Archaean crustal development in the Lewisian complex of northwest Scotland

Abstract

THE Lewisian complex of northwest Scotland is typical of many Archaean terrains and has a well documented history starting ∼2,700 Myr ago^1–6. Here we present new isotopic data that extend this history back to 3,300 Myr, and provide some insight into how the earliest continental crust may have formed. The Lewisian is dominated by tonalite, trondhjemite and granodiorite (TTG) rocks, which require a mafic lithospheric source, rather than being direct mantle melts^7–11. But the mafic and ultramafic rocks in the high-grade granulite-facies part of this terrain show little evidence of a significantly older crustal history^12,13 and precursor material to the TTG lithologies has not yet been identified. Here we show that older amphibolite material has survived at a lower metamorphic grade. Coexisting amphibolite minerals yield indistinguishable ²⁰⁷Pb–²⁰⁶Pb and ¹⁴⁷Sm–¹⁴³Nd ages of 3,310 ± 27 Myr and 3,298 ± 73 Myr, respectively. These data are consistent with an origin for much of the Lewisian terrain by the re-melting of preexisting lithosphere, with an isotopic signature similar to that of the amphibolites studied here.