Contrasting cooling histories in the Archean Minnesota River Valley Subprovince of the Superior Craton of North America with implications for secular change of metamorphic time scales

Abstract Contrasting metamorphic cooling rates in the Precambrian and Phanerozoic have been suggested to reflect secular change in the styles of orogenesis. However, very limited diffusion-chronometry-based data exist for Precambrian metamorphic terranes. Here, we present cooling rate estimates based on garnet diffusion chronometry for the Minnesota River Valley Subprovince (MRV), an Archean metamorphic terrane of the Superior Craton of North America in present day Minnesota, USA, adding important new data points for the global record of metamorphic time scales. Geochronology and thermobarometry reveal that the Morton block of the MRV experienced amphibolite-facies conditions of 610–760 °C and 3.8–6.3 kbar at ca. 2.58 Ga. The neighboring Montevideo block simultaneously underwent granulite-facies metamorphism at 710–860 °C and 4.4–7.0 kbar. Multicomponent diffusion modeling in garnet shows that the Morton block cooled slowly from its peak temperature at a rate of 1–17 °C/Ma. In contrast, the Montevideo block cooled rapidly at 36–203 °C/Ma, rates quicker than have been obtained for other Archean metamorphic terranes. The results are consistent with metamorphism driven by advective heating synchronous with the Sacred Heart Orogeny to the north of the MRV. Estimated cooling rates are compared with a compiled time series of metamorphic cooling rates throughout Earth's history. There has been a statistically significant increase in metamorphic cooling rates since 1 Ga recorded by thermochronology, but not by diffusion chronometry, although rigorous assessment of the latter is still hindered by relatively few studies. This discrepancy in interpretations based on the two methods requires more targeted investigation.