The Precambrian Shield in northeastern Alberta shows the effects of extensive cataclastic deformation. The cataclastic rocks are distributed along three cata-elastic bands that range up to one mile in width and at least 25 m in length. Seven cataclastic rock-types have been distinguished and classified; a maximum of five occur within a single band. Quantitative data for both parents and derived rocks are presented in a study of the genesis of this group of rocks.
Metamorphic rocks crystallized under conditions ranging from the upper amphibelite to the hornblende granulite facies during the Hudsonian Orogeny. During the cooling history, cataclastic deformation along confined movement zones gradually dominated over plastic deformation and recrystallization; consequently, cataclastic textures are preserved in these zones.
Early and late phases of paracrystalline cataclasis are recognized; during the early phase, the major cataclastic bands were developed under conditions of upper to middle greenschist facies, and in the late phase of cataclasis movement was localized within the existing cataclastic bands under conditions of the lower greenschist facies. Post-crystalline deformation under essentially non-metamorphic conditions gave rise to longitudinal and transverse faults that locally offset the major cataclastic bands, and has produced typical shallow-level ruptural phenomena.
Mineralogical and chemical changes are observed across the transitional contacts from parent to derived rock. The mineralogical changes reflect the pressure-temperature conditions and hydrothermal alteration during cataclasis. Chemical changes are expressed as both systematic and irregular distributions of many components, particularly SiO2 and K2O; potash metasomatism and boron metasomatism were likely effective in some of the cataclastic rocks.
A magnetic-susceptibility study shows that regions of high aeromagnetic response are produced by a high magnetite content, and that decreased magnetic response is due to hematization of magnetite.
Individual cataclastic rock-types are shown to be megascopically, microscopically and chemically homogeneous over widely separated areas. Where two cata-elastic rock-types are in juxtaposition, a narrow zone of only slight mechanical mixing separates them. Therefore, careful field mapping permits the use of both parent and cataclastically derived rocks for regional lithologie correlation.
Combined field, petrographic, and chemical study has revealed the sequence of events in the metamorphic history of this part of the Precambrian Shield:
(i) synkinematic regional granitization and intrusion,
(ii) early paracrystalline cataclasis,
(iii) late kinematic intrusion,
(iv) late paracrystalline cataclasis, and
(v) post-crystalline cataclasis.