An Occurrence of Archean Pseudotachylite from Southeastern Manitoba

Zoned veins (glass and crushed rock) of pseudotachylite occur in an Archean mylonite near Charron Lake, Manitoba. Glass occupies the centers of some veins and fusion products penetrate symmetrically distributed, crushed rock zones. The emplacement of the pseudotachylite postdates the formation of the mylonite fabric, but a later deformation affected both pseudotachylite and mylonite. The average chemical composition of the pseudotachylite veins reflects the composition of the host rocks, although the opaque, marginal crushed rock zones are enriched in iron and the fused centers are depleted in sodium. Glass is confined to crushed rock veins although not all crushed rock veins contain glass; apparently an increase in water pressure led to fusion.