Open Collections

Origin and obduction of the ophiolitic Redfern Complex on the Omineca-Intermontane Belts boundary, western Cariboo Mountains, British Columbia

University of British Columbia

The Redfern Complex is a klippe of a dismembered, metamorphosed ophiolite resting on top of the crystalline Snowshoe Group in east-central British Columbia. The Snowshoe Group belongs to the Barkerville terrane of the Omineca Crystalline Belt while the Redfern Complex, which is structurally overlain by the Triassic black phyllite, comprises the basement to Quesnel terrane rocks of the Intermontane Belt. The contiguous boundary between the two belts lies west of the Redfern Complex and is marked by the Crooked Amphibolite, a thin, highly sheared mafic amphibolite with blocks of ultramafic rock near its base. The Redfern Complex and Crooked Amphibolite are correlative assemblages evidenced by similiarities in lithology, whole rock and mineral chemistry, and structural position. Both assemblages are composed of variable amounts of hornblende-epidote-palgioclase amphibolite, dunites, cumulate layered peridotite, and serpentinite. Chemically the rocks are subalkaline, tholeiitic basalts or gabbros and depleted peridotite with relict forsteritic olivine (Fo₈₃ and Fo[sub 87-90.7]), chromian diopside, and chromite spinel. Ultramafic rocks from both assemblages show evidence of plastic deformation under mantle conditions including disolcation glide on the high temperature (010): [100] slip system and glide climb in olivine, plus dynamic recrystallization and dynamic recovery. Although the structural succession of lithologies is best defined at the locality of the Redfern Complex, variably complete structural successions across the Crooked Amphibolite were observed which indicate that the two units occupy the same structural position on top of the Hadrynian to Paleozoic metasedimentary Snowshoe Group and that both units are structurally overlain by the Triassic black phyllite. The two assemblages differ in intensity of metamorphism and degree of hydration alteration related to their relative structural positions and their size and permiability. While the Crooked Amphibolite bears the chlorite-zone assemblage actinolite + chlorite + plagioclase and related ultramafic rocks are dominantly completely serpentinized, the Redfern amphibolite metamorphic assemblage of hornblende + albite + epidote belongs to the albite-epidote amphibolite facies. In addition the large, coherent Redfern peridotite body shows evidence of complete serpentinization only at its margins and also bears an amphibolite facies assemblage of tremolite + olivine + chlorite + talc. Thus the Redfern Complex experienced more intense metamorphism because it occupied a deeper structural level than the Crooked Amphibolite. The local and regional setting of the Redfern Complex reflects a complex geologic history of multiple defomration episodes and syntectonic, locally intense metamorphism related to the emplacement of the Complex onto the Snowshoe Group along the east-vergent Redfern thrust. Prior to emplecement, the Snowshoe Group underwent one enigmatic phase of deformation and metamorphism. The intrusion of the felsic Redfern orthogneiss which has a poorly constrained U-Pb age of Devono-Mississippian may be related to this deformation. A second orthogneiss which intrudes the Redfern amphibolite and contains ultramafic xenoliths probably intruded during or soon after emplacement. Tight, east-vergent folds and a pervasive second foliation developed in the Snowshoe Group while tight folds and a pervasive foliation developed in the Redfern amphibolite and Triassic black phyllite during emplacement. Microfracturing and grain sliding predominantly on the margins of the Redfern peridotite are the only structures developed during emplacement. Metamorphism peaked after emplacement stresses had relaxed, producing sillimanite in the Snowshoe metapelites, albite-epidote amphibolite assemblages in mafic lithologies, and possibly only kyanite in the black phyllites. Post-emplacement deformation accompanied sustained but decreased temperatures of metamorphism. Large-scale, west-vergent, overturned to the east buckles fold the Redfern thrust and refold earlier structures and produced the current map patterns and distribution of foliation. Kinked metamorphic porphyroblasts and an S₂ crenulation cleavage overgrown by staurolite in the Triassic black phyllite indicate that intense deformation and fluid enhanced retrograde metamorphic reactions were localised in a structurally lower "pocket" of this unit in the core of a post-emplacement synform on the east side of the map area.

Text

2010-08-22

For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.

http://hdl.handle.net/2429/27621

Geological Sciences

University of British Columbia

Graduate