What drives metamorphism in early Archean greenstone belts? Insights from numerical modeling

Abstract

Geodynamic regimes responsible for the formation of Eoarchean-to-Mesoarchean continental crust may be investigated using numerical modeling in which deeper mantle processes are coupled with shallower processes responsible for the formation and modification of the crust. In an earlier study using a 2D coupled petrological–thermomechanical tectono-magmatic numerical model constrained by information from the geological record, experiments with initial conditions appropriate to the Eoarchean–Mesoarchean revealed multiple tectonic processes by which mostly felsic continental crust could have been formed from an initial primary mafic crust in a non-uniformitarian geodynamic regime. We use the model pressure–temperature–time (P–T–t) paths for material particles from our previous study to test the hypothesis that the range of metamorphic P–T conditions recorded in Archean granite–greenstone domains could have been generated without subduction and plate tectonics. We compare the experimental results with P–T and age data recovered from two early Archean domains, the East Pilbara Terrane (EPT), part of the Pilbara craton in Western Australia, and the Barberton greenstone belt (BGB), part of the Kapvaal craton in South Africa. The dome-and-keel structures that develop in the crust in the numerical experiment and the associated P–T–t paths at various locations within these structures are similar to the structures and P–T and age data recovered from the early Archean crust in the EPT and BGB. This study shows that metamorphism in granite–greenstone-like crust could have been the result of both vertical and horizontal tectonic processes (coupled with crustal convective overturns) that occurred over linked sites of crustal delamination and mantle upwelling. The range of thermal gradients derived from peak P–T values for markers in the experiment matches the range of apparent thermal gradients retrieved from the EPT and BGB, demonstrating that even the lowest apparent thermal gradients associated with dome-and-keel structures in greenstone belts can be achieved in the absence of subduction.