Geochemical constraints on the origin of Archaean tonalitic-trondhjemitic rocks and implications for lower crustal composition

Summary The rare earth element (REE) concentrations of high-grade trondhjemitic gneisses from the Archean Kapuskasing Structural Zone, central Ontario, are similar to REE concentrations of Archaean tonalites and trondhjemites world-wide: REE patterns are steeply fractionated (La/Yb) N = 20–100), with variable positive Eu anomalies and fractionated HREE ((Gd/Yb) N = 2–4). Calculated models for the melting of a mafic source with a flat REE pattern and variable mineralogy show that amphibole alone is not capable of producing the large amount of REE fractionation observed in Archaean tonalites; garnet amphibolite (with >20% garnet), mafic garnet granulite or eclogite are the most likely sources. The presence of garnet constrains the mafic source to be within the lower crust (deeper than 25 km) or upper mantle. Such crustal or upper mantle melting depths are not geochemically distinguishable, yet have important implications for the composition of the Archaean lower crust.