Open Collections

Geology of the Three Ladies Mountain/Mount Stevenson area, Quesnel Highland, British Columbia

University of British Columbia

In late Proterozoic to early Paleozoic time, continent-derived clastic sediment and minor carbonate of the Snowshoe Group were deposited in a continental slope to shelf environment, and shallow marine elastics and carbonates of the Cariboo Group were deposited nearer to the shore of North America. The Snowshoe Group is divided into a lower sequence of micaceous quartzite, pelite, and minor amphibolite, all interlayered with quartz diorite sheets; and an upper sequence of micaceous quartzite, pelite, and carbonate with minor calc-silicate and amphibolitic rocks. Early isoclinal (F1A) and NE-verging tight folds (F1B) formed together with a metamorphic foliation. Tight to normal, cylindroidal second phase (F2) folds, characterized by SW-vergence and NW plunge, formed during the mid-Jurassic Columbian orogeny at about the same time as accretion of suspect terranes southwest of the map area. Prograde metamorphism in the Barrovian series of amphibolite facies was synkinematic to postkinematic to F2 folding, with maximum metamorphic recrystallization outlasting deformation. Garnet-biotite geothermometry indicates temperatures of 525 ± 20°C for pelites near the kyanite to sillimanite zone isograd. Garnet-aluminosilicate-ilmenite geobarometry limits P to less than 7 kb. Grossular-anorthite-aluminosi1icate geobarometry gives P = 5.5 ± 0.7 kb. Retrograde metamorphism and F3 kink-folding occurred during uplift, followed by broad warping (F4) with NE trend. The low-angle, postmetamorphic Little River Fault emplaced chlorite to biotite zone phyllite and carbonate of the Cariboo Group, in the hanging wall, against staurolite-kyanite to sillimanite schists and gneisses of the Snowshoe Group, in the footwall, with latest movement of the hanging wall in an ESE direction. A Rb-Sr model depositional age of approximately 750 Ma, assuming an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.708, was obtained for Snowshoe Group metasedimentary rocks. Paleozoic plutonism is indicated by a Rb-Sr whole-rock isochron date of 530 ± 94 Ma with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.706, and U-Pb dates on zircon, indicating a minimum age of 335 Ma and maximum age of about 450 Ma, for quartz dioritic gneiss intrusive into the Snowshoe Group early in its deformational history, pre-F1B folding. Late- to post-metamorphic pegmatite cooled through 400-500°C at 86 ± 3 Ma. The age of the Little River Fault is bracketed between intrusion of pegmatite and a Miocene(?) erosion surface.

Text

2010-06-16

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/2429/25788

Geological Sciences

University of British Columbia

Graduate