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

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 therefore do not fully reconcile with the existing accretionary/collisional models for the Pontiac Subprovince, given differences in their predicted apparent thermal gradients, metamorphic evolution and structural patterns. Alternatively, our data more closely match predictions for a sagduction-dominated vertical process, where high heat influx at the base of the crust causes pooling/diapiric ascent of voluminous S-type plutons, bordered by sagging synclines. This process led to the development of steeply dipping tectonic fabrics and pre- to late-kinematic mineral assemblages along diverse P–T paths at contrasting yet coeval T/depth ratios. Our study provides insights into the evolution of the southeastern Superior Craton and highlights the need to apply an integrated quantitative approach to decipher its thermal structure and constrain the likely geodynamic processes that operated in the Neoarchean.