Contact metamorphism of lithic clasts in basal impact melt breccias at Manicouagan

Abstract

The 90 km diameter Manicouagan Impact Structure, located ~ 300 km North of Baie-Comeau, Quebec, is an example of a complex impact crater with a well-developed impact melt sheet. This melt sheet contains small (sub-centimetre) to large (multi-metre) clasts of variable lithologies. Some of these clasts show well-preserved shock features (e.g., planar defo1mation features in quartz), while others show evidence of extensive melting and recrystallization. Sampling of these clasts was undertaken and a variety of these were examined via optical microscopy and analytical electron microscopy. The lithologies analyzed included anorthosite, mafic gneiss, intermediate gneiss, felsic gneiss, carbonate and amphibolites. The larger (> 1 m diameter) clasts of gneiss and anorthosite retain well-preserved shock features in their core, while melting has occurred around their rims. Smaller clasts show complete recrystallization with no preservation of shock features. This evidence is used to constrain the initial melt temperature and cooling rate, as well as the source and melt-residence time of the clasts.