INTERROGATING THE RECORD OF ARCHEAN TECTONICS FROM METAMORPHISM IN THE MINNESOTA RIVER VALLEY, USA (SUPERIOR PROVINCE)

If and how plate tectonics operated in the Archean eon remains a controversial topic. The Minnesota River Valley (MRV) gneiss terrane in the southernmost part of the Superior Province contains a variety of tectonic “blocks” with diverse igneous and metamorphic rocks dating back to over 3.5 Ga. Therefore, its geologic history provides valuable insights into the tectonic processes that shaped the early continents. This study focuses on the granulite-facies Montevideo and amphibolite-facies Morton blocks of MRV, which are separated by the east-trending Yellow Medicine shear zone. We combined inverse thermobarometry calculations and phase-equilibrium modelling to constrain the pressure (P) and temperature (T) of peak metamorphism in the two blocks. The Morton amphibolite-facies metamorphic conditions are inferred to be 610–760 ℃ and 3.8–6.3 kbar, and the Montevideo granulite facies to be 800–860 ℃ and 4.4–7.0 kbar. The two blocks are likely metamorphosed together by advective heating due to plutonism at c. 2.6 Ga. We also performed zircon and monazite U–Pb geochronology and garnet diffusion chronometry to obtain ages and duration of metamorphism. The P–T–t data is used to determine the tectonic and thermal conditions of Archean metamorphism in the two blocks and whether significant lateral translation of the blocks—akin to modern plate tectonics—could have caused their juxtaposition.