Lu-Hf ages and source of mantle-derived rocks from the Saglek-Hebron Complex (Northern Labrador)

Ancient mantle-derived rocks allow us to gain insights into the geochemical evolution of Earth's early mantle. We present a 176Lu-176Hf study of the Saglek-Hebron Complex (northern Labrador, Canada), which contains some of the oldest mafic and ultramafic rocks on the planet. Low-Fe ultramafic and basaltic metavolcanic rocks yield an isochron of 3934 ± 127 Ma with a suprachondritic initial epsHf=+8.9 ± 2.7. This value implies a highly incompatible-element depleted source for these rocks, which was more depleted than the Depleted Mantle estimates at the same age. Decoupled Sm-Nd and Lu-Hf systematics can be explained by ancient source depletion during a magma ocean in the garnet stability field. High-Fe ultramafic rocks yield an isochron corresponding to an age of 3405 ± 503 Ma. Mafic intrusions were age-dated at 3875 ± 454 Ma and at 2716 ± 370 Ma. The Saglek-Hebron Complex has recorded mantle-derived magmatism for over 1 billion years.