Deep crustal oxygen isotope variations: the Wawa-Kapuskasing crustal transect, Ontario

We have studied the oxygen isotopic composition of rocks from a 100 km transect through the central Superior province of Ontario, representing progressively the shallower terrains of the Kapuskasing structural zone (KSZ), the Wawa gneiss terrane (WGT), and the Michipicoten greenstone belt (MGB). These rocks range in age from ∼2.76 to ∼2.60 Ga, and correspond to a section through approximately 20 km of crustal thickness. Equivalent lithologic types have similar range of δ18O values at each crustal level: tonalitic to granodioritic rocks: 6.4 to 9.5‰; dioritic and anorthositic rocks: 5.5 to 7.6‰; mafic gneisses: group 1 (majority): 5.7 to 7.1‰; group 2: 8.1 to 9.5‰. δ18O values exhibit a remarkable correlation with SiO2 values, similar to that observed in unaltered plutonic rocks of equivalent composition. Paragneisses have significantly higher δ18O values: 9.3 to 12.2‰. Low-grade metavolcanic and metasedimentary rocks of the MGB are 18O-enriched compared to their high-grade equivalents in the KSZ and WGT: 7.4 to 13.3‰ for mafic to felsic metavolcanic rocks; 11.4 to 14.7‰ for clastic metasediments. Coexisting minerals exhibit 18O-fractionation consistent with equilibrium, but corresponding to uniform isotopic temperatures about 553 to 584°C across the entire transect, lower than the inferred metamorphic temperatures in the highest-grade (KSZ) terrane. The lack of distinctive isotopic differences between equivalent rock types in the KSZ, WGT and MGB suggests that there is no significant gradient in δ18O with depth in the crust. The majority of mafic gneisses (group 1) appear to have been emplaced either as subaerial extrusives, intrusive sills, or, less likely, as submarine extrusives that were hydrothermally altered at high temperatures. The less abundant group 2 mafic rocks have the δ18O values typical of greenstones that were altered at low temperature by seawater, and isotopically resemble low-grade rocks in the Michipicoten and Abitibi belts. In general, no major changes in whole-rock isotopic composition appear to have occurred during granulite facies metamorphism, implying limited flux of water or CO2. The continuous linear gradient in δ18O versus SiO2 in the high-grade rocks cannot be due to differentiation of a mafic source magma. A model involving an association between mantle-derived mafic magma and 18O-enriched crustal materials is more consistent with the oxygen isotopic and the REE data.