Abstract

Recent reinterpretations of the Early Proterozoic Penokean orogeny in Minnesota invoke a plate-tectonic model with characteristics similar to Phanerozoic collisional belts. Deformation during collision produced large flattening strains (-89%) and the development of a tectonic foliation and progressive metamorphism up to amphibolite facies, as indicated by the presence of synkinematic (S1) garnets. Biotite-garnet thermometry and plagioclase-garnet-biotite-muscovite barometry give southward-increasing temperatures and pressures and indicate burial to depths of ≥21-25 km, similar to medial and internal zones of collisional orogens worldwide. Gibbs Method modeling on zoned synkinematic garnets indicates virtually no change in pressure (P) or temperature (T) during garnet growth. Growth and rotation of garnets at a point in P-T space in an actively evolving deformational and thermal regime are surprising and suggest rapid garnet growth rates and high rates of strain during orogeny. This is consistent with previous studies that indicate more rapid rates of tectonometamorphic processes in the Precambrian relative to younger orogenic belts.

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