The whole-rock oxygen isotope ratios of amphibolites from a variety of localities have been studied in an attempt to distinguish para-amphibolites from ortho-amphibolites. If para-amphibolites were derived from a pelite–carbonate mixture or any carbonate-bearing sedimentary rock, they should be enriched in 18O relative to basaltic parents. Exchange with an external oxygen reservoir acting as an isotopic buffer during metamorphism would, however, tend to obliterate initial differences between basaltic and metasedimentary parent rocks. It is found that, except for feather amphibolites from the Grenville subprovince of Ontario, rocks from a given area which are geologically distinguishable as para- or ortho-amphibolites do not display the predicted differences in 18O/16O ratios. Large differences in isotopic composition between localities are attributed in part to difference in grade of metamorphism and consequent differences in temperature of exchange with an external oxygen isotopic buffer. All samples appear to have exchanged to some extent.

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