Granites emplaced in low-grade (granite-greenstone) Archean terranes typically exhibit negative Eu anomalies, and such Eu anomalies have been transmitted to post-Archean cratonic sediments by weathering and erosion. Because average continental crust does not exhibit an Eu anomaly, a complementary reservoir with a positive Eu anomaly must exist to balance the low-grade granites. Neither the upper mantle nor average Archean lower crust appears to represent this reservoir. Archean lower crustal rocks, which are dominantly plutonic rocks of tonalitic composition, have REE distributions indicative of a mafic parentage. However, Archean high-grade granites in southern India exhibit the required positive Eu anomalies and may be representative of a widespread positive Eu anomaly reservoir in the lower crust.
Geochemical model studies indicate that Archean low- and high-grade granites may represent residual liquid and cumulate, respectively, from the fractional crystallization of a granite parent magma produced by partial melting of tonalitic rocks in the lower crust. Such melting may occur at the wave front of a CO2-rich fluid that dehydrates the lower crust and produces granulites.