Cold Deep Over Hot Shallow Crust: A Peculiar Metamorphic Architecture in the Neoarchean Metasedimentary Pontiac Subprovince, Superior Craton (Canada)

• Published in: Journal of metamorphic geology 2025-02, Vol.43 (2), p.191-224
• Main Authors: Malta, Isaac S., Guilmette, Carl, Godet, Antoine, Tinkham, Douglas K., Coldebella, Bruna, Beaudoin, Georges, LaFlamme, Crystal, Jørgensen, Taus R. C., Marsh, Jeffrey H.
• Format: Article
• Language: English
• Subjects: Accretion; Assembly; Biotite; Brittleness; Cratons; Decompression; Deformation; Deformation analysis; Depth; Dipping; Earth; Evolution; Garnet; Garnets; Heating; History; Igneous rocks; Kinematics; Kyanite; Metamorphism; Mineral assemblages; Minerals; Phase equilibria; Plutons; Predictions; Ratios; Regional development; Rock; Rocks; Sillimanite; Synclines; Tectonics; Temperature gradients; Thermal structure; Turbidites
• ISSN: 0263-4929; 1525-1314
• DOI: 10.1111/jmg.12804

ABSTRACT The Neoarchean Era is a key period in Earth's history as it witnessed a significant pulse of crustal formation corresponding to the assembly of several cratons, potentially coeval with a transition in the global tectonic regime. Neoarchean metasedimentary subprovinces of the Superior Craton, the largest unreworked Archean craton on Earth, were formed shortly before its final assembly and cratonization, thus providing valuable insights into the tectonic style, thermal state and architecture prevailing during this key period. Among these, the Pontiac Subprovince is one of the most studied, yet has a largely debated geodynamic history. In its northern extent, metamorphosed turbiditic sequences display a southward succession of index minerals—biotite, garnet, staurolite, kyanite and sillimanite—that may be indicative of a Barrovian‐like metamorphic gradient. However, the origin and evolution of this apparent gradient and its link to Neoarchean tectonics remain unclear. New mapping of metamorphic isograds and zones, petrological and microstructural analyses, whole rock and mineral chemistry analyses and phase equilibria modelling are integrated to decipher the Pontiac Subprovince tectonothermal evolution. Our analysis indicates that the peak equilibrium assemblages from the garnet, staurolite and sillimanite/melt zones developed early to late relative to the main regional deformation event (D2) and its associated steeply dipping S2 schistosity. Garnet zone rocks recorded a burial‐heating path with peak P–T conditions at 8.1–8.2 kbar and 582°C–585°C, along a low T/depth ratio of ~20°C/km. In contrast, staurolite and sillimanite/melt zone rocks followed isobaric heating and isothermal decompression paths with peak P–T conditions at 5.9–6.1 kbar and 610°C–625°C and 6 kbar and 700°C, respectively, along a moderate T/depth ratio of ~30–33°C/km. Since there is clear D2 structural continuity across the metamorphic zones over ~12 km and metamorphism occurred pre‐ to post‐D2, we interpret that the diverse P–T paths and contrasting T/depth ratios likely represent a spatially heterogeneous thermal structure developed within a single coherent structural block during and shortly after the formation of the subvertical S2 schistosity. Such features are hardly compatible with either modern inverted or continuous Barrovian sequences—known for consistent P–T evolution paths and similar T/depth ratios—or with discontinuous sequences requiring diachronicity. Our findings