An explosion breccia, 4 km east of Hopedale, Labrador, occurs in Archean migmatite forming part of the Hopedale Gneiss. The breccia, 105 by 60 m, consists of angular to well-rounded fragments compositionally similar to the country rock. Extensive development of piemontite and epidote is characteristic of the breccia. Two narrow mafic dikes post-date and fill fractures in the breccia. An electron probe analysis of piemontite and epidote shows the former contains 1.86% Mn2O3 and the latter 0.21%. A genesis of the breccia involving fluidization with subsequent development of piemontite by hydrothermal activity is suggested.