Crustal geochemistry in the Wawa–Foleyet region, Ontario
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abstract
Fifty-three rock samples from the Kapuskasing structural zone (KSZ) and 56 from the Wawa domal gneiss terrane (WGT), Ontario, have been analysed for major elements, 32 trace elements, and δ18O; δD was measured in a few samples.Average chemical compositions for the KSZ and WGT regions have been calculated from map unit averages weighted by regional abundance. Compared with estimates of the composition of the upper continental crust (UCC), the KSZ is enriched in Al, Fe, Mg, Ca, P, transition elements, Sc, and Sr; depleted in Si, B, most rare earth elements, Zr, Hf, Nb, Ta, Li, Na, K, U, Th, Ba, and Rb; but similar in composition to estimates of the lower continental crust. The WGT is closer in composition to the UCC. These data support the interpretation, on geophysical and petrological grounds, that the crust here is layered and has been uplifted, the WGT and KSZ regions representing progressively deeper levels.Igneous rock and orthogneiss δ18O values in the KSZ and WGT show good correlation with the weight percentages of SiO2. Paragneisses and clastic sediments and metavolcanics in the Michipicoten greenstone belt have higher ratios, as in other greenstone belts. Low δ18O and high δD values in most of the higher grade mafic gneisses show that they have never undergone low-temperature marine alteration. However, a few mafic gneisses with relict pillows show δ18O and δD values indicating low-temperature aqueous alteration. The δ18O and δD evidence throughout the two regions supports the view that no regional fluid homogenization took place.Errorchron ages of 2725 ± 130 Ma (Rb/Sr) and 2755 ± 110 Ma (Sm/Nd) were obtained for tonalite–granodiorite gneiss and agree within error with results from other workers. The intrusive Floranna Lake complex is 2580 ± 120 Ma (Rb/Sr), not significantly different from the age of the tonalite. Scatter in the data is to be attributed to localized late-stage alteration under low water–rock ratios by Sr-rich–Rb-poor brines. Model Nd ages suggest that there is no old crust [Formula: see text] in the region and that the 2700 Ma mantle was depleted relative to the bulk earth.
