Archean crustal evolution of the Saglek-Hebron Complex, Northern Labrador, revealed from coupled 147−146Sm-143−142Nd systematics

Abstract

The Saglek-Hebron Complex in Northern Labrador, Canada, hosts some of the oldest rocks on Earth. Recording a protracted crustal history over 1 billion years, the complex includes multiple generations of felsic rocks formed through episodic magmatic activity spanning the whole Archean Eon, between ∼3900 Ma and ∼2700 Ma. Coupled 147−146Sm-143−142Nd systematics from these granitoids show a complex crustal history, including melting of older mafic crustal precursor sources and reworking of Eoarchean felsic crust. Eoarchean granitoids are characterized by high initial ε143Nd up to +4 and μ142Nd up to +15. Combined long- and short-lived Sm-Nd isotope systematics suggest the crustal precursor source of the oldest Saglek-Hebron granitoids was derived from an early incompatible element depleted reservoir formed between 4200 Ma and 4300 Ma, evolving with a 147Sm/144Nd ratio of ∼0.265. The source involved in the formation of the Saglek-Hebron Eoarchean felsic crust appears to be younger and more depleted compared to that of the Saglek-Hebron mafic-ultramafic rocks, but similar to the source of the Eoarchean granitoids from the Itsaq gneiss complex in SW Greenland. This may suggest the formation of a Hadean highly depleted mantle reservoir at the scale of the North Atlantic Craton, perhaps resulting from the extraction of Hadean mafic crust. Paleoarchean ∼3300 Ma granitoids from the Illuilik magmatic episode exhibit initial ε143Nd between -1.6 and -1.3, which could be consistent with the reworking of the Saglek-Hebron Eoarchean felsic crust. However, the whole-rock geochemical composition of some Iluilik samples rather suggests derivation from a mafic crustal source, and thus their initial ε143Nd values imply derivation from an Hadean mafic precursor. This scenario would also account for the lower μ142Nd of ∼+6 found for the Iluilik granodiorite, compared to the older TTG, and consistent with derivation from a distinct crustal source. Most granitic rocks exhibit initial ε143Nd and μ142Nd consistent with reworking of the oldest Saglek-Hebron TTG, throughout the Archean, between ∼3700 Ma and ∼2700 Ma. Finally, coupled 147−146Sm-143−142Nd systematics suggest a shift in precursor sources at the end of the Paleoarchean, with the ∼3200 Ma Lister gneiss exhibiting more juvenile initial ε143Nd and no deviation of μ142Nd from the terrestrial standard, as opposed to most other Saglek-Hebron granitoids. This shift to more juvenile precursor material in the Paleoarchean has also been observed in other Archean cratons, suggesting a planetary-scale transition for crustal formation processes.