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Dating the mantle roots of young continental crust

¹ Danish Lithosphere Centre, Øster Voldgade 10, 1350 Copenhagen, Denmark, and School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand
² School of Earth Sciences, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK

Subcontinental lithospheric mantle xenoliths from beneath the French Massif Central contain clinopyroxene with high Lu/Hf and Hf isotope ratios (¹⁷⁶Lu/¹⁷⁷Hf = 0.1–10.8; _Hf = +40 to +2600). These Lu-Hf isotope systematics yield model ages of 313–377 Ma and apparently date melt extraction in a mantle wedge during Variscan subduction, with the residual mantle subsequently forming the stabilizing mantle keel of young European continental crust. The extremely depleted Lu-Hf mantle systematics contrast with other isotopic signatures (Sr, Pb, Nd) that have been overprinted by mantle metasomatism. The high Lu/Hf values developed in mantle clinopyroxene by melt extraction, coupled with the robustness of this system to some types of metasomatism, provide a new chronometer for directly dating crust-mantle differentiation in both young and ancient continental regions. The process of metasomatic decoupling of Nd and Hf isotopes, which has been previously recognized in Hawaiian oceanic mantle and in French Massif Central subcontinental mantle, may be responsible for the prominent displacement of terrestrial rocks to lower _Nd at a given _Hf as compared to Bulk Earth reference values obtained from analysis of chondritic meteorites.

Key Words: Lu-Hf isotopes • subcontinental lithosphere • mantle xenoliths • geochronology • Variscan orogeny • Europe

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