Abstract
Combined use of Rb–Sr, U–Pb and Sm–Nd decay schemes in geochronology provides insights into the evolution of ancient continental crust1 such as the Lewisian of north-west Scotland. We report here a Pb–Pb whole-rock isochron date of 2,950±70 Myr on amphibolite-facies grey gneisses from the northern region of the Lewisian; this is essentially the same as a Sm–Nd whole-rock isochron date (2,920±50 Myr) which was interpreted as the time of separation of igneous protoliths from the mantle2. A period of ∼240 Myr separates this date from the Scourian peak of granulite-facies metamorphism. This result indicates, for the Lewisian, that high-grade metamorphism and crustal stabilization cannot have been the result solely of thermal equilibration. More generally, it suggests that tectonic thickening was an important process during Archaean crustal evolution3 and that although whole-rock systems may remain open with respect to U–Pb until the termination of granulite-facies metamorphism, they may be closed during amphibolite-facies metamorphism.
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Whitehouse, M., Moorbath, S. Pb–Pb systematics of Lewisian gneisses—implications for crustal differentiation. Nature 319, 488–489 (1986). https://doi.org/10.1038/319488a0
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DOI: https://doi.org/10.1038/319488a0
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