (WSP) from ca. 3.4 to 2.7 Ga. The extensive WSP geochronological database, which contains 3.17–2.55 Ga plutonic rocks, allows the calculation of precise ε t Nd values and the identification of the main periods of new crustal addition at 3.4±0.1, 3.02–2.92 and 2.76–2.69 Ga. Mass-balance calculations indicate that the Western Superior Province represents a collage of Archean crusts comprising 7%, 44% and 48 wt% of crust created at 3.4, 3.0 and 2.7 Ga, respectively. Although these calculations depend on the isotopic signatures of the mantle and those of the average crust at different times, several important observations can be made: (1) the importance of pre-2.7 Ga terranes recycled in the 2.7 Ga orogen; (2) the existence of time intervals of 150–300 Ma dominated by intra-crustal processes resulting in a large range of crystallization ages (U–Pb zircon ages) that do not reflect new additions from the mantle; (3) much of the new crustal additions at 2.7 Ga were contaminated by crustal material subducted into the source region (TTG suites) or were contaminated during intrusion into the crust (sanukitoids).