Over one billion years of Archean crust evolution revealed by zircon U-Pb and Hf isotopes from the Saglek-Hebron complex

Abstract The Saglek-Hebron Complex (SHC) located in Northern Labrador, Canada, includes some of the oldest crustal rocks on Earth. This granite-greenstone terrain is dominated by trondhjemite, Mg-rich tonalite, granite, with subordinate granodiorite, and recorded over one billion years of early magmatic history between ∼3900 Ma and ∼2700 Ma. New in-situ U-Pb analyses on zircons from eighteen rock samples, suggest that six distinct magmatic events formed the felsic crust of the SHC, during the Archean Eon. Age deconvolution analysis constrains these magmatic episodes at ca. 3857 Ma, 3744 Ma, 3575 Ma, 3330 Ma, 3224 Ma and 2750 Ma, respectively defined as the Iqaluk, the Uivak I, the Uivak II, the Iluilik, the Lister and the late granite events. The 3330 Ma Iluilik represents a newly defined magmatic event, distinct from the Lister event previously recognized at ∼3200 Ma. Uranium-Pb geochronology, combined with in-situ Hf isotopic data on zircons and geochemistry of the host rocks, reveal a complex and protracted crustal history of the SHC marked by the reworking of mafic and felsic crust over one billion years. Zircons from the oldest Iqaluk and Uivak I samples yield slightly suprachondritic initial eHf values (up to ∼+4), consistent with the involvement of a long-term depleted source in the Eoarchean. The 3330 Ma Iluilik granodiorite exhibits a low initial eHf value of −6, clearly diverging from the near-chondritic Hf isotopic composition of the 3224 Ma Lister tonalites. Considering a mafic crustal precursor (with a 176Lu/177Hf ratio of ∼0.02) for trondhjemite-tonalite-granodiorite (TTG) samples, the initial Hf isotopic compositions of the SHC Eoarchean to Paleoarchean zircons would be consistent with the reworking of Hadean mafic crust. The zircon eHf vs. time trend for granite samples corresponds to a 176Lu/177Hf ratio of ∼0.01, suggesting that the granitic crust was mainly produced by crustal reworking of the oldest TTG. Similarly to the felsic crust of the Itsaq Gneiss Complex of SW Greenland, the SHC granitoids appear to record a shift to more juvenile compositions at the end of the Paleoarchean. However, contrary to what is observed in SW Greenland, almost no input of juvenile material has been recorded by the SHC Neoarchean felsic crust.