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Structure and metamorphism north of Quesnel Lake and east of Niagara Creek, Cariboo Mountains, British Columbia

University of British Columbia

An area of 15 km², located on the northeastern boundary of the Shuswap Metamorphic Complex, was mapped at a scale of 1:20,000 during the summer of 1983. A sequence of deformation and metamorphic events has been established based on petrography, structural analysis, geothermometry, and geobarometry. Four phases of deformation and related metamorphism have been recognized. Phase 1 deformation is preserved primarily as a metamorphic foliation. Rarely, isoclinal phase 1 folds of bedding were observed. Metamorphic conditions associated with this early period are not known. The major structures in the map area are second phase folds. These trend and plunge to the northwest and are southwestwardly verging. Minerals which crystallized during the main metamorphic event are syn- and post-kinematic to second phase structures. Garnet-biotite geothermometry yields temperatures of about 505°C in the kyanite-staurolite zone and - 580°C in the sillimanite zone, both at pressures of 5.0±.5 kbar. Geobarometry was based on the assemblage garnet-rutilealuminosilicate-ilmenite . Associated with phase 2 folding is movement along low-angle reverse faults, which disrupt both stratigraphy and established metamorphic zones. These faults are sheared horizons, distinguished by high strain fabrics, with extensive recrystallization. In one shear zone, garnet and biotite temperatures are systematically reset and indicate higher temperatures than in surrounding rocks. Further evidence of shear-heating includes an anomalous occurrence of sillimanite in this shear zone. Phase 3 deformation has produced upright, northerly-trending folds which post-date both metamorphic peak and reverse faulting. Third phase folds are broad and open. They are expressed strongly only at deeper structural levels. Mineral assemblages of lower greenschist facies were established prior to phase 4 deformation, and may pre-date phase 3 folds. Relatively high-angle faults disrupt phase 3 structures. In particular, the boundary of the Shuswap Metamorphic Complex, as defined by the establishment of the sillimanite zone (Reesor, 1970), is separated from adjacent kyanite-staurolite rocks to the north by such a fault. Geothermometry and geobarometry delineate a consistent temperature discontinuity of approximately 75°C across that fault. In addition to the faults, phase 4 kink folds are locally developed throughout the area.

Text

2010-05-11

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http://hdl.handle.net/2429/24583

Geological Sciences

University of British Columbia

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