NEW GEOCHEMICAL DATA FOR ARCHEAN CRUST IN THE MINNESOTA RIVER VALLEY: EVIDENCE FOR CONTINENTAL CRUSTAL RECYCLING IN THE PALEOARCHEAN?

Paleoarchean to Neoarchean rocks in the Minnesota River Valley are exposed in the structurally distinct Morton and Montevideo blocks. These contain granitoids that range in age from 3.50 – 2.60 Ga. The Morton block includes the ~3.50 Ga tonalitic Morton gneiss, a 3.35 Ga granodiorite and the 2.60 Ga Sacred Heart granite. Undated minor mafic and felsic enclaves are thought to be related to post 3.50 Ga melting events. The Montevideo block includes the 3.50 Ga trondhjemitic Montevideo gneiss, a 3.38 Ga intermediate gneiss, and a 3.14 Ga mafic enclave. Undated phases in the Montevideo block include felsic high-K metagranites from two locations associated with the 3.50 Ga gneiss.

The general major-element characteristics of the 3.50 Ga Morton and Montevideo gneisses are similar to other Archean TTG suites (i.e., >70% SiO₂, low K₂O and high Na₂O). Both the 3.50 Ga Morton and Montevideo samples have similar initial Nd and Hf isotopic compositions. The isotopic data also show that the younger rocks in both blocks were recycled from the 3.50 Ga rocks, suggesting a similar tectonic history for both blocks. However, a 3.50 Ga sample of Montevideo gneiss differs from the 3.50 Ga Morton gneiss samples in that it has a well-developed negative Eu anomaly, suggesting it formed at a shallower crustal level than the Morton gneisses, which have no significant Eu anomaly. The presence of a negative Eu anomaly in younger 3.38 Ga gneiss from the Montevideo block suggests the entire block remained at <40 km depth since its formation at ~ 3.50 Ga.

The high-K metagranites, like the 3.50 Ga Morton samples, have no Eu anomaly, implying partial melting at a crustal level below the plagioclase stability field or melting processes that consumed all available plagioclase. There are two possible scenarios for the formation of the high-K metagranites: 1) formation at the base of the block as melts of the older Montevideo material or 2) melts of cryptic >3.50 Ga felsic crust within the Montevideo block. The whole-rock trace element data potentially suggest melting of pre-3.50 Ga continental crust. However, further U-Pb age data and Nd isotopic compositions, currently being collected, will be required to assess the time of crystallization and to search for evidence of >3.50 Ga xenocrysts.