THERMAL ERASURE OF SHOCK EFFECTS: IMPACT MELT-CLAST INTERACTIONS AT MANICOUAGAN

Clasts entrained within the basal layers of the impact melt sheet at Manicouagan have been assessed for their thermal overprint on shock effects. Larger clasts (e.g., 1 m diameter) may retain any shock features within their cores, while their rims are typically annealed and are bereft of shock damage. Smaller clasts (<< 1m diameter) are commonly found completely thermally annealed with no evidence of shock being retained. The smallest size fraction shows evidence of partial to complete melting and incomplete assimilation into the bulk melt. Whilst it is difficult to equate clast populations with their source footwall lithology, some locations indicate that transport has not been significant, especially towards the periphery of melt sheet. In this respect, it may be possible to equate retained shock intensity with radial distance from the projectile contact zone, which may offer a crude constraint on shock attenuation. In other cases, the effective mixing of clasts showing distinct shock histories indicates blending over extensive radial distances. In addition, an assessment of clast volume and annealing depth versus clast diameter provides constraints on initial impact melt sheet temperatures and on the efficiency of clasts as heat sinks.