Geochemical and isotopic constraints on the origin and source of Archaean granites

Archaean granites (including quartz monzonites) can be divided into three geochemical classes based chiefly on REE distributions. Geochemical model studies, initial ⁸⁷Sr/⁸⁶Sr ratios, and O-isotope data are consistent with an origin for most Archaean granites by partial melting (approx 20%) of one of three rock types in the lower crust with short crustal residence times: tonalite-trondhjemite, high-grade gneiss (of intermediate composition), or less likely, greywacke. Only a few percent of subcontinental mantle heat is needed to account for Archaean granite magma production by partial melting of the lower crust. (Author's abstract) J.A.H.