Oxygen isotope ratios of Archean plutonic zircons from granite–greenstone belts of the Superior Province: indicator of magmatic source

Abstract

Zircons commonly preserve primary magmatic oxygen isotope ratios even through high grade metamorphic events unlike quartz which more readily loses its primary δ 18O value by subsolidus exchange. Intrusive bodies from the Superior Province, Canada have an average δ 18O(Zircon) of 5.8±0.6‰ ( n=59 rocks). Those from granite–greenstone belts have an average δ 18O(Zrc)=5.8±0.6‰ ( n=55), in contrast to those from metasedimentary belts that average δ 18O(Zrc)=6.7±0.2‰ ( n=4). The δ 18O zircon values from granite–greenstone belts indicate a maximum allowable input of 14% supracrustal, high δ 18O material into Archean magmas of the Superior Province relative to mantle δ 18O values. Igneous bodies of the Superior Province have been separated into two groups based on geochemistry: unenriched plutons and sanukitoids. Magmatic oxygen isotope ratios inferred from these zircons indicate up to a subtle but significant division of <1‰ between the two geochemical sets of rocks. This distinction is apparent only in the δ 18O(Zrc) and not in more commonly analyzed, less refractory minerals such as quartz and feldspar. Oxygen isotope data correlate with geochemical modeling of sanukitoids. The average δ 18O(Zrc) for sanukitoids is 6.4±0.2‰ in comparison to 5.5±0.4‰ for unenriched plutons. There is almost no overlap in values. The elevation of sanukitoid δ 18O(Zrc) indicates input from a high δ 18O source such as fluids from the subducting oceanic crust, melts of subducted sediments or continental crust. All of the plutons analyzed in this study have been dated by U–Pb zircon techniques, and the combination of U–Pb ages and δ 18O(Zrc) indicates little variability of δ 18O versus age during Archean plutonism in the Superior Province. Therefore, recycling of sediments and other high δ 18O sources was not a dominant process during crustal growth from 3003 to 2670 Ma in the Superior Province.