Source and evolution of 4.0 – 3.9 Ga crust revealed by U-Pb, Lu-Hf and trace element analyses of xenocrystic zircon, Superior Province

• Eoarchean zircon xenocrysts were sourced from Hadean crust in the Superior Province. • Recrystallized ca. 3.90 Ga zircons record the earliest metamorphic event on Earth. • No evidence for shock textures favour tectonically controlled metamorphism. In the Superior Province, Eoarchean magmatism sourced from partially melted Hadean mafic crust has been inferred from published ca. 3.9 Ga detrital zircon Hf isotope compositions and whole-rock 142 Nd isotopes. We report simultaneously acquired U-Pb/Lu-Hf and U-Pb/trace element analyses of 4.0 – 3.9 Ga xenocrystic zircon grains from a polyphase gneiss that provide direct evidence for Eoarchean partial melting of Hadean crust in the Hudson Bay terrane, Superior Province. Zircon domains record age groups at ca. 3.99 – 3.94 Ga, 3.92 – 3.84 Ga, 3.59 Ga, 3.48 Ga, 3.32 Ga, 3.27 Ga, 3.1 – 2.95 Ga, 2.84 Ga and 2.73 Ga. The ca. 3.99 – 3.84 Ga core domains have subchondritic Hf isotope signatures yielding an estimated mantle extraction age between 4.23 Ga and 4.18 Ga. The ca. 3.92 – 3.84 Ga zircon core and mantle domains are faintly or homogeneously zoned (ghost zonation), they inherited the Hf isotope compositions of the older ca. 3.99 – 3.94 Ga protolithic grains, and have trace element signatures consistent with expulsion of LREE over HREE and Th over U, which altogether reflect solid-state recrystallization of the older protolith zircon population. The mechanism for solid-state recrystallization requires a high-grade metamorphic event. Therefore, the ca. 3.92 – 3.84 Ga zircon population appears to record the earliest known metamorphic event on Earth, which was previously identified and inferred from detrital zircon in the Jack Hills meta-conglomerate.