The Manicouagan impact melt sheet: Evidence for isotopic homogenization with limited assimilation

The Rb–Sr, Pb–Pb and Sm–Nd isotope chemistry of impact-generated melt and target rocks has been determined from the 214 Ma, ∼90 km rim-diameter, Manicouagan impact structure of Canada. Fifty-seven samples were obtained from 8 field sites and 11 drill core sites across the impact melt sheet, located on the 55 km-diameter central island (Île René-Levasseur). The results reveal that the impact melt, though locally differentiated via fractional crystallization, was isotopically homogenized during its formation following its derivation primarily from charnockites and mesocratic gneisses. The isotopic signatures of the target rocks indicate that the protolith for the melt was the Proterozoic Manicouagan Imbricate Zone, and that the underlying Archean Gagnon Terrane was not involved in melt production, or its subsequent modification via assimilation, despite impact melt resting on Archean lithologies in the southwest sector of the melt sheet. Limited assimilation of the footwall in this sector is attributed to the relatively rapid cooling of a thinner peripheral melt body (<100 m), primarily through contact with cooler footwall and clasts. Assimilation at the base of the thicker (up to 1.4 km thick) melt sections, near the center of the structure is attributed to secondary melting of centrally uplifted anorthosite, which locally modified the original isotopic signature.