Abstract Emplacement of post-tectonic Early Palaeozoic pegmatites on Tonagh Island, Napier Complex, East Antarctica, was accompanied by the introduction of aqueous low-salinity fluids at mid- P upper-amphibolite facies conditions ( c. 8 kbar, c. 680 °C). Fluid–wall-rock interaction resulted in the development of spectacular alteration selvedges, in the immediate vicinity of the pegmatites, in adjacent Archaean orthogneisses. Archaean wall-rocks affected by the infiltration of aqueous fluids show contrasting patterns of K, Na and Ca metasomatism, which we demonstrate was fundamentally controlled by disequilibrium of invading fluids with wall-rock feldspars, rather than fluid flow up or down regional pressure or temperature gradients. Other species, such as the rare earth elements (REE; except Eu), Y, P, Rb, Th, U and Pb, for example, show significant enrichment in the metasomatized wall-rock in both examples studied. Enrichment of P (and Y), sympathetic with that of REE enrichment, is consistent with recent suggestions that REE transport in fluids may be enhanced by complexing with dissolved P and Y compounds. Dehydration and partial melting of previously unmetamorphosed sedimentary rocks, underthrust beneath the SW Napier Complex, has been long considered a viable source for felsic pegmatites (and associated fluids) observed in that region. Our results are consistent with the hypothesis that underplated sedimentary rocks were a viable source for pegmatite melt and aqueous fluids. Furthermore, as our study demonstrates a plausible relationship between Early Palaeozoic ( c. 500–530 Ma) pegmatites and fluid infiltration, we suggest that dehydration and prograde partial melting of the underthrust sedimentary rocks beneath the Napier Complex occurred, at least in part, by convergent Early Palaeozoic tectonism.