Massive and brecciated dikes in the McDougall and Despina faults, Noranda, Quebec, Canada

The McDougall and Despina faults of the central Noranda volcanic complex cut subaqueous volcanic rocks in the Archean Abitibi greenstone belt. Rhyodacitic dikes occupy the faults, along with lesser amounts of andesitic, dioritic and a mixed basaltic-rhyodacitic dike. There are two types of rhyodacitic dikes, one massive the other brecciated. Massive dikes are homogeneous and spherulitic; brecciated dikes are dominated by curved, angular fragments with a few vesicles. Both occur either alone or together in the faults. Where the two occur together they are commonly interlayered in concentric layered lobes.The faults are interpreted as fissures for pulses of nonexplosive rhyodacitic lava. Many intrusive pulses interacted with an external fluid which occupied the faults. This interaction resulted in brecciated, glassy margins and massive, crystalline pulse interiors. Magma/fluid interaction is thus invoked as the mechanism responsible both for dike brecciation and the concentric layering. The dikes are considered as intrusive analogs of extrusive rhyolitic lobe lava observed in Iceland and in Noranda.