Peel-back controlled lithospheric convergence explains the secular transitions in Archean metamorphism and magmatism

Abstract

The preserved archive of continental crust suggests that various secular geologic and geochemical transitions took place during the Mesoarchean and Neoarchean (∼3.2–2.5 Ga). These transitions, which are imprinted in the metamorphic and magmatic rock-record, include the emergence of paired metamorphism, an increasing depth of tonalite-trondhjemite-granodiorite (TTG) formation, the widespread appearance of granites and changes in the composition of mafic rock. Previous studies have argued that these secular transitions reflect secular cooling of the mantle and herald a gradual transition from pre-plate tectonic to plate tectonic regimes. However, their tectonic driver remains elusive, in part due to a lack of detailed understanding of lithospheric dynamics prevalent during this transitional period when the mantle was warmer compared to the present-day. Here, we demonstrate that lithospheric convergence driven by peel-back (/peeling) process under warmer mantle conditions – termed as peel-back convergence – may explain the late Archean secular transitions. This tectonic phenomena features large-scale peeling (a form of delamination, but not dripping) of the mantle lithosphere with or without lower crustal rocks during convergence. We simulated numerical models of peel-back convergence under Archean crust-mantle conditions and predicted the pressure-temperature (P-T) conditions of crustal metamorphism and melting. The evolution of peel-back convergent setting features juxtaposition of a colder, compressional regime with thickened crust that forms at the site of peeling, and a warmer, extensional regime with thinned crust forming behind it. The metamorphic and magmatic P-T conditions prevalent in these two tectonothermal sites replicate the features of the late Archean rock record, including the: appearance of coeval high-T/P and intermediate-T/P metamorphic rocks; greater abundance of high-T/P relative to intermediate-T/P metamorphic rocks; dominant formation of TTGs at higher pressures where garnet ± rutile is stable in the residue; reworking of pre-existing felsic rocks/sediments to produce potassic granites; and interaction of crustal melts and mantle to form hybrid granitoids. Thus, peel-back convergence can explain the thermobaric bimodality of late Archean rocks within an asymmetric tectono-thermal framework. However, unlike modern convergent plate boundaries, the asymmetry is controlled by lithospheric peeling.