Abstract
AbstractThe 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.
Highlights
Models to describe the nature of the Earth's crustal evolution during the Archean vary substantially, from horizontal lithospheric motions with subduction-like movement (Abbott et al, 1994), to vertical tectonics defined by sub-/intralithospheric diapirism (Collins, 1989; Sizova et al, 2015), extensive volcanism (Moore & Webb, 2013; Turcotte, 1989), and the formation of a single-plate lithosphere
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
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
Summary
Models to describe the nature of the Earth's crustal evolution during the Archean vary substantially, from horizontal lithospheric motions with subduction-like movement (Abbott et al, 1994), to vertical tectonics defined by sub-/intralithospheric diapirism (Collins, 1989; Sizova et al, 2015), extensive volcanism (Moore & Webb, 2013; Turcotte, 1989), and the formation of a single-plate lithosphere. A recent review of the existing metamorphic record suggests that Earth has experienced spatial and temporal changes of global crustal thermal gradients probably linked to transitions in the tectonic regime, from stagnant-lid tectonics to mobile-lid plate tectonics (Brown & Johnson, 2018). The main changes occurred at ∼2.5 Ga, ∼1.0 Ga, and 0.72 Ga, Ga which are associated with the widespread appearance of paired metamorphic belts, dominance of high T/P gradients (>775°C/GPa), and increasing abundance of low T/P gradients (
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