Formula omitted] of Archean clastic metasedimentary rocks: a petrogenetic indicator for Archean gneisses?

Abstract

The oxygen isotopic compositions of over 100 Archean clastic metasedimentary, felsic metavolcanic and gneissic rocks from selected areas within the Superior Province have been determined. δ 18 O values of low grade, immature clastic metasediments range from 8.0 to 13.3%. and hence are somewhat depleted in 18O relative to other clastic metasediments. The lower 18O content is attributed to the large proportion of unaltered rock fragments in the Archean metasediments. Felsic metavolcanics have a similar range of δ 18 O values (7.3 to 13.0%.. δ 18 O values for the middle amphibolite facies Pakwash paragneiss (8.8–11.7%.) are higher than those determined for the amphibolite to granulite facies Twilight paragneiss (7.3–9.2%.). Archean orthogneisses such as the Clay Lake gneiss (7.0–9.0%.), the Cedar Lake gneiss (7.4–8.7%.) and the Footprint gneiss (5.9-8.8%.) have δ 18 O values similar to those of the Twilight paragneiss. Therefore, at least at metamorphic grades higher than middle amphibolite facies, Archean metasedimentary gneisses may have δ 18 O values that are indistinguishable from those of Archean orthogneisses, and hence are no longer isotopically recognizable as metasedimentary rocks.