New Geochemical Data from the Manicouagan Impact Melt Sheet

Manicouagan is a complex impact structure formed in Grenvillian target rocks of the Canadian Shield, Quebec, at 214±1 Ma. It is ~100 km in diameter with a second inner ring at 55-65 km defined by the peripheral trough, which defines the Manicouagan Reservoir. Inside the reservoir, impact melt covers much of the central island. Previous studies estimated a pre-erosional melt sheet thickness of 230 metres, based on field observations. In line with previous studies of terrestrial impact melts of sub-Sudbury size, differentiation was not recognized at Manicouagan. However, recent drilling activities show that the melt sheet is thicker than previously suspected. Thicknesses vary from 50 metres to 1100 metres of macroscopically clast-free melt. Eighty-five melt sheet samples from 8 drill holes have been analyzed for major, trace and REE chemistry. SiO2 ranges from 56.24 to 62.13 wt.% (average 58.35±1.70%). This compares well to previous studies (average SiO2 value 57.75±1.21%). The greatest degree of variation occurs in hole 0608, which represents a macroscopic clast-free melt length of c. 1100 metres. It can be divided into two distinct geochemical layers, with a third transitional zone inbetween. The upper layer (UZ: quartz monzodiorite) (c. 250 metres) is relatively enriched in silica (60.64 wt.%). The lower layer (LZ: quartz monzodiorite to monzodiorite) (c. 600 metres) is enriched in CaO and MgO, whilst relatively depleted in silica (56.24 wt.%). The transitional layer (MZ: quartz monzonite) (c. 250 metres) has an average of 62.13 wt. %. The overall composition of the melt sheet (85 analyses), including hole 0608, is quartz monzodiorite. It is suggested that the large volume of melted material in the 0608 core facilitated differentiation, not seen in the shallower parts of the melt sheet. Sudbury is not the only differentiated impact melt sheet on Earth.