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

Zircons commonly preserve primary magmatic oxygen isotope ratios even throu gh high grade metamorphic events unlike quartz which more readily loses its primary delta(18)O value by subsolidus exchange. Intrusive bodies from the Superior Province, Canada have an average delta(18)O(Zircon) of 5.8 +/- 0. 6 parts per thousand (n = 59 rocks). Those from granite-greenstone belts ha ve an average delta(18)O(Zrc)=5.8 +/- 0.6 parts per thousand (n =55), in co ntrast to those from metasedimentary belts that average delta(18)O(ZrC)= 6. 7 +/- 0.2 parts per thousand (n =4). The delta(18)O zircon values from gran ite-greenstone belts indicate a maximum allowable input of 14% supracrustal , high delta(18)O material into Archean magmas of the Superior Province rel ative to mantle delta(18)O 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 sig nificant division of <1 parts per thousand between the two geochemical sets of rocks. This distinction is apparent only in the delta(18)O(ZrC) and not in more commonly analyzed, less refractory minerals such as quartz and fel dspar. Oxygen isotope data correlate with geochemical modeling of sanukitoids. The average delta(18)O(Zrc) for sanukitoids is 6.4 +/- 0.2 parts per thousand in comparison to 5.5 +/- 0.4 parts per thousand for unenriched plutons. The re is almost no overlap in values. The elevation of sanukitoid delta(18)O(Z rc) indicates input from a high delta(18)O source such as fluids from the s ubducting oceanic crust, melts of subducted sediments or continental crust. All of the plutons analyzed in this study have been dated by U-Pb zircon te chniques, and the combination of U-Pb ages and delta(18)O(ZrC) indicates li ttle variability of delta(18)O versus age during Archean plutonism in the S uperior Province. Therefore, recycling of sediments and other high delta(18 )O sources was not a dominant process during crustal growth from 3003 to 26 70 Ma in the Superior Province. (C) 1998 Elsevier Science B.V. All rights r eserved.