The role of mafic-ultramafic bodies in the origin of the early continental crust

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Major components of the Earth’s oldest continental crust are sodic rocks of the tonalite– trondhjemite–granodiorite (TTG) series, which formed through partial melting of hydrous metabasalts at different depths. However, the processes associated with TTG melt formation are still debated, and fundamental questions regarding the role and source of water in the production of such voluminous amounts of melts have yet to be answered.This project aims to better characterize the pressure–temperature–composition–time (P–T– X–t) conditions during partial melting of natural Archean mafic–ultramafic protoliths and their role in the generation of TTG melts. In particular, we aim to investigate the role and source of water during partial melting, and how the compositional diversity of the mafic–ultramafic protoliths and water availability control the composition and volume of the melts produced. The investigated samples comprise pairs of TTGs and enclosed mafic-ultramafic pods from a range of Archean terrains that differ in terms of their proposed geodynamic evolution and age. Phase equilibrium forward-modelling will be used to constrain the conditions of partial melting and the compositions of the melts produced. These results will be combined with trace element modelling and bulk-rock geochemistry of the associated TTG gneisses to compare the modelled melt compositions with those of the natural TTG gneiss samples. Additionally, U–Pb geochronology, trace element and oxygen isotope analysis of zircon will be used to i) integrate temporal and P–T constraints of the zircon-equilibrated TTG melt, ii) to provide further information on the depth of melting, and iii) to determine the source and the role of water during crustal melting. The results of this study have the potential to significantly contribute to the long-lasting discussion on Archean TTGs petrogenesis and may also provide further insights into the geodynamic setting(s) in which they formed.