Mantle heating drove short-duration Barrovian-type regional metamorphism during the Paleoproterozoic Penokean orogeny, Michigan (USA)

Barrovian-type regional metamorphism is a fundamental feature of Phanerozoic orogenesis. One of the oldest known examples of this phenomenon is recorded in the Michigamme Formation, a foredeep sequence deposited, deformed, and metamorphosed during the Paleoproterozoic (1890−1830 Ma) Penokean orogeny, Michigan (USA). The Penokean Barrovian-type regional metamorphism is manifested as a series of distinct, concentrically arranged isograd sequences (“nodes”) cored by sillimanite-grade rocks. A mafic intrusion known as the Peavy Pond Complex (PPC) is located at the center of one of the nodes, the Peavy node, suggesting a spatial relationship between mantle melting, accompanied by magmatism, and metamorphism. New U-Pb geochronology data show that the Michigamme Formation was deposited after 1846.03 ± 0.81 Ma, and gabbroic units of the PPC crystallized between 1835.41 ± 0.48 Ma and 1828.26 ± 0.49 Ma. Textural and petrologic constraints on the relative tectonic timing of PPC magmatism and Peavy node regional metamorphism suggest that the two were synchronous. Combined, the geochronology and relative timing of deformation suggest a ≤12 m.y. time window for 14−24 km of tectonic burial and subsequent peak regional metamorphism, requiring prograde metamorphic heating at an average rate ≥50 °C m.y.−1. We interpret the Barrovian-type regional metamorphism recorded in the Peavy node as resulting from advection of mantle heat into the crust during a synorogenic episode of mantle melting. The archetypal example of Barrovian regional metamorphism formed during the Ordovician Grampian orogeny (Scotland) underwent similarly rapid heating and was concurrent with large-scale, synorogenic mafic magmatism, underlining similarities in the drivers of these examples of orogenic regional metamorphism. This comparison supports growing evidence for the operation of Phanerozoic-style orogenesis during the Paleoproterozoic.