Generation of felsic crust in the Archean: A geodynamic modeling perspective

Abstract

As a consequence of secular cooling of the Earth, there is generally no modern analog to assist in understanding the tectonic style that may have operated in the Archean. Higher mantle temperatures and higher radiogenic heat production in the Archean Earth would have impacted the thickness and composition of the crust. For this reason, well-constrained numerical modeling, based on the fragmentary evidence preserved in the geological record, is the most appropriate tool to evaluate hypotheses of Archean crust formation. The main lithology of Archean gray gneiss complexes is the sodic tonalite–trondhjemite–granodiorite (TTG) suite. Melting of hydrated basalt at garnet amphibolite, granulite or eclogite facies conditions is considered to be the dominant process that generated the Archean TTGs. Taking into account geochemical signatures of possible mantle contributions to some TTGs, models proposed for the formation of Archean crust include subduction, melting at the bottom of thickened continental crust and fractional crystallization of mantle-derived melts under water-saturated conditions. We evaluated these hypotheses using a 2D coupled petrological-thermomechanical tectono-magmatic numerical model with initial conditions appropriate to the Eoarchean–Mesoarchean. Based on the result of our experiments, we identify three tectonic processes by which intermediate to felsic melts may be generated from hydrated primitive basaltic crust: (1) delamination and dripping of the lower mafic crust into the mantle; (2) local thickening of the crust; and (3) small-scale crustal overturns. In the context of a stagnant-deformable lid tectono-magmatic geodynamic regime that is terminated by short-lived subduction, we identify two distinct types of continental crust. The first type is a pristine granite–greenstone-like crust with dome-and-keel geometry formed over delaminating–upwelling mantle which is mostly subjected to vertical tectonics processes. By contrast, the second type is a reworked (accreted) crust comprising strongly deformed granite–greenstone and subduction-related sequences and subjected to both strong horizontal shortening and vertical tectonics processes. Thus, our study has identified a possible spatial and temporal transition from pristine granite–greenstone-like crust with dome-and-keel geometry to reworked (accreted) crust forming more felsic gneiss terranes in the Archean. We suggest that the contemporaneity of the proposed mechanisms can explain the variety and complexity of the Archean geological record.