Tectonics of the Isua Supracrustal Belt 1: P T X d Constraints of a Poly Metamorphic Terrane

The Eoarchean Isua supracrustal belt (ISB) has been interpreted as one of the earliest records of subduction processes, leading to the conclusion that a plate tectonic geodynamic system was likely operating since the early Archean. However, proposed tectonic models remain difficult to evaluate as our understanding of the metamorphic and structural evolution remains fragmentary. Here, we present a metamorphic study of the supracrustal rocks of the ISB. We used petrographic and microstructural observations, phase equilibria, isopleth geothermobarometry, and conventional thermometry to explore the prograde, peak, and retrograde metamorphic evolution of the northeastern ISB. Our results show that the ISB records a syn tectonic, amphibolite facies metamorphic event (M₁) with peak conditions of 550°C-600°C and 0.5-0.7 GPa. M₁ was followed by a static, lower amphibolite facies metamorphic event (M₂; <540°C and <0.5 GPa). Published constraints suggest that M₁ and M₂ occurred in the late Eoarchean (>3.5 Ga) and the Neoarchean (<2.9 Ga), respectively. These events are partially overprinted by late low temperature (<500°C) retrogression (M₃) that is most intensely developed in the northeastern part of the belt; it typically overprints some peak mineral phases while preserving the peak fabric. Our findings are consistent with spatially homogeneous syn tectonic amphibolite facies metamorphism and macroscale folding. Such features are predicted by a heat pipe tectonic model. Therefore, our findings permit the interpretation of the ISB as a record of early nonuniformitarian tectonic processes.